US20160093823A1 - Light-emitting element, display device, electronic device, and lighting device - Google Patents
Light-emitting element, display device, electronic device, and lighting device Download PDFInfo
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- US20160093823A1 US20160093823A1 US14/868,724 US201514868724A US2016093823A1 US 20160093823 A1 US20160093823 A1 US 20160093823A1 US 201514868724 A US201514868724 A US 201514868724A US 2016093823 A1 US2016093823 A1 US 2016093823A1
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- H10K50/12—OLEDs or polymer light-emitting diodes [PLED] characterised by the electroluminescent [EL] layers comprising dopants
- H10K50/121—OLEDs or polymer light-emitting diodes [PLED] characterised by the electroluminescent [EL] layers comprising dopants for assisting energy transfer, e.g. sensitization
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Definitions
- One embodiment of the present invention relates to a light-emitting element in which a light-emitting layer capable of providing light emission by application of an electric field is provided between a pair of electrodes, and also relates to a display device, an electronic device, and a lighting device including the light-emitting element.
- one embodiment of the present invention is not limited to the above technical field.
- the technical field of one embodiment of the invention disclosed in this specification and the like relates to an object, a method, or a manufacturing method.
- One embodiment of the present invention relates to a process, a machine, manufacture, or a composition of matter.
- examples of the technical field of one embodiment of the present invention disclosed in this specification include a semiconductor device, a display device, a liquid crystal display device, a light-emitting device, a lighting device, a power storage device, a storage device, a method for driving any of them, and a method for manufacturing any of them.
- EL electroluminescence
- a display device using this light-emitting element has advantages such as high visibility, no necessity of a backlight, and low power consumption. Furthermore, such a light-emitting element also has advantages in that the element can be formed to be thin and lightweight, and that response time is high.
- a light-emitting element in which an EL layer containing an organic material as the light-emitting material is provided between a pair of electrodes (e.g., an organic EL element)
- application of a voltage between the pair of electrodes causes injection of electrons from the cathode and holes from the anode into the EL layer having a light-emitting property, and thus a current flows.
- the organic material having a light-emitting property is put in an excited state, whereby light emission is obtained from the excited organic compound having a light-emitting property.
- the excited state of an organic material can be a singlet excited state or a triplet excited state, and light emission from the singlet excited state (S 1 ) is referred to as fluorescence, and light emission from the triplet excited state (T 1 ) is referred to as phosphorescence.
- a light-emitting element containing a phosphorescent material has higher emission efficiency than a light-emitting element containing a fluorescent material. Therefore, a light-emitting element containing a phosphorescent material capable of converting the triplet excited state into light emission has been actively developed in recent years.
- TADF thermally activated delayed fluorescence
- Patent Document 3 discloses a method: in a light-emitting element containing a thermally activated delayed fluorescence substance and a material emitting fluorescence, singlet excitation energy of the thermally activated delayed fluorescence substance is transferred to the material emitting fluorescence and light emission is obtained from the material emitting fluorescence.
- An object of one embodiment of the present invention is to provide a light-emitting element having high emission efficiency which contains a fluorescent material as a light-emitting material. Another object of one embodiment of the present invention is to provide a light-emitting element with high reliability. Another object of one embodiment of the present invention is to provide a light-emitting element with high emission efficiency and high reliability. Another object of one embodiment of the present invention is to provide a novel light-emitting element. Another object of one embodiment of the present invention is to provide a novel light-emitting element with high emission efficiency and low power consumption.
- One embodiment of the present invention is a light-emitting element including a pair of electrodes and an EL layer between the pair of electrodes.
- the EL layer includes a first organic compound, a second organic compound, and a guest material.
- the first organic compound has a function of emitting a thermally activated delayed fluorescence at room temperature.
- the guest material has a function of emitting fluorescence.
- a HOMO of the first organic compound has an energy level higher than or equal to an energy level of a HOMO of the second organic compound.
- a LUMO of the first organic compound has an energy level lower than or equal to an energy level of a LUMO of the second organic compound.
- Another embodiment of the present invention is a light-emitting element including a pair of electrodes and an EL layer between the pair of electrodes.
- the EL layer includes a first organic compound, a second organic compound, and a guest material.
- the first organic compound has a function of emitting a thermally activated delayed fluorescence at room temperature.
- the guest material has a function of emitting fluorescence.
- An oxidation potential of the first organic compound is lower than or equal to an oxidation potential of the second organic compound.
- a reduction potential of the first organic compound is higher than or equal to a reduction of the second organic compound.
- a difference between a singlet excitation energy level of the first organic compound and a triplet excitation energy level of the first organic compound is preferably larger than 0 eV and smaller than or equal to 0.2 eV.
- the guest material preferably emits light.
- the first organic compound include a first ⁇ -electron deficient heteroaromatic skeleton and a first ⁇ -electron rich heteroaromatic skeleton and the second organic compound include a second ⁇ -electron deficient heteroaromatic skeleton and a second ⁇ -electron rich heteroaromatic skeleton.
- the first ⁇ -electron deficient heteroaromatic skeleton include a diazine skeleton or a triazine skeleton
- the first ⁇ -electron rich heteroaromatic skeleton include any one or more of an acridine skeleton, a phenoxazine skeleton, or a 3-(9-phenyl-9H-carbazole-3-yl)-9H-carbazole skeleton
- the second ⁇ -electron deficient heteroaromatic skeleton include a pyridine skeleton or a diazine skeleton
- the second ⁇ -electron rich heteroaromatic skeleton include any one or more of a furan skeleton, a thiophene skeleton, a fluorine skeleton, and a pyrrole skeleton.
- a weight ratio of the second organic compound to the first organic compound be from 1:0.05 to 1:0.5 (the second organic compound: the first organic compound) and a weight ratio of the second organic compound to the guest material be from 1:0.001 to 1:0.01 (the second organic compound: the guest material).
- Another embodiment of the present invention is a display device which includes the light-emitting element and a color filter, a sealant, or a transistor.
- Another embodiment of the present invention is an electronic device which includes the display device and a housing or a function of a touch sensor.
- Another embodiment of the present invention is a lighting device which includes the light-emitting element in the above embodiment and a housing or a touch sensor.
- One embodiment of the present invention makes it possible to provide a light-emitting element having high emission efficiency which contains a fluorescent material as a light-emitting material.
- One embodiment of the present invention makes it possible to provide a light-emitting element with high reliability.
- One embodiment of the present invention makes it possible to provide a light-emitting element with high emission efficiency and high reliability.
- One embodiment of the present invention makes it possible to provide a novel light-emitting element.
- One embodiment of the present invention makes it possible to provide a novel light-emitting element with high emission efficiency and low power consumption.
- FIGS. 1A and 1B are schematic cross-sectional views illustrating a light-emitting element of one embodiment of the present invention.
- FIGS. 2A to 2C show correlations of energy levels in a light-emitting element of one embodiment of the present invention.
- FIGS. 3A and 3B are a schematic cross-sectional view of a light-emitting element of one embodiment of the present invention and a diagram illustrating the correlation of energy levels in a light-emitting layer.
- FIGS. 4A and 4B are a schematic cross-sectional view of a light-emitting element of one embodiment of the present invention and a diagram illustrating the correlation of energy levels in a light-emitting layer.
- FIGS. 5A and 5B are a block diagram and a circuit diagram illustrating a display device of one embodiment of the present invention.
- FIGS. 6A and 6B are perspective views of an example of a touch panel of one embodiment of the present invention.
- FIGS. 7A to 7C are cross-sectional views of examples of a display device and a touch sensor of one embodiment of the present invention.
- FIGS. 8A and 8B illustrate examples of a touch panel of one embodiment of the present invention.
- FIGS. 9A and 9B are a block diagram and a timing chart of a touch sensor of one embodiment of the present invention.
- FIG. 10 is a circuit diagram illustrating a touch sensor of one embodiment of the present invention.
- FIG. 11 is a perspective view illustrating a display module of one embodiment of the present invention.
- FIGS. 12A to 12G illustrate electronic devices of one embodiment of the present invention.
- FIG. 13 illustrates a lighting device of one embodiment of the present invention.
- FIG. 14 is a schematic cross-sectional view illustrating light-emitting elements of Examples 1 and 2.
- FIG. 15 shows transient fluorescence characteristics of a host material of Example 1.
- FIG. 16 shows current efficiency-luminance characteristics of light-emitting elements of Example 1.
- FIG. 17 shows current-voltage characteristics of light-emitting elements of Example 1.
- FIG. 18 shows external quantum efficiency-luminance characteristics of light-emitting elements of Example 1.
- FIG. 19 shows electroluminescence spectra of light emitted from light-emitting elements of Example 1.
- FIG. 20 shows current efficiency-luminance characteristics of light-emitting elements of Example 2.
- FIG. 21 shows current-voltage characteristics of light-emitting elements Example 2.
- FIG. 22 shows external quantum efficiency-luminance characteristics of light-emitting elements Example 2.
- FIG. 23 shows electroluminescence spectra of light-emitting elements of Example 2.
- FIG. 24 shows current efficiency-luminance characteristics of light-emitting elements of Example 2.
- FIG. 25 shows current-voltage characteristics of light-emitting elements of Example 2.
- FIG. 26 shows external quantum efficiency-luminance characteristics of light-emitting elements of Example 2.
- FIG. 27 shows electroluminescence spectra of light-emitting elements of Example 2.
- the terms “film” and “layer” can be interchanged with each other depending on the case or circumstances.
- the term “conductive layer” can be changed into the term “conductive film” in some cases.
- the term “insulating film” can be changed into the term “insulating layer” in some cases.
- a singlet excited state means a singlet state with excited energy.
- An S 1 level means the lowest level of the singlet excitation energy, that is, the lowest level of excited energy in a singlet excited state.
- a triplet excited state means a triplet state with excited energy.
- a T 1 level means the lowest level of the triplet excitation energy, that is, the lowest level of excited energy in a triplet excited state.
- a singlet excited state and a singlet excitation energy level mean the lowest singlet excited state and the S 1 level, respectively, in some cases.
- a triplet excited state and a triplet excitation energy level mean the lowest singlet excited state and the T 1 level, respectively, in some cases.
- a fluorescent material refers to a material that emits light in the visible light region when the level of the lowest singlet excited state (S 1 level) relaxes to the ground state.
- a phosphorescent material refers to a material that emits light in the visible light region at room temperature when the level of the lowest triplet excited state (T 1 level) relaxes to the ground state. That is, a phosphorescent material refers to a material that can convert triplet excitation energy into visible light.
- a thermally activated delayed fluorescent substance is a material which can generate a singlet excited state from a triplet excited state by reverse intersystem crossing by thermal activation.
- the thermally activated delayed fluorescent substance may include a material which can generate a singlet excited state by itself from a triplet excited state by reverse intersystem crossing, for example, a material which emits TADF.
- the thermally activated delayed fluorescent substance may include a combination of two kinds of materials which form exciplexes.
- the thermally activated delayed fluorescent substance is a material of which a triplet excited state is close to a singlet excited state.
- a material in which the difference between the energy levels of the triplet excited state and the singlet excited state is larger than 0 eV and smaller than or equal to 0.2 eV is preferably used.
- the difference between the energy levels of the triplet excited state and the singlet excited state be larger than 0 eV and smaller than or equal to 0.2 eV in a material which can generate a singlet excited state by itself from a triplet excited state by reverse intersystem crossing, for example, a material which emits TADF, or it is preferable that the difference between the levels of the triplet excited state and the singlet excited state be larger than 0 eV and smaller than or equal to 0.2 eV in exciplexes.
- thermally activated delayed fluorescence emission energy refers to an emission peak (including a shoulder) on the shortest wavelength side of thermally activated delayed fluorescence.
- phosphorescence emission energy or a triplet excitation energy refers to a phosphorescence emission peak (including a shoulder) on the shortest wavelength side of phosphorescence emission. Note that the phosphorescence emission can be observed by time-resolved photoluminescence in a low-temperature (e.g., 10 K) environment.
- room temperature refers to a temperature in a range of 0° C. to 40° C.
- FIGS. 1A and 1B and FIGS. 2A to 2C a light-emitting element according to one embodiment of the present invention will be described with reference to FIGS. 1A and 1B and FIGS. 2A to 2C .
- FIGS. 1A and 1B a structure of a light-emitting element of one embodiment of the present invention will be described with reference to FIGS. 1A and 1B .
- a light-emitting element 150 includes an EL layer 100 between a pair of electrodes (an electrode 101 and an electrode 102 ).
- the EL layer 100 includes at least a light-emitting layer 120 .
- the electrode 101 is an anode and the electrode 102 is a cathode in this embodiment, they can be interchanged for the structure of the light-emitting element 150 .
- the EL layer 100 in FIG. 1A includes a functional layer in addition to the light-emitting layer 120 .
- the functional layer is composed of a hole-injection layer 111 , a hole-transport layer 112 , an electron-transport layer 118 , and an electron-injection layer 119 .
- the structure of the EL layer 100 is not limited to the structure illustrated in FIG. 1A , and at least one selected from the hole-injection layer 111 , the hole-transport layer 112 , the electron-transport layer 118 , and the electron-injection layer 119 is included.
- the EL layer 100 may include another functional layer which can reduce a barrier to hole or electron injection, enhance a hole/electron-transport property, inhibit a hole/electron transport property, suppress a quenching phenomenon due to an electrode, and the like.
- FIG. 1B is a schematic cross-sectional view of an example of the light-emitting layer 120 in FIG. 1A .
- the light-emitting layer 120 in FIG. 1B includes an organic compound 131 , an organic compound 132 , and a guest material 133 .
- a thermally activated delayed fluorescence substance is preferably used for the organic compound 131 .
- a thermally activated delayed fluorescence substance can convert triplet excitation energy into singlet excitation energy by reverse intersystem crossing. At least part of the triplet excitation energy generated in the light-emitting layer 120 is thus converted into singlet excitation energy by the organic compound 131 .
- the singlet excitation energy is transferred to the guest material 133 and then is extracted as fluorescent emission. For this reason, a difference in energy level between singlet excitation energy and triplet excitation energy of the organic compound 131 is preferably larger than 0 eV and smaller than or equal to 0.2 eV.
- the singlet excitation energy level of the organic compound 131 is preferably higher than the singlet excitation energy level of the guest material 133
- the triplet excitation energy level of the organic compound 131 is preferably higher than the singlet excitation energy level of the guest material 133 , in which case the triplet excitation energy level of the organic compound 131 can be closer to the singlet excitation energy level.
- a wide-bandgap material is preferably used for the organic compound 132 to prevent deactivation of the organic compound 131 and the guest material 133 .
- the singlet excitation energy level of the organic compound 132 is preferably higher than the singlet excitation energy level of the organic compound 131 and that of the guest material 133
- the triplet excitation energy level of the organic compound 132 is preferably higher than the triplet excitation energy level of the organic compound 131 and that of the guest material 133 .
- the light-emitting layer 120 may contain other compounds having a function similar to the organic compound 132 .
- the guest material 133 may be a light-emitting organic material, which preferably is capable of emitting fluorescence (hereinafter also referred to as a fluorescent material).
- a fluorescent material is used as the guest material 133.
- the guest material 133 may be referred to as the fluorescent material.
- the light-emitting element 150 of one embodiment of the present invention voltage application between the pair of electrodes (the electrodes 101 and 102 ) allows electrons and holes to be injected from the cathode and the anode, respectively, into the EL layer 100 and thus current flows. By recombination of the injected electrons and holes, the guest material 133 in the light-emitting layer 120 of the EL layer 100 is brought into an excited state to provide light emission.
- FIG. 2A is a schematic diagram showing the correlation between energy levels. Note that the following explains what terms and signs in FIG. 2A represent:
- Host 1 ( 131 ): the organic compound 131 ;
- Guest ( 133 ) the guest material 133 (fluorescent material);
- T A the level of the lowest triplet excitation energy of the organic compound 131 ;
- T H the level of the lowest triplet excitation energy of the organic compound 132 ;
- T G the level of the lowest triplet excitation energy of the guest material 133 (fluorescent material).
- carriers are recombined in the guest material 133 , and the guest material 133 is brought into an excited state.
- the excited state of the guest material 133 is a singlet excited state
- fluorescence is obtained.
- the excited state of the guest material 133 is a triplet excited state
- thermal deactivation occurs.
- FIG. 2B is a schematic diagram showing the correlation between energy levels. Note that indication and numerals in FIG. 2B are similar to those in FIG. 2A .
- Carriers are recombined in the organic compound 131 , and the organic compound 131 is brought into an excited state.
- the excited state of the organic compound 131 is a single excited state and S A of the organic compound 131 is higher than S G of the guest material 133
- the singlet excitation energy of the organic compound 131 is transferred from S A of the organic compound 131 to S G of the guest material 133 as shown by a route E 1 in FIG. 2B , whereby the guest material 133 is brought into the singlet excited state. Fluorescence is obtained from the guest material 133 in the singlet excited state.
- 1 A* and 1 G* represent the singlet excitation states of the organic compound 131 and the guest material 133 , respectively, and 1 A and 1 G represent the singlet ground states of the organic compound 131 and the guest material 133 , respectively.
- the first and second processes are represented by the following general formula (G2).
- 1 A* represents the triplet excitation state of the organic compound 131 ; 1 A* and 1 G* represent the singlet excitation states of the organic compound 131 and the guest material 133 , respectively; and 1 A and 1 G represent the singlet ground states of the organic compound 131 and the guest material 133 , respectively.
- the singlet excited state ( 1 A*) of the organic compound 131 is generated from the triplet excited state ( 3 A*) of the organic compound 131 , which is a thermally activated delayed fluorescence substance, by reverse intersystem crossing. Then, excitation energy is transferred to the singlet excited state ( 1 G*) of the guest material 133 .
- the emission efficiency of the light-emitting element is decreased.
- the emission efficiency is also decreased by a decrease in efficiency of A 1 , which is the previous process where the organic compound 131 is transferred from a triplet excited state to a singlet excited state by reverse intersystem crossing.
- the energy difference between T A and S A is large particularly when T A of the organic compound 131 is lower than T G of the guest material 133 and S A ⁇ S G >T G >T A is satisfied. As a result, the reverse intersystem crossing shown by the route A 1 in FIG.
- T A is preferably higher than T G , that is, emission energy of the organic compound 131 , which is a thermally activated delayed fluorescence substance, is preferably higher than phosphorescence emission energy of the guest material 133 .
- the excitation energy is thermally deactivated also when excitation energy is transferred from T A of the organic compound 131 to T G of the guest material 133 as shown by a route E 2 in FIG. 2B . It is thus preferable that the energy transfer process shown by the route E 2 in FIG. 2B be less likely to occur because the generation efficiency of the triplet excited state of the guest material 133 can be decreased and the occurrence of thermal deactivation of excitation energy can be reduced.
- the weight percentage of the guest material 133 is preferably smaller than that of the organic compound 131 .
- their weight ratio (the organic compound 131 :the guest material 133 ) is preferably from 1:0.001 to 1:0.05, further preferably from 1:0.001 to 1:0.01.
- the direct recombination process in the guest material 133 becomes dominant, the triplet excited state of the guest material 133 very likely to occur in the light-emitting layer to cause thermal deactivation of excited energy, resulting in a decreased emission efficiency. That is, it is preferable that the probability of the ( ⁇ ) energy transfer process from a thermally activated delayed fluorescence substance be higher than that of the (or) direct recombination process in a guest material because the generation efficiency of the triplet excited state of the guest material 133 can be decreased and the occurrence of thermal deactivation of excited energy when the excited state of the guest material 133 is a triplet excited state can be reduced.
- the weight percentage of the guest material 133 be smaller than that of the organic compound 131 .
- their weight ratio is preferably from 1:0.001 to 1:0.05, further preferably from 1:0.001 to 1:0.01.
- FIG. 2C is a schematic diagram showing the correlation between energy levels. Note that indication and numerals in FIG. 2C are similar to those in FIG. 2A .
- Carriers are recombined in the organic compound 132 , and the organic compound 132 is brought into an excited state.
- the singlet excitation energy is transferred from S H of the organic compound 132 to S G of the guest material 133 , whereby the guest material 133 is brought into the singlet excited state.
- the singlet excitation energy transferred from S H of the organic compound 132 to S A of the organic compound 131 is transferred to S G of the guest material 133 through the above-described ( ⁇ ) energy transfer process from a thermally activated delayed fluorescence substance. Fluorescence is obtained from the guest material 133 in the singlet excited state.
- the organic compound 132 in this embodiment is a host material.
- 1 H*, 1 A*, and 1 G* represent the singlet excitation states of the organic compound 132 , the organic compound 131 , and the guest material 133 , respectively; and 1 H, 1 A, and 1 G represent the singlet ground states of the organic compound 132 , the organic compound 131 , and the guest material 133 , respectively.
- the energy transfer process is expressed by the following general formula (G5).
- 3 H* and 3 A* represent the triplet excitation states of the organic compounds 132 and 131 , respectively; 1 A*, and 1 G* represent the singlet excitation states of the organic compound 131 and the guest material 133 , respectively; and 1 H, 1 A, and 1 G represent the singlet ground states of the organic compound 132 , the organic compound 131 , and the guest material 133 , respectively.
- the triplet excited state ( 3 A*) of the organic compound 131 is generated from the triplet excited state ( 3 H*) of the organic compound 132 .
- the singlet excited state ( 1 A*) of the organic compound 131 is generated by reverse intersystem crossing, and then, energy is transferred to the singlet excited state ( 1 G*) of the guest material 133 .
- both the triplet excitation energy and the singlet excitation energy of the organic compound 132 are efficiently converted into the singlet excited state ( 1 G*) of the guest material 133 , and emission from the guest material 133 is possible.
- the emission efficiency of the light-emitting element is decreased.
- the emission efficiency is also decreased by a decrease in efficiency of the route A 1 , which is the previous process where the organic compound 131 is transferred from a triplet excited state to a singlet excited state by reverse intersystem crossing.
- T H of the organic compound 132 is lower than T A of the organic compound 131 , energy transfer process from T H of the organic compound 132 to T A of the organic compound 131 is unlikely to occur and reverse intersystem crossing in the organic compound 131 does not occur, leading to a decrease in generation efficiency of a singlet excitation state of the guest material 133 .
- T H of the organic compound 132 is preferably higher than T A of the organic compound 131 .
- the excitation energy is transferred from the T H of the organic compound 132 to the T G of the guest material 133 as shown by a route E 3 in FIG. 2C . Therefore, it is preferable that the energy transfer process shown by the route E 3 in FIG. 2C be less likely to occur because the generation efficiency of the triplet excited state of the guest material 133 can be decreased and the occurrence of thermal deactivation can be reduced.
- the weight percentage of the guest material 133 is preferably smaller than that of the organic compound 132 .
- their weight ratio (the organic compound 132 :the guest material 133 ) is preferably from 1:0.001 to 1:0.05, further preferably from 1:0.001 to 1:0.01.
- the probability of the (0) energy transfer process from a thermally activated delayed fluorescence substance be higher than those of the ( ⁇ ) energy transfer process from a host material and the ( ⁇ ) direct recombination process in a guest material because the generation efficiency of the triplet excited state in the light-emitting layer 120 can be decreased, that is, the occurrence of thermal deactivation can be reduced and the emission efficiency of the light-emitting element 150 can be increased.
- Carrier recombination in the organic compound 131 which is a thermally activated delayed fluorescence substance, is important for increasing the probability of the ( ⁇ ) energy transfer process from a thermally activated delayed fluorescence substance.
- the relationship of energy level between the organic compound 131 and the organic compound 132 is important to generate carrier recombination in the organic compound 131 .
- the relationship of energy level between a highest occupied molecular orbital (HOMO) and a lowest unoccupied molecular orbital (LUMO), or the relationship between an oxidation potential and a reduction potential is important.
- Carriers injected from the pair of electrodes into the EL layer 100 reach the light-emitting layer 120 , whereby they are injected into a substance included in the light-emitting layer 120 . At this time, holes and electrons tend to enter more stable HOMO and LUMO, respectively.
- the HOMO level of the organic compound 131 is higher than or equal to the HOMO level of the organic compound 132 and that the LUMO level of the organic compound 131 is lower than or equal to the LUMO level of the organic compound 132 .
- the oxidation potential of the organic compound 131 is lower than or equal to the oxidation potential of the organic compound 132 and the reduction potential of the organic compound 131 is higher than or equal to the reduction potential of the organic compound 132 .
- the weight percentage of the organic compound 131 is smaller than that of the organic compound 132 for carrier recombination in the organic compound 131 in the light-emitting layer 120 .
- the weight percentage of the organic compound 131 is preferably smaller than that of the organic compound 132 .
- the weight percentage of the organic compound 131 is preferably smaller than that of the organic compound 132 in order to suppress the energy transfer.
- their weight ratio is preferably from 1:0.05 to 1:0.5.
- Förster mechanism energy transfer does not require direct contact between molecules and energy is transferred through a resonant phenomenon of dipolar oscillation between the organic compound 131 and the guest material 133 .
- the organic compound 131 provides energy to the guest material 133 , and thus, the organic compound 131 in an excited state is put in a ground state and the guest material 133 in a ground state is put in an excited state.
- the rate constant k h* ⁇ g of Förster mechanism is expressed by Formula 1.
- ⁇ denotes a frequency
- f′ h ( ⁇ ) denotes a normalized emission spectrum of the organic compound 131 (a fluorescent spectrum in energy transfer from a singlet excited state, and a phosphorescent spectrum in energy transfer from a triplet excited state)
- ⁇ g ( ⁇ ) denotes a molar absorption coefficient of the guest material 133
- N denotes Avogadro's number
- n denotes a refractive index of a medium
- R denotes an intermolecular distance between the organic compound 131 and the guest material 133
- ⁇ denotes a measured lifetime of an excited state (fluorescence lifetime or phosphorescence lifetime)
- c denotes the speed of light
- ⁇ denotes a luminescence quantum yield (a fluorescence quantum yield in energy transfer from a singlet excited state, and a phosphorescence quantum yield in energy transfer from a triplet excited state)
- K 2 denotes a coefficient (0 to 4) of
- the organic compound 131 and the guest material 133 are close to a contact effective range where their orbitals overlap, and the organic compound 131 in an excited state and the guest material 133 in a ground state exchange their electrons, which leads to energy transfer.
- the rate constant k h* ⁇ g of Dexter mechanism is expressed by Formula 2.
- h denotes a Planck constant
- K denotes a constant having an energy dimension
- ⁇ denotes a frequency
- f′ h ( ⁇ ) denotes a normalized emission spectrum of the organic compound 131 (a fluorescent spectrum in energy transfer from a singlet excited state, and a phosphorescent spectrum in energy transfer from a triplet excited state)
- ⁇ ′ g ( ⁇ ) denotes a normalized absorption spectrum of the guest material 133
- L denotes an effective molecular radius
- R denotes an intermolecular distance between the organic compound 131 and the guest material 133 .
- the efficiency of energy transfer from the organic compound 131 to the guest material 133 is expressed by Formula 3.
- k r denotes a rate constant of a light-emission process (fluorescence in energy transfer from a singlet excited state, and phosphorescence in energy transfer from a triplet excited state) of the organic compound 131
- k n denotes a rate constant of a non-light-emission process (thermal deactivation or intersystem crossing) of the organic compound 131
- r denotes a measured lifetime of an excited state of the organic compound 131 .
- an energy transfer by Dexter mechanism is considered.
- an emission spectrum of the organic compound 131 here, a fluorescent spectrum because energy transfer from a singlet excited state is discussed
- an absorption spectrum of the guest material 133 absorption corresponding to transition from a singlet ground state to a singlet excited state
- the energy transfer efficiency can be optimized by making the emission spectrum of the organic compound 131 overlap with the absorption band of the guest material 133 which is on the longest wavelength side.
- one embodiment of the present invention provides a light-emitting element which includes the organic compound 131 having a function as an energy donor capable of efficiently transferring energy to the guest material 133 .
- the organic compound 131 is a thermally activated delayed fluorescence substance and thus has a feature that the singlet excitation energy level and the triplet excitation energy level are close to each other. Specifically, it is preferable that the organic compound 131 have a difference of larger than 0 eV and smaller than or equal to 0.2 eV between the singlet excitation energy level and the triplet excitation energy level. This enables transition (reverse intersystem crossing) of the organic compound 131 from the triplet excited state to the singlet excited state to be likely to occur.
- the generation efficiency of the singlet excited state of the organic compound 131 can be increased. Furthermore, in order to facilitate energy transfer from the singlet excited state of the organic compound 131 to the singlet excited state of the guest material 133 having a function as an energy acceptor, it is preferable that the emission spectrum of the organic compound 131 overlap with the absorption band of the guest material 133 which is on the longest wavelength side. Thus, the generation efficiency of the singlet excited state of the guest material 133 can be increased.
- the HOMO level of the organic compound 131 is higher than or equal to the HOMO level of the organic compound 132 , and the LUMO level of the organic compound 131 is lower than or equal to the LUMO level of the organic compound 132 ; or the oxidation potential of the organic compound 131 is lower than or equal to the oxidation potential of the organic compound 132 and the reduction potential of the organic compound 131 is higher than or equal to the reduction potential of the organic compound 132 , which allows recombination of carriers injected into the EL layer 100 to be performed efficiently in the organic compound 131 .
- the occurrence of thermal deactivation can be reduced and the emission efficiency can be increased.
- the organic compound 131 in the light-emitting layer 120 is composed of one kind of material. Note that another compound having a function similar to the organic compound 131 may be included in the light-emitting layer 120 . For example, in the case where the organic compound 131 is composed of one kind of material, any of the following materials can be used.
- a fullerene, a derivative thereof, an acridine derivative such as proflavine, eosin, or the like can be given.
- a metal-containing porphyrin such as a porphyrin containing magnesium (Mg), zinc (Zn), cadmium (Cd), tin (Sn), platinum (Pt), indium (In), or palladium (Pd), can be given.
- Examples of the metal-containing porphyrin include a protoporphyrin-tin fluoride complex (SnF 2 (Proto IX)), a mesoporphyrin-tin fluoride complex (SnF 2 (Meso IX)), a hematoporphyrin-tin fluoride complex (SnF 2 (Hemato IX)), a coproporphyrin tetramethyl ester-tin fluoride complex (SnF 2 (Copro III-4Me)), an octaethylporphyrin-tin fluoride complex (SnF 2 (OEP)), an etioporphyrin-tin fluoride complex (SnF 2 (Etio I)), and an octaethylporphyrin-platinum chloride complex (PtCl 2 (OEP)), which are shown in the following structural formulae.
- SnF 2 Proto IX
- a heterocyclic compound having a ⁇ -electron rich heteroaromatic ring and a ⁇ -electron deficient heteroaromatic ring such as 2-(biphenyl-4-yl)-4,6-bis(12-phenylindolo[2,3-a]carbazol-11-yl)-1,3,5-triazine (abbreviation: PIC-TRZ), 2- ⁇ 4-[3 (N-phenyl-9H-carbazol-3-yl)-9H-carbazol-9-yl]phenyl ⁇ -4,6-diphenyl-1,3,5-triazine (abbreviation: PCCzPTzn), 2-[4-(10H-phenoxazine-0-yl)phenyl]-4,6-diphenyl-1,3,5-triazine (abbreviation: PXZ-TRZ), 3-[4-(5-phenyl-5,10-dihydrophenazine-10-yl)phen
- the heterocyclic compound is preferably used because of the ⁇ -electron rich heteroaromatic ring and the ⁇ -electron deficient heteroaromatic ring, for which the electron-transport property and the hole-transport property are high.
- a substance in which the ⁇ -electron rich heteroaromatic ring is directly bonded to the ⁇ -electron deficient heteroaromatic ring is particularly preferably used because the donor property of the ⁇ -electron rich heteroaromatic ring and the acceptor property of the ⁇ -electron deficient heteroaromatic ring are both increased and the difference between the level of the singlet excited state and the level of the triplet excited state becomes small.
- the heterocyclic compound is preferably used because of the ⁇ -electron rich heteroaromatic ring and the ⁇ -electron deficient heteroaromatic ring, for which the electron-transport property and the hole-transport property are high.
- skeletons having the ⁇ -electron deficient heteroaromatic ring a diazine skeleton (a pyrimidine skeleton, a pyrazine skeleton, or a pyridazine skeleton) and a triazine skeleton have favorable stability and reliability and are particularly preferable.
- an acridine skeleton, a phenoxazine skeleton, or a 3-(9-phenyl-9H-carbazole-3-yl)-9H-carbazole skeleton has favorable stability and reliability; thus, any of the skeletons is particularly preferable.
- a substance in which the ⁇ -electron rich heteroaromatic ring is directly bonded to the ⁇ -electron deficient heteroaromatic ring is particularly preferably used because the donor property of the ⁇ -electron rich heteroaromatic ring and the acceptor property of the ⁇ -electron deficient heteroaromatic ring are both increased and the difference between the level of the singlet excited state and the level of the triplet excited state becomes small.
- the following compounds can be used as the organic compound 132 in the light-emitting layer 120 .
- the organic compound 132 functions as a host material in the light-emitting layer 120 , it preferably contains a skeleton which easily receives electrons (a skeleton having an electron-transport property) and/or a skeleton which easily receives holes (a skeleton having an hole-transport property).
- a compound, a metal complex, or the like including a ⁇ -electron deficient heteroaromatic skeleton can be used.
- a metal complex such as bis(10-hydroxybenzo[h]quinolinato)beryllium(II) (abbreviation: BeBq 2 ), bis(2-methyl-8-quinolinolato) (4-phenylphenolato)aluminum(III) (abbreviation: BAlq), bis(8-quinolinolato)zinc(II) (abbreviation: Znq), bis[2-(2-benzoxazolyl)phenolato]zinc(II) (abbreviation: ZnPBO), or bis[2-(2-benzothiazolyl)phenolato]zinc(II) (abbreviation: ZnBTZ); a heterocyclic compound having an azole skeleton such as 2-
- the heterocyclic compounds having diazine skeletons (pyrimidine, pyrazine, pyridazine) or having a pyridine skeleton are highly reliable and stable and is thus preferably used.
- the heterocyclic compounds having the skeletons have a high electron-transport property to contribute to a reduction in drive voltage.
- a compound having a skeleton which easily accepts holes (a skeleton having a hole-transport property)
- a compound having a ⁇ -electron rich heteroaromatic skeleton, an aromatic amine skeleton, or the like can be favorably used.
- Specific examples include a compound having an aromatic amine skeleton such as 2-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]spiro-9,9′-bifluorene (abbreviation: PCASF), 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB), N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine (abbreviation: TPD), 4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl (abbreviation: BSPB), 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine (abbreviation: BPAFLP), 4-phenyl
- PCBBiF N-(1,1′-biphenyl-4-yl)-9,9-dimethyl-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-9H-fluoren-2-amine
- PCBBiF a compound having a carbazole skeleton such as 1,3-bis(N-carbazolyl)benzene (abbreviation: mCP), 4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP), 3,6-bis(3,5-diphenylphenyl)-9-phenylcarbazole (abbreviation: CzTP), 3,6-di(9H-carbazol-9-yl)-9-phenyl-9H-carbazole (abbreviation: PhCzGI), 2,8-di(9H-carbazol-9-yl)-dibenzothiophene (abbreviation: Cz2DBT), or 9
- a compound including any one or more of a furan skeleton, a thiophene skeleton, a fluorine skeleton, and a pyrrole skeleton is preferable because it is stable and reliable and has a high hole-transport property to contribute to a reduction in driving voltage.
- a compound including a pyridine skeleton or a diazine skeleton (a pyrimidine skeleton, a pyrazine skeleton, and a pyridazine skeleton) as a ⁇ -electron deficient heteroaromatic skeleton and including any one or more of a furan skeleton, a thiophene skeleton, a fluorine skeleton, and a pyrrole skeleton as a ⁇ -electron rich heteroaromatic skeleton has a high carrier-transport property and thus contributes to a reduction in driving voltage.
- a compound including any of the skeletons has favorable reliability and thus is preferable. Note that an indole skeleton, a carbazole skeleton, or the 3-(9H-carbazol-9-yl)-9H-carbazole skeleton is particularly preferable as a pyrrole skeleton.
- the above-described compounds are non-limiting examples of the organic compounds 131 and 132 , and other materials may be used as long as they can transport carriers and they satisfy the following conditions: the HOMO level of the organic compound 131 is higher than or equal to the HOMO level of the organic compound 132 , and the LUMO level of the organic compound 131 is lower than or equal to the LUMO level of the organic compound 132 ; or the oxidation potential of the organic compound 131 is lower than or equal to the oxidation potential of the organic compound 132 and the reduction potential of the organic compound 131 is higher than or equal to the reduction potential of the organic compound 132 .
- a thermally activated delayed fluorescence substance may be used for the organic compound 132 .
- Table 1 shows measurement results of HOMO and LUMO levels of the above-described compounds in the thin-film state, which are non-limiting examples of the organic compounds 131 and 132 .
- Table 2 shows measurement results of the oxidation potentials and the reduction potentials of the compounds in the solution state and the HOMO and LUMO levels estimated from the results.
- Table 3 shows measurement results of the triplet excitation energy levels.
- the ionization potential of each compound was measured by a photoelectron spectrometer (AC-3, manufactured by Riken Keiki, Co., Ltd.) in the air, and the measured ionization potentials were converted into negative values.
- AC-3 photoelectron spectrometer
- an absorption spectrum of each compound in the thin-film state was measured and the absorption edge was obtained from Tauc plot with an assumption of direct transition.
- the LUMO energy in the thin-film state was calculated from the energy of the estimated bandgaps and the HOMO levels, which have been obtained.
- Electrochemical characteristics (oxidation and reduction characteristics) of each compound in the solution state were measured by cyclic voltammetry (CV). Note that an electrochemical analyzer (ALS model 600A or 600C, product of BAS Inc.) was used for the measurement. In the measurements, the potential of a working electrode with respect to the reference electrode was changed within an appropriate range, whereby the oxidation peak potential and the reduction peak potential were obtained. In addition, the HOMO and LUMO levels of each compound were calculated from the estimated redox potential of the reference electrode of ⁇ 4.94 eV and the obtained peak potentials.
- CV cyclic voltammetry
- the triplet excitation energy levels were measured by phosphorescence measurement of the compounds. The measurement was performed by using a PL microscope, LabRAM HR-PL, produced by HORIBA, Ltd., a He—Cd laser (325 nm) as excitation light, and a CCD detector at a measurement temperature of 10 K. The triplet excitation energy levels were calculated from a peak on the shortest wavelength side of the phosphorescent spectrum obtained by the measurement.
- the HOMO level of the organic compound 131 is higher than or equal to the HOMO level of the organic compound 132 , and the LUMO level of the organic compound 131 is lower than or equal to the LUMO level of the organic compound 132 ; or the oxidation potential of the organic compound 131 is lower than or equal to the oxidation potential of the organic compound 132 and the reduction potential of the organic compound 131 is higher than or equal to the reduction potential of the organic compound 132 , which are shown in Table 1 and Table 2 as an example, recombination of carriers injected into the EL layer 100 can be efficiently performed in the organic compound 131 and thus a light-emitting element with high emission efficiency can be provided.
- the guest material 133 is preferably, but not particularly limited to, an anthracene derivative, a tetracene derivative, a chrysene derivative, a phenanthrene derivative, a pyrene derivative, a perylene derivative, a stilbene derivative, an acridone derivative, a coumarin derivative, a phenoxazine derivative, a phenothiazine derivative, or the like, and for example, any of the following materials can be used.
- the examples include 5,6-bis[4-(10-phenyl-9-anthryl)phenyl]-2,2′-bipyridine (abbreviation: PAP2BPy), 5,6-bis[4′-(10-phenyl-9-anthryl)biphenyl-4-yl]-2,2′-bipyridine (abbreviation: PAPP2BPy), N,N′-diphenyl-N,N′-bis[4-(9-phenyl-9H-fluoren-9-yl)phenyl]pyrene-1,6-diamine (abbreviation: 1,6FLPAPrn), N,N′-bis(3-methylphenyl)-N,N-bis[3-(9-phenyl-9H-fluoren-9-yl)phenyl]pyrene-1,6-diamine (abbreviation: 1,6mMemFLPAPrn), N,N′-bis[4-(9H-carbazol-9-yl
- the above-described materials of the guest material 133 are non-limiting examples, other materials may be used as long as light emission (thermally activated delayed fluorescence) of the organic compound 131 which is an energy donor overlaps with an absorption band (absorption corresponding to the transition of the guest material 133 from the singlet ground state to the singlet excited state) on the longest wavelength in an absorption spectrum of the guest material 133 which is an energy accepter.
- the light-emitting layer 120 can be formed by an evaporation method (including a vacuum evaporation method), an inkjet method, a coating method, gravure printing, or the like.
- the electrode 101 and the electrode 102 have functions of injecting holes and electrons into the light-emitting layer 120 .
- the electrodes 101 and 102 can be formed using a metal, an alloy, or a conductive compound, or a mixture or a stack thereof, for example.
- a typical example of the metal is aluminum; besides, a transition metal such as silver, tungsten, chromium, molybdenum, copper, or titanium, an alkali metal such as lithium, sodium, or cesium, or a Group 2 metal such as calcium or magnesium can be used.
- a rare earth metal such as ytterbium (Yb) may be used.
- an alloy containing any of the above metals can be used as the alloy, and MgAg and AlLi can be given as examples.
- a metal oxide such as indium oxide-tin oxide (indium tin oxide) can be given. It is also possible to use an inorganic carbon-based material such as graphene as the conductive compound.
- the electrode 101 and/or the electrode 102 may be formed by stacking two or more of these materials.
- the electrode 101 and/or the electrode 102 transmits visible light.
- the electrode 101 and/or the electrode 102 is formed to a thickness that is thin enough to transmit visible light (e.g., a thickness of 1 nm to 10 nm).
- the hole-injection layer 111 has a function of reducing a barrier for hole injection from the electrode 101 to promote hole injection and is formed using a transition metal oxide, a phthalocyanine derivative, or an aromatic amine, for example.
- a transition metal oxide molybdenum oxide, vanadium oxide, ruthenium oxide, tungsten oxide, manganese oxide, or the like can be given.
- phthalocyanine derivative phthalocyanine, metal phthalocyanine, or the like can be given.
- aromatic amine a benzidine derivative, a phenylenediamine derivative, or the like can be given.
- a high molecular compound such as polythiophene or polyaniline; a typical example thereof is poly(ethylenedioxythiophene)/poly(styrenesulfonic acid), which is self-doped polythiophene.
- a layer containing a composite material of a hole-transport material and a material having a property of accepting electrons from the hole-transport material can also be used.
- a stack of a layer containing a material having an electron accepting property and a layer containing a hole-transport material may also be used. In a steady state or in the presence of an electric field, electric charge can be transferred between these materials.
- organic acceptors such as a quinodimethane derivative, a chloranil derivative, and a hexaazatriphenylene derivative can be given.
- a specific example is a compound having an electron-withdrawing group (a halogen group or a cyano group), such as 7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation: F 4 -TCNQ), chloranil, or 2,3,6,7,10,11-hexacyano-1,4,5,8,9,12-hexaazatriphenylene (abbreviation: HAT-CN).
- a transition metal oxide such as an oxide of a metal from Group 4 to Group 8 can also be used.
- vanadium oxide, niobium oxide, tantalum oxide, chromium oxide, molybdenum oxide, tungsten oxide, manganese oxide, rhenium oxide, or the like can be used.
- molybdenum oxide is more preferable because of its stability in the atmosphere, low hygroscopic property, and easiness of handling.
- a material having a property of transporting more holes than electrons can be used as the hole-transport material, and a material having a hole mobility of 1 ⁇ 10 ⁇ 6 cm 2 /Vs or higher is preferable.
- a material having a hole mobility of 1 ⁇ 10 ⁇ 6 cm 2 /Vs or higher is preferable.
- an aromatic amine, a carbazole derivative, an aromatic hydrocarbon, a stilbene derivative, or the like can be used.
- the compound including a skeleton that easily accepts holes which is described as an example of the organic compound 132 can be used.
- the hole-transport material may be a high molecular compound.
- the hole-transport layer 112 is a layer containing a hole-transport material and can be formed using any of the materials given as examples of the material of the hole-injection layer 111 .
- the HOMO level of the hole-transport layer 112 is preferably equal or close to the HOMO energy level of the hole-injection layer 111 .
- the electron-transport layer 118 has a function of transporting, to the light-emitting layer 120 , electrons injected from the electrode 102 through the electron-injection layer 119 .
- a material having a property of transporting more electrons than holes can be used as an electron-transport material, and a material having an electron mobility of 1 ⁇ 10 ⁇ 6 cm 2 /Vs or higher is preferable.
- Specific examples include a metal complex having a quinoline ligand, a benzoquinoline ligand, an oxazole ligand, or a thiazole ligand; an oxadiazole derivative; a triazole derivative; a phenanthroline derivative; a pyridine derivative; and a bipyridine derivative.
- the compound having a skeleton that easily accepts electrons which is described as an example of the organic compound 132 can be used.
- the electron-injection layer 119 has a function of reducing a barrier for electron injection from the electrode 102 to promote electron injection and can be formed using a Group 1 metal or a Group 2 metal, or an oxide, a halide, or a carbonate of any of the metals, for example.
- a composite material containing an electron-transport material (described above) and a material having a property of donating electrons to the electron-transport material can also be used.
- the material having an electron-donating property a Group 1 metal, a Group 2 metal, an oxide of any of the metals, or the like can be given.
- the hole-injection layer 111 , the hole-transport layer 112 , the electron-transport layer 118 , and the electron-injection layer 119 described above can each be formed by an evaporation method (including a vacuum evaporation method), an inkjet method, a coating method, a gravure printing method, or the like.
- an inorganic compound or a high molecular compound e.g., an oligomer, a dendrimer, or a polymer
- an inorganic compound or a high molecular compound e.g., an oligomer, a dendrimer, or a polymer
- the hole-injection layer 111 the hole-transport layer 112 , the light-emitting layer 120 , the electron-transport layer 118 , and the electron-injection layer 119 .
- the light-emitting element 150 is fabricated over a substrate of glass, plastic, or the like. As the way of stacking layers over the substrate, layers may be sequentially stacked from the electrode 101 side or sequentially stacked from the electrode 102 side.
- glass, quartz, plastic, or the like can be used for the substrate over which the light-emitting element 150 can be formed.
- a flexible substrate can be used.
- the flexible substrate is a substrate that can be bent, such as a plastic substrate made of polycarbonate or polyarylate, for example.
- a film, an inorganic film formed by evaporation, or the like can also be used. Note that materials other than these can be used as long as they can function as a support in a manufacturing process of the light-emitting element and an optical element or as long as they have a function of protecting the light-emitting element and the optical element.
- the light-emitting element 150 can be formed using a variety of substrates, for example.
- the type of substrate is not limited to a certain type.
- a semiconductor substrate e.g., a single crystal substrate or a silicon substrate
- SOI substrate SOI substrate
- glass substrate e.g., a glass substrate, a quartz substrate, a plastic substrate, a metal substrate, a stainless steel substrate, a substrate including stainless steel foil, a tungsten substrate, a substrate including tungsten foil, a flexible substrate, an attachment film, paper including a fibrous material, a base material film, or the like
- the glass substrate include a barium borosilicate glass substrate, an aluminoborosilicate glass substrate, and a soda lime glass substrate.
- Examples of the flexible substrate, the attachment film, the base film, and the like are substrates of plastics typified by polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polyether sulfone (PES), and polytetrafluoroethylene (PTFE).
- PET polyethylene terephthalate
- PEN polyethylene naphthalate
- PES polyether sulfone
- PTFE polytetrafluoroethylene
- Another example is a resin such as acrylic.
- Other examples are polypropylene, polyester, polyvinyl fluoride, polyvinyl chloride, and the like.
- Other examples are polyamide, polyimide, aramid, epoxy, an inorganic film formed by evaporation, paper, and the like.
- a flexible substrate may be used as the substrate, and the light-emitting element may be provided directly on the flexible substrate.
- a separation layer may be provided between the substrate and the light-emitting element. The separation layer can be used when part or the whole of the light-emitting element formed over the separation layer is completed, separated from the substrate, and transferred to another substrate. In such a case, the light-emitting element can be transferred to a substrate having low heat resistance or a flexible substrate as well.
- a stack including inorganic films, which are a tungsten film and a silicon oxide film, or a resin film of polyimide or the like formed over a substrate can be used, for example.
- the light-emitting element may be transferred to another substrate.
- a substrate to which the light-emitting element is transferred include, in addition to the above-described substrates, a cellophane substrate, a stone substrate, a wood substrate, a cloth substrate (including a natural fiber (e.g., silk, cotton, or hemp), a synthetic fiber (e.g., nylon, polyurethane, or polyester), a regenerated fiber (e.g., acetate, cupra, rayon, or regenerated polyester, or the like), a leather substrate, and a rubber substrate.
- a light-emitting element with high durability, a light-emitting element with high heat resistance, a lightweight light-emitting element, or a thin light-emitting element can be obtained.
- the light-emitting element 150 may be formed over an electrode electrically connected to a field-effect transistor (FET), for example, which is formed over the above-mentioned substrate, so that an active matrix display device in which the FET controls the drive of the light-emitting element 150 can be manufactured.
- FET field-effect transistor
- one embodiment of the present invention has been described. Embodiments of the present invention are described in other embodiments. Note that one embodiment of the present invention is not limited to the above examples. For example, one embodiment of the present invention is not limited to the above-described example in which the HOMO level of the organic compound 131 is higher than or equal to the HOMO level of the organic compound 132 and the LUMO level of the organic compound 131 is lower than or equal to the LUMO level of the organic compound 132 , and the example in which the oxidation potential of the organic compound 131 is lower than or equal to the oxidation potential of the organic compound 132 and the reduction potential of the organic compound 131 is higher than or equal to the reduction potential of the organic compound 132 .
- the HOMO level of the organic compound 131 is not necessarily higher than or equal to the HOMO level of the organic compound 132
- the LUMO level of the organic compound 131 is not necessarily lower than or equal to the LUMO level of the organic compound 132
- the oxidation potential of the organic compound 131 is not necessarily lower than or equal to the oxidation potential of the organic compound 132
- the reduction potential of the organic compound 131 is not necessarily higher than or equal to the reduction potential of the organic compound 132 .
- one embodiment of the present invention is not limited to the above-described example in which the organic compound 131 is a substance which exhibits thermally activated delayed fluorescence at room temperature.
- the organic compound 131 in one embodiment of the present invention may contain a substance other than the substance which exhibits thermally activated delayed fluorescence at room temperature, for example.
- the organic compound 131 in one embodiment of the present invention does not necessarily contain the substance which exhibits thermally activated delayed fluorescence at room temperature, for example.
- one embodiment of the present invention is not limited to the above example in which the weight percentage of the organic compound 131 is smaller than that of the organic compound 132 .
- the weight percentage of the organic compound 131 is not limited to be smaller than that of the organic compound 132 .
- a light-emitting element having a structure different from that described in Embodiment 1 and an emission mechanism of the light-emitting element will be described below with reference to FIGS. 3A and 3B .
- FIG. 3A is a schematic cross-sectional view of a light-emitting element 450 .
- the light-emitting element 450 illustrated in FIG. 3A includes a plurality of light-emitting units (in FIG. 3A , a light-emitting unit 441 and a light-emitting unit 442 ) between a pair of electrodes (an electrode 401 and an electrode 402 ).
- One light-emitting unit has the same structure as the EL layer 100 illustrated in FIG. 1A . That is, the light-emitting element 150 in FIG. 1A includes one light-emitting unit, while the light-emitting element 450 includes the plurality of light-emitting units.
- the electrode 401 functions as an anode and the electrode 402 functions as a cathode in the following description of the light-emitting element 450 ; however, the functions may be interchanged in the light-emitting element 450 .
- the light-emitting unit 441 and the light-emitting unit 442 are stacked, and a charge-generation layer 445 is provided between the light-emitting unit 441 and the light-emitting unit 442 .
- the light-emitting unit 441 and the light-emitting unit 442 may have the same structure or different structures.
- the EL layer 100 illustrated in FIGS. 1A and 1B be used in the light-emitting unit 441 and that a light-emitting layer containing a phosphorescent material as a light-emitting material be used in the light-emitting unit 442 .
- the light-emitting element 450 includes a light-emitting layer 443 and a light-emitting layer 444 .
- the light-emitting unit 441 includes a hole-injection layer 411 , a hole-transport layer 412 , an electron-transport layer 413 , and an electron-injection layer 414 in addition to the light-emitting layer 443 .
- the light-emitting unit 442 includes a hole-injection layer 415 , a hole-transport layer 416 , an electron-transport layer 417 , and an electron-injection layer 418 in addition to the light-emitting layer 444 .
- the charge-generation layer 445 preferably contains a composite material of an organic material and a material having an electron accepting property.
- the composite material the composite material that can be used for the hole-injection layer 111 described in Embodiment 1 may be used.
- the organic material a variety of compounds such as an aromatic amine compound, a carbazole compound, an aromatic hydrocarbon, and a high molecular compound (such as an oligomer, a dendrimer, or a polymer) can be used.
- An organic material having a hole mobility of 1 ⁇ 10 ⁇ 6 cm 2 /Vs or higher is preferably used. Note that any other material may be used as long as it has a property of transporting more holes than electrons.
- the composite material of an organic material and a material having an electron accepting property has excellent carrier-injection and carrier-transport properties, low-voltage driving or low-current driving can be realized.
- the charge-generation layer 445 can also serve as a hole-injection layer or a hole-transport layer of the light-emitting unit; thus, a hole-injection layer or a hole-transport layer does not need to be included in the light-emitting unit.
- the charge-generation layer 445 may have a stacked-layer structure of a layer containing the composite material of an organic material and a material having an electron accepting property and a layer containing another material.
- the charge-generation layer 445 may be formed using a combination of a layer containing the composite material of an organic material and a material having an electron accepting property with a layer containing one material selected from among materials having an electron donating and a compound having a high electron-transport property.
- the charge-generation layer 445 may be formed using a combination of a layer containing the composite material of an organic material and a material having an electron accepting property with a layer including a transparent conductive film.
- the charge-generation layer 445 provided between the light-emitting unit 441 and the light-emitting unit 442 may have any structure as long as electrons can be injected to the light-emitting unit on one side and holes can be injected into the light-emitting unit on the other side when a voltage is applied between the electrode 401 and the electrode 402 .
- the charge-generation layer 445 injects electrons into the light-emitting unit 441 and holes into the light-emitting unit 442 when a voltage is applied such that the potential of the electrode 401 is higher than that of the electrode 402 .
- the light-emitting element having two light-emitting units is described with reference to FIG. 3A ; however, a similar structure can be applied to a light-emitting element in which three or more light-emitting units are stacked.
- a plurality of light-emitting units partitioned by the charge-generation layer between a pair of electrodes as in the light-emitting element 450 it is possible to provide a light-emitting element which can emit light with high luminance with the current density kept low and has a long lifetime.
- a light-emitting element with low power consumption can be provided.
- the light-emitting layer 443 contains an organic compound 421 , an organic compound 422 , and a guest material 423 .
- the light-emitting layer 444 contains an organic compound 431 , an organic compound 432 , and a guest material 433 .
- the light-emitting layer 443 has a structure similar to that of the light-emitting layer 120 in FIGS. 1A and 1B . That is, the organic compound 421 , the organic compound 422 , and the guest material 423 in the light-emitting layer 443 correspond to the organic compound 131 , the organic compound 132 , and the guest material 133 in the light-emitting layer 120 , respectively.
- the guest material 433 contained in the light-emitting layer 444 is a phosphorescent material.
- the electrode 401 , the electrode 402 , the hole-injection layers 411 and 415 , the hole-transport layers 412 and 416 , the electron-transport layers 413 and 417 , and the electron-injection layers 414 and 418 correspond to the electrode 101 , the electrode 102 , the hole-injection layer 111 , the hole-transport layer 112 , the electron-transport layer 118 , and the electron-injection layer 119 in Embodiment 1, respectively. Therefore, detailed description thereof is omitted in this embodiment.
- An emission mechanism of the light-emitting layer 443 is similar to that of the light-emitting layer 120 in FIGS. 2A to 2C .
- the organic compound 431 and the organic compound 432 which are contained in the light-emitting layer 444 form exciplexes.
- the organic compound 431 serves as a host material and the organic compound 432 serves as an assist material in the description here.
- one organic compound be a material having a hole-transport property and the other organic compound be a material having an electron-transport property.
- FIG. 3B illustrates the correlation of energy levels of the organic compound 431 , the organic compound 432 , and the guest material 433 in the light-emitting layer 444 .
- Host ( 431 ) the host material (organic compound 431 );
- T PH the level of the lowest triplet excited state of the host material (organic compound 431 );
- T PG the level of the lowest triplet excited state of the guest material 433 (the phosphorescent material);
- T PE the level of the lowest triplet excited state of exciplexes.
- the level (S PE ) of the lowest singlet excited state of exciplexes, which is formed by the organic compound 432 and the organic compound 431 and the level (T PE ) of the lowest triplet excited state of exciplexes are close to each other (see E 7 in FIG. 3B ).
- the light-emitting layer 444 has the above structure, light emission from the guest material 433 (the phosphorescent material) of the light-emitting layer 444 can be efficiently obtained.
- light emitted from the light-emitting layer 443 preferably has a peak on the shorter wavelength side than light emitted from the light-emitting layer 444 .
- the luminance of a light-emitting element using the phosphorescent material emitting light with a short wavelength tends to degrade quickly.
- fluorescence is used for light emission with a short wavelength, so that a light-emitting element with less degradation of luminance can be provided.
- the light-emitting layer 443 and the light-emitting layer 444 may be made to emit light with different emission wavelengths, so that the light-emitting element can be a multicolor light-emitting element.
- the emission spectrum of the light-emitting element is formed by combining light having different emission peaks, and thus has at least two peaks.
- the above structure is also suitable for obtaining white light emission.
- white light emission can be obtained.
- white light emission with a high color rendering property that is formed of three primary colors or four or more colors can be obtained by using a plurality of light-emitting materials emitting light with different wavelengths for one of the light-emitting layers 443 and 444 or both.
- one of the light-emitting layers 443 and 444 or both may be divided into layers and each of the divided layers may contain a different light-emitting material from the others.
- a material that can be used for the light-emitting layer 120 described in Embodiment 1 may be used as a material that can be used for the light-emitting layer 443 .
- the organic compound 431 (the host material) exists in the highest proportion in weight ratio, and the guest material 433 (the phosphorescent material) is dispersed in the organic compound 431 (the host material).
- Examples of the organic compound 431 include a zinc- or aluminum-based metal complex, an oxadiazole derivative, a triazole derivative, a benzimidazole derivative, a quinoxaline derivative, a dibenzoquinoxaline derivative, a dibenzothiophene derivative, a dibenzofuran derivative, a pyrimidine derivative, a triazine derivative, a pyridine derivative, a bipyridine derivative, a phenanthroline derivative, and the like.
- Other examples are an aromatic amine, a carbazole derivative, and the like.
- the compound having a skeleton which easily accepts electrons and the compound having a skeleton which easily accepts holes, which are described in Embodiment 1, can be used.
- an iridium-, rhodium-, or platinum-based organometallic complex or metal complex can be used; in particular, an organoiridium complex such as an iridium-based ortho-metalated complex is preferable.
- an ortho-metalated ligand a 4H-triazole ligand, a 1H-triazole ligand, an imidazole ligand, a pyridine ligand, a pyrimidine ligand, a pyrazine ligand, an isoquinoline ligand, and the like can be given.
- a platinum complex having a porphyrin ligand or the like can be given.
- the organic compound 432 (the assist material) a substance which can form exciplexes together with the organic compound 431 is used.
- the organic compound 431 , the organic compound 432 , and the guest material 433 (the phosphorescent material) be selected such that the emission peak of the exciplexes overlaps with an adsorption band, specifically an adsorption band on the longest wavelength side, of a triplet metal to ligand charge transfer (MLCT) transition of the phosphorescent material.
- MLCT metal to ligand charge transfer
- the adsorption band on the longest wavelength side be a singlet absorption band.
- the compound having a skeleton which easily accepts electrons or the compound having a skeleton which easily accepts holes, which are described in Embodiment 1, can be used as the organic compound 432 .
- any material can be used as long as the material can convert triplet excitation energy into light emission.
- a thermally activated delayed fluorescence material can be given in addition to the phosphorescent material. Therefore, the term “phosphorescent material” in the description can be replaced with the term “thermally activated delayed fluorescence material”. Note that the thermally activated delayed fluorescence material is a material that can up-convert a triplet excited state into a singlet excited state (i.e., reverse intersystem crossing is possible) using a little thermal energy and efficiently exhibits light emission (fluorescence) from the singlet excited state.
- Thermally activated delayed fluorescence is efficiently obtained under the condition where the difference between the triplet excitation energy level and the singlet excitation energy level is larger than 0 eV and smaller than or equal to 0.2 eV, preferably larger than 0 eV and smaller than or equal to 0.1 eV.
- the emission colors of the light-emitting material included in the light-emitting layer 443 and the light-emitting material included in the light-emitting layer 444 are not limitation on the emission colors of the light-emitting material included in the light-emitting layer 443 and the light-emitting material included in the light-emitting layer 444 , and they may be the same or different. Light emitted from the light-emitting materials is mixed and extracted out of the element; therefore, for example, in the case where their emission colors are complementary colors, the light-emitting element can emit white light.
- the emission peak wavelength of the light-emitting material contained in the light-emitting layer 443 is preferably shorter than that of the light-emitting material contained in the light-emitting layer 444 .
- the light-emitting layers 443 and 444 can be formed by an evaporation method (including a vacuum evaporation method), an inkjet method, a coating method, gravure printing, or the like.
- FIG. 4A is a schematic cross-sectional view of a light-emitting element 452 of one embodiment of the present invention.
- the light-emitting element 452 includes a plurality of light-emitting units (in FIG. 4A , a light-emitting unit 446 and a light-emitting unit 447 ) between an electrode 401 and an electrode 402 .
- One light-emitting unit has the same structure as the EL layer 100 illustrated in FIG. 1A . That is, the light-emitting element 150 in FIG. 1A includes one light-emitting unit, while the light-emitting element 452 includes the plurality of light-emitting units.
- the electrode 401 functions as an anode and the electrode 402 functions as a cathode in the following description of this embodiment; however, the functions may be interchanged in the light-emitting element 452 .
- the light-emitting unit 446 and the light-emitting unit 447 are stacked, and a charge-generation layer 445 is provided between the light-emitting unit 446 and the light-emitting unit 447 .
- the light-emitting unit 446 and the light-emitting unit 447 may have the same structure or different structures.
- a light-emitting layer containing a fluorescent material as a light-emitting material be used in the light-emitting unit 446 and that the EL layer 100 illustrated in FIG. 1A be used in the light-emitting unit 447 .
- the light-emitting element 452 includes a light-emitting layer 448 and a light-emitting layer 449 .
- the light-emitting unit 446 includes a hole-injection layer 411 , a hole-transport layer 412 , an electron-transport layer 413 , and an electron-injection layer 414 in addition to the light-emitting layer 448 .
- the light-emitting unit 447 includes a hole-injection layer 415 , a hole-transport layer 416 , an electron-transport layer 417 , and an electron-injection layer 418 in addition to the light-emitting layer 449 .
- the light-emitting element having two light-emitting units is described with reference to FIG. 4A ; however, a similar structure can be applied to a light-emitting element in which three or more light-emitting units are stacked.
- a plurality of light-emitting units partitioned by the charge-generation layer between a pair of electrodes as in the light-emitting element 452 it is possible to provide a light-emitting element which can emit light with high luminance with the current density kept low and has a long lifetime.
- a display device with low power consumption can be provided.
- a light-emitting element with high emission efficiency can be provided.
- the light-emitting layer 448 contains a host material 461 and a guest material 462 .
- the light-emitting layer 449 contains an organic compound 471 , an organic compound 472 , and a guest material 473 .
- the light-emitting layer 449 has a structure similar to that of the light-emitting layer 120 in FIGS. 1A and 1B . That is, the organic compound 471 , the organic compound 472 , and the guest material 473 in the light-emitting layer 449 correspond to the organic compound 131 , the organic compound 132 , and the guest material 133 in the light-emitting layer 120 , respectively.
- the guest material 462 contained in the light-emitting layer 448 is a fluorescent material.
- exciton generation probability The ratio of singlet excitons to triplet excitons generated by carrier recombination (hereinafter referred to as exciton generation probability) is approximately 1:3.
- the triplet excitation energy level of the host material 461 is transferred to the triplet excitation energy level of the guest material 462 (triplet energy transfer).
- the guest material 462 in the triplet excitation energy state does not provide light emission in a visible light region because the guest material 462 is the fluorescent material.
- FIG. 4B illustrates the correlation of energy levels of the host material 461 and the guest material 462 in the light-emitting layer 448 of one embodiment of the present invention. The following explains what terms and signs in FIG. 4B represent:
- T FH the level of the lowest triplet excited state of the host material 461 ;
- T FG the level of the lowest triplet excited state of the guest material 462 (fluorescent material).
- the triplet excitation energy level of the guest material 462 (T FG in FIG. 4B ) is higher than the triplet excitation energy level of the host material 461 (T FH in FIG. 4B ).
- triplet excitons collide with each other by triplet-triplet annihilation (TTA) (see a route E 9 in FIG. 4B ), and their excitation energy are partly converted into singlet excitons having an energy at the level of the lowest singlet excited state of the host material 461 (S FH ).
- the singlet excitation energy of the host material 461 is transferred from the level of the lowest singlet excited state of the host material 461 (S FH ) to the level of the lowest singlet excited state of the guest material 462 (the fluorescent material) (S FG ) that is a level lower than S FH (see a route E 10 in FIG. 4B ).
- the guest material 462 (the fluorescent material) is brought into the singlet excited state and accordingly emits light.
- excitation energy at T F G is transferred to T FH without deactivation (see a route E 11 in FIG. 4B ), which is utilized for TTA.
- the light-emitting layer 448 has the above structure, light emission from the guest material 462 of the light-emitting layer 448 can be efficiently obtained.
- the light-emitting layer 448 and the light-emitting layer 449 may be made to emit light with different emission wavelengths, so that the light-emitting element can be a multicolor light-emitting element.
- the emission spectrum of the light-emitting element is formed by combining light having different emission peaks, and thus has at least two peaks.
- the above structure is also suitable for obtaining white light emission.
- white light emission can be obtained.
- white light emission with a high color rendering property that is formed of three primary colors or four or more colors can be obtained by using a plurality of light-emitting materials emitting light with different wavelengths for one of the light-emitting layers 448 and 449 or both.
- one of the light-emitting layers 448 and 449 or both may be divided into layers and each of the divided layers may contain a different light-emitting material from the others.
- An emission mechanism of the light-emitting layer 449 is similar to that of the light-emitting layer 120 in FIGS. 2A to 2C .
- the host material 461 is present in the highest proportion in weight ratio, and the guest material 462 (the fluorescent material) is dispersed in the host material 461 .
- the singlet excitation energy level of the host material 461 is preferably higher than the singlet excitation energy level of the guest material 462 (the fluorescent material), while the triplet excitation energy level of the host material 461 is preferably lower than the triplet excitation energy level of the guest material 462 (the fluorescent material).
- An anthracene derivative or a tetracene derivative is preferably used as the host material 461 . This is because these derivatives each have a high singlet excitation energy level and a low triplet excitation energy level.
- Specific examples include 9-phenyl-3-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: PCzPA), 3-[4-(1-naphthyl)-phenyl]-9-phenyl-9H-carbazole (abbreviation: PCPN), 9-[4-(10-phenyl-9-anthracenyl)phenyl]-9H-carbazole (abbreviation: CzPA), 7-[4-(10-phenyl-9-anthryl)phenyl]-7H-dibenzo[c,g]carbazole (abbreviation: cgDBCzPA), 6-[3-(9,10-diphenyl-2-an
- Examples of the guest material 462 include a pyrene derivative, an anthracene derivative, a triphenylene derivative, a fluorene derivative, a carbazole derivative, a dibenzothiophene derivative, a dibenzofuran derivative, a dibenzoquinoxaline derivative, a quinoxaline derivative, a pyridine derivative, a pyrimidine derivative, a phenanthrene derivative, a naphthalene derivative, and the like.
- a pyrene derivative is particularly preferable because it has a high emission quantum yield.
- pyrene derivative examples include N,N′-bis(3-methylphenyl)-N,N′-bis[3-(9-phenyl-9H-fluoren-9-yl)phenyl]pyrene-1,6-diamine (abbreviation: 1,6mMemFLPAPrn), N,N′-bis[4-(9-phenyl-9H-fluoren-9-yl)phenyl]-N,N′-diphenylpyrene-1,6-diamine (abbreviation: 1,6FLPAPrn), N,N′-bis(dibenzofuran-2-yl)-N,N′-diphenylpyrene-1,6-diamine (abbreviation: 1,6FrAPrn), N,N′-bis(dibenzothiophene-2-yl)-N,N-diphenylpyrene-1,6-diamine (abbreviation: 1,6ThAPrn), and the like.
- a material that can be used for the light-emitting layer 120 described in Embodiment 1 may be used as a material that can be used for the light-emitting layer 449 .
- the emission colors of the light-emitting material included in the light-emitting layer 448 and the light-emitting material included in the light-emitting layer 449 are not limitation on the emission colors of the light-emitting material included in the light-emitting layer 448 and the light-emitting material included in the light-emitting layer 449 , and they may be the same or different. Light emitted from the light-emitting materials is mixed and extracted out of the element; therefore, for example, in the case where their emission colors are complementary colors, the light-emitting element can emit white light.
- the emission peak wavelength of the light-emitting material contained in the light-emitting layer 448 is preferably shorter than that of the light-emitting material contained in the light-emitting layer 449 .
- the light-emitting layers 448 and 449 can be formed by an evaporation method (including a vacuum evaporation method), an inkjet method, a coating method, gravure printing, or the like.
- FIGS. 5A and 5B a display device including a light-emitting element of one embodiment of the present invention will be described with reference to FIGS. 5A and 5B .
- FIG. 5A is a block diagram illustrating the display device of one embodiment of the present invention
- FIG. 5B is a circuit diagram illustrating a pixel circuit of the display device of one embodiment of the present invention.
- the display device illustrated in FIG. 5A includes a region including pixels of display elements (the region is hereinafter referred to as a pixel portion 802 ), a circuit portion provided outside the pixel portion 802 and including circuits for driving the pixels (the portion is hereinafter referred to as a driver circuit portion 804 ), circuits having a function of protecting elements (the circuits are hereinafter referred to as protection circuits 806 ), and a terminal portion 807 . Note that the protection circuits 806 are not necessarily provided.
- a part or the whole of the driver circuit portion 804 is preferably formed over a substrate over which the pixel portion 802 is formed, in which case the number of components and the number of terminals can be reduced.
- the part or the whole of the driver circuit portion 804 can be mounted by chip-on-glass (COG) or tape automated bonding (TAB).
- the pixel portion 802 includes a plurality of circuits for driving display elements arranged in X rows (X is a natural number of 2 or more) and Y columns (Y is a natural number of 2 or more) (such circuits are hereinafter referred to as pixel circuits 801 ).
- the driver circuit portion 804 includes driver circuits such as a circuit for supplying a signal (scan signal) to select a pixel (the circuit is hereinafter referred to as a scan line driver circuit 804 a ) and a circuit for supplying a signal (data signal) to drive a display element in a pixel (the circuit is hereinafter referred to as a signal line driver circuit 804 b ).
- the scan line driver circuit 804 a includes a shift register or the like. Through the terminal portion 807 , the scan line driver circuit 804 a receives a signal for driving the shift register and outputs a signal. For example, the scan line driver circuit 804 a receives a start pulse signal, a clock signal, or the like and outputs a pulse signal.
- the scan line driver circuit 804 a has a function of controlling the potentials of wirings supplied with scan signals (such wirings are hereinafter referred to as scan lines GL_ 1 to GL_X). Note that a plurality of scan line driver circuits 804 a may be provided to control the scan lines GL_ 1 to GL_X separately. Alternatively, the scan line driver circuit 804 a has a function of supplying an initialization signal. Without being limited thereto, the scan line driver circuit 804 a can supply another signal.
- the signal line driver circuit 804 b includes a shift register or the like.
- the signal line driver circuit 804 b receives a signal (video signal) from which a data signal is derived, as well as a signal for driving the shift register, through the terminal portion 807 .
- the signal line driver circuit 804 b has a function of generating a data signal to be written to the pixel circuit 801 which is based on the video signal.
- the signal line driver circuit 804 b has a function of controlling output of a data signal in response to a pulse signal produced by input of a start pulse signal, a clock signal, or the like.
- the signal line driver circuit 804 b has a function of controlling the potentials of wirings supplied with data signals (such wirings are hereinafter referred to as data lines DL_ 1 to DL_Y).
- the signal line driver circuit 804 b has a function of supplying an initialization signal. Without being limited thereto, the signal line driver circuit 804 b can supply another signal.
- the signal line driver circuit 804 b includes a plurality of analog switches or the like, for example.
- the signal line driver circuit 804 b can output, as the data signals, signals obtained by time-dividing the video signal by sequentially turning on the plurality of analog switches.
- the signal line driver circuit 804 b may include a shift register or the like.
- a pulse signal and a data signal are input to each of the plurality of pixel circuits 801 through one of the plurality of scan lines GL supplied with scan signals and one of the plurality of data lines DL supplied with data signals, respectively.
- Writing and holding of the data signal to and in each of the plurality of pixel circuits 801 are controlled by the scan line driver circuit 804 a .
- a pulse signal is input from the scan line driver circuit 804 a through the scan line GL_m, and a data signal is input from the signal line driver circuit 804 b through the data line DL_n in accordance with the potential of the scan line GL_m.
- the protection circuit 806 shown in FIG. 5A is connected to, for example, the scan line GL between the scan line driver circuit 804 a and the pixel circuit 801 .
- the protection circuit 806 is connected to the data line DL between the signal line driver circuit 804 b and the pixel circuit 801 .
- the protection circuit 806 can be connected to a wiring between the scan line driver circuit 804 a and the terminal portion 807 .
- the protection circuit 806 can be connected to a wiring between the signal line driver circuit 804 b and the terminal portion 807 .
- the terminal portion 807 means a portion having terminals for inputting power, control signals, and video signals to the display device from external circuits.
- the protection circuit 806 is a circuit that electrically connects a wiring connected to the protection circuit to another wiring when a potential out of a certain range is applied to the wiring connected to the protection circuit.
- the protection circuits 806 are provided for the pixel portion 802 and the driver circuit portion 804 , so that the resistance of the display device to overcurrent generated by electrostatic discharge (ESD) or the like can be improved.
- ESD electrostatic discharge
- the configuration of the protection circuits 806 is not limited to that, and for example, a configuration in which the protection circuits 806 are connected to the scan line driver circuit 804 a or a configuration in which the protection circuits 806 are connected to the signal line driver circuit 804 b may be employed.
- the protection circuits 806 may be configured to be connected to the terminal portion 807 .
- the driver circuit portion 804 includes the scan line driver circuit 804 a and the signal line driver circuit 804 b is shown; however, the structure is not limited thereto.
- the scan line driver circuit 804 a may be formed and a separately prepared substrate where a signal line driver circuit is formed (e.g., a driver circuit substrate formed with a single crystal semiconductor film or a polycrystalline semiconductor film) may be mounted.
- Each of the plurality of pixel circuits 801 in FIG. 5A can have a structure illustrated in FIG. 5B , for example.
- the pixel circuit 801 illustrated in FIG. 5B includes transistors 852 and 854 , a capacitor 862 , and a light-emitting element 872 .
- One of a source electrode and a drain electrode of the transistor 852 is electrically connected to a wiring to which a data signal is supplied (hereinafter referred to as a data line DL_n).
- a gate electrode of the transistor 852 is electrically connected to a wiring to which a gate signal is supplied (hereinafter referred to as a scan line GL_m).
- the transistor 852 has a function of controlling whether to write a data signal.
- One of a pair of electrodes of the capacitor 862 is electrically connected to a wiring to which a potential is supplied (hereinafter referred to as a potential supply line VL_a), and the other is electrically connected to the other of the source electrode and the drain electrode of the transistor 852 .
- the capacitor 862 functions as a storage capacitor for storing written data.
- One of a source electrode and a drain electrode of the transistor 854 is electrically connected to the potential supply line VL_a. Furthermore, a gate electrode of the transistor 854 is electrically connected to the other of the source electrode and the drain electrode of the transistor 852 .
- One of an anode and a cathode of the light-emitting element 872 is electrically connected to a potential supply line VL_b, and the other is electrically connected to the other of the source electrode and the drain electrode of the transistor 854 .
- any of the light-emitting elements described in Embodiments 1 to 3 can be used.
- a high power supply potential VDD is supplied to one of the potential supply line VL_a and the potential supply line VL_b, and a low power supply potential VSS is supplied to the other.
- the pixel circuits 801 are sequentially selected row by row by the scan line driver circuit 804 a in FIG. 5A , for example, whereby the transistors 852 are turned on and a data signal is written.
- the transistors 852 When the transistors 852 are turned off, the pixel circuits 801 in which the data has been written are brought into a holding state. Furthermore, the amount of current flowing between the source electrode and the drain electrode of the transistor 854 is controlled in accordance with the potential of the written data signal.
- the light-emitting element 872 emits light with a luminance corresponding to the amount of flowing current. This operation is sequentially performed row by row; thus, an image is displayed.
- a light-emitting element of one embodiment of the present invention can be used for an active matrix method in which an active element is included in a pixel of a display device or a passive matrix method in which an active element is not included in a pixel of a display device.
- an active element not only a transistor but also a variety of active elements (non-linear elements) can be used.
- a metal insulator metal (MIM), a thin film diode (TFD), or the like can also be used. Since these elements can be formed with a smaller number of manufacturing steps, manufacturing cost can be reduced or yield can be improved. Alternatively, since the size of these elements is small, the aperture ratio can be improved, so that power consumption can be reduced or higher luminance can be achieved.
- the passive matrix method in which an active element (a non-linear element) is not used can also be used. Since an active element (a non-linear element) is not used, the number of manufacturing steps is small, so that manufacturing cost can be reduced or yield can be improved. Alternatively, since an active element (a non-linear element) is not used, the aperture ratio can be improved, so that power consumption can be reduced or higher luminance can be achieved, for example.
- a display device including a light-emitting element of one embodiment of the present invention and an electronic device in which the display device is provided with an input device will be described with reference to FIGS. 6A and 6B , FIGS. 7A to 7C , FIGS. 8A and 8B , FIGS. 9A and 9B , and FIG. 10 .
- a touch panel 2000 including a display device and an input device will be described as an example of an electronic device.
- a touch sensor is used as an input device.
- FIGS. 6A and 6B are perspective views of the touch panel 2000 . Note that FIGS. 6A and 6B illustrate only main components of the touch panel 2000 for simplicity.
- the touch panel 2000 includes a display device 2501 and a touch sensor 2595 (see FIG. 6B ).
- the touch panel 2000 also includes a substrate 2510 , a substrate 2570 , and a substrate 2590 .
- the substrate 2510 , the substrate 2570 , and the substrate 2590 each have flexibility. Note that one or all of the substrates 2510 , 2570 , and 2590 may be inflexible.
- the display device 2501 includes a plurality of pixels over the substrate 2510 and a plurality of wirings 2511 through which signals are supplied to the pixels.
- the plurality of wirings 2511 are led to a peripheral portion of the substrate 2510 , and parts of the plurality of wirings 2511 form a terminal 2519 .
- the terminal 2519 is electrically connected to an FPC 2509 ( 1 ).
- the substrate 2590 includes the touch sensor 2595 and a plurality of wirings 2598 electrically connected to the touch sensor 2595 .
- the plurality of wirings 2598 are led to a peripheral portion of the substrate 2590 , and parts of the plurality of wirings 2598 form a terminal.
- the terminal is electrically connected to an FPC 2509 ( 2 ). Note that in FIG. 6B , electrodes, wirings, and the like of the touch sensor 2595 provided on the back side of the substrate 2590 (the side facing the substrate 2510 ) are indicated by solid lines for clarity.
- a capacitive touch sensor can be used as the touch sensor 2595 .
- Examples of the capacitive touch sensor are a surface capacitive touch sensor and a projected capacitive touch sensor.
- Examples of the projected capacitive touch sensor are a self capacitive touch sensor and a mutual capacitive touch sensor, which differ mainly in the driving method.
- the use of a mutual capacitive type is preferable because multiple points can be sensed simultaneously.
- touch sensor 2595 illustrated in FIG. 6B is an example of using a projected capacitive touch sensor.
- touch sensor 2595 a variety of sensors that can sense proximity or touch of a sensing target such as a finger can be used as the touch sensor 2595 .
- the projected capacitive touch sensor 2595 includes electrodes 2591 and electrodes 2592 .
- the electrodes 2591 are electrically connected to any of the plurality of wirings 2598
- the electrodes 2592 are electrically connected to any of the other wirings 2598 .
- the electrodes 2592 each have a shape of a plurality of quadrangles arranged in one direction with one corner of a quadrangle connected to one corner of another quadrangle as illustrated in FIGS. 6A and 6B .
- the electrodes 2591 each have a quadrangular shape and are arranged in a direction intersecting with the direction in which the electrodes 2592 extend.
- a wiring 2594 electrically connects two electrodes 2591 between which the electrode 2592 is positioned.
- the intersecting area of the electrode 2592 and the wiring 2594 is preferably as small as possible. Such a structure allows a reduction in the area of a region where the electrodes are not provided, reducing variation in transmittance. As a result, variation in luminance of light passing through the touch sensor 2595 can be reduced.
- the shapes of the electrodes 2591 and the electrodes 2592 are not limited thereto and can be any of a variety of shapes.
- a structure may be employed in which the plurality of electrodes 2591 are arranged so that gaps between the electrodes 2591 are reduced as much as possible, and the electrodes 2592 are spaced apart from the electrodes 2591 with an insulating layer interposed therebetween to have regions not overlapping with the electrodes 2591 .
- FIG. 7A corresponds to a cross-sectional view taken along dashed-dotted line X 1 -X 2 in FIG. 6B .
- the display device 2501 includes a plurality of pixels arranged in a matrix. Each of the pixels includes a display element and a pixel circuit for driving the display element.
- a light-emitting element that emits white light as a display element
- the display element is not limited to such an element.
- light-emitting elements that emit light of different colors may be included so that the light of different colors can be emitted from adjacent pixels.
- a flexible material with a vapor permeability of lower than or equal to 1 ⁇ 10 ⁇ 5 g ⁇ m ⁇ 2 ⁇ day ⁇ 1 , preferably lower than or equal to 1 ⁇ 10 ⁇ 6 g ⁇ m ⁇ 2 ⁇ day ⁇ 1 can be favorably used.
- materials whose thermal expansion coefficients are substantially equal to each other are preferably used for the substrate 2510 and the substrate 2570 .
- the coefficients of linear expansion of the materials are preferably lower than or equal to 1 ⁇ 10 ⁇ 3 /K, further preferably lower than or equal to 5 ⁇ 10 ⁇ 5 /K, and still further preferably lower than or equal to 1 ⁇ 10 ⁇ 5 /K.
- the substrate 2510 is a stacked body including an insulating layer 2510 a for preventing impurity diffusion into the light-emitting element, a flexible substrate 2510 b , and an adhesive layer 2510 c for attaching the insulating layer 2510 a and the flexible substrate 2510 b to each other.
- the substrate 2570 is a stacked body including an insulating layer 2570 a for preventing impurity diffusion into the light-emitting element, a flexible substrate 2570 b , and an adhesive layer 2570 c for attaching the insulating layer 2570 a and the flexible substrate 2570 b to each other.
- the adhesive layer 2510 c and the adhesive layer 2570 c for example, materials that include polyester, polyolefin, polyamide (e.g., nylon, aramid), polyimide, polycarbonate, or a resin having an acrylic resin, polyurethane, an epoxy resin, or a resin having a siloxane bond such as silicone can be used.
- materials that include polyester, polyolefin, polyamide (e.g., nylon, aramid), polyimide, polycarbonate, or a resin having an acrylic resin, polyurethane, an epoxy resin, or a resin having a siloxane bond such as silicone can be used.
- a sealing layer 2560 is provided between the substrate 2510 and the substrate 2570 .
- the sealing layer 2560 preferably has a refractive index higher than that of air. In the case where light is extracted to the sealing layer 2560 side as illustrated in FIG. 7A , the sealing layer 2560 can also serve as an adhesive layer.
- a sealant may be formed in the peripheral portion of the sealing layer 2560 .
- a light-emitting element 2550 R can be provided in a region surrounded by the substrate 2510 , the substrate 2570 , the sealing layer 2560 , and the sealant.
- an inert gas such as nitrogen or argon
- a drying agent may be provided in the inert gas so as to adsorb moisture or the like.
- An ultraviolet curable resin or a heat curable resin may be used; for example, a polyvinyl chloride (PVC) based resin, an acrylic resin, a polyimide-based resin, an epoxy-based resin, a silicone-based resin, a polyvinyl butyral (PVB) based resin, or an ethylene vinyl acetate (EVA) based resin can be used.
- PVC polyvinyl chloride
- acrylic resin acrylic resin
- a polyimide-based resin an epoxy-based resin
- silicone-based resin a silicone-based resin
- EVA ethylene vinyl acetate
- an epoxy-based resin or a glass frit is preferably used as the sealant.
- a material used for the sealant a material which is impermeable to moisture or oxygen is preferably used.
- the display device 2501 includes a pixel 2502 R.
- the pixel 2502 R includes a light-emitting module 2580 R.
- the pixel 2502 R includes the light-emitting element 2550 R and a transistor 2502 t that can supply electric power to the light-emitting element 2550 R. Note that the transistor 2502 t functions as part of the pixel circuit.
- the light-emitting module 2580 R includes the light-emitting element 2550 R and a coloring layer 2567 R.
- the light-emitting element 2550 R includes a lower electrode, an upper electrode, and an EL layer between the lower electrode and the upper electrode.
- any of the light-emitting elements described in Embodiments 1 to 3 can be used, for example.
- a microcavity structure may be employed between the lower electrode and the upper electrode so as to increase the intensity of light having a specific wavelength.
- the sealing layer 2560 is provided on the light extraction side, the sealing layer 2560 is in contact with the light-emitting element 2550 R and the coloring layer 2567 R.
- the coloring layer 2567 R is positioned in a region overlapping with the light-emitting element 2550 R. Accordingly, part of light emitted from the light-emitting element 2550 R passes through the coloring layer 2567 R and is emitted to the outside of the light-emitting module 2580 R as indicated by an arrow in FIG. 7A .
- the display device 2501 includes a light-blocking layer 2567 BM on the light extraction side.
- the light-blocking layer 2567 BM is provided so as to surround the coloring layer 2567 R.
- the coloring layer 2567 R is a coloring layer having a function of transmitting light in a particular wavelength region.
- a color filter for transmitting light in a red wavelength range a color filter for transmitting light in a green wavelength range, a color filter for transmitting light in a blue wavelength range, a color filter for transmitting light in a yellow wavelength range, or the like can be used.
- Each color filter can be formed with any of various materials by a printing method, an inkjet method, an etching method using a photolithography technique, or the like.
- An insulating layer 2521 is provided in the display device 2501 .
- the insulating layer 2521 covers the transistor 2502 t .
- the insulating layer 2521 has a function of planarizing unevenness caused by the pixel circuit.
- the insulating layer 2521 may have a function of suppressing impurity diffusion. This can prevent the reliability of the transistor 2502 t or the like from being lowered by impurity diffusion.
- the light-emitting element 2550 R is formed over the insulating layer 2521 .
- a partition 2528 is provided so as to overlap with an end portion of the lower electrode of the light-emitting element 2550 R. Note that a spacer for controlling the distance between the substrate 2510 and the substrate 2570 may be formed over the partition 2528 .
- a scan line driver circuit 2503 g ( 1 ) includes a transistor 2503 t and a capacitor 2503 c . Note that the driver circuit can be formed in the same process and over the same substrate as those of the pixel circuits.
- the wirings 2511 through which signals can be supplied are provided over the substrate 2510 .
- the terminal 2519 is provided over the wirings 2511 .
- the FPC 2509 ( 1 ) is electrically connected to the terminal 2519 .
- the FPC 2509 ( 1 ) has a function of supplying a video signal, a clock signal, a start signal, a reset signal, or the like. Note that the FPC 2509 ( 1 ) may be provided with a printed wiring board (PWB).
- FIG. 7A illustrates an example of using bottom-gate transistors; however, the present invention is not limited to this example, and top-gate transistors may be used in the display device 2501 as illustrated in FIG. 7B .
- the polarity of the transistor 2502 t and the transistor 2503 t there is no particular limitation on the polarity of the transistor 2502 t and the transistor 2503 t .
- n-channel and p-channel transistors may be used, or either n-channel transistors or p-channel transistors may be used, for example.
- crystallinity of a semiconductor film used for the transistors 2502 t and 2503 t there is no particular limitation on the crystallinity of a semiconductor film used for the transistors 2502 t and 2503 t .
- an amorphous semiconductor film or a crystalline semiconductor film may be used.
- semiconductor materials include Group 13 semiconductors (e.g., a semiconductor including gallium), Group 14 semiconductors (e.g., a semiconductor including silicon), compound semiconductors (including oxide semiconductors), organic semiconductors, and the like.
- An oxide semiconductor that has an energy gap of 2 eV or more, preferably 2.5 eV or more, further preferably 3 eV or more is preferably used for one of the transistors 2502 t and 2503 t or both, so that the off-state current of the transistors can be reduced.
- the oxide semiconductors include an In—Ga oxide, an In-M-Zn oxide (M represents aluminum (Al), gallium (Ga), yttrium (Y), zirconium (Zr), lanthanum (La), cerium (Ce), tin (Sn), hafnium (Hf), or neodymium (Nd)), and the like.
- FIG. 7C corresponds to a cross-sectional view taken along dashed-dotted line X 3 -X 4 in FIG. 6B .
- the touch sensor 2595 includes the electrodes 2591 and the electrodes 2592 provided in a staggered arrangement on the substrate 2590 , an insulating layer 2593 covering the electrodes 2591 and the electrodes 2592 , and the wiring 2594 that electrically connects the adjacent electrodes 2591 to each other.
- the electrodes 2591 and the electrodes 2592 are formed using a light-transmitting conductive material.
- a light-transmitting conductive material a conductive oxide such as indium oxide, indium tin oxide, indium zinc oxide, zinc oxide, or zinc oxide to which gallium is added can be used.
- a film including graphene may be used as well.
- the film including graphene can be formed, for example, by reducing a film containing graphene oxide. As a reducing method, a method with application of heat or the like can be employed.
- the electrodes 2591 and the electrodes 2592 may be formed by, for example, depositing a light-transmitting conductive material on the substrate 2590 by a sputtering method and then removing an unnecessary portion by any of various pattern forming techniques such as photolithography.
- Examples of a material for the insulating layer 2593 are a resin such as an acrylic resin or an epoxy resin, a resin having a siloxane bond, and an inorganic insulating material such as silicon oxide, silicon oxynitride, or aluminum oxide.
- Openings reaching the electrodes 2591 are formed in the insulating layer 2593 , and the wiring 2594 electrically connects the adjacent electrodes 2591 .
- a light-transmitting conductive material can be favorably used as the wiring 2594 because the aperture ratio of the touch panel can be increased.
- a material with higher conductivity than the conductivities of the electrodes 2591 and 2592 can be favorably used for the wiring 2594 because electric resistance can be reduced.
- One electrode 2592 extends in one direction, and a plurality of electrodes 2592 are provided in the form of stripes.
- the wiring 2594 intersects with the electrode 2592 .
- Adjacent electrodes 2591 are provided with one electrode 2592 provided therebetween.
- the wiring 2594 electrically connects the adjacent electrodes 2591 .
- the plurality of electrodes 2591 are not necessarily arranged in the direction orthogonal to one electrode 2592 and may be arranged to intersect with one electrode 2592 at an angle of more than 0 degrees and less than 90 degrees.
- the wiring 2598 is electrically connected to any of the electrodes 2591 and 2592 . Part of the wiring 2598 functions as a terminal.
- a metal material such as aluminum, gold, platinum, silver, nickel, titanium, tungsten, chromium, molybdenum, iron, cobalt, copper, or palladium or an alloy material containing any of these metal materials can be used.
- an insulating layer that covers the insulating layer 2593 and the wiring 2594 may be provided to protect the touch sensor 2595 .
- connection layer 2599 electrically connects the wiring 2598 to the FPC 2509 ( 2 ).
- connection layer 2599 any of various anisotropic conductive films (ACF), anisotropic conductive pastes (ACP), or the like can be used.
- ACF anisotropic conductive films
- ACP anisotropic conductive pastes
- FIG. 8A corresponds to a cross-sectional view taken along dashed-dotted line X 5 -X 6 in FIG. 6A .
- the display device 2501 described with reference to FIG. 7A and the touch sensor 2595 described with reference to FIG. 7C are attached to each other.
- the touch panel 2000 illustrated in FIG. 8A includes an adhesive layer 2597 and an anti-reflective layer 2567 p in addition to the components described with reference to FIGS. 7A and 7C .
- the adhesive layer 2597 is provided in contact with the wiring 2594 . Note that the adhesive layer 2597 attaches the substrate 2590 to the substrate 2570 so that the touch sensor 2595 overlaps with the display device 2501 .
- the adhesive layer 2597 preferably has a light-transmitting property.
- a heat curable resin or an ultraviolet curable resin can be used for the adhesive layer 2597 .
- an acrylic resin, an urethane-based resin, an epoxy-based resin, or a siloxane-based resin can be used.
- the anti-reflective layer 2567 p is positioned in a region overlapping with pixels.
- a circularly polarizing plate can be used, for example.
- FIG. 8B is a cross-sectional view of a touch panel 2001 .
- the touch panel 2001 illustrated in FIG. 8B differs from the touch panel 2000 illustrated in FIG. 8A in the position of the touch sensor 2595 relative to the display device 2501 .
- Different parts are described in detail below, and the above description of the touch panel 2000 is referred to for the other similar parts.
- the coloring layer 2567 R is positioned in a region overlapping with the light-emitting element 2550 R.
- the light-emitting element 2550 R illustrated in FIG. 8B emits light to the side where the transistor 2502 t is provided. Accordingly, part of light emitted from the light-emitting element 2550 R passes through the coloring layer 2567 R and is emitted to the outside of the light-emitting module 2580 R as indicated by an arrow in FIG. 8B .
- the touch sensor 2595 is provided on the substrate 2510 side of the display device 2501 .
- the adhesive layer 2597 is provided between the substrate 2510 and the substrate 2590 and attaches the touch sensor 2595 to the display device 2501 .
- light may be emitted from the light-emitting element to one of upper and lower sides, or both, of the substrate.
- FIG. 9A is a block diagram illustrating the structure of a mutual capacitive touch sensor.
- FIG. 9A illustrates a pulse voltage output circuit 2601 and a current sensing circuit 2602 .
- six wirings X 1 to X 6 represent the electrodes 2621 to which a pulse voltage is applied
- six wirings Y 1 to Y 6 represent the electrodes 2622 that detect changes in current.
- FIG. 9A also illustrates capacitors 2603 that are each formed in a region where the electrodes 2621 and 2622 overlap with each other. Note that functional replacement between the electrodes 2621 and 2622 is possible.
- the pulse voltage output circuit 2601 is a circuit for sequentially applying a pulse voltage to the wirings X 1 to X 6 .
- a pulse voltage By application of a pulse voltage to the wirings X 1 to X 6 , an electric field is generated between the electrodes 2621 and 2622 of the capacitor 2603 .
- the electric field between the electrodes is shielded, for example, a change occurs in the capacitor 2603 (mutual capacitance).
- the approach or contact of a sensing target can be sensed by utilizing this change.
- the current sensing circuit 2602 is a circuit for detecting changes in current flowing through the wirings Y 1 to Y 6 that are caused by the change in mutual capacitance in the capacitor 2603 . No change in current value is detected in the wirings Y 1 to Y 6 when there is no approach or contact of a sensing target, whereas a decrease in current value is detected when mutual capacitance is decreased owing to the approach or contact of a sensing target. Note that an integrator circuit or the like is used for sensing of current values.
- FIG. 9B is a timing chart showing input and output waveforms in the mutual capacitive touch sensor illustrated in FIG. 9A .
- sensing of a sensing target is performed in all the rows and columns in one frame period.
- FIG. 9B shows a period when a sensing target is not sensed (not touched) and a period when a sensing target is sensed (touched).
- Sensed current values of the wirings Y 1 to Y 6 are shown as the waveforms of voltage values.
- a pulse voltage is sequentially applied to the wirings X 1 to X 6 , and the waveforms of the wirings Y 1 to Y 6 change in accordance with the pulse voltage.
- the waveforms of the wirings Y 1 to Y 6 change in accordance with changes in the voltages of the wirings X 1 to X 6 .
- the current value is decreased at the point of approach or contact of a sensing target and accordingly the waveform of the voltage value changes.
- FIG. 9A illustrates a passive matrix type touch sensor in which only the capacitor 2603 is provided at the intersection of wirings as a touch sensor
- an active matrix type touch sensor including a transistor and a capacitor may be used.
- FIG. 10 illustrates an example of a sensor circuit included in an active matrix type touch sensor.
- the sensor circuit in FIG. 10 includes the capacitor 2603 and transistors 2611 , 2612 , and 2613 .
- a signal G 2 is input to a gate of the transistor 2613 .
- a voltage VRES is applied to one of a source and a drain of the transistor 2613 , and one electrode of the capacitor 2603 and a gate of the transistor 2611 are electrically connected to the other of the source and the drain of the transistor 2613 .
- One of a source and a drain of the transistor 2611 is electrically connected to one of a source and a drain of the transistor 2612 , and a voltage VSS is applied to the other of the source and the drain of the transistor 2611 .
- a signal G 1 is input to a gate of the transistor 2612 , and a wiring ML is electrically connected to the other of the source and the drain of the transistor 2612 .
- the voltage VSS is applied to the other electrode of the capacitor 2603 .
- a potential for turning on the transistor 2613 is supplied as the signal G 2 , and a potential with respect to the voltage VRES is thus applied to the node n connected to the gate of the transistor 2611 . Then, a potential for turning off the transistor 2613 is applied as the signal G 2 , whereby the potential of the node n is maintained.
- a potential for turning on the transistor 2612 is supplied as the signal G 1 .
- a current flowing through the transistor 2611 that is, a current flowing through the wiring ML is changed in accordance with the potential of the node n. By sensing this current, the approach or contact of a sensing target can be sensed.
- an oxide semiconductor layer is preferably used as a semiconductor layer in which a channel region is formed.
- such a transistor is preferably used as the transistor 2613 so that the potential of the node n can be held for a long time and the frequency of operation of resupplying VRES to the node n (refresh operation) can be reduced.
- FIG. 11 a display module and electronic devices including a light-emitting element of one embodiment of the present invention will be described with reference to FIG. 11 and FIGS. 12A to 12G .
- a touch sensor 8004 connected to an FPC 8003 a display device 8006 connected to an FPC 8005 , a frame 8009 , a printed board 8010 , and a battery 8011 are provided between an upper cover 8001 and a lower cover 8002 .
- the light-emitting element of one embodiment of the present invention can be used for the display device 8006 , for example.
- the shapes and sizes of the upper cover 8001 and the lower cover 8002 can be changed as appropriate in accordance with the sizes of the touch sensor 8004 and the display device 8006 .
- the touch sensor 8004 can be a resistive touch sensor or a capacitive touch sensor and may be formed to overlap with the display device 8006 .
- a counter substrate (sealing substrate) of the display device 8006 can have a touch sensor function.
- a photosensor may be provided in each pixel of the display device 8006 so that an optical touch sensor is obtained.
- the frame 8009 protects the display device 8006 and also serves as an electromagnetic shield for blocking electromagnetic waves generated by the operation of the printed board 8010 .
- the frame 8009 may serve as a radiator plate.
- the printed board 8010 has a power supply circuit and a signal processing circuit for outputting a video signal and a clock signal.
- a power source for supplying power to the power supply circuit an external commercial power source or the battery 8011 provided separately may be used.
- the battery 8011 can be omitted in the case of using a commercial power source.
- the display module 8000 can be additionally provided with a member such as a polarizing plate, a retardation plate, or a prism sheet.
- FIGS. 12A to 12G illustrate electronic devices. These electronic devices can include a housing 9000 , a display portion 9001 , a speaker 9003 , operation keys 9005 (including a power switch or an operation switch), a connection terminal 9006 , a sensor 9007 (a sensor having a function of measuring or sensing force, displacement, position, speed, acceleration, angular velocity, rotational frequency, distance, light, liquid, magnetism, temperature, chemical substance, sound, time, hardness, electric field, current, voltage, electric power, radiation, flow rate, humidity, gradient, oscillation, odor, or infrared ray), a microphone 9008 , and the like.
- a sensor 9007 a sensor having a function of measuring or sensing force, displacement, position, speed, acceleration, angular velocity, rotational frequency, distance, light, liquid, magnetism, temperature, chemical substance, sound, time, hardness, electric field, current, voltage, electric power, radiation, flow rate, humidity, gradient, oscillation, odor, or infrared
- the electronic devices illustrated in FIGS. 12A to 12G can have a variety of functions, for example, a function of displaying a variety of data (a still image, a moving image, a text image, and the like) on the display portion, a touch sensor function, a function of displaying a calendar, date, time, and the like, a function of controlling a process with a variety of software (programs), a wireless communication function, a function of being connected to a variety of computer networks with a wireless communication function, a function of transmitting and receiving a variety of data with a wireless communication function, a function of reading a program or data stored in a memory medium and displaying the program or data on the display portion, and the like.
- a function of displaying a variety of data a still image, a moving image, a text image, and the like
- a touch sensor function a function of displaying a calendar, date, time, and the like
- a function of displaying a calendar, date, time, and the like
- the electronic devices illustrated in FIGS. 12A to 12G are not limited to those described above, and the electronic devices can have a variety of functions.
- the electronic devices may include a plurality of display portions.
- the electronic devices may have a camera or the like and a function of taking a still image, a function of taking a moving image, a function of storing the taken image in a memory medium (an external memory medium or a memory medium incorporated in the camera), a function of displaying the taken image on the display portion, or the like.
- FIGS. 12A to 12G The electronic devices illustrated in FIGS. 12A to 12G will be described in detail below.
- FIG. 12A is a perspective view of a portable information terminal 9100 .
- the display portion 9001 of the portable information terminal 9100 is flexible. Therefore, the display portion 9001 can be incorporated along a bent surface of a bent housing 9000 .
- the display portion 9001 includes a touch sensor, and operation can be performed by touching the screen with a finger, a stylus, or the like. For example, when an icon displayed on the display portion 9001 is touched, an application can be started.
- FIG. 12B is a perspective view of a portable information terminal 9101 .
- the portable information terminal 9101 functions as, for example, one or more of a telephone set, a notebook, and an information browsing system. Specifically, the portable information terminal can be used as a smartphone. Note that the speaker 9003 , the connection terminal 9006 , the sensor 9007 , and the like, which are not shown in FIG. 12B , can be positioned in the portable information terminal 9101 as in the portable information terminal 9100 shown in FIG. 12A .
- the portable information terminal 9101 can display characters and image information on its plurality of surfaces. For example, three operation buttons 9050 (also referred to as operation icons, or simply, icons) can be displayed on one surface of the display portion 9001 .
- information 9051 indicated by dashed rectangles can be displayed on another surface of the display portion 9001 .
- Examples of the information 9051 include display indicating reception of an incoming email, social networking service (SNS) message, call, and the like; the title and sender of an email and SNS message; the date; the time; remaining battery; and the reception strength of an antenna.
- the operation buttons 9050 or the like may be displayed on the position where the information 9051 is displayed.
- FIG. 12C is a perspective view of a portable information terminal 9102 .
- the portable information terminal 9102 has a function of displaying information on three or more surfaces of the display portion 9001 .
- information 9052 , information 9053 , and information 9054 are displayed on different surfaces.
- a user of the portable information terminal 9102 can see the display (here, the information 9053 ) with the portable information terminal 9102 put in a breast pocket of his/her clothes.
- a caller's phone number, name, or the like of an incoming call is displayed in a position that can be seen from above the portable information terminal 9102 .
- the user can see the display without taking out the portable information terminal 9102 from the pocket and decide whether to answer the call.
- FIG. 12D is a perspective view of a watch-type portable information terminal 9200 .
- the portable information terminal 9200 is capable of executing a variety of applications such as mobile phone calls, e-mailing, viewing and editing texts, music reproduction, Internet communication, and computer games.
- the display surface of the display portion 9001 is bent, and images can be displayed on the bent display surface.
- the portable information terminal 9200 can employ near field communication that is a communication method based on an existing communication standard. In that case, for example, mutual communication between the portable information terminal 9200 and a headset capable of wireless communication can be performed, and thus hands-free calling is possible.
- the portable information terminal 9200 includes the connection terminal 9006 , and data can be directly transmitted to and received from another information terminal via a connector. Power charging through the connection terminal 9006 is possible. Note that the charging operation may be performed by wireless power feeding without using the connection terminal 9006 .
- FIGS. 12E, 12F, and 12G are perspective views of a foldable portable information terminal 9201 .
- FIG. 12E is a perspective view illustrating the portable information terminal 9201 that is opened.
- FIG. 12F is a perspective view illustrating the portable information terminal 9201 that is being opened or being folded.
- FIG. 12G is a perspective view illustrating the portable information terminal 9201 that is folded.
- the portable information terminal 9201 is highly portable when folded. When the portable information terminal 9201 is opened, a seamless large display region is highly browsable.
- the display portion 9001 of the portable information terminal 9201 is supported by three housings 9000 joined together by hinges 9055 .
- the portable information terminal 9201 By folding the portable information terminal 9201 at a connection portion between two housings 9000 with the hinges 9055 , the portable information terminal 9201 can be reversibly changed in shape from an opened state to a folded state.
- the portable information terminal 9201 can be bent with a radius of curvature of greater than or equal to 1 mm and less than or equal to 150 mm.
- the electronic devices described in this embodiment each include the display portion for displaying some sort of data.
- the light-emitting element of one embodiment of the present invention can also be used for an electronic device which does not have a display portion.
- the structure in which the display portion of the electronic device described in this embodiment is flexible and display can be performed on the bent display surface or the structure in which the display portion of the electronic device is foldable is described as an example; however, the structure is not limited thereto and a structure in which the display portion of the electronic device is not flexible and display is performed on a plane portion may be employed.
- FIG. 13 illustrates an example in which the light-emitting element is used for an indoor lighting device 8501 . Since the light-emitting element can have a larger area, a lighting device having a large area can also be formed.
- a lighting device 8502 in which a light-emitting region has a curved surface can also be formed with the use of a housing with a curved surface.
- a light-emitting element described in this embodiment is in the form of a thin film, which allows the housing to be designed more freely. Therefore, the lighting device can be elaborately designed in a variety of ways.
- a wall of the room may be provided with a large-sized lighting device 8503 . Touch sensors may be provided in the lighting devices 8501 , 8502 , and 8503 to control the power on/off of the lighting devices.
- a lighting device 8504 which has a function as a table can be obtained.
- a lighting device which has a function as the furniture can be obtained.
- This example shows a fabrication example of light-emitting elements (light-emitting elements 1 and 2) in which a mixture of a thermally activated delayed fluorescence substance (a first organic compound), a host material (a second organic compound), and a guest material emitting fluorescence are used for a light-emitting layer.
- FIG. 14 is a schematic cross-sectional view of the light-emitting element fabricated in this example.
- Table 4 shows a detailed structure of the element.
- structures and abbreviations of compounds used here are shown below. Note that Embodiment 1 can be referred to for other compounds.
- a film of indium tin oxide containing silicon oxide (ITSO) was formed to a thickness of 110 nm over a substrate 520 by a sputtering method to form an electrode 501 .
- the area of the electrode 501 was 4 mm 2 (2 mm ⁇ 2 mm).
- UV ozone treatment was performed for 370 seconds after washing of a surface of the substrate with water and heat treatment that was performed at 200° C. for 1 hour.
- PhCzGI was deposited to a thickness of 20 nm over the hole-injection layer 531 .
- PCCzPTzn is a thermally activated delayed fluorescence substance (a first organic compound)
- 4,6mCzP2Pm is a host material (a second organic compound)
- 1,6mMemFLPAPm is a guest material.
- 4,6mCzP2Pm and bathophenanthroline (abbreviation: Bphen) were sequentially deposited to thicknesses of 15 nm each, as electron-transport layers 533 a and 533 b over the light-emitting layer 521 .
- lithium fluoride (abbreviation: LiF) was deposited to a thickness of 1 nm over the electron-transport layer 533 b.
- aluminum (Al) was deposited on the electron-injection layer 534 to a thickness of 200 nm.
- a light-emitting element was sealed by fixing a sealing substrate to the substrate 520 using a sealant for an organic EL device in a glove box under a nitrogen atmosphere.
- the sealant was applied to surround the light-emitting element, the substrate 520 and the sealing substrate were bonded to each other, irradiation with ultraviolet light a wavelength of 365 nm at 6 J/cm 2 was performed, and heat treatment was performed at 80° C. for 1 hour.
- the light-emitting element 1 was obtained.
- the light-emitting element 2 was fabricated through the same steps as those for the light-emitting element 1 except that TBP was used as a guest material for the light-emitting element 2.
- PCCzPTzn is a thermally activated delayed fluorescence substance (a first organic compound)
- 4,6mCzP2Pm is a host material (a second organic compound)
- TBP is a guest material.
- Transient fluorescent characteristics of PCCzPTzn which was the host material of the light-emitting elements in this example (the light-emitting elements 1 and 2) were measured using time-resolved emission measurement.
- the time-resolved emission measurement was performed on a thin-film sample in which PCCzPTzn was deposited over a quartz substrate to a thickness of 50 nm.
- the thin-film sample was sealed by fixing a sealing substrate to the quartz substrate over which the thin-film sample was deposited using a sealant for an organic EL device in a glove box under a nitrogen atmosphere. Specifically, after a sealant was applied to surround the thin-film over the quartz substrate and the quartz substrate was bonded to the sealing substrate, irradiation with ultraviolet light having a wavelength of 365 nm at 6 J/cm 2 and heat treatment at 80° C. for one hour were performed.
- a picosecond fluorescence lifetime measurement system (manufactured by Hamamatsu Photonics K.K.) was used for the measurement.
- the thin film was irradiated with pulsed laser, and emission of the thin film which was attenuated from the laser irradiation underwent time-resolved measurement using a streak camera to measure the lifetime of fluorescent emission of the thin film.
- a nitrogen gas laser with a wavelength of 337 nm was used as the pulsed laser.
- the thin film was irradiated with pulsed laser with a pulse width of 500 ps at a repetition rate of 10 Hz. By integrating data obtained by the repeated measurement, data with a high S/N ratio was obtained.
- the measurement was performed at room temperature (in an atmosphere kept at 23° C.).
- FIG. 15 shows transient fluorescent characteristics of PCCzPTzn obtained by the measurement.
- FIGS. 16, 17, and 18 show luminance-current density characteristics, luminance-voltage characteristics, and external quantum efficiency-luminance characteristics, respectively, of the light-emitting elements 1 and 2.
- the measurement of the light-emitting elements was performed at room temperature (in an atmosphere kept at 23° C.).
- Table 5 shows element characteristics of the light-emitting elements 1 and 2 at around 1000 cd/m 2 .
- FIG. 19 shows the electroluminescence spectra of the light-emitting elements 1 and 2 through which current flows at a current density of 2.5 mA/cm 2 . It is found from FIG. 19 that blue light emission originating from the guest material is obtained from the light-emitting elements 1 and 2.
- the light-emitting elements 1 and 2 show element characteristics of low driving voltage and high emission efficiency as shown in FIGS. 16, 17, and 18 .
- the light-emitting element 2 shows high external quantum efficiency exceeding 10% at a maximum.
- the maximum external quantum efficiency of a light-emitting element is approximately 6% on the assumption that the light extraction efficiency from the light-emitting element to the outside is 25%.
- the light-emitting elements 1 and 2 using one embodiment of the present invention exhibited higher external quantum efficiency. This is because a triplet excited state generated by recombined carriers in a thermally activated delayed fluorescence substance was converted into a single excited state by reverse intersystem crossing.
- the HOMO level of a thermally activated delayed fluorescence substance is higher than or equal to the HOMO level of a host material
- the LUMO level of the thermally activated delayed fluorescence substance is lower than or equal to the LUMO level of the host material, which are shown in Table 1 in Embodiment 1.
- the oxidation potential and the reduction potential of the thermally activated delayed fluorescence substance are lower than or equal to the oxidation potential of the host material and higher than or equal to the reduction potential of the host material, respectively, as shown in Table 2 in Embodiment 1.
- the HOMO level and the LUMO level of the thermally activated delayed fluorescence substance which are estimated from the oxidation potential and the reduction potential are higher than or equal to the HOMO level of the host material and lower than or equal to the LUMO level of the host material, respectively.
- the triplet excited energy level of 1,6mMemFLPAPrn was 1.84 eV, which was measured by the method similar to that in Embodiment 1. Therefore, as shown in Table 3 in Embodiment 1, the triplet excited energy level of the thermally activated delayed fluorescence substance (PCCzPTzn) and that of the host material (4,6mCzP2Pm) are each higher than that of the guest material.
- both the singlet excited state and the triplet excited state which are efficiently formed by carrier recombination in the thermally activated delayed fluorescence substance can be transferred efficiently to the guest material.
- the light-emitting elements 1 and 2 show high emission efficiency.
- the high emission efficiency of the light-emitting elements 1 and 2 means that the weight ratio of the host material to the thermally activated delayed fluorescence substance is preferably from 1:0.05 to 1:0.5 (host material: thermally activated delayed fluorescence substance) and the weight ratio of the host material to the guest material is preferably from 1:0.001 to 1:0.01 (host material:guest material).
- the use of a structure of one embodiment of the present invention can provide a light-emitting element with high emission efficiency.
- light-emitting elements including and not including a thermally activated delayed fluorescence substance, and light-emitting elements with different weight ratios of a host material and a guest material (light-emitting elements 3 to 5 and comparative light-emitting element 1 to 4) were fabricated.
- a schematic cross-sectional view of the light-emitting elements fabricated in this example is similar to FIG. 14 in Example 1. Details of the light-emitting elements fabricated in this example are shown in Table 6 and Table 7. In addition, structures and abbreviations of compounds used here are given below. Embodiment 1 or Example 1 may be referred to for other compounds.
- An ITSO film was formed to a thickness of 110 nm over the substrate 520 by a sputtering method to form the electrode 501 .
- the area of the electrode 501 was 4 mm 2 (2 mm ⁇ 2 mm).
- UV ozone treatment was performed for 370 seconds after washing of a surface of the substrate with water and heat treatment that was performed at 200° C. for 1 hour.
- Cz2DBT was deposited to a thickness of 20 nm over the hole-injection layer 531 .
- PCCzPTzn is a thermally activated delayed fluorescence substance (a first organic compound)
- Cz2DBT is a host material (a second organic compound)
- 1,6mMemFLPAPrn is a guest material.
- Bphen were sequentially deposited to a thickness of 30 nm each, as electron-transport layer 533 over the light-emitting layer 521 .
- LiF was deposited to a thickness of 1 nm over the electron-transport layer 533 .
- Al was deposited on the electron-injection layer 534 to a thickness of 200 nm.
- a light-emitting element was sealed by fixing a sealing substrate to the substrate 520 using a sealant for an organic EL device in a glove box under a nitrogen atmosphere.
- the sealant was applied to surround the light-emitting element, the substrate 520 and the sealing substrate were bonded to each other, irradiation with ultraviolet light having a wavelength of 365 nm at 6 J/cm 2 was performed, and heat treatment was performed at 80° C. for 1 hour. Through the above steps, the light-emitting element 3 was obtained.
- the light-emitting elements 4 and 5 and the comparative light-emitting elements 1 to 4 were fabricated through the same steps as those for the above-mentioned light-emitting element 3 except that structures of their light-emitting layers were different from the structure of the light-emitting layer of the light-emitting element 3.
- PCCzPTzn is a thermally activated delayed fluorescence substance (a first organic compound)
- Cz2DBT is a host material (a second organic compound)
- TBP is a guest material.
- the light-emitting element 4 has a structure similar to that of the light-emitting element 3 except for the guest material.
- PCCzPTzn is a thermally activated delayed fluorescence substance (a first organic compound)
- CzTAZ1 is a host material (a second organic compound)
- TBP is a guest material.
- the light-emitting element 5 has a structure similar to that of the light-emitting element 4 except for the host material.
- Cz2DBT is a host material (a second organic compound)
- 1,6mMemFLPAPrn is a guest material.
- a thermally activated delayed fluorescence substance (a first organic compound) is not used for the comparative light-emitting element 1.
- PCCzPTzn is a thermally activated delayed fluorescence substance (a first organic compound)
- Cz2DBT is a host material (a second organic compound)
- 1,6mMemFLPAPrn is a guest material.
- the comparative light-emitting element 2 has a structure similar to that of the light-emitting element 3 except for the concentration of the guest material.
- PCCzPTzn is a thermally activated delayed fluorescence substance (a first organic compound)
- Cz2DBT is a host material (a second organic compound)
- TBP is a guest material.
- the comparative light-emitting element 3 has a structure similar to that of the light-emitting element 4 except for the concentration of the guest material.
- PCCzPTzn is a thermally activated delayed fluorescence substance (a first organic compound)
- CzTAZ1 is a host material (a second organic compound)
- TBP is a guest material.
- the comparative light-emitting element 4 has a structure similar to that of the light-emitting element 5 except for the concentration of the guest material.
- FIGS. 20, 21, and 22 show current efficiency-luminance characteristics, current-voltage characteristics, and external quantum efficiency-luminance characteristics, respectively, of the light-emitting element 3 and the comparative light-emitting elements 1 and 2.
- FIG. 23 shows electroluminescence spectra when a current at a current density of 2.5 mA/cm 2 was supplied to the light-emitting element 3 and the comparative light-emitting elements 1 and 2.
- FIGS. 24, 25, and 26 show current efficiency-luminance characteristics, current-voltage characteristics, and external quantum efficiency-luminance characteristics, respectively, of the light-emitting elements 4 and 5 and the comparative light-emitting elements 3 and 4.
- FIG. 27 shows s electroluminescence spectra when a current at a current density of 2.5 mA/cm 2 was supplied to the light-emitting elements 4 and 5 and the comparative light-emitting elements 3 and 4.
- the measurements of the light-emitting elements were performed at room temperature (in an atmosphere kept at 23° C.).
- Table 8 shows the element characteristics of the light-emitting elements 3 to 5 and the comparative light-emitting element 1 to 4 at around 1000 cd/m 2 .
- the light-emitting elements 3 to 5 show high emission efficiency.
- the comparative light-emitting elements 1 to 4 do not show sufficient emission efficiency.
- Table 9 shows measurement results of the oxidation potentials and the reduction potentials of the thermally activated delayed fluorescence material (PCCzPTzn) and the host material (Cz2DBT or CzTAZ1) in the solution state and the HOMO and LUMO levels estimated from the results. Note that the measurement method is similar to that described in Embodiment 1.
- the oxidation potential and the reduction potential of the thermally activated delayed fluorescence substance are lower than or equal to the oxidation potential of the host material and higher than or equal to the reduction potential of the host material, respectively.
- the HOMO level and the LUMO level of the thermally activated delayed fluorescence substance which are estimated from the oxidation potential and the reduction potential are higher than or equal to the HOMO level of the host material and lower than or equal to the LUMO level of the host material, respectively. Therefore, both the singlet excited state and the triplet excited state which are efficiently formed by carrier recombination in the thermally activated delayed fluorescence substance can be transferred efficiently to the guest material. As a result, the light-emitting elements 3 to 5 show high emission efficiency.
- the results in which the emission efficiency of the light-emitting element 3 is higher than that of the comparative light-emitting element 2, and the emission efficiency of the comparative light-emitting element 2 is higher than that of the comparative light-emitting element 1 show that the use of PCCzPTzn as the thermally activated delayed fluorescence substance for the light-emitting layer 521 can increase the emission efficiency. This is because a triplet excited state generated by recombined carriers in PCCzPTzn, which was the thermally activated delayed fluorescence substance, was converted into a single excited state by reverse intersystem crossing.
- the emission efficiency of the light-emitting element 3 higher than that of the comparative light-emitting element 2 means that the weight ratio of the host material (Cz2DBT) to the guest material (1,6mMemFLPAPrn) in the light-emitting layer 521 is preferably from 1:0.001 to 1:0.01 (host material:guest material). This is because the concentration of a guest material that is sufficiently lower than that of a host material can suppress generation of a triplet excited state of the guest material.
- the emission efficiency of the light-emitting element 4 is higher than that of the comparative light-emitting element 3 and the emission efficiency of the light-emitting element 5 is higher than that of the comparative light-emitting element 4, which means that the weight ratio of the host material (Cz2DBT or CzTAZ1) to the guest material (TBP) in the light-emitting layer 521 is preferably from 1:0.001 to 1:0.01 (host material:guest material).
- the weight ratio of the host material to the thermally activated delayed fluorescence substance is preferably from 1:0.05 to 1:0.5 (host material:thermally activated delayed fluorescence substance) and the weight ratio of the host material to the guest material is preferably from 1:0.001 to 1:0.01 (host material:guest material).
- the use of a structure of one embodiment of the present invention can provide a light-emitting element with high emission efficiency.
- 100 EL layer, 101 : electrode, 102 : electrode, 111 : hole-injection layer, 112 : hole-transport layer, 118 : electron-transport layer, 119 : electron-injection layer, 120 : light-emitting layer, 131 : organic compound, 132 : organic compound, 133 : guest material, 150 : light-emitting element, 401 : electrode, 402 : electrode, 411 : hole-injection layer, 412 : hole-transport layer, 413 : electron-transport layer, 414 : electron-injection layer, 415 : hole-injection layer, 416 : hole-transport layer, 417 : electron-transport layer, 418 : electron-injection layer, 421 : organic compound, 422 : organic compound, 423 : guest material, 431 : organic compound, 432 : organic compound, 433 : guest material, 441 : light-emitting unit, 442 : light-emitting unit, 443 : light-in
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Abstract
Provided is a light-emitting element including a fluorescence-emitting material with high emission efficiency. The light-emitting element includes a pair of electrodes and an EL layer between the pair of electrodes. The EL layer includes a first organic compound, a second organic compound, and a guest material. The first organic compound has a function of emitting a thermally activated delayed fluorescence at room temperature. The guest material has a function of emitting fluorescence. A HOMO level of the first organic compound higher than or equal to a HOMO level of the second organic compound. A LUMO level of the first organic compound is lower than or equal to a LUMO level of the second organic compound.
Description
- One embodiment of the present invention relates to a light-emitting element in which a light-emitting layer capable of providing light emission by application of an electric field is provided between a pair of electrodes, and also relates to a display device, an electronic device, and a lighting device including the light-emitting element.
- Note that one embodiment of the present invention is not limited to the above technical field. The technical field of one embodiment of the invention disclosed in this specification and the like relates to an object, a method, or a manufacturing method. One embodiment of the present invention relates to a process, a machine, manufacture, or a composition of matter.
- Specifically, examples of the technical field of one embodiment of the present invention disclosed in this specification include a semiconductor device, a display device, a liquid crystal display device, a light-emitting device, a lighting device, a power storage device, a storage device, a method for driving any of them, and a method for manufacturing any of them.
- In recent years, research and development have been extensively conducted on light-emitting elements utilizing electroluminescence (EL). In a basic structure of such a light-emitting element, a layer containing a light-emitting substance (an EL layer) is interposed between a pair of electrodes. By application of a voltage between the electrodes of this element, light emission from the light-emitting substance can be obtained.
- Since the above light-emitting element is a self-luminous type, a display device using this light-emitting element has advantages such as high visibility, no necessity of a backlight, and low power consumption. Furthermore, such a light-emitting element also has advantages in that the element can be formed to be thin and lightweight, and that response time is high.
- In the case of a light-emitting element in which an EL layer containing an organic material as the light-emitting material is provided between a pair of electrodes (e.g., an organic EL element), application of a voltage between the pair of electrodes causes injection of electrons from the cathode and holes from the anode into the EL layer having a light-emitting property, and thus a current flows. By recombination of the injected electrons and holes, the organic material having a light-emitting property is put in an excited state, whereby light emission is obtained from the excited organic compound having a light-emitting property.
- The excited state of an organic material can be a singlet excited state or a triplet excited state, and light emission from the singlet excited state (S1) is referred to as fluorescence, and light emission from the triplet excited state (T1) is referred to as phosphorescence. The statistical generation ratio of the excited states in the light-emitting element is considered to be S1:T1=1:3. In other words, a light-emitting element containing a phosphorescent material has higher emission efficiency than a light-emitting element containing a fluorescent material. Therefore, a light-emitting element containing a phosphorescent material capable of converting the triplet excited state into light emission has been actively developed in recent years.
- As one of materials capable of partly converting the triplet excited state into light emission, a thermally activated delayed fluorescence (TADF) substance has been known. In a thermally activated delayed fluorescence substance, a singlet excited state is generated from a triplet excited state by reverse intersystem crossing, and the singlet excited state is converted into light emission.
Patent Document 1 andPatent Document 2 each disclose a thermally activated delayed fluorescence substance. - In order to increase emission efficiency of a light-emitting element using a thermally activated delayed fluorescence substance, not only efficient generation of a singlet excited state from a triplet excited state but also efficient emission from a singlet excited state, that is, high fluorescence quantum yield are important in a thermally activated delayed fluorescence substance. It is, however, difficult to design a light-emitting material that meets these two.
-
Patent Document 3 discloses a method: in a light-emitting element containing a thermally activated delayed fluorescence substance and a material emitting fluorescence, singlet excitation energy of the thermally activated delayed fluorescence substance is transferred to the material emitting fluorescence and light emission is obtained from the material emitting fluorescence. -
- [Patent Document 1] Japanese Published Patent Application No. 2004-241374
- [Patent Document 2] Japanese Published Patent Application No. 2006-24830
- [Patent Document 3] Japanese Published Patent Application No. 2014-45179
- In order to increase emission efficiency of a light-emitting element containing a thermally activated delayed fluorescence substance and a material emitting fluorescence, efficient generation of a singlet excited state from a triplet excited state is important. In addition, efficient energy transfer from an excited state of the thermally activated delayed fluorescence substance to an excited state of the material emitting fluorescence is important.
- An object of one embodiment of the present invention is to provide a light-emitting element having high emission efficiency which contains a fluorescent material as a light-emitting material. Another object of one embodiment of the present invention is to provide a light-emitting element with high reliability. Another object of one embodiment of the present invention is to provide a light-emitting element with high emission efficiency and high reliability. Another object of one embodiment of the present invention is to provide a novel light-emitting element. Another object of one embodiment of the present invention is to provide a novel light-emitting element with high emission efficiency and low power consumption.
- Note that the description of the above object does not disturb the existence of other objects. In one embodiment of the present invention, there is no need to achieve all the objects. Objects other than the above objects will be apparent from and can be derived from the description of the specification and the like.
- One embodiment of the present invention is a light-emitting element including a pair of electrodes and an EL layer between the pair of electrodes. The EL layer includes a first organic compound, a second organic compound, and a guest material. The first organic compound has a function of emitting a thermally activated delayed fluorescence at room temperature. The guest material has a function of emitting fluorescence. A HOMO of the first organic compound has an energy level higher than or equal to an energy level of a HOMO of the second organic compound. A LUMO of the first organic compound has an energy level lower than or equal to an energy level of a LUMO of the second organic compound.
- Another embodiment of the present invention is a light-emitting element including a pair of electrodes and an EL layer between the pair of electrodes. The EL layer includes a first organic compound, a second organic compound, and a guest material. The first organic compound has a function of emitting a thermally activated delayed fluorescence at room temperature. The guest material has a function of emitting fluorescence. An oxidation potential of the first organic compound is lower than or equal to an oxidation potential of the second organic compound. A reduction potential of the first organic compound is higher than or equal to a reduction of the second organic compound.
- In the above structure, a difference between a singlet excitation energy level of the first organic compound and a triplet excitation energy level of the first organic compound is preferably larger than 0 eV and smaller than or equal to 0.2 eV.
- In the above structure, the guest material preferably emits light.
- In the above structure, it is preferable that the first organic compound include a first π-electron deficient heteroaromatic skeleton and a first π-electron rich heteroaromatic skeleton and the second organic compound include a second π-electron deficient heteroaromatic skeleton and a second π-electron rich heteroaromatic skeleton.
- In the above structure, it is preferable that the first π-electron deficient heteroaromatic skeleton include a diazine skeleton or a triazine skeleton, the first π-electron rich heteroaromatic skeleton include any one or more of an acridine skeleton, a phenoxazine skeleton, or a 3-(9-phenyl-9H-carbazole-3-yl)-9H-carbazole skeleton, the second π-electron deficient heteroaromatic skeleton include a pyridine skeleton or a diazine skeleton, and the second π-electron rich heteroaromatic skeleton include any one or more of a furan skeleton, a thiophene skeleton, a fluorine skeleton, and a pyrrole skeleton.
- In the above structure, it is preferable that a weight ratio of the second organic compound to the first organic compound be from 1:0.05 to 1:0.5 (the second organic compound: the first organic compound) and a weight ratio of the second organic compound to the guest material be from 1:0.001 to 1:0.01 (the second organic compound: the guest material).
- Another embodiment of the present invention is a display device which includes the light-emitting element and a color filter, a sealant, or a transistor. Another embodiment of the present invention is an electronic device which includes the display device and a housing or a function of a touch sensor. Another embodiment of the present invention is a lighting device which includes the light-emitting element in the above embodiment and a housing or a touch sensor.
- One embodiment of the present invention makes it possible to provide a light-emitting element having high emission efficiency which contains a fluorescent material as a light-emitting material. One embodiment of the present invention makes it possible to provide a light-emitting element with high reliability. One embodiment of the present invention makes it possible to provide a light-emitting element with high emission efficiency and high reliability. One embodiment of the present invention makes it possible to provide a novel light-emitting element. One embodiment of the present invention makes it possible to provide a novel light-emitting element with high emission efficiency and low power consumption.
- Note that the description of these effects does not disturb the existence of other effects. One embodiment of the present invention does not necessarily achieve all the objects listed above. Other effects will be apparent from and can be derived from the description of the specification, the drawings, the claims, and the like.
-
FIGS. 1A and 1B are schematic cross-sectional views illustrating a light-emitting element of one embodiment of the present invention. -
FIGS. 2A to 2C show correlations of energy levels in a light-emitting element of one embodiment of the present invention. -
FIGS. 3A and 3B are a schematic cross-sectional view of a light-emitting element of one embodiment of the present invention and a diagram illustrating the correlation of energy levels in a light-emitting layer. -
FIGS. 4A and 4B are a schematic cross-sectional view of a light-emitting element of one embodiment of the present invention and a diagram illustrating the correlation of energy levels in a light-emitting layer. -
FIGS. 5A and 5B are a block diagram and a circuit diagram illustrating a display device of one embodiment of the present invention. -
FIGS. 6A and 6B are perspective views of an example of a touch panel of one embodiment of the present invention. -
FIGS. 7A to 7C are cross-sectional views of examples of a display device and a touch sensor of one embodiment of the present invention. -
FIGS. 8A and 8B illustrate examples of a touch panel of one embodiment of the present invention. -
FIGS. 9A and 9B are a block diagram and a timing chart of a touch sensor of one embodiment of the present invention. -
FIG. 10 is a circuit diagram illustrating a touch sensor of one embodiment of the present invention. -
FIG. 11 is a perspective view illustrating a display module of one embodiment of the present invention. -
FIGS. 12A to 12G illustrate electronic devices of one embodiment of the present invention. -
FIG. 13 illustrates a lighting device of one embodiment of the present invention. -
FIG. 14 is a schematic cross-sectional view illustrating light-emitting elements of Examples 1 and 2. -
FIG. 15 shows transient fluorescence characteristics of a host material of Example 1. -
FIG. 16 shows current efficiency-luminance characteristics of light-emitting elements of Example 1. -
FIG. 17 shows current-voltage characteristics of light-emitting elements of Example 1. -
FIG. 18 shows external quantum efficiency-luminance characteristics of light-emitting elements of Example 1. -
FIG. 19 shows electroluminescence spectra of light emitted from light-emitting elements of Example 1. -
FIG. 20 shows current efficiency-luminance characteristics of light-emitting elements of Example 2. -
FIG. 21 shows current-voltage characteristics of light-emitting elements Example 2. -
FIG. 22 shows external quantum efficiency-luminance characteristics of light-emitting elements Example 2. -
FIG. 23 shows electroluminescence spectra of light-emitting elements of Example 2. -
FIG. 24 shows current efficiency-luminance characteristics of light-emitting elements of Example 2. -
FIG. 25 shows current-voltage characteristics of light-emitting elements of Example 2. -
FIG. 26 shows external quantum efficiency-luminance characteristics of light-emitting elements of Example 2. -
FIG. 27 shows electroluminescence spectra of light-emitting elements of Example 2. - Embodiments of the present invention will be explained below with reference to the drawings. However, the present invention is not limited to description to be given below, and it is to be easily understood that modes and details thereof can be variously modified without departing from the purpose and the scope of the present invention. Accordingly, the present invention should not be interpreted as being limited to the content of the embodiments below.
- Note that the position, the size, the range, or the like of each structure illustrated in drawings and the like is not accurately represented in some cases for simplification. Therefore, the disclosed invention is not necessarily limited to the position, the size, the range, or the like disclosed in the drawings and the like.
- Ordinal numbers such as “first” and “second” in this specification and the like are used for convenience and do not denote the order of steps or the stacking order of layers in some cases. Therefore, for example, description can be made even when “first” is replaced with “second” or “third”, as appropriate. In addition, the ordinal numbers in this specification and the like are not necessarily the same as those which specify one embodiment of the present invention.
- In the description of modes of the present invention in this specification and the like with reference to the drawings, the same components in different diagrams are commonly denoted by the same reference numeral in some cases.
- In this specification and the like, the terms “film” and “layer” can be interchanged with each other depending on the case or circumstances. For example, the term “conductive layer” can be changed into the term “conductive film” in some cases. Also, the term “insulating film” can be changed into the term “insulating layer” in some cases.
- Note that in this specification and the like, a singlet excited state means a singlet state with excited energy. An S1 level means the lowest level of the singlet excitation energy, that is, the lowest level of excited energy in a singlet excited state. A triplet excited state means a triplet state with excited energy. A T1 level means the lowest level of the triplet excitation energy, that is, the lowest level of excited energy in a triplet excited state. Note that in this specification and the like, a singlet excited state and a singlet excitation energy level mean the lowest singlet excited state and the S1 level, respectively, in some cases. A triplet excited state and a triplet excitation energy level mean the lowest singlet excited state and the T1 level, respectively, in some cases.
- In this specification and the like, a fluorescent material refers to a material that emits light in the visible light region when the level of the lowest singlet excited state (S1 level) relaxes to the ground state. A phosphorescent material refers to a material that emits light in the visible light region at room temperature when the level of the lowest triplet excited state (T1 level) relaxes to the ground state. That is, a phosphorescent material refers to a material that can convert triplet excitation energy into visible light.
- In this specification and the like, a thermally activated delayed fluorescent substance is a material which can generate a singlet excited state from a triplet excited state by reverse intersystem crossing by thermal activation. The thermally activated delayed fluorescent substance may include a material which can generate a singlet excited state by itself from a triplet excited state by reverse intersystem crossing, for example, a material which emits TADF. Alternatively, the thermally activated delayed fluorescent substance may include a combination of two kinds of materials which form exciplexes.
- It also can be said that the thermally activated delayed fluorescent substance is a material of which a triplet excited state is close to a singlet excited state. Specifically, a material in which the difference between the energy levels of the triplet excited state and the singlet excited state is larger than 0 eV and smaller than or equal to 0.2 eV is preferably used. That is, it is preferable that the difference between the energy levels of the triplet excited state and the singlet excited state be larger than 0 eV and smaller than or equal to 0.2 eV in a material which can generate a singlet excited state by itself from a triplet excited state by reverse intersystem crossing, for example, a material which emits TADF, or it is preferable that the difference between the levels of the triplet excited state and the singlet excited state be larger than 0 eV and smaller than or equal to 0.2 eV in exciplexes.
- In this specification and the like, thermally activated delayed fluorescence emission energy refers to an emission peak (including a shoulder) on the shortest wavelength side of thermally activated delayed fluorescence. In this specification and the like, phosphorescence emission energy or a triplet excitation energy refers to a phosphorescence emission peak (including a shoulder) on the shortest wavelength side of phosphorescence emission. Note that the phosphorescence emission can be observed by time-resolved photoluminescence in a low-temperature (e.g., 10 K) environment.
- Note that in this specification and the like, “room temperature” refers to a temperature in a range of 0° C. to 40° C.
- In this embodiment, a light-emitting element according to one embodiment of the present invention will be described with reference to
FIGS. 1A and 1B andFIGS. 2A to 2C . - First, a structure of a light-emitting element of one embodiment of the present invention will be described with reference to
FIGS. 1A and 1B . - A light-emitting
element 150 includes anEL layer 100 between a pair of electrodes (anelectrode 101 and an electrode 102). TheEL layer 100 includes at least a light-emittinglayer 120. Although theelectrode 101 is an anode and theelectrode 102 is a cathode in this embodiment, they can be interchanged for the structure of the light-emittingelement 150. - The
EL layer 100 inFIG. 1A includes a functional layer in addition to the light-emittinglayer 120. The functional layer is composed of a hole-injection layer 111, a hole-transport layer 112, an electron-transport layer 118, and an electron-injection layer 119. Note that the structure of theEL layer 100 is not limited to the structure illustrated inFIG. 1A , and at least one selected from the hole-injection layer 111, the hole-transport layer 112, the electron-transport layer 118, and the electron-injection layer 119 is included. TheEL layer 100 may include another functional layer which can reduce a barrier to hole or electron injection, enhance a hole/electron-transport property, inhibit a hole/electron transport property, suppress a quenching phenomenon due to an electrode, and the like. -
FIG. 1B is a schematic cross-sectional view of an example of the light-emittinglayer 120 inFIG. 1A . The light-emittinglayer 120 inFIG. 1B includes anorganic compound 131, anorganic compound 132, and aguest material 133. - A thermally activated delayed fluorescence substance is preferably used for the
organic compound 131. A thermally activated delayed fluorescence substance can convert triplet excitation energy into singlet excitation energy by reverse intersystem crossing. At least part of the triplet excitation energy generated in the light-emittinglayer 120 is thus converted into singlet excitation energy by theorganic compound 131. The singlet excitation energy is transferred to theguest material 133 and then is extracted as fluorescent emission. For this reason, a difference in energy level between singlet excitation energy and triplet excitation energy of theorganic compound 131 is preferably larger than 0 eV and smaller than or equal to 0.2 eV. In addition, the singlet excitation energy level of theorganic compound 131 is preferably higher than the singlet excitation energy level of theguest material 133, and the triplet excitation energy level of theorganic compound 131 is preferably higher than the singlet excitation energy level of theguest material 133, in which case the triplet excitation energy level of theorganic compound 131 can be closer to the singlet excitation energy level. - A wide-bandgap material is preferably used for the
organic compound 132 to prevent deactivation of theorganic compound 131 and theguest material 133. In other words, the singlet excitation energy level of theorganic compound 132 is preferably higher than the singlet excitation energy level of theorganic compound 131 and that of theguest material 133, and the triplet excitation energy level of theorganic compound 132 is preferably higher than the triplet excitation energy level of theorganic compound 131 and that of theguest material 133. The light-emittinglayer 120 may contain other compounds having a function similar to theorganic compound 132. - The
guest material 133 may be a light-emitting organic material, which preferably is capable of emitting fluorescence (hereinafter also referred to as a fluorescent material). An example in which a fluorescent material is used as theguest material 133 will be described. Note that theguest material 133 may be referred to as the fluorescent material. - First, an emission mechanism of the light-emitting
element 150 will be described. - In the light-emitting
element 150 of one embodiment of the present invention, voltage application between the pair of electrodes (theelectrodes 101 and 102) allows electrons and holes to be injected from the cathode and the anode, respectively, into theEL layer 100 and thus current flows. By recombination of the injected electrons and holes, theguest material 133 in the light-emittinglayer 120 of theEL layer 100 is brought into an excited state to provide light emission. - Note that light emission from the
guest material 133 can be obtained through the following three processes: - (α) direct recombination process in a guest material;
(β) energy transfer process from a thermally activated delayed fluorescence substance; and
(γ) energy transfer process from a host material. - First, the direct recombination process in the
guest material 133 is explained with reference toFIG. 2A , which is a schematic diagram showing the correlation between energy levels. Note that the following explains what terms and signs inFIG. 2A represent: - Host1 (131): the
organic compound 131; - Host2 (132): the
organic compound 132; - Guest (133): the guest material 133 (fluorescent material);
- SA: the level of the lowest singlet excitation energy of the
organic compound 131; - TA: the level of the lowest triplet excitation energy of the
organic compound 131; - SH: the level of the lowest singlet excitation energy of the
organic compound 132; - TH: the level of the lowest triplet excitation energy of the
organic compound 132; - SG: the level of the lowest singlet excitation energy of the guest material 133 (fluorescent material); and
- TG: the level of the lowest triplet excitation energy of the guest material 133 (fluorescent material).
- As shown in
FIG. 2A , carriers (electrons and holes) are recombined in theguest material 133, and theguest material 133 is brought into an excited state. In the case where the excited state of theguest material 133 is a singlet excited state, fluorescence is obtained. In contrast, in the case where the excited state of theguest material 133 is a triplet excited state, thermal deactivation occurs. - In the (α) direct recombination process in a guest material, high emission efficiency can be obtained from the singlet excited state of the
guest material 133 when the fluorescence quantum efficiency of theguest material 133 is high. However, the triplet excited state of theguest material 133 does not contribute to light emission. - <<(β) Energy Transfer Process from a Thermally Activated Delayed Fluorescence Substance>>
- Next, the energy transfer process of the
organic compound 131 and theguest material 133 is described with reference toFIG. 2B , which is a schematic diagram showing the correlation between energy levels. Note that indication and numerals inFIG. 2B are similar to those inFIG. 2A . - Carriers are recombined in the
organic compound 131, and theorganic compound 131 is brought into an excited state. In the case where the excited state of theorganic compound 131 is a single excited state and SA of theorganic compound 131 is higher than SG of theguest material 133, the singlet excitation energy of theorganic compound 131 is transferred from SA of theorganic compound 131 to SG of theguest material 133 as shown by a route E1 inFIG. 2B , whereby theguest material 133 is brought into the singlet excited state. Fluorescence is obtained from theguest material 133 in the singlet excited state. - Note that since direct transition of the
guest material 133 from a singlet ground state to a triplet excited state is forbidden, energy transfer from theorganic compound 131 in the singlet excited state to theguest material 133 in the triplet excited state is unlikely to be a main energy transfer process; therefore, the description is omitted. In other words, energy transfer from theorganic compound 131 in the singlet excited state to theguest material 133 in the singlet excited state as shown in the following general formula (G1) is important. -
1 A*+ 1 G→ 1 A+ 1 G (G1) - Note that in the general formula (G1), 1A* and 1G* represent the singlet excitation states of the
organic compound 131 and theguest material 133, respectively, and 1A and 1G represent the singlet ground states of theorganic compound 131 and theguest material 133, respectively. - In the case where the
organic compound 131 is brought into a triplet excitation state, fluorescence is obtained through the following two processes. - Since the
organic compound 131 is a thermally activated delayed fluorescence substance, excitation energy is transferred from TA to SA of theorganic compound 131 by reverse intersystem crossing (upconversion) as shown by a route A1 inFIG. 2B . This is the first process. - Subsequently, in the case where SA of the
organic compound 131 is higher than SG of theguest material 133, excitation energy is transferred from SA of theorganic compound 131 to SG of theguest material 133 as shown by the route E1 inFIG. 2B , whereby theguest material 133 is brought into the singlet excited state. This is the second process. Fluorescence is obtained from theguest material 133 in the singlet excited state. - The first and second processes are represented by the following general formula (G2).
-
3 A*+ 1 G→(reverse intersystem crossing)→1 A*+ 1 G→ 1 A+ 1 G* (G2) - Note that in the general formula (G2), 1A* represents the triplet excitation state of the
organic compound 131; 1A* and 1G* represent the singlet excitation states of theorganic compound 131 and theguest material 133, respectively; and 1A and 1G represent the singlet ground states of theorganic compound 131 and theguest material 133, respectively. - As represented by the general formula (G2), the singlet excited state (1A*) of the
organic compound 131 is generated from the triplet excited state (3A*) of theorganic compound 131, which is a thermally activated delayed fluorescence substance, by reverse intersystem crossing. Then, excitation energy is transferred to the singlet excited state (1G*) of theguest material 133. - When all the energy transfer processes described in the (β) Energy transfer process from a thermally activated delayed fluorescence substance occur efficiently, both the triplet excitation energy and the singlet excitation energy of the
organic compound 131 are efficiently converted into the singlet excited state (1G*) of theguest material 133, leading to high-efficiency light emission. - However, if the
organic compound 131 releases excitation energy as light or heat and is deactivated before the excitation energy is transferred from the singlet excited state of theorganic compound 131 to the singlet excited state of theguest material 133, the emission efficiency of the light-emitting element is decreased. In addition, the emission efficiency is also decreased by a decrease in efficiency of A1, which is the previous process where theorganic compound 131 is transferred from a triplet excited state to a singlet excited state by reverse intersystem crossing. The energy difference between TA and SA is large particularly when TA of theorganic compound 131 is lower than TG of theguest material 133 and SA≧SG>TG>TA is satisfied. As a result, the reverse intersystem crossing shown by the route A1 inFIG. 2B is unlikely to occur; accordingly, the efficiency of the subsequent energy transfer process shown by the route E1 is decreased to lower efficiency for generating a singlet excited state of theguest material 133. Thus, TA is preferably higher than TG, that is, emission energy of theorganic compound 131, which is a thermally activated delayed fluorescence substance, is preferably higher than phosphorescence emission energy of theguest material 133. - The excitation energy is thermally deactivated also when excitation energy is transferred from TA of the
organic compound 131 to TG of theguest material 133 as shown by a route E2 inFIG. 2B . It is thus preferable that the energy transfer process shown by the route E2 inFIG. 2B be less likely to occur because the generation efficiency of the triplet excited state of theguest material 133 can be decreased and the occurrence of thermal deactivation of excitation energy can be reduced. Thus, the weight percentage of theguest material 133 is preferably smaller than that of theorganic compound 131. Specifically, their weight ratio (the organic compound 131:the guest material 133) is preferably from 1:0.001 to 1:0.05, further preferably from 1:0.001 to 1:0.01. - Note that when the direct recombination process in the
guest material 133 becomes dominant, the triplet excited state of theguest material 133 very likely to occur in the light-emitting layer to cause thermal deactivation of excited energy, resulting in a decreased emission efficiency. That is, it is preferable that the probability of the (β) energy transfer process from a thermally activated delayed fluorescence substance be higher than that of the (or) direct recombination process in a guest material because the generation efficiency of the triplet excited state of theguest material 133 can be decreased and the occurrence of thermal deactivation of excited energy when the excited state of theguest material 133 is a triplet excited state can be reduced. Thus, as mentioned above, it is preferable that the weight percentage of theguest material 133 be smaller than that of theorganic compound 131. Specifically, their weight ratio (the organic compound 131:the guest material 133) is preferably from 1:0.001 to 1:0.05, further preferably from 1:0.001 to 1:0.01. - <<(γ) Energy Transfer Process from a Host Material>>
- Next, the energy transfer process from the
organic compound 132 to theorganic compound 131 or theguest material 133 is described with reference toFIG. 2C , which is a schematic diagram showing the correlation between energy levels. Note that indication and numerals inFIG. 2C are similar to those inFIG. 2A . - Carriers are recombined in the
organic compound 132, and theorganic compound 132 is brought into an excited state. In the case where the excited state of theorganic compound 132 is a single excited state and SH of theorganic compound 132 is higher than SA of theorganic compound 131 and SG of theguest material 133, the singlet excitation energy is transferred from SH of theorganic compound 132 to SG of theguest material 133, whereby theguest material 133 is brought into the singlet excited state. Alternatively, the singlet excitation energy transferred from SH of theorganic compound 132 to SA of theorganic compound 131 is transferred to SG of theguest material 133 through the above-described (β) energy transfer process from a thermally activated delayed fluorescence substance. Fluorescence is obtained from theguest material 133 in the singlet excited state. Note that theorganic compound 132 in this embodiment is a host material. - Note that since direct transition of the
guest material 133 from a singlet ground state to a triplet excited state is forbidden, energy transfer from theorganic compound 132 in the singlet excited state to theguest material 133 in the triplet excited state is unlikely to be a main energy transfer process; therefore, a description thereof is omitted. In other words, energy transfer from theorganic compound 132 in the singlet excited state to theguest material 133 in the singlet excited state as shown in the following general formula (G3) or (G4) is possible. -
1 H*+ 1 A+ 1 G→ 1 H+ 1 A+ 1 G* (G3) -
1 H*+ 1 A+ 1 G→H+ 1 A*+ 1 G→H+ 1 A+ 1 G* (G4) - Note that in the general formula (G3) or (G4), 1H*, 1A*, and 1G* represent the singlet excitation states of the
organic compound 132, theorganic compound 131, and theguest material 133, respectively; and 1H, 1A, and 1G represent the singlet ground states of theorganic compound 132, theorganic compound 131, and theguest material 133, respectively. - In the case where the exited state of the
organic compound 132 is the triplet excited state, when TH of theorganic compound 132 is higher than TA of theorganic compound 131 and SA of theorganic compound 131 is higher than SG of theguest material 133, fluorescence is obtained through the following process. - First, energy is transferred from TH of the
organic compound 132 to TA of theorganic compound 131. - Subsequently, as described in the (β) energy transfer process from a thermally activated delayed fluorescence substance, energy is transferred from SA of the
organic compound 131 to SG of theguest material 133 through the reverse intersystem crossing (the route A1) in theorganic compound 131, which is a thermally activated delayed fluorescence substance, so that fluorescence is obtained from theguest material 133 in the singlet excited state. - The energy transfer process is expressed by the following general formula (G5).
-
3 H*+ 1 A+ 1 G→ 1 H+ 3 A*+ 1 G→(reverse intersystem crossing)→1 H+ 1 A*+ 1 G→ 1 H+ 1 A+ 1 G* (G5) - Note that in the general formula (G5), 3H* and 3A* represent the triplet excitation states of the
organic compounds organic compound 131 and theguest material 133, respectively; and 1H, 1A, and 1G represent the singlet ground states of theorganic compound 132, theorganic compound 131, and theguest material 133, respectively. - As represented by the general formula (G5), the triplet excited state (3A*) of the
organic compound 131 is generated from the triplet excited state (3H*) of theorganic compound 132. Immediately after that, the singlet excited state (1A*) of theorganic compound 131 is generated by reverse intersystem crossing, and then, energy is transferred to the singlet excited state (1G*) of theguest material 133. - When all the energy transfer processes described above in the (γ) energy transfer process from a host material occur efficiently, both the triplet excitation energy and the singlet excitation energy of the
organic compound 132 are efficiently converted into the singlet excited state (1G*) of theguest material 133, and emission from theguest material 133 is possible. - However, if the
organic compound 132 releases excitation energy as light or heat and is deactivated before the excitation energy is transferred from the singlet excited state of theorganic compound 132 to the singlet excited state of theguest material 133, the emission efficiency of the light-emitting element is decreased. In addition, the emission efficiency is also decreased by a decrease in efficiency of the route A1, which is the previous process where theorganic compound 131 is transferred from a triplet excited state to a singlet excited state by reverse intersystem crossing. Particularly when TH of theorganic compound 132 is lower than TA of theorganic compound 131, energy transfer process from TH of theorganic compound 132 to TA of theorganic compound 131 is unlikely to occur and reverse intersystem crossing in theorganic compound 131 does not occur, leading to a decrease in generation efficiency of a singlet excitation state of theguest material 133. Thus, TH of theorganic compound 132 is preferably higher than TA of theorganic compound 131. - In the case where excitation energy is transferred from the TH of the
organic compound 132 to the TG of theguest material 133 as shown by a route E3 inFIG. 2C , the excitation energy is also thermally deactivated. Therefore, it is preferable that the energy transfer process shown by the route E3 inFIG. 2C be less likely to occur because the generation efficiency of the triplet excited state of theguest material 133 can be decreased and the occurrence of thermal deactivation can be reduced. Thus, the weight percentage of theguest material 133 is preferably smaller than that of theorganic compound 132. Specifically, their weight ratio (the organic compound 132:the guest material 133) is preferably from 1:0.001 to 1:0.05, further preferably from 1:0.001 to 1:0.01. - As described above, although part of excitation energy is converted into fluorescence of the
guest material 133 in the (γ) energy transfer process from a host material, there is a possibility of thermal deactivation in the routes E2 and E3 inFIG. 2C . Thus, it is preferable that the probability of the (0) energy transfer process from a thermally activated delayed fluorescence substance be higher than those of the (γ) energy transfer process from a host material and the (α) direct recombination process in a guest material because the generation efficiency of the triplet excited state in the light-emittinglayer 120 can be decreased, that is, the occurrence of thermal deactivation can be reduced and the emission efficiency of the light-emittingelement 150 can be increased. Carrier recombination in theorganic compound 131, which is a thermally activated delayed fluorescence substance, is important for increasing the probability of the (β) energy transfer process from a thermally activated delayed fluorescence substance. - The relationship of energy level between the
organic compound 131 and theorganic compound 132 is important to generate carrier recombination in theorganic compound 131. In particular, the relationship of energy level between a highest occupied molecular orbital (HOMO) and a lowest unoccupied molecular orbital (LUMO), or the relationship between an oxidation potential and a reduction potential is important. - Carriers injected from the pair of electrodes into the
EL layer 100 reach the light-emittinglayer 120, whereby they are injected into a substance included in the light-emittinglayer 120. At this time, holes and electrons tend to enter more stable HOMO and LUMO, respectively. Thus, what is important for carrier recombination in theorganic compound 131, which is a thermally activated delayed fluorescence substance, is that the HOMO level of theorganic compound 131 is higher than or equal to the HOMO level of theorganic compound 132 and that the LUMO level of theorganic compound 131 is lower than or equal to the LUMO level of theorganic compound 132. It is also important that the oxidation potential of theorganic compound 131 is lower than or equal to the oxidation potential of theorganic compound 132 and the reduction potential of theorganic compound 131 is higher than or equal to the reduction potential of theorganic compound 132. - In such a structure, exciplexes are unlikely to be formed between the
organic compounds - Although carriers can easily transfer between adjacent molecules of the
organic compound 131 in the light-emittinglayer 120, carriers easily transfer also to a functional layer (e.g., the hole-transport layer 112 and the electron-transport layer 118) other than the light-emittinglayer 120. It is thus preferable that the weight percentage of theorganic compound 131 is smaller than that of theorganic compound 132 for carrier recombination in theorganic compound 131 in the light-emittinglayer 120. In addition, in order to suppress energy transfer between excited-state molecules and ground-state molecules of theorganic compound 131, the weight percentage of theorganic compound 131 is preferably smaller than that of theorganic compound 132. In the case where a molecule of theorganic compound 131 is adjacent to a molecule of theguest material 133, there is a possibility of energy transfer from a triplet excitation state of theorganic compound 131 to a triplet excitation state of theguest material 133. Thus, the weight percentage of theorganic compound 131 is preferably smaller than that of theorganic compound 132 in order to suppress the energy transfer. Specifically, their weight ratio (the organic compound 132:the organic compound 131) is preferably from 1:0.05 to 1:0.5. - Next, factors controlling the above-described processes of intermolecular energy transfer between the
organic compound guest material 133 are described. As mechanisms of the intermolecular energy transfer, two mechanisms, i.e., Förster mechanism (dipole-dipole interaction) and Dexter mechanism (electron exchange interaction), have been proposed. Although intermolecular energy transfer between theorganic compound 131 and theguest material 133 is described here, the same is applied to intermolecular energy transfer between theorganic compound 132 and theguest material 133. - In Förster mechanism, energy transfer does not require direct contact between molecules and energy is transferred through a resonant phenomenon of dipolar oscillation between the
organic compound 131 and theguest material 133. By the resonant phenomenon of dipolar oscillation, theorganic compound 131 provides energy to theguest material 133, and thus, theorganic compound 131 in an excited state is put in a ground state and theguest material 133 in a ground state is put in an excited state. Note that the rate constant kh*→g of Förster mechanism is expressed byFormula 1. -
- In
Formula 1, ν denotes a frequency, f′h(ν) denotes a normalized emission spectrum of the organic compound 131 (a fluorescent spectrum in energy transfer from a singlet excited state, and a phosphorescent spectrum in energy transfer from a triplet excited state), εg(ν) denotes a molar absorption coefficient of theguest material 133, N denotes Avogadro's number, n denotes a refractive index of a medium, R denotes an intermolecular distance between theorganic compound 131 and theguest material 133, τ denotes a measured lifetime of an excited state (fluorescence lifetime or phosphorescence lifetime), c denotes the speed of light, φ denotes a luminescence quantum yield (a fluorescence quantum yield in energy transfer from a singlet excited state, and a phosphorescence quantum yield in energy transfer from a triplet excited state), and K2 denotes a coefficient (0 to 4) of orientation of a transition dipole moment between theorganic compound 131 and theguest material 133. Note that K2=2/3 in random orientation. - In Dexter mechanism, the
organic compound 131 and theguest material 133 are close to a contact effective range where their orbitals overlap, and theorganic compound 131 in an excited state and theguest material 133 in a ground state exchange their electrons, which leads to energy transfer. Note that the rate constant kh*→g of Dexter mechanism is expressed byFormula 2. -
- In
Formula 2, h denotes a Planck constant, K denotes a constant having an energy dimension, ν denotes a frequency, f′h(ν) denotes a normalized emission spectrum of the organic compound 131 (a fluorescent spectrum in energy transfer from a singlet excited state, and a phosphorescent spectrum in energy transfer from a triplet excited state), ε′g(ν) denotes a normalized absorption spectrum of theguest material 133, L denotes an effective molecular radius, and R denotes an intermolecular distance between theorganic compound 131 and theguest material 133. - Here, the efficiency of energy transfer from the
organic compound 131 to the guest material 133 (energy transfer efficiency φET) is expressed byFormula 3. InFormula 3, kr denotes a rate constant of a light-emission process (fluorescence in energy transfer from a singlet excited state, and phosphorescence in energy transfer from a triplet excited state) of theorganic compound 131, kn denotes a rate constant of a non-light-emission process (thermal deactivation or intersystem crossing) of theorganic compound 131, and r denotes a measured lifetime of an excited state of theorganic compound 131. -
- According to
Formula 3, it is found that the energy transfer efficiency φET can be increased by increasing the rate constant kh*→g of energy transfer so that another competing rate constant kr+kn(=1/τ) becomes relatively small. - In both the energy transfer processes of the general formulae (G1) and (G2), since energy is transferred from the singlet excited state (1A*) of the
organic compound 131 to the singlet excited state (1G*) of theguest material 133, energy transfers by both Förster mechanism (Formula 1) and Dexter mechanism (Formula 2) occur. - First, an energy transfer by Förster mechanism is considered. When τ is eliminated from
Formula 1 andFormula 3, it can be said that the energy transfer efficiency φET is higher when the quantum yield φ (here, a fluorescence quantum yield because energy transfer from a singlet excited state is discussed) is higher. However, in practice, a more important factor is that the emission spectrum of the organic compound 131 (here, a fluorescent spectrum because energy transfer from a singlet excited state is discussed) largely overlaps with the absorption spectrum of the guest material 133 (absorption corresponding to the transition from the singlet ground state to the singlet excited state). Note that it is preferable that the molar absorption coefficient of theguest material 133 be also high. This means that the emission spectrum of theorganic compound 131 overlaps with the absorption band of theguest material 133 which is on the longest wavelength side. - Next, an energy transfer by Dexter mechanism is considered. According to
Formula 2, in order to increase the rate constant kh*→g, it is preferable that an emission spectrum of the organic compound 131 (here, a fluorescent spectrum because energy transfer from a singlet excited state is discussed) largely overlap with an absorption spectrum of the guest material 133 (absorption corresponding to transition from a singlet ground state to a singlet excited state). - The above description suggests that the energy transfer efficiency can be optimized by making the emission spectrum of the
organic compound 131 overlap with the absorption band of theguest material 133 which is on the longest wavelength side. - In view of this, one embodiment of the present invention provides a light-emitting element which includes the
organic compound 131 having a function as an energy donor capable of efficiently transferring energy to theguest material 133. Theorganic compound 131 is a thermally activated delayed fluorescence substance and thus has a feature that the singlet excitation energy level and the triplet excitation energy level are close to each other. Specifically, it is preferable that theorganic compound 131 have a difference of larger than 0 eV and smaller than or equal to 0.2 eV between the singlet excitation energy level and the triplet excitation energy level. This enables transition (reverse intersystem crossing) of theorganic compound 131 from the triplet excited state to the singlet excited state to be likely to occur. Therefore, the generation efficiency of the singlet excited state of theorganic compound 131 can be increased. Furthermore, in order to facilitate energy transfer from the singlet excited state of theorganic compound 131 to the singlet excited state of theguest material 133 having a function as an energy acceptor, it is preferable that the emission spectrum of theorganic compound 131 overlap with the absorption band of theguest material 133 which is on the longest wavelength side. Thus, the generation efficiency of the singlet excited state of theguest material 133 can be increased. - In addition, in the light-emitting
element 150 of one embodiment of the present invention, the HOMO level of theorganic compound 131 is higher than or equal to the HOMO level of theorganic compound 132, and the LUMO level of theorganic compound 131 is lower than or equal to the LUMO level of theorganic compound 132; or the oxidation potential of theorganic compound 131 is lower than or equal to the oxidation potential of theorganic compound 132 and the reduction potential of theorganic compound 131 is higher than or equal to the reduction potential of theorganic compound 132, which allows recombination of carriers injected into theEL layer 100 to be performed efficiently in theorganic compound 131. Thus, the occurrence of thermal deactivation can be reduced and the emission efficiency can be increased. - Next, components of a light-emitting element of one embodiment of the present invention are described in detail.
- The
organic compound 131 in the light-emittinglayer 120 is composed of one kind of material. Note that another compound having a function similar to theorganic compound 131 may be included in the light-emittinglayer 120. For example, in the case where theorganic compound 131 is composed of one kind of material, any of the following materials can be used. - First, a fullerene, a derivative thereof, an acridine derivative such as proflavine, eosin, or the like can be given. Furthermore, a metal-containing porphyrin, such as a porphyrin containing magnesium (Mg), zinc (Zn), cadmium (Cd), tin (Sn), platinum (Pt), indium (In), or palladium (Pd), can be given. Examples of the metal-containing porphyrin include a protoporphyrin-tin fluoride complex (SnF2(Proto IX)), a mesoporphyrin-tin fluoride complex (SnF2(Meso IX)), a hematoporphyrin-tin fluoride complex (SnF2(Hemato IX)), a coproporphyrin tetramethyl ester-tin fluoride complex (SnF2(Copro III-4Me)), an octaethylporphyrin-tin fluoride complex (SnF2(OEP)), an etioporphyrin-tin fluoride complex (SnF2(Etio I)), and an octaethylporphyrin-platinum chloride complex (PtCl2(OEP)), which are shown in the following structural formulae.
- Alternatively, a heterocyclic compound having a π-electron rich heteroaromatic ring and a π-electron deficient heteroaromatic ring, such as 2-(biphenyl-4-yl)-4,6-bis(12-phenylindolo[2,3-a]carbazol-11-yl)-1,3,5-triazine (abbreviation: PIC-TRZ), 2-{4-[3 (N-phenyl-9H-carbazol-3-yl)-9H-carbazol-9-yl]phenyl}-4,6-diphenyl-1,3,5-triazine (abbreviation: PCCzPTzn), 2-[4-(10H-phenoxazine-0-yl)phenyl]-4,6-diphenyl-1,3,5-triazine (abbreviation: PXZ-TRZ), 3-[4-(5-phenyl-5,10-dihydrophenazine-10-yl)phenyl]-4,5-diphenyl-1,2,4-triazole (abbreviation: PPZ-3TPT), 3-(9,9-dimethyl-9H-acridin-10-yl)-9H-xanthen-9-one (abbreviation: ACRXTN), bis[4-(9,9-dimethyl-9,10-dihydroacridine)phenyl]sulfone (abbreviation: DMAC-DPS), or 10-phenyl-10H, 10′H-spiro[acridin-9,9′-anthracen]-10′-one (abbreviation: ACRSA) shown in the following structural formulae, can be used as the organic compound 131. The heterocyclic compound is preferably used because of the π-electron rich heteroaromatic ring and the π-electron deficient heteroaromatic ring, for which the electron-transport property and the hole-transport property are high. Note that a substance in which the π-electron rich heteroaromatic ring is directly bonded to the π-electron deficient heteroaromatic ring is particularly preferably used because the donor property of the π-electron rich heteroaromatic ring and the acceptor property of the π-electron deficient heteroaromatic ring are both increased and the difference between the level of the singlet excited state and the level of the triplet excited state becomes small. The heterocyclic compound is preferably used because of the π-electron rich heteroaromatic ring and the π-electron deficient heteroaromatic ring, for which the electron-transport property and the hole-transport property are high. Among skeletons having the π-electron deficient heteroaromatic ring, a diazine skeleton (a pyrimidine skeleton, a pyrazine skeleton, or a pyridazine skeleton) and a triazine skeleton have favorable stability and reliability and are particularly preferable. Among skeletons having the π-electron rich heteroaromatic ring, an acridine skeleton, a phenoxazine skeleton, or a 3-(9-phenyl-9H-carbazole-3-yl)-9H-carbazole skeleton has favorable stability and reliability; thus, any of the skeletons is particularly preferable. Note that a substance in which the π-electron rich heteroaromatic ring is directly bonded to the π-electron deficient heteroaromatic ring is particularly preferably used because the donor property of the π-electron rich heteroaromatic ring and the acceptor property of the π-electron deficient heteroaromatic ring are both increased and the difference between the level of the singlet excited state and the level of the triplet excited state becomes small.
- The following compounds can be used as the
organic compound 132 in the light-emittinglayer 120. Because theorganic compound 132 functions as a host material in the light-emittinglayer 120, it preferably contains a skeleton which easily receives electrons (a skeleton having an electron-transport property) and/or a skeleton which easily receives holes (a skeleton having an hole-transport property). - As the compound containing a skeleton which easily accepts electrons (a skeleton having an electron-transport property), a compound, a metal complex, or the like including a π-electron deficient heteroaromatic skeleton can be used. Specific examples include a metal complex such as bis(10-hydroxybenzo[h]quinolinato)beryllium(II) (abbreviation: BeBq2), bis(2-methyl-8-quinolinolato) (4-phenylphenolato)aluminum(III) (abbreviation: BAlq), bis(8-quinolinolato)zinc(II) (abbreviation: Znq), bis[2-(2-benzoxazolyl)phenolato]zinc(II) (abbreviation: ZnPBO), or bis[2-(2-benzothiazolyl)phenolato]zinc(II) (abbreviation: ZnBTZ); a heterocyclic compound having an azole skeleton such as 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation: PBD), 3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole (abbreviation: TAZ), 9-[4-(4,5-diphenyl-4H-1,2,4-triazol-3-yl)phenyl]-9H-carbazole (abbreviation: CzTAZ1), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene (abbreviation: OXD-7), 9-[4-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl]-9H-carbazole (abbreviation: CO11), 2,2′,2″-(1,3,5-benzenetriyl)tris(1-phenyl-1H-benzimidazole) (abbreviation: TPBI), or 2-[3-(dibenzothiophene-4-yl)phenyl]-1-phenyl-1H-benzimidazole (abbreviation: mDBTBIm-II); a heterocyclic compound having a diazine skeleton such as 2-[3-(dibenzothiophene-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation: 2mDBTPDBq-II), 2-[3′-(dibenzothiophene-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline (abbreviation: 2mDBTBPDBq-II), 2-[3′-(9H-carbazol-9-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline (abbreviation: 2mCzBPDBq), 2-[3-(3,9′-bi-9H-carbazol-9-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation: 2mCzCzPDBq), 4,6-bis[3-(9H-carbazol-9-yl)phenyl]pyrimidine (abbreviation; 4,6mCzP2Pm), 4,6-bis[3-(phenanthren-9-yl)phenyl]pyrimidine (abbreviation: 4,6mPnP2Pm), or 4,6-bis[3-(4-dibenzothienyl)phenyl]pyrimidine (abbreviation: 4,6mDBTP2Pm-II); a heterocyclic compound having a triazine skeleton such as 2-{4-[3-(N-phenyl-9H-carbazol-3-yl)-9H-carbazol-9-yl]phenyl}-4,6-diphenyl-1,3,5-triazine (abbreviation: PCCzPTzn); and a heterocyclic compound having a pyridine skeleton such as 3,5-bis[3-(9H-carbazol-9-yl)phenyl]pyridine (abbreviation: 35DCzPPy) or 1,3,5-tri[3-(3-pyridyl)phenyl]benzene (abbreviation: TmPyPB). Among the heterocyclic compounds, the heterocyclic compounds having diazine skeletons (pyrimidine, pyrazine, pyridazine) or having a pyridine skeleton are highly reliable and stable and is thus preferably used. In addition, the heterocyclic compounds having the skeletons have a high electron-transport property to contribute to a reduction in drive voltage.
- As the compound having a skeleton which easily accepts holes (a skeleton having a hole-transport property), a compound having a π-electron rich heteroaromatic skeleton, an aromatic amine skeleton, or the like can be favorably used. Specific examples include a compound having an aromatic amine skeleton such as 2-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]spiro-9,9′-bifluorene (abbreviation: PCASF), 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB), N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine (abbreviation: TPD), 4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl (abbreviation: BSPB), 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine (abbreviation: BPAFLP), 4-phenyl-3′-(9-phenylfluoren-9-yl)triphenylamine (abbreviation: mBPAFLP), 4-phenyl-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PCBA1BP), 4,4′-diphenyl-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PCBBi1BP), 4-(1-naphthyl)-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PCBANB), 4,4′-di(1-naphthyl)-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PCBNBB), 9,9-dimethyl-N-phenyl-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]fluoren-2-amine (abbreviation: PCBAF), or N-phenyl-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]spiro-9,9′-bifluoren-2-amine (abbreviation: PCBASF);
- N-(1,1′-biphenyl-4-yl)-9,9-dimethyl-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-9H-fluoren-2-amine (abbreviation: PCBBiF), a compound having a carbazole skeleton such as 1,3-bis(N-carbazolyl)benzene (abbreviation: mCP), 4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP), 3,6-bis(3,5-diphenylphenyl)-9-phenylcarbazole (abbreviation: CzTP), 3,6-di(9H-carbazol-9-yl)-9-phenyl-9H-carbazole (abbreviation: PhCzGI), 2,8-di(9H-carbazol-9-yl)-dibenzothiophene (abbreviation: Cz2DBT), or 9-phenyl-9H-3-(9-phenyl-9H-carbazol-3-yl)carbazole (abbreviation: PCCP); a compound having a thiophene skeleton such as 4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzothiophene) (abbreviation: DBT3P-II), 2,8-diphenyl-4-[4-(9-phenyl-9H-fluoren-9-yl)phenyl]dibenzothiophene (abbreviation: DBTFLP-III), or 4-[4-(9-phenyl-9H-fluoren-9-yl)phenyl]-6-phenyldibenzothiophene (abbreviation: DBTFLP-IV); and a compound having a furan skeleton such as 4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzofuran) (abbreviation: DBF3P-II) or 4-{3-[3-(9-phenyl-9H-fluoren-9-yl)phenyl]phenyl}dibenzofuran (abbreviation: mmDBFFLBi-II). Among the above-described compounds, a compound including any one or more of a furan skeleton, a thiophene skeleton, a fluorine skeleton, and a pyrrole skeleton is preferable because it is stable and reliable and has a high hole-transport property to contribute to a reduction in driving voltage.
- In addition, among the above-described compounds, a compound including a pyridine skeleton or a diazine skeleton (a pyrimidine skeleton, a pyrazine skeleton, and a pyridazine skeleton) as a π-electron deficient heteroaromatic skeleton and including any one or more of a furan skeleton, a thiophene skeleton, a fluorine skeleton, and a pyrrole skeleton as a π-electron rich heteroaromatic skeleton has a high carrier-transport property and thus contributes to a reduction in driving voltage. In addition, a compound including any of the skeletons has favorable reliability and thus is preferable. Note that an indole skeleton, a carbazole skeleton, or the 3-(9H-carbazol-9-yl)-9H-carbazole skeleton is particularly preferable as a pyrrole skeleton.
- Note that the above-described compounds are non-limiting examples of the
organic compounds organic compound 131 is higher than or equal to the HOMO level of theorganic compound 132, and the LUMO level of theorganic compound 131 is lower than or equal to the LUMO level of theorganic compound 132; or the oxidation potential of theorganic compound 131 is lower than or equal to the oxidation potential of theorganic compound 132 and the reduction potential of theorganic compound 131 is higher than or equal to the reduction potential of theorganic compound 132. In addition, a thermally activated delayed fluorescence substance may be used for theorganic compound 132. - Table 1 shows measurement results of HOMO and LUMO levels of the above-described compounds in the thin-film state, which are non-limiting examples of the
organic compounds - The structures and abbreviations of these compounds are shown below.
-
TABLE 1 HOMO(eV) LUMO(eV) in thin- in thin- Abbreviation film state film state Organic compound PCCzPTzn −5.86 −3.01 131 (First organic PXZ-TRZ −5.63 −3.13 compound)) Organic compound 2mDBTBPDBq-II −6.17 −3.07 132 (Second organic compound) 2mCzBPDBq −5.78 −2.67 2mCzCzPDBq −5.95 −2.88 4,6mDBTP2Pm-II −6.36 −2.87 4,6mCzP2Pm −6.23 −2.77 35DCzPPy −6.21 −2.73 -
TABLE 2 HOMO(eV) LUMO(eV) Oxidation Reduction estimated from estimated from potential(V) potential(V) oxidation potential oxidation potential Abbreviation in solution state in solution state in solution state in solution state Organic compound PCCzPTzn 0.70 −1.97 −5.64 −2.97 131 (First organic PXZ-TRZ 0.39 −1.95 −5.33 −2.99 compound) Organic compound 2mDBTBPDBq-II 1.28 −2.00 −6.22 −2.94 132 (Second organic 2mCzBPDBq 0.97 −1.99 −5.91 −2.95 compound) 2mCzCzPDBq 0.80 −1.97 −5.74 −2.97 4,6mDBTP2Pm-II 1.28 −2.12 −6.22 −2.83 4,6mCzP2Pm 0.95 −2.06 −5.89 −2.88 35DCzPPy 0.96 −2.56 −5.90 −2.39 -
TABLE 3 Triplet excitation Abbreviation energy level(T1) (eV) Organic compound PCCzPTzn 2.53 131 (First organic compound) Organic compound 2mDBTBPDBq-II 2.41 132 (Second organic 2mCzCzPDBq 2.42 compound) 4,6mDBTP2Pm-II 2.62 4,6mCzP2Pm 2.70 35DCzPPy 2.75 - To obtain the HOMO level of each compound in the thin-film state, the ionization potential of each compound was measured by a photoelectron spectrometer (AC-3, manufactured by Riken Keiki, Co., Ltd.) in the air, and the measured ionization potentials were converted into negative values. In addition, to estimate the optical bandgap of each compound in the solid state, an absorption spectrum of each compound in the thin-film state was measured and the absorption edge was obtained from Tauc plot with an assumption of direct transition. The LUMO energy in the thin-film state was calculated from the energy of the estimated bandgaps and the HOMO levels, which have been obtained.
- Electrochemical characteristics (oxidation and reduction characteristics) of each compound in the solution state were measured by cyclic voltammetry (CV). Note that an electrochemical analyzer (ALS model 600A or 600C, product of BAS Inc.) was used for the measurement. In the measurements, the potential of a working electrode with respect to the reference electrode was changed within an appropriate range, whereby the oxidation peak potential and the reduction peak potential were obtained. In addition, the HOMO and LUMO levels of each compound were calculated from the estimated redox potential of the reference electrode of −4.94 eV and the obtained peak potentials.
- The triplet excitation energy levels were measured by phosphorescence measurement of the compounds. The measurement was performed by using a PL microscope, LabRAM HR-PL, produced by HORIBA, Ltd., a He—Cd laser (325 nm) as excitation light, and a CCD detector at a measurement temperature of 10 K. The triplet excitation energy levels were calculated from a peak on the shortest wavelength side of the phosphorescent spectrum obtained by the measurement.
- With the use of compounds which satisfy the following condition: the HOMO level of the
organic compound 131 is higher than or equal to the HOMO level of theorganic compound 132, and the LUMO level of theorganic compound 131 is lower than or equal to the LUMO level of theorganic compound 132; or the oxidation potential of theorganic compound 131 is lower than or equal to the oxidation potential of theorganic compound 132 and the reduction potential of theorganic compound 131 is higher than or equal to the reduction potential of theorganic compound 132, which are shown in Table 1 and Table 2 as an example, recombination of carriers injected into theEL layer 100 can be efficiently performed in theorganic compound 131 and thus a light-emitting element with high emission efficiency can be provided. - In addition, with the use of compounds such that the triplet excitation energy level of the
organic compound 132 is higher than that of theorganic compound 131, which are shown in Table 3 as an example, energy can be easily transferred from the triplet excitation energy level of theorganic compound 132 to that of theorganic compound 131. Thus, the (γ) energy transfer process from a host material becomes easily occur and a light-emitting element with high emission efficiency can be provided. - In the light-emitting
layer 120, the guest material 133 (fluorescent material) is preferably, but not particularly limited to, an anthracene derivative, a tetracene derivative, a chrysene derivative, a phenanthrene derivative, a pyrene derivative, a perylene derivative, a stilbene derivative, an acridone derivative, a coumarin derivative, a phenoxazine derivative, a phenothiazine derivative, or the like, and for example, any of the following materials can be used. - The examples include 5,6-bis[4-(10-phenyl-9-anthryl)phenyl]-2,2′-bipyridine (abbreviation: PAP2BPy), 5,6-bis[4′-(10-phenyl-9-anthryl)biphenyl-4-yl]-2,2′-bipyridine (abbreviation: PAPP2BPy), N,N′-diphenyl-N,N′-bis[4-(9-phenyl-9H-fluoren-9-yl)phenyl]pyrene-1,6-diamine (abbreviation: 1,6FLPAPrn), N,N′-bis(3-methylphenyl)-N,N-bis[3-(9-phenyl-9H-fluoren-9-yl)phenyl]pyrene-1,6-diamine (abbreviation: 1,6mMemFLPAPrn), N,N′-bis[4-(9H-carbazol-9-yl)phenyl]-N,N′-diphenylstilbene-4,4′-diamine (abbreviation: YGA2S), 4-(9H-carbazol-9-yl)-4′-(10-phenyl-9-anthryl)triphenylamine (abbreviation: YGAPA), 4-(9H-carbazol-9-yl)-4′-(9,10-diphenyl-2-anthryl)triphenylamine (abbreviation: 2YGAPPA), N,9-diphenyl-N-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine (abbreviation: PCAPA), perylene, 2,5,8,11-tetra(tert-butyl)perylene (abbreviation: TBP), 4-(10-phenyl-9-anthryl)-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PCBAPA), N,N″-(2-tert-butylanthracene-9,10-diyldi-4,1-phenylene)bis[N,N′,N′-triphenyl-1,4-phenylenediamine] (abbreviation: DPABPA), N,9-diphenyl-N-[4-(9,10-diphenyl-2-anthryl)phenyl]-9H-carbazol-3-amine (abbreviation: 2PCAPPA), N-[4-(9,10-diphenyl-2-anthryl)phenyl]-N,N′,N-triphenyl-1,4-phenylenediamine (abbreviation: 2DPAPPA), N,N,N,N′,N″,N″,N″,N″-octaphenyldibenzo[g,p]chrysene-2,7,10,15-tetraamine (abbreviation: DBC1), coumarin 30, N-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine (abbreviation: 2PCAPA), N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,9-diphenyl-9H-carbazol-3-amine (abbreviation: 2PCABPhA), N-(9,10-diphenyl-2-anthryl)-N,N′,N-triphenyl-1,4-phenylenediamine (abbreviation: 2DPAPA), N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,N′,N′-triphenyl-1,4-phenylenediamine (abbreviation: 2DPABPhA), 9,10-bis(1,1′-biphenyl-2-yl)-N-[4-(9H-carbazol-9-yl)phenyl]-N-phenylanthracen-2-amine (abbreviation: 2YGABPhA), N,N,9-triphenylanthracen-9-amine (abbreviation: DPhAPhA), coumarin 6, coumarin 545T, N,N′-diphenylquinacridone (abbreviation: DPQd), 5,6,11,12-tetraphenylnaphthacene (common name: rubrene), 5,12-bis(1,1′-biphenyl-4-yl)-6,11-diphenyltetracene (abbreviation: BPT), 2-(2-{2-[4-(dimethylamino)phenyl]ethenyl}-6-methyl-4H-pyran-4-ylidene)propanedinitrile (abbreviation: DCM1), 2-{2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5H-benzo[i]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile (abbreviation: DCM2), N,N,N′,N′-tetrakis(4-methylphenyl)tetracene-5,11-diamine (abbreviation: p-mPhTD), 7,14-diphenyl-N,N,N′,N′-tetrakis(4-methylphenyl)acenaphtho[1,2-a]fluoranthene-3,10-diamine (abbreviation: p-mPhAFD), 2-{2-isopropyl-6-[2-(1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile (abbreviation: DCJTI), 2-{2-tert-butyl-6-[2-(1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethe nyl]-4H-pyran-4-ylidene}propanedinitrile (abbreviation: DCJTB), 2-(2,6-bis {2-[4-(dimethylamino)phenyl]ethenyl}-4H-pyran-4-ylidene)propanedinitrile (abbreviation: BisDCM), 2-{2,6-bis[2-(8-methoxy-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile (abbreviation: BisDCJTM), and 5,10,15,20-tetraphenylbisbenzo[5,6]indeno[1,2,3-cd: 1′,2′,3′-lm]perylene.
- Because the above-described materials of the
guest material 133 are non-limiting examples, other materials may be used as long as light emission (thermally activated delayed fluorescence) of theorganic compound 131 which is an energy donor overlaps with an absorption band (absorption corresponding to the transition of theguest material 133 from the singlet ground state to the singlet excited state) on the longest wavelength in an absorption spectrum of theguest material 133 which is an energy accepter. - Note that the light-emitting
layer 120 can be formed by an evaporation method (including a vacuum evaporation method), an inkjet method, a coating method, gravure printing, or the like. - Next, details of other components of the light-emitting
element 150 inFIG. 1A are described. - The
electrode 101 and theelectrode 102 have functions of injecting holes and electrons into the light-emittinglayer 120. Theelectrodes Group 2 metal such as calcium or magnesium can be used. As the transition metal, a rare earth metal such as ytterbium (Yb) may be used. An alloy containing any of the above metals can be used as the alloy, and MgAg and AlLi can be given as examples. As the conductive compound, a metal oxide such as indium oxide-tin oxide (indium tin oxide) can be given. It is also possible to use an inorganic carbon-based material such as graphene as the conductive compound. As described above, theelectrode 101 and/or theelectrode 102 may be formed by stacking two or more of these materials. - Light emitted from the light-emitting
layer 120 is extracted through theelectrode 101 and/or theelectrode 102. Therefore, at least one of theelectrodes electrode 101 and/or theelectrode 102 is formed to a thickness that is thin enough to transmit visible light (e.g., a thickness of 1 nm to 10 nm). - The hole-
injection layer 111 has a function of reducing a barrier for hole injection from theelectrode 101 to promote hole injection and is formed using a transition metal oxide, a phthalocyanine derivative, or an aromatic amine, for example. As the transition metal oxide, molybdenum oxide, vanadium oxide, ruthenium oxide, tungsten oxide, manganese oxide, or the like can be given. As the phthalocyanine derivative, phthalocyanine, metal phthalocyanine, or the like can be given. As the aromatic amine, a benzidine derivative, a phenylenediamine derivative, or the like can be given. It is also possible to use a high molecular compound such as polythiophene or polyaniline; a typical example thereof is poly(ethylenedioxythiophene)/poly(styrenesulfonic acid), which is self-doped polythiophene. - As the hole-
injection layer 111, a layer containing a composite material of a hole-transport material and a material having a property of accepting electrons from the hole-transport material can also be used. Alternatively, a stack of a layer containing a material having an electron accepting property and a layer containing a hole-transport material may also be used. In a steady state or in the presence of an electric field, electric charge can be transferred between these materials. As examples of the material having an electron-accepting property, organic acceptors such as a quinodimethane derivative, a chloranil derivative, and a hexaazatriphenylene derivative can be given. A specific example is a compound having an electron-withdrawing group (a halogen group or a cyano group), such as 7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation: F4-TCNQ), chloranil, or 2,3,6,7,10,11-hexacyano-1,4,5,8,9,12-hexaazatriphenylene (abbreviation: HAT-CN). Alternatively, a transition metal oxide such as an oxide of a metal fromGroup 4 toGroup 8 can also be used. Specifically, vanadium oxide, niobium oxide, tantalum oxide, chromium oxide, molybdenum oxide, tungsten oxide, manganese oxide, rhenium oxide, or the like can be used. In particular, molybdenum oxide is more preferable because of its stability in the atmosphere, low hygroscopic property, and easiness of handling. - A material having a property of transporting more holes than electrons can be used as the hole-transport material, and a material having a hole mobility of 1×10−6 cm2/Vs or higher is preferable. Specifically, an aromatic amine, a carbazole derivative, an aromatic hydrocarbon, a stilbene derivative, or the like can be used. The compound including a skeleton that easily accepts holes which is described as an example of the
organic compound 132 can be used. Furthermore, the hole-transport material may be a high molecular compound. - The hole-
transport layer 112 is a layer containing a hole-transport material and can be formed using any of the materials given as examples of the material of the hole-injection layer 111. In order that the hole-transport layer 112 has a function of transporting holes injected into the hole-injection layer 111 to the light-emittinglayer 120, the HOMO level of the hole-transport layer 112 is preferably equal or close to the HOMO energy level of the hole-injection layer 111. - The electron-
transport layer 118 has a function of transporting, to the light-emittinglayer 120, electrons injected from theelectrode 102 through the electron-injection layer 119. A material having a property of transporting more electrons than holes can be used as an electron-transport material, and a material having an electron mobility of 1×10−6 cm2/Vs or higher is preferable. Specific examples include a metal complex having a quinoline ligand, a benzoquinoline ligand, an oxazole ligand, or a thiazole ligand; an oxadiazole derivative; a triazole derivative; a phenanthroline derivative; a pyridine derivative; and a bipyridine derivative. The compound having a skeleton that easily accepts electrons which is described as an example of theorganic compound 132 can be used. - The electron-
injection layer 119 has a function of reducing a barrier for electron injection from theelectrode 102 to promote electron injection and can be formed using aGroup 1 metal or aGroup 2 metal, or an oxide, a halide, or a carbonate of any of the metals, for example. Alternatively, a composite material containing an electron-transport material (described above) and a material having a property of donating electrons to the electron-transport material can also be used. As the material having an electron-donating property, aGroup 1 metal, aGroup 2 metal, an oxide of any of the metals, or the like can be given. - Note that the hole-
injection layer 111, the hole-transport layer 112, the electron-transport layer 118, and the electron-injection layer 119 described above can each be formed by an evaporation method (including a vacuum evaporation method), an inkjet method, a coating method, a gravure printing method, or the like. - Besides the above-mentioned materials, an inorganic compound or a high molecular compound (e.g., an oligomer, a dendrimer, or a polymer) may be used for the hole-
injection layer 111, the hole-transport layer 112, the light-emittinglayer 120, the electron-transport layer 118, and the electron-injection layer 119. - The light-emitting
element 150 is fabricated over a substrate of glass, plastic, or the like. As the way of stacking layers over the substrate, layers may be sequentially stacked from theelectrode 101 side or sequentially stacked from theelectrode 102 side. - Note that, for example, glass, quartz, plastic, or the like can be used for the substrate over which the light-emitting
element 150 can be formed. Alternatively, a flexible substrate can be used. The flexible substrate is a substrate that can be bent, such as a plastic substrate made of polycarbonate or polyarylate, for example. A film, an inorganic film formed by evaporation, or the like can also be used. Note that materials other than these can be used as long as they can function as a support in a manufacturing process of the light-emitting element and an optical element or as long as they have a function of protecting the light-emitting element and the optical element. - The light-emitting
element 150 can be formed using a variety of substrates, for example. The type of substrate is not limited to a certain type. As the substrate, a semiconductor substrate (e.g., a single crystal substrate or a silicon substrate), an SOI substrate, a glass substrate, a quartz substrate, a plastic substrate, a metal substrate, a stainless steel substrate, a substrate including stainless steel foil, a tungsten substrate, a substrate including tungsten foil, a flexible substrate, an attachment film, paper including a fibrous material, a base material film, or the like can be used, for example. Examples of the glass substrate include a barium borosilicate glass substrate, an aluminoborosilicate glass substrate, and a soda lime glass substrate. Examples of the flexible substrate, the attachment film, the base film, and the like are substrates of plastics typified by polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polyether sulfone (PES), and polytetrafluoroethylene (PTFE). Another example is a resin such as acrylic. Other examples are polypropylene, polyester, polyvinyl fluoride, polyvinyl chloride, and the like. Other examples are polyamide, polyimide, aramid, epoxy, an inorganic film formed by evaporation, paper, and the like. - Alternatively, a flexible substrate may be used as the substrate, and the light-emitting element may be provided directly on the flexible substrate. Alternatively, a separation layer may be provided between the substrate and the light-emitting element. The separation layer can be used when part or the whole of the light-emitting element formed over the separation layer is completed, separated from the substrate, and transferred to another substrate. In such a case, the light-emitting element can be transferred to a substrate having low heat resistance or a flexible substrate as well. For the above separation layer, a stack including inorganic films, which are a tungsten film and a silicon oxide film, or a resin film of polyimide or the like formed over a substrate can be used, for example.
- In other words, after the light-emitting element is formed using a substrate, the light-emitting element may be transferred to another substrate. Examples of a substrate to which the light-emitting element is transferred include, in addition to the above-described substrates, a cellophane substrate, a stone substrate, a wood substrate, a cloth substrate (including a natural fiber (e.g., silk, cotton, or hemp), a synthetic fiber (e.g., nylon, polyurethane, or polyester), a regenerated fiber (e.g., acetate, cupra, rayon, or regenerated polyester, or the like), a leather substrate, and a rubber substrate. By using such a substrate, a light-emitting element with high durability, a light-emitting element with high heat resistance, a lightweight light-emitting element, or a thin light-emitting element can be obtained.
- The light-emitting
element 150 may be formed over an electrode electrically connected to a field-effect transistor (FET), for example, which is formed over the above-mentioned substrate, so that an active matrix display device in which the FET controls the drive of the light-emittingelement 150 can be manufactured. - In this embodiment, one embodiment of the present invention has been described. Embodiments of the present invention are described in other embodiments. Note that one embodiment of the present invention is not limited to the above examples. For example, one embodiment of the present invention is not limited to the above-described example in which the HOMO level of the
organic compound 131 is higher than or equal to the HOMO level of theorganic compound 132 and the LUMO level of theorganic compound 131 is lower than or equal to the LUMO level of theorganic compound 132, and the example in which the oxidation potential of theorganic compound 131 is lower than or equal to the oxidation potential of theorganic compound 132 and the reduction potential of theorganic compound 131 is higher than or equal to the reduction potential of theorganic compound 132. Depending on circumstances or conditions, in one embodiment of the present invention, the HOMO level of theorganic compound 131 is not necessarily higher than or equal to the HOMO level of theorganic compound 132, and the LUMO level of theorganic compound 131 is not necessarily lower than or equal to the LUMO level of theorganic compound 132. The oxidation potential of theorganic compound 131 is not necessarily lower than or equal to the oxidation potential of theorganic compound 132, and the reduction potential of theorganic compound 131 is not necessarily higher than or equal to the reduction potential of theorganic compound 132. Alternatively, one embodiment of the present invention is not limited to the above-described example in which theorganic compound 131 is a substance which exhibits thermally activated delayed fluorescence at room temperature. Depending on circumstances or conditions, theorganic compound 131 in one embodiment of the present invention may contain a substance other than the substance which exhibits thermally activated delayed fluorescence at room temperature, for example. Alternatively, depending on circumstances or conditions, theorganic compound 131 in one embodiment of the present invention does not necessarily contain the substance which exhibits thermally activated delayed fluorescence at room temperature, for example. Alternatively, one embodiment of the present invention is not limited to the above example in which the weight percentage of theorganic compound 131 is smaller than that of theorganic compound 132. Depending on circumstances or conditions, the weight percentage of theorganic compound 131 is not limited to be smaller than that of theorganic compound 132. - The structure described above in this embodiment can be combined with any of the structures described in the other embodiments as appropriate.
- In this embodiment, a light-emitting element having a structure different from that described in
Embodiment 1 and an emission mechanism of the light-emitting element will be described below with reference toFIGS. 3A and 3B . -
FIG. 3A is a schematic cross-sectional view of a light-emittingelement 450. - The light-emitting
element 450 illustrated inFIG. 3A includes a plurality of light-emitting units (inFIG. 3A , a light-emittingunit 441 and a light-emitting unit 442) between a pair of electrodes (anelectrode 401 and an electrode 402). One light-emitting unit has the same structure as theEL layer 100 illustrated inFIG. 1A . That is, the light-emittingelement 150 inFIG. 1A includes one light-emitting unit, while the light-emittingelement 450 includes the plurality of light-emitting units. Note that theelectrode 401 functions as an anode and theelectrode 402 functions as a cathode in the following description of the light-emittingelement 450; however, the functions may be interchanged in the light-emittingelement 450. - In the light-emitting
element 450 illustrated inFIG. 3A , the light-emittingunit 441 and the light-emittingunit 442 are stacked, and a charge-generation layer 445 is provided between the light-emittingunit 441 and the light-emittingunit 442. Note that the light-emittingunit 441 and the light-emittingunit 442 may have the same structure or different structures. For example, it is preferable that theEL layer 100 illustrated inFIGS. 1A and 1B be used in the light-emittingunit 441 and that a light-emitting layer containing a phosphorescent material as a light-emitting material be used in the light-emittingunit 442. - That is, the light-emitting
element 450 includes a light-emittinglayer 443 and a light-emittinglayer 444. The light-emittingunit 441 includes a hole-injection layer 411, a hole-transport layer 412, an electron-transport layer 413, and an electron-injection layer 414 in addition to the light-emittinglayer 443. The light-emittingunit 442 includes a hole-injection layer 415, a hole-transport layer 416, an electron-transport layer 417, and an electron-injection layer 418 in addition to the light-emittinglayer 444. - The charge-
generation layer 445 preferably contains a composite material of an organic material and a material having an electron accepting property. For the composite material, the composite material that can be used for the hole-injection layer 111 described inEmbodiment 1 may be used. As the organic material, a variety of compounds such as an aromatic amine compound, a carbazole compound, an aromatic hydrocarbon, and a high molecular compound (such as an oligomer, a dendrimer, or a polymer) can be used. An organic material having a hole mobility of 1×10−6 cm2/Vs or higher is preferably used. Note that any other material may be used as long as it has a property of transporting more holes than electrons. Since the composite material of an organic material and a material having an electron accepting property has excellent carrier-injection and carrier-transport properties, low-voltage driving or low-current driving can be realized. Note that when a surface of a light-emitting unit on the anode side is in contact with the charge-generation layer 445 as that of the light-emittingunit 442, the charge-generation layer 445 can also serve as a hole-injection layer or a hole-transport layer of the light-emitting unit; thus, a hole-injection layer or a hole-transport layer does not need to be included in the light-emitting unit. - The charge-
generation layer 445 may have a stacked-layer structure of a layer containing the composite material of an organic material and a material having an electron accepting property and a layer containing another material. For example, the charge-generation layer 445 may be formed using a combination of a layer containing the composite material of an organic material and a material having an electron accepting property with a layer containing one material selected from among materials having an electron donating and a compound having a high electron-transport property. Furthermore, the charge-generation layer 445 may be formed using a combination of a layer containing the composite material of an organic material and a material having an electron accepting property with a layer including a transparent conductive film. - The charge-
generation layer 445 provided between the light-emittingunit 441 and the light-emittingunit 442 may have any structure as long as electrons can be injected to the light-emitting unit on one side and holes can be injected into the light-emitting unit on the other side when a voltage is applied between theelectrode 401 and theelectrode 402. For example, inFIG. 3A , the charge-generation layer 445 injects electrons into the light-emittingunit 441 and holes into the light-emittingunit 442 when a voltage is applied such that the potential of theelectrode 401 is higher than that of theelectrode 402. - The light-emitting element having two light-emitting units is described with reference to
FIG. 3A ; however, a similar structure can be applied to a light-emitting element in which three or more light-emitting units are stacked. With a plurality of light-emitting units partitioned by the charge-generation layer between a pair of electrodes as in the light-emittingelement 450, it is possible to provide a light-emitting element which can emit light with high luminance with the current density kept low and has a long lifetime. A light-emitting element with low power consumption can be provided. - When the structure of the
EL layer 100 shown inFIGS. 1A and 1B is applied to at least one of the plurality of units, a light-emitting element with high emission efficiency can be provided. - The light-emitting
layer 443 contains anorganic compound 421, anorganic compound 422, and aguest material 423. The light-emittinglayer 444 contains anorganic compound 431, anorganic compound 432, and aguest material 433. - In this embodiment, the light-emitting
layer 443 has a structure similar to that of the light-emittinglayer 120 inFIGS. 1A and 1B . That is, theorganic compound 421, theorganic compound 422, and theguest material 423 in the light-emittinglayer 443 correspond to theorganic compound 131, theorganic compound 132, and theguest material 133 in the light-emittinglayer 120, respectively. In the following description, theguest material 433 contained in the light-emittinglayer 444 is a phosphorescent material. Note that theelectrode 401, theelectrode 402, the hole-injection layers transport layers transport layers injection layers electrode 101, theelectrode 102, the hole-injection layer 111, the hole-transport layer 112, the electron-transport layer 118, and the electron-injection layer 119 inEmbodiment 1, respectively. Therefore, detailed description thereof is omitted in this embodiment. - An emission mechanism of the light-emitting
layer 443 is similar to that of the light-emittinglayer 120 inFIGS. 2A to 2C . - Next, an emission mechanism of the light-emitting
layer 444 will be described. - The
organic compound 431 and theorganic compound 432 which are contained in the light-emittinglayer 444 form exciplexes. Theorganic compound 431 serves as a host material and theorganic compound 432 serves as an assist material in the description here. - Although it is acceptable as long as the combination of the
organic compound 431 and theorganic compound 431 can form exciplexes in the light-emittinglayer 444, it is preferred that one organic compound be a material having a hole-transport property and the other organic compound be a material having an electron-transport property. -
FIG. 3B illustrates the correlation of energy levels of theorganic compound 431, theorganic compound 432, and theguest material 433 in the light-emittinglayer 444. The following explains what terms and signs inFIG. 3B represent: - Host (431): the host material (organic compound 431);
- Assist (432): the assist material (organic compound 432);
- Guest (433): the guest material 433 (phosphorescent material);
- SPH: the level of the lowest singlet excited state of the host material (organic compound 431);
- TPH: the level of the lowest triplet excited state of the host material (organic compound 431);
- TPG: the level of the lowest triplet excited state of the guest material 433 (the phosphorescent material);
- SPE: the level of the lowest singlet excited state of exciplexes; and
- TPE: the level of the lowest triplet excited state of exciplexes.
- The level (SPE) of the lowest singlet excited state of exciplexes, which is formed by the
organic compound 432 and theorganic compound 431 and the level (TPE) of the lowest triplet excited state of exciplexes are close to each other (see E7 inFIG. 3B ). - Both energies of SPE and TPE of exciplexes are then transferred to the level (TPG) of the lowest triplet excited state of the guest material 433 (the phosphorescent material); thus, light emission is obtained (see E8 in
FIG. 3B ). - The above-described processes through a route E7 and a route E8 may be referred to as exciplex-triplet energy transfer (ExTET) in this specification and the like.
- When one of the
organic compounds layer 444 exist as exciplexes. The band gap of the exciplex is narrower than that of each of theorganic compounds - When the light-emitting
layer 444 has the above structure, light emission from the guest material 433 (the phosphorescent material) of the light-emittinglayer 444 can be efficiently obtained. - Note that light emitted from the light-emitting
layer 443 preferably has a peak on the shorter wavelength side than light emitted from the light-emittinglayer 444. The luminance of a light-emitting element using the phosphorescent material emitting light with a short wavelength tends to degrade quickly. In view of the above, fluorescence is used for light emission with a short wavelength, so that a light-emitting element with less degradation of luminance can be provided. - Furthermore, the light-emitting
layer 443 and the light-emittinglayer 444 may be made to emit light with different emission wavelengths, so that the light-emitting element can be a multicolor light-emitting element. In that case, the emission spectrum of the light-emitting element is formed by combining light having different emission peaks, and thus has at least two peaks. - The above structure is also suitable for obtaining white light emission. When the light-emitting
layer 443 and the light-emittinglayer 444 emit light of complementary colors, white light emission can be obtained. - In addition, white light emission with a high color rendering property that is formed of three primary colors or four or more colors can be obtained by using a plurality of light-emitting materials emitting light with different wavelengths for one of the light-emitting
layers layers - Next, materials that can be used for the light-emitting
layers - <Material that can be Used for Light-Emitting
Layer 443> - A material that can be used for the light-emitting
layer 120 described inEmbodiment 1 may be used as a material that can be used for the light-emittinglayer 443. - <Material that can be Used for Light-Emitting
Layer 444> - In the light-emitting
layer 444, the organic compound 431 (the host material) exists in the highest proportion in weight ratio, and the guest material 433 (the phosphorescent material) is dispersed in the organic compound 431 (the host material). - Examples of the organic compound 431 (the host material) include a zinc- or aluminum-based metal complex, an oxadiazole derivative, a triazole derivative, a benzimidazole derivative, a quinoxaline derivative, a dibenzoquinoxaline derivative, a dibenzothiophene derivative, a dibenzofuran derivative, a pyrimidine derivative, a triazine derivative, a pyridine derivative, a bipyridine derivative, a phenanthroline derivative, and the like. Other examples are an aromatic amine, a carbazole derivative, and the like. In addition, the compound having a skeleton which easily accepts electrons and the compound having a skeleton which easily accepts holes, which are described in
Embodiment 1, can be used. - As the guest material 433 (the phosphorescent material), an iridium-, rhodium-, or platinum-based organometallic complex or metal complex can be used; in particular, an organoiridium complex such as an iridium-based ortho-metalated complex is preferable. As an ortho-metalated ligand, a 4H-triazole ligand, a 1H-triazole ligand, an imidazole ligand, a pyridine ligand, a pyrimidine ligand, a pyrazine ligand, an isoquinoline ligand, and the like can be given. As the metal complex, a platinum complex having a porphyrin ligand or the like can be given.
- As the organic compound 432 (the assist material), a substance which can form exciplexes together with the
organic compound 431 is used. In that case, it is preferable that theorganic compound 431, theorganic compound 432, and the guest material 433 (the phosphorescent material) be selected such that the emission peak of the exciplexes overlaps with an adsorption band, specifically an adsorption band on the longest wavelength side, of a triplet metal to ligand charge transfer (MLCT) transition of the phosphorescent material. This makes it possible to provide a light-emitting element with drastically improved emission efficiency. Note that in the case where a thermally activated delayed fluorescence material is used instead of the phosphorescent material, it is preferable that the adsorption band on the longest wavelength side be a singlet absorption band. Specifically, the compound having a skeleton which easily accepts electrons or the compound having a skeleton which easily accepts holes, which are described inEmbodiment 1, can be used as theorganic compound 432. - As the light-emitting material contained in the light-emitting
layer 444, any material can be used as long as the material can convert triplet excitation energy into light emission. As an example of the material that can convert triplet excitation energy into light emission, a thermally activated delayed fluorescence material can be given in addition to the phosphorescent material. Therefore, the term “phosphorescent material” in the description can be replaced with the term “thermally activated delayed fluorescence material”. Note that the thermally activated delayed fluorescence material is a material that can up-convert a triplet excited state into a singlet excited state (i.e., reverse intersystem crossing is possible) using a little thermal energy and efficiently exhibits light emission (fluorescence) from the singlet excited state. Thermally activated delayed fluorescence is efficiently obtained under the condition where the difference between the triplet excitation energy level and the singlet excitation energy level is larger than 0 eV and smaller than or equal to 0.2 eV, preferably larger than 0 eV and smaller than or equal to 0.1 eV. - There is no limitation on the emission colors of the light-emitting material included in the light-emitting
layer 443 and the light-emitting material included in the light-emittinglayer 444, and they may be the same or different. Light emitted from the light-emitting materials is mixed and extracted out of the element; therefore, for example, in the case where their emission colors are complementary colors, the light-emitting element can emit white light. In consideration of the reliability of the light-emitting element, the emission peak wavelength of the light-emitting material contained in the light-emittinglayer 443 is preferably shorter than that of the light-emitting material contained in the light-emittinglayer 444. - Note that the light-emitting
layers - Note that the structure described above in this embodiment can be combined with any of the structures described in the other embodiments as appropriate.
- In this embodiment, a light-emitting element having a structure different from those described in
Embodiment 1 andEmbodiment 2 will be described below with reference toFIGS. 4A and 4B . -
FIG. 4A is a schematic cross-sectional view of a light-emittingelement 452 of one embodiment of the present invention. - The light-emitting
element 452 includes a plurality of light-emitting units (inFIG. 4A , a light-emittingunit 446 and a light-emitting unit 447) between anelectrode 401 and anelectrode 402. One light-emitting unit has the same structure as theEL layer 100 illustrated inFIG. 1A . That is, the light-emittingelement 150 inFIG. 1A includes one light-emitting unit, while the light-emittingelement 452 includes the plurality of light-emitting units. Note that theelectrode 401 functions as an anode and theelectrode 402 functions as a cathode in the following description of this embodiment; however, the functions may be interchanged in the light-emittingelement 452. - In the light-emitting
element 452 illustrated inFIG. 4A , the light-emittingunit 446 and the light-emittingunit 447 are stacked, and a charge-generation layer 445 is provided between the light-emittingunit 446 and the light-emittingunit 447. Note that the light-emittingunit 446 and the light-emittingunit 447 may have the same structure or different structures. For example, it is preferable that a light-emitting layer containing a fluorescent material as a light-emitting material be used in the light-emittingunit 446 and that theEL layer 100 illustrated inFIG. 1A be used in the light-emittingunit 447. - That is, the light-emitting
element 452 includes a light-emittinglayer 448 and a light-emittinglayer 449. The light-emittingunit 446 includes a hole-injection layer 411, a hole-transport layer 412, an electron-transport layer 413, and an electron-injection layer 414 in addition to the light-emittinglayer 448. The light-emittingunit 447 includes a hole-injection layer 415, a hole-transport layer 416, an electron-transport layer 417, and an electron-injection layer 418 in addition to the light-emittinglayer 449. - The light-emitting element having two light-emitting units is described with reference to
FIG. 4A ; however, a similar structure can be applied to a light-emitting element in which three or more light-emitting units are stacked. With a plurality of light-emitting units partitioned by the charge-generation layer between a pair of electrodes as in the light-emittingelement 452, it is possible to provide a light-emitting element which can emit light with high luminance with the current density kept low and has a long lifetime. A display device with low power consumption can be provided. - When the structure of the
EL layer 100 shown inFIG. 1A is applied to at least one of the plurality of units, a light-emitting element with high emission efficiency can be provided. - The light-emitting
layer 448 contains ahost material 461 and aguest material 462. The light-emittinglayer 449 contains anorganic compound 471, anorganic compound 472, and aguest material 473. - In this embodiment, the light-emitting
layer 449 has a structure similar to that of the light-emittinglayer 120 inFIGS. 1A and 1B . That is, theorganic compound 471, theorganic compound 472, and theguest material 473 in the light-emittinglayer 449 correspond to theorganic compound 131, theorganic compound 132, and theguest material 133 in the light-emittinglayer 120, respectively. In the following description, theguest material 462 contained in the light-emittinglayer 448 is a fluorescent material. - First, an emission mechanism of the light-emitting
layer 448 will be described. - In the light-emitting
layer 448, an excited state is generated by recombination of carriers. Because the amount of thehost material 461 is large as compared to theguest material 462, thehost material 461 is brought into an excited state by the exciton generation. The ratio of singlet excitons to triplet excitons generated by carrier recombination (hereinafter referred to as exciton generation probability) is approximately 1:3. - First, a case where the triplet excitation energy level of the
host material 461 is higher than the triplet excitation energy level of theguest material 462 will be described below. - The triplet excitation energy level of the
host material 461 is transferred to the triplet excitation energy level of the guest material 462 (triplet energy transfer). However, theguest material 462 in the triplet excitation energy state does not provide light emission in a visible light region because theguest material 462 is the fluorescent material. Thus, it is difficult to use the triplet excitation energy of thehost material 461 for light emission. Therefore, when the triplet excitation energy level of thehost material 461 is higher than the triplet excitation energy level of theguest material 462, it is difficult to use more than approximately 25% of injected carriers for light emission. -
FIG. 4B illustrates the correlation of energy levels of thehost material 461 and theguest material 462 in the light-emittinglayer 448 of one embodiment of the present invention. The following explains what terms and signs inFIG. 4B represent: - Host (461): the
host material 461; - Guest (462): the guest material 462 (fluorescent material);
- SFH: the level of the lowest singlet excited state of the
host material 461; - TFH: the level of the lowest triplet excited state of the
host material 461; - SFG: the level of the lowest singlet excited state of the guest material 462 (fluorescent material); and
- TFG: the level of the lowest triplet excited state of the guest material 462 (fluorescent material).
- As illustrated in
FIG. 4B , the triplet excitation energy level of the guest material 462 (TFG inFIG. 4B ) is higher than the triplet excitation energy level of the host material 461 (TFH inFIG. 4B ). - In addition, as illustrated in
FIG. 4B , triplet excitons collide with each other by triplet-triplet annihilation (TTA) (see a route E9 inFIG. 4B ), and their excitation energy are partly converted into singlet excitons having an energy at the level of the lowest singlet excited state of the host material 461 (SFH). The singlet excitation energy of thehost material 461 is transferred from the level of the lowest singlet excited state of the host material 461 (SFH) to the level of the lowest singlet excited state of the guest material 462 (the fluorescent material) (SFG) that is a level lower than SFH (see a route E10 inFIG. 4B ). Thus, the guest material 462 (the fluorescent material) is brought into the singlet excited state and accordingly emits light. - Because the triplet excitation energy level of the
host material 462 is lower than the triplet excitation energy level of the guest material, excitation energy at TFG is transferred to TFH without deactivation (see a route E11 inFIG. 4B ), which is utilized for TTA. - When the light-emitting
layer 448 has the above structure, light emission from theguest material 462 of the light-emittinglayer 448 can be efficiently obtained. - Note that the light-emitting
layer 448 and the light-emittinglayer 449 may be made to emit light with different emission wavelengths, so that the light-emitting element can be a multicolor light-emitting element. In that case, the emission spectrum of the light-emitting element is formed by combining light having different emission peaks, and thus has at least two peaks. - The above structure is also suitable for obtaining white light emission. When the light-emitting
layer 448 and the light-emittinglayer 449 emit light of complementary colors, white light emission can be obtained. - In addition, white light emission with a high color rendering property that is formed of three primary colors or four or more colors can be obtained by using a plurality of light-emitting materials emitting light with different wavelengths for one of the light-emitting
layers layers - An emission mechanism of the light-emitting
layer 449 is similar to that of the light-emittinglayer 120 inFIGS. 2A to 2C . - Next, materials that can be used for the light-emitting
layers - <Material that can be Used for Light-Emitting
Layer 448> - In the light-emitting
layer 448, thehost material 461 is present in the highest proportion in weight ratio, and the guest material 462 (the fluorescent material) is dispersed in thehost material 461. The singlet excitation energy level of thehost material 461 is preferably higher than the singlet excitation energy level of the guest material 462 (the fluorescent material), while the triplet excitation energy level of thehost material 461 is preferably lower than the triplet excitation energy level of the guest material 462 (the fluorescent material). - An anthracene derivative or a tetracene derivative is preferably used as the
host material 461. This is because these derivatives each have a high singlet excitation energy level and a low triplet excitation energy level. Specific examples include 9-phenyl-3-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: PCzPA), 3-[4-(1-naphthyl)-phenyl]-9-phenyl-9H-carbazole (abbreviation: PCPN), 9-[4-(10-phenyl-9-anthracenyl)phenyl]-9H-carbazole (abbreviation: CzPA), 7-[4-(10-phenyl-9-anthryl)phenyl]-7H-dibenzo[c,g]carbazole (abbreviation: cgDBCzPA), 6-[3-(9,10-diphenyl-2-anthryl)phenyl]-benzo[b]naphtho[1,2-d]furan (abbreviation: 2mBnfPPA), and 9-phenyl-10-{4-(9-phenyl-9H-fluoren-9-yl)biphenyl-4′-yl}anthracene (abbreviation: FLPPA). Besides, 5,12-diphenyltetracene, 5,12-bis(biphenyl-2-yl)tetracene, and the like can be given. - Examples of the guest material 462 (the fluorescent material) include a pyrene derivative, an anthracene derivative, a triphenylene derivative, a fluorene derivative, a carbazole derivative, a dibenzothiophene derivative, a dibenzofuran derivative, a dibenzoquinoxaline derivative, a quinoxaline derivative, a pyridine derivative, a pyrimidine derivative, a phenanthrene derivative, a naphthalene derivative, and the like. A pyrene derivative is particularly preferable because it has a high emission quantum yield. Specific examples of the pyrene derivative include N,N′-bis(3-methylphenyl)-N,N′-bis[3-(9-phenyl-9H-fluoren-9-yl)phenyl]pyrene-1,6-diamine (abbreviation: 1,6mMemFLPAPrn), N,N′-bis[4-(9-phenyl-9H-fluoren-9-yl)phenyl]-N,N′-diphenylpyrene-1,6-diamine (abbreviation: 1,6FLPAPrn), N,N′-bis(dibenzofuran-2-yl)-N,N′-diphenylpyrene-1,6-diamine (abbreviation: 1,6FrAPrn), N,N′-bis(dibenzothiophene-2-yl)-N,N-diphenylpyrene-1,6-diamine (abbreviation: 1,6ThAPrn), and the like. Any of the fluorescent materials described in
Embodiment 1 can be used. - <Material that can be Used for Light-Emitting
Layer 449> - A material that can be used for the light-emitting
layer 120 described inEmbodiment 1 may be used as a material that can be used for the light-emittinglayer 449. - There is no limitation on the emission colors of the light-emitting material included in the light-emitting
layer 448 and the light-emitting material included in the light-emittinglayer 449, and they may be the same or different. Light emitted from the light-emitting materials is mixed and extracted out of the element; therefore, for example, in the case where their emission colors are complementary colors, the light-emitting element can emit white light. In consideration of the reliability of the light-emitting element, the emission peak wavelength of the light-emitting material contained in the light-emittinglayer 448 is preferably shorter than that of the light-emitting material contained in the light-emittinglayer 449. - Note that the light-emitting
layers - Note that the above-described structure can be combined with any of the structures in this embodiment and the other embodiments.
- In this embodiment, a display device including a light-emitting element of one embodiment of the present invention will be described with reference to
FIGS. 5A and 5B . -
FIG. 5A is a block diagram illustrating the display device of one embodiment of the present invention, andFIG. 5B is a circuit diagram illustrating a pixel circuit of the display device of one embodiment of the present invention. - The display device illustrated in
FIG. 5A includes a region including pixels of display elements (the region is hereinafter referred to as a pixel portion 802), a circuit portion provided outside thepixel portion 802 and including circuits for driving the pixels (the portion is hereinafter referred to as a driver circuit portion 804), circuits having a function of protecting elements (the circuits are hereinafter referred to as protection circuits 806), and aterminal portion 807. Note that theprotection circuits 806 are not necessarily provided. - A part or the whole of the
driver circuit portion 804 is preferably formed over a substrate over which thepixel portion 802 is formed, in which case the number of components and the number of terminals can be reduced. When a part or the whole of thedriver circuit portion 804 is not formed over the substrate over which thepixel portion 802 is formed, the part or the whole of thedriver circuit portion 804 can be mounted by chip-on-glass (COG) or tape automated bonding (TAB). - The
pixel portion 802 includes a plurality of circuits for driving display elements arranged in X rows (X is a natural number of 2 or more) and Y columns (Y is a natural number of 2 or more) (such circuits are hereinafter referred to as pixel circuits 801). Thedriver circuit portion 804 includes driver circuits such as a circuit for supplying a signal (scan signal) to select a pixel (the circuit is hereinafter referred to as a scanline driver circuit 804 a) and a circuit for supplying a signal (data signal) to drive a display element in a pixel (the circuit is hereinafter referred to as a signalline driver circuit 804 b). - The scan
line driver circuit 804 a includes a shift register or the like. Through theterminal portion 807, the scanline driver circuit 804 a receives a signal for driving the shift register and outputs a signal. For example, the scanline driver circuit 804 a receives a start pulse signal, a clock signal, or the like and outputs a pulse signal. The scanline driver circuit 804 a has a function of controlling the potentials of wirings supplied with scan signals (such wirings are hereinafter referred to as scan lines GL_1 to GL_X). Note that a plurality of scanline driver circuits 804 a may be provided to control the scan lines GL_1 to GL_X separately. Alternatively, the scanline driver circuit 804 a has a function of supplying an initialization signal. Without being limited thereto, the scanline driver circuit 804 a can supply another signal. - The signal
line driver circuit 804 b includes a shift register or the like. The signalline driver circuit 804 b receives a signal (video signal) from which a data signal is derived, as well as a signal for driving the shift register, through theterminal portion 807. The signalline driver circuit 804 b has a function of generating a data signal to be written to thepixel circuit 801 which is based on the video signal. In addition, the signalline driver circuit 804 b has a function of controlling output of a data signal in response to a pulse signal produced by input of a start pulse signal, a clock signal, or the like. Furthermore, the signalline driver circuit 804 b has a function of controlling the potentials of wirings supplied with data signals (such wirings are hereinafter referred to as data lines DL_1 to DL_Y). Alternatively, the signalline driver circuit 804 b has a function of supplying an initialization signal. Without being limited thereto, the signalline driver circuit 804 b can supply another signal. - The signal
line driver circuit 804 b includes a plurality of analog switches or the like, for example. The signalline driver circuit 804 b can output, as the data signals, signals obtained by time-dividing the video signal by sequentially turning on the plurality of analog switches. The signalline driver circuit 804 b may include a shift register or the like. - A pulse signal and a data signal are input to each of the plurality of
pixel circuits 801 through one of the plurality of scan lines GL supplied with scan signals and one of the plurality of data lines DL supplied with data signals, respectively. Writing and holding of the data signal to and in each of the plurality ofpixel circuits 801 are controlled by the scanline driver circuit 804 a. For example, to thepixel circuit 801 in the m-th row and the n-th column (m is a natural number of less than or equal to X, and n is a natural number of less than or equal to Y), a pulse signal is input from the scanline driver circuit 804 a through the scan line GL_m, and a data signal is input from the signalline driver circuit 804 b through the data line DL_n in accordance with the potential of the scan line GL_m. - The
protection circuit 806 shown inFIG. 5A is connected to, for example, the scan line GL between the scanline driver circuit 804 a and thepixel circuit 801. Alternatively, theprotection circuit 806 is connected to the data line DL between the signalline driver circuit 804 b and thepixel circuit 801. Alternatively, theprotection circuit 806 can be connected to a wiring between the scanline driver circuit 804 a and theterminal portion 807. Alternatively, theprotection circuit 806 can be connected to a wiring between the signalline driver circuit 804 b and theterminal portion 807. Note that theterminal portion 807 means a portion having terminals for inputting power, control signals, and video signals to the display device from external circuits. - The
protection circuit 806 is a circuit that electrically connects a wiring connected to the protection circuit to another wiring when a potential out of a certain range is applied to the wiring connected to the protection circuit. - As illustrated in
FIG. 5A , theprotection circuits 806 are provided for thepixel portion 802 and thedriver circuit portion 804, so that the resistance of the display device to overcurrent generated by electrostatic discharge (ESD) or the like can be improved. Note that the configuration of theprotection circuits 806 is not limited to that, and for example, a configuration in which theprotection circuits 806 are connected to the scanline driver circuit 804 a or a configuration in which theprotection circuits 806 are connected to the signalline driver circuit 804 b may be employed. Alternatively, theprotection circuits 806 may be configured to be connected to theterminal portion 807. - In
FIG. 5A , an example in which thedriver circuit portion 804 includes the scanline driver circuit 804 a and the signalline driver circuit 804 b is shown; however, the structure is not limited thereto. For example, only the scanline driver circuit 804 a may be formed and a separately prepared substrate where a signal line driver circuit is formed (e.g., a driver circuit substrate formed with a single crystal semiconductor film or a polycrystalline semiconductor film) may be mounted. - Each of the plurality of
pixel circuits 801 inFIG. 5A can have a structure illustrated inFIG. 5B , for example. - The
pixel circuit 801 illustrated inFIG. 5B includestransistors capacitor 862, and a light-emittingelement 872. - One of a source electrode and a drain electrode of the
transistor 852 is electrically connected to a wiring to which a data signal is supplied (hereinafter referred to as a data line DL_n). A gate electrode of thetransistor 852 is electrically connected to a wiring to which a gate signal is supplied (hereinafter referred to as a scan line GL_m). - The
transistor 852 has a function of controlling whether to write a data signal. - One of a pair of electrodes of the
capacitor 862 is electrically connected to a wiring to which a potential is supplied (hereinafter referred to as a potential supply line VL_a), and the other is electrically connected to the other of the source electrode and the drain electrode of thetransistor 852. - The
capacitor 862 functions as a storage capacitor for storing written data. - One of a source electrode and a drain electrode of the
transistor 854 is electrically connected to the potential supply line VL_a. Furthermore, a gate electrode of thetransistor 854 is electrically connected to the other of the source electrode and the drain electrode of thetransistor 852. - One of an anode and a cathode of the light-emitting
element 872 is electrically connected to a potential supply line VL_b, and the other is electrically connected to the other of the source electrode and the drain electrode of thetransistor 854. - As the light-emitting
element 872, any of the light-emitting elements described inEmbodiments 1 to 3 can be used. - Note that a high power supply potential VDD is supplied to one of the potential supply line VL_a and the potential supply line VL_b, and a low power supply potential VSS is supplied to the other.
- In the display device including the
pixel circuits 801 inFIG. 5B , thepixel circuits 801 are sequentially selected row by row by the scanline driver circuit 804 a inFIG. 5A , for example, whereby thetransistors 852 are turned on and a data signal is written. - When the
transistors 852 are turned off, thepixel circuits 801 in which the data has been written are brought into a holding state. Furthermore, the amount of current flowing between the source electrode and the drain electrode of thetransistor 854 is controlled in accordance with the potential of the written data signal. The light-emittingelement 872 emits light with a luminance corresponding to the amount of flowing current. This operation is sequentially performed row by row; thus, an image is displayed. - A light-emitting element of one embodiment of the present invention can be used for an active matrix method in which an active element is included in a pixel of a display device or a passive matrix method in which an active element is not included in a pixel of a display device.
- In the active matrix method, as an active element (a non-linear element), not only a transistor but also a variety of active elements (non-linear elements) can be used. For example, a metal insulator metal (MIM), a thin film diode (TFD), or the like can also be used. Since these elements can be formed with a smaller number of manufacturing steps, manufacturing cost can be reduced or yield can be improved. Alternatively, since the size of these elements is small, the aperture ratio can be improved, so that power consumption can be reduced or higher luminance can be achieved.
- As a method other than the active matrix method, the passive matrix method in which an active element (a non-linear element) is not used can also be used. Since an active element (a non-linear element) is not used, the number of manufacturing steps is small, so that manufacturing cost can be reduced or yield can be improved. Alternatively, since an active element (a non-linear element) is not used, the aperture ratio can be improved, so that power consumption can be reduced or higher luminance can be achieved, for example.
- The structure described in this embodiment can be combined with any of the structures described in the other embodiments or examples as appropriate.
- In this embodiment, a display device including a light-emitting element of one embodiment of the present invention and an electronic device in which the display device is provided with an input device will be described with reference to
FIGS. 6A and 6B ,FIGS. 7A to 7C ,FIGS. 8A and 8B ,FIGS. 9A and 9B , andFIG. 10 . - In this embodiment, a
touch panel 2000 including a display device and an input device will be described as an example of an electronic device. In addition, an example in which a touch sensor is used as an input device will be described. -
FIGS. 6A and 6B are perspective views of thetouch panel 2000. Note thatFIGS. 6A and 6B illustrate only main components of thetouch panel 2000 for simplicity. - The
touch panel 2000 includes adisplay device 2501 and a touch sensor 2595 (seeFIG. 6B ). Thetouch panel 2000 also includes asubstrate 2510, asubstrate 2570, and asubstrate 2590. Thesubstrate 2510, thesubstrate 2570, and thesubstrate 2590 each have flexibility. Note that one or all of thesubstrates - The
display device 2501 includes a plurality of pixels over thesubstrate 2510 and a plurality ofwirings 2511 through which signals are supplied to the pixels. The plurality ofwirings 2511 are led to a peripheral portion of thesubstrate 2510, and parts of the plurality ofwirings 2511 form aterminal 2519. The terminal 2519 is electrically connected to an FPC 2509(1). - The
substrate 2590 includes thetouch sensor 2595 and a plurality ofwirings 2598 electrically connected to thetouch sensor 2595. The plurality ofwirings 2598 are led to a peripheral portion of thesubstrate 2590, and parts of the plurality ofwirings 2598 form a terminal. The terminal is electrically connected to an FPC 2509(2). Note that inFIG. 6B , electrodes, wirings, and the like of thetouch sensor 2595 provided on the back side of the substrate 2590 (the side facing the substrate 2510) are indicated by solid lines for clarity. - As the
touch sensor 2595, a capacitive touch sensor can be used. Examples of the capacitive touch sensor are a surface capacitive touch sensor and a projected capacitive touch sensor. - Examples of the projected capacitive touch sensor are a self capacitive touch sensor and a mutual capacitive touch sensor, which differ mainly in the driving method. The use of a mutual capacitive type is preferable because multiple points can be sensed simultaneously.
- Note that the
touch sensor 2595 illustrated inFIG. 6B is an example of using a projected capacitive touch sensor. - Note that a variety of sensors that can sense proximity or touch of a sensing target such as a finger can be used as the
touch sensor 2595. - The projected
capacitive touch sensor 2595 includeselectrodes 2591 andelectrodes 2592. Theelectrodes 2591 are electrically connected to any of the plurality ofwirings 2598, and theelectrodes 2592 are electrically connected to any of theother wirings 2598. - The
electrodes 2592 each have a shape of a plurality of quadrangles arranged in one direction with one corner of a quadrangle connected to one corner of another quadrangle as illustrated inFIGS. 6A and 6B . - The
electrodes 2591 each have a quadrangular shape and are arranged in a direction intersecting with the direction in which theelectrodes 2592 extend. - A
wiring 2594 electrically connects twoelectrodes 2591 between which theelectrode 2592 is positioned. The intersecting area of theelectrode 2592 and thewiring 2594 is preferably as small as possible. Such a structure allows a reduction in the area of a region where the electrodes are not provided, reducing variation in transmittance. As a result, variation in luminance of light passing through thetouch sensor 2595 can be reduced. - Note that the shapes of the
electrodes 2591 and theelectrodes 2592 are not limited thereto and can be any of a variety of shapes. For example, a structure may be employed in which the plurality ofelectrodes 2591 are arranged so that gaps between theelectrodes 2591 are reduced as much as possible, and theelectrodes 2592 are spaced apart from theelectrodes 2591 with an insulating layer interposed therebetween to have regions not overlapping with theelectrodes 2591. In this case, it is preferable to provide, between twoadjacent electrodes 2592, a dummy electrode electrically insulated from these electrodes because the area of regions having different transmittances can be reduced. - Next, the
display device 2501 will be described in detail with reference toFIG. 7A .FIG. 7A corresponds to a cross-sectional view taken along dashed-dotted line X1-X2 inFIG. 6B . - The
display device 2501 includes a plurality of pixels arranged in a matrix. Each of the pixels includes a display element and a pixel circuit for driving the display element. - In the following description, an example of using a light-emitting element that emits white light as a display element will be described; however, the display element is not limited to such an element. For example, light-emitting elements that emit light of different colors may be included so that the light of different colors can be emitted from adjacent pixels.
- For the
substrate 2510 and thesubstrate 2570, for example, a flexible material with a vapor permeability of lower than or equal to 1×10−5 g·m−2·day−1, preferably lower than or equal to 1×10−6 g·m−2·day−1 can be favorably used. Alternatively, materials whose thermal expansion coefficients are substantially equal to each other are preferably used for thesubstrate 2510 and thesubstrate 2570. For example, the coefficients of linear expansion of the materials are preferably lower than or equal to 1×10−3/K, further preferably lower than or equal to 5×10−5/K, and still further preferably lower than or equal to 1×10−5/K. - Note that the
substrate 2510 is a stacked body including an insulatinglayer 2510 a for preventing impurity diffusion into the light-emitting element, aflexible substrate 2510 b, and anadhesive layer 2510 c for attaching the insulatinglayer 2510 a and theflexible substrate 2510 b to each other. Thesubstrate 2570 is a stacked body including an insulatinglayer 2570 a for preventing impurity diffusion into the light-emitting element, aflexible substrate 2570 b, and anadhesive layer 2570 c for attaching the insulatinglayer 2570 a and theflexible substrate 2570 b to each other. - For the
adhesive layer 2510 c and theadhesive layer 2570 c, for example, materials that include polyester, polyolefin, polyamide (e.g., nylon, aramid), polyimide, polycarbonate, or a resin having an acrylic resin, polyurethane, an epoxy resin, or a resin having a siloxane bond such as silicone can be used. - A
sealing layer 2560 is provided between thesubstrate 2510 and thesubstrate 2570. Thesealing layer 2560 preferably has a refractive index higher than that of air. In the case where light is extracted to thesealing layer 2560 side as illustrated inFIG. 7A , thesealing layer 2560 can also serve as an adhesive layer. - A sealant may be formed in the peripheral portion of the
sealing layer 2560. With the use of the sealant, a light-emittingelement 2550R can be provided in a region surrounded by thesubstrate 2510, thesubstrate 2570, thesealing layer 2560, and the sealant. Note that an inert gas (such as nitrogen or argon) may be used instead of thesealing layer 2560. A drying agent may be provided in the inert gas so as to adsorb moisture or the like. An ultraviolet curable resin or a heat curable resin may be used; for example, a polyvinyl chloride (PVC) based resin, an acrylic resin, a polyimide-based resin, an epoxy-based resin, a silicone-based resin, a polyvinyl butyral (PVB) based resin, or an ethylene vinyl acetate (EVA) based resin can be used. For example, an epoxy-based resin or a glass frit is preferably used as the sealant. As a material used for the sealant, a material which is impermeable to moisture or oxygen is preferably used. - The
display device 2501 includes apixel 2502R. Thepixel 2502R includes a light-emittingmodule 2580R. - The
pixel 2502R includes the light-emittingelement 2550R and atransistor 2502 t that can supply electric power to the light-emittingelement 2550R. Note that thetransistor 2502 t functions as part of the pixel circuit. The light-emittingmodule 2580R includes the light-emittingelement 2550R and acoloring layer 2567R. - The light-emitting
element 2550R includes a lower electrode, an upper electrode, and an EL layer between the lower electrode and the upper electrode. As the light-emittingelement 2550R, any of the light-emitting elements described inEmbodiments 1 to 3 can be used, for example. - A microcavity structure may be employed between the lower electrode and the upper electrode so as to increase the intensity of light having a specific wavelength.
- In the case where the
sealing layer 2560 is provided on the light extraction side, thesealing layer 2560 is in contact with the light-emittingelement 2550R and thecoloring layer 2567R. - The
coloring layer 2567R is positioned in a region overlapping with the light-emittingelement 2550R. Accordingly, part of light emitted from the light-emittingelement 2550R passes through thecoloring layer 2567R and is emitted to the outside of the light-emittingmodule 2580R as indicated by an arrow inFIG. 7A . - The
display device 2501 includes a light-blocking layer 2567BM on the light extraction side. The light-blocking layer 2567BM is provided so as to surround thecoloring layer 2567R. - The
coloring layer 2567R is a coloring layer having a function of transmitting light in a particular wavelength region. For example, a color filter for transmitting light in a red wavelength range, a color filter for transmitting light in a green wavelength range, a color filter for transmitting light in a blue wavelength range, a color filter for transmitting light in a yellow wavelength range, or the like can be used. Each color filter can be formed with any of various materials by a printing method, an inkjet method, an etching method using a photolithography technique, or the like. - An insulating
layer 2521 is provided in thedisplay device 2501. The insulatinglayer 2521 covers thetransistor 2502 t. Note that the insulatinglayer 2521 has a function of planarizing unevenness caused by the pixel circuit. The insulatinglayer 2521 may have a function of suppressing impurity diffusion. This can prevent the reliability of thetransistor 2502 t or the like from being lowered by impurity diffusion. - The light-emitting
element 2550R is formed over the insulatinglayer 2521. Apartition 2528 is provided so as to overlap with an end portion of the lower electrode of the light-emittingelement 2550R. Note that a spacer for controlling the distance between thesubstrate 2510 and thesubstrate 2570 may be formed over thepartition 2528. - A scan
line driver circuit 2503 g(1) includes atransistor 2503 t and acapacitor 2503 c. Note that the driver circuit can be formed in the same process and over the same substrate as those of the pixel circuits. - The
wirings 2511 through which signals can be supplied are provided over thesubstrate 2510. The terminal 2519 is provided over thewirings 2511. The FPC 2509(1) is electrically connected to theterminal 2519. The FPC 2509(1) has a function of supplying a video signal, a clock signal, a start signal, a reset signal, or the like. Note that the FPC 2509(1) may be provided with a printed wiring board (PWB). - In the
display device 2501, transistors with any of a variety of structures can be used.FIG. 7A illustrates an example of using bottom-gate transistors; however, the present invention is not limited to this example, and top-gate transistors may be used in thedisplay device 2501 as illustrated inFIG. 7B . - In addition, there is no particular limitation on the polarity of the
transistor 2502 t and thetransistor 2503 t. For these transistors, n-channel and p-channel transistors may be used, or either n-channel transistors or p-channel transistors may be used, for example. Furthermore, there is no particular limitation on the crystallinity of a semiconductor film used for thetransistors Group 14 semiconductors (e.g., a semiconductor including silicon), compound semiconductors (including oxide semiconductors), organic semiconductors, and the like. An oxide semiconductor that has an energy gap of 2 eV or more, preferably 2.5 eV or more, further preferably 3 eV or more is preferably used for one of thetransistors - Next, the
touch sensor 2595 will be described in detail with reference toFIG. 7C .FIG. 7C corresponds to a cross-sectional view taken along dashed-dotted line X3-X4 inFIG. 6B . - The
touch sensor 2595 includes theelectrodes 2591 and theelectrodes 2592 provided in a staggered arrangement on thesubstrate 2590, an insulatinglayer 2593 covering theelectrodes 2591 and theelectrodes 2592, and thewiring 2594 that electrically connects theadjacent electrodes 2591 to each other. - The
electrodes 2591 and theelectrodes 2592 are formed using a light-transmitting conductive material. As a light-transmitting conductive material, a conductive oxide such as indium oxide, indium tin oxide, indium zinc oxide, zinc oxide, or zinc oxide to which gallium is added can be used. Note that a film including graphene may be used as well. The film including graphene can be formed, for example, by reducing a film containing graphene oxide. As a reducing method, a method with application of heat or the like can be employed. - The
electrodes 2591 and theelectrodes 2592 may be formed by, for example, depositing a light-transmitting conductive material on thesubstrate 2590 by a sputtering method and then removing an unnecessary portion by any of various pattern forming techniques such as photolithography. - Examples of a material for the insulating
layer 2593 are a resin such as an acrylic resin or an epoxy resin, a resin having a siloxane bond, and an inorganic insulating material such as silicon oxide, silicon oxynitride, or aluminum oxide. - Openings reaching the
electrodes 2591 are formed in the insulatinglayer 2593, and thewiring 2594 electrically connects theadjacent electrodes 2591. A light-transmitting conductive material can be favorably used as thewiring 2594 because the aperture ratio of the touch panel can be increased. Moreover, a material with higher conductivity than the conductivities of theelectrodes wiring 2594 because electric resistance can be reduced. - One
electrode 2592 extends in one direction, and a plurality ofelectrodes 2592 are provided in the form of stripes. Thewiring 2594 intersects with theelectrode 2592. -
Adjacent electrodes 2591 are provided with oneelectrode 2592 provided therebetween. Thewiring 2594 electrically connects theadjacent electrodes 2591. - Note that the plurality of
electrodes 2591 are not necessarily arranged in the direction orthogonal to oneelectrode 2592 and may be arranged to intersect with oneelectrode 2592 at an angle of more than 0 degrees and less than 90 degrees. - The
wiring 2598 is electrically connected to any of theelectrodes wiring 2598 functions as a terminal. For thewiring 2598, a metal material such as aluminum, gold, platinum, silver, nickel, titanium, tungsten, chromium, molybdenum, iron, cobalt, copper, or palladium or an alloy material containing any of these metal materials can be used. - Note that an insulating layer that covers the insulating
layer 2593 and thewiring 2594 may be provided to protect thetouch sensor 2595. - A
connection layer 2599 electrically connects thewiring 2598 to the FPC 2509(2). - As the
connection layer 2599, any of various anisotropic conductive films (ACF), anisotropic conductive pastes (ACP), or the like can be used. - Next, the
touch panel 2000 will be described in detail with reference toFIG. 8A .FIG. 8A corresponds to a cross-sectional view taken along dashed-dotted line X5-X6 inFIG. 6A . - In the
touch panel 2000 illustrated inFIG. 8A , thedisplay device 2501 described with reference toFIG. 7A and thetouch sensor 2595 described with reference toFIG. 7C are attached to each other. - The
touch panel 2000 illustrated inFIG. 8A includes anadhesive layer 2597 and ananti-reflective layer 2567 p in addition to the components described with reference toFIGS. 7A and 7C . - The
adhesive layer 2597 is provided in contact with thewiring 2594. Note that theadhesive layer 2597 attaches thesubstrate 2590 to thesubstrate 2570 so that thetouch sensor 2595 overlaps with thedisplay device 2501. Theadhesive layer 2597 preferably has a light-transmitting property. A heat curable resin or an ultraviolet curable resin can be used for theadhesive layer 2597. For example, an acrylic resin, an urethane-based resin, an epoxy-based resin, or a siloxane-based resin can be used. - The
anti-reflective layer 2567 p is positioned in a region overlapping with pixels. As theanti-reflective layer 2567 p, a circularly polarizing plate can be used, for example. - Next, a touch panel having a structure different from that illustrated in
FIG. 8A will be described with reference toFIG. 8B . -
FIG. 8B is a cross-sectional view of atouch panel 2001. Thetouch panel 2001 illustrated inFIG. 8B differs from thetouch panel 2000 illustrated inFIG. 8A in the position of thetouch sensor 2595 relative to thedisplay device 2501. Different parts are described in detail below, and the above description of thetouch panel 2000 is referred to for the other similar parts. - The
coloring layer 2567R is positioned in a region overlapping with the light-emittingelement 2550R. The light-emittingelement 2550R illustrated inFIG. 8B emits light to the side where thetransistor 2502 t is provided. Accordingly, part of light emitted from the light-emittingelement 2550R passes through thecoloring layer 2567R and is emitted to the outside of the light-emittingmodule 2580R as indicated by an arrow inFIG. 8B . - The
touch sensor 2595 is provided on thesubstrate 2510 side of thedisplay device 2501. - The
adhesive layer 2597 is provided between thesubstrate 2510 and thesubstrate 2590 and attaches thetouch sensor 2595 to thedisplay device 2501. - As illustrated in
FIG. 8A or 8B , light may be emitted from the light-emitting element to one of upper and lower sides, or both, of the substrate. - Next, an example of a method for driving a touch panel will be described with reference to
FIGS. 9A and 9B . -
FIG. 9A is a block diagram illustrating the structure of a mutual capacitive touch sensor.FIG. 9A illustrates a pulsevoltage output circuit 2601 and acurrent sensing circuit 2602. Note that inFIG. 9A , six wirings X1 to X6 represent theelectrodes 2621 to which a pulse voltage is applied, and six wirings Y1 to Y6 represent theelectrodes 2622 that detect changes in current.FIG. 9A also illustratescapacitors 2603 that are each formed in a region where theelectrodes electrodes - The pulse
voltage output circuit 2601 is a circuit for sequentially applying a pulse voltage to the wirings X1 to X6. By application of a pulse voltage to the wirings X1 to X6, an electric field is generated between theelectrodes capacitor 2603. When the electric field between the electrodes is shielded, for example, a change occurs in the capacitor 2603 (mutual capacitance). The approach or contact of a sensing target can be sensed by utilizing this change. - The
current sensing circuit 2602 is a circuit for detecting changes in current flowing through the wirings Y1 to Y6 that are caused by the change in mutual capacitance in thecapacitor 2603. No change in current value is detected in the wirings Y1 to Y6 when there is no approach or contact of a sensing target, whereas a decrease in current value is detected when mutual capacitance is decreased owing to the approach or contact of a sensing target. Note that an integrator circuit or the like is used for sensing of current values. -
FIG. 9B is a timing chart showing input and output waveforms in the mutual capacitive touch sensor illustrated inFIG. 9A . InFIG. 9B , sensing of a sensing target is performed in all the rows and columns in one frame period.FIG. 9B shows a period when a sensing target is not sensed (not touched) and a period when a sensing target is sensed (touched). Sensed current values of the wirings Y1 to Y6 are shown as the waveforms of voltage values. - A pulse voltage is sequentially applied to the wirings X1 to X6, and the waveforms of the wirings Y1 to Y6 change in accordance with the pulse voltage. When there is no approach or contact of a sensing target, the waveforms of the wirings Y1 to Y6 change in accordance with changes in the voltages of the wirings X1 to X6. The current value is decreased at the point of approach or contact of a sensing target and accordingly the waveform of the voltage value changes.
- By detecting a change in mutual capacitance in this manner, the approach or contact of a sensing target can be sensed.
- Although
FIG. 9A illustrates a passive matrix type touch sensor in which only thecapacitor 2603 is provided at the intersection of wirings as a touch sensor, an active matrix type touch sensor including a transistor and a capacitor may be used.FIG. 10 illustrates an example of a sensor circuit included in an active matrix type touch sensor. - The sensor circuit in
FIG. 10 includes thecapacitor 2603 andtransistors - A signal G2 is input to a gate of the
transistor 2613. A voltage VRES is applied to one of a source and a drain of thetransistor 2613, and one electrode of thecapacitor 2603 and a gate of thetransistor 2611 are electrically connected to the other of the source and the drain of thetransistor 2613. One of a source and a drain of thetransistor 2611 is electrically connected to one of a source and a drain of thetransistor 2612, and a voltage VSS is applied to the other of the source and the drain of thetransistor 2611. A signal G1 is input to a gate of thetransistor 2612, and a wiring ML is electrically connected to the other of the source and the drain of thetransistor 2612. The voltage VSS is applied to the other electrode of thecapacitor 2603. - Next, the operation of the sensor circuit in
FIG. 10 will be described. First, a potential for turning on thetransistor 2613 is supplied as the signal G2, and a potential with respect to the voltage VRES is thus applied to the node n connected to the gate of thetransistor 2611. Then, a potential for turning off thetransistor 2613 is applied as the signal G2, whereby the potential of the node n is maintained. - Then, mutual capacitance of the
capacitor 2603 changes owing to the approach or contact of a sensing target such as a finger, and accordingly the potential of the node n is changed from VRES. - In reading operation, a potential for turning on the
transistor 2612 is supplied as the signal G1. A current flowing through thetransistor 2611, that is, a current flowing through the wiring ML is changed in accordance with the potential of the node n. By sensing this current, the approach or contact of a sensing target can be sensed. - In each of the
transistors transistor 2613 so that the potential of the node n can be held for a long time and the frequency of operation of resupplying VRES to the node n (refresh operation) can be reduced. - The structure described in this embodiment can be combined with any of the structures described in the other embodiments or examples as appropriate.
- In this embodiment, a display module and electronic devices including a light-emitting element of one embodiment of the present invention will be described with reference to
FIG. 11 andFIGS. 12A to 12G . - In a
display module 8000 inFIG. 11 , atouch sensor 8004 connected to anFPC 8003, adisplay device 8006 connected to anFPC 8005, aframe 8009, a printedboard 8010, and abattery 8011 are provided between anupper cover 8001 and alower cover 8002. - The light-emitting element of one embodiment of the present invention can be used for the
display device 8006, for example. - The shapes and sizes of the
upper cover 8001 and thelower cover 8002 can be changed as appropriate in accordance with the sizes of thetouch sensor 8004 and thedisplay device 8006. - The
touch sensor 8004 can be a resistive touch sensor or a capacitive touch sensor and may be formed to overlap with thedisplay device 8006. A counter substrate (sealing substrate) of thedisplay device 8006 can have a touch sensor function. A photosensor may be provided in each pixel of thedisplay device 8006 so that an optical touch sensor is obtained. - The
frame 8009 protects thedisplay device 8006 and also serves as an electromagnetic shield for blocking electromagnetic waves generated by the operation of the printedboard 8010. Theframe 8009 may serve as a radiator plate. - The printed
board 8010 has a power supply circuit and a signal processing circuit for outputting a video signal and a clock signal. As a power source for supplying power to the power supply circuit, an external commercial power source or thebattery 8011 provided separately may be used. Thebattery 8011 can be omitted in the case of using a commercial power source. - The
display module 8000 can be additionally provided with a member such as a polarizing plate, a retardation plate, or a prism sheet. -
FIGS. 12A to 12G illustrate electronic devices. These electronic devices can include ahousing 9000, adisplay portion 9001, aspeaker 9003, operation keys 9005 (including a power switch or an operation switch), aconnection terminal 9006, a sensor 9007 (a sensor having a function of measuring or sensing force, displacement, position, speed, acceleration, angular velocity, rotational frequency, distance, light, liquid, magnetism, temperature, chemical substance, sound, time, hardness, electric field, current, voltage, electric power, radiation, flow rate, humidity, gradient, oscillation, odor, or infrared ray), amicrophone 9008, and the like. - The electronic devices illustrated in
FIGS. 12A to 12G can have a variety of functions, for example, a function of displaying a variety of data (a still image, a moving image, a text image, and the like) on the display portion, a touch sensor function, a function of displaying a calendar, date, time, and the like, a function of controlling a process with a variety of software (programs), a wireless communication function, a function of being connected to a variety of computer networks with a wireless communication function, a function of transmitting and receiving a variety of data with a wireless communication function, a function of reading a program or data stored in a memory medium and displaying the program or data on the display portion, and the like. Note that functions that can be provided for the electronic devices illustrated inFIGS. 12A to 12G are not limited to those described above, and the electronic devices can have a variety of functions. Although not illustrated inFIGS. 12A to 12G , the electronic devices may include a plurality of display portions. The electronic devices may have a camera or the like and a function of taking a still image, a function of taking a moving image, a function of storing the taken image in a memory medium (an external memory medium or a memory medium incorporated in the camera), a function of displaying the taken image on the display portion, or the like. - The electronic devices illustrated in
FIGS. 12A to 12G will be described in detail below. -
FIG. 12A is a perspective view of aportable information terminal 9100. Thedisplay portion 9001 of theportable information terminal 9100 is flexible. Therefore, thedisplay portion 9001 can be incorporated along a bent surface of abent housing 9000. In addition, thedisplay portion 9001 includes a touch sensor, and operation can be performed by touching the screen with a finger, a stylus, or the like. For example, when an icon displayed on thedisplay portion 9001 is touched, an application can be started. -
FIG. 12B is a perspective view of aportable information terminal 9101. Theportable information terminal 9101 functions as, for example, one or more of a telephone set, a notebook, and an information browsing system. Specifically, the portable information terminal can be used as a smartphone. Note that thespeaker 9003, theconnection terminal 9006, thesensor 9007, and the like, which are not shown inFIG. 12B , can be positioned in theportable information terminal 9101 as in theportable information terminal 9100 shown inFIG. 12A . Theportable information terminal 9101 can display characters and image information on its plurality of surfaces. For example, three operation buttons 9050 (also referred to as operation icons, or simply, icons) can be displayed on one surface of thedisplay portion 9001. Furthermore,information 9051 indicated by dashed rectangles can be displayed on another surface of thedisplay portion 9001. Examples of theinformation 9051 include display indicating reception of an incoming email, social networking service (SNS) message, call, and the like; the title and sender of an email and SNS message; the date; the time; remaining battery; and the reception strength of an antenna. Instead of theinformation 9051, theoperation buttons 9050 or the like may be displayed on the position where theinformation 9051 is displayed. -
FIG. 12C is a perspective view of aportable information terminal 9102. Theportable information terminal 9102 has a function of displaying information on three or more surfaces of thedisplay portion 9001. Here,information 9052,information 9053, andinformation 9054 are displayed on different surfaces. For example, a user of theportable information terminal 9102 can see the display (here, the information 9053) with theportable information terminal 9102 put in a breast pocket of his/her clothes. Specifically, a caller's phone number, name, or the like of an incoming call is displayed in a position that can be seen from above theportable information terminal 9102. Thus, the user can see the display without taking out theportable information terminal 9102 from the pocket and decide whether to answer the call. -
FIG. 12D is a perspective view of a watch-typeportable information terminal 9200. Theportable information terminal 9200 is capable of executing a variety of applications such as mobile phone calls, e-mailing, viewing and editing texts, music reproduction, Internet communication, and computer games. The display surface of thedisplay portion 9001 is bent, and images can be displayed on the bent display surface. Theportable information terminal 9200 can employ near field communication that is a communication method based on an existing communication standard. In that case, for example, mutual communication between theportable information terminal 9200 and a headset capable of wireless communication can be performed, and thus hands-free calling is possible. Theportable information terminal 9200 includes theconnection terminal 9006, and data can be directly transmitted to and received from another information terminal via a connector. Power charging through theconnection terminal 9006 is possible. Note that the charging operation may be performed by wireless power feeding without using theconnection terminal 9006. -
FIGS. 12E, 12F, and 12G are perspective views of a foldableportable information terminal 9201.FIG. 12E is a perspective view illustrating theportable information terminal 9201 that is opened.FIG. 12F is a perspective view illustrating theportable information terminal 9201 that is being opened or being folded.FIG. 12G is a perspective view illustrating theportable information terminal 9201 that is folded. Theportable information terminal 9201 is highly portable when folded. When theportable information terminal 9201 is opened, a seamless large display region is highly browsable. Thedisplay portion 9001 of theportable information terminal 9201 is supported by threehousings 9000 joined together by hinges 9055. By folding theportable information terminal 9201 at a connection portion between twohousings 9000 with thehinges 9055, theportable information terminal 9201 can be reversibly changed in shape from an opened state to a folded state. For example, theportable information terminal 9201 can be bent with a radius of curvature of greater than or equal to 1 mm and less than or equal to 150 mm. - The electronic devices described in this embodiment each include the display portion for displaying some sort of data. Note that the light-emitting element of one embodiment of the present invention can also be used for an electronic device which does not have a display portion. The structure in which the display portion of the electronic device described in this embodiment is flexible and display can be performed on the bent display surface or the structure in which the display portion of the electronic device is foldable is described as an example; however, the structure is not limited thereto and a structure in which the display portion of the electronic device is not flexible and display is performed on a plane portion may be employed.
- The structure described in this embodiment can be combined with any of the structures described in the other embodiments or examples as appropriate.
- In this embodiment, examples of lighting devices in which the light-emitting element of one embodiment of the present invention is used will be described with reference to
FIG. 13 . -
FIG. 13 illustrates an example in which the light-emitting element is used for anindoor lighting device 8501. Since the light-emitting element can have a larger area, a lighting device having a large area can also be formed. In addition, alighting device 8502 in which a light-emitting region has a curved surface can also be formed with the use of a housing with a curved surface. A light-emitting element described in this embodiment is in the form of a thin film, which allows the housing to be designed more freely. Therefore, the lighting device can be elaborately designed in a variety of ways. Furthermore, a wall of the room may be provided with a large-sized lighting device 8503. Touch sensors may be provided in thelighting devices - Moreover, when the light-emitting element is used on the surface side of a table, a
lighting device 8504 which has a function as a table can be obtained. When the light-emitting element is used as part of other furniture, a lighting device which has a function as the furniture can be obtained. - In this manner, a variety of lighting devices to which the light-emitting element is applied can be obtained. Note that such lighting devices are also embodiments of the present invention.
- The structure described in this embodiment can be combined with any of the structures described in the other embodiments as appropriate.
- This example shows a fabrication example of light-emitting elements (light-emitting
elements 1 and 2) in which a mixture of a thermally activated delayed fluorescence substance (a first organic compound), a host material (a second organic compound), and a guest material emitting fluorescence are used for a light-emitting layer.FIG. 14 is a schematic cross-sectional view of the light-emitting element fabricated in this example. Table 4 shows a detailed structure of the element. In addition, structures and abbreviations of compounds used here are shown below. Note thatEmbodiment 1 can be referred to for other compounds. -
TABLE 4 Reference Thickness Layer numeral (nm) Material Weight ratio Light-emitting Electrode 502 200 Al — element 1Electron- 534 1 LiF — injection layer Electron- 533b 15 Bphen — transport layer 533a 15 4,6mCzP2Pm — Light-emitting 521 30 PCCzPTzn:4,6mCzP2Pm:1,6mMemFLPAPrn 0.3:0.7:0.0025 layer Hole- transport 532 20 PhCzGI — layer Hole- injection 531 70 DBT3P-II:MoO3 1:0.5 layer Electrode 501 110 ITSO — Light-emitting Electrode 502 200 Al — element 2Electron- 534 1 LiF — injection layer Electron- 533b 15 Bphen — transport layer 533a 15 4,6mCzP2Pm — Light-emitting 521 30 PCCzPTzn:4,6mCzP2Pm:TBP 0.3:0.7:0.0025 layer Hole- transport 532 20 PhCzGI — layer Hole- injection 531 70 DBT3P-II:MoO3 1:0.5 layer Electrode 501 110 ITSO — - Fabrication methods of the light-emitting
elements - A film of indium tin oxide containing silicon oxide (ITSO) was formed to a thickness of 110 nm over a
substrate 520 by a sputtering method to form anelectrode 501. Note that the area of theelectrode 501 was 4 mm2 (2 mm×2 mm). - Then, as pretreatment for forming the light-emitting element over the
substrate 520, UV ozone treatment was performed for 370 seconds after washing of a surface of the substrate with water and heat treatment that was performed at 200° C. for 1 hour. - Next, the
substrate 520 was fixed to a substrate holder inside a vacuum evaporation apparatus reduced to approximately 1×10−4 Pa with theelectrode 501 side down. Then, as a hole-injection layer 531, DBT3P-II and molybdenum oxide (MoO3) were deposited on theelectrode 501 by co-evaporation in a weight ratio of DBT3P-II:MoO3=1:0.5 to a thickness of 70 nm. - As a hole-
transport layer 532, PhCzGI was deposited to a thickness of 20 nm over the hole-injection layer 531. - As a light-emitting
layer 521, PCCzPTzn, 4,6mCzP2Pm, and 1.6mMemFLPAPrn were deposited over the hole-transport layer 532 by co-evaporation such that the deposited layer has a weight ratio of PCCzPTzn:4,6mCzP2Pm:1,6mMemFLPAPrn=0.3:0.7:0.0025 and a thickness of 30 nm. Note that in the light-emittinglayer 521, PCCzPTzn is a thermally activated delayed fluorescence substance (a first organic compound), 4,6mCzP2Pm is a host material (a second organic compound), and 1,6mMemFLPAPm is a guest material. - Next, 4,6mCzP2Pm and bathophenanthroline (abbreviation: Bphen) were sequentially deposited to thicknesses of 15 nm each, as electron-
transport layers layer 521. - Then, as the electron-
injection layer 534, lithium fluoride (abbreviation: LiF) was deposited to a thickness of 1 nm over the electron-transport layer 533 b. - As an
electrode 502, aluminum (Al) was deposited on the electron-injection layer 534 to a thickness of 200 nm. - Through the above steps, the components over the
substrate 520 were formed. Note that a resistance heating method was used for the above deposition process. - Next, a light-emitting element was sealed by fixing a sealing substrate to the
substrate 520 using a sealant for an organic EL device in a glove box under a nitrogen atmosphere. - Specifically, the sealant was applied to surround the light-emitting element, the
substrate 520 and the sealing substrate were bonded to each other, irradiation with ultraviolet light a wavelength of 365 nm at 6 J/cm2 was performed, and heat treatment was performed at 80° C. for 1 hour. - Through the above steps, the light-emitting
element 1 was obtained. - The light-emitting
element 2 was fabricated through the same steps as those for the light-emittingelement 1 except that TBP was used as a guest material for the light-emittingelement 2. - In other words, as the light-emitting
layer 521 of the light-emittingelement 2, PCCzPTzn, 4,6mCzP2Pm, and TBP were deposited by co-evaporation such that the deposited layer has a weight ratio of PCCzPTzn:4,6mCzP2Pm:TBP=0.3:0.7:0.0025 and a thickness of 30 nm. Note that in the light-emittinglayer 521, PCCzPTzn is a thermally activated delayed fluorescence substance (a first organic compound), 4,6mCzP2Pm is a host material (a second organic compound), and TBP is a guest material. - Transient fluorescent characteristics of PCCzPTzn which was the host material of the light-emitting elements in this example (the light-emitting
elements 1 and 2) were measured using time-resolved emission measurement. - The time-resolved emission measurement was performed on a thin-film sample in which PCCzPTzn was deposited over a quartz substrate to a thickness of 50 nm. The thin-film sample was sealed by fixing a sealing substrate to the quartz substrate over which the thin-film sample was deposited using a sealant for an organic EL device in a glove box under a nitrogen atmosphere. Specifically, after a sealant was applied to surround the thin-film over the quartz substrate and the quartz substrate was bonded to the sealing substrate, irradiation with ultraviolet light having a wavelength of 365 nm at 6 J/cm2 and heat treatment at 80° C. for one hour were performed.
- A picosecond fluorescence lifetime measurement system (manufactured by Hamamatsu Photonics K.K.) was used for the measurement. In this measurement, the thin film was irradiated with pulsed laser, and emission of the thin film which was attenuated from the laser irradiation underwent time-resolved measurement using a streak camera to measure the lifetime of fluorescent emission of the thin film. A nitrogen gas laser with a wavelength of 337 nm was used as the pulsed laser. The thin film was irradiated with pulsed laser with a pulse width of 500 ps at a repetition rate of 10 Hz. By integrating data obtained by the repeated measurement, data with a high S/N ratio was obtained. The measurement was performed at room temperature (in an atmosphere kept at 23° C.).
-
FIG. 15 shows transient fluorescent characteristics of PCCzPTzn obtained by the measurement. - The attenuation curve shown in
FIG. 15 was fitted withFormula 4. -
- In
Formula 4, L and t represent normalized emission intensity and elapsed time, respectively. The attenuation curve was able to be fitted when n was 1 to 3. This fitting results show that the emission component of the PCCzPTzn thin-film sample contains a fluorescent component having an emission lifetime of 0.015 ps and a delayed fluorescence component having an emission lifetime of 1.5 is. In other words, it is found that PCCzPTzn is a thermally activated delayed fluorescence substance exhibiting delayed fluorescent at room temperature. -
FIGS. 16, 17, and 18 show luminance-current density characteristics, luminance-voltage characteristics, and external quantum efficiency-luminance characteristics, respectively, of the light-emittingelements - Table 5 shows element characteristics of the light-emitting
elements -
TABLE 5 CIE Current Power External Voltage Current density chromaticity Luminance efficiency efficiency quantum (V) (mA/cm2) (x, y) (cd/m2) (cd/A) (lm/W) efficiency (%) Light-emitting 3.5 8.49 (0.18, 0.27) 995 11.7 10.5 6.09 element 1Light-emitting 3.4 7.10 (0.18, 0.29) 924 13.0 12.0 6.70 element 2 -
FIG. 19 shows the electroluminescence spectra of the light-emittingelements FIG. 19 that blue light emission originating from the guest material is obtained from the light-emittingelements - In addition, the light-emitting
elements FIGS. 16, 17, and 18 . In particular, the light-emittingelement 2 shows high external quantum efficiency exceeding 10% at a maximum. In the case where a fluorescent substance is used as a guest material and energy only from a singlet excited state is used for emission, the maximum external quantum efficiency of a light-emitting element is approximately 6% on the assumption that the light extraction efficiency from the light-emitting element to the outside is 25%. However, the light-emittingelements - In addition, in the light-emitting
elements Embodiment 1. In addition, the oxidation potential and the reduction potential of the thermally activated delayed fluorescence substance are lower than or equal to the oxidation potential of the host material and higher than or equal to the reduction potential of the host material, respectively, as shown in Table 2 inEmbodiment 1. Thus, the HOMO level and the LUMO level of the thermally activated delayed fluorescence substance which are estimated from the oxidation potential and the reduction potential are higher than or equal to the HOMO level of the host material and lower than or equal to the LUMO level of the host material, respectively. - The triplet excited energy level of 1,6mMemFLPAPrn was 1.84 eV, which was measured by the method similar to that in
Embodiment 1. Therefore, as shown in Table 3 inEmbodiment 1, the triplet excited energy level of the thermally activated delayed fluorescence substance (PCCzPTzn) and that of the host material (4,6mCzP2Pm) are each higher than that of the guest material. - Therefore, in the light-emitting
elements elements - The high emission efficiency of the light-emitting
elements - As described above, the use of a structure of one embodiment of the present invention can provide a light-emitting element with high emission efficiency.
- In this example, light-emitting elements including and not including a thermally activated delayed fluorescence substance, and light-emitting elements with different weight ratios of a host material and a guest material (light-emitting
elements 3 to 5 and comparative light-emittingelement 1 to 4) were fabricated. A schematic cross-sectional view of the light-emitting elements fabricated in this example is similar toFIG. 14 in Example 1. Details of the light-emitting elements fabricated in this example are shown in Table 6 and Table 7. In addition, structures and abbreviations of compounds used here are given below.Embodiment 1 or Example 1 may be referred to for other compounds. -
TABLE 6 Reference Thickness Layer numeral (nm) Material Weight ratio Light-emitting Electrode 502 200 Al — element 3Electron- 534 1 LiF — injection layer Electron- 533 30 Bphen — transport layer Light-emitting 521 30 PCCzPTzn:Cz2DBT:1,6mMemFLPAPrn 0.1:0.9:0.005 layer Hole- transport 532 20 Cz2DBT — layer Hole- injection 531 70 DBT3P-II:MoO3 1:0.5 layer Electrode 501 110 ITSO — Comparative Electrode 502 200 Al — light-emitting Electron- 534 1 LiF — element 1injection layer Electron- 533 30 Bphen — transport layer Light-emitting 521 30 Cz2DBT:1,6mMemFLPAPrn 1:0.05 layer Hole- transport 532 20 Cz2DBT — layer Hole- injection 531 70 DBT3P-II:MoO3 1:0.5 layer Electrode 501 110 ITSO — Comparative Electrode 502 200 Al — light-emitting Electron- 534 1 LiF — element 2injection layer Electron- 533 30 Bphen — transport layer Light-emitting 521 30 PCCzPTzn:Cz2DBT:1,6mMemFLPAPrn 0.1:0.9:0.05 layer Hole- transport 532 20 Cz2DBT — layer Hole- injection 531 70 DBT3P-II:MoO3 1:0.5 layer Electrode 501 110 ITSO — -
TABLE 7 Reference Thickness Layer numeral (nm) Material Weight ratio Light-emitting Electrode 502 200 Al — element 4 Electron- 534 1 LiF — injection layer Electron- 533 30 Bphen — transport layer Light-emitting 521 30 PCCzPTzn:Cz2DBT:TBP 0.1:0.9:0.005 layer Hole-transport 532 20 Cz2DBT — layer Hole-injection 531 70 DBT3P-II:MoO3 1:0.5 layer Electrode 501 110 ITSO — Light-emitting Electrode 502 200 Al — element 5 Electron- 534 1 LiF — injection layer Electron- 533 30 Bphen — transport layer Light-emitting 521 30 PCCzPTzn:CzTAZ1:TBP 0.1:0.9:0.005 layer Hole-transport 532 20 Cz2DBT — layer Hole-injection 531 70 DBT3P-II:MoO3 1:0.5 layer Electrode 501 110 ITSO — Comparative Electrode 502 200 Al — light-emitting Electron- 534 1 LiF — element 3 injection layer Electron- 533 30 Bphen — transport layer Light-emitting 521 30 PCCzPTzn:Cz2DBT:TBP 0.1:0.9:0.05 layer Hole-transport 532 20 Cz2DBT — layer Hole-injection 531 70 DBT3P-II:MoO3 1:0.5 layer Electrode 501 110 ITSO — Comparative Electrode 502 200 Al — light-emitting Electron- 534 1 LiF — element 4 injection layer Electron- 533 30 Bphen — transport layer Light-emitting 521 30 PCCzPTznCzTAZ1:TBP 0.1:0.9:0.05 layer Hole-transport 532 20 Cz2DBT — layer Hole-injection 531 70 DBT3P-II:MoO3 1:0.5 layer Electrode 501 110 ITSO — - A method for fabricating the light-emitting
elements 3 to 5 and the comparative light-emittingelements 1 to 4 will be described. - An ITSO film was formed to a thickness of 110 nm over the
substrate 520 by a sputtering method to form theelectrode 501. Note that the area of theelectrode 501 was 4 mm2 (2 mm×2 mm). - Then, as pretreatment for forming the light-emitting element over the
substrate 520, UV ozone treatment was performed for 370 seconds after washing of a surface of the substrate with water and heat treatment that was performed at 200° C. for 1 hour. - Next, the
substrate 520 was fixed to a substrate holder inside a vacuum evaporation apparatus reduced to approximately 1×10−4 Pa with theelectrode 501 side down. Then, as a hole-injection layer 531, DBT3P-II and MoO3 were deposited on theelectrode 501 by co-evaporation in a weight ratio of DBT3P-II:MoO3=1:0.5 to a thickness of 70 nm. - As a hole-
transport layer 532, Cz2DBT was deposited to a thickness of 20 nm over the hole-injection layer 531. - As a light-emitting
layer 521, PCCzPTzn, Cz2DBT, and 1.6mMemFLPAPrn were deposited over the hole-transport layer 532 by co-evaporation such that the deposited layer has a weight ratio of PCCzPTzn:Cz2DBT: 1,6mMemFLPAPrn=0.1:0.9:0.005 and a thickness of 30 nm. Note that in the light-emittinglayer 521, PCCzPTzn is a thermally activated delayed fluorescence substance (a first organic compound), Cz2DBT is a host material (a second organic compound), and 1,6mMemFLPAPrn is a guest material. - Next, Bphen were sequentially deposited to a thickness of 30 nm each, as electron-
transport layer 533 over the light-emittinglayer 521. - Then, as the electron-
injection layer 534, LiF was deposited to a thickness of 1 nm over the electron-transport layer 533. - As an
electrode 502, Al was deposited on the electron-injection layer 534 to a thickness of 200 nm. - Through the above steps, the components over the
substrate 520 were formed. Note that a resistance heating method was used for the above deposition process. - Next, a light-emitting element was sealed by fixing a sealing substrate to the
substrate 520 using a sealant for an organic EL device in a glove box under a nitrogen atmosphere. - Specifically, the sealant was applied to surround the light-emitting element, the
substrate 520 and the sealing substrate were bonded to each other, irradiation with ultraviolet light having a wavelength of 365 nm at 6 J/cm2 was performed, and heat treatment was performed at 80° C. for 1 hour. Through the above steps, the light-emittingelement 3 was obtained. - The light-emitting
elements elements 1 to 4 were fabricated through the same steps as those for the above-mentioned light-emittingelement 3 except that structures of their light-emitting layers were different from the structure of the light-emitting layer of the light-emittingelement 3. - As the light-emitting
layer 521 of the light-emittingelement 4, PCCzPTzn, Cz2DBT, and TBP were deposited by co-evaporation such that the deposited layer has a weight ratio of PCCzPTzn:Cz2DBT:TBP=0.1:0.9:0.005 and a thickness of 30 nm. Note that in the light-emittinglayer 521, PCCzPTzn is a thermally activated delayed fluorescence substance (a first organic compound), Cz2DBT is a host material (a second organic compound), and TBP is a guest material. In other words, the light-emittingelement 4 has a structure similar to that of the light-emittingelement 3 except for the guest material. - As the light-emitting
layer 521 of the light-emittingelement 5, PCCzPTzn, CzTAZ1, and TBP were deposited by co-evaporation such that the deposited layer has a weight ratio of PCCzPTzn:CzTAZ1:TBP=0.1:0.9:0.005 and a thickness of 30 nm. Note that in the light-emittinglayer 521, PCCzPTzn is a thermally activated delayed fluorescence substance (a first organic compound), CzTAZ1 is a host material (a second organic compound), and TBP is a guest material. In other words, the light-emittingelement 5 has a structure similar to that of the light-emittingelement 4 except for the host material. - As the light-emitting
layer 521 of the comparative light-emittingelement 1, Cz2DBT and 1.6mMemFLPAPrn were deposited by co-evaporation such that the deposited layer has a weight ratio of Cz2DBT:1,6mMemFLPAPrn=1:0.05 and a thickness of 30 nm. Note that in the light-emittinglayer 521, Cz2DBT is a host material (a second organic compound) and 1,6mMemFLPAPrn is a guest material. In other words, a thermally activated delayed fluorescence substance (a first organic compound) is not used for the comparative light-emittingelement 1. - As the light-emitting
layer 521 of the comparative light-emittingelement 2, PCCzPTzn, Cz2DBT, and 1.6mMemFLPAPrn were deposited by co-evaporation such that the deposited layer has a weight ratio of PCCzPTzn:Cz2DBT:1,6mMemFLPAPrn=0.1:0.9:0.05 and a thickness of 30 nm. Note that in the light-emittinglayer 521, PCCzPTzn is a thermally activated delayed fluorescence substance (a first organic compound), Cz2DBT is a host material (a second organic compound), and 1,6mMemFLPAPrn is a guest material. In other words, the comparative light-emittingelement 2 has a structure similar to that of the light-emittingelement 3 except for the concentration of the guest material. - As the light-emitting
layer 521 of the comparative light-emittingelement 3, PCCzPTzn, Cz2DBT, and TBP were deposited by co-evaporation such that the deposited layer has a weight ratio of PCCzPTzn:Cz2DBT:TBP=0.1:0.9:0.05 and a thickness of 30 nm. Note that in the light-emittinglayer 521, PCCzPTzn is a thermally activated delayed fluorescence substance (a first organic compound), Cz2DBT is a host material (a second organic compound), and TBP is a guest material. In other words, the comparative light-emittingelement 3 has a structure similar to that of the light-emittingelement 4 except for the concentration of the guest material. - As the light-emitting
layer 521 of the comparative light-emittingelement 4, PCCzPTzn, CzTAZ1, and TBP were deposited by co-evaporation such that the deposited layer has a weight ratio of PCCzPTzn:CzTAZ1: TBP=0.1:0.9:0.05 and a thickness of 30 nm. Note that in the light-emittinglayer 521, PCCzPTzn is a thermally activated delayed fluorescence substance (a first organic compound), CzTAZ1 is a host material (a second organic compound), and TBP is a guest material. In other words, the comparative light-emittingelement 4 has a structure similar to that of the light-emittingelement 5 except for the concentration of the guest material. -
FIGS. 20, 21, and 22 show current efficiency-luminance characteristics, current-voltage characteristics, and external quantum efficiency-luminance characteristics, respectively, of the light-emittingelement 3 and the comparative light-emittingelements FIG. 23 shows electroluminescence spectra when a current at a current density of 2.5 mA/cm2 was supplied to the light-emittingelement 3 and the comparative light-emittingelements FIGS. 24, 25, and 26 show current efficiency-luminance characteristics, current-voltage characteristics, and external quantum efficiency-luminance characteristics, respectively, of the light-emittingelements elements FIG. 27 shows s electroluminescence spectra when a current at a current density of 2.5 mA/cm2 was supplied to the light-emittingelements elements - Table 8 shows the element characteristics of the light-emitting
elements 3 to 5 and the comparative light-emittingelement 1 to 4 at around 1000 cd/m2. -
TABLE 8 Current CIE Current Power External Voltage density chromaticity Luminance efficiency efficiency quantum (V) (mA/cm2) (x, y) (cd/m2) (cd/A) (lm/W) efficiency (%) Light-emitting 4.2 1.05 (0.16, 0.19) 81 7.7 5.8 5.52 element 3Light-emitting 4.0 1.05 (0.16, 0.23) 103 9.8 7.7 6.22 element 4Light-emitting 3.8 0.83 (0.17, 0.26) 89 10.7 8.9 6.17 element 5Comparative 4.4 2.35 (0.14, 0.17) 94 4.0 2.9 3.19 light-emitting element 1Comparative 4.8 2.16 (0.14, 0.19) 104 4.8 3.2 3.56 light-emitting element 2Comparative 4.2 1.77 (0.15, 0.25) 87 4.9 3.7 2.98 light-emitting element 3Comparative 4.2 2.00 (0.16, 0.26) 75 3.7 2.8 2.19 light-emitting element 4 - It is found from the emission spectra in
FIG. 23 andFIG. 27 that blue light emission originating from the guest material is obtained from the light-emittingelements 3 to 5 and the comparative light-emittingelement 1 to 4. - As shown in
FIGS. 20 to 22 andFIGS. 24 to 26 , the light-emittingelements 3 to 5 show high emission efficiency. In contrast, the comparative light-emittingelements 1 to 4 do not show sufficient emission efficiency. - Table 9 shows measurement results of the oxidation potentials and the reduction potentials of the thermally activated delayed fluorescence material (PCCzPTzn) and the host material (Cz2DBT or CzTAZ1) in the solution state and the HOMO and LUMO levels estimated from the results. Note that the measurement method is similar to that described in
Embodiment 1. -
TABLE 9 HOMO(eV) LUMO(eV) Oxidation Reduction estimated from estimated from potential(V) potential(V) oxidation potential oxidation potential Abbreviation in solution state in solution state in solution state in solution state Thermally activated PCCzPTzn 0.70 −1.97 −5.64 −2.97 delayed fluorescence substance Host material Cz2DBT 0.92 −2.62 −5.86 −2.33 CzTAZ1 1.00 −2.70 −5.94 −2.24 - As shown in Table 9, in the light-emitting
elements 3 to 5, the oxidation potential and the reduction potential of the thermally activated delayed fluorescence substance are lower than or equal to the oxidation potential of the host material and higher than or equal to the reduction potential of the host material, respectively. Thus, the HOMO level and the LUMO level of the thermally activated delayed fluorescence substance which are estimated from the oxidation potential and the reduction potential are higher than or equal to the HOMO level of the host material and lower than or equal to the LUMO level of the host material, respectively. Therefore, both the singlet excited state and the triplet excited state which are efficiently formed by carrier recombination in the thermally activated delayed fluorescence substance can be transferred efficiently to the guest material. As a result, the light-emittingelements 3 to 5 show high emission efficiency. - In addition, the results in which the emission efficiency of the light-emitting
element 3 is higher than that of the comparative light-emittingelement 2, and the emission efficiency of the comparative light-emittingelement 2 is higher than that of the comparative light-emittingelement 1 show that the use of PCCzPTzn as the thermally activated delayed fluorescence substance for the light-emittinglayer 521 can increase the emission efficiency. This is because a triplet excited state generated by recombined carriers in PCCzPTzn, which was the thermally activated delayed fluorescence substance, was converted into a single excited state by reverse intersystem crossing. - The emission efficiency of the light-emitting
element 3 higher than that of the comparative light-emittingelement 2 means that the weight ratio of the host material (Cz2DBT) to the guest material (1,6mMemFLPAPrn) in the light-emittinglayer 521 is preferably from 1:0.001 to 1:0.01 (host material:guest material). This is because the concentration of a guest material that is sufficiently lower than that of a host material can suppress generation of a triplet excited state of the guest material. - The emission efficiency of the light-emitting
element 4 is higher than that of the comparative light-emittingelement 3 and the emission efficiency of the light-emittingelement 5 is higher than that of the comparative light-emittingelement 4, which means that the weight ratio of the host material (Cz2DBT or CzTAZ1) to the guest material (TBP) in the light-emittinglayer 521 is preferably from 1:0.001 to 1:0.01 (host material:guest material). - Thus, the weight ratio of the host material to the thermally activated delayed fluorescence substance is preferably from 1:0.05 to 1:0.5 (host material:thermally activated delayed fluorescence substance) and the weight ratio of the host material to the guest material is preferably from 1:0.001 to 1:0.01 (host material:guest material).
- As described above, the use of a structure of one embodiment of the present invention can provide a light-emitting element with high emission efficiency.
- 100: EL layer, 101: electrode, 102: electrode, 111: hole-injection layer, 112: hole-transport layer, 118: electron-transport layer, 119: electron-injection layer, 120: light-emitting layer, 131: organic compound, 132: organic compound, 133: guest material, 150: light-emitting element, 401: electrode, 402: electrode, 411: hole-injection layer, 412: hole-transport layer, 413: electron-transport layer, 414: electron-injection layer, 415: hole-injection layer, 416: hole-transport layer, 417: electron-transport layer, 418: electron-injection layer, 421: organic compound, 422: organic compound, 423: guest material, 431: organic compound, 432: organic compound, 433: guest material, 441: light-emitting unit, 442: light-emitting unit, 443: light-emitting layer, 444: light-emitting layer, 445: charge-generation layer, 446: light-emitting unit, 447: light-emitting unit, 448: light-emitting layer, 449: light-emitting layer, 450: light-emitting element, 452: light-emitting element, 461: host material, 462: guest material, 471: organic compound, 472: organic compound, 473: guest material, 501: electrode, 502: electrode, 520: substrate, 521: light-emitting layer, 531: hole-injection layer, 532: hole-transport layer, 533: electron-transport layer, 533 a: electron-transport layer, 533 b: electron-transport layer, 534: electron-injection layer, 801: pixel circuit, 802: pixel portion, 804: driver circuit portion, 804 a: scan line driver circuit, 804 b: signal line driver circuit, 806: protective circuit, 807: terminal portion, 852: transistor, 854: transistor, 862: capacitor, 872: light-emitting element, 2000: touch panel, 2001: touch panel, 2501: display device, 2502R: pixel, 2502 t: transistor, 2503 c: capacitor, 2503 g(1): scan line driver circuit, 2503 t: transistor, 2509: FPC, 2510: substrate, 2510 a: insulating layer, 2510 b: flexible substrate, 2510 c: adhesive layer, 2511: wiring, 2519: terminal, 2521: insulating layer, 2528: partition, 2550R: light-emitting element, 2560: sealing layer, 2567BM: light-blocking layer, 2567 p: anti-reflective layer, 2567R: coloring layer, 2570: substrate, 2570 a: insulating layer, 2570 b: flexible substrate, 2570 c: adhesive layer, 2580R: light-emitting module, 2590: substrate, 2591: electrode, 2592: electrode, 2593: insulating layer, 2594: wiring, 2595: touch sensor, 2597: adhesive layer, 2598: wiring, 2599: connection layer, 2601: pulse voltage output circuit, 2602: current sensing circuit, 2603: capacitor, 2611: transistor, 2612: transistor, 2613: transistor, 2621: electrode, 2622: electrode, 8000: display module, 8001: upper cover, 8002: lower cover, 8003: FPC, 8004: touch sensor, 8005: FPC, 8006: display device, 8009: frame, 8010: printed board, 8011: battery, 8501: lighting device, 8502: lighting device, 8503: lighting device, 8504: lighting device, 9000: housing, 9001: display portion, 9003: speaker, 9005: operation key, 9006: connection terminal, 9007: sensor, 9008: microphone, 9050: operation button, 9051: information, 9052: information, 9053: information, 9054: information, 9055: hinge, 9100: portable information terminal, 9101: portable information terminal, 9102: portable information terminal, 9200: portable information terminal, 9201: portable information terminal.
- This application is based on Japanese Patent Application serial no. 2014-200355 filed with Japan Patent Office on Sep. 30, 2014, the entire contents of which are hereby incorporated by reference.
Claims (18)
1. A light-emitting element comprising:
a pair of electrodes; and
an EL layer between the pair of electrodes,
wherein the EL layer comprises a first organic compound, a second organic compound, and a guest material,
wherein the first organic compound has a function of emitting a thermally activated delayed fluorescence at room temperature,
wherein the guest material has a function of emitting fluorescence,
wherein a HOMO level of the first organic compound is higher than or equal to a HOMO level of the second organic compound, and
wherein a LUMO level of the first organic compound is lower than or equal to a LUMO level of the second organic compound.
2. The light-emitting element according to claim 1 , wherein a difference between a singlet excitation energy level of the first organic compound and a triplet excitation energy level of the first organic compound is larger than 0 eV and smaller than or equal to 0.2 eV.
3. The light-emitting element according to claim 1 , wherein the guest material emits light.
4. The light-emitting element according to claim 1 ,
wherein the first organic compound comprises a first π-electron deficient heteroaromatic skeleton and a first π-electron rich heteroaromatic skeleton, and
wherein the second organic compound comprises a second π-electron deficient heteroaromatic skeleton and a second π-electron rich heteroaromatic skeleton.
5. The light-emitting element according to claim 4 ,
wherein the first π-electron deficient heteroaromatic skeleton comprises a diazine skeleton or a triazine skeleton,
wherein the first π-electron rich heteroaromatic skeleton comprises any one or more of an acridine skeleton, a phenoxazine skeleton, or a 3-(9-phenyl-9H-carbazole-3-yl)-9H-carbazole skeleton,
wherein the second π-electron deficient heteroaromatic skeleton comprises a pyridine skeleton or a diazine skeleton, and
wherein the second π-electron rich heteroaromatic skeleton comprises any one or more of a furan skeleton, a thiophene skeleton, a fluorine skeleton, and a pyrrole skeleton.
6. The light-emitting element according to claim 1 ,
wherein a weight ratio of the second organic compound to the first organic compound is from 1:0.05 to 1:0.5, and
wherein a weight ratio of the second organic compound to the guest material is from 1:0.001 to 1:0.01.
7. A display device comprising:
the light-emitting element according to claim 1 ; and
a color filter, a sealant, or a transistor.
8. An electronic device comprising:
the display device according to claim 7 , and
a housing or a function of a touch sensor.
9. A lighting device comprising:
the light-emitting element according to claim 1 , and
a housing or a function of a touch sensor.
10. A light-emitting element comprising:
a pair of electrodes; and
an EL layer between the pair of electrodes,
wherein the EL layer comprises a first organic compound, a second organic compound, and a guest material,
wherein the first organic compound has a function of emitting a thermally activated delayed fluorescence at room temperature,
wherein the guest material has a function of emitting fluorescence,
wherein an oxidation potential of the first organic compound is lower than or equal to an oxidation potential of the second organic compound, and
wherein a reduction potential of the first organic compound is higher than or equal to a reduction of the second organic compound.
11. The light-emitting element according to claim 10 , wherein a difference between a singlet excitation energy level of the first organic compound and a triplet excitation energy level of the first organic compound is larger than 0 eV and smaller than or equal to 0.2 eV.
12. The light-emitting element according to claim 10 , wherein the guest material emits light.
13. The light-emitting element according to claim 10 ,
wherein the first organic compound comprises a first π-electron deficient heteroaromatic skeleton and a first π-electron rich heteroaromatic skeleton, and
wherein the second organic compound comprises a second π-electron deficient heteroaromatic skeleton and a second π-electron rich heteroaromatic skeleton.
14. The light-emitting element according to claim 13 ,
wherein the first π-electron deficient heteroaromatic skeleton comprises a diazine skeleton or a triazine skeleton,
wherein the first π-electron rich heteroaromatic skeleton comprises any one or more of an acridine skeleton, a phenoxazine skeleton, or a 3-(9-phenyl-9H-carbazole-3-yl)-9H-carbazole skeleton,
wherein the second π-electron deficient heteroaromatic skeleton comprises a pyridine skeleton or a diazine skeleton, and
wherein the second π-electron rich heteroaromatic skeleton comprises any one or more of a furan skeleton, a thiophene skeleton, a fluorine skeleton, and a pyrrole skeleton.
15. The light-emitting element according to claim 10 ,
wherein a weight ratio of the second organic compound to the first organic compound is from 1:0.05 to 1:0.5, and
wherein a weight ratio of the second organic compound to the guest material is from 1:0.001 to 1:0.01.
16. A display device comprising:
the light-emitting element according to claim 10 ; and
a color filter, a sealant, or a transistor.
17. An electronic device comprising:
the display device according to claim 16 , and
a housing or a function of a touch sensor.
18. A lighting device comprising:
the light-emitting element according to claim 10 , and
a housing or a function of a touch sensor.
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US17/094,374 US11557742B2 (en) | 2014-09-30 | 2020-11-10 | Light-emitting element and display device including compound having function of emitting TADF at room temperature |
US17/980,869 US20230095777A1 (en) | 2014-09-30 | 2022-11-04 | Light-emitting element, display device, electronic device, and lighting device |
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US17/980,869 Pending US20230095777A1 (en) | 2014-09-30 | 2022-11-04 | Light-emitting element, display device, electronic device, and lighting device |
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