JP2005100977A - Electroluminescent element and light-emitting device using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To lower drive voltage of a dope-type element applying a light-emitting layer with a little guest material added to a host material, especially, to lower the drive voltage as well as improve color purity of a dope-type element with a red-light-emitting material having an electron-attracting group as a guest material, and especially, to lower the drive voltage and improve the color purity as well of a dope-type element with the red-light-emitting material having an electron-attracting group. <P>SOLUTION: An organic compound having a hole transport property is used as a host material 521 for an electroluminescent element having a light-emitting layer 513 containing a host material 521 and a guest material 522 with an electron-attracting group. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an electroluminescent element having an anode, a cathode, and a layer containing an organic compound that can emit light by applying an electric field (hereinafter referred to as “electroluminescent layer”). In particular, the present invention relates to an electroluminescent element that emits red light.

  An electroluminescent element using an organic compound as a light emitter is an element that emits light when an electric field is applied and an electric current flows. The light emission mechanism is such that when a voltage is applied with an electroluminescent layer sandwiched between electrodes, electrons injected from the cathode and holes injected from the anode recombine in the electroluminescent layer to cause excited molecules (hereinafter referred to as “excited molecules”). , Which is referred to as “excited molecule”) and is said to emit light when the excited molecule returns to the ground state. As the excited state, a singlet excited state and a triplet excited state are known, and it is considered that light emission is possible through either excited state.

  In such an electroluminescent element, the electroluminescent layer is usually formed of a thin film of about 100 to 200 nm. In addition, since the electroluminescent device is a self-luminous device in which light is emitted from the electroluminescent layer itself, a backlight as used in a conventional liquid crystal display is not necessary. Therefore, it is a great advantage that it can be made extremely thin and light.

  For example, in an electroluminescent layer of about 100 nm, the time from carrier injection to recombination is about several tens of nanoseconds considering the carrier mobility, and the process from carrier recombination to light emission is Even if included, light emission occurs on the order of microseconds or less. Therefore, one of the features is that the response speed is very fast.

  Furthermore, since an electroluminescent element using an organic compound as a light emitter is a carrier injection type element, it is not necessary to apply a high-voltage AC voltage unlike an inorganic EL element, and a low direct current of about several volts to several tens of volts. It can be driven by voltage.

  As described above, an electroluminescent element using an organic compound as a light emitter has characteristics such as thin and light weight, high-speed response, and direct current low voltage driving, and is attracting attention as a next-generation flat panel display element. In particular, a light-emitting device in which such electroluminescent elements are arranged in a matrix has an advantage in that it has a wide viewing angle and excellent visibility as compared with a conventional liquid crystal display device.

  By the way, when these electroluminescent elements are applied to a flat panel display or the like, it is necessary to control the emission color to a desired color. In recent years, as a method for controlling the emission color of an electroluminescent element, in particular, a desired light emission derived from a guest material can be obtained by applying a light emitting layer in which a small amount of a guest material (also referred to as a dopant material) is added to a host material. A technique of obtaining a color (hereinafter referred to as “doping method”) is actively used (see, for example, Patent Document 1).

  The doping method typified by Patent Document 1 can suppress concentration quenching of light emitting molecules and obtain high brightness and high efficiency. Therefore, it is an effective method for emitting a red light emitting material that is particularly easy to quench the concentration. is there. For example, in the following Non-Patent Document 1, various 4-dicyanomethylene-4H-pyran derivatives, which are red light emitting materials, are synthesized and used as guest materials.

  However, many of the electroluminescent elements (hereinafter referred to as “doped elements”) to which such a doping method is applied have a demerit that the drive voltage increases. In particular, it is known that the tendency is remarkable in a doped element in which a red light emitting material is added as a guest material (see, for example, Non-Patent Document 2).

  In addition, in a doped device, not only the guest material but also the host material often emits light, so that the emission color cannot be controlled well, and as a result, the color purity of the emission may deteriorate. This is considered to be a phenomenon that occurs when there is a large difference between the excitation energy of the host material and the excitation energy of the guest material. This phenomenon is often seen in doped devices in which a red light emitting material is added as a guest material. is there. This phenomenon is said to be eliminated by further adding an assist dopant material having an excitation energy located between the host material and the guest material (see, for example, Non-Patent Document 3).

  However, in the method of Non-Patent Document 3, an assist dopant material must be further added in addition to the host material and guest material. Therefore, if an element is manufactured by a vacuum evaporation method, ternary co-evaporation using three evaporation sources is required, and the element manufacturing process becomes complicated. Therefore, a problem also occurs in the reproducibility of the element.

As described above, in the doped element, problems such as an increase in driving voltage or inability to control the emitted color and deterioration in color purity have occurred, and countermeasures are desired.
Japanese Patent No. 2814435 C. H. Chen, 3 others, Macromolecular Symposia, No. 125, 49-58 (1997) Yoshiharu Sato, Society of Applied Physics, Journal of Organic Molecules and Bioelectronics, Vol. 11, no. 1 (2000), 86-99 Yuji Hamada, 4 others, Applied Physics Letters, Vol. 75, no. 12, 1682-1684 (1999)

  An object of the present invention is to reduce the driving voltage in a doped element. In particular, an object of the present invention is to reduce the driving voltage in a doped element to which a red light emitting material is added as a guest material.

  Another object of the present invention is to reduce the driving voltage and improve the color purity in the doped element. In particular, in a doped element in which a red light emitting material is added as a guest material, it is an object to reduce driving voltage and improve color purity.

  The inventors of the present invention paid attention to the fact that the drive voltage is particularly increased in the electroluminescent device to which the 4-dicyanomethylene-4H-pyran derivative used in Patent Document 1 is added as a guest material. And it was thought that the cause of the drive voltage rise was the electron withdrawing group contained in the 4-dicyanomethylene-4H-pyran derivative.

  Based on this consideration, the present inventors have made extensive studies and found that the driving voltage can be reduced by adopting the following configuration in the electroluminescent element to which the guest material having an electron-withdrawing group is added. It was.

  That is, in the present invention, in an electroluminescent element having a light emitting layer containing a host material and a guest material having an electron withdrawing group, and an electron transport layer provided in contact with the light emitting layer, the host material has a hole transport property. It is an organic compound having The host material may be anything as long as it is an organic compound having a hole transport property, but an organic compound having an aromatic amine skeleton is particularly preferable.

  The above-described configuration is effective for guest materials having various electron-withdrawing groups, but is particularly effective for guest materials into which a cyano group, a halogeno group, or a carbonyl group is introduced. For example, it is effective for a guest material having a 4-dicyanomethylene-4H-pyran skeleton.

  Furthermore, many guest materials having an electron-withdrawing group often emit light in a yellow to red region due to the effect of the substituent. Accordingly, the present invention is particularly characterized in that the peak wavelength of the emission spectrum of the guest material described above is not less than 560 nm and not more than 700 nm.

  By the way, the above-described configuration is extremely effective in reducing the driving voltage. However, not only that, but also the configuration in which the electron transport layer provided in contact with the light-emitting layer emits light and the color purity is further improved. It is also possible to take.

  That is, another configuration of the present invention is characterized in that, in the above-described configuration, the ionization potential of the host material is greater by 0.3 eV or more than the ionization potential of the electron transporting material constituting the electron transport layer. At this time, the ionization potential of the host material is preferably 5.1 eV or less. Alternatively, the ionization potential of the electron transporting material is preferably 5.6 eV or more.

  In addition, according to another configuration of the present invention, holes can be trapped between the light-emitting layer and the electron transport layer in the above-described configuration, and more energy than the electron transport material constituting the electron transport layer. A hole trap region made of a hole trap material having a small gap value is provided. The hole trap material preferably has a smaller ionization potential than the host material and the electron transport material in order to trap holes more effectively. In addition, if the hole trap region is made thick, current may not flow easily. Therefore, the hole trap region is preferably a layer of 5 nm or less or an island.

The hole trap material is preferably an aromatic hydrocarbon compound having 18 or more carbon atoms typified by tetracene, perylene, rubrene or the like, or a carbon allotrope typified by fullerene (C ₆₀ ).

  By the way, the present inventors have found that when a light emitting layer is formed by adding a guest material having an electron withdrawing group to a host material, the peak wavelength of the light emission spectrum of the element changes depending on the dipole moment of the molecule of the host material. I found it. Specifically, this is a phenomenon in which the peak wavelength of the emission spectrum is blue-shifted as the dipole moment of the molecule of the host material is smaller. Therefore, when a red light emitting device is manufactured using a guest material having an electron withdrawing group and emitting red light, if a host material having a small dipole moment is used, light emission of orange or yellow may occur. Such a host material may not be suitable.

  Therefore, according to the present invention, in the electroluminescence device having the above-described configuration, the peak wavelength of the emission spectrum of the guest material exists in the red region of 600 nm to 700 nm and the dipole moment of the molecule of the host material is 4 Debye or more. It is characterized by that.

  The electroluminescent elements as described above have a feature that the driving voltage is low and the color purity is good depending on the configuration. Therefore, if these electroluminescent elements are used, a light emitting device with low power consumption and good color purity is used. Can be produced. Accordingly, the present invention includes a light emitting device having the electroluminescent element of the present invention.

  Note that the light emitting device in this specification refers to an image display device or a light emitting device using an electroluminescent element as a light emitting element. Also, a module in which a connector, for example, an anisotropic conductive film, a TAB (Tape Automated Bonding) tape or a TCP (Tape Carrier Package) is attached to an electroluminescent element, a module in which a printed wiring board is provided at the end of a TAB tape or TCP Alternatively, a module in which an IC (integrated circuit) is directly mounted on an electroluminescent element by a COG (Chip On Glass) method is included in the light emitting device.

  By implementing the present invention, it is possible to reduce the driving voltage in the doped element. In particular, in a doped element to which a red light emitting material is added as a guest material, the driving voltage can be reduced.

  Further, by implementing the present invention, it is possible to reduce the driving voltage and improve the color purity in the doped element. In particular, in a doped element to which a red light emitting material is added as a guest material, the driving voltage can be reduced and the color purity can be improved.

  First, FIG. 1A shows a band diagram of a laminated structure often used in an electroluminescent element, that is, a structure in which a hole transport layer and an electron transport layer are laminated. As shown in FIG. 1A, holes are transmitted through the HOMO level of the hole transporting material in the hole transporting layer 101, and electrons are transmitted through the LUMO level of the electron transporting material in the electron transporting layer 102 to lubricate the electrons. Since these carriers are transported, the recombination region 103 a of these carriers is in the vicinity of the interface between the hole transport layer 101 and the electron transport layer 102.

  Conventionally, a guest material having an electron withdrawing group has been added to the electron transport layer 102 based on the structure of FIG. A band diagram in that case is shown in FIG. Since the guest material having an electron withdrawing group has a very large electron affinity due to the strong electron withdrawing property, the LUMO level 104 is at a low position as shown in FIG. An electron trap level is formed.

  In this case, in the region 105 doped with a guest material having an electron-withdrawing group, electrons do not move easily due to the deep electron trap level, and the carrier recombination region 103b moves away from the vicinity of the interface of the stacked structure to transport electrons. It is expected to spread to the layer 102 side. Then, since the electron transport layer composed of the electron transport material is in a state that must carry holes (broken arrows in the figure), the current hardly flows and the carriers recombine and emit light. The present inventors thought that the voltage (i.e., the drive voltage) was increased.

Actually, 4,4′-bis [N- (1-naphthyl) -N-phenyl-amino] -biphenyl (abbreviation: α-NPD), which is a hole transporting material, is transported to the hole transport layer, and electron transport is performed to the electron transport layer. Tris (8-quinolinolato) aluminum (abbreviation: Alq ₃ ) as a functional material and 4-dicyanomethylene-2,6-bis [p- (N-carbazolyl) styryl as a guest material having a cyano group as an electron withdrawing group When 4H-pyran (abbreviation: BisDCCz) is used, a band diagram as shown in FIG. 1C is obtained, and therefore, the driving voltage rises due to a very deep electron trap level (−3.3 eV). is expected.

  In addition, the value of the HOMO level shown in FIG. 1C (a negative value, the absolute value of which corresponds to the ionization potential) is measured using a photoelectron spectrometer AC-2 (manufactured by Riken Keiki Co., Ltd.). The value of the ionization potential in the thin film state of the material was measured and calculated by converting the value into a negative value. In addition, the LUMO level value is obtained by measuring the absorption spectrum of the thin film of each material using an ultraviolet / visible spectrophotometer (manufactured by JASCO Corp.), obtaining the value of the energy gap from the absorption edge, and determining the HOMO level value. Calculated by adding to the value.

  On the other hand, the basic concept of the present invention is a structure in which a light emitting layer is formed by adding a guest material having an electron withdrawing group to the hole transport layer in order to avoid the above-described phenomenon. The band diagram in that case is shown in FIG. 201 is a hole transport layer, 202 is an electron transport layer, and 205 is a region where a guest material having an electron withdrawing group is added to the hole transport layer, that is, a light emitting layer.

  In the case of the configuration shown in FIG. 2, electrons pass through the electron transport layer 202 and are then trapped in the LUMO level 204 of the guest material in the vicinity of the interface 203 with the electron transport layer 202 in the light emitting layer 205. However, in this configuration, since the host material in the light emitting layer 205 is a hole transporting material used for the hole transport layer 201, unlike the case of FIG. 1, the transport of holes is easy. That is, even if electrons are trapped in the vicinity of the interface 203 and cannot move, holes are easily transported to the vicinity of the interface 203, so that carrier recombination can be facilitated. As a result, the current flows more easily than in the case of FIG. 1, and the drive voltage can be reduced.

  In FIG. 2, the same hole transporting material is used for both the hole transporting layer 201 and the light emitting layer 205, but different hole transporting materials may be used.

Here, as a hole transporting material that can be used as a host material of the light-emitting layer 205, an organic compound having an aromatic amine skeleton is preferable. In addition to the α-NPD described above, 4,4′-bis [N -(3-Methylphenyl) -N-phenyl-amino] -biphenyl (abbreviation: TPD), 4,4 ′, 4 ″ -tris (N, N-diphenyl-amino) -triphenylamine (abbreviation: TDATA) 4,4 ′, 4 ″ -tris [N- (3-methylphenyl) -N-phenyl-amino] -triphenylamine (abbreviation: MTDATA), 4,4 ′, 4 ″ -tris [N— (1-naphthyl) -N-phenyl-amino] -triphenylamine (abbreviation: 1-TNATA) and the like can be given. Tris (5-diphenylamino-8-hydroxyquinolinato) aluminum (abbreviation: Al (daq) ₃ ), bis (5-diphenylamino-8-hydroxyquinolinato) zinc, which is a metal complex having an aromatic amine skeleton (Abbreviation: Zn (daq) ₃ ), tris (1-phenylpyrazole) cobalt (III) (abbreviation: Co (PPZ) ₃ ), tris (1- (4-methylphenyl) pyrazole, which is a kind of organometallic complex Cobalt (III) (abbreviation: Co (m-PPZ) ₃ ) and the like also exhibit hole transport properties.

  On the other hand, as the guest material having an electron-withdrawing group in the light-emitting layer 205, a light-emitting material having an electron-withdrawing group such as a cyano group, a halogeno group, or a carbonyl group can be used. As the light-emitting material having a cyano group, for example, besides coumarin 337, 4- (dicyanomethylene) -2- [p- (dimethylamino) styryl] -6-methyl-4H-pyran (abbreviation: DCM1), 4- ( Dicyanomethylene) -2-methyl-6- (9-julolidyl) ethynyl-4H-pyran (abbreviation: DCM2), 4- (dicyanomethylene) -2,6-bis [p- (dimethylamino) styryl] -4H- Examples include light-emitting materials having a 4-dicyanomethylene-4H-pyran skeleton such as pyran (abbreviation: BisDCM) and BisDCCz described above. Typical examples of the light emitting material having a halogeno group include light emitting materials having a haloalkyl group such as coumarin 152 and coumarin 153. Examples of the luminescent material having a carbonyl group include a luminescent material having an ester group such as coumarin 314, a luminescent material having an acyl group such as coumarin 334, and a luminescent material having a carboxyl group such as coumarin 343 and coumarin-3-carboxylic acid. Materials.

Examples of the electron transporting material that forms the electron transport layer 202 include Alq ₃ , tris (5-methyl-8-quinolinolato) aluminum (abbreviation: Almq ₃ ), bis (2-methyl-8-quinolinolato) − 4-phenylphenolato-aluminum (abbreviation: BAlq), tris (8-quinolinolato) gallium (abbreviation: Gaq ₃ ), bis (2-methyl-8-quinolinolato) -4-phenylphenolato-gallium (abbreviation: BGaq) Bis (10-hydroxybenzo [h] -quinolinato) beryllium abbreviation: BeBq ₂ ), bis [2- (2-hydroxyphenyl) benzoxazolate] zinc (abbreviation: Zn (BOX) ₂ ), bis [2- And metal complexes such as (2-hydroxyphenyl) benzothiazolate] zinc (abbreviation: Zn (BTZ) ₂ ). In addition to metal complexes, 2- (4-biphenylyl) -5- (4-tert-butylphenyl) -1,3,4-oxadiazole (abbreviation: PBD), 1,3-bis [5 -(P-tert-butylphenyl) -1,3,4-oxadiazol-2-yl] benzene (abbreviation: OXD-7), 3- (4-tert-butylphenyl) -4-phenyl-5 (4-Biphenylyl) -1,2,4-triazole (abbreviation: TAZ), 3- (4-tert-butylphenyl) -4- (4-ethylphenyl) -5- (4-biphenylyl) -1,2 , 4-triazole (abbreviation: p-EtTAZ), bathophenanthroline (abbreviation: BPhen), bathocuproin (abbreviation: BCP), and the like can be used.

  By the way, in the structure as shown in FIG. 2, the electron transport layer 202 may emit light depending on the combination of materials. This is because, since the recombination region is in the vicinity of the interface 203, holes partially enter the electron transport layer 202 and the material of the electron transport layer 202 is excited. When this phenomenon occurs, not only the guest material originally intended to emit light but also the material of the electron transport layer 202 emits light, which causes a decrease in color purity.

  Therefore, as one of the more preferable configurations of the present invention, a configuration as shown in FIG. 3 can be considered. That is, the difference (barrier 206) between the ionization potential of the host material (hole transport material) of the light-emitting layer 205 and the ionization potential of the electron transport material of the electron transport layer 202 may be increased. Specifically, the barrier 206 may be set to 0.3 eV or more. With such a structure, a phenomenon in which holes enter the electron transport layer 202 can be prevented, so that a phenomenon in which the electron transport material of the electron transport layer 202 is excited to emit light can be suppressed.

  In order to increase the barrier 206, the ionization potential of the host material (hole transport material) of the light-emitting layer 205 may be decreased, or the ion transport potential of the electron transport material of the electron transport layer 202 may be increased.

Usually, many electron transporting materials have an ionization potential of about 5.4 eV or higher (for example, Alq ₃ which is a typical electron transporting material is 5.4 eV). Therefore, it is sufficient that the ionization potential of the host material (hole transport material) is 5.1 eV or less. Specifically, the above-described TDATA, MTDATA, 1-TNATA, Al (daq) ₃ ), Zn (daq) ₃ and the like. For example, the ionization potential of 1-TNATA is 5.0 eV.

  Conversely, many hole transporting materials have an ionization potential of about 5.3 eV or less (for example, α-NPD which is a typical hole transporting material is 5.3 eV). Therefore, it is sufficient that the ionization potential of the electron transporting material is 5.6 eV or more. Specifically, the above-described BAlq, BGaq, PBD, OXD-7, TAZ, p-EtTAZ, BPhen, BCP, and the like. For example, the ionization potential of BAlq is 5.6 eV.

  Further, as another preferred configuration of the present invention, a configuration as shown in FIG. 4 is also conceivable. That is, a hole made of a hole trap material that can trap holes between the light emitting layer 205 and the electron transport layer 202 and has a smaller energy gap value than the electron transport material constituting the electron transport layer 202. A trap region 207 is provided. With such a configuration, a phenomenon in which holes enter the electron transport layer 202 can be prevented, and even if the hole trap material is excited, the excitation energy is transferred to the electron transport material of the electron transport layer 202. The phenomenon of moving can be prevented. Therefore, the phenomenon that the electron transporting material of the electron transport layer 202 is excited to emit light can be suppressed.

If a hole trap material having a smaller ionization potential than the host material of the light emitting layer 205 and the electron transport material of the electron transport layer 202 is used, holes can be trapped effectively as shown by arrows in FIG. can do. However, a hole trap can be used as long as it is a material that can prevent holes from entering the electron transport layer 202 and suppress emission of light from the electron transport material of the electron transport layer 202 even if not necessarily configured as described above. Can be a material. As a specific example of the hole trap material, an aromatic hydrocarbon compound having 18 or more carbon atoms represented by tetracene, pentacene, perylene, coronene, rubrene and the like is preferable because of its low ionization potential. In addition, carbon allotropes such as fullerene (C ₆₀ ), carbon nanotubes, and diamond-like carbon (abbreviation: DLC) are suitable because of their small energy gaps.

  If the hole trap region is too thick, depending on the material of the hole trap material, the electron flow may be hindered, or the hole trap material itself may be excited to emit light. In particular, in order to avoid the adverse effect of the hole trap material emitting light, the hole trap region has a layer thickness of 5 nm or less in consideration of the distance at which the Forster type energy transfer from the hole trap material to the guest material is sufficiently possible. Preferably there is.

  Further, from the viewpoint of 5 nm or less, the hole trap region may be formed in an island shape instead of a layer shape. As a method for forming the island structure, a known method may be used. For example, as disclosed in JP-A-2001-267077, the average film thickness on the film thickness monitor is made smaller than the film thickness of the monomolecular film. The method of vacuum-depositing a material is mentioned.

  By the way, as described above, when a red light emitting guest material having an electron withdrawing group is added to a host material to form a light emitting layer and a red light emitting element is manufactured, a host material having a small dipole moment is used. As compared with the case of using a host material having a large dipole moment, the light emission is blue-shifted, and in some cases, red with good color purity may not be achieved. This is considered to be a kind of solvent effect.

Alq ₃ conventionally used as a host material has two types of structural isomers and is usually referred to as fac type. The dipole moment was calculated using a commercially available molecular orbital calculation program WinMOPAC3.5 (manufactured by Fujitsu Limited) and found to be 9.398 debye (by the way, another structural isomer of mer type The dipole moment is 5.788 debye). A device in which a guest material that emits red light having an electron-attracting group with Alq ₃ as a host material is added to a material that has a higher dipole moment but has a smaller dipole moment (specifically, a material that is smaller than 4 debyes). It has been experimentally found that the device emits red light better than the element used as the material.

This indicates that the magnitude of the dipole moment of the host material is important in order to achieve red light emission with good color purity. However, since Alq ₃ is an electron transporting material, it is not suitable as a host material in the present invention.

Therefore, when a red light emitting element is manufactured using the present invention, it is preferable to use a material having a large dipole moment and exhibiting hole transport properties such as Alq ₃ . Many organic compounds having a normal aromatic amine skeleton have a small dipole moment, but such as Al (daq) ₃ , Zn (daq) ₃ , Co (PPZ) ₃ , and Co (m-PPZ) ₃ described above. A metal complex exhibiting hole transport properties is suitable because of its large dipole moment. For example, when the dipole moment of Al (daq) ₃ was calculated, it was 9.221 debye for the fac type (by the way, the dipole moment of the mer type which is another structural isomer is 4.639 debye). is there).

  Therefore, in the present invention, when the peak wavelength of the emission spectrum of the guest material is in the red region of 600 nm or more and 700 nm or less in the electroluminescent element having the above-described configuration, the dipole moment of the molecule of the host material is 4 Debye or more. Is preferred.

  Next, embodiments of the electroluminescent element of the present invention will be described in detail below. The electroluminescent layer of the electroluminescent element of the present invention may include at least the above-described light emitting layer and electron transport layer. That is, layers (hole injection layer, hole transport layer, electron transport layer, electron injection layer) showing functions other than light emission as known in conventional electroluminescent elements may be appropriately combined.

[Embodiment 1]
In Embodiment 1, an element structure of an electroluminescent element having a hole injection layer, a hole transport layer, a light emitting layer, an electron transport layer, and an electron injection layer will be described with reference to FIG. FIG. 5 illustrates an electroluminescent element in which a first electrode 501 is formed over a substrate 500, an electroluminescent layer 502 is formed over the first electrode 501, and a second electrode 503 is formed thereon.

  In addition, as a material used for the board | substrate 500 here, what is used for the conventional electroluminescent element should just be used, For example, what consists of glass, quartz, a transparent plastic, etc. can be used.

  In the first embodiment, the first electrode 501 functions as an anode, and the second electrode 503 functions as a cathode.

  That is, the first electrode 501 is preferably formed using an anode material, and a metal, an alloy, a conductive compound, a mixture thereof, or the like having a high work function (specifically, 4.0 eV or more) is preferably used. Specifically, in addition to ITO (indium tin oxide), IZO (indium zinc oxide) in which 2 to 20% zinc oxide (ZnO) is mixed with indium oxide, gold (Au), platinum (Pt), nickel (Ni), tungsten (W), chromium (Cr), molybdenum (Mo), iron (Fe), cobalt (Co), copper (Cu), palladium (Pd), or metal nitride (TiN), etc. Can be mentioned.

  On the other hand, as the cathode material used for the second electrode 503, it is preferable to use a metal, an alloy, a conductive compound, a mixture thereof, or the like with a low work function (specifically, 3.8 eV or less). Specifically, metals belonging to Group 1 or Group 2 of the element periodic rule, that is, alkali metals such as Li and Cs, and alkaline earth metals such as Mg, Ca, and Sr, and alloys containing them (Mg: Ag, Al: Li), rare earth metals such as Er and Yb, and alloys containing these. However, the second electrode 503 can be formed of a metal / conductive inorganic compound such as Al, Ag, or ITO by applying an electron injection layer described later.

  Note that the first electrode 501 and the second electrode 503 can be formed by an evaporation method, a sputtering method, or the like. The film thickness is preferably 10 to 500 nm.

  In the electroluminescent element of the present invention, light generated by recombination of carriers in the electroluminescent layer 502 is emitted from one or both of the first electrode 501 and the second electrode 503 to the outside. That is, when light is emitted from the first electrode 501, the first electrode 501 is formed of a light-transmitting material, and when light is emitted from the second electrode 503 side, the second electrode 501 is formed. The electrode 503 is formed of a light-transmitting material.

  The electroluminescent layer 502 is formed by stacking a plurality of layers. In Embodiment Mode 1, the hole injection layer 511, the hole transport layer 512, the light emitting layer 513, the electron transport layer 514, and the electron injection layer 515 are stacked. It is formed by doing. These layers can be formed by a vacuum deposition method or a wet coating method.

As a hole-injecting material that can be used for the hole-injecting layer 511, porphyrin-based compounds are effective as long as they are organic compounds, such as phthalocyanine (abbreviation: H ₂ -Pc), copper phthalocyanine (abbreviation: Cu-Pc), and the like. Can be used. In addition, as an organic compound, 4,4′-bis [N- {4- (N, N-bis (3-methylphenyl) amino) phenyl} -N-phenylamino] biphenyl (abbreviation: DNTPD) and the like Can be used. In addition, there is a material obtained by chemically doping a conductive polymer compound. Polyethylenedioxythiophene (abbreviation: PEDOT) doped with polystyrene sulfonic acid (abbreviation: PSS), polyaniline (abbreviation: PAni), or the like may be used. it can. Also effective are inorganic semiconductor layers such as VO _x and MoO _x and ultrathin films of inorganic insulators such as Al ₂ O ₃ .

Examples of the hole transporting material that can be used for the hole transport layer 512 include α-NPD, TPD, TDATA, MTDATA, 1-TNATA, Al (daq) ₃ , Zn (daq) ₃ , and Co (PPZ). ₃ , Co (m-PPZ) _{3 and the} like.

  The light-emitting layer 513 includes a host material 521 that exhibits hole transportability and a guest material 522 having an electron-withdrawing group. As the host material 521 exhibiting hole transportability, the above-described hole transportable material may be applied and may be the same as or different from the hole transportable material of the hole transport layer 512. Examples of the guest material having an electron withdrawing group include DCM1, DCM2, BisDCM, BisDCCz, Coumarin 337, Coumarin 152, Coumarin 153, Coumarin 314, Coumarin 334, Coumarin 343, Coumarin-3-carboxylic acid and the like described above. .

Examples of the electron transporting material that can be used for the electron transporting layer 514 include Alq ₃ , Almq ₃ , BAlq, Gaq ₃ , BGaq, BeBq ₂ , Zn (BOX) ₂ , Zn (BTZ) ₂ , PBD, OXD-7, TAZ, p-EtTAZ, BPhen, BCP and the like can be mentioned.

As the electron injecting material that can be used for the electron injecting layer 515, the above-described electron transporting material can be used. In addition, an ultra-thin film of an insulator such as an alkali metal halide such as LiF or CsF, an alkaline earth halide such as CaF ₂ , or an alkali metal oxide such as Li ₂ O is often used. In addition, alkali metal complexes such as lithium acetylacetonate (abbreviation: Li (acac) and 8-quinolinolato-lithium (abbreviation: Liq) are also effective. Furthermore, the above-described electron transporting materials, Mg, Li, Cs, and the like are also effective. A layer mixed with a metal having a small work function can also be used as the electron injection layer 515.

  As described above, the electroluminescent element of the present invention including the light-emitting layer 513 including the host material 521 having hole transportability and the guest material 522 having an electron-withdrawing group and the electron-transport layer 514 provided in contact with the light-emitting layer 513 is obtained. Can be produced.

[Embodiment 2]
In Embodiment 2, a structure in which a hole trap region is added to the element structure disclosed in Embodiment 1 will be described with reference to FIGS. In FIG. 6, the reference numerals of FIG. 5 are cited.

  As shown in FIG. 6, in the second embodiment, a hole trap region 516 is provided between the light emitting layer 513 and the electron transport layer 514. In FIG. 6, it is formed in an island shape, but may be a layer shape of 5 nm or less.

As the hole trap material 523 constituting the hole trap region 516, tetracene, pentacene, perylene, coronene, rubrene, fullerene (C ₆₀ ), carbon nanotube, DLC, or the like can be used as described above.

  As described above, the light-emitting layer 513 including the host material 521 having a hole-transport property and the guest material 522 having an electron-withdrawing group, the electron-transport layer 514 provided in contact with the light-emitting layer 513, and the light-emitting layer 513 An electroluminescent element of the present invention in which a hole trap region 516 is provided between the electron transport layer 514 and the electron transport layer 514 can be manufactured. With this configuration, a phenomenon in which the electron transporting material of the electron transporting layer 514 emits light can be suppressed.

  In Example 1, an example of manufacturing the electroluminescent element of the present invention shown in FIG. 5 is specifically illustrated.

  First, a first electrode (anode) 501 is formed over a glass substrate 500 having an insulating surface. ITO, which is a transparent conductive film, was used as a material, and a film thickness of 110 nm was formed by a sputtering method. The shape of the anode 501 was 2 mm × 2 mm.

  After the substrate on which the anode 501 is thus formed is washed and dried, the electroluminescent layer 502 is formed on the anode 501. First, the substrate on which the anode 501 is formed is fixed to the substrate holder of the vacuum deposition apparatus with the surface on which the anode 501 is formed facing down, and Cu—Pc is deposited to a thickness of 20 nm by a vacuum deposition method using a resistance heating method. The film was formed. This becomes the hole injection layer 511. Next, α-NPD, which is a hole transporting material, was formed with a film thickness of 25 nm by the same method to form a hole transporting layer 512.

  Further, α-NPD which is a hole transporting material is used as the host material 521, BisDCM is used as the guest material 522 having an electron withdrawing group, and the light emitting layer 513 is formed by performing co-evaporation so that the BisDCM concentration becomes 2 wt%. Formed. The film thickness was 15 nm.

Next, Alq ₃ which is an electron transporting material was formed to have a thickness of 75 nm by a vacuum evaporation method, and an electron transporting layer 514 was obtained. Further, 1 nm of CaF ₂ was formed as the electron injection layer 515 by vacuum deposition. The above is the electroluminescent layer 502, and the total film thickness is 136 nm.

  Finally, a second electrode (cathode) 503 is formed. In this example, aluminum (Al) was formed to 200 nm by a vacuum evaporation method using resistance heating to form a cathode 503.

When a voltage of 10 V was applied to the electroluminescent device of the present invention produced as described above, a current flowed at a current density of 6.83 mA / cm ² , and light was emitted with a luminance of 127 cd / m ² . The peak wavelength of the emission spectrum was 642 nm.

[Comparative Example 1]
On the other hand, a conventional electroluminescent device having a light emitting layer in which a guest material having an electron withdrawing group was added to an electron transporting material was manufactured and compared with Example 1. The element structure of this comparative example is shown in FIG.

  Similar to Example 1, an electroluminescent layer 702 is formed on a glass substrate 700 on which 110 nm of ITO is formed as an anode 701. First, after cleaning and drying the substrate, the surface on which the anode 701 is formed is fixed to the substrate holder of the vacuum evaporation apparatus with the surface facing down, and Cu—Pc is deposited to a thickness of 20 nm by vacuum evaporation using a resistance heating method. A film was formed. This becomes the hole injection layer 711. Next, α-NPD, which is a hole transporting material, was formed to a thickness of 40 nm by the same method as the hole transport layer 712.

Further, Alq ₃ which is an electron transporting material is used as the host material 721, BisDCM is used as the guest material 722 having an electron withdrawing group, and co-evaporation is performed so that the concentration of BisDCM becomes 2 wt%. A light emitting layer 713 was formed. The film thickness was 15 nm.

Next, Alq ₃ which is an electron transporting material was formed to a thickness of 60 nm by a vacuum evaporation method, and an electron transport layer 714 was formed. Further, 1 nm of CaF ₂ was formed as the electron injection layer 715 by vacuum deposition. The above is the electroluminescent layer 702, and the total film thickness is 136 nm, which is the same as that in Example 1.

  Finally, the cathode 703 is formed. As in Example 1, aluminum (Al) was formed to a thickness of 200 nm by a vacuum evaporation method using resistance heating to form a cathode 703.

When a voltage of 10 V was applied to the conventional electroluminescent device fabricated as described above, a current flowed at a current density of 2.84 mA / cm ² , and light was emitted with a luminance of 27.1 cd / m ² . The peak wavelength of the emission spectrum was 666 nm.

  From the above results, the electroluminescent device to which the present invention was applied was able to lower the driving voltage, although the peak wavelength of the emission spectrum was slightly blue shifted. The current-voltage characteristics of Example 1 and Comparative Example 1 are shown in FIG. As can be seen from FIG. 10, it can be seen that by applying the present invention, the current easily flows as intended.

  In Example 2, an element using a host material different from that in Example 1 is specifically illustrated.

The element structure is the structure shown in FIG. 5, and the substrate 500, the anode 501, and the cathode 503 are configured in the same manner as in Example 1. The electroluminescent layer 502 is made of 20 nm of CuPc as the hole injection layer 511, 30 nm of α-NPD as the hole transport layer 512, and 1 wt. Of 2,3-bis (4-diphenylaminophenyl) quinoxaline (abbreviation: TPAQn) as the light emitting layer 513. A layer to which BisDCM was added at a ratio of 30% was formed, Alq ₃ was formed to be 20 nm as an electron transport layer, and CaF ₂ was formed to be ₂ nm as an electron injection layer. The total film thickness is 102 nm. Since TPAQn is a bipolar material, it has hole transportability.

When a voltage of 10 V was applied to the electroluminescent device of the present invention produced as described above, a current flowed at a current density of 283 mA / cm ² , and light was emitted with a luminance of 1350 cd / m ² . The peak wavelength of the emission spectrum was 616 nm.

[Comparative Example 2]
On the other hand, a conventional electroluminescent device having a light emitting layer in which a guest material having an electron withdrawing group was added to an electron transporting material was manufactured and compared with Example 2. The element structure of this comparative example was the same as that shown in FIG.

As in the second embodiment, the substrate 700, the anode 701, and the cathode 703 are exactly the same as the second embodiment. The electroluminescent layer 702 is made of 20 nm of CuPc as the hole injection layer 711, 30 nm of α-NPD as the hole transport layer 712, and 30 nm of BiSDC added to BAl as an electron transport material at a ratio of 1 wt% as the light emitting layer 713. Then, 20 nm of Alq ₃ was formed as the electron transport layer, and 2 nm of CaF ₂ was formed as the electron injection layer. The total film thickness is 102 nm, which is the same as in Example 2.

When a voltage of 10 V was applied to the conventional electroluminescent device fabricated as described above, current flowed at a current density of 4.23 mA / cm ² , and light was emitted with a luminance of 30.9 cd / m ² . The peak wavelength of the emission spectrum was 619 nm.

  From the above results, the electroluminescence device to which the present invention is applied has the peak wavelength of the emission spectrum substantially the same as the conventional one, and the driving voltage can be lowered. The current-voltage characteristics of Example 2 and Comparative Example 2 are shown in FIG. As can be seen from FIG. 11, by applying the present invention, it can be seen that the current easily flows as intended.

  In this embodiment, a light-emitting device having the electroluminescent element of the present invention in a pixel portion will be described with reference to FIG. 8A is a top view illustrating the light-emitting device, and FIG. 8B is a cross-sectional view taken along line A-A ′ in FIG. 8A. Reference numeral 801 indicated by a dotted line denotes a drive circuit portion (source side drive circuit), 802 denotes a pixel portion, and 803 denotes a drive circuit portion (gate side drive circuit). Further, reference numeral 804 denotes a sealing substrate, and 805 denotes a sealing material. An inner side surrounded by the sealing material 805 is a space 807.

  Reference numeral 808 denotes a wiring for transmitting signals input to the source side driver circuit 801 and the gate side driver circuit 803, and a video signal, a clock signal, and a start signal from an FPC (flexible printed circuit) 809 serving as an external input terminal. Receive a reset signal, etc. Although only the FPC is shown here, a printed wiring board (PWB) may be attached to the FPC. The light-emitting device in this specification includes not only a light-emitting device body but also a state in which an FPC or a PWB is attached thereto.

  Next, a cross-sectional structure is described with reference to FIG. A driver circuit portion and a pixel portion are formed over the element substrate 810. Here, a source side driver circuit 801 which is a driver circuit portion and a pixel portion 802 are shown.

  Note that the source side driver circuit 801 is a CMOS circuit in which an n-channel TFT 823 and a p-channel TFT 824 are combined. The TFT forming the driving circuit may be formed by a known CMOS circuit, PMOS circuit or NMOS circuit. In this embodiment mode, a driver integrated type in which a driver circuit is formed over a substrate is shown; however, this is not always necessary, and the driver circuit may be formed outside the substrate.

  The pixel portion 802 is formed of a plurality of pixels including a switching TFT 811, a current control TFT 812, and a first electrode 813 electrically connected to the drain thereof. Note that an insulator 814 is formed to cover an end portion of the first electrode 813. Here, a positive photosensitive acrylic resin film is used.

  In order to improve the coverage, a curved surface having a curvature is formed at the upper end portion or the lower end portion of the insulator 814. For example, in the case where positive photosensitive acrylic is used as a material for the insulator 814, it is preferable that only the upper end portion of the insulator 814 have a curved surface having a curvature radius (0.2 μm to 3 μm). As the insulator 814, either a negative type that becomes insoluble in an etchant by photosensitive light or a positive type that becomes soluble in an etchant by light can be used.

  An electroluminescent layer 816 and a second electrode 817 are formed over the first electrode 813, respectively. Here, as a material used for the first electrode 813 functioning as an anode, a material having a high work function is preferably used. For example, ITO (Indium Tin Oxide) film, Indium Zinc Oxide (IZO) film, Titanium nitride film, Chromium film, Tungsten film, Zn film, Pt film, etc., as well as titanium nitride and aluminum as main components And a three-layer structure of a titanium nitride film, a film containing aluminum as its main component, and a titanium nitride film can be used. Note that with a stacked structure, resistance as a wiring is low, good ohmic contact can be obtained, and a function as an anode can be obtained.

  The electroluminescent layer 816 is formed by an evaporation method using an evaporation mask or an inkjet method. The configuration of the electroluminescent layer 816 may be the configuration of the electroluminescent layer shown in Example 1 or Example 2, for example.

Further, as a material used for the second electrode (cathode) 817 formed on the electroluminescent layer 816, a material having a low work function (Al, Ag, Li, Ca, or alloys thereof MgAg, MgIn, AlLi, CaF) is used. ₂ or CaN). Note that in the case where light generated in the electroluminescent layer 816 is transmitted through the second electrode 817, as the second electrode (cathode) 817, for example, a thin metal film and a transparent conductive film (ITO, And a method using a stack with IZO, zinc oxide (ZnO), or the like.

  Further, the sealing substrate 804 is bonded to the element substrate 810 with the sealant 805, whereby the electroluminescent element 818 is provided in the space 807 surrounded by the element substrate 810, the sealing substrate 804, and the sealant 805. ing. Note that the space 807 includes a structure filled with a sealant 805 in addition to a case where the space is filled with an inert gas (nitrogen, argon, or the like).

  Note that an epoxy-based resin is preferably used for the sealant 805. Moreover, it is desirable that these materials are materials that do not transmit moisture and oxygen as much as possible. In addition to a glass substrate and a quartz substrate, a plastic substrate made of FRP (Fiberglass-Reinforced Plastics), PVF (polyvinyl fluoride), Mylar, polyester, acrylic, or the like can be used as a material used for the sealing substrate 804.

  As described above, a light-emitting device having the electroluminescent element of the present invention can be obtained.

  For example, various electric appliances including a light-emitting device having the electroluminescent element of the present invention as a display portion can be provided.

  As an electric appliance manufactured using a light emitting device having an electroluminescent element of the present invention, a video camera, a digital camera, a goggle type display, a navigation system, an acoustic reproduction device (car audio, audio component, etc.), a notebook personal computer, Reproducing a recording medium such as a game machine, a portable information terminal (mobile computer, cellular phone, portable game machine, electronic book, etc.), an image reproducing apparatus (specifically, Digital Versatile Disc (DVD)) equipped with a recording medium, A device provided with a display device capable of displaying the image). Specific examples of these electric appliances are shown in FIG.

  FIG. 9A illustrates a display device, which includes a housing 9101, a support base 9102, a display portion 9103, a speaker portion 9104, a video input terminal 9105, and the like. It is manufactured by using a light emitting device having the electroluminescent element of the present invention for the display portion 9103. The display device includes all information display devices such as a computer, a TV broadcast receiver, and an advertisement display.

  FIG. 9B shows a laptop personal computer, which includes a main body 9201, a housing 9202, a display portion 9203, a keyboard 9204, an external connection port 9205, a pointing mouse 9206, and the like. It is manufactured by using a light emitting device having the electroluminescent element of the present invention for the display portion 9203.

  FIG. 9C illustrates a mobile computer, which includes a main body 9301, a display portion 9302, a switch 9303, operation keys 9304, an infrared port 9305, and the like. It is manufactured by using a light emitting device having the electroluminescent element of the present invention for the display portion 9302.

  FIG. 9D illustrates a portable image reproducing device (specifically, a DVD reproducing device) provided with a recording medium, which includes a main body 9401, a housing 9402, a display portion A 9403, a display portion B 9404, and a recording medium (such as a DVD). A reading unit 9405, operation keys 9406, a speaker unit 9407, and the like are included. The display portion A9403 mainly displays image information, and the display portion B9404 mainly displays character information. The display portion A9403 is manufactured by using a light-emitting device having the electroluminescent element of the present invention for the display portions A9403 and B9404. Note that an image reproducing device provided with a recording medium includes a home game machine and the like.

  FIG. 9E illustrates a goggle type display, which includes a main body 9501, a display portion 9502, and an arm portion 9503. It is manufactured by using a light emitting device having an electroluminescent element of the present invention for the display portion 9502.

  FIG. 9F illustrates a video camera, which includes a main body 9601, a display portion 9602, a housing 9603, an external connection port 9604, a remote control reception portion 9605, an image receiving portion 9606, a battery 9607, an audio input portion 9608, operation keys 9609, and an eyepiece. Part 9610 and the like. The display device 9602 is manufactured using the light-emitting device having the electroluminescent element of the present invention.

  Here, FIG. 9G shows a cellular phone, which includes a main body 9701, a housing 9702, a display portion 9703, an audio input portion 9704, an audio output portion 9705, operation keys 9706, an external connection port 9707, an antenna 9708, and the like. It is manufactured by using a light emitting device having the electroluminescent element of the present invention for the display portion 9703. Note that the display portion 9703 can suppress power consumption of the mobile phone by displaying white characters on a black background.

  In Example 5, an example of manufacturing an electroluminescent element using a guest material having an electron-withdrawing group different from those in Examples 1 and 2 is specifically illustrated. The element structure was as shown in FIG.

  First, the anode 501 is formed over the glass substrate 500 having an insulating surface. Using indium tin oxide (ITSO) to which silicon oxide was added as a material, a film having a thickness of 110 nm was formed by a sputtering method. The shape of the anode 501 was 2 mm × 2 mm.

  After the substrate on which the anode 501 is thus formed is washed and dried, the electroluminescent layer 502 is formed on the anode 501. First, the substrate on which the anode 501 is formed is fixed to the substrate holder of the vacuum evaporation apparatus with the surface on which the anode 501 is formed facing downward, and 4,4′-bis [N] is obtained by vacuum evaporation using a resistance heating method. -[4- {N, N-bis (3-methylphenyl) amino} phenyl] -N-phenylamino] biphenyl (abbreviation: DNTPD) was formed to a thickness of 50 nm. This becomes the hole injection layer 511. Next, α-NPD, which is a hole transporting material, was formed to a thickness of 10 nm by the same method to form a hole transport layer 512.

  Further, α-NPD that is a hole transporting material is used as the host material 521, and coumarin 153 is used as the guest material 522 having an electron-withdrawing group, and co-evaporation is performed so that the concentration of the coumarin 153 becomes 0.5 mass%. A light emitting layer 513 was formed. The film thickness was 30 nm. Note that since the coumarin 153 has a trifluoromethyl group which is a kind of haloalkyl group, it is a compound containing a halogeno group which is an electron-withdrawing group.

Next, Alq ₃ which is an electron transporting material was formed to a thickness of 30 nm by a vacuum evaporation method, and an electron transport layer 514 was obtained. Further, 1 nm of CaF ₂ was formed as the electron injection layer 515 by vacuum deposition. The above is the electroluminescent layer 502, and the total film thickness is 121 nm.

  Finally, the cathode 503 is formed. In this example, aluminum (Al) was formed to a thickness of 150 nm by a vacuum evaporation method using resistance heating to form a cathode 503.

When a voltage of 6.0 V was applied to the electroluminescent device of the present invention produced as described above, a current flowed at a current density of 51.3 mA / cm ² and light was emitted at a luminance of 1590 cd / m ² . The peak wavelength of the emission spectrum was 518 nm.

[Comparative Example 3]
On the other hand, a conventional electroluminescent device having a light emitting layer in which a guest material having an electron withdrawing group was added to an electron transporting material was manufactured and compared with Example 5. The element structure was the same as that of Example 5 except for the light emitting layer 513.

The light emitting layer 513 in this comparative example uses Alq ₃ which is an electron transporting material as a host material, and coumarin 153 which is the same as that in Example 5 as a guest material having an electron withdrawing group, and the concentration of the coumarin 153 is 0.5 mass%. It formed by performing co-evaporation so that it might become. The film thickness was 30 nm. The total thickness of the electroluminescent layer 502 is 121 nm, which is the same as that of the fifth embodiment.

When a voltage of 6.0 V was applied to the conventional electroluminescent device manufactured as described above, a current flowed at a current density of 18.8 mA / cm ² , and light was emitted at a luminance of 1250 cd / m ² . The peak wavelength of the emission spectrum was 530 nm.

  From the above results, in the electroluminescent device to which the present invention was applied, the peak wavelength of the emission spectrum was slightly blue shifted, but the driving voltage could be lowered. The current-voltage characteristics of Example 5 and Comparative Example 3 are shown in FIG. As can be seen from FIG. 12, it can be seen that application of the present invention facilitates the flow of current.

 In Example 6, an example of manufacturing an electroluminescent element using a guest material having an electron-withdrawing group different from those in Examples 1 and 2 is specifically illustrated. The element structure was as shown in FIG.

  First, the anode 501 is formed over the glass substrate 500 having an insulating surface. ITSO was used as a material, and a film thickness of 110 nm was formed by a sputtering method. The shape of the anode 501 was 2 mm × 2 mm.

  After the substrate on which the anode 501 is thus formed is washed and dried, the electroluminescent layer 502 is formed on the anode 501. First, the substrate on which the anode 501 is formed is fixed to the substrate holder of the vacuum deposition apparatus with the surface on which the anode 501 is formed facing down, and DNTPD is formed to a thickness of 50 nm by a vacuum deposition method using a resistance heating method. Filmed. This becomes the hole injection layer 511. Next, α-NPD, which is a hole transporting material, was formed to a thickness of 10 nm by the same method to form a hole transport layer 512.

  Further, α-NPD that is a hole transporting material is used as the host material 521, and coumarin 153 is used as the guest material 522 having an electron-withdrawing group, and co-evaporation is performed so that the concentration of the coumarin 153 becomes 0.5 mass%. A light emitting layer 513 was formed. The film thickness was 30 nm. Since coumarin 153 has a trifluoromethyl group which is a kind of haloalkyl group, it is a compound containing a halogeno group which is an electron-withdrawing group.

Next, BAlq which is an electron transporting material was formed to 10 nm, Alq ₃ was formed to 20 nm by a vacuum evaporation method, and an electron transporting layer 514 was obtained. Further, 1 nm of CaF ₂ was formed as the electron injection layer 515 by vacuum deposition. The above is the electroluminescent layer 502, and the total film thickness is 121 nm.

  Finally, the cathode 503 is formed. In this example, aluminum (Al) was formed to a thickness of 150 nm by a vacuum evaporation method using resistance heating to form a cathode 503.

When a voltage of 6.0 V was applied to the electroluminescent device of the present invention produced as described above, a current flowed at a current density of 30.3 mA / cm ² , and light was emitted with a luminance of 659 cd / m ² . The peak wavelength of the emission spectrum was 499 nm.

[Comparative Example 4]
On the other hand, a conventional electroluminescent device having a light emitting layer in which a guest material having an electron withdrawing group was added to an electron transporting material was manufactured and compared with Example 6. The element structure was the same as that of Example 6 except for the light emitting layer 513 and the electron transport layer 514.

The light emitting layer 513 in this comparative example uses BAlq which is an electron transport material as a host material, and the same coumarin 153 as in Example 6 as a guest material having an electron withdrawing group, and the concentration of the coumarin 153 is 0.5 mass%. It formed by performing co-evaporation so that it might become. The film thickness was 30 nm. The electron transport layer 514 was formed by vacuum-depositing Alq ₃ with a thickness of 30 nm. The total thickness of the electroluminescent layer 502 is 121 nm, which is the same as that of the sixth embodiment.

When a voltage of 6.0 V was applied to the conventional electroluminescent device manufactured as described above, a current flowed at a current density of 0.262 mA / cm ² , and light was emitted with a luminance of 14.4 cd / m ² . The peak wavelength of the emission spectrum was 517 nm.

  From the above results, in the electroluminescent device to which the present invention was applied, the peak wavelength of the emission spectrum was slightly blue shifted, but the driving voltage could be lowered. The current-voltage characteristics of Example 6 and Comparative Example 4 are shown in FIG. As can be seen from FIG. 13, it can be seen that applying the present invention facilitates the flow of current.

  In Example 7, an example of manufacturing an electroluminescent element using a guest material having an electron withdrawing group different from those in Examples 1, 2, 5, and 6 is specifically illustrated. The element structure was as shown in FIG.

  First, the anode 501 is formed over the glass substrate 500 having an insulating surface. ITSO was used as a material, and a film thickness of 110 nm was formed by a sputtering method. The shape of the anode 501 was 2 mm × 2 mm.

  After the substrate on which the anode 501 is thus formed is washed and dried, the electroluminescent layer 502 is formed on the anode 501. First, the substrate on which the anode 501 is formed is fixed to the substrate holder of the vacuum deposition apparatus with the surface on which the anode 501 is formed facing down, and DNTPD is formed to a thickness of 50 nm by a vacuum deposition method using a resistance heating method. Filmed. This becomes the hole injection layer 511. Next, α-NPD, which is a hole transporting material, was formed to a thickness of 10 nm by the same method to form a hole transport layer 512.

  Further, α-NPD which is a hole transporting material is used as the host material 521, and coumarin 334 is used as the guest material 522 having an electron withdrawing group, and co-evaporation is performed so that the concentration of the coumarin 334 becomes 0.5 mass%. A light emitting layer 513 was formed. The film thickness was 30 nm. Note that coumarin 334 is a compound including a carbonyl group that is an electron-withdrawing group because it has an acetyl group that is a kind of acyl group.

Next, BAlq which is an electron transporting material was formed to 10 nm, Alq ₃ was formed to 20 nm by a vacuum evaporation method, and an electron transporting layer 514 was obtained. Further, 1 nm of CaF ₂ was formed as the electron injection layer 515 by vacuum deposition. The above is the electroluminescent layer 502, and the total film thickness is 121 nm.

  Finally, the cathode 503 is formed. In this example, aluminum (Al) was formed to a thickness of 150 nm by a vacuum evaporation method using resistance heating to form a cathode 503.

When a voltage of 6.0 V was applied to the electroluminescent device of the present invention produced as described above, a current flowed at a current density of 11.0 mA / cm ² and light was emitted at a luminance of 220 cd / m ² . The peak wavelength of the emission spectrum was 477 nm.

[Comparative Example 5]
On the other hand, a conventional electroluminescent device having a light emitting layer in which a guest material having an electron withdrawing group was added to an electron transporting material was manufactured and compared with Example 7. The element structure was the same as that of Example 7 except for the light emitting layer 513 and the electron transport layer 514.

The light emitting layer 513 in this comparative example uses BAlq which is an electron transport material as a host material, and the same coumarin 334 as in Example 7 as a guest material having an electron withdrawing group, and the concentration of coumarin 334 is 0.5 mass%. It formed by performing co-evaporation so that it might become. The film thickness was 30 nm. The electron transport layer 514 was formed by vacuum-depositing Alq ₃ with a thickness of 30 nm. The total thickness of the electroluminescent layer 502 is 121 nm, which is the same as that of the seventh embodiment.

When a voltage of 6.0 V was applied to the conventional electroluminescent device produced as described above, a current flowed at a current density of 1.07 mA / cm ² , and light was emitted with a luminance of 42.5 cd / m ² . The peak wavelength of the emission spectrum was 485 nm.

  From the above results, in the electroluminescent device to which the present invention was applied, the peak wavelength of the emission spectrum was slightly blue shifted, but the driving voltage could be lowered. The current-voltage characteristics of Example 7 and Comparative Example 5 are shown in FIG. As can be seen from FIG. 14, it can be seen that applying the present invention facilitates the flow of current.

  As described above, the applicable range of the light-emitting device having the electroluminescent element of the present invention is so wide that the light-emitting device can be applied to electric appliances in various fields.

The figure which shows the band diagram of the conventional electroluminescent element. The figure which shows the band diagram of the electroluminescent element of this invention. The figure which shows the band diagram of the electroluminescent element of this invention. The figure which shows the band diagram of the electroluminescent element of this invention. The figure which shows the element structure of the electroluminescent element of this invention. The figure which shows the element structure of the electroluminescent element of this invention. The figure which shows the element structure of the conventional electroluminescent element. 4A and 4B illustrate a light-emitting device using an electroluminescent element of the present invention. The figure explaining the electric appliance using the light-emitting device of this invention. The figure which shows the current-voltage characteristic of Example 1 and Comparative Example 1. The figure which shows the current-voltage characteristic of Example 2 and Comparative Example 2. FIG. The figure which shows the current-voltage characteristic of Example 5 and Comparative Example 3. The figure which shows the current-voltage characteristic of Example 6 and Comparative Example 4. The figure which shows the current-voltage characteristic of Example 7 and Comparative Example 5. FIG.

Explanation of symbols

500 substrate 501 first electrode 502 electroluminescent layer 503 second electrode 511 hole injection layer 512 hole transport layer 513 light emission layer 514 electron transport layer 515 electron injection layer 521 host material 522 guest material

Claims (16)

  In an electroluminescent element having a light emitting layer containing a host material and a guest material having an electron withdrawing group, and an electron transport layer provided in contact with the light emitting layer, the host material is an organic compound having a hole transporting property. There is provided an electroluminescent element.   2. The electroluminescent device according to claim 1, wherein the host material is an organic compound having an aromatic amine skeleton.   3. The electroluminescent device according to claim 1, wherein the electron withdrawing group is a cyano group, a halogeno group, or a carbonyl group.   3. The electroluminescent device according to claim 1, wherein the guest material is an organic compound having a 4-dicyanomethylene-4H-pyran skeleton.   5. The electroluminescent device according to claim 1, wherein a peak wavelength of an emission spectrum of the guest material is not less than 560 nm and not more than 700 nm.   6. The electroluminescent device according to claim 1, wherein an ionization potential of the host material is 0.3 eV or more larger than an ionization potential of an electron transport material constituting the electron transport layer. An electroluminescent element characterized by the above.   7. The electroluminescent device according to claim 6, wherein the ionization potential of the host material is 5.1 eV or less.   The electroluminescent device according to claim 6, wherein an ionization potential of the electron transporting material is 5.6 eV or more.   6. The electroluminescent device according to claim 1, wherein holes can be trapped between the light emitting layer and the electron transport layer, and the electron transport layer is configured. An electroluminescent element comprising a hole trap region made of a hole trap material having an energy gap value smaller than that of an electron transporting material.   10. The electroluminescent device according to claim 9, wherein the hole trap material has an ionization potential smaller than that of the host material and the electron transport material.   11. The electroluminescent device according to claim 9, wherein the hole trap region has a layer shape of 5 nm or less.   11. The electroluminescent device according to claim 9, wherein the hole trap region is formed in an island shape.   The electroluminescence device according to any one of claims 9 to 12, wherein the hole trap material is an aromatic hydrocarbon compound having 18 or more carbon atoms or a carbon allotrope. element.   14. The electroluminescent element according to claim 1, wherein the peak wavelength of the emission spectrum of the guest material is 600 nm or more and 700 nm or less, and the dipole moment of the molecule of the host material is 4 An electroluminescent element characterized by being Debye or higher.   A light-emitting device using the electroluminescent element according to claim 1. An electric appliance using the light-emitting device according to claim 15.

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