US20150333283A1 - Light-emitting element, light-emitting device, display device, electronic device, and lighting device - Google Patents

Light-emitting element, light-emitting device, display device, electronic device, and lighting device Download PDF

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US20150333283A1
US20150333283A1 US14/709,008 US201514709008A US2015333283A1 US 20150333283 A1 US20150333283 A1 US 20150333283A1 US 201514709008 A US201514709008 A US 201514709008A US 2015333283 A1 US2015333283 A1 US 2015333283A1
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light
emitting
layer
emitting element
electrode
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Takahiro Ishisone
Satoshi Seo
Yusuke Nonaka
Nobuharu Ohsawa
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Semiconductor Energy Laboratory Co Ltd
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Semiconductor Energy Laboratory Co Ltd
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Assigned to SEMICONDUCTOR ENERGY LABORATORY CO., LTD. reassignment SEMICONDUCTOR ENERGY LABORATORY CO., LTD. ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: OHSAWA, NOBUHARU, SEO, SATOSHI, ISHISONE, TAKAHIRO, NONAKA, YUSUKE
Publication of US20150333283A1 publication Critical patent/US20150333283A1/en
Priority to US15/911,225 priority Critical patent/US10686153B2/en
Priority to US16/878,759 priority patent/US11158832B2/en
Priority to US17/408,588 priority patent/US11864403B2/en
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    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
    • H10K50/00—Organic light-emitting devices
    • H10K50/10—OLEDs or polymer light-emitting diodes [PLED]
    • H10K50/11—OLEDs or polymer light-emitting diodes [PLED] characterised by the electroluminescent [EL] layers
    • H10K50/12—OLEDs or polymer light-emitting diodes [PLED] characterised by the electroluminescent [EL] layers comprising dopants
    • H01L51/504—
    • H01L51/5016—
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
    • H10K50/00—Organic light-emitting devices
    • H10K50/10—OLEDs or polymer light-emitting diodes [PLED]
    • H10K50/11—OLEDs or polymer light-emitting diodes [PLED] characterised by the electroluminescent [EL] layers
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
    • H10K50/00—Organic light-emitting devices
    • H10K50/10—OLEDs or polymer light-emitting diodes [PLED]
    • H10K50/11—OLEDs or polymer light-emitting diodes [PLED] characterised by the electroluminescent [EL] layers
    • H10K50/125—OLEDs or polymer light-emitting diodes [PLED] characterised by the electroluminescent [EL] layers specially adapted for multicolour light emission, e.g. for emitting white light
    • H10K50/13—OLEDs or polymer light-emitting diodes [PLED] characterised by the electroluminescent [EL] layers specially adapted for multicolour light emission, e.g. for emitting white light comprising stacked EL layers within one EL unit
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
    • H10K50/00—Organic light-emitting devices
    • H10K50/10—OLEDs or polymer light-emitting diodes [PLED]
    • H10K50/11—OLEDs or polymer light-emitting diodes [PLED] characterised by the electroluminescent [EL] layers
    • H10K50/125—OLEDs or polymer light-emitting diodes [PLED] characterised by the electroluminescent [EL] layers specially adapted for multicolour light emission, e.g. for emitting white light
    • H10K50/13—OLEDs or polymer light-emitting diodes [PLED] characterised by the electroluminescent [EL] layers specially adapted for multicolour light emission, e.g. for emitting white light comprising stacked EL layers within one EL unit
    • H10K50/131—OLEDs or polymer light-emitting diodes [PLED] characterised by the electroluminescent [EL] layers specially adapted for multicolour light emission, e.g. for emitting white light comprising stacked EL layers within one EL unit with spacer layers between the electroluminescent layers
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
    • H10K50/00—Organic light-emitting devices
    • H10K50/80—Constructional details
    • H10K50/805—Electrodes
    • H01L2251/5376—
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L2924/00—Indexing scheme for arrangements or methods for connecting or disconnecting semiconductor or solid-state bodies as covered by H01L24/00
    • H01L2924/10—Details of semiconductor or other solid state devices to be connected
    • H01L2924/11—Device type
    • H01L2924/12—Passive devices, e.g. 2 terminal devices
    • H01L2924/1204—Optical Diode
    • H01L2924/12044—OLED
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
    • H10K2101/00—Properties of the organic materials covered by group H10K85/00
    • H10K2101/10—Triplet emission
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
    • H10K2101/00—Properties of the organic materials covered by group H10K85/00
    • H10K2101/27—Combination of fluorescent and phosphorescent emission
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
    • H10K2101/00—Properties of the organic materials covered by group H10K85/00
    • H10K2101/40—Interrelation of parameters between multiple constituent active layers or sublayers, e.g. HOMO values in adjacent layers
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
    • H10K85/00—Organic materials used in the body or electrodes of devices covered by this subclass
    • H10K85/30—Coordination compounds
    • H10K85/341—Transition metal complexes, e.g. Ru(II)polypyridine complexes
    • H10K85/342—Transition metal complexes, e.g. Ru(II)polypyridine complexes comprising iridium
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
    • H10K85/00—Organic materials used in the body or electrodes of devices covered by this subclass
    • H10K85/60—Organic compounds having low molecular weight
    • H10K85/631—Amine compounds having at least two aryl rest on at least one amine-nitrogen atom, e.g. triphenylamine
    • H10K85/633—Amine compounds having at least two aryl rest on at least one amine-nitrogen atom, e.g. triphenylamine comprising polycyclic condensed aromatic hydrocarbons as substituents on the nitrogen atom

Definitions

  • One embodiment of the present invention relates to a light-emitting element, and a light-emitting device, a display device, an electronic device, and a lighting device each including a light-emitting element.
  • the technical field of one embodiment of the present invention also includes a semiconductor device including the light-emitting element and its manufacturing method.
  • a light-emitting element in which a layer containing an organic compound is provided between a pair of electrodes and a light-emitting device including the light-emitting element are called an organic electroluminescent element and an organic electroluminescent device, respectively.
  • Organic electroluminescent devices can be used for display devices, lighting devices, and the like (see Patent Document 1, for example).
  • An object of one embodiment of the present invention is to improve the emission efficiency of a light-emitting element.
  • Another object of one embodiment of the present invention is to provide a light-emitting element and a semiconductor device including the light-emitting element. Note that the description of these objects does not disturb the existence of other objects. In one embodiment of the present invention, there is no need to achieve all the objects. Other objects will be apparent from and can be derived from the description of the specification, the drawings, the claims, and the like.
  • One embodiment of the present invention is a light-emitting element which includes a first electrode, a second electrode over the first electrode, a first light-emitting layer, and a second light-emitting layer.
  • the first light-emitting layer and the second light-emitting layer are both provided between the first electrode and the second electrode and have regions which overlap with each other.
  • the first light-emitting layer contains a first host material and a first light-emitting material
  • the second light-emitting layer contains a second host material and a second light-emitting material.
  • the first light-emitting material is a fluorescent material
  • the second light-emitting material is a phosphorescent material.
  • the level of the lowest triplet excited state (T 1 level) of the first light-emitting material is higher than the T 1 level of the first host material.
  • Another embodiment of the present invention is a light-emitting element which includes a first electrode, a second electrode over the first electrode, a first light-emitting unit, and a second light-emitting unit.
  • the first light-emitting unit and the second light-emitting unit are both provided between the first electrode and the second electrode and have regions which overlap with each other.
  • An interlayer is provided between the first light-emitting unit and the second light-emitting unit.
  • the first light-emitting unit includes a first light-emitting layer and a second light-emitting layer which overlap with each other, and the second light-emitting unit includes a third light-emitting layer.
  • the first light-emitting layer contains a first host material and a first light-emitting material
  • the second light-emitting layer contains a second host material and a second light-emitting material
  • the third light-emitting layer contains a third host material and a third light-emitting material.
  • the first light-emitting material is a fluorescent material
  • the second light-emitting material is a phosphorescent material
  • the third light-emitting material is a fluorescent material or a phosphorescent material.
  • the T 1 level of the first light-emitting material is higher than the T 1 level of the first host material.
  • 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 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.
  • the first host material is present in the highest proportion by weight; in the second light-emitting layer, the second host material; and in the third light-emitting layer, the third host material.
  • the T 1 level of the second host material is preferably higher than that of the first host material.
  • a region of the first light-emitting layer and a region of the second light-emitting layer may be in contact with each other.
  • the first light-emitting layer and the second light-emitting layer may be separated from each other.
  • a layer in which a hole-transport material and an electron-transport material are mixed or a layer containing a bipolar material may be provided between the first light-emitting layer and the second light-emitting layer.
  • the hole-transport material or the electron-transport material may be the same as the second host material.
  • the bipolar material may be the same as the second host material.
  • the second light-emitting layer may be provided over the first light-emitting layer; alternatively, the first light-emitting layer may be provided over the second light-emitting layer.
  • the second light-emitting unit may be provided over the first light-emitting unit; alternatively, the first light-emitting unit may be provided over the second light-emitting unit.
  • One embodiment of the present invention is a light-emitting device which includes a plurality of light-emitting elements having the above structure and a transistor or a substrate.
  • One embodiment of the present invention is an electronic device which includes the light-emitting device having the above structure.
  • One embodiment of the present invention is a lighting device which includes the light-emitting device having the above structure and a housing or a support.
  • a light-emitting device refers to an image display device or a light source used for an image display device.
  • the category of the light-emitting device includes a module in which a connector such as a flexible printed circuit (FPC) or a tape carrier package (TCP) is attached to a light-emitting device, a module in which a printed wiring board is provided on the tip of a TCP, and a module in which an integrated circuit (IC) is directly mounted on a light-emitting device by a chip on glass (COG) method.
  • a connector such as a flexible printed circuit (FPC) or a tape carrier package (TCP)
  • TCP tape carrier package
  • COG chip on glass
  • a light-emitting element a light-emitting device, an electronic device, or a lighting device having high efficiency
  • a light-emitting element having high efficiency
  • 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 effects.
  • 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 to 1D illustrate structural examples of a light-emitting element of one embodiment of the present invention.
  • FIG. 2 illustrates a light emission mechanism of a light-emitting element of one embodiment of the present invention.
  • FIGS. 3A to 3C illustrate structural examples of a light-emitting element of one embodiment of the present invention.
  • FIGS. 4A and 4B illustrate a structural example of a light-emitting device of one embodiment of the present invention.
  • FIGS. 5A and 5B illustrate a structural example of a light-emitting device of one embodiment of the present invention.
  • FIGS. 6A to 6D illustrate examples of an electronic device of one embodiment of the present invention.
  • FIGS. 7A and 7B illustrate an example of an electronic device of one embodiment of the present invention.
  • FIG. 8 illustrates examples of a lighting device of one embodiment of the present invention.
  • FIGS. 9A and 9B are schematic views of a light-emitting element 1 and a light-emitting element 2 in Example 1 and 2.
  • FIG. 10 shows a voltage—luminance curve of the light-emitting element 1 in Example 1.
  • FIG. 11 shows a luminance—current efficiency curve and a luminance—external quantum efficiency curve of the light-emitting element 1 in Example 1.
  • FIG. 12 shows a luminance—power efficiency curve of the light-emitting element 1 in Example 1.
  • FIG. 13 shows an electroluminescence spectrum of the light-emitting element 1 in Example 1.
  • FIG. 14 shows a voltage—luminance curve of the light-emitting element 2 in Example 2.
  • FIG. 15 shows a luminance—current efficiency curve and a luminance—external quantum efficiency curve of the light-emitting element 2 in Example 2.
  • FIG. 16 shows a luminance—power efficiency curve of the light-emitting element 2 in Example 2.
  • FIG. 17 shows an electroluminescence spectrum of the light-emitting element 2 in Example 2.
  • FIGS. 18A and 18B are schematic views of light-emitting elements 3 to 6 (LEEs 3 to 6 ) in Reference Example 1.
  • FIG. 19 shows luminance—current efficiency curves of the light-emitting elements 3 to 6 in Reference Example 1.
  • FIG. 20 shows voltage—luminance curves of the light-emitting elements 3 to 6 in Reference Example 1.
  • FIG. 21 shows luminance—external quantum efficiency curves of the light-emitting elements 3 to 6 in Reference Example 1.
  • FIG. 22 shows electroluminescence spectra of the light-emitting elements 3 to 6 in Reference Example 1.
  • FIG. 23 shows results of reliability tests of the light-emitting elements 3 to 6 in Reference Example 1.
  • the light-emitting element includes a first electrode 100 , a second electrode 102 , and a first light-emitting layer 120 and a second light-emitting layer 122 provided therebetween.
  • the first light-emitting layer 120 and the second light-emitting layer 122 overlap with each other.
  • the first electrode 100 serves as an anode and the second electrode 102 serves as a cathode.
  • the first electrode 100 has a function of injecting holes into the first light-emitting layer 120 and the second light-emitting layer 122
  • the second electrode 102 has a function of injecting electrons into the first light-emitting layer 120 and the second light-emitting layer 122
  • These electrodes can be formed using a metal, an alloy, a conductive compound, a mixture or a stack of such materials, or the like.
  • Typical examples of the metal are aluminum (Al) and silver (Ag); besides, a transition metal such as tungsten, chromium, molybdenum, copper, or titanium, an alkali metal such as lithium (Li) or cesium, or a Group 2 metal such as calcium or magnesium (Mg) can be used.
  • the transition metal a rare earth metal may be used.
  • An alloy containing any of the above metals can be used, and MgAg and AlLi can be given as examples.
  • 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.
  • the first electrode 100 and/or the second electrode 102 may be formed by stacking two or more of these materials.
  • the first electrode 100 , the second electrode 102 , or part thereof is formed to a thickness that is thin enough to transmit visible light.
  • the specific thickness is 1 nm or more and 10 nm or less.
  • the first light-emitting layer 120 contains a first host material and a first light-emitting material, and the first light-emitting material is a fluorescent material.
  • the first host material is present in the highest proportion by weight, and the first light-emitting material is dispersed in the first host material.
  • the T 1 level of the first light-emitting material is higher than the T 1 level of the first host material.
  • the S 1 level of the first host material is preferably higher than the S 1 level of the first light-emitting material. The light emission mechanism of the first light-emitting layer 120 will be described later.
  • An anthracene derivative or a tetracene derivative is preferably used as the first host material. This is because these derivatives each have a high S 1 level and a low T 1 level.
  • Specific examples include 9-phenyl-3-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (PCzPA), 3-[4-(1-naphthyl)-phenyl]-9-phenyl-9H-carbazole (PCPN) 9-[4-(10-phenyl-9-anthracenyl)phenyl]-9H-carbazole (CzPA), 7-[4-(10-phenyl-9-anthryl)phenyl]-7H-dibenzo[c,g]carbazole (cgDBCzPA), 6-[3-(9,10-diphenyl-2-anthryl)phenyl]-benzo[b]naphtho[1,2-d]furan (2mB
  • Examples of the first light-emitting 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, and a naphthalene derivative.
  • 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 (1,6mMemFLPAPrn), N,N′-bis[4-(9-phenyl-9H-fluoren-9-yl)phenyl]-N,N′-diphenylpyrene-1,6-diamine (1,6FLPAPrn), N,N′-bis(dibenzofuran-2-yl)-N,N′-diphenylpyrene-1,6-diamine (1,6FrAPrn), and N,N′-bis(dibenzothiophen-2-yl)-N,N′-diphenylpyrene-1,6-diamine (1,6ThAPrn).
  • the second light-emitting layer 122 contains a second host material and a second light-emitting material, and the second light-emitting material is a phosphorescent material.
  • the second host material is present in the highest proportion by weight, and the second light-emitting material is dispersed in the second host material.
  • the T 1 level of the second host material is preferably higher than the T 1 level of the second light-emitting 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.
  • 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, or the like can be given.
  • a platinum complex having a porphyrin ligand or the like can be given.
  • Examples of the second 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, and a phenanthroline derivative.
  • Other examples are an aromatic amine and a carbazole derivative.
  • the second light-emitting layer 122 may further contain an additive which can form an exciplex (i.e., a heteroexcimer) together with the second host material.
  • an additive which can form an exciplex (i.e., a heteroexcimer) together with the second host material.
  • the second host material, the additive, and the second light-emitting material be selected so that the emission peak of the exciplex 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 second light-emitting material.
  • MLCT metal-to-ligand charge transfer
  • the emission colors of the first light-emitting material and the second light-emitting material are not limitation on the emission colors of the first light-emitting material and the second light-emitting material, 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 first light-emitting material is preferably shorter than that of the second light-emitting material. For example, it is preferable that the first light-emitting material emit blue light and that the second light-emitting material emit green, yellow, or red light.
  • the second light-emitting layer 122 may have a structure in which a plurality of layers is stacked. In this case, different structures or different materials may be used for the plurality of layers.
  • first light-emitting layer 120 and the second light-emitting layer 122 can be formed by an evaporation method (including a vacuum evaporation method), an inkjet method, a coating method, gravure printing, or the like.
  • the light-emitting element of one embodiment of the present invention may include another layer besides the first light-emitting layer 120 and the second light-emitting layer 122 .
  • the light-emitting element may include a hole-injection layer, a hole-transport layer, an electron-blocking layer, a hole-blocking layer, an electron-transport layer, or an electron-injection layer.
  • each of these layers may be formed of a plurality of layers. These layers can reduce a carrier injection barrier, improve the carrier transport property, or suppress a quenching phenomenon by an electrode, thereby contributing to an improvement in emission efficiency or a reduction in drive voltage.
  • FIG. 1A includes a hole-injection layer 124 , a hole-transport layer 126 , an electron-transport layer 128 , and an electron-injection layer 130 besides the first light-emitting layer 120 and the second light-emitting layer 122 .
  • all layers provided between the first electrode 100 and the second electrode 102 is collectively defined as an EL layer.
  • a stack including the hole-injection layer 124 , the hole-transport layer 126 , the first light-emitting layer 120 , the second light-emitting layer 122 , the electron-transport layer 128 , and the electron-injection layer 130 corresponds to an EL layer.
  • the hole-injection layer 124 has a function of reducing a barrier for hole injection from the first electrode 100 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, a 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 a doped polythiophene.
  • a mixed layer containing 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. Electric charge can be transferred between these materials in the presence or absence of an electric field.
  • organic acceptors such as a quinodimethane derivative, a chloranil derivative, and a hexaazatriphenylene derivative can be given.
  • a specific example is a material 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 preferable because it is stable in the air, has a low hygroscopic property, and is easily handled.
  • 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 hole-transport material may be a high molecular compound.
  • the hole-transport layer 126 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 124 .
  • the highest occupied molecular orbital (HOMO) level of the hole-transport layer 126 is preferably equal or close to the HOMO level of the hole-injection layer 124 .
  • the electron-injection layer 130 has a function of reducing a barrier for electron injection from the second 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 the metal, for example.
  • a mixed layer containing an electron-transport material (described later) and a material having a property of donating electrons to the electron-transport material can also be used.
  • a Group 1 metal, a Group 2 metal, an oxide of the metal, or the like can be given.
  • the electron-transport layer 128 has a function of transporting, to the second light-emitting layer 122 , electrons injected from the second electrode 102 through the electron-injection layer 130 .
  • 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 hole-injection layer 124 , the hole-transport layer 126 , the electron-injection layer 130 , and the electron-transport layer 128 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 second light-emitting layer 122 is provided over the first light-emitting layer 120 ; however, one embodiment of the present invention is not limited to this structure. As illustrated in FIG. 1B , the first light-emitting layer 120 may be positioned over the second light-emitting layer 122 .
  • FIG. 2 illustrates a correlation between energy levels of the first host material and the first light-emitting material.
  • the symbols in FIG. 2 denote as follows:
  • T 1(h) the level of the lowest triplet excited state of the first host material
  • T 1(g) the level of the lowest triplet excited state of the first light-emitting material.
  • the first light-emitting layer 120 contains the first host material and the first light-emitting material whose T 1 level is higher than that of the first host material. That is, T 1(g) is higher than T 1(h) . Furthermore, in the first light-emitting layer 120 , the first host material is present in a larger amount than the first light-emitting material.
  • FIG. 2 shows the energy levels of two molecules of the first host material and one molecule of the first light-emitting material.
  • excited states are formed by carrier recombination. Since the first host material is present in a larger amount than the first light-emitting material, most of the excited states are excited states of the first host material.
  • the ratio of the singlet excited state to the triplet excited state produced by carrier recombination (hereinafter, exciton generation probability) is approximately 1:3. That is, the singlet excited state with S 1(h) and the triplet excited state with T 1(h) are generated in the proportion of approximately 1 to 3.
  • S 1(g) is lower than S 1(h)
  • light emission can be obtained in the following manner: energy is rapidly transferred from the first host material in the singlet excited state to the first light-emitting material (singlet energy transfer: Process (a)), a singlet excited state of the first light-emitting material is produced, and the singlet excited state relaxes to the ground state through a radiative process (Process (b)).
  • T 1(h) is higher than T 1(g)
  • energy is rapidly transferred from the first host material in the triplet excited state to the first light-emitting material (triplet energy transfer), so that a triplet excited state of the first light-emitting material is formed.
  • the first light-emitting material is a fluorescent material
  • its triplet excited state does not provide light emission in the visible light region. Consequently, the triplet excited state of the first host material cannot be utilized for light emission.
  • T 1(h) is higher than T 1(g)
  • only the light emission through Process (a) can be used; as a result, no more than approximately 25% of injected carriers can be used for light emission.
  • T 1(g) is higher than T 1(h) as shown in FIG. 2 . Therefore, triplet energy transfer (Process (c)) from the first host material to the first light-emitting material does not occur or is negligible.
  • a relaxation process Process (d)
  • TTA triplet-triplet annihilation
  • the singlet excited state of the first light-emitting material is formed through the following two processes: (1) Process (a) through which energy is transferred from the singlet excited state of the first host material generated directly by carrier recombination and (2) Process (e) through which energy is transferred from the singlet excited state of the first host material generated by TTA.
  • Process (a) through which energy is transferred from the singlet excited state of the first host material generated directly by carrier recombination
  • Process (e) through which energy is transferred from the singlet excited state of the first host material generated by TTA.
  • T 1(g) is higher than T 1(h) as in the light-emitting element of one embodiment of the present invention
  • both the processes can be utilized; therefore, an emission efficiency exceeding the exciton generation probability can be achieved, and a light-emitting element with high efficiency can be provided.
  • a light-emitting element of one embodiment of the present invention will be described with reference to FIG. 1C .
  • the light-emitting element in this embodiment is different from the light-emitting element in Embodiment 1 in that a separation layer 135 is provided between the first light-emitting layer 120 and the second light-emitting layer 122 .
  • the separation layer 135 is in contact with the first light-emitting layer 120 and the second light-emitting layer 122 .
  • the structures of the other layers are similar to those in Embodiment 1; therefore, description thereof is omitted.
  • the separation layer 135 is provided to prevent energy transfer by the Dexter mechanism (particularly triplet energy transfer) from the second host material in an excited state or the second light-emitting material in an excited state which is generated in the second light-emitting layer 122 to the first host material or the first light-emitting material in the first light-emitting layer 120 . Therefore, the thickness of the separation layer may be approximately several nanometers, specifically 0.1 nm or more and 20 nm or less, 1 nm or more and 10 nm or less, or 1 nm or more and 5 nm or less.
  • the separation layer 135 may contain a single material or both a hole-transport material and an electron-transport material.
  • a bipolar material may be used.
  • the bipolar material here refers to a material in which the ratio between the electron mobility and the hole mobility is 100 or less.
  • the hole-transport material, the electron-transport material, or the like given as an example in Embodiment 1 can be used.
  • at least one of materials contained in the separation layer 135 may be the same as the second host material. This facilitates the manufacture of the light-emitting element and reduces the drive voltage.
  • At least one of materials contained in the separation layer 135 may have a higher T 1 level than the second host material.
  • the recombination region can be adjusted by adjusting the mixed ratio of the hole-transport material and the electron-transport material in the separation layer 135 , whereby the emission color can be controlled.
  • the recombination region can be shifted from the first electrode 100 side to the second electrode 102 side by increasing the proportion of the hole-transport material in the separation layer 135 .
  • the contribution of the second light-emitting layer 122 to light emission can be increased.
  • the recombination region can be shifted from the second electrode 102 side to the first electrode 100 side, so that the contribution of the first light-emitting layer 120 to light emission can be increased.
  • the emission color of the light-emitting element as a whole can be changed by adjusting the recombination region.
  • the hole-transport material and the electron-transport material may form an exciplex in the separation layer 135 , which effectively prevents exciton diffusion. Specifically, energy transfer from the second host material in an excited state or the second light-emitting material in an excited state to the first host material or the first light-emitting material can be prevented.
  • the first light-emitting layer 120 may be positioned over the second light-emitting layer 122 as illustrated in FIG. 1D .
  • the second light-emitting layer 122 is provided over the hole-transport layer 126
  • the first light-emitting layer 120 is provided over the second light-emitting layer 122 with the separation layer 135 interposed therebetween.
  • the light-emitting element in this embodiment includes the first electrode 100 , the second electrode 102 , and a first light-emitting unit 140 - 1 and a second light-emitting unit 140 - 2 provided therebetween.
  • the first light-emitting unit 140 - 1 and the second light-emitting unit 140 - 2 overlap with each other with an interlayer 150 provided therebetween.
  • the first electrode 100 serves as an anode and the second electrode 102 serves as a cathode.
  • Components denoted by the same reference numerals or names as those in Embodiments 1 and 2, such as the first electrode 100 and the second electrode 102 are similar to those in Embodiments 1 and 2; therefore, detailed description thereof is omitted.
  • the first light-emitting unit 140 - 1 includes the first light-emitting layer 120 and the second light-emitting layer 122 .
  • the structures and materials of these layers are similar to those in Embodiment 1. Therefore, although the second light-emitting layer 122 is provided over the first light-emitting layer 120 in the light-emitting element in the FIG. 3A , the first light-emitting layer 120 may be provided over the second light-emitting layer 122 .
  • the hole-injection layer 124 , a hole-transport layer 126 - 1 , and an electron-transport layer 128 - 1 may be further provided.
  • the separation layer 135 may be provided between the first light-emitting layer 120 and the second light-emitting layer 122 as described in Embodiment 2.
  • the interlayer 150 has a function of injecting electrons into the first light-emitting unit 140 - 1 and injecting holes into the second light-emitting unit 140 - 2 when a voltage is applied between the first electrode 100 and the second electrode 102 .
  • the interlayer 150 it is preferable that the interlayer 150 be capable of transmitting visible light and have a visible light transmittance of 40% or higher.
  • the interlayer 150 includes a first layer 150 - 1 and a second layer 150 - 2 .
  • the first layer 150 - 1 is provided on the first light-emitting unit 140 - 1 side
  • the second layer 150 - 2 is provided on the second light-emitting unit 140 - 2 side.
  • the first layer 150 - 1 can be formed using a Group 1 metal or a Group 2 metal, or a compound thereof (e.g., an oxide, a halide, or a carbonate), for example.
  • a mixed layer containing the electron-transport material described in Embodiment 1 and a material having a property of donating electrons to the electron-transport material can also be used.
  • a layer containing the transition metal oxide described in Embodiment 1 can be used as the second layer 150 - 2 . It is also possible to use a mixed layer containing a hole-transport material and a material having a property of accepting electrons from the hole-transport material or a stack of a layer containing a material having an electron accepting property and a layer containing a hole-transport material. Specifically, the mixed layer or the stack which is described in Embodiment 1 and can be used as the hole-injection layer 124 can be used.
  • a buffer layer may be provided between the first layer 150 - 1 and the second layer 150 - 2 .
  • the buffer layer can prevent a material of the first layer 150 - 1 and a material of the second layer 150 - 2 from reacting with each other at the interface.
  • the buffer layer contains an electron-transport material, examples of which include a perylene derivative and a nitrogen-containing condensed aromatic compound.
  • the interlayer 150 can be formed by an evaporation method (including a vacuum evaporation method), an inkjet method, a coating method, a gravure printing method, or the like.
  • the second light-emitting unit 140 - 2 includes a third light-emitting layer 132 .
  • the third light-emitting layer 132 contains a third host material and a third light-emitting material, and the third light-emitting material is a fluorescent material or a phosphorescent material.
  • the third host material is present in the highest proportion by weight, and the third light-emitting material is dispersed in the third host material.
  • As the third host material a material similar to the first host material or the second host material described in Embodiment 1 can be used.
  • the third host material may be the same as or different from the first host material or the second host material.
  • the S 1 level of the third host material is preferably higher than that of the third light-emitting material.
  • the T 1 level of the third host material is preferably higher than that of the third light-emitting material.
  • the third light-emitting material a material similar to the first light-emitting material or the second light-emitting material described in Embodiment 1 can be used.
  • the third light-emitting material may be the same as or different from the first light-emitting material or the second light-emitting material.
  • the first light-emitting material, the second light-emitting material, and the third light-emitting material are used to provide light in the three primary colors of red, blue, and green, whereby white light with high color rendering properties can be extracted from the light-emitting element.
  • the second light-emitting unit 140 - 2 further includes a hole-transport layer 126 - 2 , an electron-transport layer 128 - 2 , and the electron-injection layer 130 . Layers similar to those in Embodiment 1 can be used as these layers.
  • the light-emitting element described in this embodiment can have a current efficiency which is twice or more that of the light-emitting elements described in Embodiments 1 and 2 at substantially the same current density; thus, a light-emitting element with high efficiency can be achieved.
  • the light-emitting unit (the first light-emitting unit 140 - 1 ) including the first light-emitting layer 120 and the second light-emitting layer 122 is formed on the first electrode 100 side; however, as illustrated in FIG. 3B , the first light-emitting unit 140 - 1 may be formed on the second electrode 102 side. Also in this case, the separation layer 135 may be provided between the first light-emitting layer 120 and the second light-emitting layer 122 as in Embodiment 2.
  • embodiments of the present invention also include a light-emitting element illustrated in FIG. 3C in which n (n is an integer of 3 or more) light-emitting units ( 140 - 1 to 140 - n ) are stacked.
  • interlayers 150 ( 1 ) to 150 ( n - 1 )) are provided between the respective adjacent light-emitting units.
  • At least one of the n light-emitting units has a structure similar to that of the first light-emitting unit, and at least another one of the n light-emitting units has a structure similar to that of the second light-emitting unit.
  • FIG. 4A is a top view of the light-emitting device
  • FIG. 4B is a cross-sectional view taken along line A-A′ in FIG. 4A .
  • the light-emitting device includes a source side driver circuit 403 , a pixel portion 402 , and gate side driver circuits 404 a and 404 b over an element substrate 401 .
  • Reference numeral 406 denotes a sealing substrate
  • reference numeral 405 denotes a sealant.
  • a region 418 is surrounded by the sealant 405 .
  • a glass substrate, a quartz substrate, or a flexible substrate formed of fiber reinforced plastic (FRP), poly(vinyl fluoride) (PVF), a polyester, an acrylic resin, or the like can be used.
  • a wiring 407 is a lead wiring for receiving a variety of signals from an FPC 408 and transmitting them to the source side driver circuit 403 and the gate side driver circuits 404 a and 404 b.
  • a printed wiring board (PWB) may be attached to the FPC.
  • FIG. 4B illustrates part of the source side driver circuit 403 and one pixel in the pixel portion 402 .
  • a CMOS circuit in which an n-channel transistor 409 and a p-channel transistor 410 are combined is formed; however, a circuit different from the CMOS circuit, such as a PMOS circuit or an NMOS circuit, may be provided.
  • the source side driver circuit 403 and the gate side driver circuits 404 a and 404 b may be partly or entirely formed not over the substrate but outside the substrate.
  • the transistors may be staggered transistors or inverted staggered transistors.
  • a semiconductor layer for forming the transistors may be formed using any material as long as it exhibits semiconductor characteristics; for example, a Group 14 element such as silicon or germanium, a compound such as gallium arsenide or indium phosphide, or an oxide such as zinc oxide or tin oxide can be used.
  • a Group 14 element such as silicon or germanium
  • a compound such as gallium arsenide or indium phosphide
  • an oxide such as zinc oxide or tin oxide
  • oxide exhibiting semiconductor characteristics oxide
  • the semiconductor layer may be crystalline or amorphous. Specific examples of a crystalline semiconductor include a single crystal semiconductor, a polycrystalline semiconductor, and a microcrystalline semiconductor.
  • the pixel portion 402 includes a plurality of pixels each including a switching transistor 411 , a current controlling transistor 412 , and a first electrode 413 electrically connected to the current controlling transistor 412 .
  • An insulator 414 is formed to cover an end portion of the first electrode 413 .
  • a light-emitting element 417 which has the structure of the light-emitting element described in Embodiment 1, 2, or 3 is provided in an opening portion of the insulator 414 . That is, the light-emitting element 417 includes the first electrode 413 , an EL layer 415 , and a second electrode 416 ; the EL layer 415 includes at least a first light-emitting layer and a second light-emitting layer and may further include a third light-emitting layer. Note that a plurality of light-emitting elements is formed in the pixel portion 402 ; some of them may have a structure different from the structures of the light-emitting elements described in Embodiments 1 to 3.
  • the sealing substrate 406 and the element substrate 401 are bonded to each other by the sealant 405 , and the light-emitting element 417 is provided in the region 418 .
  • the region 418 is filled with an inert gas or a resin and/or a drying agent.
  • An epoxy-based resin or glass frit is preferably used as the sealant 405 .
  • FIG. 5A is a perspective view of the light-emitting device
  • FIG. 5B is a cross-sectional view taken along line X-Y in FIG. 5A .
  • the light-emitting device includes a substrate 551 , a first electrode 552 , a second electrode 556 , and an EL layer 555 , and the EL layer 555 includes the first light-emitting layer 120 and the second light-emitting layer 122 described in Embodiment 1, 2, or 3.
  • Part of the first electrode 552 is covered with an insulating layer 553 , and a partition layer 554 is provided over the insulating layer 553 .
  • the width of the partition layer 554 increases with distance from the substrate 551 . In other words, a cross section of the partition layer 554 in the short side direction is trapezoidal, and the base in contact with the insulating layer 553 is shorter than the upper side. Accordingly, a defect of the light-emitting element due to crosstalk can be prevented.
  • Examples of the electronic device are a television device, a computer, a camera (a digital camera or a digital video camera), a digital photo frame, a mobile phone, a portable information terminal, a game machine, and an audio reproducing device. Specific examples of these electronic devices are illustrated in FIGS. 6A to 6D .
  • FIG. 6A illustrates an example of a television device.
  • a display portion 6103 is incorporated in a housing 6101 .
  • a light-emitting device including the light-emitting element described in Embodiment 1, 2, or 3 is provided.
  • FIG. 6B illustrates an example of a computer.
  • the computer includes a main body 6201 , a housing 6202 , a display portion 6203 , a keyboard 6204 , an external connection port 6205 , a pointing device 6206 , and the like.
  • a light-emitting device including the light-emitting element described in Embodiment 1, 2, or 3 is provided.
  • FIG. 6C illustrates an example of a smart watch.
  • the smart watch includes a housing 6302 , a display panel 6304 , operation buttons 6311 and 6312 , a connection terminal 6313 , a band 6321 , a clasp 6322 , and the like.
  • a light-emitting device including the light-emitting element described in Embodiment 1, 2, or 3 is provided.
  • the display panel 6304 has a non-rectangular display region and can display an icon 6305 indicating time, another icon 6306 , and the like.
  • FIG. 6D illustrates an example of a mobile phone.
  • a mobile phone 6400 is provided with a display portion 6402 incorporated in a housing 6401 , an operation button 6403 , an external connection port 6404 , a speaker 6405 , a microphone 6406 , and the like.
  • a light-emitting device including the light-emitting element described in Embodiment 1, 2, or 3 is provided.
  • the display portion 6402 is provided with a touch panel; when a user touches the display portion 6402 with his or her finger or the like, the user can operate the mobile phone 6400 or input data to the mobile phone 6400 .
  • An image sensor may be mounted on the display portion 6402 to provide an imaging function.
  • FIGS. 7A and 7B illustrate an example of a foldable tablet terminal.
  • the tablet terminal is open (unfolded).
  • the tablet terminal includes a housing 730 , a display portion 731 a, a display portion 731 b, a display mode switch 734 , a power switch 735 , a power-saving mode switch 736 , a clasp 733 , an operation switch 738 , and the like.
  • a light-emitting device including the light-emitting element described in Embodiment 1, 2, or 3 is provided in the display portion 731 a and the display portion 731 b.
  • the display portion 731 a and the display portion 731 b can be partly or entirely a touch panel region 732 a and a touch panel region 732 b, respectively, and a variety of operations such as data input may be performed by touching an operation key 737 or an operation switch 739 displayed thereon.
  • the display mode switch 734 the display can be switched between a portrait mode, a landscape mode, and the like, and between monochrome display and color display, for example.
  • the power-saving mode switch 736 the luminance of display can be optimized in accordance with the amount of external light detected by an optical sensor incorporated in the tablet terminal.
  • the display portion 731 a and the display portion 731 b may have different areas.
  • the display portion 731 a and the display portion 731 b may have different display specifications; for example, one may have higher resolution than the other.
  • the tablet terminal is closed (folded), and a solar cell 750 , a charge and discharge control circuit 752 , a battery 754 , a DCDC converter 756 , and the like are illustrated.
  • the solar cell 750 can supply power to the tablet terminal. Note that the solar cell 750 may be provided on one side or both sides of the housing 730 .
  • the light-emitting device has a considerably wide application range and can be used for electronic devices in a variety of fields.
  • FIG. 8 illustrates a lighting device 801 on the ceiling, a lighting device 803 on the wall, a lighting device 802 on a curved surface, and a lighting device 804 on furniture such as a table.
  • the lighting device 801 includes a housing 821 and a light-emitting device 811 provided in the housing 821 .
  • the lighting device 802 includes a support 822 and a light-emitting device 812 on the support 822 .
  • As the lighting device 803 a light-emitting device 813 is provided on the wall.
  • the lighting device 804 includes a support 824 and a light-emitting device 814 on the support 824 .
  • the light-emitting element described Embodiment 1, 2, or 3 can be used for the light-emitting devices included in these lighting devices.
  • FIG. 9A is a schematic view of a light-emitting element (light-emitting element 1 ) fabricated in this example, Table 1 shows the detailed structure of the element, and structures and abbreviations of compounds used here are given below.
  • Second electrode 102 200 Al — Electron-injection layer 130 1 LiF — Electron-transport layer 128(2) 15 Bphen — 128(1) 10 2mDBTBPDBq II — Second light-emitting layer 122(2) 20 2mDBTBPDBq-II:PCBBiF:Ir(tBuppm) 2 (acac) 0.7:0.3:0.05 122(1) 10 2mDBTBPDBq-II:PCBBiF:Ir(tppr) 2 (dpm) 0.5:0.5:0.05 First light-emitting layer 120 20 cgDBCzPA:PCzPA:1,6mMemFLPAPrn 0.3:0.7:0.05 Hole-transport layer 126 20 PCPPn — Hole-injection layer 124 30 DBT3P-II:MoO 3 2:1 First electrode 100 110 ITSO —
  • Indium tin oxide containing silicon oxide indium tin oxide doped with SiO 2 : ITSO which was formed over a glass substrate to have a thickness of 110 nm and an area of 4 mm 2 (2 mm ⁇ 2 mm) was used as the first electrode 100 .
  • DBT3P-II 1,3,5-tri(dibenzothiophen-4-yl)benzene
  • 2mDBTBPDBq-II and bathophenanthroline (Bphen) were sequentially deposited by evaporation to a thickness of 10 nm and 15 nm, respectively, so that electron-transport layers 128 ( 1 ) and 128 ( 2 ) were formed.
  • lithium fluoride (LiF) was deposited by evaporation to a thickness of 1 nm to form the electron-injection layer 130 .
  • aluminum (Al) was deposited by evaporation to a thickness of 200 nm to form the second electrode 102 .
  • a counter glass substrate was fixed to the glass substrate using a sealant in a nitrogen atmosphere. In this manner, the light-emitting element 1 was obtained.
  • FIG. 10 to FIG. 13 show initial characteristics of the light-emitting element 1 .
  • the light-emitting element 1 starts emitting light at around 2.4 V and its luminance exceeds 8000 cd/m 2 at a voltage of 5.0 V, which indicates that it can be driven at a low voltage.
  • the current efficiency and the external quantum efficiency are 20.4 cd/A and 10.7%, respectively (see FIG. 11 ), and the power efficiency is 16.9 lm/W (see FIG. 12 ).
  • the light-emitting element 1 contains a blue-emitting fluorescent material (1,6mMemFLPAPrn) in the first light-emitting layer 120 , a red-emitting phosphorescent material (Ir(tppr) 2 (dpm)) in the first layer 122 ( 1 ) of the second light-emitting layer 122 , and a green-emitting phosphorescent material (Ir(tBuppm) 2 (acac)) in the second layer 122 ( 2 ) of the second light-emitting layer 122 .
  • FIG. 13 shows an electroluminescence spectrum of the light-emitting element at 500 cd/m 2 .
  • FIG. 9B is a schematic view of a light-emitting element (light-emitting element 2 ) fabricated in this example
  • Table 2 shows the detailed structure of the element, and structures and abbreviations of compounds used here are given below. Note that the structures and abbreviations of the compounds used for the light-emitting element 1 described in Example 1 are omitted.
  • Second electrode 102 200 Ag — 1 Ag:Mg 0.6:0.2 a Electron-injection layer 130 1 LiF — Electron-transport layer 128(2) 15 Bphen — 128(1) 10 2mDBTBPDBq II — Second light-emitting layer 122 20 2mDBTBPDBq-II:PCBBiF:Ir(ppm-dmp) 2 (acac) 0.8:0.2:0.05 Separation layer 135 2 2mDBTBPDBq-II:PCBBiF 0.6:0.4 First light-emitting layer 120 10 PCzPA:1,6mMemFLPAPrn 1:0.05 Hole-transport layer 126 20 PCPPn — Hole-injection layer 124 30 DBT3P-II:MoO 3 2:1 First electrode 100 110 IT SO — a Volume ratio.
  • 2mDBTBPDBq-II and Bphen were sequentially deposited by evaporation to a thickness of 10 nm and 15 nm, respectively, so that the electron-transport layers 128 ( 1 ) and 128 ( 2 ) were formed.
  • lithium fluoride was deposited by evaporation to a thickness of 1 nm to form the electron-injection layer 130 .
  • sealing was performed in a manner similar to that of the light-emitting element 1 . In this manner, the light-emitting element 2 was obtained.
  • FIG. 14 to FIG. 17 show initial characteristics of the light-emitting element 2 .
  • the light-emitting element 2 starts to emit light at around 2.6 V and its luminance exceeds 10000 cd/m 2 at a voltage of 4.3 V, which indicates that it can be driven at a low voltage.
  • the current efficiency and the external quantum efficiency are 55.3 cd/A and 17.2%, respectively (see FIG. 15 ), and the power efficiency is 52.6 lm/W (see FIG. 16 ).
  • the light-emitting element 2 contains a blue-emitting fluorescent material (1,6mMemFLPAPrn) in the first light-emitting layer 120 and a yellow-emitting phosphorescent material (Ir(ppm-dmp) 2 (acac)) in the second light-emitting layer 122 .
  • FIG. 17 shows an electroluminescence spectrum of the light-emitting element 2 at 1000 cd/m 2 . A peak is observed in each of the blue and yellow wavelength regions, which indicates that light is concurrently emitted from these two light-emitting materials.
  • the light-emitting elements of this example show a high current efficiency and can be operated at a low drive voltage.
  • the structures described in this example can be used in appropriate combination with any of the embodiments and the other example.
  • FIG. 18A is a schematic cross-sectional view of the light-emitting elements 3 and 5
  • FIG. 18B is a schematic cross-sectional view of the light-emitting elements 4 and 6 .
  • first electrode 1100 As a first electrode 1100 , an ITSO film was formed over a glass substrate to a thickness of 110 nm.
  • the electrode area of the first electrode 1100 was 4 mm 2 (2 mm ⁇ 2 mm).
  • BPAFLP 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine
  • 2mDBTBPDBq-II is a host material
  • PCBBiF is an additive which can form an exciplex together with the host material
  • Ir(tBuppm) 2 (acac) is a light-emitting material.
  • an electron-transport layer 1128 ( 1 ), an electron-transport layer 1128 ( 2 ), and an electron-injection layer 1130 2mDBTBPDBq-II, Bphen, and LiF were sequentially deposited on the light-emitting layer 1122 ( 2 ) by evaporation to a thickness of 20 nm, 10 nm, and 1 nm, respectively.
  • a second electrode 1102 aluminum (Al) was formed on the electron-injection layer 1130 to a thickness of 200 nm.
  • the light-emitting element 3 was fabricated over the glass substrate. Note that in the above deposition process, evaporation was all performed by a resistance heating method.
  • the light-emitting element 3 was sealed by fixing a sealing substrate to the glass substrate using a sealant for an organic EL device in a glove box containing a nitrogen atmosphere. Specifically, the sealant was applied to surround the light-emitting element, the glass substrate and the sealing substrate were bonded to each other, irradiation with 365-nm ultraviolet light 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 element 4 was fabricated by the same method as the light-emitting element 3 , except for the following steps.
  • 2mDBTBPDBq-II is a host material
  • PCBBiF is an additive which can form an exciplex together with the host material
  • Ir(mpmppm) 2 (acac) is a light-emitting material.
  • 2mDBTBPDBq-II and Bphen were sequentially deposited on the light-emitting layer 1122 by evaporation to a thickness of 15 nm and 20 nm, respectively.
  • the light-emitting element 5 was fabricated by the same method as the light-emitting element 3 , except for the following steps.
  • 2mDBTBPDBq-II is a host material
  • PCBBiF is an additive which can form an exciplex together with the host material
  • Ir(dppm) 2 (acac) is a light-emitting material.
  • the light-emitting element 6 was fabricated by the same method as the light-emitting element 3 , except for the following steps.
  • an ITSO film was formed over a glass substrate to a thickness of 70 nm.
  • 2mDBTBPDBq-II is a host material
  • PCBBiF is an additive which can form an exciplex together with the host material
  • Ir(tppr) 2 (dpm) is a light-emitting material.
  • 2mDBTBPDBq-II and Bphen were sequentially deposited on the light-emitting layer 1122 by evaporation to a thickness of 20 nm and 15 nm, respectively.
  • FIG. 19 shows luminance—current efficiency curves of the fabricated light-emitting elements 3 to 6 .
  • FIG. 20 shows their voltage—luminance curves.
  • FIG. 21 shows their luminance—external quantum efficiency curves. The measurements of the light-emitting elements were performed at room temperature (in an atmosphere kept at 23° C.).
  • Table 4 shows the device characteristics of the light-emitting elements 3 to 6 at around 1000 cd/m 2 . Note that the external quantum efficiency in FIG. 21 and Table 4 was calculated in consideration of light distribution characteristics.
  • FIG. 22 shows electroluminescence spectra obtained by supplying the light-emitting elements 3 to 6 with a current at a current density of 2.5 mA/cm 2 .
  • the light-emitting element 3 emits green light
  • the light-emitting element 4 emits yellow light
  • the light-emitting element 5 emits orange light
  • the light-emitting element 6 emits red light.
  • the light-emitting elements 3 to 6 emit light with high current efficiency and high external quantum efficiency at a low drive voltage. Furthermore, the efficiency of the light-emitting elements 3 to 6 decreases only slightly even in a high luminance region, and the high emission efficiency is maintained.
  • Results of reliability tests of the light-emitting elements 3 to 6 are shown in FIG. 23 .
  • the light-emitting elements 3 to 6 were driven under the conditions where the initial luminance of the light-emitting elements was set to 5000 cd/m 2 and the current density was constant.
  • the time (LT90) taken for the luminance of the light-emitting elements 3 to 6 to decrease to 90% of the initial luminance was estimated: the light-emitting element 3 , 1000 hours; the light-emitting element 4 , 1300 hours; the light-emitting element 5 , 2800 hours; and the light-emitting element 6 , 560 hours.
  • the above results prove the high reliability of the elements.
  • the light-emitting layer 1122 contains an additive capable of forming an exciplex together with a host material and the exciplex is utilized as an energy transfer medium, regardless of emission color, green- to red-emissive light-emitting elements with high current efficiency, high external quantum efficiency, and a low drive voltage are obtained. Moreover, highly reliable light-emitting elements are obtained.
  • the obtained mixture was subjected to extraction with dichloromethane and purified by silica gel column chromatography (developing solvent: dichloromethane), whereby 1.6 g of 4-chloro-6-phenylpyrimidine were obtained (yield: 23%, a pale yellow solid).
  • the microwave irradiation in this reference example was performed using a microwave synthesis system (Discover, manufactured by CEM Corporation).
  • the obtained mixture was subjected to extraction with dichloromethane and purified by silica gel column chromatography (developing solvent:ethyl acetate and hexane in a ratio of 1:5), whereby 0.50 g of Hppm-dmp were obtained (yield: 23%, a pale yellow oily substance).
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US20180261788A1 (en) 2018-09-13
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US20210391554A1 (en) 2021-12-16
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