JP2005108665A - Light-emitting device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a light-emitting device having good light-emitting efficiency and durability. <P>SOLUTION: The light-emitting device includes a hole transport layer, at least two layers of light-emitting layers with, and an electron transport layer between a pair of electrodes. All the light-emitting layers contain phosphorescence luminescent compounds and host compounds, respectively, and also, the density of the phosphorescence luminescent compound of the light-emitting layer at a cathode side is higher than that at an anode side in the light-emitting device. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a light emitting device having good luminous efficiency and durability.

  In general, a light-emitting element includes a light-emitting layer and a pair of counter electrodes sandwiching the layer. In light emission, when an electric field is applied between both electrodes, electrons are injected from the cathode and holes are injected from the anode. This is a phenomenon in which energy is released as light when the electrons and holes recombine in the light emitting layer and the energy level returns from the conduction band to the valence band.

As such a light-emitting element, for example, there is an organic light-emitting element using an organic compound in a light-emitting layer, but its luminous efficiency is very low as compared with a fluorescent tube. Many of the current organic light-emitting devices use fluorescence obtained from singlet excitons, and the use of phosphorescence obtained from triplet excitons has been studied to improve luminous efficiency. For example, phosphorescent organic light-emitting devices using iridium phenylpyridine complexes have been reported (see, for example, Non-Patent Document 1 and Patent Document 1). According to these reports, it has been reported that the conventional organic light-emitting element using fluorescence has a luminous efficiency of 2 to 3 times.
However, the phosphorescent organic light-emitting element has poor durability during driving for practical use, and a light-emitting element having both excellent luminous efficiency and durability is strongly desired.
US Pat. No. 6,303,238 Applied Physics Letter, 1999, 75, 4

  The objective of this invention is providing the light emitting element excellent in luminous efficiency and durability.

According to the present invention, a light-emitting element having the following configuration is provided, and the above object of the present invention is provided.
<1> A light emitting device having a hole transport layer, at least two light emitting layers, and an electron transport layer between a pair of electrodes,
A light-emitting element, wherein all the light-emitting layers each contain a phosphorescent compound and a host compound, and the concentration of the phosphorescent compound is higher in the light-emitting layer on the anode side than in the light-emitting layer on the cathode side.
<2> The light emitting device according to <1>, wherein the concentration of the phosphorescent compound in the light emitting layer is 0.01% by mass or more and 20% by mass or less in each light emitting layer.
<3> The light-emitting device according to <1> or <2>, wherein the electron transport material contained in the electron transport layer is an aromatic heterocyclic compound having one or more hetero atoms in the molecule.

  According to the present invention, each of at least two light emitting layers contains the same or different phosphorescent compound and host compound, and the concentration of the phosphorescent compound in the light emitting layer is changed from the light emitting layer on the cathode side to the light emission on the anode side. By sequentially increasing the thickness toward the layer, a light-emitting element having both light emission efficiency and durability can be obtained, and in particular, a high-luminance light-emitting element with greatly improved durability can be provided. The light emitting device of the present invention can be effectively used for a surface light source such as a full color display, a backlight and an illumination light source, a light source array such as a printer, and the like.

The light-emitting element of the present invention is a light-emitting element having an organic layer including a hole transport layer, at least two light-emitting layers, and an electron transport layer between a pair of electrodes, and all the light-emitting layers are the same or Including different phosphorescent compounds and host compounds. The concentration of the phosphorescent compound in the light emitting layer is higher in the light emitting layer on the anode side than the light emitting layer on the cathode side, that is, the concentration of the phosphorescent compound in the light emitting layer is higher than that in the anode side. It is a light emitting element that is gradually increasing toward the light emitting layer on the side.
Here, “the concentration of the phosphorescent compound in the light emitting layer is increasing sequentially from the light emitting layer on the cathode side to the light emitting layer on the anode side” specifically means that there are, for example, three light emitting layers. The phosphorescent compound in the light emitting layer closest to the cathode side has the lowest concentration, the phosphorescent compound in the light emitting layer closest to the anode side has the highest concentration, and the phosphorescent compound in the light emitting layer located in the middle It means a state in which the density is in the middle of both.

  The phosphorescent compound is a compound that can emit light from triplet excitons, and is not particularly limited, but is preferably an orthometalated metal complex or a porphyrin metal complex.

  Orthometalated metal complexes include, for example, Akio Yamamoto, “Organic Metal Chemistry: Fundamentals and Applications”, pages 150 and 232; It is a general term for a group of compounds described in pages 71 to 77, pages 135 to 146, and Springer-Verlag (published in 1987). The organic layer containing an orthometalated metal complex is advantageous in that it has high luminance and excellent luminous efficiency.

There are various ligands that form ortho-metalated metal complexes, which are also described in the above-mentioned documents. Among them, preferred ligands include 2-phenylpyridine derivatives, 7,8-benzo Examples include quinoline derivatives, 2- (2-thienyl) pyridine derivatives, 2- (1-naphthyl) pyridine derivatives, and 2-phenylquinoline derivatives. These derivatives may have a substituent as necessary.
The orthometalated metal complex may have other ligands in addition to the above ligands.

The orthometalated metal complexes used in the present invention are described in Inorg.Chem., 1991, 30, 1685, 1988, 27, 3464, 1994, 33, 545, Inorg.Chim.Acta. 1991, 181, 245, J. Organomet. Chem., 1987, 335, 293, J. Am. Chem. Soc., 1985, 107, 1431, etc. Can be synthesized.
A compound that emits light from triplet excitons among ortho-metalated metal complexes can be used as a phosphorescent compound in the present invention.

Of the porphyrin metal complexes, a porphyrin platinum complex is preferred.
A phosphorescent compound may be used individually by 1 type, and may use 2 or more types together.

  The host compound is not particularly limited as long as it is a compound capable of transferring exciton energy to the light emitting material, and can be appropriately selected according to the purpose. Specifically, the carbazole derivative, triazole derivative, oxazole derivative, oxadiazole derivative. , Imidazole derivatives, polyarylalkane derivatives, pyrazoline derivatives, pyrazolone derivatives, phenylenediamine derivatives, arylamine derivatives, amino-substituted chalcone derivatives, styrylanthracene derivatives, fluorenone derivatives, hydrazone derivatives, stilbene derivatives, silazane derivatives, aromatic tertiary amine compounds , Styrylamine compounds, aromatic dimethylidene compounds, porphyrin compounds, anthraquinodimethane derivatives, anthrone derivatives, diphenylquinone derivatives, thiopyrandioxide Hetero derivatives, carbodiimide derivatives, fluorenylidenemethane derivatives, distyrylpyrazine derivatives, heterocyclic tetracarboxylic anhydrides such as naphthaleneperylene, phthalocyanine derivatives, metal complexes of 8-quinolinol derivatives, metal phthalocyanines, benzoxazoles and benzothiazoles Various metal complexes represented by metal complexes as ligands, polysilane compounds, poly (N-vinylcarbazole) derivatives, aniline copolymers, thiophene oligomers, conductive polymer oligomers such as polythiophene, polythiophene derivatives, polyphenylene derivatives, And high molecular compounds such as polyphenylene vinylene derivatives and polyfluorene derivatives. These host compounds may be used individually by 1 type, and may use 2 or more types together.

  Although the number of light emitting layers is not particularly limited, it is preferably 2 or more and 20 or less, more preferably 2 or more and 10 or less, still more preferably 2 or more and 5 or less, and particularly preferably, for device preparation. 3 or more and 5 or less layers.

As described above, in the light-emitting element of the present invention, the concentration of the phosphorescent compound in the light-emitting layer is gradually increased from the light-emitting layer on the cathode side toward the light-emitting layer on the anode side.
The case where there are five light emitting layers will be described as an example. The element configuration in this case is, for example, anode / hole transport layer / first light emitting layer / second light emitting layer / third light emitting layer / fourth light emitting layer / fifth light emitting layer / electron transport layer / electron injection layer / cathode. Is mentioned. In this element configuration, for example, the phosphorescent compound concentration of the first light-emitting layer that is the light-emitting layer closest to the anode is 20% by mass, and then sequentially, the second light-emitting layer is 15% by mass and the third light-emitting layer is 10% by mass. The concentration of the phosphorescent compound in the light emitting layer is changed from the light emitting layer on the cathode side to the light emitting layer on the anode side by adjusting the content to 5% by mass in the fourth light emitting layer and 0.01% by mass in the fifth light emitting layer. Can be increased sequentially. That is, the light-emitting element of the present invention can be manufactured in which the phosphorescent compound concentration in the light-emitting layer is higher in the light-emitting layer on the anode side than in the light-emitting layer on the cathode side.

The concentration of the phosphorescent compound in the light emitting layer is preferably 0.01% by mass or more and 20% by mass or less, and more preferably 0.01% by mass or more and 10% by mass or less in any light emitting layer. If the concentration of the phosphorescent compound becomes higher than this, the light emission efficiency decreases due to concentration quenching. When the concentration of the phosphorescent compound is lower than this, the probability of energy transfer from the host material to the phosphorescent material is lowered, and the light emission efficiency is lowered.
Further, the concentration difference between the phosphorescent compounds in adjacent light emitting layers is preferably 1% by mass or more and 20% by mass or less, more preferably 1% by mass or more and 10% by mass, and further preferably 1% by mass or more and 5% by mass or less. 2 mass% or more and 5 mass% or less are especially preferable.

  The thickness of each light emitting layer is preferably 1 to 50 nm, more preferably 2 to 40 nm. When the thickness of each light emitting layer exceeds 50 nm, the driving voltage may increase, and when it is less than 1 nm, the light emitting element may be short-circuited.

With the element configuration of the present invention, it is possible to achieve an improvement in luminous efficiency and an improvement in durability. The reason for this is not clear, but it is considered that the above structure reduces the rate of holes penetrating through the light emitting layer and can effectively excite carriers and use them for generation.
Furthermore, by increasing the concentration of the phosphorescent compound in each light-emitting layer sequentially from the cathode side to the anode side, the concentration is increased on the hole transport layer side to assist hole injection, and the electron It is considered that the penetration of holes into the electron transport layer can be prevented by lowering the concentration on the transport layer side.

  You may add a hole transport material and an electron transport material to the light emitting layer of this invention as needed.

As the hole transport material added to the light emitting layer as necessary, either a low molecular hole transport material or a high molecular hole transport material can be used, and examples thereof include the following materials.
Carbazole derivatives, triazole derivatives, oxazole derivatives, oxadiazole derivatives, imidazole derivatives, polyarylalkane derivatives, pyrazoline derivatives, pyrazolone derivatives, phenylenediamine derivatives, arylamine derivatives, amino-substituted chalcone derivatives, styrylanthracene derivatives, fluorenone derivatives, hydrazone derivatives , Stilbene derivatives, silazane derivatives, aromatic tertiary amine compounds, styrylamine compounds, aromatic dimethylidene compounds, porphyrin compounds, polysilane compounds, poly (N-vinylcarbazole) derivatives, aniline copolymers, thiophene oligomers, Conductive polymer oligomer such as polythiophene, polythiophene derivative, polyphenylene derivative, polyphenylene vinylene derivative, polyfluorene Polymeric compounds such as derivatives. These may be used individually by 1 type and may use 2 or more types together.

The electron transport material added to the light emitting layer as necessary is not particularly limited, and examples thereof include the following materials.
Triazole derivatives, oxazole derivatives, oxadiazole derivatives, fluorenone derivatives, anthraquinodimethane derivatives, anthrone derivatives, diphenylquinone derivatives, thiopyrandioxide derivatives, carbodiimide derivatives, fluorenylidenemethane derivatives, distyrylpyrazine derivatives, naphthaleneperylene, etc. Various metal complexes represented by metal tetracyclic anhydrides, metal complexes of phthalocyanine derivatives, 8-quinolinol derivatives, metal phthalocyanines, metal complexes having benzoxazole and benzothiazole as ligands, aniline copolymers, Polymer compounds such as thiophene oligomers, conductive polymer oligomers such as polythiophene, polythiophene derivatives, polyphenylene derivatives, polyphenylene vinylene derivatives, polyfluorene derivatives, etc. It can be mentioned. These may be used individually by 1 type and may use 2 or more types together.

  Each light emitting layer is formed by a dry film formation method such as a vapor deposition method or a sputtering method, a dipping method, a spin coating method, a dip coating method, a casting method, a die coating method, a roll coating method, a bar coating method or a gravure coating method. In any case, the film can be suitably formed.

The light emitting device of the present invention will be described in detail below.
-Organic layer-
-Composition of organic layer-
There is no restriction | limiting in particular as a formation position in the light emitting element of an organic layer, Although it can select suitably according to the use and objective of this light emitting element, It is preferable to form on an anode or a cathode. In this case, the organic layer is formed on the front surface or one surface on the anode or the cathode.
There is no restriction | limiting in particular about the shape of a organic layer, a magnitude | size, thickness, etc., According to the objective, it can select suitably.

The specific layer structure of the light-emitting device of the present invention including the organic layer is as follows: anode / hole transport layer / multiple light-emitting layers / electron transport layer / cathode, anode / hole transport layer / multiple light-emitting layers / electron transport layer / Electron injection layer / cathode, anode / hole injection layer / hole transport layer / multiple emission layer / electron transport layer / cathode, anode / hole injection layer / hole transport layer / multiple emission layer / electron transport layer / electron injection Layer / cathode and the like.
In any case, a phosphorescent compound is contained in the light emitting layer, and light emission is usually extracted from the anode which is a transparent electrode.

--Electron transport layer--
In the present invention, an electron transport layer containing an electron transport material is provided.
The electron transport material is not limited as long as it has either a function of transporting electrons or a function of blocking holes injected from the anode, and an electron transport material can be preferably used. it can.
In the present invention, an aromatic heterocyclic compound having one or more hetero elements in the molecule is preferable.

  The electron transport material used in the present invention preferably has an azole skeleton among aromatic heterocyclic compounds having one or more hetero elements in the molecule. The compound having an azole skeleton is a compound having two or more atoms other than carbon atoms and hydrogen atoms in the basic skeleton, and may be monocyclic or condensed. The heterocyclic skeleton is preferably an aromatic heterocyclic ring having at least two atoms selected from N, O, and S atoms, more preferably an aromatic heterocyclic ring having at least one N atom in the skeleton. An aromatic heterocycle having two or more N atoms in the skeleton. In addition, the heteroatom may be in a condensed position or a non-condensed position.

  Examples of the compound having a heterocyclic skeleton containing two or more heteroatoms include pyrazole, imidazole, pyrazine, pyrimidine, indazole, purine, phthalazine, naphthyridine, quinoxaline, quinazoline, cinnoline, pteridine, perimidine, phenanthroline, pyrroloimidazole, pyrrolotriazole , Pyrazoloimidazole, pyrazolotriazole, pyrazolopyrimidine, pyrazolotriazine, imidazoimidazole, imidazopyridazine, imidazopyridine, imidazopyrazine, triazolopyridine, benzimidazole, naphthimidazole, benzoxazole, naphthoxazole, benzothiazole, naphthothiazole Benzotriazole, tetrazaindene, triazine, etc., preferably imidazopyri Jin, imidazopyridine, imidazopyrazine, a compound having benzimidazole, naphthoimidazole imidazole, benzoxazole, naphthoxazole, benzothiazole, a compound or a triazine skeleton having a condensed azole skeleton such naphthothiazole, particularly preferably imidazopyridine.

  The compound having an azole skeleton is preferably a compound represented by the following general formula (1).

In the general formula (1), R represents a hydrogen atom or a substituent. Examples of the substituent include alkyl groups such as methyl group, ethyl group, i-propyl group, and n-propyl group, and aryl groups such as phenyl group and naphthyl group which may have a substituent.
X is O, S or N—R ^a (R ^a is a hydrogen atom, an aliphatic hydrocarbon group (eg, methyl group, ethyl group, i-propyl group, n-propyl group), aryl group (eg, phenyl group, A naphthyl group, an anthranyl group) or a heterocyclic group (for example, a thienyl group or a pyridyl group).
Q represents an atomic group necessary for binding to N and X to form a heterocycle.
R and X, and R and Q may be combined to form a ring, if possible.

  Specific examples of the electron transport material used in the present invention are shown below, but the present invention is not limited thereto.

  The compounds represented by the general formula (1) used in the present invention are disclosed in JP-B Nos. 44-23025, 48-8842, JP-A-53-6331, JP-A-10-92578, US Patent 3,449,255, 5,766,779, J. Pat. Am. Chem. Soc. 94, 2414 (1972), Helv. Chim. Acta, 63, 413 (1980), Liebigs Ann. Chem. , 1423 (1982) and the like.

  By using these compounds represented by the general formula (1) as an electron transport material in the electron transport layer of the present invention, the light emission efficiency and durability can be further improved.

  The thickness of the electron transport layer is preferably 10 to 200 nm and more preferably 20 to 80 nm in order to avoid an increase in driving voltage and a short circuit of the light emitting element.

  In the present invention, a hole blocking layer can be provided between the light emitting layer and the electron transport layer. The hole blocking layer is a layer that suppresses the penetration of holes from the light emitting layer to the cathode side, and a material that has an electron transporting property and a large ionization potential can be used. These can be suitably selected from electron transport materials.

In the present invention, an electron injection layer can be provided between the electron transport layer and the cathode.
The electron injection layer is a layer that facilitates injection of electrons from the cathode into the electron transport layer. Specifically, lithium salts such as lithium fluoride, lithium chloride, and lithium bromide, sodium fluoride, sodium chloride, fluoride An alkali metal salt such as cesium, an insulating metal oxide such as lithium oxide, aluminum oxide, indium oxide, and magnesium oxide can be preferably used. The thickness of the electron injection layer is 0.1 to 5 nm.

--- Hole transport layer--
In the present invention, a hole transport layer containing a hole transport material can be provided as necessary.
The hole transport material is not limited as long as it has either a function of transporting holes or a function of blocking electrons injected from the cathode, and the hole transport material is preferably used. be able to.
The thickness of the hole transport layer is preferably 10 to 200 nm and more preferably 20 to 80 nm in order to avoid an increase in driving voltage and a short circuit of the light emitting element.

In the present invention, a hole injection layer can be provided between the hole transport layer and the anode.
The hole injection layer is a layer that facilitates injection of holes from the anode into the hole transport layer, and specifically, a material having a small ionization potential is preferably used among the hole transport materials. For example, a phthalocyanine compound, a porphyrin compound, a starburst type triarylamine compound, etc. can be mentioned, It can use suitably.
The thickness of the hole injection layer is 1 to 30 nm.

--Formation of organic layer--
Organic layers such as the electron transport layer and the hole transport layer are formed by dry film formation methods such as vapor deposition and sputtering, dipping, spin coating, dip coating, casting, die coating, roll coating, and bar coating. The film can be suitably formed by any of wet film forming methods such as gravure coating.
In addition, selection of the kind of these film forming methods can be suitably performed according to the material of this organic layer.

  When the film is formed by a wet film forming method, the film can be appropriately dried after film formation. The drying conditions are not particularly limited, but a temperature in a range that does not damage the coated layer is adopted. be able to.

-Base material-
The material of the base material is preferably a material that does not transmit moisture or a material that has a very low moisture transmission rate, and is preferably a material that does not scatter or attenuate light emitted from the organic layer, for example, YSZ (zirconia stable) Yttrium), inorganic materials such as glass, polyesters such as polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, polystyrene, polycarbonate, polyethersulfone, polyarylate, allyl diglycol carbonate, polyimide, polycycloolefin, norbornene resin, poly And organic materials such as synthetic resins such as (chlorotrifluoroethylene). In the case of an organic material, it is preferable to be excellent in heat resistance, dimensional stability, solvent resistance, electrical insulation, workability, low air permeability, low moisture absorption, and the like. Among these, when the material of the transparent electrode is indium tin oxide (ITO) that is suitably used as the material of the transparent electrode, a material having a small difference in lattice constant from the indium tin oxide (ITO) is preferable. . These materials may be used alone or in combination of two or more.

  There is no restriction | limiting in particular about the shape of a base material, a structure, a magnitude | size, It can select suitably according to the use, the objective, etc. of a light emitting element. In general, the shape is a plate. The structure may be a single layer structure, a laminated structure, may be formed of a single member, or may be formed of two or more members.

  The substrate may be colorless and transparent or colored and transparent, but is preferably colorless and transparent in that it does not scatter or attenuate the light emitted from the light emitting layer.

The substrate is preferably provided with a moisture permeation preventing layer (gas barrier layer) on the front surface or back surface (transparent electrode side).
As a material for the moisture permeation preventive layer (gas barrier layer), inorganic materials such as silicon nitride and silicon oxide are preferably used. The moisture permeation preventing layer (gas barrier layer) can be formed by, for example, a high frequency sputtering method.
The base material may be further provided with a hard coat layer, an undercoat layer or the like, if necessary.

-Anode-
As the anode, it is usually only necessary to have a function as an anode for supplying holes to the organic layer, and there is no particular limitation on the shape, structure, size, etc., depending on the use and purpose of the light emitting device. , Can be appropriately selected from known electrodes.

  As a material of the anode, for example, a metal, an alloy, a metal oxide, an organic conductive compound, or a mixture thereof can be suitably cited, and a material having a work function of 4.0 eV or more is preferable. Specific examples include semiconductive metal oxides such as tin oxide (ATO, FTO) doped with antimony or fluorine, tin oxide, zinc oxide, indium oxide, indium tin oxide (ITO), indium zinc oxide (IZO), etc. Metals such as gold, silver, chromium and nickel, and mixtures or laminates of these metals and conductive metal oxides, inorganic conductive materials such as copper iodide and copper sulfide, organics such as polyaniline, polythiophene and polypyrrole Examples thereof include conductive materials and laminates of these with ITO.

  The anode may be, for example, a printing method, a wet method such as a coating method, a physical method such as a vacuum deposition method, a sputtering method, or an ion plating method, a chemical method such as a CVD method or a plasma CVD method, or the like. It can be formed on the substrate according to a method appropriately selected in consideration of suitability. For example, when ITO is selected as the anode material, the anode can be formed according to a direct current or high frequency sputtering method, a vacuum deposition method, an ion plating method, or the like. Moreover, when selecting an organic electroconductive compound as a material of an anode, it can carry out according to the wet film forming method.

  There is no restriction | limiting in particular as a formation position of the anode in the light emitting element of this invention, Although it can select suitably according to the use and objective of this light emitting element, It is preferable to form on a board | substrate. In this case, the anode may be formed on the entire one surface of the substrate or may be formed on a part thereof.

  The patterning of the anode may be performed by chemical etching such as photolithography, or may be performed by physical etching using a laser or the like, or may be performed by vacuum deposition or sputtering with a mask overlapped. It may be performed by a lift-off method or a printing method.

The thickness of the anode can be appropriately selected according to the above materials and cannot be generally defined, but is usually 10 nm to 50 μm, and preferably 50 nm to 20 μm.
The resistance value of the anode is preferably 10 ³ Ω / □ or less, and more preferably 10 ² Ω / □ or less.
The anode is preferably transparent in order to extract light emitted from the anode side, and the transmittance is preferably 60% or more, and more preferably 70% or more. This transmittance can be measured according to a known method using a spectrophotometer. In this case, the anode may be colorless and transparent or colored and transparent.

  The anode is described in detail in “New Development of Transparent Electrode Film”, published by CMC (1999), supervised by Yutaka Sawada, and these can be applied to the present invention. When using a plastic substrate having low heat resistance, an anode formed using ITO or IZO at a low temperature of 150 ° C. or lower is preferable.

-Cathode-
As the cathode, it is usually only necessary to have a function as a cathode for injecting electrons into the organic layer, and there are no particular restrictions on the shape, structure, size, etc., depending on the use and purpose of the light-emitting element, It can select suitably from well-known electrodes.

  Examples of the material for the cathode include metals, alloys, metal oxides, electrically conductive compounds, and mixtures thereof, and those having a work function of 4.5 eV or less are preferable. Specific examples include alkali metals (eg, Li, Na, K, Cs, etc.), alkaline earth metals (eg, Mg, Ca, etc.), gold, silver, lead, aluminum, sodium-potassium alloys, lithium-aluminum alloys, magnesium. -Rare earth metals such as silver alloys, indium, ytterbium, and the like. These may be used alone, but two or more can be suitably used in combination from the viewpoint of achieving both stability and electron injection.

Among these, alkali metals and alkalinity metals are preferable from the viewpoint of electron injection properties, and materials mainly composed of aluminum are preferable from the viewpoint of excellent storage stability.
The material mainly composed of aluminum is aluminum alone, or an alloy or mixture of aluminum and 0.01 to 10% by weight of alkali metal or alkaline earth metal (for example, lithium-aluminum alloy, magnesium-aluminum alloy, etc.). Say.

  The cathode material is described in detail in JP-A-2-15595 and JP-A-5-121172.

  There is no restriction | limiting in particular in the formation method of a cathode, It can carry out according to a well-known method. For example, suitability with the above materials from among wet methods such as printing methods, coating methods, physical methods such as vacuum deposition methods, sputtering methods and ion plating methods, chemical methods such as CVD and plasma CVD methods, etc. It can be formed according to a method appropriately selected in consideration. For example, when a metal or the like is selected as the cathode material, one or more of them can be simultaneously or sequentially performed according to a sputtering method or the like.

  The cathode patterning may be performed by chemical etching such as photolithography, physical etching by a laser, or the like, or by vacuum deposition or sputtering with a mask overlapped. It may be performed by a lift-off method or a printing method.

There is no restriction | limiting in particular as a formation position in the light emitting light emitting element of a cathode, Although it can select suitably according to the use and purpose of this light emitting element, forming in an organic layer is preferable. In this case, the cathode may be formed on the entire organic layer or a part thereof.
Further, a dielectric layer made of an alkali metal or alkaline earth metal fluoride or the like may be inserted between the cathode and the organic layer with a thickness of 0.1 to 5 nm.
The dielectric layer can be formed by, for example, a vacuum deposition method, a sputtering method, an ion plating method, or the like.

The thickness of the cathode can be appropriately selected depending on the material and cannot be generally defined, but is usually 10 nm to 5 μm, and preferably 50 nm to 1 μm.
The cathode may be transparent or opaque. The transparent cathode can be formed by thinly forming the cathode material to a thickness of 1 to 10 nm and further laminating the transparent conductive material such as ITO or IZO.

-Other layers-
There is no restriction | limiting in particular as another layer, According to the objective, it can select suitably, For example, a protective layer etc. are mentioned.
As the protective layer, for example, those described in JP-A-7-85974, JP-A-7-192866, JP-A-8-22891, JP-A-10-275682, JP-A-10-106746 are suitable. Can be mentioned.
In the light emitting laminate comprising a substrate, an anode, an organic layer, and a cathode, the protective layer is formed on the outermost surface, for example, when the substrate, the anode, the organic layer, and the cathode are laminated in this order on the cathode. When the substrate, the cathode, the organic layer, and the anode are laminated in this order, they are formed on the anode.
The shape, size, thickness, and the like of the protective layer can be selected as appropriate, and as the material, it is possible to suppress the penetration or transmission of substances that can deteriorate the light emitting element such as moisture and oxygen into the light emitting element. If it has the function to perform, there will be no restriction | limiting in particular, For example, silicon oxide, silicon dioxide, germanium oxide, germanium dioxide, etc. are mentioned.

  The method for forming the protective layer is not particularly limited. For example, the vacuum deposition method, the sputtering method, the reactive sputtering method, the molecular send epitaxy method, the cluster ion beam method, the ion plating method, the plasma polymerization method, the plasma CVD method, Examples include laser CVD, thermal CVD, and coating.

Furthermore, in the present invention, it is also preferable to provide a sealing layer for the purpose of preventing moisture and oxygen from entering each layer in the light emitting laminate.
Examples of the material for the sealing layer include a copolymer containing tetrafluoroethylene and at least one comonomer, a fluorine-containing copolymer having a cyclic structure in the copolymer main chain, polyethylene, polypropylene, polymethyl methacrylate, and polyimide. , Polyurea, polytetrafluoroethylene, polychlorotrifluoroethylene, polydichlorodifluoroethylene, chlorotrifluoroethylene and dichlorodifluoroethylene, two or more copolymers, a water-absorbing substance having a water absorption of 1% or more, water absorption Moisture-proof material with a rate of 0.1% or less, metals such as In, Sn, Pb, Au, Cu, Ag, Al, Tl, Ni, MgO, SiO, SiO ₂ , Al ₂ O ₃ , GeO, NiO, CaO, _{BaO, Fe 2 O 3, Y} 2 O 3, TiO metal oxides such as _2, MgF _2, LiF, Al _3, CaF _2, perfluoroalkanes, perfluoro amines, liquid fluorinated carbon perfluoro ether, which moisture and oxygen in the liquid fluorinated carbon was dispersed adsorbent for adsorbing, like It is done.

The light-emitting element of the present invention obtains light emission by applying a direct current (which may include an alternating current component as necessary) voltage (usually 2 to 40 volts) or a direct current between the anode and the cathode. Can do.
Regarding the driving of the light emitting element of the present invention, JP-A-2-148687, JP-A-6-301355, JP-A-5-29080, JP-A-7-134558, JP-A-8-234485, JP-A-241047. , U.S. Pat. Nos. 5,828,429, 6,023,308, Japanese Patent 2,784,615, and the like can be used.

  The use of the light-emitting element of the present invention is not limited, but it can be effectively used for a surface light source such as a full-color display, a backlight, and an illumination light source, and a light source array such as a printer.

  Examples of the light emitting device of the present invention will be described below, but the present invention is not limited to these examples.

(Example 1)
An ITO target (indium: tin = 95: 5) having a SnO ₂ content of 10% by mass was introduced into a vacuum chamber using a glass plate of 0.5 mm thickness and 2.5 cm square as the substrate. (Molar ratio)) was used to form an ITO thin film (thickness 0.2 μm) as a transparent electrode (anode) by DC magnetron sputtering (conditions: substrate temperature 250 ° C., oxygen pressure 1 × 10 ⁻³ Pa). . The surface resistance of the ITO thin film was 10Ω / □.
Next, the substrate on which the transparent electrode was formed was placed in a cleaning container and subjected to IPA cleaning, and then UV-ozone treatment was performed for 30 minutes.

On this anode, 0.03 μm of N, N′-dinaphthyl-N, N′-diphenylbenzidine was provided at a rate of 1 nm / second by vacuum deposition as a hole transport layer.
On top of this, tris (2-phenylpyridyl) iridium as a phosphorescent compound and 4,4′-N, N′-dicarbazolebiphenyl as a host compound were co-deposited at a deposition rate of 1/5. A first light-emitting layer having a (2-phenylpyridyl) iridium concentration of 20% by mass was formed to a thickness of 0.006 μm.
Furthermore, tris (2-phenylpyridyl) iridium concentration is obtained by using tris (2-phenylpyridyl) iridium which is a phosphorescent compound and 4,4′-N, N′-dicarbazolebiphenyl as a host compound. However, the second light-emitting layer is 0.006 μm, the third light-emitting layer is 0.006 μm, and the second to fifth light-emitting layers are 15% by mass, 10% by mass, 5% by mass, and 0.01% by mass, respectively. The fourth light emitting layer was provided at 0.006 μm, and the fifth light emitting layer was provided at 0.006 μm.

Furthermore, 2,2 ′, 2 ″-(1,3,5-benzenetriyl) tris [3- (2-methylphenyl) -3H-imidazo [4,5-b] is used as an electron transport material. Pyridine] (Compound 27) was deposited at a rate of 1 nm / sec to provide an electron transport layer of 0.050 μm.
Furthermore, LiF was vapor-deposited as an electron injection layer at a rate of 1 nm / second to provide a 0.002 μm electron injection layer.

  Further, a patterned mask (a mask having a light emission area of 5 mm × 5 mm) was placed on the electron injection layer, and aluminum was deposited by 0.25 μm in a vapor deposition apparatus to form a back electrode (cathode).

Aluminum lead wires were respectively connected from the transparent electrode (functioning as an anode) and the back electrode to form a light emitting laminate.
The light emitting laminate obtained here was placed in a glove box substituted with nitrogen gas. In a glove box, 10 mg of calcium oxide powder as a moisture adsorbent was placed in a stainless steel sealing cover provided with a recess on the inside, and fixed with an adhesive tape. Sealing was performed using this sealing cover and an ultraviolet curable adhesive (XNR5516HV, manufactured by Chiba Nagase) as an adhesive. Thus, the light emitting element of Example 1 was produced.

Using the light emitting device, evaluation was performed by the following method.
Using a source measure unit type 2400 manufactured by Toyo Technica, direct voltage was applied to the light emitting element to emit light, and the initial light emitting performance was measured. The maximum luminance at that time was L _max , and the voltage when L _max was obtained was V _max . Further, the luminous efficiency at ₂₀₀₀ Cd / m ² is shown in Table 1 as the external quantum efficiency (η ₂₀₀₀ ).
In addition, a driving durability test was performed at an initial luminance of 2000 Cd / m ² , and the test results are shown in Table 1 with the time when the luminance was halved as half time (T _1/2 ).

(Example 2)
In Example 1, an element was prepared by the same method as in Example 1 except that the light emitting layer was deposited under the following conditions, and evaluated in the same manner as in Example 1. The test results are shown in Table 1.

<Evaporation conditions for light emitting layer>
As a phosphorescent compound, tris (2-phenylpyridyl) iridium and a host compound, 4,4′-N, N′-dicarbazolebiphenyl was co-deposited at a deposition rate of 1/10, and tris (2- A first light-emitting layer having a phenylpyridyl) iridium concentration of 10% by mass was formed into a film of 0.006 μm.
Furthermore, tris (2-phenylpyridyl) iridium concentration is obtained by using tris (2-phenylpyridyl) iridium which is a phosphorescent compound and 4,4′-N, N′-dicarbazolebiphenyl as a host compound. However, the second light emitting layer is 0.006 μm, the third light emitting layer is 0.006 μm, and the fourth light emitting layer is 8 mass%, 6 mass%, 4 mass%, and 1 mass%, respectively. The light emitting layer was provided with 0.006 μm, and the fifth light emitting layer was provided with 0.006 μm.

(Example 3)
In Example 1, an element was prepared by the same method as in Example 1 except that the light emitting layer was deposited under the following conditions, and evaluated in the same manner as in Example 1. The test results are shown in Table 1.

<Evaporation conditions for light emitting layer>
As a phosphorescent compound, tris (2-phenylpyridyl) iridium and a host compound, 4,4′-N, N′-dicarbazolebiphenyl was co-deposited at a deposition rate of 1/10, and tris (2- A first light-emitting layer having a phenylpyridyl) iridium concentration of 10% by mass was formed to a thickness of 0.01 μm.
Furthermore, tris (2-phenylpyridyl) iridium concentration is obtained by using tris (2-phenylpyridyl) iridium which is a phosphorescent compound and 4,4′-N, N′-dicarbazolebiphenyl as a host compound. However, the second and third light-emitting layers were respectively provided with a second light-emitting layer of 0.01 μm and a third light-emitting layer of 0.01 μm so as to be 5% by mass and 0.01% by mass, respectively.

Example 4
In Example 1, the layers up to the light emitting layer were formed by the same method as in Example 1. Thereafter, ((1,1′-biphenyl) -4-orato) bis (2-methyl-8-quinolinolato N1, O8) aluminum (Balq) was vapor-deposited at a rate of 1 nm / second as a hole blocking material. A hole block transport layer of 010 μm was provided.
Further, tris (8-hydroxyquinolinato) aluminum was deposited thereon as an electron transporting material at a rate of 1 nm / second to provide a 0.040 μm electron transporting layer.
Furthermore, LiF was vapor-deposited as an electron injection layer at a rate of 1 nm / second to provide a 0.002 μm electron injection layer.

  Further, a patterned mask (a mask having a light emission area of 5 mm × 5 mm) was placed on the electron injection layer, and aluminum was deposited by 0.25 μm in a deposition apparatus to form a back electrode.

Aluminum lead wires were respectively connected from the anode and the cathode to form a light emitting laminate.
The light emitting laminate obtained here was placed in a glove box substituted with nitrogen gas. In a glove box, 10 mg of calcium oxide powder as a moisture adsorbent was placed in a stainless steel sealing cover provided with a recess on the inside, and fixed with an adhesive tape. Sealing was performed using this sealing cover and an ultraviolet curable adhesive (XNR5516HV, manufactured by Chiba Nagase) as an adhesive. Thus, the light-emitting element of Example 4 was created.
Then, it evaluated by the same method as Example 1. The results are shown in Table 1.

(Comparative Example 1)
In Example 1, an element was prepared by the same method as in Example 1 except that the light emitting layer was deposited under the following conditions, and evaluated in the same manner as in Example 1. The test results are shown in Table 1.

<Evaporation conditions for light emitting layer>
As a phosphorescent compound tris (2-phenylpyridyl) iridium and a host compound, 4,4′-N, N′-dicarbazolebiphenyl was co-evaporated at a deposition rate of 1/20, and tris (2- A single light-emitting layer having a phenylpyridyl) iridium concentration of 5% by mass was formed into 0.03 μm.

  From the results shown in Table 1, the concentration of the phosphorescent compound in the light emitting layer increases in order from the light emitting layer on the cathode side to the light emitting layer on the anode side. It can be seen that the light emitting efficiency and durability are remarkably superior to those of the comparative light emitting device of Comparative Example 1 composed of a uniform single light emitting layer.

Claims (3)

A light emitting device having a hole transport layer, at least two light emitting layers, and an electron transport layer between a pair of electrodes,
A light-emitting element, wherein all the light-emitting layers each contain a phosphorescent compound and a host compound, and the concentration of the phosphorescent compound is higher in the light-emitting layer on the anode side than in the light-emitting layer on the cathode side.   2. The light-emitting element according to claim 1, wherein the concentration of the phosphorescent compound in the light-emitting layer is 0.01% by mass or more and 20% by mass or less in each light-emitting layer.   3. The light emitting device according to claim 1, wherein the electron transport material contained in the electron transport layer is an aromatic heterocyclic compound having one or more hetero atoms in the molecule.