JP7582403B2 - INK COMPOSITION, LIGHT-EMITTING LAYER AND ELECTROLUMINESCENT DEVICE - Google Patents

Description

The present invention relates to an ink composition containing quantum dots and a solvent, and a light-emitting layer and an electroluminescent device made using the ink composition.

Electroluminescent devices (EL, ElectroLuminescence) have attracted attention as surface-emitting devices that are lightweight, thin, consume little power, and have excellent freedom of shape. Electroluminescent devices have many excellent features, such as high-brightness emission, fast response, wide viewing angle, thin and lightweight, and high resolution, and their application to flat panel displays and lighting is being considered, with the use of quantum dots as one type of electroluminescent device attracting attention.

Quantum dots are nanoscale semiconductor particles in which charge carriers and excitons are confined in all three dimensions, and are also called semiconductor nanoparticles. Quantum dots exhibit intermediate behavior between atomic or molecular behavior and macroscopic solid behavior such as bulk form, and the effective band gap increases with decreasing particle size. That is, as the size of quantum dots decreases, their absorption and emission shift to shorter wavelengths, i.e., from red to blue. In addition, by controlling the combination of the composition and particle size of the quantum dots, a wide spectrum from the infrared region to the ultraviolet region can be obtained, and by further controlling the distribution of particle size, a spectrum with a narrow half-width and excellent color purity can be obtained.
In recent years, taking advantage of these characteristics, electroluminescent devices using quantum dots as the light-emitting material have been proposed.

Generally, quantum dots are treated as a quantum dot-containing composition dispersed in water or an organic solvent, and are processed into a film by a wet method such as coating, printing, etc. Therefore, among the multiple layers constituting an electroluminescent device, the lower layer of the light-emitting layer formed of the quantum dot-containing composition is damaged by the solvent contained in the composition, and therefore there are limitations on the selection of materials and organic solvents constituting the lower layer.
In particular, electroluminescent elements often have a hole transport layer below the light-emitting layer, and when a quantum dot-containing composition containing an aromatic organic solvent is applied onto a hole transport layer made of an aromatic low molecule or aromatic polymer, significant damage occurs to the hole transport layer.

In response to the above-mentioned problems, Patent Document 1 discloses a technology for improving characteristics by utilizing the fact that the solvent in the quantum dot-containing composition dissolves the lower layer, forming a mixed layer. Patent Document 2 also describes an ink in which quantum dots having a triphenylamine skeleton and ligands with phosphine oxide as an adsorption group are dispersed in diethylene glycol dibutyl ether.

International Publication No. 2015/105027 International Publication No. 2019/065346

However, the invention of Patent Document 1 does not necessarily lead to an improvement depending on the combination of the lower layer and the organic solvent, and in many cases, it worsens. Furthermore, there is a problem that the reproducibility of the mixed layer is poor and the element characteristics are not stable. Furthermore, Patent Document 2 combines a ligand having an adsorption group of phosphine oxide with diethylene glycol dibutyl ether, and there is a problem that the element characteristics and the ink jet ejection property of the ink are poor.
In other words, the object of the present invention is to provide an electroluminescent element that allows for a reduction in driving voltage without impeding the injection of electrical energy into quantum dots, and that has excellent luminous efficiency and durability while suppressing damage to underlying layers due to lamination, a light-emitting layer thereof, and an ink composition for forming them.

As a result of extensive research into solving the above problems, we discovered that the above problems could be solved by the embodiment shown below, and thus completed the present invention.

An embodiment of the present invention relates to an ink composition comprising quantum dots and a solvent, wherein the quantum dots comprise a carrier transporting ligand having at least one functional group selected from the group consisting of a sulfanyl group and a carboxyl group, the solvent comprises at least one solvent represented by the following general formulas (1) to (4), and the total amount of the solvents represented by the following general formulas (1) to (4) is 50 mass% or more with respect to the total amount of the solvent.
General formula (1): R ³⁶ CO(OR ³⁷ ) _p OR ³⁸
General formula (2): R ³⁹ CO(OR ⁴⁰ ) _q OCOR ⁴¹
General formula (3): R ⁴² (OR ⁴³ ) _r OR ⁴⁴
General formula (4): R ⁴⁵ (OR ⁴⁶ ) _s OR ⁴⁷
[In general formulas (1) to (4),
R ³⁶ , R ³⁸ , R ³⁹ , R ⁴¹ , R ⁴² and R ⁴⁴ each independently represent an alkyl group having 1 to 4 carbon atoms, R ³⁷ , R ⁴⁰ and R ⁴³ each independently represent an alkylene group having 2 to 4 carbon atoms, R ⁴⁵ and R ⁴⁷ each independently represent a hydrogen atom or an alkyl group having 1 to 6 carbon atoms, at least one of R ⁴⁵ and R ⁴⁷ is a hydrogen atom, R ⁴⁶ represents an alkylene group having 1 to 10 carbon atoms, and p, q, r and s each independently represent an integer of 1 to 6.]

［一般式（５）中、Ｌ^１は、直接結合又は炭素数１～５のアルキレン基であり、Ｌ^２は、炭素数１～５のアルキレン基であり、Ｒ^４８は、炭素数１～５のアルキル基であり、ｎは、０～４の整数である。］ Another embodiment of the present invention relates to the ink composition described above, wherein the carrier transporting ligand is a compound having a group represented by general formula (5).
General formula (5)

[In general formula (5), L ¹ is a direct bond or an alkylene group having 1 to 5 carbon atoms, L ² is an alkylene group having 1 to 5 carbon atoms, R ⁴⁸ is an alkyl group having 1 to 5 carbon atoms, and n is an integer of 0 to 4.]

Another embodiment of the present invention relates to the ink composition described above for use in an inkjet system.

Another embodiment of the present invention relates to a light-emitting layer formed using the ink composition described above.

Another embodiment of the present invention relates to an electroluminescent device having a configuration in which an anode, a hole transport layer, the above-described light-emitting layer, and a cathode are laminated in this order on a substrate, and the light-emitting layer is provided in contact with the hole transport layer.

The present invention makes it possible to reduce the driving voltage without impeding the injection of electrical energy into the quantum dots, and to provide an electroluminescent element with excellent luminous efficiency and durability, in which damage to the lower layers due to lamination is suppressed, as well as the light-emitting layer thereof, and an ink composition for forming them.

<Ink composition>
The ink composition of the present invention comprises quantum dots and a solvent, the quantum dots comprise a carrier transporting ligand having at least one functional group selected from the group consisting of a sulfanyl group and a carboxyl group, the solvent comprises a solvent belonging to a specific structure, and the total amount of the solvents belonging to the specific structure is 50 mass% or more relative to the total amount of the solvent.
By using such an ink composition, the light-emitting layer and electroluminescent element formed using the composition can reduce the driving voltage without impeding the injection of electrical energy into the quantum dots, and damage to the lower layers due to lamination is suppressed, thereby exhibiting excellent light-emitting efficiency and durability.
The present invention will be described in detail below.

<Quantum dots>
The quantum dots in the present invention are composed of semiconductor nanoparticles of about several nm to several tens of nm in size and a material called a ligand. The ligand is mainly physically or chemically adsorbed onto the surface of the semiconductor nanoparticles, and in addition to imparting dispersibility in a solvent, it also plays a role in protecting the surface of the semiconductor nanoparticles.

<Semiconductor nanoparticles>
Examples of materials for semiconductor nanoparticles include carbon (C) (amorphous carbon, graphite, graphene, carbon nanotubes, etc.), silicon (Si), germanium (Ge), tin (Sn), and other elements in group IV of the periodic table, elements in group V of the periodic table such as phosphorus (P) (black phosphorus), elements in group VI of the periodic table such as selenium (Se) and tellurium (Te), tin oxide (IV), boron nitride (BN), boron phosphide (BP), boron arsenide (BAs), aluminum nitride (AlN), Compounds of elements in Group III of the periodic table and elements in Group V of the periodic table, such as aluminum phosphide (AlP), aluminum arsenide (AlAs), aluminum antimonide (AlSb), gallium nitride (GaN), gallium phosphide (GaP), gallium arsenide (GaAs), gallium antimonide (GaSb), indium nitride (InN), indium phosphide (InP), indium arsenide (InAs), and indium antimonide (InSb), aluminum sulfide (Al ₂ S ₃ ), aluminum selenide (Al ₂ Se ₃ ), gallium sulfide (Ga ₂ S ₃ ), gallium selenide (GaSe, Ga ₂ Se ₃ ), gallium telluride (GaTe, Ga ₂ Te ₃ ), indium oxide (In ₂ O ₃ ), indium sulfide (In ₂ S ₃ , InS), indium selenide (In ₂ Se ₃ ), indium telluride (In ₂ Te ₃ compounds of Group III elements of the periodic table and Group VI elements of the periodic table, such as zinc oxide (ZnO), zinc sulfide (ZnS), zinc selenide (ZnSe), zinc telluride (ZnTe), cadmium oxide (CdO), cadmium sulfide (CdS), cadmium selenide (CdSe), cadmium telluride (CdTe), mercury sulfide (HgS), mercury selenide (HgSe), and mercury telluride (HgTe); compounds of Group II elements of the periodic table and Group VI elements of the periodic table, such as copper(I) oxide (Cu ₂ O);
and compounds of Group I elements and Group VII elements of the periodic table, such as copper chloride (I) (CuCl), copper bromide (I) (CuBr), copper iodide (I) (CuI), silver chloride (AgCl), and silver bromide (AgBr). Two or more of these may be used in combination, if necessary.
These semiconductors may contain elements other than the constituent elements, and in the case of III-V group semiconductors, they may be alloys such as INGaP and INGaN. Semiconductor nanoparticles doped with rare earth elements or transition metal elements may also be used, such as ZnS:Mn, ZnS:Tb, ZnS:Ce, and _LaPO4 :Ce.

Among these, silicon (Si), germanium (Ge), gallium nitride (GaN), gallium phosphide (GaP), gallium arsenide (GaAs), indium nitride (InN), indium phosphide ₍ InP ₎ , indium arsenide (InAs), gallium selenide (GaSe, _Ga2Se3 ), indium sulfide ( _In2S3 Preferably used are alloy systems such as InS, zinc oxide (ZnO), zinc sulfide (ZnS), zinc selenide (ZnSe), zinc telluride (ZnTe), cadmium oxide (CdO), cadmium sulfide (CdS), cadmium selenide (CdSe), cadmium telluride (CdTe), InGaP, and InGaN, more preferably indium phosphide (InP), cadmium selenide (CdSe), zinc sulfide (ZnS), and zinc selenide (ZnSe), and even more preferably those having InP as a core and ZnS and/or ZnSe as a shell.

Furthermore, perovskite crystals can be suitably used as the material for the semiconductor nanoparticles. The perovskite crystals have a composition represented by the following formula (I) and have a three-dimensional crystal structure.
Formula (I): _AQX3
[In formula (I), A is a monovalent cation of at least one amine compound selected from the group consisting of methylammonium (CH ₃ NH ₂ ) and formamidinium (NH ₂ CHNH), or a monovalent cation of at least one alkali metal element selected from the group consisting of rubidium (Rb), cesium (Ce), and francium (Fr); Q is a divalent cation of at least one metal element selected from the group consisting of lead (Pb) and tin (Sn); and X is a monovalent anion of at least one halogen element selected from the group consisting of iodine (I), bromine (Br), and chlorine (Cl)]

The semiconductor nanoparticles preferably have a core-shell structure. Core-shell type semiconductor nanoparticles have a structure in which the core structure is covered with a material made of a component different from the material forming the core. By selecting a semiconductor with a large band gap for the shell, excitons (electron-hole pairs) generated by photoexcitation are confined within the core. As a result, the probability of non-radiative transitions on the particle surface is reduced, improving the quantum yield of light emission and the stability of the fluorescent properties. The shell may have multiple layers. Furthermore, the boundaries between the core and the shell, and between one shell and another shell, may be clear or may have a gradient structure in which the shells are gradually joined together by providing a concentration gradient. Furthermore, the shell may cover only a part of the core, or the entire core.

The average particle size of the semiconductor nanoparticles, including the core and shell, is usually 0.5 nm to 100 nm, preferably 1 to 50 nm, and more preferably 1 to 15 nm.

The average particle size referred to here refers to a value obtained by observing semiconductor nanoparticles with a transmission electron microscope, measuring the size of 30 randomly selected particles, and adopting the average value. In this case, since the semiconductor nanoparticles are accompanied by a ligand described below, a scanning transmission electron microscope equipped with an energy dispersive X-ray analysis is used to identify the semiconductor material portion, and the particle size is measured by utilizing the fact that the semiconductor nanoparticle portion is imaged darker than the ligand described below due to the difference in electron density in the transmission electron microscope image. The shape of the semiconductor nanoparticles is not limited to spherical, but may be rod-shaped, disk-shaped, or other shapes.

<Ligand>
The quantum dots of the present invention are characterized by including a carrier transporting ligand having at least one functional group selected from the group consisting of a sulfanyl group and a carboxyl group. The ligand is used in the surface treatment of semiconductor nanoparticles and covers at least a part of the surface of the semiconductor nanoparticles. Conventional ligands such as alkylthiols are insulating substances, and therefore have a problem of inhibiting injection of electric energy into the quantum dots. However, the present invention includes a carrier transporting ligand having the transport properties of holes and electrons, which are carriers, and thus reduces the driving voltage without inhibiting injection of electric energy into the quantum dots, and can exhibit excellent luminous efficiency and durability.

Surface treatment methods include, for example, a method in which a ligand is added during the synthesis of semiconductor nanoparticles to chemically adsorb the ligand to the semiconductor nanoparticles in advance for surface treatment; a method in which semiconductor nanoparticles or semiconductor nanoparticles that have been surface-treated with another ligand are stirred in a solvent to chemically adsorb, physically adsorb, or chemically bond the ligand to the surface of the semiconductor nanoparticles; and a method in which the solvent is removed from semiconductor nanoparticles or semiconductor nanoparticles that have been surface-treated with another ligand by centrifugal sedimentation or the like, and then the semiconductor nanoparticles are redispersed in a solvent containing the ligand and surface-treated by chemical adsorption, physically adsorb, or chemically bond.

<Carrier transporting ligand>
The carrier transporting ligand in the present invention has at least one functional group selected from the group consisting of a sulfanyl group and a carboxyl group, which is an adsorption site to the semiconductor nanoparticles, and an aromatic skeleton portion. The aromatic skeleton has π electrons, which are more mobile than σ electrons, in a conjugated form, and therefore has carrier transportability because it is easy to transfer carriers such as holes and electrons. Examples of carrier transportability include hole transportability and electron transportability, and the carrier transporting ligand may be a hole transporting ligand having hole transportability, an electron transporting ligand having electron transportability, or a ligand having both hole transportability and electron transportability. Hereinafter, the ligand having both hole transportability and electron transportability is included in either a hole transporting ligand or an electron transporting ligand. Whether the carrier transportability is either hole transportability or electron transportability is determined by the aromatic skeleton and the ionization potential.
The hole transporting ligand and the electron transporting ligand will be described below.

[Hole-transporting ligand]
As described above, the hole transporting ligand has at least one functional group selected from the group consisting of a sulfanyl group and a carboxyl group, which is an adsorption site onto the surface of the semiconductor nanoparticle, and a hole transporting site having the function of transporting holes. The adsorption site and the hole transporting site do not need to be completely separate, and for example, part of the hole transporting site may serve as the adsorption site.

The hole transporting portion may be a portion having a structure consisting of a known hole transporting compound and its derivatives used as an organic electroluminescent device material, and is not particularly limited. Examples of the hole transporting portion include, but are not limited to, tertiary aromatic amines, thiophene oligomers, thiophene polymers, pyrrole oligomers, pyrrole polymers, phenylene vinylene oligomers, phenylene vinylene polymers, vinyl carbazole oligomers, vinyl carbazole polymers, fluorene oligomers, fluorene polymers, phenylene ethene oligomers, phenylene ethene polymers, phenylene oligomers, phenylene polymers, acetylene oligomers, acetylene polymers, phthalocyanines, phthalocyanine derivatives, porphyrins, and porphyrin derivatives. In addition, the following compounds and their derivatives can also be used as the hole transporting portion.

Furthermore, preferred structures of the hole transport ligand include those represented by the following general formula (6), general formula (7), or general formula (8).

General formula (6)

[In general formula (6), X ¹ is a hydrogen atom, an alkyl group which may have a substituent, an aryl group which may have a substituent, or an acyl group which may have a substituent, and R ¹ to R ⁸ are each independently a hydrogen atom, an alkyl group which may have a substituent, an aryl group which may have a substituent, an alkoxy group which may have a substituent, an aryloxy group which may have a substituent, an acyl group which may have a substituent, an amino group which may have a substituent, a (poly)organosiloxane group which may have a substituent, a carboxyl group, or a sulfanyl group, and at least one of X ¹ and R ¹ to R ⁸ is a group having a carboxyl group or a group having a sulfanyl group.]

General formula (7)

[In general formula (7), R ¹¹ to R ²⁵ each independently represent a hydrogen atom, an alkyl group which may have a substituent, an aryl group which may have a substituent, an alkoxy group which may have a substituent, an aryloxy group which may have a substituent, an acyl group which may have a substituent, a (poly)organosiloxane group which may have a substituent, a nitro group, an amino group which may have a substituent, a carboxyl group, or a sulfanyl group, and at least one of R ¹¹ to R ²⁵ is a group having a carboxyl group or a group having a sulfanyl group.]

General formula (8)

[In general formula (8), X ² is a hydrogen atom, an alkyl group which may have a substituent, an aryl group which may have a substituent (excluding a phenyl group), or an acyl group which may have a substituent, and R ²⁶ to R ³⁵ are each independently a hydrogen atom, an alkyl group which may have a substituent, an aryl group which may have a substituent, an alkoxy group which may have a substituent, an aryloxy group which may have a substituent, a (poly)organosiloxane group which may have a substituent, an amino group which may have a substituent, an acyl group which may have a substituent, a carboxyl group, or a sulfanyl group, and at least one of X ² and R ²⁶ to R ³⁵ is a group having a carboxyl group or a group having a sulfanyl group.]
The substituents in the above general formulas (6) to (8) will be described below.

The alkyl group may be linear, branched or cyclic, and may have some hydrogen atoms removed to form a double bond or a triple bond. In order to increase the content of semiconductor fine particles in the composition, it is preferable that the molecular weight of the treatment agent is small, and the alkyl group preferably has 1 to 30 carbon atoms.
Examples of the alkyl group include linear alkyl groups such as methyl, ethyl, hexyl, dodecyl, and eicosyl groups; branched alkyl groups such as 2-ethylhexyl groups; cyclic alkyl groups such as cyclohexyl and 3-cyclohexylpropyl groups; alkenyl groups such as ethenyl and dodecene groups; and alkynyl groups such as prop-2-yn-1- and propargyl groups.

The aryl group is a residue in which one hydrogen atom has been formally removed from an aromatic ring, and more preferably a residue in which one hydrogen atom has been formally removed from an aromatic ring having 6 to 30 carbon atoms.
Examples of the aromatic ring include a benzene ring, a naphthalene ring, a pyrene ring, a pyridine ring, a pyrimidine ring, a pyridazine ring, a pyrazine ring, a quinoline ring, a furan ring, a pyrrole ring, a thiophene ring, an imidazole ring, a pyrazole ring, an oxazole ring, a thiazole ring, an indole ring, a benzimidazole ring, a benzofuran ring, a purine ring, an acridine ring, a phenothiazine ring, a biphenyl group, and a terphenyl group.

An alkoxy group is a functional group to which an alkyl group is bonded via an ether bond, and the alkyl group has the same meaning as the alkyl group in the above general formulas (6) to (8).

An aryloxy group is a functional group to which an aryl group is bonded via an ether bond, and the aryl group has the same meaning as the aryl group in the above general formulas (6) to (8).

An acyl group is a functional group in which an alkyl group or an aryl group is bonded via a carbonyl group, and the alkyl group and the aryl group have the same meaning as the alkyl group and the aryl group in the above general formulas (6) to (8).

The (poly)organosiloxane group is a residue obtained by formally removing one hydrogen atom from a siloxane compound, and the silicon of the siloxane compound may be substituted with an alkyl group. The alkyl group that may be substituted has the same meaning as the alkyl group in the above general formulas (6) to (8). Examples of the siloxane compound include pentamethyldisiloxane, heptaethyltrisiloxane, undecamethyltetrasiloxane, trimethylsiloxydimethylsilanol, 1,1,3,3,tetramethyl-1,3-dihydroxydisiloxane, and 1,1,3,3,5,5-hexamethyl-1,5-dihydroxytrisiloxane.
Since this makes it possible to increase the content of semiconductor fine particles in the ink composition, it is preferable that the molecular weight of the ligand is small, and the number of repeating siloxane bonds is preferably 4 or less.

The amino group which may have a substituent includes an amino group and a substituted amino group, and examples of the substituent of the substituted amino group include an alkyl group, an aryl group, and an acyl group. The above alkyl group, aryl group, and acyl group are the same as the alkyl group, aryl group, and acyl group in the above general formulas (6) to (8).

In addition, the alkyl group, aryl group, alkoxy group, amino group, aryloxy group and acyl group in the general formulae (6) to (8) may have a substituent.
Examples of the substituent that may be possessed include a halogeno group, a cyano group, a hydroxy group, a nitro group, an alkoxy group, an alkyl group, an aralkyl group, an aryl group, an aryloxy group, a silyloxy group, a heterocyclic oxy group, an acyloxy group, a carbamoyloxy group, an alkoxycarbonyloxy group, an aryloxycarbonyloxy group, an amino group (including an anilino group), an acylamino group, an aminocarbonylamino group, an alkoxycarbonylamino group, an aryloxycarbonylamino group, a sulfamoylamino group, an alkyl and arylsulfonylamino group, a sulfanyl group, an alkylthio group, an arylthio group, a heterocyclic thio group, an alkyl and arylsulfinyl group, an alkyl and arylsulfonyl group, an acyl group, an aryloxycarbonyl group, an alkoxycarbonyl group, a carbamoyl group, an imido group, a phosphino group, a phosphinyl group, a phosphinyloxy group, a phosphinylamino group, a silyl group, or a group containing a siloxane bond.
The substituent that may be possessed in the present invention may be a monovalent group obtained by combining the above-mentioned monovalent substituent and, if necessary, an alkylene group, an alkenylene group, an arylene group, a heteroarylene group, an ether bond, an ester bond, a sulfide bond, a carbonyl group, an imino group, or a divalent linking group obtained by combining them.

The group having a carboxyl group or a sulfanyl group includes, in addition to the carboxyl group and the sulfanyl group, an alkyl group, an aryl group, an alkoxy group, an aryloxy group, an amino group, a (poly)organosiloxane group, or an acyl group having a carboxyl group or a sulfanyl group as a substituent. Here, the carboxyl group and the sulfanyl group, as well as the alkyl group, the aryl group, the alkoxy group, the aryloxy group, the (poly)organosiloxane group, the acyl group, or the nitro group having a carboxyl group or a sulfanyl group as a substituent, are the same as the groups in the above general formulae (6) to (8).

Examples of alkyl groups having a carboxyl group or aryl groups having a carboxyl group include, for example, 2-carboxyethyl groups, 2,2-dicarboxyethyl groups, 2-carboxylethenyl groups, 2-cyano-2-carboxylethenyl groups, 2-trifluoromethyl-2-carboxylethenyl groups, and 2-fluoro-2-carboxylethenyl groups.

Examples of alkyl groups having a sulfanyl group, aryl groups having a sulfanyl group, or acyl groups having a sulfanyl group include the [(3-sulfanylpropanoyl)oxy]ethyl group, the [(2-sulfanylacetyl)oxy]ethyl group, the 4-(sulfanylmethyl)phenyl group, the [(3-sulfanylpropanoyl)oxy]propyl group, the 3-sulfanyl[(3-sulfanylpropanoyl)oxy]ethyl group, and the {[(2-sulfanylethoxy)propanoyl]oxy}ethyl group.

Specific examples of ligands represented by general formulas (6) to (8) are shown below, but are not limited to these. In addition, in the following structural formulas, when cis- and trans-type geometric isomers exist, they may be either the cis type, the trans type, or a mixture of cis- and trans-type isomers. The same applies to syn-anti geometric isomerism.

[Electron transporting ligand]
As described above, the electron transport ligand has at least one functional group selected from the group consisting of a sulfanyl group and a carboxyl group, which is an adsorption site onto the surface of the semiconductor nanoparticle, and an electron transport site having the function of transporting electrons. The adsorption site and the electron transport site do not need to be completely separate, and for example, part of the electron transport site may serve as the adsorption site.

Examples of the electron transport moiety include moieties having a structure consisting of known electron transport compounds and derivatives thereof used as organic electroluminescent device materials, and are not particularly limited thereto. Examples include oxadiazole, oxadiazole derivatives, oxazole, oxazole derivatives, isoxazole, isoxazole derivatives, thiazole, thiazole derivatives, isothiazole, isothiazole derivatives, thiadiazole, thiadiazole derivatives, 1,2,3-triazole, 1,2,3-triazole derivatives, 1,3,5-triazine, 1,3,5-triazine derivatives, quinoxaline, quinoxaline derivatives, pyrrole oligomers, pyrrole polymers, phenylene vinylene oligomers, phenylene vinylene polymers, vinyl carbazole oligomers, vinyl carbazole polymers, fluorene oligomers, fluorene polymers, phenylene ethyne oligomers, phenylene ethyne polymers, phenylene oligomers, phenylene polymers, thiophene oligomers, thiophene polymers, acetylene polymers, and acetylene oligomer derivatives. Additionally, the following compounds and their derivatives can also be used as electron transport moieties.

The above-mentioned hole transporting moieties and electron transporting moieties contain the same structure, but this is because they can be either electron transporting moieties or hole transporting moieties. Whether a ligand behaves as a hole transporting or electron transporting moiety is determined by the relative HOMO-LUMO relationship between the hole transporting ligand and the electron transporting ligand, and the relative HOMO-LUMO relationship with other compounds used in the electroluminescent device, and even for a specific structure, the HOMO or LUMO changes depending on the presence of electron-withdrawing or electron-donating substituents. For the above reasons, some structures can be either hole transporting moieties or electron transporting moieties, and are therefore described as both.

In the hole transporting ligand and the electron transporting ligand, the hole transporting moiety and the adsorption moiety, or the electron transporting moiety and the adsorption moiety, may be directly bonded or may be bonded via a linking group such as an alkylene group. In this case, the alkylene group may be a branched or straight-chain alkylene group, and is preferably a straight-chain alkylene group. In addition, the alkylene group preferably has a carbon number of about 1 to 5, and specific examples include a methylene group, an ethylene group, an n-propylene group, an n-butylene group, and an n-pentylene group.

The carrier transporting ligand of the present invention preferably has a substituent shown in the following general formula (5).

General formula (5)

[In general formula (5), L ¹ is a direct bond or an alkylene group having 1 to 5 carbon atoms, L ² is an alkylene group having 1 to 5 carbon atoms, R ⁴⁸ is an alkyl group having 1 to 5 carbon atoms, and n is an integer of 0 to 4.]

Examples of the alkylene group having 1 to 5 carbon atoms in L ¹ and L ² in general formula (5) include a methylene group, an ethylene group, an n-propylene group, an isopropylene group, an n-butylene group, an isobutylene group, a sec-butylene group, a tert-butylene group, and an n-pentylene group. Among these, an alkylene group having 1 to 3 carbon atoms is preferred.
^L2 's contained in the repeating unit may be the same or different in the repeating structure.

In ^R48 of general formula (5), examples of the alkyl group having 1 to 5 carbon atoms include a methyl group, an ethyl group, an n-propyl group, an isopropyl group, an n-butyl group, an isobutyl group, a sec-butyl group, a tert-butyl group, and an n-pentyl group. Among these, an alkyl group having 1 to 3 carbon atoms is preferable.

<Solvent>
The ink composition of the present invention is characterized in that it contains at least one solvent selected from the following general formulas (1) to (4), and the total amount of the solvents of the general formulas (1) to (4) is 50 mass % or more based on the total amount of the solvents:
General formula (1): R ³⁶ CO(OR ³⁷ ) _p OR ³⁸
General formula (2): R ³⁹ CO(OR ⁴⁰ ) _q OCOR ⁴¹
General formula (3): R ⁴² (OR ⁴³ ) _r OR ⁴⁴
General formula (4): R ⁴⁵ (OR ⁴⁶ ) _s OR ⁴⁷
[In general formulas (1) to (4),
R ³⁶ , R ³⁸ , R ³⁹ , R ⁴¹ , R ⁴² and R ⁴⁴ each independently represent an alkyl group having 1 to 4 carbon atoms, R ³⁷ , R ⁴⁰ and R ⁴³ each independently represent an alkylene group having 2 to 4 carbon atoms, R ⁴⁵ and R ⁴⁷ each independently represent a hydrogen atom or an alkyl group having 1 to 6 carbon atoms, at least one of R ⁴⁵ and R ⁴⁷ is a hydrogen atom, R ⁴⁶ represents an alkylene group having 1 to 10 carbon atoms, and p to s each independently represent an integer of 1 to 6.]

By containing a predetermined amount or more of the solvent belonging to the above specific structure, the inkjet printability is excellent, and a smooth and defect-free light-emitting layer is obtained. In addition, these solvents hardly dissolve aromatic low molecular weight compounds or high molecular weight compounds, and furthermore, have excellent wettability to the lower layer containing an aromatic compound, so that even when the light-emitting layer is laminated on the lower layer containing an aromatic compound, the interface between the lower layer and the light-emitting layer is not disturbed, and the light-emitting layer can be formed in close contact with the lower layer. Therefore, the light-emitting layer and the electroluminescent element formed by using the ink composition of the present invention can reduce the driving voltage without impeding the injection of electric energy into the quantum dots, and damage to the lower layer due to lamination is suppressed, so that they show excellent light-emitting efficiency and durability.

Specific examples of solvents that fall under the general formula (1) include ethylene glycol monomethyl ether acetate, ethylene glycol monoethyl ether acetate, ethylene glycol monobutyl ether acetate, diethylene glycol monomethyl ether acetate, diethylene glycol monoethyl ether acetate, diethylene glycol monobutyl ether acetate, propylene glycol monomethyl ether acetate, dipropylene glycol monomethyl ether acetate, ethylene glycol monomethyl ether propionate, ethylene glycol monoethyl ether propionate, ethylene glycol monobutyl ether propionate, diethylene glycol monomethyl ether propionate, diethylene glycol Examples of glycol monoacetates include glycol monoethyl ether propionate, diethylene glycol monobutyl ether propionate, propylene glycol monomethyl ether propionate, dipropylene glycol monomethyl ether propionate, ethylene glycol monomethyl ether butyrate, ethylene glycol monoethyl ether butyrate, ethylene glycol monobutyl ether butyrate, diethylene glycol monomethyl ether butyrate, diethylene glycol monoethyl ether butyrate, diethylene glycol monobutyl ether butyrate, propylene glycol monomethyl ether butyrate, dipropylene glycol monomethyl ether butyrate, and methoxybutyl acetate.

Specific examples of solvents that fall under the general formula (2) include ethylene glycol diacetate, diethylene glycol diacetate, propylene glycol diacetate, dipropylene glycol diacetate, ethylene glycol acetate propionate, ethylene glycol acetate butyrate, ethylene glycol propionate butyrate, ethylene glycol dipropionate, ethylene glycol acetate dibutyrate, diethylene glycol acetate propionate, diethylene glycol acetate butyrate, diethylene glycol propionate butyrate, and diethylene glycol di Examples of glycol diacetates include propionate, diethylene glycol acetate dibutyrate, propylene glycol acetate propionate, propylene glycol acetate butyrate, propylene glycol propionate butyrate, propylene glycol dipropionate, propylene glycol acetate dibutyrate, dipropylene glycol acetate propionate, dipropylene glycol acetate butyrate, dipropylene glycol propionate butyrate, dipropylene glycol dipropionate, and dipropylene glycol acetate dibutyrate.

Specific examples of solvents that fall under the general formula (3) include glycol dialkyl ethers such as ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, diethylene glycol methyl ethyl ether, diethylene glycol methyl butyl ether, triethylene glycol dimethyl ether, triethylene glycol diethyl ether, triethylene glycol methyl butyl ether, tetraethylene glycol dimethyl ether, dipropylene glycol dimethyl ether, dipropylene glycol diethyl ether, and tripropylene glycol dimethyl ether.

Specific examples of the solvent represented by the general formula (4) include ethylene glycol monomethyl ether, ethylene glycol monobutyl ether, ethylene glycol monoisopropyl ether, ethylene glycol monohexyl ether, ethylene glycol mono 2-ethylhexyl ether, propylene glycol monomethyl ether (1-methoxy-2-propanol), propylene glycol monobutyl ether, propylene glycol n-propyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monobutyl ether, diethylene glycol monoisopropyl ether, diethylene glycol monohexyl ether, diethylene glycol mono 2-ethylhexyl ether, and dipropylene glycol monomethyl ether. glycol monoalkyl ethers such as dipropylene glycol monoethyl ether, dipropylene glycol n-propyl ether, dipropylene glycol monobutyl ether, triethylene glycol monomethyl ether, triethylene glycol monoethyl ether, triethylene glycol monobutyl ether, tripropylene glycol monomethyl ether, tripropylene glycol monobutyl ether, 3-methoxy-3-methylbutanol, and 3-methoxybutanol; and alkanediols such as ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, dipropylene glycol, 1,3-butylene glycol, 1,2-hexanediol, 2-ethyl-1,3-hexanediol, and 2,5-dimethylhexanediol.

In the present invention, the solvent may contain a conventionally known solvent other than those represented by the general formulas (1) to (4). Such a solvent is not particularly limited, but examples thereof include aromatic hydrocarbons such as toluene, o-xylene, m-xylene, p-xylene, methylene, cyclohexylbenzene, and tetralin; halogenated aromatic hydrocarbons such as chlorobenzene, dichlorobenzene, and trichlorobenzene; aromatic ethers such as 1,2-dimethoxybenzene, 1,3-dimethoxybenzene, anisole, phenetole, 2-methoxytoluene, 3-methoxytoluene, 4-methoxytoluene, 2,3-dimethylanisole, and 2,4-dimethylanisole; and aromatic ethers such as phenyl acetate, phenyl propionate, methyl benzoate, ethyl benzoate, propyl benzoate, and n-butyl benzoate. alicyclic alcohols such as methyl ethyl ketone, cyclohexanone, cyclooctanone, etc.; aliphatic alcohols such as butanol, hexanol, etc.; aliphatic esters such as ethyl acetate, n-butyl acetate, ethyl lactate, n-butyl lactate, etc.; and aliphatic hydrocarbons such as n-hexane, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane, 2,3-dimethylbutane, n-heptane, n-decane, 2,2,4-trimethylpentane, cyclohexane, methylcyclohexane, mineral spirits, etc.
These solvents may be used alone or in combination of two or more.

The ink composition of the present invention may contain other known luminescent materials and known additives, as described below, within the scope of not impairing the effects of the present invention. Known additives are not particularly limited, but examples thereof include defoamers, leveling agents, and thickeners.

Furthermore, the ink composition of the present invention can be suitably used as an inkjet ink, particularly in the inkjet method.
When the ink composition of the present invention is used as an inkjet ink, the viscosity of the inkjet ink at 25°C is preferably 2 to 100 mPa·s, more preferably 2 to 40 mPa·s, and even more preferably 4 to 20 mPa·s. This viscosity range allows stable ejection characteristics to be exhibited from heads having a normal frequency of 4 to 10 KHz to high-frequency heads of 10 to 50 KHz. A viscosity of 2 mPa·s or more is preferred because no decrease in ejection followability is observed in high-frequency heads. A viscosity of 100 mPa·s or less is preferred because the ejection properties are good and the ejection stability is excellent.

When the ink composition is used in an inkjet system, the surface tension of the inkjet ink is preferably 20 to 40 mN/m, more preferably 24 to 35 mN/m. If it is 20 mN/m or more, the ink can form droplets, and if it is 40 mN/m or less, the ink can be stably ejected from the head.

In addition, for piezo heads, the ink composition preferably has an electrical conductivity of 10 μS/cm or less, and is an ink that does not cause electrical corrosion inside the head. In addition, for continuous types, it is necessary to adjust the electrical conductivity using an electrolyte, and in this case it is preferable to adjust the electrical conductivity to 0.5 mS/cm or more.

<Light-emitting layer>
The light-emitting layer of the present invention can be formed using the above-mentioned ink composition of the present invention.
The method for forming the light-emitting layer is preferably a wet film formation method, such as a coating method, an inkjet method, a dip coating method, a die coating method, a spray coating method, a spin coating method, a roll coater method, a wet coating method, a screen printing method, a flexographic printing method, a screen printing method, or an LB method.

The thickness of the light-emitting layer is preferably 1 to 500 nm, more preferably 5 to 100 nm, and even more preferably 5 to 50 nm. If it is more than 500 nm, it becomes difficult to pass electricity through it, and if it is less than 1 nm, the quantum dots become almost sparse and do not function as a light-emitting layer.

The light-emitting layer is preferably smooth, since it can provide an electroluminescent device with no uneven light emission and excellent durability. For example, it is preferable that there are no aggregates of quantum dots in the film. The parameter indicating the smoothness is arithmetic mean roughness Ra, which is also an index of surface roughness. The Ra of the light-emitting layer of the present invention is preferably 50 nm or less, more preferably 10 nm or less. The device for measuring Ra is DECTAC6M (manufactured by Veeco), which is a stylus-type surface shape measuring device, and the like.
Furthermore, the quantum dot particles forming the light-emitting layer may be arranged in a self-organized manner, and such a film is preferable because it has excellent carrier transport properties.
Furthermore, the ionization potential of the light-emitting layer is preferably 5.0 to 6.5 eV. Outside this range, it becomes difficult to efficiently retain and recombine carriers in the light-emitting layer, and the light-emitting efficiency is likely to decrease. The ionization potential can be measured by photoelectron yield spectroscopy or the like, for example, using PYS-202 manufactured by Sumitomo Heavy Industries Mechatronics Co., Ltd.

<Electroluminescent Device>
The electroluminescence device of the present invention has a structure in which an anode, the above-mentioned light-emitting layer of the present invention, and a cathode are laminated in this order on a substrate.
For the purpose of facilitating the injection of holes or electrons into the light-emitting layer or facilitating the recombination of holes and electrons in the light-emitting layer, it is preferable that the light-emitting layer further has a hole injection layer, a hole transport layer, a hole blocking layer, an electron injection layer, etc. In addition, an interlayer layer may be inserted between the light-emitting layer and the anode, adjacent to the light-emitting layer, and serving to isolate the light-emitting layer from the anode, or the light-emitting layer from the hole injection layer or the hole transport layer.
Therefore, representative preferred element configurations are: (1) anode/hole injection layer/light emitting layer/cathode; (2) anode/hole injection layer/hole transport layer/light emitting layer/cathode; (3) anode/hole injection layer/light emitting layer/electron injection layer/cathode; (4) anode/hole injection layer/hole transport layer/light emitting layer/electron injection layer/cathode; (5) anode/hole injection layer/light emitting layer/hole blocking layer/electron injection layer/cathode; (6) anode/hole injection layer/hole transport layer/light emitting layer/cathode (7) anode/light-emitting layer/hole-blocking layer/electron injection layer/cathode, (8) anode/light-emitting layer/electron injection layer/cathode, (9) anode/hole injection layer/hole transport layer/interlayer layer/light-emitting layer/cathode, (10) anode/hole injection layer/interlayer layer/light-emitting layer/electron injection layer/cathode, (11) anode/hole injection layer/hole transport layer/interlayer layer/light-emitting layer/electron injection layer/cathode, and the like.

Each of the above-mentioned layers such as the hole injection layer, the hole transport layer, the light emitting layer, the hole blocking layer, the electron injection layer, and the interlayer layer may be formed by a layer structure of two or more layers, or several layers may be repeatedly laminated.As an example of such a structure, in recent years, there is an element structure called "multi-photon emission" in which some layers of the above-mentioned multi-layered electroluminescence are multi-layered in order to improve the light extraction efficiency, and for example, a method of forming a plurality of electroluminescence elements by stacking them in series using a charge generation layer is included.

The electroluminescent device of the present invention preferably has a configuration in which an anode, a hole transport layer, the above-mentioned light-emitting layer of the present invention, and a cathode are laminated in this order, and the light-emitting layer is provided in contact with the hole transport layer.

[Hole transport layer and hole injection layer]
The hole transport layer is made of a material that exhibits excellent hole injection and hole transport effects for the light emitting layer and has excellent thin film formability. As such a material, a material having high hole mobility and small ionization energy, typically 5.5 eV or less, is preferably used.
The hole transport layer is provided between the anode and the light emitting layer, and may have a multi-layer structure in which a plurality of hole transport materials are laminated. In the case of a multi-layer structure, the hole transport layer in contact with the anode may be particularly called the hole injection layer, and each material may be particularly called the hole injection material or the hole transport material. For the hole injection layer, a material having excellent adhesion to the anode interface is used.
The hole transport layer is preferably a material that transports holes to the light emitting layer at a lower electric field strength, and more preferably has a hole mobility of at least 10 ^-6 cm ² /V·sec when an electric field of, for example, 10 ⁴ to 10 ⁶ V/cm is applied. The hole injection material and hole transport material are not particularly limited as long as they have the above-mentioned preferable properties, and any material may be selected from those conventionally used as charge transport materials for holes in photoconductive materials and known materials used in hole transport layers or hole injection layers of organic EL devices.

Examples of such hole injection materials and hole transport materials include triazole derivatives, oxadiazole derivatives, imidazole derivatives, polyarylalkane derivatives, pyrazoline derivatives and pyrazolone derivatives, phenylenediamine derivatives, arylamine derivatives, amino-substituted chalcone derivatives, oxazole derivatives, styrylanthracene derivatives, fluorenone derivatives, hydrazone derivatives, stilbene derivatives, silazane derivatives, polysilane-based and aniline-based copolymers, aromatic dimethylidene-based compounds, conductive polymer oligomers, and in particular thiophene oligomers.

Preferred examples of hole injection materials and hole transport materials include porphyrin compounds, aromatic tertiary amine compounds, and styrylamine compounds, such as 4,4'-bis(N-(1-naphthyl)-N-phenylamino)biphenyl, which has two condensed aromatic rings in the molecule, and 4,4',4"-tris(N-(3-methylphenyl)-N-phenylamino)triphenylamine, which has three triphenylamine units linked in a starburst configuration. Preferred hole injection materials include phthalocyanine derivatives such as copper phthalocyanine and hydrogen phthalocyanine.

Materials that can be used for the hole transport layer or hole injection layer include inorganic semiconductor compounds such as p-type Si and p-type SiC, and inorganic oxides such as molybdenum oxide (MnO _x ), vanadium oxide (VO _x ), ruthenium oxide (RuO _x ), copper oxide (CuO _x ), tungsten oxide (WO _x ), iridium oxide (IrO _x ), and doped versions thereof.

Specific examples of aromatic tertiary amine derivatives include N,N'-diphenyl-N,N'-(3-methylphenyl)-1,1'-biphenyl-4,4'-diamine, N,N,N',N'-(4-methylphenyl)-1,1'-phenyl-4,4'-diamine, N,N,N',N'-(4-methylphenyl)-1,1'-biphenyl-4,4'-diamine, N
,N'-diphenyl-N,N'-dinaphthyl-1,1'-biphenyl-4,4'-diamine, N,N'-(methylphenyl)-N,N'-(4-n-butylphenyl)-phenanthrene-9,10-diamine, N,N-bis(4-di-4-tolylaminophenyl)-4-phenyl-cyclohexane, N,N'-bis(4'-diphenylamino-4-biphenylyl)-N,N'-diphenylbenzidine, N,N'-bis(4'-diphenylamino-4-phenyl)- Examples thereof include N,N'-diphenylbenzidine, N,N'-bis(4'-diphenylamino-4-phenyl)-N,N'-di(1-naphthyl)benzidine, N,N'-bis(4'-phenyl(1-naphthyl)amino-4-phenyl)-N,N'-diphenylbenzidine, and N,N'-bis(4'-phenyl(1-naphthyl)amino-4-phenyl)-N,N'-di(1-naphthyl)benzidine, and these can be used as either a hole injection material or a hole transport material.

Particularly preferred examples of hole injection materials and hole transport materials are shown in Tables 1 and 2.

The hole transport layer or hole injection layer can be formed by forming a thin film of the above-mentioned compound using a known method such as vacuum deposition, spin coating, casting, or the Langmuir-Blodgett method (LB method). There is no particular limit to the total thickness of the hole transport layer and hole injection layer, but it is usually 5 nm to 5 μm.

Examples of materials used for the interlayer layer include polyvinylcarbazole and its derivatives, polyarylene derivatives having aromatic amines in the side chain or main chain, arylamine derivatives, triphenyldiamine derivatives, and other polymers containing aromatic amines. In addition, examples of the method for forming the interlayer layer include a method of forming a film from a solution when a high molecular weight material is used.

The interlayer layer can be formed from a solution by a known wet film formation method, such as a coating method such as spin coating, casting, microgravure coating, gravure coating, bar coating, roll coating, wire bar coating, dip coating, spray coating, screen printing, flexographic printing, offset printing, inkjet printing, capillary coating, or nozzle coating.

The optimum thickness of the interlayer layer varies depending on the material used, and can be selected so that the driving voltage and luminous efficiency are adequate. It is usually 1 nm to 1 μm, preferably 2 to 500 nm, and more preferably 5 to 200 nm.

[Electron injection layer]
For the electron injection layer, an electron injection material is used that can form an electron injection layer that exhibits excellent electron injection effect for the light emitting layer and has excellent adhesion to the cathode interface and excellent thin film formability. Examples of such electron injection materials include metal complex compounds, nitrogen-containing five-membered ring derivatives, fluorenone derivatives, anthraquinodimethane derivatives, diphenoquinone derivatives, thiopyran dioxide derivatives, perylene tetracarboxylic acid derivatives, fluorenylidenemethane derivatives, anthrone derivatives, silole derivatives, triarylphosphine oxide derivatives, polyquinoline and its derivatives, polyquinoxaline and its derivatives, polyfluorene and its derivatives, calcium acetylacetonate, sodium acetate, etc. In addition, inorganic/organic composite materials in which a metal such as cesium is doped into bathophenanthroline (Proceedings of the Society of Polymer Science, Vol. 50, No. 4, p. 660, published in 2001) and Proceedings of the 50th Joint Conference of Applied Physics, No. No. 3, p. 1402, published in 2003, BCP, TPP, T5MPyTZ, and the like are given as examples of the electron injection material, but the material is not particularly limited to these, so long as it can form a thin film necessary for element production, can inject electrons from the cathode, and can transport electrons.

Among the above electron injection materials, preferred are metal complex compounds, nitrogen-containing five-membered ring derivatives, silole derivatives, and triarylphosphine oxide derivatives. Preferred metal complex compounds are metal complexes of 8-hydroxyquinoline or its derivatives. Specific examples of metal complexes of 8-hydroxyquinoline or its derivatives include tris(8-hydroxyquinolinato)aluminum, tris(2-methyl-8-hydroxyquinolinato)aluminum, tris(4-methyl-8-hydroxyquinolinato)aluminum, tris(5-methyl-8-hydroxyquinolinato)aluminum, tris(5-phenyl-8-hydroxyquinolinato)aluminum, bis(8-hydroxyquinolinato)(1-naphtholate)aluminum, bis(8-hydroxyquinolinato)(2-naphtholate)aluminum, bis(8-hydroxyquinolinato)(phenolate)aluminum, and bis(8-hydroxyquinolinato)(4-cyano-1-naphtholate)aluminum. aluminum complex compounds such as bis(4-methyl-8-hydroxyquinolinato)(1-naphtholate)aluminum, bis(5-methyl-8-hydroxyquinolinato)(2-naphtholate)aluminum, bis(5-phenyl-8-hydroxyquinolinato)(phenolate)aluminum, bis(5-cyano-8-hydroxyquinolinato)(4-cyano-1-naphtholate)aluminum, bis(8-hydroxyquinolinato)chloroaluminum, and bis(8-hydroxyquinolinato)(o-cresolate)aluminum; tris(8-hydroxyquinolinato)gallium, tris(2-methyl-8-hydroxyquinolinato)gallium, tris(4-methyl-8-hydroxyquinolinato) gallium, tris(5-methyl-8-hydroxyquinolinato)gallium, tris(2-methyl-5-phenyl-8-hydroxyquinolinato)gallium, bis(2-methyl-8-hydroxyquinolinato)(1-naphtholate)gallium, bis(2-methyl-8-hydroxyquinolinato)(2-naphtholate)gallium, bis(2-methyl-8-hydroxyquinolinato)(phenolate)gallium, bis(2-methyl-8-hydroxyquinolinato)(4-cyano-1-naphtholate)gallium, bis(2,4-dimethyl-8-hydroxyquinolinato)(1-naphtholate)gallium, bis(2,5-dimethyl-8-hydroxyquinolinato)(2-naphtholate)gallium, bis(2-methyl gallium complex compounds such as bis(2-methyl-8-hydroxyquinolinato)(o-cresolate)gallium, bis(2-methyl-8-hydroxyquinolinato)(5-phenyl-8-hydroxyquinolinato)(phenolate), bis(2-methyl-5-cyano-8-hydroxyquinolinato)(4-cyano-1-naphtholate)gallium, bis(2-methyl-8-hydroxyquinolinato)chlorogallium, and bis(2-methyl-8-hydroxyquinolinato)(o-cresolate)gallium; and other metal complex compounds such as 8-hydroxyquinolinato lithium, bis(8-hydroxyquinolinato)copper, bis(8-hydroxyquinolinato)manganese, bis(10-hydroxybenzo[h]quinolinato)beryllium, bis(8-hydroxyquinolinato)zinc, and bis(10-hydroxybenzo[h]quinolinato)zinc.

Preferable nitrogen-containing five-membered ring derivatives include oxazole derivatives, thiazole derivatives, oxadiazole derivatives, thiadiazole derivatives, and triazole derivatives. Specific examples thereof include 2,5-bis(1-phenyl)-1,3,4-oxazole, 2,5-bis(1-phenyl)-1,3,4-thiazole, 2,5-bis(1-phenyl)-1,3,4-oxadiazole, 2-(4'-tert-butylphenyl)-5-(4"-biphenyl)1,3,4-oxadiazole, 2,5-bis(1-naphthyl)-1,3,4-oxadiazole, 1,4-bis[2-(5-phenyloxadiazolyl)]benzene, 1,
Examples of such bis[2-(5-phenyloxadiazolyl)-4-tert-butylbenzene] include 4-bis[2-(5-phenyloxadiazolyl)-4-tert-butylbenzene], 2-(4'-tert-butylphenyl)-5-(4"-biphenyl)-1,3,4-thiadiazole, 2,5-bis(1-naphthyl)-1,3,4-thiadiazole, 1,4-bis[2-(5-phenylthiadiazolyl)]benzene, 2-(4'-tert-butylphenyl)-5-(4"-biphenyl)-1,3,4-triazole, 2,5-bis(1-naphthyl)-1,3,4-triazole, and 1,4-bis[2-(5-phenyltriazolyl)]benzene.

Additionally, materials that can be used for the electron injection layer include inorganic oxides such as zinc oxide (ZnO _x ), titanium oxide (TiO _x ), and doped versions thereof.

Specific examples of particularly preferred oxadiazole derivatives, triazole derivatives, and silole derivatives are shown in Tables 3 to 5. In the tables, Ph represents a phenyl group.

(Oxadiazole Derivatives)

(Triazole Derivatives)

(Silole derivatives)

Furthermore, as a type of electron injection layer, there is a hole blocking layer that prevents holes that have passed through the light emitting layer from reaching the electron injection layer and has excellent thin film forming properties. For this, a hole blocking material is used that has significantly inferior hole transport properties compared to electron transport properties. Examples of such hole blocking materials include aluminum complex compounds such as bis(8-hydroxyquinolinato)(4-phenylphenolato)aluminum, gallium complex compounds such as bis(2-methyl-8-hydroxyquinolinato)(4-phenylphenolato)gallium, and nitrogen-containing condensed aromatic compounds such as 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP).

The light-emitting layer of the electroluminescent device preferably has the following functions:
Injection function: the ability to inject holes from the anode or hole injection layer and to inject electrons from the cathode or electron injection layer when an electric field is applied. Transport function: the ability to move injected charges (electrons and holes) by the force of an electric field. Light-emitting function: the ability to provide a site for the recombination of electrons and holes, leading to light emission. However, there may be differences in the ease with which holes and electrons are injected, and there may be differences in the transport ability, which is represented by the mobility of holes and electrons.

The light-emitting layer constituting the electroluminescent device of the present invention is formed from the ink composition of the present invention containing quantum dots as a light-emitting material, and may further contain a known light-emitting material, such as a benzothiazole-based, benzimidazole-based, or benzoxazole-based fluorescent brightener, a metal chelated oxinoid compound, or a styrylbenzene-based compound.
Examples of the metal chelated oxinoid compounds include 8-hydroxyquinoline-based metal complexes such as tris(8-quinolinol)aluminum, and dilithium epinetridione.

When a known light-emitting material is further added to the light-emitting layer of the present invention, or a light-emitting layer using a known light-emitting material is further added to the light-emitting layer of the present invention, the structure of the element is not particularly limited, but the following can be used. An element in which the energy level of each layer of the laminated structure of the electroluminescent element is specified and light is emitted using tunnel injection (European Patent No. 0390551). An element that also uses tunnel injection and describes a white light-emitting element as an example (Japanese Patent Laid-Open No. 3-230584). An element in which a two-layer light-emitting layer is described (Japanese Patent Laid-Open Nos. 2-220390 and 2-216790). An element in which the light-emitting layer is divided into multiple parts and each part is composed of a material with a different emission wavelength (Japanese Patent Laid-Open No. 4-51491). A configuration in which a blue light-emitting body (fluorescence peak 380-480 nm) and a green light-emitting body (480-580 nm) are laminated together, and further containing a red phosphor (JP Patent Publication No. 6-207170). A configuration in which the blue light-emitting layer contains a blue fluorescent dye, and the green light-emitting layer has a region containing a red fluorescent dye, and further contains a green phosphor (JP Patent Publication No. 7-142169).

[anode]
The material used for the anode is preferably a metal, alloy, electrically conductive compound, or a mixture thereof having a large work function (4 eV or more). Specific examples of such electrode materials include metals such as Au, and conductive materials such as CuI, ITO, SnO ₂ , and ZnO. To form the anode, these electrode materials can be formed into a thin film by a method such as deposition or sputtering. When light emitted from the light-emitting layer is extracted from the anode, it is desirable for the anode to have a characteristic such that the transmittance of the anode for light emission is greater than 10%. In addition, the sheet resistance of the anode is preferably several hundred Ω/□ or less. The thickness of the anode is usually selected within a range of 10 nm to 1 μm, preferably 10 to 200 nm, depending on the material.

[cathode]
The material used for the cathode is a metal, alloy, electrically conductive compound, or a mixture thereof having a small work function (4 eV or less). Specific examples of such electrode materials include sodium, sodium-potassium alloy, magnesium, lithium, magnesium-silver alloy, aluminum/aluminum oxide, aluminum-lithium alloy, indium, and rare earth metal. The cathode can be produced by forming a thin film of these electrode materials by a method such as vapor deposition or sputtering. Here, when light emitted from the light-emitting layer is taken out from the cathode, the transmittance of the cathode for light emission is preferably greater than 10%. In addition, the sheet resistance of the cathode is preferably several hundred Ω/□ or less, and the thickness of the cathode is usually 10 nm to 1 μm, preferably 50 to 200 nm.

The method for producing an electroluminescent device is to form an anode, a light-emitting layer, a hole injection layer if necessary, and an electron injection layer if necessary, using the above-mentioned materials and methods, and finally to form a cathode. It is also possible to produce an electroluminescent device in the reverse order from the cathode to the anode.

The electroluminescent element is fabricated on a light-transmitting substrate. The light-transmitting substrate is a substrate that supports the electroluminescent element, and its light transmittance in the visible region of 400 to 700 nm is desirably 50% or more, preferably 90% or more, and it is further preferable to use a smooth substrate.

These substrates are not particularly limited as long as they have mechanical and thermal strength and are transparent, and may be flexible film or sheet-like substrates. For example, glass plates and synthetic resin plates are preferably used as substrates. Examples of glass plates include plates made of soda-lime glass, barium-strontium-containing glass, lead glass, aluminosilicate glass, borosilicate glass, barium borosilicate glass, quartz, etc. Examples of synthetic resin plates include plates made of polycarbonate resin, acrylic resin, polyethylene terephthalate resin, polyether sulfide resin, polysulfone resin, etc.

The layers of the electroluminescent device can be formed by dry deposition methods such as vacuum deposition, electron beam irradiation, sputtering, plasma, and ion plating, or wet deposition methods such as spin coating, dipping, and flow coating. Laser Induced Thermal Imaging (LITI), printing (offset printing, flexographic printing, gravure printing, and screen printing), inkjet printing, and other methods can also be used. However, the light-emitting layer is preferably formed by a wet deposition method.

In addition, organic layers formed of organic compounds other than the anode, cathode and light-emitting layer, such as the hole injection layer, hole transport layer or electron injection layer, are preferably films formed by molecular deposition. Here, the film formed by molecular deposition refers to a thin film formed by deposition from a material compound in a gas phase state or a film formed by solidifying a material compound in a solution or liquid phase state. Usually, this molecular deposition film can be distinguished from a thin film (molecularly accumulated film) formed by the LB method by differences in aggregation structure, higher-order structure and functional differences resulting therefrom. In addition, an organic layer can also be formed by dissolving a binder such as a resin and a material compound in a solvent to form a solution, and then thinning the solution by a spin coating method or the like. The thickness of each layer is not particularly limited, but if the thickness is too thick, a large applied voltage is required to obtain a certain light output, resulting in poor efficiency, and conversely, if the thickness is too thin, pinholes and the like are generated, making it difficult to obtain sufficient light emission brightness even when an electric field is applied. Therefore, the thickness of each layer is suitable to be in the range of 1 nm to 1 μm, but more preferably in the range of 10 nm to 0.2 μm.

In addition, in order to improve the stability of the electroluminescent element against temperature, humidity, atmosphere, etc., a protective layer may be provided on the surface of the element, or the entire element may be covered or sealed with a resin or the like. In particular, when covering or sealing the entire element, a photocurable resin that hardens when exposed to light is preferably used.

The current applied to the electroluminescent element of the present invention is usually direct current, but pulse current or alternating current may also be used. There are no particular limitations on the current and voltage values as long as they are within a range that does not destroy the element, but considering the power consumption and lifespan of the element, it is desirable to emit light efficiently with as little electrical energy as possible.

The electroluminescent element of the present invention can be driven by not only the passive matrix method but also the active matrix method. In addition, the method of extracting light from the electroluminescent element of the present invention can be applied not only to bottom emission, in which light is extracted from the anode side, but also to top emission, in which light is extracted from the cathode side.

The main methods of full coloring of the electroluminescent element of the present invention include a three-color coating method, a color conversion method, and a color filter method. The three-color coating method includes a deposition method using a shadow mask, an inkjet method, and a printing method. A laser thermal transfer method (also called a Laser Induced Thermal Imaging, LITI method) can also be used. The color conversion method is a method in which a blue-emitting light-emitting layer is used to convert the light into green and red, which have longer wavelengths than blue, through a color conversion (CCM) layer in which a fluorescent dye is dispersed. The color filter method is a method in which a white-emitting organic EL element is used to extract three primary colors of light through a liquid crystal color filter, but in addition to these three primary colors, a part of the white light is extracted as it is and used for light emission, thereby increasing the luminous efficiency of the entire element.

Furthermore, electroluminescent elements may adopt a microcavity structure. This is a structure in which an electroluminescent layer is sandwiched between an anode and a cathode, and the emitted light causes multiple interference between the anode and the cathode. However, by appropriately selecting the optical characteristics such as the reflectance and transmittance of the anode and the cathode, and the film thickness of the organic and/or inorganic layer sandwiched between them, the multiple interference effect can be actively utilized to control the emission wavelength extracted from the element. This also makes it possible to improve the chromaticity of the emitted light.

<Production of quantum dot dispersion>
(Quantum dot dispersion: QD-1)
1.35 parts of indium acetate, 1.30 parts of zinc octanoate, 4.65 parts of stearic acid, and 92.7 parts of 1-octadecene were placed in a flask and heated to 100° C. Then, the contents were distilled off under reduced pressure at that temperature.
Next, the flask was filled with argon gas and heated to 300° C. 23 parts of a hexane solution containing 1.16 parts of tris(trimethylsilyl)phosphine, which had been separately prepared in a glove box without water, was poured into the flask, and the mixture was reacted at 280° C. for 9 minutes, followed by quenching to obtain a solution.
15 parts of the obtained solution, 46 parts of 1-octadecene, and 11.5 parts of oleylamine were placed in a flask and stirred for 15 minutes, after which 4.3 parts of zinc dibenzyldithiocarbamate was added. After the atmosphere in the flask was replaced with argon, the temperature was raised to 170°C over 45 minutes and the temperature was maintained at 170°C for 2 hours. After that, the mixture was cooled, acetone was added, and a precipitated paste was obtained by centrifugation.
The resulting precipitated paste was dispersed in toluene, 3.7 parts of dodecanethiol was added, and the mixture was stirred at room temperature for 24 hours. After concentrating under reduced pressure, acetone was added, and the mixture was centrifuged to obtain a precipitated paste.
The precipitated paste was then dispersed in toluene to a solid content of 10%, to obtain a quantum dot dispersion QD-1 that was surface-treated with dodecanethiol. The semiconductor particles contained in the quantum dots had an InP core and a ZnS shell.

(Quantum dot dry product: QDS-1)
The quantum dot dispersion liquid QD-1 was diluted with toluene to a solid concentration of 0.5%. Next, a 1.0% toluene solution of the compound (a) shown in Table 6, which is a hole transport ligand, was added to the quantum dot dispersion liquid QD-1 in an amount of 5 times the amount, and the mixture was stirred for 12 hours. After purification by reprecipitation using hexane and ethanol, the mixture was dried under reduced pressure to obtain a dried quantum dot QDS-1 surface-treated with the compound (a), which is a hole transport ligand.

(Quantum dot dry matter: QDS-2 to 8)
Quantum dot dry products QDS-2 to 8 were obtained in the same manner as QDS-1, except that compounds (b) to (h) shown in Table 6 were used instead of compound (a).

<Production of Ink Composition>
[Example 1]: Ink composition 1
The quantum dot dried product QDS-1 was dispersed in ethylene glycol monobutyl ether acetate (hereinafter, BGAc) to a solid concentration of 0.1%, and filtered through a 1 μm filter to obtain an ink composition 1.

[Examples 2 to 7]: Ink compositions 2 to 7
Ink compositions 2 to 7 were obtained in the same manner as ink composition 1, except that the quantum dot dried materials QDS-2 to 7 were used instead of the quantum dot dried material QDS-1.

[Example 8]: Ink composition 8
Ink composition 8 was obtained in the same manner as ink composition 1, except that diethylene glycol diethyl ether (hereinafter, DEDG) was used instead of BGAc.

[Example 9]: Ink composition 9
Ink composition 9 was obtained in the same manner as ink composition 1, except that a mixed solvent of DEDG and triethylene glycol monoethyl ether (hereinafter, ETG) in a mass ratio of 9:1 was used instead of BGAc.

[Example 10]: Ink composition 10
Ink composition 10 was obtained in the same manner as ink composition 1, except that a mixed solvent of BGAc and propylene glycol diacetate (hereinafter, PGDA) in a mass ratio of 8:2 was used instead of BGAc.

[Example 11]: Ink composition 11
Ink composition 11 was obtained in the same manner as ink composition 1, except that a mixed solvent of BGAc and toluene in a mass ratio of 5:5 was used instead of BGAc.

[Comparative Example 1]: Ink composition 12
The quantum dot dispersion QD-1 was diluted with toluene to a solids concentration of 0.1% to obtain ink composition 12.

[Comparative Example 2]: Ink composition 13
Ink composition 12 was obtained in the same manner as ink composition 1, except that toluene was used instead of BGAc.

[Comparative Example 3]: Ink composition 14
Ink composition 14 was obtained in the same manner as ink composition 1, except that the quantum dot dried material QDS-8 was used instead of the quantum dot dried material QDS-1.

[Comparative Example 4]: Ink composition 15
Ink composition 15 was obtained in the same manner as ink composition 1, except that a mixed solvent of BGAc and toluene in a mass ratio of 4:6 was used instead of BGAc.

<Evaluation of Ink Composition>
The resulting ink compositions were subjected to the following evaluations, and the results are shown in Table 7.

(Evaluation of electroluminescence device performance)
The preparation and evaluation of the electroluminescence device will be described below. The deposition was carried out in a vacuum of 10 ⁻⁶ Torr without temperature control such as heating or cooling of the substrate. The device was evaluated using an electroluminescence device with a light-emitting element area of 2 mm×2 mm.
First, PEDOT/PSS (poly(3,4-ethylenedioxy)-2,5-thiophene/polystyrenesulfonic acid, CLEVIOS (registered trademark) P VP CH8000 manufactured by Heraeus) was applied by spin coating onto a washed glass plate with an ITO electrode, and dried at 110° C. for 20 minutes to obtain a hole injection layer having a thickness of 35 nm. On the hole injection layer, a chloroform solution having a solid content concentration of the following compound (i) of 0.5% was applied by spin coating, and dried under reduced pressure to form a hole transport layer having a thickness of 50 nm.
Next, a solid pattern was ink-jet printed on the hole transport layer using the ink composition obtained under the printing conditions described below to form a light-emitting layer having a thickness of 20 nm.
Next, the following compound (j) was deposited on the light-emitting layer to form an electron transport layer having a thickness of 30 nm. Lithium fluoride was deposited on the electron transport layer to a thickness of 0.5 nm, and aluminum (hereinafter, Al) was deposited on the electron transport layer to a thickness of 200 nm to form an electrode, thereby obtaining an electroluminescent device.

The electroluminescent device thus obtained was measured for driving voltage (V) and external quantum efficiency (%) when driven at a current density of 10 (mA/ ^cm2 ). In addition, the luminance after continuous driving for 10 hours under the condition of an initial luminance of 500 (cd/ ^m2 ) was measured, and the relative luminance (luminance after 10 hours/initial luminance) was calculated.
The electroluminescent element using the ink composition of Example 1 had an emission spectrum with a peak wavelength of 605 nm and a full width at half maximum of 30 nm. Furthermore, for a sample in which the light-emitting layer was formed in the same manner, the arithmetic mean roughness Ra was measured using a DECTAC6M manufactured by Veeco Corporation, and the result was that the Ra was 10 nm or less. Furthermore, a sample in which only the light-emitting layer was formed on an ITO substrate was prepared, and the ionization potential was measured using a PYS-202 manufactured by Sumitomo Heavy Industries Mechatronics Co., Ltd., and the result was that it was 5.9 eV.

Inkjet printing conditions
Printing machine: Dimatix MaterialsPrinter
Cartridge: 10 Dimatix Materials Cartridges, 10 pL Substrate temperature: 30° C.
Drying after printing: 40°C for 20 minutes

The abbreviations in Table 7 are as follows:
BGAc: Ethylene glycol monobutyl ether acetate (corresponding to general formula (1))

DEDG: Diethylene glycol diethyl ether (corresponding to general formula (3))

ETG: triethylene glycol monoethyl ether (corresponding to general formula (4))

PGDA: propylene glycol diacetate (corresponding to general formula (2))

As described above, Examples 1 to 11 showed superior electroluminescence device characteristics compared to Comparative Examples 1 to 4. In Comparative Example 2, since toluene was used instead of the solvent of the present invention, it is assumed that damage occurred to the hole transport layer, and unevenness and bright spots were present on the light-emitting surface of the electroluminescence device. Since deterioration of device characteristics, which is believed to be due to damage to the hole transport layer, was also confirmed in Comparative Example 4, it can be confirmed that the ratio of the solvent is also important. In addition, in Comparative Example 3, it is believed that the electroluminescence device characteristics were affected due to poor inkjet ejection properties. In fact, numerous bright and dark spots were generated on the light-emitting surface. It is hypothesized that this is because the adsorption of the phosphine oxide moiety to the semiconductor nanoparticles is unstable compared to the carboxyl group and sulfanyl group, which are the adsorption groups of the ligand of the present invention, and therefore some of the quantum dots aggregate. As described above, the electroluminescence device of the present invention can reduce the driving voltage, and has excellent luminescence efficiency, stable luminescence efficiency at driving current density, and high durability. Therefore, the organic EL element can be used as a flat panel display such as a wall-mounted television, various flat light emitters, and further as a light source for a copying machine or a printer, a light source for a liquid crystal display or an instrument, a display board, a marker light, etc.

Claims (4)

An ink composition comprising quantum dots and a solvent,
the quantum dots contain a carrier transporting ligand (excluding those having a fluorene skeleton or an oxofluorene skeleton) having at least one functional group selected from the group consisting of a sulfanyl group and a carboxyl group;
The ink composition comprises at least one solvent represented by the following general formulas (1) to (4), and the total amount of the solvents represented by the following general formulas (1) to (4) is 50 mass % or more based on the total amount of the solvent.
General formula (1): R ³⁶ CO(OR ³⁷ ) _p OR ³⁸
General formula (2): R ³⁹ CO(OR ⁴⁰ ) _q OCOR ⁴¹
General formula (3): R ⁴² (OR ⁴³ ) _r OR ⁴⁴
General formula (4): R ⁴⁵ (OR ⁴⁶ ) _s OR ⁴⁷
[In general formulas (1) to (4),
R ³⁶ , R ³⁸ , R ³⁹ , R ⁴¹ , R ⁴² and R ⁴⁴ each independently represent an alkyl group having 1 to 4 carbon atoms, R ³⁷ , R ⁴⁰ and R ⁴³ each independently represent an alkylene group having 2 to 4 carbon atoms, R ⁴⁵ and R ⁴⁷ each independently represent a hydrogen atom or an alkyl group having 1 to 6 carbon atoms, at least one of R ⁴⁵ and R ⁴⁷ is a hydrogen atom, R ⁴⁶ represents an alkylene group having 1 to 10 carbon atoms, and p, q, r and s each independently represent an integer of 1 to 6.] The ink composition according to claim 1 , which is used in an ink jet system. A light-emitting layer formed by using the ink composition according to claim 1 or 2 . 1. An electroluminescent device comprising an anode, a hole transport layer, the light emitting layer according to claim 3 , and a cathode laminated in this order on a substrate,
The electroluminescent device comprises the light-emitting layer provided in contact with the hole transport layer.