Light-emitting device and display device

Abstract

This light-emitting element (1) comprises an electron transport layer (11ET) that can achieve carrier balance without compromising the high hole injection capacity of nickel oxide nanoparticles. The light-emitting element (1) includes a first electrode (10), a functional layer (11), and a second electrode (12). The functional layer (11) includes a hole injection layer (11HI), a light-emitting layer (11EM), and an electron transport layer (11ET). The hole injection layer (11HI) includes zinc oxide nanoparticles, and the electron transport layer (11ET) includes composite zinc oxide nanoparticles. The composite zinc oxide nanoparticles include zinc oxide carrier particles that support zinc oxide nanoparticles doped with metal atoms as the dopant.

Description

[Technical field]

[0001] The present disclosure relates to a light-emitting device and a display device. [Background technology]

[0002] In recent years, various display devices equipped with light-emitting elements have been developed. In particular, display devices equipped with QLEDs (Quantum dot Light Emitting Diodes) or OLEDs (Organic Light Emitting Diodes) have attracted a great deal of attention because of their ability to achieve low power consumption, thinness, high image quality, and the like.

[0003] For example, Patent Document 1 describes a display screen using hybrid light-emitting diodes, which includes a backplane and an anode located on the surface of the backplane, in which a red quantum dot light-emitting diode (red QLED), a green quantum dot light-emitting diode (green QLED), and a blue organic light-emitting diode (blue OLED) are respectively arranged in a first region, a second region, and a third region on the surface of the anode, the second region and the third region do not overlap with each other, and a cathode is arranged on the surfaces of the red QLED, the green QLED, and the blue OLED.

[0004] Patent Document 1 describes the use of NiO-NPs (nickel oxide nanoparticles) as a hole injection layer (HIL) and ZnO as an electron transport layer (ETL) to improve the reliability of quantum dot light-emitting diodes (QLEDs). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. WO2020-041993 Summary of the Invention [Problem to be solved by the invention]

[0006] The nickel oxide nanoparticles used in the light-emitting device described in Patent Document 1 have high hole injection capability as a material for the hole injection layer (hole transport layer (HTL)), but in contrast, ZnO has low electron transport capability as a material for the electron transport layer (ETL). This makes it difficult to achieve a carrier balance between the hole transport layer and the electron transport layer, resulting in a problem of low external quantum efficiency (EQE).

[0007] The present disclosure has been made in consideration of the above-mentioned problems in the conventional art, and has an object to provide a light-emitting device and a display device that are equipped with a novel electron transport layer, which can achieve carrier balance while using nickel oxide nanoparticles in the hole transport layer without impairing the high hole injection ability of the nickel oxide nanoparticles. [Means for solving the problem]

[0008] In order to solve the above problems, a light-emitting element according to one embodiment of the present disclosure includes a first electrode, a second electrode, and a functional layer provided between the first electrode and the second electrode, the functional layer including a hole transport layer, a light-emitting layer, and an electron transport layer, the hole transport layer including nickel oxide nanoparticles, the electron transport layer including composite zinc oxide nanoparticles, and the composite zinc oxide nanoparticles including zinc oxide carrier particles carrying zinc oxide nanoparticles doped with metal atoms as a dopant.

[0009] In addition, in order to solve the above-mentioned problems, a display device according to one embodiment of the present disclosure includes a substrate and a plurality of light-emitting elements on the substrate, the light-emitting elements including a first electrode, a second electrode, and a functional layer provided between the first electrode and the second electrode, the functional layer including a hole transport layer, a light-emitting layer, and an electron transport layer, the hole transport layer including nickel oxide nanoparticles, the electron transport layer including composite zinc oxide nanoparticles, and the composite zinc oxide nanoparticles including zinc oxide carrier particles supporting zinc oxide nanoparticles doped with metal atoms as a dopant. Effect of the Invention

[0010] According to one aspect of the present disclosure, it is possible to provide a light-emitting element and a display device having a novel electron transport layer that can achieve carrier balance while using nickel oxide nanoparticles in a hole transport layer without impairing the high hole injection ability of the nickel oxide nanoparticles. [Brief description of the drawings]

[0011] [Figure 1] FIG. 1 is a cross-sectional view showing a schematic configuration of a light-emitting device 1 according to an embodiment of the present disclosure. [Diagram 2] FIG. 2 is a diagram illustrating an outline of MgZnO@ZnO-NPs contained in an electron transport layer 11ET included in a light-emitting element 1 according to an embodiment of the present disclosure. [Diagram 3] (a) is a cross-sectional view showing a schematic configuration of a red light-emitting element 1R of one embodiment of the present disclosure, (b) is a cross-sectional view showing a schematic configuration of a green light-emitting element 1G provided in a display device of one embodiment of the present disclosure, and (c) is a cross-sectional view showing a schematic configuration of a blue light-emitting element 1B provided in a display device of one embodiment of the present disclosure. [Figure 4] FIG. 1 is a plan view showing a schematic configuration of a display area of ​​a display device 100 according to an embodiment of the present disclosure. [Diagram 5] 1 is a cross-sectional view showing a schematic configuration of a display area of ​​a display device 100 according to an embodiment of the present disclosure. [Figure 6] 1 is a scheme outlining a method for producing composite zinc oxide nanoparticles (MgZnO@ZnO-NPs) contained in a hole transport layer in a light-emitting element according to an embodiment of the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] One embodiment of the present disclosure will be described below with reference to FIG.

[0013] In this specification, unless otherwise specified, "A to B" expressing a numerical range means "A or more, and B or less."

[0014] In addition, in this specification, "nanoparticles" may be abbreviated as "NPs."

[0015] The light-emitting element 1 of the present embodiment is applied to a quantum dot light-emitting element including a quantum dot light-emitting diode (QLED), and further to a light-emitting device (display device) including the quantum dot light-emitting element of the present embodiment and an array substrate.

[0016] [Light-emitting element 1] First, a configuration of a light-emitting device 1 including a quantum dot light-emitting diode (QLED) in an embodiment of the present disclosure will be described with reference to Fig. 1. Fig. 1 is a cross-sectional view showing a schematic configuration of the light-emitting device 1 in this embodiment.

[0017] 1, the light-emitting element 1 in this embodiment includes a hole injection layer (HIL) 11HI, a hole transport layer (HTL) 11HT, an emission layer (EML) 11EM, an electron transport layer (ETL) 11ET, and a second electrode 12 serving as a cathode, in this order on a first electrode 10 serving as an anode. The functional layer 11 includes the hole injection layer 11HI, the hole transport layer 11HT, the emission layer 11EM, and the electron transport layer 11ET, and is provided between the first electrode 10 and the second electrode 12.

[0018] The light-emitting element 1 shown in Fig. 1 may be of a top emission type or a bottom emission type. In the light-emitting element 1, the second electrode 12, which is a cathode, is disposed in a layer above the first electrode 10, which is an anode, and the first electrode 10, which is an anode, is formed from an electrode material that reflects visible light, and the second electrode 12, which is a cathode, is formed from an electrode material that transmits visible light. To make the light-emitting element 1 of the bottom emission type, the second electrode 12, which is a cathode, is disposed in a layer above the first electrode 10, which is an anode, and the first electrode 10, which is an anode, is formed from an electrode material that transmits visible light, and the second electrode 12, which is a cathode, is formed from an electrode material that reflects visible light.

[0019] The electrode material that reflects visible light is not particularly limited as long as it can reflect visible light and has electrical conductivity, and examples thereof include metal materials such as Al, Cu, Au, Mg, Li, and Ag, alloys of the metal materials, laminates of the metal materials and transparent metal oxides (e.g., indium tin oxide, indium zinc oxide, indium gallium zinc oxide, etc.), and laminates of the alloys and the transparent metal oxides.

[0020] On the other hand, the electrode material that transmits visible light is not particularly limited as long as it can transmit visible light and has electrical conductivity, and examples thereof include transparent metal oxides (e.g., indium tin oxide, indium zinc oxide, indium gallium zinc oxide, etc.), thin films made of metal materials such as Al and Ag, and nanowires made of metal materials such as Al and Ag.

[0021] The first electrode 10 and the second electrode 12 can be formed by a general electrode forming method, for example, a physical vapor deposition (PVD) method such as a vacuum deposition method, a sputtering method, an EB deposition method, an ion plating method, or a chemical vapor deposition (CVD) method. The patterning method of the first electrode 10 and the second electrode 12 is not particularly limited as long as it can form a desired pattern with high accuracy, but specific examples include a photolithography method and an inkjet method.

[0022] In one embodiment, the light-emitting element 1 may have a forward stack structure, but is not limited thereto, and may have an inverted stack structure. The light-emitting element 1 having a forward stack structure includes a first electrode 10 as an anode and a second electrode 12 as a cathode provided as an upper layer above the first electrode 10. The functional layer 11 including the light-emitting layer provided between the first electrode 10 as an anode and the second electrode 12 as a cathode can be configured, for example, by stacking a hole injection layer, a hole transport layer, a red light-emitting layer, an electron transport layer, and an electron injection layer in order from the first electrode 10 side. Although not shown, the light-emitting element having an inverted stack structure includes a first electrode as a cathode and a second electrode as an anode provided as an upper layer above the first electrode. The functional layer including the light-emitting layer provided between the first electrode as a cathode and the second electrode as an anode can be configured, for example, by stacking an electron injection layer, an electron transport layer, a green light-emitting layer, a hole transport layer, and a hole injection layer in order from the first electrode side. Furthermore, in one embodiment, regardless of whether the light-emitting element has a forward stack structure or an inverted stack structure, the functional layer provided in the light-emitting element only needs to contain nickel oxide nanoparticles in the hole transport layer or hole injection layer. In this embodiment, an example will be described in which a hole injection layer containing nickel oxide nanoparticles is provided, but the hole injection layer is not a required component.

[0023] [Hole Injection Layer] The light-emitting element may include a plurality of hole transport layers from the viewpoint of increasing the hole injection efficiency. The light-emitting element 1 according to one embodiment includes a hole injection layer 11HI and a hole transport layer 11HT, and the hole injection layer 11HI injects holes from the first electrode 10, which is an anode, into the hole transport layer 11HT. The hole injection layer is also a layer that transports holes from the first electrode side, which is an anode, to the light-emitting layer side, and is therefore sometimes described as one aspect of the hole transport layer. The hole injection layer 11HI includes an inorganic material as a hole injection material, and the inorganic material includes nickel oxide nanoparticles.

[0024] The hole injection layer 11HI may be formed by applying a dispersion liquid obtained by dispersing nickel oxide nanoparticles in a polar solvent such as water, ethanol, or dimethyl sulfoxide to the sub-pixels of the light-emitting elements 1 collectively by a spin coating method, or may be applied to each sub-pixel by an inkjet method, etc. The dispersion liquid of nickel oxide nanoparticles may contain a dispersing material such as thiol or amine mixed therein.

[0025] The nickel oxide nanoparticles have a positive surface charge and the zeta potential of the particles themselves may be positive. The nickel oxide nanoparticles may have a ligand, as in the quantum dots that may be contained in the light-emitting layer 11EM and the composite nickel oxide particles contained in the electron transport layer 11ET described below. Alternatively, the dispersion of nickel oxide nanoparticles may contain a ligand as a dispersion material.

[0026] The hole injection layer 11HI containing nickel oxide nanoparticles may be formed by electrophoretic deposition, for example. In the electrophoretic deposition, a dispersion liquid containing nickel oxide nanoparticles is applied onto a first electrode serving as an anode, and a voltage is applied between a counter electrode (not shown) and the first electrode serving as an anode, thereby forming a hole injection layer in which nickel oxide nanoparticles are deposited.

[0027] D50, or so-called median diameter, of the nickel oxide nanoparticles contained in the hole-injection layer 11HI is determined, for example, as a cumulative distribution based on volume, and may be within the range of 10 nm to 500 nm, and preferably within the range of 10 nm to 30 nm.

[0028] The nickel oxide nanoparticles may be, for example, NiO-NPs. Alternatively, the hole injection layer 11HI may be formed by sputtering.

[0029] [Hole transport layer] The hole transport layer 11HT transports holes injected from the hole injection layer 11HI to the light emitting layer 11EM. The hole transport layer 11HT is a layer formed on the hole injection layer 11HI. The material used for the hole transport layer 11HT is not particularly limited as long as it is a hole transport material that can stabilize the transport of holes to the light emitting layer 11EM. The hole transport material in the hole transport layer 11HT is preferably one having high hole mobility. Furthermore, the hole transport material is preferably a material (electron blocking material) that can prevent the penetration of electrons that have moved from the second electrode 12, which is the cathode. This is because the recombination efficiency of holes and electrons in the light emitting layer 11EM can be increased. The hole transport layer 11HT can be formed using a hole transport material such as NiO-NP, poly-TPD, polyvinylcarbazole (PVK), or poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4'-(N-(4-sec-butylphenyl))diphenylamine)] (TFB), etc. As described above, the hole transport layer 11HT can be a layer that prevents electrons transported from the electron transport layer 11ET to the light emitting layer 11EM from escaping to the first electrode 10.

[0030] The hole transport layer 11HT may be applied collectively to the sub-pixels of the multiple light-emitting elements 1 by dip coating or spin coating using a dispersion liquid containing a hole transport material dispersed therein, or may be applied separately to each sub-pixel by an inkjet method or the like.

[0031] [Light-emitting layer] The light-emitting layer 11EM emits light by recombination of holes transported from the first electrode 10, which is an anode, and electrons transported from the second electrode 12, which is a cathode. In the present embodiment, the light-emitting layer 11EM is a quantum dot light-emitting layer including quantum dots (QDs: semiconductor nanoparticles) of each color as a light-emitting material, but is not limited thereto and may be an OLED (organic light-emitting diode).

[0032] The quantum dot (QD) may have, for example, a core structure, a core / shell structure, a core / shell / shell structure, or a shell structure with a continuously changing core / ratio. When the quantum dot (QD) has a core structure, a ligand is provided on the surface of the core, and when the quantum dot (QD) has a structure having a shell, a ligand is provided on the surface of the shell. The core part can be composed of, for example, Si, C, etc. in the case of a single element system, for example, CdSe, CdS, CdTe, InP, GaP, InN, ZnSe, ZnS, ZnTe, etc. in the case of a binary element system, for example, CdSeTe, GaInP, ZnSeTe, etc. in the case of a ternary element system, for example, CdSeTe, GaInP, ZnSeTe, etc. in the case of a quaternary element system, for example, AIGS, etc. In the case of a binary system, the shell portion can be composed of, for example, CdS, CdTe, CdSe, ZnS, ZnSe, ZnTe, etc., and in the case of a ternary system, the shell portion can be composed of, for example, CdSSe, CdTeSe, CdSTe, ZnSSe, ZnSTe, ZnTeSe, AIP, etc.

[0033] The quantum dot (QD) refers to a nanoscale semiconductor crystal having unique optical properties according to quantum mechanics. The shape of the quantum dot (QD) is not particularly limited as long as it satisfies the above optical properties, and is not limited to a spherical three-dimensional shape (circular cross-sectional shape). For example, it may be a polygonal cross-sectional shape, a rod-like three-dimensional shape, a branch-like three-dimensional shape, a three-dimensional shape with unevenness on the surface, or a combination thereof.

[0034] The light-emitting layer 11EM can be formed by applying a dispersion liquid in which quantum dots are dispersed in a solvent such as hexane, toluene, octadecane, cyclododecene, or phenylcyclohexane to each subpixel by spin coating, inkjet, or the like. The quantum dots may have a ligand on their surface, and the quantum dot dispersion liquid may contain a dispersing material such as thiol or amine. In addition, the light-emitting layer 11EM containing quantum dots may be deposited on the hole transport layer 11HT by electrophoretic deposition, utilizing the surface potential of the quantum dots.

[0035] [Electron transport layer] FIG. 2 is a diagram for explaining the outline of MgZnO@ZnO-NPs included in the electron transport layer 11ET included in the light-emitting element 1 according to an embodiment of the present disclosure. The MgZnO@ZnO-NPs illustrated in FIG. 2 is formed by supporting zinc oxide nanoparticles (MgZnO-NPs) doped with magnesium (Mg) as a dopant on zinc oxide support particles (ZnO-NPs). When the zinc oxide nanoparticles doped with magnesium (Mg) as a dopant are supported on the surface of nanoparticle-shaped zinc oxide support particles as illustrated in FIG. 2, the composite zinc oxide nanoparticles may be referred to as MgZnO-supported ZnO nanoparticles or MgZnO@ZnO-NPs. In MgZnO@ZnO-NPs, it is intended that ZnO-NPs are support particles, and MgZnO-NPs are particles supported on the support particles.

[0036] As shown in FIG. 2, MgZnO@ZnO-NPs is expected to have a structure in which MgZnO-NPs supported on ZnO-NPs form unevenness on the surface of ZnO-NPs while the surface of ZnO-NPs is exposed. In this respect, the structure of MgZnO@ZnO-NPs is expected to differ from the core / shell structure, core / shell / shell structure, and shell structure with continuously changed core / ratio that the above-mentioned quantum dots (QDs) may have. In addition, the composite zinc oxide nanoparticles exemplified by MgZnO@ZnO-NPs in FIG. 2 may have a ligand coordinated to its surface. When the ZnO-NPs / ligand shown in FIG. 2 has an organic ligand, the ligand may be intended to be a ligand in which a part of the ligand as a dispersion material is coordinated to the surface of ZnO-NPs. In addition, when the ZnO-NPs / ligand shown in FIG. 2 has an inorganic ligand, the ZnO-NPs / ligand may be intended to be ZnO-NPs in which a core / shell structure is formed by an inorganic ligand. The ZnO-NPs / ligand may be provided with inorganic ligands and may further be provided with organic ligands.

[0037] The dispersion containing MgZnO@ZnO-NPs (MgZnO@ZnO-NPs dispersion) may contain a ligand as a dispersion material, and examples of the dispersion material include organic ligands that are not coordinated to MgZnO@ZnO-NPs. Examples of the ligands that are not coordinated to MgZnO@ZnO-NPs include ZnO support particles described below and ligands similar to those used in doped ZnO nanoparticles.

[0038] The electron transport layer 11ET transports electrons from the second electrode 12, which is the cathode, to the light-emitting layer 11EM by using an electron transport material. The composite zinc oxide nanoparticles (composite ZnO nanoparticles) have a higher energy level of the conduction band minimum (CBM) corresponding to the lowest unoccupied molecular orbital (LUMO) than zinc oxide nanoparticles (ZnO nanoparticles) that do not contain a dopant, because the doped zinc oxide nanoparticles are supported on the surface of the zinc oxide support particles (ZnO support particles). Therefore, the electron transport layer 11ET can achieve carrier balance with the hole injection layer 11HI without impairing the hole injection ability of the nickel oxide nanoparticles contained in the hole injection layer 11HI.

[0039] The dopant doped into the ZnO nanoparticles is not limited to magnesium (Mg) as long as it is a dopant that can increase the energy level of the CBM in ZnO. Examples of such dopants include metal atoms such as aluminum (Al) and lithium (Li). ZnO containing magnesium (Mg), aluminum (Al), or lithium (Li) as a dopant may be referred to as MgZnO, AlZnO, LiZnO, etc., respectively. In addition, ZnO nanoparticles doped with a dopant may be simply referred to as "doped ZnO nanoparticles".

[0040] In the electron transport material, the mass ratio of ZnO carrier particles to doped ZnO nanoparticles is preferably 1:2 to 5:1, and more preferably 3:1 to 5:1. In the mass ratio of ZnO carrier particles to doped ZnO nanoparticles of 1:2 to 5:1, the larger the mass ratio of ZnO carrier particles, the better the carrier balance, the better the voltage current response in the light-emitting device, and the higher the external quantum efficiency (EQE) of the light-emitting device.

[0041] The median diameter (D50) of the ZnO carrier particles is preferably larger than the D50 of the doped ZnO nanoparticles, and is preferably 6 to 10 times the D50 of the doped ZnO nanoparticles. This allows the doped ZnO nanoparticles to be successfully supported on the surface of the ZnO carrier particles by van der Waals forces. In this specification, for convenience, the ZnO carrier particles are described as ZnO carrier particles to distinguish them from doped ZnO nanoparticles, but the ZnO carrier particles may be ZnO nanoparticles that are not substantially doped with a dopant.

[0042] The D50 of the ZnO carrier particles is determined as a cumulative distribution based on volume, and is preferably within the range of 10 nm to 60 nm, and more preferably within the range of 10 nm to 20 nm. The smaller the D50 of the ZnO carrier particles, the more effectively the light from the light-emitting layer 11EM can be transmitted to the second electrode 12 side, which is the cathode. Therefore, the first electrode 10, which is the anode, is formed of an electrode material that reflects visible light, Cathode If the second electrode 12, which is the first electrode 10, is formed of an electrode material that transmits visible light, it can be suitably used as a top-emission type light-emitting element. Note that, as described above, if the first electrode 10 is formed of an electrode material that transmits visible light and the second electrode 12 is formed of an electrode material that reflects visible light, it can be used as a bottom-emission type light-emitting element. The D50 of the ZnO carrier particles and nanoparticles, including the D50 of the doped ZnO nanoparticles described below, can be evaluated by dynamic light scattering.

[0043] The ZnO carrier particles may have a ligand. The ligand coordinated to the ZnO carrier particles may be an organic compound having a functional group and a hydrocarbon group. The functional group of the ligand coordinates to the ZnO carrier particles, and the hydrocarbon group may have a function of increasing the dispersion stability of the ZnO carrier particles in polar solvents and alcohol-based solvents, which will be described later. The functional group of the ligand may be, for example, an amino group, a thiol group, a carboxyl group, a hydroxyl group, a phosphonyl group, or the like, which can be coordinated to the surface of the ZnO carrier particles. The ligand may have multiple functional groups, such as diamine and dithiol, or may have multiple types of functional groups, such as carbamate and thiol-carboxylic acid. The hydrocarbon group of the ligand may be a linear or branched hydrocarbon group, and may be an unsaturated hydrocarbon group, a saturated hydrocarbon group, or an aromatic hydrocarbon group. The ligand may be a ligand known as a capping ligand. The ZnO carrier particles may have multiple types of ligands coordinated to them. Examples of the ligand include oleylamine, octanethiol, tributylphosphine oxide, etc. Examples of the ligand include an anion portion constituting a quaternary ammonium salt such as tetrabutylammonium tetrafluoroborate, and the anion portion may be coordinated to the ZnO carrier particle as a ligand. In addition, the ligand of the ZnO carrier particle may be a metal chalcogenide compound other than ZnO, and may be an inorganic ligand such as zinc sulfide (ZnS).

[0044] The D50 of the doped ZnO nanoparticles, determined as a cumulative distribution on a volume basis, may be in the range of 5 nm to 15 nm, preferably in the range of 5 nm to 10 nm.

[0045] In addition, the doped ZnO nanoparticles may have a ligand. The ligand coordinated to the doped ZnO nanoparticles may be selected according to the type of the doped ZnO nanoparticles. In addition, the ligand coordinated to the doped ZnO nanoparticles may be the same as the ligand that the ZnO carrier particles have, so the description thereof will be omitted.

[0046] Composite ZnO nanoparticles, including MgZnO-supported ZnO nanoparticles, can be obtained by mixing a dispersion of ZnO carrier particles with a dispersion of doped ZnO nanoparticles to prepare a mixed dispersion, and drying the mixed dispersion, thereby supporting the MgZnO nanoparticles on the ZnO carrier particles.

[0047] The dispersion of ZnO carrier particles includes ZnO carrier particles and an organic solvent, and may further include a ligand that coordinates with the ZnO carrier particles. The organic solvent included in the dispersion may include, for example, an alcohol-based solvent such as ethanol. The dispersion of ZnO carrier particles may include a known dispersing material as a ligand.

[0048] The dispersion of doped ZnO nanoparticles includes doped ZnO nanoparticles and an organic solvent, and may further include a ligand that coordinates with the doped ZnO particles. The organic solvent included in the dispersion may include, for example, an alcohol-based solvent such as ethanol. The dispersion of doped ZnO nanoparticles may include a known dispersion material as a ligand, similar to the dispersion of ZnO carrier particles.

[0049] In each of the dispersion of ZnO carrier particles and the dispersion of doped ZnO nanoparticles, the type of ligand coordinated to the ZnO carrier particles and the type of ligand coordinated to the doped ZnO nanoparticles may be different from each other or may be the same. Here, it is preferable that each of the dispersion of ZnO carrier particles and the dispersion of doped ZnO nanoparticles is a dispersion containing an organic solvent compatible with each other or the same organic solvent. By making the organic solvent of the dispersion of doped ZnO nanoparticles and the dispersion of ZnO carrier particles compatible with each other, the dispersion stability of the ZnO carrier particles with ligands coordinated and the doped ZnO nanoparticles with ligands coordinated can be improved. This can prevent unintended clouding that occurs when the ZnO carrier particles support the doped ZnO nanoparticles in the mixed dispersion.

[0050] The concentration of the composite ZnO nanoparticles contained in the mixed dispersion may be appropriately adjusted so that an electron transport layer having a desired film thickness can be formed.

[0051] By drying the mixed dispersion obtained by mixing the dispersions, the ligands of the ZnO carrier particles can be slipped through, and the doped ZnO nanoparticles can be supported on the ZnO carrier particles. This makes it possible to obtain composite ZnO particles. The obtained composite ZnO nanoparticles may be dispersed in an organic solvent, such as hexanol or octanol, to a desired concentration, thereby obtaining a dispersion of composite ZnO nanoparticles. The composite ZnO nanoparticles can be stabilized by being reversed micellized in the organic solvent by the ligands coordinated to the ZnO carrier particles and / or the ligands coordinated to the doped ZnO nanoparticles. Thereafter, the dispersion of the composite ZnO nanoparticles can be applied, for example, by spinner coating or inkjet coating, to form an electron transport layer 11ET having high flatness. After applying the dispersion of the composite ZnO nanoparticles, it is sufficient to heat-dry the layer at a temperature of, for example, 25 to 110°C, preferably 25 to 80°C. After the electron transport layer 11ET is heat-dried, the electron transport layer 11ET may be washed or rinsed with an organic solvent.

[0052] Alternatively, composite ZnO nanoparticles may be deposited on light-emitting layer 11EM by electrophoretic deposition to form electron-transporting layer 11ET containing composite ZnO nanoparticles.

[0053] The electron transport layer 11ET formed as described above may have a thickness within a range of 20 nm to 200 nm, and preferably within a range of 50 nm to 150 nm. This provides an advantage that the directionality of light emitted from the light emitting layer 11EM via the electron transport layer 11ET can be improved by the cavity effect. From another perspective, the thickness of the electron transport layer 11ET is 50 nm or more, so that damage to the light emitting layer 11EM can be reduced when the second electrode 12 is formed by, for example, an ITO sputtering process.

[0054] In the conventional method, the electron transport layer of the light-emitting device is formed by a layer containing a metal oxide such as zinc oxide (ZnO) and a layer of zinc oxide doped with metal atoms (M x Zinc y In some cases, the light-emitting layer is formed from a layer of a metal oxide such as ZnO. As mentioned above, when the light-emitting layer contains quantum dots, when the two-layer electron transport layer is formed, the light-emitting layer that has been formed once is exposed to the solvent of the dispersion liquid for forming the electron transport layer and heated at least twice. As a result, the flatness of the two-layer electron transport layer is reduced, and the electron movement in the electron transport layer is biased, increasing the possibility that the light-emitting layer will not emit light uniformly. In addition, since the metal oxide of the nanoparticles in the electron transport layer is distributed unevenly, when a cathode is applied on the electron transport layer, there is a possibility that the light-emitting layer, which is the layer below the electron transport layer, will be thermally damaged.

[0055] In this regard, one advantage of the light-emitting element 1 according to one embodiment is that the electron transport layer 11ET not only achieves carrier balance with the hole injection layer 11HI due to its high CBM, but also is formed without compromising the uniformity of the light-emitting layer 11EM and the electron transport layer 11ET themselves by applying composite ZnO nanoparticles at once as the electron transport material of the electron transport layer 11ET.

[0056] In addition, the electron transport layer 11ET can achieve a carrier balance without impairing the high hole injection ability of the hole injection layer 11HI containing nickel oxide nanoparticles, and therefore the light-emitting device 1 can improve the voltage-current response and the external quantum efficiency.

[0057] In the light-emitting element 1 according to one embodiment, the light-emitting layer 11EM can emit any one of red light, green light, and blue light. Here, the red light is light having a central emission wavelength in a wavelength band of more than 600 nm and not more than 780 nm. The green light is light having a central emission wavelength in a wavelength band of more than 500 nm and not more than 600 nm. Furthermore, the blue light is light having a central emission wavelength in a wavelength band of 400 nm or more and not more than 500 nm. In one embodiment, the multiple types of quantum dots are a combination of red quantum dots, green quantum dots, and blue quantum dots, but this combination is not necessarily required.

[0058] Figure 3 (a) shows a red light-emitting element 1R having a functional layer 11R including a red light-emitting layer 11REM that emits red light, Figure 3 (b) shows a green light-emitting element 1G having a functional layer 11G including a green light-emitting layer 11GEM that emits green light, and Figure 3 (c) shows a blue light-emitting element 1B having a functional layer 11B including a blue light-emitting layer 11BEM that emits blue light.

[0059] In the present embodiment, the red light emitting element 1R, the green light emitting element 1G, and the blue light emitting element 1B are described as QLEDs (quantum dot light emitting diodes) as an example, but are not limited thereto. A part of the red light emitting element 1R, the green light emitting element 1G, and the blue light emitting element 1B may be a QLED, and a part of the remaining part of the red light emitting element 1R, the green light emitting element 1G, and the blue light emitting element 1B may be an OLED. Furthermore, the red light emitting element 1R, the green light emitting element 1G, and the blue light emitting element 1B may be an OLED (organic light emitting diode). In addition, when the red light emitting element 1R, the green light emitting element 1G, and the blue light emitting element 1B are QLEDs, the light emitting layer of each light emitting element is, for example, a light emitting layer containing quantum dots formed by a coating method or an inkjet method, and when the red light emitting element 1R, the green light emitting element 1G, and the blue light emitting element 1B are OLEDs, the light emitting layer of each light emitting element is, for example, an organic light emitting layer formed by a deposition method.

[0060] The red light emitting element 1R includes a first electrode 10R, a functional layer 11R including a red light emitting layer 11REM, and a second electrode 12, the green light emitting element 1G includes a first electrode 10G, a functional layer 11G including a green light emitting layer 11GEM, and a second electrode 12, and the blue light emitting element 1B includes a first electrode 10B, a functional layer 11B including a blue light emitting layer 11BEM, and a second electrode 12. G The insulating bank 13 (resin layer) covering each edge of the first electrode 10B can be formed, for example, by applying an organic material such as polyimide or acrylic and then patterning it on a planarizing film described later using a photolithography method.

[0061] In addition, functional layers 11R, 11G, and 11B shown in Figures 3 (a) to (c) may each include a hole injection layer formed in the same process using the same material, a hole transport layer formed in the same process using the same material, and an electron transport layer formed in the same process using the same material.

[0062] [Display device] A display device according to an embodiment of the present disclosure includes a plurality of light-emitting elements having a functional layer including a hole transport layer containing nickel oxide nanoparticles, a light-emitting layer, and an electron transport layer containing composite ZnO nanoparticles, in this order, between a first electrode and a second electrode, and the light-emitting elements are provided on a substrate. In the following, a plurality of light-emitting elements emitting light with different emission peak wavelengths will be described, but the present invention is not limited thereto, and each of the plurality of light-emitting elements may be a light-emitting element emitting light of the same color. In one embodiment, the functional layer of the light-emitting element in the display device may include nickel oxide nanoparticles in the hole transport layer or the hole injection layer, and in this embodiment, a case where a hole injection layer containing nickel oxide nanoparticles is provided will be described as an example, but the hole injection layer is not an essential component.

[0063] Fig. 4 is a plan view showing a schematic configuration of the display device 100 of embodiment 1. Fig. 5 is a cross-sectional view showing a schematic configuration of the display area DA of the display device 100 according to an embodiment of the present disclosure.

[0064] As shown in Fig. 4, the display device 100 includes a frame area NDA and a display area DA. The display area DA of the display device 100 includes a plurality of pixels PIX, each of which includes a red subpixel RSP, a green subpixel GSP, and a blue subpixel BSP. In the embodiment of the present disclosure, a case in which one pixel PIX is composed of a red subpixel RSP, a green subpixel GSP, and a blue subpixel BSP will be described as an example, but the present disclosure is not limited to this. For example, one pixel PIX may include subpixels of other colors in addition to the red subpixel RSP, the green subpixel GSP, and the blue subpixel BSP.

[0065] The red subpixel RSP, green subpixel GSP, and blue subpixel BSP shown in Fig. 4 each include a red light emitting element 1R, a green light emitting element 1G, or a blue light emitting element 1B shown in Fig. 3(a) to (c). More specifically, the red subpixel RSP shown in Fig. 5 includes a red light emitting element 1R, the green subpixel GSP includes a green light emitting element 1G, and the blue subpixel BSP includes a blue light emitting element 1B. Note that a control circuit including a transistor TR and a light emitting element provided for each of the red subpixel RSP, green subpixel GSP, and blue subpixel BSP are collectively referred to as a subpixel circuit.

[0066] 5, in the display region DA of the display device 100, a barrier layer 3, a thin film transistor layer 4 including a transistor TR, a red light emitting element 1R, a green light emitting element 1G, a blue light emitting element 1B, a bank 13 (transparent resin layer), a sealing layer 6, and a functional film 30 are provided on a substrate 20 in this order from the substrate 20 side. Note that a substrate having a barrier layer 3, a thin film transistor layer 4 including a transistor TR, and a plurality of first electrodes 10R, 10G, and 10B provided on the substrate 20 in this order from the substrate 20 side as shown in FIG. 5 is referred to as a substrate with first electrodes (active matrix substrate) 2.

[0067] The red subpixel RSP provided in the display area DA of the display device 100 includes a red light emitting element 1R (first light emitting element), the green subpixel GSP provided in the display area DA of the display device 100 includes a green light emitting element 1G (second light emitting element), and the blue subpixel BSP provided in the display area DA of the display device 100 includes a blue light emitting element 1B (third light emitting element). The red light emitting element 1R included in the red subpixel RSP includes a first electrode 10R, a functional layer 11R including a red light emitting layer, and a second electrode 12, the green light emitting element 1G included in the green subpixel GSP includes a first electrode 10G, a functional layer 11G including a green light emitting layer, and a second electrode 12, and the blue light emitting element 1B included in the blue subpixel BSP includes a first electrode 10B, a functional layer 11B including a blue light emitting layer, and a second electrode 12.

[0068] The substrate 20 may be, for example, a resin substrate made of a resin material such as polyimide, or a glass substrate. In this embodiment, in order to make the display device 100 a flexible display device, a case where a resin substrate made of a resin material such as polyimide is used as the substrate 20 will be described as an example, but the present invention is not limited to this. When the display device 100 is a non-flexible display device, a glass substrate can be used as the substrate 20.

[0069] The barrier layer 3 is a layer that prevents foreign substances such as water and oxygen from penetrating into the transistor TR, the red light-emitting element 1R, the green light-emitting element 1G, and the blue light-emitting element 1B, and can be composed of, for example, a silicon oxide film, a silicon nitride film, or a silicon oxynitride film, or a laminated film of these, formed by a CVD method.

[0070] The transistor TR portion of the thin film transistor layer 4 including the transistor TR includes the semiconductor film SEM and the doped semiconductor films SEM' and SEM'', an inorganic insulating film 21, a gate electrode G, an inorganic insulating film 22, an inorganic insulating film 23, a source electrode S, a drain electrode D, and a planarization film 24, and the portion of the thin film transistor layer 4 including the transistor TR other than the transistor TR portion includes the inorganic insulating film 21, the inorganic insulating film 22, the inorganic insulating film 23, and the planarization film 24.

[0071] The semiconductor films SEM, SEM', and SEM'' may be made of, for example, low temperature polysilicon (LTPS) or an oxide semiconductor (for example, an In-Ga-Zn-O based semiconductor). In this embodiment, the transistor TR has a top gate structure as an example, but is not limited to this and the transistor TR may have a bottom gate structure.

[0072] The gate electrode G and the source electrode S and drain electrode D can be formed of a single layer or a multilayer film of a metal containing at least one of aluminum, tungsten, molybdenum, tantalum, chromium, titanium, and copper, for example.

[0073] The inorganic insulating film 21, the inorganic insulating film 22, and the inorganic insulating film 23 can be formed, for example, by a silicon oxide film, a silicon nitride film, a silicon oxynitride film, or a laminated film of these films formed by a CVD method.

[0074] The planarizing film 24 can be made of a coatable organic material such as polyimide or acrylic.

[0075] The red light emitting element 1R includes a first electrode 10R above the planarization film 24, a functional layer 11R including a red light emitting layer, and a second electrode 12. The green light emitting element 1G includes a first electrode 10G above the planarization film 24, a functional layer 11G including a green light emitting layer, and a second electrode 12. The blue light emitting element 1B includes a first electrode 10B above the planarization film 24, a functional layer 11B including a blue light emitting layer, and a second electrode 12. G The insulating bank 13 (transparent resin layer) covering the edges of the first electrode 10B can be formed, for example, by applying an organic material such as polyimide or acrylic and then patterning it by photolithography.

[0076] The sealing layer 6 is a light-transmitting film, and can be composed of, for example, an inorganic sealing film 26 covering the second electrode 12, an organic film 27 upper than the inorganic sealing film 26, and an inorganic sealing film 28 upper than the organic film 27. The sealing layer 6 prevents foreign matter such as water and oxygen from penetrating into the red light-emitting element 1R, the green light-emitting element 1G, and the blue light-emitting element 1B.

[0077] The inorganic sealing film 26 and the inorganic sealing film 28 are inorganic films, and may be formed, for example, by a silicon oxide film, a silicon nitride film, a silicon oxynitride film, or a laminated film of these films, which are formed by a CVD method. The organic film 27 is a light-transmitting organic film having a planarizing effect, and may be formed, for example, by a coatable organic material such as acrylic. The organic film 27 may be formed, for example, by an inkjet method. In the present embodiment, the sealing layer 6 is formed of two inorganic films and one organic film provided between the two inorganic films, but the lamination order of the two inorganic films and the one organic film is not limited to this. Furthermore, the sealing layer 6 may be formed only of an inorganic film, may be formed only of an organic film, may be formed of one inorganic film and two organic films, or may be formed of two or more inorganic films and two or more organic films.

[0078] The functional film 30 is, for example, a film having at least one of an optical compensation function and a protection function.

[0079] As described above, the light-emitting element according to the first aspect of the present disclosure includes a first electrode, a second electrode, and a functional layer disposed between the first electrode and the second electrode, the functional layer including the hole transport layer, the light-emitting layer, and the electron transport layer, the hole transport layer including nickel oxide nanoparticles, the electron transport layer including composite zinc oxide nanoparticles, and the composite zinc oxide nanoparticles including zinc oxide carrier particles carrying zinc oxide nanoparticles doped with a dopant.

[0080] This allows the light-emitting element to use nickel oxide nanoparticles in the hole transport layer while maintaining the high hole injection ability of the nickel oxide nanoparticles and achieving carrier balance in the electron transport layer.

[0081] In addition, in the light-emitting device according to aspect 2 of the present disclosure, in the above aspect 1, the dopant is preferably a metal atom selected from the group consisting of Mg, Li, and Al.

[0082] This makes it possible to raise the energy level of the CBM in the electron transport layer, and to achieve a carrier balance between the hole transport layer and the electron transport layer without impairing the high hole injection ability of the nickel oxide nanoparticles.

[0083] In addition, in the light-emitting device according to aspect 3 of the present disclosure, in the above aspect 1 or 2, a mass ratio of the zinc oxide support particles and the zinc oxide nanoparticles doped with the dopant contained in the electron transport layer may be within a range of 2:1 to 1:5.

[0084] This makes it possible to obtain a light emitting device having high external quantum efficiency.

[0085] In addition, in the light-emitting device according to aspect 4 of the present disclosure, in any one of aspects 1 to 3 above, the median diameter (D50) of the zinc oxide support particles is preferably larger than the median diameter (D50) of the zinc oxide nanoparticles doped with the dopant.

[0086] This allows the zinc oxide support particles to successfully support the zinc oxide nanoparticles doped with the dopant.

[0087] In addition, in the light emitting device according to aspect 5 of the present disclosure, in any one of aspects 1 to 4 above, the median diameter (D50) of the zinc oxide support particles may be within a range of 10 nm to 60 nm.

[0088] This also allows the zinc oxide support particles to successfully support the zinc oxide nanoparticles doped with the dopant, and allows the light generated in the light-emitting layer to successfully transmit from the reflective anode to the transparent cathode.

[0089] In addition, in the light-emitting device according to aspect 6 of the present disclosure, in any one of aspects 1 to 5, the median diameter (D50) of the zinc oxide nanoparticles doped with the dopant may be within a range of 5 nm to 15 nm.

[0090] This also makes it possible to successfully support zinc oxide nanoparticles containing metal atoms as dopants on zinc oxide support particles.

[0091] A display device according to a seventh aspect of the present disclosure includes a substrate, and a plurality of the light-emitting elements according to any one of the first to sixth aspects on the substrate.

[0092] According to an eighth aspect of the present disclosure, in the display device of the seventh aspect, each of the plurality of light-emitting elements may be a light-emitting element that emits light of the same color.

[0093] A display device according to one aspect 8 of the present disclosure, in accordance with aspect 7 above, may be such that the plurality of light-emitting elements include a first light-emitting element, a second light-emitting element, and a third light-emitting element, the first light-emitting element having a first light-emitting layer as the light-emitting layer, the second light-emitting element having a second light-emitting layer as the light-emitting layer having a different emission peak wavelength from the first light-emitting layer, and the third light-emitting element having a third light-emitting layer as the light-emitting layer having a different emission peak wavelength from the first light-emitting layer and the second light-emitting layer.

[0094] The display devices of the sixth to eighth aspects can be display devices equipped with light-emitting elements having high external quantum efficiency.

[0095] The present disclosure is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present disclosure. EXAMPLES

[0096] An embodiment of the present disclosure is described below.

[0097] Light-emitting devices of Examples 1 and 2 and Comparative Examples 1 and 2 each having a different ETL were fabricated, and their photoelectric properties were evaluated.

[0098] (Preparation of ETL-forming composition) As shown below, compositions of Examples 1 and 2 and Comparative Examples 1 and 2 having different mass ratios of ZnO:MgZnO were prepared. Example 1: MgZnO@ZnO-NPs (ZnO:MgZnO mass ratio=3:1) Example 2: MgZnO@ZnO-NPs (ZnO:MgZnO mass ratio=5:1) Comparative Example 1: ZnO-NPs Comparative example 2; MgZnO-NPs For Example 1, a dispersion of MgZnO@ZnO-NPs was prepared according to the scheme shown in FIG. 6. First, an ethanol dispersion of MgZnO-NPs (concentration 20 mg / mL, D50 = 5 nm, MgZnO) was added to an ethanol dispersion of ZnO-NPs (concentration 5 wt%, D50 = 11 nm, ZnO) while stirring with a magnetic stirrer so that ZnO:MgZnO was at a predetermined mass ratio. This resulted in a mixed dispersion in which ZnO and MgZnO were uniformly dispersed. Then, an evaporator was used to remove the ethanol contained in the mixed dispersion to obtain a solid content. Octanol was added to the obtained solid content and stirred to obtain a redispersion in which ZnO carrying MgZnO was reversed micellized in octanol. The obtained redispersion was centrifuged at 4000 rpm for 3 minutes or more to confirm that there was no precipitate in the redispersion, thereby obtaining a stable dispersion of nanoparticles. The redispersion liquid was filtered using a syringe filter having a pore size of 0.22 μm to obtain the composition of Example 1 containing MgZnO@ZnO-NPs.

[0099] Next, a composition of Example 2 was obtained according to the same procedure as in Example 1, except that the mass ratio of ZnO:MgZnO was changed from 3:1 to 5:1. Subsequently, a composition of Comparative Example 1 was obtained according to the same procedure as in Example 1, except that only the ethanol dispersion of ZnO-NPs was used. Similarly, a composition of Comparative Example 2 was obtained according to the same procedure as in Example 1, except that only the ethanol dispersion of MgZnO-NPs was used.

[0100] (Creating light-emitting elements) Using each of the compositions of Examples 1 and 2 and Comparative Examples 1 and 2, a light-emitting device was formed having a cathode, an electron transport layer (ETL), an emissive layer (EML), a hole transport layer (HTL), a hole injection layer (HIL), and an anode.

[0101] The materials for forming each layer are as follows: Cathode: ITO ETL: Examples 1 and 2, Comparative Examples 1 and 2 (film thickness 90 nm) EML:QD HTL: Poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4'-(N-(4-sec-butylphenyl))diphenylamine)] (TFB) HIL:NiO-NPs (film thickness 75nm) Anode: ITO / Ag / ITO The composition of Example 1 was spin-coated on the EML of a substrate having an anode, HIL, HTL, and EML formed in this order from the backplane, and heated at 80° C. for 15 minutes to form an ETL. A cathode was then formed on the ETL by sputtering, thereby producing the light-emitting device of Example 1. The light-emitting devices of Example 2 and Comparative Examples 1 and 2 were then produced following the same procedure as in Example 1.

[0102] For each of the light-emitting devices of Examples 1 and 2 and Comparative Examples 1 and 2, the photoelectric characteristics (current density-external quantum efficiency (EQE)) were evaluated.

[0103] The evaluation results of the photoelectric properties (current density-external quantum efficiency (EQE)) are shown in Table 1. As shown in Table 1, when the current density is the same, the light-emitting device having an ETL of ZnO nanoparticles carrying MgZnO (MgZnO@ZnO-NPs), such as the light-emitting devices of Examples 1 and 2, has a higher external quantum efficiency than the light-emitting device having an ETL of ZnO-NPs of a single composition, such as Comparative Example 1, or MgZnO-NPs, such as Comparative Example 2.

[0104] [Table 1] [Explanation of symbols]

[0105] 1 Light emitting element 1R Red light emitting element (light emitting element) 1G Green light emitting element (light emitting element) 1B Blue light-emitting element (light-emitting element) 10, 10R, 10G, 10B First electrode (anode) 11 Functional Layer 11HI Hole injection layer (hole transport layer) 11HT Hole transport layer 11EM Light Emitting Layer 11REM Red light emitting layer 11GEM Green light-emitting layer 11BEM Blue light-emitting layer 11ET electron transport layer 12 Second electrode (cathode) 13 Bank (transparent resin layer) 100 display device 3. Barrier layer 4 Thin Film Transistor Layer 6 Sealing layer 20 Substrate 21, 22, 23 Inorganic insulating film 24 Planarization film 26, 28 Inorganic sealing film 27 Organic film 30 Functional Film PIX RSP Red sub-pixel GSP Green sub-pixel BSP Blue subpixel TR Transistor G gate electrode D drain electrode S source electrode DA display area NDA Frame Area

Claims

1. A first electrode; A second electrode; A functional layer provided between the first electrode and the second electrode, the functional layer includes a hole transport layer, a light emitting layer, and an electron transport layer; the hole transport layer comprises nickel oxide nanoparticles; the electron transport layer comprises composite zinc oxide nanoparticles; The composite zinc oxide nanoparticles are a light-emitting device comprising zinc oxide carrier particles carrying zinc oxide nanoparticles doped with metal atoms as a dopant.

2. The light-emitting device of claim 1 , wherein the metal atom is selected from the group consisting of Mg, Li, and Al.

3. 3. The light-emitting element according to claim 1, wherein the mass ratio of the zinc oxide carrier particles to the zinc oxide nanoparticles doped with the dopant in the composite zinc oxide nanoparticles is within a range of 2:1 to 1:

5.

4. 3. The light-emitting element according to claim 1, wherein a median diameter (D50) of the zinc oxide support particles is larger than a median diameter (D50) of the zinc oxide nanoparticles doped with the dopant.

5. 3. The light-emitting device according to claim 1, wherein the median diameter (D50) of the zinc oxide support particles is within a range of 10 to 60 nm.

6. 3. The light-emitting element according to claim 1, wherein the median diameter (D50) of the zinc oxide nanoparticles doped with the dopant is within a range of 5 to 15 nm.

7. A substrate; A display device comprising a plurality of the light-emitting elements according to claim 1 or 2 on the substrate.

8. The display device according to claim 7 , wherein each of the plurality of light-emitting elements emits light of the same color.

9. The plurality of light emitting elements include a first light emitting element, a second light emitting element, and a third light emitting element, The first light emitting element includes a first light emitting layer as the light emitting layer, the second light emitting element includes, as the light emitting layer, a second light emitting layer having an emission peak wavelength different from that of the first light emitting layer, The display device according to claim 7 , wherein the third light emitting element includes, as the light emitting layer, a third light emitting layer having a different emission peak wavelength from the first light emitting layer and the second light emitting layer.

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