Light-emitting element 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

Light-emitting element and display device

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

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

[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 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).

[0005] International Publication No. WO2020-041993

[0006] The nickel oxide nanoparticles used in the light-emitting device described in Patent Document 1 have a high hole injection capability as a material for a hole injection layer (hole transport layer (HTL)), but in contrast, ZnO has a low electron transport capability as a material for an 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 conventional problems, and its purpose is to provide a light-emitting device and a display device that are equipped with a novel electron transport layer that 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.

[0008] In order to solve the above problems, a light-emitting element according to one aspect 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 supporting 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 aspect 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.

[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 uses nickel oxide nanoparticles in a hole transport layer while maintaining the high hole injection ability of the nickel oxide nanoparticles and achieving carrier balance.

[0011] 1A is a cross-sectional view showing a schematic configuration of a light-emitting element 1 according to an embodiment of the present disclosure. It is a diagram illustrating an outline of MgZnO@ZnO-NPs contained in an electron transport layer 11ET included in the light-emitting element 1 according to an embodiment of the present disclosure. (a) is a cross-sectional view showing a schematic configuration of a red light-emitting element 1R according to an embodiment of the present disclosure. (b) is a cross-sectional view showing a schematic configuration of a green light-emitting element 1G included in a display device according to an embodiment of the present disclosure. (c) is a cross-sectional view showing a schematic configuration of a blue light-emitting element 1B included in a display device according to an embodiment of the present disclosure. It is a plan view showing a schematic configuration of a display region of a display device 100 according to an embodiment of the present disclosure. It is a cross-sectional view showing a schematic configuration of a display region of a display device 100 according to an embodiment of the present disclosure. It is a scheme illustrating an outline of a method for producing composite zinc oxide nanoparticles (MgZnO@ZnO-NPs) included in a hole transport layer in a light-emitting element according to an embodiment of the present disclosure.

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

[0013] Unless otherwise specified in this specification, "A to B" representing 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 quantum dot light-emitting elements including quantum dot light-emitting diodes (QLEDs), 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 device 1] First, the configuration of a light-emitting device 1 including a quantum dot light-emitting diode (QLED) according to 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 according to the present embodiment.

[0017] 1 , the light-emitting element 1 in this embodiment includes, on a first electrode 10 serving as an anode, a hole injection layer (HIL: Hole Injection Layer) 11HI, a hole transport layer (HTL: Hole Transport Layer) 11HT, an emission layer (EML: Emission Layer) 11EM, an electron transport layer (ETL: Electron Transport Layer) 11ET, and a second electrode 12 serving as a cathode, in this order. 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 a top-emission type or a bottom-emission type. In the light-emitting element 1, the second electrode 12 serving as a cathode is disposed above the first electrode 10 serving as an anode, the first electrode 10 serving as an anode is formed from an electrode material that reflects visible light, and the second electrode 12 serving as a cathode is formed from an electrode material that transmits visible light. To make the light-emitting element 1 a bottom-emission type, the second electrode 12 serving as a cathode is disposed above the first electrode 10 serving as an anode, the first electrode 10 serving as an anode is formed from an electrode material that transmits visible light, and the second electrode 12 serving as 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 conductivity, and examples thereof include metal materials such as Al, Cu, Au, Mg, Li, and Ag, alloys of the above metal materials, laminates of the above metal materials and transparent metal oxides (e.g., indium tin oxide, indium zinc oxide, indium gallium zinc oxide, etc.), and laminates of the above alloys and the above 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 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 formation method, for example, a physical vapor deposition (PVD) method such as vacuum deposition, sputtering, EB deposition, or ion plating, or a chemical vapor deposition (CVD) method. The first electrode 10 and the second electrode 12 can be patterned by any method that can accurately form a desired pattern, but specific examples include photolithography and inkjet printing.

[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 serving as an anode and a second electrode 12 serving as a cathode provided above the first electrode 10. The functional layer 11 including the light-emitting layer provided between the first electrode 10 serving as an anode and the second electrode 12 serving as a cathode can be formed, 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 this order from the first electrode 10 side. Furthermore, although not shown, the light-emitting element having an inverted stack structure includes a first electrode serving as a cathode and a second electrode serving as an anode provided above the first electrode. The functional layer including the light-emitting layer provided between the first electrode serving as a cathode and the second electrode serving as an anode can be formed, 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 this 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 multiple hole transport layers to increase 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. The hole injection layer 11HI injects holes from the first electrode 10, which is the anode, into the hole transport layer 11HT. The hole injection layer also transports holes from the first electrode, which is the anode, to the light-emitting layer, 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 by applying the dispersion liquid to each sub-pixel separately by an inkjet method, etc. The dispersion liquid of nickel oxide nanoparticles may be mixed with a dispersing material such as thiol or amine.

[0025] The nickel oxide nanoparticles have a positive surface charge and may have a positive zeta potential. The nickel oxide nanoparticles may have a ligand, similar to the quantum dots that may be contained in the light-emitting layer 11EM and the composite nickel oxide particles that are contained in the electron transport layer 11ET, which will be described later. Alternatively, the nickel oxide nanoparticle dispersion may contain a ligand as a dispersing material.

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

[0027] The 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 on a volume basis, and may be in the range of 10 nm to 500 nm, and preferably in 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 preferably has high hole mobility. Furthermore, the hole transport material is preferably a material (electron blocking material) that can prevent electrons moving from the second electrode 12, which is the cathode, from passing through. 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). As described above, the hole transport layer 11HT can also 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 to the sub-pixels of the light-emitting elements 1 collectively 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 due to 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 this embodiment, the light-emitting layer 11EM is a quantum dot light-emitting layer that includes quantum dots (QDs: semiconductor nanoparticles) of each color as a light-emitting material, but is not limited to this and may also be an OLED (organic light-emitting diode).

[0032] The quantum dots (QDs) may have, for example, a core structure, a core / shell structure, a core / shell / shell structure, or a shell structure with a continuously varying core / shell ratio. When the quantum dots (QDs) have a core structure, a ligand is provided on the surface of the core, and when the quantum dots (QDs) have a shell structure, a ligand is provided on the surface of the shell. The core portion may be composed of, for example, Si, C, etc. in the case of a unicomponent system, for example, CdSe, CdS, CdTe, InP, GaP, InN, ZnSe, ZnS, ZnTe, etc. in the case of a ternary system, for example, CdSeTe, GaInP, ZnSeTe, etc., or, for example, AIGS, etc. in the case of a quaternary system. 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] Quantum dots (QDs) refer to nanoscale semiconductor crystals with unique optical properties based on quantum mechanics. The shape of the quantum dots (QDs) 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, they may have a polygonal cross-sectional shape, a rod-like three-dimensional shape, a branch-like three-dimensional shape, or a three-dimensional shape with an uneven surface, or a combination thereof.

[0034] The light-emitting layer 11EM can be formed by applying a dispersion of quantum dots in a solvent such as hexane, toluene, octadecane, cyclododecene, or phenylcyclohexane to each subpixel using a spin coating method, inkjet printing, or the like. The quantum dots may have ligands on their surfaces, and the quantum dot dispersion may contain a dispersing material such as a thiol or an amine. Alternatively, 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 illustrating an 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 are formed by supporting zinc oxide nanoparticles (MgZnO-NPs) doped with magnesium (Mg) as a dopant on zinc oxide support particles (ZnO-NPs). When composite zinc oxide nanoparticles in which zinc oxide nanoparticles doped with magnesium (Mg) as a dopant are supported on the surfaces of nanoparticulate zinc oxide support particles, as illustrated in FIG. 2, the composite zinc oxide nanoparticles may also be referred to as MgZnO-supported ZnO nanoparticles or MgZnO@ZnO-NPs. In the case of MgZnO@ZnO-NPs, it is intended that the ZnO-NPs are the support particles, and the MgZnO-NPs are particles supported on the support particles.

[0036] As shown in Figure 2, MgZnO@ZnO-NPs are expected to have a structure in which the MgZnO-NPs supported on the ZnO-NPs form irregularities on the surface of the ZnO-NPs, while the surface of the 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 a continuously changing core / shell ratio that the above-mentioned quantum dots (QDs) may have. Furthermore, the composite zinc oxide nanoparticles exemplified by MgZnO@ZnO-NPs in Figure 2 may have a ligand coordinated to its surface. When the ZnO-NPs / ligand shown in Figure 2 has an organic ligand, it may be intended that a portion of the ligand as a dispersed material is coordinated to the surface of the ZnO-NPs. 2 is provided with an inorganic ligand, the ZnO-NPs / ligand may be intended to be ZnO-NPs in which a core / shell structure is formed by the inorganic ligand. The ZnO-NPs / ligand may be provided with an inorganic ligand and may further be provided with an organic ligand.

[0037] The dispersion containing MgZnO@ZnO-NPs (MgZnO@ZnO-NPs dispersion) may contain a ligand as a dispersing material, and examples of the dispersing 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 the same ligands as those used in the ZnO support particles and doped ZnO nanoparticles described below.

[0038] The electron transport layer 11ET transports electrons from the second electrode 12, which is the cathode, to the light-emitting layer 11EM using an electron transport material. Composite zinc oxide nanoparticles (composite ZnO nanoparticles) have doped zinc oxide nanoparticles supported on the surfaces of zinc oxide support particles (ZnO support particles), so that the energy level of the conduction band minimum (CBM), which corresponds to the lowest unoccupied molecular orbital (LUMO), is higher than that of zinc oxide nanoparticles (ZnO nanoparticles) that do not contain a dopant. 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 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 is sometimes referred to as MgZnO, AlZnO, LiZnO, etc., respectively. ZnO nanoparticles doped with a dopant are sometimes 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. When the mass ratio of ZnO carrier particles to doped ZnO nanoparticles is 1:2 to 5:1, the larger the mass ratio of ZnO carrier particles, the better the carrier balance, which can improve the voltage-current response of the light-emitting device and increase the external quantum efficiency (EQE) of the light-emitting device.

[0041] The median diameter (D50) of the ZnO support 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 support particles by van der Waals forces. For convenience, the ZnO support particles are referred to herein as ZnO support particles to distinguish them from doped ZnO nanoparticles, but the ZnO support particles may be ZnO nanoparticles that are substantially not doped with a dopant.

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

[0043] The ZnO support particles may include a ligand. Examples of the ligand coordinated to the ZnO support particles include organic compounds having a functional group and a hydrocarbon group. The functional group of the ligand coordinates to the ZnO support particles, and the hydrocarbon group can enhance the dispersion stability of the ZnO support particles in polar solvents, alcohol-based solvents, and the like, as described below. Examples of functional groups possessed by the ligand include functional groups capable of coordinating to the surface of the ZnO support particles, such as amino groups, thiol groups, carboxyl groups, hydroxyl groups, and phosphonyl groups. The ligand may have multiple functional groups, such as diamines and dithiols, or multiple types of functional groups, such as carbamates and thiol-carboxylic acids. The hydrocarbon group possessed by 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 support particles may have multiple types of ligands coordinated thereto. Examples of the ligand include oleylamine, octanethiol, tributylphosphine oxide, etc. Examples of the ligand include an anion moiety constituting a quaternary ammonium salt such as tetrabutylammonium tetrafluoroborate, and the anion moiety may be coordinated to the ZnO support particle as a ligand. Alternatively, the ligand provided on the ZnO support particle may be any 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] The doped ZnO nanoparticles may also have a ligand. The ligand coordinated to the doped ZnO nanoparticles can be selected depending on the type of the doped ZnO nanoparticles. The ligand coordinated to the doped ZnO nanoparticles may be the same as the ligands of the ZnO support particles, and therefore, the description thereof will be omitted.

[0046] Composite ZnO nanoparticles, including MgZnO-supported ZnO nanoparticles, can be obtained by mixing a dispersion of ZnO support 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 support particles.

[0047] The dispersion of ZnO support particles contains ZnO support particles and an organic solvent, and may further contain a ligand that coordinates with the ZnO support particles. Examples of the organic solvent contained in the dispersion include alcohol-based solvents such as ethanol. The dispersion of ZnO support particles may contain a known dispersing material as a ligand.

[0048] The dispersion of doped ZnO nanoparticles contains doped ZnO nanoparticles and an organic solvent, and may further contain a ligand that coordinates with the doped ZnO particles. Examples of the organic solvent contained in the dispersion include alcoholic solvents such as ethanol. The dispersion of doped ZnO nanoparticles, like the dispersion of ZnO support particles, may contain a known dispersing material as a ligand.

[0049] In the dispersion of ZnO support particles and the dispersion of doped ZnO nanoparticles, the type of ligand coordinated to the ZnO support particles and the type of ligand coordinated to the doped ZnO nanoparticles may be different or the same. Here, the dispersion of ZnO support particles and the dispersion of doped ZnO nanoparticles are preferably dispersions containing mutually compatible organic solvents or the same organic solvent. The compatibility of the organic solvents of the dispersion of doped ZnO nanoparticles and the dispersion of ZnO support particles can enhance the dispersion stability of the ligand-coordinated ZnO support particles and the ligand-coordinated doped ZnO nanoparticles. This can prevent unintended clouding of the mixed dispersion when the ZnO support particles support the doped ZnO nanoparticles.

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

[0051] By drying the mixed dispersion obtained by mixing the respective dispersions, the ligands of the ZnO support particles are evacuated, allowing the doped ZnO nanoparticles to be supported on the ZnO support particles. This allows composite ZnO particles to be obtained. The obtained composite ZnO nanoparticles can be dispersed to a desired concentration in an organic solvent, such as hexanol or octanol, to obtain a dispersion of composite ZnO nanoparticles. The composite ZnO nanoparticles can be stabilized by reverse micellization in the organic solvent by the ligands coordinated to the ZnO support particles and / or the ligands coordinated to the doped ZnO nanoparticles. The composite ZnO nanoparticle dispersion can then be applied, for example, by spin coating or inkjet coating, to form an electron transport layer 11ET with high flatness. After applying the composite ZnO nanoparticle dispersion, the substrate can be dried by heating, for example, at a temperature of 25 to 110°C, preferably 25 to 80°C. After the electron transport layer 11ET is dried by heating, the electron transport layer 11ET may be washed or rinsed with an organic solvent.

[0052] Alternatively, the electron transport layer 11ET containing composite ZnO nanoparticles may be formed by depositing composite ZnO nanoparticles on the light-emitting layer 11EM by electrophoretic deposition.

[0053] The electron transport layer 11ET formed as described above preferably has a thickness in the range of 20 nm to 200 nm, and more preferably in the range of 50 nm to 150 nm. This provides a cavity effect, which enhances the directivity of light emitted from the light-emitting layer 11EM via the electron transport layer 11ET. From another perspective, having the electron transport layer 11ET have a thickness of 50 nm or more can mitigate damage to the light-emitting layer 11EM when forming the second electrode 12, for example, by an ITO sputtering process.

[0054] In a conventional method, the electron transport layer of a light-emitting device is formed by a layer containing a metal oxide such as zinc oxide (ZnO) and a layer containing a metal-doped zinc oxide (M ｘ Zn ｙIn some cases, the light-emitting layer contains quantum dots, and the two-layer electron transport layer is exposed to the solvent of the dispersion liquid for forming the electron transport layer and heated at least twice. As a result, the two-layer electron transport layer becomes less flat, which increases the likelihood of uneven electron movement in the electron transport layer and non-uniform light emission. Furthermore, the metal oxide nanoparticles in the electron transport layer are unevenly distributed, which may cause thermal damage to the light-emitting layer below the electron transport layer when a cathode is applied on the electron transport layer.

[0055] In this regard, one of the advantages of the light-emitting element 1 according to one embodiment is that the electron transport layer 11ET not only achieves a carrier balance with the hole injection layer 11HI due to a high CBM, but also that the electron transport layer 11ET is formed without impairing 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] Furthermore, 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, thereby improving the voltage-current response of the light-emitting element 1 and increasing 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, red light refers to light having a central emission wavelength in a wavelength band of more than 600 nm and not more than 780 nm. Green light refers to light having a central emission wavelength in a wavelength band of more than 500 nm and not more than 600 nm. Furthermore, blue light refers to 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] (a) of Figure 3 shows a red light-emitting element 1R having a functional layer 11R including a red light-emitting layer 11REM that emits red light, (b) of Figure 3 shows a green light-emitting element 1G having a functional layer 11G including a green light-emitting layer 11GEM that emits green light, and (c) of Figure 3 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 quantum dot light-emitting diodes (QLEDs) as an example, but the present invention is not limited thereto. A portion of the red light-emitting element 1R, the green light-emitting element 1G, and the blue light-emitting element 1B may be QLEDs, and the remaining portions of the red light-emitting element 1R, the green light-emitting element 1G, and the blue light-emitting element 1B may be OLEDs (organic light-emitting diodes). 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 layers included in the light-emitting elements of each color are light-emitting layers containing quantum dots formed by, for example, a coating method or an inkjet method. 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 layers included in the light-emitting elements of each color are organic light-emitting layers formed by, for example, a vapor 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. 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. Note that the insulating bank 13 (resin layer) covering each edge of the first electrode 10R, the first electrode 10B, and the first electrode 10B can be formed by applying an organic material such as polyimide or acrylic and then patterning it on a planarizing film (described later) by photolithography.

[0061] Furthermore, the functional layers 11R, 11G, and 11B shown in Figures 3(a) to 3(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 each having a functional layer between a first electrode and a second electrode, the functional layer including, in this order, a hole transport layer containing nickel oxide nanoparticles, a light-emitting layer, and an electron transport layer containing composite ZnO nanoparticles, and the light-emitting elements are disposed on a substrate. Hereinafter, a plurality of light-emitting elements emitting light with different emission peak wavelengths will be described, but this is not limited thereto, and each of the plurality of light-emitting elements may be light-emitting elements emitting the same color. Furthermore, in one embodiment, the functional layer included in the light-emitting element in the display device may include nickel oxide nanoparticles in the hole transport layer or hole injection layer. 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 a required component.

[0063] Fig. 4 is a plan view showing a schematic configuration of the display device 100 according to 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 region NDA and a display region DA. The display region 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 configured with a red subpixel RSP, a green subpixel GSP, and a blue subpixel BSP will be described as an example, but this is not limiting. 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 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, as shown in Fig. 5. The light-emitting elements and control circuits including transistors TR provided for each of the red subpixel RSP, green subpixel GSP, and blue subpixel BSP are collectively referred to as subpixel circuits.

[0066] 5, in a display area DA of the display device 100, a barrier layer 3, a thin-film transistor layer 4 including a transistor TR, red light-emitting elements 1R, green light-emitting elements 1G, blue light-emitting elements 1B, and 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 (active matrix substrate) 2 provided with first electrodes.

[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. 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, since the display device 100 is 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. If 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, a silicon oxynitride film, or a laminated film of these, formed by the CVD method.

[0070] The transistor TR portion of the thin film transistor layer 4 including the transistor TR includes a semiconductor film SEM and 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 and 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 case where the transistor TR has a top-gate structure will be described as an example, but the present invention is not limited to this, and the transistor TR may also 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 films 21, 22 and 23 can be formed of, for example, 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.

[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, and 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. Note that the insulating bank 13 (transparent resin layer) covering each edge of the first electrode 10R, the first electrode 10B, and the first electrode 10B can be formed 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 that covers the second electrode 12, an organic film 27 that is above the inorganic sealing film 26, and an inorganic sealing film 28 that is above the organic film 27. The sealing layer 6 prevents foreign substances 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 films 26 and 28 are each an inorganic film, and may be formed, for example, by a CVD method using a silicon oxide film, a silicon nitride film, a silicon oxynitride film, or a laminated film thereof. The organic film 27 is a light-transmitting organic film with a planarizing effect, and may be formed, for example, using a coatable organic material such as acrylic. The organic film 27 may also be formed, for example, by an inkjet method. In this embodiment, the sealing layer 6 is formed of two inorganic films and one organic film disposed between the two inorganic films. However, the stacking order of the two inorganic films and one organic film is not limited to this. Furthermore, the sealing layer 6 may be formed solely of an inorganic film, solely of an organic film, one inorganic film and two organic films, or two or more inorganic films and two or more organic films.

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

[0079] As described above, the light-emitting element according to aspect 1 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 device 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-mentioned aspect 1, it is preferable that the dopant is 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 maintain the high hole injection ability of the nickel oxide nanoparticles, thereby achieving a carrier balance between the hole transport layer and the electron transport layer.

[0083] In addition, in the light-emitting element according to aspect 3 of the present disclosure, in the above-mentioned aspect 1 or 2, the mass ratio of the zinc oxide support particles to 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 with high external quantum efficiency.

[0085] Furthermore, in the light-emitting element according to aspect 4 of the present disclosure, in any one of aspects 1 to 3 above, it is preferable that the median diameter (D50) of the zinc oxide support particles is 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] Furthermore, in the light-emitting device according to aspect 5 of the present disclosure, in any one of aspects 1 to 4, the median diameter (D50) of the zinc oxide support particles may be in the 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] Furthermore, 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 in the range of 5 nm to 15 nm.

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

[0091] A display device according to a seventh aspect of the present disclosure includes a substrate and a plurality of 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 is, in the above-mentioned aspect 7, a display device in which 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 Aspects 6 to 8 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.

[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 (mass ratio of ZnO:MgZnO = 3:1) Example 2: MgZnO@ZnO-NPs (mass ratio of ZnO:MgZnO = 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 ZnO-NPs (concentration 5 wt%, D50 = 11 nm, ZnO) was stirred with a magnetic stirrer to achieve a predetermined ZnO:MgZnO mass ratio, while an ethanol dispersion of MgZnO-NPs (concentration 20 mg / mL, D50 = 5 nm, MgZnO) was added. This resulted in a mixed dispersion in which ZnO and MgZnO were uniformly dispersed. Subsequently, an evaporator was used to remove the ethanol contained in the mixed dispersion, yielding a solid. Octanol was added to the obtained solid, and the mixture was stirred to obtain a redispersion of ZnO supported on MgZnO in octanol as reverse micelles. The obtained redispersion was centrifuged at 4000 rpm for 3 minutes or more, and the absence of precipitates in the redispersion was confirmed, yielding a stable dispersion of nanoparticles. The re-dispersion liquid was filtered using a syringe filter with 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 an 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 an ethanol dispersion of MgZnO-NPs was used.

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

[0101] The materials used to form 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 75 nm) Anode: ITO / Ag / ITO On a substrate having an anode, HIL, HTL, and EML formed in this order from the backplane, the composition of Example 1 was spin-coated onto the EML, and the substrate was heated at 80°C for 15 minutes to form an ETL. Thereafter, a cathode was formed on the ETL by sputtering, thereby producing the light-emitting device of Example 1. Light-emitting devices of Example 2 and Comparative Examples 1 and 2 were then produced according to the same procedure as in Example 1.

[0102] The light-emitting devices of Examples 1 and 2 and Comparative Examples 1 and 2 were evaluated for their photoelectric characteristics (current density-external quantum efficiency (EQE)).

[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 devices having an ETL of ZnO nanoparticles carrying MgZnO (MgZnO@ZnO-NPs), such as the light-emitting devices of Examples 1 and 2, have higher external quantum efficiency than light-emitting devices having an ETL of ZnO-NPs with a single composition, such as Comparative Example 1, or MgZnO-NPs, such as Comparative Example 2.

[0104]

[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 Pixel RSP Red sub-pixel GSP Green sub-pixel BSP Blue sub-pixel 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.