Light-emitting element, display device, dispersion of metal oxide nanoparticles, metal oxide nanoparticles, and method for producing same

Abstract

A light-emitting element (10) comprises an anode (2), a cathode (6), a light-emitting layer (EM) disposed between the anode (2) and the cathode (6), and a hole injection layer (3) disposed between the anode (2) and the light-emitting layer (EM). The hole injection layer (3) contains a plurality of metal oxide nanoparticles including a plurality of Ni atoms, a plurality of oxygen atoms, and a plurality of Mg atoms. In at least some unit lattices among a plurality of unit lattices in the crystal structure of the metal oxide nanoparticles, a plurality of Ni atoms and a plurality of oxygen atoms are arranged to surround one Mg atom.

Description

Light-emitting element, display device, dispersion of metal oxide nanoparticles, metal oxide nanoparticles and method for producing the same

[0001] The present disclosure relates to a light-emitting element, a display device, a dispersion of metal oxide nanoparticles, metal oxide nanoparticles, and a method for producing metal oxide nanoparticles.

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

[0003] Patent Document 1 describes a light-emitting device having a hole injection layer made of nickel oxide.

[0004] Japanese Patent Publication No. 2012-23388

[0005] The hole injection layer made of nickel oxide described in Patent Document 1 and the hole injection layer made of nickel oxide nanoparticles commonly used in the fields of OLEDs and QLEDs have a problem in that satisfactory electrical characteristics cannot be obtained. Furthermore, in light-emitting elements including such hole injection layers and display devices including such light-emitting elements, the driving voltage increases, resulting in a problem of a deterioration in electrical characteristics, which indicate the relationship between driving voltage and luminance or between driving voltage and current density.

[0006] One aspect of the present disclosure aims to provide metal oxide nanoparticles with improved electrical properties and a method for producing the same, a metal oxide nanoparticle dispersion containing metal oxide nanoparticles with improved electrical properties, and a light-emitting element and a display device with improved electrical properties.

[0007] In order to solve the above-mentioned problems, the metal oxide nanoparticles of the present disclosure contain a plurality of Ni atoms, a plurality of oxygen atoms, and a plurality of Mg atoms, and in at least some of the unit cells of the crystal structure, the plurality of Ni atoms and the plurality of oxygen atoms are arranged so as to surround one Mg atom.

[0008] In order to solve the above-mentioned problems, the dispersion of metal oxide nanoparticles of the present disclosure contains the metal oxide nanoparticles and a solvent.

[0009] In order to solve the above-mentioned problems, the light-emitting element of the present disclosure includes an anode, a cathode, a light-emitting layer provided between the anode and the cathode, and a hole injection layer provided between the anode and the light-emitting layer, wherein the hole injection layer includes a plurality of metal oxide nanoparticles each including a plurality of Ni atoms, a plurality of oxygen atoms, and a plurality of Mg atoms, and in at least a portion of the unit cells of the crystal structure of the metal oxide nanoparticles, the plurality of Ni atoms and the plurality of oxygen atoms are arranged so as to surround one Mg atom.

[0010] In order to solve the above-mentioned problems, the display device of the present disclosure includes the light-emitting element.

[0011] In order to solve the above-mentioned problems, the method for producing metal oxide nanoparticles of the present disclosure includes the following steps: a first step of preparing a mixed solution containing a plurality of first precursors containing Ni atoms, a plurality of second precursors containing Mg atoms, and a first solvent; a second step of adjusting the pH of the mixed solution to produce a suspension; a third step of stirring the suspension; a fourth step of centrifuging the stirred suspension obtained in the third step to obtain a precipitate; a fifth step of adding a second solvent to the precipitate obtained in the fourth step and centrifuging again; a sixth step of drying the precipitate recovered by centrifugation in the fifth step; a seventh step of pulverizing the dried precipitate obtained in the sixth step; and an eighth step of heat-treating the pulverized powder obtained in the seventh step.

[0012] According to one aspect of the present disclosure, it is possible to provide metal oxide nanoparticles with improved electrical properties and a method for producing the same, a metal oxide nanoparticle dispersion containing metal oxide nanoparticles with improved electrical properties, and a light-emitting element and a display device with improved electrical properties.

[0013] 1 is a plan view showing a schematic configuration of a display device of embodiment 1. FIG. 2 is a cross-sectional view showing a schematic configuration of a light-emitting device provided in the display device of embodiment 1. FIG. 3 is a diagram for explaining a method for manufacturing metal oxide nanoparticles containing a plurality of Ni atoms, a plurality of oxygen atoms, and a plurality of Mg atoms, which are contained in a hole injection layer provided in the light-emitting device shown in FIG. 2. FIG. 4 is a diagram comparing a unit lattice of the crystal structure of metal oxide nanoparticles containing a plurality of Ni atoms, a plurality of oxygen atoms, and a plurality of Mg atoms, which are contained in a hole injection layer provided in the light-emitting device shown in FIG. 2, with a unit lattice of the crystal structure of conventional nickel oxide nanoparticles. FIG. 5 is a diagram showing X-ray diffraction (XRD) spectra of conventional nickel oxide nanoparticles and metal oxide nanoparticles containing a plurality of Ni atoms, a plurality of oxygen atoms, and a plurality of Mg atoms, which are contained in a hole injection layer provided in the light-emitting device shown in FIG. 2. FIG. 6 is a band diagram of conventional nickel oxide nanoparticles and metal oxide nanoparticles containing a plurality of Ni atoms, a plurality of oxygen atoms, and a plurality of Mg atoms, which are contained in a hole injection layer provided in the light-emitting device shown in FIG. 2.

[0014] The following describes an embodiment of the present disclosure with reference to Figures 1 to 6. For the sake of convenience, components having the same functions as those described in a specific embodiment will be denoted by the same reference numerals, and their description may be omitted.

[0015] First Embodiment FIG. 1 is a plan view showing a schematic configuration of a display device 1 according to a first embodiment.

[0016] As shown in Fig. 1, the display device 1 includes a frame area NDA and a display area DA. The display area DA of the display device 1 includes a plurality of pixels PIX, each of which includes a red subpixel RSP, a green subpixel GSP, and a blue subpixel BSP. In this embodiment, 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.

[0017] FIG. 2 is a cross-sectional view showing a schematic configuration of the light-emitting element 10 provided in the display device 1 of the first embodiment.

[0018] The red subpixel RSP, green subpixel GSP, and blue subpixel BSP provided in the display area DA of the display device 1 each include a light-emitting element 10 shown in Fig. 2. Specifically, the red subpixel RSP includes a red light-emitting element in which the light-emitting layer EM provided in the light-emitting element 10 shown in Fig. 2 is a red light-emitting layer, the green subpixel GSP includes a green light-emitting element in which the light-emitting layer EM provided in the light-emitting element 10 shown in Fig. 2 is a green light-emitting layer, and the blue subpixel BSP includes a blue light-emitting element in which the light-emitting layer EM provided in the light-emitting element 10 shown in Fig. 2 is a blue light-emitting layer.

[0019] As shown in FIG. 2 , the light-emitting element 10 includes an anode 2, a cathode 6, an emitting layer EM provided between the anode 2 and the cathode 6, and a hole injection layer 3 provided between the anode 2 and the emitting layer EM. In this embodiment, a case in which the light-emitting element 10 further includes a hole transport layer 4 between the hole injection layer 3 and the emitting layer EM is described as an example, but the present invention is not limited thereto. In the case in which the light-emitting element 10 includes a hole injection layer 3 with improved electrical properties, as described below, only the hole injection layer 3 may be provided between the anode 2 and the emitting layer EM. In addition, in this embodiment, a case in which the light-emitting element 10 includes an electron transport layer 5 between the cathode 6 and the emitting layer EM is described as an example, but the present invention is not limited thereto. An electron injection layer may further be provided between the electron transport layer 5 and the cathode 6, or only an electron injection layer may be provided between the cathode 6 and the emitting layer EM. Both the electron transport layer 5 and the electron injection layer may be omitted as appropriate.

[0020] Fig. 3 is a diagram illustrating a method for producing metal oxide nanoparticles containing a plurality of Ni atoms, a plurality of oxygen atoms, and a plurality of Mg atoms, which are contained in the hole injection layer 3 of the light-emitting element 10 shown in Fig. 2 . Fig. 4 is a diagram comparing the unit cell of the crystal structure of metal oxide nanoparticles containing a plurality of Ni atoms, a plurality of oxygen atoms, and a plurality of Mg atoms, which are contained in the hole injection layer 3 of the light-emitting element 10 shown in Fig. 2 , with the unit cell of the crystal structure of conventional nickel oxide nanoparticles. Fig. 5 is a diagram showing X-ray diffraction (XRD) spectra of conventional nickel oxide nanoparticles and metal oxide nanoparticles containing a plurality of Ni atoms, a plurality of oxygen atoms, and a plurality of Mg atoms, which are contained in the hole injection layer 3 of the light-emitting element 10 shown in Fig. 2 . Fig. 6 is a band diagram of conventional nickel oxide nanoparticles and metal oxide nanoparticles containing a plurality of Ni atoms, a plurality of oxygen atoms, and a plurality of Mg atoms, which are contained in the hole injection layer 3 of the light-emitting element 10 shown in Fig. 2 .

[0021] The hole injection layer 3 provided in the light-emitting element 10 shown in FIG. 2 contains a plurality of metal oxide nanoparticles containing a plurality of Ni atoms, a plurality of oxygen atoms, and a plurality of Mg atoms. In at least some of the unit cells of the crystal structure of the metal oxide nanoparticles containing a plurality of Ni atoms, a plurality of oxygen atoms, and a plurality of Mg atoms, a plurality of Ni atoms and a plurality of oxygen atoms are arranged to surround one Mg atom, as shown on the right side of FIG. 4 . Meanwhile, the left side of FIG. 4 shows a unit cell of the crystal structure of conventional nickel oxide nanoparticles having substantially the same particle size as the metal oxide nanoparticles containing a plurality of Ni atoms, a plurality of oxygen atoms, and a plurality of Mg atoms shown on the right side of FIG. 4 . As can be seen by comparing the left and right views of FIG. 4 , the metal oxide nanoparticles containing a plurality of Ni atoms, a plurality of oxygen atoms, and a plurality of Mg atoms can incorporate one Mg atom into the center of the unit cell while substantially maintaining the crystal structure of the conventional nickel oxide nanoparticles. Therefore, in metal oxide nanoparticles containing multiple Ni atoms, multiple oxygen atoms, and multiple Mg atoms, the incorporation of Mg atoms can improve electrical properties, as described below. In at least some of the unit cells of the crystal structure of metal oxide nanoparticles containing multiple Ni atoms, multiple oxygen atoms, and multiple Mg atoms, multiple Ni atoms and multiple oxygen atoms are arranged to surround one Mg atom. This can be confirmed by the fact that, as shown in Figure 5 (described below), the position of the peak intensity in the X-ray diffraction (XRD) spectrum of metal oxide nanoparticles containing multiple Ni atoms, multiple oxygen atoms, and multiple Mg atoms is unchanged from the position of the peak intensity in the X-ray diffraction (XRD) spectrum of conventional nickel oxide nanoparticles containing a unit cell of the crystal structure shown on the left side of Figure 4. In the process of producing metal oxide nanoparticles containing multiple Ni atoms, multiple oxygen atoms, and multiple Mg atoms, the amount of Mg incorporated (doping amount) can be determined appropriately.In metal oxide nanoparticles containing multiple Ni atoms, multiple oxygen atoms, and multiple Mg atoms, as the amount of Mg incorporated (doping amount) increases, the proportion of unit cells in which multiple Ni atoms and multiple oxygen atoms are arranged to surround one Mg atom increases among the multiple unit cells in the crystal structure of the metal oxide nanoparticles containing multiple Ni atoms, multiple oxygen atoms, and multiple Mg atoms. In at least some of the unit cells of the crystal structure of the metal oxide nanoparticles containing a plurality of Ni atoms, a plurality of oxygen atoms, and a plurality of Mg atoms contained in the hole injection layer 3 of the light-emitting element 10 shown in Fig. 2, the plurality of Ni atoms and the plurality of oxygen atoms may be arranged so as to surround one Mg atom. Therefore, among the unit cells of the crystal structure of the metal oxide nanoparticles containing a plurality of Ni atoms, a plurality of oxygen atoms, and a plurality of Mg atoms, there may be a unit cell in which no Mg atoms are present, and in each of the unit cells of the crystal structure of the metal oxide nanoparticles containing a plurality of Ni atoms, a plurality of oxygen atoms, and a plurality of Mg atoms, the plurality of Ni atoms and the plurality of oxygen atoms may be arranged so as to surround one Mg atom. Note that defects of the Ni atoms, oxygen atoms, and Mg atoms may occur in the unit cells.

[0022] A method for producing metal oxide nanoparticles containing a plurality of Ni atoms, a plurality of oxygen atoms, and a plurality of Mg atoms, which are contained in the hole injection layer 3, will be described below with reference to FIG.

[0023] In the first step (S1) shown in FIG. 3 , a mixed solution containing a plurality of first precursors containing Ni atoms, a plurality of second precursors containing Mg atoms, and a first solvent (solvent) is prepared. In this embodiment, a case where nickel nitrate hexahydrate containing Ni atoms is used as the first precursor, magnesium acetate tetrahydrate containing Mg atoms is used as the second precursor, and water (e.g., pure water) is used as the first solvent is described as an example, but is not limited thereto. In this embodiment, pure water is preferably used as the water, and the pure water may contain a trace amount of deuterium. Note that pure water refers to tap water or industrial water that has been treated in some way to remove impurities, such as ion-exchanged water (deionized water). The metal oxide nanoparticles (Example 1) containing multiple Ni atoms, multiple oxygen atoms, and multiple Mg atoms shown in Figures 5 and 6 are obtained by preparing a mixed solution using 7.3 g of nickel nitrate hexahydrate, 0.5 g of magnesium acetate tetrahydrate, and 20 ml of water. In this case, the Mg doping amount is 10 mol%, and the Ni ion concentration is 1.25 M. As described above, in this embodiment, the case where the Mg doping amount is 10 mol% is described as an example, but this is not limited thereto. Note that, considering the film formability when forming the hole injection layer 3 using the metal oxide nanoparticle dispersion, the Mg doping amount is preferably less than 20 mol%, more preferably 15 mol% or less, and most preferably 10 mol% or less. Then, in the second step (S2) shown in Figure 3, the pH (pH) of the mixed solution is adjusted to produce a suspension. The pH of the mixed solution is preferably adjusted to a value between 10 and 12 by adding, for example, a sodium hydroxide aqueous solution (NaOH aqueous solution) dropwise to the mixed solution, thereby producing a green suspension. In this embodiment, the pH is adjusted to 10, but the present invention is not limited to this.By adjusting the pH (pH) to 10 or more and less than 12, it is possible to increase the proportion of unit cells in which multiple Ni atoms and multiple oxygen atoms are arranged so as to surround one Mg atom in multiple unit cells of the crystal structure of metal oxide nanoparticles containing multiple Ni atoms, multiple oxygen atoms, and multiple Mg atoms. Then, in the third step (S3) shown in FIG. 3, the suspension obtained in the second step (S2) shown in FIG. 3 is stirred. When stirring the suspension, for example, ultrasonic waves may be applied while stirring. The stirring time can be appropriately determined while observing the degree of stirring of the suspension. Thereafter, in the fourth step (S4) shown in FIG. 3, the stirred suspension obtained in the third step (S3) shown in FIG. 3 is centrifuged to obtain a precipitate. The precipitate (e.g., Mg-Ni-(OH) ２ ) was green. Then, in the fifth step (S5) shown in FIG. 3, the precipitate (e.g., Mg—Ni—(OH) ２ ) and centrifugal separation is performed again. In this embodiment, water (e.g., pure water) is used as the second solvent, but the second solvent is not limited to this. In this embodiment, the fifth step (S5) shown in FIG. 3 is a water washing step in which water (e.g., pure water) is added to the precipitate obtained in the fourth step (S4) shown in FIG. 3 and centrifugal separation is performed again. In the sixth step (S6) shown in FIG. 3, the precipitate (e.g., Mg—Ni—(OH) ２ The precipitate may be dried by, for example, heat-treating the precipitate at 80° C. Then, in the seventh step (S7) shown in FIG. 3, the dried precipitate (for example, Mg—Ni—(OH) ２ ) is pulverized. Since the precipitate after drying is green clay-like (lump), it is pulverized using a pulverizer (for example, a hand mill) until the average particle size is about 10 nm, and then it is made into a powder state. Thereafter, in the eighth step (S8) shown in FIG. 3, the pulverized powder (for example, Mg—Ni—(OH) ２) is heat-treated. The heat treatment is preferably performed in air at a temperature of, for example, 250°C or higher and 350°C or lower, and the heat treatment time is preferably, for example, 1 hour or higher and 8 hours or lower. In this embodiment, the heat treatment is performed in air at 300°C for 5 hours to obtain metal oxide nanoparticles containing multiple Ni atoms, multiple oxygen atoms, and multiple Mg atoms. By performing the heat treatment at a temperature of 250°C or higher and 350°C or lower, it is possible to increase the proportion of unit cells in which multiple Ni atoms and multiple oxygen atoms are arranged to surround one Mg atom in the multiple unit cells of the crystal structure of the metal oxide nanoparticles containing multiple Ni atoms, multiple oxygen atoms, and multiple Mg atoms.

[0024] As shown in FIG. 5 , in the X-ray diffraction (XRD) spectrum results for the conventional nickel oxide nanoparticles (Comparative Example 1) and the metal oxide nanoparticles (Example 1) produced by the above-described method and containing a plurality of Ni atoms, a plurality of oxygen atoms, and a plurality of Mg atoms with a doping amount of Mg of 10 mol %, which are contained in the hole injection layer 3 provided in the light-emitting element 10 shown in FIG. 2 , there is no change in the position of the peak intensity. This shows that the metal oxide nanoparticles (Example 1) containing a plurality of Ni atoms, a plurality of oxygen atoms, and a plurality of Mg atoms are able to incorporate Mg atoms into the central portion of the unit lattice while substantially maintaining the crystal structure of the conventional nickel oxide nanoparticles (Comparative Example 1). As described above, the metal oxide nanoparticles containing multiple Ni atoms, multiple oxygen atoms, and multiple Mg atoms (Example 1) have a relatively large amount of Mg incorporated, that is, when the proportion of unit cells in which multiple Ni atoms and multiple oxygen atoms are arranged so as to surround one Mg atom is relatively high among the multiple unit cells of the crystal structure of the metal oxide nanoparticles containing multiple Ni atoms, multiple oxygen atoms, and multiple Mg atoms is relatively high. Even in this case, it can be confirmed that the crystal structure of the conventional nickel oxide nanoparticles (Comparative Example 1) can be largely maintained.

[0025] 6, the energy level of the conduction band minimum (CBM) of the conventional nickel oxide nanoparticles (Comparative Example 1) is −1.59 eV, the energy level of the valence band maximum (VBM) of the conventional nickel oxide nanoparticles (Comparative Example 1) is −5.29 eV, and the band gap of the conventional nickel oxide nanoparticles (Comparative Example 1) is 3.7 eV. On the other hand, the energy level of the conduction band minimum (CBM) of the metal oxide nanoparticles (Example 1) containing a plurality of Ni atoms, a plurality of oxygen atoms, and a plurality of Mg atoms produced by the above-mentioned method is −1.50 eV, the energy level of the valence band maximum (VBM) of the metal oxide nanoparticles (Example 1) containing a plurality of Ni atoms, a plurality of oxygen atoms, and a plurality of Mg atoms produced by the above-mentioned method is −5.40 eV, and the band gap of the metal oxide nanoparticles (Example 1) containing a plurality of Ni atoms, a plurality of oxygen atoms, and a plurality of Mg atoms produced by the above-mentioned method is 3.9 eV. The metal oxide nanoparticles (Example 1) containing multiple Ni atoms, multiple oxygen atoms, and multiple Mg atoms produced by the above-mentioned method had an average particle size of about 10 nm, and the average particle size of conventional nickel oxide nanoparticles (Comparative Example 1) also had an average particle size of about 10 nm.

[0026] In metal oxide nanoparticles (Example 1) containing multiple Ni atoms, multiple oxygen atoms, and multiple Mg atoms with a doping level of 10 mol% Mg, the band gap can be widened to 3.9 eV and the energy level of the valence band edge (VBM) can be deepened to −5.40 eV by incorporating Mg atoms into the central portion of the unit cell while substantially maintaining the crystal structure of conventional nickel oxide nanoparticles. Metal oxide nanoparticles (Example 1) containing multiple Ni atoms, multiple oxygen atoms, and multiple Mg atoms incorporate a relatively large amount of Mg, but by further increasing the amount of Mg incorporated compared to Example 1 described above, the band gap can be widened to more than 3.9 eV and the energy level of the valence band edge (VBM) can be deepened to more than −5.40 eV. On the other hand, the amount of Mg incorporated may be reduced compared to that of Example 1 described above. In this case, the band gap can be greater than 3.7 eV but smaller than 3.9 eV, and the energy level of the valence band upper limit (VBM) can be deeper than -5.29 eV but shallower than -5.40 eV. The absolute value of the difference between the vacuum level and the energy level of the valence band upper limit (VBM) is the same as the value of the ionization potential, and the absolute value of the difference between the vacuum level and the energy level of the conduction band lower limit (CBM) is the same as the value of the electron affinity. The band gap of metal oxide nanoparticles containing multiple Ni atoms, multiple oxygen atoms, and multiple Mg atoms is preferably greater than 3.7 eV, more preferably 3.9 eV or greater. The ionization potential of metal oxide nanoparticles containing multiple Ni atoms, multiple oxygen atoms, and multiple Mg atoms is preferably greater than 5.29 eV, more preferably 5.40 eV or greater. The hole injection layer 3 can be formed using a dispersion of metal oxide nanoparticles containing the above-mentioned metal oxide nanoparticles containing a plurality of Ni atoms, a plurality of oxygen atoms, and a plurality of Mg atoms, and a solvent. In this case, water (e.g., pure water) may be used as the solvent, but other solvents may also be used. The band gap of the hole injection layer 3 is preferably greater than 3.7 eV, and more preferably 3.9 eV or greater.The ionization potential value of the hole injection layer 3 is preferably greater than 5.29 eV, and more preferably greater than 5.40 eV. With this configuration, the energy level of the valence band upper limit (VBM) of the hole injection layer 3 can be brought closer to the energy level of the valence band upper limit (VBM) of the light-emitting layer EM, allowing holes supplied from the anode 2 to be injected into the light-emitting layer EM with a smaller injection barrier. As described above, metal oxide nanoparticles containing multiple Ni atoms, multiple oxygen atoms, and multiple Mg atoms and the hole injection layer 3 can improve electrical characteristics. A light-emitting element 10 including a hole injection layer 3 with improved electrical characteristics and a display device 1 including the light-emitting element 10 can suppress an increase in driving voltage and improve electrical characteristics that indicate the relationship between driving voltage and luminance or between driving voltage and current density.

[0027] The hole transport layer 4 may be formed using an organic material such as poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4'-(N-(4-sec-butylphenyl))diphenylamine)] (TFB), N,N'-bis(4-butylphenyl)-N,N'-bis(phenyl)-benzidine (poly-TPD) or polyvinylcarbazole (PVK), or may be formed using an inorganic material. In this embodiment, the hole transport layer 4 was formed using the organic material described above.

[0028] The electron transport layer 5 may be formed using an organic material such as 2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi), or an inorganic material such as ZnO nanoparticles or nanoparticles of an oxide containing Zn and Mg.

[0029] The electron injection layer can be formed using, for example, an alkali metal or alkaline earth metal such as aluminum, strontium, calcium, lithium, cesium, magnesium oxide, aluminum oxide, strontium oxide, lithium oxide, lithium fluoride, magnesium fluoride, strontium fluoride, calcium fluoride, barium fluoride, cesium fluoride, polymethyl methacrylate polystyrene sodium sulfonate, an oxide of an alkali metal or alkaline earth metal, a fluoride of an alkali metal or alkaline earth metal, or an organic complex of an alkali metal.

[0030] The light-emitting element 10 shown in Fig. 2 may be a top-emission type or a bottom-emission type. In this embodiment, the light-emitting element 10 shown in Fig. 2 is described as an example of a forward stack structure in which the cathode 6 is disposed as an upper layer than the anode 2, i.e., a light-emitting element in which the hole injection layer 3, the hole transport layer 4, the light-emitting layer EM, the electron transport layer 5, and the cathode 6 are stacked in this order from the anode 2 side. However, the light-emitting element 10 is not limited to this, and although not shown, the light-emitting element 10 may also be a light-emitting element in an inverted stack structure in which the anode 2 is disposed as an upper layer than the cathode 6, i.e., a light-emitting element in which the electron transport layer 5, the light-emitting layer EM, the hole transport layer 4, the hole injection layer 3, and the anode 2 are stacked in this order from the cathode 6 side. As in this embodiment, to make a light-emitting element with a forward stack structure a top emission type, the anode 2 may be formed from an electrode material that reflects visible light, and the cathode 6 may be formed from an electrode material that transmits visible light, while to make a light-emitting element with a forward stack structure a bottom emission type, the anode 2 may be formed from an electrode material that transmits visible light, and the cathode 6 may be formed from an electrode material that reflects visible light. On the other hand, to make a light-emitting element with an inverted stack structure a top emission type, the cathode 6 may be formed from an electrode material that reflects visible light, and the anode 2 may be formed from an electrode material that transmits visible light, while to make a light-emitting element with an inverted stack structure a bottom emission type, the cathode 6 may be formed from an electrode material that transmits visible light, and the anode 2 may be formed from an electrode material that reflects visible light.

[0031] The electrode material that reflects visible light is not particularly limited as long as it can reflect visible light and has electrical conductivity. Examples of the electrode material that reflects visible light include metal materials such as Al, 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.

[0032] 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.

[0033] The light-emitting layer EM included in the light-emitting element 10 shown in Fig. 2 may be a light-emitting layer containing quantum dots or a light-emitting layer containing an organic light-emitting material. The light-emitting element 10 including a light-emitting layer containing quantum dots is a QLED (Quantum Dot Light Emitting Diode), and the light-emitting element 10 including a light-emitting layer containing an organic light-emitting material is an OLED (Organic Light Emitting Diode).

[0034] Quantum dots, such as red-emitting quantum dots contained in a red-emitting layer, green-emitting quantum dots contained in a green-emitting layer, and blue-emitting quantum dots contained in a blue-emitting layer, 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. The shell may partially cover the core, but more preferably completely cover the core. The core of the quantum dot may include, for example, one or more selected from Si, Ge, CdSe, CdS, CdTe, InP, GaP, InN, ZnSe, ZnS, ZnTe, CdSeTe, GaInP, and ZnSeTe. The shell of the quantum dot may include, for example, one or more selected from CdS, ZnS, CdSSe, CdTeSe, CdSTe, ZnSSe, ZnSTe, ZnTeSe, and AgInP (AIP), and may be selected to have a lattice constant close to that of the core and a larger band gap than the core.

[0035] [Additional Notes] 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. Furthermore, new technical features can be formed by combining the technical means disclosed in each embodiment.

[0036] The present disclosure can be utilized in light-emitting devices, display devices, dispersions of metal oxide nanoparticles, metal oxide nanoparticles, and methods for producing metal oxide nanoparticles.

[0037] REFERENCE SIGNS LIST 1 display device 2 anode 3 hole injection layer 4 hole transport layer 5 electron transport layer 6 cathode 10 light emitting element EM light emitting layer PIX pixel RSP red sub-pixel GSP green sub-pixel BSP blue sub-pixel DA display area NDA frame area

Claims

1. Metal oxide nanoparticles comprising a plurality of Ni atoms, a plurality of oxygen atoms, and a plurality of Mg atoms, wherein the plurality of Ni atoms and the plurality of oxygen atoms are arranged so as to surround one Mg atom in at least some of the unit cells of a crystal structure.

2. The metal oxide nanoparticles of claim 1, having a band gap greater than 3.7 eV.

3. Metal oxide nanoparticles according to claim 1 or 2, having an ionization potential value of greater than 5.29 eV.

4. The metal oxide nanoparticles according to claim 1, having a band gap of 3.9 eV or more.

5. The metal oxide nanoparticles according to claim 1 or 4, having an ionization potential value of 5.40 eV or more.

6. A dispersion of metal oxide nanoparticles comprising the metal oxide nanoparticles according to any one of claims 1 to 5 and a solvent.

7. A light-emitting device comprising: an anode; a cathode; a light-emitting layer provided between the anode and the cathode; and a hole injection layer provided between the anode and the light-emitting layer, wherein the hole injection layer comprises a plurality of metal oxide nanoparticles each including a plurality of Ni atoms, a plurality of oxygen atoms, and a plurality of Mg atoms, and wherein the plurality of Ni atoms and a plurality of oxygen atoms are arranged to surround one Mg atom in at least some of the unit cells of a crystal structure of the metal oxide nanoparticles.

8. The light-emitting device according to claim 7, wherein the band gap of the hole injection layer is greater than 3.7 eV.

9. The light-emitting device according to claim 7 or 8, wherein the value of the ionization potential of the hole injection layer is greater than 5.29 eV.

10. The light-emitting device according to claim 7, wherein the band gap of said hole injection layer is 3.9 eV or more.

11. The light-emitting device according to claim 7 or 10, wherein the ionization potential of the hole injection layer is 5.40 eV or more.

12. The light-emitting element according to any one of claims 7 to 11, wherein only the hole injection layer is provided between the anode and the light-emitting layer.

13. A display device comprising the light-emitting element according to any one of claims 7 to 12.

14. A method for producing metal oxide nanoparticles, comprising: a first step of preparing a mixed solution containing a plurality of first precursors containing Ni atoms, a plurality of second precursors containing Mg atoms, and a first solvent; a second step of adjusting the pH of the mixed solution to produce a suspension; a third step of stirring the suspension; a fourth step of centrifuging the stirred suspension obtained in the third step to obtain a precipitate; a fifth step of adding a second solvent to the precipitate obtained in the fourth step and centrifuging again; a sixth step of drying the precipitate recovered by centrifugation in the fifth step; a seventh step of pulverizing the dried precipitate obtained in the sixth step; and an eighth step of heat-treating the pulverized powder obtained in the seventh step.

15. The method for producing metal oxide nanoparticles according to claim 14, wherein the first precursor is nickel nitrate hexahydrate, the second precursor is magnesium acetate tetrahydrate, and the first solvent and the second solvent are water.

16. The method for producing metal oxide nanoparticles according to claim 14 or 15, wherein in the second step, the pH is adjusted to 10 or more and less than 12.

17. A method for producing metal oxide nanoparticles according to any one of claims 14 to 16, wherein the eighth step involves heat treatment at a temperature of 250°C or higher and 350°C or lower.

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