Light-emitting element, display device, dispersion of metal oxide nanoparticles, metal oxide nanoparticles, and production method therefor

Abstract

This light-emitting element (10) comprises: an anode (2); a cathode (6); a light-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 light-emitting layer (EM), wherein the hole injection layer (3) contains metal oxide nanoparticles that are composed of a plurality of Ni atoms, a plurality of oxygen atoms, and a plurality of Mg atoms or a plurality of Co atoms the number of which is smaller than the number of the plurality of Ni atoms.

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 inventors of the present disclosure have found that, in the case of a hole injection layer made of nickel oxide as described in Patent Document 1 or a hole injection layer made of nickel oxide nanoparticles that is commonly used in the fields of OLEDs and QLEDs, a satisfactorily high transmittance cannot be obtained and only a relatively high refractive index can be obtained. In the case of a light-emitting device including such an electron injection layer, there is a problem that the light extraction efficiency becomes low.

[0006] An aspect of the present disclosure aims to provide a light-emitting element and a display device with high light extraction efficiency, metal oxide nanoparticles and a dispersion of the metal oxide nanoparticles that can be used in a process of forming a hole injection layer provided in the light-emitting element, and a method for producing the metal oxide nanoparticles.

[0007] 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 metal oxide nanoparticles composed of a plurality of Ni atoms, a plurality of oxygen atoms, and a plurality of Mg atoms or a plurality of Co atoms in a number smaller than the number of the Ni atoms.

[0008] In order to solve the above-mentioned problems, the display device of the present disclosure includes the light-emitting element.

[0009] In order to solve the above-mentioned problems, the metal oxide nanoparticles of the present disclosure are composed of a plurality of Ni atoms, a plurality of oxygen atoms, and a plurality of Mg atoms or a plurality of Co atoms in a number smaller than the number of the Ni atoms.

[0010] 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.

[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 or Co atoms, and a first solvent, wherein the number of the Mg atoms or the number of the Co atoms is less than the number of the Ni atoms; 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 a light-emitting element and a display device having high light extraction efficiency, metal oxide nanoparticles and a dispersion of the metal oxide nanoparticles that can be used in a process of forming a hole injection layer provided in the light-emitting element, and a method for producing the metal oxide nanoparticles.

[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 comparing the transmittance of a hole injection layer provided in the light-emitting device shown in FIG. 2 with the transmittance of a hole injection layer of a comparative example; FIG. 4 is a diagram comparing the transmittance at a wavelength of 440 nm of a hole injection layer provided in the light-emitting device shown in FIG. 2 with the transmittance at a wavelength of 440 nm of a hole injection layer of a comparative example; FIG. 5 is a diagram comparing the transmittance at a wavelength of 520 nm of a hole injection layer provided in the light-emitting device shown in FIG. 2 with the transmittance at a wavelength of 520 nm of a hole injection layer of a comparative example; FIG. 6 is a diagram comparing the transmittance at a wavelength of 660 nm of a hole injection layer provided in the light-emitting device shown in FIG. 2 with the transmittance at a wavelength of 660 nm of a hole injection layer of a comparative example; FIG. 7 is a diagram comparing the refractive index of a hole injection layer provided in the light-emitting device shown in FIG. 2 with the refractive index of a hole injection layer of a comparative example; and FIG. 8 is a diagram for explaining a method for producing metal oxide nanoparticles contained in the 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 8. 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 this is not limited thereto, and the hole transport layer 4 can be omitted as appropriate. 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 this is not limited thereto, and the light-emitting element 10 can further include an electron injection layer between the electron transport layer 5 and the cathode 6, or can include only an electron injection layer between the cathode 6 and the emitting layer EM, or both the electron transport layer 5 and the electron injection layer can be omitted as appropriate.

[0020] The hole injection layer 3 provided in the light-emitting element 10 shown in FIG. 2 contains metal oxide nanoparticles composed of a plurality of Ni atoms, a plurality of oxygen atoms, and a plurality of Mg atoms or a plurality of Co atoms in a number less than the number of Ni atoms. In this embodiment, a case where the metal oxide nanoparticles contained in the hole injection layer 3 are composed of a plurality of Ni atoms, a plurality of Mg atoms in a number less than the number of Ni atoms, and a plurality of oxygen atoms is described as an example, but this is not limited thereto. The metal oxide nanoparticles contained in the hole injection layer 3 may be composed of a plurality of Ni atoms, a plurality of Co atoms in a number less than the number of Ni atoms, and a plurality of oxygen atoms. In this embodiment, a case where the hole injection layer 3 is composed of metal oxide nanoparticles composed of a plurality of Ni atoms, a plurality of Mg atoms in a number less than the number of Ni atoms, and a plurality of oxygen atoms is described as an example, but this is not limited thereto. The hole injection layer 3 may also contain metal oxide nanoparticles composed of a plurality of Ni atoms, a plurality of Mg atoms in a number less than the number of Ni atoms, and a plurality of oxygen atoms.

[0021] The number of specific atoms (Ni atoms, Mg atoms, and Co atoms) contained in the metal oxide nanoparticles may be determined as the concentration per unit volume or per unit area of ​​the elements corresponding to the specific atoms, and may be confirmed, for example, using a transmission electron microscope (TEM) and energy dispersive X-ray spectroscopy (EDX), time-of-flight secondary ion mass spectrometry (TOF-SIMS), or X-ray photoelectron spectroscopy (XPS).

[0022] As described above, the metal oxide nanoparticles contained in the hole injection layer 3 provided in the light-emitting element 10 shown in Fig. 2 are composed of a plurality of Ni atoms, a plurality of Mg atoms in a number smaller than the number of Ni atoms, and a plurality of oxygen atoms, and contain NiO and MgO. The fact that the metal oxide nanoparticles contained in the hole injection layer 3 contain NiO and MgO can be confirmed using, for example, FT-IR.

[0023] Furthermore, the value of ((number of Mg atoms) / (number of Mg atoms+number of Ni atoms))×100% in the metal oxide nanoparticles contained in the hole injection layer 3 provided in the light-emitting element 10 shown in FIG. 2 is preferably 0.1% or more and 30% or less, and may be 0.1% or more and 10% or less.

[0024] 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.

[0025] 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.

[0026] 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.

[0027] 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.

[0028] 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.

[0029] 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.

[0030] 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).

[0031] 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.

[0032] Fig. 3 is a graph comparing the transmittance of the hole injection layer 3 provided in the light-emitting element 10 shown in Fig. 2 with the transmittance of a hole injection layer of a comparative example. Fig. 4 is a graph comparing the transmittance at a wavelength of 440 nm of the hole injection layer 3 provided in the light-emitting element 10 shown in Fig. 2 with the transmittance at a wavelength of 440 nm of a hole injection layer of a comparative example. Fig. 5 is a graph comparing the transmittance at a wavelength of 520 nm of the hole injection layer 3 provided in the light-emitting element 10 shown in Fig. 2 with the transmittance at a wavelength of 520 nm of a hole injection layer of a comparative example. Fig. 6 is a graph comparing the transmittance at a wavelength of 660 nm of the hole injection layer 3 provided in the light-emitting element 10 shown in Fig. 2 with the transmittance at a wavelength of 660 nm of a hole injection layer of a comparative example.

[0033] The hole injection layer 3 shown in FIG. 3 is an example in which the value of ((number of Mg atoms) / (number of Mg atoms + number of Ni atoms)) × 100% of the metal oxide nanoparticles contained in the hole injection layer 3 is 0.8%, and a thickness of 18.8 nm was obtained by one coating, a thickness of 52.3 nm was obtained by two coatings, and a thickness of 63.3 nm was obtained by three coatings. Note that each of the above-mentioned predetermined thicknesses can be obtained by spin coating one to three times using a dispersion of metal oxide nanoparticles in which the value of ((number of Mg atoms) / (number of Mg atoms + number of Ni atoms)) × 100% is 0.8%. Each of the first to third spin coatings was performed using a dispersion (e.g., concentration: 0.3 mol / L or more and 0.4 mol / L or less) of metal oxide nanoparticles composed of a plurality of Ni atoms, a plurality of Mg atoms in a number less than the number of Ni atoms, and a plurality of oxygen atoms dispersed in a solvent, such as water (e.g., pure water), using a spin coater (e.g., rotation speed: 3000 rpm). 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). In this embodiment, metal oxide nanoparticles were used that were composed of a plurality of Ni atoms, a plurality of Mg atoms (the number of Mg atoms being less than the number of Ni atoms), and a plurality of oxygen atoms dispersed in water (e.g., pure water). The metal oxide nanoparticles had a value of ((number of Mg atoms) / (number of Mg atoms+number of Ni atoms))×100% of 0.8%. A method for producing such metal oxide nanoparticles will be described later with reference to FIG. 8 . In this embodiment, a case in which the light-emitting element 10 has a hole injection layer 3 with a thickness of 63.3 nm obtained by three coatings will be described as an example. However, the present invention is not limited to this. The light-emitting element 10 may have a hole injection layer 3 with a thickness of 52.3 nm obtained by two coatings, or a hole injection layer 3 with a thickness of 18.8 nm obtained by one coating. The hole injection layer 3 may be formed to an optimal thickness in consideration of the luminous efficiency of the light-emitting element 10.In this embodiment, metal oxide nanoparticles having an average particle size of approximately 10 nm are used as the metal oxide nanoparticles contained in the hole injection layer 3. However, this is not limited thereto. For example, metal oxide nanoparticles having an average particle size of 15 nm or less can be suitably used. Metal oxide nanoparticles having an average particle size of 10 nm or more but not more than 15 nm can also be used, or metal oxide nanoparticles having an average particle size of 5 nm or less can also be used. Furthermore, metal oxide nanoparticles having an average particle size greater than 15 nm can also be used. The average particle size of the metal oxide nanoparticles is the particle size of metal oxide nanoparticles at a particle size cumulative 50% (D50) measured by DLS (dynamic light scattering) using the above-mentioned dispersion of metal oxide nanoparticles.

[0034] In the case of the hole injection layer 3 (coated once) having a thickness of 18.8 nm shown in FIG. 3, a transmittance of 94% or more was obtained at a wavelength of 440 nm as shown in FIG. 4, a transmittance of 95% or more was obtained at a wavelength of 520 nm as shown in FIG. 5, and a transmittance of 96% or more was obtained at a wavelength of 660 nm as shown in FIG. 6. The transmittance of the hole injection layer 3 having a thickness of 18 nm or more and 20 nm or less composed of the metal oxide nanoparticles was 94% or more at a wavelength of 440 nm, 95% or more at a wavelength of 520 nm, and 96% or more at a wavelength of 660 nm. In the case of the hole injection layer 3 (coated twice) having a thickness of 52.3 nm shown in FIG. 3, a transmittance of 87% or more was obtained at a wavelength of 440 nm as shown in FIG. 4, a transmittance of 89% or more was obtained at a wavelength of 520 nm as shown in FIG. 5, and a transmittance of 92% or more was obtained at a wavelength of 660 nm as shown in FIG. 6. In the case of the hole injection layer 3 (coated three times) having a thickness of 63.3 nm as shown in FIG. 3, a transmittance of 84% or more could be obtained at a wavelength of 440 nm as shown in FIG. 4, a transmittance of 85% or more could be obtained at a wavelength of 520 nm as shown in FIG. 5, and a transmittance of 88% or more could be obtained at a wavelength of 660 nm as shown in FIG. 6.

[0035] On the other hand, the hole injection layer of the comparative example shown in FIG. 3 is a hole injection layer made of nickel oxide nanoparticles commonly used in the fields of OLEDs and QLEDs. A single coating resulted in a thickness of 16.1 nm, two coatings resulted in a thickness of 47.9 nm, and three coatings resulted in a thickness of 55.7 nm. Each of the first to third spin coatings was performed using a spin coater (e.g., rotation speed: 3000 rpm) using a dispersion of nickel oxide nanoparticles (e.g., concentration: 0.3 mol / L or more and 0.4 mol / L or less) dispersed in a solvent, such as water (e.g., pure water). Note that, although nickel oxide nanoparticles with an average particle size of approximately 10 nm were used as the nickel oxide nanoparticles contained in the hole injection layer of the comparative example shown in FIG. 3 , this is not limiting. The nickel oxide nanoparticles contained in the hole injection layer of the comparative example shown in FIG. 3 can be nickel oxide nanoparticles with an average particle size substantially equal to the average particle size of the metal oxide nanoparticles contained in the hole injection layer 3 described above. The average particle size of the nickel oxide nanoparticles is the particle size of the nickel oxide nanoparticles at a particle size-based cumulative 50% (D50) measured by DLS (dynamic light scattering) using the above-mentioned dispersion of nickel oxide nanoparticles.

[0036] In the case of the hole injection layer (single-coated) of the comparative example having a thickness of 16.1 nm shown in FIG. 3 , although the thickness is thinner than the hole injection layer 3 (single-coated) having a thickness of 18.8 nm shown in FIG. 3 , at a wavelength of 440 nm, the transmittance is 93.15%, which is 1.03% lower than the transmittance of the hole injection layer 3 (single-coated) of 94.18%, as shown in FIG. 4 ; at a wavelength of 520 nm, the transmittance is 94.42%, which is 0.74% lower than the transmittance of the hole injection layer 3 (single-coated) of 95.16%, as shown in FIG. 5 ; and at a wavelength of 660 nm, the transmittance is 96.13%, which is 0.40% lower than the transmittance of the hole injection layer 3 (single-coated) of 96.53%, as shown in FIG. 6 . In the case of the hole injection layer (two-coated) of the comparative example having a thickness of 47.9 nm shown in FIG. 3 , although the thickness is thinner than the hole injection layer 3 (two-coated) having a thickness of 52.3 nm shown in FIG. 3 , at a wavelength of 440 nm, the transmittance is 85.49%, which is 1.80% lower than the 87.29% transmittance of the hole injection layer 3 (two-coated), as shown in FIG. 4 ; at a wavelength of 520 nm, the transmittance is 87.95%, which is 1.20% lower than the 89.15% transmittance of the hole injection layer 3 (two-coated), as shown in FIG. 5 ; and at a wavelength of 660 nm, the transmittance is 91.51%, which is 0.56% lower than the 92.07% transmittance of the hole injection layer 3 (two-coated), as shown in FIG. 6 . In the case of the hole injection layer (coated three times) of the comparative example having a thickness of 55.7 nm shown in FIG. 3 , although the thickness is thinner than the hole injection layer 3 (coated three times) having a thickness of 63.3 nm shown in FIG. 3 , at a wavelength of 440 nm, as shown in FIG. 4 , the transmittance is 81.39%, which is 2.84% lower than the transmittance of the hole injection layer 3 (coated three times), which is 84.23%. As shown in FIG. 5 , at a wavelength of 520 nm, the transmittance is 83.13%, which is 2.18% lower than the transmittance of the hole injection layer 3 (coated three times), which is 85.31%. As shown in FIG. 6 , at a wavelength of 660 nm, the transmittance is 86.85%, which is 1.54% lower than the transmittance of the hole injection layer 3 (coated three times).

[0037] From the above, the transmittance of the hole injection layer 3 having a thickness of 18 nm to 64 nm and composed of the metal oxide nanoparticles having an average particle size of 10 nm to 15 nm is higher at wavelengths of 440 nm, 520 nm, and 660 nm than that of a layer (hole injection layer) having the corresponding thickness and composed of nickel oxide nanoparticles having an average particle size of 10 nm to 15 nm. For example, the transmittance of the hole injection layer 3 having a thickness of 20 nm is higher at wavelengths of 440 nm, 520 nm, and 660 nm than that of a layer (hole injection layer) composed of the nickel oxide nanoparticles having a thickness of 20 nm. Note that, when measuring the average particle size of the metal oxide nanoparticles or nickel oxide nanoparticles after forming the hole injection layer, for example, a transmission electron microscope (TEM) or a scanning transmission electron microscope (STEM) may be used to determine the average particle size of multiple metal oxide nanoparticles or nickel oxide nanoparticles present in the cross section of the hole injection layer. Before forming the hole injection layer, the particle size of the metal oxide nanoparticles measured by DLS (dynamic light scattering) using a dispersion of the metal oxide nanoparticles or nickel oxide nanoparticles at a particle size-based cumulative 50% (D50) is approximately the same as the average particle size of the multiple metal oxide nanoparticles or nickel oxide nanoparticles present in the cross section of the hole injection layer, as determined using a transmission electron microscope (TEM) or a scanning transmission electron microscope (STEM) after forming the hole injection layer, as described above.

[0038] Therefore, the light emitting device 10 including the hole injection layer 3 can have a higher light extraction efficiency than a light emitting device including a comparative hole injection layer of the same film thickness made of nickel oxide nanoparticles.

[0039] FIG. 7 is a diagram comparing the refractive index of the hole injection layer 3 provided in the light emitting element 10 shown in FIG. 2 with the refractive index of a hole injection layer of a comparative example.

[0040] The refractive index of the hole injection layer 3 (coated three times) shown in FIG. 7 is the result of measuring the refractive index using the hole injection layer 3 (coated three times) having a thickness of 63.3 nm shown in FIG. 3 , and the refractive index of the hole injection layer (coated three times) of the comparative example shown in FIG. 7 is the result of measuring the refractive index using the hole injection layer (coated three times) of the comparative example having a thickness of 55.7 nm shown in FIG. 3 .

[0041] As shown in FIG. 7 , the refractive index of the hole injection layer 3 (coated three times) having a thickness of 63.3 nm is lower than the refractive index of the hole injection layer (coated three times) having a thickness of 55.7 nm of the comparative example at wavelengths of 440 nm, 520 nm, and 660 nm. Note that the change in refractive index due to the difference in film thickness is not shown in Figure 7 because there is almost no change in the refractive index when the film thickness difference is about 50 nm or less. However, the refractive index of the hole injection layer 3 (two-coated) with a film thickness of 52.3 nm and the hole injection layer 3 (single-coated) with a film thickness of 18.8 nm, whose transmittance is shown in Figure 3, is approximately the same as the refractive index of the hole injection layer 3 (three-coated) with a film thickness of 63.3 nm shown in Figure 7. The refractive index of the hole injection layer (two-coated) with a film thickness of 47.9 nm and the hole injection layer (single-coated) with a film thickness of 16.1 nm, whose transmittance is shown in Figure 3, is approximately the same as the refractive index of the hole injection layer (three-coated) with a film thickness of 55.7 nm shown in Figure 7. Therefore, the refractive index of the hole injection layer 3 with a film thickness of 18 nm or more and 64 nm or less composed of the metal oxide nanoparticles is less than 1.8 at wavelengths of 440 nm, 520 nm, and 660 nm.

[0042] From the above, the refractive index of the hole injection layer 3 composed of the metal oxide nanoparticles with an average particle size of 10 nm to 15 nm and having a thickness of 18 nm to 64 nm is lower than the refractive index of a layer (hole injection layer) with the same thickness composed of nickel oxide nanoparticles with an average particle size of 10 nm to 15 nm at wavelengths of 440 nm, 520 nm, and 660 nm. For example, the refractive index of the hole injection layer 3 with a thickness of 20 nm is lower than the refractive index of a layer composed of the nickel oxide nanoparticles with a thickness of 20 nm at wavelengths of 440 nm, 520 nm, and 660 nm. As described above, the hole injection layer 3 having a thickness of 18 nm to 64 nm and made of the metal oxide nanoparticles with an average particle size of 10 nm to 15 nm has a higher transmittance at wavelengths of 440 nm, 520 nm, and 660 nm than the transmittance of a layer of the same thickness made of nickel oxide nanoparticles with an average particle size of 10 nm to 15 nm, and a lower refractive index at wavelengths of 440 nm, 520 nm, and 660 nm than the refractive index of a layer of the same thickness made of nickel oxide nanoparticles with an average particle size of 10 nm to 15 nm.

[0043] In this embodiment, since the light-emitting element 10 shown in FIG. 2 is a top-emission type, the anode 2 provided in the light-emitting element 10 shown in FIG. 2 is formed from the above-mentioned electrode material that reflects visible light, and light emitted from the light-emitting layer EM is reflected by the anode 2 and enters the interface between the hole injection layer 3 and the hole transport layer 4. As described above, in this embodiment, the hole transport layer 4 is 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). Therefore, the refractive index of the hole transport layer 4 is approximately 1.5 to 1.7 over the entire wavelength range of 380 nm to 780 nm, which is the visible light range. When the refractive index of the hole injection layer 3 is N1 and the refractive index of the hole transport layer 4 is N2, the critical angle of total reflection θc at the interface between the hole injection layer 3 and the hole transport layer 4 can be calculated from sin θc = N2 / N1. The conditions for total reflection to occur at the interface between the hole injection layer 3 and the hole transport layer 4 are satisfied when N1 (refractive index of the hole injection layer 3) > N2 (refractive index of the hole transport layer 4) and the angle of incidence θi at the interface between the hole injection layer 3 and the hole transport layer 4 > the critical angle of total reflection θc.

[0044] When a comparative example hole injection layer made of nickel oxide nanoparticles having a refractive index greater than N2, the refractive index of the hole transport layer 4, and having a large difference from N2, the refractive index of the hole transport layer 4, is used, the value of N2 / N1 in sinθc=N2 / N1 becomes small, and the total reflection critical angle θc also becomes small. Therefore, in this case, of the light components reflected by the anode 2 and incident on the interface between the comparative example hole injection layer and hole transport layer 4, the light components having an incident angle θi at the interface that exceeds the total reflection critical angle θc increase, resulting in a decrease in the light extraction efficiency of the light emitting element.

[0045] On the other hand, when the above-described hole injection layer 3 is used, which has a refractive index larger than N2, the refractive index of the hole transport layer 4, but a small difference from N2, the refractive index of the hole transport layer 4, the value of N2 / N1 in sinθc=N2 / N1 can be made closer to 1, and the critical angle of total reflection θc can be made larger. Therefore, in this case, of the light components reflected by the anode 2 and incident on the interface between the hole injection layer 3 and the hole transport layer 4, the light components having an incident angle θi at the interface that exceeds the critical angle of total reflection θc can be reduced, and the light extraction efficiency of the light-emitting element can be improved.

[0046] In the present embodiment, the case where the value of ((number of Mg atoms) / (number of Mg atoms + number of Ni atoms)) × 100% of the metal oxide nanoparticles contained in the hole injection layer 3 is 0.8% has been described as an example. However, the value is not particularly limited as long as the metal oxide nanoparticles contained in the hole injection layer 3 are composed of a plurality of Ni atoms, a plurality of oxygen atoms, and a plurality of Mg atoms in a number smaller than the number of Ni atoms. For example, the value of ((number of Mg atoms) / (number of Mg atoms + number of Ni atoms)) × 100% of the metal oxide nanoparticles contained in the hole injection layer 3 may be, for example, 0.1%, 0.7%, 1%, 10%, or 30%. Furthermore, the metal oxide nanoparticles contained in the hole injection layer 3 may be composed of a plurality of Ni atoms, a plurality of oxygen atoms, and a plurality of Co atoms in a number smaller than the number of Ni atoms.

[0047] FIG. 8 is a diagram illustrating a method for producing metal oxide nanoparticles contained in the hole injection layer 3 provided in the light emitting element 10 shown in FIG.

[0048] 8, a mixed solution is prepared that includes a plurality of first precursors containing Ni atoms, a plurality of second precursors containing Mg atoms or Co atoms, and a first solvent, wherein the number of Mg atoms or the number of Co atoms is less than the number of Ni atoms. 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. However, the present invention is not limited thereto, and cobalt(II) sulfate or cobalt(II) nitrate containing Co atoms may also be used as the second precursor. In this embodiment, water (e.g., pure water) is used as the first solvent, but a solvent other than water (e.g., pure water) may be used. Then, in the second step (S2) shown in FIG. 8 , the pH of the mixed solution is adjusted to produce a suspension. The pH of the mixed solution may be adjusted to 12 by, for example, adding a sodium hydroxide aqueous solution (NaOH aqueous solution) dropwise to the mixed solution to produce a green suspension. Then, in the third step (S3) shown in FIG. 8, the suspension obtained in the second step (S2) shown in FIG. 8 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. 8, the stirred suspension obtained in the third step (S3) shown in FIG. 8 is centrifuged to obtain a precipitate. The precipitate (for example, Mg—Ni—(OH) ２ ) was green. Then, in the fifth step (S5) shown in FIG. 8, the precipitate (e.g., Mg—Ni—(OH) ２) and centrifuging 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. 8 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. 8 and centrifuging again. In the sixth step (S6) shown in FIG. 8, the precipitate (e.g., Mg—Ni—(OH) ２ ) is dried. The drying of the precipitate may be performed by, for example, heat-treating the precipitate at 80°C. Then, in a seventh step (S7) shown in Fig. 8, 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. 8, the pulverized powder (for example, Mg—Ni—(OH) ２) is heat-treated. The heat treatment can be performed in air at a temperature of 200°C or higher and 600°C or lower for 1 hour or higher and 8 hours or lower. In this embodiment, metal oxide nanoparticles containing NiO and MgO are obtained by performing heat treatment in air at a temperature of 500°C for 5 hours. In the first step (S1) shown in FIG. 8 described above, the mixed solution may be prepared so that the value of ((number of Mg atoms) / (number of Mg atoms + number of Ni atoms)) × 100% is 0.1% or higher and 30% or lower, or 0.1% or higher and 10% or lower. In this embodiment, in the first step (S1) shown in FIG. 8 , a mixed solution of the first precursor (nickel nitrate hexahydrate), the second precursor (magnesium acetate tetrahydrate), and the first solvent (water (e.g., pure water)) was prepared so that the concentration ratio of Mg ions to Ni ions was 8:992. After the eighth step (S8) shown in FIG. 8 , metal oxide nanoparticles were obtained in which the value of ((number of Mg atoms) / (number of Mg atoms + number of Ni atoms)) × 100% was 0.8%. Furthermore, the obtained metal oxide nanoparticles were dispersed in a solvent, such as water (e.g., pure water), to obtain a dispersion of metal oxide nanoparticles (e.g., concentration: 0.3 mol / L or more and 0.4 mol / L or less).

[0049] The metal oxide nanoparticles produced by the method for producing metal oxide nanoparticles shown in FIG. 8 are composed of a plurality of Ni atoms, a plurality of oxygen atoms, and a plurality of Mg atoms or a plurality of Co atoms in a number less than the number of Ni atoms. In this embodiment, metal oxide nanoparticles composed of a plurality of Ni atoms, a plurality of oxygen atoms, and a plurality of Mg atoms in a number less than the number of Ni atoms can be obtained. Furthermore, the metal oxide nanoparticles composed of a plurality of Ni atoms, a plurality of oxygen atoms, and a plurality of Mg atoms in a number less than the number of Ni atoms include NiO and MgO. Furthermore, for metal oxide nanoparticles composed of a plurality of Ni atoms, a plurality of oxygen atoms, and a plurality of Mg atoms in a number less than the number of Ni atoms, the value of ((number of Mg atoms) / (number of Mg atoms + number of Ni atoms)) × 100% may be 0.1% or more and 30% or less, or 0.1% or more and 10% or less.

[0050] The hole injection layer 3 provided in the light-emitting element 10 shown in Fig. 2 can be formed using a dispersion of metal oxide nanoparticles containing metal oxide nanoparticles each composed of a plurality of Ni atoms, a plurality of oxygen atoms, and a plurality of Mg atoms or a plurality of Co atoms in a number less than the number of Ni atoms, and a solvent. The solvent can be, for example, water (e.g., pure water) or an alcohol-based solvent.

[0051] [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.

[0052] 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.

[0053] 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. 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 contains metal oxide nanoparticles composed of a plurality of Ni atoms, a plurality of oxygen atoms, and a plurality of Mg atoms or a plurality of Co atoms in a number smaller than the number of the Ni atoms.

2. The light-emitting element according to claim 1, wherein the metal oxide nanoparticles are composed of the plurality of Ni atoms, the plurality of Mg atoms, and the plurality of oxygen atoms.

3. The light-emitting device according to claim 2, wherein the metal oxide nanoparticles include NiO and MgO.

4. The light-emitting element according to claim 2 or 3, wherein the value of ((number of Mg atoms) / (number of Mg atoms+number of Ni atoms))×100% in the metal oxide nanoparticles is 0.1% or more and 30% or less.

5. The light-emitting element according to claim 2 or 3, wherein the value of ((number of Mg atoms) / (number of Mg atoms+number of Ni atoms))×100% in the metal oxide nanoparticles is 0.1% or more and 10% or less.

6. The light-emitting element according to any one of claims 2 to 5, wherein the hole injection layer made of the metal oxide nanoparticles and having a thickness of 18 nm or more and 20 nm or less has a transmittance of 94% or more at a wavelength of 440 nm, 95% or more at a wavelength of 520 nm, and 96% or more at a wavelength of 660 nm.

7. A light-emitting element according to any one of claims 2 to 5, wherein the refractive index of the hole injection layer made of the metal oxide nanoparticles and having a film thickness of 18 nm or more and 64 nm or less is less than 1.8 at wavelengths of 440 nm, 520 nm, and 660 nm.

8. The light-emitting element according to any one of claims 2 to 5, wherein the hole injection layer composed of the metal oxide nanoparticles having an average particle size of 10 nm or more and 15 nm or less and having a film thickness of 18 nm or more and 64 nm or less has a higher transmittance at wavelengths of 440 nm, 520 nm, and 660 nm than the transmittance of a layer of the same film thickness composed of nickel oxide nanoparticles having an average particle size of 10 nm or more and 15 nm or less, and a lower refractive index at wavelengths of 440 nm, 520 nm, and 660 nm than the refractive index of a layer of the same film thickness.

9. The light-emitting device according to any one of claims 1 to 8, further comprising a hole-transporting layer between the hole-injecting layer and the light-emitting layer.

10. A display device comprising a light-emitting element according to any one of claims 1 to 9.

11. Metal oxide nanoparticles composed of a plurality of Ni atoms, a plurality of oxygen atoms, and a plurality of Mg atoms or a plurality of Co atoms in a number less than the number of the Ni atoms.

12. The metal oxide nanoparticles according to claim 11, which are composed of the plurality of Ni atoms, the plurality of Mg atoms, and the plurality of oxygen atoms.

13. The metal oxide nanoparticles of claim 12, comprising NiO and MgO.

14. Metal oxide nanoparticles according to claim 12 or 13, wherein the value of ((number of Mg atoms) / (number of Mg atoms+number of Ni atoms)) x 100% is 0.1% or more and 30% or less.

15. Metal oxide nanoparticles according to claim 12 or 13, wherein the value of ((number of Mg atoms) / (number of Mg atoms+number of Ni atoms)) x 100% is 0.1% or more and 10% or less.

16. A dispersion of metal oxide nanoparticles comprising the metal oxide nanoparticles according to any one of claims 11 to 15 and a solvent.

17. 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 or Co atoms, and a first solvent, wherein the number of Mg atoms or the number of Co atoms is less than the number of Ni atoms; 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.

18. The method for producing metal oxide nanoparticles according to claim 17, 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.

19. The method for producing metal oxide nanoparticles according to claim 18, wherein in the first step, the mixed solution is prepared so that the value of ((number of Mg atoms) / (number of Mg atoms + number of Ni atoms)) x 100% is 0.1% or more and 30% or less.

20. The method for producing metal oxide nanoparticles described in claim 18, wherein in the first step, the mixed solution is prepared so that the value of ((number of Mg atoms) / (number of Mg atoms + number of Ni atoms)) x 100% is 0.1% or more and 10% or less.

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