Light-emitting element, display device, nanoparticle dispersion liquid, and method for producing light-emitting element

Abstract

The light-emitting element (ED) includes an anode (EA), a cathode (EC), a light-emitting layer (EM) located between the anode (EA) and the cathode (EC), and a light scattering layer (SL) located between the light-emitting layer (EM) and the anode (EA) and containing a hole-transporting metal oxide.

Description

Technical Field

[0001] The present disclosure relates to a light-emitting element, a display device, a nanoparticle dispersion liquid, and a method for manufacturing a light-emitting element. Background Art

[0002] As an existing light-emitting element, a quantum dot light-emitting diode including a hole injection layer containing NiO (nickel oxide) is known. Prior Art Documents Patent Documents

[0003] Patent Document 1: US Patent Application Publication No. 2021 / 0091325 Patent Document 2: Chinese Patent Application Publication No. 105895829 Summary of the Invention Technical Problem to be Solved by the Invention

[0004] The existing light-emitting element has a problem of low light extraction efficiency. Technical Solution for Solving the Technical Problem

[0005] A light-emitting element according to one embodiment of the present disclosure includes: an anode and a cathode; a light-emitting layer located between the anode and the cathode; and a light-scattering layer located between the light-emitting layer and the anode and containing a hole-transporting metal oxide. Advantageous Effects

[0006] According to one embodiment of the present disclosure, the light extraction efficiency of the light-emitting element can be improved. Brief Description of the Drawings

[0007] Figure 1 It is a schematic diagram showing a configuration example of a display device according to an embodiment. Figure 2 It is a cross-sectional view showing a configuration example of a display device according to an embodiment. Figure 3 It is a cross-sectional view showing a configuration example of a light-emitting element according to an embodiment. Figure 4 It is a cross-sectional view showing a configuration example of a light-emitting element according to an embodiment. Figure 5 It is a cross-sectional view showing a configuration example of a display device according to an embodiment. Figure 6 It is a schematic cross-sectional view showing a configuration example of a light-emitting element according to an embodiment. Figure 7 It is a graph showing the relationship between the inclination of the upper surface of the light-scattering layer and the extraction efficiency. Figure 8 It is a cross-sectional view showing another example of a light-emitting element according to an embodiment. Figure 9A It is a conceptual diagram of a light-scattering layer. Figure 9B It is a conceptual diagram of a light-scattering layer with a layer thickness greater than Figure 9A Figure 9C It is a conceptual diagram of a light-scattering layer with a layer thickness greater than Figure 9B Figure 10 It is a cross-sectional view showing another configuration example of the light-emitting element of the embodiment. Figure 11 It is a flowchart showing the manufacturing method of the light-emitting element according to the present embodiment. Figure 12 It is a flowchart showing the manufacturing method of the light-scattering layer according to the present embodiment. Figure 13 It is a cross-sectional SEM image of an example of a light-emitting element. Figure 14 It is a cross-sectional SEM image of another example of a light-emitting element. Figure 15 It is a cross-sectional SEM image of another example of a light-emitting element. Figure 16 It is Figure 13 STEM image of the light-emitting element Figure 17 It is Figure 14 STEM image of the light-emitting element Figure 18 It is Figure 15 STEM image of the light-emitting element Figure 19 It is a graph showing the relationship between the number of coating times and the layer thickness during the formation of the light-scattering layer. Figure 20A It is a graph showing the voltage-current density characteristics of the light-emitting element when the layer thickness (number of coating times) of the light-scattering layer is changed. Figure 20B It is a graph showing the voltage-current density characteristics of the light-emitting element when the layer thickness (number of coating times) of the light-scattering layer is changed. Figure 21A It is a graph showing the voltage-luminance characteristics of the light-emitting element when the layer thickness (number of coating times) of the light-scattering layer is changed. Figure 21B It is a graph showing the voltage-luminance characteristics of the light-emitting element when the layer thickness (number of coating times) of the light-scattering layer is changed. Figure 22A It is a graph showing the current density-EQE characteristics of the light-emitting element when the layer thickness (number of coating times) of the light-scattering layer is changed. Figure 22BIt is a graph showing the current density - EQE characteristics of a light - emitting element when the layer thickness (coating times) of the light - scattering layer is changed. Figure 23A It is a graph showing the current density - luminance characteristics of a light - emitting element when the layer thickness (coating times) of the light - scattering layer is changed. Figure 23B It is a graph showing the current density - luminance characteristics of a light - emitting element when the layer thickness (coating times) of the light - scattering layer is changed. Figure 24 It is a graph showing the temporal change of the relative luminance of a light - emitting element when the layer thickness (coating times) of the light - scattering layer is changed. Figure 25 It is a graph showing the temporal change of the relative luminance of a light - emitting element when the solvent of the nanoparticle dispersion is changed. Detailed implementation mode

[0008] Figure 1 It is a schematic diagram showing a configuration example of a display device according to an embodiment. Figure 2 It is a cross - sectional view showing a configuration example of a display device according to an embodiment. As Figure 1 shown, the display device 10 includes: a display unit DA including a plurality of sub - pixels SP, a first driver X1 and a second driver X2 for driving the plurality of sub - pixels SP, and a display control unit DC for controlling the first driver X1 and the second driver X2. The sub - pixel SP includes a light - emitting element ED and a pixel circuit PC connected to the light - emitting element ED. The pixel circuit PC may also be connected to a scan signal line GL, a data signal line DL, and a light - emission control line EL. The scan signal line GL and the light - emission control line EL may also be connected to the first driver X1, and the data signal line DL is connected to the second driver X2.

[0009] The display device 10 includes a pixel circuit substrate 13, a light - emitting element layer 14, and a sealing layer 15. The pixel circuit substrate 13 includes a substrate 11 and a pixel circuit layer 12. The substrate 11 may use a glass substrate, a resin substrate, etc. The substrate 11 may also be flexible. The pixel circuit layer 12 includes, for example, a plurality of pixel circuits PC arranged in a matrix. The pixel circuit PC may include a pixel capacitor for writing a grayscale signal, a transistor for controlling the current value of the light - emitting element ED according to the grayscale signal, a transistor connected to the scan signal line GL and the data signal line DL, and a transistor connected to the light - emission control line EL. As Figure 2As shown in the figure, the light-emitting element layer 14 may include an anode EA, an edge covering film JF covering the edge of the anode EA, a first functional layer FK, a light-emitting layer EM, a second functional layer SK, and a cathode EC in this order from the pixel circuit substrate 13 side. The first functional layer FK has a hole-transporting function, and the second functional layer SK has an electron-transporting function. The light-emitting element layer 14 may include a light-emitting element RD (ED) including a light-emitting layer RE (EM) that emits red light, a light-emitting element GD (ED) including a light-emitting layer GE (EM) that emits green light, and a light-emitting element BD (ED) including a light-emitting layer BE (EM) that emits blue light. The sealing layer 15 includes an inorganic insulating film such as a silicon nitride film and a silicon oxide film, and prevents foreign substances (such as water and oxygen) from invading the light-emitting element layer 14.

[0010] Figure 3 It is a cross-sectional view showing a configuration example of the light-emitting element according to the embodiment. As Figure 3 shown, the light-emitting element ED includes an anode EA and a cathode EC facing each other, and a light-scattering layer SL containing a hole-transporting metal oxide located between the anode EA and the cathode EC. In the light-emitting element ED, the light-scattering layer SL can transport holes supplied from the anode EA to the light-emitting layer EM side, and scatter light (incident light) incident at a small angle with respect to a plane (for example, the substrate surface AF) perpendicular to the thickness direction of the light-emitting element ED on the upper surface, so as to become scattered light at a large angle with respect to the substrate surface. Thereby, the light extraction efficiency of the light-emitting element ED can be improved. On the upper surface of the light-scattering layer SL, Rayleigh scattering (for example, Rayleigh scattering of visible light) can be generated by surface irregularities, or diffuse reflection of light including scattering and reflection can be generated by surface irregularities.

[0011] When the light-emitting element ED is a top-emission type (a structure in which light is extracted on the side opposite to the pixel circuit substrate 13), it is sufficient to set the anode EA as a light-reflecting electrode and the cathode EC as a light-transmitting electrode, and the scattered light traveling downward from the light-scattering layer SL is reflected by the anode EA. When the light-emitting element ED is a bottom-emission type (a structure in which light is extracted toward the pixel circuit substrate 13 side), it is sufficient to set the anode EA as a light-transmitting electrode and the cathode EC as a light-reflecting electrode, and the scattered light traveling upward from the light-scattering layer SL is reflected by the cathode EC. When the anode EA is used as a light-reflecting electrode, for example, a laminate of a lower-layer Ag (silver) film or Al (aluminum) film and an upper-layer ITO (Indium Tin Oxide) film can be formed. When the anode EA is set as a light-transmitting electrode, for example, it can be set as an ITO film.

[0012] From the viewpoint of taking into account both the hole transport function (reliability) and the light scattering function of the light scattering layer SL, the upper surface roughness of the light scattering layer SL is preferably 5 [nm] to 70 [nm]. The upper surface roughness may be the roughness of the surface of the light scattering layer SL that is far from the pixel circuit substrate 13. The upper surface roughness of the light scattering layer SL is defined as the difference (Dt - Db) between the maximum thickness Dt and the minimum thickness Db when observing a cross section parallel to the thickness direction of the light scattering layer SL. The field of view may be over the entire width of the light scattering layer SL or any one of dividing the entire width of the light scattering layer SL into multiple regions (setting the width of each region as the unit length L). In this case, the average value of the multiple upper surface roughnesses corresponding to the multiple regions may be 5 [nm] to 70 [nm]. The unit length L is, for example, 500 [nm], but a value corresponding to the total width of the light scattering layer SL may be used. It is desired to observe the upper surface roughness of the light scattering layer SL through cross-sectional observation of a cross-sectional SEM image. As described later, the upper surface roughness can be observed not only through a cross-sectional SEM image but also through an image of a STEM image. However, since the STEM image also observes depth information, it is difficult to determine the upper surface roughness. Therefore, when cross-sectioning the upper surface roughness, cross-sectional SEM images are preferentially observed compared to STEM images.

[0013] The light scattering layer SL may also include a base portion BS on the anode EA side and a light scattering portion SS (surface unevenness) on the light emitting layer EM side. The thickness of the base portion BS may be 45 [%] or more of the upper surface roughness of the light scattering layer SL. The thickness of the base portion BS can be defined as the minimum thickness Db of the light scattering layer SL in the unit length L. The light scattering portion SS (surface unevenness) may also have a shape that is uneven in the plane (random shape). The light scattering portion SS is constituted by surface unevenness on the upper surface side of the light scattering layer SL, and the upper surface roughness may be the maximum height difference of the surface unevenness.

[0014] The atomic percentage of carbon atoms in the light scattering layer SL may be 0 [%] to 5 [%]. The light scattering layer SL may also be configured not to contain an organic ligand. That is, it is preferable that substantially no carbon from the organic matter remains on the light scattering layer SL. In this way, the organic matter does not hinder hole conduction in the light scattering layer SL, and the hole transport function is improved. As a result, the hole injection effect on the light emitting layer EM is improved, and generally, the carrier balance of a light emitting element with an excess of electrons is improved. The applied voltage to the light emitting element ED is also reduced. Furthermore, it is possible to avoid the phenomenon that the solubility in an organic solvent becomes large due to the organic ligand, and it is possible to eliminate the defect that the light scattering layer SL dissolves (film reduction) in the process of forming the upper layer (for example, coating a solution containing an organic solvent).

[0015] A hole transport layer YL containing an organic material may be provided between the light scattering layer SL and the light-emitting layer EM. By setting the upper end of the valence band (VBM) of the hole transport layer YL to an energy level between the VBM of the light scattering layer SL and the VBM of the light-emitting layer EM, the injection barrier of holes is reduced, and the hole transport function of the first functional layer FK is improved. In addition, the VBM can also be replaced with the energy level of the highest occupied molecular orbital (HOMO). In addition, by forming the hole transport layer YL by coating, the surface roughness of the upper surface of the hole transport layer YL can be made smaller than the surface roughness of the light scattering layer SL. Through the hole transport layer YL, the light-emitting layer EM is separated from defects and dipoles contained in the metal oxide of the light scattering layer SL, thereby reducing the deactivation of excitons generated by recombination.

[0016] The refractive index of the hole transport layer YL can also be smaller than that of the light scattering layer SL. In this case, the effective refractive index of the light incident from the light-emitting layer EM to the light scattering layer SL becomes larger. Regarding Rayleigh scattering, since the larger the refractive index of the scattering center, the stronger the scattered light, the light extraction efficiency can be improved. The refractive index difference between the light scattering layer SL and the hole transport layer YL in contact with the cathode EC side of the light scattering layer SL can be 0.5 or more. The refractive index ns of the light scattering layer SL is generally 2.0 or more for a powdery material, and the refractive index ny of the hole transport layer YL containing an organic compound is generally less than 1.5 (for example, the refractive index of poly-TPD is 1.46), so ns - ny ≥ 0.5.

[0017] The lower surface of the hole transport layer YL can be in the shape following the upper surface of the light scattering layer SL, and the layer thickness of the hole transport layer YL can be larger than the surface roughness of the upper surface of the light scattering layer SL. In this way, the surface unevenness of the light scattering layer SL can be filled by the hole transport layer YL, and the flatness of the light-emitting layer EM formed on the hole transport layer YL can be improved. As the material (organic material) of the hole transport layer YL, TFB, PVK, poly-TPD, TPD, TAPC, TcTa, α-NPD, CBP, m-MTDATA can be cited.

[0018] The surface roughness of the upper surface of the light scattering layer SL can be 45 [nm] or less. In this way, blue light with a wavelength of 450 nm or less can be efficiently Rayleigh scattered. The surface roughness of the upper surface of the light scattering layer SL can be 30 [nm] or less. In this way, the hole transport layer YL is applicable to the case of 30 [nm] or less. The surface roughness can be 15 [nm] or more. In this way, by strongly diffusely reflecting light with a sufficiently large roughness, the light extraction efficiency can be further improved.

[0019] The light scattering layer SL may contain at least one of nickel oxide, copper oxide, vanadium oxide, molybdenum oxide, and tungsten oxide as a hole-transporting metal oxide. Copper oxide can be Cu 2O (cuprous oxide), or it may be CuO (copper oxide). The thickness of the light scattering layer SL may also be 47 [nm] or more. The thickness of the light scattering layer SL may also be the average value of the maximum thickness Dt per unit length (L) as described above.

[0020] Figure 4 It is a cross-sectional view showing a configuration example of a light-emitting element according to an embodiment. The light scattering layer SL may have a polycrystalline structure. The light scattering layer SL may have a plurality of nanoparticles NP. The nanoparticles NP may be metal oxide particles having a particle diameter (outer diameter) of 1.0 [nm] to 100 [nm]. When the nanoparticles NP are not spherical, for example, when they can be isometrically deformed into a spherical shape, the diameter in that case can be set as the particle diameter. For example, the value of the diameter of a circle equivalent to the cross-sectional area of the nanoparticles NP calculated from the cross-sectional view of the nanoparticles NP can be set as the particle diameter of the nanoparticles NP. The shape of the nanoparticles NP is not limited to a spherical three-dimensional shape (circular cross-sectional shape). For example, it may be a polygonal cross-sectional shape, a rod-shaped three-dimensional shape, a branched three-dimensional shape, a three-dimensional shape having irregularities on the surface, or a combination of these shapes. The nanoparticles NP may contain one or more of nickel oxide, copper oxide, vanadium oxide, molybdenum oxide, and tungsten oxide. The nanoparticles NP may also be composed of nickel oxide. Transition metal oxides generally have a compositional deviation. Therefore, nickel oxide can be represented by NiOx (x = 0.5 to 1.5), for example. The particle diameter (for example, median diameter) of the nanoparticles NP in the light scattering layer SL may be 4.0 [nm] or more. In this case, even in a state where the organic ligand is not coordinated, it is easily dispersed in the solution and agglomeration is less likely to occur. The particle diameter of the nanoparticles NP can be confirmed by observing images such as cross-sectional TEM (Transmission Electron Microscope), STEM (Scanning Transmission Electron Microscope), and SEM (Scanning Electron Microscope) of the coating solution. Specifically, calculate the particle diameter distribution of all the nanoparticles NP (for example, 10 to 100) in the field of view. For example, in the particle diameter distribution, if the particle diameter (D50: median diameter) at which the cumulative number reaches 50% is 4 nm or more, the particle diameter of the nanoparticles NP in the nanoparticle group Nx can be regarded as 4 nm or more. In the particle diameter distribution, it is more preferable that the particle diameter (D10) at which the cumulative number reaches 10% is 4 nm or more (90% of the nanoparticles have a particle diameter of 4 nm or more). Preferably, all the particles are 4 nm or more. The particle diameter of the nanoparticles NP is preferably the median diameter, second preferably the mode diameter, and second preferably the average diameter.

[0021] When the light-scattering layer SL is a nickel oxide nanoparticle layer containing nickel oxide nanoparticles NX, the surface roughness of the upper surface of the light-scattering layer SL can be made greater than the particle diameter of the nanoparticles NP in the nanoparticle group NX and less than 1 / 10 of the emission peak wavelength of the light-emitting layer EM. In this way, while ensuring the hole transport (injection) function, the light extraction efficiency of Rayleigh scattering can be improved. The upper surface roughness can be the difference between the maximum thickness and the minimum thickness in the cross-section of the light-scattering layer, or the arithmetic mean roughness Ra. The particle diameter of the nanoparticles NP is preferably the median particle diameter, but can also be the mode diameter or the average diameter. As Figure 4 shown, a hole transport layer YL containing an organic material can be provided between the light-scattering layer SL (nickel oxide nanoparticle layer) containing the nickel oxide nanoparticle group NX and the light-emitting layer EM. In this case, the surface roughness of the hole transport layer YL is smaller than that of the nickel oxide nanoparticle layer (light-scattering layer SL). In other words, the surface irregularities (light-scattering portion SS) generated by the surface nickel oxide nanoparticle aggregate can be flattened by the hole transport layer YL.

[0022] The light-emitting layer EM can also contain quantum dots Q. The quantum dots Q can be fine particles having a particle diameter (outer diameter) of 1.0 [nm] to 100 [nm], and the shape can be a sphere or a non-sphere. The shape of the quantum dots Q only needs to satisfy the above particle diameter range, and there is no particular limitation, and it is not limited to a spherical shape (circular cross-sectional shape). For example, it can be a polygonal cross-sectional shape, a rod-shaped three-dimensional shape, a branched three-dimensional shape, a three-dimensional shape having irregularities on the surface, or a combination of these shapes. The quantum dots Q can be composed of a semiconductor material. The semiconductor material can have a certain bandgap or can be a material that generates electroluminescence. The wavelength region of electroluminescence can be any one of the red region, the green region, and the blue region. The quantum dots Q can be any one of II-VI group semiconductor crystals such as MgS, MgSe, MgTe, CaS, CaSe, CaTe, SrS, SrSe, SrTe, BaS, BaSe, BaTe, ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, HgSe, HgTe and / or III-V group semiconductor crystals such as GaAs, GaP, InN, InAs, InP, InSb and / or IV group semiconductor crystals such as Si, Ge. The light-emitting layer EM can also contain an inorganic matrix material (for example, an inorganic compound having a bandgap larger than that of the quantum dots Q) that fills the gaps between multiple quantum dots Q. The inorganic matrix material can contain at least one of zinc sulfide and magnesium sulfide.

[0023] Figure 5 is a cross-sectional view showing a configuration example of a display device according to an embodiment. As Figure 1 and Figure 5As shown, the display device 10 may also include a first sub-pixel RP that emits red light, a second sub-pixel GP that emits green light, and a third sub-pixel BP that emits blue light. The first sub-pixel RP includes a light-emitting element RD (ED), the second sub-pixel GP includes a light-emitting element GD (ED), and the third sub-pixel BP includes a light-emitting element BD (ED). In Figure 5 it may also be the relationship that the surface roughness of the light-scattering layer RS (SL) of the first sub-pixel RP > the surface roughness of the light-scattering layer GS (SL) of the second sub-pixel GP > the surface roughness of the light-scattering layer BS (SL) of the third sub-pixel BP. In this way, the intensity of Rayleigh scattering is optimized for each sub-pixel (color), and the light extraction efficiency can be further improved.

[0024] Figure 6 is a cross-sectional view showing a configuration example of a light-emitting element layer according to an embodiment. On the upper surface (light-scattering portion SS) of the light-scattering layer SL, there may also be a region SA having an inclination of 16 degrees or more and 43 degrees or less with respect to a plane parallel to the anode EA. In this case, light reflected at a small angle with respect to the substrate surface can be made into light having a large angle with respect to the substrate surface. Thereby, the light extraction efficiency of the light-emitting element ED can be improved.

[0025] When considering the loss caused by total reflection at the glass / air interface, if the angle of the inclined surface is set within the range of 16 degrees or more and 43 degrees or less, the extraction efficiency of light emitted downward (from the light-emitting layer EM toward the anode EA) can be 70% or more (theoretical value). Figure 7 is a coordinate diagram showing the relationship between the inclination of the upper surface of the light-scattering layer and the extraction efficiency. Generally, the extraction efficiency γ when light travels from the first medium (refractive index n1) to the second medium (refractive index n2 < n1) can be calculated as shown in the following formula 1: However, the light is emitted equally in all directions. Here, the denominator 2π is the sum of the solid angles of a hemispherical surface, and θ C is the critical angle, satisfying sinθ C = n 2 / n 1 , so the extraction efficiency γ is expressed by the following formula 2. For example, when the first medium is glass and the second medium is air, [Equation 1] [Equation 2]

[0026] Consider the extraction efficiency of light irradiated in the direction opposite to the extraction direction (downward). Assume that light is reflected at a certain interface and directed toward the extraction direction (upward), and the interface is inclined at an angle α from the horizontal plane. Here, α is 45° or less. When reflecting, if considering that the incident angle (the inclination angle of the light with respect to the normal of the extraction surface) changes from θ to abs(θ - 2α), the extraction efficiency of the downward light is expressed by the following formula (3). [Equation 3] Here, R = min(θ C + 2α, π / 2), L = max(2α - θ C , 0). Therefore, the extraction efficiency of the entire light directed downward becomes Figure 7 As shown, if the angle of the inclined surface of the light-emitting layer EM side (upper surface) of the light-scattering layer SL is within the range of 16 degrees or more and 43 degrees or less, the extraction efficiency of the light irradiated downward can be 70% or more.

[0027] Figure 8 is a cross-sectional view showing another configuration of the light-emitting element according to the embodiment. As Figure 8 shown, the light-emitting layer EM can be formed in contact with the light-scattering layer SL. In this case, an interface between the light-scattering layer SL and the light-emitting layer EM is generated. The layer thickness of the light-emitting layer EM can be larger than the surface roughness of the upper surface of the light-scattering layer SL. The surface roughness of the upper surface of the light-scattering layer SL can be 15 [nm] or less. In this way, it is suitable for the case where the thickness of the light-emitting layer EM is 15 [nm] or less. The refractive index of the light-scattering layer SL is greater than that of the light-emitting layer EM, and the refractive index difference between the light-scattering layer SL and the light-emitting layer EM can be 0.5 or more, and the light-emitting layer EM is in contact with the cathode EC side of the light-scattering layer SL.

[0028] As Figure 3 and Figure 4 and Figure 8 shown, the structure (positive structure) in which the anode EA, the light-scattering layer SL, and the light-emitting layer EM are sequentially formed on the pixel circuit substrate 13 has the following advantages: By coating and forming the light-scattering layer SL using a specified method, it is possible to form a desired (capable of balancing light scattering and hole transportability) roughness on the upper surface, and the heat during firing of the light-scattering layer SL does not affect the light-emitting layer EM.

[0029] In Figure 4In the light-emitting element ED, a light-scattering layer SL is formed of nickel oxide nanoparticles NP. The upper surface roughness of the light-scattering layer SL is large enough to cause light at a small angle with respect to the substrate surface to scatter on the upper surface of the light-scattering layer SL, and the upper surface roughness is not greater than the degree at which current concentration is significant. Specifically, the upper surface roughness of the light-scattering layer SL is 5 [nm] to 70 [nm]. If the layer thickness of the light-scattering layer SL is increased, even if there is upper surface roughness, the change ratio of the layer thickness becomes smaller, so that the influence on characteristics such as current concentration can be reduced.

[0030] Figures 9A - 9C It is a conceptual diagram of light-scattering layers with different layer thicknesses. Figure 9A In the light-scattering layer SL, there is a region where the layer thickness is zero. In other words, Figure 9A the light-scattering layer SL does not have a base. Sometimes it is difficult to inject holes into the region where the layer thickness is zero, and the hole transport function is reduced. Figure 9B In, although there is no region where the layer thickness is zero, the change ratio of the layer thickness is large, and current concentration occurs in the region where the layer thickness is small. Figure 9C In, since the thickness Db of the base is appropriate and the change ratio of the layer thickness is small enough, current concentration is not likely to occur. Therefore, good EQE, uniform light emission, and high reliability can be achieved.

[0031] Generally, if the layer thickness of the hole injection layer is too thick, the driving voltage increases, and it also has an adverse effect on EQE. However, in the case where the light-scattering layer SL is formed of nickel oxide nanoparticles NP, if the layer thickness becomes larger, a phenomenon that the driving voltage gradually decreases is observed. When the layer thickness of the light-scattering layer SL is small, as Figure 9A shown, there are regions where nickel oxide nanoparticles NP do not exist, which hinders hole injection. But as Figure 9B and Figure 9C shown, it can be considered that if the layer thickness is increased, the regions where nanoparticles NP do not exist disappear, and hole injection increases. In the configuration of Figure 9C where good EQE is achieved, Db / (Dt - Db) ≥ 0.45 can be satisfied. In addition, Db / (Dt - Db) is the ratio of the thickness of the base BS to the upper surface roughness.

[0032] In this embodiment, by optimizing the upper surface roughness and layer thickness of the light scattering layer SL, the adverse effect of reliability on EQE is successfully minimized, and the extraction efficiency is improved. The light scattering layer SL is originally a metal oxide layer with hole transport properties required for driving the light-emitting element, and is thus also preferable in terms of cost. As will be described later, it has been clarified that by repeatedly coating a nanoparticle dispersion liquid containing nickel oxide nanoparticles NP and a specific solvent to form the light scattering layer SL, the layer thickness and surface roughness of the light scattering layer SL can be optimized. Specifically, a light scattering layer SL with a layer thickness of 47 nm or more and an upper surface roughness of 5 [nm] to 70 [nm] is achieved.

[0033] The nickel oxide nanoparticles NP of the light scattering layer SL are not coordinated with an organic ligand. Therefore, the hole injection function of the light scattering layer SL is improved, and the carrier balance is improved. Since no organic ligand is coordinated in the nanoparticles NP, an increase in the driving voltage accompanying the thickening of the light scattering layer SL is also suppressed.

[0034] If a nanoparticle layer coordinated with an organic ligand is used in the hole injection layer, the solubility in an organic solvent increases. Therefore, when coating the hole transport layer and the light-emitting layer as the upper layers, the hole injection layer (nanoparticle layer) may reduce in film thickness. Especially when the hole injection layer includes an upper surface roughness, the portions thinned due to the upper surface roughness are further shaved off, which may become a factor for reducing EQE due to current concentration. In Figure 4 the structure, no organic ligand is coordinated in the nanoparticles NP of the light scattering layer SL, and the solubility in an organic solvent is small, so such an adverse situation can be avoided.

[0035] A hole transport layer YL containing an organic material is formed between the light scattering layer SL containing nickel oxide nanoparticles NP and the light-emitting layer EM. The upper surface roughness of the hole transport layer YL is smaller than that of the light scattering layer SL containing nickel oxide nanoparticles NP. Therefore, by coating the hole transport layer YL, the lower surface roughness of the light-emitting layer EM (quantum dot layer) is smaller than the upper surface roughness of the light scattering layer SL. Thereby, the decrease in reliability caused by current concentration and the decrease in EQE caused by an increase in reactive current can be minimized. The hole transport layer YL is composed of an organic material, and generally has a refractive index smaller than that of the light scattering layer SL containing nickel oxide nanoparticles NP. Therefore, the intensity of scattered light caused by Rayleigh scattering can be increased.

[0036] Figure 10 It is a cross-sectional view showing another structure of the light-emitting element according to the embodiment. Figure 10The light-emitting element ED may sequentially include a cathode EC, a second functional layer (electron transport layer) SK, a light-emitting layer EM, a first functional layer FK, and an anode EA starting from the pixel circuit substrate 13 side. The first functional layer FK includes a light-scattering layer SL containing a hole-transporting metal oxide and a hole-transporting layer YL located on the light-scattering layer SL. In the light-emitting element ED, the light-scattering layer SL can transport holes supplied from the anode EA toward the light-emitting layer EM side, and scatter light (incident light) incident at a small angle with respect to a plane (e.g., the substrate plane AF) perpendicular to the thickness direction of the light-emitting element ED on the upper surface, becoming scattered light at a large angle with respect to the substrate plane. Thereby, the light extraction efficiency of the light-emitting element ED can be improved.

[0037] Figure 11 It is a flowchart showing a manufacturing method of a light-emitting element according to the present embodiment. Figure 12 It is a flowchart showing a method for forming a light-scattering layer according to the present embodiment. As Figure 11 shown, the manufacturing method of the light-emitting element of the present embodiment includes a step of forming an anode (S10), a step of forming a light-scattering layer (S20), a step of forming a light-emitting layer (S30), and a step of forming a cathode (S40). In the step of forming the light-scattering layer (S20), as Figure 12 shown, the following steps are performed: a step of coating a nanoparticle dispersion liquid on a base layer (e.g., an anode), the nanoparticle dispersion liquid dispersing a plurality of nanoparticles containing nickel oxide and containing 10% or more of water by volume and having a surface tension of 15 mN / m to 45 mN / m at 20°C; and a step of heating the coated nanoparticle dispersion liquid (S24). The step of coating the nanoparticle dispersion liquid after heating and coating the nanoparticle dispersion liquid may be repeated multiple times. The heating temperature is preferably 100°C or higher, more preferably 150°C or higher. In this way, the residual amount of the dispersion solvent and the inorganic ligand contained in the nanoparticle dispersion liquid in the light-scattering layer can be reduced, and the hole-transporting function of the light-scattering layer can be improved. In addition, when the steps S22 and S24 are repeated multiple times, it is difficult for the lower layer (nanoparticle layer) to dissolve in the second and subsequent steps S22 (the step of coating the nanoparticle dispersion liquid on the nanoparticle layer), and film loss can be suppressed. The nanoparticle dispersion liquid may contain nickel oxide nanoparticles, water (H 2 O), and at least one of 2-methoxyethanol, 1,4-dioxane, 2-butanol, 1-butanol, pyridine, acetic acid, and acetylacetone. The base layer on which the nanoparticle dispersion liquid is coated is not limited to the anode, and may be, for example, a hole injection layer. 〔Example〕 (Production of nickel oxide nanoparticles) 0.05 mmol of nickel nitrate hexahydrate (Ni(NO 3 ) 2·6H2O) and 20 mL of pure water were placed in a medicine bottle. After stirring, an aqueous sodium hydroxide solution with a concentration of 10 mol / L was added until the pH was above 9 to obtain a green precipitate. Pure water was added to the green precipitate for centrifugation, and the washing process of removing the supernatant was repeated 3 times. After the drying process, a green powder was obtained. The green powder was calcined at 270 °C for 2 hours to obtain black nickel oxide nanoparticles (powder).

[0038] By changing the pH of the solution after adding the aqueous sodium hydroxide solution, the particle size of the nickel oxide nanoparticles can be controlled. Nickel oxide nanoparticles were prepared at several pH values, and powder X-ray diffraction measurements were performed on each nickel oxide nanoparticle. The crystallite diameter D (nm) was calculated according to the following Scherrer formula. D (nm) = 0.89λ (nm) / B (deg) cosθ Among them, λ (nm) is the wavelength of X-rays, and the CuKα line with λ (nm) = 0.154 was used in this measurement. B (deg) is the line width of the X-ray diffraction peak, and θ is the diffraction angle.

[0039] Furthermore, nickel oxide nanoparticles with each crystallite diameter were dispersed in an aqueous solvent, and it was tested whether the solution could pass through a 0.45 μm filter. Furthermore, for the passed solution, particle size distribution measurement was performed, and the median diameter D50 (nm) of the nickel oxide nanoparticles dispersed in the aqueous solvent was calculated. These results are shown in the following table.

[0040] [Table 1] It can be seen that compared with the case where the crystallite diameter is 4 nm or more, when the crystallite diameter is less than 4 nm, very intense particle aggregation occurs when dispersed in an aqueous solvent. It is considered that this is because the organic ligands that contribute to dispersion are not coordinated on the surface of the nickel oxide nanoparticles. Therefore, if the particle size of the nickel oxide nanoparticles becomes too small, the aggregation with surrounding nickel oxide particles to reduce the surface area becomes stronger. The intense aggregation makes the roughness of the nickel oxide nanoparticle layer during coating extremely large, which causes a decrease in EQE due to an increase in the ineffective current of the light-emitting element and a decrease in reliability due to current concentration. Therefore, for nickel oxide nanoparticles without coordinated organic ligands, it is preferable to set the particle size (for example, the median particle size) to 4 nm or more.

[0041] In this example, nickel oxide nanoparticles obtained by making the pH of the solution after adding the aqueous sodium hydroxide solution 10 were dispersed in a solution in which water and 2-methoxyethanol were mixed at a volume ratio of 1:1 to prepare a 15 mg / mL nickel oxide nanoparticle solution (nanoparticle dispersion).

[0042] (Fabrication of Light-Emitting Element) On a substrate (e.g., a pixel circuit substrate), an ITO film (e.g., with a film thickness of 30 nm) is formed by sputtering to form an anode. Subsequently, in the atmosphere, according to the desired layer thickness, the coating (e.g., spin coating) of nickel oxide nanoparticle solution (nanoparticle dispersion) and the heating of the coating solution (e.g., 150 - 200 °C) are repeated 1, 2, 3, 4, 5 times to form nickel oxide nanoparticle layers (light scattering layers SL) with thicknesses (maximum thickness Dt) of 31 nm, 36 nm, 47 nm, 54 nm, and 62 nm respectively. Subsequently, in N 2 atmosphere, a Poly-TPD film (hole transport layer YL) with a film thickness of, for example, 30 nm is formed by coating a chlorobenzene solution of poly[bis(4-phenyl)(4-butylphenyl)amine (Poly-TPD) (e.g., spin coating) and volatilizing the solvent (chlorobenzene). Subsequently, in N 2 atmosphere, a solution in which luminescent quantum dots (e.g., phosphor particles of InP emitting red light) are dispersed in an octane solvent is coated by spin coating to form a quantum dot layer (emission layer EM) with a layer thickness of, for example, 13 nm. Subsequently, in N 2 atmosphere, a solution in which nanoparticles of magnesium zinc oxide (e.g., Mg 0.15 Zn 0.85 O) (e.g., with a particle size of 5 nm) are dispersed in an ethanol solvent is coated by spin coating to form an electron transport layer (second functional layer SK) with a layer thickness of, for example, 60 nm. Subsequently, an Ag film (cathode EC) with a film thickness of, for example, 65 nm is formed by vacuum evaporation. Subsequently, in N 2 atmosphere, the substrate and the light-emitting element on the substrate are sealed by a sealing member. The coating method of the nickel oxide nanoparticle solution is not limited to spin coating. Inkjet printing can also be used.

[0043] A self-assembled monolayer (SAM) can be formed between the light scattering layer SL and the hole transport layer YL. The self-assembled monolayer can be formed, for example, by coating a solution obtained by dispersing 2-(3,6-dimethoxy-9H-carbazol-9-yl)ethyl]phosphonic acid (MeO-2PACz) in an ethanol solvent (on the light scattering layer) and volatilizing the solvent based on baking. In this case, multiple self-organized monolayers are arranged along the surface irregularities of the light scattering layer SL.

[0044] The anode EA is, in addition to ITO (indium tin oxide), IZO (indium zinc oxide), SnO 2 (tin oxide), FTO (fluorine-doped tin oxide), etc. In the electron transport layer, in addition to undoped ZnO (zinc oxide), ZnO (zinc oxide) doped with at least one of Li, Al, Ti, Ga, Zr, TiO 2 (titanium oxide), ZrO 2(zirconia), etc. The hole transport layer uses TFB, PVK, etc.

[0045] The nickel oxide nanoparticle solution (nanoparticle dispersion) involved in this embodiment contains 10% or more of water by volume ratio, and the surface tension can be 15 mN / m to 45 mN / m at 20°C. By adding highly polar water to the nickel oxide nanoparticle solution, the dispersion of nickel oxide nanoparticles can be achieved even without the coordination of organic ligands. By mixing with 2-methoxyethanol, the surface tension of the solution is 45 mN / m or less, the wettability to the substrate is improved, and the surface roughness during coating will not become extremely large. Therefore, the roughness suitable for the hole-transporting light-scattering layer located under the light-emitting layer can be formed. The viscosity of the nickel oxide nanoparticle solution is preferably 4 mPa·s or more at 20°C. In this case, since the surface tension of the nickel oxide nanoparticle solution is not as large as that of water, it can also be ejected by the inkjet method. The nickel oxide nanoparticle solution preferably does not contain organic ligands. It can be confirmed by FTIR (Fourier Transform Infrared Spectroscopy) etc. that the nickel oxide nanoparticle solution does not contain organic ligands. For example, if the nickel oxide nanoparticle solution is measured by FTIR, no peak caused by C-H stretching from 2800 cm -1 to 3000 cm -1 is observed, it can be regarded as not containing organic ligands. The nickel oxide nanoparticle solution preferably contains nitrate ions. Nitrate ions promote the dispersion of nickel oxide nanoparticles in the solvent (water), and agglomeration of nanoparticles is not likely to occur. As a result, the solution stability is improved, and the surface roughness during coating formation will not become extremely large. Since the boiling point of nitric acid is as low as 83°C, nitrate ions volatilize during the coating of the nickel oxide nanoparticle solution or during the heating after coating, and will not affect the characteristics of the light-emitting element. It can be confirmed by FTIR etc. that the nickel oxide nanoparticle solution contains nitrate ions. In order to prevent the residue of nitrate ions, the heating temperature after solution coating is preferably 100°C or higher, more preferably 150°C or higher.

[0046] The boiling point of the nickel oxide nanoparticle solution can be 150°C or lower. If the boiling point of the nickel oxide nanoparticle solution exceeds 150°C, when forming the nickel oxide nanoparticle layer, the water responsible for dispersing the nickel oxide nanoparticles volatilizes first, and the nickel oxide nanoparticles may agglomerate violently in the remaining solvent. When the boiling point is 150°C or lower, the volatilization of water first can be suppressed.

[0047] In this embodiment, it is preferable to repeat the coating of the nickel oxide nanoparticle solution and the heating of the coating solution more than 2 times, and more preferably more than 3 times (described later). In the case where multiple coatings are not performed, a region with a layer thickness of zero is formed on the light scattering layer SL, and holes are not easily injected. By performing multiple coatings, there is basically no region with a layer thickness of zero on the light scattering layer SL, and the hole transport function is improved. Specifically, when the light scattering layer SL is inspected for 500 nm or more in a direction perpendicular to the layer thickness direction, the ratio of the total length of the zero layer thickness region to the inspection length is preferably 0.02 or less. In addition, by performing multiple coatings, a light scattering layer SL with a more uniform upper surface roughness can be achieved, rather than a specific part having a significantly larger upper surface roughness than other parts.

[0048] Figure 13 It is a cross-sectional SEM image of an example of a light-emitting element. Figure 13 The light-emitting element uses ITO (indium tin oxide) in the anode EA, nickel oxide nanoparticles in the light scattering layer SL, Poly-TPD as an organic material in the hole transport layer YL, magnesium zinc oxide in the second functional layer (electron transport layer) SK, and Ag (silver) in the cathode EC. Figure 13 The light scattering layer SL of is formed by coating a nickel oxide nanoparticle solution (nanoparticle dispersion liquid) once, and there is a position A1 where nickel oxide nanoparticles do not exist, and the layer thickness (Db) of position A1 is zero. The layer thickness (Dt) of the position A2 where the thickness of the light scattering layer SL is the largest is 28.5 nm, and the upper surface roughness of the light scattering layer SL is about 28.5 nm. The thickness Y1 of the hole transport layer YL at position A1 is 46.8 nm, and the thickness Y2 at position A2 is 20.9 nm. Therefore, the sum of the thickness of the light scattering layer SL and the thickness of the hole transport layer YL is 46.8 nm at position A1 and 49.4 nm at position A2, and the upper surface roughness of the hole transport layer YL is about 49.4 - 46.8 = 2.6 nm. In this way, the upper surface roughness of the hole transport layer YL is smaller than that of the light scattering layer SL. By providing a hole transport layer YL that is flatter than the light scattering layer SL, although the light scattering layer SL has a large upper surface roughness, Figure 13 the flatness of the light-emitting layer EM is achieved in the entire observation region (unit length L).

[0049] Figure 14 It is a cross-sectional SEM image of another example of a light-emitting element. The constituent materials of the light-emitting element are the same as those of Figure 13 the same. Figure 14 The light scattering layer SL of is formed by coating the nickel oxide nanoparticle solution twice, and the layer thickness (Db) of the position A3 where the thickness of the light scattering layer SL is the smallest is 10.0 nm. The layer thickness (Dt) of the position A4 where the thickness of the light scattering layer SL is the largest is 31.6 nm, and the upper surface roughness of the light scattering layer SL is about 21.6 nm.Figure 14 Among them, the upper surface roughness of the hole transport layer YL (the difference between the sum of the thickness of the light scattering layer SL and the thickness of the hole transport layer YL at position A3 and the sum of the thickness of the light scattering layer SL and the thickness of the hole transport layer YL at position A4) is smaller than the upper surface roughness of the light scattering layer SL. Thus, although the light scattering layer SL has a large upper surface roughness, by providing the hole transport layer YL, flatness of the light emitting layer EM is achieved throughout Figure 14 the entire observation region (unit length L).

[0050] Figure 15 is a cross-sectional SEM image of another example of a light emitting device. The constituent materials of the light emitting device are the same as Figure 13 those. Figure 15 The light scattering layer SL of is formed by coating a nickel oxide nanoparticle solution three times. The layer thickness (Db) at position A5 where the light scattering layer SL has the smallest thickness is 15.9 nm. The layer thickness (Dt) at position A6 where the light scattering layer SL has the largest thickness is 37.4 nm, and the upper surface roughness of the light scattering layer SL is about 21.5 nm. Figure 15 Among them, the upper surface roughness of the hole transport layer YL (the difference between the sum of the thickness of the light scattering layer SL and the thickness of the hole transport layer YL at position A5 and the sum of the thickness of the light scattering layer SL and the thickness of the hole transport layer YL at position A6) is smaller than the upper surface roughness of the scattering layer SL. Thus, although the light scattering layer SL has a large upper surface roughness, by providing the hole transport layer YL, flatness of the light emitting layer EM is achieved throughout Figure 15 the entire observation region (unit length L). Further, in Figure 15 the light scattering layer SL of, there is a region SA having an inclination angle of 20 degrees with respect to the substrate surface F on the upper surface.

[0051] Figure 16 is Figure 13 a STEM image of the light emitting device of. Figure 16 Among them, the layer thickness (Db) at position A7 where the light scattering layer SL has the smallest thickness is 10.5 nm, and the layer thickness (Dt) at position A8 where the light scattering layer SL has the largest thickness is 31.1 nm. Figure 16 The STEM image of shows larger layer thicknesses of the light scattering layer SL, the hole transport layer YL, etc. compared to the SEM image of Figure 13 because depth information is also observed in STEM (different from SEM).

[0052] Figure 17 is Figure 14 a STEM image of the light emitting device of. Figure 17 Among them, the layer thickness (Db) at position A9 where the light scattering layer SL has the smallest thickness is 22.2 nm, and the layer thickness (Dt) at position A10 where the light scattering layer SL has the largest thickness is 35.8 nm.

[0053] Figure 18 is Figure 15 a STEM image of a light-emitting element. In Figure 18 , the layer thickness (Db) at position A11 where the thickness of the light-scattering layer SL is the smallest is 24.7 nm, and the layer thickness (Dt) at position A12 where the thickness of the light-scattering layer SL is the largest is 47.3 nm.

[0054] Figure 19 is a graph showing the relationship between the number of coating times and the layer thickness during the formation of the light-scattering layer. One coating is equivalent to a layer thickness of 31 nm, two coatings are equivalent to a layer thickness of 36 nm, three coatings are equivalent to a layer thickness of 47 nm, four coatings are equivalent to a layer thickness of 54 nm, and five coatings are equivalent to a layer thickness of 62 nm. The layer thickness refers to the thickness (Dt) of the thickest part. In the case where the light-scattering layer SL is formed by one coating of the nanoparticle dispersion liquid, as Figure 13 and Figure 16 and Figure 19 shown, the Db (the thickness of the bottom of the thinnest part) of the light-scattering layer SL is 0 nm, and the Dt (the thickness of the thickest part) is about 31 nm. Therefore, the surface roughness of the upper surface of the light-scattering layer SL is 31 nm - 0 nm = 31 nm. The ratio of the thickness of the base BS to the surface roughness is 0%. That is, in this case, the light-scattering layer is in the state shown in Figure 9A shown.

[0055] In the case where the light-scattering layer SL is formed by two coatings of the nanoparticle dispersion liquid, as Figure 14 and Figure 17 and Figure 19 shown, the Db (the thickness of the base of the thinnest part) of the light-scattering layer SL is about 10 nm, and the Dt (the thickness of the thickest part) is about 36 nm. Therefore, the surface roughness of the upper surface of the light-scattering layer SL is 36 nm - 10 nm = 26 nm. The ratio of the thickness of the base BS to the surface roughness is 10 / 26 = 38%. In this case, the light-scattering layer is in the state where the ratio of the thickness of the base BS to the surface roughness is relatively small Figure 9B shown.

[0056] In the case where the light-scattering layer SL is formed by three coatings of the nanoparticle dispersion liquid, as Figure 15 and Figure 18 and Figure 19 shown, the Db (the thickness of the base of the thinnest part) of the light-scattering layer SL is about 16 nm, and the Dt (the thickness of the thickest part) is about 47 nm. Therefore, the surface roughness of the upper surface of the light-scattering layer SL is 47 nm - 16 nm = 31 nm. The ratio of the thickness of the base BS to the surface roughness is 16 / 31 = 52%. In this case, the light-scattering layer is in the state where the ratio of the thickness of the base BS to the surface roughness is large enoughFigure 9C The state shown

[0057] In the case where the light-scattering layer SL is formed by coating the nanoparticle dispersion four times, as Figure 19 shown, the Db (thickness at the base of the thinnest part) of the light-scattering layer SL is about 25 nm, and the Dt (thickness of the thickest part) is about 54 nm. Therefore, the surface roughness of the upper surface of the light-scattering layer SL is 54 nm - 25 nm = 29 nm. The ratio of the thickness of the base BS to the surface roughness of the upper surface is 25 / 29 = 86%. In this case, the light-scattering layer is in the state where the ratio of the thickness of the base BS to the surface roughness of the upper surface is sufficiently large, Figure 9C the state shown

[0058] In the case where the light-scattering layer SL is formed by coating the nanoparticle dispersion five times, as Figure 19 shown, the Db (thickness of the base of the thinnest part) of the light-scattering layer SL is approximately 32 nm, and the Dt (thickness of the thickest part) is approximately 62 nm. Therefore, the surface roughness of the upper surface of the light-scattering layer SL is 62 nm - 32 nm = 30 nm. The ratio of the thickness of the base BS to the surface roughness of the upper surface is 32 / 30 = 107%. In this case, the light-scattering layer is in the state where the ratio of the thickness of the base BS to the surface roughness of the upper surface is sufficiently large, Figure 9C the state shown

[0059] Figure 20A , Figure 20B is a graph showing the voltage-current density characteristics of the light-emitting element when the layer thickness (coating times) of the light-scattering layer is changed. Figure 21A , Figure 21B is a graph showing the voltage-luminance characteristics of the light-emitting element when the layer thickness (coating times) of the light-scattering layer is changed. Figure 22A , Figure 22B is a graph showing the current density-EQE characteristics of the light-emitting element when the layer thickness (coating times) of the light-scattering layer is changed. Figure 23A , Figure 23B is a graph showing the current density-luminance characteristics of the light-emitting element when the layer thickness (coating times) of the light-scattering layer is changed. Figure 24 is a graph showing the change over time (reliability) of the relative luminance of the light-emitting element when the layer thickness (coating times) of the light-scattering layer is changed.

[0060] It is known that when the layer thickness (Dt) of the light scattering layer SL (nickel oxide nanoparticle layer) is 47 nm or more, both the EQE and reliability increase sharply, and the driving voltage also decreases. This is because when the nickel oxide nanoparticle layer is thin, holes are formed in part of the nickel oxide nanoparticle layer due to the surface roughness on the upper surface, or when no holes are formed, the change ratio of the layer thickness is large. As a result, it causes a decrease in hole injection or a decrease in reliability. Therefore, when the layer thickness becomes 47 nm or more, there are no holes, and the change ratio of the layer thickness becomes sufficiently small, resulting in a sharp increase in characteristics including EQE and reliability.

[0061] If the coating times of the nickel oxide nanoparticle solution are changed from 1 time to 2 times, the EQE and reliability of the light-emitting element are significantly improved. That is, if a base is provided on the light scattering layer SL, the EQE and reliability are significantly improved. This is because no holes are formed, so hole injection is improved and the carrier balance is improved. If the coating times of the nickel oxide nanoparticle solution are changed from 2 times to 3 times, the EQE and reliability of the light-emitting element are significantly improved. That is, if the thickness of the base BS relative to the surface roughness of the light scattering layer SL is changed from 38% to 52%, the EQE and reliability are significantly improved. Therefore, it is considered critical that the thickness (Db) of the base BS is 45% or more of the surface roughness (Dt - Db) of the light scattering layer SL. This is because in this case, no holes are formed in the light scattering layer SL (nickel oxide nanoparticle layer), and the change ratio of the layer thickness is small enough, so adverse effects such as current concentration are difficult to occur.

[0062] As Figure 20A shown, by increasing the thickness of the light scattering layer SL from 1 coating to 3 coatings, the voltage (required to obtain a specified current density) decreases instead. After 3 coatings, even if the thickness of the light scattering layer SL is increased, the voltage hardly changes ( Figure 20B ). As Figure 22A shown, by increasing the thickness of the light scattering layer SL from 1 coating to 3 coatings, the EQE of the light-emitting element is improved. After 3 coatings, even if the thickness of the light scattering layer SL is increased, the EQE hardly changes ( Figure 22B ). As Figure 24 shown, from 1 coating to 3 coatings, the reliability of the light-emitting element is improved by increasing the thickness of the light scattering layer SL, but after 3 coatings, even if the thickness of the light scattering layer SL is increased, the reliability hardly changes. Thus, it is preferable to repeat the coating of the nickel oxide nanoparticle solution and the heating of the coating liquid 2 times or more, and more preferably 3 times or more.

[0063] Figure 25It is a graph showing the change over time of the relative brightness of a light-emitting element when the solvent of the nanoparticle dispersion is changed. When a nanoparticle dispersion (concentration 15 mg / mL) with a mixed solution of water and 2-MeOEtOH (2-methoxyethanol) as the solvent is coated three times to form a light-scattering layer with a thickness of 47 nm, compared with the case where a nanoparticle dispersion (concentration 30 mg / mL) with only water as the solvent is coated three times to form a light-scattering layer with a thickness of 49 nm, the decrease in relative brightness due to the change over time is improved. From this, it can be seen that by using a mixed solution of water and an organic solvent as the solvent of the nanoparticle dispersion, the reliability of the light-emitting element is improved. It is considered that by mixing an organic solvent in the coating solution to make the surface tension 45 mN / m or less, the wettability with respect to the substrate is improved, the upper surface roughness is extremely large when coating the nickel oxide nanoparticle solution, and current concentration becomes difficult.

[0064] In the evaluation of nickel oxide nanoparticles, the light-scattering layer, the light-emitting element, etc., the following measuring instruments are used. · Powder X-ray Diffraction (XRD) instrument: MiniFlex II manufactured by Rigaku · Particle size distribution measuring instrument: Nanotrac wave II manufactured by MicrotracBEL · Current-voltage characteristic measuring instrument: 2400 manufactured by Kethley · Brightness and spectrum measuring instrument: MCPD7000 manufactured by Otsuka Electronics · Constant current life evaluation device: EAS-10R manufactured by System Giken

[0065] The above-described embodiments are for illustrative and explanatory purposes and are not intended to be limiting. Based on these illustrations and explanations, those skilled in the art should understand that various modification methods can be carried out. Explanation of reference numerals

[0066] 11 Substrate 13 Pixel circuit substrate EA Anode EC Cathode SL Light-scattering layer YL Hole transport layer BS Base SS Light-scattering part EM Light-emitting layer ED Light-emitting element NP Nanoparticle

Claims

1. A light-emitting element, characterized in that, comprising: an anode and a cathode; a light-emitting layer located between the anode and the cathode; and a light-scattering layer located between the light-emitting layer and the anode and containing a hole-transporting metal oxide.

2. The light-emitting element according to claim 1, characterized in that, the surface roughness of the upper surface of the light-scattering layer is 5 nm to 70 nm.

3. The light-emitting element according to claim 1 or 2, characterized in that, the light-scattering layer includes a base portion on the anode side and a light-scattering portion on the light-emitting layer side.

4. The light-emitting element according to claim 3, characterized in that, the thickness of the base portion is 45% or more of the surface roughness of the upper surface of the light-scattering layer.

5. The light-emitting element according to any one of claims 1 to 4, characterized in that, the atomic percentage of carbon atoms in the light-scattering layer is 0% to 5%.

6. The light-emitting element according to any one of claims 1 to 5, characterized in that, the light-scattering layer does not contain an organic ligand.

7. The light-emitting element according to any one of claims 1 to 6, characterized in that, there is a region on the upper surface of the light-scattering layer having an inclination angle of 16 degrees or more and 43 degrees or less with respect to a plane parallel to the anode.

8. The light-emitting element according to any one of claims 1 to 7, characterized in that, a hole-transporting layer containing an organic material is provided between the light-scattering layer and the light-emitting layer.

9. The light-emitting element according to claim 8, characterized in that, the surface roughness of the upper surface of the hole-transporting layer is smaller than the surface roughness of the upper surface of the light-scattering layer.

10. The light-emitting element according to claim 8 or 9, characterized in that, the refractive index of the hole-transporting layer is smaller than the refractive index of the light-scattering layer.

11. The light-emitting element according to any one of claims 8 to 10, characterized in that, the lower surface of the hole-transporting layer follows the shape of the upper surface of the light-scattering layer.

12. The light-emitting element according to any one of claims 8 to 11, characterized in that, the layer thickness of the hole-transporting layer is larger than the surface roughness of the upper surface of the light-scattering layer.

13. The light-emitting element according to any one of claims 1 to 12, characterized in that, the metal oxide contains at least one of nickel oxide, copper oxide, vanadium oxide, molybdenum oxide, and tungsten oxide.

14. The light-emitting element according to claim 13, characterized in that, the metal oxide is nickel oxide, and the thickness of the light-scattering layer is 47 nm or more.

15. The light-emitting element according to any one of claims 1 to 14, characterized in that, the light-scattering layer has a polycrystalline structure.

16. The light-emitting element according to claim 13 or 14, characterized in that, the light-scattering layer has a plurality of nanoparticles, and the plurality of nanoparticles are each composed of nickel oxide.

17. The light-emitting element according to claim 16, characterized in that, the median particle size of the plurality of nanoparticles is 4 nm or more.

18. The light-emitting element according to any one of claims 1 to 17, characterized in that, Rayleigh scattering occurs in the light-scattering layer.

19. The light-emitting element according to any one of claims 1 to 18, characterized in that, it further includes a substrate, and the anode is closer to the substrate than the cathode.

20. The light-emitting element according to any one of claims 1 to 19, characterized in that, the light-emitting layer contains quantum dots.

21. The light-emitting element according to any one of claims 1 to 7, characterized in that, the light-emitting element has an interface between the light-scattering layer and the light-emitting layer.

22. The light-emitting element according to claim 21, characterized in that, the layer thickness of the light-emitting layer is larger than the surface roughness of the upper surface of the light-scattering layer.

23. The light-emitting element according to any one of claims 1 to 7, characterized in that, a hole transport layer is provided between the light-scattering layer and the light-emitting layer, the light-scattering layer contains nickel oxide nanoparticles, and the hole transport layer contains an organic material.

24. The light-emitting element according to claim 23, characterized in that, the surface roughness of the upper surface of the light-scattering layer is 15 nm to 45 nm.

25. The light-emitting element according to claim 23 or 24, characterized in that, the organic material is at least one of TFB, PVK, poly-TPD, TPD, TAPC, TcTa, α-NPD, CBP, and m-MTDATA.

26. The light-emitting element according to any one of claims 1 to 25, characterized in that, the refractive index difference between the light-scattering layer and the layer in contact with the cathode side of the light-scattering layer is 0.5 or more.

27. A light-emitting element, characterized in that, it includes: an anode and a cathode; a light-emitting layer, which is located between the anode and the cathode and contains quantum dots; a nickel oxide nanoparticle layer, which is located between the anode and the light-emitting layer and contains a group of nickel oxide nanoparticles, the surface roughness of the upper surface of the nickel oxide nanoparticle layer is larger than the particle diameter of the nickel oxide nanoparticles in the group of nickel oxide nanoparticles and smaller than one-tenth of the emission peak wavelength of the light-emitting layer; and a hole transport layer, which is located between the nickel oxide nanoparticle layer and the light-emitting layer and contains an organic material, and the surface roughness of the upper surface of the hole transport layer is smaller than that of the nickel oxide nanoparticle layer.

28. A display device, characterized in that, it includes a first sub-pixel that emits red light, a second sub-pixel that emits green light, and a third sub-pixel that emits blue light, and the first sub-pixel, the second sub-pixel, and the third sub-pixel respectively include the light-emitting element according to any one of claims 1 to 27.

29. The display device according to claim 28, characterized in that, the surface roughness of the upper surface of the light-scattering layer of the first sub-pixel > the surface roughness of the upper surface of the light-scattering layer of the second sub-pixel > the surface roughness of the upper surface of the light-scattering layer of the third sub-pixel.

30. A nanoparticle dispersion liquid, characterized in that, the nanoparticle dispersion liquid disperses a plurality of nanoparticles containing nickel oxide, contains 10% or more of water by volume ratio, and has a surface tension of 15 mN / m to 45 mN / m at 20°C.

31. The nanoparticle dispersion liquid according to claim 30, It is characterized in that the viscosity of the nanoparticle dispersion is 4 mPa·s or more at 20°C.

32. The nanoparticle dispersion according to claim 30 or 31, It is characterized in that the nanoparticle dispersion does not contain an organic ligand.

33. The nanoparticle dispersion according to any one of claims 30 to 32, It is characterized in that the nanoparticle dispersion contains nitrate ions.

34. The nanoparticle dispersion according to any one of claims 30 to 33, It is characterized in that the boiling point of the nanoparticle dispersion is 150°C or less.

35. The nanoparticle dispersion according to any one of claims 30 to 34, It is characterized in that the nanoparticle dispersion contains at least one of 1,4-dioxane, 2-butanol, 1-butanol, pyridine, acetic acid, 2-methoxyethanol, and acetylacetone.

36. A method for manufacturing a light-emitting element, It is characterized in that comprises: a step of coating a nanoparticle dispersion on a base layer, the nanoparticle dispersion being dispersed with a plurality of nanoparticles containing nickel oxide, containing 10% or more of water by volume, and having a surface tension of 15 mN / m to 45 mN / m at 20°C; and a step of heating the coated nanoparticle dispersion.

37. The method for manufacturing a light-emitting element according to claim 36, It is characterized in that the following step is repeated two or more times: the step of heating the nanoparticle dispersion coated after the nanoparticle dispersion is coated.

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