Light-emitting element, display device, and light-emitting element manufacturing method

Abstract

A light-emitting element (2) comprises a first light-emitting layer (23) and an electron accumulation layer (24). The first light-emitting layer includes at least one quantum dot (41). The electron accumulation layer is adjacent to the first light-emitting layer and includes an inorganic compound having a higher electron affinity than that of the material of the quantum dot.

Description

Light-emitting element, display device, and method for manufacturing the same

[0001] The present disclosure relates to a light-emitting device that includes quantum dots as a light-emitting material, a display device that includes the light-emitting device, and a method for manufacturing the light-emitting device.

[0002] Patent Document 1 discloses a light-emitting device in which the light-emitting layer contains quantum dots as a light-emitting material.

[0003] Japanese Patent Application Publication No. 2009-88276

[0004] In a light-emitting element that includes quantum dots as a light-emitting material in the light-emitting layer, such as the light-emitting element described in Patent Document 1, an excess of electrons in the light-emitting layer may cause a decrease in light-emitting efficiency or deterioration of the light-emitting layer.

[0005] A light-emitting element according to one aspect of the present disclosure includes an anode, a cathode facing the anode, a first light-emitting layer located between the anode and the cathode and including at least one quantum dot, and an electron accumulation layer adjacent to the first light-emitting layer on the anode side or the cathode side and including an inorganic compound having a larger electron affinity than a material of the quantum dot.

[0006] A method for manufacturing a light-emitting element according to one aspect of the present disclosure includes forming an anode, forming a cathode facing the anode, forming a first light-emitting layer located between the anode and the cathode and including at least one quantum dot, and forming an electron accumulation layer adjacent to the first light-emitting layer on the anode side or the cathode side and including an inorganic compound having a larger electron affinity than a material of the quantum dot.

[0007] In a light-emitting device containing quantum dots as a light-emitting material in a light-emitting layer, the excess electrons in the light-emitting layer are reduced, thereby improving the light-emitting efficiency or reducing the deterioration of the light-emitting layer.

[0008] FIG. 1 is a schematic side cross-sectional view of a display device according to embodiment 1, and a band diagram of each functional layer. FIG. 2 is a schematic view of a display device according to embodiment 1. FIG. 3 is a graph showing an example of the relationship between the diameter of semiconductor nanoparticles and the band gap. FIG. 4 is a band diagram of a first light-emitting layer and an electron accumulation layer according to embodiment 1. FIG. 5 is a flowchart of a method for manufacturing a display device according to embodiment 1. FIG. 6 is a graph showing the relationship between the external quantum efficiency and the current density of a light-emitting element according to a comparative embodiment. FIG. 7 is a graph showing the relationship between the external quantum efficiency and the current density of light-emitting elements according to an example and a comparative example. FIG. 8 is a schematic side cross-sectional view of a display device according to embodiment 2. FIG. 9 is a schematic side cross-sectional view of a display device according to embodiment 3. FIG. 10 is a schematic side cross-sectional view of a display device according to embodiment 4. FIG. 11 is a schematic side cross-sectional view of a display device according to embodiment 5. FIG. 12 is a band diagram of a first light-emitting layer and an electron accumulation layer according to embodiment 5. FIG. 13 is a schematic side cross-sectional view of a display device according to embodiment 6.

[0009] [Embodiment 1] Hereinafter, an embodiment of the present disclosure will be described with reference to the drawings. Note that in each drawing, similar configurations are assigned the same reference numerals, and their description will be omitted. Furthermore, in this disclosure, for simplicity of illustration, components assigned the same reference numerals may be shown at different scales or with different hatching depending on the drawing. However, the components shown in each drawing of the present disclosure are merely examples, and the scales are not limited to those shown in the drawings. Furthermore, in this disclosure, components assigned the same reference numerals have similar configurations as described above. Furthermore, in each drawing of the present disclosure, when two components have substantially the same shape but different compositions, etc., they may be assigned different reference numerals but with the same hatching.

[0010] <Overview of Display Device> Fig. 2 is a schematic diagram of a display device 1 according to this embodiment. The display device 1 is a device that can be used, for example, as a display for a television, a smartphone, or the like. The display device 1 includes a display section DA including a plurality of sub-pixels, and a driver circuit DR that drives the plurality of sub-pixels. Each of the plurality of sub-pixels includes a light-emitting element 2 (described below) and a pixel circuit PC that drives the light-emitting element 2. The display device 1 performs display on the display section DA by controlling light emission from each of the plurality of light-emitting elements 2 formed in the display section DA via the driver circuit DR and the pixel circuit PC.

[0011] The structure of the display unit DA of the display device 1, particularly the structure of the light-emitting element 2, will be described in more detail with reference to Fig. 1. Fig. 1 shows a schematic side cross-sectional view CS of the display device 1 according to an embodiment of the present disclosure, and band diagrams B1 and B2 of each functional layer of the light-emitting element 2, which will be described later.

[0012] The schematic cross-sectional side views of the display device 1 in the present disclosure, including the schematic cross-sectional side view CS in FIG. 1, show a cross section perpendicular to the display surface of the display device 1 and passing through the light-emitting element 2. In other words, the cross section of the display device 1 shown in the schematic cross-sectional views in the present disclosure is, for example, a cross section parallel to the film thickness direction DT of the electron accumulation layer 24 described below. Furthermore, the band diagrams in the present disclosure, including the band diagram B1 and the band diagram B2, all show a case where the level at infinity is located on the upper side as viewed in the drawing. Furthermore, in both of the band diagrams in the present disclosure, including the band diagram B1 and the band diagram B2, the anode 21 described below is located on the left side as viewed in the drawing, and the cathode 26 described below is located on the right side as viewed in the drawing.

[0013] As shown in Fig. 1, the display device 1 according to this embodiment includes a display section DA that includes the above-described plurality of light-emitting elements 2 and a substrate 3. The display device 1 includes the light-emitting elements 2 on the substrate 3, and as shown in Fig. 2 in particular, the display device 1 includes a plurality of light-emitting elements 2. The display device 1 has a structure in which the layers of the light-emitting elements 2 are stacked on the substrate 3 on which, for example, TFTs (Thin Film Transistors) (not shown) are formed as pixel circuits PC. In this specification, the direction from the light-emitting elements 2 of the display device 1 to the substrate 3 is referred to as the "downward direction," and the direction opposite to the downward direction is referred to as the "upward direction."

[0014] <Light-emitting element: overview> The light-emitting element 2 includes an anode 21, a hole transport layer 22, a first light-emitting layer 23, an electron accumulation layer 24, an electron transport layer 25, and a cathode 26, in this order from the substrate 3 side. The anode 21 is electrically connected to the TFT of the substrate 3. Therefore, the anode 21 and the cathode 26 face each other, and the first light-emitting layer 23 is located between the anode 21 and the cathode 26.

[0015] As will be described later, each of the layers in the light-emitting element 2, from the hole transport layer 22 to the electron transport layer 25, has a function of transporting and storing charges or a function of emitting light in the light-emitting element 2. For this reason, in the present disclosure, each of the layers, from the hole transport layer 22 to the electron transport layer 25, may be referred to as a functional layer.

[0016] As described above, band diagram B1 and band diagram B2 show the band gaps of each functional layer from the hole transport layer 22 to the electron transport layer 25. Band diagram B1 shows the band gap of each functional layer before the functional layers are stacked, particularly before the functional layers are electrically connected to each other. In other words, band diagram B1 shows the band gap of each functional layer alone. On the other hand, band diagram B2 shows the band gap of each functional layer after the functional layers are stacked, particularly after the functional layers are electrically connected to each other. In other words, band diagram B2 shows the band gap of each functional layer after the transfer of charges between the functional layers that occurs as the functional layers are stacked is complete.

[0017] <Light-emitting element: electrodes> The anode 21 and the cathode 26 contain a conductive material and are electrically connected to the hole transport layer 22 and the electron transport layer 25, respectively.

[0018] At least one of the anode 21 and the cathode 26 is a transparent electrode that transmits visible light. Examples of transparent electrodes include ITO (indium tin oxide), IZO (indium zinc oxide), ZnO, AZO (aluminum-doped zinc oxide, also known as ZAO), BZO (boron-doped zinc oxide), and FTO (fluorine-doped tin oxide). Either the anode 21 or the cathode 26 may contain a metal material. As the metal material, Al, Cu, Au, Ag, or Mg, or an alloy thereof, which have high visible light reflectance, are preferred. The anode 21 and the cathode 26 may be formed by sputtering or the like, or may be patterned by dry etching or the like.

[0019] <Light-emitting element: charge transport layer> The hole transport layer 22 is a layer containing a hole transport material that transports holes from the anode 21 to the first light-emitting layer 23. In this embodiment, the material of the hole transport layer 22 can be an organic or inorganic material that has been conventionally used in light-emitting elements including quantum dots. For example, conductive compounds such as polyvinylcarbazole (PVK), [N,N'-bis(4-butylphenyl)-N,N'-bis(phenyl)-benzidine] (TPD), 4,4'-bis(carbazol-9-yl)biphenyl (CBP), polyphenylenevinylene (PPV), a composite of poly(3,4-ethylenedioxythiophene) (PEDOT) and polystyrene sulfonic acid (PSS) (PEDOT-PSS), and poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4'-(N-4-sec-butylphenyl)diphenylamine)]) (TFB) can be used as organic materials for the hole transport layer 22. Examples of inorganic materials for the hole transport layer 22 include molybdenum oxide, NiO, and Cr. ２ O ３ , MgO, MgZnO, LaNiO ３ , MoO ３ , or W.O. ３ In particular, as the material for the hole transport layer 22, a material having a large electron affinity and ionization potential is suitable.

[0020] The electron transport layer 25 is a layer containing an electron transport material that transports electrons from the cathode 26 to the electron accumulation layer 24. The material of the electron transport layer 25 can be an organic or inorganic material that has been conventionally used in light-emitting devices containing quantum dots. For example, the electron transport layer 25 can be made of zinc oxide (ZnO), zinc magnesium oxide (ZnMgO), titanium oxide (TiO), and tungsten oxide (WO ３ ), or may contain an inorganic nanoparticle material that is nanoparticles of these inorganic materials. Alternatively, the electron transport layer 25 may contain an organic material as the electron transport material, such as tris(8-quinolinol)aluminum complex (Alq3), bathocuproine (BCP), or (2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole) (t-Bu-PBD). Note that, as the inorganic material of the electron transport layer 25, metal oxides such as ZnO, ZAO, ITO, InGaZnO, or electride may be used. In particular, a material with a small electron affinity is suitable as the material of the electron transport layer 25.

[0021] In this embodiment, the hole transport layer 22 and the electron transport layer 25 can be formed by vacuum deposition, sputtering, or a coating method using a colloidal solution using the above-mentioned materials.

[0022] <Light-emitting element: electron storage layer> The electron storage layer 24 is a functional layer that temporarily stores electrons injected from the electron transport layer 25 until the electrons are injected into the first light-emitting layer 23. The mechanism by which the electron storage layer 24 stores injected electrons will be described in detail later. The electron storage layer 24 according to this embodiment is adjacent to the first light-emitting layer 23 on the cathode 26 side of the first light-emitting layer 23. In this disclosure, "two adjacent layers" may refer to a configuration in which the two layers are in contact with each other, or a configuration in which a thin film is located between the two layers. In this disclosure, when "two adjacent layers" and a thin film is located between the two layers, the film may have a thickness of 4 nm or less. For example, to reduce the inhibition of electron injection due to cracks occurring in the electron storage layer 24, the electron storage layer 24 may be a continuous layer without cracks. In addition, when a cross section of the electron storage layer 24 in the in-plane direction DP is observed, the electron storage layer 24 may have a thickness of 1000 nm or less. ２ In this case, the electron accumulation layer 24 may be considered to be a continuous layer in that portion.

[0023] The electron accumulation layer 24 according to this embodiment contains an inorganic compound having a larger electron affinity than the quantum dot material contained in the first light-emitting layer 23, which will be described in detail later. In particular, the electron accumulation layer 24 according to this embodiment contains at least one inorganic nanoparticle 31 having the inorganic compound.

[0024] The inorganic compound contained in the inorganic nanoparticles 31 according to this embodiment may include, for example, a II-VI compound. In this case, the inorganic compound may have defects in group VI atoms. An inorganic compound that includes a II-VI compound and also has defects in group VI atoms becomes an n-type semiconductor. However, the present invention is not limited to this, and the inorganic compound contained in the inorganic nanoparticles 31 may also include an intrinsic semiconductor.

[0025] The inorganic compound contained in the inorganic nanoparticles 31 may include, for example, at least one of ZnS, ZnSe, and ZnTe. The inorganic nanoparticles 31 may include a mixture of the above-mentioned inorganic compounds, or may include one type of particle and a shell surrounding the particle.

[0026] The particle diameter D31 of the inorganic nanoparticles 31 may be at least twice the exciton Bohr radius of the inorganic compound contained in the inorganic nanoparticles 31. The exciton Bohr radius of the inorganic compound is a value obtained by converting the atomic radius of hydrogen in the Bohr atomic model using the relative dielectric constant and reduced mass of the inorganic compound. The exciton Bohr radius a of the inorganic compound Ｂ ＊ is expressed by the following formula:

[0027] In the above formula, the value 0.053 (nm) corresponds to the approximate value of the atomic radius of hydrogen in Bohr's atomic model. Also, in the above formula, ε is the relative dielectric constant of the inorganic compound, m ０ is the mass of an electron, and μ is the reduced mass of the inorganic compound. The reduced mass μ of an inorganic compound is expressed by the following formula:

[0028] In the above formula, m ｅ is the effective mass of an electron in an inorganic compound, and m ｈ is the effective mass of a hole in an inorganic compound. When one of the effective mass of an electron and the effective mass of a hole is sufficiently light compared to the other, the effective mass of the lighter electron is approximately equal to the reduced mass μ.

[0029] When the particle diameter D31 of the inorganic nanoparticles 31 is equal to or greater than twice the exciton Bohr radius of the inorganic compound, the quantum effect of the inorganic nanoparticles 31 is reduced, specifically, the effect of confining charges inside the inorganic nanoparticles 31 is reduced. Therefore, with the above configuration, the electron accumulation layer 24 reduces the confinement of holes and electrons inside the inorganic nanoparticles 31, thereby reducing the probability of generating excitons due to recombination of holes and electrons.

[0030] As will be described in detail later, a reduction in the probability of generating excitons in the inorganic nanoparticles 31 reduces the probability of the deactivation process occurring in the electron accumulation layer 24, leading to an increase in the probability of the light emission process occurring in the quantum dots of the first light-emitting layer 23, which will be described later. Therefore, with the above configuration, the electron accumulation layer 24 improves the luminous efficiency of the light-emitting element 2.

[0031] The inorganic nanoparticles 31 may have a spherical or non-spherical shape. The particle diameter D31 of the inorganic nanoparticles 31 may be measured by performing cross-sectional observation of the electron accumulation layer 24 in the film thickness direction DT. In the cross-sectional observation, the particle diameter D31 of each inorganic nanoparticle 31 may be considered to be the same as the diameter of a circle having the same area as the cross-sectional area of ​​the inorganic nanoparticles 31. The cross-sectional observation may be performed by analyzing images obtained by capturing a transmission electron microscope (TEM) image of the cross section of each layer.

[0032] In the present disclosure, the thickness direction DT of the electron accumulation layer 24 may be, for example, the stacking direction of the layers of the light-emitting element 2, the direction connecting the upper surface of the anode 21 and the lower surface of the cathode 26, or the light extraction direction of the light-emitting element 2. Alternatively, the thickness direction DT may be substantially the same as the normal direction to the upper surface of the electron accumulation layer 24, the lower surface of the electron accumulation layer 24, or the upper surface of the substrate 3. Furthermore, an in-plane direction DP of the electron accumulation layer 24, which will be described later, may be a direction substantially perpendicular to the thickness direction DT.

[0033] In this embodiment, it is sufficient that the particle diameter D31 of at least one inorganic nanoparticle 31 is equal to or greater than twice the exciton Bohr radius of the inorganic compound contained in the inorganic nanoparticle 31. This allows the electron accumulation layer 24 to achieve the above-described effects for at least the inorganic nanoparticle 31.

[0034] However, the present invention is not limited to this, and from the viewpoint of further improving the luminous efficiency of the light-emitting element 2, the average value of the particle diameter D31 of the inorganic nanoparticles 31 may be equal to or greater than twice the exciton Bohr radius of the inorganic compound of the inorganic nanoparticles 31. Here, the average value of the particle diameter D31 may be measured by measuring the particle diameter D31 of 20 particles in the above-mentioned cross-sectional observation and calculating the average.

[0035] Furthermore, in this embodiment, the particle diameter D31 of at least one inorganic nanoparticle 31 may be equal to or greater than the particle diameter of a quantum dot 41 (described later) or the particle diameter of a core 41C of the quantum dot 41. With the above configuration, the light-emitting element 2 reduces the charge confinement effect inside the inorganic nanoparticle 31 and improves the luminous efficiency for the same reason as described above.

[0036] <Light-emitting element: first light-emitting layer: quantum dot> The first light-emitting layer 23 includes at least one quantum dot 41 as a light-emitting semiconductor nanoparticle. The first light-emitting layer 23 according to this embodiment includes a plurality of quantum dots 41. The quantum dot 41 has, for example, a core 41C and a shell 41S positioned around the core 41C.

[0037] The quantum dot 41 according to this embodiment is, for example, a luminescent semiconductor nanoparticle that emits light due to excitons generated by the recombination of injected electrons and holes. For example, the recombination of electrons and holes in the quantum dot 41 occurs mainly in the core 41C. The core 41C is a luminescent material having a valence band level and a conduction band level, and emits light due to the recombination of holes in the valence band level and electrons in the conduction band level. The quantum dot emits light with a narrow spectrum due to the quantum confinement effect, enabling emission of relatively deep chromaticity. Furthermore, the shell 41S functions to suppress the occurrence of defects or dangling bonds in the core 41C and reduce the recombination of carriers undergoing a deactivation process. To efficiently obtain the quantum confinement effect in the quantum dot 41, the particle size of the core 41C may be less than twice the exciton Bohr radius of the material of the core 41C.

[0038] The quantum dots 41 may have a spherical or non-spherical shape. The particle size of the quantum dots 41 may be measured by the same method as the particle size D31 of the inorganic nanoparticles 31.

[0039] The core 41C is made of ZnSe, ZnTe, and mixed crystal materials thereof, InP, InGaP, GaInN, or CsPbI. ３ The material may include perovskites such as CdTe, or CIGS such as Cu(InGa)Se.

[0040] Any atom in the shell 41S may be located in the same group in the periodic table as any atom in the inorganic compound of the electron accumulation layer 24. In particular, the inorganic compound and the shell 41S may contain the same material. The shell 41S may contain, for example, at least one of ZnS, ZnSe, and ZnTe, or may contain a plurality of stacked layers of these materials. In particular, from the viewpoint of improving the efficiency of charge injection into the core 41C via the shell 41S, the shell 41S may contain ZnTe, ZnSe, and ZnS stacked in this order from the core 41C side.

[0041] <Light-emitting element: first light-emitting layer: particle size of core> The wavelength of light emitted from the quantum dots 41 can be controlled by controlling the particle size of the cores 41C. This is because the band gap of the cores 41C can be controlled by controlling the particle size of the cores 41C. Therefore, by controlling the particle size of the cores 41C of the quantum dots 41, the wavelength of light emitted by the light-emitting element 2 including the first light-emitting layer 23 containing the quantum dots 41 can be controlled.

[0042] The relationship between the diameter of a semiconductor nanoparticle including core 41C and the band gap of the semiconductor will be described in detail with reference to Fig. 3. Fig. 3 is a graph showing the relationship between the diameter and band gap of a semiconductor nanoparticle. In the graph of Fig. 3, the horizontal axis represents the diameter (unit: nm) of the semiconductor nanoparticle, and the vertical axis represents the band gap (unit: eV) of the semiconductor nanoparticle.

[0043] As shown in the graph of Figure 3, in general, the smaller the particle size of nanoparticles containing semiconductors, the larger the band gap becomes, because the quantum effect in the nanoparticles becomes stronger as the particle size of the nanoparticles becomes smaller.

[0044] In general, the band gap of nanoparticles containing semiconductors decreases with increasing particle size, but once the particle size exceeds a certain value, the band gap no longer changes significantly. This is because, when the particle size of nanoparticles exceeds a certain value, the quantum effect of the nanoparticles decreases and they behave like bulks. For example, in the graph of Figure 3, the particle size at the right end of the curve for each semiconductor material is near the minimum particle size at which the quantum effect of nanoparticles containing that semiconductor material becomes sufficiently small that they behave like bulks. Furthermore, this particle size corresponds to approximately twice the exciton Bohr radius of each semiconductor material.

[0045] The value obtained by dividing the difference between the maximum and minimum particle sizes of the cores 41C of the quantum dots 41 in the first light-emitting layer 23 by the average particle size of the cores 41C of the quantum dots 41 in the first light-emitting layer 23 may be 0.05 or less. Furthermore, the difference between the maximum and minimum particle sizes of the quantum dots 41 in the first light-emitting layer 23 may be 5% or less of the average particle size of the quantum dots 41. Alternatively, the value obtained by dividing the difference between the maximum and minimum particle sizes of the quantum dots 41 in the first light-emitting layer 23 by the average particle size of the quantum dots 41 in the first light-emitting layer 23 may be 0.15 or less.

[0046] The above configuration reduces the difference between the particle size of the quantum dots 41 contained in the first light-emitting layer 23 and the particle size of the cores 41C of the quantum dots 41. In this case, the first light-emitting layer 23 reduces the change in wavelength of the light obtained from the quantum dots 41, regardless of the quantum dots 41. Therefore, the above configuration enables the light-emitting element 2 to improve the chromaticity of the light obtained from the first light-emitting layer 23.

[0047] In this embodiment, the particle sizes of the quantum dots 41 or cores 41C in the first light-emitting layer 23 may be confirmed by observing the cross section of the first light-emitting layer 23 along the film thickness direction DT and confirming the particle sizes of 20 quantum dots 41 or cores 41C. In this case, the maximum and minimum particle sizes of the quantum dots 41 or cores 41C in the first light-emitting layer 23 may be the maximum and minimum particle sizes of the 20 quantum dots 41 or cores 41C.

[0048] In this embodiment, the boundary between the first light-emitting layer 23 and the electron accumulation layer 24 may be confirmed by observing a cross section passing through the first light-emitting layer 23 and the electron accumulation layer 24 and confirming the concentration of the material at each position on the cross section. The concentration of the material at each position described above may be measured, for example, by performing TEM-EDX on the cross section.

[0049] For example, in the cross section, the portion where the concentration of the material of the first light-emitting layer 23, particularly the material of the quantum dots 41, is 80% or more may be defined as the first light-emitting layer 23, and the portion where the concentration is less than 80% may be defined as the electron accumulation layer 24, and the boundary between the first light-emitting layer 23 and the electron accumulation layer 24 may be confirmed. Alternatively, in the cross section, the portion where the concentration of the material of the quantum dots 41 decreases by 80% or more may be considered to be the boundary between the first light-emitting layer 23 and the electron accumulation layer 24.

[0050] For example, in the cross section, the portion where the concentration of the material of the electron accumulation layer 24, particularly the material of the inorganic nanoparticles 31, is 80% or more may be defined as the electron accumulation layer 24, and the portion where the concentration is less than 80% may be defined as the first light-emitting layer 23, and the boundary between the first light-emitting layer 23 and the electron accumulation layer 24 may be confirmed. Alternatively, in the cross section, the portion where the concentration of the material of the inorganic nanoparticles 31 decreases by 80% or more may be considered to be the boundary between the first light-emitting layer 23 and the electron accumulation layer 24.

[0051] In addition to the above-described boundary, the boundary between the electron accumulation layer 24 and the layer in contact with the electron accumulation layer 24 may be realized by checking the concentration of the material at each position on a cross section passing through the electron accumulation layer 24 and the layer in contact with the electron accumulation layer 24 using a method corresponding to the above-described method.

[0052] <Light-emitting element: Supplementary notes> The light-emitting element 2 may include a hole injection layer between the anode 21 and the hole transport layer 22, and may include an electron injection layer between the cathode 26 and the electron transport layer 25. Furthermore, the light-emitting element 2 may include an intermediate layer between the hole transport layer 22 and the first light-emitting layer 23, or between the electron transport layer 25 and the electron accumulation layer 24. These hole injection layer, electron injection layer, and intermediate layer may all be formed by the same method as the hole transport layer 22 or the electron transport layer 25.

[0053] The display device 1 according to this embodiment includes a light-emitting element 2 having an anode 21 located closer to the substrate 3 than the first light-emitting layer 23. However, the configuration of the display device 1 according to this embodiment is not limited to this. For example, the display device 1 may include a light-emitting element 2 having a cathode 26 located closer to the substrate 3 than the first light-emitting layer 23. In this case, the light-emitting element 2 may include, in this order from the substrate 3 side, the cathode 26, an electron transport layer 25, an electron accumulation layer 24, the first light-emitting layer 23, a hole transport layer 22, and an anode 21. In this case, the cathode 26 may be formed in an island shape for each sub-pixel and electrically connected to the pixel circuit PC of each sub-pixel, while the anode 21 may be formed in common to a plurality of sub-pixels.

[0054] 1 , the uppermost level of the band gap of the first light-emitting layer 23, particularly the first light-emitting layer 23 including the quantum dots 41, is the uppermost level of the band gap of the quantum dots 41. The uppermost level of the band gap of the quantum dots 41 is located higher than the uppermost level of the band gap of the electron accumulation layer 24, particularly the inorganic compound of the inorganic nanoparticles 31 of the electron accumulation layer 24. This corresponds to the electron accumulation layer 24 including inorganic nanoparticles 31 having an inorganic compound with a larger electron affinity than the material of the quantum dots 41.

[0055] The lower end of the band gap of the first light-emitting layer 23, particularly the first light-emitting layer 23 including the quantum dots 41, is the lower end of the band gap of the quantum dots 41. The lower end of the band gap of the quantum dots 41 may be substantially the same as the lower end of the band gap of the electron accumulation layer 24, as shown in the band diagram B1 of FIG. 1 . This configuration can be realized, for example, by using the same material for the shells 41S of the quantum dots 41 in the first light-emitting layer 23 and the same inorganic compound for the inorganic nanoparticles 31 in the electron accumulation layer 24.

[0056] In this embodiment, the particle size of the quantum dots 41 is small enough to generate the quantum effect in the quantum dots 41, as described above, and therefore the band gap of the quantum dots 41 is large, as described above with reference to Fig. 3. Therefore, even when the material of the shell 41S and the inorganic compound of the inorganic nanoparticles 31 are the same, the band gap of the first light-emitting layer 23 is larger than the band gap of the electron accumulation layer 24.

[0057] On the other hand, in general, when the band gap of the quantum dot 41 increases due to the above-mentioned quantum effect, the lower level of the band gap of the quantum dot 41 hardly changes, while the upper level of the band gap of the quantum dot 41 changes significantly. This is because the effective mass of the electron and the effective mass of the hole in the quantum dot 41 are significantly different.

[0058] Therefore, by making the particle size of the quantum dots 41 sufficiently small relative to the particle size D31 of the inorganic nanoparticles 31, it is possible to realize the respective band gaps of the first light-emitting layer 23 and the electron accumulation layer 24 shown in the band diagram B1 of Fig. 1. Therefore, the respective band gaps of the first light-emitting layer 23 and the electron accumulation layer 24 shown in the band diagram B1 of Fig. 1 can be realized even when, for example, the inorganic compound of the inorganic nanoparticles 31 and the shell 41S of the quantum dots 41 contain the same material.

[0059] Also, for example, in this embodiment, the inorganic compound contained in the inorganic nanoparticles 31 is an n-type semiconductor. In this case, as shown in the band diagram B1 of FIG. 1 , the Fermi level 24F of the electron accumulation layer 24 is biased toward the upper end level of the band gap of the electron accumulation layer 24 with respect to the midpoint between the upper end level and the lower end level. On the other hand, for example, in this embodiment, each portion of the quantum dots 41 is assumed to be an intrinsic semiconductor. In this case, as shown in the band diagram B1 of FIG. 1 , the Fermi level 23F of the first light-emitting layer 23 is approximately the same as the midpoint between the upper end level and the lower end level of the band gap of the first light-emitting layer 23.

[0060] In the above case, the difference between the Fermi level 23F and the Fermi level 24F is smaller than when the inorganic compound of the inorganic nanoparticles 31 and each portion of the quantum dots 41 are both intrinsic semiconductors. Assume that the first light-emitting layer 23 and the electron accumulation layer 24 are stacked to enable charge transfer between the first light-emitting layer 23 and the electron accumulation layer 24. Even in this case, in this embodiment, the difference between the Fermi level 23F and the Fermi level 24F is small, so that no significant change, such as curvature, occurs in the band gaps of the first light-emitting layer 23 and the electron accumulation layer 24, as shown in the band diagram B2 of FIG. 1 .

[0061] <Storage of Electrons in Electron Storage Layer> The mechanism of electron storage in the electron storage layer 24 will be described in detail with reference to Fig. 4. Fig. 4 is a band diagram of the first light-emitting layer 23 and the electron storage layer 24 according to this embodiment.

[0062] In particular, band diagrams B3, B4, and B5 in Fig. 4 are band diagrams of the first light-emitting layer 23 according to this embodiment and the electron accumulation layer 24 including inorganic nanoparticles 31 having an n-type inorganic semiconductor compound. As will be described later, band diagrams B6, B7, and B8 in Fig. 4 are band diagrams of the first light-emitting layer 23 according to this embodiment and the electron accumulation layer 24 including inorganic nanoparticles 31 having an intrinsic inorganic semiconductor compound.

[0063] 4 are band diagrams of the first light-emitting layer 23 and the electron accumulation layer 24 before lamination, and band diagrams B4 and B7 are band diagrams of the first light-emitting layer 23 and the electron accumulation layer 24 after lamination. Furthermore, band diagrams B5 and B8 are band diagrams of the first light-emitting layer 23 and the electron accumulation layer 24 when the light-emitting element 2 in which the first light-emitting layer 23 and the electron accumulation layer 24 are laminated is driven. Band diagrams B5 and B8 also show holes H injected from the anode 21 and electrons E injected from the cathode 26.

[0064] As described above, when the inorganic compound of the inorganic nanoparticles 31 of the electron accumulation layer 24 is an n-type semiconductor, there is no significant change in the band gap between the first light-emitting layer 23 and the electron accumulation layer 24 before and after lamination, as shown in band diagrams B3 and B4 in Fig. 4. When the light-emitting element 2 is driven, an electric field between the anode 21 and the cathode 26 causes a tilt in the band gap between the first light-emitting layer 23 and the electron accumulation layer 24 such that the cathode 26 side is upward, as shown in band diagram B5 in Fig. 4.

[0065] When the light-emitting element 2 is driven, holes H are injected from the anode 21 toward the first light-emitting layer 23, and electrons E are injected from the cathode 26 toward the first light-emitting layer 23. Therefore, as shown in the band diagram B8 in FIG. 4 , the holes H reach the lower end level side of the first light-emitting layer 23, and the electrons E reach the upper end level side of the electron accumulation layer 24.

[0066] Generally, in a field injection light-emitting device, the injection barrier of holes from the first layer to the second layer corresponds to the energy obtained by subtracting the electron affinity of the second layer from the electron affinity of the first layer, which corresponds to the difference in energy between the uppermost energy levels of the first layer and the second layer.

[0067] As described above, the electron accumulation layer 24 contains an inorganic compound having a larger electron affinity than the material of the quantum dots 41, and therefore the uppermost energy level of the electron accumulation layer 24 is located below B5 in the band diagram relative to the uppermost energy level of the first light-emitting layer 23. Therefore, in the light-emitting element 2 according to this embodiment, the injection barrier of electrons from the electron accumulation layer 24 to the first light-emitting layer 23 becomes large when the light-emitting element 2 is driven.

[0068] Therefore, the electrons E injected into the electron accumulation layer 24 are less likely to be injected into the first light-emitting layer 23 and are accumulated in the electron accumulation layer 24. This reduces the density of the electrons E injected from the electron accumulation layer 24 into the first light-emitting layer 23, and reduces the excess of electrons in the first light-emitting layer 23.

[0069] <Reducing Excess Electrons in the Light-Emitting Layer> Reducing the excess electrons in the first light-emitting layer 23 reduces the probability of occurrence of processes that do not contribute to the light emission of the quantum dots 41, such as the generation of Auger electrons due to interactions between electrons, in the first light-emitting layer 23. Furthermore, Auger electrons generally have high energy and may attack and deteriorate the quantum dots 41 in the first light-emitting layer 23 or layers located in the vicinity of the first light-emitting layer 23. Therefore, reducing Auger electrons in the first light-emitting layer 23 reduces deterioration of the first light-emitting layer 23 and layers located in the vicinity of the first light-emitting layer 23, improving the reliability of each layer of the light-emitting element 2.

[0070] Therefore, the light-emitting element 2 according to this embodiment improves the light-emitting efficiency or extends the lifespan by reducing the excess electrons in the first light-emitting layer 23. The display device 1 including the light-emitting element 2 achieves power saving or a long lifespan.

[0071] Furthermore, in this embodiment, the particle diameter D31 of the inorganic nanoparticles 31 is at least twice the exciton Bohr radius of the inorganic compound of the inorganic nanoparticles 31. Therefore, the inorganic nanoparticles 31 of the electron accumulation layer 24 are less likely to exhibit a quantum effect that confines holes and electrons therein, such as the cores 41C of the quantum dots 41. Therefore, the electron accumulation layer 24 reduces the confinement of electrons E or holes H accumulated in the electron accumulation layer 24 in the inorganic nanoparticles 31, thereby reducing the generation of Auger electrons or excitons in the electron accumulation layer 24. Therefore, the light-emitting element 2 according to this embodiment, with the above-described configuration, further reduces the excess electrons in the first light-emitting layer 23, thereby further improving the luminous efficiency or extending the lifetime.

[0072] In this embodiment, the inorganic compound of the inorganic nanoparticles 31 has an n-type semiconductor. Therefore, as described above, even when the first light-emitting layer 23 and the electron accumulation layer 24 are stacked, the band gaps of the first light-emitting layer 23 and the electron accumulation layer 24 are unlikely to be curved. In this case, in the light-emitting element 2, a decrease in the efficiency of hole injection from the anode 21 to the first light-emitting layer 23 due to the band gap curvature of the first light-emitting layer 23 is unlikely to occur.

[0073] Therefore, with the above-described configuration, the light-emitting element 2 improves the hole density relative to the electron density in the first light-emitting layer 23, further reducing the excess electrons in the first light-emitting layer 23. In particular, the inorganic compound contained in the inorganic nanoparticles 31 according to this embodiment includes a II-VI compound and has defects in VI group atoms. This allows the electron accumulation layer 24 to realize an n-type inorganic semiconductor compound with a simple configuration.

[0074] The electron accumulation layer 24 is adjacent to the first light-emitting layer 23 on the cathode 26 side of the first light-emitting layer 23. Therefore, the electron accumulation layer 24 can accumulate electrons E injected from the cathode 26 side in the electron accumulation layer 24 before they are injected into the first light-emitting layer 23. Therefore, with the above-described configuration, the light-emitting element 2 further reduces the excess electrons in the first light-emitting layer 23.

[0075] <Inorganic Compound of Intrinsic Semiconductor> Even when the inorganic compound of the inorganic nanoparticles 31 of the electron accumulation layer 24 is an intrinsic semiconductor, the electron accumulation layer 24 can accumulate electrons E from the cathode 26. For example, when the inorganic compound of the inorganic nanoparticles 31 of the electron accumulation layer 24 is an intrinsic semiconductor, the Fermi level 24F of the electron accumulation layer 24 is approximately the same as the midpoint between the upper and lower end levels of the band gap of the electron accumulation layer 24, as shown in the band diagram B6 of FIG.

[0076] Accordingly, when the first light-emitting layer 23 and the electron accumulation layer 24 are stacked, the band gap of the first light-emitting layer 23 is curved downward on the anode 21 side, as shown in the band diagram B7 of Fig. 4. This is because electrons flow from the first light-emitting layer 23 to the electron accumulation layer 24 so that the energy difference between the Fermi level 23F of the first light-emitting layer 23 and the Fermi level 24F of the electron accumulation layer 24 becomes smaller.

[0077] On the other hand, even when the first light-emitting layer 23 and the electron accumulation layer 24 are stacked, there is no significant change in the band gap of the first light-emitting layer 23 on the cathode 26 side or in the band gap of the electron accumulation layer 24, as shown in the band diagram B7 of Fig. 4. This is because, due to the difference in mobility between electrons and holes, electrons are more likely to flow from the first light-emitting layer 23 to the electron accumulation layer 24 than holes to flow from the electron accumulation layer 24 to the first light-emitting layer 23 as the first light-emitting layer 23 and the electron accumulation layer 24 are stacked. For this reason, even when the inorganic compound is an intrinsic semiconductor and the first light-emitting layer 23 and the electron accumulation layer 24 are stacked, the upper end level of the band gap of the first light-emitting layer 23 is located above the upper end level of the band gap of the electron accumulation layer 24.

[0078] When the light-emitting element 2 is driven, the electric field between the anode 21 and the cathode 26 tilts the band gap between the first light-emitting layer 23 and the electron accumulation layer 24 so that the cathode 26 side is positioned upward, as shown in B8 in the band diagram of Fig. 4 . In this case, too, electrons E are accumulated in the electron accumulation layer 24 for the same reason as described above. Therefore, in the above case, too, the light-emitting element 2 reduces the electron excess in the first light-emitting layer 23.

[0079] In particular, when the inorganic compound of the electron accumulation layer 24 contains an intrinsic semiconductor, the need for introducing specific atomic defects into the inorganic compound can be reduced, and the inorganic compound can be easily realized. In addition, when the inorganic compound has few specific atomic defects, the occurrence of deactivation processes such as recombination of electrons or electrons and holes at the defects is reduced.

[0080] <Method of Manufacturing Display Device> A method of manufacturing the display device 1 according to this embodiment will be described with reference to Fig. 5. Fig. 5 is a flowchart of the method of manufacturing the display device 1 according to this embodiment.

[0081] In the manufacturing method of the display device 1 according to this embodiment, first, a substrate 3 is prepared (step S1). The substrate 3 may be formed by forming pixel circuits and drivers for driving the light-emitting elements 2 of each sub-pixel on a substrate such as a glass substrate or a film substrate.

[0082] Next, the anode 21 is formed on the substrate 3 (step S2). The anode 21 may be formed by forming a thin conductive film common to a plurality of sub-pixels by the above-described method, and then patterning the thin film for each sub-pixel.

[0083] Next, the hole transport layer 22 is formed on the anode 21 by the method described above (step S3). The hole transport layer 22 may be formed in common for a plurality of subpixels, or may be patterned for each subpixel. In particular, in step S3 according to this embodiment, the hole transport layer 22 may be formed by forming a thin film containing a polymer organic monomer by coating or the like, and polymerizing or crosslinking the monomer by heat-treating the thin film.

[0084] Next, the first light-emitting layer 23 is formed on the hole transport layer 22 (step S4). In forming the first light-emitting layer 23, for example, quantum dots 41 are first synthesized and dispersed in a solvent such as octane together with an organic ligand, etc., to prepare a quantum dot dispersion. The quantum dots 41 may be synthesized by, for example, a heating method, hot injection, a microwave-assisted method, a continuous flow method, etc.

[0085] Next, the quantum dot dispersion is applied onto the hole transport layer 22 by various coating methods such as an inkjet method, and then the quantum dot dispersion is heated and dried to volatilize the solvent in the quantum dot dispersion, thereby forming the first light-emitting layer 23 containing a plurality of quantum dots 41.

[0086] The first light-emitting layer 23 may be patterned for each subpixel after the above method. Furthermore, when the display device 1 includes light-emitting elements 2 that emit light of different colors for each subpixel, the first light-emitting layer 23 may be formed by repeatedly performing the above method and patterning while changing the emission color of the quantum dots 41.

[0087] Next, the electron accumulation layer 24 is formed on the first light-emitting layer 23 (step S5). In forming the electron accumulation layer 24, for example, first, inorganic nanoparticles 31 are synthesized and dispersed in a solvent such as octane together with an organic ligand, etc., to prepare an inorganic nanoparticle dispersion. The inorganic nanoparticles 31 may be synthesized by the same method as that for synthesizing the quantum dots 41, except that, for example, the reaction time is prolonged or the reaction temperature is increased to increase the particle size.

[0088] In this case, for example, by increasing the temperature of the material in the synthesis of the inorganic nanoparticles 31, defects of a specific atom may be formed in the inorganic compound of the inorganic nanoparticles 31. For example, when the inorganic nanoparticles 31 contain sulfide, by increasing the temperature of the sulfur source in the synthesis of the inorganic nanoparticles 31, inorganic nanoparticles 31 containing sulfide with defects of sulfur atoms may be synthesized.

[0089] Next, the inorganic nanoparticle dispersion is applied onto the first light-emitting layer 23 by various coating methods such as an inkjet method, and then the inorganic nanoparticle dispersion is heated and dried to volatilize the solvent of the inorganic nanoparticle dispersion, thereby forming the electron accumulation layer 24 containing a plurality of inorganic nanoparticles 31.

[0090] Since the electron accumulation layer 24 contains a plurality of inorganic nanoparticles 31 as a member having an inorganic compound, the electron accumulation layer 24 can be easily formed by applying and drying the above-described dispersion liquid of the inorganic nanoparticles 31. Therefore, since the electron accumulation layer 24 contains the inorganic nanoparticles 31, the electron accumulation layer 24 realizes a reduction in cost or a shortened takt time in the method of manufacturing the light-emitting element 2.

[0091] Next, the electron transport layer 25 is formed (step S6). In forming the electron transport layer 25, nanoparticles containing an electron transport material are synthesized, for example, by the method described above. Next, the synthesized nanoparticles are dispersed in a solvent such as methanol, ethanol, 2-propanol, or butanol to prepare a nanoparticle dispersion. Next, the electron transport layer 25 may be formed by applying the nanoparticle dispersion onto the electron accumulation layer 24 and drying it.

[0092] Next, the cathode 26 is formed (step S7). The cathode 26 may be formed by forming a thin conductive film common to a plurality of sub-pixels using the method described above. In this manner, the light-emitting element 2 is formed on the substrate 3, completing the manufacture of the display device 1. The above method makes it possible to manufacture a light-emitting element 2 that improves luminous efficiency or extends the lifespan by reducing the excess electrons in the first light-emitting layer 23.

[0093] In step S3, when the hole transport layer 22 is formed by polymerization or crosslinking of a polymer organic monomer, the reaction of the monomer may continue in the hole transport layer 22 even after the formation of the hole transport layer 22, particularly after the formation of the light-emitting element 2. In this case, cracks may occur in the hole transport layer 22 due to the reaction of the monomer after the formation of the light-emitting element 2.

[0094] In this embodiment, after the hole transport layer 22 is formed, the electron accumulation layer 24 is formed by applying a dispersion of inorganic nanoparticles 31 and heating and drying it. Therefore, in the method for manufacturing the light-emitting element 2 according to this embodiment, a step of heating each layer on the substrate 3 again is performed after the hole transport layer 22 is formed. This promotes the reaction of the monomers in the already formed hole transport layer 22 in the step of forming the electron accumulation layer 24, making it easier to terminate the reaction. As a result, the above method reduces the progress of the reaction of the monomers in the hole transport layer 22 of the light-emitting element 2 after manufacture, thereby reducing deterioration of the hole transport layer 22.

[0095] On the other hand, heating in forming the electron accumulation layer 24 is performed in a state where a layer including the first light-emitting layer 23 is formed on the hole transport layer 22. Therefore, even when heating in forming the electron accumulation layer 24, the hole transport layer 22 is not directly exposed to the atmosphere, and deterioration of the hole transport layer 22 due to heating of the hole transport layer 22 is reduced.

[0096] Therefore, the above method promotes the reaction of the monomers in the hole transport layer 22 and makes it easier to terminate the reaction, while not directly exposing the hole transport layer 22 to the atmosphere due to heating during the formation of the electron accumulation layer 24. Therefore, the above method reduces both deterioration of the hole transport layer 22 during the formation of the electron accumulation layer 24 and deterioration of the hole transport layer 22 after the light-emitting element 2 is manufactured.

[0097] <Characteristics of Light-Emitting Element: Comparative Example> The characteristics of the light-emitting element 2 according to this embodiment will be described in comparison with the characteristics of the light-emitting elements according to Comparative Example 1 and Comparative Example 2.

[0098] The light-emitting device according to Comparative Example 1 differs in configuration from the light-emitting device 2 according to this embodiment only in that the first light-emitting layer 23 contains a bulk inorganic semiconductor material as a light-emitting material and does not include the electron accumulation layer 24. The light-emitting device according to Comparative Example 1 is, for example, a micro LED.

[0099] The light-emitting device according to Comparative Example 2 differs in configuration from the light-emitting device 2 according to this embodiment only in that it does not include the electron accumulation layer 24. Therefore, the first light-emitting layer 23 of the light-emitting device according to Comparative Example 2 includes quantum dots 41 as a light-emitting material.

[0100] <Characteristics of Light-Emitting Element: abc Model> Light-emitting elements according to each of Comparative Form 1 and Comparative Form 2 were manufactured, and the luminance was measured while changing the applied voltage, thereby measuring the change in the external quantum efficiency (EQE) of each light-emitting element in response to the change in the current density of the current flowing between the electrodes. The results are summarized in the graphs of FIG. 6.

[0101] 6 shows a graph G1 showing the relationship between EQE and current density in a light-emitting element according to Comparative Example 1, and a graph G2 showing the relationship between EQE and current density in a light-emitting element according to Comparative Example 2. In graphs G1 and G2, the horizontal axis represents the current density in each light-emitting element, and the vertical axis represents the EQE in each light-emitting element. However, in graphs G1 and G2, the EQE is normalized so that the maximum value is 1. The unit of current density in graph G1 is A / cm. ２ On the other hand, the unit of the current density in graph G2 is mA / cm ２is.

[0102] In the light-emitting elements according to the comparative examples, the relationship of the EQE of the light-emitting element to the current density in the light-emitting element corresponds to the abc model, in which the proportional relationship of the EQE value of the light-emitting element to the current density changes depending on whether the current density in the light-emitting element is in the low region, the medium region, or the high region.

[0103] Specifically, as shown in graphs G1 and G2 in Figure 6, the proportional relationship of the EQE of each light-emitting element to the current density in each light-emitting element changes depending on whether the current density falls in the low region RA, the middle region RB, or the high region RC.

[0104] In each comparative example, as the current density in the light-emitting element increases in the low region RA, the number of electrons and holes injected into the light-emitting layer of the light-emitting element increases. This rapidly increases the probability of recombination occurring in the light-emitting layer, and as a result, the EQE also rapidly increases. Specifically, when the current density flowing through each light-emitting element is included in the low region RA, the EQE of each light-emitting element increases in proportion to the current density.

[0105] In each comparative example, as the current density in the light-emitting device increases in the middle region RB, the density of electrons injected into the light-emitting layer becomes higher than the density of holes. This is because the mobility of electrons is higher than the mobility of holes, and therefore the efficiency of electron injection from the cathode to the light-emitting layer tends to be higher than the efficiency of hole injection from the anode to the light-emitting layer.

[0106] Therefore, as the current density in the light-emitting element increases in the middle region RB, the probability of the recombination process in the light-emitting layer saturates, and further, the probability of the process of generating Auger electrons that do not contribute to the light emission of the light-emitting layer increases. As a result, the EQE reaches a maximum once and then gradually begins to decrease. Specifically, when the current density flowing through each light-emitting element is included in the middle region RB, the EQE of each light-emitting element changes in proportion to the square of the current density, and the coefficient is negative.

[0107] In each comparative example, as the current density in the light-emitting element increases in the high region RC, the probability of the Auger electron generation process in the light-emitting layer further increases, and as a result, the EQE of each light-emitting element further decreases. Specifically, when the current density flowing through each light-emitting element is included in the high region RC, the EQE of each light-emitting element decreases in proportion to the cube of the current density.

[0108] From the above, in order to drive the light-emitting element according to each comparative example while increasing the EQE as much as possible, it is necessary to drive the light-emitting element so that the current density in the light-emitting element falls within the middle region RB.

[0109] The light-emitting element of Comparative Example 1 and the light-emitting element of Comparative Example 2 differ greatly in the current density values ​​in the low, medium, and high regions.

[0110] In Comparative Example 1, approximately 3 A / cm ２ More than 15A / cm ２ The following current densities are included in the middle region RB. When the current density in the light-emitting device according to Comparative Example 1 is included in the middle region RB, the carrier density in the light-emitting layer of the light-emitting device is approximately 10 １８ / cm ３ From 10 １９ / cm ３ Therefore, when the light-emitting element according to Comparative Example 1 is driven at a current density included in the middle region RB, carriers are injected into the light-emitting layer at a sufficient density. Therefore, the light-emitting element according to Comparative Example 1 can maximize the EQE as much as possible and obtain sufficient brightness for use as a light-emitting element in, for example, a display device.

[0111] However, as described above, the light-emitting element according to Comparative Example 1 includes a bulk inorganic light-emitting material in the light-emitting layer. The light-emitting layer including the bulk inorganic light-emitting material must be formed by a method that makes it difficult to control the film formation position and film thickness, such as crystal growth. Furthermore, the light-emitting layer including the bulk inorganic light-emitting material tends to produce light with a wider spectral width and lower chromaticity than a light-emitting layer including quantum dots as the light-emitting material.

[0112] <Light-emitting device characteristics: generation of trion Auger electrons> On the other hand, in Comparative Example 2, the current density in the middle region RB is approximately 2 mA / cm ２ 6mA / cm or more ２ Therefore, when the current density in the light-emitting device according to Comparative Example 2 is included in the middle region RB, the carrier density in the light-emitting layer of the light-emitting device is approximately 10 ２０ / cm ３ In this case, when the light-emitting element according to Comparative Example 2 is driven at a current density included in the middle region RB, the density of carriers injected into the light-emitting layer becomes insufficient, making it difficult to obtain sufficient luminance for use as a light-emitting element in, for example, a display device.

[0113] The light-emitting layer of the light-emitting element according to Comparative Example 2 contains quantum dots as a light-emitting material. Therefore, recombination of electrons and holes injected into the light-emitting layer is likely to occur inside the quantum dots, which have a high confinement effect. As a result, when an excess of electrons occurs in the light-emitting layer of the light-emitting element according to Comparative Example 2, further electrons may be injected into the quantum dots where recombination of electrons and holes has occurred, and may be confined inside the quantum dots.

[0114] In this case, the extra electrons injected into the quantum dot may become Auger electrons by being excited by the energy of excitons generated in the quantum dot. Auger electrons generated by the above process are generally called trion Auger electrons. The generation of such trion Auger electrons is more likely to occur inside quantum dots, which have the above-mentioned strong carrier confinement effect, and also occurs at a relatively low electron density.

[0115] Therefore, in the light-emitting layer of the light-emitting element according to Comparative Form 2, generation of Auger electrons is dominant in the process of charge interaction in the light-emitting layer, even though the carrier density is lower than in the light-emitting layer of the light-emitting element according to Comparative Form 1. Therefore, the current density in the middle region RB of the light-emitting element according to Comparative Form 2 is lower than that of the light-emitting element according to Comparative Form 1.

[0116] In order to obtain sufficient luminance using the light-emitting element according to Comparative Example 2, it is necessary to increase the current density in the light-emitting element to the high region RC and drive the light-emitting element. However, as described above, when the current density in the light-emitting element is increased to the high region RC, the EQE is significantly reduced. Therefore, it is difficult for the light-emitting element according to Comparative Example 2 to ensure luminance and maintain the EQE at the same time.

[0117] <Characteristics of Light-Emitting Device: Comparison Between Examples and Comparative Examples> In contrast, the light-emitting device 2 according to this embodiment reduces the excess electrons in the first light-emitting layer 23 for the reasons described above. Therefore, the light-emitting device 2 efficiently reduces the probability of occurrence of the generation process of Auger electrons, particularly trion Auger electrons, in the first light-emitting layer 23 during operation. Therefore, the light-emitting device 2 can obtain sufficient brightness while reducing the decrease in EQE even when driven by increasing the current density to about the high region RC.

[0118] The characteristics of the light-emitting element 2 according to this embodiment and the characteristics of the light-emitting element according to Comparative Example 2 are evaluated by comparing the characteristics of the light-emitting elements according to each of Examples 1 and 2 with the characteristics of the light-emitting element according to the comparative example. The light-emitting elements according to Examples 1 and 2 were manufactured by the above-described method for forming the light-emitting element 2. The inorganic compound contained in the inorganic nanoparticles 31 of the electron accumulation layer 24 of the light-emitting element according to Example 1 was an n-type semiconductor, and the inorganic compound contained in the inorganic nanoparticles 31 of the electron accumulation layer 24 of the light-emitting element according to Example 2 was an intrinsic semiconductor. The light-emitting element according to the comparative example was manufactured by the same method as the light-emitting elements according to each Example, except that the electron accumulation layer 24 was not formed.

[0119] For each of the light-emitting elements according to Example 1, Example 2, and Comparative Example, the luminance was measured while changing the applied voltage, and the change in EQE of each light-emitting element according to the change in the current density of the current flowing between the electrodes was measured, and the results were summarized in the graph of FIG.

[0120] In the graph of Fig. 7, the horizontal axis represents the current density in each light-emitting element, and the vertical axis represents the EQE in each light-emitting element. However, in the graph of Fig. 7, the EQE is normalized so that the maximum value in the light-emitting element according to Example 1 is 1. In the graph of Fig. 7, the measurement results of Example 1 are shown by a solid line, the measurement results of Example 2 by a dashed line, and the measurement results of the comparative example by a dashed line.

[0121] 7, the maximum EQE values ​​of the light-emitting elements according to Examples 1 and 2 are greater than the maximum EQE value of the light-emitting element according to the comparative example. This is thought to be because the excess electrons in the first light-emitting layer 23 of the light-emitting elements according to each Example are reduced, thereby reducing the probability of occurrence of processes such as the generation of Auger electrons that do not contribute to light emission from the first light-emitting layer 23.

[0122] 7, the EQE of the light-emitting element according to each example does not decrease significantly compared to the EQE of the light-emitting element according to the comparative example even when the current density increases in the high region RC. This is thought to be because the excess electrons in the first light-emitting layer 23 of the light-emitting element according to each example are reduced, thereby suppressing an increase in the probability of the generation process of trion Auger electrons even when the carrier density in the first light-emitting layer 23 is increased.

[0123] 7 , the EQE of the light-emitting element according to Example 1 is further improved compared to the EQE of the light-emitting element according to Example 2. This is because, in the light-emitting element according to Example 1, the band gap of the first light-emitting layer 23 does not bend, and the efficiency of hole injection from the anode 21 to the first light-emitting layer 23 increases, resulting in an increased probability of recombination of holes and electrons in the first light-emitting layer 23.

[0124] As a result, the light-emitting elements according to the examples have a higher maximum EQE and suppress a decrease in EQE due to an increase in current density compared to the light-emitting elements according to the comparative examples. Therefore, the light-emitting elements according to the examples have an increased luminous efficiency and can efficiently obtain sufficient brightness for use as a light-emitting element in, for example, a display device.

[0125] 8 is a schematic side cross-sectional view of a display device 1 according to this embodiment. The display device 1 according to this embodiment has the same configuration as the display device 1 according to the previous embodiment, except that the electron accumulation layer 24 includes a continuous film 32 instead of a plurality of inorganic nanoparticles 31.

[0126] The continuous film 32 is a continuous film containing the same inorganic compound as the inorganic compound of the inorganic nanoparticles 31 according to the previous embodiment. ２ The electron storage layer 24 includes a portion having the above area. In this case, the electron storage layer 24 may be considered to have a continuous film 32 of an inorganic compound in that portion.

[0127] Therefore, the electron accumulation layer 24 including the continuous film 32 accumulates electrons injected from the cathode 26 side by the same mechanism as that described in the previous embodiment. Therefore, for the same reason as described above, the light-emitting element 2 according to this embodiment reduces the excess electrons in the first light-emitting layer 23, thereby improving the luminous efficiency or extending the lifetime.

[0128] The film thickness T32 of the continuous film 32 according to this embodiment in the film thickness direction DT is at least twice the exciton Bohr radius of the inorganic compound contained in the continuous film 32. Therefore, the quantum effect does not occur strongly in the continuous film 32, and the continuous film 32 behaves as a bulk inorganic compound.

[0129] Therefore, the electron accumulation layer 24 according to this embodiment reduces the electrons E accumulated in the electron accumulation layer 24 from being confined in the continuous film 32, and reduces the generation of Auger electrons in the electron accumulation layer 24. Therefore, the light-emitting element 2 according to this embodiment has the above-described configuration, which further reduces the excess electrons in the first light-emitting layer 23, thereby improving the light-emitting efficiency or extending the lifetime.

[0130] Furthermore, the thickness T32 of the continuous film 32 is substantially uniform regardless of the position in the planar view of the display device 1. In the present disclosure, the thickness T32 of the continuous film 32 may be considered to be substantially uniform when the difference between the maximum and minimum values ​​of the thickness T32 of the continuous film 32 is 6 nm or less. Therefore, for example, the thickness T32 of the continuous film 32 according to this embodiment is equal to or greater than twice the exciton Bohr radius of the inorganic compound contained in the continuous film 32, regardless of the position in the planar view of the display device 1. For example, the maximum and minimum values ​​of the thickness T32 of the continuous film 32 may be the maximum and minimum values ​​obtained by searching for the thickness T32 in a range of 100 nm or more, preferably 1 μm or more, in the in-plane direction DP during cross-sectional observation of the electron accumulation layer 24 along the thickness direction DT.

[0131] With the above configuration, the electron accumulation layer 24 allows the accumulation of electrons injected from the cathode 26 and reduces electron confinement, regardless of the position in the planar view of the display device 1. Therefore, the light-emitting element 2 according to this embodiment can average the efficiency of charge injection into the first light-emitting layer 23 regardless of the position in the planar view of the display device 1, thereby further improving the light-emitting efficiency. Furthermore, for the same reason, the light-emitting element 2 according to this embodiment can reduce the occurrence of strong light emission only at a certain position in the first light-emitting layer 23, averaging out deterioration of the first light-emitting layer 23, and as a result, the life of the light-emitting element 2 as a whole can be extended.

[0132] Furthermore, in this embodiment, at least one of the minimum value and average value of the film thickness T32 of the continuous film 32, in other words, at least one of the minimum film thickness and average film thickness of the continuous film 32, may be equal to or greater than the particle diameter of the quantum dots 41 or the particle diameter of the cores 41C of the quantum dots 41. With the above configuration, the light-emitting element 2 reduces the charge confinement effect in the continuous film 32 and improves the light-emitting efficiency for the same reasons as described above.

[0133] The display device 1 according to this embodiment may be manufactured by the same method as the display device 1 according to the previous embodiment, except for the method for manufacturing the electron accumulation layer 24. In this embodiment, the electron accumulation layer 24 may be formed by applying a dispersion containing a precursor of an inorganic compound such as xanthogenic acid and then heating and drying the dispersion to convert the precursor into an inorganic compound. Alternatively, the electron accumulation layer 24 may be formed by a film formation method using a Successive Ionic-Layer Adsorption and Reaction (SILAR) method. When the electron accumulation layer 24 is formed by the SILAR method, the electron accumulation layer 24 may be formed by, for example, adsorbing the precursor onto the first light-emitting layer 23 and then converting the precursor into an inorganic compound through a chemical reaction.

[0134] [Embodiment 3] <Continuous Film with Varying Thickness> Figure 9 is a schematic side cross-sectional view of a display device 1 according to this embodiment. The display device 1 according to this embodiment has the same configuration as the display device 1 according to the previous embodiment, except that the electron accumulation layer 24 includes a continuous film 33 instead of the continuous film 32. The continuous film 33 has the same configuration as the continuous film 32 according to the previous embodiment, except that the film thickness T33 in the film thickness direction DT varies depending on the position in the planar view of the display device 1. In other words, the continuous film 33 contains the same inorganic compound as the continuous film 32. For example, in this embodiment, the difference between the maximum and minimum values ​​of the film thickness T33 of the continuous film 33 may exceed 6 nm.

[0135] In this embodiment, too, the film thickness T33 of any portion of the continuous film 33 is equal to or greater than twice the exciton Bohr radius of the inorganic compound contained in the continuous film 33. This allows the electron accumulation layer 24 to reduce the electron confinement effect at least in that portion. However, from the viewpoint of further reducing the electron confinement effect, the average value of the film thickness T33 of the continuous film 33 may be equal to or greater than twice the exciton Bohr radius, and further, the minimum value of the film thickness T33 of the continuous film 33 may be equal to or greater than twice the exciton Bohr radius. For example, the average value of the film thickness T33 of the continuous film 33 may be the average value obtained by searching for film thicknesses T33 in a range of 100 nm or more, preferably 1 μm or more, in the in-plane direction DP during cross-sectional observation of the electron accumulation layer 24 along the film thickness direction DT.

[0136] The display device 1 according to this embodiment may be manufactured by the same method as the display device 1 according to the previous embodiment. However, in this embodiment, a material having high liquid repellency against the inorganic compound precursor of the electron accumulation layer 24 may be used for the first light-emitting layer 23. Also, in this embodiment, the film formation rate of the electron accumulation layer 24 may be increased. In this case, differences in the thickness T33 of the continuous film 33 of the electron accumulation layer 24 are likely to occur. Even in this case, the light-emitting element 2 according to this embodiment can reduce the excess electrons in the first light-emitting layer 23 for the same reasons as described above. Therefore, the light-emitting element 2 according to this embodiment reduces the excess electrons in the first light-emitting layer 23, while improving the degree of freedom in the material of the first light-emitting layer 23 or shortening the takt time for forming the electron accumulation layer 24.

[0137] 10 is a schematic side cross-sectional view of a display device 1 according to this embodiment. The display device 1 according to this embodiment has the same configuration as the display device 1 according to the previous embodiment, except that the electron accumulation layer 24 includes a continuous film 34 instead of the continuous film 33.

[0138] The continuous film 34 has a first portion 51 and a second portion 52 in any cross section in the thickness direction DT of the continuous film 34. The second portion 52 is discontinuous with the first portion 51 in the cross section. Therefore, in the light-emitting element 2 according to this embodiment, there may be a portion where the first light-emitting layer 23 and the electron transport layer 25 are in direct contact with each other.

[0139] The first portion 51 and the second portion 52 may be discontinuous in the cross section. In other words, the first portion 51 and the second portion 52 may be continuous in a portion not including the cross section. On the other hand, the first portion 51 and the second portion 52 may be spaced apart from each other in a plan view. For example, the first portion 51 and the second portion 52 may be positioned like islands.

[0140] In this embodiment, the thickness T34 of any portion of the continuous film 34 is equal to or greater than twice the exciton Bohr radius of the inorganic compound contained in the continuous film 34. This allows the electron accumulation layer 24 to reduce the electron confinement effect at least in that portion. However, from the viewpoint of further reducing the electron confinement effect, the average value of the thickness T34 of the continuous film 34 in at least one of the first portion 51 and the second portion 52 may be equal to or greater than twice the exciton Bohr radius.

[0141] The display device 1 according to this embodiment may be manufactured by the same method as the display device 1 according to the previous embodiment. In this embodiment, a material having even higher liquid repellency against the inorganic compound precursor of the electron accumulation layer 24 may be used for the first light-emitting layer 23. In addition, in this embodiment, the film formation speed of the electron accumulation layer 24 may be further improved. Therefore, the light-emitting element 2 according to this embodiment further improves the degree of freedom in the material of the first light-emitting layer 23, or further shortens the takt time for forming the electron accumulation layer 24, while reducing the excess electrons in the first light-emitting layer 23.

[0142] Furthermore, the electron accumulation layer 24 according to this embodiment may have granular portions. In this case, in the present disclosure, these portions may be regarded as inorganic nanoparticles 31. For example, when a cross section of the electron accumulation layer 24 in the in-plane direction DP is observed, if the inorganic compound is 1000 nm thick, the inorganic nanoparticles 31 may be formed. ２ When the electron accumulation layer 24 includes a portion having an area of ​​less than 100 μm, the portion may be considered to include inorganic nanoparticles 31. In other words, the electron accumulation layer 24 according to the present disclosure may include both inorganic nanoparticles having an inorganic compound and a continuous film.

[0143] [Embodiment 5] <Electron Accumulation Layer Located on the Anode Side> Figure 11 is a schematic side cross-sectional view of a display device 1 according to this embodiment. The display device 1 according to this embodiment has the same configuration as the display device 1 according to the above-described embodiments, except that the electron accumulation layer 24 is adjacent to the first light-emitting layer 23 on the anode 21 side of the first light-emitting layer 23. In this embodiment, as shown in Figure 11, a case will be described in which the electron accumulation layer 24 includes a plurality of inorganic nanoparticles 31. However, the configuration of the electron accumulation layer 24 according to this embodiment is not limited to this, and the electron accumulation layer 24 may have the same configuration as the electron accumulation layer 24 according to any of the above-described embodiments, except for its position in the light-emitting element 2.

[0144] The mechanism of electron accumulation in the electron accumulation layer 24 according to this embodiment will be described in detail with reference to Fig. 12. Fig. 12 is a band diagram of the first light-emitting layer 23 and the electron accumulation layer 24 according to this embodiment.

[0145] In particular, band diagrams B9, B10, and B11 in Fig. 12 are band diagrams of the first light-emitting layer 23 according to this embodiment and the electron accumulation layer 24 containing inorganic nanoparticles 31 having an n-type inorganic semiconductor compound. Band diagrams B12, B13, and B14 in Fig. 4 are band diagrams of the first light-emitting layer 23 according to this embodiment and the electron accumulation layer 24 containing inorganic nanoparticles 31 having an intrinsic inorganic semiconductor compound. Band diagrams B11 and B14 in Fig. 12 also show the band gap of the hole transport layer 22.

[0146] 12 are band diagrams of the first light-emitting layer 23 and the electron accumulation layer 24 before lamination, and band diagrams B10 and B13 are band diagrams of the first light-emitting layer 23 and the electron accumulation layer 24 after lamination. Furthermore, band diagrams B11 and B14 are band diagrams of the first light-emitting layer 23 and the electron accumulation layer 24 when the light-emitting element 2 in which the first light-emitting layer 23 and the electron accumulation layer 24 are laminated is driven.

[0147] As described above, when the inorganic compound of the inorganic nanoparticles 31 of the electron accumulation layer 24 is an n-type semiconductor, there is no significant change in the band gap between the first light-emitting layer 23 and the electron accumulation layer 24 before and after lamination, as shown in band diagrams B9 and B10 in Fig. 12. When the light-emitting element 2 is driven, an electric field between the anode 21 and the cathode 26 causes a tilt in the band gap between the first light-emitting layer 23 and the electron accumulation layer 24 such that the cathode 26 side is upward, as shown in band diagram B11 in Fig. 12.

[0148] When the light-emitting element 2 is driven, holes H are injected from the anode 21 toward the first light-emitting layer 23, and electrons E are injected from the cathode 26 toward the first light-emitting layer 23. Therefore, as shown in the band diagram B11 in FIG. 12 , the holes H reach the lower end level side of the electron accumulation layer 24, and the electrons E reach the upper end level side of the first light-emitting layer 23.

[0149] As described above, the electron accumulation layer 24 contains an inorganic compound having a larger electron affinity than the material of the quantum dots 41. Therefore, the upper end level of the band gap of the electron accumulation layer 24 is located lower, and as a result, the barrier to electron injection from the electron accumulation layer 24 to the hole transport layer 22 tends to be large. Therefore, even when the electrons E injected into the first light-emitting layer 23 flow into the electron accumulation layer 24, the electrons E are accumulated in the electron accumulation layer 24. In particular, when the upper end level of the band gap of the electron accumulation layer 24 is located lower than the upper end level of the band gap of the hole transport layer 22, the electron accumulation layer 24 is more likely to accumulate electrons E.

[0150] In addition, the particle diameter D31 of the inorganic nanoparticles 31 in the electron accumulation layer 24 is at least twice the exciton Bohr radius of the inorganic compound of the inorganic nanoparticles 31. Therefore, even when electrons accumulate in the electron accumulation layer 24, the electron confinement effect in the inorganic nanoparticles 31 is unlikely to occur. As a result, even when holes H are injected into the electron accumulation layer 24 from the anode 21 side, the probability that the holes H will be injected into the inorganic nanoparticles 31 and recombine with electrons E is reduced.

[0151] Therefore, in the light-emitting element 2 according to this embodiment, the probability that holes H from the anode 21 are injected into the first light-emitting layer 23 via the electron accumulation layer 24 increases. Therefore, in the light-emitting element 2 according to this embodiment, the density of holes H in the first light-emitting layer 23 is improved, and the excess electrons are reduced.

[0152] As described above, any atom contained in the inorganic compound of the inorganic nanoparticle 31 and any atom contained in the shell 41S of the quantum dot 41 may be located in the same group on the periodic table. In this case, as described above, the difference in the lower end level of the band gap between the electron accumulation layer 24 and the first light-emitting layer 23 is reduced. Furthermore, if the inorganic compound of the inorganic nanoparticle 31 and the shell 41S of the quantum dot 41 contain the same material, this difference is further reduced. With this configuration, the injection barrier of holes H from the electron accumulation layer 24 to the first light-emitting layer 23 is reduced, which in turn improves the hole density in the first light-emitting layer 23 and reduces the electron excess in the first light-emitting layer 23.

[0153] When the inorganic compound of the inorganic nanoparticles 31 of the electron accumulation layer 24 is an intrinsic semiconductor, the Fermi level 24F of the electron accumulation layer 24 is approximately the midpoint between the upper and lower end levels of the band gap of the electron accumulation layer 24, as shown in the band diagram B12 of Fig. 12 . Accordingly, when the first light-emitting layer 23 and the electron accumulation layer 24 are stacked, for the same reason as described above, the band gap of the first light-emitting layer 23 is curved so that the cathode 26 side moves downward, as shown in the band diagram B13 of Fig. 12 . On the other hand, even when the first light-emitting layer 23 and the electron accumulation layer 24 are stacked, for the same reason as described above, no significant change occurs in the anode 21 side of the band gap of the first light-emitting layer 23 or the band gap of the electron accumulation layer 24.

[0154] When the light-emitting element 2 is driven, the electric field between the anode 21 and the cathode 26 tilts the band gap between the first light-emitting layer 23 and the electron accumulation layer 24 so that the cathode 26 side is positioned upward, as shown in B14 in the band diagram of Fig. 12. In this case as well, electrons E are accumulated in the electron accumulation layer 24 for the same reason as described above. Therefore, in the above case as well, the light-emitting element 2 reduces the electron excess in the first light-emitting layer 23.

[0155] In particular, in this embodiment, when the inorganic compound of the electron accumulation layer 24 contains an intrinsic semiconductor, the upper and lower end levels of the band gap of the first light-emitting layer 23 shift upward on the anode 21 side of the first light-emitting layer 23. Therefore, in this embodiment, when the inorganic compound of the electron accumulation layer 24 contains an intrinsic semiconductor, the inflow of electrons E from the first light-emitting layer 23 to the electron accumulation layer 24 is reduced, and the injection barrier of holes H from the electron accumulation layer 24 to the first light-emitting layer 23 is reduced. Therefore, with the above configuration, the light-emitting element 2 according to this embodiment further reduces the excess of electrons in the first light-emitting layer 23.

[0156] The display device 1 according to this embodiment may be manufactured by the same method as the display device 1 according to any of the above-described embodiments, except that the order of forming the first light-emitting layer 23 and the electron accumulation layer 24 is reversed. In this embodiment, when the electron accumulation layer 24 includes a continuous film of any of the above-described inorganic compounds, the continuous film may have a substantially uniform thickness like the above-described continuous film 32. In this case, the flatness of the first light-emitting layer 23 formed above the electron accumulation layer 24 is improved, and the efficiency of charge injection into the quantum dots 41 in the first light-emitting layer 23 is improved.

[0157] 13 is a schematic side cross-sectional view of a display device 1 according to this embodiment. The display device 1 according to this embodiment differs from the display device 1 according to embodiment 1 only in that a second light-emitting layer 27 is located between the electron accumulation layer 24 and the electron transport layer 25.

[0158] In other words, the second light-emitting layer 27 is located between the anode 21 and the cathode 26, and is located on the opposite side of the electron accumulation layer 24 from the first light-emitting layer 23. The electron accumulation layer 24 is adjacent to the second light-emitting layer 27 on the cathode 26 side, in other words, on the opposite side from the first light-emitting layer 23.

[0159] The second light-emitting layer 27 includes at least one quantum dot 41, and may include a plurality of quantum dots 41, for example, as shown in Fig. 13. The quantum dots 41 included in the second light-emitting layer 27 have the same structure as the quantum dots 41 included in the first light-emitting layer 23. In other words, the second light-emitting layer 27 has the same structure as the first light-emitting layer 23 except for its position in the light-emitting element 2.

[0160] 13 shows a schematic cross-sectional view of the display device 1 in which the electron accumulation layer 24 contains a plurality of inorganic nanoparticles 31. However, the configuration of the electron accumulation layer 24 according to this embodiment is not limited to this, and the electron accumulation layer 24 may have the same configuration as the electron accumulation layer 24 according to any of the above-described embodiments.

[0161] In this embodiment, too, for the same reason as described above, electrons from the cathode 26 accumulate in the electron accumulation layer 24, thereby reducing the excess of electrons in the first light-emitting layer 23. Furthermore, since the light-emitting element 2 according to this embodiment includes the second light-emitting layer 27, even if the anode 21 flows into the electron accumulation layer 24 beyond the first light-emitting layer 23, the holes may be further injected into the second light-emitting layer 27. This is because, as described above, the inorganic compound in the electron accumulation layer 24 has a low electron confinement effect, and therefore the holes may reach the second light-emitting layer 27 without recombining with electrons.

[0162] Holes injected into the second light-emitting layer 27 via the electron accumulation layer 24 may recombine with electrons injected into the second light-emitting layer 27 from the cathode 26 via the electron transport layer 25 at the quantum dots 41 contained in the second light-emitting layer 27. Therefore, the light-emitting element 2 according to this embodiment can obtain light emission not only from the first light-emitting layer 23 but also from the second light-emitting layer 27, thereby further improving the luminous efficiency. Note that the second light-emitting layer 27 includes quantum dots 41 having the same configuration as the quantum dots 41 in the first light-emitting layer 23. Therefore, even when both the first light-emitting layer 23 and the second light-emitting layer 27 emit light, a decrease in the chromaticity of the light from the light-emitting element 2 is unlikely to occur.

[0163] The display device 1 according to this embodiment may be manufactured by the same method as the display device 1 according to any of the above-described embodiments, except that the second light-emitting layer 27 is formed after the electron storage layer 24. The second light-emitting layer 27 may be formed by the same method as the first light-emitting layer 23.

[0164] 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.

[0165] REFERENCE SIGNS LIST 1 Display device 2 Light-emitting element 21 Anode 23 First light-emitting layer 24 Electron accumulation layer 26 Cathode 27 Second light-emitting layer 31 Inorganic nanoparticles 32 Continuous film 41 Quantum dots 41C Core 41S Shell 51 First part 52 Second part

Claims

1. A light-emitting element comprising: an anode; a cathode facing the anode; a first light-emitting layer located between the anode and the cathode and including at least one quantum dot; and an electron accumulation layer adjacent to the first light-emitting layer on the anode side or the cathode side, the electron accumulation layer including an inorganic compound having a larger electron affinity than a material of the quantum dot.

2. The light-emitting device according to claim 1, wherein the electron accumulation layer comprises at least one inorganic nanoparticle having the inorganic compound.

3. The light-emitting device according to claim 2, wherein the particle size of at least one of said inorganic nanoparticles is at least twice the exciton Bohr radius of said inorganic compound.

4. The light-emitting device according to claim 2 or 3, wherein the particle size of at least one of said inorganic nanoparticles is equal to or greater than the particle size of said quantum dot or the particle size of said quantum dot core.

5. The light-emitting device according to any one of claims 1 to 4, wherein the electron accumulation layer includes a continuous film having the inorganic compound.

6. The light-emitting device according to claim 5, wherein the thickness of any part of the continuous film is at least twice the exciton Bohr radius of the inorganic compound.

7. The light-emitting device according to claim 5 or 6, wherein at least one of the average thickness and the minimum thickness of said continuous film is equal to or greater than the particle diameter of said quantum dots or the particle diameter of said quantum dot cores.

8. The light-emitting device according to any one of claims 5 to 7, wherein the continuous film has a substantially uniform thickness.

9. The light-emitting device according to any one of claims 5 to 7, wherein the thickness of the continuous film varies depending on the position.

10. A light-emitting element described in any one of claims 5 to 7, wherein the continuous film has, in any cross section in the film thickness direction of the continuous film, a first portion and a second portion that is discontinuous with the first portion.

11. The light-emitting device according to any one of claims 1 to 10, wherein the inorganic compound contains an n-type semiconductor.

12. The light-emitting device according to any one of claims 1 to 11, wherein the inorganic compound contains a II-VI compound and has defects of group VI atoms.

13. The light-emitting device according to any one of claims 1 to 12, wherein the inorganic compound comprises an intrinsic semiconductor.

14. A light-emitting element according to any one of claims 1 to 13, wherein the quantum dot has a core and a shell surrounding the core, and any atom contained in the inorganic compound and any atom contained in the shell are located in the same group on the periodic table.

15. The light-emitting device according to claim 14, wherein the inorganic compound and the shell comprise the same material.

16. A light-emitting element according to claim 14 or 15, wherein the first light-emitting layer contains a plurality of quantum dots, and the difference between the maximum and minimum particle sizes of the cores in the first light-emitting layer divided by the average particle size of the cores in the first light-emitting layer is 0.05 or less.

17. The light-emitting device described in claim 16, wherein the difference between the maximum particle size and the minimum particle size of the quantum dots in the first light-emitting layer is 5% or less of the average particle size of the quantum dots, or the value obtained by dividing the difference between the maximum particle size and the minimum particle size of the quantum dots in the first light-emitting layer by the average particle size of the quantum dots in the first light-emitting layer is 0.15 or less.

18. The light-emitting element according to any one of claims 1 to 17, wherein the electron storage layer is adjacent to the first light-emitting layer on the cathode side of the first light-emitting layer.

19. The light-emitting element according to any one of claims 1 to 17, wherein the electron accumulation layer is adjacent to the first light-emitting layer on the anode side of the first light-emitting layer.

20. A light-emitting device according to any one of claims 1 to 19, further comprising a second light-emitting layer that includes at least one quantum dot and is located between the anode and the cathode and on the opposite side of the electron storage layer from the first light-emitting layer, wherein the electron storage layer is adjacent to the second light-emitting layer on the opposite side from the first light-emitting layer.

21. A display device comprising a light-emitting element according to any one of claims 1 to 20.

22. A method for manufacturing a light-emitting element, comprising: forming an anode; forming a cathode facing the anode; forming a first light-emitting layer located between the anode and the cathode and including at least one quantum dot; and forming an electron accumulation layer adjacent to the first light-emitting layer on the anode side or the cathode side and including an inorganic compound having a larger electron affinity than a material of the quantum dot.

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