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

Abstract

A light-emitting layer (23) of this light-emitting element (2) includes first quantum dots (31) and second quantum dots (32). The first quantum dots each have a first core (31C) and a first shell (31S) positioned around the first core, and the second quantum dots each have a second core (32C) and a second shell (32S) positioned around the second core. The first core and the second core have the same particle diameter, and the second shell has a greater thickness than the first shell.

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 a light-emitting layer contains quantum dots as a light-emitting material, each quantum dot having a core and a shell surrounding the core.

[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 comprises an anode, a cathode facing the anode, and a light-emitting layer located between the anode and the cathode, wherein the light-emitting layer includes at least one first quantum dot and at least one second quantum dot, wherein the first quantum dot has a first core and a first shell located around the first core, and the second quantum dot has a second core having approximately the same particle diameter as the first core, and a second shell located around the second core and having a thickness greater than that of the first shell.

[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, and forming a light-emitting layer located between the anode and the cathode, wherein the light-emitting layer includes at least one first quantum dot and at least one second quantum dot, the first quantum dot having a first core and a first shell located around the first core, and the second quantum dot having a second core having a particle size substantially the same as that of the first core, and a second shell located around the second core and having a thickness greater than that of the first shell.

[0007] The electron excess in the light-emitting layer of the light-emitting element is reduced.

[0008] 1 is a schematic side cross-sectional view of a display device according to embodiment 1, and a schematic diagram showing first quantum dots and second quantum dots. FIG. 2 is a schematic diagram of a display device according to embodiment 1. FIG. 3 is a band diagram of quantum dots for explaining a change in the band gap of quantum dots depending on the shell thickness of the quantum dots according to embodiment 1. FIG. 4 is a graph showing an example of the relationship between the diameter of semiconductor nanoparticles and the band gap. FIG. 5 is a band diagram of first quantum dots and second quantum dots according to embodiment 1. FIG. 6 is a flowchart of a method for manufacturing a display device according to embodiment 1. FIG. 7 is a flowchart of a method for forming a light-emitting layer according to embodiment 1. FIG. 8 is a schematic diagram of a quantum dot dispersion according to embodiment 1. FIG. 9 is a cross-sectional view of a process in a part of a method for manufacturing a display device according to embodiment 1. FIG. 10 is a schematic side cross-sectional view of a display device according to embodiment 2. FIG. 11 is a flowchart of a method for manufacturing a display device according to embodiment 2. FIG. 12 is a schematic side cross-sectional view of a display device according to embodiment 3. FIG. 13 is a graph showing the relationship between external quantum efficiency and current density of a light-emitting element according to a comparative embodiment. FIG. 14 is a graph showing the relationship between external quantum efficiency and current density of light-emitting elements according to examples and comparative examples.

[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] <Display Device: Overview> 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 or a smartphone. The display device 1 includes a display unit 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.

[0011] In particular, the display device 1 includes a red subpixel XR, a green subpixel XG, and a blue subpixel XB in a display area DA. The red subpixel XR includes a red light-emitting element 2R that emits red light, the green subpixel XG includes a green light-emitting element 2G that emits green light, and the blue subpixel XB includes a blue light-emitting element 2B that emits blue light, as light-emitting elements 2.

[0012] The display device 1 performs display on the display area DA by controlling light emission from each of the plurality of light-emitting elements 2 formed in the display area DA via the driver circuit DR and the pixel circuit PC. In particular, the display device 1 may perform full-color display by controlling red light from the red sub-pixel XR, green light from the green sub-pixel XG, and blue light from the blue sub-pixel XB.

[0013] <Light-Emitting Element: Overview> 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 101 of the display device 1 according to this embodiment, a schematic view 102 of a first quantum dot 31 (described later), and a schematic view 103 of a second quantum dot 32 (described later). In particular, the schematic side cross-sectional views according to the present disclosure, including schematic side cross-sectional view 101, all show cross sections perpendicular to the display surface of the display device and passing through the light-emitting element. Furthermore, schematic views 102 and 103 show cross sections passing through the core and shell of each quantum dot (described later).

[0014] 1, the display device 1 according to this embodiment includes, in a display section DA, the above-described plurality of light-emitting elements 2 and a substrate 3, and in particular, the plurality of light-emitting elements 2 on the substrate 3. The display device 1 has a structure in which, for example, layers of the light-emitting elements 2 are stacked on the substrate 3 on which 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."

[0015] The light-emitting element 2 includes an anode 21, a hole transport layer 22, a light-emitting layer 23, an electron transport layer 24, and a cathode 25, in this order from the substrate 3 side. In other words, the light-emitting element 2 includes the anode 21, the cathode 25 facing the anode 21, and the light-emitting layer 23 located between the anode 21 and the cathode 25. The light-emitting element 2 also includes the electron transport layer 24 between the light-emitting layer 23 and the cathode 25. The anode 21 of the light-emitting element 2 is electrically connected to, for example, a TFT serving as a pixel circuit PC of the substrate 3.

[0016] The structure of each layer of the light emitting element 2 will be described in more detail below.

[0017] <Light-emitting element: electrodes> The anode 21 and the cathode 25 contain a conductive material and are electrically connected to the hole transport layer 22 and the electron transport layer 24, respectively. The anode 21 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 cathode 25 may be formed in common for multiple sub-pixels.

[0018] At least one of the anode 21 and the cathode 25 is a transparent electrode that transmits visible light. Examples of transparent electrodes that can be used 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). The transparent electrode may be formed by a sputtering method or the like. Alternatively, either the anode 21 or the cathode 25 may contain a metal material, preferably Al, Cu, Au, Ag, or Mg, or an alloy thereof, which have high reflectivity for visible light.

[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 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, examples of the organic material for the hole transport layer 22 include 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), or poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4'-(N-4-sec-butylphenyl)diphenylamine)]) (TFB). Other organic materials that can be used for the hole transport layer 22 include polytriarylamine semiconductor (PTAA) and [dipyrazino[2,3-f:2',3'-h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile] (HAT-CN). Other inorganic materials that can be used for the hole transport layer 22 include molybdenum oxide (MoO ３ ), nickel oxide (NiO), chromium oxide (Cr ２ O ３ ), magnesium oxide (MgO), lanthanum nickel oxide (LaNiO ３ ), or tungsten oxide (WO３ Metal oxides such as ZnO, ZnS, ZnO ...

[0020] The electron transport layer 24 is a layer containing an electron transport material that transports electrons from the cathode 25 to the light-emitting layer 23. In particular, in this embodiment, the electron transport layer 24 is in contact with the light-emitting layer 23. The material of the electron transport layer 24 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 24 can be made of an electron transport material such as 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 24 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 the inorganic material of the electron transport layer 24 may be a metal oxide, such as ZnO, ZAO, ITO, InGaZnO, or electride.

[0021] In this embodiment, the hole transport layer 22 and the electron transport layer 24 can be formed by vacuum deposition, sputtering, or a coating method using a colloidal solution using the above-mentioned materials. The light-emitting element 2 may also include a hole injection layer between the anode 21 and the hole transport layer 22, or an electron injection layer between the cathode 25 and the electron transport layer 24. The hole injection layer and the electron injection layer may both be formed by the same method as the hole transport layer 22 or the electron transport layer 24.

[0022] 1 , the light-emitting layer 23 includes, as light-emitting quantum dots, at least one first quantum dot 31 and at least one second quantum dot 32. In this embodiment, the light-emitting layer 23 includes a plurality of first quantum dots 31 and a plurality of second quantum dots 32. In the present disclosure, the light-emitting quantum dots including the first quantum dot 31 and the second quantum dots 32 may be simply referred to as "quantum dots."

[0023] As shown in schematic diagram 102 of Fig. 1 , the first quantum dot 31 has a first core 31C and a first shell 31S located around the first core 31C. Also, as shown in schematic diagram 103 of Fig. 1 , the second quantum dot 32 has a second core 32C and a second shell 32S located around the second core 32C. In the present disclosure, "around the core" refers to a region that is in contact with the core but excludes the core.

[0024] In the light-emitting layer 23 according to this embodiment, at least one first quantum dot 31 and at least one second quantum dot 32 may be adjacent to each other. For example, when a cross-sectional observation in any direction of the light-emitting layer 23 confirms that the first quantum dot 31 and the second quantum dot 32 are in contact with each other, the first quantum dot 31 and the second quantum dot 32 may be considered to be adjacent to each other. Furthermore, when the cross-sectional observation confirms that the distance between the first quantum dot 31 and the second quantum dot 32 is 3 nm or less, the first quantum dot 31 and the second quantum dot 32 may be considered to be adjacent to each other. In this case, charge transfer is likely to occur between the first quantum dot 31 and the second quantum dot 32.

[0025] The quantum dots according to this embodiment are, for example, luminescent semiconductor nanoparticles that emit light due to excitons generated by the recombination of injected electrons and holes. For example, the recombination of electrons and holes in quantum dots occurs primarily in the core. The quantum dot core 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, making it possible to obtain light with a relatively deep chromaticity. Furthermore, the shell functions to suppress the occurrence of defects or dangling bonds in the core and reduce the recombination of carriers undergoing a deactivation process. From the viewpoint of efficiently obtaining the quantum confinement effect in the quantum dots, the particle size of the quantum dot core may be less than approximately twice the exciton Bohr radius of the core material.

[0026] The core and shell materials of the quantum dots may each contain materials used in conventionally known core and shell materials. The quantum dots may have a core / shell structure of, for example, InP / ZnS, CdSe / ZnS, CdSe / ZnSe, CdSe / CdS, ZnSe / ZnS, or a I-III-V chalcopyrite-based material / ZnS, including CuInGaS (CIGS). Alternatively, the quantum dots may contain InZnP, CdSeTe, or ZnSeTe. The core of the quantum dot may also contain CuInZnS, CuInS, CuGaS, AgInS, or ZnAgInS. The shell may be formed from multiple layers containing multiple different materials; in other words, the quantum dots disclosed herein may have a shell with multiple layers.

[0027] The quantum dots have a particle size of approximately 1 to 100 nm. The quantum dots may have a spherical or non-spherical shape. In the present disclosure, the particle size of the quantum dots may be measured by observing a cross section of the light-emitting layer 23 in the film thickness direction. In this cross-sectional observation, the particle size of each quantum dot may be considered to be the same as the diameter of a circle having the same area as the cross-sectional area of ​​the quantum dot. 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. Furthermore, the particle size of the quantum dot core and the thickness of the quantum dot shell may also be measured by the cross-sectional observation. When the quantum dot has a shell with multiple layers, the shell thickness of the quantum dot may be the sum of the thicknesses of all the shells.

[0028] As will be described in detail later, in this embodiment, the particle size of the first core 31C of the first quantum dot 31 and the particle size of the second core 32C of the second quantum dot 32 are substantially the same. On the other hand, in this embodiment, the thickness of the first shell 31S of the first quantum dot 31 is different from the thickness of the second shell 32S of the second quantum dot 32. In particular, the thickness of the second shell 32S is greater than the thickness of the first shell 31S. Therefore, the particle size of the second quantum dot 32 is greater than the particle size of the first quantum dot 31.

[0029] Suppose that observation of the cross section of the light-emitting layer 23 reveals that the core diameters of two quantum dots are the same and that the shell thickness of one quantum dot is smaller than that of the other quantum dot. In this case, of the two quantum dots, the one with the smaller shell thickness may be considered to be the first quantum dot 31, and the other may be considered to be the second quantum dot 32. In this embodiment, the shell thickness of a quantum dot may be half the value obtained by subtracting the core diameter from the diameter of the quantum dot.

[0030] Furthermore, suppose that by observing the cross section of the light-emitting layer 23, it is confirmed that the core diameters of two quantum dots are the same and that one quantum dot is smaller than the other. In this case, the difference in diameter between the two quantum dots can be considered to be twice the difference in shell thickness. In other words, if the core diameters of two quantum dots are the same, the difference in shell thickness between the two quantum dots can be half the difference in diameter between the two quantum dots. In the above case, the one of the two quantum dots with the smaller diameter can be considered the first quantum dot 31, and the other can be considered the second quantum dot 32.

[0031] Assume that the above cross-sectional observation initially identifies a pair of first quantum dots 31 and second quantum dots 32, and in particular the thickness of the first shell 31S of the first quantum dot 31 and the thickness of the second shell 32S of the second quantum dot 32. In this case, the shell thickness of another quantum dot may be measured and compared with the thickness of the first shell 31S and the thickness of the second shell 32S already confirmed above, thereby determining whether the other quantum dot is the first quantum dot 31 or the second quantum dot 32.

[0032] For example, if the shell thickness of a quantum dot newly confirmed by the above cross-sectional observation is less than the midpoint between the thicknesses of the first shell 31S and the second shell 32S whose thicknesses were initially confirmed, the quantum dot may be considered to be the first quantum dot 31. Alternatively, if the shell thickness of the quantum dot is equal to or greater than the midpoint between the thicknesses of the first shell 31S and the second shell 32S, the quantum dot may be considered to be the second quantum dot 32.

[0033] Assume that a plurality of first quantum dots 31 and a plurality of second quantum dots 32 are confirmed by any of the above methods. In this case, the particle size value of the first quantum dots 31 may be a value obtained by measuring the particle size values ​​of 20 quantum dots in the cross section that are deemed to correspond to the first quantum dots 31 by any of the above methods and calculating the average. Similarly, the particle size value of the second quantum dots 32 may be a value obtained by measuring the particle size values ​​of 20 quantum dots in the cross section that are deemed to correspond to the second quantum dots 32 by any of the above methods and calculating the average. Note that if the number of quantum dots that are deemed to correspond to the first quantum dots 31 or the number of quantum dots that are deemed to correspond to the second quantum dots 32 that can be confirmed by any of the above methods is less than 20, the above measurements may be performed for the maximum number of confirmed quantum dots.

[0034] In the present disclosure, there may be cases where the first quantum dots 31 and the second quantum dots 32 cannot be distinguished from each other by observing the cross section of the light-emitting layer 23 and checking the particle size or shell size relationship of each quantum dot using a cross-sectional image, etc. In such cases, the first quantum dots 31 and the second quantum dots 32 are distinguished from each other by the following method.

[0035] In the present disclosure, it is assumed that in a region where multiple quantum dots having the same core diameter are present as determined by the cross-sectional observation, the difference in shell thickness between any two or more quantum dots contained in the region is less than 0.25 nm. Alternatively, it is assumed that the difference in particle diameter between any two or more quantum dots contained in the region is less than 0.5 nm. In this case, the particle diameters or shell thicknesses of these quantum dots may be considered to be the same. Furthermore, it is assumed that the difference in shell thickness between all quantum dots contained in the region is less than 0.25 nm. Alternatively, it is assumed that the difference in particle diameter between all quantum dots contained in the region is less than 0.5 nm. In this case, it is assumed that the particle diameters or shell thicknesses of the quantum dots contained in the region may be considered to be the same.

[0036] Furthermore, suppose that the cross-sectional observation reveals that the difference in shell thickness between at least two of the quantum dots is 0.25 nm or more in a region where multiple quantum dots with the same core diameter are present. In this case, the shell thicknesses of the at least two quantum dots may be considered to be different, and the one of the two quantum dots with the smaller shell thickness may be considered the first quantum dot 31, and the other may be considered the second quantum dot 32. Furthermore, when confirming that the shell thicknesses of at least two of the quantum dots are different, it is preferable to confirm that the difference in shell thickness between the at least two quantum dots is 0.5 nm or more, 1.0 nm or more, 1.5 nm or more, or 2.0 nm or more. However, as described above, whether a quantum dot whose shell thickness has been newly confirmed is the first quantum dot 31 or the second quantum dot 32 may be confirmed by comparing the shell thickness with the intermediate value between the thicknesses of the first shell 31S and the second shell 32S whose thicknesses were initially confirmed.

[0037] In addition, in a region where multiple quantum dots having the same core diameter are present, the cross-sectional observation may reveal that the difference in particle diameter between at least two of the quantum dots is 0.5 nm or more. In this case, the particle diameters of the at least two quantum dots, and therefore the shell thicknesses, may be considered to be different. Of the two quantum dots, the one with the smaller particle diameter may be considered the first quantum dot 31, and the other may be considered the second quantum dot 32. When confirming that the particle diameters of at least two of the quantum dots are different, it may be preferable to confirm that the difference in particle diameter between the at least two quantum dots is 1.0 nm or more, 2.0 nm or more, 3.0 nm or more, or 4.0 nm or more. Furthermore, in the above confirmation, the difference in particle diameter between at least two of the quantum dots may be determined by selecting 20 or more pairs of two quantum dots from the other quantum dots and averaging the difference in particle diameter between the quantum dots in each pair.

[0038] For example, suppose that the cross-sectional observation identifies two quantum dots: a specific quantum dot and an arbitrary quantum dot adjacent to the specific quantum dot, in other words, located within a distance of 3 nm. Here, it is confirmed that the core diameters of the two quantum dots are the same, but the particle diameters or shell thicknesses of the two quantum dots are different, for example, the particle diameters of the two quantum dots differ by 0.5 nm or more. In this case, the particle diameters or shell thicknesses of the specific quantum dot and the quantum dot adjacent to the specific quantum dot may be considered to be different. In this case, the quantum dot with the smaller particle diameter may be considered the first quantum dot 31, and the other may be considered the second quantum dot 32.

[0039] On the other hand, if it is confirmed that the core diameters of the two quantum dots are the same and that the diameters or shell thicknesses of the two quantum dots are also the same, the diameters or shell thicknesses of the quantum dots adjacent to the given quantum dot may be considered to be the same. In this case, these quantum dots may be considered to be either the first quantum dot 31 or the second quantum dot 32.

[0040] The wavelength of light emitted from quantum dots can be controlled by their particle size. In particular, because quantum dots have a core / shell structure, the wavelength of light emitted from the quantum dots can be controlled by controlling the particle size of the core. Therefore, the wavelength of light emitted by the light-emitting element 2 can be controlled by controlling the particle size of the quantum dots. Therefore, when it is difficult to measure the particle size of the quantum dot core by the above-mentioned cross-sectional observation, the core diameter of the quantum dot may be calculated from the central wavelength of the light emitted by the quantum dot. In this case, the quantum dots may be made to emit light by applying a voltage to the light-emitting element, or by irradiating the quantum dots with ultraviolet light or the like.

[0041] The core diameter of a quantum dot can be measured by measuring the wavelength of light emitted from the quantum dot. ＊This may be achieved by calculating the core diameter using the following Brus' formula:

[0042] Here, in the above formula, E ｇ ｂｕｌｋ is the bulk band gap energy of the core material of the quantum dot. In the above formula, h is the reduced Planck constant [eVs], and is expressed as h = h / 2π using Planck's constant h. Furthermore, in the above formula, r is the radius of the core [m], e is the elementary charge [C], m ０ is the rest mass of the electron [kg], m ｅ ＊ is the effective mass of the electron [-], m ｈ ＊ is the effective mass of the hole [-], ε is the relative permittivity [-], ε ０ is the dielectric constant of vacuum [F / m]. The bulk band gap of the core material, E ｇ ｂｕｌｋ may be calculated by identifying the material contained in the core by SIMS or TEM-EDX on the cross section described above.

[0043] For example, suppose the cores of the quantum dots are InP cores that emit blue light. In this case, if the difference in the core diameters of any two quantum dots calculated from the central wavelengths of the light emitted by the quantum dots is less than 0.5 nm, the core diameters of the two quantum dots may be considered to be the same. Alternatively, the central wavelengths of the light emitted by any two quantum dots may be measured, the difference between the central wavelengths of the quantum dots may be compared, and if the difference is less than 5 nm, the core diameters of the two quantum dots may be considered to be the same. Furthermore, if the difference in the core diameters of any two quantum dots calculated from the half-widths of the wavelengths of the light emitted by the quantum dots is 1.0 nm or less, the core diameters of the two quantum dots may be considered to be the same. Alternatively, the half-widths of the wavelengths of the light emitted by any two quantum dots may be measured, the difference between the central wavelengths of the quantum dots may be measured, and if the difference is less than 0.5 nm, the core diameters of the two quantum dots may be considered to be the same.

[0044] In this embodiment, the particle diameter DC1 of the first core 31C and the particle diameter DC2 of the second core 32C are substantially the same. Here, the particle diameter DC1 and the particle diameter DC2 may be values ​​measured for each of the cores of the first quantum dot 31 and the second quantum dot 32 using the same method as the method for measuring the particle diameter of the quantum dots described above.

[0045] For example, the central wavelength of light emitted by at least one first quantum dot 31 and the central wavelength of light emitted by at least one second quantum dot 32 are substantially the same. In this case, the particle size of the first core 31C and the particle size of the second core 32C may be considered to be substantially the same. However, in the present disclosure, "the central wavelengths of the two lights are substantially the same" does not necessarily mean that the central wavelengths of the two lights are strictly the same. For example, in the present disclosure, "the central wavelengths of the two lights are substantially the same" may mean that the central wavelength of one of the two lights is located within the half-width of the central wavelength of the emission spectrum of the other light.

[0046] Furthermore, the difference between the particle diameter DC2 of the second core 32C and the particle diameter DC1 of the first core 31C is 10% or less between the adjacent first quantum dot 31 and second quantum dot 32. In this case, the particle diameters of the first core 31C and the second core 32C may be considered to be approximately the same.

[0047] Furthermore, the difference between the particle diameter DC1 of at least one first core 31C and the particle diameter DC2 of at least one second core 32C divided by the average value of the particle diameter DC1 of at least one first core 31C and the particle diameter DC2 of at least one second core 32C is 0.05 or less. In this case, the particle diameters of the first core 31C and the second core 32C may be considered to be approximately the same.

[0048] In this case, the light emitting element 2 reduces the difference in wavelength between the light obtained from the first quantum dots 31 and the second quantum dots 32, thereby improving the chromaticity of the emitted light.

[0049] On the other hand, the thickness DS2 of the second shell 32S is greater than the thickness DS1 of the first shell 31S. In the present disclosure, the thicknesses DS1 and DS2 may be calculated from the difference between the particle diameter D1 of the first quantum dot 31 and the particle diameter DC1 of the first core 31C, and the difference between the particle diameter D2 of the second quantum dot 32 and the particle diameter DC2 of the second core 32C. For example, the thickness DS1 may be half the value obtained by subtracting the particle diameter DC1 from the particle diameter D1. Also, in the present disclosure, the shell of the quantum dot may be formed in an island shape on the outer surface of the core. In this case, the thickness of the shell may be the largest value among the thicknesses at each position.

[0050] The thickness DS1 of the first shell 31S and the thickness DS2 of the second shell 32S may be 0.5 nm or more from the viewpoint of improving the effect of protecting the cores of the first quantum dots 31 and the second quantum dots 32. Furthermore, the thickness DS1 of the first shell 31S and the thickness DS2 of the second shell 32S may be 8 nm or less from the viewpoint of improving the efficiency of injecting charges into the cores of the first quantum dots 31 and the second quantum dots 32.

[0051] Due to the difference between the thickness DS1 of the first shell 31S and the thickness DS2 of the second shell 32S, the particle diameter D2 of at least one second quantum dot 32 may be 5% or more larger than the particle diameter D1 of at least one first quantum dot 31. Furthermore, the value obtained by dividing the difference between the maximum and minimum particle diameters of the first quantum dot 31 and the second quantum dot 32 by the average particle diameter of the first quantum dot 31 and the second quantum dot 32 may be 0.15 or more. The value obtained by dividing the difference between the particle diameter D1 of at least one first quantum dot 31 and the particle diameter D2 of at least one second quantum dot 32 by the average value of the particle diameter D1 of at least one first quantum dot 31 and the particle diameter D2 of at least one second quantum dot 32 may be 0.05 or more. In this case, the difference between the band gap of the first quantum dot 31 and the band gap of the second quantum dot 32, which will be described later, becomes sufficiently large, thereby further reducing the excess of electrons in the light-emitting layer 23, which will be described later. In the present disclosure, the band gap of a given material corresponds to the difference between the ionization potential and the electron affinity of the material. The ionization potential of a given material corresponds to the energy difference between the vacuum level and the upper end of the valence band of the material, and the electron affinity of a given material corresponds to the energy difference between the vacuum level and the lower end of the conduction band of the material.

[0052] In the present disclosure, it is assumed that the maximum, minimum, and average particle diameters of each quantum dot are confirmed when 20 or more first quantum dots 31 and 20 or more second quantum dots 32 are searched for in a cross-sectional observation of the light-emitting layer 23. In this case, the maximum, minimum, and average values ​​may be the maximum, minimum, and average particle diameters of the first quantum dots 31 and the second quantum dots 32, respectively.

[0053] In this embodiment, the plurality of first quantum dots 31 and the plurality of second quantum dots 32 may be distributed approximately uniformly in at least a portion of the light-emitting layer 23. For example, in the present disclosure, the value obtained by dividing the difference between the total area of ​​the first quantum dots 31 and the total area of ​​the second quantum dots 32 in a cross-sectional observation of the light-emitting layer 23 in any direction by the sum of the total areas of the first quantum dots 31 and the second quantum dots 32 may be 0.40 or less. In this case, the first quantum dots 31 and the second quantum dots 32 may be considered to be distributed approximately uniformly in the portion of the light-emitting layer 23 including the cross-section. Also, for example, in the present disclosure, the value obtained by dividing the difference between the total number of first quantum dots 31 and the total number of second quantum dots 32 in a cross-sectional observation of the light-emitting layer 23 in any direction by the sum of the total numbers of the first quantum dots 31 and the second quantum dots 32 may be 0.40 or less. In this case, the first quantum dots 31 and the second quantum dots 32 may be considered to be distributed approximately uniformly in the portion of the light-emitting layer 23 including the cross-section.

[0054] The specific structure of each layer of the light-emitting element 2 according to this embodiment may be confirmed by checking the composition at the interface of each layer and its position in the film thickness direction, for example, by performing EDX (energy dispersive X-ray spectroscopy) on a cross section of each layer of the light-emitting element 2 using a SEM (scanning electron microscope).

[0055] <Other Configurations of Display Device and Light-Emitting Device> The light-emitting layer 23 may have an additive containing at least one of a metal sulfide and a metal oxide around the first quantum dots 31 and the second quantum dots 32. The additive may fill at least one of the spaces between the plurality of first quantum dots 31, the spaces between the plurality of second quantum dots 32, and the spaces between the first quantum dots 31 and the second quantum dots 32. The additive may protect each part of the light-emitting layer 23, including the first quantum dots 31 and the second quantum dots 32, from the intrusion of foreign matter such as moisture into the light-emitting layer 23 or from deterioration due to heat.

[0056] 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 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 includes a light-emitting element 2 having a cathode 25 located closer to the substrate 3 than the light-emitting layer 23. In this case, the light-emitting element 2 may include, in this order from the substrate 3 side, the cathode 25, the electron transport layer 24, the light-emitting layer 23, the hole transport layer 22, and the anode 21. In this case, the cathode 25 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.

[0057] <Relationship between Shell Thickness and Energy of Each Level of Quantum Dot> When the shell thickness of a quantum dot having a core / shell structure changes, the energy of each level of the quantum dot changes. This will be explained with reference to FIG. 3. FIG. 3 is a band diagram of the quantum dot in this embodiment, illustrating the difference in the quantum dot level due to the difference in shell thickness of the quantum dot. Note that all of the band diagrams according to this embodiment, including FIG. 3, are band diagrams in which the level at infinity is located on the upper side.

[0058] In the band diagrams according to this embodiment, including Figure 3, the dotted lines represent the Fermi levels of the components shown in the band diagrams. In this embodiment, the first quantum dots 31 and the second quantum dots 32 are substantially intrinsic semiconductors; in other words, the Fermi levels of the first quantum dots 31 and the second quantum dots 32 are located substantially in the center of their respective bandgaps. Furthermore, in each of the band diagrams in this disclosure, the anode 21 is located on the left side of the page, and the cathode 25 is located on the right side.

[0059] Band diagrams B1 and B2 in Fig. 3 show the band diagrams of the first quantum dot 31 and the second quantum dot 32, respectively. In Fig. 3, the infinite levels in both band diagrams are assumed to be at the same position. The thickness of the band gap in the horizontal direction on the paper in each band diagram in Fig. 3 represents the difference in particle size between the quantum dots.

[0060] 3 , the lower level of the band gap is almost the same between the first quantum dot 31 and the second quantum dot 32. This means that the ionization potential does not change significantly between the first quantum dot 31 and the second quantum dot 32. On the other hand, the upper level of the first quantum dot 31 is located higher than the upper level of the second quantum dot 32. This means that the electron affinity of the first quantum dot 31 is smaller than the electron affinity of the second quantum dot 32.

[0061] The relationship between the diameter of semiconductor nanoparticles, including the first quantum dots 31 and the second quantum dots 32, and the band gap of the semiconductor will be described in detail with reference to Fig. 4. Fig. 4 is a graph showing the relationship between the diameter and band gap of semiconductor nanoparticles. In the graph of Fig. 4, the horizontal axis represents the diameter (unit: nm) of the semiconductor nanoparticles, and the vertical axis represents the band gap (unit: eV) of the semiconductor nanoparticles.

[0062] As shown in the graph of Figure 4, 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.

[0063] 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 4, 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.

[0064] As described above, the particle diameter DC1 of the first core 31C and the particle diameter DC2 of the second core 32C are substantially the same, while the thickness DS2 of the second shell 32S is larger than the thickness DS1 of the first shell 31S. Therefore, the particle diameter D2 of the second quantum dots 32 is larger than the particle diameter D1 of the first quantum dots 31. Therefore, the quantum effect of the second quantum dots 32 is smaller than that of the first quantum dots 31, and the band gap of the second quantum dots 32 is smaller than that of the first quantum dots 31.

[0065] On the other hand, since the effective mass of electrons and the effective mass of holes in semiconductors such as the first quantum dot 31 and the second quantum dot 32 are generally significantly different, the lower level of the band gap does not change significantly between the first quantum dot 31 and the second quantum dot 32. Therefore, as shown in FIG.

[0066] In this embodiment, as described above, the Fermi levels of the first quantum dot 31 and the second quantum dot 32 are located approximately in the center of their respective band gaps. Therefore, the Fermi level of the first quantum dot 31 is located above the Fermi level of the second quantum dot 32 in the band diagram.

[0067] <Example of Levels of First Quantum Dot and Second Quantum Dot> The energy levels of the first quantum dot 31 and the second quantum dot 32 according to this embodiment will be described with reference to Fig. 5. Fig. 5 is a band diagram of the first quantum dot 31 and the second quantum dot 32.

[0068] 5 show the respective band gaps of the first quantum dots 31 and the second quantum dots 32 when the first quantum dots 31 are located closer to the anode 21 than the second quantum dots 32. Also, band diagrams B6, B7, and B8 in Fig. 5 show the respective band gaps of the first quantum dots 31 and the second quantum dots 32 when the first quantum dots 31 are located closer to the cathode 25 than the second quantum dots 32.

[0069] Band diagrams B3 and B6 are band diagrams of the first quantum dot 31 and the second quantum dot 32, respectively, when the distance between the first quantum dot 31 and the second quantum dot 32 is large enough that no charge transfer occurs between them.

[0070] Band diagrams B4 and B7 are band diagrams of the first quantum dot 31 and the second quantum dot 32, respectively, when the first quantum dot 31 and the second quantum dot 32 are adjacent to each other. The first quantum dot 31 and the second quantum dot 32 being adjacent to each other means that the first quantum dot 31 and the second quantum dot 32 are close enough to each other that charge transfer between them occurs naturally.

[0071] When the first quantum dot 31 and the second quantum dot 32 are adjacent to each other, charge transfer occurs between the first quantum dot 31 and the second quantum dot 32 in a direction in which their Fermi levels approach each other, resulting in a bend in the band gap between the first quantum dot 31 and the second quantum dot 32, as shown in band diagrams B4 and B7.

[0072] In particular, in the example shown in band diagram B4, the energy level of the first quantum dot 31 on the cathode 25 side shifts upward, and the energy level of the second quantum dot 32 on the anode 21 side shifts downward. On the other hand, in the example shown in band diagram B7, the energy level of the first quantum dot 31 on the cathode 25 side shifts downward, and the energy level of the second quantum dot 32 on the anode 21 side shifts upward.

[0073] Band diagrams B5 and B8 are band diagrams of the first quantum dots 31 and the second quantum dots 32 when a voltage is applied to the light-emitting element 2 in the states shown in band diagrams B4 and B7, respectively. In this case, due to the electric field between the anode 21 and the cathode 25, the band gaps of the first quantum dots 31 and the second quantum dots 32 shift upward on the cathode 25 side relative to the anode 21, as shown in band diagrams B5 and B8.

[0074] As a result, as shown in band diagrams B5 and B8, near the interface between the first quantum dot 31 and the second quantum dot 32, the uppermost energy level of the second quantum dot 32 moves higher than the uppermost energy level of the first quantum dot 31. Meanwhile, near the interface, the energy difference between the lowermost energy level of the first quantum dot 31 and the uppermost energy level of the second quantum dot 32 is relatively small.

[0075] Band diagrams B5 and B8 also show holes H injected from the anode 21 into the light-emitting layer 23 and electrons E injected from the cathode 25 into the light-emitting layer 23 upon application of a voltage to the light-emitting element 2. Generally, electrons are transported more efficiently from the cathode 25 to the anode 21 than holes due to the difference in effective mass or mobility between holes and electrons.

[0076] Therefore, in general, in light-emitting devices, an excess of electrons may occur in the light-emitting layer, where the concentration of electrons is excessively higher than the concentration of holes. The excess of electrons in the light-emitting layer causes electrons to flow out of the light-emitting layer toward the anode. Furthermore, the excess of electrons does not contribute to the light emission of the light-emitting layer, and also causes the generation of Auger electrons, which have high energy and can cause deterioration of the light-emitting layer or various parts in the vicinity of the light-emitting layer, as described below.

[0077] In the examples according to band diagrams B5 and B8, the top level of the second quantum dot 32 is higher than the top level of the first quantum dot 31 near the interface between the first quantum dot 31 and the second quantum dot 32. This creates a barrier to the injection of electrons E injected into the light-emitting layer 23 from the second quantum dot 32 to the first quantum dot 31, which in turn makes it easier for the electrons E to remain near the interface between the first quantum dot 31 and the second quantum dot 32. This reduces the outflow of electrons from the light-emitting layer 23 to the anode 21 in the examples according to band diagrams B5 and B8.

[0078] Furthermore, in the examples according to band diagrams B5 and B8, the energy difference between the lowest energy levels of the first quantum dots 31 and the second quantum dots 32 is relatively small near the interface. Therefore, the holes H injected into the light-emitting layer 23 are efficiently injected into the first quantum dots 31 and the second quantum dots 32, thereby improving the transport efficiency of the holes H in the light-emitting layer 23. Therefore, in the examples according to band diagrams B5 and B8, the hole concentration in the light-emitting layer 23 is improved, thereby reducing the electron excess in the light-emitting layer 23.

[0079] <Reduction of Excess Electrons> As described above, the light-emitting element 2 including the light-emitting layer 23 according to this embodiment improves the excess electrons in the light-emitting layer 23. The light-emitting element 2 that improves the excess electrons in the light-emitting layer 23 reduces the outflow of electrons from the light-emitting layer 23 and reduces deterioration of the light-emitting layer 23 and various components in the vicinity of the light-emitting layer 23, thereby improving luminous efficiency and reliability. The display device 1 including the light-emitting element 2 achieves reduced power consumption and a longer lifespan. Particularly for the reasons described above, when the first quantum dots 31 and the second quantum dots 32 are adjacent to each other, the light-emitting element 2 further reduces the excess electrons in the light-emitting layer 23.

[0080] The light-emitting element 2 includes an electron transport layer 24 between the light-emitting layer 23 and the cathode 25, which improves the efficiency of transporting electrons from the cathode 25 to the light-emitting layer 23. In general, light-emitting elements including an electron transport layer tend to have a more pronounced excess of electrons in the light-emitting layer. Therefore, the light-emitting element 2 according to this embodiment more efficiently reduces the excess of electrons in the light-emitting layer 23.

[0081] The light-emitting layer 23 according to this embodiment contains a plurality of first quantum dots 31 and a plurality of second quantum dots 32 that are distributed approximately uniformly. Therefore, in the light-emitting layer 23, the number of pairs of adjacent first quantum dots 31 and second quantum dots 32 increases compared to when only the first quantum dots 31 or only the second quantum dots 32 are unevenly located in a certain region. Therefore, with the above configuration, the number of pairs of first quantum dots 31 and second quantum dots 32 that reduce the above-described excess electrons increases, and the excess electrons in the light-emitting layer 23 are further reduced.

[0082] <Manufacturing Method of Display Device: Up to Formation of Hole Transport Layer> A manufacturing method of the display device 1 according to this embodiment will be described with reference to Fig. 6. Fig. 6 is a flowchart of the manufacturing method of the display device 1 according to this embodiment.

[0083] 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.

[0084] Next, an anode 21 is formed on the substrate 3 (step S2). The anode 21 may be formed by forming a thin conductive film common to multiple subpixels using the method described above, and then patterning the thin film for each subpixel. Next, a hole transport layer 22 is formed on the anode 21 using the method described above (step S3). The hole transport layer 22 may be formed common to multiple subpixels, or may be patterned for each subpixel. Between the formation of the anode 21 and the formation of the hole transport layer 22, a hole injection layer may be formed on the anode 21 using the same method as for forming the hole transport layer 22, except for the material. In this case, the hole transport layer 22 may be formed on the hole injection layer.

[0085] <Display Device Manufacturing Method: Formation of Light-Emitting Layer: Preparation of Quantum Dot Dispersion> Next, the light-emitting layer 23 is formed on the hole transport layer 22 (step S4). The method for forming the light-emitting layer 23 will be described in more detail with reference to Fig. 7. Fig. 7 is a flowchart of the method for forming the light-emitting layer 23 according to this embodiment.

[0086] In the method for forming the light-emitting layer 23, first, the first quantum dots 31 and the second quantum dots 32 are synthesized (step S41). The synthesis of the first quantum dots 31 and the second quantum dots 32 may be performed by, for example, a heating method, a hot injection method, a microwave-assisted method, a continuous flow method, or the like.

[0087] In step S41, each quantum dot may be synthesized by synthesizing a core of the quantum dot and then forming a shell around the core. In this case, the synthesis of the second quantum dot 32 may be performed by the same method as the synthesis of the first quantum dot 31, except that the thickness of the shell formed around the core is larger. For example, in forming the second shell 32S around the second core 32C, the reaction time may be longer or the reaction temperature may be higher than in forming the first shell 31S around the first core 31C.

[0088] Next, a quantum dot dispersion liquid is prepared (step S42). The quantum dot dispersion liquid according to this embodiment will be described with reference to Fig. 8. Fig. 8 is a schematic diagram of a quantum dot dispersion liquid 50 according to this embodiment.

[0089] 8, the quantum dot dispersion 50 is a dispersion in which the first quantum dots 31 and the second quantum dots 32 are dispersed in a solvent 51 such as ethanol. The quantum dot dispersion 50 may be prepared by adding the first quantum dots 31 and the second quantum dots 32 to the solvent 51. The quantum dot dispersion 50 may further contain a dispersed precursor of the above-mentioned adduct.

[0090] <Display Device Manufacturing Method: Formation of Light-Emitting Layer: Application and Drying of Quantum Dot Dispersion Liquid> Returning to FIG. 7 , following step S42, quantum dot dispersion liquid 50 is applied (step S43). The method for forming the light-emitting layer 23 from step S43 onward will be described with reference to FIG. 9 . FIG. 9 is a process cross-sectional view for explaining in more detail part of the method for forming the light-emitting layer 23 according to this embodiment, particularly step S43 onward.

[0091] 9 , before the start of the method for forming the light-emitting layer 23, the anode 21 and the hole transport layer 22 have been formed in this order on the substrate 3. Here, in step S43, as shown in step S43 of Fig. 9 , the quantum dot dispersion liquid 50 is applied onto the hole transport layer 22. In other words, when the substrate is a laminated body from the substrate 3 to the hole transport layer 22, the application of the quantum dot dispersion liquid 50 is the application of the quantum dot dispersion liquid 50 onto the substrate. The application of the quantum dot dispersion liquid 50 may be achieved by various application methods, including conventionally known application methods such as an inkjet method.

[0092] Next, the applied quantum dot dispersion liquid 50 is dried (step S44). For example, in step S44, each layer on the substrate 3, including the applied quantum dot dispersion liquid 50, is heated, for example, at 130°C for 5 minutes. As a result, as shown in step S44 in Fig. 9, the solvent 51 of the quantum dot dispersion liquid 50 is volatilized to form the light-emitting layer 23. If the quantum dot dispersion liquid 50 contains a precursor of an adduct, the precursor may be converted to the adduct in the quantum dot dispersion liquid 50 in step S44, thereby forming the light-emitting layer 23 containing the adduct.

[0093] In the display device 1, if the emission color of the light-emitting elements 2 differs for each subpixel as described above, the application and drying of the quantum dot dispersion liquid 50 and the patterning of the light-emitting layer 23 may be repeated in forming the light-emitting layer 23. In this case, the emission colors of the first quantum dots 31 and the second quantum dots 32 dispersed in the quantum dot dispersion liquid 50 may be changed depending on the emission color of the light-emitting elements 2 located in the subpixels that form the light-emitting layer 23. In this way, light-emitting elements 2 of emission colors corresponding to the red subpixel XR, the green subpixel XG, and the blue subpixel XB may be formed.

[0094] 6 , after the light-emitting layer 23 is formed, the electron transport layer 24 is formed on the light-emitting layer 23 by the above-described method (step S5). The electron transport layer 24 may be formed in common for a plurality of sub-pixels, or may be patterned for each sub-pixel.

[0095] Next, the cathode 25 is formed (step S6). The cathode 25 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, and the manufacture of the display device 1 is completed. According to the method described above, it is possible to manufacture a light-emitting element 2 in which the excess electrons in the light-emitting layer 23 are reduced.

[0096] Furthermore, according to the above method, the light-emitting layer 23 can be formed using the quantum dot dispersion liquid 50 containing both the first quantum dots 31 and the second quantum dots 32 dispersed therein. Therefore, according to the above method, it is not necessary to separately prepare a quantum dot dispersion liquid containing the first quantum dots 31 dispersed therein and a quantum dot dispersion liquid containing the second quantum dots 32 dispersed therein, thereby more simply manufacturing the light-emitting element 2. The light-emitting layer 23 in which the plurality of first quantum dots 31 and the plurality of second quantum dots 32 are distributed approximately uniformly can be simply formed by the above method.

[0097] 10 is a schematic side cross-sectional view of a display device 4 according to this embodiment. The display device 4 according to this embodiment has the same configuration as the display device 1 according to the previous embodiment, except that each sub-pixel includes a light-emitting element 5 instead of the light-emitting element 2. The light-emitting element 5 has the same configuration as the light-emitting element 2, except that each sub-pixel includes a light-emitting layer 26 instead of the light-emitting layer 23.

[0098] The light-emitting layer 26 includes a first light-emitting layer 41 and a second light-emitting layer 42 stacked in this order from the anode 21 side. In other words, in the light-emitting layer 26, the first light-emitting layer 41 is located closer to the anode 21 than the second light-emitting layer 42.

[0099] The first light-emitting layer 41 includes a plurality of first quantum dots 31. The second light-emitting layer 42 includes a plurality of second quantum dots 32. Therefore, the first quantum dots 31 and the second quantum dots 32 are adjacent to each other near the boundary between the first light-emitting layer 41 and the second light-emitting layer 42 in the light-emitting layer 26.

[0100] Note that the first light-emitting layer 41 and the second light-emitting layer 42 in this embodiment are merely components provided for the sake of convenience in order to more clearly explain the configuration of the light-emitting layer 26 in the present disclosure. In other words, the light-emitting layer 26 does not need to include a clearly distinguishable first light-emitting layer 41 and second light-emitting layer 42. For example, in this embodiment, it may be confirmed that the light-emitting layer 26 includes a portion of each of the first light-emitting layer 41 including the first quantum dots 31 and the second light-emitting layer 42 including the second quantum dots 32. In this case, it may be considered that the light-emitting layer 26 includes the first light-emitting layer 41 and the second light-emitting layer 42 without confirming a clear boundary between the first light-emitting layer 41 and the second light-emitting layer 42.

[0101] In this manner, in the present disclosure, it is assumed that the positional relationship between each part of the light-emitting layer 26, such as the first quantum dots 31 and the second quantum dots 32, can be confirmed, for example, by observing a cross section of the light-emitting layer 26. In this case, unless otherwise specified, the specific configuration of the light-emitting layer 26 in the present disclosure is not particularly limited.

[0102] For example, the boundary between the first light-emitting layer 41 and the second light-emitting layer 42 may be located approximately at the center in the thickness direction of the light-emitting layer 26, or may be located closer to the anode 21 or the cathode 25 than the center. Furthermore, at least one of the first quantum dots 31 and the second quantum dots 32 may be located on the boundary between the first light-emitting layer 41 and the second light-emitting layer 42.

[0103] Alternatively, there may be no clear boundary between the first light-emitting layer 41 and the second light-emitting layer 42, as long as at least a portion of the second light-emitting layer 42 is located closer to the cathode 25 than at least a portion of the first light-emitting layer 41. In addition, the light-emitting layer 26 may include, depending on the position in a planar view of the light-emitting element 5, a portion including the first light-emitting layer 41 and the second light-emitting layer 42, and a portion including a uniform mixture of the first quantum dots 31 and the second quantum dots 32.

[0104] The light-emitting element 5 according to this embodiment includes a light-emitting layer 26 that includes first quantum dots 31 and second quantum dots 32 adjacent to the first quantum dots 31. Therefore, for the same reasons as described above, the light-emitting element 5 reduces the excess electrons in the light-emitting layer 26. The display device 4 including the light-emitting element 5 achieves reduced power consumption and a longer lifespan.

[0105] Furthermore, the light-emitting layer 26 included in the light-emitting element 5 includes a first light-emitting layer 41 including the first quantum dots 31 and a second light-emitting layer 42 including the second quantum dots 32, stacked together. Therefore, the light-emitting element 2 can have the first quantum dots 31 and the second quantum dots 32 adjacent to each other at the interface between the first light-emitting layer 41 and the second light-emitting layer 42 with a simple configuration. Therefore, the light-emitting element 5 reduces the electron excess in the light-emitting layer 26 more efficiently while reducing the complexity of its configuration.

[0106] In particular, the light-emitting layer 26 includes the first light-emitting layer 41 located closer to the anode 21 than the second light-emitting layer 42. Therefore, the band gaps of the first quantum dots 31 and the second quantum dots 32 adjacent to each other near the interface between the first light-emitting layer 41 and the second light-emitting layer 42 are as shown in the band diagram B4 of FIG. 5 described above. Therefore, when the light-emitting element 5 is driven, the band gaps of the first quantum dots 31 and the second quantum dots 32 are as shown in the band diagram B5 of FIG. 5 described above. Therefore, for the same reasons as described above, the light-emitting element 5 reduces the outflow of electrons from the light-emitting layer 23 to the anode 21 side, improves the hole concentration in the light-emitting layer 23, and further reduces the excess electrons in the light-emitting layer 23.

[0107] <Method of Forming First Light-Emitting Layer and Second Light-Emitting Layer> A method of manufacturing the display device 4 according to this embodiment will be described with reference to Fig. 11. Fig. 11 is a flowchart of the method of manufacturing the display device 4 according to this embodiment.

[0108] The manufacturing method of the display device 4 according to this embodiment may be the same as the manufacturing method of the display device 1 according to the previous embodiment from step S1 to step S3, whereby the anode 21 and the hole transport layer 22 are formed on the substrate 3 upon completion of step S3.

[0109] In this embodiment, the first light-emitting layer 41 is then formed (step S7). The first light-emitting layer 41 may be formed, for example, by synthesizing the first quantum dots 31, preparing a first quantum dot dispersion in which the first quantum dots 31 are dispersed, and applying and drying the first quantum dot dispersion.

[0110] The synthesis of the first quantum dots 31 in this embodiment may be the same as step S41 in the previous embodiment. The first quantum dot dispersion in this embodiment may be the same as the quantum dot dispersion 50 in the previous embodiment, except that it does not include the second quantum dots 32. The application and drying of the first quantum dot dispersion in this embodiment may be the same as steps S43 and S44, except that the quantum dot dispersion 50 is replaced with the first quantum dot dispersion.

[0111] Following the formation of the first light-emitting layer 41, the second light-emitting layer 42 is formed (step S8). The second light-emitting layer 42 may be formed, for example, by synthesizing the second quantum dots 32, preparing a second quantum dot dispersion in which the second quantum dots 32 are dispersed, and applying and drying the second quantum dot dispersion.

[0112] The synthesis of the second quantum dots 32 in this embodiment may be the same as step S41 in the previous embodiment. The second quantum dot dispersion in this embodiment may be the same as the quantum dot dispersion 50 in the previous embodiment, except that it does not include the first quantum dots 31. The application and drying of the second quantum dot dispersion in this embodiment may be the same as steps S43 and S44, except that the quantum dot dispersion 50 is replaced with the second quantum dot dispersion and that the second quantum dot dispersion is applied onto the first light-emitting layer 41.

[0113] Next, steps S5 and S6 in the manufacturing method of the display device 1 according to the previous embodiment may be performed to form the electron transport layer 24 and the cathode 25. This forms the light-emitting element 5 according to this embodiment on the substrate 3, completing the manufacturing method of the display device 4.

[0114] According to the method for manufacturing the light-emitting element 5 of this embodiment, the first light-emitting layer 41 and the second light-emitting layer 42 are formed in separate steps, which allows the light-emitting layer 26 to be formed by a simple method. Therefore, according to the above method, the light-emitting element 5 that further reduces the excess electrons in the light-emitting layer 26 can be manufactured more simply.

[0115] 12 is a schematic side cross-sectional view of a display device 6 according to this embodiment. The display device 6 according to this embodiment has the same configuration as the display device 4 according to the previous embodiment, except that each sub-pixel includes a light-emitting element 7 instead of the light-emitting element 5. The light-emitting element 7 has the same configuration as the light-emitting element 5, except that each sub-pixel includes a light-emitting layer 27 instead of the light-emitting layer 26.

[0116] The light-emitting layer 27 includes a second light-emitting layer 42 and a first light-emitting layer 41 stacked in this order from the anode 21 side. In other words, in the light-emitting layer 27, the first light-emitting layer 41 is located closer to the cathode 25 than the second light-emitting layer 42. The first light-emitting layer 41 and the second light-emitting layer 42 according to this embodiment each have the same configuration as the first light-emitting layer 41 and the second light-emitting layer 42 according to the previous embodiment.

[0117] Therefore, the light-emitting element 7 according to this embodiment includes a light-emitting layer 27 that includes the first quantum dots 31 and the second quantum dots 32. For the same reasons as described above, the light-emitting element 7 reduces the excess electrons in the light-emitting layer 27. The display device 6 including the light-emitting element 7 achieves power saving and a longer lifespan.

[0118] Furthermore, the light-emitting layer 27 included in the light-emitting element 7 includes a stack of a first light-emitting layer 41 including the first quantum dots 31 and a second light-emitting layer 42 including the second quantum dots 32. Therefore, for the same reasons as those described above, the light-emitting element 7 more efficiently reduces the excess electrons in the light-emitting layer 27 while reducing the complexity of its configuration.

[0119] In particular, the light-emitting layer 27 includes the first light-emitting layer 41 closer to the cathode 25 than the second light-emitting layer 42. Therefore, the band gaps of the first quantum dots 31 and the second quantum dots 32 adjacent to each other near the interface between the first light-emitting layer 41 and the second light-emitting layer 42 are as shown in the band diagram B7 of FIG. 5 described above. Therefore, when the light-emitting element 7 is driven, the band gaps of the first quantum dots 31 and the second quantum dots 32 are as shown in the band diagram B8 of FIG. 5 described above. Therefore, for the same reasons as described above, the light-emitting element 7 further reduces the outflow of electrons from the light-emitting layer 27 to the anode 21 side, improves the hole concentration in the light-emitting layer 27, and ultimately reduces the excess electrons in the light-emitting layer 27.

[0120] The display device 6 according to this embodiment may be manufactured by the same method as the method for manufacturing the display device 4 according to the previous embodiment, except that the order of execution of step S7 and step S8 is reversed.

[0121] <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.

[0122] 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 it includes a light-emitting layer containing a bulk inorganic semiconductor material as a light-emitting material instead of the light-emitting layer 23. The light-emitting device according to Comparative Example 1 is, for example, a micro LED.

[0123] The light-emitting device of Comparative Example 2 differs in configuration from the light-emitting device 2 of this embodiment in that it includes a light-emitting layer containing only first quantum dots 31 as a light-emitting material, instead of the light-emitting layer 23. Therefore, the light-emitting layer of the comparative embodiment does not include second quantum dots 32. In other words, the light-emitting layer of the light-emitting device of Comparative Example 2 includes a plurality of quantum dots having shells of approximately the same thickness. Except for the above, the light-emitting device of Comparative Example 2 has the same configuration as the light-emitting device 2 of this embodiment.

[0124] <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. 13 .

[0125] 13 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 ２ is.

[0126] 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.

[0127] Specifically, as shown in graphs G1 and G2 in Figure 13, 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.

[0128] 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.

[0129] In each comparative example, as the current density in the light-emitting device increases in the middle region RB, the concentration of electrons injected into the light-emitting layer becomes higher than the concentration 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 is more likely to be increased than the efficiency of hole injection from the anode to the light-emitting layer.

[0130] 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.

[0131] 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.

[0132] 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.

[0133] 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.

[0134] 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 concentration in the light-emitting layer of the light-emitting device is approximately 10 １８ / cm ３ From 10 １９ / cm３ 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 concentration. Therefore, the light-emitting element according to Comparative Example 1 can obtain sufficient brightness for use as a light-emitting element in, for example, a display device, while maximizing the EQE as much as possible.

[0135] 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.

[0136] <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 concentration 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 concentration 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.

[0137] 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.

[0138] 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 concentration.

[0139] 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.

[0140] 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.

[0141] <Characteristics of Light-Emitting Device: Comparison Between Examples and Comparative Examples> In contrast, the light-emitting device according to each embodiment reduces the excess electrons in the light-emitting layer for the reasons described above. Therefore, the light-emitting device according to each embodiment efficiently reduces the probability of occurrence of the generation process of Auger electrons, particularly trion Auger electrons, in the light-emitting layer during operation. Therefore, the light-emitting device according to each embodiment can obtain sufficient brightness while reducing the decrease in EQE, even when the light-emitting device is operated by increasing the current density to the level of the high region RC described above.

[0142] The characteristics of the light-emitting device according to each embodiment and the characteristics of the light-emitting device according to Comparative Example 2 are evaluated by comparing the characteristics of the light-emitting device according to the Examples with the characteristics of the light-emitting device according to the Comparative Example. The light-emitting device according to Example 1 was manufactured by the method for forming light-emitting device 2 described above. The light-emitting device according to Example 2 was manufactured by the method for forming light-emitting device 7 described above. The light-emitting device according to the Comparative Example was manufactured by the same method as the light-emitting device according to the Examples, except that the light-emitting layer 23 did not include second quantum dots 32.

[0143] For the light-emitting elements of each example and comparative example, the luminance was measured while changing the applied voltage, and the change in EQE of each light-emitting element in response 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 Figure 14.

[0144] In the graph of Fig. 14, 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. 14, the EQE is normalized so that the maximum value in the light-emitting elements according to the examples is 1. In the graph of Fig. 14, the measurement results of Example 1 are shown by a dashed line, the measurement results of Example 2 by a solid line, and the measurement results of the comparative example by a dashed line.

[0145] 14, the maximum EQE of the light-emitting element according to each example is greater than the maximum EQE of the light-emitting element according to the comparative example. This is thought to be because the excess electrons in the light-emitting layer of the light-emitting element 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 light-emitting layer.

[0146] 14, 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 light-emitting layer 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 concentration in the light-emitting layer is increased.

[0147] 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.

[0148] 14 , the EQE of the light-emitting device according to Example 2 is greater than that of the light-emitting device according to Example 1. This is because the hole concentration in the light-emitting layer of the light-emitting device according to Example 2 is higher than that of the light-emitting device according to Example 1, thereby increasing the probability of generating excitons in the light-emitting layer of the light-emitting device according to Example 2. Therefore, the light-emitting device according to Example 2 has a greater improvement in luminous efficiency than the light-emitting device according to Example 1.

[0149] 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.

[0150] REFERENCE SIGNS LIST 1 display device 2 light-emitting element 21 anode 23 light-emitting layer 24 electron transport layer 25 cathode 31 first quantum dot 31C first core 31S first shell 32 second quantum dot 32C second core 32S second shell 41 first light-emitting layer 42 second light-emitting layer

Claims

1. A light-emitting device comprising: an anode; a cathode facing the anode; and a light-emitting layer located between the anode and the cathode, wherein the light-emitting layer includes at least one first quantum dot and at least one second quantum dot, wherein the first quantum dot has a first core and a first shell located around the first core, and the second quantum dot has a second core having approximately the same diameter as the first core, and a second shell located around the second core and having a thickness greater than that of the first shell.

2. The light-emitting device according to claim 1, wherein at least one of the first quantum dots and at least one of the second quantum dots are adjacent to each other.

3. The light-emitting element according to claim 1 or 2, wherein the thickness of the first shell and the thickness of the second shell are 0.5 nm or more and 8 nm or less.

4. A light-emitting element according to any one of claims 1 to 3, wherein the central wavelength of light emitted by at least one of the first quantum dots is substantially the same as the central wavelength of light emitted by at least one of the second quantum dots.

5. A light-emitting element described in any one of claims 1 to 4, wherein the difference between the particle size of at least one of the first cores and the particle size of at least one of the second cores divided by the average value of the particle size of at least one of the first cores and the particle size of at least one of the second cores is 0.05 or less.

6. A light-emitting element described in any one of claims 1 to 5, wherein the difference between the particle size of at least one of the first quantum dots and the particle size of at least one of the second quantum dots divided by the average particle size of at least one of the first quantum dots and the particle size of at least one of the second quantum dots is 0.05 or more.

7. A light-emitting device according to any one of claims 1 to 6, wherein the particle size of at least one of the second quantum dots is 5% or more larger than the particle size of at least one of the first quantum dots.

8. A light-emitting element described in any one of claims 1 to 7, wherein the difference between the maximum and minimum particle sizes of the first quantum dots and the second quantum dots divided by the average particle size of the first quantum dots and the second quantum dots is 0.15 or more.

9. The light-emitting device according to any one of claims 1 to 8, wherein the light-emitting layer includes a plurality of the first quantum dots and a plurality of the second quantum dots.

10. The light-emitting element according to claim 9, wherein the plurality of first quantum dots and the plurality of second quantum dots are distributed substantially uniformly in at least a portion of the light-emitting layer.

11. The light-emitting element according to claim 9 or 10, wherein the light-emitting layer comprises a first light-emitting layer including a plurality of the first quantum dots and a second light-emitting layer including a plurality of the second quantum dots, stacked together.

12. The light-emitting device according to claim 11, wherein the first light-emitting layer is located closer to the anode than the second light-emitting layer.

13. The light-emitting device according to claim 11, wherein the first light-emitting layer is located closer to the cathode than the second light-emitting layer.

14. The light-emitting device according to any one of claims 1 to 13, further comprising an electron transport layer between the light-emitting layer and the cathode.

15. A display device comprising the light-emitting element according to any one of claims 1 to 14.

16. A method for manufacturing a light-emitting device, the method comprising: forming an anode; forming a cathode facing the anode; and forming a light-emitting layer located between the anode and the cathode, wherein the light-emitting layer includes at least one first quantum dot and at least one second quantum dot, the first quantum dot having a first core and a first shell located around the first core, and the second quantum dot having a second core having approximately the same particle size as the first core, and a second shell located around the second core and having a thickness greater than a thickness of the first shell.

17. The method for manufacturing a light-emitting element according to claim 16, wherein the formation of the light-emitting layer includes: preparing a quantum dot dispersion liquid in which the first quantum dots and the second quantum dots are dispersed; applying the quantum dot dispersion liquid; and drying the applied quantum dot dispersion liquid.

18. A method for manufacturing a light-emitting element according to claim 16, wherein the formation of the light-emitting layer includes: forming a first light-emitting layer containing the first quantum dots; and forming a second light-emitting layer laminated on the first light-emitting layer and containing the second quantum dots.

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