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

Abstract

A light-emitting layer (23) of a light-emitting element (2) includes a first quantum dot (31), a second quantum dot (32), a first halogen atom, and a second halogen atom having a larger atomic number than that of the first halogen atom. The first halogen atom is included in the first quantum dot or is adjacent to the first quantum dot. The second halogen atom is included in the second quantum dot or is adjacent to the second quantum dot.

Description

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

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

[0002] Patent Document 1 discloses a light-emitting device in which 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 includes 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, at least one second quantum dot, a first halogen atom, and a second halogen atom having an atomic number greater than that of the first halogen atom, wherein the first halogen atom is contained in the first quantum dot or adjacent to the first quantum dot, and the second halogen atom is contained in the second quantum dot or adjacent to the second quantum dot.

[0006] A method for manufacturing a light-emitting element according to one aspect of the present disclosure includes forming an anode, forming a cathode facing the anode, 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, at least one second quantum dot, a first halogen atom, and a second halogen atom having an atomic number greater than that of the first halogen atom, wherein the first halogen atom is contained in the first quantum dot or adjacent to the first quantum dot, and the second halogen atom is contained in the second quantum dot or adjacent to the second quantum dot.

[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 according to embodiment 1, illustrating a difference in energy level of the quantum dots depending on the type of halogen atom adjacent to or contained in the quantum dots. FIG. 4 is a band diagram of the first quantum dots and the second quantum dots according to embodiment 1. FIG. 5 is another band diagram of the first quantum dots and the 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 graph showing the relationship between external quantum efficiency and current density of a light-emitting element according to a comparative embodiment. FIG. 11 is a graph showing the relationship between external quantum efficiency and current density of light-emitting elements according to each of an example and a comparative example. FIG. 12 is a schematic side cross-sectional view of a display device according to embodiment 2. FIG. 13 is a flowchart of a method for manufacturing a display device according to embodiment 2.

[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] In other words, the display device 1 includes, as light-emitting elements 2, a first light-emitting element that emits light having a first wavelength and a second light-emitting element that emits light having a second wavelength shorter than the first wavelength. Here, the first light-emitting element may be a red light-emitting element 2R, and the second light-emitting element may be a green light-emitting element 2G. Alternatively, the first light-emitting element may be a green light-emitting element 2G, and the second light-emitting element may be a blue light-emitting element 2B. Furthermore, the first light-emitting element may be a red light-emitting element 2R, and the second light-emitting element may be a blue light-emitting element 2B.

[0014] <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 the 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, the schematic views 102 and 103 show cross sections passing through the core, shell, and halide of each quantum dot (described later).

[0015] 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."

[0016] 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.

[0017] The structure of each layer of the light emitting element 2 will be described in more detail below.

[0018] <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.

[0019] 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.

[0020] <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 (WO ３ Metal oxides such as ZnO, ZnS, ZnO ...

[0021] 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.

[0022] 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.

[0023] 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."

[0024] As shown in a 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 a 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.

[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 approximately two times or less the exciton Bohr radius of the core material.

[0026] The quantum dots may contain materials used for the core and shell, respectively, that are conventionally known. 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.

[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 performing a cross-sectional observation of the light-emitting layer 23 in the film thickness direction. In the 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 particle size of the quantum dots may be measured by measuring the particle size of 20 quantum dots in the cross-sectional observation and calculating the average. 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.

[0028] 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, by controlling the particle size of the quantum dots, the wavelength of light emitted by the light-emitting element 2 can be controlled.

[0029] In this embodiment, the particle diameter of the first core 31C and the particle diameter of the second core 32C may be approximately the same. In this case, the light-emitting element 2 reduces the difference in wavelength of light obtained from the first quantum dot 31 and the second quantum dot 32, thereby improving the chromaticity of the emitted light. In the present disclosure, "the particle diameters of the two cores are approximately the same" does not necessarily mean that the particle diameters of the two cores are strictly the same. For example, the difference between the particle diameter of at least one first core 31C and the particle diameter of at least one second core 32C divided by the average value of the particle diameters of at least one first core 31C and at least one second core 32C may be 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.

[0030] In other words, the central wavelength of the light emitted by at least one first quantum dot 31 and the central wavelength of the light emitted by at least one second quantum dot 32 may be substantially the same. 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.

[0031] 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.

[0032] 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.

[0033] <Light-emitting element: light-emitting layer: halogen atoms> As shown in schematic cross-sectional side view 101 of Figure 1, light-emitting layer 23 includes first halide 41 having a first halogen atom and second halide 42 having a second halogen atom. Therefore, light-emitting layer 23 includes first halogen atoms and second halogen atoms. Since light-emitting layer 23 includes first halide 41 and second halide 42, light-emitting element 2 can easily achieve light-emitting layer 23 including first halogen atoms and second halogen atoms.

[0034] In this disclosure, the term "atom" does not only mean that an atom exists as a single atom. In this disclosure, the term "atom" also includes an atom that exists in the form of a molecule having two or more atoms, including the atom and another atom, an atom that exists in the form of a complex, an atom that exists in the form of a compound, or an atom that exists in the form of an ion. In this disclosure, the term "atom" does not limit the form in which other atoms exist. In other words, a halogen atom includes an atom that exists in the form of a compound having a halogen atom, and an atom that exists in the form of a halogen ion. Regardless of the form in which a halogen atom exists, if its presence in a substance can be identified by analysis, the substance may be considered to contain a halogen atom.

[0035] Here, the second halogen atom has a larger atomic number than the first halogen atom. For example, the first halogen atom includes a fluorine atom, a chlorine atom, or a bromine atom, and the second halogen atom includes a chlorine atom, a bromine atom, or an iodine atom. For example, the first halide 41 is zinc fluoride (ZnF ２ ), zinc chloride (ZnCl ２ ), or zinc bromide (ZnBr ２ The second halide 42 may also include zinc chloride (ZnCl ２ ), zinc bromide (ZnBr ２ ), zinc iodide (ZnI ２) may be contained. In this way, any of the above-mentioned halogen atoms can be contained in the light-emitting layer 23 as halogen atoms contained in a halide. In particular, halogen atoms contained in a halide are more stable than simple halogen atoms, and therefore, the above-mentioned configuration of the light-emitting element 2 reduces deterioration of each part of the light-emitting layer 23.

[0036] As shown in the schematic cross-sectional side view 101 of Fig. 1 , the first halide 41 is adjacent to the first quantum dot 31, in other words, the first halogen atom is adjacent to the first quantum dot 31. Alternatively, as shown in the schematic view 102 of Fig. 1 , the first halide 41 is included in the first quantum dot 31, in other words, the first halogen atom is included in the first quantum dot 31. In particular, the first halide 41 may be adjacent to the first shell 31S, in other words, the first halogen atom may be adjacent to the first shell 31S.

[0037] 1, the second halide 42 is adjacent to the second quantum dot 32, in other words, the second halogen atom is adjacent to the second quantum dot 32. Alternatively, as shown in the schematic diagram 103 of FIG. 1, the second halide 42 is included in the second quantum dot 32, in other words, the second halogen atom is included in the second quantum dot 32. In particular, the second halide 42 may be included in the second shell 32S, in other words, the second halogen atom may be included in the second shell 32S.

[0038] In other words, in this embodiment, the first quantum dot 31 may include a first halogen atom, and the second quantum dot 32 may include a second halogen atom. Furthermore, in this embodiment, the first quantum dot 31 may be adjacent to the first halogen atom, and the second quantum dot 32 may be adjacent to the second halogen atom.

[0039] For example, when a cross-sectional observation in any direction of the light-emitting layer 23 confirms that a quantum dot and a halide are in contact with each other, the quantum dot and a halogen atom in the halide may be considered to be adjacent to each other. Furthermore, when the cross-sectional observation confirms that the distance between the quantum dot and the halide is 3 nm or less, the quantum dot and a halogen atom in the halide may be considered to be adjacent to each other. Furthermore, when the cross-sectional observation confirms that a portion of a quantum dot is located closer to the periphery of the quantum dot than the halide, the halide and the halogen atom in the halide may be considered to be included in the quantum dot.

[0040] 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).

[0041] In this embodiment, the halogen atoms contained in the first quantum dots 31 or adjacent to the first quantum dots 31 are mainly first halogen atoms. Furthermore, the halogen atoms contained in the second quantum dots 32 or adjacent to the second quantum dots 32 are mainly second halogen atoms. For example, in this embodiment, the ratio of the first halogen atoms contained in the first quantum dots 31 or adjacent to the first quantum dots 31 to the halogen atoms contained in the first quantum dots 31 or adjacent to the first quantum dots 31 may be 60% or more. Furthermore, in this embodiment, the ratio of the second halogen atoms contained in the second quantum dots 32 or adjacent to the second quantum dots 32 to the halogen atoms contained in the second quantum dots 32 may be 40% or more.

[0042] Furthermore, in this embodiment, the halogen atoms contained in the first quantum dot 31 or adjacent to the first quantum dot 31 may be only the first halogen atoms. Furthermore, the halogen atoms contained in the second quantum dot 32 or adjacent to the second quantum dot 32 may be only the second halogen atoms. Here, it is assumed that the proportion of the specified halogen atoms contained in or adjacent to a given quantum dot according to the present disclosure to all halogen atoms contained in or adjacent to the given quantum dot is 60% or more. In this case, the halogen atoms contained in or adjacent to the given quantum dot may be considered to be only the specified halogen atoms.

[0043] <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. In this case, the additive may also contain a first halide 41 and a second halide 42. The additive may protect each portion 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.

[0044] 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.

[0045] <Relationship between Halogen Atoms and the Energy of Each Level of a Quantum Dot> When a halogen atom is adjacent to a quantum dot or when a quantum dot contains a halogen atom, the energy of each level of the quantum dot changes. This change also depends on the type of halogen atom. This will be explained with reference to FIG. 3.

[0046] 3 is a band diagram of a quantum dot for explaining the difference in the energy level of the quantum dot depending on the type of halogen atom adjacent to or contained in the quantum dot in this embodiment. Note that all of the band diagrams according to this embodiment, including FIG. 3, are band diagrams in which the energy level at infinity is located on the upper side. In addition, in the band diagrams according to this embodiment, including FIG. 3, the dotted lines represent the Fermi levels of the components shown in the band diagram.

[0047] The band gaps in Figure 3 all show the band gaps of any of the quantum dots according to this embodiment, and only the type of halogen atoms adjacent to or contained in the quantum dots differs. Band diagrams B1, B2, B3, and B4 in Figure 3 show band diagrams of quantum dots in which the halogen atoms adjacent to or contained in the quantum dots are fluorine, chlorine, bromine, and iodine, respectively. In Figure 3, the infinite energy levels in all band diagrams are assumed to be at the same position.

[0048] As shown in Figure 3, the band gap of a quantum dot does not change significantly depending on the type of halogen atom adjacent to or contained in the quantum dot. On the other hand, both the upper and lower energy levels of the quantum dot move upward in the band diagram as the atomic weight of the halogen atom adjacent to or contained in the quantum dot increases. This indicates that the ionization potential and electron affinity of the quantum dot decrease as the atomic weight of the halogen atom adjacent to or contained in the quantum dot increases.

[0049] <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. 4. Fig. 4 is a band diagram of the first quantum dot 31 and the second quantum dot 32. In each of the band diagrams in Fig. 4, the anode 21 is located on the left side of the page, and the cathode 25 is located on the right side.

[0050] Band diagram B5 is a band diagram for each of the first quantum dot 31 and the second quantum dot 32 when the distance between the first quantum dot 31 and the second quantum dot 32 is large enough to prevent charge transfer between them. As described above, the second halogen atom has a larger atomic number than the first halogen atom. Therefore, as shown in band diagram B5 of FIG. 4, the levels of the second quantum dot 32 are located higher in band diagram B5 than the levels of the first quantum dot 31.

[0051] The band diagram B6 is a band diagram of the first quantum dot 31 and the second quantum dot 32 when the first quantum dot 31 and the second quantum dot 32 are adjacent to each other. In the band diagram B6, the first quantum dot 31 is positioned closer to the anode 21 than the second quantum dot 32.

[0052] 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. As a result, as shown in band diagram B6, a bend occurs in the band gap between the first quantum dot 31 and the second quantum dot 32. In particular, in the example shown in band diagram B6, the energy level on the cathode 25 side of the first quantum dot 31 moves downward, and the energy level on the anode 21 side of the second quantum dot 32 moves upward.

[0053] Band diagram B7 is a band diagram of each 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 state shown in band diagram B6. In this case, due to the electric field between the anode 21 and the cathode 25, the band gaps of each 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 diagram B7.

[0054] As a result, as shown in the band diagram B7, 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.

[0055] The band diagram B7 also shows holes H injected from the anode 21 into the light-emitting layer 23 by application of a voltage to the light-emitting element 2, and electrons E injected from the cathode 25 into the light-emitting layer 23. 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.

[0056] 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.

[0057] In the example according to band diagram B7, 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 example according to band diagram B7.

[0058] Furthermore, in the example according to band diagram B7, the energy difference between the bottom level of the first quantum dot 31 and the top level of the second quantum dot 32 is relatively small near the interface. Therefore, the holes H injected into the light-emitting layer 23 are efficiently injected from the first quantum dot 31 to the second quantum dot 32, thereby improving the transport efficiency of the holes H in the light-emitting layer 23. Therefore, in the example according to band diagram B7, the hole concentration in the light-emitting layer 23 is improved, thereby reducing the electron excess in the light-emitting layer 23.

[0059] <Another Example of Energy Levels of First Quantum Dot and Second Quantum Dot> Another example of 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 another band diagram of the first quantum dot and the second quantum dot. In each of the band diagrams in Fig. 5, the anode 21 is located on the left side and the cathode 25 is located on the right side.

[0060] 5 , the energy levels of the second quantum dot 32 are located higher than the energy levels of the first quantum dot 31.

[0061] Band diagram B9 is a band diagram of the first quantum dot 31 and the second quantum dot 32 when they are adjacent to each other. In band diagram B9, the first quantum dot 31 is positioned closer to the cathode 25 than the second quantum dot 32. In the case shown in band diagram B9, the energy level of the first quantum dot 31 on the anode 21 side shifts downward, and the energy level of the second quantum dot 32 on the cathode 25 side shifts upward.

[0062] Band diagram B10 is a band diagram of each of the first quantum dot 31 and the second quantum dot 32 when a voltage is applied to the light-emitting element 2 in the state shown in band diagram B9. As shown in band diagram B10, 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 further upward 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 smaller than when no voltage is applied to the light-emitting element 2.

[0063] Therefore, in the example according to band diagram B10, electrons E are more likely to remain on the anode 21 side of the first quantum dot 31 in the vicinity of the interface between the first quantum dot 31 and the second quantum dot 32. As a result, in the example according to band diagram B10, the outflow of electrons from the light-emitting layer 23 to the anode 21 side is further reduced. Furthermore, in the example according to band diagram B10, holes H are efficiently injected from the second quantum dot 32 to the first quantum dot 31 even in the vicinity of the interface, thereby improving the transport efficiency of holes H in the light-emitting layer 23. Therefore, also in the example according to band diagram B10, the hole concentration in the light-emitting layer 23 is improved, thereby reducing the electron excess in the light-emitting layer 23.

[0064] <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.

[0065] The ratio of the first halogen atoms contained in or adjacent to the first quantum dots 31 to the halogen atoms contained in or adjacent to the first quantum dots 31 is 60% or more. At the same time, the ratio of the second halogen atoms contained in or adjacent to the second quantum dots 32 to the halogen atoms contained in or adjacent to the second quantum dots 32 is 40% or more. In this case, the difference in band gap between the first quantum dots 31 and the second quantum dots 32 becomes larger, and as described above, the band gap between the first quantum dots 31 and the second quantum dots 32 further improves the electron excess in the light-emitting layer 23. Therefore, with the above configuration, the light-emitting element 2 further improves the electron excess in the light-emitting layer 23, thereby improving luminous efficiency and reliability.

[0066] In particular, the halogen atoms contained in the first quantum dots 31 or adjacent to the first quantum dots 31 are only first halogen atoms, and the halogen atoms contained in the second quantum dots 32 or adjacent to the second quantum dots 32 are only second halogen atoms. In this case, the band gap between the first quantum dots 31 and the second quantum dots 32 is in a state that further improves the electron excess in the light-emitting layer 23. Therefore, with the above configuration, the light-emitting element 2 further improves the electron excess in the light-emitting layer 23, thereby improving the luminous efficiency and reliability.

[0067] 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.

[0068] From the viewpoint of efficiently achieving the reduction of the excess electrons in the light-emitting layer 23, the concentration of halogen atoms in the light-emitting layer 23 is set to 10 １６ atoms / cm ３ In order to reduce the influence of halogen atoms in the light-emitting layer 23 on the generation of excitons in the light-emitting layer 23, the concentration of halogen atoms in the light-emitting layer 23 may be 10 or more. ２０ atoms / cm ３ It may be the following:

[0069] In this embodiment, by having a halogen atom adjacent to or contained in the shell of each quantum dot, the influence of the halogen atom on the generation of excitons in the core is reduced in the light-emitting element 2. Therefore, with the above configuration, the light-emitting element 2 improves the luminous efficiency while reducing the electron excess.

[0070] <Selection of Halogen Atoms According to Emitted Color> In this embodiment, the type of halogen atom contained in each light-emitting layer 23 may be changed according to the emitted color of the light-emitting element 2. In particular, in this embodiment, the atomic number of the first halogen atom in the first light-emitting element may be larger than the atomic number of the first halogen atom in the second light-emitting element. At the same time, the atomic number of the second halogen atom in the first light-emitting element may be larger than the atomic number of the second halogen atom in the second light-emitting element. In other words, among the light-emitting elements 2 included in the display device 1, a certain light-emitting element 2 may have in its light-emitting layer 23 halogen atoms with a larger atomic number than a light-emitting element 2 that emits light with a shorter wavelength than the certain light-emitting element 2.

[0071] More specifically, red light-emitting element 2R may include bromine atoms as the first halogen atoms and iodine atoms as the second halogen atoms in light-emitting layer 23. Green light-emitting element 2G may include chlorine atoms as the first halogen atoms and bromine atoms as the second halogen atoms in light-emitting layer 23. Blue light-emitting element 2B may include fluorine atoms as the first halogen atoms and chlorine atoms as the second halogen atoms in light-emitting layer 23.

[0072] Generally, quantum dots have different bandgaps depending on the emitted color, and the shorter the wavelength of the emitted light, the larger the bandgap. Therefore, the overall electrical resistance of a light-emitting device that contains quantum dots as a light-emitting material generally tends to increase as the wavelength of the light emitted by the quantum dots becomes shorter.

[0073] In general, the shorter the emission color of a quantum dot, the smaller the electron affinity, i.e., the upper level tends to move upward in the band diagram. On the other hand, the ionization potential of a quantum dot generally does not change significantly regardless of the emission color, i.e., the lower level tends to be less likely to change in the band diagram regardless of the emission color of the quantum dot.

[0074] Therefore, in general, in a light-emitting device containing quantum dots as a light-emitting material, the shorter the wavelength of the emitted light, the higher the upper level of the band gap of the quantum dots moves in the band diagram, which in turn increases the barrier for injecting charges into the light-emitting layer. Therefore, in general, in the above-mentioned light-emitting device, the shorter the wavelength of the emitted light, the more efficiently the electrical resistance can be reduced by reducing the energy of each band gap level in the band diagram.

[0075] With the above configuration, the display device 1 can shift the upper end level of the band gap of the quantum dots of the light-emitting element 2 that emits light of a shorter wavelength further downward in the band diagram, thereby more efficiently reducing the overall electrical resistance value of the light-emitting element 2. Therefore, with the above configuration, the display device 1 can more simply level out the electrical resistance value that varies depending on the emission color of the light-emitting element 2.

[0076] <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.

[0077] 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.

[0078] 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.

[0079] <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.

[0080] 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.

[0081] In step S41, each quantum dot may be synthesized by synthesizing a core of each quantum dot and then forming a shell around the core. In this case, the shell may contain halogen atoms as the shell is formed. In this case, the shell may be synthesized by converting the shell precursor, a halide, and the core into a dispersion in an appropriate solvent. The shell may be formed around the core so that the halide is trapped within the shell. The first quantum dot 31 and the second quantum dot 32 may be synthesized by the same method, except for whether the halogen atoms contained in the halide included in each shell are first halogen atoms or second halogen atoms.

[0082] 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.

[0083] 8 , quantum dot dispersion 50 is a dispersion in which first quantum dots 31, second quantum dots 32, first halide 41, and second halide 42 are dispersed in a solvent 51 such as ethanol. Quantum dot dispersion 50 may be prepared by adding first quantum dots 31, second quantum dots 32, first halide 41, and second halide 42 to solvent 51. Quantum dot dispersion 50 may further contain a dispersed precursor of the above-mentioned adduct.

[0084] <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.

[0085] 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.

[0086] 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.

[0087] In the display device 1, when 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 repeatedly performed 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. This may form light-emitting elements 2 that emit light of colors corresponding to the red subpixel XR, the green subpixel XG, and the blue subpixel XB, respectively. In this case, the type of halide contained in the shell of each quantum dot and the type of halide added to the quantum dot dispersion liquid 50 may be changed depending on the emission color of the subpixel.

[0088] 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.

[0089] 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.

[0090] 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.

[0091] <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.

[0092] 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.

[0093] The light-emitting device according to Comparative Example 2 differs in configuration from the light-emitting device 2 according to this embodiment in that it includes a light-emitting layer containing only first quantum dots 31 as a light-emitting material, instead of light-emitting layer 23. Therefore, the light-emitting layer according to Comparative Example 2 does not include second quantum dots 32. Furthermore, the light-emitting layer according to Comparative Example 2 does not include first halide 41 or second halide 42. Except for the above, the light-emitting device according to Comparative Example 2 has the same configuration as the light-emitting device 2 according to this embodiment.

[0094] <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. 10.

[0095] 10 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.

[0096] 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.

[0097] Specifically, as shown in graphs G1 and G2 in Figure 10, 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.

[0098] 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.

[0099] 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.

[0100] 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.

[0101] 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.

[0102] 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.

[0103] 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.

[0104] 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 / cm ３ Therefore, when the light-emitting element according to Comparative Example 1 is driven at a current density included in the middle region RB, carriers are injected into the light-emitting layer at a sufficient 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.

[0105] 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.

[0106] <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.

[0107] 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.

[0108] 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.

[0109] 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.

[0110] 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.

[0111] <Characteristics of Light-Emitting Device: Comparison Between Examples and Comparative Examples> In contrast, the light-emitting device 2 according to this embodiment reduces the excess electrons in the light-emitting layer 23 for the reasons described above. Therefore, the light-emitting device 2 efficiently reduces the probability of occurrence of the process of generating Auger electrons, particularly trion Auger electrons, in the light-emitting layer 23 during operation. Therefore, even when the light-emitting device 2 is operated by increasing the current density to about the high region RC, it is possible to obtain sufficient brightness while reducing the decrease in EQE.

[0112] The characteristics of the light-emitting device 2 according to this 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 example and the characteristics of the light-emitting device according to the comparative example. The light-emitting device according to the example was manufactured by the above-described method for forming light-emitting device 2. The light-emitting device according to the comparative example was manufactured by the same method as the light-emitting device according to the example, except that the light-emitting layer 23 did not contain the first halide 41 and the second halide 42.

[0113] For the light-emitting elements of each of the examples and comparative examples, the luminance was measured while changing the applied voltage, thereby measuring 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, and the results were summarized in the graph of Figure 11.

[0114] In the graph of Fig. 11, 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. 11, the EQE is normalized so that the maximum value for the light-emitting element according to the example is 1. In the graph of Fig. 11, the measurement results for the example are shown by a solid line, and the measurement results for the comparative example are shown by a dashed line.

[0115] 11, the maximum value of the EQE of the light-emitting element according to the example is greater than the maximum value of the 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 23 of the light-emitting element according to the example are reduced, thereby reducing the probability of occurrence of processes such as the generation of Auger electrons that do not contribute to the light emission of the light-emitting layer 23.

[0116] 11, the EQE of the light-emitting device according to the example does not decrease significantly compared to the EQE of the light-emitting device 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 23 of the light-emitting device according to the 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 23 is increased.

[0117] As a result, the light-emitting device according to the example has a higher maximum EQE value and suppresses the decrease in EQE caused by an increase in current density compared to the light-emitting device according to the comparative example. Therefore, the light-emitting device according to the example has an increased luminous efficiency and can efficiently obtain sufficient brightness for use as a light-emitting device in, for example, a display device.

[0118] 12 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 27 instead of the light-emitting layer 23.

[0119] The light-emitting layer 27 includes a first light-emitting layer 61 and a second light-emitting layer 62 stacked in this order from the anode 21 side. In other words, in the light-emitting layer 27, the first light-emitting layer 61 is located closer to the anode 21 than the second light-emitting layer 62.

[0120] The first light-emitting layer 61 includes a plurality of first quantum dots 31. Therefore, the first light-emitting layer 61 includes a first halogen atom adjacent to or contained in the first quantum dots 31. The second light-emitting layer 62 includes a plurality of second quantum dots 32. Therefore, the second light-emitting layer 62 includes a second halogen atom adjacent to or contained in the second quantum dots 32.

[0121] Note that the first light-emitting layer 61 and the second light-emitting layer 62 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 27 in the present disclosure. In other words, the light-emitting layer 27 does not need to include a clearly distinguishable first light-emitting layer 61 and second light-emitting layer 62. For example, in this embodiment, it may be confirmed that the light-emitting layer 27 includes a portion of each of the first light-emitting layer 61 including the first quantum dots 31 and the first halide 41 and the second light-emitting layer 62 including the second quantum dots 32 and the second halide 42. In this case, it may be considered that the light-emitting layer 27 includes the first light-emitting layer 61 and the second light-emitting layer 62 without confirming a clear boundary between the first light-emitting layer 61 and the second light-emitting layer 62.

[0122] In this manner, in the present disclosure, it is assumed that the positional relationships of the various parts of the light-emitting layer 27, such as the first quantum dots 31, the second quantum dots 32, the first halide 41, and the second halide 42, can be confirmed by, for example, observing a cross section of the light-emitting layer 27. In this case, unless otherwise specified, the specific configuration of the light-emitting layer 27 in the present disclosure is not particularly limited.

[0123] 12 , the boundary between the first light-emitting layer 61 and the second light-emitting layer 62 may be located at approximately the center in the film thickness direction of the light-emitting layer 27, or may be located closer to the anode 21 or the cathode 25 than the center. In addition, at least one of the first quantum dots 31, the second quantum dots 32, the first halide 41, and the second halide 42 may be located on the boundary between the first light-emitting layer 61 and the second light-emitting layer 62.

[0124] Alternatively, there may be no clear boundary between the first light-emitting layer 61 and the second light-emitting layer 62, as long as at least a portion of the second light-emitting layer 62 is located closer to the cathode 25 than at least a portion of the first light-emitting layer 61. In addition, the light-emitting layer 27 may include, depending on the position in the planar view of the light-emitting element 5, a portion including the first light-emitting layer 61 and the second light-emitting layer 62, and a portion including a uniform mixture of the first quantum dots 31 and the second quantum dots 32.

[0125] The light-emitting element 5 according to this embodiment includes a light-emitting layer 27 including first quantum dots 31 adjacent to or containing first halogen atoms, and second quantum dots 32 adjacent to or containing second halogen atoms. Therefore, for the same reasons as described above, the light-emitting element 5 reduces excess electrons in the light-emitting layer 27. The display device 4 including the light-emitting element 5 achieves reduced power consumption and a longer lifespan.

[0126] Furthermore, the light-emitting layer 27 included in the light-emitting element 5 includes a first light-emitting layer 61 including the first quantum dots 31 and a second light-emitting layer 62 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 61 and the second light-emitting layer 62 with a simple configuration. Therefore, the light-emitting element 5 reduces the electron excess in the light-emitting layer 27 more efficiently while reducing the complexity of its configuration.

[0127] In particular, the light-emitting layer 27 includes the first light-emitting layer 61 located closer to the anode 21 than the second light-emitting layer 62. 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 61 and the second light-emitting layer 62 are as shown in the band diagram B6 of FIG. 4 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 B7 of FIG. 4 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.

[0128] <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. 13. Fig. 13 is a flowchart of the method of manufacturing the display device 4 according to this embodiment.

[0129] 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.

[0130] In this embodiment, the first light-emitting layer 61 is then formed (step S7). The first light-emitting layer 61 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 and the first halide 41 are dispersed, and applying and drying the first quantum dot dispersion.

[0131] 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 and the second halide 42. 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.

[0132] Following the formation of the first light-emitting layer 61, the second light-emitting layer 62 is formed (step S8). The second light-emitting layer 62 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 and the second halide 42 are dispersed, and applying and drying the second quantum dot dispersion.

[0133] 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 and the first halide 41. 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 61.

[0134] 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.

[0135] According to the manufacturing method of the light-emitting device 5 of this embodiment, the first light-emitting layer 61 and the second light-emitting layer 62 are formed in separate steps, thereby enabling the light-emitting layer 27 to be formed by a simple method. Furthermore, according to the above method, the first light-emitting layer 61 can be formed from a first quantum dot dispersion liquid that does not contain the second quantum dots 32 and the second halide 42, and the second light-emitting layer 62 can be formed from a second quantum dot dispersion liquid that does not contain the first quantum dots 31 and the first halide 41. Therefore, according to the above method, compared to the above-described method of forming the light-emitting layer 23, it is easier to arrange the first halide 41 adjacent to the first quantum dots 31 and the second halide 42 adjacent to the second quantum dots 32 in the light-emitting layer 27. Therefore, according to the above method, it is easier to manufacture a light-emitting device 5 that further reduces the electron excess in the light-emitting layer 27.

[0136] 14 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 28 instead of the light-emitting layer 27.

[0137] The light-emitting layer 28 includes a second light-emitting layer 62 and a first light-emitting layer 61 stacked in this order from the anode 21 side. In other words, in the light-emitting layer 28, the first light-emitting layer 61 is located closer to the cathode 25 than the second light-emitting layer 62. The first light-emitting layer 61 and the second light-emitting layer 62 according to this embodiment each have the same configuration as the first light-emitting layer 61 and the second light-emitting layer 62 according to the previous embodiment.

[0138] Therefore, the light-emitting element 7 according to this embodiment includes a light-emitting layer 28 including first quantum dots 31 adjacent to or containing first halogen atoms, and second quantum dots 32 adjacent to or containing second halogen atoms. Therefore, for the same reasons as described above, the light-emitting element 7 reduces the excess electrons in the light-emitting layer 28. The display device 6 including the light-emitting element 7 achieves power saving and a long lifespan.

[0139] Furthermore, the light-emitting layer 28 included in the light-emitting element 7 includes a stack of a first light-emitting layer 61 including the first quantum dots 31 and a second light-emitting layer 62 including the second quantum dots 32. Therefore, for the same reasons as described above, the light-emitting element 7 more efficiently reduces the excess electrons in the light-emitting layer 28 while reducing the complexity of its configuration.

[0140] In particular, the light-emitting layer 28 includes the first light-emitting layer 61 closer to the cathode 25 than the second light-emitting layer 62. 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 61 and the second light-emitting layer 62 are as shown in the band diagram B9 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 B10 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 28 to the anode 21 side, improves the hole concentration in the light-emitting layer 28, and ultimately reduces the excess electrons in the light-emitting layer 28.

[0141] 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.

[0142] 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.

[0143] 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 halide 42 second halide 50 quantum dot dispersion 61 first light-emitting layer 62 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 comprises at least one first quantum dot, at least one second quantum dot, a first halogen atom, and a second halogen atom having an atomic number greater than that of the first halogen atom, wherein the first halogen atom is contained in the first quantum dot or adjacent to the first quantum dot, and the second halogen atom is contained in the second quantum dot or adjacent to the second quantum dot.

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 described in claim 1 or 2, wherein the ratio of the first halogen atoms contained in the first quantum dot or adjacent to the first quantum dot to the halogen atoms contained in the first quantum dot or adjacent to the first quantum dot is 60% or more, and the ratio of the second halogen atoms contained in the second quantum dot or adjacent to the second quantum dot to the halogen atoms contained in the second quantum dot or adjacent to the second quantum dot is 40% or more.

4. The light-emitting element according to claim 3, wherein the halogen atoms contained in or adjacent to the first quantum dot are only the first halogen atoms, and the halogen atoms contained in or adjacent to the second quantum dot are only the second halogen atoms.

5. A light-emitting element according to any one of claims 1 to 4, wherein the first quantum dots contain the first halogen atoms, and the second quantum dots contain the second halogen atoms.

6. The light-emitting element according to claim 5, wherein the first quantum dot is adjacent to the first halogen atom, and the second quantum dot is adjacent to the second halogen atom.

7. The light-emitting element according to any one of claims 1 to 6, wherein the first halogen atom includes a fluorine atom, a chlorine atom, or a bromine atom, and the second halogen atom includes a chlorine atom, a bromine atom, or an iodine atom.

8. The concentration of halogen atoms in the light-emitting layer is 10 １６ atoms / cm ３ That's it, 10 ２０ atoms / cm ３ 8. The light-emitting device according to claim 1, wherein:

9. The light-emitting element according to any one of claims 1 to 8, wherein the light-emitting layer contains a first halide having the first halogen atom and a second halide having the second halogen atom.

10. A light-emitting element according to any one of claims 1 to 9, wherein the central wavelength of light emitted by at least one of the first quantum dots is approximately the same as the central wavelength of light emitted by at least one of the second quantum dots.

11. A light-emitting element described in any one of claims 1 to 10, wherein the first quantum dot has a first core and a first shell located around the first core, the second quantum dot has a second core and a second shell located around the second core, the first halogen atom is contained in the first shell or adjacent to the first shell, and the second halogen atom is contained in the second shell or adjacent to the second shell.

12. The light-emitting device according to claim 11, wherein the particle diameter of the first core and the particle diameter of the second core are substantially the same.

13. A light-emitting element according to claim 11 or 12, 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.

14. The light-emitting element according to any one of claims 1 to 13, wherein the light-emitting layer includes a plurality of the first quantum dots and a plurality of the second quantum dots.

15. The light-emitting element according to claim 14, 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.

16. The light-emitting element according to claim 14 or 15, wherein the light-emitting layer comprises a first light-emitting layer containing a plurality of the first quantum dots and a second light-emitting layer containing a plurality of the second quantum dots, stacked together.

17. The light-emitting device according to claim 16, wherein the first light-emitting layer is located closer to the anode than the second light-emitting layer.

18. The light-emitting device according to claim 16, wherein the first light-emitting layer is located closer to the cathode than the second light-emitting layer.

19. The light-emitting device according to any one of claims 1 to 18, further comprising an electron transport layer between the light-emitting layer and the cathode.

20. A display device comprising a light-emitting element according to any one of claims 1 to 19.

21. A display device as described in claim 20, comprising as the light-emitting elements a first light-emitting element that emits light having a first wavelength and a second light-emitting element that emits light having a second wavelength that is shorter than the first wavelength, wherein the atomic number of the first halogen atom in the first light-emitting element is larger than the atomic number of the first halogen atom in the second light-emitting element, and the atomic number of the second halogen atom in the first light-emitting element is larger than the atomic number of the second halogen atom in the second light-emitting element.

22. 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 comprises at least one first quantum dot, at least one second quantum dot, a first halogen atom, and a second halogen atom having an atomic number greater than that of the first halogen atom, the first halogen atom being contained in the first quantum dot or adjacent to the first quantum dot, and the second halogen atom being contained in the second quantum dot or adjacent to the second quantum dot.

23. The method for manufacturing a light-emitting element according to claim 22, wherein the formation of the light-emitting layer includes: preparing a quantum dot dispersion liquid in which the first quantum dots, the second quantum dots, the first halogen atoms, and the second halogen atoms are dispersed; applying the quantum dot dispersion liquid; and drying the applied quantum dot dispersion liquid.

24. A method for manufacturing a light-emitting element as described in claim 22 or 23, wherein the formation of the light-emitting layer includes: forming a first light-emitting layer containing the first quantum dots and the first halogen atoms; and forming a second light-emitting layer stacked on the first light-emitting layer and containing the second quantum dots and the second halogen atoms.

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