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

Abstract

A light-emitting element (2G) comprises: a light-emitting layer (24G) that is positioned between an anode (21G) and a cathode (26); and a hole transport layer (23G) that is adjacent to the light-emitting layer on the anode side. The light-emitting layer includes first quantum dots (41G) that have a first shell (43G). The hole transport layer includes second quantum dots (31) that have a second shell (33) that has a smaller film thickness than the first shells.

Description

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

[0001] The present disclosure relates to a light-emitting element, a display device including the light-emitting element, and a method for manufacturing the light-emitting element.

[0002] Patent Literature 1 discloses a light-emitting device that includes quantum dots as a light-emitting material in its light-emitting layer, each having a core and a shell surrounding the core. Quantum dots emit light by excitons generated by confining electrons and holes injected into the core and recombining them inside the core through the quantum effect. The shell protects the core and reduces core degradation.

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

[0004] Injection of carriers, including electrons and holes, from outside the quantum dot into the core of a quantum dot having a core and a shell as described in Patent Document 1 is achieved mainly by causing the carriers to tunnel through the shell. Therefore, if the thickness of the shell is increased in order to improve the protection effect of the core in the quantum dot described in Patent Document 1, the efficiency of carrier injection into the core decreases, which in turn reduces the luminescence efficiency of the quantum dot.

[0005] A light-emitting element according to one aspect of the present disclosure comprises an anode, a cathode facing the anode, a light-emitting layer including first quantum dots each having a first core and a first shell surrounding the first core and positioned between the anode and the cathode, a hole transport material, and a second quantum dot having a second core and a second shell surrounding the second core and having a thickness smaller than that of the first shell, the second quantum dot emitting light of approximately the same wavelength as that of light emitted by the first quantum dot, and a hole transport layer adjacent to the anode side of the light-emitting layer.

[0006] A display device according to one aspect of the present disclosure includes a substrate, a first light-emitting element on the substrate, and a second light-emitting element on the substrate that emits light of a color different from the emission color of the first light-emitting element, wherein the first light-emitting element includes a first anode, a first cathode facing the first anode, a first light-emitting layer that is located between the first anode and the first cathode, and a first quantum dot that has a first core and a first shell located around the first core and a first shell, and the first light-emitting layer is located between the first anode and the first cathode, and a first hole transport layer that includes a second quantum dot that has a first core and a second shell located around the second core and has a thickness smaller than that of the first shell, and that emits light of approximately the same wavelength as the wavelength of light emitted by the first quantum dot, and the display device includes a first hole transport layer that is adjacent to the first anode side of the first light-emitting layer.

[0007] A method for manufacturing a light-emitting element according to one aspect of the present disclosure includes forming an anode, forming a cathode facing the anode, forming a light-emitting layer between the anode and the cathode, the light-emitting layer including first quantum dots each having a first core and a first shell surrounding the first core, the light-emitting layer including a hole transport material, a second quantum dot having a second core and a second shell surrounding the first core and having a thickness smaller than that of the first shell, the second quantum dot emitting light of approximately the same wavelength as that of light emitted by the first quantum dot, and forming a hole transport layer in contact with the anode side of the light-emitting layer.

[0008] In a light-emitting element containing quantum dots in the light-emitting layer, the light-emitting efficiency is improved by improving the protection effect of the quantum dot cores and the efficiency of carrier injection into the cores, thereby achieving power saving and a longer life for a display device equipped with the light-emitting element.

[0009] Fig. 1 is a schematic side cross-sectional view of a display device according to embodiment 1. Fig. 2 is a schematic plan view of a display device according to embodiment 1. Fig. 3 is a schematic enlarged view of a side cross-section of a light-emitting element included in the display device according to embodiment 1. Fig. 4 is another schematic enlarged view of a side cross-section of a light-emitting element included in the display device according to embodiment 1. Fig. 5 is a flowchart of a manufacturing method of the display device according to embodiment 1. Fig. 6 is a schematic side cross-sectional view of a display device according to embodiment 2.

[0010] [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, even for components with different hatching, components assigned the same reference numerals have similar configurations as described above.

[0011] <Display Device> Fig. 2 is a schematic diagram of a display device 1 according to this embodiment. The display device 1 is a device that can be used, for example, as a display for a television or a smartphone. The display device 1 includes a display section DA including a plurality of sub-pixels, and a driver circuit DR that drives the plurality of sub-pixels. Each of the plurality of sub-pixels includes a light-emitting element 2 (described below) and a pixel circuit PC that drives the light-emitting element 2. The display device 1 also includes a substrate (described below) on which the pixel circuit PC is formed for each sub-pixel, and includes the light-emitting element 2 on the substrate, for example.

[0012] In particular, the display device 1 includes a red subpixel XR, a green subpixel XG, and a blue subpixel XB in a display section 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, respectively, on a substrate.

[0013] 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.

[0014] In other words, the display device 1 includes, as light-emitting elements 2, a green light-emitting element 2G as a first light-emitting element and a red light-emitting element 2R as a second light-emitting element that emits light of a color different from that of the first light-emitting element, on a substrate. Furthermore, the display device 1 includes, as light-emitting elements 2, a blue light-emitting element 2B as a third light-emitting element that emits light of a color different from both the light emitted by the first light-emitting element and the light emitted by the second light-emitting element, on a substrate. However, in this embodiment, the second light-emitting element may be the blue light-emitting element 2B, and the third light-emitting element may be the red light-emitting element 2R. In other words, one of the second light-emitting element and the third light-emitting element is the red light-emitting element 2R, and the other is the blue light-emitting element 2B. Note that, in this embodiment, blue light is light having a central emission wavelength in a wavelength band of, for example, 380 nm to 500 nm. Furthermore, green light is light having a central emission wavelength in a wavelength band of, for example, greater than 500 nm and less than 600 nm. Furthermore, red light is light having a central emission wavelength in a wavelength band of greater than 600 nm and less than 780 nm.

[0015] <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 is a schematic side cross-sectional view of the display device 1 according to this embodiment. FIG. 1 is a cross-sectional view taken along the arrow I-I line shown in FIG. 2. In particular, FIG. 1 shows a portion of a side cross-section of the display unit DA of the display device 1 according to this embodiment, taken along a plane perpendicular to a planar direction DP parallel to the upper surface of the substrate 3 (described later) and passing through the red light-emitting element 2R, the green light-emitting element 2G, and the blue light-emitting element 2B. In particular, the side cross-section shown in FIG. 1 shows a plane parallel to the stacking direction DT of the red light-emitting element 2R, the green light-emitting element 2G, and the blue light-emitting element 2B.

[0016] As shown in Fig. 1, the display device 1 according to this embodiment includes a display section DA that includes the above-described plurality of light-emitting elements 2 and a substrate 3. The display device 1 includes the light-emitting elements 2 on the substrate 3, and as shown in Fig. 2 in particular, the display device 1 includes a plurality of light-emitting elements 2. The display device 1 has a structure in which the layers of the light-emitting elements 2 are stacked on the substrate 3 on which, for example, TFTs (Thin Film Transistors) (not shown) are formed as pixel circuits PC. In this specification, the direction from the light-emitting elements 2 of the display device 1 to the substrate 3 is referred to as the "downward direction," and the direction opposite to the downward direction is referred to as the "upward direction."

[0017] The light-emitting element 2 includes, in order from the substrate 3 side, an anode 21, a hole injection layer 22, a hole transport layer 23, a light-emitting layer 24, an electron transport layer 25, and a cathode 26. In other words, the anode 21 and the cathode 26 face each other, and the light-emitting layer 24 is located between the anode 21 and the cathode 26. In this embodiment, the hole transport layer 23 is adjacent to the light-emitting layer 24 on the anode 21 side. In the present disclosure, "two layers adjacent to each other" means that at least parts of the two layers are in direct contact with each other, or that the two layers are adjacent to each other via a thin film of 5 nm or less.

[0018] Furthermore, the light-emitting element 2 includes a bank BK on the substrate 3. The bank BK includes an insulating resin material, such as polyimide. The bank BK partitions the anode 21, the hole injection layer 22, the hole transport layer 23, and the light-emitting layer 24 of the light-emitting element 2 into red sub-pixels XR, green sub-pixels XG, and blue sub-pixels XB, respectively, in a plan view of the substrate 3.

[0019] In particular, in this embodiment, the anode 21 is partitioned by banks BK into an anode 21R for the red subpixel XR, an anode 21G for the green subpixel XG, and an anode 21B for the blue subpixel XB. The hole injection layer 22 is also partitioned by banks BK into a hole injection layer 22R for the red subpixel XR, a hole injection layer 22G for the green subpixel XG, and a hole injection layer 22B for the blue subpixel XB. The hole transport layer 23 is also partitioned by banks BK into a hole transport layer 23R for the red subpixel XR, a hole transport layer 23G for the green subpixel XG, and a hole transport layer 23B for the blue subpixel XB. Additionally, the light-emitting layer 24 is partitioned by banks BK into a red light-emitting layer 24R for the red subpixel XR, a green light-emitting layer 24G for the green subpixel XG, and a blue light-emitting layer 24B for the blue subpixel XB.

[0020] In this embodiment, the electron transport layer 25 and the cathode 26 are formed in common for the plurality of sub-pixels described above. However, the electron transport layer 25 and the cathode 26 may also be partitioned for each sub-pixel by the bank BK.

[0021] Therefore, the green light-emitting element 2G, which is the first light-emitting element according to this embodiment, includes an anode 21G as a first anode, a hole injection layer 22G, a hole transport layer 23G as a first hole transport layer, and a green light-emitting layer 24G as a first light-emitting layer. Furthermore, the green light-emitting element 2G includes, of the electron transport layer 25 and the cathode 26, portions that overlap with the green sub-pixel XG in a plan view of the substrate 3, as the electron transport layer 25 and the cathode 26 of the green light-emitting element 2G.

[0022] The red light-emitting element 2R, which is the second light-emitting element according to this embodiment, includes an anode 21R as a second anode, a hole injection layer 22R, a hole transport layer 23R as a second hole transport layer, and a red light-emitting layer 24R as a second light-emitting layer. Furthermore, the red light-emitting element 2R includes, of the electron transport layer 25 and the cathode 26, portions that overlap with the red sub-pixel XR in a plan view of the substrate 3, as the electron transport layer 25 and the cathode 26 of the red light-emitting element 2R.

[0023] The blue light-emitting element 2B, which is the third light-emitting element according to this embodiment, includes an anode 21B as a third anode, a hole injection layer 22B, a hole transport layer 23B as a third hole transport layer, and a blue light-emitting layer 24B as a third light-emitting layer. Furthermore, the blue light-emitting element 2B includes, of the electron transport layer 25 and the cathode 26, portions that overlap with the blue sub-pixel XB in a plan view of the substrate 3, as the electron transport layer 25 and the cathode 26 of the blue light-emitting element 2B.

[0024] <Light-emitting element: anode and cathode> At least one of the anode 21 and the cathode 26 is a transparent electrode that transmits visible light. Examples of the transparent electrode include ITO, InZnO, and SnO. ２ Alternatively, FTO or the like may be used. Either the anode 21 or the cathode 26 may be a reflective electrode. The reflective electrode may contain a metal material that has a high reflectivity for visible light, and the metal material may be, for example, Al, Ag, Cu, or Au alone or an alloy of these.

[0025] <Light-emitting element: hole injection layer> The hole injection layer 22 is a layer that injects holes from the anode 21 into the light-emitting layer 24. As a material for the hole injection layer 22, an organic or inorganic material having a hole transport property that has been conventionally used in light-emitting elements including quantum dots can be used. The hole injection layer 22 is made of nanoparticles of nickel oxide (NiO), [2-(9H-carbazol-9-yl)ethyl]phosphonic acid (2PACz), [2-(3,6-dimethoxy-9H-carbazol-9-yl)ethyl]phosphonic acid (MeO-2PACz), [2-(3,6-dimethyl-9H-carbazol-9-yl)ethyl]phosphonic acid (Me-2PACz), [2-(3,6-dichloro-9H-carbazol-9-yl)ethyl]phosphonic acid (Cl-2PACz), [2-(3,6-dibromo-9H-carbazol-9-yl)ethyl]phosphonic acid (Br-2PACz), The phosphonic acid may comprise a self-assembled monolayer such as [4-(9H-carbazol-9-yl)butyl]phosphonic acid (4PACz), [4-(3,6-dimethoxy-9H-carbazol-9-yl)butyl]phosphonic acid (MeO-4PACz), [4-(3,6-dimethyl-9H-carbazol-9-yl)butyl]phosphonic acid (Me-4PACz), [4-(3,6-dichloro-9H-carbazol-9-yl)butyl]phosphonic acid (Cl-4PACz), or [4-(3,6-dibromo-9H-carbazol-9-yl)butyl]phosphonic acid (Br-4PACz). Other examples of the material for the hole injection layer 22 include a composite of poly(3,4-ethylenedioxythiophene) (PEDOT) and polystyrene sulfonic acid (PSS) (abbreviated as "PEDOT:PSS"), CuSCN (copper thiocyanate), and molybdenum oxide (MoO ３ ), tungsten oxide (WO ３ ), vanadium oxide (V ２ O ５ ), cuprous oxide (Cu ２ Examples of the material include nickel oxide (NiO), cupric oxide (CuO), etc. Furthermore, the hole injection layer 22 may contain bulk NiO (nickel oxide) instead of nanoparticles. Note that these materials may be used alone or in combination of two or more.

[0026] <Light-emitting element: hole transport layer: green quantum dots> The hole transport layer 23 is a layer that transports holes injected from the anode 21 into the hole injection layer 22 toward the light-emitting layer 24. In this embodiment, the hole transport layer 23G of the hole transport layer 23 includes a plurality of quantum dots 31 as second quantum dots. The quantum dots 31 have a core / shell structure including a core 32 as a second core and at least one shell 33 as a second shell located around the core 32. In this disclosure, the term "surrounding the core" refers to a region that is in contact with the core but excludes the core. The shell 33 may have a plurality of layers extending from the center of the core 32 to the periphery.

[0027] The quantum dots 31 in the hole transport layer 23 have a function of transporting holes injected from the anode 21 to the green light-emitting layer 24G by hopping conduction or the like. The shells 33 of the quantum dots 31 may have a function of protecting the cores 32, for example, by compensating for defects in the cores 32.

[0028] In the present disclosure, "quantum dot" refers to a dot having a maximum width of 100 nm or less. For example, the shape of the quantum dots 31 is not particularly limited as long as it satisfies the above-mentioned maximum width, and is not limited to a spherical three-dimensional shape (circular cross-sectional shape). The shape of the quantum dots 31 may be, for example, a polygonal cross-sectional shape, a rod-like three-dimensional shape, a branch-like three-dimensional shape, a three-dimensional shape with an uneven surface, or a combination thereof.

[0029] The quantum dots 31 are typically made of a semiconductor. The semiconductor preferably has a certain band gap. The semiconductor is a material capable of emitting light and may include, for example, at least one of the materials described below. The semiconductor may include, for example, at least one selected from the group consisting of a II-VI compound, a III-V compound, a chalcogenide, and a perovskite compound. Note that a II-VI compound refers to a compound containing a II element and a VI element, and a III-V compound refers to a compound containing a III element and a V element. Furthermore, a II element may include a group 2 element and a group 12 element, a group III element may include a group 3 element and a group 13 element, a group V element may include a group 5 element and a group 15 element, and a group VI element may include a group 6 element and a group 16 element.

[0030] The II-VI compound includes, for example, at least one selected from the group consisting of MgS, MgSe, MgTe, CaS, CaSe, CaTe, SrS, SrSe, SrTe, BaS, BaSe, BaTe, ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, HgSe, and HgTe.

[0031] The III-V compound includes, for example, at least one selected from the group consisting of GaN, GaAs, GaP, GaSb, InN, InAs, InP, and InSb. In particular, the quantum dots 31 may typically include InP.

[0032] Chalcogenides are compounds containing a Group VI A(16) element, such as CdS or CdSe. Chalcogenides may also include mixed crystals thereof.

[0033] Perovskite compounds are, for example, compounds of the general formula CsPbX ３ , CsSnX ３ , C.H. ３ NH ３ PbX ３ , or C.H. ３ NH ３ SnX ３ The constituent element X includes at least one element selected from the group consisting of Cl, Br, and I, for example.

[0034] Here, the numbering of element groups using Roman numerals is based on the old IUPAC (International Union of Pure and Applied Chemistry) system or the old CAS (Chemical Abstracts Service) system, and the numbering of element groups using Arabic numerals is based on the current IUPAC system.

[0035] From the viewpoint of reducing aggregation of the quantum dots 31 in the hole transport layer 23G, the hole transport layer 23G may contain an organic ligand that coordinates to the surface of the quantum dots 31. In the present disclosure, when a cross section of a layer containing quantum dots is observed and it is confirmed that the distance between the quantum dots and the ligand is 2 nm or less, it may be considered that the ligand is coordinated to the quantum dot. Alternatively, when it is confirmed that the quantum dots and the ligand are in contact with each other, it may be considered that the ligand is coordinated to the quantum dot.

[0036] In the present embodiment, an example has been described in which the hole transport layer 23G of the green light-emitting element 2G contains quantum dots 31, but this is not limiting. For example, the hole transport layer of a light-emitting element that emits light of another color, including at least one of the hole transport layer 23R of the red light-emitting element 2R and the hole transport layer 23B of the blue light-emitting element 2B, may also contain quantum dots 31.

[0037] <Light-Emitting Element: Hole Transport Layer: Hole Transport Material> The hole transport layer 23G includes a hole transport material 34G having hole transport properties as a first hole transport material. The hole transport material 34G may be an organic or inorganic material having hole transport properties that has been conventionally used in light-emitting elements including quantum dots. Examples of the hole transport material 34G include poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4'-(N-4-sec-butylphenyl))diphenylamine)] (abbreviated as "TFB"), poly[N,N'-bis(4-butylphenyl)-N,N'-bis(phenyl)benzidine] (abbreviated as "poly-TPD"), polyvinylcarbazole (abbreviated as "PVK"), and poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (abbreviated as "PTAA"). The hole transport layer 23G may be formed around the quantum dots 31, in other words, in contact with the quantum dots 31, while the hole transport layer 23G may have a hole transport material 34G in the region excluding the quantum dots 31.

[0038] Furthermore, the hole transport layer 23R and the hole transport layer 23B contain a hole transport material 34R and a hole transport material 34B as the second and third hole transport materials, respectively. The hole transport material 34R and the hole transport material 34B may contain the same material as the hole transport material 34G. In particular, the hole transport material 34G, the hole transport material 34R, and the hole transport material 34B may contain the same material. This simplifies or reduces the cost of the process for forming the hole transport layer 23 in the display device 1, or simplifies the design of each light-emitting element by standardizing the profile of the hole transport layer 23 of each light-emitting element.

[0039] On the other hand, when hole transport layer 23G includes quantum dots 31 as second quantum dots, hole transport layer 23R and hole transport layer 23B may not include quantum dots such as quantum dots 31. In the present disclosure, "not including quantum dots" means not including a component that meets the above-described definition of "quantum dot," and does not limit the inclusion of, for example, other semiconductors. Hole transport layer 23R or hole transport layer 23B that does not include quantum dots simplifies the formation process or reduces the cost of formation.

[0040] <Light-emitting element: electron transport layer> The electron transport layer 25 is a layer that transports electrons injected from the cathode 26 to the light-emitting layer 24. The electron transport layer 25 according to this embodiment may have nanoparticles as an electron transport material. The electron transport layer 25 may also contain a ligand that can be coordinated to the nanoparticles. For example, the nanoparticles may be zinc oxide (ZnO), zinc oxide (ZnO) doped with at least one of Li, Mg, Al, Ti, Ga, and Zr, or titanium oxide (TiO ２ ), or zirconium oxide (ZrO ２ ) nanoparticles. In the present disclosure, the chemical formulas are representative examples. In addition, in the present disclosure, the composition ratios described in the chemical formulas do not necessarily have to be stoichiometric, in which the composition of the actual compound is the same as the chemical formula.

[0041] The electron transport material contained in the electron transport layer 25 is not limited to nanoparticles. For example, the electron transport layer 25 may use, as the electron transport material, an organic or inorganic material having electron transport properties that has been conventionally employed in light-emitting devices containing quantum dots. The electron transport material may include, for example, 2,2',2"-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (abbreviated as "TPBi"), 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (abbreviated as "BCP"), 4,7-diphenyl-1,10-phenanthroline (abbreviated as "Bphen"), or the like. Alternatively, the electron transport layer 25 may use, as the electron transport material, bulk zinc oxide (ZnO) that is not nanoparticles, zinc oxide (ZnO), titanium oxide (TiO ２ ), or zirconium oxide (ZrO ２ The bulk zinc oxide (ZnO) may be doped with at least one of Li, Mg, Al, Ti, Ga, and Zr. The electron transport material may contain only one of the above-mentioned materials, or may contain two or more of them as appropriate.

[0042] <Light-emitting element: light-emitting layer> The light-emitting layer 24 includes quantum dots into which holes are injected from the anode 21 via the hole injection layer 22 and the hole transport layer 23, and electrons are injected from the cathode 26 via the electron transport layer 25. In this embodiment, the quantum dots included in the light-emitting layer 24 have a core that emits light upon recombination of the holes and electrons injected into the quantum dot, and a shell located around the core.

[0043] The green light-emitting layer 24G includes, as first quantum dots, green quantum dots 41G that emit green light. The green quantum dots 41G have a core 42G as a first core and a shell 43G as a first shell located around the core 42G.

[0044] The red light-emitting layer 24R includes, as a third quantum dot, a red quantum dot 41R that emits red light. The red quantum dot 41R may have a core 42R and a shell 43R surrounding the core 42R. The blue light-emitting layer 24B includes, as a fourth quantum dot, a blue quantum dot 41B that emits blue light. The blue quantum dot 41B may have a core 42B and a shell 43B surrounding the core 42B.

[0045] The green quantum dot 41G may have the same configuration as the quantum dot 31, except that the thickness of the shell 43G is greater than the thickness of the shell 33, as described below. Therefore, the quantum dot 31 and the green quantum dot 41G may emit green light, which is light of approximately the same wavelength. In the present disclosure, "the wavelengths of the two lights are approximately the same" means that the difference in the center wavelengths of the two lights is 30 nm or less. The wavelength of the light emitted by the quantum dot 31 can be obtained, for example, by irradiating the quantum dot 31 with ultraviolet light or the like to photoexcite the quantum dot 31 and confirming the wavelength of the light obtained.

[0046] The red quantum dots 41R emit red light, and the blue quantum dots 41B emit blue light. The thicknesses of the shells 43R and 43B may be smaller than the thickness of the shells 43G. Furthermore, from the viewpoint of improving the efficiency of carrier injection into the cores of the red quantum dots 41R and the blue quantum dots 41B, the thicknesses of the shells 43R and 43B may be larger than the thickness of the shell 33G.

[0047] Generally, the wavelength of light emitted by a quantum dot having a core / shell structure depends on the particle size of the core, and in particular, the smaller the particle size of the core, the shorter the wavelength of light emitted by the quantum dot. Therefore, the particle size of core 42G may be approximately the same as that of core 32, the particle size of core 42R may be larger than that of core 32, and the particle size of core 42B may be smaller than that of core 32.

[0048] The materials of the green quantum dots 41G, the red quantum dots 41R, and the blue quantum dots 41B may include the same materials as those described above that may be included in the quantum dots 31. In particular, the core of the green quantum dot 41G may typically include InP.

[0049] <Interface between Green Light-Emitting Layer and Hole Transport Layer> The hole transport layer 23G and the green light-emitting layer 24G according to this embodiment will be described in more detail with reference to Fig. 3 and Fig. 4. Fig. 3 and Fig. 4 are schematic enlarged views showing the vicinity of the interface between the hole transport layer 23G and the green light-emitting layer 24G in the side cross section of the green light-emitting element 2G of the display device 1, particularly enlarged views of the region E shown in Fig. 1.

[0050] Fig. 3 shows the vicinity of the boundary surface BP between the hole transport layer 23G and the green light-emitting layer 24G when the boundary surface BP is substantially parallel to a planar direction DP that is parallel to the upper surface of the substrate 3, and Fig. 4 shows the vicinity of the boundary surface BP when the boundary surface BP is not flat. Figs. 3 and 4 show the case where the hole transport layer 23G and the green light-emitting layer 24G are in contact with each other.

[0051] As shown in Figures 3 and 4, the quantum dots 31 in the hole transport layer 23G and the green quantum dots 41G in the green light-emitting layer 24G are also located near the boundary plane BP. Here, as shown in Figures 3 and 4, some of the quantum dots 31 may be located in both the hole transport layer 23G and the green light-emitting layer 24G. In other words, in a cross section along the stacking direction DT, at least one quantum dot 31 intersects with the boundary plane BP. In Figures 3 and 4, the portion of the boundary plane BP that intersects with the quantum dot 31 is indicated by a dotted line. In the above cross section, the boundary plane BP may intersect only with the shell 33 of the quantum dot 31, or may also intersect with the core 32.

[0052] When a thin film is located between the hole transport layer 23G and the green light-emitting layer 24G, the quantum dots 31 may intersect the boundary between the thin film and the hole transport layer 23G in the cross section. In other words, in the cross section along the stacking direction DT, at least one quantum dot 31 may intersect the boundary between the hole transport layer 23G and a layer with which the hole transport layer 23G is in contact on the side of the green light-emitting layer 24G. Furthermore, in the cross section, the quantum dots 31 may also intersect the boundary between the thin film and the green light-emitting layer 24G.

[0053] The position of the boundary surface BP may be confirmed by observing the cross section and confirming the concentration of the material at each position on the cross section. A means for confirming the concentration of the material at each position as described above is, for example, performing EDX (energy dispersive X-ray spectroscopy) on the cross section using a TEM (transmission electron microscope).

[0054] For example, in the above cross section, the portion where the concentration of the material of the hole transport layer 23, particularly the material of the quantum dots 31, is 80% or more is regarded as the hole transport layer 23G, and the portion where the concentration is less than 80% is regarded as the green light-emitting layer 24G. Thus, in this embodiment, the boundary between the hole transport layer 23G and the green light-emitting layer 24G may be defined as the interface BP. Alternatively, the portion of the cross section where the concentration of the material of the quantum dots 31 decreases by 80% or more and is located on the cathode 26 side may be defined as the interface BP between the hole transport layer 23G and the green light-emitting layer 24G.

[0055] Meanwhile, in the cross section, the portion where the concentration of the material of the green light-emitting layer 24G, particularly the material of the green quantum dots 41G, is 80% or more is regarded as the green light-emitting layer 24G, and the portion where the concentration is less than 80% is regarded as the hole transport layer 23G. Thus, in this embodiment, the boundary between the hole transport layer 23G and the green light-emitting layer 24G may be referred to as the interface BP. Alternatively, the portion of the cross section where the concentration of the material of the green quantum dots 41G is reduced by 80% or more and located on the anode 21G side may be referred to as the interface BP between the hole transport layer 23G and the green light-emitting layer 24G.

[0056] 3 and 4, the shell 43G has a thickness T1, and the shell 33 has a thickness T2. The thickness T1 is greater than the thickness T2. The thickness T1 may be 4 nm or more, and the thickness T2 may be 2 nm or less.

[0057] Since the thickness T1 is greater than the thickness T2, the green light emitting element 2G can more firmly protect the cores 42G of the green quantum dots 41G contained in the green light emitting layer 24G, thereby improving the reliability of the green light emitting layer 24G and achieving improved luminous efficiency or longer life.

[0058] Here, since the hole transport layer 23G contains the quantum dots 31, the green light emitting element 2G realizes the transport of holes from the hole transport layer 23G to the green light emitting layer 24G mainly by hopping conduction via the quantum dots 31.

[0059] In the green light-emitting element 2G, the thickness T1 is greater than the thickness T2, which increases the difference in ionization potential between the hole transport material 34G contained in the hole transport layer 23G and the green quantum dots 41G. For the same reason, in the green light-emitting element 2G, the band bending between the hole transport layer 23G and the green light-emitting layer 24G increases the difference in ionization potential between the hole transport layer 23G and the green light-emitting layer 24G at the boundary between the hole transport layer 23G and the green light-emitting layer 24G. Furthermore, the large thickness T1 reduces the probability of holes tunneling from the outside of the green quantum dots 41G to the cores 42G.

[0060] However, as described above, the transport of holes from the hole transport layer 23G to the green light-emitting layer 24G is achieved mainly by hopping conduction via the quantum dots 31. Therefore, in the green light-emitting element 2G, a decrease in hole injection from the hole transport layer 23G to the green light-emitting layer 24G that occurs due to an increase in the difference in ionization potential between the material of the hole transport layer 23G and the material of the green light-emitting layer 24G can be suppressed. For the same reason, in the green light-emitting element 2G, a decrease in the efficiency of hole injection into the green quantum dot 41G that occurs due to a decrease in the probability of hole tunneling from the outside of the green quantum dot 41G to the core 42G can be suppressed.

[0061] Therefore, the green light-emitting element 2G improves the reliability of the green light-emitting layer 24G while improving the efficiency of hole injection from the hole transport layer 23G to the green quantum dots 41G. Therefore, the green light-emitting element 2G achieves a long life while improving luminous efficiency. The display device 1 including the green light-emitting element 2G achieves power saving and a long life.

[0062] In this embodiment, when the quantum dots 31 emit light of the same wavelength as the green quantum dots 41G, the green light-emitting element 2G can reduce the effects of, for example, a decrease in chromaticity caused by the light emission of the quantum dots 31 during operation. Furthermore, when the quantum dots 31 emit light of the same wavelength as the green quantum dots 41G, the difference in ionization potential between the quantum dots 31 and the green quantum dots 41G may be reduced. In this case, the green light-emitting element 2G improves the efficiency of hole injection from the hole transport layer 23G to the green light-emitting layer 24G by hopping conduction via the quantum dots 31. In particular, the shell 33 of the quantum dots 31 and the shell 43G of the green quantum dots 41G may contain the same material. This reduces the difference in ionization potential between the quantum dots 31 and the green quantum dots 41G, thereby improving the efficiency of hole transport from the hole transport layer 23G to the green light-emitting layer 24G by hopping conduction via the quantum dots 31.

[0063] However, in the green light-emitting element 2G, holes injected into the hole transport layer 23G are injected into the green light-emitting layer 24G mainly by hopping conduction via the quantum dots 31. Also, in the green light-emitting element 2G, electrons transported from the cathode 26 toward the hole transport layer 23G are consumed for recombination with holes in the cores 42G of the green quantum dots 41G in the green light-emitting layer 24G, and therefore electrons are less likely to be injected into the hole transport layer 23G. Therefore, the green light-emitting element 2G reduces light emission from the quantum dots 31 contained in the hole transport layer 23G. Therefore, the green light-emitting element 2G may include quantum dots 31 in the hole transport layer 23G that emit light of a different color from the color of light emitted by the green quantum dots 41G.

[0064] Because the thickness T2 is smaller than the thickness T1, the hole confinement effect inside the shell 33 of the quantum dot 31 is reduced. As a result, the band gap of the shell 33 of the quantum dot 31 is larger than the band gap of the shell 43G of the green quantum dot 41G. Accordingly, the ionization potential of the shell 33 of the quantum dot 31 is larger than the ionization potential of the shell 43G of the green quantum dot 41G. Therefore, the green light-emitting element 2G further reduces the barrier to hole injection from the shell 33 of the quantum dot 31 to the shell 43G of the green quantum dot 41G.

[0065] The film thickness of each of the shells 33 and 43G may be confirmed, for example, by observing samples of the quantum dots 31 in the hole transport layer 23G and the green quantum dots 41G in the green light-emitting layer 24G and confirming the distribution or proportion of materials in the samples.

[0066] In the above confirmation, for example, samples of the quantum dots 31 and the green quantum dots 41G may be obtained by dissolving the hole transport layer 23G and the green light-emitting layer 24G in appropriate solvents. The samples may be dropped onto a grid of a scanning transmission electron microscope (STEM) and observed using the STEM. Alternatively, cross sections passing through the hole transport layer 23G and the green light-emitting layer 24G may be exposed to prepare samples, and EDX may be performed on the cross sections using a TEM.

[0067] As a result of the above, for example, the material distributed near the center of each of the quantum dots 31 and the green quantum dots 41G in the sample may be identified as the core material. Next, the elements present in the range excluding the core may be identified for each of the quantum dots 31 and the green quantum dots 41G. Next, the range excluding the identified core may be regarded as the shell, and the film thickness of each of the shells 33 and 43G may be determined from the thickness of the shell.

[0068] Alternatively, the total elemental ratio of the core material to the shell material located in the area excluding the core may be calculated for each of the quantum dots 31 and green quantum dots 41G in the sample. The volume ratio of the core to the shell may then be calculated from the total elemental ratio of the core material and the total elemental ratio of the shell material. The thickness of the shell may then be calculated from the volume ratio, assuming that each of the quantum dots 31 and green quantum dots 41G in the sample has a spherical or cubic core / shell structure.

[0069] Here, the core and shell may share some common elements. In this case, the core material may be identified by identifying the elements distributed throughout each quantum dot and the elements distributed only near the center of each quantum dot. For example, if Se is distributed only near the center of the quantum dot and Zn is distributed throughout the quantum dot, the quantum dot may be identified as containing ZnSe as its core material.

[0070] <Summary> The thickness T1 of the shell 43G of the green quantum dot 41G contained in the green light-emitting layer 24G is greater than the thickness T2 of the shell 33 of the quantum dot 31 contained in the hole-transport layer 23G. Therefore, in the green light-emitting element 2G, the core 42G of the green quantum dot 41G can be protected more strongly than the core 32 of the quantum dot 31. Protection of the core 42G of the green quantum dot 41G, which contributes more to the emission of the green light-emitting element 2G than the core 32 of the quantum dot 31, contributes more strongly to improving the reliability of the green light-emitting element 2G.

[0071] Furthermore, both the hole transport layer 23G and the green light-emitting layer 24G contain quantum dots with a core / shell structure. Therefore, in the green light-emitting element 2G, holes injected into the quantum dots 31 in the hole transport layer 23G can be injected into the green quantum dots 41G in the green light-emitting layer 24G adjacent to the hole transport layer 23G by hopping conduction via the shells 33 and 43G. Therefore, in the green light-emitting element 2G, the quantum dots 31 contained in the hole transport layer 23G can suppress a reduction in the efficiency of hole injection from the outside of the green quantum dots 41G to the cores 42G, which would be caused by the shells 43G having a thickness T1 greater than the thickness T2.

[0072] Furthermore, the shell 33 of the quantum dots 31 has a thickness T2 that is smaller than the thickness T1. Therefore, in the green light-emitting element 2G, the quantum dots 31 contained in the hole-transport layer 23G function as a material that transports holes from the hole-transport layer 23G to the green light-emitting layer 24G by hopping conduction. Therefore, the green light-emitting element 2G including the hole-transport layer 23G improves the efficiency of hole transport to the green light-emitting layer 24G via the quantum dots 31 in the hole-transport layer 23G. Furthermore, because the quantum dots 31 contained in the hole-transport layer 23G do not contribute to light emission that could cause deterioration of the quantum dots 31, deterioration of the quantum dots 31 is reduced even when the thickness of the shell 33 is small. Furthermore, because the thickness T1 is greater than the thickness T2, the shell 43G more strongly protects the cores 42G and reduces deterioration of the green quantum dots 41G, thereby improving the luminous efficiency of the green quantum dots 41G in the green light-emitting element 2G.

[0073] As described above, the green light emitting element 2G improves the reliability of the green light emitting layer 24G and also improves the light emitting efficiency, and the display device 1 including the green light emitting element 2G achieves power saving and a longer life.

[0074] The quantum dots 31 and the green quantum dots 41G may emit green light having substantially the same wavelength. In this case, even when light is emitted from the quantum dots 31, the green light-emitting element 2G can extract and utilize the light generated in the quantum dots 31 while suppressing a decrease in chromaticity. Therefore, the green light-emitting element 2G can utilize electrons that have flowed out from the green light-emitting layer 24G to the hole transport layer 23G for light emission in the quantum dots 31. Furthermore, when the quantum dots 31 and the green quantum dots 41G emit light having substantially the same wavelength, the efficiency of hole injection from the hole transport layer 23G to the green light-emitting layer 24G due to hopping conduction via the quantum dots 31 is improved. This further improves the efficiency of hole injection from the hole transport layer 23G to the green light-emitting layer 24G in the green light-emitting element 2G.

[0075] The shell 43G, having a thickness T1 greater than the thickness of the shell 43R of the red quantum dot 41R and the thickness of the shell 43B of the blue quantum dot 41B, provides more robust protection for the core 42G. Generally, InP is used for the core of a quantum dot that emits green light, and InP tends to deteriorate more easily than materials used for the core of a quantum dot that emits red or blue light. Therefore, by positioning the shell 43G having a thickness T1 around the core 42G of the green quantum dot 41G, which tends to be made of a material that deteriorates easily, the green light-emitting element 2G can reduce deterioration of the green quantum dot 41G. By including the green light-emitting element 2G according to this embodiment, the display device 1 can reduce deterioration of green light-emitting elements, which generally tend to have low reliability, thereby improving display quality or achieving a longer lifespan.

[0076] The hole transport layer 23R and the hole transport layer 23B do not contain quantum dots. Therefore, the configurations of the hole transport layer 23R and the hole transport layer 23B are simplified in the red light-emitting element 2R and the blue light-emitting element 2B. Furthermore, the red light-emitting element 2R and the blue light-emitting element 2B suppress a decrease in chromaticity due to the emission of quantum dots contained in the hole transport layer 23R and the hole transport layer 23B. In this case, the film thickness of the shell 43R of the red quantum dot 41R and the film thickness of the shell 43B of the blue quantum dot 41B may be smaller than the film thickness of the shell 43G of the green quantum dot 41G. This improves the efficiency of hole injection into the red quantum dots 41R and the blue quantum dots 41B, respectively, without relying on hopping conduction via the quantum dots contained in each hole transport layer.

[0077] <Additional Note> In this embodiment, the hole transport layer 23G may be in contact with the anode 21G side of the green light-emitting layer 24G. In this case, the quantum dots 31 in the hole transport layer 23G and the green quantum dots 41G in the green light-emitting layer 24G are closer to each other. With this configuration, the green light-emitting element 2G further improves the efficiency of hole injection from the quantum dots 31 to the green quantum dots 41G via hopping conduction from the shell 33 to the shell 43G.

[0078] When the hole transport layer 23G includes quantum dots 31 that intersect with the boundary plane BP in a cross section along the stacking direction DT, the quantum dots 31 are closer to the green quantum dots 41G in the green light-emitting layer 24G. With the above configuration, the green light-emitting element 2G further improves the efficiency of hole injection from the quantum dots 31 to the green quantum dots 41G via hopping conduction from the shell 33 to the shell 43G.

[0079] The probability that carriers injected into the core of a quantum dot with a core / shell structure will escape to the outside of the quantum dot is determined by the wave function of the carriers. This wave function also depends on the thickness of the shell and the band level. In particular, the probability that carriers in the core will escape to the outside of the quantum dot decreases exponentially with increasing shell thickness.

[0080] Generally, when the shell thickness of a quantum dot having a core / shell structure is 3 nm or more, carriers injected into the core hardly escape to the outside of the quantum dot. Furthermore, in quantum dots having a core / shell structure that are used as light-emitting materials, the shell thickness is generally about 2 nm or more but less than 3 nm, from the viewpoint of facilitating the injection of carriers into the core from the outside of the quantum dot by tunneling through the shell.

[0081] The shell 43G of the green quantum dot 41G has a thickness T1 of 4 nm or more, which is at least twice the lower limit of the shell thickness of the commonly used quantum dots described above and exceeds the upper limit of 3 nm. Therefore, a green light-emitting element 2G having a green light-emitting layer 24G including the green quantum dots 41G reduces the outflow of carriers injected into the cores 42G to the outside of the green quantum dots 41G. Therefore, the green light-emitting element 2G further improves the probability of recombination of electrons and holes in the green quantum dots 41G. Furthermore, the shell 43G having a thickness T1 of 4 nm or more further protects the cores 42G and further reduces degradation of the green quantum dots 41G. From the viewpoint of suppressing a decrease in the efficiency of carrier injection into the green quantum dots 41G, the thickness T1 of the shell 43G may be 8 nm or less.

[0082] On the other hand, the shell 33 of the quantum dot 31 has a thickness T2 that is equal to or less than the lower limit of the thickness of the shell of a quantum dot generally used as described above. Therefore, the green light-emitting element 2G including the hole transport layer 23G including the quantum dots 31 allows the injected holes to more efficiently flow out of the quantum dots 31 and be injected into the green light-emitting layer 24G.

[0083] The hole transport layer 23G may contain 1 atm % or more of halogen atoms. In this disclosure, "atom" does not necessarily mean the existence of a single atom. In this disclosure, "atom" also includes atoms that exist in the form of a molecule containing two or more atoms, including the atom and another atom, atoms that exist in the form of a complex, atoms that exist in the form of a compound, or atoms that exist in the form of an ion. In this disclosure, "atom" does not limit the form of existence of other atoms. That is, a halogen atom includes atoms that exist in the form of a compound containing a halogen atom and atoms that exist in the form of a halogen ion. Regardless of the form of halogen atoms, if their presence in a substance can be identified by analysis, the substance may be considered to contain halogen atoms.

[0084] When the hole transport layer 23G contains 1 atm % or more of halogen atoms, the energy of the highest part of the valence band of the quantum dots 31 is reduced by approximately 0.1 eV. In other words, when the hole transport layer 23G contains 1 atm % or more of halogen atoms, the ionization potential of the quantum dots 31 is increased by approximately 0.1 eV. Therefore, with the above configuration, the green light-emitting element 2G reduces the difference in ionization potential between the hole transport layer 23G and the green light-emitting layer 24G, thereby improving the efficiency of hole injection into the green quantum dots 41G in the green light-emitting layer 24G via the quantum dots 31 in the hole transport layer 23G. Note that, from the viewpoint of reducing the outflow of electrons from the green light-emitting layer 24G to the hole transport layer 23G side via the halogen atoms, the halogen atoms in the hole transport layer 23G may be 10 atm % or less.

[0085] <Manufacturing Method> A manufacturing method for the display device 1 according to this embodiment will be described with reference to Fig. 5. Fig. 5 is a flowchart of the manufacturing method for the display device 1 according to this embodiment.

[0086] 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.

[0087] Next, the anode 21 is formed on the substrate 3 (step S2). The anode 21 may be formed by forming a thin conductive film common to a plurality of sub-pixels by the above-described method, and then patterning the thin film for each sub-pixel.

[0088] Next, a bank BK is formed on the substrate 3 (step S3). The bank BK may be formed by applying an insulating resin material containing, for example, polyimide onto the substrate 3 and patterning the resin material. In this case, the resin material may contain, for example, a photosensitive resin, and the patterning of the resin material may be performed by photolithography.

[0089] Next, the hole injection layer 22 is formed on the anode 21 by the method described above (step S4). The hole injection layer 22 may be formed in common for a plurality of subpixels, or may be patterned for each subpixel. In particular, in step S4 according to this embodiment, the hole injection layer 22 may be formed by forming a thin film containing a polymer organic monomer by coating or the like, and polymerizing or crosslinking the monomer by heat-treating the thin film.

[0090] Next, the hole transport layer 23 is formed on the hole injection layer 22 by the method described above (step S5). The hole transport layer 23 may be formed by repeatedly applying a dispersion liquid containing a hole transport material, heating it, and patterning it for each emission color of the subpixel. The patterning of the hole transport material may be performed by a lift-off method in which a photosensitive resin is applied and patterned in advance, a resin layer is formed in areas other than the position where the resin will be formed, a thin film of the hole transport material is formed, and then the resin layer is removed.

[0091] However, in this embodiment, at least one subpixel, particularly the green subpixel XG, is provided with a hole transport layer 23G containing quantum dots 31 and a hole transport material 34G. The quantum dots 31 may be synthesized by a conventionally known method as long as the shell 33 has a thickness T2. The hole transport layer 23G may be formed by applying, heating, and patterning a dispersion liquid containing the hole transport material 34G mixed with the quantum dots 31.

[0092] Next, the light-emitting layer 24 is formed on the hole transport layer 23 (step S6). The light-emitting layer 24 may be formed by synthesizing quantum dots of the luminescent color corresponding to each sub-pixel, applying a dispersion liquid containing the quantum dots, heating it, and patterning it. The light-emitting layer 24 may be patterned using the lift-off method described above.

[0093] In this embodiment, at least one subpixel, particularly the green subpixel XG, has a green light-emitting layer 24G including green quantum dots 41G with a shell thickness greater than that of the quantum dots 31 contained in the hole-transport layer 23G. The green quantum dots 41G may be synthesized by the same method as the quantum dots 31, except that the time required for synthesizing the shell is increased compared to the quantum dots 31, or the ratio of the shell material added to the solution during shell synthesis is increased. In this way, green quantum dots 41G may be synthesized having a shell 43G with a thickness T1 greater than the thickness T2 of the shell 33 of the quantum dot 31.

[0094] Next, an electron transport layer 25 is formed on the light-emitting layer 24 (step S7), and further a cathode 26 is formed thereon (step S8). Steps S7 to S8 may be performed by the same method as the method for forming each layer described above.

[0095] As a result of the above, the red light-emitting element 2R, the green light-emitting element 2G, and the blue light-emitting element 2B are formed on the substrate 3, completing the manufacture of the display device 1. By using the above method, it is possible to manufacture the green light-emitting element 2G with improved reliability of the green light-emitting layer 24G and improved luminous efficiency. The display device 1 manufactured by the above method achieves power saving and a long life.

[0096] [Embodiment 2] <Light-emitting layer having halogen atoms> Figure 6 is a schematic side cross-sectional view of a display device 1 according to this embodiment, and in particular shows a side cross-section of the display device 1 at the same position as the side cross-section of the display device 1 shown in Figure 1. The display device 1 according to this embodiment has the same configuration as the display device 1 according to the previous embodiment, except for the configuration of the green light-emitting layer 24G included in the green light-emitting element 2G. The green light-emitting layer 24G according to this embodiment has the same configuration as the green light-emitting layer 24G according to the previous embodiment, except that it further includes a halide 51.

[0097] Therefore, in this embodiment, the green light-emitting element 2G also includes quantum dots 31 in the hole transport layer 23G and green quantum dots 41G in the green light-emitting layer 24G. Therefore, for the same reasons as described above, the green light-emitting element 2G according to this embodiment improves luminous efficiency and reliability. A display device 1 including this green light-emitting element 2G achieves power saving and a longer life.

[0098] The halide 51 may have, for example, a fluorine atom, a chlorine atom, a bromine atom, or an iodine atom as a halogen atom. Furthermore, the hole transport layer 23G according to this embodiment has a halogen atom. In particular, the hole transport layer 23G may contain the halide 51 as a substance having a halogen atom. Furthermore, in this embodiment, at least one of the red light-emitting layer 24R and the blue light-emitting layer 24B may contain the halide 51.

[0099] The hole transport layer 23G and the green light-emitting layer 24G may contain halogen atoms of the same element. In this case, the concentration of halogen atoms in the hole transport layer 23G may be greater than the concentration of halogen atoms in the green light-emitting layer 24G. In this case, the ionization potential of the quantum dots 31 becomes greater than the ionization potential of the green quantum dots 41G, further reducing the barrier to hole injection from the quantum dots 31 to the green quantum dots 41G. In particular, from the viewpoint of further reducing the barrier to hole injection from the quantum dots 31 to the green quantum dots 41G, the concentration of halogen atoms in the hole transport layer 23G may be greater than the concentration of halogen atoms in the green light-emitting layer 24G by 1 atm % or more.

[0100] The concentration of halogen atoms in the hole transport layer 23G and the concentration of halogen atoms in the green light-emitting layer 24G may be substantially the same. The term "substantially the same" does not necessarily mean that the concentrations of halogen atoms are strictly the same, but may also include, for example, a case where the difference in the concentration of halogen atoms is 1 atm% or less. In this case, the electronegativity of the halogen atoms in the hole transport layer 23G may be greater than the electronegativity of the halogen atoms in the green light-emitting layer 24G. In this case, the ionization potential of the quantum dots 31 is greater than the ionization potential of the green quantum dots 41G, further reducing the barrier to hole injection from the quantum dots 31 to the green quantum dots 41G. For example, if the halogen atoms in the green light-emitting layer 24G are chlorine atoms, bromine atoms, or iodine atoms, the halogen atoms in the hole transport layer 23G may be fluorine atoms, chlorine atoms, or bromine atoms, respectively.

[0101] The concentration of halogen atoms in the hole transport layer 23G may be greater than the concentration of halogen atoms in the green light-emitting layer 24G, and the electronegativity of the halogen atoms in the hole transport layer 23G may be greater than the electronegativity of the halogen atoms in the green light-emitting layer 24G. In this case, the ionization potential of the quantum dots 31 becomes greater than the ionization potential of the green quantum dots 41G, further reducing the barrier to hole injection from the quantum dots 31 to the green quantum dots 41G.

[0102] The display device 1 according to this embodiment may be manufactured by the same method as the method for manufacturing the display device 1 according to the previous embodiment. In particular, in this embodiment, a substance having a halogen atom, such as halide 51, may be added to the material in each of the steps of forming the hole transport layer 23G and the green light-emitting layer 24G.

[0103] 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.

[0104] DESCRIPTION OF SYMBOLS 1 Display device 2 Light-emitting element 2G Green light-emitting element (first light-emitting element) 2R Red light-emitting element (second light-emitting element) 2B Blue light-emitting element (third light-emitting element) 3 Substrate 21 Anode 23G Hole transport layer (first hole transport layer) 23R Hole transport layer (second hole transport layer) 23B Hole transport layer (third hole transport layer) 24G Green light-emitting layer (first light-emitting layer) 24R Red light-emitting layer (second light-emitting layer) 24B Blue light-emitting layer (third light-emitting layer) 26 Cathode 31 Quantum dot (second quantum dot) 32 Core (second core) 33 Shell (second shell) 34G Hole transport material (first hole transport material) 34R Hole transport material (second hole transport material) 34B Hole transport material (third hole transport material) 41G Green quantum dot (first quantum dot) 41R Red quantum dot (second quantum dot) 41B Blue quantum dot (third quantum dot) 42G Core (first core) 43G Shell (first shell)

Claims

1. A light-emitting device comprising: an anode; a cathode facing the anode; a light-emitting layer located between the anode and the cathode, the light-emitting layer including first quantum dots each having a first core and a first shell surrounding the first core; and a hole-transporting layer adjacent to the anode side of the light-emitting layer, the hole-transporting layer including second quantum dots each having a hole-transporting material and a second shell surrounding the second core and having a thickness smaller than that of the first shell.

2. The light-emitting device according to claim 1, wherein the second quantum dots emit light having substantially the same wavelength as the wavelength of light emitted by the first quantum dots.

3. The light-emitting device according to claim 1 or 2, wherein the hole transport layer is in contact with the anode side of the light-emitting layer.

4. A light-emitting element according to any one of claims 1 to 3, wherein, in a cross section along the stacking direction, at least one of the second quantum dots intersects with the boundary surface between the hole transport layer and a layer that contacts the hole transport layer on the side of the light-emitting layer.

5. The light-emitting element according to any one of claims 1 to 4, wherein the film thickness of the first shell is 4 nm or more.

6. The light-emitting element according to any one of claims 1 to 5, wherein the second shell has a thickness of 2 nm or less.

7. The light-emitting device according to any one of claims 1 to 6, wherein the hole transport layer contains 1 atm % or more of halogen atoms.

8. The light-emitting device according to claim 7, wherein the light-emitting layer contains halogen atoms of the same element as those contained in the hole-transporting layer, and the concentration of halogen atoms in the hole-transporting layer is greater than the concentration of halogen atoms in the light-emitting layer.

9. The light-emitting device according to claim 7, wherein the concentration of halogen atoms in the light-emitting layer and the concentration of halogen atoms in the hole transport layer are substantially the same, and the electronegativity of the halogen atoms in the hole transport layer is greater than the electronegativity of the halogen atoms in the light-emitting layer.

10. The light-emitting device according to claim 7, wherein the concentration of halogen atoms in the hole transport layer is greater than the concentration of halogen atoms in the light-emitting layer, and the electronegativity of the halogen atoms in the hole transport layer is greater than the electronegativity of the halogen atoms in the light-emitting layer.

11. The light-emitting device according to any one of claims 1 to 10, wherein the first quantum dot emits green light.

12. A display device comprising the light-emitting element according to any one of claims 1 to 11.

13. A display device comprising: a substrate; a first light-emitting element on the substrate; and a second light-emitting element on the substrate that emits light of a color different from the emission color of the first light-emitting element, wherein the first light-emitting element comprises: a first anode; a first cathode facing the first anode; a first light-emitting layer that includes first quantum dots having a first core and a first shell surrounding the first core and is located between the first anode and the first cathode; and a first hole transport layer that includes second quantum dots having a first core and a second shell surrounding the second core and having a thickness smaller than that of the first shell, and is adjacent to the first anode side of the first light-emitting layer.

14. The display device according to claim 13, wherein the second quantum dots emit light having substantially the same wavelength as the wavelength of light emitted by the first quantum dots.

15. The display device according to claim 13 or 14, wherein the second light-emitting element comprises: a second anode; a second cathode facing the second anode; a second light-emitting layer including third quantum dots and located between the second anode and the second cathode; and a second hole transport layer including a second hole transport material and in contact with the second light-emitting layer on the side of the second anode.

16. The display device of claim 15, wherein the first hole transport material and the second hole transport material comprise the same material.

17. The display device according to claim 15 or 16, wherein the second hole transport layer does not contain quantum dots.

18. The display device according to any one of claims 13 to 17, wherein the first hole transport layer is in contact with the first anode side of the first light-emitting layer.

19. A display device according to any one of claims 13 to 18, wherein, in a cross section along the stacking direction, at least one of the second quantum dots intersects the boundary surface between the first hole transport layer and a layer that contacts the first hole transport layer on the side of the first light-emitting layer.

20. The display device according to any one of claims 13 to 19, wherein the film thickness of the first shell is 4 nm or more.

21. The display device according to any one of claims 13 to 20, wherein the second shell has a thickness of 2 nm or less.

22. The display device according to any one of claims 13 to 21, wherein the first hole transport layer contains 1 atm % or more of halogen atoms.

23. The display device according to claim 22, wherein the first light-emitting layer contains halogen atoms of the same element as those contained in the first hole transport layer, and the concentration of halogen atoms in the first hole transport layer is greater than the concentration of halogen atoms in the first light-emitting layer.

24. The display device according to claim 22, wherein the concentration of halogen atoms in the first light-emitting layer and the concentration of halogen atoms in the first hole transport layer are substantially the same, and the electronegativity of the halogen atoms in the first hole transport layer is greater than the electronegativity of the halogen atoms in the first light-emitting layer.

25. The display device of claim 22, wherein the concentration of halogen atoms in the first hole transport layer is greater than the concentration of halogen atoms in the first light-emitting layer, and the electronegativity of the halogen atoms in the first hole transport layer is greater than the electronegativity of the halogen atoms in the first light-emitting layer.

26. The display device according to any one of claims 13 to 25, wherein the first quantum dots emit green light.

27. The display device described in claim 15, further comprising: a third light-emitting element on the substrate that emits light of a color different from both the emission color of the first light-emitting element and the emission color of the second light-emitting element, wherein the third light-emitting element comprises: a third anode; a third cathode facing the third anode; a third light-emitting layer including fourth quantum dots and positioned between the third anode and the third cathode; and a third hole transport layer including a third hole transport material and in contact with the third light-emitting layer on the side of the third anode.

28. The display device of claim 27, wherein the first hole transport material, the second hole transport material, and the third hole transport material comprise the same material.

29. The display device of claim 27 or 28, wherein the third hole transport layer does not contain quantum dots.

30. A display device according to any one of claims 27 to 29, wherein the first light-emitting element is a green light-emitting element, and one of the second light-emitting element and the third light-emitting element is a red light-emitting element and the other is a blue light-emitting element.

31. A method for manufacturing a light-emitting device, comprising: forming an anode; forming a cathode facing the anode; forming a light-emitting layer between the anode and the cathode, the light-emitting layer including first quantum dots having a first core and a first shell surrounding the first core; and forming a hole-transport layer in contact with the anode side of the light-emitting layer, the hole-transport layer including second quantum dots having a hole-transport material, a second core, and a second shell surrounding the first core and having a thickness smaller than that of the first shell.

32. The method for manufacturing a light-emitting device according to claim 31, wherein the second quantum dots emit light having substantially the same wavelength as the wavelength of light emitted by the first quantum dots.

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