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

Abstract

A light-emitting element (11) comprises a light-emitting layer (24) located between an anode (21) and a cathode (26). The light-emitting layer has a plurality of quantum dots (50) and a matrix material (51). The matrix material contains an oxide of a Group 14 element, or a ternary oxide having a first Group 14 element and a second Group 14 element, and fills a space at least between two quantum dots. The matrix material has a first portion, and a second portion that is located closer to the cathode side than the first portion and has a lower concentration of oxygen atoms than the first portion or a smaller band gap than the first portion.

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] In a light-emitting device having a light-emitting layer containing quantum dots (semiconductor nanoparticles) as a light-emitting material, reactive current may occur when carriers pass through the light-emitting layer without being injected into the quantum dots. The generation of such reactive current not only reduces the light-emitting efficiency of the light-emitting device, but also causes deterioration of the quantum dots or surrounding layers of the light-emitting layer, resulting in reduced reliability of the light-emitting device. Patent Document 1 discloses a light-emitting device in which the light-emitting layer contains multiple quantum dots with different shell thicknesses, thereby improving carrier confinement in the quantum dots.

[0003] Japanese Patent No. 6233417

[0004] The light-emitting device disclosed in Patent Document 1 can be said to be configured to suppress reactive current by reducing the outflow of carriers injected into the quantum dots to the outside of the quantum dots. Therefore, it is difficult to reduce the reactive current flowing between the quantum dots in the light-emitting device disclosed in Patent Document 1.

[0005] A light-emitting element according to one aspect of the present disclosure includes an anode, a cathode, and a light-emitting layer located between the anode and the cathode. The light-emitting layer has a plurality of quantum dots and a matrix material. The matrix material contains an oxide of a Group 14 element and fills spaces between at least two of the quantum dots. The matrix material has a first portion and a second portion that is located closer to the cathode than the first portion and has a lower concentration of oxygen atoms than the first portion.

[0006] A light-emitting element according to another aspect of the present disclosure includes an anode, a cathode, and a light-emitting layer located between the anode and the cathode. The light-emitting layer has a plurality of quantum dots and a matrix material. The matrix material includes a ternary oxide having a first Group 14 element and a second Group 14 element, and fills spaces between at least two of the quantum dots. The matrix material has a first portion and a second portion located closer to the cathode than the first portion, the second portion having a smaller band gap than the first portion, and a ratio of the atomic concentration of the first Group 14 element to the atomic concentration of the second Group 14 element different from that of the first portion.

[0007] A method for manufacturing a light-emitting element according to one aspect of the present disclosure includes forming an anode, forming a cathode, and forming a light-emitting layer located between the anode and the cathode and having a plurality of quantum dots and a matrix material, wherein the formation of the light-emitting layer includes forming the matrix material that contains an oxide of a Group 14 element and fills spaces between at least two of the quantum dots, and the formation of the matrix material includes forming a first portion and forming a second portion that is located closer to the cathode than the first portion and has a lower concentration of oxygen atoms than the first portion.

[0008] A method for manufacturing a light-emitting element according to another aspect of the present disclosure includes forming an anode, forming a cathode, and forming a light-emitting layer located between the anode and the cathode, the light-emitting layer having a plurality of quantum dots and a matrix material, wherein the formation of the light-emitting layer includes forming the matrix material, the matrix material including a ternary oxide having a first Group 14 element and a second Group 14 element, and filling spaces between at least two of the quantum dots, and the formation of the matrix material includes forming a first portion and forming a second portion having a smaller band gap than the first portion and a different ratio of atomic concentrations of the first Group 14 element to the second Group 14 element from that of the first portion.

[0009] The reactive current flowing between the quantum dots is reduced, and the light-emitting efficiency of the light-emitting device is further improved.

[0010] FIG. 1 is a diagram showing a schematic side cross-sectional view of a display device according to embodiment 1, and a schematic diagram showing a matrix material filling spaces between quantum dots. FIG. 2 is a schematic plan view of a display device according to embodiment 1. FIG. 3 is a schematic band diagram of each layer of a light-emitting element according to embodiment 1. FIG. 4 is a flowchart illustrating an example of a manufacturing method of a display device according to embodiment 1. FIG. 5 is a schematic plan view of a quantum dot layer according to an example. FIG. 6 is a graph showing the height at each position on the top surface of a quantum dot layer according to an example. FIG. 7 is a schematic side cross-sectional view of a display device according to embodiment 2. FIG. 8 is a schematic side cross-sectional view of a display device according to embodiment 3.

[0011] [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.

[0012] <Display Device> Fig. 2 is a schematic plan view 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 and a frame unit NA formed around the periphery of the display unit DA. The display device 1 performs display on the display unit DA by controlling light emission from each of a plurality of light-emitting elements (described later) formed in the display unit DA. Drivers and the like for driving each of the plurality of light-emitting elements of the display unit DA may be formed in the frame unit NA.

[0013] The display unit DA of the display device 1 according to this embodiment may include a plurality of subpixels, including a red subpixel, a green subpixel, and a blue subpixel. A light-emitting element (described later) is formed in each subpixel, and each light-emitting element individually emits light. This allows the display device 1 to perform display by individually controlling the light emitted from the plurality of light-emitting elements of the display unit DA using, for example, a driver or the like formed in the frame portion NA.

[0014] <Light-emitting element: overview> The structure of the display unit DA of the display device 1 according to this embodiment will be described in more detail with reference to Fig. 1. Fig. 1 is a schematic side cross-sectional view 101 of the display device 1 according to this embodiment, and schematic views 102 and 103 showing a matrix material 51 that fills spaces between quantum dots 50, which will be described later. In this disclosure, the direction from the substrate 20 to the cathode 26, which will be described later, of the display device 1 may be referred to as "upper," and the opposite direction may be referred to as "lower."

[0015] 2, and is a diagram showing a cross section passing through the light emitting element 11 in a plan view of the substrate 20 of the display device 1 according to the present embodiment. Note that all of the schematic cross sectional side views of the display device in the present disclosure show a cross section of the display device corresponding to the cross section shown in the schematic cross sectional side view 101.

[0016] Schematic diagrams 102 and 103 are diagrams respectively showing two examples of a set P of two quantum dots 50 and a region (space) K therebetween, as shown in the schematic cross-sectional side view 101. In particular, schematic diagrams 102 and 103 are diagrams respectively showing sets P1 and P2, which are examples of sets of quantum dots 50A and 50B.

[0017] As shown in the schematic side cross-sectional view 101, the display device 1 includes a light-emitting element 11. In this embodiment, the light-emitting element 11 includes a substrate 20. For example, the substrate 20 may be formed at a position overlapping the display section DA and the frame section NA in a plan view of the display device 1, and the light-emitting element 11 may be considered to include a portion of the substrate 20 that overlaps with the display section DA in a plan view of the display device 1. In other words, the substrate 20 may be formed across the display section DA and the frame section NA in a plan view of the display device 1. The top surface of the substrate 20 may be approximately parallel to the display surface of the display device 1; in other words, the plan view of the substrate 20 may be approximately the same as the plan view of the display device 1.

[0018] Furthermore, the light-emitting element 11 includes, in order from the substrate 20 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 light-emitting layer 24 is located between the anode 21 and the cathode 26, and the electron transport layer 25 is located between the light-emitting layer 24 and the cathode 26.

[0019] In this embodiment, at least one light-emitting element 11 includes an anode 21 closer to the substrate 20 than the light-emitting layer 24. However, this embodiment is not limited to this, and the light-emitting element 11 may include, in this order from the substrate 20 side, a cathode, an electron transport layer, a light-emitting layer, a hole transport layer, a hole injection layer, and an anode. Furthermore, the light-emitting element 11 may include an electron injection layer between the electron transport layer 25 and the cathode 26.

[0020] The light-emitting element 11 may be formed individually for each of the plurality of sub-pixels described above. The display device 1 may also include a driver or the like (not shown) at a position overlapping the frame portion NA of the substrate 20 in a plan view. The substrate 20 may also include a pixel circuit (not shown) corresponding to each sub-pixel. The pixel circuit may be electrically connected to the anode 21 of the light-emitting element 11. The display device 1 may control light emission from each light-emitting element 11 by controlling the application of a voltage to the anode 21 by each pixel circuit through the control of the driver or the like.

[0021] <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 transparent electrodes 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.

[0022] <Light-emitting element: charge transport layer> The hole injection layer 22 is a layer that injects holes from the anode 21 toward the light-emitting layer 24. Organic or inorganic materials having hole transport properties that have been conventionally used in light-emitting elements including quantum dots can be used as the material for the hole injection layer 22. The hole injection layer 22 may contain nickel oxide (NiO) nanoparticles. The hole injection layer 22 may be formed of any of [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), [4- [4-(3,6-dimethoxy-9H-carbazol-9-yl)butyl]phosphonic acid (4PACz), [4-(3,6-dimethyl-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), [4-(3,6-dibromo-9H-carbazol-9-yl)butyl]phosphonic acid (Br-4PACz), or the like. 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.

[0023] The hole transport layer 23 is a layer that transports holes injected from the anode 21 into the hole injection layer 22 to the light-emitting layer 24. The hole transport layer 23 can be made of an organic or inorganic material having hole transport properties that has been conventionally used in light-emitting devices containing quantum dots. Examples of materials for the hole transport layer 23 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"). These materials may also be used alone, or two or more types may be mixed or laminated as appropriate.

[0024] 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 include nanoparticles 30 as an electron transport material. The electron transport layer 25 may also include a ligand that can coordinate to the nanoparticles 30.

[0025] For example, the nanoparticles 30 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.

[0026] The electron transport material contained in the electron transport layer 25 is not limited to the nanoparticles 30. 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.

[0027] <Light-emitting element: light-emitting layer: quantum dots> The light-emitting layer 24 includes, in this order from the anode 21 side, a first light-emitting layer 40 and a second light-emitting layer 41. Both the first light-emitting layer 40 and the second light-emitting layer 41 have a plurality of quantum dots 50 as light-emitting materials. The quantum dots 50 have a core / shell structure including a core and at least one shell surrounding the core. The shell may have multiple layers extending from the center of the core to the periphery. The first light-emitting layer 40 and the second light-emitting layer 41 may include an organic ligand capable of coordinating with the outermost shell of the quantum dots 50.

[0028] Holes from the anode 21 and electrons from the cathode 26 are injected into the core of the quantum dot 50, and the recombination of the holes and electrons generates excitons, which emit light. The shell of the quantum dot 50 may have a function of protecting the core, such as compensating for defects in the core. Alternatively, the quantum dot 50 may have any of various other conventionally known structures.

[0029] 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 50 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 50 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.

[0030] The quantum dots 50 are typically made of a semiconductor. The semiconductor may have a certain band gap. The semiconductor may be any material capable of emitting light and may include at least the materials described below. The semiconductor may be capable of emitting blue, green, and red light, respectively. 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.

[0031] 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.

[0032] 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.

[0033] Chalcogenides are compounds containing a Group VI A(16) element, such as CdS or CdSe. Chalcogenides may also include mixed crystals thereof.

[0034] 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.

[0035] 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.

[0036] In this embodiment, the concentration of the quantum dots 50 in the first light-emitting layer 40 is higher than the concentration of the quantum dots 50 in the second light-emitting layer 41. Therefore, in the light-emitting layer 24, the concentration of the quantum dots 50 monotonically decreases in the direction from the anode 21 to the cathode 26.

[0037] For example, the light-emitting layer 24 may have a portion where the concentration of the quantum dots 50 monotonically decreases in the direction from the anode 21 to the cathode 26, or may monotonically decrease throughout the entire light-emitting layer 24. Hereinafter, as an example, a case where the concentration monotonically decreases throughout the entire light-emitting layer 24 will be described, but the present invention is not necessarily limited to this configuration.

[0038] In this disclosure, the case where the concentration of a material in each portion of the light-emitting layer 24 "monotonically decreases" is described as an example, but this configuration is not necessarily limited to this. In this disclosure, unless inconsistent, the "monotonically decreasing" concentration of the material does not exclude a case where the concentration has a monotonically decreasing portion or a case where the concentration monotonically decreases throughout the entire light-emitting layer 24. Furthermore, the "monotonically decreasing" concentration at the boundary between the light-emitting layer 24 and an adjacent layer may refer to a change in concentration that is unrelated to a desired concentration configuration. In other words, since the change in oxygen atom concentration at the boundary is not necessarily steep, the region 1.2 nm or less from the interface may be excluded from the light-emitting layer 27.

[0039] In the present disclosure, the concentration of a material in each part of the light-emitting layer 24 refers to, for example, the ratio of the number of regions occupied by the material in the cross section of the light-emitting layer 24 or the area ratio. In addition, in the present disclosure, the phrase "the concentration of the material monotonically decreases" in each part of the light-emitting layer 24 refers to the concentration gradually or stepwise decreasing, and does not limit the presence of parts where the concentration of the material is approximately the same. In addition, in the present disclosure, the phrase "the concentration of the material monotonically increases" in each part of the light-emitting layer 24 refers to the concentration gradually or stepwise increasing, and does not limit the presence of parts where the concentration of the material is approximately the same. Here, the phrase "the part where the concentration of the material is approximately the same" refers to the part where the concentration of the material is approximately the same at 500 nm when observing the cross section of the light-emitting layer 24. ２ The difference in the number or area ratio of the regions occupied by a specific material in the light-emitting layer 24 may be measured by observing a cross section of the light-emitting layer 24 by energy dispersive X-ray spectroscopy (EDX) using a transmission electron microscope (TEM), for example.

[0040] The area ratio of the quantum dots 50 in the cross section of the first light-emitting layer 40 may be 60% or more from the viewpoint of reducing reactive current, which is caused by carriers not being injected into the quantum dots and not contributing to light emission. Furthermore, this area ratio may be 90% or less from the viewpoint of enhancing the protective effect of the matrix material 51 on the quantum dots 50, which will be described later. Furthermore, the area ratio of the quantum dots 50 in the cross section of the second light-emitting layer 41 may be 5% or more, or may be 60% or less from the viewpoint of enhancing the protective effect of the matrix material 51 on the quantum dots 50 against foreign matter entering from the cathode 26 side, which will be described later.

[0041] <Light-emitting element: Light-emitting layer: Matrix material> The light-emitting layer 24 includes a matrix material 51 that fills the spaces between at least two quantum dots 50. The matrix material 51 includes a first matrix material 52 as a first portion and a second matrix material 53 as a second portion. In particular, the first light-emitting layer 40 includes the first matrix material 52 of the matrix material 51, and the second light-emitting layer 41 includes the second matrix material 53 of the matrix material 51. In other words, the first light-emitting layer 40 includes a plurality of quantum dots 50 and the first matrix material 52 that fills the spaces between at least two of the quantum dots 50, and the second light-emitting layer 41 includes a plurality of quantum dots 50 and the second matrix material 53 that fills the spaces between at least two of the quantum dots 50. Therefore, the matrix material 51 includes the first matrix material 52 as a first portion and the second matrix material 53 that is located closer to the cathode 26 than the first matrix material 52.

[0042] Note that the matrix material 51 filling the space between two quantum dots 50 means that it fills at least the region K between the quantum dots 50A and 50B, as shown in the schematic diagram 102 of the set P1 in Fig. 1 . Region K is a region surrounded by two lines (common circumscribing lines) tangent to the peripheries of the quantum dots 50A and 50B and the opposing peripheries of the quantum dots 50A and 50B in the cross section of the light-emitting layer 24. Therefore, as shown in the schematic diagram 103 of the set P2 in Fig. 1 , region K can exist even if the quantum dots 50A and 50B are close to each other, and the matrix material 51 fills region K.

[0043] The matrix material 51 filling the gap between the two quantum dots 50 does not necessarily mean that the region K between the quantum dots 50A and 50B is entirely made of the matrix material 51. For example, the region K between the quantum dots 50A and 50B may contain a material, such as a ligand, different from the material of the matrix material 51. Specifically, the light-emitting layer 24 may contain an organic ligand that is added to improve the dispersibility of the quantum dots 50 in a solution used for coating and that coordinates to the outer surfaces of the quantum dots 50 in the solution. In this case, from the viewpoint of improving the reliability of the light-emitting layer 24, for example, the weight ratio of the organic ligand to the total weight including the region K may be less than 5%. Furthermore, the light-emitting layer 24 may include voids in the matrix material 51 where no matrix material 51 is present. In this case, from the viewpoint of improving the reliability of the light-emitting layer 24, for example, the area of ​​the voids in the cross section of the light-emitting layer 24 may be 15% or less of the area occupied by the matrix material 51 in the cross section.

[0044] The matrix material 51 may fill the region of the light-emitting layer 24 other than the region where the quantum dots 50 are located. For example, the outer edge (upper and lower surfaces) of the light-emitting layer 24 may be covered with the matrix material 51. Alternatively, the light-emitting layer 24 may be configured so that a portion of the matrix material 51 extends from the outer edge thereof, and the quantum dots 50 are positioned away from the outer edge. The outer edge of the light-emitting layer 24 may not be formed solely by the matrix material 51, and some of the quantum dots 50 may be exposed from the matrix material 51. The matrix material 51 may refer to the portion of the light-emitting layer 24 other than the portion where the quantum dots 50 are located.

[0045] The matrix material 51 may contain a plurality of quantum dots 50. The matrix material 51 may be formed so as to fill a space formed between two quantum dots 50. The plurality of quantum dots 50 may be embedded in the matrix material 51 at intervals.

[0046] The matrix material 51 is formed in a thickness of 1000 nm along a plane direction perpendicular to the film thickness direction. ２The light-emitting layer 24 may include a continuous film having an area of ​​100 μm or more. The continuous film may be a film that is not separated in one plane by materials other than the material that constitutes the continuous film. The continuous film may be an integrated film that is connected without interruption by chemical bonds of the matrix material 51. However, even when the matrix material 51 includes a continuous film, the light-emitting layer 24 may include voids in the matrix material 51 where no matrix material 51 is present. Even in this case, from the viewpoint of improving the reliability of the light-emitting layer 24, for example, the area of ​​the voids in the cross section of the light-emitting layer 24 may be 15% or less of the area occupied by the matrix material 51 in the cross section.

[0047] The concentration of the matrix material 51 in the light-emitting layer 24 is, for example, the area ratio of the matrix material 51 in the cross section of the light-emitting layer 24. This concentration may be 10% or more and 90% or less, or 30% or more and 70% or less, when observed from the cross section. This concentration may be measured, for example, from the area ratio of an image obtained by observing the cross section.

[0048] The light-emitting layer 24 may be composed of a plurality of quantum dots 50 and a matrix material 51. When the light-emitting layer 24 is analyzed, the intensity of carbon detected due to the chain structure may be below noise. Furthermore, the proportion of carbon detected in the light-emitting layer 24 may be 5% or less, 1% or less, or even none at all. When quantum dots 50 coordinated with organic ligands are used in the light-emitting layer 24 as in known techniques, the carbon chain of the organic ligand may decompose, or the organic ligand itself may become detached from the quantum dots, over long periods of operation. In this case, the quantum dots 50 may deteriorate, resulting in a decrease in brightness. As disclosed herein, by filling the quantum dots 50 with a matrix material 51, the quantum dots 50 can be protected without using organic ligands. Therefore, the display device 1 according to this embodiment can achieve high reliability; in other words, it can suppress a decrease in brightness over long periods of operation of the light-emitting element 11.

[0049] The matrix material 51 contains an oxide of a Group 14 element. In addition, the second matrix material 53, which is the second part of the matrix material 51, has a lower concentration of oxygen atoms than the first matrix material 52, which is the first part. In particular, in the matrix material 51, the concentration of oxygen atoms may monotonically decrease in the direction from the anode 21 toward the cathode 26. The oxide of the Group 14 element contained in the matrix material 51 is, for example, silicon oxide (SiO ２ ), and germanium oxide (GeO ２ ) may also be included.

[0050] In this disclosure, "atom" does not only mean that an atom exists as a single atom. In this disclosure, "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. However, in this disclosure, "atom" does not limit the form of existence of other atoms. In other words, an oxygen atom includes an atom that exists in the form of a compound having an oxygen atom, and an oxygen atom that exists in the form of an oxygen ion. Regardless of the form of existence of an oxygen atom, if its presence in a substance can be identified by analysis, the substance may be considered to contain an oxygen atom.

[0051] For example, the matrix material 51 includes a ternary oxide having a first Group 14 element and a second Group 14 element. In this case, the band gap of the second portion, that is, the second matrix material 53, of the matrix material 51 is smaller than the band gap of the first portion, that is, the first matrix material 52. Furthermore, the ratio of the atomic concentration of the first Group 14 element to the atomic concentration of the second Group 14 element in the matrix material 51 is different between the first matrix material 52 and the second matrix material 53.

[0052] In particular, in the matrix material 51, the band gap may monotonically decrease in the direction from the anode 21 to the cathode 26. Furthermore, in the direction from the anode 21 to the cathode 26, the ratio of the atomic concentration of the second Group 14 element to the atomic concentration of the first Group 14 element may monotonically increase or decrease.

[0053] In the present disclosure, the phrase "the band gap of the material monotonically increases or decreases" in each portion of the light-emitting layer 24 refers to the band gap gradually or stepwise increasing or decreasing, and does not limit the material from having portions with approximately the same band gap. Here, the phrase "portions with approximately the same band gap" refers to portions where the difference in band gap between portions is within 0.04 eV.

[0054] For example, the first Group 14 element may be silicon. In this case, the concentration of silicon atoms in the second matrix material 53 may be lower than the concentration of silicon atoms in the first matrix material 52. Alternatively, the second Group 14 element may be germanium. In this case, the concentration of germanium atoms in the second matrix material 53 may be higher than the concentration of germanium atoms in the first matrix material 52.

[0055] Furthermore, the ternary oxide contained in the matrix material 51 may have both silicon atoms and germanium atoms. For example, the matrix material 51 may contain silicon germanium oxide as the ternary oxide. More specifically, when 0<x<1, the matrix material 51 may contain Si ｘ Ge １－ｘ O ２ In this case, the value of x in the matrix material 51 may decrease monotonically from the anode 21 to the cathode 26. However, the matrix material 51 may partially contain a binary oxide, for example, instead of a ternary oxide. In particular, the matrix material 51 may contain silicon oxide (SiO ２ ), and germanium oxide (GeO ２ The matrix material 51 may contain both a ternary oxide and a binary oxide.

[0056] Even when the matrix material 51 contains the above-described ternary oxide, the concentration of oxygen atoms in the second matrix material 53 may be lower than the concentration of oxygen atoms in the first matrix material 52. Furthermore, even when the matrix material 51 contains the above-described ternary oxide, the concentration of oxygen atoms in the first matrix material 52 may monotonically decrease in the direction from the anode 21 to the cathode 26.

[0057] In this embodiment, the boundary between the light-emitting layer 24 and the electron transport layer 25 may be confirmed by observing a cross section passing through the light-emitting layer 24 and the electron transport layer 25 and confirming the sum of the atomic concentrations of the Group 14 elements at each position on the cross section. For example, the boundary between the light-emitting layer 24 and the electron transport layer 25 may be confirmed by defining the portion of the cross section where the sum of the atomic concentrations of the Group 14 elements is 15% or more as the light-emitting layer 24 and the portion where the sum is less than 15% as the electron transport layer 25. Alternatively, the portion of the cross section where the sum of the atomic concentrations of the Group 14 elements decreases by 15% or more may be considered to be the boundary between the light-emitting layer 24 and the electron transport layer 25. Note that, in addition to oxygen atoms, if there are atoms with higher concentrations in only one of the light-emitting layer 24 and the electron transport layer 25, the portion where the concentration of the atom changes by 15% or more may be considered to be the boundary between the light-emitting layer 24 and the electron transport layer 25. Therefore, as long as the above conditions are met, even a portion near which quantum dots 50 are not confirmed may be considered to be part of the light-emitting layer 24. Note that the change in the sum of the atomic concentrations of the Group 14 elements is not necessarily steep at the interface between the light-emitting layer 24 and the electron transport layer 25. For this reason, a region 1.2 nm or less from the interface determined above toward the anode 21 may be excluded from the light-emitting layer 24, or may be included in the electron transport layer 25. Furthermore, the sum of the atomic concentrations of the Group 14 elements in the light-emitting layer 24 may refer to the sum of the atomic concentrations of the Group 14 elements in the matrix material 51 of the light-emitting layer 24 excluding the quantum dots 50.

[0058] Note that the first light-emitting layer 40 and the second light-emitting layer 41 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 24 in the present disclosure. In other words, the light-emitting layer 24 does not need to include a clearly distinguishable first light-emitting layer 40 and second light-emitting layer 41. For example, in this embodiment, it may be confirmed that the light-emitting layer 24 includes a portion of each of the first light-emitting layer 40 including the first matrix material 52 as a first portion and the second light-emitting layer 41 including the second matrix material 53 as a second portion. In this case, it may be considered that the light-emitting layer 24 includes the first light-emitting layer 40 and the second light-emitting layer 41 without confirming a clear boundary between the first light-emitting layer 40 and the second light-emitting layer 41.

[0059] In this manner, in the present disclosure, it is assumed that the positional relationship of each part of the light-emitting layer 24, such as the quantum dots 50, the first matrix material 52, and the second matrix material 53, can be confirmed, for example, by observing a cross section of the light-emitting layer 24. In this case, unless otherwise specified, the specific configuration of the light-emitting layer 24 in the present disclosure is not particularly limited.

[0060] For example, the boundary between the first light-emitting layer 40 and the second light-emitting layer 41 may be located approximately at the center in the film thickness direction of the light-emitting layer 24, or may be located closer to the anode 21 or the cathode 26 than the center. Quantum dots 50 may also be located on the boundary between the first light-emitting layer 40 and the second light-emitting layer 41.

[0061] <Consideration of the density of free electrons in the matrix material> As described above, the concentration of oxygen atoms in the second matrix material 53 is assumed to be lower than the concentration of oxygen atoms in the first matrix material 52. In this case, the density of atomic defects of oxygen atoms in the first matrix material 52 is lower than the density of atomic defects of oxygen atoms in the second matrix material 53. Therefore, in the matrix material 51, the density of atomic defects of oxygen atoms monotonically increases in the direction from the anode 21 to the cathode 26. The difference in the concentration of oxygen atoms at each position in the matrix material 51 in the direction from the anode 21 to the cathode 26 may correspond to the difference in the density of defects of oxygen atoms in the matrix material 51 at each position.

[0062] In the present disclosure, the density of atomic defects of oxygen atoms in each part of the matrix material 51 refers to, for example, the ratio of the number of regions occupied by the atomic defects in the cross section of the matrix material 51 or the area ratio. In addition, in the present disclosure, the phrase "the density of atomic defects monotonically increases" in each part of the matrix material 51 refers to the density increasing gradually or stepwise, and does not limit the existence of parts where the density of atomic defects is approximately the same. In addition, in the present disclosure, the phrase "the density of atomic defects monotonically decreases" in each part of the matrix material 51 refers to the density increasing gradually or stepwise, and does not limit the existence of parts where the density of atomic defects is approximately the same. Here ... parts where the density of atomic defects in the matrix material 51 is approximately the same" refers to parts where the density of atomic defects is approximately the same within a 500 nm range when observing the cross section of the matrix material 51. ２ The atomic defect area indicates a portion where the difference in the number ratio or area ratio of the regions occupied by the atomic defect in the region of the matrix material 51 is within 5%. The number ratio or area ratio of the regions occupied by the specific atomic defect in the matrix material 51 may be measured by observing a cross section of the matrix material 51 using, for example, a TEM.

[0063] The concentration of oxygen atoms in the matrix material 51, particularly the density of atomic defects of oxygen atoms, correlates with the density of free electrons at each position in the matrix material 51. This is because the concentration of free electrons possessed by the matrix material 51 changes depending on the concentration of oxygen atoms in the matrix material 51. In particular, when defects of oxygen atoms occur in the matrix material 51 and are activated, two free electrons are generated per defect near the defect.

[0064] Here, in order to consider the density of free electrons on the hole transport layer 23 side of the light-emitting layer 24, we will consider the hole density at the interface between the hole transport layer 23 and the light-emitting layer 24. For example, when the hole transport layer 23 contains an organic material as a hole transport material, the current flowing through the hole transport layer 23 is a space-charge limited current. Therefore, the hole density p near the interface between the hole transport layer 23 and the light-emitting layer 24 is expressed by the following formula:

[0065] In the above formula, e is the elementary charge, ε ０is the dielectric constant of a vacuum, ε ｒ is the relative dielectric constant of the hole transport layer 23, J is the current density of the hole transport layer 23, L is the film thickness of the hole transport layer 23, and μ is the hole mobility of the hole transport layer 23. In the present disclosure, the vicinity of the interface refers to a region within 1.2 nm from the interface in the film thickness direction.

[0066] For example, when the light emitting element 11 is driven, the current density J flowing through the hole transport layer 23 is set to 10 mA / cm ２ Furthermore, the relative dielectric constant ε of the hole transport layer 23 is ｒ is 3.5, the film thickness L is 30 nm, and the hole mobility μ is 10 －４ cm ２ In this case, from the above formula, the hole density p near the interface between the hole transport layer 23 and the light emitting layer 24 is 1.4×10 １６ cm －３ This becomes:

[0067] When free electrons are present near the interface between the hole transport layer 23 and the light emitting layer 24, even when holes are injected from the hole transport layer 23 into the light emitting layer 24, recombination with free electrons outside the quantum dots 50 located near the interface may occur, preventing the light emission process. On the other hand, when the hole density near the interface between the hole transport layer 23 and the light emitting layer 24 exceeds the density of free electrons near the interface, even when recombination of electrons and holes occurs at the interface, excess holes are generated and are more likely to be injected into the quantum dots 50. Therefore, from the viewpoint of improving the efficiency of hole injection from the hole transport layer 23 to the light emitting layer 24, the hole density near the interface between the hole transport layer 23 and the light emitting layer 24 is required to be higher than the density of free electrons near the interface.

[0068] Therefore, in order to improve the efficiency of hole injection from the hole transport layer 23 to the light emitting layer 24, the density of free electrons in the matrix material 51 is set to 1×10 in a region within 1.2 nm in the film thickness direction from the end face of the light emitting layer 24 on the anode 21 side. １６ cm －３ In other words, in a region within 1.2 nm from the interface between the hole transport layer 23 and the first light-emitting layer 40 in the film thickness direction of the first light-emitting layer 40, the density of free electrons in the first matrix material 52 in the first light-emitting layer 40 may be 1×10１６ cm －３ It may be the following:

[0069] Here, when the matrix material 51 has a polycrystalline or amorphous structure, the activation rate of the matrix material 51 is not high, and is considered to be about 1%. Therefore, the activation rate of the matrix material 51 in the light-emitting layer 24 is set to 1%, in other words, one out of every 100 oxygen atom defects in the matrix material 51 is activated to generate two free electrons. In this case, in a region within 1.2 nm in the film thickness direction from the end face of the light-emitting layer 24 on the anode 21 side, the oxygen atom defect density of the matrix material 51 is 5×10 １７ cm －３ With the above configuration, the density of free electrons in the matrix material 51 in the region may be 1×10 １６ cm －３ This can improve the efficiency of hole injection from the hole transport layer 23 to the light-emitting layer 24. Furthermore, since the free electron density of the matrix material 51 is low, the resistivity of the matrix material 51 increases. Therefore, reactive current that does not contribute to light emission, which occurs when carriers flow through the matrix material 51 and are not injected into the quantum dots 50, is reduced. Therefore, the above configuration improves the light-emitting efficiency of the light-emitting element 11.

[0070] Next, the density of free electrons on the electron transport layer 25 side of the light-emitting layer 24 will be considered. If free electrons are present near the interface between the electron transport layer 25 and the light-emitting layer 24, the free electrons will move within the light-emitting layer 24 toward the anode 21 when the light-emitting element 11 is driven. Therefore, if the density of free electrons on the electron transport layer 25 side of the light-emitting layer 24 is high, a greater proportion of free electrons will flow between the quantum dots 50 to be injected into them. Furthermore, if the density of atomic defects in the matrix material 51 in the light-emitting layer 24 is high, the crystallinity of the matrix material 51 will decrease, which may result in a decrease in electron mobility in the light-emitting layer 24 and ultimately a decrease in the electron transport ability of the light-emitting layer 24. Furthermore, if the density of atomic defects in the matrix material 51 in the light-emitting layer 24 is high and the crystallinity of the matrix material 51 is decreased, the effect of the matrix material 51 in protecting the quantum dots may be reduced.

[0071] On the other hand, if the density of free electrons near the interface between the electron transport layer 25 and the light-emitting layer 24 is low, the concentration of electrons injected from the electron transport layer 25 will decrease, which may result in an excess of holes in the light-emitting layer 24 and an increase in the driving voltage of the light-emitting element 11.

[0072] Therefore, from the viewpoint of improving the efficiency of electron injection from the electron transport layer 25 to the light-emitting layer 24 while suppressing the flow of electrons between the quantum dots 50, the density of free electrons near the interface between the electron transport layer 25 and the light-emitting layer 24 is required to be within a predetermined range.

[0073] In order to improve the efficiency of electron injection from the electron transport layer 25 to the light emitting layer 24, it is preferable that the density of free electrons in the light emitting layer 24 near the interface between the light emitting layer 24 and the electron transport layer 25 is equal to or higher than the density of free electrons in the electron transport layer 25. For example, when the nanoparticles 30 in the electron transport layer 25 are zinc oxide nanoparticles, the density of free electrons in the electron transport layer 25 is 1×10 １８ cm －３ Therefore, in order to improve the efficiency of electron injection from the electron transport layer 25 to the light emitting layer 24, the density of free electrons in the matrix material 51 is set to about 1×10 in a region within 1.2 nm in the film thickness direction from the end face of the light emitting layer 24 on the cathode 26 side. １８ cm －３ In other words, in a region within 1.2 nm from the interface between the electron transport layer 25 and the second light-emitting layer 41 in the film thickness direction of the second light-emitting layer 41, the density of free electrons in the second matrix material 53 in the second light-emitting layer 41 is 1×10 １８ cm －３ It may be more than that.

[0074] Here, as in the above, the activation rate of the matrix material 51 in the light-emitting layer 24 is assumed to be 1%. In this case, in a region within 1.2 nm in the film thickness direction from the end face of the light-emitting layer 24 on the cathode 26 side, the defect density of oxygen atoms in the matrix material 51 is 5×10 １９ cm －３ With the above configuration, the density of free electrons in the matrix material 51 in the region may be 1×10 １８ cm －３As a result, the efficiency of electron injection from the electron transport layer 25 to the light emitting layer 24 can be improved.

[0075] From the above, in the region within 1.2 nm in the film thickness direction from the end face of the light-emitting layer 24 on the cathode 26 side, the defect density of oxygen atoms in the matrix material 51 is 2×10 ２１ cm －３ In other words, in a region within 1.2 nm from the interface between the electron transport layer 25 and the second light-emitting layer 41 in the film thickness direction of the second light-emitting layer 41, the defect density of oxygen atoms in the second matrix material 53 in the second light-emitting layer 41 may be 2×10 ２１ cm －３ With the above-described configuration, it is possible to reduce the decrease in the electron transporting ability of the light-emitting layer 24 and improve the efficiency of electron injection from the electron transport layer 25 to the light-emitting layer 24 .

[0076] As in the above, the activation rate of the matrix material 51 in the light-emitting layer 24 is set to 1%. In this case, the density of free electrons in the matrix material 51 is 4×10 in a region within 1.2 nm in the film thickness direction from the end face of the light-emitting layer 24 on the cathode 26 side. １９ cm －３ With the above configuration, the defect density of oxygen atoms in the matrix material 51 in the region may be 2×10 ２１ cm －３ This can reduce the deterioration of the crystallinity of the matrix material 51 of the light-emitting layer 24 and the resulting deterioration of the electron transport ability. Furthermore, the above-mentioned configuration can reduce the deterioration of the crystallinity of the matrix material 51, so that the light-emitting layer 24 can reduce the deterioration of the protective effect of the matrix material 51 on the quantum dots.

[0077] In addition, the density of free electrons in the matrix material 51 in a region within 1.2 nm in the film thickness direction from the end face of the light-emitting layer 24 on the anode 21 side may be 10% or less of the density of free electrons in a region within 1.2 nm in the film thickness direction from the end face of the light-emitting layer 24 on the cathode 26 side. In other words, the density of free electrons in the first matrix material 52 of the first light-emitting layer 40 may be one-tenth or less of the density of free electrons in the second matrix material 53 of the second light-emitting layer 41. In this case, the efficiency of hole injection and the efficiency of electron injection into the light-emitting layer 24 can be improved in both directions, thereby reducing the drive voltage of the light-emitting element 11 and improving the energy efficiency of the light-emitting layer 24. Furthermore, the carrier balance in the light-emitting layer 24 can be adjusted, improving the luminous efficiency of the light-emitting element 11.

[0078] As a result, the density of free electrons in the matrix material 51 increases in the direction from the anode 21 to the cathode 26. Therefore, for the reasons described above, the light-emitting element 11 can improve the efficiency of hole injection and the efficiency of electron injection into the light-emitting layer 24 in both directions. This allows the light-emitting element 11 to reduce the driving voltage and improve the energy efficiency of the light-emitting layer 24. Furthermore, the light-emitting element 11 can also adjust the carrier balance in the light-emitting layer 24, improving light-emitting efficiency. The display device 1 including the light-emitting element 11 achieves power saving. When the concentration of oxygen atoms in the matrix material 51 monotonically decreases in the direction from the anode 21 to the cathode 26, the light-emitting element 11 further improves its light-emitting efficiency.

[0079] Furthermore, when the concentration of quantum dots 50 in the light-emitting layer 24 monotonically decreases in the direction from the anode 21 to the cathode 26, the quantum dots 50 that may be deteriorated by foreign matter from the cathode 26 side can be reduced on the cathode 26 side of the light-emitting layer 24. Furthermore, with the above-described configuration, the light-emitting layer 24 increases the effective film thickness of the second matrix material 53 located closer to the cathode 26 than the first matrix material 52, thereby enhancing the protective effect of the light-emitting layer 24. In addition, the light-emitting layer 24 efficiently transports electrons, which have higher mobility than holes, from the cathode 26 side to the anode 21 side, thereby efficiently obtaining light emission from the quantum dots 50 located on the anode 21 side.

[0080] Furthermore, because electron mobility in semiconductors is generally higher than hole mobility, in the light-emitting layer 24 containing quantum dots 50, light is primarily emitted from the quantum dots 50 located on the anode 21 side. Therefore, with the above-described configuration, the first matrix material 52 on the anode 21 side of the light-emitting layer 24 has a low free electron density, resulting in high resistance. This allows the light-emitting layer 24 to reduce reactive current that does not contribute to light emission, which occurs when carriers flow through the first matrix material 52 without entering the quantum dots 50. Therefore, with the above-described configuration, the light-emitting element 11 can more efficiently emit light from the quantum dots 50. Furthermore, defects in the matrix material 51 are more likely to form on the surface than inside. Therefore, with the above-described configuration, the average distance between the defects and the quantum dots 50 can be increased.

[0081] In addition, the second light-emitting layer 41 can effectively increase the thickness of the matrix material 51 filling the spaces between the quantum dots 50 compared to the first light-emitting layer 40, thereby enhancing the protective effect of the matrix material 51 on the quantum dots 50. Therefore, the light-emitting element 11 can more efficiently protect the light-emitting layer 24 from foreign matter such as moisture and oxygen that infiltrate from the cathode 26 side, or heat that propagates from the cathode 26 side.

[0082] Therefore, with the above-described configuration, the light emitting element 11 can improve the luminous efficiency while more efficiently enhancing the protective effect of the matrix material 51 of the light emitting layer 24 on the quantum dots 50 .

[0083] When the light-emitting element 11 includes an electron transport layer 25 having a plurality of nanoparticles 30 between the light-emitting layer 24 and the cathode 26, the efficiency of electron transport from the cathode 26 to the light-emitting layer 24 via the electron transport layer 25 increases. Therefore, the above configuration may cause an excess of electrons or an increase in reactive current in the light-emitting layer 24. Furthermore, since gaps may be present between the plurality of nanoparticles 30 in the electron transport layer 25, foreign matter that has infiltrated into the light-emitting element 11 from the cathode 26 side can easily reach the light-emitting layer 24 by passing between the nanoparticles 30.

[0084] In this embodiment, the light-emitting layer 24 includes a matrix material 51 that reduces the occurrence of excess electrons and reactive current in the light-emitting layer 24 and reduces degradation of the quantum dots 50 due to foreign matter. Therefore, the light-emitting element 11 improves the efficiency of electron injection from the cathode 26 to the light-emitting layer 24 while also improving the efficiency of hole injection, thereby improving the light-emitting efficiency and reducing degradation of the quantum dots 50, thereby improving the reliability of the light-emitting layer 24.

[0085] The light-emitting element 11 according to this embodiment includes an anode 21 on the substrate 20 side. Generally, the substrate 20 is less susceptible to infiltration of foreign matter such as moisture than the layers between the electrodes of the light-emitting element 11. Therefore, foreign matter is more likely to infiltrate the light-emitting element 11 from the cathode 26 side, which is the side opposite the substrate 20. Furthermore, since the electron transport layer 25, which is located closer to the cathode 26 than the light-emitting layer 24, includes nanoparticles 30, foreign matter that infiltrates the light-emitting element 11 from the cathode 26 side can easily reach the light-emitting layer 24 through the nanoparticles 30, as described above. Therefore, with the above-described configuration, the light-emitting element 11 can more efficiently enhance the protective effect of the quantum dots 50 provided by the matrix material 51 of the light-emitting layer 24. Because glass substrates are less susceptible to infiltration of foreign matter such as moisture than film substrates, the substrate 20 is preferably a glass substrate.

[0086] <Band Diagram of Each Part of the Light-Emitting Element> The band gap of each part of the light-emitting element 11 according to this embodiment will be described with reference to FIG. 3. FIG. 3 is a schematic band diagram showing an example of the band gap of each part of the light-emitting element 11 according to this embodiment. Note that the band diagram in FIG. 3 has a vacuum level on the upper side within the plane of the paper. The left and right directions of the band diagram in FIG. 3 represent the thickness direction in the display direction of the display device 3, with the left side of the plane of the paper being the anode 21 side and the right side being the cathode 26 side.

[0087] The band diagram of Fig. 3 shows the Fermi levels of the anode 21 and the cathode 26. The band gaps of the hole injection layer 22, the hole transport layer 23, and the electron transport layer 25 are also shown. In particular, the band gap of the nanoparticles 30 is shown as the band gap of the electron transport layer 25.

[0088] 3 shows the band gap of the first light-emitting layer 40 and the second light-emitting layer 41 as the band gap of the light-emitting layer 24. In particular, the band diagram of FIG. 3 shows the band gap of the quantum dots 50, the band gap of the first matrix material 52, and the band gap of the second matrix material 53.

[0089] Here, in the first light-emitting layer 40 and the second light-emitting layer 41, the matrix material 51 fills the spaces between the quantum dots 50. Therefore, in the band diagram of FIG. 3 , the band gaps of the first light-emitting layer 40 and the second light-emitting layer 41 can be illustrated such that the band gaps of the matrix material 51 are located on both ends of the band gap of the quantum dots 50.

[0090] 3 , the band gap of the second matrix material 53 is smaller than the band gap of the first matrix material 52. In other words, in this embodiment, the band gap of the matrix material 51 monotonically decreases in the direction from the anode 21 to the cathode 26. The gradient of the band gap of the matrix material 51 described above may be realized by a gradient of the atomic concentration of the Group 14 element in the matrix material 51 described above.

[0091] In particular, the electron affinity of the matrix material 51 monotonically increases in the direction from the anode 21 to the cathode 26. In the band diagram of FIG. 3, the electron affinity of each part corresponds to the distance from the vacuum level to the upper end of the band gap. Therefore, in the band diagram of FIG. 3, the lower the upper end of the band gap of a certain layer is located, the greater the electron affinity of that layer. In other words, the larger the band gap of a certain layer, the smaller the electron affinity of that layer tends to be.

[0092] The electron injection barrier from the first layer to the second layer corresponds to the electron affinity of the second layer minus the electron affinity of the first layer, so in this embodiment, a barrier exists for electron injection from the second matrix material 53 to the first matrix material 52.

[0093] Therefore, the light-emitting element 11 according to this embodiment suppresses the movement of electrons through the matrix material 51 in the direction from the cathode 26 to the anode 21. Therefore, the light-emitting element 11 can increase the ratio of carriers injected into the quantum dots 50 to carriers contributing to the reactive current flowing through the matrix material 51.

[0094] As described above, the light-emitting element 11 according to this embodiment suppresses the movement of electrons injected from the electron transport layer 25 between the quantum dots 50 in the light-emitting layer 24, thereby reducing the reactive current in the light-emitting layer 24 and improving the light-emitting efficiency and reliability.

[0095] Since the matrix material 51 contains the above-described ternary oxide, the band gap of the matrix material 51 can be easily designed by adjusting the atomic concentrations of the first Group 14 element and the second Group 14 element. For example, in the direction from the anode 21 to the cathode 26, the ratio of the atomic concentration of the second Group 14 element to the atomic concentration of the first Group 14 element in the matrix material 51 may monotonically increase or decrease. This allows the light-emitting element 11 to easily design the above-described band gap in the light-emitting layer 24.

[0096] In particular, when the matrix material 51 contains a ternary oxide containing silicon atoms and germanium atoms, it is possible to enhance the protection effect for the quantum dots 50 while increasing the efficiency of carrier injection into the quantum dots 50. By lowering the concentration of silicon atoms or increasing the concentration of germanium atoms in the second matrix material 53 compared to the first matrix material 52, the above-mentioned band gap design can be more easily realized.

[0097] <Light Emitting Element: Manufacturing Method> A method for manufacturing the light emitting element 11 according to this embodiment will be described with reference to Fig. 4. Fig. 4 is a flowchart showing the method for manufacturing the light emitting element 11 according to this embodiment.

[0098] 4, in the method for manufacturing the light-emitting element 11 according to this embodiment, first, a substrate 20 is prepared (step S1). The substrate 20 may be a glass substrate, a film substrate, or the like, on which a pixel circuit is formed for each sub-pixel. The substrate 20 may also be formed with a driver in the frame portion NA and wiring between the driver and each pixel circuit.

[0099] Next, the anode 21 is formed on the substrate 20 (step S2). The anode 21 may be formed by depositing a thin film of a metal material on the substrate 20 by sputtering or the like. The anode 21 may be formed so as to be electrically connected to the pixel circuit, or may be patterned for each sub-pixel. For example, the anode 21 may be formed by forming a 30 nm thick ITO film on the substrate 20 by sputtering.

[0100] Next, a hole injection layer 22 is formed on the anode 21 (step S3). In step S32, for example, a solution in which nickel oxide nanoparticles are dispersed at 15 mg / mL in a solvent mixture of equal volumes of water and 2-methoxyethanol may be applied to the anode 21 by spin coating and baked at 200°C. This process may be performed only once, or may be repeated two to five times. Next, a solution in which MeO-2PACz is dispersed in an ethanol solvent may be applied to the nickel oxide nanoparticle layer by spin coating under a nitrogen atmosphere, and the solvent may then be volatilized by baking. This may form a layered structure of a nickel oxide nanoparticle layer and a self-assembled monolayer of MeO-2PACz, forming the hole injection layer 22.

[0101] Next, a hole transport layer 23 is formed on the hole injection layer 22 (step S4). In step S4, a solution in which poly-TPD is dispersed in a chlorobenzene solvent may be applied to the self-assembled monolayer by spin coating in a nitrogen atmosphere, and the solvent may then be evaporated by baking. This may result in a poly-TPD film having a thickness of 30 nm being formed on the self-assembled monolayer, thereby forming the hole transport layer 23. In step S3, a TFB film, a PVK film, or a PTAA film may be formed instead of the poly-TPD film.

[0102] Next, the light-emitting layer 24 is formed on the hole transport layer 23. In this embodiment, an example will be described in which the light-emitting layer 24 is formed by forming the first light-emitting layer 40 and then forming the second light-emitting layer 41 on the first light-emitting layer 40.

[0103] In the process of forming the light-emitting layer 24 according to this embodiment, first, a first solution synthesized in advance in a separate process is applied to the hole transport layer 23 by a spin coating method or the like (step S5). The first solution is a mixed solution containing the quantum dots 50 and a first inorganic precursor that is a precursor of the first matrix material 52.

[0104] The first inorganic precursor includes, for example, an oxygen source and a group 14 element atom source for the first matrix material 52. For example, when the first matrix material 52 includes silicon oxide, the first inorganic precursor may include ((3-mercaptopropyl)trimethoxysilane) (MPS) represented by the following chemical formula as a silicon source.

[0105] Alternatively, when the first matrix material 52 includes silicon oxide, the first inorganic precursor may include tetramethyl orthosilicate (TMOS) as a silicon source, as shown in the following chemical formula:

[0106] When the first inorganic precursor contains MPS, the MPS may be coordinated to the outermost surface of the quantum dots 50 via a sulfur atom in the carbon chain. Furthermore, for example, when the first matrix material 52 contains germanium oxide, the first inorganic precursor may contain tetramethoxygermanium, represented by the following chemical formula, as a germanium source.

[0107] When the first matrix material 52 includes a ternary oxide, the first inorganic precursor may include an atomic source of a first Group 14 element and an atomic source of a second Group 14 element. For example, when the first matrix material 52 includes a ternary oxide including silicon atoms and germanium atoms, the first inorganic precursor may include both the silicon source and the germanium source described above.

[0108] The concentration ratio of the first Group 14 element atoms to the second Group 14 element atoms in the ternary oxide of the first matrix material 52 may be adjusted by adjusting the concentration ratio of the first Group 14 element atom source to the second Group 14 element atom source contained in the first inorganic precursor. For example, the concentration ratio of silicon atoms to germanium atoms in the ternary oxide of the first matrix material 52 may be adjusted by adjusting the concentration ratio of the silicon source to the germanium source contained in the first inorganic precursor.

[0109] Next, the first solution applied on the hole transport layer 23 is heated (step S6). For example, the heating in step S6 may be performed at a first temperature of 150° C. Specifically, in step S6, the first solution applied on the hole transport layer 23 may be heated in an atmosphere of 150° C. for 30 minutes.

[0110] As a result, the solvent of the first solution volatilizes, and the first inorganic precursor in the first solution is converted to form a first matrix material 52. Here, the first inorganic precursor in the first solution is converted by heating in step S6, and the first matrix material 52 is successively formed around the quantum dots 50 in the first solution. Therefore, in step S6, the first matrix material 52 is formed so as to fill the spaces between the multiple quantum dots 50. In this way, the first light-emitting layer 40 is formed, which includes the multiple quantum dots 50 and the first matrix material 52 filling the spaces between the quantum dots 50.

[0111] Next, a second solution synthesized in advance in a separate process is applied onto the first light-emitting layer 40 by spin coating or the like (step S7). The second solution is a mixed solution containing a plurality of quantum dots 50 and a second inorganic precursor that is a precursor of the second matrix material 53.

[0112] The second inorganic precursor includes, for example, an oxygen source and a Group 14 element atom source for the second matrix material 53. In particular, the second inorganic precursor may include the same material as the first inorganic precursor. However, when the first matrix material 52 includes a ternary oxide, the second inorganic precursor may have a different concentration ratio of the first Group 14 element atom source to the second Group 14 element atom source compared to the first inorganic precursor. For example, the ratio of the germanium source concentration to the silicon source concentration in the second inorganic precursor may be higher than the ratio of the germanium source concentration to the silicon source concentration in the first inorganic precursor. In this embodiment, the second matrix material 53 may be formed having a lower silicon atom concentration and a higher germanium atom concentration than the first matrix material 52.

[0113] The concentration of the quantum dots 50 relative to the concentration of the second inorganic precursor in the second solution is lower than the concentration of the quantum dots 50 relative to the concentration of the first inorganic precursor in the first solution. This makes it possible to make the concentration of the quantum dots 50 in the second light-emitting layer 41 formed by the method described below lower than the quantum dots 50 in the first light-emitting layer 40, while making the amount of the second solution applied in step S8 substantially the same as the amount of the first solution applied in step S6.

[0114] Next, the second solution applied on the first light-emitting layer 40 is heated (step S8). For example, the heating in step S8 may be performed at a second temperature of 200° C. that is higher than the first temperature. Specifically, in step S8, the second solution applied on the first light-emitting layer 40 may be heated in an atmosphere of 200° C. for 30 minutes.

[0115] As a result, the solvent of the second solution volatilizes and the second inorganic precursor in the second solution is converted to form the second matrix material 53. As a result, the second light-emitting layer 41 is formed, which includes the plurality of quantum dots 50 and the second matrix material 53 filling the spaces between the quantum dots 50, similar to the first light-emitting layer 40.

[0116] Note that both steps S6 and S8 include a process of heating a solution containing a precursor having an atomic source of a Group 14 element of the matrix material 51 and an oxygen source. Here, heating the solution may cause the oxygen source contained in the precursor in the solution to volatilize together with the solvent. Furthermore, the amount of volatilization of the oxygen source tends to increase as the heating temperature of the solution increases.

[0117] Here, the heating temperature of the second solution in step S8 is the second temperature, which is higher than the first temperature, which is the heating temperature of the first solution in step S6. Therefore, the proportion of the oxygen source volatilized from the second solution in step S8 is higher than the proportion of the oxygen source volatilized from the first solution in step S6.

[0118] As a result, the density of atomic defects of oxygen atoms in the second matrix material 53 formed in step S8 is higher than the density of atomic defects of oxygen atoms in the first matrix material 52 formed in step S6. Therefore, the above steps form the light-emitting layer 24 having the matrix material 51 in which the density of atomic defects of oxygen atoms monotonically increases in the direction from the anode 21 to the cathode 26. In other words, the above steps form the light-emitting layer 24 having the matrix material 51 in which the concentration of oxygen atoms monotonically decreases in the direction from the anode 21 to the cathode 26.

[0119] However, in this embodiment, the first inorganic precursor and the second inorganic precursor contain both the atomic source of the first Group 14 element and the atomic source of the second Group 14 element, resulting in the formation of a light-emitting layer 24 having a matrix material 51 containing a ternary oxide. Also, in this embodiment, the first inorganic precursor and the second inorganic precursor contain different concentrations of the atomic source of the first Group 14 element and the atomic source of the second Group 14 element. In other words, the second matrix material 53 of the formed light-emitting layer 24 has a smaller band gap and a different ratio of the atomic concentration of the first Group 14 element and the atomic concentration of the second Group 14 element compared to the first matrix material 52. In this case, the oxygen atom concentrations of the first inorganic precursor and the second inorganic precursor may be equal, and therefore the heating temperatures in steps S6 and S8 may be the same.

[0120] Following the formation of the light-emitting layer 24, an electron transport layer 25 is formed on the light-emitting layer 24 (step S9). In step S9, a solution in which zinc oxide nanoparticles 30 are dispersed in an ethanol solvent may be applied to the light-emitting layer 24 by spin coating or the like under a nitrogen atmosphere, and the solution may be dried to form an electron transport layer 25 with a film thickness of 60 nm. The zinc oxide nanoparticles 30 may be doped with at least one of Li, Mg, Al, Ti, Ga, and Zr. Alternatively, the nanoparticles 30 may be titanium oxide or zirconium oxide nanoparticles.

[0121] Next, a cathode 26 is formed on the electron transport layer 25 (step S10). In step S10, the cathode 26 may be formed by vacuum deposition of a 50 nm-thick silver thin film. Thus, the light-emitting element 11 is manufactured. The manufacture of the display device 1 may be completed upon completion of the manufacture of the light-emitting element 11 described above. Alternatively, a sealing layer or the like for sealing or protecting the light-emitting element 11 may be formed following the manufacture of the light-emitting element 11. According to the above method, a light-emitting element 11 with improved luminous efficiency due to reduced reactive current in the light-emitting layer 24 can be formed, and a display device 1 including the light-emitting element 11 can be manufactured.

[0122] The effects of the light emitting device 11 according to this embodiment will be described in more detail by fabricating quantum dot layers according to examples and measuring the physical properties of the quantum dot layers.

[0123] In the examples, blue-emitting quantum dots were first dispersed in a nonpolar solvent such as octane at a concentration of 2 mg / mL to prepare a quantum dot dispersion. Next, MPS and TMOS were added to the dispersion to a concentration of 0.4 mol / L and 0.04 mol / L, respectively, to prepare a first solution. Next, in the examples, a halogen source such as zinc chloride was dissolved in a polar solvent such as DMF to a concentration of 0.4 mol / L to prepare a second solution.

[0124] Next, equal amounts of the first solution and the second solution were mixed and vigorously stirred for 12 hours in a container with a stir bar. As a result, the quantum dots were transferred from the layer of the first solution to the interface between the first and second solutions. The layer containing the quantum dots was extracted from the mixed solution and centrifuged at 4000 rpm for 5 minutes to precipitate the quantum dots. Next, all the solvent was removed from the solution containing the precipitate, and the precipitate was dispersed in toluene to obtain a coating solution of the desired concentration, specifically a coating solution containing quantum dots at a concentration of 20 mg / mL.

[0125] Next, the coating solution prepared by the above method was applied to a glass substrate by spin coating in a nitrogen atmosphere and baked at 100°C for 30 minutes to form a quantum dot layer with a thickness of approximately 30 nm on the glass substrate.

[0126] 5 is a schematic plan view of a quantum dot layer 24A according to an example, measured by observation with a differential interference contrast microscope. As shown in FIG. 5, the quantum dot layer 24A includes a matrix material 51A having the same configuration as the matrix material 51 according to the present embodiment. As shown in FIG. 5, a plurality of cracks 24C are present in the matrix material 51A on the upper surface of the quantum dot layer 24A.

[0127] The height of each position on a specific straight line on the top surface of the quantum dot layer 24A was measured by observing the top surface of the quantum dot layer 24A with a step gauge. Figure 6 is a graph showing the height at each position on the top surface of the quantum dot layer 24A measured using the above method. In the graph of Figure 6, the horizontal axis represents the distance (unit: μm) from a certain predetermined position, and the vertical axis represents the height (unit: Å) of the top surface of the quantum dot layer 24A. The height shown in the graph of Figure 6 was calculated by defining the height of the top surface of the quantum dot layer 24A at a predetermined position as 0 Å, with negative values ​​being used when the height is lower than that height and positive values ​​being used when the height is higher than that height.

[0128] 6, multiple cracks 24C have occurred on the upper surface of the quantum dot layer 24A, resulting in unevenness on the upper surface of the quantum dot layer 24A. It is believed that these cracks 24C in the quantum dot layer 24A were generated during the heating step of the coating solution, in which the precursor is converted into the matrix material 51A. As such, the surface of the matrix material 51A containing an oxide containing a Group 14 element is prone to have uneven shapes such as the cracks 24C during the formation step.

[0129] Therefore, in the light-emitting layer 24 of the light-emitting element 11, a crack like the above-described crack 24C may occur on the upper surface on the cathode 26 side. In this case, foreign matter such as oxygen or moisture can easily penetrate into the inside of the light-emitting layer 24 through the crack.

[0130] Meanwhile, as described above, in this embodiment, the concentration of the quantum dots 50 in the light-emitting layer 24 monotonically decreases in the direction from the anode 21 to the cathode 26. In other words, in the light-emitting layer 24 according to this embodiment, the concentration of the quantum dots 50 on the cathode 26 side is lower than the concentration of the quantum dots 50 on the anode 21 side. As a result, even if a crack occurs in the surface of the light-emitting layer 24 on the cathode 26 side, the average distance between the crack and the quantum dots 50 becomes longer. Therefore, with the above configuration, even if a crack occurs in the surface of the light-emitting layer 24 on the cathode 26 side, the light-emitting element 11 reduces deterioration of the quantum dots 50 due to foreign matter seeping through the crack.

[0131] [Embodiment 2] <Matrix Material Layer> A display device 2 according to this embodiment will be described with reference to Fig. 7. Fig. 7 is a schematic side cross-sectional view of the display device 2 according to this embodiment. The display device 2 according to this embodiment has the same configuration as the display device 1 according to the previous embodiment, except that it includes a light-emitting element 12 instead of the light-emitting element 11. The light-emitting element 12 has the same configuration as the light-emitting element 11 according to the previous embodiment, except that it includes a light-emitting layer 27 instead of the light-emitting layer 24.

[0132] The light-emitting layer 27 includes, in order from the anode 21 side, a first light-emitting layer 40 and a matrix material layer 42. The first light-emitting layer 40 according to this embodiment has the same configuration as the first light-emitting layer 40 according to the previous embodiment. The matrix material layer 42 according to this embodiment differs from the second light-emitting layer 41 according to the previous embodiment in that it includes only a second matrix material 53 and does not include quantum dots 50. The second matrix material 53 included in the matrix material layer 42 is part of the matrix material 51 and is continuous with the first matrix material 52 included in the first light-emitting layer 40 on the cathode 26 side. In other words, the first matrix material 52 and the second matrix material 53 include a continuous film of the matrix material 51.

[0133] In other words, the light-emitting layer 27 has a first light-emitting layer 40 as a quantum dot layer including quantum dots 50 and a first matrix material 52 as a first portion of the matrix material 51. The light-emitting layer 27 also has a matrix material layer 42 including a second matrix material 53 as a second portion of the matrix material 51. Here, the matrix material layer 42 includes only the matrix material 51 out of the quantum dots 50 and the matrix material 51. As long as the above configuration is satisfied, the matrix material layer 42 may include a material different from the quantum dots 50 and the matrix material 51.

[0134] Therefore, in this embodiment as well, the light-emitting layer 27 has a plurality of quantum dots 50 and also has, as the matrix material 51, a first matrix material 52 and a second matrix material 53 in this order from the anode 21 side.

[0135] Therefore, in this embodiment as well, the matrix material 51 contains an oxide of a Group 14 element and fills the spaces between at least two quantum dots 50. In this case, the concentration of oxygen atoms in the second matrix material 53 is lower than the concentration of oxygen atoms in the first matrix material 52.

[0136] Alternatively, the matrix material 51 contains a ternary oxide having a first Group 14 element and a second Group 14 element, and fills the spaces between at least two quantum dots 50. In this case, the band gap of the second matrix material 53 is smaller than the band gap of the first matrix material 52, and the ratio of the atomic concentration of the first Group 14 element to the atomic concentration of the second Group 14 element differs between the first matrix material 52 and the second matrix material 53.

[0137] With the above configuration, the light-emitting element 12 suppresses the movement of electrons injected from the electron transport layer 25 in the light-emitting layer 27 between the quantum dots 50 for the same reasons as those described for the light-emitting element 11. Therefore, the light-emitting element 12 reduces the reactive current in the light-emitting layer 27, improving the luminous efficiency and reliability.

[0138] In particular, in this embodiment, the light-emitting layer 27 of the light-emitting element 12 has a matrix material layer 42 that does not include quantum dots 50. Therefore, the light-emitting layer 27 does not include quantum dots 50, which may be deteriorated by foreign matter from the cathode 26 side, in the matrix material layer 42 on the cathode 26 side. Furthermore, since the matrix material layer 42 does not include quantum dots 50, the effective film thickness of the second matrix material 53 is increased, thereby enhancing the protective effect of the light-emitting layer 27. In addition, the light-emitting layer 27 efficiently transports electrons, which have higher mobility than holes, from the matrix material layer 42 to the first light-emitting layer 40, thereby obtaining light emission from the quantum dots 50 in the first light-emitting layer 40. Therefore, the light-emitting element 12 can improve its luminous efficiency while more efficiently enhancing the protective effect of the matrix material 51 of the light-emitting layer 27 on the quantum dots 50.

[0139] The thickness D42 of the matrix material layer 42 may be 1.2 nm or more, or may be 6 nm or more, so that the matrix material layer 42 has a thickness approximately twice or more the unit lattice of the second matrix material 53, thereby efficiently enhancing the protection effect of the light-emitting layer 27.

[0140] If the thickness of the matrix material layer 42 is large, there is a possibility that the driving voltage of the light emitting element 12 increases. For this reason, in this embodiment, in a region within 1.2 nm in the thickness direction from the end face of the light emitting layer 27 on the cathode 26 side, the density of free electrons in the matrix material 51 is set to 1×10 １８ cm －３ The above-described configuration is preferable because it is possible to sufficiently increase the free electron density in the region, and therefore the light-emitting layer 27 can reduce the resistivity of the matrix material 51 in the region, thereby minimizing an increase in the driving voltage of the light-emitting element 12 and increasing the protective effect of the quantum dots 50.

[0141] In this embodiment, the boundary between the light-emitting layer 27 and the electron transport layer 25 may also be confirmed by observing a cross section passing through the light-emitting layer 27 and the electron transport layer 25 and confirming the composition of the material at each position on the cross section. In this embodiment, for example, a portion where the concentration of at least one atom contained in the second matrix material 53 is 15% or less may be considered to be the boundary between the light-emitting layer 27 and the electron transport layer 25. Alternatively, a portion where the concentration of at least one atom contained in the second matrix material 53 decreases by 15% or more may be considered to be the boundary between the light-emitting layer 27 and the electron transport layer 25. Note that if there is an atom with a high concentration in only one of the second matrix material 53 and the electron transport layer 25, the portion where the concentration of that atom changes by 15% or more may be considered to be the boundary between the light-emitting layer 27 and the electron transport layer 25. The criteria for confirming the boundary are given priority in the order of their description; in other words, an earlier description takes precedence over a later description.

[0142] In other words, even if the light-emitting layer 27 has a portion that does not contain quantum dots 50, such as the matrix material layer 42, the portion where the composition of the second matrix material 53 can be confirmed can be considered to be included in the matrix material layer 42, and therefore in the light-emitting layer 27.

[0143] The light-emitting element 12 according to this embodiment may be manufactured by the same method as the manufacturing method of the light-emitting element 11 according to the previous embodiment, in accordance with the flowchart shown in FIG. 4 , except for the material of the second solution applied onto the first light-emitting layer 40 in step S7. In this embodiment, the second solution contains only the second inorganic precursor out of the quantum dots 50 and the second inorganic precursor, which is a precursor of the second matrix material 53. For example, the second solution may contain other materials except for the quantum dots 50 and the second inorganic precursor. As a result, in step S8, a matrix material layer 42 having the second matrix material 53 but not the quantum dots 50 is formed on the first light-emitting layer 40.

[0144] <Multiple Quantum Dot Layers> The display device 3 according to this embodiment will be described with reference to Fig. 8. Fig. 8 is a schematic side cross-sectional view of the display device 3 according to this embodiment. The display device 3 according to this embodiment has the same configuration as the display device 2 according to the previous embodiment, except that it includes a light-emitting element 13 instead of the light-emitting element 12. The light-emitting element 13 has the same configuration as the light-emitting element 12 according to the previous embodiment, except that it includes a light-emitting layer 28 instead of the light-emitting layer 27.

[0145] The light-emitting layer 28 includes, in order from the anode 21 side, a first light-emitting layer 40, a second light-emitting layer 43, a third light-emitting layer 44, and a matrix material layer 45. The first light-emitting layer 40 has the same configuration as the first light-emitting layer 40 according to each of the above-described embodiments. The second light-emitting layer 43 includes a plurality of quantum dots 50 and a second matrix material 54 filling the spaces between at least two of the quantum dots 50. The third light-emitting layer 44 includes a plurality of quantum dots 50 and a third matrix material 55 filling the spaces between at least two of the quantum dots 50. The matrix material layer 45 includes, instead of the second matrix material 53, a fourth matrix material 56 that is part of the matrix material 51 and is continuous with the third matrix material 55 on the cathode 26 side of the third matrix material 55.

[0146] The quantum dots 50 contained in the second light-emitting layer 43 and the third light-emitting layer 44 have the same configuration as the quantum dots 50 according to the above-described embodiments. The second matrix material 54, the third matrix material 55, and the fourth matrix material 56 have the same configuration as the first matrix material 52 according to the above-described embodiments, except for the concentration of oxygen atoms or the concentration ratio of metal atoms in the ternary oxide.

[0147] Therefore, the light-emitting layer 28 includes quantum dots 50 and matrix materials 51, which are a first matrix material 52, a second matrix material 54, a third matrix material 55, and a fourth matrix material 56. In particular, the light-emitting layer 28 includes a first light-emitting layer 40, a second light-emitting layer 43, and a third light-emitting layer 44 as quantum dot layers each including the quantum dots 50 and the matrix material 51. The light-emitting layer 28 also includes a matrix material layer 45 that includes only the matrix material 51 out of the quantum dots 50 and the matrix material 51.

[0148] In this embodiment, the concentration of oxygen atoms gradually decreases in the order of the first matrix material 52, the second matrix material 54, the third matrix material 55, and the fourth matrix material 56. In other words, also in this embodiment, the concentration of oxygen atoms gradually decreases in the matrix material 51 in the direction from the anode 21 toward the cathode 26.

[0149] In this case, the density of atomic defects of oxygen atoms increases in the order of the first matrix material 52, the second matrix material 54, the third matrix material 55, and the fourth matrix material 56. In other words, also in this embodiment, the density of atomic defects of oxygen atoms increases in the matrix material 51 in the direction from the anode 21 toward the cathode 26.

[0150] Alternatively, the band gap decreases in the order of the first matrix material 52, the second matrix material 54, the third matrix material 55, and the fourth matrix material 56. In this case, the matrix material 51 contains a ternary oxide, and the ratio of the atomic concentration of the first Group 14 element to the atomic concentration of the second Group 14 element differs among the first matrix material 52, the second matrix material 54, the third matrix material 55, and the fourth matrix material 56.

[0151] For the same reasons as those described in the above embodiments, the light-emitting device 13 according to this embodiment suppresses the movement of electrons injected from the electron transport layer 25 in the light-emitting layer 28 between the quantum dots 50. Therefore, the light-emitting device 13 reduces the reactive current in the light-emitting layer 28, and improves the light-emitting efficiency and reliability.

[0152] The concentration of the quantum dots 50 in the light-emitting layer 28 decreases in the order of the first light-emitting layer 40, the second light-emitting layer 43, and the third light-emitting layer 44. Furthermore, as described above, the matrix material layer 45 does not contain quantum dots 50. Therefore, for the same reasons as those described in the above embodiments, the light-emitting element 13 can improve the luminous efficiency while more efficiently enhancing the protective effect of the matrix material 51 of the light-emitting layer 28 on the quantum dots 50.

[0153] In particular, the light-emitting element 13 according to this embodiment can reduce the decrease in light-emitting efficiency even when the efficiency of hole injection into the light-emitting layer 28 decreases due to deterioration of each component. For example, assume that, at the time of shipment of the display device 3, the recombination of electrons and holes in the light-emitting layer 28 occurs in the quantum dots 50 near the second light-emitting layer 43, and the quantum dots 50 in the second light-emitting layer 43 mainly emit light. In this case, the mobility and injection efficiency of holes from the hole injection layer 22 to the light-emitting layer 28 may decrease due to deterioration caused by operation of the display device 3 or aging as each component of the light-emitting element 13 deteriorates. This is generally because the durability of the hole injection layer 22 or the hole transport layer 23 is inferior to the durability of the electron transport layer 25. This is particularly noticeable when an organic material is used for the hole transport layer 23 and an inorganic material is used for the electron transport layer 25. As a result, the recombination of electrons and holes in the light-emitting layer 28 may occur in the quantum dots 50 closer to the anode 21 than the second light-emitting layer 43. Even in this case, recombination of electrons and holes in the light-emitting layer 28 can occur in the quantum dots 50 near the first light-emitting layer 40, causing the quantum dots 50 in the first light-emitting layer 40 to emit light, thereby reducing a decrease in the light-emitting efficiency of the light-emitting element 13. In other words, even if hole injection into the light-emitting layer 28 deteriorates due to deterioration of each layer of the light-emitting element 13, the light-emitting position is simply shifted toward the anode 21, and a decrease in the light-emitting efficiency of the light-emitting element 13 due to a deterioration in the carrier balance in the light-emitting layer 28 can be suppressed. Therefore, the light-emitting layer 28 can improve the reliability of the light-emitting element 13.

[0154] The manufacturing method of the light emitting element 13 according to this embodiment can be the same as the manufacturing method of the light emitting element 12 according to the previous embodiment, except for the method of forming the light emitting layer 28. In this embodiment, the first light emitting layer 40, the second light emitting layer 43, and the third light emitting layer 44 of the light emitting layer 28 may be formed by repeatedly performing the above-described steps S5 and S6.

[0155] However, in this embodiment, the concentration of the quantum dots 50 in the solution to be applied in step S5 is gradually decreased, and the heating temperature of the solution in step S6 is gradually increased, and steps S5 and S6 are repeatedly performed. Alternatively, the concentration ratio of the atomic source of the first Group 14 element to the atomic source of the second Group 14 element in the precursor in the solution to be applied in step S5 is varied. This makes it possible to form the above-described first light-emitting layer 40, second light-emitting layer 43, and third light-emitting layer 44. Note that the concentration of the quantum dots 50 in the solution to be applied does not necessarily have to be changed, and may be approximately the same.

[0156] Furthermore, in this embodiment, the matrix material layer 45 may be formed by performing steps S7 and S8 according to the previous embodiment. However, in this embodiment, the heating temperature of the solution in step S8 is higher than the heating temperature of the solution in step S6 described above, and steps S7 and S8 are performed. Alternatively, the concentration ratio of the atomic source of the first Group 14 element to the atomic source of the second Group 14 element in the precursor in step S8 is made different from the concentration ratio in each of steps S6 described above. This allows the formation of the matrix material layer 45 described above, and the light-emitting layer 28.

[0157] 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.

[0158] REFERENCE SIGNS LIST 1 Display device 11 Light-emitting element 20 Substrate 21 Anode 24 Light-emitting layer 25 Electron transport layer 26 Cathode 30 Nanoparticles 50 Quantum dots 51 Matrix material 52 First matrix material (first part) 53 Second matrix material (second part)

Claims

1. A light-emitting device comprising: an anode; a cathode; and a light-emitting layer located between the anode and the cathode, wherein the light-emitting layer has a plurality of quantum dots and a matrix material, wherein the matrix material contains an oxide of a Group 14 element and fills spaces between at least two of the quantum dots, and the matrix material has a first portion and a second portion located closer to the cathode than the first portion and having a lower concentration of oxygen atoms than the first portion.

2. The light-emitting device according to claim 1, wherein the concentration of oxygen atoms in said matrix material monotonically decreases in the direction from said anode to said cathode.

3. A light-emitting device comprising: an anode; a cathode; and a light-emitting layer located between the anode and the cathode, wherein the light-emitting layer has a plurality of quantum dots and a matrix material, wherein the matrix material contains a ternary oxide having a first Group 14 element and a second Group 14 element and fills spaces between at least two of the quantum dots, and the matrix material has a first portion and a second portion located closer to the cathode than the first portion, the second portion having a smaller band gap than the first portion, and the ratio of the atomic concentration of the first Group 14 element to the atomic concentration of the second Group 14 element being different from that of the first portion.

4. The light-emitting device according to claim 3, wherein the band gap of the matrix material monotonically decreases in the direction from the anode to the cathode, and the ratio of the atomic concentration of the second Group 14 element to the atomic concentration of the first Group 14 element monotonically increases or monotonically decreases in the direction from the anode to the cathode.

5. The light-emitting device according to claim 3 or 4, wherein the first Group 14 element is silicon, and the concentration of silicon atoms in the second portion is lower than the concentration of silicon atoms in the first portion.

6. A light-emitting element according to any one of claims 3 to 5, wherein the second Group 14 element is germanium, and the concentration of germanium atoms in the second portion is higher than the concentration of germanium atoms in the first portion.

7. The light-emitting element according to any one of claims 3 to 6, wherein the ternary oxide contains both silicon atoms and germanium atoms.

8. A light-emitting element according to any one of claims 3 to 7, wherein the concentration of oxygen atoms in the second portion is lower than the concentration of oxygen atoms in the first portion.

9. The light-emitting device according to claim 8, wherein the concentration of oxygen atoms in said matrix material monotonically decreases in the direction from said anode to said cathode.

10. The light-emitting device according to any one of claims 1 to 9, wherein the concentration of the quantum dots in the light-emitting layer monotonically decreases in the direction from the anode to the cathode.

11. A light-emitting element according to any one of claims 1 to 10, wherein the light-emitting layer includes a quantum dot layer containing the quantum dots and the matrix material, and a matrix material layer located closer to the cathode than the quantum dot layer and containing only the matrix material of the quantum dots and the matrix material.

12. The light-emitting device according to claim 11, wherein the matrix material layer has a thickness of 1.2 nm or more.

13. The light-emitting device of any one of claims 1 to 12, further comprising an electron transport layer having a plurality of nanoparticles and positioned between the light-emitting layer and the cathode.

14. A display device comprising: a substrate; and a plurality of light-emitting elements according to any one of claims 1 to 13 on the substrate.

15. The display device according to claim 14, wherein at least one of the light-emitting elements has the anode on the substrate side of the light-emitting layer.

16. A method for manufacturing a light-emitting element, comprising: forming an anode; forming a cathode; and forming a light-emitting layer located between the anode and the cathode and having a plurality of quantum dots and a matrix material, wherein the formation of the light-emitting layer comprises forming the matrix material containing an oxide of a Group 14 element and filling spaces between at least two of the quantum dots, and the formation of the matrix material comprises forming a first portion; and forming a second portion located closer to the cathode than the first portion and having a lower concentration of oxygen atoms than the first portion.

17. A method for manufacturing a light-emitting element, comprising: forming an anode; forming a cathode; and forming a light-emitting layer located between the anode and the cathode and having a plurality of quantum dots and a matrix material, wherein the formation of the light-emitting layer comprises forming the matrix material, which contains a ternary oxide having a first Group 14 element and a second Group 14 element, and fills spaces between at least two of the quantum dots, and the formation of the matrix material comprises: forming a first portion; and forming a second portion, which has a smaller band gap than the first portion and a different ratio of atomic concentration of the first Group 14 element to the atomic concentration of the second Group 14 element from that of the first portion.

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