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

Abstract

A light-emitting element (2) comprises an emissive layer (23) located between an anode (21) and a cathode (25). The emissive layer contains a plurality of quantum dots (30) and an inorganic matrix (31) which fills the space in between the plurality of quantum dots and which contains a metal oxide. The inorganic matrix includes a second part (42) and a first part (41) containing the metal oxide. The ratio of oxygen atoms to metal atoms in the second part is lower than the ratio of oxygen atoms to metal atoms in the first part, and is equal to or higher than the ratio of oxygen atoms to metal atoms in metal oxides that are stoichiometric.

Description

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

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

[0002] Non-Patent Document 1 discloses a quantum dot structure having quantum dots (semiconductor nanoparticles) in a matrix containing silicon oxide (silica) as a light emitter used in a light-emitting device. The structure having quantum dots in a matrix protects the quantum dots with the matrix, improving the reliability of the quantum dots.

[0003] Cong Shen, Yanqing Zhu et al. Blue-Emitting InP / GaP / ZnS Quantum Dots with Enhanced Stability by Siloxane Capping: Implication for Electroluminescent Devices. ACS Appl. Nano Mater. 2022.5.2, pp. 2801-2811.

[0004] In a light-emitting element having a structure such as that described in Non-Patent Document 1 in its light-emitting layer, while the quantum dots can be protected by the matrix, the matrix may inhibit the injection of charges from each electrode into the quantum dots, which may increase the resistance of the light-emitting element or reduce the light-emitting efficiency of the light-emitting element.

[0005] A light-emitting element according to one aspect of the present disclosure includes an anode, a cathode facing the anode, and a light-emitting layer located between the anode and the cathode. The light-emitting layer includes a plurality of quantum dots and an inorganic matrix having a metal oxide and filling spaces between the quantum dots. The inorganic matrix includes a first portion having the metal oxide, and a second portion having the metal oxide, the second portion having a ratio of oxygen atoms to metal atoms that is lower than the ratio of oxygen atoms to metal atoms in the first portion and equal to or greater than the ratio of oxygen atoms to metal atoms in the metal oxide, which is stoichiometric.

[0006] A method for manufacturing a light-emitting element according to one aspect of the present disclosure is a method for manufacturing a light-emitting element including an anode, a cathode facing the anode, and a light-emitting layer located between the anode and the cathode, the method including forming the light-emitting layer, wherein the light-emitting layer includes a plurality of quantum dots and an inorganic matrix having a metal oxide and filling spaces between the plurality of quantum dots, the inorganic matrix including a first portion having the metal oxide, and a second portion having the metal oxide, the second portion having the metal oxide and having a ratio of oxygen atoms to metal atoms that is lower than the ratio of oxygen atoms to metal atoms in the first portion and equal to or greater than the ratio of oxygen atoms to metal atoms in the metal oxide, which is stoichiometric.

[0007] According to a configuration according to one aspect of the present disclosure, a light-emitting element can be designed to improve the efficiency of injection of charges from each electrode into the quantum dots while protecting the quantum dots.

[0008] 1 is a schematic side cross-sectional view of a display device according to embodiment 1. FIG. 2 is a schematic view of a display device according to embodiment 1. FIG. 3 is a schematic enlarged view of a cross section of a light-emitting layer according to embodiment 1. FIG. 4 is a schematic view showing an inorganic matrix filling spaces between quantum dots according to embodiment 1. FIG. 5 is a schematic view showing an example of the structure of an inorganic matrix according to embodiment 1. FIG. 6 is a schematic view showing a reaction process of precursors according to embodiment 1. FIG. 7 is a schematic view showing an example of the structure of a metal oxide produced by a reaction of precursors according to embodiment 1. FIG. 8 is a schematic view showing another example of the structure of a metal oxide produced by a reaction of precursors according to embodiment 1. FIG. 9 is a flowchart showing a method for forming a light-emitting layer according to embodiment 1. FIG. 10 is a process side view showing a mixing step in the method for forming a light-emitting layer according to embodiment 1. FIG. 11 is a schematic view showing a first dispersion according to embodiment 1. FIG. 12 is a process cross-sectional view showing a part of the method for forming a light-emitting layer according to embodiment 1. FIG. 13 is a schematic side cross-sectional view of a display device according to embodiment 2. FIG. 14 is a schematic enlarged view of a cross section of a light-emitting layer according to embodiment 2. FIG. 15 is a flowchart showing a method for forming a light-emitting layer according to embodiment 2. FIG. 16 is a schematic view showing a third dispersion according to embodiment 2. FIG. 17 is a process cross-sectional view showing a part of the method for forming a light-emitting layer according to embodiment 2. FIG. 18 is a flowchart showing a method for forming a light-emitting layer according to embodiment 3. FIG. 19 is a schematic view showing a fourth dispersion according to embodiment 3. It is a schematic diagram showing a fifth dispersion liquid according to Embodiment 3. It is a process cross-sectional view showing a part of a method for forming a light-emitting layer according to Embodiment 3. It is a schematic side cross-sectional view of a display device according to Embodiment 4.

[0009] [Embodiment 1] <Display Device: Overview> Hereinafter, embodiments 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 and may be hatched differently depending on the drawing. However, the components shown in each drawing of the present disclosure are merely examples, and the scales are not limited to those shown in the drawings. Furthermore, in this disclosure, even for components with different hatching, components assigned the same reference numerals have similar configurations as described above.

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

[0011] The structure of the display unit DA of the display device 1, particularly the structure of the light-emitting element 2, will be described in more detail with reference to Fig. 1. Fig. 1 is a schematic side cross-sectional view of the display device 1 according to an embodiment of the present disclosure, particularly showing a cross section perpendicular to the display surface of the display device 1 and passing through the light-emitting element 2. Note that the cross-sectional views of each process of the display device in the present disclosure and the cross-sectional views of each process described below show cross sections corresponding to the cross section of the display device 1 shown in Fig. 1.

[0012] 1, the display device 1 according to this embodiment includes a display section DA that includes the above-described plurality of light-emitting elements 2 and a substrate 3, and in particular includes the plurality of light-emitting elements 2 on the substrate 3. The display device 1 has a structure in which the layers of the light-emitting elements 2 are stacked on the substrate 3 on which, for example, TFTs (Thin Film Transistors) (not shown) are formed as pixel circuits PC. In this specification, the direction from the light-emitting elements 2 of the display device 1 to the substrate 3 is referred to as the "downward direction," and the direction opposite to the downward direction is referred to as the "upward direction."

[0013] The light-emitting element 2 includes an anode 21, a hole transport layer 22, a light-emitting layer 23, an electron transport layer 24, and a cathode 25, in this order from the substrate 3 side. The anode 21 is electrically connected to the TFT of the substrate 3.

[0014] <Outline of Light-Emitting Element> The structure of each layer of the light-emitting element 2 will be described in more detail below.

[0015] The anode 21 and the cathode 25 comprise conductive materials and are electrically connected to the hole transport layer 22 and the electron transport layer 24, respectively.

[0016] At least one of the anode 21 and the cathode 25 is a transparent electrode that transmits visible light. Examples of transparent electrodes that can be used include ITO (indium tin oxide), IZO (indium zinc oxide), ZnO, AZO (aluminum-doped zinc oxide, also known as ZAO), BZO (boron-doped zinc oxide), and FTO (fluorine-doped tin oxide). The transparent electrode may be formed by a sputtering method or the like. Alternatively, either the anode 21 or the cathode 25 may contain a metal material, preferably Al, Cu, Au, Ag, or Mg, or an alloy thereof, which have high reflectivity for visible light.

[0017] The hole transport layer 22 is a layer containing a hole transport material that transports holes from the anode 21 to the light-emitting layer 23. The material of the hole transport layer 22 can be an organic or inorganic material that has been conventionally used in light-emitting devices including quantum dots. For example, the hole transport layer 22 may contain at least one of polyvinylcarbazole (PVK) and [N,N'-bis(4-butylphenyl)-N,N'-bis(phenyl)-benzidine] (TPD) as the hole transport material. As the organic material for the hole transport layer 22, conductive compounds such as 4,4'-bis(carbazol-9-yl)biphenyl (CBP), polyphenylene vinylene (PPV), a composite of poly(3,4-ethylenedioxythiophene) (PEDOT) and polystyrene sulfonic acid (PSS) (PEDOT-PSS), and poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4'-(N-4-sec-butylphenyl)diphenylamine)]) (TFB) can be used. As the inorganic material for the hole transport layer 22, molybdenum oxide, NiO, Cr ２ O ３ , MgO, MgZnO, LaNiO ３ , MoO ３ , or W.O. ３ In particular, as the material for the hole transport layer 22, a material having a large electron affinity and ionization potential is suitable.

[0018] The electron transport layer 24 is a layer containing an electron transport material that transports electrons from the cathode 25 to the light-emitting layer 23. The material of the electron transport layer 24 can be an organic or inorganic material that has been conventionally used in light-emitting devices containing quantum dots. For example, the electron transport layer 24 can be made of zinc oxide (ZnO), zinc magnesium oxide (ZnMgO), titanium oxide (TiO), and tungsten oxide (WO ３ ), or may contain an inorganic nanoparticle material that is a nanoparticle of these inorganic materials. Alternatively, the electron transport layer 24 may contain an organic material as the electron transport material, such as tris(8-quinolinol)aluminum complex (Alq3), bathocuproine (BCP), or (2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole) (t-Bu-PBD). Note that, as the inorganic material of the electron transport layer 24, metal oxides such as ZnO, ZAO, ITO, InGaZnO, or electride may be used. In particular, a material with a small electron affinity is suitable as the material of the electron transport layer 24.

[0019] In this embodiment, the hole transport layer 22 and the electron transport layer 24 can be formed by vacuum deposition, sputtering, or a coating method using a colloidal solution using the above-mentioned materials. The light-emitting element 2 may also include a hole injection layer between the anode 21 and the hole transport layer 22, or an electron injection layer between the cathode 25 and the electron transport layer 24. The light-emitting element 2 may also include an intermediate layer between the hole transport layer 22 and the light-emitting layer 23, or between the electron transport layer 24 and the light-emitting layer 23. The hole injection layer, electron injection layer, and intermediate layer may all be formed by the same method as the hole transport layer 22 or the electron transport layer 24.

[0020] <Light-emitting layer: quantum dots> The light-emitting layer 23 according to this embodiment will be described in more detail with reference to Fig. 3 in addition to Fig. 1. Fig. 3 is an enlarged schematic view of the light-emitting layer 23 in the cross section shown in Fig. 1, and in particular, an enlarged view of region E1 shown in Fig. 1.

[0021] The light-emitting layer 23 includes a plurality of quantum dots 30 and an inorganic matrix 31 .

[0022] The quantum dots 30 may each have a core / shell structure, including a core and a shell surrounding the core. In this embodiment, the quantum dots 30 are, for example, luminescent semiconductor nanoparticles that emit light due to excitons generated by the recombination of injected electrons and holes. For example, the recombination of electrons and holes in the quantum dots 30 occurs primarily in the core. The core of the quantum dot 30 is a luminescent material that has a valence band level and a conduction band level and emits light due to the recombination of holes in the valence band level and electrons in the conduction band level. The quantum dots 30 emit light with a narrow spectrum due to the quantum confinement effect, enabling the production of light with a relatively deep chromaticity. Furthermore, the shell functions to suppress the occurrence of defects or dangling bonds in the core and reduce the recombination of carriers undergoing a deactivation process.

[0023] The core and shell materials of the quantum dots 30 may each contain materials used for the core and shell materials of conventionally known core / shell quantum dots. The quantum dots 30 may have a core / shell structure such as InP / ZnS, CdSe / ZnS, CdSe / ZnSe, CdSe / CdS, ZnSe / ZnS, or CIGS / ZnS. The shell may be formed of multiple layers containing multiple different materials.

[0024] The quantum dots 30 have a particle size of about 1 to 100 nm. The wavelength of light emitted from the quantum dots 30 can be controlled by the particle size. In particular, since the quantum dots 30 have a core / shell structure, the wavelength of light emitted from the quantum dots 30 can be controlled by controlling the particle size of the core. Therefore, by controlling the particle size of the quantum dots 30, the wavelength of light emitted by the display device 1 can be controlled.

[0025] <Light-Emitting Layer: Inorganic Matrix> In this embodiment, the inorganic matrix 31 fills the spaces between the quantum dots 30. The inorganic matrix 31 filling the spaces between the quantum dots 30 will be described in more detail with further reference to FIG. 4 . Schematic diagrams 401 and 402 in FIG. 4 are schematic diagrams showing the inorganic matrix 31 filling the spaces between the quantum dots 30. In particular, schematic diagrams 401 and 402 are respectively diagrams showing two examples of a set P of two quantum dots 30 and the region (space) K between them, as shown in FIG. 3 . In particular, schematic diagrams 401 and 402 are respectively diagrams showing set P1 and set P2, which are examples of sets of quantum dots 30A and quantum dots 30B.

[0026] In this specification, "the inorganic matrix 31 fills the spaces between the quantum dots 30" means that the inorganic matrix 31 fills at least the region K between the quantum dots 30A and 30B, as shown in the schematic diagram 401 of the set P1 in Fig. 4. The region K is a region surrounded by two lines (common circumscribing lines) tangent to the peripheries of the quantum dots 30A and 30B and the opposing peripheries of the quantum dots 30A and 30B in the cross section of the light-emitting layer 23. Therefore, as shown in the schematic diagram 402 of the set P2 in Fig. 4, the region K can exist even if the quantum dots 30A and 30B are close to each other, and the inorganic matrix 31 fills the region K.

[0027] The phrase "the inorganic matrix 31 fills the spaces between the quantum dots 30" does not necessarily mean that the region K between the quantum dots 30A and 30B is entirely composed of the inorganic matrix 31. For example, the region K between the quantum dots 30A and 30B may contain a material, such as a ligand, that is different from the material of the inorganic matrix 31. Specifically, for example, the light-emitting layer 23 may contain an organic ligand that is added to improve the dispersibility of the quantum dots 30 in a dispersion liquid used for coating and that coordinates to the outer surfaces of the quantum dots 30 in the dispersion liquid. In this case, in the light-emitting layer 23, from the viewpoint of improving the reliability of the light-emitting layer 23, for example, the weight ratio of the organic ligand to the total weight including the region K may be less than 5%.

[0028] 1 and 3 , the inorganic matrix 31 may fill the regions of the light-emitting layer 23 other than the regions where the quantum dots 30 are present. For example, the outer edges (upper and lower surfaces) of the light-emitting layer 23 may be covered with the inorganic matrix 31. Alternatively, the light-emitting layer 23 may be configured such that a portion of the inorganic matrix 31 extends from the outer edge thereof, with the quantum dots 30 positioned away from the outer edge. The outer edge of the light-emitting layer 23 may not be formed solely by the inorganic matrix 31, and some of the quantum dots 30 may be exposed from the inorganic matrix 31. The inorganic matrix 31 may refer to the portion of the light-emitting layer 23 other than the regions where the quantum dots 30 are present.

[0029] The inorganic matrix 31 is formed at a position in the thickness direction of the light-emitting layer 23 so as to have a thickness of 1000 nm in a plane perpendicular to the thickness direction. ２ In the light-emitting layer 23, the quantum dots 30 may be encapsulated in a continuous film of the inorganic matrix 31.

[0030] For example, when 60% or more of the surface of 80% or more of the quantum dots 30 constituting the light-emitting layer 23 is in contact with the continuous film of the inorganic matrix 31, the quantum dots 30 contained in the light-emitting layer 23 can be said to be encapsulated in the inorganic matrix 31. In this way, the light-emitting layer 23 containing the quantum dots 30 encapsulated in the inorganic matrix 31 has improved luminescence properties and a longer lifespan.

[0031] The light-emitting layer 23 has a thickness of 1000 nm or less in a plane direction perpendicular to the thickness direction at any position in the thickness direction. ２ The light-emitting layer 23 may contain one or more quantum dots 30 per layer. In this case, the light-emitting layer 23 generally contains quantum dots 30 at a concentration sufficient to function as a light-emitting layer of a light-emitting element.

[0032] The inorganic matrix 31 may be located, for example, around the entire periphery of the quantum dot 30. For example, as shown in Fig. 3, in any cross section passing through any quantum dot 30, the inorganic matrix 31 may be located around the entire periphery of the quantum dot 30. Here, "the inorganic matrix 31 is located around the entire periphery of the quantum dot 30" may mean that the inorganic matrix 31 is located around 90% or more of the periphery of the quantum dot 30. Furthermore, as shown in Fig. 3, the surface of the quantum dot 30 and the inorganic matrix 31 may be in contact with each other.

[0033] The band gap of the inorganic matrix 31 may be wider than the band gap of the constituent material of the quantum dots 30. When the quantum dots 30 have a core and a shell surrounding the core, the band gap of the inorganic matrix 31 may be wider than the band gap of the constituent material of the shell.

[0034] In this embodiment, the inorganic matrix 31 includes a metal oxide. Here, the metal oxide in this specification includes oxides having atoms that are sometimes generally called semimetal atoms, including silicon (Si), boron (B), germanium (Ge), arsenic (As), antimony (Sb), and tellurium (Te) as metal atoms.

[0035] In this disclosure, the term "atom" does not only mean that an atom exists as a single atom. In this disclosure, the term "atom" also includes an atom that exists in the form of a molecule having two or more atoms, including the atom and another atom, an atom that exists in the form of a complex, an atom that exists in the form of a compound, or an atom that exists in the form of an ion. However, the term "atom" in this disclosure does not limit the form of existence of other atoms. In other words, a metal atom includes an atom that exists in the form of a compound having a metal atom, and an atom that exists in the form of a metal ion. Regardless of the form of existence of a metal atom, if its presence in a substance can be identified by analysis, the substance may be considered to contain the metal atom.

[0036] In particular, the inorganic matrix 31 is, for example, silicon oxide (SiO ２) as a metal oxide. Alternatively, the inorganic matrix 31 may contain, for example, titanium oxide (TiO ２ ), aluminum oxide (Al ２ O ３ ), boron oxide (B ２ O ３ ), phosphorus oxide (P ２ O ５ ), germanium oxide (GeO ２ ), hafnium oxide (HfO ２ ), zinc oxide (ZnO), zirconium oxide (ZrO ２ ), tellurium oxide (TeO ２ ), bismuth oxide (Bi ２ O ３ ), vanadium oxide (V ２ O ５ ), antimony oxide (Sb ２ O ５ The compound may include one or more selected from the group consisting of lead oxide (PbO), lead oxide (PbO), and copper oxide (CuO). The chemical formulas written in parentheses after the compound names are representative examples.

[0037] The chemical formulas of the metal oxides described above are based on the case where the metal oxides have a stoichiometric composition ratio. However, a portion of the inorganic matrix 31 according to this embodiment contains a metal oxide with a composition ratio different from that of the stoichiometric metal oxide. In particular, the metal oxide contained in the inorganic matrix 31 according to this embodiment has a ratio of oxygen atoms to metal atoms that is equal to or greater than the ratio of oxygen atoms to metal atoms in the stoichiometric metal oxide.

[0038] In this specification, the stoichiometric metal oxide may refer to a metal oxide having a composition ratio that can exist most stably among the chemical formulas that can be obtained from the ratio of the valences of the metal atoms and oxygen atoms of the metal oxide. For example, silicon oxide as a metal oxide is SiO ２ and SiO as the chemical formula, but unless otherwise specified, in this specification, stoichiometric silicon oxide is SiO ２Similarly, stoichiometric titanium oxide refers to silicon oxide having the same composition ratio as TiO ２ , stoichiometric aluminum oxide is Al ２ O ３ , stoichiometric boron oxide is B ２ O ３ , stoichiometric phosphorus oxide is P ２ O ５ , stoichiometric germanium oxide is GeO ２ Hafnium oxide, which is stoichiometric, is HfO ２ , stoichiometric zinc oxide is ZnO, stoichiometric zirconium oxide is ZrO ２ , stoichiometric tellurium oxide is TeO ２ Bismuth oxide, which is stoichiometric, is Bi ２ O ３ , stoichiometric vanadium oxide is V ２ O ５ , stoichiometric antimony oxide is Sb ２ O ５ Stoichiometric lead oxide refers to PbO, and stoichiometric copper oxide refers to a metal oxide having the same composition ratio as CuO.

[0039] <Metal Oxide Structure> Specific examples of the structure of the metal oxide contained in the inorganic matrix 31 will be described with reference to Figs. 5 to 8. Fig. 5 is a schematic diagram showing an example of the structure of the metal oxide contained in the inorganic matrix 31 according to this embodiment. Fig. 6 is a schematic diagram showing the reaction process of the precursor according to this embodiment. Figs. 7 and 8 are schematic diagrams showing examples of the structure of the metal oxide obtained by the reaction of the precursor described above.

[0040] 5 shows an example of the structure of the inorganic matrix 31 when the inorganic matrix 31 contains silicon oxide as the metal oxide 60. As will be described later, this embodiment will be described assuming that the inorganic matrix 31 is formed from a precursor containing tetramethyl orthosilicate (TMOS) represented by the following formula (1). However, this embodiment is not limited to this, and the precursor of the inorganic matrix 31 may also be, for example, tetraethyl silicate (TEOS).

[0041] 5, the metal oxide 60 contained in the inorganic matrix 31 has a two-dimensional network structure having Si—O—Si bonds. In addition, in the metal oxide 60, some of the Si—O—Si bonds are substituted with methoxy groups (CH ３ Therefore, the metal oxide 60 has unsubstituted portions NS where methoxy groups are concentrated between Si atoms that are not bonded to each other by Si—O—Si bonds.

[0042] The metal oxide 60 shown in Fig. 5 is formed by the reaction of TMOS 61, which is a precursor of the metal oxide 60, as shown in Fig. 6. The reaction of TMOS 61 and the structure of the metal oxide produced thereby will be described with reference to Figs. 6 to 8.

[0043] For example, two TMOS 61 shown in the schematic diagram 601 of FIG. 6 are water (H ２ 6 , the hydroxyl groups of the two derivatives 62 react with each other to form a condensate 63 having an Si—O—Si bond by dehydration condensation, as shown in schematic diagram 603.

[0044] As the above reaction progresses, a compound having the chemical formula [SiO ３ ・(2CH ３ )] ｎHere, n is an integer representing the degree of condensation. The condensation product 64 has a structure that is one-dimensionally extended by the Si-O-Si bond. Furthermore, as the above reaction progresses, substitution of methoxy groups with hydroxyl groups and dehydration condensation between the hydroxyl groups also progresses within the condensation product 64, and finally silicon oxide 65 (SiO ２ The silicon oxide 65 has a two-dimensional network structure formed by Si—O—Si bonds.

[0045] Here, by stopping the above reaction after the condensate 64 is produced but before the silicon oxide 65 is produced, a two-dimensional network structure is formed in part by Si—O—Si bonds, while some methoxy groups remain in the condensate 64. In this way, the metal oxide 60 shown in FIG.

[0046] Here, the ratio of oxygen atoms to silicon atoms in TMOS 61, which is the precursor of metal oxide 60, is 4, and the ratio of oxygen atoms to silicon atoms in condensate 64 is 3. Furthermore, silicon oxide 65 is stoichiometric, and the ratio of oxygen atoms to silicon atoms in silicon oxide 65 is 2.

[0047] Therefore, the ratio of oxygen atoms to metal atoms in the condensate 64 can be said to be the average value of the ratio of oxygen atoms to metal atoms in the TMOS 61 and the ratio of oxygen atoms to metal atoms in the stoichiometric silicon oxide 65. Furthermore, during the reaction process in which silicon oxide 65 is produced from the condensate 64, the ratio of oxygen atoms to silicon atoms decreases from 3 to 2. Therefore, the ratio of oxygen atoms to metal atoms in the metal oxide 60 is equal to or less than the above average value, and in particular, is between 2 and 3.

[0048] <Distance Between Unsubstituted Portions> Because the number of unsubstituted portions NS in the metal oxide 60 corresponds to the number of unreacted methoxy groups in the condensate 64, the density of the unsubstituted portions NS in the metal oxide 60 decreases as the reaction of the condensate 64 progresses. Therefore, the average distance L between adjacent unsubstituted portions NS in the metal oxide 60 shown in Figure 5 increases as the reaction of the condensate 64 progresses. The length of the average distance L will be considered below.

[0049] For example, the distance between the oxygen atom and the silicon atom of the Si—O—Si bond in the metal oxide 60 is defined as d. ２ In this case, the distance d is about 0.16 nm. In addition, in the metal oxide 60, the ratio of the unsubstituted portion NS to the total number of Si—O—Si bonds is r. In this case, when the metal oxide of the inorganic matrix 31 is SiO ｘ Then x=2(1+r).

[0050] Here, it is assumed that the silicon atoms and oxygen atoms in the metal oxide 60 are on a certain plane, and furthermore, a square SQ on the metal oxide 60 shown in Figure 5 is assumed on the plane. The square SQ has a side length of d, and one of the four vertices is located at the center of the silicon atom, and the other two are located at the centers of the oxygen atoms adjacent to the silicon atom. In this case, the area of ​​the square SQ is d ２ The number of oxygen atoms contained in the square SQ is 1 / 2. Therefore, the number of unsubstituted portions NS contained in the square SQ is r / 2.

[0051] From this, the average value of the area including one unsubstituted portion NS in the metal oxide 60 is d ２ / (r / 2) = 2d ２ Therefore, since the square root of the area corresponds to the average distance L, L = d(2 / r) １／２ It can also be seen from this formula that the smaller the ratio r, in other words, the more the reaction of the condensation product 64 progresses and the smaller the ratio of the unsubstituted portion NS, the longer the average distance L becomes.

[0052] 1 and 3, the inorganic matrix 31 includes a first portion 41 and a second portion 42 each having a metal oxide. In this embodiment, the ratio of oxygen atoms to metal atoms in the second portion 42 is lower than the ratio of oxygen atoms to metal atoms in the first portion 41. For example, when x and y are real numbers, the inorganic matrix 31 includes silicon oxide as a metal oxide, and the silicon oxide included in the first portion 41 is SiO ｘ The silicon oxide contained in the second portion 42 is SiO ｙ If x is smaller than y, then y is lower than x.

[0053] The ratio of oxygen atoms to metal atoms in the metal oxide of the inorganic matrix 31 of the light-emitting layer 23 according to this embodiment may be confirmed by: Method 1: measuring the ratio of metal atoms to oxygen atoms in the thickness direction of the light-emitting layer 23 using SIMS (secondary ion mass spectroscopy). Alternatively, Method 2: measuring the ratio of metal atoms to oxygen atoms in the thickness direction of the light-emitting layer 23 by performing ion sputtering on the light-emitting layer 23 and XPS (X-ray photoelectron spectroscopy) to confirm the ratio. Alternatively, Method 3: performing AES (Auger electron spectroscopy) on a cross section of the light-emitting layer 23 to measure the ratio of metal atoms to oxygen atoms at a target location to confirm the ratio. Alternatively, Method 4: measuring the ratio of metal atoms to oxygen atoms at a target location on a cross section of the light-emitting layer 23 by EDX (energy dispersive X-ray spectroscopy) using an SEM (scanning electron microscope) or a TEM (transmission electron microscope) to confirm the ratio. When using TEM in Method 4, processing the cross section of the light-emitting layer 23 using FIB (focused ion beam) may be performed in combination. The methods for checking the ratio are prioritized in the order of Method 1 to Method 4 described above, and if the ratio can be checked by the first method, the check by the second method may be omitted.

[0054] In this embodiment, the first portion 41 is located closer to the anode 21 than the second portion 42. In particular, the light-emitting layer 23 may include a first light-emitting layer 23A including the first portion 41, and a second light-emitting layer 23B located closer to the cathode 25 than the first light-emitting layer 23A and including the second portion 42. In this case, some of the quantum dots 30 may be located at the boundary between the first light-emitting layer 23A and the second light-emitting layer 23B, in other words, they may be included in both the first light-emitting layer 23A and the second light-emitting layer 23B.

[0055] <Light-emitting layer: additional notes> The quantum dots 30 in the light-emitting layer 23 may be regularly arranged in the light-emitting layer 23, or may be included in the light-emitting layer 23 in a disordered manner. In the light-emitting layer 23 shown in FIG. 1 , an inorganic matrix 31 (described later) is formed between two quantum dots 30, and the quantum dots 30 are not in contact with each other. However, this is not limiting, and the light-emitting layer 23 may include two or more quantum dots 30 that are in contact with each other. The film thickness of the light-emitting layer 23 may be approximately 1 nm to 100 nm.

[0056] As will be described later, when the light-emitting layer 23 is formed from a quantum dot dispersion liquid containing the quantum dots 30, a step of heating the quantum dot dispersion liquid may be included. Therefore, in this embodiment, from the viewpoint of the heat resistance of the light-emitting element 2, all of the layers provided in the light-emitting element 2, from the anode 21 to the cathode 25, may be formed of inorganic layers.

[0057] <Injection of Charges into the Light-Emitting Layer> In this embodiment, the light-emitting layer 23 has a metal oxide in the inorganic matrix 31 that fills the spaces between the quantum dots 30. The inorganic matrix 31 also has a first portion 41 and a second portion 42 that has a lower ratio of oxygen atoms to metal atoms than the first portion 41. Therefore, in the metal oxide contained in the inorganic matrix 31, the ratio of oxygen atoms to metal atoms varies depending on the position.

[0058] When the ratio of oxygen atoms to metal atoms in the metal oxide of inorganic matrix 31 is close to the ratio in a stoichiometric metal oxide, the number of bonds between metal atoms and oxygen atoms increases, and the structure formed by the metal oxide becomes denser. Therefore, second portion 42, in which the ratio of oxygen atoms to metal atoms is closer to the ratio in a stoichiometric metal oxide than first portion 41, better prevents foreign substances such as moisture from reaching quantum dots 30, improving the protective effect of quantum dots 30.

[0059] On the other hand, when the ratio of oxygen atoms to metal atoms is high in the metal oxide contained in the inorganic matrix 31, the number of portions where oxygen atoms that are not bonded to metal atoms are concentrated increases, such as the unsubstituted portion NS described above. Since oxygen atoms with relatively high electronegativity are concentrated in these portions, charge is more likely to propagate in these portions compared to portions where metal atoms and oxygen atoms are bonded.

[0060] When the ratio of oxygen atoms to metal atoms in the metal oxide contained in the inorganic matrix 31 is high, the distance between two adjacent unsubstituted portions NS becomes shorter. The shorter the distance between two adjacent unsubstituted portions NS, the higher the probability of charge tunneling between the unsubstituted portions NS. Therefore, when the ratio of oxygen atoms to metal atoms in the metal oxide contained in the inorganic matrix 31 is high, charge transfer becomes easier in the metal oxide.

[0061] As described above, the light-emitting layer 23 can improve the transfer of charges in the first portion 41 while protecting the quantum dots 30 in the second portion 42. Therefore, the light-emitting element 2 including the light-emitting layer 23 can be designed to improve the efficiency of injection of charges from each electrode into the quantum dots 30 while protecting the quantum dots 30, thereby achieving both a longer lifespan and higher luminous efficiency. A display device 1 including the light-emitting element 2 that achieves both a longer lifespan and higher luminous efficiency achieves a longer lifespan or lower power consumption.

[0062] In particular, in the light-emitting layer 23 according to this embodiment, the first portion 41 is located closer to the anode 21 than the second portion 42. Therefore, the light-emitting element 2 including the light-emitting layer 23 can improve the injection efficiency of holes injected from the anode 21 side to the quantum dots 30 via the first portion 41, while protecting the quantum dots 30 located on the cathode 25 side of the light-emitting layer 23 with the second portion 42.

[0063] In general, due to the difference in mobility between holes and electrons, the light-emitting layer of a light-emitting device including quantum dots tends to have an excess of electrons, where the concentration of electrons is in excess of the concentration of injected holes. The excess of electrons in the light-emitting layer can reduce the efficiency of exciton generation in the light-emitting layer and can also increase the generation process of Auger electrons, which do not contribute to light emission and can cause deterioration of the light-emitting layer and surrounding layers.

[0064] The light-emitting element 2 according to this embodiment has the first portion 41 on the anode 21 side of the light-emitting layer 23, thereby increasing the efficiency of hole injection into the light-emitting layer 23 and reducing electron excess. Furthermore, the light-emitting element 2 has the second portion 42 on the cathode 25 side of the light-emitting layer 23, thereby protecting the quantum dots 30 on the cathode 25 side of the light-emitting layer 23. In particular, the second portion 42 efficiently reduces deterioration of the quantum dots 30 located on the cathode 25 side of the light-emitting layer 23, which is caused by Auger electrons resulting from electrons injected from the cathode 25. Therefore, the light-emitting element 2 according to this embodiment efficiently achieves both improved luminous efficiency of the light-emitting layer 23 and protection of the quantum dots 30.

[0065] <Additional note on the ratio of oxygen atoms to metal atoms in the inorganic matrix> The inorganic matrix 31 contains SiO as a metal oxide. ｘ In this case, as described above, a metal oxide having the above structure can be obtained from TMOS as a precursor, and SiO can be obtained from TMOS. ２ In particular, when x is 3 or less, a two-dimensional network structure is formed in at least a part of the metal oxide, which can further improve the protection effect of the quantum dots 30.

[0066] As described above, in the metal oxide of the inorganic matrix 31, considering that charge transfer is likely to occur via the unsubstituted portions NS, the shorter the average distance L between the unsubstituted portions NS, the more efficiently charge transfer occurs in the inorganic matrix 31. In particular, when the average distance L is 1 nm or less, charge tunnels more efficiently between the unsubstituted portions NS.

[0067] Therefore, d = 0.16 nm, L = d(2 / r) １／２ , and L≦1 nm, r is found to be r≧0.051. ｘ In this case, x=2(1+r) holds as described above. Therefore, from the viewpoint of efficiently generating charge tunneling between the unsubstituted portions NS and improving charge movement in the inorganic matrix 31, the value of x may be 2.1 or more.

[0068] <Method for Manufacturing Display Device> The display device 1 according to this embodiment may be manufactured by forming multiple light-emitting elements 2 on a substrate 3 on which a driver circuit DR, a pixel circuit PC, and the like are separately formed. In the method for manufacturing the light-emitting element 2 according to this embodiment, the layers other than the light-emitting layer 23 may be formed by any method, using common materials and common film-forming processes. For example, each layer can be formed by applying a solution containing the materials for each layer dispersed therein using a spin coating method, a slit coating method, or the like, and then drying the solution. For example, each layer can be formed by preparing a solution in which the materials for each layer are mixed with a solvent for viscosity control, and then printing the solution using an inkjet method, a screen printing method, or the like. Printing methods are suitable for producing light-emitting panels and display panels because they enable uniform deposition of each layer over a large area.

[0069] The manufacturing method of the light-emitting element 2 according to this embodiment includes a method for forming the light-emitting layer 23, which will be described later. The method for forming the light-emitting layer 23 according to this embodiment will be described with reference to FIGS. 9 to 12. FIG. 9 is a flowchart showing the method for forming the light-emitting layer 23 according to this embodiment. FIG. 10 is a side view showing a mixing step in the method for forming the light-emitting layer 23 according to this embodiment. FIG. 11 is a schematic view showing a first dispersion liquid, which will be described later, used in the method for forming the light-emitting layer 23 according to this embodiment. FIG. 12 is a cross-sectional view showing a step of a part of the method for forming the light-emitting layer 23 according to this embodiment.

[0070] In the method for forming the light-emitting layer 23 according to this embodiment, first, a first dispersion and a second dispersion are prepared (step S1). The preparation of the first dispersion and the second dispersion includes a mixing step of a dispersion in which the quantum dots 30 are dispersed and a dispersion in which the precursor of the inorganic matrix 31 is dispersed. This mixing step will be described with reference to Fig. 10. Fig. 10 according to this embodiment describes a method for preparing the first dispersion by the mixing step.

[0071] In the method for preparing the first dispersion according to this embodiment, prior to the mixing step, dispersions LA and LB are prepared as shown in step S1-1 of Fig. 10. In the method for preparing the first dispersion according to this embodiment, for example, commercially available blue quantum dots are used as the quantum dots 30, and zinc chloride (ZnCl) is used as the halogen source. ２ ) powder is prepared. The blue quantum dots have a core of ZnSeTe and a shell of ZnS, and may be coordinated with an organic ligand different from the precursor of the inorganic matrix 31 described later. 3-(mercaptopropyl)trimethoxysilane (MPS) and TMOS shown in the following formula (2) are prepared as precursors of the inorganic matrix 31. Octane (C ８ H １８ ) and N,N-dimethylformamide (DMF).

[0072] In this embodiment, ZnCl ２ , MPS, and TMOS to prepare 10 ml of Dispersion LA. Dispersion LA may contain ZnCl ２The mixture may be 10 ml of a solution having a concentration of 0.02 to 0.2 mol / L, MPS of 0.2 mol / L, and TMOS of 0.02 mol / L.

[0073] In this embodiment, the quantum dots 30 coordinated with the organic ligands described above may be dispersed in octane to a concentration of 1 mg / ml to prepare 10 ml of dispersion LB. Since octane and DMF have different polarities, when dispersion LA and dispersion LB are allowed to stand in a container, the liquid in the container separates into a layer of dispersion LA and a layer of dispersion LB.

[0074] In the mixing step, the dispersion liquids LA and LB are thoroughly stirred and mixed. The stirring may be performed for 1 to 48 hours, preferably for approximately 10 to 20 hours. During the stirring process, some of the ligands coordinated to the quantum dots 30 are replaced by MPS from the organic ligands. This makes the quantum dots 30 more easily dispersible in DMF than in octane, and as shown in step S1-2 of FIG. 10 , the quantum dots 30 are transferred from the dispersion liquid LB to the dispersion liquid LA. In particular, MPS coordinates to the quantum dots 30 using thiol groups as coordinating functional groups. As described below, the MPS coordinated to the quantum dots 30 reacts with TMOS to form an inorganic matrix, which allows the MPS to coordinate to the quantum dots 30 more firmly than when an organic ligand is coordinated to the quantum dots 30. Therefore, the light-emitting layer 23 formed by the method according to this embodiment can more effectively protect the quantum dots 30. The organic ligands do not have to be completely replaced with MPS, and some of the organic ligands (e.g., less than 5% by weight) may remain in the ligands coordinated to the quantum dots 30. A moderate amount of remaining organic ligands prevents aggregation of the quantum dots 30 in the dispersion liquid or suppresses the occurrence of cracks in the light-emitting layer 23.

[0075] Furthermore, in the dispersion liquid LA, MPS and TMOS reacted according to the reaction process described with reference to FIG. 6 , synthesizing silicon oxide around the quantum dots 30. In particular, in the MPS coordinated to the quantum dots 30, the trimethoxysilane group located on the side opposite the quantum dots 30 reacts with other MPS or TMOS. Therefore, as shown in step S1-2 of FIG. 10 , a quantum dot structure 50 including quantum dots 30 is synthesized by mixing the dispersion liquid LA and the dispersion liquid LB. For example, the quantum dot structure 50 has a metal oxide 51 encapsulating the quantum dots 30. Note that in step S1-2, the quantum dot structure 50 may migrate to the interface between the dispersion liquid LA and the dispersion liquid LB.

[0076] Next, the quantum dot structure 50 is extracted from the dispersion liquid LA by centrifuging the dispersion liquid LA at 4000 rpm for 5 minutes. When centrifuging the dispersion liquid LA, a poor solvent such as ethyl acetate or acetone may be added to the dispersion liquid LA to precipitate the quantum dot structure 50. Next, the precipitate containing the quantum dot structure 50 is dispersed in toluene. Here, ZnCl as a halogen source is added to the dispersion liquid. ２ and TMOS as a precursor of the inorganic matrix 31 to prepare a first dispersion.

[0077] The first dispersion liquid prepared by the above mixing process will be described with reference to FIG. 11 . FIG. 11 shows the first dispersion liquid L1 poured into a container C. The first dispersion liquid L1 contains, for example, a quantum dot structure 50, a precursor 52 of an inorganic matrix 31, a halogen source 53, and a solvent 54 containing toluene. As described above, the quantum dot structure 50 has quantum dots 30 and a metal oxide 51 encapsulating the quantum dots 30. Note that the quantum dot structure 50 may have a thiol group and a carbon chain between the quantum dots 30 and the metal oxide 51, which are derived from the thiol group of MPS and the carbon chain bonded to the thiol group.

[0078] The second dispersion may be prepared by the same method as the first dispersion described above, except that the concentration of the halogen source in the dispersion LA, particularly the concentration of the halogen source relative to the precursor of the inorganic matrix 31, is higher than that in the first dispersion described above. The higher concentration of the halogen source in the dispersion LA allows the reaction of MPS and TMOS to proceed more quickly in the mixing step. Accordingly, the conversion rate of the precursor to the metal oxide in the preparation of the second dispersion is higher than that in the preparation of the first dispersion described above. Therefore, the metal oxide of the quantum dot structures contained in the prepared second dispersion has a lower ratio of oxygen atoms to metal atoms than the metal oxide 51 of the quantum dot structures 50 contained in the first dispersion.

[0079] 9 , after the preparation of the first dispersion L1 and the second dispersion, the first dispersion L1 is applied to the substrate (step S2), which is a laminate including the substrate 3 and each layer formed on the substrate 3 up to the step of forming the light-emitting layer 23. The steps from step S2 onwards will be described in more detail with reference to FIG.

[0080] 12, at the start of execution of step S2, an anode 21 and a hole transport layer 22 are formed in this order on the substrate 3. In this case, in step S2, as shown in step S2-2 in FIG.

[0081] Next, each portion of the substrate 3 containing the applied first dispersion liquid L1 is heated, for example, at 100° C. for 30 minutes to react the precursor 52 in the first dispersion liquid L1 and convert the precursor 52 in the first dispersion liquid L1 into a metal oxide (step S3). Here, the metal oxide formed by converting the precursor 52 in the first dispersion liquid L1 in step S3 may have the same ratio of oxygen atoms to metal atoms as the metal oxide 51 of the quantum dot structure 50 in the first dispersion liquid L1. Note that the ratio of oxygen atoms to metal atoms of the metal oxide formed in step S3 and the metal oxide 51 of the quantum dot structure 50 do not need to be completely the same; for example, if the difference between the ratios is 5% or less, the ratios of oxygen atoms to metal atoms of the two may be considered to be the same.

[0082] 12 , the first portion 41 is formed from the metal oxide 51 of the quantum dot structure 50 and the metal oxide formed by conversion of the precursor 52. In this way, the first light-emitting layer 23A is formed, which includes the quantum dots 30 and the first portion 41 filling the spaces between the quantum dots 30.

[0083] As described above, the first portion 41 is formed by converting at least a portion of the precursor 52 in the first dispersion liquid L1 into a metal oxide, and therefore the ratio of oxygen atoms to metal atoms in the metal oxide in the first portion 41 is less than the ratio of oxygen atoms to metal atoms in the precursor 52 in the first dispersion liquid L1.

[0084] Next, the second dispersion is applied to the substrate, which is a laminate including the substrate 3 through the first light-emitting layer 23A (step S4). Next, the portions of the substrate 3 containing the applied second dispersion are heated to react the precursors in the second dispersion and convert them into metal oxides (step S5). The metal oxide formed by converting the precursors in the second dispersion in step S5 may have the same ratio of oxygen atoms to metal atoms as the metal oxide of the quantum dot structure in the second dispersion.

[0085] In other words, the conversion rate of the precursor in the second dispersion to the metal oxide in step S5 is higher than the conversion rate of the precursor in the first dispersion to the metal oxide in step S3. Therefore, the metal oxide formed in step S5 has a lower ratio of oxygen atoms to metal atoms than the metal oxide contained in the first portion 41. In other words, the ratio of oxygen atoms to metal atoms in the metal oxide in the second portion 42 is lower than the ratio of oxygen atoms to metal atoms in the precursor in the second dispersion.

[0086] 12, the second portions 42 are formed. As a result, the second light-emitting layer 23B including the quantum dots 30 and the second portions 42 filling the spaces between the quantum dots 30 is formed, and the formation of the light-emitting layer 23 is completed.

[0087] The above-described method can produce the light-emitting element 2 including the light-emitting layer 23 according to this embodiment. In particular, the method can appropriately design the ratio of oxygen atoms to metal atoms in the metal oxide contained in the inorganic matrix 31 to be formed by changing parameters such as heating conditions in each step of converting the precursor into a metal oxide.

[0088] 13 is a schematic side cross-sectional view of a display device 4 according to this embodiment. Compared to the display device 1 according to the previous embodiment, the display device 4 according to this embodiment includes a light-emitting element 5 instead of the light-emitting element 2. Compared to the light-emitting element 2, the light-emitting element 5 includes a light-emitting layer 26 instead of the light-emitting layer 23.

[0089] The light-emitting layer 26 will be described in more detail with reference to Fig. 14. Fig. 14 is an enlarged schematic view of the light-emitting layer 26 in the cross section shown in Fig. 13, and in particular of a region E2 shown in Fig. 13.

[0090] The light-emitting layer 26 includes a plurality of quantum dots 30 and an inorganic matrix 31. The inorganic matrix 31 has a first portion 41 and a second portion 42. The light-emitting layer 26 differs from the light-emitting layer 23 only in the positional relationship between the first portion 41 and the second portion 42.

[0091] 14 , in the light-emitting layer 26, the second portions 42 are located closer to the quantum dots 30 than the first portions 41. In particular, in the light-emitting layer 26, the second portions 42 are located around each quantum dot 30. More specifically, the second portions 42 may encapsulate each quantum dot 30. Furthermore, the first portions 41 are located around each second portion 42. More specifically, the first portions 41 may encapsulate each second portion 42. Therefore, the light-emitting layer 26 can also be considered to include a plurality of quantum dot structures, each having a quantum dot 30 and a second portion 42 located around the quantum dot 30, and a first portion 41 filling the spaces between the plurality of quantum dot structures.

[0092] Except for the above, the display device 4 according to this embodiment may have the same configuration as the display device 1 according to the previous embodiment.

[0093] In the light-emitting layer 26 according to this embodiment, the second portions 42 are located closer to each quantum dot 30 than the first portions 41. In particular, in the light-emitting layer 26, the second portions 42 are located around each quantum dot 30, and the first portions 41, which have a higher ratio of oxygen atoms to metal atoms than the second portions 42, are located around the second portions 42.

[0094] Therefore, the light-emitting layer 26 can protect each quantum dot 30 by the second portion 42, which is closer to each quantum dot 30 than the first portion 41. Furthermore, the light-emitting layer 26 more efficiently transports carriers from each charge transport layer by the first portion 41. Note that although the second portion 42 is located between each quantum dot 30 and the first portion 41 in the light-emitting layer 26, carriers transported from the first portion 41 toward each quantum dot 30 are injected into each quantum dot 30 by tunneling through the second portion 42. Therefore, the light-emitting element 5 including the light-emitting layer 26 according to this embodiment can more efficiently achieve both protection of the quantum dots 30 and improvement in the efficiency of injection of charges from each electrode into the quantum dots 30.

[0095] The method for manufacturing the light-emitting element 5 according to this embodiment differs from the method for manufacturing the light-emitting element 2 described above only in that the method for forming the light-emitting layer 23 is replaced with a method for forming the light-emitting layer 26, which will be described later. The method for forming the light-emitting layer 26 according to this embodiment will be described in detail with reference to FIGS. 15 to 17 . FIG. 15 is a flowchart showing the method for forming the light-emitting layer 26 according to this embodiment. FIG. 16 is a schematic diagram showing a third dispersion liquid, which will be described later, used in the method for forming the light-emitting layer 26 according to this embodiment. FIG. 17 is a cross-sectional view showing a process of a part of the method for forming the light-emitting layer 26 according to this embodiment.

[0096] In the method for forming the light-emitting layer 26 according to this embodiment, first, a third dispersion is prepared (step S6). The preparation of the third dispersion includes a mixing step of a dispersion in which the quantum dots 30 are dispersed and a dispersion in which the precursor of the inorganic matrix 31 is dispersed. In particular, the preparation of the third dispersion may be performed by the same method as the preparation of the first dispersion and the second dispersion, for example, except that the metal oxide 51 formed around each quantum dot in the mixing step is the second portion 42.

[0097] Specifically, in step S6, for example, the dispersion liquid LA is ２ Alternatively, the dispersion LB may be 10 ml of a mixed solution having a concentration of 0.6 mol / L of MPS, 0.2 mol / L of MPS, and 0.002 mol / L of TMOS. In addition, in step S6, for example, the dispersion LB may be the same quantum dot dispersion LB as the dispersion LB in step S1 described above.

[0098] By mixing the dispersion liquid LA and the dispersion liquid LB described above, second portions 42 are formed around the quantum dots 30. As a result, in the mixing process of step S6, a quantum dot structure 55 (described below) is synthesized, which includes the quantum dots 30 and the second portions 42 located around the quantum dots 30. For example, the quantum dot structure 55 has the second portions 42 that encapsulate the quantum dots 30. In other words, step S6 includes the formation of the quantum dot structure 55, and the formation of the quantum dot structure 55 includes the formation of second portions around the multiple quantum dots 30 by converting a precursor into a metal oxide.

[0099] Next, the quantum dot structure 55 extracted from the dispersion liquid LA was dispersed in toluene by the same method as described above, and ZnCl ２ and TMOS as a precursor of the inorganic matrix 31 to prepare a third dispersion.

[0100] The third dispersion liquid prepared by the above mixing process will be described with reference to FIG. 16 . FIG. 16 shows the third dispersion liquid L3 poured into the container C. The third dispersion liquid L3 contains, for example, a quantum dot structure 55, a precursor 52 of the inorganic matrix 31, a halogen source 53, and a solvent 54 containing toluene. As described above, the quantum dot structure 55 has quantum dots 30 and a second portion 42 encapsulating the quantum dots 30. Here, for example, ZnCl as the halogen source 53 in the third dispersion liquid L3. ２ The concentration of ZnCl in the dispersion liquid LA in step S6 ２ The concentration is lower than that of the quantum dot structure 55. The quantum dot structure 55 may have a thiol group and a carbon chain between the quantum dots 30 and the second portion 42, which are derived from the thiol group of MPS and the carbon chain bonded to the thiol group. When the MPS coordinated to the quantum dots 30 reacts with TMOS to form the second portion 42, the MPS coordinates to the quantum dots 30 more firmly than when an organic ligand coordinates to the quantum dots 30. Therefore, the light-emitting layer 23 formed by the method according to this embodiment can more strongly protect the quantum dots 30. The organic ligand does not have to be completely replaced by MPS; some of the ligands coordinated to the quantum dots 30 may contain residual organic ligands (e.g., less than 5% by weight). Remaining an appropriate amount of organic ligand prevents aggregation of the quantum dots 30 in the dispersion or suppresses cracking in the light-emitting layer 23.

[0101] 15 , after the preparation of the third dispersion L3, the third dispersion L3 is applied to the substrate (step S7), which is a laminate including the substrate 3 and each layer formed on the substrate 3 up to the step of forming the light-emitting layer 23. The steps from step S7 onwards will be described in more detail with reference to FIG.

[0102] For example, as shown in step S7-1 in Fig. 17, at the start of execution of step S7, an anode 21 and a hole transport layer 22 are formed in this order on the substrate 3. In this case, in step S7, as shown in step S7-2 in Fig. 17, a third dispersion L3 is applied to the upper surface of the hole transport layer 22. Step S7 may be performed in the same manner as step S2, except that the dispersion to be applied is changed from the first dispersion L1 to the third dispersion L3.

[0103] Next, each portion of the substrate 3 containing the applied third dispersion L3 is heated, for example, at 100° C. for 30 minutes by the same method as in step S3, thereby causing the precursor 52 in the third dispersion L3 to react and convert the precursor 52 in the third dispersion L3 into a metal oxide (step S8).

[0104] As described above, ZnCl 2 is used as the halogen source 53 in the third dispersion L3. ２ The concentration of ZnCl in the dispersion liquid LA in step S6 ２ Therefore, in the conversion of the precursor in the dispersion liquid LA to the metal oxide in the formation of the quantum dot structure 55, in other words, the conversion to the second portion 42, the conversion rate of the precursor is higher than the conversion of the precursor 52 in the third dispersion liquid L3 to the metal oxide.

[0105] Therefore, the metal oxide formed by converting the precursor 52 in the third dispersion liquid L3 in step S8 has a higher ratio of oxygen atoms to metal atoms compared to the second portion 42. Therefore, in step S8, the metal oxide formed around the quantum dot structures 55, in other words, around the second portion 42, becomes the first portion 41. In other words, step S8 includes forming the first portion 41 around each quantum dot structure 55. In this way, the light-emitting layer 26 according to this embodiment is formed.

[0106] The above-described method can produce a light-emitting element 5 including the light-emitting layer 26 according to this embodiment. In particular, according to the above-described method, by changing parameters such as heating conditions in each step of converting the precursor into a metal oxide, it is possible to appropriately design the ratio of oxygen atoms to metal atoms in the metal oxide contained in the formed inorganic matrix 31. Furthermore, according to the above-described method, the light-emitting layer 26 can be formed by preparing and applying the third dispersion L3 and converting the precursor 52 in the third dispersion L3, thereby eliminating the steps of preparing and applying other dispersions and simplifying the process of forming the light-emitting layer 26.

[0107] <Evaluation of Light-Emitting Device> The light-emitting device 5 according to this embodiment is evaluated by manufacturing light-emitting devices according to each of the example, comparative example 1, and comparative example 2, and comparing the characteristics of these light-emitting devices.

[0108] The light-emitting element according to the example has the same configuration as the light-emitting element 5 according to the present embodiment, and was manufactured by the same manufacturing method as the light-emitting element 5 according to the present embodiment. The light-emitting elements according to comparative examples 1 and 2 have the same configuration as the light-emitting element according to the example, except for the light-emitting layer. The light-emitting layers of the light-emitting elements according to comparative examples 1 and 2 differ from the light-emitting layer 26 only in that they contain only the first part 41 as the inorganic matrix 31 and do not contain the second part 42. The light-emitting layers of the light-emitting elements according to comparative examples 1 and 2 were formed by the same method as the method for forming the light-emitting layer 26 according to the previous embodiment, except that only the first dispersion liquid L1 was prepared in step S1, and steps S4 and S5 were not performed.

[0109] In the light-emitting layer 26 of the light-emitting element according to the example, the first portion 41 is made of SiO ｘ and the second portion 42 includes SiO ｙ In the example, x=3 and y=2.2. In the light-emitting layers of the light-emitting devices according to Comparative Examples 1 and 2, the first portion 41 contained SiO ｘ In Comparative Example 1, x=2, and in Comparative Example 2, x=3.5.

[0110] The light emitting efficiency and reliability of each of the light emitting devices according to Example, Comparative Example 1, and Comparative Example 2 were evaluated and summarized in Table 1 below.

[0111] In Table 1, the "External quantum efficiency" column indicates the efficiency of each light-emitting device at 25 mA / cm ２ The column "Lifetime" shows the external quantum efficiency of each light-emitting element when driven at a current density of 25 mA / cm. In other words, it shows the luminous efficiency of each light-emitting element. ２ The graph shows the time it takes for the luminance of each light-emitting element to reach 50% of the initial luminance when driven at a current density of 1000 kJ / s.

[0112] As shown in Table 1, the external quantum efficiency of the light-emitting device according to the example is nearly twice as high as that of the light-emitting device according to comparative example 1, and is slightly higher than that of the light-emitting device according to comparative example 2. In addition, as shown in Table 1, the lifetime of the light-emitting device according to the example is nearly three times longer than that of the light-emitting device according to comparative example 2, and the difference between the lifetime and that of the light-emitting device according to comparative example 1 is relatively small. This is thought to be because the inorganic matrix 31 of the light-emitting layer 26 of the light-emitting device according to the example has both the first portion 41 and the second portion 42, and therefore the inorganic matrix 31 protects the quantum dots 30 while improving the efficiency of injection of charges from each electrode into the quantum dots 30.

[0113] [Embodiment 3] <Another Manufacturing Example> The display device 4 according to this embodiment has the same configuration as the display device 4 according to the previous embodiment, and differs only in the method for manufacturing the light-emitting element 5. The method for manufacturing the light-emitting element 5 according to this embodiment differs from the method for manufacturing the light-emitting element 5 according to the previous embodiment only in the method for forming the light-emitting layer 26. The method for forming the light-emitting layer 26 according to this embodiment will be described in detail with reference to FIGS. 18 to 21. FIG. 18 is a flowchart showing the method for forming the light-emitting layer 26 according to this embodiment. FIGS. 19 and 20 are schematic diagrams showing a fourth dispersion liquid and a fifth dispersion liquid, described below, used in the method for forming the light-emitting layer 26 according to this embodiment, respectively. FIG. 21 is a cross-sectional view showing a process of a part of the method for forming the light-emitting layer 26 according to this embodiment.

[0114] In the method for forming the light-emitting layer 26 according to this embodiment, first, a fourth dispersion and a fifth dispersion are prepared (step S9). The fourth dispersion may be prepared by the same method as the method for preparing the third dispersion L3 according to the previous embodiment, except that after extracting the quantum dot structures 55 from the dispersion LA, only the quantum dot structures 55 are added to the solvent 54. For example, as shown in FIG. 19 , the fourth dispersion L4 is a dispersion in which the quantum dot structures 55 are dispersed in the solvent 54. The fifth dispersion has the same configuration as the third dispersion L3, except that it does not contain the quantum dot structures 55. The fifth dispersion may be prepared by adding a precursor 52 and a halogen source 53 to the solvent 54. For example, as shown in FIG. 20 , the fifth dispersion L5 is a dispersion in which the precursor 52 and the halogen source 53 are dispersed in the solvent 54.

[0115] Following the preparation of the fourth dispersion L4 and the fifth dispersion L5, the fourth dispersion L4 is applied to a laminate including the substrate 3 and each layer formed on the substrate 3 up to the step of forming the light-emitting layer 26 (step S10). Each step from step S10 onwards will be described in more detail with reference to FIG.

[0116] 21, the fourth dispersion L4 is applied to the upper surface of the hole transport layer 22. Step S10 may be performed in the same manner as step S7, except that the dispersion to be applied is changed from the third dispersion L3 to the fourth dispersion L4.

[0117] Next, the fourth dispersion L4 is dried by, for example, heating the laminate on the substrate 3 containing the applied fourth dispersion L4 (step S11). As a result, as shown in step S11 in Fig. 21 , only the solvent 54 of the applied fourth dispersion L4 volatilizes, forming a quantum dot structure layer 26A containing a plurality of quantum dot structures 55. Note that in step S11, the plurality of quantum dot structures 55 accumulate on the hole transport layer 22 side due to their own weight. Therefore, the density of the quantum dots 30 in the light-emitting layer 26 formed by the method according to this embodiment may be higher than the density of the quantum dots 30 in the light-emitting layer 26 formed by the method according to the previous embodiment.

[0118] In particular, in the light-emitting layer 26 according to this embodiment, the quantum dots 30 may be unevenly distributed closer to the hole transport layer 22 than in the light-emitting layer 26 according to the previous embodiment. With the above configuration, the quantum dots 30 in the light-emitting layer 26 are closer to the hole transport layer 22, and therefore the light-emitting element 5 improves the efficiency of hole injection from the hole transport layer 22 to the quantum dots 30, and further reduces the excess electrons in the light-emitting layer 26.

[0119] Next, the fifth dispersion liquid L5 is applied onto the quantum dot structure layer 26A (step S12). As a result, as shown in step S12 of Fig. 21, the precursor 52 and the halogen source 53 in the fifth dispersion liquid L5 penetrate between the quantum dot structures 55 of the quantum dot structure layer 26A. Therefore, at the completion of step S12, the precursor 52 and the halogen source 53 are located around each quantum dot structure 55 of the quantum dot structure layer 26A.

[0120] Next, the quantum dot structure layer 26A and each portion of the substrate 3 containing the fifth dispersion L5 applied onto the quantum dot structure layer 26A are heated, for example, at 100° C. for 30 minutes by the same method as in step S3, thereby causing the precursor 52 in the fifth dispersion L5 to react and convert the precursor 52 in the fifth dispersion L5 into a metal oxide (step S13).

[0121] As described above, the method for forming the quantum dot structure 55 is the same as the method for forming the quantum dot structure 55 described in the previous embodiment. ２ The concentration of ZnCl as the halogen source 53 in the third dispersion L3 ２The concentration of the precursor in the fifth dispersion liquid L5 is the same as that in the fifth dispersion liquid L6. Therefore, in the present embodiment, the conversion rate of the precursor to metal oxide in the formation of the quantum dot structures 55, in other words, the conversion rate of the precursor to the second portions 42, is higher than the conversion rate of the precursor 52 in the fifth dispersion liquid L5 to metal oxide. Therefore, in step S13, the metal oxide formed around the quantum dot structures 55, in other words, around the second portions 42, becomes the first portions 41. In other words, step S13 includes the formation of the first portions 41 around each of the quantum dot structures 55. In this way, the light-emitting layer 26 according to this embodiment is formed.

[0122] 22 is a schematic side cross-sectional view of a display device 6 according to this embodiment. Compared to the display device 1 described above, the display device 6 according to this embodiment includes a light-emitting element 7 instead of the light-emitting element 2. Compared to the light-emitting element 2, the light-emitting element 7 includes a light-emitting layer 27 instead of the light-emitting layer 23.

[0123] The light-emitting layer 27 includes a plurality of quantum dots 30 and an inorganic matrix 31. The light-emitting layer 27 also includes a first light-emitting layer 27A, a second light-emitting layer 27B located closer to the cathode 25 than the first light-emitting layer 27A, and a third light-emitting layer 27C located between the first light-emitting layer 27A and the second light-emitting layer 27B, which are stacked one on top of the other.

[0124] The first light-emitting layer 27A and the second light-emitting layer 27B each have the same configuration as the first light-emitting layer 23A and the second light-emitting layer 23B described above. The third light-emitting layer 27C has the same configuration as the first light-emitting layer 23A, except that it has a third portion 43 instead of the first portion 41. The third portion 43 is a part of the inorganic matrix 31, and in other words, it has a metal oxide. The ratio of oxygen atoms to metal atoms in the third portion 43 is lower than the ratio of oxygen atoms to metal atoms in the first portion 41 and higher than the ratio of oxygen atoms to metal atoms in the second portion 42.

[0125] Except for the above, the display device 6 according to this embodiment has the same configuration as the display device 1 described above.

[0126] The light-emitting layer 27 according to this embodiment has, as part of the inorganic matrix 31, a third portion 43 in which the ratio of oxygen atoms to metal atoms is between the ratio in the first portion 41 and the ratio in the second portion 42. Therefore, the light-emitting element 7 including the light-emitting layer 27 can more efficiently achieve a configuration that achieves both a longer life and high luminous efficiency.

[0127] In particular, in this embodiment, the inorganic matrix 31 contained in the light-emitting layer 27 has, in order from the anode 21 side toward the cathode 25 side, a first portion 41, a third portion 43, and a second portion 42. Therefore, in the inorganic matrix 31 according to this embodiment, the ratio of oxygen atoms to metal atoms gradually decreases from the anode 21 side toward the cathode 25 side. Therefore, the light-emitting element 7 including the light-emitting layer 27 more efficiently achieves both a longer lifespan and higher luminous efficiency.

[0128] The manufacturing method of the display device 6 according to this embodiment differs from the manufacturing method of the display device 1 described above only in the manufacturing method of the light-emitting element 7. The manufacturing method of the light-emitting element 7 according to this embodiment differs from the manufacturing method of the light-emitting element 2 described above only in that it includes a method of forming the light-emitting layer 27 instead of the method of forming the light-emitting layer 23. In particular, the method of forming the light-emitting layer 27 is performed by a method in which some of the steps of the method of forming the light-emitting layer 23 are modified.

[0129] Specifically, in the method for forming the light-emitting layer 27, a sixth dispersion liquid containing a quantum dot structure having quantum dots and a metal oxide encapsulating the quantum dots is prepared in step S1. The sixth dispersion liquid has the same composition as the first dispersion liquid except that the ratio of oxygen atoms to metal atoms in the metal oxide of the quantum dot structure to be dispersed is between the ratios in the first dispersion liquid and the second dispersion liquid.

[0130] The method for forming the light-emitting layer 27 also includes, between steps S3 and S4, a step of forming a third light-emitting layer 27C by applying the sixth dispersion and converting the precursor in the sixth dispersion to a metal oxide. The step of forming the third light-emitting layer 27C may be performed by the same method as steps S2 and S3, or steps S4 and S5, except that the metal oxide of the precursor in the sixth dispersion is converted to form the third portion.

[0131] 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.

[0132] REFERENCE SIGNS LIST 1 Display device 2 Light-emitting element 3 Substrate 21 Anode 23 Light-emitting layer 25 Cathode 30 Quantum dots 31 Inorganic matrix 41 First part 42 Second part 51 Metal oxide 52 Precursor

Claims

1. A light-emitting element comprising: an anode; a cathode facing the anode; and a light-emitting layer located between the anode and the cathode, wherein the light-emitting layer includes a plurality of quantum dots and an inorganic matrix having a metal oxide filling spaces between the plurality of quantum dots, wherein the inorganic matrix includes a first portion having the metal oxide, and a second portion having the metal oxide, the second portion having the metal oxide and having a ratio of oxygen atoms to metal atoms that is lower than the ratio of oxygen atoms to metal atoms in the first portion and is equal to or greater than the ratio of oxygen atoms to metal atoms in the metal oxide, which is stoichiometric.

2. The light-emitting device according to claim 1, wherein the first portion is located closer to the anode than the second portion.

3. The light-emitting device according to claim 1, wherein the second portion is located closer to the quantum dot than the first portion.

4. The light-emitting device according to claim 3, wherein the second portion is positioned around the quantum dot, and the first portion is positioned around the second portion.

5. The light-emitting element according to any one of claims 1 to 4, wherein the metal oxide includes silicon oxide.

6. The metal oxide is SiO ｘ The light-emitting element according to claim 5 , wherein x is 2 or more and 3 or less.

7. The light-emitting device according to claim 6, wherein x is 2.1 or more.

8. A light-emitting element described in any one of claims 1 to 7, wherein the inorganic matrix contains the metal oxide and includes a third portion in which the ratio of oxygen atoms to metal atoms is lower than the ratio of oxygen atoms to metal atoms in the first portion and is higher than the ratio of oxygen atoms to metal atoms in the second portion.

9. The light-emitting element according to claim 8, wherein the third portion is located between the first portion and the second portion.

10. A display device comprising a plurality of light-emitting elements according to any one of claims 1 to 9.

11. A method for manufacturing a light-emitting element including an anode, a cathode facing the anode, and a light-emitting layer located between the anode and the cathode, the method including forming the light-emitting layer, the light-emitting layer including a plurality of quantum dots and an inorganic matrix having a metal oxide and filling spaces between the plurality of quantum dots, the inorganic matrix including a first portion having the metal oxide, and a second portion having the metal oxide, the second portion having the metal oxide, the ratio of oxygen atoms to metal atoms being lower than the ratio of oxygen atoms to metal atoms in the first portion and equal to or greater than the ratio of oxygen atoms to metal atoms in the metal oxide, which is stoichiometric.

12. A method for manufacturing a light-emitting element as described in claim 11, wherein the formation of the light-emitting layer includes: forming a first light-emitting layer including a plurality of the quantum dots and the first portion; and forming a second light-emitting layer including a plurality of the quantum dots and the second portion.

13. A method for manufacturing a light-emitting element as described in claim 12, wherein the formation of the first light-emitting layer includes: applying a first dispersion containing a plurality of the quantum dots and a precursor of the metal oxide to a substrate; and converting the precursor in the applied first dispersion to the metal oxide; and the formation of the second light-emitting layer includes: applying a second dispersion containing a plurality of the quantum dots and the precursor to a substrate; and converting the precursor in the applied second dispersion to the metal oxide.

14. A method for producing a light-emitting element as described in claim 13, wherein the conversion rate of the precursor to the metal oxide in the second dispersion is higher than the conversion rate of the precursor to the metal oxide in the first dispersion.

15. A method for producing a light-emitting element as described in claim 13 or 14, wherein the ratio of oxygen atoms to metal atoms in each of the first and second parts is less than the ratio of oxygen atoms to metal atoms in the metal oxide precursor.

16. A method for manufacturing a light-emitting element as described in claim 15, wherein the ratio of oxygen atoms to metal atoms in each of the first and second parts is equal to or less than the average value of the ratio of oxygen atoms to metal atoms in the precursor and the ratio of oxygen atoms to metal atoms in the metal oxide, which is stoichiometric.

17. The method for manufacturing a light-emitting element described in claim 13, wherein forming the light-emitting layer includes forming a plurality of quantum dot structures including the quantum dot and the second portion around the quantum dot, and forming the first portion around each of the quantum dot structures.

18. The method of claim 17, wherein forming the quantum dot structure includes converting the precursor to the metal oxide around a plurality of the quantum dots.

19. A method for manufacturing a light-emitting element as described in claim 18, wherein forming the light-emitting layer includes preparing a third dispersion containing the formed quantum dot structure and the precursor, and applying the third dispersion to a substrate, and forming the first part includes converting the precursor in the applied third dispersion to the metal oxide, and the conversion rate of the precursor to the metal oxide in the formation of the quantum dot structure is higher than the conversion of the precursor to the metal oxide in the third dispersion.

20. A method for manufacturing a light-emitting element as described in claim 18, wherein the formation of the light-emitting layer includes: preparing a fourth dispersion containing the formed quantum dot structures; forming a quantum dot structure layer by applying the fourth dispersion to a substrate; applying a fifth dispersion containing the precursor onto the quantum dot structure layer; and converting the precursor in the applied fifth dispersion to the metal oxide, wherein the conversion rate of the precursor to the metal oxide in the formation of the quantum dot structures is higher than the conversion of the precursor to the metal oxide in the fifth dispersion.

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