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

Abstract

A red light-emitting element (5R) comprises an anode (22), a cathode (25), and a red light-emitting layer (24REM) provided between the anode (22) and the cathode (25). The red light-emitting layer (24REM) includes: a continuous film (MR) containing a metal sulfide that serves as a matrix; and a plurality of quantum dots (QD) embedded in the continuous film (MR). In a cross-section, of a portion of the red light-emitting layer (24REM), including cross-sections of a prescribed number of quantum dots (QD) among said plurality of quantum dots (QD), the area C of the cross-section of the portion of the red light-emitting layer (24REM) is smaller than twice the area B obtained by adding up the respective areas of the cross-sections of the prescribed number of quantum dots (QD).

Description

[Technical Field]

[0001] This disclosure relates to a light-emitting element, a display device, a method for manufacturing a light-emitting element, and a method for forming a light-emitting layer. [Background technology]

[0002] In recent years, QLEDs (Quantum dot Light Emitting Diodes), which are light-emitting elements containing quantum dots, and display devices equipped with QLEDs have attracted considerable attention due to their ability to achieve lower power consumption, thinner designs, and higher image quality.

[0003] Furthermore, in the field of QLEDs, research is actively being conducted on the light-emitting layer incorporated into QLEDs and methods for forming the light-emitting layer.

[0004] For example, Patent Document 1 discloses quantum dots on which a fluoride-containing ligand or fluoride anion is bonded to the surface. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2020-180278 (Published November 5, 2020) [Overview of the project] [Problems that the invention aims to solve]

[0006] The invention described in Patent Document 1 had the problem of low durability and luminous efficiency of the light-emitting element.

[0007] One aspect of this disclosure aims to provide a light-emitting element, a display device, a method for manufacturing a light-emitting element, and a method for forming a light-emitting layer, which can improve durability and luminous efficiency. [Means for solving the problem]

[0008] The light-emitting element of this disclosure solves the above-mentioned problems, A-scatter, Cathode and, A light-emitting layer is provided between the anode and the cathode, The light-emitting layer comprises a matrix containing a metal sulfide and a plurality of quantum dots embedded in the matrix. In a cross-section of a portion of the light-emitting layer that includes the cross-sections of a predetermined number of quantum dots among the plurality of quantum dots, the area C of the cross-section of the portion of the light-emitting layer is less than twice the area B of the combined area of ​​each of the cross-sections of the predetermined number of quantum dots.

[0009] The display device of this disclosure solves the aforementioned problems, Includes the aforementioned light-emitting element.

[0010] The present disclosure provides a method for manufacturing a light-emitting element to solve the above-mentioned problems. A method for manufacturing a light-emitting element, which includes the step of forming the light-emitting layer, The step of forming the light-emitting layer is: A step of preparing a quantum dot solution comprising the plurality of quantum dots, the metal sulfide precursor, and a solvent, The steps include applying the quantum dot solution, The process includes a heat treatment step of thermally decomposing the metal sulfide precursor to form a matrix containing the metal sulfide, In the step of preparing the quantum dot solution, the amount of the plurality of quantum dots and the amount of the metal sulfide precursor contained in the quantum dot solution are adjusted so that the value of ((weight of the light-emitting layer - weight of the quantum dots) / weight of the light-emitting layer) × 100% in the light-emitting layer after the heat treatment step is greater than 3.3% and less than 35%.

[0011] The method for forming the light-emitting layer of this disclosure solves the above-mentioned problems. A step of preparing a quantum dot solution comprising multiple quantum dots, a metal sulfide precursor, and a solvent, The steps include applying the quantum dot solution, A method for forming a light-emitting layer, comprising: a heat treatment step of thermally decomposing a precursor of the metal sulfide to form a matrix containing the metal sulfide. In the step of preparing the quantum dot solution, the amounts of the plurality of quantum dots and the precursor of the metal sulfide contained in the quantum dot solution are adjusted such that the value of ((weight of the light-emitting layer - weight of the quantum dots) / weight of the light-emitting layer)×100% in the light-emitting layer after the heat treatment step is greater than 3.3% and less than 35%.

[0012] In order to solve the above problems, a method for manufacturing a light-emitting device according to the present disclosure A step of forming one of the anode and the cathode; A step of forming a light-emitting layer by the method for forming the light-emitting layer; A step of forming the other of the anode and the cathode.

Advantages of the Invention

[0013] According to one aspect of the present disclosure, it is possible to provide a light-emitting device, a display device, a method for manufacturing a light-emitting device, and a method for forming a light-emitting layer that can improve durability and luminous efficiency.

Brief Description of the Drawings

[0014] [Figure 1] It is a plan view showing a schematic configuration of a display device of the present embodiment. [Figure 2] It is a cross-sectional view showing a schematic configuration of a display area of a display device of the present embodiment. [Figure 3] It is a cross-sectional view showing a schematic configuration of a red light-emitting device provided in the display device of the present embodiment. [Figure 4] It is a cross-sectional view showing a schematic configuration of a red light-emitting layer of a red light-emitting device provided in the display device of the present embodiment. [Figure 5] It is a view showing a case where, in a red light-emitting layer of a red light-emitting device provided in the display device of the present embodiment, the material of the shell of the quantum dots and the continuous film containing the metal sulfide contained in the red light-emitting layer are the same material. [Figure 6]This figure shows another red light-emitting layer that can be provided in the red light-emitting element of the display device of this embodiment, in which the material of the quantum dot shell and the continuous film containing metal sulfide included in the red light-emitting layer are different materials. [Figure 7] This figure shows examples of several types of quantum dot solutions, each containing a plurality of quantum dots, a metal sulfide precursor, and a solvent, which can be used in the process of forming the red light-emitting layer of a red light-emitting element provided in the display device of this embodiment. [Figure 8] This diagram schematically shows the band levels of the hole transport layer, the red light-emitting layer, and the electron transport layer provided in the red light-emitting element of the display device of this embodiment. [Figure 9] This figure explains why the red light-emitting element of the display device in this embodiment can achieve good external quantum efficiency (EQE). [Figure 10] This diagram schematically shows the band levels of the hole transport layer, red light-emitting layer, and electron transport layer in a comparative example red light-emitting device. [Figure 11] This diagram explains why a good external quantum efficiency (EQE) cannot be achieved in the comparative example, the red light-emitting element. [Figure 12] This figure shows the relationship between voltage and current density for Electron Only Devices (EODs) containing a red light-emitting layer formed using a quantum dot solution with a concentration of 0.02 M, and for Electron Only Devices (EODs) containing a red light-emitting layer formed using a quantum dot solution with a concentration of 0.08 M, among the various types of quantum dot solutions shown in Figure 7. [Figure 13] This figure shows the relationship between voltage and current density for Hole Only Devices (HODs) containing a red light-emitting layer formed using a quantum dot solution with a concentration of 0.02 M, and Hole Only Devices (HODs) containing a red light-emitting layer formed using a quantum dot solution with a concentration of 0.08 M, among the various types of quantum dot solutions shown in Figure 7. [Figure 14]This figure shows the degree of brightness reduction due to quantum dot charge-up when a red light-emitting element containing a red light-emitting layer formed using a quantum dot solution with a concentration of 0.01 M, one of several types of quantum dot solutions shown in Figure 7, and a comparative example red light-emitting element shown in Figure 10 are intermittently driven. [Figure 15] This figure shows the photoluminescence intensity (PL) at various wavelengths for a red light-emitting layer formed using a quantum dot solution with a concentration of 0.01 M, one of several types of quantum dot solutions shown in Figure 7, and a red light-emitting layer formed using a comparative example quantum dot solution with a concentration of 0.16 M. [Figure 16] This figure shows the relationship between the weight ratio of continuous films containing metal sulfides as a medium in multiple types of red light-emitting layers, which are thin films formed using multiple types of quantum dot solutions shown in Figure 7, and the internal quantum yield (PLQY: photoluminescence quantum yield). [Figure 17] Figure 7 shows the emission peak wavelength, internal quantum yield (PLQY), maximum external quantum efficiency (MAXEQE), and brightness degradation rate after 5 hours of operation for several types of red light-emitting devices, including a red light-emitting layer formed using several types of quantum dot solutions. [Figure 18] This figure shows the degree of change in brightness and voltage over time of a red light-emitting element, which includes a red light-emitting layer formed using a quantum dot solution with a concentration of 0.04 M, among several types of quantum dot solutions shown in Figure 7. [Figure 19] This figure shows the degree of change in brightness and voltage over time of a red light-emitting element, which includes a red light-emitting layer formed using a quantum dot solution with a concentration of 0.08 M, among the various types of quantum dot solutions shown in Figure 7. [Figure 20] This figure illustrates the process of forming a red light-emitting layer, which is included in the manufacturing method of the red light-emitting element provided in the display device of this embodiment. [Modes for carrying out the invention]

[0015] The embodiments of this disclosure will be described below with reference to Figures 1 to 20. For the sake of convenience, in the following description, components having the same function as those described in a particular embodiment will be denoted by the same reference numerals, and their descriptions may be omitted.

[0016] Figure 1 is a plan view showing the schematic configuration of the display device 1 of this embodiment.

[0017] As shown in Figure 1, the display device 1 comprises a frame area NDA and a display area DA. The display area DA of the display device 1 is provided with a plurality of pixels PIX, each of which includes a red subpixel RSP, a green subpixel GSP, and a blue subpixel BSP. In this embodiment, the case in which one pixel PIX is composed of a red subpixel RSP, a green subpixel GSP, and a blue subpixel BSP is described as an example, but it is not limited to this. For example, one pixel PIX may include subpixels of other colors in addition to the red subpixel RSP, green subpixel GSP, and blue subpixel BSP.

[0018] Figure 2 is a cross-sectional view showing a schematic configuration of the display area DA of the display device 1 of this embodiment.

[0019] As shown in Figure 2, in the display area DA of the display device 1, a barrier layer 3, a thin-film transistor layer 4 including a transistor TR, a red light-emitting element 5R, a green light-emitting element 5G, a blue light-emitting element 5B and a bank 23, a sealing layer 6, and a functional film 39 are provided on the substrate 12 in this order from the substrate 12 side.

[0020] The red subpixel RSP provided in the display area DA of the display device 1 includes a red light-emitting element 5R, the green subpixel GSP provided in the display area DA of the display device 1 includes a green light-emitting element 5G, and the blue subpixel BSP provided in the display area DA of the display device 1 includes a blue light-emitting element 5B.

[0021] The substrate 12 may be a resin substrate made of a resin material such as polyimide, or it may be a glass substrate. In this embodiment, since the display device 1 is a flexible display device, the case in which a resin substrate made of a resin material such as polyimide is used as the substrate 12 will be described as an example, but it is not limited to this. When the display device 1 is a non-flexible display device, a glass substrate can be used as the substrate 12.

[0022] The barrier layer 3 is a layer that prevents foreign substances such as water and oxygen from entering the transistor TR, the red light-emitting element 5R, the green light-emitting element 5G, and the blue light-emitting element 5B. For example, it can be composed of a silicon oxide film, a silicon nitride film, or a silicon oxynitride film, or a laminate of these, formed by the CVD method.

[0023] The transistor TR portion of the thin film transistor layer 4, which includes the transistor TR, comprises a semiconductor film SEM and a doped semiconductor film SEM'·SEM'', an inorganic insulating film 16, a gate electrode G, an inorganic insulating film 18, an inorganic insulating film 20, a source electrode S and a drain electrode D, and a planarization film 21. The portion of the thin film transistor layer 4, which includes the transistor TR, other than the transistor TR portion, comprises the inorganic insulating film 16, the inorganic insulating film 18, the inorganic insulating film 20, and the planarization film 21.

[0024] The semiconductor film SEM·SEM'·SEM'' may be composed of, for example, low-temperature polysilicon (LTPS) or oxide semiconductors (e.g., In-Ga-Zn-O semiconductors). In this embodiment, the case where the transistor TR has a top-gate structure is given as an example, but it is not limited to this, and the transistor TR may have a bottom-gate structure.

[0025] The gate electrode G, source electrode S, and drain electrode D can be made of a single-layer or multilayer film of a metal containing, for example, at least one of aluminum, tungsten, molybdenum, tantalum, chromium, titanium, or copper.

[0026] The inorganic insulating film 16, inorganic insulating film 18, and inorganic insulating film 20 can be composed of, for example, a silicon oxide film, a silicon nitride film, a silicon oxynitride film, or a laminate of these, formed by the CVD method.

[0027] The planarization film 21 can be made of, for example, a coatable organic material such as polyimide or acrylic, but it may also be formed of an inorganic film.

[0028] The red light-emitting element 5R included in the red subpixel RSP includes an anode 22 located above the planarization film 21, a functional layer 24R including a red light-emitting layer, and a cathode 25. The green light-emitting element 5G included in the green subpixel GSP includes an anode 22 located above the planarization film 21, a functional layer 24G including a green light-emitting layer, and a cathode 25. The blue light-emitting element 5B included in the blue subpixel BSP includes an anode 22 located above the planarization film 21, a functional layer 24B including a blue light-emitting layer, and a cathode 25. The insulating bank 23 covering the edge of the anode 22 can be formed, for example, by coating an organic material such as polyimide or acrylic and then patterning it by photolithography, but it may also be formed from an inorganic film.

[0029] In this embodiment, the case in which the red light-emitting element 5R, green light-emitting element 5G, and blue light-emitting element 5B have a forward-facing structure will be described as an example, but the embodiment is not limited to this, and the red light-emitting element 5R, green light-emitting element 5G, and blue light-emitting element 5B may also have an inverted-facing structure. The red light-emitting element 5R, which has a forward-facing structure, comprises an anode 22 and a cathode 25 provided as an upper layer above the anode 22. The functional layer 24R, which includes a red light-emitting layer provided between the anode 22 and the cathode 25, can be constructed, for example, by stacking a hole injection layer, a hole transport layer, a red light-emitting layer, an electron transport layer, and an electron injection layer in that order from the anode 22 side. Of the functional layer 24R including the red light-emitting layer, one or more layers other than the red light-emitting layer, such as the hole injection layer, hole transport layer, electron transport layer, and electron injection layer, may be appropriately omitted. In this embodiment, the functional layer 24R, which includes the red light-emitting layer 24REM, is described as being constructed by stacking the hole transport layer 24HT, the red light-emitting layer 24REM, and the electron transport layer 24ET in that order from the anode 22 side, but the embodiment is not limited to this.

[0030] A red light-emitting element, which has an inverse stack structure (not shown), comprises a cathode and an anode provided as an upper layer above the cathode. The functional layer, which includes the red light-emitting layer 24REM provided between the cathode and the anode, can be constructed, for example, by stacking an electron injection layer, an electron transport layer, the red light-emitting layer 24REM, a hole transport layer, and a hole injection layer in that order from the cathode side. Of the functional layer including the red light-emitting layer 24REM, one or more layers other than the red light-emitting layer 24REM, such as the electron injection layer, electron transport layer, hole transport layer, and hole injection layer, may be appropriately omitted. The above describes a red light-emitting element, but a green light-emitting element or a blue light-emitting element can be described and understood in exactly the same way. That is, by replacing the "red" part in each of the descriptions here with "green" or "blue," and replacing the "R" part of the symbol with "G" or "B," they can be described and understood in exactly the same way.

[0031] The material used for the hole injection layer is not particularly limited as long as it is a hole-injection material that can stabilize the injection of holes into the light-emitting layer. For example, a composite of poly(3,4-ethylenedioxythiophene) (PEDOT) and polystyrene sulfonic acid (PSS) (PEDOT:PSS) can be used.

[0032] As materials used for the hole transport layer, for example, organic materials such as poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4'-(N-(4-sec-butylphenyl))diphenylamine)] (TFB), N,N'-bis(4-butylphenyl)-N,N'-bis(phenyl)-benzidine (poly-TPD), or polyvinylcarbazole (PVK) may be used, or nanoparticles with hole-transporting properties such as NiO particles may be used. In this embodiment, the case in which N,N'-bis(4-butylphenyl)-N,N'-bis(phenyl)-benzidine (poly-TPD) is used as the hole transport layer 24HT will be described as an example, but the invention is not limited to this.

[0033] As the material used for the electron transport layer, for example, an organic material such as 2,2′,2”-(1,3,5-benzintriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) may be used, or electron-transporting nanoparticles such as ZnO particles may be used. In this embodiment, the electron transport layer 24ET is an oxide particle containing Zn and Mg, for example, Zn 0.9 Mg 0.1 We will explain using the case of using O particles as an example, but this is not the only way to explain.

[0034] The material used for the electron injection layer is not particularly limited as long as it is an electron-injection material that can stabilize the injection of electrons into the light-emitting layer. For example, alkali metals or alkaline earth metals such as aluminum, strontium, calcium, lithium, cesium, magnesium oxide, aluminum oxide, strontium oxide, lithium oxide, lithium fluoride, magnesium fluoride, strontium fluoride, calcium fluoride, barium fluoride, cesium fluoride, polymethyl methacrylate, sodium polystyrene sulfonate, alkali metals or alkaline earth metals, oxides of alkali metals or alkaline earth metals, fluorides of alkali metals or alkaline earth metals, and organic complexes of alkali metals can be used.

[0035] Furthermore, in this embodiment, we will describe, as an example, the case in which each of the functional layers 24R including a red light-emitting layer, functional layer 24G including a green light-emitting layer, and functional layer 24B including a blue light-emitting layer is provided with a hole transport layer and an electron transport layer formed using the same material and in the same process, but we are not limited to this. For example, the hole injection layers included in each of the functional layers 24R, 24G, and 24B may be formed using the same material and in the same process, or they may be formed using different materials. Alternatively, the hole injection layers included in two of the functional layers 24R, 24G, and 24B may be formed using the same material and in the same process, while the hole injection layer included in the remaining functional layer may be formed using a different material and in a separate process. Furthermore, for example, the hole transport layers contained in each functional layer 24R, 24G, and 24B may be formed from different materials. Alternatively, the hole transport layers in two of the functional layers 24R, 24G, and 24B may be formed using the same material and the same process, while the hole transport layer in the remaining functional layer may be formed using a different material and a separate process. Similarly to the hole transport layers, the electron transport layers can also be appropriately selected to use the same material and process in any desired combination of functional layers 24R, 24G, and 24B.

[0036] In this embodiment, the case in which the red light-emitting element 5R, green light-emitting element 5G, and blue light-emitting element 5B are all QLEDs (quantum dot light-emitting diodes) is described as an example, but the invention is not limited to this, and it is sufficient if one or more of the red light-emitting element 5R, green light-emitting element 5G, and blue light-emitting element 5B are QLEDs, and the rest are OLEDs.

[0037] In this embodiment, when the red light-emitting element 5R, green light-emitting element 5G, and blue light-emitting element 5B are all QLEDs, the light-emitting layer of each color light-emitting element includes quantum dots. The quantum dots may have, for example, a core structure, a core / shell structure, a core / shell / shell structure, or a shell structure with a continuously changing core / ratio. The shell may cover a portion of the core, but it is preferable that it completely covers the core. The core material of a quantum dot can be composed of crystals of group II-VI semiconductors such as MgS, MgSe, MgTe, CaS, CaSe, CaTe, BaS, BaSe, BaTe, ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, PbS, PbSe, HgS, HgSe, HgTe, etc.; crystals of group III-V semiconductors such as GaAs, GaP, InN, InAs, InP, InSb, etc.; crystals of group III-VI semiconductors such as Ga3S2, Ga2Se3, In2S3, In2Se3, etc.; crystals of group I-III-VI semiconductors such as CuInGaS, AgInGaS, CuInGaS, AgInGaZnS, CuInGaSe, AgInGaSe, etc.; crystals of group IV semiconductors such as C, Si, etc.; and crystals of perovskite semiconductors such as CsPbI3, CsPbBr3, CsPbCl3, etc. The shell material is selected from the same group of materials as the core material, and it is preferable that the shell material has a lattice constant similar to that of the core material and a larger band gap than the core material.

[0038] If the quantum dots described above include organic or inorganic ligands (e.g., halogen ligands) placed on or near the surface of the quantum dots, then the term "quantum dot" means one that includes organic or inorganic ligands. Therefore, if the quantum dots include ligands such as organic or inorganic ligands, then the red light-emitting layer 24REM, the green light-emitting layer, and the blue light-emitting layer each also include ligands.

[0039] Furthermore, in the quantum dot, the maximum width excluding the ligand portion is 100 nm or less. Also, the shape of the quantum dot excluding the ligand portion is not particularly restricted and is not limited to a spherical three-dimensional shape (circular cross-sectional shape), as long as it satisfies the above maximum width. For example, it may be a polygonal cross-sectional shape, a rod-shaped three-dimensional shape, a branch-shaped three-dimensional shape, a three-dimensional shape with irregularities on the surface, or a combination thereof.

[0040] The red light-emitting element 5R, green light-emitting element 5G, and blue light-emitting element 5B shown in Figure 2 may be either top-emission or bottom-emission types. Since the red light-emitting element 5R, green light-emitting element 5G, and blue light-emitting element 5B have a sequential stacking structure in which the cathode 25 is positioned above the anode 22, to make it a top-emission type, the anode 22 should be formed from an electrode material that reflects visible light and the cathode 25 should be formed from an electrode material that transmits visible light. To make it a bottom-emission type, the anode 22 should be formed from an electrode material that transmits visible light and the cathode 25 should be formed from an electrode material that reflects visible light. On the other hand, if the red light-emitting element, green light-emitting element, and blue light-emitting element have an inverted stack structure in which the anode 22 is positioned above the cathode 25, then to make it a top-emission type, the cathode 25 should be formed from an electrode material that reflects visible light and the anode 22 should be formed from an electrode material that transmits visible light. To make it a bottom-emission type, the cathode 25 should be formed from an electrode material that transmits visible light and the anode 22 should be formed from an electrode material that reflects visible light.

[0041] The electrode material that reflects visible light is not particularly limited as long as it can reflect visible light and is conductive, but examples include metallic materials such as Al, Mg, Li, and Ag, or alloys of the metallic materials, or laminates of the metallic material and transparent metal oxides (e.g., indium tin oxide, indium zinc oxide, indium gallium zinc oxide, etc.), or laminates of the alloy and the transparent metal oxide.

[0042] On the other hand, the electrode material that transmits visible light is not particularly limited as long as it can transmit visible light and is conductive, but examples include transparent metal oxides (e.g., indium tin oxide, indium zinc oxide, indium gallium zinc oxide, etc.), thin films made of metal materials such as Al and Ag, or nanowires made of metal materials such as Al and Ag.

[0043] As for the film deposition method for the anode 22 and cathode 25, general electrode formation methods can be used, such as physical vapor deposition (PVD) methods including vacuum deposition, sputtering, EB deposition, and ion plating, or chemical vapor deposition (CVD). Furthermore, as for the patterning method for the anode 22 and cathode 25, there are no particular limitations as long as it can accurately form the desired pattern, but specifically, photolithography and inkjet methods can be mentioned.

[0044] The sealing layer 6 is a light-transmitting film and can be composed of, for example, an inorganic sealing film 26 covering the cathode 25, an organic film 27 above the inorganic sealing film 26, and an inorganic sealing film 28 above the organic film 27. The sealing layer 6 prevents foreign substances such as water and oxygen from penetrating the red light-emitting element 5R, the green light-emitting element 5G, and the blue light-emitting element 5B.

[0045] The inorganic sealing film 26 and the inorganic sealing film 28 are each inorganic films, and can be composed of, for example, silicon oxide films, silicon nitride films, or silicon oxynitride films formed by CVD, or laminated films thereof. The organic film 27 is a translucent organic film with a planarization effect, and can be composed of, for example, a coatable organic material such as acrylic. The organic film 27 may be formed by, for example, an inkjet method. In this embodiment, the sealing layer 6 was described as being formed of two inorganic films and one organic film provided between two inorganic films, but the stacking order of the two inorganic films and one organic film is not limited to this. Furthermore, the sealing layer 6 may be composed of inorganic films only, or of organic films only, or of one inorganic film and two organic films, or of two or more inorganic films and two or more organic films.

[0046] The functional film 39 is, for example, a film having at least one of the following functions: optical compensation function, touch sensor function, and protective function.

[0047] Figure 3 is a cross-sectional view showing the schematic configuration of the red light-emitting element 5R provided in the display device 1 of this embodiment.

[0048] As shown in Figure 3, the red light-emitting element 5R comprises an anode 22, a cathode 25, and a red light-emitting layer 24REM provided between the anode 22 and the cathode 25.

[0049] In this embodiment, the red light-emitting element 5R includes a functional layer 24REM which contains a red light-emitting layer 24 R The device is constructed by stacking a hole transport layer 24HT, a red light-emitting layer 24REM, and an electron transport layer 24ET in this order from the anode 22 side.

[0050] As shown in Figure 3, the red light-emitting layer 24REM includes a continuous film MR containing a metal sulfide and a plurality of quantum dots QD embedded within the continuous film MR. In this embodiment, the case in which all quantum dots QD are embedded within the continuous film MR of the red light-emitting layer 24REM is described as an example, but the red light-emitting layer 24REM is not limited to this as long as it includes a plurality of quantum dots QD embedded within the continuous film MR. The continuous film MR may also be a matrix.

[0051] A continuous film MR is a continuous matrix. A matrix refers to a component that contains and holds other materials, and can be rephrased as a substrate, base material, or filler. A continuous matrix may be a component that contains and holds multiple quantum dots (QDs). The matrix may be filled in the light-emitting layer. The matrix only needs to fill the region (space) between two adjacent quantum dots (QDs). The matrix may also fill the region (space) in the light-emitting layer other than the multiple quantum dots (QDs). When we say that a matrix is ​​filled between multiple quantum dots (QDs), it means that the region between two adjacent quantum dots (QDs) is filled with the matrix, and it is sufficient to know that. As long as the matrix is ​​filled in at least the region between two adjacent quantum dots (QDs), the desired effect will be achieved, so it is not necessarily required to know that the matrix is ​​filled between all (more than two) quantum dots (QDs) within a certain range. The outer edges (top and bottom surfaces) of the light-emitting layer may be covered with the matrix. Furthermore, the luminescent layer may have a matrix portion extending from its outer edge, with the quantum dot group positioned away from the outer edge. The outer edge of the luminescent layer may not be formed solely of the matrix, and a portion of the quantum dot group may be exposed from the matrix. The matrix may refer to the portion of the luminescent layer excluding the quantum dot group. If the luminescent layer contains multiple quantum dots (QDs) embedded in the matrix, the matrix may be formed to partially or completely fill the space formed between two adjacent quantum dots (QDs). There may be voids in the luminescent layer. If the luminescent layer contains multiple quantum dots (QDs) embedded in the matrix, the luminescent layer may have multiple quantum dots (QDs), and the matrix may partially or completely fill the region other than the multiple quantum dots (QDs). Each of the two adjacent quantum dots (QDs) included in the multiple quantum dots (QDs) may be embedded in the matrix at intervals.

[0052] A continuous matrix, i.e., a continuous film MR, has an area of ​​1000 nm in the light-emitting layer. 2Any matrix that is continuous within the above area range is acceptable. A continuous film MR, i.e., a continuous matrix, means a film that is not divided by any material other than the material constituting the continuous film MR, i.e., the continuous matrix, in a single plane. In other words, it is sufficient if it is connected in some part, and this does not exclude cases where quantum dots or other materials are included in between. Divide refers to a case where it is completely separated by another layer with no connecting parts whatsoever. A continuous film MR, i.e., a continuous matrix, may be a single film-like structure that is continuously connected by chemical bonds of the constituent materials without interruption. The continuous matrix may be made of the same material as the shell of the quantum dot QD. In that case, the average distance between adjacent cores of two adjacent quantum dots QD (inter-core distance) is preferably 3 nm or more, and may be 5 nm or more. The inter-core distance is the average distance between 20 adjacent cores in a space containing 20 cores. The inter-core distance should be kept wider than the distance when shells are in contact with each other. Note that when observing the cross-section of a particle, the size of the cross-sectional area varies depending on the cutting position of the particle. Therefore, the core diameter can be considered to be the diameter of a circle with the same area as the average cross-sectional area of ​​the top 10% of particles with the largest cross-sectional areas.

[0053] Figure 4 is a cross-sectional view showing the schematic configuration of the red light-emitting layer 24REM of the red light-emitting element 5R provided in the display device 1 of this embodiment.

[0054] As shown in Figure 4, in a cross-section of a portion of the red light-emitting layer 24REM that includes the cross-sections of a predetermined number of quantum dots QD among the multiple quantum dots QD contained in the red light-emitting layer 24REM, the area C of the cross-section of the portion of the red light-emitting layer 24REM is less than twice the area B of the combined area of ​​the cross-sections of each of the predetermined number of quantum dots QD.

[0055] In this embodiment, as shown in Figure 4, when comparing the area A of the cross-section of the continuous film MR containing the metal sulfide (hatched area in Figure 4) with the combined area B of the cross-sections of the 52 quantum dots QD in a portion of the red light-emitting layer 24REM that includes, for example, the cross-section of 52 quantum dots QD, area A is smaller than area B. Furthermore, if we define area C as the area of ​​the portion of the red light-emitting layer 24REM that includes the cross-sections of 52 quantum dots QD, that is, the area of ​​the portion of the cross-section of the entire light-emitting layer including the cross-sections of the quantum dots QD and the matrix, then area C is smaller than twice the combined area B of the cross-sections of the 52 quantum dots QD. Here, the case of a portion of the cross-section of the red light-emitting layer 24REM containing 52 quantum dots QD has been used as an example, but the invention is not limited to this. There may be 50 or more quantum dots, 50 quantum dots, or 100 quantum dots. As shown in Figure 4, the size of the cross-section of the 52 quantum dots (QDs) varies depending on the cutting position, even if the quantum dots (QDs) have the same particle size. Furthermore, for example, when observing the cross-section of the red luminescence layer 24REM with a transmission electron microscope (TEM), if a discernible void is present, the cross-sectional area of ​​this void is not included in the area A of the cross-section of the continuous film MR containing the metal sulfide.

[0056] In this embodiment, we will explain, as an example, the case in which the area A of the cross-section of the continuous film MR containing metal sulfides is smaller than the combined area B of a predetermined number, for example, 50 or more quantum dots QD, in any cross-section of the red light-emitting layer 24REM. However, we are not limited to this, and the red light-emitting layer 24REM may include a cross-section of a predetermined number, for example, 50 or more quantum dots QD, obtained by cutting a portion of the red light-emitting layer 24REM, where the area A of the cross-section of the continuous film MR containing metal sulfides is smaller than the combined area B of a predetermined number, for example, 50 or more quantum dots QD. The fact that the red light-emitting layer 24REM has a portion of cross-sections where "the area A of the cross-section of the continuous film MR containing metal sulfides is smaller than a predetermined number, for example, the combined area B of the cross-sectional areas of 50 or more quantum dots QD" means that the red light-emitting layer 24REM includes a portion where "the area A of the cross-section of the continuous film MR containing metal sulfides is smaller than a predetermined number, for example, the combined area B of the cross-sectional areas of 50 or more quantum dots QD". The inventors of this disclosure have confirmed that if the red light-emitting layer 24REM includes a predetermined number of cross-sections where "the area A of the cross-section of the continuous film MR containing metal sulfides is smaller than the predetermined number, for example, the combined area B of the cross-sectional areas of 50 or more quantum dots QD", then improvements in carrier balance and external quantum efficiency (EQE) and suppression of brightness reduction due to quantum dot charge-up can be achieved. Therefore, the red light-emitting layer 24REM may include, for example, a predetermined number of cross-sections containing 50 quantum dots QD, where the area A of the cross-section of the continuous film MR containing the metal sulfide is smaller than a predetermined number, for example, the combined area B of the cross-sections of 50 quantum dots QD, or it may include a cross-section containing 100 quantum dots QD, where the area A of the cross-section of the continuous film MR containing the metal sulfide is smaller than the combined area B of the cross-sections of 100 quantum dots QD.Furthermore, the area of ​​a portion of the red light-emitting layer 24REM containing the cross-sections of 52 quantum dots (QDs) may be calculated, for example, by observing a portion of the red light-emitting layer 24REM containing the cross-sections of 52 quantum dots (QDs) shown in Figure 4 with a transmission electron microscope (TEM), and then calculating the area obtained by subtracting voids discernible by the transmission electron microscope (TEM) from the value of the width in the left-right direction of the portion of the red light-emitting layer 24REM shown in Figure 4 multiplied by the height in the vertical direction of the portion of the red light-emitting layer 24REM shown in Figure 4.

[0057] In a cross-section of a portion of the red light-emitting layer 24REM that includes the cross-sections of 50 or more quantum dots QD among the multiple quantum dots QD contained in the red light-emitting layer 24REM, the area A of the cross-section of the continuous film MR containing the metal sulfide, i.e., the area A obtained from area C - area B, is preferably 3.4% or more and 27.0% or less of the area B which is the sum of the areas of each of the cross-sections of the predetermined number, for example, 50 or more quantum dots QD.

[0058] A portion of the red light-emitting layer 24REM containing a predetermined number of quantum dots (QDs), such as 50 or more, as shown in Figure 4, may be observed with a transmission electron microscope (TEM). In this case, the TEM image may include a first region with a higher electron density through which the irradiated electron beam has passed, and a second region with a lower electron density. The area of ​​the first region may be defined as the area A of the cross-section of the continuous film MR containing metal sulfide, calculated from area C - area B, and the area of ​​the second region may be defined as the area B, which is the sum of the cross-sectional areas of each of the 52 quantum dots (QDs).

[0059] For example, by adjusting the display settings of a transmission electron microscope (TEM), regions containing heavier elements can be displayed in lighter colors because the irradiated electron beam is less likely to penetrate, resulting in lower electron density, while regions containing lighter elements can be displayed in darker colors because the irradiated electron beam is more likely to penetrate, resulting in higher electron density.

[0060] Since quantum dots (QDs) contain heavier elements than Zn, such as Se, In, and Cd (e.g., ZnSe, InP, and CdS), area B, which is the area of ​​the second region with a lighter color and lower electron density, may be determined from the area of ​​the region displayed in a lighter color in a transmission electron microscope (TEM) image. On the other hand, continuous films (MRs) containing metal sulfides (e.g., ZnS) contain Zn, which is a lighter element than Se, so area A, which is the area of ​​the first region with a darker color and higher electron density, may be determined from the area of ​​the region displayed in a darker color in a transmission electron microscope (TEM) image.

[0061] Alternatively, after determining the composition of the continuous film MR containing quantum dots (QD) and metal sulfides in the red light-emitting layer 24REM by elemental analysis of the red light-emitting layer 24REM, X-ray diffraction may be performed on a portion of the cross-section of the red light-emitting layer 24REM that includes a predetermined number of cross-sections, for example, 50 or more quantum dots (QD), as shown in Figure 4. A portion of the red light-emitting layer 24REM containing cross-sections of 50 or more quantum dots (QDs) includes a first region having an intrinsic diffraction peak due to X-ray diffraction of metal sulfides and a second region having an intrinsic diffraction peak due to X-ray diffraction of quantum dots (QDs). The area of ​​the first region in the portion of the red light-emitting layer 24REM determined based on the intrinsic diffraction peak due to X-ray diffraction of metal sulfides may be taken as the area A of the cross-section of the continuous film MR containing metal sulfides, calculated from area C - area B. The area of ​​the second region in the portion of the red light-emitting layer 24REM determined based on the intrinsic diffraction peak due to X-ray diffraction of quantum dots (QDs) may be taken as the area B, which is the sum of the areas of each of the cross-sections of the 50 or more quantum dots (QDs).

[0062] Furthermore, in a cross-section of a portion of the red light-emitting layer 24REM that includes the cross-section of 50 or more quantum dots (QDs) as shown in Figure 4, the number-average inter-quantum dot distance of the 50 or more quantum dots (QDs) is preferably 1 nm or more and 5 nm or less. The number-average inter-quantum dot distance is a value that can be calculated by finding the distance between N adjacent quantum dots (where N is a natural number of 2 or more) and the quantum dots with the shortest inter-quantum dot distance in a cross-section of a portion of the red light-emitting layer 24REM that includes the cross-section of 50 or more quantum dots (QDs) as shown in Figure 4, and then calculating (the sum of the N shortest distances / N).

[0063] Figure 5 shows a red light-emitting layer 24REM of a red light-emitting element 5R provided in the display device 1 of this embodiment, where the material of the shell SH of the quantum dot QD and the continuous film MR containing metal sulfide, which are included in the red light-emitting layer 24REM, are the same material.

[0064] As shown in Figure 5, in this embodiment, the quantum dot QD contained in the red light-emitting layer 24REM includes a core CO and a shell SH, and the shell SH of the quantum dot QD is composed of a metal sulfide. The explanation will take as an example the case in which both the metal sulfide contained in the continuous film MR and the metal sulfide constituting the shell SH of the quantum dot QD are ZnS, but is not limited to this. The metal sulfide contained in the continuous film MR and the metal sulfide constituting the shell SH of the quantum dot QD may be different metal sulfides. The metal sulfide may contain one or more metal elements selected from Zn, Mg, and Ga, and may be any of zinc sulfide (e.g., ZnS), magnesium zinc sulfide (e.g., ZnMgS), gallium sulfide (e.g., GaS), zinc selenide sulfide (e.g., ZnSeS), zinc telluride sulfide (e.g., ZnTeS), magnesium sulfide (e.g., MgS), and zinc gallium sulfide (e.g., ZnGaS). The band gap of a continuous film MR containing a metal sulfide matrix is ​​larger than the band gap of the core CO of a quantum dot (QD).

[0065] As shown in this embodiment in Figure 5, when the shell SH of a quantum dot (QD) is composed of a metal sulfide, for example, if both the metal sulfide contained in the continuous film MR and the metal sulfide constituting the shell SH of the quantum dot (QD) are ZnS, then the shell SH of the quantum dot (QD) composed of the metal sulfide is formed continuously with the continuous film MR and can therefore be considered as part of the continuous film MR.

[0066] Figure 6 shows another red light-emitting layer 24REM' that can be provided in the red light-emitting element 5R of the display device 1 of this embodiment, in which the material of the shell SH' of the quantum dot QD' and the continuous film MR containing metal sulfide are different materials.

[0067] As shown in Figure 6, the quantum dots QD' contained in the red light-emitting layer 24REM' consist of a core CO and a shell SH'. The shell SH' of the quantum dots QD' is composed of materials other than metal sulfides, and the band gap of the continuous film MR containing the metal sulfide matrix is ​​larger than the band gap of the core CO of the quantum dots QD'.

[0068] As shown in Figure 6, if the shell SH' of a quantum dot QD' is composed of something other than a metal sulfide, then the shell SH' of the quantum dot QD' does not contain a metal sulfide, and therefore it is a quantum dot QD.

[0069] Figure 7 shows an example of several types of quantum dot solutions, each containing multiple quantum dots (QD), a metal sulfide precursor, and a solvent, which can be used in the process of forming the red light-emitting layer (REM) of the red light-emitting element (5R) provided in the display device 1 of this embodiment.

[0070] As shown in Figure 7, in this embodiment, zinc ethylxanthogenic acid, which decomposes after heat treatment to form ZnS, is used as a precursor of the metal sulfide as an example, but the invention is not limited to this. As a precursor of the metal sulfide, a metal complex may be used in which one or more atoms selected from thiourea, N-methylthiourea, 1,3-dimethylthiourea, N,N'-dimethylthiourea, tetramethylthiourea, thioacetamide, and xanthogenic acid are coordinated to a metal atom. Furthermore, the precursor of the metal sulfide consists of a metal source and a sulfur source, the metal source being one or more selected from metal acetates, metal nitrates, and metal halogen salts, and the sulfur source being one or more selected from thiourea, N-methylthiourea, 1,3-dimethylthiourea, N,N'-dimethylthiourea, tetramethylthiourea, and thioacetamide.

[0071] In this embodiment, we will describe, as an example, a case in which a quantum dot (QD) is used in which the core is composed of InP, the shell is composed of ZnS, and the ligand is a halogen ligand, but we are not limited to this.

[0072] In this embodiment, the case in which N,N-dimethylformamide is used as the solvent is described as an example, but the invention is not limited to this, and for example, dimethyl sulfoxide may also be used.

[0073] The quantum dot solution with a concentration [M] of 0.08 shown in Figure 7 is a solution prepared by mixing quantum dots QD, zinc ethylxanthogenic acid, and N,N-dimethylformamide such that the concentration of quantum dots QD is 15 mg / ml and the concentration of zinc ethylxanthogenic acid, a precursor of metal sulfides, is 24.64 mg / ml. In the red luminescent layer 24REM formed using the quantum dot solution with a concentration [M] of 0.08, the weight ratio of quantum dots QD is 64.6%, and the weight ratio of the continuous film MR (medium) is 35.4%.

[0074] The quantum dot solution with a concentration [M] of 0.04 shown in Figure 7 is a solution prepared by mixing quantum dots QD, zinc ethylxanthogenic acid, and N,N-dimethylformamide such that the concentration of quantum dots QD is 15 mg / ml and the concentration of zinc ethylxanthogenic acid, a precursor of metal sulfides, is 12.32 mg / ml. In the red light-emitting layer 24REM formed using the quantum dot solution with a concentration [M] of 0.04, the weight ratio of quantum dots QD is 78.5%, and the weight ratio of the continuous film MR (medium) is 21.5%.

[0075] The quantum dot solution with a concentration [M] of 0.02 shown in Figure 7 is a solution prepared by mixing quantum dots QD, zinc ethylxanthogenic acid, and N,N-dimethylformamide such that the concentration of quantum dots QD is 15 mg / ml and the concentration of zinc ethylxanthogenic acid, a precursor of metal sulfides, is 6.16 mg / ml. In the red luminescent layer 24REM formed using the quantum dot solution with a concentration [M] of 0.02, the weight ratio of quantum dots QD is 88.0%, and the weight ratio of the continuous film MR (medium) is 12.0%.

[0076] The quantum dot solution with a concentration [M] of 0.01 shown in Figure 7 is a solution prepared by mixing quantum dots QD, zinc ethylxanthogenic acid (a precursor of metal sulfides), and N,N-dimethylformamide, such that the concentration of quantum dots QD is 15 mg / ml and the concentration of zinc ethylxanthogenic acid (a precursor of metal sulfides) is 3.08 mg / ml. In the red light-emitting layer 24REM formed using the quantum dot solution with a concentration [M] of 0.01, the weight ratio of quantum dots QD is 93.6%, and the weight ratio of the continuous film MR (medium) is 6.4%.

[0077] The quantum dot solution with a concentration [M] of 0.005 shown in Figure 7 is a solution prepared by mixing quantum dots QD, zinc ethylxanthogenic acid, and N,N-dimethylformamide such that the concentration of quantum dots QD is 15 mg / ml and the concentration of zinc ethylxanthogenic acid, a precursor of metal sulfides, is 1.54 mg / ml. In the red light-emitting layer 24REM formed using the quantum dot solution with a concentration [M] of 0.005, the weight ratio of quantum dots QD is 96.7%, and the weight ratio of the continuous film MR (medium) is 3.3%.

[0078] Regardless of which of the quantum dot solutions with concentrations [M] of 0.08, 0.04, 0.02, 0.01, and 0.005 shown in Figure 7 above is used to form the red light-emitting layer 24REM, in a portion of the red light-emitting layer 24REM that includes the cross-sections of 50 or more quantum dots QD, the area A of the cross-section of the continuous film MR containing the metal sulfide is smaller than the combined area B of the cross-sections of each of the 50 or more quantum dots QD.

[0079] Figure 8 is a schematic diagram showing the band levels of the hole transport layer 24HT, the red light-emitting layer 24REM, and the electron transport layer 24ET provided in the red light-emitting element 5R of the display device 1 of this embodiment.

[0080] In a portion of the red light-emitting layer 24REM provided on the red light-emitting element 5R, including the cross-sections of 50 or more quantum dots QD, the area A of the cross-section of the continuous film MR containing the metal sulfide is smaller than the combined area B of the cross-sections of each of the 50 or more quantum dots QD.

[0081] With this configuration, as shown in Figure 8, holes H are injected from the upper valence band level (-5.3eV) of the hole transport layer 24HT into the upper valence band level (-5.6eV) of the core CO of the quantum dot QD and the upper valence band level (-6.8eV) of the shell SH of the quantum dot QD, and electrons E are injected from the lower conduction band level (-3.5eV) of the electron transport layer 24ET into the lower conduction band level (-3.6eV) of the core CO of the quantum dot QD and the lower conduction band level (-3.1eV to -3.3eV) of the shell SH of the quantum dot QD, thus ensuring a good carrier balance between holes H and electrons E.

[0082] Figure 9 is a diagram illustrating the reason why good external quantum efficiency (EQE) can be achieved in the red light-emitting element 5R of the display device 1 in this embodiment.

[0083] In the case of the red light-emitting element 5R, as described above, a good carrier balance between holes H and electrons E is ensured. As shown in Figure 9, holes H and electrons E recombine at the center of the red light-emitting layer 24REM and emit light as excitons, thus achieving a good external quantum efficiency (EQE). The red light-emitting element 5R improves durability and luminescence efficiency.

[0084] Figure 10 schematically shows the band levels of the hole transport layer 24HT, the red light-emitting layer 74REM, and the electron transport layer 24ET provided in the comparative example red light-emitting element 50R.

[0085] The red light-emitting layer 74REM provided in the comparative example red light-emitting element 50R is a layer formed using, for example, a quantum dot solution with a concentration [M] of 0.3. The quantum dot solution with a concentration [M] of 0.3 is a solution obtained by mixing quantum dots QD, zinc ethylxanthogenic acid, and N,N-dimethylformamide such that the concentration of quantum dots QD is 15 mg / ml and the concentration of zinc ethylxanthogenic acid, a precursor of metal sulfides, is 92.4 mg / ml. In the red light-emitting layer 74REM formed using the quantum dot solution with a concentration [M] of 0.3, the weight ratio of quantum dots QD is 32.7%, and the weight ratio of the continuous film MR (medium) is 67.3%. The concentration of zinc ethylxanthogenic acid, a precursor of metal sulfides, becomes 30.8 mg / ml after decomposition following heat treatment.

[0086] In a portion of the red light-emitting layer 74REM that includes the cross-sections of 50 or more quantum dots (QDs) among the multiple quantum dots (QDs) contained in the red light-emitting layer 74REM, the area A of the cross-section of the continuous film MR containing the metal sulfide is larger than the combined area B of the cross-sections of the 50 or more quantum dots (QDs).

[0087] In a comparative example, a red light-emitting element 50R, in a cross-section of a portion of the red light-emitting layer 74REM that includes the cross-sections of 50 or more quantum dots QDs, the area A of the cross-section of the continuous film MR containing the metal sulfide is larger than the combined area B of the cross-sections of the 50 or more quantum dots QDs.

[0088] With this configuration, as shown in Figure 10, holes H are mainly injected from the upper valence band level (-5.3eV) of the hole transport layer 24HT to the upper valence band level (-6.8eV) of the shell SH of the quantum dot QD, where the level difference is relatively large at 1.5eV. However, they are less likely to be injected into the upper valence band level (-5.6eV) of the core CO of the quantum dot QD, where the level difference is relatively small at 0.3eV, due to the influence of the continuous film MR containing metal sulfides, which occupies a large portion of the red light-emitting layer 74REM. On the other hand, electrons E are injected from the lower conduction band level (-3.5eV) of the electron transport layer 24ET into the lower conduction band level (-3.6eV) of the quantum dot QD core CO and the lower conduction band level (-3.1eV to -3.3eV) of the quantum dot QD shell SH. As a result, in the red light-emitting layer 74REM, there is an excess of electrons E, and the carrier balance between holes H and electrons E is disrupted.

[0089] Figure 11 illustrates the reason why good external quantum efficiency (EQE) cannot be achieved in the comparative example, the red light-emitting element 50R.

[0090] As shown in Figure 11, in the red light-emitting layer 74REM, which is in an excess state of electrons E, two electrons E and one hole H form a trion state. This trion state does not emit light and is thermally deactivated, making it impossible to achieve good external quantum efficiency (EQE).

[0091] Figure 12 shows the relationship between voltage and current density for Electron Only Devices (EODs) containing a red light-emitting layer 24REM formed using a quantum dot solution with a concentration of 0.02 M, and for Electron Only Devices (EODs) containing a red light-emitting layer 24REM formed using a quantum dot solution with a concentration of 0.08 M, among the various types of quantum dot solutions shown in Figure 7.

[0092] Figure 13 shows the relationship between voltage and current density for Hole Only Devices (HODs) containing a red light-emitting layer 24REM formed using a quantum dot solution with a concentration of 0.02 M, and for Hole Only Devices (HODs) containing a red light-emitting layer 24REM formed using a quantum dot solution with a concentration of 0.08 M, among the various types of quantum dot solutions shown in Figure 7.

[0093] Figure 17 shows the emission peak wavelength, internal quantum yield (PLQY), maximum external quantum efficiency (MAX EQE), and brightness degradation rate after 5 hours of operation for multiple types of red light-emitting elements 5R, including a red light-emitting layer 24REM formed using multiple types of quantum dot solutions shown in Figure 7.

[0094] As shown in Figure 12, when the voltage applied to the Electron Only Device (EOD) is kept constant, and the amount of electrons E injected into an Electron Only Device (EOD) containing a red light-emitting layer 24REM formed using a quantum dot solution with a concentration of 0.02 M and an Electron Only Device (EOD) containing a red light-emitting layer 24REM formed using a quantum dot solution with a concentration of 0.08 M are compared, it can be confirmed that when the concentration increases fourfold, the amount of electrons E injected decreases by 30%.

[0095] On the other hand, as shown in Figure 13, when the voltage applied to the Hole Only Device (HOD) is set to a certain value, and the amount of holes H injected into a Hole Only Device (HOD) containing a red light-emitting layer 24REM formed using a quantum dot solution with a concentration of 0.02 M is compared with that of a Hole Only Device (HOD) containing a red light-emitting layer 24REM formed using a quantum dot solution with a concentration of 0.08 M, it can be confirmed that when the concentration increases fourfold, the amount of holes H injected decreases by 70%.

[0096] As described above, it can be seen that as the concentration increases, the decrease in the amount of holes H injected becomes significantly larger than the decrease in the amount of electrons E injected. This can also be understood from the band levels shown in Figures 8 and 10.

[0097] From the perspective of ensuring an even better carrier balance between holes H and electrons E and achieving an even better external quantum efficiency (EQE), it is preferable to form the red light-emitting layer 24REM using a quantum dot solution with a concentration [M] of 0.08, 0.04, 0.02, or 0.01, as shown in Figure 17, which exhibits a relatively high maximum EQE of 6.8% or more.

[0098] Figure 14 shows the degree of brightness reduction due to charge-up of quantum dots (QD) when a red light-emitting element 5R, which includes a red light-emitting layer 24REM formed using a quantum dot solution with a concentration of 0.01 M from among the various types of quantum dot solutions shown in Figure 7, and a comparative example red light-emitting element 50R shown in Figure 10 are intermittently driven.

[0099] As shown in Figure 14, when the red light-emitting element 5R and the comparative example red light-emitting element 50R are intermittently driven by repeatedly alternating between a driving period (a period of continuous power supply) and a non-driving period (a period of no power supply), the comparative example red light-emitting element 50R exhibits a significant temporary decrease in brightness during the driving period (a period of continuous power supply) compared to the red light-emitting element 5R. This temporary decrease in brightness that occurs during the driving period (a period of continuous power supply) is due to the charge-up of the quantum dots (QDs) and is a reversible decrease in brightness that can be resolved by applying a reverse voltage or leaving the device unpowered.

[0100] Figure 18 shows the degree of change in brightness and voltage over time of a red light-emitting element 5R, which includes a red light-emitting layer 24REM formed using a quantum dot solution with a concentration of 0.04 M, one of the multiple types of quantum dot solutions shown in Figure 7.

[0101] Figure 19 shows the degree of change in brightness and voltage with respect to the operating time of a red light-emitting element 5R, which includes a red light-emitting layer 24REM formed using a quantum dot solution with a concentration of 0.08 M, one of the multiple types of quantum dot solutions shown in Figure 7.

[0102] The initial brightness degradation rate (e.g., brightness degradation rate up to 1 hour) of the red light-emitting element 5R containing the red light-emitting layer 24REM formed using the quantum dot solution with a concentration of 0.04 M shown in Figure 18 is significantly suppressed compared to the initial brightness degradation rate (e.g., brightness degradation rate up to 1 hour) of the red light-emitting element 5R containing the red light-emitting layer 24REM formed using the quantum dot solution with a concentration of 0.08 M shown in Figure 19.

[0103] Furthermore, the initial increase in driving voltage (for example, the increase in driving voltage up to 1 hour) of the red light-emitting element 5R containing the red light-emitting layer 24REM formed using the quantum dot solution with a concentration of 0.04 M shown in Figure 18 is significantly suppressed compared to the initial increase in driving voltage (for example, the increase in driving voltage up to 1 hour) of the red light-emitting element 5R containing the red light-emitting layer 24REM formed using the quantum dot solution with a concentration of 0.08 M shown in Figure 19.

[0104] Based on the above, considering the initial luminance reduction rate (e.g., luminance reduction rate up to 1 hour), the initial increase in driving voltage (e.g., the increase in driving voltage up to 1 hour), and the luminance reduction rate after 5 hours of operation as shown in Figure 17, it is preferable to form the red light-emitting layer 24REM using a quantum dot solution with a concentration [M] of 0.04, 0.02, 0.01, or 0.005 from among the multiple types of quantum dot solutions shown in Figure 7.

[0105] Figure 15 shows the photoluminescence intensity (PL) at each wavelength for a red light-emitting layer 24REM formed using a quantum dot solution with a concentration of 0.01 M, one of the multiple types of quantum dot solutions shown in Figure 7, and a comparative example red light-emitting layer formed using a quantum dot solution with a concentration of 0.16 M.

[0106] As shown in Figure 15, the emission peak wavelength of the red light-emitting layer formed using a quantum dot solution with a concentration of 0.16 M, which is a comparative example, is 632 nm, while the emission peak wavelength of the red light-emitting layer 24REM formed using a quantum dot solution with a concentration of 0.01 M is 634 nm. A shift to the longer wavelength side of the emission peak wavelength is observed with decreasing concentration, but the amount of the shift is good, at about 2 nm.

[0107] This shift of the emission peak wavelength toward longer wavelengths is thought to be due to an increase in Förster resonance energy transfer (FRET). As shown in Figure 17, the emission peak wavelengths of the red emission layers 24REM formed using quantum dot solutions with concentrations of 0.02 M, 0.04 M, and 0.08 M, respectively, are all 632 nm, and no shift of the emission peak wavelength toward longer wavelengths is observed. This is thought to be because, in the case of the red emission layers 24REM formed using quantum dot solutions with concentrations of 0.02 M, 0.04 M, and 0.08 M, respectively, the distance between the cores of the quantum dots (QD) can be ensured to a certain level or higher, thereby suppressing Förster resonance energy transfer (FRET). On the other hand, in the case of the red emission layer 24REM formed using a quantum dot solution with a concentration of 0.005 M, the distance between the cores of the quantum dots (QD) cannot be ensured to a certain level or higher, and it is thought that the shift of the emission peak wavelength toward longer wavelengths occurs due to an increase in Förster resonance energy transfer (FRET).

[0108] Based on the above, when considering the shift of the emission peak wavelength toward longer wavelengths, it is preferable to form the red emission layer 24REM using a quantum dot solution with a concentration [M] of 0.08, 0.04, 0.02, or 0.01 from among the multiple types of quantum dot solutions shown in Figure 7.

[0109] Figure 16 shows the relationship between the weight ratio of continuous film MR containing metal sulfides, which are the medium in multiple types of red light-emitting layers 24REM, which are thin films formed using multiple types of quantum dot solutions shown in Figure 7, and the internal quantum yield (PLQY: photoluminescence quantum yield).

[0110] As shown in Figures 16 and 17, in the case of a red light-emitting element 5R containing a red light-emitting layer 24REM formed using a quantum dot solution with a concentration [M] of 0.005 from among the multiple types of quantum dot solutions shown in Figure 7, the weight ratio of ZnS medium in the red light-emitting layer 24REM was 3.3%, and the internal quantum yield (PLQY) was a good value of 60%. Furthermore, in each of the red light-emitting elements 5R containing a red light-emitting layer 24REM formed using quantum dot solutions with concentrations [M] of 0.01, 0.02, 0.04, and 0.08, respectively, the weight ratio of ZnS medium in the red light-emitting layer 24REM was 6.4% to 35.4%, and the internal quantum yield (PLQY) was a good value of 74% to 76%.

[0111] Based on the above, when considering the internal quantum yield (PLQY), it is preferable to form the red light-emitting layer 24REM using a quantum dot solution with a concentration [M] of 0.08, 0.04, 0.02, 0.01, or 0.005 from among the multiple types of quantum dot solutions shown in Figure 7.

[0112] As shown in Figure 17, when considering the suppression of emission peak wavelength shift, internal quantum yield (PLQY), maximum external quantum efficiency (MAX EQE), and brightness reduction rate after 5 hours of operation, it is preferable that the ZnS medium weight ratio in the red light-emitting layer 24REM is greater than 3.3% and less than 35%. For example, it is preferable to form the red light-emitting layer 24REM using a quantum dot solution with a concentration [M] of 0.04, 0.02, or 0.01 from among the multiple types of quantum dot solutions shown in Figure 7. In the case of a red light-emitting element 5R including a red light-emitting layer 24REM formed using a quantum dot solution with a concentration [M] of 0.04, 0.02, or 0.01, the emission peak wavelength shift can be suppressed to 0-2 nm, the internal quantum yield (PLQY) can be achieved to 74%-76%, the maximum external quantum efficiency (MAX EQE) can be achieved to 7.6%-7.9%, and the brightness reduction rate after 5 hours of operation can be suppressed to 28%-44%.

[0113] When the weight ratio of ZnS medium in the red light-emitting layer 24REM is greater than 3.3% and less than 35%, in a cross-section of a portion of the red light-emitting layer 24REM that includes the cross-sections of a predetermined number of quantum dots QD, for example, 50 or more quantum dots QD, the area A of the cross-section of the continuous film MR containing the metal sulfide matrix is ​​3.4% or more and 27.0% or less of the combined area B of the cross-sections of each of the predetermined number of quantum dots QD, for example, 50 or more quantum dots QD.

[0114] Furthermore, when the weight ratio of ZnS medium in the red light-emitting layer 24REM is greater than 3.3% and less than 35%, the volume of the continuous film MR containing the metal sulfide matrix in the red light-emitting layer 24REM is between 3.6% and 37.1% of the volume of the red light-emitting layer 24REM.

[0115] The preferred volume of the continuous film MR containing the metal sulfide relative to the volume of the red light-emitting layer 24REM described above can be calculated, for example, as follows. For example, as a core material for quantum dots (QD), the density is 4.81 g / cm³. 3 InP, which is used as a shell material for quantum dots (QDs), has a density of 4.09 g / cm³. 3 Assuming that ZnS is used, the core diameter of the quantum dot (QD) is 3.2 nm, and the shell thickness of the quantum dot (QD) is 0.5 nm, then the average density of the quantum dot (QD) is 4.41 g / cm³. 3 This can be achieved. When ZnS medium is mixed with 15 mg of quantum dots (QD) at the lower limit of 0.005 M, the weight of the ZnS medium becomes 3.3%, and the volume of the ZnS medium becomes 3.6%. On the other hand, when ZnS medium is mixed with 15 mg of quantum dots (QD) at the upper limit of 0.08 M, the weight of the ZnS medium becomes 35.4%, and the volume of the ZnS medium becomes 37.1%.

[0116] Also, the preferable area A of the cross-section of the continuous film MR containing metal sulfide with respect to the area B obtained by combining the areas of the cross-sections of each of a predetermined number, for example, 50 or more quantum dots QD, described above, can be calculated as follows. Assuming the volume of the quantum dot QD is V(QD) and the volume of the ZnS medium is V(MR), the volume V(24REM) of the red light-emitting layer 24REM is V(QD) + V(MR). When the thickness of the sample (the thickness in the depth direction of FIG. 4) during observation with a transmission electron microscope (TEM) is the radius r of the quantum dot QD, the cross-sectional area of the quantum dot QD is ((V(QD) + V(MR)) / r). Also, assuming the radius of the quantum dot QD is r, the volume V(QD) of each of the n quantum dots QD is 4 / 3 × π × r 3 results in, and the maximum cross-sectional area of the quantum dot QD is πr 2 However, the sum of the cross-sectional areas of the n quantum dots QD cut at random positions is 0.66nπr 2 results in. Therefore, when the volume of the ZnS medium is 3.6% of the red light-emitting layer 24REM, the cross-sectional area of the ZnS medium is 3.4% of the red light-emitting layer 24REM, and when the volume of the ZnS medium is 37.1% of the red light-emitting layer 24REM, the cross-sectional area of the ZnS medium is 27.0% of the entire red light-emitting layer 24REM.

[0117] Also, an image of a cross-section of a transmission electron microscope (TEM) of a part of the red light-emitting layer 24REM including the cross-sections of 50 or more quantum dots QD among the plurality of quantum dots QD corresponds to the area of the cross-section of the continuous film MR containing metal sulfide, and includes a first region with a higher electron density through which the irradiated electron beam has passed, and a second region corresponding to the area obtained by combining the areas of the cross-sections of each of the 50 or more quantum dots QD and having a lower electron density. The red light-emitting layer 24REM has a width L orthogonal to the cross-section of the red light-emitting layer 24REM, and the value F obtained by combining the product D of the area of the first region and the width L and the product E of the area of the second region and the width L is the volume of a part of the red light-emitting layer 24REM. The product D may be regarded as the volume of the continuous film MR containing metal sulfide, which is the matrix in a part of the red light-emitting layer 24REM.

[0118] Furthermore, the red light-emitting layer 24REM includes a first region having an intrinsic diffraction peak due to X-ray diffraction of metal sulfides and a second region having an intrinsic diffraction peak due to X-ray diffraction of quantum dots QD. The area of ​​the first region in the cross-section of the red light-emitting layer 24REM, determined based on the intrinsic diffraction peak due to X-ray diffraction of the metal sulfides, is area G. The area of ​​the second region in the cross-section of the red light-emitting layer 24REM, determined based on the intrinsic diffraction peak due to X-ray diffraction of the quantum dots, is area H. The red light-emitting layer 24REM has a width L perpendicular to the cross-section of the red light-emitting layer 24REM. The sum of the product I of area G and width L and the product J of area H and width L is defined as the volume of a portion of the red light-emitting layer 24REM, and the product I may be defined as the volume of a continuous film MR containing metal sulfides, which is the matrix in a portion of the red light-emitting layer 24REM.

[0119] Figure 20 is a diagram illustrating the process of forming the red light-emitting layer 24REM included in the manufacturing method of the red light-emitting element 5R provided in the display device 1 of this embodiment.

[0120] As shown in Figure 20, the step of forming the red light-emitting layer 24REM included in the manufacturing method of the red light-emitting element 5R provided in the display device 1 includes the steps of: preparing a quantum dot solution (S1) containing a plurality of quantum dots QD, a metal sulfide precursor, and a solvent; applying the quantum dot solution (S2); and a heat treatment step (S3) in which the metal sulfide precursor is thermally decomposed to form a continuous film MR containing the metal sulfide, which is the matrix.

[0121] In the step (S1) for preparing the quantum dot solution shown in Figure 20, the amounts of the plurality of quantum dots and the amount of the metal sulfide precursor contained in the quantum dot solution are adjusted so that the value of ((weight of red light-emitting layer 24REM - weight of quantum dots QD) / weight of red light-emitting layer 24REM) × 100% in the red light-emitting layer 24REM after the heat treatment step (S3) is greater than 3.3% and less than 35%.

[0122] Although not shown in the figures, the method for manufacturing the red light-emitting element 5R includes the steps of forming one of the anode 22 and cathode 25, forming the red light-emitting layer 24REM described above, and forming the other of the anode 22 and cathode 25.

[0123] In this embodiment, the configuration and manufacturing method of the red light-emitting element 5R were described using it as an example, but the same configuration and manufacturing method can be applied to the green light-emitting element 5G and the blue light-emitting element 5B.

[0124] [Additional Notes] This disclosure is not limited to the embodiments described above, 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 this disclosure. Furthermore, new technical features can be formed by combining the technical means disclosed in each embodiment. [Industrial applicability]

[0125] This disclosure can be used in light-emitting elements, display devices, methods for manufacturing light-emitting elements, and methods for forming light-emitting layers. [Explanation of Symbols]

[0126] 1 Display device 3. Barrier layer 4 Thin-film transistor layer 5R Red Light-Emitting Device (Light-Emitting Device) 5G Green Light-Emitting Device (Light-Emitting Device) 5B Blue light-emitting element (light-emitting element) 6. Sealing layer 12 circuit boards 16, 18, 20 Inorganic insulating film 21 Planarization film 22 Anodes 24R red light-emitting layer (functional layer) Functional layer including 24G green light-emitting layer 24B Functional layer containing a blue light-emitting layer 24REM, 24REM' Red light-emitting layer (light-emitting layer) 24HT Hole Transport Layer 24ET electron transport layer 25 Cathode 26, 28 Inorganic sealing film 27 Organic film 39 Functional Films QD, QD' Quantum dots CO2 quantum dot core SH, SH' Quantum dot shells MR continuous membrane (matrix) PIX pixels RSP (Red Subpixel) GSP Green Subpixel BSP (Blue Subpixel) DA display area NDA frame area

Claims

1. A-scatter, Cathode and, A light-emitting layer is provided between the anode and the cathode, The light-emitting layer comprises a matrix containing a metal sulfide and a plurality of quantum dots embedded in the matrix. A light-emitting element in which, in a cross-section of a portion of the light-emitting layer that includes the cross-sections of a predetermined number of quantum dots among the plurality of quantum dots, the area C of the cross-section of the portion of the light-emitting layer is less than twice the area B of the combined area of ​​the cross-sections of each of the predetermined number of quantum dots.

2. The light-emitting element according to claim 1, wherein the area A obtained from area C - area B is 3.4% or more and 27.0% or less of area B.

3. The transmission electron microscope (TEM) image of the cross-section of the light-emitting layer includes a first region where the electron density transmitted by the irradiated electron beam is higher, and a second region where the electron density is lower. The area of ​​the first region is the area A obtained from area C minus area B, The light-emitting element according to claim 1 or 2, wherein the area of ​​the second region is the area B.

4. The cross-section of the light-emitting layer includes a first region having an intrinsic diffraction peak due to X-ray diffraction of the metal sulfide, and a second region having an intrinsic diffraction peak due to X-ray diffraction of the plurality of quantum dots. The area of ​​the first region in the cross-section of the light-emitting layer, determined based on the intrinsic diffraction peaks of the metal sulfide by X-ray diffraction, is the area A obtained from area C - area B. The light-emitting element according to claim 1 or 2, wherein the area of ​​the second region in the cross-section of the light-emitting layer, determined based on the intrinsic diffraction peaks of the plurality of quantum dots by X-ray diffraction, is the area B.

5. A-scatter, Cathode and, A light-emitting layer is provided between the anode and the cathode, The light-emitting layer comprises a matrix containing a metal sulfide and a plurality of quantum dots embedded in the matrix. A light-emitting element in which the volume of the matrix in the light-emitting layer is 3.6% or more and 37.1% or less of the volume of the light-emitting layer.

6. A transmission electron microscope (TEM) image of a cross-section of a portion of the light-emitting layer, including the cross-sections of a predetermined number of quantum dots among the plurality of quantum dots, comprises a first region corresponding to the area of ​​the matrix cross-section, where the electron density transmitted by the irradiated electron beam is higher, and a second region corresponding to the combined area of ​​the cross-sectional areas of the predetermined number of quantum dots, where the electron density is lower. The light-emitting layer has a width L perpendicular to the cross-section of the light-emitting layer, The sum of the product D of the area of ​​the first region and the width L and the product E of the area of ​​the second region and the width L, F, is the volume of a portion of the light-emitting layer. The light-emitting element according to claim 5, wherein the product D is the volume of the matrix in a part of the light-emitting layer.

7. The light-emitting layer includes a first region having an intrinsic diffraction peak due to X-ray diffraction of the metal sulfide, and a second region having an intrinsic diffraction peak due to X-ray diffraction of the plurality of quantum dots. The light-emitting layer is determined based on the intrinsic diffraction peaks of the metal sulfide obtained by X-ray diffraction. The area of ​​the first region on the surface is area G, The area of ​​the second region on a certain surface of the light-emitting layer, determined based on the intrinsic diffraction peaks of the plurality of quantum dots due to X-ray diffraction, is area H. The light-emitting layer has a width L perpendicular to a certain surface of the light-emitting layer, The volume of the light-emitting layer is the sum of the product I of the area G and the width L and the product J of the area H and the width L. The light-emitting element according to claim 5, wherein the product I is the volume of the matrix in the light-emitting layer.

8. A light-emitting element according to any one of claims 1, 2, 5 to 7, wherein in a cross-section of a portion of the light-emitting layer including the cross-section of a predetermined number of quantum dots among the plurality of quantum dots, the number-average inter-quantum dot distance of the predetermined number of quantum dots is 1 nm or more and 5 nm or less.

9. Each of the plurality of quantum dots includes a core and a shell, Each of the shells of the plurality of quantum dots is composed of a metal sulfide. The light-emitting element according to any one of claims 1, 2, 5 to 7, wherein the band gap of the matrix is ​​greater than the band gap of each of the cores of the plurality of quantum dots.

10. Each of the plurality of quantum dots includes a core and a shell, Each of the shells of the plurality of quantum dots is composed of a material other than a metal sulfide. The light-emitting element according to any one of claims 1, 2, 5 to 7, wherein the band gap of the matrix is ​​greater than the band gap of each of the cores of the plurality of quantum dots.

11. The light-emitting element according to any one of claims 1, 2, 5 to 7, wherein the metal sulfide comprises one or more metal elements selected from Zn, Mg, and Ga.

12. The light-emitting element according to claim 11, wherein the metal sulfide is any one of zinc sulfide, magnesium zinc sulfide, gallium sulfide, zinc selenide sulfide, zinc telluride sulfide, magnesium sulfide, and zinc gallium sulfide.

13. The light-emitting element according to claim 12, wherein the metal sulfide is zinc sulfide.

14. A display device comprising a light-emitting element according to any one of claims 1, 2, 5 to 7.

15. A step of preparing a quantum dot solution comprising multiple quantum dots, a metal sulfide precursor, and a solvent, The steps include applying the quantum dot solution, A method for forming an luminescent layer, comprising a heat treatment step of thermally decomposing the metal sulfide precursor to form a matrix containing the metal sulfide, A method for forming an emissive layer, wherein in the step of preparing the quantum dot solution, the amount of the plurality of quantum dots and the amount of the metal sulfide precursor contained in the quantum dot solution are adjusted such that the value of ((weight of emissive layer - weight of quantum dots) / weight of emissive layer) × 100% in the emissive layer after the heat treatment step is greater than 3.3% and less than 35%.

16. A step of forming one of the anode and cathode, A step of forming an emissive layer by the method for forming an emissive layer described in claim 15, A method for manufacturing a light-emitting element, comprising the step of forming the other of the anode and the cathode.

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