Display device, nanomaterial and dispersion liquid, and method for manufacturing red display device

Abstract

The purpose of the present invention is to provide a display device capable of selecting a combination of preferred materials of an electron transport layer and a hole transport layer, and a method for manufacturing the same. The present invention is a display device which is provided with a display region having a light-emitting element, and which is characterized in that: the light-emitting element is arranged on the display region; the light-emitting element has a first electrode, a light-emitting layer, a second electrode, an electron transport layer or a hole transport layer formed between the first electrode and the light-emitting layer, and a hole transport layer or an electron transport layer formed between the second electrode and the light-emitting layer, and is formed by laminating these layers. The electron transport layer contains at least any one of ZnO, MgZnO, and SnO, and the hole transport layer or the hole injection layer contains at least any one of NiO and MoO.

Description

Technical Field

[0001] This invention relates to display devices using quantum dots. Background Technology

[0002] The inventions related to organic EL (organic electroluminescence) are disclosed in the following patent documents.

[0003] Organic EL devices are constructed by stacking an anode, a hole injection layer, a hole transport layer, a light-emitting layer, an electron transport layer, an electron injection layer, and a cathode on a substrate. Such organic EL devices are formed from organic compounds and emit light through excitons generated by the recombination of electrons and holes injected into the organic compound.

[0004] Existing technical documents Patent documents Patent Document 1: Japanese Patent Application Publication No. 2017-45650 Summary of the Invention

[0005] The problem that the invention aims to solve However, the invention described in Patent Document 1 does not specify any preferred combination of materials for the electron transport layer and hole transport layer associated with the absorption edge.

[0006] The present invention was made in view of the above circumstances, and its object is to provide a display device capable of selecting a preferred combination of materials for the electron transport layer and the hole transport layer.

[0007] Methods for solving problems This invention relates to a display device, characterized in that it is a display device having a display area, wherein the display area has a light-emitting element, the light-emitting element having a first electrode, a light-emitting layer, a second electrode, an electron transport layer or a hole transport layer formed between the first electrode and the light-emitting layer, and a hole transport layer or an electron transport layer formed between the second electrode and the light-emitting layer, and the two layers are stacked together, wherein the electron transport layer comprises at least one of ZnO, MgZnO or SnO, and the hole transport layer or hole injection layer comprises at least one of NiO or MoO.

[0008] The present invention is characterized in that the material used in the electronic transmission layer constituting the above-described display device has a slope greater than that of the body material when the Tauc plot of the absorption edge is made.

[0009] The present invention is characterized in that the polydispersity index of the dispersion of the material described above is less than 0.1 in the DLS measurement.

[0010] Invention Effects According to the display device of the present invention, by combining the materials constituting the electron transport layer and the hole transport layer, the absorption edge can be greatly increased. Attached Figure Description

[0011] Figure 1 This is a partial top view of the display device in this embodiment.

[0012] Figure 2 It is Figure 1 The image shows a magnified cross-sectional view of a display area of ​​the display device.

[0013] Figure 3 It means and Figure 2 Cross-sectional views of different thin-film transistor structures.

[0014] Figure 4A This is a cross-sectional view of the light-emitting element in the first embodiment. Figure 4B This is an energy level diagram of each layer in the display device of the first embodiment.

[0015] Figure 5 is a schematic diagram of the quantum dot in this embodiment.

[0016] Figure 6 and Figure 1 Cross-sectional views of light-emitting elements in different implementations.

[0017] Figure 7A This is an energy level diagram using quantum dots with a core-shell structure. Figure 7B This is an energy level diagram when using quantum dots with a structure where the core is not covered by a shell.

[0018] Figure 8A This is a cross-sectional view of a light-emitting element different from that in Figure 4. Figure 8B yes Figure 8A Energy level diagram of each layer in the light-emitting element.

[0019] Figure 9 shows a cross-sectional view of a light-emitting element in a different embodiment than Figure 8.

[0020] Figure 10A This is an energy level diagram using quantum dots with a core-shell structure. Figure 10B This is an energy level diagram when using quantum dots with a structure where the core is not covered by a shell.

[0021] Figure 11 This is a schematic diagram illustrating the process of forming an inorganic layer using inkjet printing.

[0022] Figure 12 These are coating photographs of an embodiment.

[0023] Figure 13 These are PYS measurement data for Cd-based green quantum dots.

[0024] Figure 14 These are PYS measurement data.

[0025] Figure 15 This is an energy level diagram of each layer in the light-emitting element used in the experiment.

[0026] Figure 16 This is a graph showing the relationship between the current value and EQE of EL and PL light emitters using green quantum dots.

[0027] Figure 17 The graphs show the relationship between the current value and EQE of EL and PL light emitters using red quantum dots, and further show the relationship between the current value and EQE of EL light emitters using blue quantum dots.

[0028] Figure 18 This represents the energy band gap Eg and the energy E at the lower end of the conduction band of each layer in the light-emitting element used in the experiment. CB Energy E at the upper end of the valence band VB The curves and energy level diagrams of each layer.

[0029] Figure 19 ZnO is used in the electron transport layer (ETL). X (Li) and ZnO X UV data for (K).

[0030] Figure 20 ZnO is used in the electron transport layer (ETL). X (Li) and ZnO X PL data for (K).

[0031] Figure 21 ZnO is used in the electron transport layer (ETL). X (Li) and ZnO X PYS data for (K).

[0032] Figure 22 This is the synthesis process of ZnO.

[0033] Figure 23 This is the synthesis process of MgZnO.

[0034] Figure 24 This is the synthesis process of SnOx.

[0035] Figure 25 This is the synthesis process for NiOx.

[0036] Figure 26 This is the synthesis process of MnOx.

[0037] Figure 27A It is a graph representing the UV spectrum.

[0038] Figure 27B It is a curve representing the band gap energy.

[0039] Figure 28 This is the synthesis process of NiO.

[0040] Figure 29 This is the XRD pattern of NiO.

[0041] Figure 30 This represents the UV spectrum of NiO.

[0042] Figure 31 This is a curve representing the band gap energy of NiO (Tauc plot).

[0043] Figure 32 This is the XANES spectrum used in Ni valence evaluation.

[0044] Figure 33 It is a graph showing the relationship between particle size and scattering intensity distribution in Examples 10 to 12. Detailed Implementation

[0045] Hereinafter, one embodiment of the present invention (hereinafter referred to as "Embodiment") will be described in detail. Furthermore, the present invention is not limited to the following embodiment, and various modifications can be made within its scope.

[0046] like Figure 1 As shown, multiple display areas 2 are arranged in a matrix in the display device 1. Among the display areas 2, there are three types: a red emitting area 2a that emits red light, a green emitting area 2b that emits green light, and a blue emitting area 2c that emits blue light. These three emitting areas 2a, 2b, and 2c are arranged, for example, in the row direction to form a group, constituting a pixel in the color display.

[0047] Light-emitting elements 3 are formed in each of the light-emitting regions 2a, 2b, and 2c. The layer structure of the light-emitting elements 3 will be described later. A thin-film transistor (TFT) 4 is connected to each light-emitting element 3. The light-emitting element 3 is, for example, a top-emitting type.

[0048] Figure 2The thin-film transistor 4 shown is an n-channel (n-ch) TFT, constructed by stacking a gate 4a, a channel layer 4b, a gate insulating film (not shown), a drain 4c, and a source 4d on a substrate 5. The material of the channel layer 4b is not limited, but it is an N-type semiconductor, preferably an oxide semiconductor. As an oxide semiconductor, In-Ga-Zn-O based semiconductors are preferred. In-Ga-Zn-O based semiconductors have high mobility and low leakage current, making them a preferred choice for thin-film transistors. Alternatively, Poly-Si can also be preferred. Figure 2 The thin-film transistor 4 shown is a top-contact bottom-gate type, but it can also be a bottom-contact bottom-gate type.

[0049] The source electrode 4d is connected to the power supply line, and the drain electrode 4c is connected to the light-emitting element 3.

[0050] Alternatively, thin-film transistor 4 could also be Figure 3 The top grid type is shown. (As shown in the image) Figure 3 As shown, a channel layer 4b is formed on the substrate 5, and the surface of the channel layer 4b is covered by a gate insulating film 4e. Then, a gate 4a is formed on the surface of the gate insulating film 4e. Figure 3 As shown, the surface of the gate 4a is covered by an insulating film 4f. Furthermore, multiple through-holes are formed, penetrating the gate insulating film 4e and the insulating film 4f and communicating with the channel layer 4b. The drain 4c and source 4d are formed through each through-hole. Moreover, the surfaces of the drain 4c and source 4d are covered by a protective film 7. Additionally, a transparent electrode communicating with the drain 4c and source 4d is formed on the surface of the protective film 7. Figure 3 The transparent electrode 8 shown is connected to the drain electrode 4c.

[0051] Figure 3 The channel layer 4b of the thin-film transistor 4 shown is an N-type semiconductor, preferably an oxide semiconductor. As an oxide semiconductor, an In-Ga-Zn-O based semiconductor is preferred.

[0052] like Figure 2 As shown, the display device 1 has a structure in which a thin film transistor 4 and a light-emitting element 3 are located between a pair of substrates 5 and 6. A sealing resin (not shown) is provided in a frame shape between each substrate 5 and 6, and the substrates 5 and 6 are connected by the sealing resin.

[0053] The structure of the light-emitting element 3 will be described below. Figure 4A This is a cross-sectional view of the light-emitting element in the first embodiment. Figure 4B This is an energy level diagram of each layer in the display device of the first embodiment.

[0054] like Figure 4AAs shown, the light-emitting element 3 is configured to have: a substrate 10; a cathode 15 formed on the substrate; an electron transport layer (ETL) 14 formed on the cathode 15; a light-emitting layer (EML) 13 formed on the electron transport layer 14; a hole transport layer (HTL) 12 formed on the light-emitting layer 13; and an anode 11 formed on the hole transport layer 12.

[0055] When a voltage is applied between the electrodes of the light-emitting element 3 in this embodiment, holes are injected from the anode 11 and electrons are injected from the cathode 15. Figure 4B Energy level models for hole transport layer 12, luminescent layer 13, and electron transport layer 14 are shown respectively. Figure 4B As shown, holes transported from hole transport layer 12 are injected from the HOMO level of hole transport layer 12 into the HOMO level of light-emitting layer 13. On the other hand, electrons transported from electron transport layer 14 are injected from the LUMO level of electron transport layer 14 into the LUMO level of light-emitting layer 13. Then, holes and electrons recombine in light-emitting layer 13, and the quantum dots in light-emitting layer 13 become excited, enabling light emission from the excited quantum dots.

[0056] In this embodiment, the light-emitting layer 13 is formed of an inorganic layer containing quantum dots.

[0057] (Quantum dot) The composition and material of quantum dots are not limited. For example, in this embodiment, quantum dots are nanoparticles with a particle size of about several nm to tens of nm.

[0058] For example, quantum dots are formed from CdS, CdSe, ZnS, ZnSe, ZnSeS, ZnTe, ZnTeS, InP, (Zn)AgInS2, (Zn)CuInS2, etc. Due to the toxicity of Cd, its use is restricted in various countries. Therefore, it is preferable that quantum dots do not contain RoHS-restricted substances such as Cd and Pb.

[0059] like Figure 5A As shown, preferably, a plurality of organic ligands 21 are coordinated on the surface of the quantum dot 20. This suppresses the aggregation of the quantum dots 20 and allows them to exhibit the desired optical properties. The ligands that can be used for the reaction are not particularly limited; for example, the following ligands can be cited as representative ligands.

[0060] Aliphatic primary amines, oleylamines: C 18 H 35 NH2, stearyl (octadecyl)amine: C 18 H 37NH2, dodecyl (lauryl)amine: C 12 H 25 NH2, decylamine: C 10 H 21 NH2, Octylamine: C8H 17 NH2 Fatty acids, oleic acid: C 17 H 33 COOH, stearic acid: C 17 H 35 COOH, palmitic acid: C 15 H 31 COOH, myristic acid: C 13 H 27 COOH, lauryl (dodecanoic acid): C 11 H 23 COOH, Decanoic acid: C9H 19 COOH, Caprylic acid: C7H 15 COOH Thiol series, octadecylthiol: C 18 H 37 SH, hexadecylthiol: C 16 H 33 SH, tetradecylthiol: C 14 H 29 SH, dodecylthiol: C 12 H 25 SH, decanethiol: C 10 H 21 SH, Octanethiol: C8H 17 SH Phosphine series, trioctylphosphine: (C8H 17 3P, triphenylphosphine: (C6H5)3P, tributylphosphine: (C4H9)3P Phosphine oxide series, trioctylphosphine oxide: (C8H 17 3P=O, Triphenylphosphine oxide: (C6H5)3P=O, Tributylphosphine oxide: (C4H9)3P=O In addition, in this embodiment, a short ligand is preferably used as the organic ligand 21. Although not limited, 3-mercaptopropionic acid (MPA) can be used as the organic ligand 21.

[0061] The ligands of the quantum dots 20 contained in the quantum dot layer are preferably shorter than those of the quantum dots 20 formed by liquid-phase synthesis.

[0062] Thus, by using short ligands in the ligands of the quantum dots 20 contained in the quantum dot layer, the roughness of the quantum dot layer can be reduced, and the extraction efficiency of electrons and holes can be improved. On the other hand, by using long ligands in the ligands when forming quantum dots 20 using liquid-phase synthesis, the dispersibility and film-forming properties can be improved.

[0063] Alternatively, after synthesizing quantum dots 20 with long ligands by liquid-phase synthesis, the composition containing quantum dots 20 can be replaced with short ligands (e.g., 3-mercaptopropionic acid) before or after coating.

[0064] in addition, Figure 5B The quantum dot 20 shown is a core-shell structure having a core 20a and a shell 20b covering the surface of the core 20a. For example... Figure 5B As shown, preferably, a plurality of organic ligands 21 are coordinated on the surface of the quantum dot 20. Figure 5B The core 20a of the quantum dot 20 shown is Figure 5A The nanoparticles are shown. Therefore, the core 20a is formed, for example, from the materials listed above. The material of the shell 20b is not limited, and it can be formed, for example, from zinc sulfide (ZnS). Like the core 20a, the shell 20b is preferably free of RoHS-restricted substances such as cadmium (Cd) and phosphorus (Pb).

[0065] Alternatively, the shell 20b can also be in a state of solid solution dissolved on the surface of the core 20a. Figure 5B In the diagram, the boundary between the core 20a and the shell 20b is shown by a dashed line. However, this means that the boundary between the core 20a and the shell 20b may or may not be confirmed by analysis; either case is acceptable. Furthermore, although not shown, it is preferable to have a buffer layer between the core 20a and the shell 20b. The buffer layer is a region where at least some or all of the elements constituting the core 20a are mixed with at least some or all of the elements constituting the shell 20b.

[0066] (Emitting layer 13) The light-emitting layer 13 may be formed solely of the quantum dots listed above, or it may contain quantum dots and other fluorescent materials. Alternatively, the light-emitting layer 13 may be formed by coating quantum dots dissolved in a solvent using, for example, an inkjet printing method, or a small amount of solvent may remain in the light-emitting layer 13.

[0067] Formed in Figure 1 The light-emitting layer 13 of the light-emitting element 3 in the red light-emitting region 2a shown contains red quantum dots that emit red fluorescence. Additionally, in the formation of... Figure 1 The light-emitting layer 13 of the light-emitting element 3 in the green light-emitting region 2b shown contains green quantum dots that emit green fluorescence. Additionally, in the formation of... Figure 1 The light-emitting layer 13 of the light-emitting element 3 in the blue light-emitting region 2c shown contains blue quantum dots that emit blue fluorescence.

[0068] In addition, the wavelength of blue light emission is preferably around 450nm. In this way, by adjusting the emission to not emit light with a wavelength shorter than 450nm, health risks can be suppressed.

[0069] The light-emitting layer 13 can be formed using existing thin film formation methods such as inkjet printing or vacuum evaporation, as listed above.

[0070] (Hole transport layer 12) The hole transport layer 12 is composed of an inorganic or organic material capable of transporting holes. The hole transport layer 12 is preferably composed of an inorganic material, preferably containing at least one of NiO or MoO. The hole transport layer 12 is particularly preferably formed from NiO nanoparticles. Alternatively, Al2O3 may be mixed into NiO in the hole transport layer 12, for example. Li, Mg, Al, etc., may also be doped into the metal oxide. Furthermore, the hole transport layer 12 may also be an inorganic material other than an inorganic oxide. In this embodiment, the oxygen vacancies of the metal oxide used in the hole transport layer 12 are preferably smaller than those of the bulk material.

[0071] "Bulk material" refers to a block containing the metal oxide, regardless of size or shape.

[0072] The hole transport layer 12, like the light-emitting layer 13, can be formed by printing methods such as inkjet printing, or by existing thin film technologies such as vacuum evaporation.

[0073] (Electron transport layer 14) The electron transport layer 14 is composed of an inorganic or organic substance capable of transporting electrons. The electron transport layer 14 is preferably composed of an inorganic substance, such as preferably containing at least one of ZnO, MgZnO, or SnO. Two or more of these substances may also be selected. The electron transport layer 14 is particularly preferably composed of ZnO. X Nanoparticles are formed. Alternatively, Li, Mg, Al, Mn, etc., can be doped into the metal oxide. Furthermore, the electron transport layer 14 may also contain inorganic materials other than inorganic oxides (e.g., CsPbBr3). X is not limited, but is approximately 0.8 to 1.2. In this embodiment, the oxygen vacancies in the metal oxide used in the electron transport layer 14 are preferably smaller than those in the bulk material.

[0074] Like the light-emitting layer 13, the electron transport layer 14 can be formed by printing methods such as inkjet printing with a solvent containing nanoparticles, or by existing thin film technologies such as vacuum evaporation with a solvent containing nanoparticles.

[0075] (Anode 11) In this embodiment, the material of the anode 11 is not limited. For example, the anode 11 is preferably made of metals such as Au and Ag, Cu, Si, O2, or ZnO. X The anode 11 is formed from an isoconductive transparent material, indium-tin oxide (ITO). The anode 11 is preferably formed from ITO. The anode 11 can be formed as a thin film on the substrate 10 using methods such as evaporation or sputtering.

[0076] In this embodiment, since the light is extracted from the anode 11 side, the anode 11 needs to be transparent, preferably a thin metal film with excellent transmittance such as Ag or a metal oxide with excellent transmittance.

[0077] (Cathode 15) In this embodiment, the material of the cathode 15 is not limited. For example, the cathode 15 can be made of indium-tin oxide (ITO), metals, alloys, conductive compounds, and mixtures thereof as electrode materials. For example, the cathode 15 is formed of ITO. Furthermore, the cathode 15 is formed, for example, via a non-transmissive metal layer formed on the substrate 10. Thus, the light-emitting element 3 can be configured as a top-emitting element.

[0078] The cathode 15 can be formed into a thin film by using methods such as vapor deposition or sputtering to deposit these electrode materials.

[0079] (Substrate 10) In this embodiment, the material of the substrate 10 is not limited. The substrate 10 can be formed of, for example, glass or plastic. Specifically, the substrate 10 is formed of, for example, glass, quartz or a transparent resin film.

[0080] The substrate 10 can be either a rigid or flexible substrate; either is acceptable, but using a flexible substrate allows it to be flexible. Transparent resin films can be, for example, polyesters such as polyethylene terephthalate (PET) and polyethylene naphthalate (PEN), polyethylene, polypropylene, cellophane, cellulose diacetate, and cellulose triacetate (TAC).

[0081] exist Figure 2 In the display device 1, by making both substrates 5 and 6 flexible substrates, the display device 1 can be made flexible. Furthermore, substrates 5 and 6 can also be made of the same material as substrate 10. Substrate 5 can also serve as substrate 10.

[0082] In this embodiment, all layers from cathode 15 to anode 11—namely, cathode 15, electron transport layer 14, light-emitting layer 13, hole transport layer 12, and anode 11—can be formed entirely of inorganic layers. By forming all layers with inorganic layers, the same coating and drying equipment can be used for film formation, simplifying the manufacturing process. Furthermore, the HOMO energy level relationship from anode 11 to hole transport layer 12 and light-emitting layer 13 can be optimized, as can the LUMO energy level relationship from cathode 15 to electron transport layer 14 and light-emitting layer 13, improving carrier balance compared to using organic compounds.

[0083] In the first embodiment shown in Figure 4, the hole injection layer and electron injection layer are formed without being separated from the transport layer, which reduces the number of layers. That is, the transport layer can be configured to also serve as the injection layer. However, in this embodiment, the inorganic hole injection layer and electron injection layer can also be located between each electrode and each transport layer.

[0084] Figure 6A This is a cross-sectional view of the light-emitting element according to the second embodiment. Figure 6A In this structure, a cathode 15, an electron transport layer 14, a light-emitting layer 13, a hole transport layer 12, a hole injection layer (HIL) 16, and an anode 11 are sequentially stacked on a substrate 10. Figure 6A In, with Figure 4A Unlike the previous one, a hole injection layer 16 is included between the anode 11 and the hole transport layer 12.

[0085] Figure 6B This is a cross-sectional view of the light-emitting element according to the third embodiment. Figure 6B In this structure, a cathode 15, an electron injection layer (EIL) 18, an electron transport layer 14, a light-emitting layer 13, a hole transport layer 12, and an anode 11 are sequentially stacked on a substrate 10. Figure 6B In, with Figure 4A Unlike other electron injection layers, an electron injection layer 18 is included between the electron transport layer 14 and the cathode 15.

[0086] Figure 6C This is a cross-sectional view of the light-emitting element according to the fourth embodiment. Figure 6C In this structure, a cathode 15, an electron injection layer 18, an electron transport layer 14, a light-emitting layer 13, a hole transport layer 12, a hole injection layer 16, and an anode 11 are sequentially stacked on a substrate 10. Figure 6B In, with Figure 4A Unlike the previous method, a hole injection layer 16 is included between the anode 11 and the hole transport layer 12, and an electron injection layer 18 is included between the electron transport layer 14 and the cathode 15.

[0087] The materials of the hole injection layer 16 and the electron injection layer 18 are not limited; they can be inorganic or organic. However, it is preferred that the hole injection layer 16 and the electron injection layer 18 be formed from inorganic layers, so that all layers from the anode 11 to the cathode 15 can be formed from inorganic layers. The materials of the hole injection layer 16 and the electron injection layer 18 can be selected in various ways based on the energy level model.

[0088] In this embodiment, the layer between the cathode 15 and the light-emitting layer 13 is preferably an electron transport layer 14, or an electron injection layer 18, or a layer that serves as both an electron injection layer and an electron transport layer, or a layer formed by stacking the electron transport layer 14 and the electron injection layer 18.

[0089] In this embodiment, the layer between the anode 11 and the light-emitting layer 13 is preferably a hole transport layer 12, or a hole injection layer 16, or a layer that serves as both a hole injection layer and a hole transport layer, or a layer formed by stacking the hole transport layer 12 and the hole injection layer 16.

[0090] In addition, in order to Figure 2 In the case of the structure of the display device 1 shown, the thin-film transistor 4 is, for example, a bottom-gate type, and the drain 4c is connected to the cathode 15 of the light-emitting element 3. In this case, the cathode 15 is formed without overlapping with the drain 4c, so the drain 4c can also be used as the cathode 15. Thus, the connection between the light-emitting element 3, the thin-film transistor 4, and the ground wire can be properly made.

[0091] In this embodiment, the hole transport layer 12, the light-emitting layer 13, and the electron transport layer 14 can all be made into inorganic layers formed of nanoparticles. In this case, each layer can be formed easily and with uniform film thickness by printing methods such as inkjet printing. As a result, the luminous efficiency can be effectively improved.

[0092] When the quantum dots used in the light-emitting layer 13 of this embodiment have a core-shell structure, they become... Figure 7A The energy level diagram shown indicates that the energy levels of the shell could potentially act as a barrier for the recombination of holes and electrons. Therefore, as... Figure 7B As shown, it is preferable to use quantum dots that do not utilize a shell to cover the surface of the core (the surface of the core is exposed: the material constituting the quantum dot is uniform from the center of the quantum dot to the surface). By using such quantum dots, the energy barrier for the recombination of holes and electrons disappears, enabling efficient recombination of holes and electrons and improving luminescence efficiency. Furthermore, to improve electron transport efficiency and hole transport efficiency, such as... Figure 5A As shown, it is preferable to coordinate the organic ligand 21 to the surface of the quantum dot 20.

[0093] In addition, in this embodiment, such as Figure 8AAs shown, the light-emitting element 3 may also be configured to have: a substrate 10; an anode 11 formed on the substrate; a hole transport layer (HTL) 12 formed on the anode 11; an emitter layer (EML) 13 formed on the hole transport layer 12; an electron transport layer (ETL) 14 formed on the emitter layer 13; and a cathode 15 formed on the electron transport layer 14.

[0094] The materials of each layer are as described above. However, in Figure 8A In this embodiment, the anode 11 constitutes the first electrode, and the cathode 15 constitutes the second electrode. The light-emitting element 3 in this embodiment is a top-emitting type; therefore, the anode 11 is formed of a very thin transmissive material such as Ag, and the cathode 15 is preferably formed of ITO on a non-transmissive metal layer, for example. Thus, light is reflected at the cathode 15, and light can be extracted from the surface side (the side opposite to the thin-film transistor) that serves as the anode 11.

[0095] Figure 9A Is with Figure 8A Cross-sectional views of light-emitting elements in different implementations. Figure 9A In this structure, an anode 11, a hole injection layer (HIL) 16, a hole transport layer 12, a light-emitting layer 13, an electron transport layer 14, and a cathode 15 are sequentially stacked on a substrate 10. Figure 9A In, with Figure 8A Unlike the previous one, a hole injection layer 16 is included between the anode 11 and the hole transport layer 12.

[0096] Figure 9B Is with Figure 8A Cross-sectional views of light-emitting elements in different implementations. Figure 9B In this structure, an anode 11, a hole transport layer 12, a light-emitting layer 13, an electron transport layer 14, an electron injection layer (EIL) 18, and a cathode 15 are sequentially stacked on a substrate 10. Figure 9B In, with Figure 8A Unlike other electron injection layers, an electron injection layer 18 is included between the electron transport layer 14 and the cathode 15.

[0097] Figure 9C This is a cross-sectional view of the light-emitting element according to the fourth embodiment. Figure 9C In this structure, an anode 11, a hole injection layer 16, a hole transport layer 12, a light-emitting layer 13, an electron transport layer 14, an electron injection layer 18, and a cathode 15 are sequentially stacked on a substrate 10. Figure 9C In, with Figure 8AUnlike the previous method, a hole injection layer 16 is included between the anode 11 and the hole transport layer 12, and an electron injection layer 18 is included between the electron transport layer 14 and the cathode 15.

[0098] When the quantum dots used in the light-emitting layer 13 of the embodiments shown in Figures 8 and 9 have a core-shell structure, it becomes Figure 10A The energy level diagram shown indicates that the energy levels of the shell could potentially act as a barrier for the recombination of holes and electrons. Therefore, as... Figure 10B As shown, by using quantum dots whose nuclei are not covered by a shell, the energy barrier for the recombination of holes and electrons disappears, enabling efficient recombination and improving luminescence efficiency. Furthermore, to improve electron and hole transport efficiency, such as... Figure 5A As shown, it is preferable to coordinate the organic ligand 21 to the surface of the quantum dot 20.

[0099] The light-emitting elements of the embodiments shown in Figures 8 and 9 are conventional ELs, while the light-emitting elements of the embodiments shown in Figures 4 and 6 are constructed by reverse stacking conventional ELs. In the light-emitting elements of the embodiments shown in Figures 8 and 9, it is preferable to use a p-channel (p-ch) TFT as the thin-film transistor; therefore, it is preferable to form the channel layer with a p-type semiconductor.

[0100] In this embodiment, at least one of the following layers can be formed by inkjet printing: the layer between the cathode 15 and the light-emitting layer 13, the light-emitting layer 13, and the layer between the light-emitting layer 13 and the anode 11. For example... Figure 11 As shown, a mask 30 is disposed on a substrate 10, and an inorganic layer 31 is printed by inkjet printing within the space disposed in the mask 30, i.e., multiple coating areas 30a. At this time, in order to make the sidewalls 30b of the mask 30 hydrophobic, the surface of the sidewalls 30b is fluorinated. This suppresses the affinity between the ink and the surface of the sidewalls 30b, suppresses defects such as depressions on the surface of the printed inorganic layer 31, and improves the flatness of the surface of the inorganic layer 31.

[0101] This embodiment is a top-emitting type, and in the inverted EL-type light-emitting element 3 shown in Figures 4 and 6, carrier balance can be appropriately improved. Furthermore, the layer between the cathode 15 and the light-emitting layer 13 (electron transport layer 14, or electron transport layer 14 and electron injection layer 18) and the light-emitting layer 13 can be formed by coating. Alternatively, the layer between the light-emitting layer 13 and the anode 11 (hole transport layer 12, or hole transport layer 12 and hole injection layer 16) can be formed by vapor deposition or coating. This simplifies the manufacturing process of the light-emitting element.

[0102] Figure 1The display device 1 shown is one example; the arrangement of the red light-emitting area 2a, the green light-emitting area 2b, and the blue light-emitting area 2c can also be... Figure 1 Other than the arrangement. Alternatively, it can be a display device with only one of the following light-emitting areas: red light-emitting area 2a, green light-emitting area 2b, and blue light-emitting area 2c, or with light-emitting areas of two colors.

[0103] In a display device that uses quantum dots as described in this embodiment, quantum dots can be configured as point light sources or as surface light sources. Depending on the substrate selected, curved light sources or flexible products can also be realized.

[0104] Furthermore, according to this embodiment, it is possible to develop products with features that were previously difficult to achieve, such as lighting with color mixing properties equal to sunlight, lighting that is gentle on the eyes, and lighting that is optimized for plant factories.

[0105] Thus, display devices using quantum dots can achieve thinness and lightness, high degree of freedom in configuration such as curved surfaces, overall surface illumination, natural light emission that is not dazzling even when viewed directly, and minimal shadow formation. Furthermore, they consume less power and have a longer lifespan. For example, in organic EL display devices, the quantum dot display device using this embodiment excels in color rendering, luminescence, product lifespan, and product price.

[0106] As a display device using quantum dots according to this embodiment, it can be used as a PL (Plastic Photon) emitter in parallel with an EL (Elastic Electron) emitter. Furthermore, in a display device using quantum dots, a hybrid light-emitting element that combines an EL emitter and a PL emitter can be realized. For example, a PL emitter can be superimposed on the surface of an EL emitter, and the emission wavelength can be changed by utilizing the quantum dots contained in the PL emitter through the emission from quantum dots excited in the EL emitter. The EL emitter is a stacked structure of the aforementioned light-emitting element, and the PL emitter is, for example, a sheet-like wavelength conversion component in which multiple quantum dots are dispersed in resin. Such a hybrid configuration can be achieved using quantum dots.

[0107] Furthermore, in this embodiment, in order to balance the large-area application of quantum dot display devices and the reduction of manufacturing costs, inkjet printing, spin coating, and dispenser methods are preferred for coating.

[0108] Furthermore, while the above describes a top-emitting type embodiment, a bottom-emitting type is also possible.

[0109] Example The effects of the present invention will be described below through embodiments. Furthermore, the implementation of the present invention is not limited to any of the following embodiments.

[0110] Prepare the samples shown in Table 1 below and investigate the inkjet droplet properties. Furthermore, "Abs10" in the table refers to an absorbance of 10% in the state with dispersed quantum dots, and "Abs20" refers to an absorbance of 20% in the state with dispersed quantum dots.

[0111] In Table 1, the "dropping" column shows a sample with an "O" indicating proper dropping and a sample with an "X" indicating improper dropping.

[0112] In Table 1, the samples of "Red QD" and "Green QD" are applied to the luminescent layer. Additionally, the samples of "Polyvinylcarbazole" are applied to the hole injection layer. The samples of "Zinc oxide nanoparticles" are applied to the electron transport layer or electron injection layer.

[0113] As shown in Table 1, IPA and propylene glycol are not preferred solvents for zinc oxide nanoparticles and need to be changed. Solvents with "droplet addition" marked as 0 in Table 1 can be used appropriately, with hydrophilic solvents being preferred. For example, alcohol-based solvents can be used as hydrophilic solvents.

[0114] Figure 12 It is ZnO X The image shows the coating process using ethoxyethanol:EG = 7:3 as a solvent, achieved through inkjet printing. (Example image follows) Figure 12 As shown, a good coating condition was obtained.

[0115] In addition, the adverse effects on the EPDM (ethylene propylene diene monomer) inside the inkjet head were also investigated. As shown in Table 1, depending on the samples, some samples showed cap deformation or adverse effects on the EPDM. Therefore, it is advisable to consider the effects on the EPDM when using it.

[0116] (Experiment on the shell thickness dependence of quantum dots) In the experiment, quantum dots (green QDs) for each sample shown in Table 1 were fabricated, possessing... Figure 4A In a display device with light-emitting elements, the relationship between shell thickness and external quantum efficiency (EQE) was investigated.

[0117] As shown in Table 2, a correlation was observed between shell thickness and EQE. Although not limited, the shell thickness is 0.1 nm or more and 4.0 nm or less, preferably 0.5 nm or more and 3.5 nm or less, more preferably 1.0 nm or more and 3.0 nm or less, and even more preferably 1.3 nm or more and 2.5 nm or less.

[0118] Furthermore, if the relationship between quantum dot layer thickness and EQE is investigated, there is a tendency for EQE to increase with a certain degree of quantum dot layer thickness. While the quantum dot layer thickness is not limited, it is preferably 5 nm or more and 50 nm or less, more preferably 10 nm or more and 45 nm or less, more preferably 15 nm or more and 40 nm or less, even more preferably 20 nm or more and 40 nm or less, and even more preferably 25 nm or more and 40 nm or less.

[0119] In addition, PYS measurements were performed on Cd-based green quantum dots. Figure 13 The circular markers shown are experimental data for the core only in Example 1, and the square markers in Example 2 are experimental data for the case where the core is covered with a shell.

[0120] The ionization potential can be measured using photoelectron yield spectroscopy (PYS). For example, it can be measured using devices such as the AC-2 and AC-3 from Riken Keiki Co., Ltd.

[0121] like Figure 13 As shown, the energy of the rise is different in Example 1 and Example 2. It is approximately 6.1 eV in Example 1 and approximately 7.1 eV in Example 2.

[0122] Figure 14 The results are PYS measurements of Cd-based green quantum dots from Examples 3 and 4, which have different shell thicknesses. Example 4 is thicker than Example 3. It is evident that the energy rise differs between Examples 3 and 4. It is approximately 7.1 eV in Example 3 and approximately 8.1 eV in Example 4.

[0123] (Experiment on the current dependence of EQE) Figure 15 This is an energy level diagram of each layer in the light-emitting element used in the experiment. Figure 16 This is a graph showing the relationship between the current value and EQE of EL and PL light emitters using red quantum dots. Additionally, Figure 17 This is a graph showing the relationship between the current value and EQE of an EL (Elastic Optical Cell) and a PL (Plastic Optical Cell) using red quantum dots, and further, a graph showing the relationship between the current value and EQE of an EL (Elastic Optical Cell) using blue quantum dots. Figure 16 In Examples 5 and 6 shown, the shell thickness is different. The shell thickness of Example 5 is thicker than that of Example 6. Additionally, in... Figure 17 Among them, the shell thickness of Example 7 is the thickest, while the shell thickness of Examples 8 and 9 decreases sequentially.

[0124] like Figure 16 , Figure 17As shown, in EL and PL light emitters, an increase in EQE is observed up to approximately 20mA. On the other hand, in the red device, an increase in EQE is observed even when the current value exceeds 20mA. Furthermore, as... Figure 16 , Figure 17 As shown, it was observed that the thicker the shell, the higher the EQE.

[0125] (About ZnO) X (Experimental study on the synthesis of) Figure 18 This represents the energy band gap Eg and the energy E at the lower end of the conduction band of each layer in the light-emitting element used in the experiment. CB Energy E at the upper end of the valence band VB The curves and energy level diagrams of each layer. Figure 18 The L1 or L2 shown uses ZnO X (Li). Here, Li can be doped or undoped. While not limited, X is approximately 0.8~1.2. For example... Figure 18 As shown, if ZnO is used in the electron injection layer and electron transport layer (ETL)... X Using ZnO X (Li) can widen the band gap. It is speculated that this can be achieved through ZnO. X (Li) has the effect of reducing particle size. Figure 18 The PVK layer shown is the hole injection layer, and B1, B2, G(H), G(I3), and R(F) are the light-emitting layers (EL layers). ZnO X L1, L2, and L4 are electron injection layers. It can be seen that when B1 or B2 is used in the light-emitting layer, ZnO can be used in the electron injection layer. X However, when G(H), G(I3), and R(F) are used in the light-emitting layer, L2 or L4 is preferred in the electron injection layer. L2 and L4 are ZnO. X (Li).

[0126] Especially when using a light-emitting layer (EL layer) with a shallow conduction band, ZnO X (Li) is effective when applied to electron injection layers and electron transport layers.

[0127] Although there are no restrictions, ZnO X (Li) can be generated by mixing a zinc acetate-ethanol solution with a LiOH·4H2O-ethanol solution after stirring at a specified temperature and time, followed by centrifugation and washing.

[0128] Figures 19 to 21 ZnO is used in the electron transport layer (ETL). X (Li) and ZnO X(K) UV (bandgap), PL, and PYS data. ZnO X (K) is generated catalytically using KOH, without K or Li doping. It is known that in ZnO... X (Li) and ZnO X In (K), both UV and PL data show deviations. On the other hand, it can be seen that in PYS, ZnO... X (Li) and ZnO X (K) shows almost no deviation, and the energy increase remains almost constant.

[0129] In this way, it is possible to propose ZnO as an electron injection / electron transport layer for quantum dot-based EL devices, with band gaps controlled by various particle sizes. X Or, doped ZnO with defect and bandgap control achieved by adding different types of dopants. X .

[0130] Nevertheless, in cases where a balance between the recombination of electrons and holes that generate light cannot be achieved, it is preferable to have a thin insulating layer between the EL layer and the electron injection layer, or to use ZnO, in order to achieve this balance adjustment. X It integrates with the molecule, thereby adding the function of hole blocking. Here, the integrated layer refers, for example, to ZnO. x Integration with T2T (2,4,6-tris(biphenyl-3-yl)-1,3,5-triazine, i.e., 2,4,6-tris(biphenyl-3-yl)-1,3,5-triazine). Although not specified, X is approximately 0.8~1.2.

[0131] Furthermore, it is known that ZnO x It not only functions as an electron injection / electron transport layer, but through ozone treatment, it can also function as a hole injection / hole transport layer. That is, it was discovered that by treating ZnO... x Ozone treatment can improve the transmission capacity of holes.

[0132] In this embodiment, the electron transport layer is formed from ZnO, MgZnO, or SnO. The synthesis process for these metal oxides will be described below.

[0133] Figure 22 This is the synthesis process of ZnO. For example... Figure 22 As shown, Zn(OAc)2·2H2O (6.22 g, 28.3 mmol) and 0.5 wt% hydrated methanol (198.8 g, 251 ml) were mixed in a flask, and the temperature was raised to 60 °C.

[0134] Next, KOH / 0.5wt% hydrated methanol was added to the flask at a rate of 13 ml per minute, and this addition was continued for 10 minutes. Then, the temperature was maintained at 60°C for 2 hours.

[0135] Next, after cooling, the mixture was transferred to two centrifuge tubes and centrifuged at 7000 pm for 5 minutes. Then, the supernatant was drained from each tube, and the mixture was sonicated before being centrifuged again at 7000 pm for 5 minutes. This process of draining the supernatant, sonicating, and centrifuging was repeated once more.

[0136] Next, drain the supernatant and proceed with the sonication process. During this time, between draining the supernatant and the sonication process, add 5 ml of ethanol and 0.5 ml of aminoethanol to each centrifuge tube. Then, store in the dark overnight.

[0137] Next, transfer the contents to two other centrifuge tubes, add approximately 35-40 ml of ethyl acetate to each tube, and centrifuge at 5500 pm for 5 minutes. Then, drain the supernatant from each centrifuge tube, transfer it to a G-BOX, and disperse it using a vortex mixer.

[0138] Next, the sample was transferred through a 0.54 μm filter to a 50 ml tube, from which 0.7 ml was aliquoted, and the remainder was stored in a G-BOX. The aliquoted 0.7 ml was used for concentration determination, and the concentration and dilution ratio were calculated. Based on this calculation, the G-BOX was adjusted with dehydrated ethanol to achieve the calculated dilution ratio. The completed G-BOX was then stored. 1.6 ml was aliquoted from this completed G-BOX for DLS, UV, PL, and concentration analysis.

[0139] Figure 23 This is the synthesis process of MgZnO. For example... Figure 23 As shown, first, anhydrous ethanol (30 mL) was prepared, and Zn(OAc)₂·2H₂O (560 mg, 2.55 mmol) and Mg(OAc)₂·4H₂O (96.5 mg, 0.45 mmol) were added to it. The mixture was stirred at room temperature for 30 minutes. Next, KOH (309 mg, 5.5 mmol) / anhydrous ethanol (20 mL) was added. The mixture was then stirred at room temperature for 1 hour. Next, hexane (80 mL) was added. The mixture became cloudy and white. Ethanolamine (1 mL) was added to the mixture until dissolved, and hexane was further added to precipitate the solid, which was then dispersed with ethanol. This yielded MgZnO with a particle size of approximately 4.2 nm. The band gap energy E of MgZnO is shown. g It is 3.79 eV.

[0140] Figure 24 This is the synthesis process for SnOx. For example... Figure 24As shown, prepare ethylene glycol (100 mL), and mix it with SnCl4·5H2O (11.57 g, 0.33 mol), acetic acid (10 mL), and a 30% tetramethylammonium hydroxide aqueous solution (25 mL). Stir at 50°C for 30 minutes. Then, raise the temperature to 160°C and stir for 4 hours.

[0141] Next, wash with ethanol-ethyl acetate, and add 2 ml of ethanolamine to disperse it in ethanol.

[0142] In this embodiment, the hole transport layer 45 is formed of NiO or MoO. The synthesis process of these metal oxides will be described below.

[0143] Figure 25 This is the synthesis process for NiOx. For example... Figure 25 As shown, 100 mL of DMSO was prepared, and Ni(NO3)2·6H2O (10 mmol) and KOH (673 mg, 12 mmol) / 100 mL of anhydrous ethanol were mixed into it. The mixture was stirred at room temperature for 30 minutes. Next, after washing with ethanol, 150 mL of DMSO was mixed in and stirred at 160 °C for 4 hours. Then, the mixture was washed with ethanol-ethyl acetate, and 0.2 mL of ethanolamine was added to disperse it in ethanol. A dark gray precipitate was obtained.

[0144] Figure 26 This is the synthesis procedure for MoOx. In a sealed tube, 1-octadecene and octanoic acid (total volume 60 mL, 1-octadecene:octanoic acid volume ratio = 1:9), MoO2(acac)2 (244.62 mg, 0.75 mmol), and acetic acid (10 mL) are mixed and stirred at 50°C for 30 minutes. Next, the temperature is raised to 180°C and stirred for 40 minutes. Then, n-octane (250 mL) is added, yielding a deep blue precipitate. This precipitate is then dispersed in ethanol.

[0145] According to the synthesis process of this embodiment, metal oxides that do not produce oxidation defects can be obtained. Moreover, by using the electron transport layer and hole transport layer of this embodiment, the absorption edge in the infrared region can be effectively increased dramatically.

[0146] Figure 27A This is a graph representing the UV spectrum. Figure 27B This is a graph representing the band gap energy. In the experiment, the Mg doping amount of MgZnO was varied. Furthermore, the MgZnO used in the experiment was freshly synthesized MgZnO dispersed in DEGME. As shown in Figure 27 and Table 3, the band gap energy E increases with increasing Mg doping amount. g Increase.

[0147] Figure 28 This is the synthesis process for NiO. Additionally, it shows the synthesis process for NiO. Figure 25 Different synthesis processes. That is, such as Figure 28 As shown, Ni(NO3)2·6H2O, dimethyl sulfoxide, and KOH / anhydrous ethanol solution were mixed and stirred at room temperature for 30 minutes. This yielded a green precipitate. This precipitate was mixed with DMSO and kept at 160°C for 3 hours, followed by washing with ethanol / ethyl acetate. Then, ethanolamine was added to disperse it in ethanol.

[0148] Figure 29 This is the XRD pattern of NiO. The NiO used in the experiment was obtained through... Figure 28 It is obtained through a synthetic process. For example... Figure 29 NiO was observed. On the other hand, Ni was not observed, indicating that appropriate synthesis was carried out.

[0149] Figure 30 Indicates the UV spectrum of NiO, Figure 31 It is a curve representing the band gap energy (Tauc plot).

[0150] like Figure 31 As shown, the slope of the absorbing edge in the Tauc plot is greater than that of the bulk material. This indicates that the absorbing edge can be dramatically increased. Figure 31 As shown, when using nanoparticles, the Tauc plot exhibits a smaller tail and a more rapid ascent compared to the bulk material. In this embodiment, all electron transport materials are semiconductors; therefore, among all electron transport materials, the slope of the Tauc plot is greater than that of the bulk material.

[0151] In addition, such as Figure 31 As shown, within regions A and B enclosed by the tangent representing the slope, the plotted curve, and the horizontal axis, region A of the nanoparticles is smaller than region B of the bulk material. The smaller this region, the fewer oxygen vacancies, thus proving that the oxygen vacancies in the nanoparticles are smaller than those in the bulk material.

[0152] <Regarding the Ni valence of NiO> Using standard samples with known valences (NiO: divalent, LiNiO2: trivalent), a standard curve was constructed to plot the rising position of the XANES spectrum and the valence of Ni. In the experiment, BL08W was used as the beamline, and transmission method was employed for the determination.

[0153] Next, the Ni valence was evaluated using the XANES spectrum of NiO synthesized using the synthesis method of this embodiment. The experimental results are shown below. Figure 32 .

[0154] like Figure 32As shown, the XANES spectrum shifts due to the change in valence. Furthermore, it is known that since the XANES spectrum of the embodiment exists between the divalent NiO and trivalent LiNiO2, the Ni valence of the NiO in the embodiment is between 2 and 3. Because it is closer to the divalent NiO side, the Ni valence of the embodiment is greater than 2 and less than 2.5, preferably 2.1 or higher and 2.4 or lower. Additionally, in Figure 32 In the embodiment, the Ni valence is predicted to be around 2.2. This proves that NiO in this embodiment is a hole carrier.

[0155] Furthermore, the polydispersity index of the metal oxide dispersion used in the electron transport layer of this embodiment is less than 0.1 during DLS measurement. Dynamic light scattering (DLS) is suitable for nanoparticles dispersed in a liquid. DLS measurement is a method for calculating particle size by measuring the diffusion rate of nanoparticles moving due to Brownian motion.

[0156] To represent the width of the particle size distribution, the polydispersity index (PDI) is used. In the experiment, ZnO particles were fabricated as a metal oxide used in the electron transport layer. Figure 33 Table 4 shows the relationship between particle size and scattering intensity distribution in each embodiment. As shown in Table 4, in Examples 10 to 12, the polydispersity index was less than 0.1. Therefore, it can be seen that in each embodiment, all measured particles had approximately the same particle size.

[0157] Metal oxides such as ZnO or NiO exist in the form of films in display devices. In this embodiment, the polydispersity index is made less than 0.1 as described above to improve dispersibility, but the performance of the film can be defined by the surface roughness after film formation.

[0158] That is, the surface roughness Ra (arithmetic mean roughness) of the film is preferably less than 1.0 nm. The surface roughness (Ra) can be analyzed by AFM measurement or the like.

[0159] Industrial availability According to the present invention, a light-emitting element containing quantum dots can be applied to a display device, and excellent light-emitting characteristics can be obtained.

[0160] This application is based on Japan Patent Application No. 2023-170557, filed on September 29, 2023. Its entire contents are contained herein.

Claims

1. A display device, characterized in that, It is a display device with a display area, wherein, The display area has a light-emitting element, which has a first electrode, a light-emitting layer, a second electrode, an electron transport layer or a hole transport layer formed between the first electrode and the light-emitting layer, and a hole transport layer or an electron transport layer formed between the second electrode and the light-emitting layer, and is formed by stacking these components. The electron transport layer comprises at least one of ZnO, MgZnO, or SnO. The hole transport layer or hole injection layer comprises at least one of NiO or MoO.

2. The display device according to claim 1, characterized in that, The oxygen defects in the metal oxides constituting the electron transport layer and the hole transport layer are smaller than those in the bulk material.

3. The display device according to claim 1, characterized in that, The luminescent layer contains quantum dots, and the quantum dots contain organic ligands on their surface, the organic ligands containing 3-mercaptopropionic acid.

4. The display device according to claim 1, characterized in that, The light-emitting layer contains quantum dots, and the quantum dots contain ligands on their surface, wherein the ligands have 2 or more but less than 5 carbon atoms.

5. A nanoparticle material, characterized in that, It is a material used in the electronic transmission layer constituting the display device of claim 1, and the slope of its absorption edge when plotted using Tauc is greater than that of the body material.

6. The dispersion of the material according to claim 5, characterized in that, The polydispersity index during DLS measurement was less than 0.

1.

7. The method for manufacturing the display device according to claim 1, characterized in that, In the process of forming the light-emitting layer, ligands are exchanged after coating the composition containing quantum dots.

8. The method for manufacturing a display device according to claim 7, characterized in that, When synthesizing the quantum dots using a liquid-phase synthesis method, the quantum dots with ligand lengths are synthesized and then replaced with shorter ligands before or after coating the composition containing the quantum dots.

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