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

Abstract

The purpose of the present invention is to provide: a display device in which a suitable combination of materials for an electron transport layer and a hole transport layer can be selected; and a method for manufacturing same. The present invention relates to a display device having a display region, characterized in that the display region has a light-emitting element in which 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 are stacked, wherein the electron transport layer contains at least one of ZnO, MgZnO, or SnO, and the hole transport layer contains NiO or MoO.

Description

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

[0001] The present invention relates to a display device using quantum dots.

[0002] The following patent documents disclose inventions relating to organic electroluminescence (EL).

[0003] An organic EL element is constructed by laminating 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 an organic EL element is made of an organic compound, and emits light by excitons generated by recombination of electrons and holes injected into the organic compound.

[0004] JP 2017-45650 A

[0005] However, the invention described in Patent Document 1 does not disclose any preferred combination of materials for the electron transport layer and the hole transport layer in relation to the absorption edge. The present invention has been made in view of this point, and an object of the present invention is to provide a display device that allows selection of a preferred combination of materials for the electron transport layer and the hole transport layer.

[0006] The present invention provides a display device having a display area, wherein the display area includes light-emitting elements stacked one on top of the other, each of which includes 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, wherein the electron transport layer contains at least one of ZnO, MgZnO, and SnO, and the hole transport layer or the hole injection layer contains at least one of NiO and MoO.

[0007] The present invention is characterized in that the material used in the electron transport layer constituting the display device described above has a Tauch plot of the absorption edge with a slope greater than that of the bulk material. The present invention is characterized in that the dispersion of the material described above has a polydispersity index of less than 0.1 when measured by DLS.

[0008] According to the display device of the present invention, the absorption edge can be sharply extended by the combination of materials constituting the electron transport layer and the hole transport layer.

[0009] 1 is a partial plan view of a display device of the present embodiment; FIG. 2 is a partially enlarged cross-sectional view showing an enlarged view of one display region of the display device shown in FIG. 1; and FIG. 3 is a cross-sectional view showing the structure of a thin film transistor different from that of FIG. 2. FIG. 4A is a cross-sectional view of a light-emitting element of the first embodiment, and FIG. 4B is an energy level diagram of each layer in the display device of the first embodiment. It is a schematic diagram of quantum dots of the present embodiment. It is a cross-sectional view of a light-emitting element of an embodiment different from that of FIG. 1. FIG. 7A is an energy level diagram when quantum dots with a core-shell structure are used, and FIG. 7B is an energy level diagram when quantum dots with a structure in which the core is not covered with a shell are used. FIG. 8A is a cross-sectional view of a light-emitting element different from that of FIG. 4, and FIG. 8B is an energy level diagram of each layer in the light-emitting element of FIG. 8A. It is a cross-sectional view of a light-emitting element of an embodiment different from that of FIG. 8. FIG. 10A is an energy level diagram when quantum dots with a core-shell structure are used, and FIG. 10B is an energy level diagram when quantum dots with a structure in which the core is not covered with a shell are used. It is a schematic diagram showing a process of forming an inorganic layer by an inkjet method. It is a photograph of a coating of an example. 1 shows PYS measurement data for Cd-based green quantum dots. 2 shows PYS measurement data. 3 shows an energy level diagram for each layer in the light-emitting device used in the experiment. 4 shows a graph showing the relationship between the current value and EQE for an EL light-emitting body and a PL light-emitting body using green quantum dots. 5 shows a graph showing the relationship between the current value and EQE for an EL light-emitting body and a PL light-emitting body using red quantum dots, and a graph showing the relationship between the current value and EQE for an EL light-emitting body using blue quantum dots. 6 shows the energy band gap Eg and the energy E at the bottom of the conduction band for each layer in the light-emitting device used in the experiment. ＣＢ , the energy at the top of the valence band E ＶＢ ZnO used in the electron transport layer (ETL) Ｘ (Li) and ZnO Ｘ (K) UV data. ZnO used in the electron transport layer (ETL) Ｘ (Li) and ZnO Ｘ(K) PL data of ZnO used in the electron transport layer (ETL) Ｘ (Li) and ZnO Ｘ 1 is a PYS data of (K). ZnO synthesis flow. MgZnO synthesis flow. SnOx synthesis flow. NiOx synthesis flow. MnOx synthesis flow. Graph showing UV spectrum. Graph showing band gap energy. NiO synthesis flow. XRD of NiO. UV spectrum of NiO. Graph (Tauk plot) showing band gap energy of NiO. XANES spectrum in Ni valence evaluation. Graph showing the relationship between particle size and scattering intensity distribution in Examples 10 to 12.

[0010] An embodiment of the present invention (hereinafter abbreviated as "embodiment") will be described in detail below. Note that the present invention is not limited to the following embodiment, and various modifications can be made within the scope of the gist thereof.

[0011] 1, a display device 1 has a plurality of display regions 2 arranged in a matrix. The display regions 2 include three types of regions: a red light-emitting region 2a that emits red light, a green light-emitting region 2b that emits green light, and a blue light-emitting region 2c that emits blue light. These three light-emitting regions 2a, 2n, and 2c are arranged, for example, in the row direction to form a set, and each set constitutes one pixel (picture element) in a color display.

[0012] A light-emitting element 3 is formed in each of the light-emitting regions 2a, 2b, and 2c. The layer structure of the light-emitting element 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-emission type.

[0013] The thin film transistor 4 shown in FIG. 2 is an n-channel TFT, and is configured by stacking a gate electrode 4a, a channel layer 4b, a gate insulating film (not shown), a drain electrode 4c, a source electrode 4d, and the like on a substrate 5. While the material of the channel layer 4b is not critical, an N-type semiconductor, such as an oxide semiconductor, is preferably used. An In—Ga—Zn—O-based semiconductor is preferably used as the oxide semiconductor. In—Ga—Zn—O-based semiconductors have high mobility and low leakage current, making them suitable for use as thin film transistors. Poly-Si is also preferably used. The thin film transistor 4 shown in FIG. 2 is a top-contact / bottom-gate type, but may also be a bottom-contact / bottom-gate type. The source electrode 4d is connected to a power supply line, and the drain electrode 4c is connected to the light-emitting element 3.

[0014] The thin film transistor 4 may also be a top-gate type as shown in FIG. 3. As shown in FIG. 3, a channel layer 4b is formed on a substrate 5, and the surface of the channel layer 4b is covered with a gate insulating film 4e. A gate electrode 4a is formed on the surface of the gate insulating film 4e. As shown in FIG. 3, the surface of the gate electrode 4a is covered with an insulating film 4f. A plurality of through-holes are formed through the gate insulating film 4e and the insulating film 4f to reach the channel layer 4b, and a drain electrode 4c and a source electrode 4d are formed through each through-hole. Furthermore, the surfaces of the drain electrode 4c and the source electrode 4d are covered with a protective film 7. Transparent electrodes connected to the drain electrode 4c and the source electrode 4d are formed on the surface of the protective film 7. A transparent electrode 8 shown in FIG. 3 is connected to the drain electrode 4c.

[0015] 3 is an N-type semiconductor, and an oxide semiconductor is preferably used as the channel layer 4b of the thin film transistor 4. As the oxide semiconductor, an In-Ga-Zn-O based semiconductor is preferably used.

[0016] As shown in FIG. 2, the display device 1 has a structure in which a thin-film transistor 4 and a light-emitting element 3 are interposed between a pair of substrates 5 and 6, and a sealing resin (not shown) is provided in a frame shape between each of the substrates 5 and 6, and the substrates 5 and 6 are connected to each other via the sealing resin.

[0017] The following describes the structure of the light-emitting element 3. Fig. 4A is a cross-sectional view of the light-emitting element in the first embodiment, and Fig. 4B is an energy level diagram of each layer in the display device of the first embodiment.

[0018] As shown in FIG. 4A , the light-emitting element 3 includes a substrate 10, a cathode 15 formed on the substrate, an electron transport layer (ETL) 14 formed on the cathode 15, an emitter layer (EML) 13 formed on the electron transport layer 14, a hole transport layer (HTL) 12 formed on the emitter layer 13, and an anode 11 formed on the hole transport layer 12.

[0019] When a voltage is applied between the electrodes of the light-emitting element 3 of this embodiment, holes are injected from the anode 11 and electrons are injected from the cathode 15. FIG. 4B shows energy level models of the hole transport layer 12, the light-emitting layer 13, and the electron transport layer 14. As shown in FIG. 4B , holes transported through the hole transport layer 12 are injected from the HOMO level of the hole transport layer 12 to the HOMO level of the light-emitting layer 13. Meanwhile, electrons transported from the electron transport layer 14 are injected from the LUMO level of the electron transport layer 14 to the LUMO level of the light-emitting layer 13. The holes and electrons then recombine in the light-emitting layer 13, exciting the quantum dots in the light-emitting layer 13, allowing light to be emitted from the excited quantum dots. In this embodiment, the light-emitting layer 13 is formed of an inorganic layer containing quantum dots.

[0020] (Quantum Dots) The structure and material of the quantum dots are not limited, but for example, the quantum dots in this embodiment are nanoparticles having a particle size of about several nanometers to several tens of nanometers.

[0021] For example, quantum dots include CdS, CdSe, ZnS, ZnSe, ZnSeS, ZnTe, ZnTeS, InP, (Zn)AgInS ２ , (Zn)CuInS ２The use of Cd is restricted in various countries due to its toxicity, so it is preferable that the quantum dots do not contain substances restricted by RoHS, such as Cd and Pb.

[0022] 5A, it is preferable that a large number of organic ligands 21 are coordinated to the surface of the quantum dots 20. This makes it possible to suppress aggregation of the quantum dots 20, thereby enabling the desired optical properties to be exhibited. There are no particular limitations on the ligands that can be used in the reaction, but the following ligands are representative examples:

[0023] Aliphatic primary amine, oleylamine: C １８ H ３５ NH ２ , stearyl(octadecyl)amine: C １８ H ３７ NH ２ , dodecyl(lauryl)amine: C １２ H ２５ NH ２ , decylamine: C １０ H ２１ NH ２ , octylamine: C ８ H １７ NH ２

[0024] 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: C ９ H １９ COOH, octanoic acid: C ７ H １５ COOH

[0025] Thiol-based, octadecanethiol: C １８ H ３７ SH, hexanedecanethiol: C １６ H ３３ SH, tetradecanethiol: C １４ H２９ SH, dodecanethiol: C １２ H ２５ SH, decanethiol: C １０ H ２１ SH, octanethiol: C ８ H １７ SH phosphine system, trioctylphosphine: (C ８ H １７ ) ３ P, triphenylphosphine: (C ６ H ５ ) ３ P, tributylphosphine: (C ４ H ９ ) ３ P

[0026] Phosphine oxide series, trioctylphosphine oxide: (C ８ H １７ ) ３ P═O, triphenylphosphine oxide: (C ６ H ５ ) ３ P═O, tributylphosphine oxide: (C ４ H ９ ) ３ P=O

[0027] In this embodiment, it is preferable to use a short ligand as the organic ligand 21. Although not limited thereto, the organic ligand 21 can be 3-mercaptopropionic acid (MPA).

[0028] The ligands of the quantum dots 20 contained in the quantum dot layer are preferably shorter than the ligands when the quantum dots 20 are formed by liquid phase synthesis.

[0029] In this way, by using short ligands for the quantum dots 20 contained in the quantum dot layer, the roughness of the quantum dot layer can be reduced and the electron and hole extraction efficiency can be improved. On the other hand, by using long ligands when forming the quantum dots 20 by liquid phase synthesis, the dispersion and film formation properties can be improved.

[0030] Alternatively, quantum dots 20 with long ligands may be synthesized by a liquid phase synthesis method, and then the composition containing the quantum dots 20 may be replaced with a short ligand (e.g., 3-mercaptopropionic acid) before or after application.

[0031] The quantum dot 20 shown in FIG. 5B has a core-shell structure having a core 20a and a shell 20b covering the surface of the core 20a. As shown in FIG. 5B, it is preferable that a large number of organic ligands 21 are coordinated to the surface of the quantum dot 20. The core 20a of the quantum dot 20 shown in FIG. 5B is the nanoparticle shown in FIG. 5A. Therefore, the core 20a is formed of, for example, one of the materials listed above. The material of the shell 20b is not particularly limited, but it may be formed of, for example, zinc sulfide (ZnS). Like the core 20a, it is preferable that the shell 20b does not contain RoHS-regulated substances such as cadmium (Cd) and Pb.

[0032] The shell 20b may be in a state of being solid-solutioned on the surface of the core 20a. In Fig. 5B, the boundary between the core 20a and the shell 20b is shown by a dotted line, but this means that the boundary between the core 20a and the shell 20b may or may not be confirmed by analysis. 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 in which at least some or all of the elements constituting the core 20a and at least some or all of the elements constituting the shell 20b are mixed.

[0033] (Light-emitting layer 13) The light-emitting layer 13 may be formed solely from the quantum dots described above, or may contain quantum dots and another fluorescent substance. The light-emitting layer 13 may also be formed by applying quantum dots dissolved in a solvent, for example, by an inkjet method, and some solvent components may remain in the light-emitting layer 13.

[0034] The light-emitting layer 13 of the light-emitting element 3 formed in the red light-emitting region 2a shown in Fig. 1 contains red quantum dots that fluoresce red. The light-emitting layer 13 of the light-emitting element 3 formed in the green light-emitting region 2b shown in Fig. 1 contains green quantum dots that fluoresce green. The light-emitting layer 13 of the light-emitting element 3 formed in the blue light-emitting region 2c shown in Fig. 1 contains blue quantum dots that fluoresce blue.

[0035] The wavelength of the blue light emission is preferably about 450 nm. In this way, by adjusting the wavelength so as not to emit light with wavelengths shorter than 450 nm, health risks can be reduced. The light-emitting layer 13 can be formed using an existing thin film formation method such as the inkjet method or vacuum deposition method described above.

[0036] (Hole transport layer 12) The hole transport layer 12 is made of an inorganic or organic material that has the function of transporting holes. The hole transport layer 12 is preferably made of an inorganic material, and preferably contains at least one of NiO and MoO. In particular, the hole transport layer 12 is preferably formed of NiO nanoparticles. The hole transport layer 12 may also be made of, for example, NiO with Al. ２ O ３ It is also possible to mix metal oxides with other metals. Metal oxides may be doped with Li, Mg, Al, or the like. The hole transport layer 12 may also be made of an inorganic material other than an inorganic oxide. In this embodiment, the oxygen deficiency of the metal oxide used in the hole transport layer 12 is preferably smaller than that of the bulk material. The term "bulk material" refers to a mass containing the metal oxide, and the size and shape are not important.

[0037] The hole transport layer 12, like the light emitting layer 13, can be formed by a printing method such as an ink jet method, or by an existing thin film technique such as a vacuum deposition method.

[0038] (Electron Transport Layer 14) The electron transport layer 14 is made of an inorganic or organic material having the function of transporting electrons. The electron transport layer 14 is preferably made of an inorganic material, and preferably contains at least one of ZnO, MgZnO, and SnO, for example. Two or more of these may also be selected. The electron transport layer 14 is particularly preferably made of ZnO. Ｘ The electron transport layer 14 is preferably formed of nanoparticles of metal oxides. The metal oxides may be doped with Li, Mg, Al, Mn, etc. The electron transport layer 14 may also be formed of inorganic substances other than inorganic oxides (e.g., CsPbBr ３ X is not limited to, but is about 0.8 to 1.2. However, in this embodiment, it is preferable that the oxygen deficiency of the metal oxide used in the electron transport layer 14 is smaller than that of the bulk material.

[0039] The electron transport layer 14 can be formed by applying a solvent containing nanoparticles by a printing method such as an ink jet method, or by using an existing thin film technology such as a vacuum deposition method, similar to the light emitting layer 13 .

[0040] (Anode 11) In this embodiment, the material of the anode 11 is not limited. For example, the anode 11 may be made of a metal such as Au or Ag, or CuISnO ２ , ZnO Ｘ Preferably, the anode 11 is formed of a conductive transparent material such as ZnO, or indium-tin composite oxide (ITO). Of these, the anode 11 is preferably formed of ITO. The anode 11 can be formed as a thin film on the substrate 10 by vapor deposition, sputtering, or the like of these electrode materials.

[0041] In this embodiment, since light is extracted from the anode 11 side, the anode 11 needs to be transparent, and is preferably a thin metal film having excellent transparency such as Ag or a metal oxide having excellent transparency as described above.

[0042] (Cathode 15) In this embodiment, the material of the cathode 15 is not limited, but for example, indium-tin composite oxide (ITO), metals, alloys, electrically conductive compounds, and mixtures thereof can be used as the electrode material for the cathode 15. For example, the cathode 15 is formed of ITO. The cathode 15 is formed, for example, via a non-transparent metal layer formed on the substrate 10. This allows the light-emitting element 3 to be a top emitter. The cathode 15 can be formed as a thin film of these electrode materials by a method such as vapor deposition or sputtering.

[0043] (Substrate 10) In this embodiment, the material of the substrate 10 is not limited, but the substrate 10 can be formed of, for example, glass, plastic, etc. Specifically, the substrate 10 is formed of, for example, glass, quartz, or a transparent resin film.

[0044] The substrate 10 may be either a rigid substrate or a flexible substrate, but flexibility can be achieved by using a flexible substrate. The transparent resin film may be, for example, polyester such as polyethylene terephthalate (PET) or polyethylene naphthalate (PEN), polyethylene, polypropylene, cellophane, cellulose diacetate, or cellulose triacetate (TAC).

[0045] 2, by using flexible substrates for both the substrates 5 and 6, the display device 1 can be made flexible. The substrates 5 and 6 can also be made of the same material as the substrate 10. The substrate 5 can also serve as the substrate 10.

[0046] In this embodiment, all layers from the cathode 15 to the anode 11, i.e., the cathode 15, the electron transport layer 14, the light-emitting layer 13, the hole transport layer 12, and the anode 11, can be formed from inorganic layers. By forming all layers from inorganic layers in this way, films can be formed using the same coating / drying equipment, etc., and the manufacturing process can be simplified. Furthermore, the magnitude relationship of the HOMO levels from the anode 11 to the hole transport layer 12 and the light-emitting layer 13 can be optimized, and the magnitude relationship of the LUMO levels from the cathode 15 to the electron transport layer 14 and the light-emitting layer 13 can be optimized, resulting in an improved carrier balance compared to when an organic compound is used.

[0047] In the first embodiment shown in Fig. 4, the hole injection layer and the electron injection layer are not formed separately from the transport layer, thereby reducing the number of layers. That is, the transport layer can also function as the injection layer. However, in this embodiment, an inorganic hole injection layer or an inorganic electron injection layer can also be interposed between each electrode and each transport layer.

[0048] Fig. 6A is a cross-sectional view of a light-emitting device according to a second embodiment. In Fig. 6A, a cathode 15, an electron transport layer 14, an emitting layer 13, a hole transport layer 12, a hole injection layer (HIL) 16, and an anode 11 are stacked in this order on a substrate 10. Unlike Fig. 4A, Fig. 6A includes a hole injection layer 16 between the anode 11 and the hole transport layer 12.

[0049] Fig. 6B is a cross-sectional view of a light-emitting element according to a third embodiment. In Fig. 6B, 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 stacked in this order on a substrate 10. Unlike Fig. 4A, Fig. 6B includes an electron injection layer 18 between the electron transport layer 14 and the cathode 15.

[0050] Fig. 6C is a cross-sectional view of a light-emitting device according to a fourth embodiment. In Fig. 6C, 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 stacked in this order on a substrate 10. In Fig. 6B, unlike Fig. 4A, a hole injection layer 16 is included between the anode 11 and the hole transport layer 12, and an electron injection layer 18 is further included between the electron transport layer 14 and the cathode 15.

[0051] The hole injection layer 16 and the electron injection layer 18 may be made of any material, either inorganic or organic, but it is preferable to form the hole injection layer 16 and the electron injection layer 18 from inorganic layers, since this allows all layers from the anode 11 to the cathode 15 to be made of inorganic layers. The materials for the hole injection layer 16 and the electron injection layer 18 are selected from various materials based on an energy level model.

[0052] In this embodiment, the layer between the cathode 15 and the light-emitting layer 13 is preferably the electron transport layer 14, the electron injection layer 18, a layer serving as both the electron injection layer and the electron transport layer, or a layer in which the electron transport layer 14 and the electron injection layer 18 are stacked.

[0053] In this embodiment, the layer between the anode 11 and the light-emitting layer 13 is preferably the hole transport layer 12, the hole injection layer 16, a layer serving as both the hole injection layer and the hole transport layer, or a layer in which the hole transport layer 12 and the hole injection layer 16 are stacked.

[0054] 2, the thin film transistor 4 is, for example, a bottom gate type, and the drain electrode 4c is connected to the cathode 15 of the light-emitting element 3. In this case, the cathode 15 is not formed on top of the drain electrode 4c, and the drain electrode 4c can also serve as the cathode 15. This allows the light-emitting element 3 to be appropriately connected to the thin film transistor 4 and the ground line.

[0055] In this embodiment, the hole transport layer 12, the light-emitting layer 13, and the electron transport layer 14 can all be inorganic layers formed of nanoparticles. In this case, each layer can be printed by an inkjet method or the like, and each layer can be easily formed with a uniform film thickness. This can effectively improve the luminous efficiency.

[0056] If the quantum dots used in the light-emitting layer 13 of this embodiment have a core-shell structure, the energy level diagram shown in FIG. 7A is obtained, and the energy level of the shell may act as a barrier to the recombination of holes and electrons. For this reason, it is preferable to use quantum dots in which the surface of the core is not covered with a shell (the surface of the core is exposed; the material constituting the quantum dot is uniform from the center to the surface of the quantum dot), as shown in FIG. 7B. By using such quantum dots, the energy barrier for the recombination of holes and electrons is eliminated, allowing for efficient recombination of holes and electrons, thereby improving the luminous efficiency. In addition, to improve the electron transport efficiency and hole transport efficiency, it is preferable to coordinate organic ligands 21 to the surface of the quantum dots 20, as shown in FIG. 5A.

[0057] In addition, in this embodiment, as shown in FIG. 8A , the light-emitting element 3 may be configured to include a substrate 10, an anode 11 formed on the substrate, a hole transport layer (HTL: Hole Transport Layer) 12 formed on the anode 11, an emitter layer (EML: Emitter Layer) 13 formed on the hole transport layer 12, an electron transport layer (ETL: Electron Transport Layer) 14 formed on the emitter layer 13, and a cathode 15 formed on the electron transport layer 14.

[0058] The materials of each layer are as described above. However, in Fig. 8A, the anode 11 constitutes the first electrode, and the cathode 15 constitutes the second electrode. Since the light-emitting element 3 of this embodiment is a top-emission type, the anode 11 is preferably formed of a very thin transparent material such as Ag, and the cathode 15 is preferably formed of, for example, ITO on a non-transparent metal layer. This allows light to be reflected by the cathode 15 and extracted from the front side, which is the anode 11 side (the side opposite the thin-film transistor).

[0059] Fig. 9A is a cross-sectional view of a light-emitting device of an embodiment different from that of Fig. 8A. In Fig. 9A, 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 stacked in this order on a substrate 10. Unlike Fig. 8A, Fig. 9A includes a hole injection layer 16 between the anode 11 and the hole transport layer 12.

[0060] Fig. 9B is a cross-sectional view of a light-emitting device of an embodiment different from that of Fig. 8A. In Fig. 9B, an anode 11, a hole transport layer 12, an emitting layer 13, an electron transport layer 14, an electron injection layer (EIL) 18, and a cathode 15 are stacked in this order on a substrate 10. Unlike Fig. 8A, Fig. 9B includes an electron injection layer 18 between the electron transport layer 14 and the cathode 15.

[0061] 9C is a cross-sectional view of a light-emitting element according to a fourth embodiment. In FIG. 9C, 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 stacked in this order on a substrate 10. Unlike FIG. 8A, in FIG. 9C, the hole injection layer 16 is included between the anode 11 and the hole transport layer 12, and the electron injection layer 18 is further included between the electron transport layer 14 and the cathode 15.

[0062] When the quantum dots used in the light-emitting layer 13 of the embodiment shown in Figures 8 and 9 have a core-shell structure, the energy level diagram shown in Figure 10A is obtained, and the energy level of the shell may act as a barrier to the recombination of holes and electrons. Therefore, by using quantum dots in which the surface of the core is not covered with a shell, as shown in Figure 10B, the energy barrier during recombination of holes and electrons is eliminated, allowing holes and electrons to recombine efficiently, thereby improving the light-emitting efficiency. In order to improve the electron transport efficiency and hole transport efficiency, it is preferable to coordinate organic ligands 21 on the surface of the quantum dots 20, as shown in Figure 5A.

[0063] The light-emitting elements of the embodiments shown in Figures 8 and 9 are conventional EL elements, while the light-emitting elements of the embodiments shown in Figures 4 and 6 have a configuration in which conventional EL elements are invertedly stacked. In the light-emitting elements of the embodiments shown in Figures 8 and 9, it is preferable that the thin-film transistor is a p-ch TFT, and therefore it is preferable that the channel layer is formed of a P-type semiconductor.

[0064] In this embodiment, at least one of the layers 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 can be formed by the inkjet method. As shown in FIG. 11 , a mask 30 is placed on the substrate 10, and an inorganic layer 31 is printed by the inkjet method in multiple application regions 30a, which are spaces defined in the mask 30. At this time, the surface of the sidewall 30b of the mask 30 is fluorine-treated to impart water repellency, for example. This suppresses the affinity of the ink with the surface of the sidewall 30b, thereby suppressing defects such as depressions in the surface of the printed inorganic layer 31 and enabling the inorganic layer 31 to have a higher degree of planarization.

[0065] This embodiment is a top-emission type, and can appropriately improve the carrier balance in the inverted EL type light-emitting element 3 shown in FIGS. 4 and 6. Furthermore, the layers between the cathode 15 and the light-emitting layer 13 (the electron transport layer 14, or the electron transport layer 14 and the electron injection layer 18), and the light-emitting layer 13 can be formed by coating. Furthermore, the layers between the light-emitting layer 13 and the anode 11 (the hole transport layer 12, or the hole transport layer 12 and the hole injection layer 16) can be formed by vapor deposition or coating. This simplifies the manufacturing process of the light-emitting element.

[0066] The display device 1 shown in Fig. 1 is an example, and the arrangement of the red light-emitting region 2a, the green light-emitting region 2b, and the blue light-emitting region 2c may be other than that shown in Fig. 1. Furthermore, it is also possible to provide a display device having light-emitting regions of only one color among the red light-emitting region 2a, the green light-emitting region 2b, and the blue light-emitting region 2c, or light-emitting regions of two colors.

[0067] In a display device using quantum dots, as in this embodiment, the quantum dots can be configured as either a point light source or a surface light source, and by selecting the substrate, curved light sources and flexible products can also be realized.

[0068] Furthermore, according to this embodiment, it is possible to develop distinctive products such as lighting with color mixing properties equivalent to that of sunlight, which has been difficult to achieve until now, lighting that is easy on the eyes, and lighting optimized for plant factories.

[0069] As described above, display devices using quantum dots have a high degree of freedom in placement, such as being thin and light, and being able to be formed on curved surfaces. They can emit light from the entire surface, and they can achieve natural light emission that is not dazzling even when viewed directly and does not create shadows. Furthermore, they consume less power and have a longer lifespan. For example, display devices using quantum dots according to this embodiment are superior to organic EL display devices in terms of color rendering, light emission, product lifespan, and product price.

[0070] In the display device using quantum dots of this embodiment, they can be used as PL emitters in parallel with EL emitters. Furthermore, in the display device using quantum dots, a hybrid light-emitting device can be realized in which an EL emitter and a PL emitter are stacked. For example, a PL emitter is stacked on the surface of an EL emitter, and the quantum dots excited by the EL emitter emit light, and the emission wavelength can be changed by the quantum dots contained in the PL emitter. The EL emitter has a stacked structure of the above-mentioned light-emitting elements, and the PL emitter is, for example, a sheet-like wavelength conversion material in which multiple quantum dots are dispersed in a resin. Such a hybrid configuration can be realized by using quantum dots.

[0071] In this embodiment, in order to achieve both a large area and a reduced manufacturing cost for the quantum dot display device, it is preferable to use an inkjet printing method, a spin coater method, or a dispenser method as the coating method. Although the above describes a top emission type example, a bottom emission type may also be used.

[0072] The effects of the present invention will be explained below by way of examples of the present invention, but the embodiments of the present invention are not limited to the following examples.

[0073] The samples shown in Table 1 below were prepared and their inkjet dropability was examined. Note that "Abs10" in the table indicates an absorbance of 10% when the quantum dots are dispersed, and "Abs20" indicates an absorbance of 20% when the quantum dots are dispersed.

[0074]

[0075] In the "Dropping" column shown in Table 1, ◯ indicates a sample in which the droplets were dropped appropriately, and × indicates a sample in which the droplets were not dropped properly.

[0076] In Table 1, the "red QD" and "green QD" samples are applied to the light-emitting layer. The "polyvinyl carbazole" samples are applied to the hole injection layer. The "zinc oxide nanoparticle" samples are applied to the electron transport layer and electron injection layer.

[0077] As shown in Table 1, IPA and propylene glycol are not suitable solvents for zinc oxide nanoparticles, and it was found that they must be changed. Solvents marked with a circle in the "Dropping" column in Table 1 can be used as appropriate, but hydrophilic solvents are preferred. For example, alcohol-based solvents can be used as hydrophilic solvents.

[0078] FIG. 12 shows the ZnO Ｘ 12 is a photograph of the state of the coating applied by the inkjet method using a solvent of ethoxyethanol:EG=7:3. As shown in FIG. 12, a good coating state was obtained.

[0079] We also investigated the adverse effects on EPDM (ethylene propylene diene rubber) inside the inkjet head. As shown in Table 1, some samples caused cap deformation or adverse effects on the EPDM. Therefore, when using EPDM, it is recommended to consider the effects on the EPDM.

[0080] (Experiment on Shell Thickness Dependence of Quantum Dots) In the experiment, quantum dots (green QDs) of each sample shown in Table 1 were manufactured, and the relationship between shell thickness and external quantum efficiency (EQE) was investigated using a display device equipped with the light-emitting element of FIG. 4A.

[0081]

[0082] As shown in Table 2, a correlation was observed between shell thickness and EQE. Although not limited thereto, the shell thickness was 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.

[0083] Furthermore, when the relationship between the quantum dot layer thickness and the EQE was examined, it was found that a certain quantum dot layer thickness tends to increase the EQE. Although the quantum dot layer thickness is not limited, the quantum dot layer thickness was 5 nm to 50 nm, preferably 10 nm to 45 nm, more preferably 15 nm to 40 nm, even more preferably 20 nm to 40 nm, and still more preferably 25 nm to 40 nm.

[0084] 13, the PYS measurement of the Cd-based green quantum dots was performed. The circles in FIG. 13 represent experimental data for Example 1, which is a core only, and the squares in Example 2, which are cores coated with a shell.

[0085] Photoelectron yield spectroscopy (PYS) can measure ionization potential, for example, using an AC-2 or AC-3 spectrometer manufactured by Riken Keiki Co., Ltd.

[0086] 13, it was found that the rising energy differed between Example 1 and Example 2. In Example 1, it was about 6.1 eV, and in Example 2, it was about 7.1 eV.

[0087] 14 shows the PYS measurement results for Cd-based green quantum dots in Examples 3 and 4, which have different shell thicknesses. Example 4 is thicker than Example 3. It was found that the rise energy differs between Examples 3 and 4. In Example 3, it was approximately 7.1 eV, and in Example 4, it was approximately 8.1 eV.

[0088] (Experiment on current dependence of EQE) Figure 15 is an energy level diagram of each layer in the light-emitting element used in the experiment. Figure 16 is a graph showing the relationship between the current value and EQE for an EL emitter and a PL emitter using red quantum dots. Figure 17 is a graph showing the relationship between the current value and EQE for an EL emitter and a PL emitter using red quantum dots, and a graph showing the relationship between the current value and EQE for an EL emitter using blue quantum dots. Examples 5 and 6 shown in Figure 16 have different shell thicknesses. Example 5 has a thicker shell thickness than Example 6. Figure 17 also shows that Example 7 has the thickest shell thickness, followed by Examples 8 and 9, in that order.

[0089] As shown in Figures 16 and 17, the EQE increased for the EL and PL emitters up to about 20 mA. On the other hand, the EQE increased for the red device even at currents of 20 mA or more. Furthermore, as shown in Figures 16 and 17, the EQE increased as the shell thickness increased.

[0090] (ZnO Ｘ FIG. 18 shows the energy band gap Eg and the conduction band minimum E of each layer in the light-emitting element used in the experiment. ＣＢ , the energy at the top of the valence band E ＶＢ 18 is a graph showing the energy level of each layer, and L1 or L2 shown in FIG. Ｘ (Li) was used. Here, Li may or may not be doped. Although not limited thereto, X is about 0.8 to 1.2. As shown in FIG. 18, ZnO used in the electron injection layer and electron transport layer (ETL) Ｘ ZnO Ｘ It was found that the band gap can be widened by using (Li). ＸPVK shown in FIG. 18 is a hole injection layer, B1, B2, G(H), G(I3), and R(F) are light-emitting layers (EL layers), and ZnO Ｘ , L2, and L4 are electron injection layers. When B1 or B2 is used in the light emitting layer, ZnO is used in the electron injection layer. Ｘ However, when G(H), G(I3), or R(F) is used in the light-emitting layer, it is preferable to use L2 or L4 in the electron injection layer. Ｘ (Li).

[0091] In particular, when a light-emitting layer (EL layer) with a shallow conduction band is used, ZnO Ｘ It is effective to apply (Li) to the electron injection layer or electron transport layer.

[0092] Including, but not limited to, ZnO Ｘ (Li) was obtained by stirring a zinc acetate-ethanol solution at a predetermined temperature for a predetermined time, followed by adding LiOH.4H ２ O-ethanol solution can be mixed and stirred, centrifuged, washed, etc. to produce the product.

[0093] 19 to 21 show ZnO applied to the electron transport layer (ETL). Ｘ (Li) and ZnO Ｘ (K) UV (bandgap), PL, and PYS data. Ｘ (K) is produced using KOH as a catalyst and is not doped with K or Li. Ｘ (Li) and ZnO Ｘ It was found that there was a discrepancy in the UV and PL data between PYS and ZnO (K). Ｘ (Li) and ZnO Ｘ It was found that there was almost no difference between (K) and (K), and the rising energy was almost the same.

[0094] In this way, ZnO with a controlled band gap by various particle sizes can be used as the electron injection / transport layer of an EL device using quantum dots. Ｘ By adding doping species, defect control and band gap control can be achieved. Ｘcan be proposed.

[0095] However, if the balance between the electrons and holes that generate light cannot be achieved, a thin insulating layer may be interposed between the EL layer and the electron injection layer, or ZnO Ｘ It is preferable to add a hole blocking function by integrating the molecules with the ZnO. ｘ and T2T (2,4,6-tris(biphenyl-3-yl)-1,3,5-triazine). Although not limited thereto, X is about 0.8 to 1.2.

[0096] Also, ZnO ｘ It has been found that ZnO can be used not only as an electron injection / transport layer but also as a hole injection / transport layer by performing ozone treatment or the like. ｘ It has been found that the hole transport capability is improved by performing ozone treatment on the electron transport layer. In this embodiment, the electron transport layer is formed of ZnO, MgZnO, or SnO. The synthesis flow of these metal oxides is described below.

[0097] Figure 22 shows the synthesis flow of ZnO. As shown in Figure 22, Zn(OAc) ２ ・2H ２ O (6.22 g, 28.3 mmmol) and 0.5 wt% hydrous methanol (198.8 g, 251 ml) were mixed and the temperature was raised to 60°C. Next, KOH / 0.5 wt% hydrous methanol was added to the flask at a rate of 13 ml per minute, and this addition was continued for 10 minutes. The temperature was then maintained at 60°C for 2 hours.

[0098] After cooling, the mixture was transferred to two centrifuge tubes and centrifuged at 7000 pm for 5 minutes. The supernatant was then removed from each tube, and the mixture was sonicated and centrifuged again at 7000 pm for 5 minutes. The process of removing the supernatant, sonicating, and centrifugation was repeated once more.

[0099] Then, the supernatant was removed and ultrasonication was performed, and between the removal of the supernatant and the ultrasonication, ethanol (5 ml) and aminoethanol (0.5 ml) were added to each centrifuge tube, and the tubes were then stored overnight in a dark place.

[0100] Next, the mixture was transferred to two other centrifuge tubes, and about 35 to 40 ml of ethyl acetate was added to each tube, followed by centrifugation at 5500 rpm for 5 minutes. The supernatant was then discarded from each tube, and the mixture was transferred to a G-BOX and dispersed using a vortex mixer.

[0101] Next, the solution was passed through a 0.54 μm filter and transferred to a 50 ml tube, from which 0.7 ml was taken, and the remainder was stored in a G-BOX. The concentration of the taken 0.7 ml was measured, and the concentration and dilution ratio were calculated. Based on this calculation, the dilution ratio was adjusted to the calculated value using dehydrated ethanol in the G-BOX. The completed G-BOX was then stored. A 1.6 ml aliquot was taken from this completed G-BOX, and DLS, UV value, PL value, and concentration were analyzed.

[0102] 23 shows the synthesis flow of MgZnO. As shown in FIG. 23, first, anhydrous ethanol (30 mL) was prepared, and Zn(OAc) ２ ・2H ２ O (560 mg, 2.55 mmol) and Mg (OAc) ２ ・4H ２ O (96.5 mg, 0.45 mmol) was added and stirred at room temperature for 30 minutes. Subsequently, KOH (309 mg, 5.5 mmol) / absolute ethanol (20 mL) was added. Then, stirring was continued at room temperature for 1 hour. Subsequently, hexane (80 mL) was added. This resulted in the mixture becoming cloudy white. Ethanolamine (1 mL) was added to this mixture until it dissolved, and further hexane was added to precipitate the solid, which was then dispersed in ethanol. This resulted in the production of MgZnO with a particle size of approximately 4.2 nm. The band gap energy E of MgZnO was ｇ was 3.79 eV.

[0103] Figure 24 shows the synthesis flow of SnOx. As shown in Figure 24, ethylene glycol (100 mL) was prepared, and SnCl４ 5H ２ O (11.57 g, 0.33 mol), acetic acid (10 mL), and a 30% aqueous solution of tetramethylammonium hydroxide (25 mL) were mixed and stirred at 50°C for 30 minutes. The temperature was then raised to 160°C and stirring was continued for 4 hours. The mixture was then washed with ethanol-ethyl acetate, and 2 mL of ethanolamine was added, followed by dispersion in ethanol. In this embodiment, the hole transport layer 45 is formed from NiO or MoO. The synthesis flow for these metal oxides is described below.

[0104] Figure 25 shows the synthesis flow of NiOx. As shown in Figure 25, DMSO (100 mL) was prepared and Ni(NO ３ ) ２ ・6H ２ O (10 mmol) and KOH (673 mg, 12 mmol) in absolute ethanol (100 mL) were mixed and stirred at room temperature for 30 minutes. Subsequently, after washing with ethanol, DMSO (150 mL) was mixed and stirred at a temperature of 160°C for 4 hours. Next, it was washed with ethanol-ethyl acetate, and ethanolamine (0.2 mL) was added and dispersed in ethanol. A dark gray precipitate was obtained.

[0105] 26 shows the synthesis flow of MoOx. 1-octadecene and octanoic acid (total volume 60 mL, volume ratio of 1-octadecene:octanoic acid = 1:9) and MoOx were placed in a sealed tube. ２ (acac) ２ (244.62 mg, 0.75 mmol) and acetic acid (10 mL) were mixed and stirred at 50°C for 30 minutes. The temperature was then raised to 180°C and stirred for 40 minutes, and n-octane (250 mL) was added to obtain a dark blue precipitate. The precipitate was dispersed in ethanol.

[0106] According to the synthesis flow of this embodiment, a metal oxide free from oxidation defects can be obtained. Furthermore, by using the electron transport layer and hole transport layer of this embodiment, it is possible to effectively and sharply extend the absorption edge in the infrared region.

[0107] FIG. 27A is a graph showing the UV spectrum, and FIG. 27B is a graph showing the band gap energy. In the experiment, the amount of Mg doping in MgZnO was changed. Note that the MgZnO used in the experiment was immediately after synthesis and dispersed in DEGME. As shown in FIG. 27 and Table 3, as the amount of Mg doping increased, the band gap energy E ｇ was found to increase.

[0108]

[0109] Fig. 28 shows a synthesis flow of NiO. Note that this is a synthesis flow different from that of Fig. 25. That is, as shown in Fig. 28, ３ ) ２ ・6H ２ O, dimethyl sulfoxide, and KOH / absolute ethanol solution were mixed and stirred at room temperature for 30 minutes. This resulted in a green precipitate. This was mixed with DMSO, kept at 160°C for 3 hours, and washed with ethanol / ethyl acetate. Ethanolamine was then added and dispersed in ethanol.

[0110] Figure 29 shows the XRD of NiO. The NiO used in the experiment was obtained using the synthesis flow shown in Figure 28. As shown in Figure 29, NiO was observed. However, Ni was not observed, indicating that the NiO was properly synthesized. Figure 30 shows the UV spectrum of NiO, and Figure 31 is a graph (Tauk plot) showing the band gap energy.

[0111] As shown in FIG. 31, the slope of the Tauch plot of the absorption edge was found to be greater than that of the bulk material. This indicated that the absorption edge could be sharply extended. As shown in FIG. 31, the Tauch plot for nanoparticles showed a steeper rise with a smaller tail than that of the bulk material. Because all of the electron transport materials in this example were semiconductors, the slope of the Tauch plot for all electron transport materials was greater than that of the bulk material. Furthermore, as shown in FIG. 31, it was found that the regions A and B enclosed by the tangent line indicating the slope, the plot curve, and the horizontal axis were smaller in region A for nanoparticles than in region B for bulk material. The smaller this region, the smaller the oxygen vacancy, proving that nanoparticles have smaller oxygen vacancies than bulk materials.

[0112] <Regarding Ni valence of NiO> Standard samples with known valence (NiO: divalent, LiNiO ２ A calibration curve of the rise position of the XANES spectrum and the Ni valence was created using the BL08W beamline and the transmission method for the measurement.

[0113] Next, the Ni valence was evaluated from the XANES spectrum of NiO prepared by the synthesis method of this example. The experimental results are shown in FIG.

[0114] As shown in Figure 32, it was found that the XANES spectrum shifted due to the change in valence. ２ Since the XANES spectrum of the example exists between the trivalent and trivalent states, the Ni valence of the NiO of the example is between 2 and 3, leaning toward divalent NiO. Therefore, it was found that the Ni valence of the example is greater than 2 and less than 2.5, preferably 2.1 or more and 2.4 or less. In the example of Figure 32, the Ni valence is predicted to be about 2.2. This proves that the NiO of the example is a hole carrier.

[0115] Furthermore, the polydispersity index of the metal oxide dispersion used in the electron transport layer of this example was less than 0.1 when measured by DLS. Dynamic light scattering (DLS) is applied to nanoparticles suspended and dispersed in a liquid. DLS measurement is a method for calculating particle diameter by measuring the diffusion rate of nanoparticles moving by Brownian motion.

[0116] The polydispersity index (PDI) is used to represent the width of the particle size distribution. In the experiments, ZnO particles were prepared as the metal oxide used in the electron transport layer. Figure 33 is a graph showing the relationship between particle size and scattering intensity distribution in each example. As shown in Table 4, in Examples 10 to 12, the polydispersity index was less than 0.1. This indicates that all particles measured in each example had approximately the same particle size.

[0117]

[0118] Metal oxides such as ZnO and NiO exist as films in display devices. In this embodiment, the polydispersity index is set to less than 0.1 as described above to improve dispersibility, but the performance of the film can be defined as the surface roughness after film formation.

[0119] 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.

[0120] According to the present invention, a light-emitting element including quantum dots can be applied to a display device, and excellent light-emitting characteristics can be obtained.

[0121] This application is based on Japanese Patent Application No. 2023-170557, filed September 29, 2023, the entire contents of which are incorporated herein by reference.

Claims

1. A display device having a display area, the display area having light-emitting elements stacked together, 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, the electron transport layer containing at least one of ZnO, MgZnO, and SnO, and the hole transport layer or hole injection layer containing at least one of NiO, and MoO.

2. The display device according to claim 1, wherein the oxygen deficiency of the metal oxide constituting the electron transport layer and the hole transport layer is smaller than that of the bulk material.

3. The display device according to claim 1, wherein the light-emitting layer contains quantum dots, the quantum dots contain organic ligands on the surfaces thereof, and the organic ligands contain 3-mercaptopropionic acid.

4. The display device according to claim 1, wherein the light-emitting layer contains quantum dots, the quantum dots contain ligands on their surfaces, and the number of carbon atoms in the ligand is 2 or more and 5 or less.

5. A nanoparticle material used in the electron transport layer constituting the display device according to claim 1, characterized in that the slope of the Tauch plot of the absorption edge is greater than that of the bulk material.

6. A dispersion of the material of claim 5, characterized in that the polydispersity index, as measured by DLS, is less than 0.

1.

7. A method for manufacturing a display device according to claim 1, characterized in that in the step of forming the light-emitting layer, after applying a composition containing quantum dots, ligands are exchanged.

8. The method for manufacturing a display device according to claim 7, characterized in that, when the quantum dots are synthesized by a liquid phase synthesis method, the quantum dots are synthesized using long ligands, and then the ligands are replaced with short ligands before or after applying a composition containing the quantum dots.

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