Display device

Abstract

To provide a display device including quantum dots.SOLUTION: The present invention relates to a display device including a display region. The display region includes a light emitting element in which a first electrode, a layer between the first electrode and a light emitting layer, the light emitting layer, a layer between the light emitting layer and a second electrode, and the second electrode are stacked in this order on a substrate. The light emitting layer is formed of an inorganic layer including quantum dots. The light emitting element is a top emission type. One of the first electrode and the second electrode is an anode and the other is a cathode, a layer formed of nanoparticles of NiO is included in a layer between the anode and the light emitting layer. A thin film transistor connected to the light emitting element is a n-ch TFT.SELECTED DRAWING: Figure 2

Description

[Technical Field]

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

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

[0003] An organic EL device 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 device is made of an organic compound and emits light by excitons generated by recombination of electrons and holes injected into the organic compound. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2017-45650 A Summary of the Invention [Problem to be solved by the invention]

[0005] Recently, progress has been made in the development of light-emitting devices using quantum dots. Quantum dots are nanoparticles composed of hundreds to thousands of atoms and having a particle size of several to tens of nanometers. Quantum dots are also called fluorescent nanoparticles, semiconductor nanoparticles, or nanocrystals. Quantum dots have the advantage that their emission wavelength can be varied by adjusting the particle size and composition of the nanoparticles. Light-emitting devices using quantum dots can be made thin and achieve surface emission, similar to organic EL devices.

[0006] However, the stacked structure of top-emission type light-emitting elements using quantum dots and the structure of a display device using the light-emitting elements have not yet been established.

[0007] The present invention has been made in view of the above points, and has an object to provide a display device equipped with light-emitting elements containing quantum dots. [Means for solving the problem]

[0008] The present invention is a display device having a display area, wherein the display area has a light-emitting element in which a first electrode, a layer between the first electrode and a light-emitting layer, the light-emitting layer, a layer between the light-emitting layer and a second electrode, and the second electrode are stacked in this order on a substrate, the light-emitting layer being formed of an inorganic layer containing quantum dots, the light-emitting element being a top-emission type, one of the first electrode and the second electrode being an anode and the other being a cathode, a layer formed of NiO nanoparticles being included between the anode and the light-emitting layer, and a thin-film transistor connected to the light-emitting element being an n-ch TFT.

[0009] In the present invention, it is preferable that the oxide semiconductor of the thin film transistor is formed of an In-Ga-Zn-O based semiconductor. In the present invention, it is preferable that the light emitting element is laminated with a wavelength conversion member in which a plurality of quantum dots are dispersed in a resin.

[0010] The present invention is preferably applied to lighting for a plant factory.In the present invention, it is preferable that the drain electrode of the n-ch TFT also serves as the cathode.

[0011] In the present invention, the display device is preferably flexible. In the present invention, it is preferable that the layer between the first electrode and the light-emitting layer and the light-emitting layer are formed by coating, and the layer between the light-emitting layer and the second electrode is formed by vapor deposition or coating. [Effects of the Invention]

[0012] According to the display device of the present invention, it is possible to optimize the stack structure of the quantum dot-containing light-emitting element used in the display device. Furthermore, in the present invention, all layers from the cathode to the anode can be formed of inorganic layers. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 2 is a partial plan view of the display device of the present embodiment. [Figure 2] 2 is a partially enlarged cross-sectional view showing one display area of ​​the display device shown in FIG. 1. FIG. [Figure 3] 3 is a cross-sectional view showing a structure of a thin film transistor different from that shown in FIG. 2. [Figure 4] 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. [Figure 5] FIG. 2 is a schematic diagram of a quantum dot according to the present embodiment. [Figure 6] FIG. 2 is a cross-sectional view of a light-emitting element according to an embodiment different from that shown in FIG. [Figure 7] 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. [Figure 8] 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. [Figure 9] 9 is a cross-sectional view of a light-emitting element according to an embodiment different from that shown in FIG. 8. FIG. [Figure 10] 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. [Figure 11] 1A to 1C are schematic diagrams illustrating a process of forming an inorganic layer by an inkjet method. [Figure 12] 1 is a photograph of an application example. [Figure 13] This is PYS measurement data for Cd-based green quantum dots. [Figure 14] This is PYS measurement data. [Figure 15] FIG. 2 is an energy level diagram of each layer in the light-emitting element used in the experiment. [Figure 16] 1 is a graph showing the relationship between current and EQE for an EL emitter and a PL emitter using green quantum dots. [Figure 17] 1 is a graph showing the relationship between current and EQE for an EL emitter and a PL emitter using red quantum dots, and a graph showing the relationship between current and EQE for an EL emitter using blue quantum dots. [Figure 18] 1 is a graph showing the energy band gap Eg, the energy at the bottom of the conduction band ECB, and the energy at the top of the valence band EVB of each layer in the light-emitting element used in the experiment, and an energy level diagram of each layer. [Figure 19] UV data for ZnOX(Li) and ZnOX(K) used in the electron transport layer (ETL). [Figure 20] This shows PL data of ZnOX(Li) and ZnOX(K) used in the electron transport layer (ETL). [Figure 21] This is PYS data for ZnOX(Li) and ZnOX(K) used in the electron transport layer (ETL). DETAILED DESCRIPTION OF THE INVENTION

[0014] 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 of the present invention.

[0015] 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.

[0016] 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 of a top-emission type.

[0017] 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. The material of the channel layer 4b is not critical, but 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.

[0018] The source electrode 4 d is connected to a power supply line, and the drain electrode 4 c is connected to the light emitting element 3 .

[0019] 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 4 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. 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.

[0020] 3 is an N-type semiconductor, and an oxide semiconductor is preferably used for the channel layer 4b of the thin film transistor 4. As the oxide semiconductor, an In-Ga-Zn-O based semiconductor is preferably used.

[0021] As shown in Figure 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.

[0022] The following describes the structure of the light-emitting element 3. Figure 4A is a cross-sectional view of the light-emitting element in the first embodiment, and Figure 4B is an energy level diagram of each layer in the display device of the first embodiment.

[0023] 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.

[0024] 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, causing the quantum dots in the light-emitting layer 13 to enter an excited state, and light can be emitted from the excited quantum dots.

[0025] In this embodiment, the light-emitting layer 13 is formed of an inorganic layer containing quantum dots.

[0026] (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 nm to several tens of nm.

[0027] For example, quantum dots are formed from CdS, CdSe, ZnS, ZnSe, ZnSeS, ZnTe, ZnTeS, InP, (Zn)AgInS2, (Zn)CuInS2, etc. Because of the toxicity of Cd, its use is restricted in various countries, so it is preferable that quantum dots do not contain Cd.

[0028] As shown in Fig. 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: Aliphatic primary amine, oleylamine: C 18 H 35 NH2, stearyl (octadecyl)amine: C 18 H 37 NH2, 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: CH 19 COOH, octanoic acid: CH 15COOH Thiol, octadecanethiol: C 18 H 37 SH, hexanedecanethiol: C 16 H 33 SH, tetradecanethiol: C 14 H 29 SH, dodecanethiol: C 12 H 25 SH, decanethiol: C 10 H 21 SH, Octanethiol: CH 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

[0029] 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 is preferably formed of zinc sulfide (ZnS), for example. Like the core 20a, it is preferable that the shell 20b does not contain cadmium (Cd).

[0030] The shell 20b may be in a state of being solid-solution 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 it does not matter whether the boundary between the core 20a and the shell 20b can be confirmed by analysis or not.

[0031] (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 by, for example, an inkjet method, and some solvent components may remain in the light-emitting layer 13.

[0032] 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.

[0033] The wavelength of the blue light emitted is preferably about 450 nm. By adjusting the wavelength so that light with a wavelength shorter than 450 nm is not emitted, health risks can be reduced.

[0034] The light-emitting layer 13 can be formed by using an existing thin film formation method such as the ink-jet method or vacuum deposition method mentioned above.

[0035] (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 is preferably formed of an inorganic oxide such as NiO or WO3. The hole transport layer 12 is particularly preferably formed of NiO nanoparticles. The hole transport layer 12 can also be formed by mixing NiO with Al2O3, for example. Metal oxides may also 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.

[0036] 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.

[0037] (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, such as ZnO. X , Ti-O, Sn-O, VO x It is preferable that the electron transport layer 14 is made of an inorganic oxide such as ZnO, Mo—O, etc. Two or more of these may be selected. X Preferably, the electron transport layer 14 is formed of nanoparticles of the metal oxide. The metal oxide may be doped with Li, Mg, Al, Mn, or the like. The electron transport layer 14 may also be made of an inorganic substance other than an inorganic oxide (e.g., CsPbBr3, etc.). X is not limited to, but is about 0.8 to 1.2.

[0038] The electron transport layer 14, like the light-emitting layer 13, can be formed by printing a solvent containing nanoparticles using a printing method such as an inkjet method, or by using an existing thin film technology such as a vacuum deposition method.

[0039] (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, CuISnO2, or 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 other methods of forming the electrode material.

[0040] In this embodiment, since light is extracted from the anode 11 side, the anode 11 must be transparent, and is preferably a thin metal film with excellent transparency such as Ag or a metal oxide with excellent transparency as described above.

[0041] (Cathode 15) In this embodiment, the material of the cathode 15 is not limited, but for example, indium-tin composite oxide (ITO), metal, alloy, electrically conductive compound, and mixtures thereof can be used as the electrode material for the cathode 15. For example, the cathode 15 is formed of ITO. Note that 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.

[0042] The cathode 15 can be formed as a thin film of these electrode materials by vapor deposition, sputtering, or other methods.

[0043] (Substrate 10) In this embodiment, the material of the substrate 10 is not limited, but the substrate 10 can be made of, for example, glass, plastic, etc. Specifically, the substrate 10 is made 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, it is possible to provide flexibility to the display device 1. 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, the same coating and drying equipment can be used for film formation, simplifying the manufacturing process. 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 improved carrier balance compared to when organic compounds are used.

[0047] In the first embodiment shown in Fig. 4, the hole injection layer and 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, inorganic hole injection layers and electron injection layers can also be interposed between each electrode and each transport layer.

[0048] Fig. 6A is a cross-sectional view of a light-emitting element 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 device 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 a hole transport layer 12, a hole injection layer 16, a layer serving as both a hole injection layer and a 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 will be 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 core surface is not covered with a shell (the core surface 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. The use of such quantum dots eliminates the energy barrier during recombination of holes and electrons, allowing for efficient recombination of holes and electrons, thereby improving luminous efficiency. To improve 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 15 is preferably formed of a very thin transparent material such as Ag, and the cathode 11 is preferably formed of, for example, ITO on a non-transparent metal layer. This allows light to be reflected by the cathode 11 and extracted from the front surface side, which is the anode 15 side (the side opposite to 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] Fig. 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, Fig. 9C includes a hole injection layer 16 between the anode 11 and the hole transport layer 12, and further includes an electron injection layer 18 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 for the recombination of holes and electrons is eliminated, allowing holes and electrons to recombine efficiently, thereby improving the luminous efficiency. Note that, 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 preventing problems such as depressions in the surface of the printed inorganic layer 31 and increasing the flatness of the surface of the inorganic layer 31.

[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 are thin and light, and can be formed on curved surfaces, allowing for a high degree of freedom in placement. They can emit light from the entire surface, providing 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, luminescence, product lifespan, and product price.

[0070] In a display device using quantum dots according to this embodiment, the quantum dots can be used as a PL light emitter in parallel with an EL light emitter. Furthermore, in a display device using quantum dots, a hybrid light-emitting device can be realized in which an EL light emitter and a PL light emitter are stacked. For example, a PL light emitter is stacked on the surface of an EL light emitter, and the quantum dots excited by the EL light emitter emit light, and the emission wavelength can be changed by the quantum dots contained in the PL light emitter. The EL light emitter has a stacked structure of the above-described light-emitting elements, and the PL light 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 larger area for the display device using quantum dots and a reduced manufacturing cost, it is preferable to use an inkjet printing method, a spin coater method, or a dispenser method as the coating method. [Example]

[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 dispensability was examined. Note that "Abs10" in Table 1 indicates an absorbance of 10% when quantum dots are dispersed, and "Abs20" indicates an absorbance of 20% when quantum dots are dispersed.

[0074] [Table 1]

[0075] In the "Dropping" column in Table 1, ◯ indicates a sample that was dropped properly, and × indicates a sample that experienced a dropping failure.

[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, and the "zinc oxide nanoparticle" sample is 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 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] Figure 12 shows the ZnO X 12 is a photograph of the state of coating by inkjet method using ethoxyethanol:EG=7:3 as the solvent. As shown in Figure 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 had adverse effects on EPDM. Therefore, when using EPDM, it is advisable to consider the effects on EPDM.

[0080] (Experiment on the shell thickness dependence of quantum dots) In the experiment, quantum dots (green QDs) of each sample shown in Table 2 were manufactured, and the relationship between shell thickness and external quantum efficiency (EQE) was investigated using a bottom-bottom emission display device equipped with the light-emitting element shown in Figure 4A. [Table 2]

[0081] 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.

[0082] Furthermore, when the relationship between the thickness (diameter) of the quantum dots and the EQE was investigated, it was found that a certain thickness of the quantum dots tends to increase the EQE. Although the thickness of the quantum dots is not limited, the thickness of the quantum dots 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 even more preferably 25 nm to 40 nm.

[0083] 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 the core only, and the squares in Example 2, which is the core coated with a shell.

[0084] Photoelectron Yield Spectroscopy (PYS) can measure ionization potential. For example, it can be measured using Riken Keiki's AC-2 and AC-3 instruments.

[0085] 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.

[0086] Figure 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 Example 3 and Example 4. In Example 3, it was approximately 7.1 eV, and in Example 4, it was approximately 8.1 eV.

[0087] (Experiment on current dependence of EQE) FIG. 15 is an energy level diagram of each layer in the light-emitting device used in the experiment. FIG. 16 is a graph showing the relationship between the current value and the EQE for an EL emitter and a PL emitter using red quantum dots. FIG. 17 is a graph showing the relationship between the current value and the EQE for an EL emitter and a PL emitter using red quantum dots, and a graph showing the relationship between the current value and the EQE for an EL emitter using blue quantum dots. Example 5 and Example 6 shown in FIG. 16 have different shell thicknesses. Example 5 has a thicker shell thickness than Example 6. Also, in FIG. 17, Example 7 has the thickest shell thickness, followed by Example 8 and Example 9, in that order.

[0088] As shown in Figures 16 and 17, the EQE of the EL and PL emitters increased up to about 20 mA. On the other hand, the EQE of the red device increased even at currents above 20 mA. Furthermore, as shown in Figures 16 and 17, the EQE increased as the shell thickness increased.

[0089] (ZnO X (Experiment on the synthesis of FIG. 18 shows the energy band gap Eg and the conduction band minimum energy E CB , the energy at the top of the valence band E VB 18. The graph showing the energy level of each layer and the energy level diagram of each layer are shown in FIG. X ZnO (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 (ETL) and electron transport layer X ZnO X It was found that the band gap can be widened by using (Li). X It is presumed that the (Li) has the effect of reducing the particle diameter. PVK shown in Figure 18 is the hole injection layer, B1, B2, G(H), G(I3), and R(F) are the 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. L2 and L4 are ZnO X (Li).

[0090] In particular, when using a light-emitting layer (EL layer) with a shallow conduction band, ZnO X It is effective to apply (Li) to the electron injection layer or electron transport layer.

[0091] Non-limiting examples include ZnO X (Li) can be produced by stirring a zinc acetate-ethanol solution at a specified temperature for a specified time, then mixing it with a LiOH·4H2O-ethanol solution, stirring, centrifuging, washing, etc.

[0092] 19 to 21 show ZnO applied to the electron injection layer (ETL). X (Li) and ZnO X UV (band gap), PL, and PYS data for (K). ZnO X (K) is produced using KOH as a catalyst and is not doped with K or Li. ZnO 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.

[0093] In this way, we have developed ZnO with various particle sizes and controlled band gaps as the electron injection / transport layer for quantum dot-based EL devices. X By adding doping species, we can control defects and band gaps to produce doped ZnO. X can be proposed.

[0094] However, if the balance between the electrons and holes that generate light cannot be achieved, a thin insulating layer can be inserted between the EL layer and the electron injection layer, or a 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). X is, but is not limited to, approximately 0.8 to 1.2.

[0095] In addition, 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. x We found that the hole transport capacity was improved by treating the material with ozone. [Industrial Applicability]

[0096] 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.

[0097] This application is based on Japanese Patent Application No. 2017-215801, filed November 8, 2017, the entire contents of which are incorporated herein by reference.

Claims

1. A display device having a display area, the display region has a light-emitting element in which a first electrode, a layer between the first electrode and a light-emitting layer, the light-emitting layer, a layer between the light-emitting layer and a second electrode, and the second electrode are stacked in this order on a substrate; the light-emitting layer is formed of an inorganic layer containing quantum dots, and the light-emitting element is a top-emission type; one of the first electrode and the second electrode is an anode and the other is a cathode; a layer formed of NiO nanoparticles between the anode and the light-emitting layer; The display device is characterized in that the thin film transistor connected to the light emitting element is an n-ch TFT.

2. 2. The display device according to claim 1, wherein the oxide semiconductor of the thin film transistor is formed of an In--Ga--Zn--O based semiconductor.

3. 3. The display device according to claim 1, wherein a wavelength conversion member in which a plurality of quantum dots are dispersed in a resin is laminated on the light emitting element.

4. 4. The display device according to claim 3, which is used for lighting a plant factory.

5. 5. The display device according to claim 1, wherein the drain electrode of the n-ch TFT also serves as the cathode.

6. 6. The display device according to claim 1, wherein the display device is flexible.

7. 7. The display device according to claim 1, wherein the layer between the first electrode and the light-emitting layer and the light-emitting layer are formed by coating, and the layer between the light-emitting layer and the second electrode is formed by vapor deposition or coating.

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