Display device

Abstract

To provide a display device including a quantum dot.SOLUTION: A display device includes 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 on the substrate in this order. The light-emitting layer is formed of an inorganic layer including a quantum dot. The light-emitting element is of a top-emission type. The quantum dot has a core-shell structure. The shell thickness is 1.3 nm to 2.5 nm. A thin film transistor connected to the light-emitting element is preferably an n-ch TFT.SELECTED DRAWING: Figure 1

Description

Technical Field

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

Background Art

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

[0003] An organic EL element is configured 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 formed 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

Summary of the Invention

Problems to be Solved by the Invention

[0005] By the way, in recent years, the development of light emitting elements using quantum dots has been progressing. Quantum dots are nanoparticles composed of about several hundred to several thousand atoms and having a particle size of about several nm to several tens of nm. Quantum dots are also called fluorescent nanoparticles, semiconductor nanoparticles, or nanocrystals. Quantum dots have the characteristic that the emission wavelength can be variously changed depending on the particle size and composition of the nanoparticles. A light emitting element using quantum dots can achieve thinning and surface emission, similar to an organic EL element.

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

[0007] The present invention has been made in view of such a point, and an object thereof is to provide a display device including a light-emitting element containing quantum dots.

Means for Solving the Problems

[0008] The present invention is a display device including a display region, and the display region has a light-emitting element laminated in the order of On the TFT, an anode formed of ITO, a hole injection layer containing PEDOT:PSS, a hole transport layer containing polyvinylcarbazole, a light-emitting layer, an electron injection layer containing ZnOx(Li) (x is 0.8 to 1.2), and a cathode formed of Al , the light-emitting layer is formed of an inorganic layer containing quantum dots, the light-emitting element is a top emission type, the quantum dots have a core-shell structure, the shell is formed of ZnS, the shell thickness is 1.3 nm or more and 2.5 nm or less, and the film thickness of the light-emitting layer is 25 nm or more and 37 nm or less uniformly , and a voltage is applied between the anode and the cathode to perform surface light emission.

[0009] This In the invention, the light-emitting element and the thin-film transistor are interposed between a pair of substrates, and it is preferable that a sealing resin is provided between the substrates and the substrates are connected via the sealing resin.

[0010] In the present invention, it is preferable that the thin-film transistor connected to the light-emitting element is an n-ch TFT.

[0011] In the present invention, it is preferable that the display device has flexibility.

Effects of the Invention

[0012] According to the display device of the present invention, the laminated structure of the light-emitting element containing quantum dots used in the display device can be optimized. Further, in the present invention, all layers from the cathode to the anode can be formed of inorganic layers.

Brief Description of the Drawings

[0013]

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Mode for Carrying Out the Invention

[0014] Hereinafter, an embodiment of the present invention (hereinafter abbreviated as "embodiment") will be described in detail. Note that the present invention is not limited to the following embodiments, and various modifications can be made within the scope of the gist.

[0015] As shown in FIG. 1, in the display device 1, a plurality of display regions 2 are arranged in a matrix. In the display region 2, there are three types: 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, 2c are, for example, arranged side by side in the row direction to form a set, and constitute one pixel in color display.

[0016] In each of the light-emitting regions 2a, 2b, 2c, a light-emitting element 3 is formed. The layer structure of the light-emitting element 3 will be described later. A thin film transistor (TFT: Thin Film Transistor) 4 is connected to each light-emitting element 3. The light-emitting element 3 is a top emission type.

[0017] The thin-film transistor 4 shown in FIG. 2 is an n-ch TFT, and on a substrate 5, a gate electrode 4a, a channel layer 4b, a gate insulating film (not shown), a drain electrode 4c, a source electrode 4d, etc. are laminated and configured. Although the material of the channel layer 4b is not questioned, it is an N-type semiconductor, and an oxide semiconductor is preferably used. As the oxide semiconductor, an In-Ga-Zn-O-based semiconductor is preferably used. The In-Ga-Zn-O-based semiconductor has high mobility and low leakage current, and can be suitably used as a thin-film transistor. Also, Poly-Si can also be preferably used. The thin-film transistor 4 shown in FIG. 2 is a top-contact bottom-gate type, but it may also be a bottom-contact bottom-gate type.

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

[0019] Also, the thin-film transistor 4 may 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. And on the surface of the gate insulating film 4e, a gate electrode 4a is formed. As shown in FIG. 3, the surface of the gate electrode 4a is covered with an insulating film 4f. Also, a plurality of through holes penetrating the gate insulating film 4e and the insulating film 4f and leading to the channel layer 4b are formed, and through each through hole, a drain electrode 4c and a source electrode 4d are respectively formed. Further, the surfaces of the drain electrode 4c and the source electrode 4d are covered with a protective film 7. Also, a transparent electrode leading to the drain electrode 4c and the source electrode 4d is formed on the surface of the protective film 7. The transparent electrode 8 shown in FIG. 3 leads to the drain electrode 4c.

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

[0021] As shown in FIG. 2, the display device 1 has a structure in which the thin film transistor 4 and the light emitting element 3 are interposed between a pair of substrates 5 and 6. A sealing resin (not shown) is provided in a frame shape between the substrates 5 and 6, and the substrates 5 and 6 are connected via the sealing resin.

[0022] Hereinafter, the structure of the light emitting element 3 will be described. 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.

[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, a light emitting layer (EML) 13 formed on the electron transport layer 14, a hole transport layer (HTL) 12 formed on the light emitting layer 13, and an anode 11 formed on the hole transport layer 12.

[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 the energy level models of the hole transport layer 12, the light emitting layer 13, and the electron transport layer 14, respectively. As shown in FIG. 4B, the 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. On the other hand, the 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. Then, the holes and electrons recombine in the light emitting layer 13, the quantum dots in the light emitting layer 13 are excited, and light can be obtained 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) Although the composition and material of the quantum dots are not limited, for example, the quantum dots in the present embodiment are nanoparticles having a particle size of about several nm to several tens of nm.

[0027] For example, the quantum dots are formed of CdS, CdSe, ZnS, ZnSe, ZnSeS, ZnTe, ZnTeS, InP, (Zn)AgInS2, (Zn)CuInS2, etc. Since Cd is regulated in its use in each country due to its toxicity, it is preferable that the 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 on the surface of the quantum dots 20. Thereby, aggregation of the quantum dots 20 can be suppressed, and the intended optical properties can be exhibited. The ligands that can be used in the reaction are not particularly limited, and for example, the following ligands can be mentioned as representative ones. Aliphatic primary amine-based, 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 acid, 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, lauric (dodecane) acid: C 11 H 23 COOH, decanoic acid: C9H 19 COOH, octanoic acid: C7H 15 COOH Thiol-based, octadecanethiol: C 18 H 37 SH, hexadecanethiol: C 16 H33 SH, Tetradecanethiol: C 14 H 29 SH, Dodecanethiol: C 12 H 25 SH, Decanethiol: C 10 H 21 SH, Octanethiol: C8H 17 SH Phosphine-based, Trioctylphosphine: (C8H 17 )3P, Triphenylphosphine: (C6H5)3P, Tributylphosphine: (C4H9)3P Phosphine oxide-based, Trioctylphosphine oxide: (C8H 17 )3P=O, Triphenylphosphine oxide: (C6H5)3P=O, Tributylphosphine oxide: (C4H9)3P=O

[0029] Also, the quantum dot 20 shown in Fig. 5B has a core-shell structure having a core 20a and a shell 20b coated on the surface of the core 20a. As shown in Fig. 5B, it is preferable that a large number of organic ligands 21 are coordinated on 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, the materials listed above. Although the material of the shell 20b is not limited, it is formed of, for example, zinc sulfide (ZnS) or the like. Similar to the core 20a, the shell 20b preferably does not contain cadmium (Cd).

[0030] Note that the shell 20b may be in a state of being solid-solubilized 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, which indicates that the boundary between the core 20a and the shell 20b may or may not be confirmed by analysis.

[0031] (Light-emitting layer 13) The light-emitting layer 13 may be formed only of the quantum dots listed above, or may contain quantum dots and another fluorescent substance. Further, the light-emitting layer 13 can be formed by applying quantum dots dissolved in a solvent, for example, by an inkjet method, and a solvent component may remain in the light-emitting layer 13 to some extent.

[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. Further, 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. Further, 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] Note that the wavelength of blue light emission is preferably about 450 nm. By adjusting so as not to emit light with a wavelength shorter than 450 nm in this way, the health risk can be suppressed.

[0034] The light-emitting layer 13 can be formed using an existing thin film formation method such as the inkjet method or the vacuum evaporation method listed above.

[0035] (Hole transport layer 12) The hole transport layer 12 is made of an inorganic substance or an organic substance having a function of transporting holes. The hole transport layer 12 is preferably made of an inorganic substance, and is preferably formed of an inorganic oxide such as NiO or WO3, for example. The hole transport layer 12 is particularly preferably formed of NiO nanoparticles. Further, for example, Al2O3 or the like can be mixed with NiO in the hole transport layer 12. Further, Li, Mg, Al, etc. may be doped into the metal oxide. Further, the hole transport layer 12 may be an inorganic substance other than an inorganic oxide.

[0036] Similar to the light-emitting layer 13, the hole transport layer 12 can be formed by a printing method such as an inkjet method, or by an existing thin film technique such as a vacuum evaporation method.

[0037] (Electron transport layer 14) The electron transport layer 14 is made of an inorganic substance or an organic substance having a function of transporting electrons. The electron transport layer 14 is preferably made of an inorganic substance, for example, ZnO X , Ti - O, Sn - O, V - O x , and is preferably formed of inorganic oxides such as Mo - O. Two or more of these can also be selected. The electron transport layer 14 is particularly preferably formed of nanoparticles of ZnO X . Further, the metal oxide may be doped with Li, Mg, Al, Mn, etc. Further, the electron transport layer 14 may be an inorganic substance other than inorganic oxides (for example, CsPbBr3, etc.). X is not limited, but is about 0.8 to 1.2

[0038] Similar to the light - emitting layer 13, the electron transport layer 14 can be formed by a printing method such as an ink - jet method using a solvent containing nanoparticles, or by an existing thin - film technology such as a vacuum evaporation method

[0039] (Anode 11) In this embodiment, the material of the anode 11 is not limited. For example, the anode 11 is preferably formed of a metal such as Au, Ag, a conductive transparent material such as CuISnO2, ZnO X , etc., or a composite oxide of indium - tin (ITO). Among these, the anode 11 is preferably formed of ITO. The anode 11 can be formed as a thin film on the substrate 10 by methods such as evaporation or sputtering of these electrode materials

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

[0041] (Cathode 15) In this embodiment, the material of the cathode 15 is not limited. For example, the cathode 15 can use indium-tin composite oxide (ITO), metal, alloy, electrically conductive compound, and mixtures thereof as electrode materials. For example, the cathode 15 is formed of ITO. Note that the cathode 15 is formed, for example, via a non-permeable metal layer formed on the substrate 10. Thereby, the light-emitting element 3 can be a top emission type.

[0042] The cathode 15 can be formed into a thin film by methods such as vapor deposition or sputtering using these electrode materials.

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

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

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

[0046] In this embodiment, all the layers from the cathode 15 to the anode 11, that is, the cathode 15, the electron transport layer 14, the light-emitting layer 13, the hole transport layer 12, and the anode 11 can all be formed of inorganic layers. By forming all the layers of inorganic layers in this way, film formation can be carried out using the same coating and drying apparatus, etc., and the manufacturing process can be simplified. Further, 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, 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, and the carrier balance can be improved compared with the case of using organic compounds.

[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, and the number of layers can be reduced. That is, a configuration can be adopted in which the transport layer also serves as the injection layer. However, in this embodiment, it is also possible to interpose an inorganic hole injection layer or electron injection layer between each electrode and each transport layer.

[0048] FIG. 6A is a cross-sectional view of a light-emitting element of the second embodiment. In FIG. 6A, on a substrate 10, a cathode 15, an electron transport layer 14, a light-emitting layer 13, a hole transport layer 12, a hole injection layer (HIL) 16, and an anode 11 are laminated in this order. In FIG. 6A, different from FIG. 4A, a hole injection layer 16 is included between the anode 11 and the hole transport layer 12.

[0049] FIG. 6B is a cross-sectional view of a light-emitting element of the third embodiment. In FIG. 6B, on a substrate 10, 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 laminated in this order. In FIG. 6B, different from FIG. 4A, an electron injection layer 18 is included 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 FIGS. 8 and 9 have a core-shell structure, the energy level diagram shown in FIG. 10A is obtained, and the energy level of the shell may become a barrier for the recombination of holes and electrons. Therefore, as shown in FIG. 10B, by using quantum dots whose core surface is not covered with a shell, the energy barrier during the recombination of holes and electrons is eliminated, and holes and electrons can be efficiently recombined, making it possible to improve the light-emitting efficiency. In addition, in order to improve the electron transport efficiency and the hole transport efficiency, as shown in FIG. 5A, it is preferable to coordinate an organic ligand 21 on the surface of the quantum dots 20.

[0063] The light-emitting elements of the embodiments shown in FIGS. 8 and 9 are Conventional EL, and the light-emitting elements of the embodiments shown in FIGS. 4 and 6 have a structure in which Conventional EL is inversely laminated. In the light-emitting elements of the embodiments shown in FIGS. 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 layer between the cathode 15 and the light-emitting layer 13, the light-emitting layer 13, and the layer between the light-emitting layer 13 and the anode 11 can be formed by an inkjet method. As shown in FIG. 11, a mask 30 is disposed on the substrate 10, and an inorganic layer 31 is printed by an inkjet method in a plurality of coating regions 30a which are spaces provided in the mask 30. At this time, the surface of the side wall 30b of the mask 30 is fluorinated, for example, to make the side wall 30b have water repellency. Thereby, the affinity between the side wall 30b surface of the ink can be suppressed, and problems such as the surface of the printed inorganic layer 31 being recessed can be suppressed, and the flatness of the surface of the inorganic layer 31 can be increased.

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

[0066] The display device 1 shown in FIG. 1 is an example, and the arrangement order 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. Also, it is possible to use a display device having only one color light-emitting region or two color light-emitting regions among the red light-emitting region 2a, the green light-emitting region 2b, and the blue light-emitting region 2c.

[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, a curved surface light source or a flexible product can also be realized.

[0068] Also, according to this embodiment, it is possible to develop characteristic products such as illumination having a color mixing property equal to sunlight, which has been difficult to realize so far, eye-friendly illumination, and illumination optimized for a plant factory.

[0069] Thus, in a display device using quantum dots, it is thin and light, can be formed on a curved surface, etc., has a high degree of freedom in arrangement, can emit light over the entire surface, realizes natural light emission that is not dazzling even when viewed directly and is difficult to form shadows. Furthermore, it consumes less power and has a long lifespan. For example, compared with an organic EL display device, the display device using the quantum dots of this embodiment is superior in terms of color rendering property, light-emitting property, product lifespan, and product price.

[0070] As a display device using quantum dots according to this embodiment, it can be used as a PL light emitter in parallel with an EL light emitter. Further, in a display device using quantum dots, a hybrid light-emitting element in which an EL light emitter and a PL light emitter are stacked can be realized. For example, a PL light emitter is overlaid on the surface of the EL light emitter, and the EL light emitter can change the emission wavelength with quantum dots contained in the PL light emitter by the emission from the excited quantum dots. The EL light emitter has the stacked structure of the above-described light-emitting element, and as the PL light emitter, for example, it is a sheet-like wavelength conversion member in which a plurality of quantum dots are dispersed in a resin. Such a hybrid configuration can be realized by using quantum dots.

[0071] Note that in this embodiment, in order to achieve both a large area and a reduction in manufacturing cost of a display device using quantum dots, it is preferable to use an inkjet printing method, a spin coater method, or a dispenser method as the coating method.

Example

[0072] Hereinafter, the effects of the present invention will be described with reference to examples of the present invention. Note that the embodiments of the present invention are not limited by the following examples in any way.

[0073] Each sample shown in Table 1 below was prepared, and the droplet property by inkjet was examined. Note that "Abs10" shown in Table 1 indicates an absorbance of 10% in a state where quantum dots are dispersed, and "Abs20" indicates an absorbance of 20% in a state where quantum dots are dispersed.

[0074]

Table 1

[0075] In the "Droplet" column shown in Table 1, ○ indicates a sample in which droplets were appropriately dropped, and × indicates a sample in which droplet failure occurred.

[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 equipped with the light-emitting device shown in Figure 4A. Top The relationship between shell thickness and external quantum efficiency (EQE) was investigated in emission type displays. [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 element used in the experiment. FIG. 16 is a graph showing the relationship between the current value and the EQE of the EL light-emitting body and the PL light-emitting body using red quantum dots. Further, FIG. 17 is a graph showing the relationship between the current value and the EQE of the EL light-emitting body and the PL light-emitting body using red quantum dots, and also a graph showing the relationship between the current value and the EQE of the EL light-emitting body using blue quantum dots. In Examples 5 and 6 shown in FIG. 16, the shell thickness is different. Example 5 has a thicker shell thickness than Example 6. Also, in FIG. 17, Example 7 has the thickest shell thickness, and the shell thickness decreases in the order of Example 8 and Example 9.

[0088] As shown in FIGS. 16 and 17, in the EL light-emitting body and the PL light-emitting body, an increase in EQE was observed up to about 20 mA. On the other hand, in the red device, an increase in EQE was also observed even when the current value was 20 mA or more. Also, as shown in FIGS. 16 and 17, the thicker the shell thickness, the more the increase in EQE was observed.

[0089] (Synthesis experiment of ZnO X ) FIG. 18 is a graph showing the energy band gap Eg, the energy E of the lower end of the conduction band CB , and the energy E of the upper end of the valence band VB of each layer in the light-emitting element used in the experiment, and an energy level diagram of each layer. ZnO X (Li) was used for L1 or L2 shown in FIG. 18. Here, Li may or may not be doped. Although not limited, X is about 0.8 to 1.2. As shown in FIG. 18, it was found that using ZnO X for the electron injection layer (ETL) and the ZnO used for the electron transport layer X (Li) can widen the band gap. It is presumed that ZnO X (Li) has an effect of reducing the particle diameter. PVK shown in FIG. 18 is a hole injection layer, and B1, B2, G(H), G(I3), R(F) are light-emitting layers (EL layers), and ZnO X , L2, and L4 are electron injection layers. When B1 or B2 is used for the light-emitting layer, ZnO is used for the electron injection layerX Although it can be used, when G(H), G(I3), or R(F) is used in the light-emitting layer, it was found that it is preferable to use L2 or L4 in the electron injection layer. L2 and L4 are ZnO X (Li).

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

[0091] Although not limited, ZnO X (Li) can be produced by stirring a zinc acetate-ethanol solution at a predetermined temperature and time, then mixing and stirring a LiOH·4H2O-ethanol solution, and obtaining it through centrifugation, washing, etc.

[0092] Figures 19 to 21 are the UV (bandgap), PL, and PYS data of ZnO X (Li) and ZnO X (K) applied to the electron injection layer (ETL). ZnO X (K) is produced using KOH catalytically, and neither K nor Li is doped. ZnO X (Li) and ZnO X (K) were both found to have deviations in the UV and PL data. On the other hand, in PYS, ZnO X (Li) and ZnO X (K) showed almost no deviation, and it was found that the rising energy hardly changed.

[0093] Thus, ZnO with its bandgap controlled at various particle sizes as the electron injection / transport layer of an EL device using quantum dots X and doped ZnO with defect control and bandgap control by adding doping species X can be proposed.

[0094] However, when the balance of the recombination of electrons and holes that generate light cannot be achieved, in order to adjust the balance, a thin insulating layer may be interposed between the EL layer and the electron injection layer, or ZnO X is preferably integrated with a molecule to add a function of blocking holes. Here, the integrated layer refers to, for example, the integration of ZnO x and T2T (2,4,6-tris(biphenyl-3-yl)-1,3,5-triazine). Although not limited, X is about 0.8 to 1.2.

[0095] Also, ZnO x is known to have a function that 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. That is, by performing ozone treatment on ZnO x it has been found that the hole transport ability is improved.

Industrial Applicability

[0096] According to the present invention, a light-emitting device 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 on November 8, 2017. The entire content is incorporated herein by reference.

Claims

1. A display device having a display area, wherein the display area has a light-emitting element laminated in the order of an anode formed of ITO, a hole injection layer containing PEDOT:PSS, a hole transport layer containing polyvinylcarbazole, a light-emitting layer, an electron injection layer containing ZnOx(Li) (x is 0.8 to 1.2), and a cathode formed of Al on a TFT, the light-emitting layer is formed of an inorganic layer containing quantum dots, and the light-emitting element is of a top emission type, the quantum dots have a core-shell structure, the shell is formed of ZnS, and the shell thickness is 1.3 nm or more and 2.5 nm or less, the film thickness of the light-emitting layer is uniform at 25 nm or more and 37 nm or less, a voltage is applied between the anode and the cathode, and surface light emission occurs, characterizing the display device.

2. a structure in which the light-emitting element and the thin-film transistor are interposed between a pair of substrates, a sealing resin is provided between the substrates, and the substrates are connected via the sealing resin, characterizing the display device according to Claim 1.

3. The display device according to Claim 1, wherein the thin-film transistor connected to the light-emitting element is an n-ch TFT.

4. The display device according to Claim 3, wherein the oxide semiconductor of the thin-film transistor is formed of an In-Ga-Zn-O-based semiconductor.

5. The display device according to any one of Claims 1 to 4, wherein the display device has flexibility.

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