Patent Publication Number: US-2021184074-A1

Title: Display device

Description:
TECHNICAL FIELD 
     The present invention relates to a display device using quantum dots. 
     BACKGROUND ART 
     JP 2017-045650 A (PTL 1) discloses an invention relating to organic electro-luminescence (EL). 
     An organic EL device has a structure in which an anode, a hole injection layer, a hole transport layer, an emitting layer, an electron transport layer, an electron injection layer, and a cathode are stacked on a substrate. Such an organic EL device is formed from an organic compound and emits light from excitons formed by the recombination of electrons and holes injected into the organic compound. 
     CITATION LIST 
     Patent Literature 
     PTL 1: JP 2017-045650 A 
     SUMMARY OF INVENTION 
     Technical Problem 
     In recent years, light emitting devices using quantum dots are being developed. Quantum dots are nanoparticles made of around several hundreds to several thousands of atoms, each having a particle diameter of around several nanometers to several tens of nanometers. Quantum dots are also referred to as fluorescent nanoparticles, semiconductor nanoparticles, or nanocrystals. The emission wavelength of quantum dots may be variously changed depending on the particle diameter and the composition of the nanoparticles. As with an organic EL device, a light emitting device using quantum dots makes it possible to obtain a thinner device and surface emission. 
     However, the layered structure of bottom emission light emitting devices using quantum dots has not yet been established. 
     The present invention is made in consideration of the above, and seeks to provide a display device having a light emitting device that includes quantum dots. 
     Solution to Problem 
     A display device according to the present invention includes a display area. The display area has a light emitting device in which a first electrode, a layer between the first electrode and an emitting layer, the emitting layer, a layer between the emitting layer and a second electrode, and the second electrode are stacked in this order on a substrate. The emitting layer is formed of an inorganic layer containing quantum dots, and the light emitting device is a bottom emission device. 
     In an aspect of the present invention, all the layers from the first electrode to the second electrode are preferably each formed of an inorganic layer. 
     In another aspect of the present invention, the layer between the first electrode and the emitting layer, the emitting layer, and the layer between the emitting layer and the second electrode are preferably each constituted by the inorganic layer formed from nanoparticles. 
     In yet another aspect of the present invention, the display device is preferably flexible. 
     In yet another aspect of the present invention, the quantum dots preferably have a structure in which a surface of a core is not covered by a shell. 
     In yet another aspect of the present invention, at least one of the layer between the first electrode and the emitting layer, the emitting layer, and the layer between the emitting layer and the second electrode is formed by an inkjet process. 
     In yet another aspect of the present invention, all the layers situated between the first electrode and the second electrode are preferably formed by coating. 
     Advantageous Effects of Invention 
     In a display device of the present invention, the layered structure of light emitting devices containing quantum dots, used in a display device can be optimized. Further, in the present invention, all the layers from the anode to the cathode can be formed of inorganic layers. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       In the accompanying drawings: 
         FIG. 1  is a partial plan view of a display device according to one embodiment; 
         FIG. 2  is a partial enlarged cross-sectional view illustrating one of display areas of the display device depicted in  FIG. 1  that is enlarged; 
         FIG. 3  is a cross-sectional view illustrating the structure of a thin film transistor different from one in  FIG. 2 ; 
         FIG. 4A  is a cross-sectional view of a light emitting device of Embodiment 1, and  FIG. 4B  is an energy level diagram of each layer in a display device of Embodiment 1; 
         FIG. 5  is a schematic view of a quantum dot according to one embodiment; 
         FIGS. 6A to 6C  are cross-sectional views each illustrating a light emitting device according to an embodiment different from that in  FIG. 1 ; 
         FIG. 7A  is an energy level diagram in the case of using quantum dots having a core-shell structure, and  FIG. 7B  is an energy level diagram in the case of using quantum dots having a structure in which a core is not covered by a shell; 
         FIG. 8A  is a cross-sectional view of a light emitting device different from one in  FIG. 4 , and  FIG. 8B  is an energy level diagram of each layer in the light emitting device of  FIG. 8A ; 
         FIGS. 9A to 9C  are cross-sectional views each illustrating a light emitting device according to an embodiment different from that in  FIG. 8 ; 
         FIG. 10A  is an energy level diagram in the case of using quantum dots having a core-shell structure, and  FIG. 10B  is an energy level diagram in the case of using quantum dots having a structure in which a core is not covered by a shell; 
         FIG. 11  is a schematic view illustrating a step of forming an inorganic layer by the inkjet process; 
         FIG. 12  is a photograph showing an application in Examples; 
         FIG. 13  shows PYS measurement data of Cd-based green quantum dots; 
         FIG. 14  shows PYS measurement data; 
         FIG. 15  is an energy level diagram of each layer in the light emitting device used in an experiment; 
         FIG. 16  is a graph illustrating the relationship between the current value and the EQE of an EL emitter and a PL emitter using green quantum dots; 
         FIG. 17  shows plots illustrating the relationship between the current value and the EQE of an EL emitter and a PL emitter using red quantum dots; and a plot illustrating the relationship between the current value and the EQE of an EL emitter using blue quantum dots; 
         FIG. 18  presents a graph illustrating the energy band gap Eg of each layer in the light emitting device used in an experiment, the energy E CB  at the bottom of the conduction band, and the energy E VB  at the top of the conduction band, and an energy level diagram of the layers; 
         FIG. 19  shows the UV data of ZnO X (Li) and ZnO X (K) used in an electron transport layer (ETL); 
         FIG. 20  shows the PL data of ZnO X (Li) and ZnO X (K) used in an electron transport layer (ETL); 
         FIG. 21  shows the PYS data of ZnO X (Li) and ZnO X (K) used in an electron transport layer (ETL); and 
         FIG. 22A  and  FIG. 22B  are schematic views illustrating the structures for improving the outcoupling efficiency of bottom emission devices. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments of the present invention (hereinafter simply referred to as “embodiments”) will now be described in detail. Note that the present invention is not limited to the following embodiments, and various modifications may be made without departing from the spirit of the present invention. 
     As illustrated in  FIG. 1 , a plurality of display areas  2  are arranged in a matrix in the display device  1 . The display areas  2  fall into three types: red emission regions  2   a  emitting red light, green emission regions  2   b  emitting green light, and blue emission regions  2   c  emitting blue light. These three emission regions  2   a ,  2   b , and  2   c  are for example arranged in a row direction to form a set constituting one pixel for color display. 
     In each of the emission regions  2   a ,  2   b , and  2   c , a light emitting device  3  is formed. The layer structure of the light emitting device  3  will be described below. A thin film transistor (TFT)  4  is connected to each light emitting device  3 . The light emitting devices  3  are bottom emission devices. 
     As illustrated in  FIG. 2 , the thin film transistor  4  is built in such a manner that a gate electrode  4   a , a channel layer  4   b , a gate insulating film (not shown), a drain electrode  4   c , a source electrode  4   d , etc. are stacked on a substrate  5 . The channel layer  4   b  is formed from a P-type semiconductor, for example, amorphous silicon; however, the material of the layer is not limited to this. The thin film transistor  4  depicted in  FIG. 2  has a top-contact bottom-gate configuration; alternatively, the thin film transistor  4  may have a bottom-contact bottom-gate configuration. 
     The source electrode  4   d  is connected to a power supply line, and the drain electrode  4   c  is connected to the light emitting device  3 . 
     Further, the thin film transistor  4  may have a top-gate configuration depicted in  FIG. 3 . As illustrated in  FIG. 3 , the channel layer  4   b  is formed on the substrate  5 , and the surface of the channel layer  4   b  is covered with the gate insulating film  4   e . The gate electrode  4   a  is formed on the surface of the gate insulating film  4   e . As illustrated in  FIG. 3 , the surface of the gate electrode  4   a  is covered with an insulating film  4   f . Further, a plurality of through holes extending through the gate insulating film  4   e  and the insulating film  4   f  to reach the channel layer  4   b , and the drain electrode  4   c  and the source electrode  4   d  are formed in the respective through holes. Further, the surfaces of the drain electrode  4   c  and the source electrode  4   d  are covered with a protective film  7 . Further, a transparent electrode connected to the drain electrode  4   c  and the source electrode  4   d  is formed on the surface of the protective film  7 . The transparent electrode  8  depicted in  FIG. 3  is connected to the drain electrode  4   c.    
     The channel layer  4   b  of the thin film transistor  4  shown in  FIG. 3  is formed from, for example, P—Si. 
     As illustrated in  FIG. 2 , the display device  1  has a structure in which the thin film transistor  4  and the light emitting device  3  are interposed between a pair of substrates  5  and  6 , and, although not shown, sealing resin is provided in a frame pattern between the substrates  5  and  6  so that the substrates  5  and  6  are connected with the sealing resin therebetween. 
     Hereinafter, the structure of the light emitting device  3  will be described.  FIG. 4A  is a cross-sectional view of a light emitting device of Embodiment 1, and  FIG. 4B  is an energy level diagram of each layer in a display device of Embodiment 1. 
     As shown in  FIG. 4A , the light emitting device  3  is configured to have a substrate  10 , an anode  11  formed on the substrate, a hole transport layer (HTL)  12  formed on the anode  11 , an emitting layer (EML)  13  formed on the hole transport layer  12 , an electron transport layer (ETL)  14  formed on the emitting layer  13 , and a cathode  15  formed on the electron transport layer  14 . In this embodiment, the anode  11  is built as a first electrode, and the cathode  15  is built as a second electrode. 
     When a voltage is applied to between the electrodes of the light emitting device  3  in 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 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  into the HOMO level of the emitting layer  13 . On the other hand, electrons transported through the electron transport layer  14  are injected from the LUMO level of the electron transport layer  14  into the LUMO level of the emitting layer  13 . The holes and electrons are recombined in the emitting layer  13 , which promotes quantum dots in the emitting layer  13  to the excited state, thus light emission from the excited quantum dots can be achieved. 
     In this embodiment, the emitting layer  13  is formed of an inorganic layer containing quantum dots. 
     (Quantum Dot) 
     For example, quantum dots in this embodiment are nanoparticles having a particle diameter of around several nanometers to several tens of nanometers; however, the structure and the material of the quantum dots are not limited to those. 
     For example, quantum dots are formed from CdS, CdSe, ZnS, ZnSe, ZnSeS, ZnTe, ZnTeS, InP, (Zn)AgInS 2 , (Zn)CuInS 2 , etc. Because of the toxicity of Cd, the use of Cd is restricted in many countries; thus, quantum dots are preferably free of Cd. 
     As shown in  FIG. 5A , many organic ligands  21  are preferably placed on the surface of a quantum dot  20 . This can inhibit aggregation of quantum dots  20 , resulting in the target optical properties. The ligands available for the reaction are not particularly limited; for example, the following ligands can be given as typical examples. Aliphatic primary amines: 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 , Aliphatic 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, lauric (dodecanoic) acid: C 11 H 23 COOH, decanoic acid: C 9 H 19 COOH, octanoic acid: C 7 H 15 COOH Thiols: octadecanethiol: C 18 H 37 SH, hexadecanethiol: C 16 H 33 SH, tetradecanethiol: C 14 H 29 SH, dodecanethiol: C 12 H 25 SH, decanethiol: C 10 H 21 SH, octanethiol: C 8 H 17 SH Phosphines: trioctylphosphine: (C 8 H 17 ) 3 P, triphenylphosphine: (C 6 H 5 ) 3 P, tributylphosphine: (C 4 H 9 ) 3 P Phosphine oxides: 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 
     A quantum dot  20  depicted in  FIG. 5B  has a core-shell structure having a core  20   a  and a shell  20   b  covering the surface of the core  20   a . As shown in  FIG. 5B , many organic ligands  21  are preferably placed on the surface of the quantum dot  20 . The core  20   a  of the quantum dot  20  shown in  FIG. 5B  is the nanoparticle shown in  FIG. 5A . Accordingly, the core  20   a  is formed for example from the materials listed above. The shell  20   b  is formed from, for example, zinc sulfide (ZnS); however, the material of the shell  20   b  is not limited to this. As with the core  20   a , the shell  20   b  is preferably free of cadmium (Cd). 
     The shell  20   b  may be in a condition of being a solid solution on the surface of the core  20   a . In  FIG. 5B , the boundary between the core  20   a  and the shell  20   b  is indicated by a dotted line, and this means that the boundary between the core  20   a  and the shell  20   b  may or may not be identified by an analysis. 
     (Emitting layer  13 ) 
     The emitting layer  13  may be formed from the above-mentioned quantum dots alone; alternatively, the emitting layer  13  may contain the quantum dots and another luminescent material other than the quantum dots. Further, the emitting layer  13  may be formed by applying quantum dots dissolved in a solvent, for example, by the inkjet process. Here, a slight amount of the solvent component may be left in the emitting layer  13 . 
     Red quantum dots emitting red light are contained in the emitting layer  13  of the light emitting devices  3  formed in the red emission regions  2   a  depicted in  FIG. 1 . Further, green quantum dots emitting green light are contained in the emitting layer  13  in the light emitting devices  3  formed in the green emission regions  2   b  depicted in  FIG. 1 . Further, blue quantum dots emitting blue light are contained in the emitting layer  13  in the light emitting devices  3  formed in the blue emission regions  2   c  depicted in  FIG. 1 . 
     Note that the wavelength of blue emission is preferably around 450 nm. Thus, health risks can be reduced by adjustments such that light of a wavelength shorter than 450 nm is not emitted can. 
     The emitting layer  13  can be formed by an existing thin film formation method such as the inkjet process and vacuum deposition mentioned above. 
     (Hole Transport Layer  12 ) 
     The hole transport layer  12  is made from an inorganic material or an organic material having hole transporting functions. The hole transport layer  12  is preferably made from an inorganic material, for example, is preferably formed from an inorganic oxide such as NiO or WO 3 . In particular, the hole transport layer  12  is preferably formed from nanoparticles of NiO. Further, for use in the hole transport layer  12 , for example, Al 2 O 3  or the like may be mixed in NiO. And, a metal oxide may be doped with Li, Mg, Al, etc. Further, the hole transport layer  12  may be of an inorganic material other than inorganic oxides. 
     As with the emitting layer  13 , the hole transport layer  12  can be formed by a printing process such as the inkjet process, or may be formed by an existing thin film technique such as vacuum deposition. 
     (Electron Transport Layer  14 ) 
     The electron transport layer  14  is made from an inorganic material or an organic material having electron transporting functions. The electron transport layer  14  is preferably made from an inorganic material, for example, is preferably formed from an inorganic oxide such as ZnO X , Ti—O, Zn—O, Sn—O, V—O, or Mo—O. Two or more of these materials may be selected as materials. In particular, the electron transport layer  14  is preferably formed from nanoparticles of ZnO X . And, a metal oxide may be doped with Li, Mg, Al, Mn, etc. Further, the electron transport layer  14  may be made of an inorganic material (for example, CsPbBr 3  etc.) other than inorganic oxides. X is, but not limited to, around 0.8 to 1.2. 
     As with the emitting layer  13 , the electron transport layer  14  can be formed by applying a solvent containing nanoparticles by a printing process such as the inkjet process, or may be formed by an existing thin film technique such as vacuum deposition. 
     (Anode  11 ) 
     In this embodiment, the material for forming the anode  11 , is preferably for example, but not limited to, a compound oxide of indium-tin (ITO), a metal such as Au, a conductive transparent material such as CuISnO 2 , or ZnO X . Of those, the anode  11  is preferably formed from ITO. The anode  11  can be formed as a thin film of the electrode material on the substrate  10  by a method such as vapor deposition or sputtering. 
     In this embodiment, since a structure in which light is given off from the substrate  10  side is used, the anode  11  needs to be a transparent electrode, and is preferably one of the metal oxides mentioned above or an extremely thin metal film. 
     (Cathode  15 ) 
     In this embodiment, for example, a metal, an alloy, an electrically conductive compound, and a mixture of those can be used as an electrode material of the cathode  15 ; however, the material of the cathode  15  is not limited to those. Examples of the electrode material include Al, Mg, Li, and mixtures of these. Of those, the cathode  15  is preferably formed from Al. 
     The cathode  15  can be formed as a thin film of the electrode material by a method such as vapor deposition or sputtering. 
     (Substrate  10 ) 
     In this embodiment, the substrate  10  can be formed of, for example, glass or plastic; however, the material of the substrate  10  is not limited to these. In this embodiment, since a structure in which light is given off from the substrate  10  side (thin film transistor side) is used, the substrate  10  is preferably a transparent substrate. Examples of the transparent substrate include, for example, glass, quartz, and transparent resin films. 
     The substrate  10  may either be a rigid substrate or a flexible substrate; when a flexible substrate is used, a flexible device can be obtained. Examples of the material of the transparent film include, for example, polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polyester, polyethylene, polypropylene, cellophane, cellulose diacetate, and cellulose triacetate (TAC). 
     In the display device  1  in  FIG. 2 , using flexible substrates for the substrates  5  and  6 , the display device  1  can be flexible. Note that the substrates  5  and  6  can also be formed using a similar material to that of the substrate  10 . The substrate  5  can also serve as the substrate  10 . 
     In this embodiment, all the layers from the anode  11  to the cathode  15 , that is, all the anode  11 , the hole transport layer  12 , the emitting layer  13 , the electron transport layer  14 , and the cathode  15  can each be formed of an inorganic layer. Forming all the layers from inorganic layers allow all the layers to be formed using the same coating/drying apparatuses, etc. and facilitates the production process. Further, the high-low relationships of the HOMO levels of the anode  11 , the hole transport layer  12 , and the emitting layer  13  can be optimized; the high-low relationships of the LUMO levels of the cathode  15 , the electron transport layer  14 , and the emitting layer  13  can be optimized. This improves the carrier balance as compared with the case of using organic compounds. 
     In Embodiment 1 illustrated in  FIG. 4 , a hole injection layer and an electron injection layer are not formed separately from the transport layers, thus the number of the layers can be reduced. Namely, the transport layers also serve as injection layers. Note however that in this embodiment, a hole injection layer and an electron injection layer made of an inorganic material may be interposed between the electrodes and the transport layers. 
       FIG. 6A  is a cross-sectional view of a light emitting device of Embodiment 2. In  FIG. 6A , the anode  11 , a hole injection layer (HIL)  16 , the hole transport layer  12 , the emitting layer  13 , the electron transport layer  14 , and the cathode  15  are stacked in this order on the substrate  10 . Unlike in  FIG. 4A , in  FIG. 6A , the hole injection layer  16  is placed between the anode  11  and the hole transport layer  12 . 
       FIG. 6B  is a cross-sectional view of a light emitting device of Embodiment 3. In  FIG. 6B , the anode  11 , the hole transport layer  12 , the emitting layer  13 , the electron transport layer  14 , an electron injection layer (EIL)  18 , and the cathode  15  are stacked in this order on the substrate  10 . Unlike in  FIG. 4A , in  FIG. 6B , the electron injection layer  18  is placed between the electron transport layer  14  and the cathode  15 . 
       FIG. 6C  is a cross-sectional view of a light emitting device of Embodiment 4. In  FIG. 6C , the anode  11 , the hole injection layer  16 , the hole transport layer  12 , the emitting layer  13 , the electron transport layer  14 , the electron injection layer  18 , and the cathode  15  are stacked in this order on the substrate  10 . Unlike in  FIG. 4A , in  FIG. 6C , the hole injection layer  16  is placed between the anode  11  and the hole transport layer  12 , and in addition, the electron injection layer  18  is placed between the electron transport layer  14  and the cathode  15 . 
     The material of the hole injection layer  16  and the electron injection layer  18  may be either an inorganic material or an organic material. Forming the hole injection layer  16  and the electron injection layer  18  from inorganic layers is preferred because all the layers from the anode  11  to the cathode  15  can each be formed of an inorganic layer. However, the material of the injection layers is not limited to this. The material of the hole injection layer  16  and the electron injection layer  18  is selected from various kinds of materials depending on the energy level models. 
     In this embodiment, the layer between the anode  11  and the emitting layer  13  is preferably a layer serving as the hole transport layer  12 , the hole injection layer  16 , or 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. 
     In this embodiment, the layer between the cathode  15  and the light emitting layer  13  is preferably a layer serving as the electron transport layer  14 , the electron injection layer  18 , or 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. 
     Note that in the structure of the display device  1  depicted in  FIG. 2 , the thin film transistor  4  has, for example, a bottom-gate configuration, and the drain electrode  4   c  is connected to the anode  11  of the light emitting device  3 . Here, the drain electrode  4   c  can be made to serve as the anode  11  without forming the anode  11  superposed on the drain electrode  4   c . This allows the light emitting device  3  to be suitably connected with the thin film transistor  4  and the ground line. 
     In this embodiment, the hole transport layer  12 , the emitting layer  13 , and the electron transport layer  14  can all be inorganic layers formed from nanoparticles. In such a case, each layer can be formed by printing by the inkjet process or the like, thus the layers can be formed easily and formed to be uniform in thickness. This can effectively improve emission efficiency. 
     When the quantum dots used in the emitting layer  13  of this embodiment have a core-shell structure, the energy level diagram presented in  FIG. 7A  is obtained, and the energy level of the shell would be as a barrier to the recombination of holes and electrons. Accordingly, a quantum dot of which surface is not covered with a shell (the surface of the core is exposed, or the material forming the quantum dot is uniform from the center of the quantum dot to the surface thereof) as shown in  FIG. 7B  is preferably used. Using such quantum dots eliminates the energy barrier to the recombination of hole and electrons, and allows holes and electrons to be efficiently recombined, thus the light emission efficiency can be improved. With a view to improving the electron transportation efficiency and the hole transportation efficiency, the organic ligands  21  are preferably placed on the surface of each quantum dot  20  as illustrated in  FIG. 5A . 
     Further, in this embodiment, as illustrated in  FIG. 8A , the light emitting device  3  may be configured to have the substrate  10 , the cathode  15  formed on the substrate, the electron transport layer (ETL)  14  formed on the cathode  15 , the emitting layer (EML)  13  formed on the electron transport layer  14 , the hole transport layer (HTL)  12  formed on the light emitting layer  13 , and the anode  11  formed on the hole transport layer  12 . 
     The materials of the layers are as described above. Note that in  FIG. 8A , the anode  11  forms a second electrode, and the cathode  15  forms a first electrode. Since the light emitting device  3  of this embodiment is a bottom emission device, the cathode  15  is preferably formed as a transparent electrode of ITO etc., and the anode  11  is preferably made of an opaque material such as Al that can reflect light. This allows light to be reflected at the anode  11  and light to be given off from the cathode  15  side (thin film transistor side). 
       FIG. 9A  is a cross-sectional view of a light emitting device according to a different embodiment from  FIG. 8A . In  FIG. 9A , the cathode  15 , the electron transport layer  14 , the emitting layer  13 , the hole transport layer  12 , the hole injection layer (HIL)  16 , and the anode  11  are stacked in this order on the substrate  10 . Unlike in  FIG. 8A , in  FIG. 9A , the hole injection layer  16  is placed between the anode  11  and the hole transport layer  12 . 
       FIG. 9B  is a cross-sectional view of a light emitting device according to a different embodiment from  FIG. 8A . In  FIG. 9B , the cathode  15 , the electron injection layer (EIL)  18 , the electron transport layer  14 , the emitting layer  13 , the hole transport layer  12 , and the anode  11  are stacked in this order on the substrate  10 . Unlike in  FIG. 8A , in FIG.  9 B, the electron injection layer  18  is placed between the electron transport layer  14  and the cathode  15 . 
       FIG. 9C  is a cross-sectional view of a light emitting device according to a different embodiment from  FIG. 8A . In  FIG. 9C , the cathode  15 , the electron injection layer  18 , the electron transport layer  14 , the emitting layer  13 , the hole transport layer  12 , the hole injection layer  16 , and the anode  11  are stacked in this order on the substrate  10 . Unlike in  FIG. 8A , in  FIG. 9C , the hole injection layer  16  is placed between the anode  11  and the hole transport layer  12 , and in addition, the electron injection layer  18  is placed between the electron transport layer  14  and the cathode  15 . 
     When the quantum dots used in the emitting layer  13  of this embodiment in  FIG. 8A  to  FIG. 9C  have a core-shell structure, the energy level diagram presented in  FIG. 10A  is obtained, and the energy level of the shell would be as a barrier to the recombination of holes and electrons. Accordingly, using quantum dots in which the surface of the core is not covered with a shell as depicted in  FIG. 10B  eliminates the energy barrier to the recombination of hole and electrons, and allows holes and electrons to be efficiently recombined, thus the light emission efficiency can be improved. With a view to improving the electron transportation efficiency and the hole transportation efficiency, the organic ligands  21  are preferably placed on the surface of each quantum dot  20  as illustrated in  FIG. 5A . 
     The light emitting devices of the embodiments shown in  FIG. 8A  to  FIG. 9C  are inverted EL devices having a layered structure opposite to ones in  FIGS. 4A and 4B  and  FIGS. 6A to 6C . The thin film transistor is preferably an n-ch TFT, and an In—Ga—Zn—O-based semiconductor can be preferably used in the channel layer. Alternatively. Poly-Si can also be preferably used. 
     As described above, since the light emitting device  3  of this embodiment is a bottom emission device, the first electrode on the substrate  10  side is a transparent electrode, and the second electrode away from the substrate  10  is preferably formed from an opaque material (preferably a metal) that is excellent in the light reflectivity. 
     In this embodiment, at least one of the layer between the anode  11  and the emitting layer  13 , the emitting layer  13 , and the layer between the emitting layer  13  and the cathode  15  can be formed by the inkjet process. As illustrated in  FIG. 11 , a mask  30  is placed on the substrate  10 , and an inorganic layer  31  is printed in a plurality of application regions  30   a  provided in the mask  30  by the inkjet process. Here, the surfaces of the sidewalls  30   b  of the rises of the mask  30  are, for example, subjected to fluorination to impart water repellency to the sidewalls  30   b . This can reduce the affinity of the surface of the sidewalls  30   b  for ink and prevents defects such as dents in the surface of the printed inorganic layer  31 , and thus can increase the flatness of the surface of the inorganic layer  31 . 
     This embodiment involves bottom emission devices, and can improve the carrier balance in the conventional EL light emitting devices  3  depicted in  FIGS. 4A and 4B  and  FIGS. 6A to 6C . Besides, all the layers situated between the anode  11  and the cathode  15  can be formed by coating (application). Specifically, in the structures in  FIG. 4A  and  FIG. 8A , the hole transport layer  12 , the emitting layer  13 , and the electron transport layer  14  can all be formed by coating. Further, in the structures in  FIG. 6A  and  FIG. 9A , the hole injection layer  16 , the hole transport layer  12 , the emitting layer  13 , and the electron transport layer  14  can all be formed by coating. Further, in the structures in  FIG. 6B  and  FIG. 9B , the hole transport layer  12 , the emitting layer  13 , the electron transport layer  14 , and the electron injection layer  18  can all be formed by coating. This can facilitate the production process of the light emitting devices. 
     The display device  1  depicted in  FIG. 1  is an example, and the arrangement of the red emission regions  2   a , the green emission regions  2   b , and the blue emission regions  2   c  may be different from that in  FIG. 1 . Further, of the red emission regions  2   a , the green emission regions  2   b , and the blue emission regions  2   c , only the emission regions of one color or the emission regions of two colors may be included in the display device. 
     As in this embodiment, in a display device using quantum dots, the quantum dots can be used to build either a point light source or a surface light source, and a curved light source or a flexible product may also be obtained by selecting a suitable substrate. 
     Further, according to this embodiment, distinctive products such as lightings producing a mixture of colors comparable to that of sunlight which has been hardly obtained, lightings producing light easy on the eyes, and lightings optimized for plant factories can be developed. 
     Thus, display devices using quantum dots provide a high degree of flexibility in the design; for example, the devices can be formed to be thin, lightweight, and curved. Further, the devices can produce natural light not dazzling in the eyes that produces less shadows. In addition, the devices consume less power and have a long life. For example, display devices using quantum dots of this embodiment are superior to organic EL display devices in terms of color rendering properties, emission properties, product life, and product price. 
     A display device using quantum dots of this embodiment can be used as a PL emitter as well as an EL emitter. Further, for a display device using quantum dots, a hybrid light emitting device in which an EL emitter and a PL emitter are stacked can be obtained. For example, a PL emitter is superposed on a surface of an EL emitter, and the emission wavelength of the light emitted by excited quantum dots in the EL emitter can be changed using the quantum dots contained in the PL emitter. The EL emitter is a light emitting device having a layered structure described above, and the PL emitter is, for example, a sheet-like wavelength converting member in which a plurality of quantum dots are dispersed in a resin. Such a hybrid structure can be obtained with the use of quantum dots. 
     Note that in this embodiment, in order to both increase the area of the display device using quantum dots and reduce the production cost, the inkjet printing process, the spin coating process, or the dispensing process is preferably used as a method for applying the quantum dots. 
     EXAMPLES 
     The effects of the present invention will be described using Examples of the present invention. Note that the embodiments of the present invention are not limited to the following examples in any way. 
     The samples shown in Table 1 below were prepared to investigate the drop characteristics in the inkjet process. Note that in Table 1, “Abs10” refers to a sample exhibiting an absorbance of 10% with the quantum dots being dispersed, and “Abs20” refers to a sample exhibiting an absorbance of 20% with the quantum dots being dispersed. 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
             
            
               
                   
                   
                   
                 Viscosity 
                 R.T. 
                 Ejection 
                 Mass 
                   
               
               
                 Sample 
                 Solvent 
                 SG 
                 (mPa · s) 
                 (° C.) 
                 number 
                 (g) 
                 Mass/drop 
               
               
                   
               
               
                 Green QD Abs10 
                 Cyclododecene 
                 0.87 
                   
                 23.5 
                 10 million 
                 0.0407 
                 4.07E−09 
               
               
                 Red QD Abs20 
                 Cyclododecene 
                 0.87 
                   
                 25.6 
                 10 million 
                 0.0397 
                 3.97E−09 
               
               
                 Red QD Abs20 
                 Tetradecane 
                 0.77 
                   
                 26.1 
                 10 million 
                 0.0303 
                 3.03E−09 
               
               
                 Green QD Abs10 
                 Tetradecane 
                   
                   
                   
                 10 million 
                 Cancelled 
               
               
                 Green QD Abs10 
                 Octadecene 
                 0.79 
                 3.12 
                 25.0 
                 10 million 
                 0.0447 
                 4.47E−09 
               
               
                 Red QD Abs20 
                 Octadecene 
                 0.79 
                 2.97 
                 26.7 
                 10 million 
                 0.0449 
                 4.49E−09 
               
               
                 Polyvinylcarbazole 
                 Dimethoxybenzene 
                 1.08 
                   
                 26.7 
                 10 million 
                 0.0579 
                 5.79E−09 
               
               
                 Polyvinylcarbazole 
                 Dimethoxybenzene:Cyclo- 
               
               
                   
                 hexylbenzene 1:1 
               
               
                 Polyvinylcarbazole 
                 Dimethoxybenzene:Cyclo- 
                 1.13 
                   
                 26.5 
                 10 million 
                 0.0527 
                 5.27E−09 
               
               
                   
                 hexylbenzene 2:1 
               
               
                 Zinc oxide 
                 IPA:Propylene 
               
               
                 nanoparticles 
                 glycol 1:1 
               
               
                 Solvent only 
                 Tetradecane 
                 0.77 
                 1.60 
                 23.8 
                  1 million 
                 0.0040 
                 4.00E−09 
               
               
                   
                 Tetradecane 
                   
                   
                   
                  3 million 
                 0.0126 
                 4.20E−09 
               
               
                   
                 Tetradecane 
                   
                   
                   
                   
                 0.0109 
                 3.63E−09 
               
               
                   
                 Tetradecane 
                   
                   
                   
                 10 million 
                 0.0362 
                 3.62E−09 
               
               
                 Solvent only 
                 ODE(1-Octadecene) 
                 0.79 
                 3.10 
                 23.0 
                 10 million 
                 0.0385 
                 3.85E−09 
               
               
                 Solvent only 
                 Decahydronaphthalene 
                 0.88 
                 2.20 
                 23.0 
                 10 million 
                 0.0430 
                 4.30E−09 
               
               
                 Solvent only 
                 Cyclododecene 
                 0.87 
                 4.20 
                 23.0 
                 10 million 
                 0.0514 
                 5.14E−09 
               
               
                 Solvent only 
                 Ethylene glycol 
                 1.10 
                 20.50 
                 21.7 
                 Failed 
                 — 
                 — 
               
               
                 Solvent only 
                 Ethylene glycol 
                 0.94 
                 7.40 
                 25.0 
                 10 million 
                 0.0500 
                 5.00E−09 
               
               
                   
                 1:IPA 1 
               
               
                 Solvent only 
                 Ethylene glycol 
                 1.02 
                 12.80 
                 22.7 
                 10 million 
                 0.0481 
                 4.81E−09 
               
               
                   
                 3:IPA 1 
               
               
                 Solvent only 
                 Ethylene glycol 
                 0.97 
                 10.30 
                 22.3 
                 10 million 
                 0.0500 
                 5.00E−09 
               
               
                   
                 3:IPA 2 
               
               
                 Solvent only 
                 Phenylcyclohexane 
                 0.94 
                 2.29 
                 26.3 
                 10 million 
                 0.0444 
                 4.44E−09 
               
               
                 Solvent only 
                 Dichlorobenzene 
                 1.30 
                 1.62 
                 25.5 
                 Failed 
                 — 
                 — 
               
               
                 Solvent only 
                 n-Octane 
                 0.70 
                 Unmeasurable 
                 24.3 
                 Failed 
                 — 
                 — 
               
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                 Volume/ 
                   
                 Adverse 
                   
               
               
                   
                   
                 drop 
                   
                 effect on 
               
               
                 Sample 
                 Solvent 
                 (pL) 
                 Drop 
                 EPDM 
                 Notes 
               
               
                   
               
               
                 Green QD Abs10 
                 Cyclododecene 
                 4.68 
                 + 
               
               
                 Red QD Abs20 
                 Cyclododecene 
                 4.56 
                 + 
               
               
                 Red QD Abs20 
                 Tetradecane 
                 3.94 
                 + 
                 Cap deformation 
                 Head clogged 
               
               
                 Green QD Abs10 
                 Tetradecane 
                   
                 + 
               
               
                 Green QD Abs10 
                 Octadecene 
                 5.66 
                 + 
               
               
                 Red QD Abs20 
                 Octadecene 
                 5.68 
                 + 
                   
                 Previous head 
               
               
                   
                   
                   
                   
                   
                 replacement 
               
               
                 Polyvinylcarbazole 
                 Dimethoxybenzene 
                 5.34 
                 + 
                 Head replacement 
                 Clogging after 
               
               
                   
                   
                   
                   
                 twice 
                 ethanol cleaning/ 
               
               
                   
                   
                   
                   
                   
                 Clogging of damper 
               
               
                   
                   
                   
                   
                   
                 filter 
               
               
                 Polyvinylcarbazole 
                 Dimethoxybenzene:Cyclo- 
                   
                 − 
                 Malformation of drops 
               
               
                   
                 hexylbenzene 1:1 
                   
                   
                 (2 drops) 
               
               
                 Polyvinylcarbazole 
                 Dimethoxybenzene:Cyclo- 
                 4.66 
                 + 
               
               
                   
                 hexylbenzene 2:1 
               
               
                 Zinc oxide 
                 IPA:Propylene 
                   
                 − 
                 Defective drops 
               
               
                 nanoparticles 
                 glycol 1:1 
                   
                   
                 Possibly high viscosity 
               
               
                 Solvent only 
                 Tetradecane 
                 4.60 
                 + 
               
               
                   
                 Tetradecane 
                 4.83 
                 + 
               
               
                   
                 Tetradecane 
                 4.18 
                 + 
               
               
                   
                 Tetradecane 
                 4.16 
                 + 
               
               
                 Solvent only 
                 ODE(1-Octadecene) 
                 4.87 
                 + 
                 Cap deformation 
               
               
                 Solvent only 
                 Decahydronaphthalene 
                 4.89 
                 + 
                 Affected 
               
               
                 Solvent only 
                 Cyclododecene 
                 5.91 
                 + 
               
               
                 Solvent only 
                 Ethylene glycol 
                 — 
                 − 
                 Not affected 
               
               
                 Solvent only 
                 Ethylene glycol 
                 5.32 
                 + 
                 Not affected 
                 IPA specific 
               
               
                   
                 1:IPA 1 
                   
                   
                   
                 gravity: 0.78 
               
               
                 Solvent only 
                 Ethylene glycol 
                 4.72 
                 + 
                 Not affected 
               
               
                   
                 3:IPA 1 
               
               
                 Solvent only 
                 Ethylene glycol 
                 5.15 
                 + 
                 Not affected 
               
               
                   
                 3:IPA 2 
               
               
                 Solvent only 
                 Phenylcyclohexane 
                 4.72 
                 + 
               
               
                 Solvent only 
                 Dichlorobenzene 
                 — 
                 − 
                 Affected 
               
               
                 Solvent only 
                 n-Octane 
                 — 
                 − 
                 Cap deformation 
               
               
                   
               
            
           
         
       
     
     The “+” signs shown in the “drop” column in Table 1 correspond to the samples that had been appropriately dropped and the “−” signs correspond to the samples that were failed to be appropriately dropped. 
     In Table 1, the samples of “Red QD” and “Green QD” are used in emitting layers. Further, the samples of “polyvinylcarbazole” are used in hole injection layers. The sample of “Zinc oxide nanoparticles” is used in an electron transport layer or an electron injection layer. 
     Table 1 shows that IPA and propylene glycol were not preferred as the solvent for zinc oxide nanoparticles, and another solvent had to be used. The solvents corresponding to the “+” signs in the “drop” column shown in Table 1 can be appropriately used; however, hydrophilic solvents are preferred. For example, as a hydrophilic solvent, an alcohol-based solvent can be used. 
       FIG. 12  is a photograph showing a state where the application was performed by the inkjet process using ZnO X  dissolved in ethoxyethanol:EG=7:3 as a solvent. As shown in  FIG. 12 , a good application state was achieved. 
     Further, adverse effects on ethylene propylene diene monomer rubber (EPDM) inside an inkjet head was also investigated. As shown in Table 1, in some samples, cap deformation occurred or EPDM was adversely affected. This demonstrated that the effect on EPDM was preferably considered in the case of using EPDM. 
     (Experiment on Shell Thickness Dependence of Quantum Dot) 
     In this experiment, quantum dots (green QDs) of the samples shown in Table 2 were prepared. For a bottom emission display device including the light emitting device using the quantum dots in  FIG. 4A , the relationship between the shell thickness and the external quantum efficiency (EQE) was investigated. 
     
       
         
           
               
               
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                   
                   
                   
                 Shell total 
                   
                   
                   
                   
                 QD layer 
               
               
                 No. 
                 Core 
                 Coating 1 
                 Coaring 2 
                 Coating 3 
                 (mm) 
                 CV (%) 
                 EQE (%) 
                 QY (%) 
                 FWHM 
                 thickness 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 1 
                 GC_A 
                 I_A 
                 II_A 
                 III_A Thick 1 
                 1.8 
                 17.2 
                 — 
                 86 
                 38 
                 — 
               
               
                 2 
                 GC_A 
                 I_A 
                 II_A 
                 III_A Thick 2 
                 2.4 
                 18.8 
                 4.5 
                 80 
                 39 
                 37 
               
               
                 3 
                 GC_A 
                 I_A 
                 II_A 
                 III_A Thick 3 
                 3.1 
                 15.2 
                 0.9 
                 43 
                 39 
                 23 
               
               
                 4 
                 GC_A 
                 I_A 
                 II_A 
                 — 
                 1.2 
                 12.4 
                 0.5 
                 85 
                 30 
                 15 
               
               
                 5 
                 GC_A 
                 I_A 
                 II_B 
                 — 
                 1.8 
                 21.9 
                 2 
                 88 
                 32 
                 25 
               
               
                 6 
                 GC_A 
                 I_A 
                 II_C 
                 — 
                 2   
                 18.9 
                 1.2 
                 82 
                 38 
                 27 
               
               
                 7 
                 GC_A 
                 — 
                 — 
                 — 
                 Reference 
                 10.3 
                 — 
                 13 
                 30 
               
               
                 8 
                 GC_A 
                 I_B Thin 
                 — 
                 — 
                 0.2 
                 16.8 
                   
                 10 
                 38 
               
               
                 9 
                 GC_A 
                 I_B Thin 
                 — 
                 — 
                 0.9 
                 12.5 
                   
                 91 
                 34 
               
               
                 10 
                 GC_A 
                 I_B 
                 — 
                 — 
                 1.3 
                 12.9 
                 6 
                 86 
                 33 
                 36 
               
               
                 11 
                 GC_A 
                 I_B 
                 II_D 
                 — 
                 1.4 
                 14.5 
                 6 
                 93 
                 33 
                 31 
               
               
                   
               
            
           
         
       
     
     As shown in Table 2, a correlation was found between the shell thickness and the EQE. The shell thickness was, but not limited to, 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, still more preferably 1.3 nm or more and 2.5 nm or less. 
     Further, the relationship between the quantum dot thickness (diameter) and the EQE was investigated, and the EQE was found to tend to be higher when the quantum dots had a certain level of thickness. The quantum dot thickness was, but not limited to, 5 nm or more and 50 nm or less, preferably 10 nm or more and 45 nm or less, more preferably 15 nm or more and 40 nm or less, still more preferably 20 nm or more and 40 nm or less, yet more preferably 25 nm or more and 40 nm or less. 
     Further, Cd-based green quantum dots were subjected to PYS measurements. In  FIG. 13 , the circle marks represent experimental data of the quantum dots each constituted by a core alone in Example 1, and the square marks represent experimental data of the quantum dots constituted by a core coated with a shell in Example 2. 
     The photo-electron yield spectroscopy (PYS) can measure the ionization potential. For example, the measurement can be performed using a system named AC-2/AC-3 manufactured by RIKEN KEIKI CO., LTD. 
     As shown in  FIG. 13 , the energy of the rising edge in Example 1 was found to be different from that in Example 2. The energy rose at approximately 6.1 eV in Example 1, and the energy rose at approximately 7.1 eV in Example 2. 
       FIG. 14  shows the PYS measurement results of the Cd-based green quantum dots with different shell thicknesses in Example 3 and Example 4. The shell thickness was larger in Example 4 than in Example 3. The energy of the rising edge in Example 3 was found to be different from that in Example 4. The energy rose at approximately 7.1 eV in Example 3, and the energy rose at approximately 8.1 eV in Example 4. 
     (Experiment on Current Dependence of EQE) 
       FIG. 15  is an energy level diagram of each layer in the light emitting device used in an experiment.  FIG. 16  is a graph illustrating the relationship between the current value and the EQE of an EL emitter and a PL emitter using red quantum dots; Further,  FIG. 17  shows plots illustrating the relationship between the current value and the EQE of an EL emitter and a PL emitter using red quantum dots; and a plot illustrating the relationship between the current value and the EQE of an EL emitter using blue quantum dots. The shell thicknesses in Example 5 and Example 6 in  FIG. 16  were different. The shell thickness was larger in Example 5 than in Example 6. Further, in  FIG. 17 , the shell thickness was largest in Example 7, and the shell thickness was smaller in Example 8 and Example 9 in this order. 
     As shown in  FIG. 16  and  FIG. 17 , for the EL emitter and the PL emitter, the EQE increased to the point around 20 mA. On the other hand, in the red emission device, the EQE increased even when the current value was 20 mA or more. As shown in  FIG. 16  and  FIG. 17 , a larger shell thickness resulted in a larger increase in the EQE. 
     (Experiment on ZnO X  Synthesis) 
       FIG. 18  presents a graph illustrating the energy band gap Eg of each layer in the light emitting device used in an experiment, the energy E CB  at the bottom of the conduction band, and the energy E CB  at the top of the conduction band, and an energy level diagram of the layers. ZnO X (Li) was used as L 1  or L 2  in  FIG. 18 . Here, Li may or may not be added. X is, but not limited to, around 0.8 to 1.2. As shown in  FIG. 18 , it was found that use of ZnO X (Li) as ZnO X  used for the electron injection layer (ETL) and the electron transport layer increased the band gap. ZnO X (Li) is presumed to have an effect of reducing the particle diameter. PVK shown in  FIG. 18  corresponds to a hole injection layer; B 1 , B 2 , G(H), G( 13 ), and R(F) correspond to emitting layers (EL layers); and ZnO X , L 2 , and L 4  correspond to electron injection layers. When B 1  or B 2  is used for the emitting layer, ZnO X  can be used for the electron injection layer; however, it was found that when G(H), G(I3), or R(F) was used for the emitting layer, L 2  or L 4  was preferably used for the electron injection layer. L 2  and L 4  are ZnO X (Li). 
     In particular, when an emitting layer (EL layer) with a shallow conduction band is used, it is advantageous to use ZnO X (Li) for the electron injection layer or the electron transport layer. 
     ZnO X (Li) can be, but not exclusively, prepared by stirring a zinc acetate-ethanol solution at a predetermined temperature for a predetermined time, and then mixing and stirring a LiOH.4H 2 O-ethanol solution, followed by centrifugal separation, cleaning, etc. 
       FIG. 19  to  FIG. 21  show the UV (band gap), the PL spectra, and the PYS data of ZnO X (Li) and ZnO X (K) used for the electron injection layer (ETL). ZnO X (K) was produced using KOH catalytically and was not doped with K or Li. ZnO X (Li) and ZnO X (K) were found to show differences in the UV data and the PL data. On the other hand, the PYS data of ZnO X (Li) and ZnO X (K) hardly differed, and the rising edge energies were almost the same. 
     Thus, ZnO X  of which band gap is controlled by selecting from different particle diameters to be used for an electron injection/transport layer of an EL device using quantum dots, and doped ZnO X  for which defects are controlled and the band gap is controlled by adding doping species can be proposed. 
     When the recombination of electrons and holes for producing light cannot still be brought into balance, hole blocking functions are preferably added by interposing a thick insulating layer between the EL layer and the electron injection layer or integrating ZnO X  with molecules thereby adjusting the balance. Here, the integrated layer involves the integration of, for example, ZnO X  with T2T (2,4,6-tris(biphenyl-3-yl)-1,3,5-triazine). X is, but not limited to, around 0.8 to 1.2. 
     Further, ZnO X  is known to have functions suitable for use in not only the electron injection/transport layer but also in a hole injection/transportation layer by performing ozone treatment or the like. Specifically, performing ozone treatment on ZnO X  was found to improve the hole transporting performance. 
     (Consideration of Outcoupling Efficiency) 
     With a view to improving the outcoupling efficiency of a bottom emission device, the refractive index may be optimized to increase the outcoupling efficiency by a factor of approximately 1.5 to 2 as shown in  FIG. 22 . The numeric values given in  FIG. 22A  and  FIG. 22B  show the refractive indices of the layers. 
     In  FIG. 22A , a glass substrate (refractive index: approximately 1.6) was placed on the underside of an ITO substrate (refractive index: 1.8 to 2.0), and a lens made of resin or the like with a refractive index of approximately 1.6 was placed on the underside of the glass substrate. 
     In particular, as illustrated in  FIG. 22B , scattering effects can be obtained by placing one or more kinds of members having a high refractive index (higher than 1.5 and lower than 1.8), for example, a resin layer or glass is placed between the ITO electrode and the glass substrate, thus the outcoupling efficiency is expected to be further improved. 
     The EQE can be improved by improving the outcoupling efficiency. 
     INDUSTRIAL APPLICABILITY 
     According to the present invention, a light emitting device containing quantum dots can be used for a display device, and excellent emission properties can be obtained. 
     This application is based on Japanese patent application No. 2017-215800 filed on Nov. 8, 2017, the content of which is hereby incorporated in its entirety.