Patent Publication Number: US-2022223809-A1

Title: Light emitting element and display device

Description:
TECHNICAL FIELD 
     The disclosure relates to a light-emitting element and a display device. 
     BACKGROUND ART 
     The light-emitting element described in PTL 1 includes an electron transport layer between a quantum dot layer and a positive electrode. 
     CITATION LIST 
     Patent Literature 
     
         
         PTL 1: JP 2012-23388 A 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     In the light-emitting element of PTL 1, a difference in the electron affinity between the quantum dot layer and the electron transport layer increases depending on a material of the quantum dot layer. As a result, a barrier to electrons heading from the electron transport layer towards the quantum dot layer increases. Accordingly, electrons are less likely to be injected from the electron transport layer to the quantum dot layer, and the electron density in the quantum dot layer decreases. As a result, recombination rate between electrons and positive holes in the quantum dot layer decreases, and luminous efficiency (external quantum efficiency) of the light-emitting element decreases. 
     An object of the light-emitting element according to an aspect of the disclosure is to increase the luminous efficiency of the light-emitting element by reducing the barrier to electrons between the electron transport layer and the quantum dot layer. 
     Solution to Problem 
     A light-emitting element according to an embodiment of the disclosure includes a positive electrode, a negative electrode, a quantum dot layer provided between the positive electrode and the negative electrode and including quantum dots, and a first electron transport layer provided in contact with the quantum dot layer between the quantum dot layer and the negative electrode and containing a compound having a composition of ZnMO, constituent elements M in the composition being elements of at least one of Co, Rh, and Ir. 
     Advantageous Effects of Disclosure 
     According to a light-emitting element according to an aspect of the disclosure, luminous efficiency of the light-emitting element can be increased by reducing a barrier to electrons between an electron transport layer and a quantum dot layer. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic cross-sectional view illustrating a display device of the disclosure. 
         FIG. 2  is a cross-sectional view schematically illustrating a light-emitting element according to a first embodiment. 
         FIG. 3  is a table exemplifying materials used in a second electron transport layer of the light-emitting element according to the first embodiment. 
         FIG. 4  is an energy diagram illustrating an example of electron affinity and ionization potential of each layer between a positive electrode and a negative electrode of a light-emitting element according to an example 1-1. 
         FIG. 5  is a table showing compositions and characteristic values of light-emitting elements according to respective examples and a comparative example. 
         FIG. 6  is an energy diagram illustrating an example of electron affinity and ionization potential of each layer between a positive electrode and a negative electrode of a light-emitting element according to an example 2. 
         FIG. 7  is an energy diagram illustrating an example of electron affinity and ionization potential of each layer between a positive electrode and a negative electrode of the light-emitting element according to an example 3. 
         FIG. 8  is a cross-sectional view schematically illustrating a light-emitting element according to a comparative example. 
         FIG. 9  is an energy diagram illustrating an example of electron affinity and ionization potential of each layer between a positive electrode and a negative electrode according to the light-emitting element of the comparative example. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, exemplary embodiments and examples of the disclosure will be described with reference to the drawings. Note that duplicate items in each of the embodiments and the examples are omitted as appropriate. In the following, descriptions of upper and lower correspond to those in each drawing. 
     First Embodiment 
       FIG. 1  is a cross-sectional view schematically illustrating a display device  10  including a light-emitting element  20  according to a first embodiment. The display device  10  of the present embodiment is a quantum-dot light emitting diode (QLED) panel in which the light-emitting elements  20  including quantum dots are provided on a first film  11  having flexibility and a resin layer  12 . 
     The display device  10  has a structure in which the resin layer  12 , a barrier layer  13 , a thin film transistor layer  14  (hereinafter, thin film transistor is referred to as TFT) including TFTs, a light-emitting element layer  15  including the light-emitting elements  20  and cover films  151 , a sealing layer  16 , and a second film  17  are layered in this order on the first film  11 , as illustrated in  FIG. 1 . 
     The first film  11  is a support member in the display device  10  having flexibility. The first film  11  can be formed of a material having flexibility such as PET. Note that in a case where the display device  10  is configured to have no flexibility, a substrate formed of a hard material such as glass may be used instead of the first film  11 . 
     The resin layer  12  is provided between the first film  11  and the barrier layer  13 . The resin layer  12  is a layer for partially removed when a support substrate (not illustrated) is peeled from the barrier layer  13 . The resin layer  12  can be formed of, for example, a resin material such as polyimide. Note that the resin layer  12  may have a multilayer structure in which a plurality of resin films are layered, or may have a multi-layer structure in which an inorganic film is interposed between the plurality of resin films. Note that in the case in which the display device  10  is configured to have no flexibility, the resin layer  12  need not be provided. 
     The barrier layer  13  is a layer for preventing foreign matters such as water and oxygen from entering the TFT layer  14  and the light-emitting element layer  15 . The barrier layer  13  is a single layer or a multi-layer insulating film, and can be formed of an insulating material, such as silicon oxide, silicon nitride, or silicon oxynitride. 
     The TFT layer  14  includes a semiconductor film  141 , a gate insulating film  142  above the semiconductor film  141 , a gate electrode GE and a gate wiring line (not illustrated) above the gate insulating film  142 , a first insulating film  143  above the gate electrode GE and the gate wiring line, a capacitance electrode CE above the first insulating film  143 , a second insulating film  144  above the capacitance electrode CE, a source wiring line SW and a drain wiring line DW (not illustrated) above the second insulating film  144 , and a flattening film  145  above the source wiring line SW and the drain wiring line DW. 
     The TFT includes the semiconductor film  141 , the gate insulating film  142 , the gate electrode GE, the first insulating film  143 , and the second insulating film  144 . A source region and a drain region (not illustrated) of the semiconductor film  141  are regions in which high concentration doping is performed on an upper face of the semiconductor film  141 , and function as a source electrode and a drain electrode. The source wiring line SW and the drain wiring line DW are connected to the source region and the drain region, respectively via contact holes passing through the gate insulating film  142 , the first insulating film  143 , and the second insulating film  144 . The gate electrode GE is connected to a gate wiring line (not illustrated). 
     The semiconductor film  141  can be formed of, for example, a semiconductor material such as low-temperature polysilicon (LTPS) or an oxide semiconductor (for example, an In—Ga—Zn—O based semiconductor). The gate electrode GE, the gate wiring line, the capacitance electrode CE, the drain wiring line DW, and the source wiring line SW are single layer or multi-layer conductive films, and can be formed of a metal material such as Al, W, Mo, Ta, Cr, Ti, or Cu. 
     The first insulating film  143  and the second insulating film  144  are single layer or multi-layer insulating films, and can be formed of an insulating material, such as silicon oxide or silicon nitride. 
     The flattening film  145  is a film layered on the TFT for flattening irregularities formed by the TFT. The flattening film  145  can facilitate layering the light-emitting element  15  thereon. The flattening film  145  can be formed of, for example, an organic material such as polyimide and acrylic. 
     Note that although a structure of the TFT included in the TFT layer  14  is illustrated as a top gate type in  FIG. 1 , the structure of the TFT may be a bottom gate type or a double gate type. The TFT is a switching element that controls light emission of the light-emitting element  20 . One TFT is connected to one light-emitting element  20 . In  FIG. 1 , the drain region of the TFT and a positive electrode  21  of the light-emitting element  20  are connected to each other via the contact hole formed in the flattening film  145  and the drain wiring line DW provided in the TFT layer  14 . 
     The light-emitting element layer  15  includes a plurality of the light-emitting elements  20  and the cover films  151 . The plurality of light-emitting elements  20  is arranged in a matrix in a display region of an image in the display device  10 . Note that  FIG. 1  illustrates a structure in which the plurality of light-emitting elements  20  shares one negative electrode  27 . However, the shape of the negative electrode  27  is not limited thereto. For example, a structure may be such that each light-emitting element  20  has a separate negative electrode  27 .  FIG. 1  illustrates a structure in which each light-emitting element  20  has a separate positive electrode  21 . However, the shape of the positive electrode  21  is not limited thereto. For example, a structure may be such that the plurality of light-emitting elements  20  shares one positive electrode  21 . 
     The cover films  151  are provided between the plurality of light-emitting elements  20  so as to cover a side surface of each light-emitting element  20  and an edge of each positive electrode  21 . The cover film  151  is provided in a lattice pattern in the display region. The cover film  151  is an insulating film, and can be formed of an organic material such as polyimide or acrylic. 
     The sealing layer  16  is a layer for sealing the light-emitting element layer  15  to prevent foreign matters such as water and oxygen from entering the TFT layer  14  and the light-emitting element layer  15 .  FIG. 1  illustrates a case in which the sealing layer  16  has a three-layer structure including a first sealing film  161  covering the negative electrode  27 , a second sealing film  162  covering the first sealing film  161 , and a third sealing film  163  covering the second sealing film  162 . However, the sealing layer  16  is not limited to the three-layer structure. For example, the sealing layer  16  may have a structure of any number of layers including a single layer. 
     For example, the first sealing film  161  and the third sealing film  163  are single layer or multi-layer inorganic insulating films, and can be formed of an inorganic material such as a silicon oxide film, a silicon nitride film, and a silicon oxynitride film. The second sealing film  162  is, for example, a transparent organic film, and can be formed of a transparent organic material such as acrylic. 
     The second film  17  can be formed of, for example, a PET film. As a result, the display device  10  having flexibility can be realized. Note that in the case where the display device  10  has no flexibility, a substrate formed of a hard material such as glass may be used instead of the second film  17 . 
     Among the first film  11  and the second film  17 , for a film provided on a side from which light emitted from the light-emitting element  20  is output in other words a film provided on a side to be the display region of the display device  10 , a function film having, for example, an optical compensation function, a touch sensor function, and a protection function can be used. 
     The light-emitting element  20  of the present embodiment illustrates a configuration in which light is output from the positive electrode  21  side among the positive electrode  21  and the negative electrode  27  to the outside of the display device  10 . Thus, in the configuration of the display device  10 , the first film  11 , the resin layer  12 , the barrier layer  13 , and the TFT layer  14  provided in a direction in which light is output from the light-emitting element  20  to the outside of the display device  10  is preferably formed of a high transparent material. In the configuration of the display device  10 , at least one of the sealing layer  16  and the second film  17  provided on the negative electrode  27  side, which is the opposite side, among the positive electrode  21  and the negative electrode  27  preferably has a reflecting function. 
     Note that the light-emitting element  20  may be configured to output light from the negative electrode  27  side among the positive electrode  21  and the negative electrode  27  to the outside of the display device  10 . In this case, in the configuration of the display device  10 , the sealing layer  16  and the second film  17  provided in a direction in which light is output from the light-emitting element  20  to the outside of the display device  10  is preferably formed of a high transparent material. In the configuration of the display device  10 , at least one of the first film  11 , the resin layer  12 , the barrier layer  13 , and the TFT layer  14  provided on the positive electrode  21  side which is the opposite side among the positive electrode  21  and the negative electrode  27  preferably has the reflecting function. 
     Although not illustrated, an electronic circuit board and a power circuit board (for example, an IC chip or FPC) are disposed outside the display region of the display device  10 . A plurality of the TFTs and the light-emitting elements  20  described above are arranged on a plane to configure the display region of the display device  10 . Power is supplied from each of the above-described circuits to the plurality of TFTs and light-emitting elements  20  arranged on the plane, and each operation is controlled by each of the circuits. Thus, a screen display of the display device  10  is performed. 
     To make the display device  10  of the present embodiment, first, the resin layer  12  is formed on the support substrate (resin layer  12  forming step). Next, the barrier layer  13  is formed. Next, the TFT layer  14  including the TFTs is formed. Next, the light-emitting element layer  15  including the bottom-emitting type light-emitting elements  20  is formed. Next, the sealing layer  16  is formed. Next, the second film  17  is bonded to the sealing layer  16 . 
     Next, the resin layer  12  is partially removed by irradiating the resin layer  12  with laser light or the like transmitted through the support substrate, and the support substrate is peeled from the resin layer  12  (support substrate peeling step). Next, the first film  11  is bonded to the lower face of the resin layer  12  from which the support substrate has been peeled (bonding step). Next, the layered body including the first film  11 , the resin layer  12 , the barrier layer  13 , the TFT layer  14 , the light-emitting element layer  15 , the sealing layer  16 , and the second film  17  is divided to obtain a plurality of individual pieces. Next, the electronic circuit board is disposed on a portion of a non-display region outside the display region. Note that these steps are performed by a manufacturing apparatus of the display device  10 . 
     In the case where manufacturing the display device  10  having no flexibility, the resin layer  12  forming step, the support substrate peeling step, and the first film  11  bonding step are not necessary, for example, the first film  11  may be replaced with a glass substrate or the like, and steps subsequent to the barrier layer  13  forming step may be performed on the glass substrate. A method corresponding to the material of each layer, such as an application method, sputtering, a photolithography method, or CVD can be appropriately used as a method for layering each layer in the above steps. 
       FIG. 2  is a cross-sectional view schematically illustrating the light-emitting element  20  according to the first embodiment. The light-emitting element  20  of the present embodiment includes the positive electrode  21 , a hole injection layer  22 , a hole transport layer  23 , a quantum dot layer  24 , a first electron transport layer  25 , a second electron transport layer  26 , and the negative electrode  27 , and is configured by layering thereof in this order. Note that in the present embodiment, a direction from the light-emitting element  20  to the positive electrode  21  is referred to as a downward direction, and a direction from the positive electrode  21  toward the light-emitting element  20  is referred to as an upward direction. Details of the light-emitting element  20  will be described below. 
     The positive electrode  21  is an electrode for injecting positive holes into the quantum dot layer  24 . The negative electrode  27  is an electrode for injecting electrons into the quantum dot layer  24 . The positive electrode  21  and the negative electrode  27  can be formed of a conductive material. The positive electrode  21  is in contact with hole injection layer  22 . The negative electrode  27  is in contact with the second electron transport layer  26 . 
     One of the positive electrode  21  and the negative electrode  27  is a light-transmissive electrode and the other is a non-light-transmissive electrode. The light-transmissive electrode can be formed of a conductive material such as ITO, IZO, ZnO, AZO, BZO, or FTO. The non-light-transmissive electrode can be formed of a metal material having high light reflectivity such as Al, Cu, Au, Ag, Mg, or alloys thereof. By using the material having high light reflectivity as the non-light-transmissive electrode, the light emitted by the quantum dot layer  24  can be reflected to a direction in which light is output from the light-emitting element  20 . 
     In the present embodiment, the light emitted by the quantum dot layer  24  is reflected by the negative electrode  27  and transmitted through the positive electrode  21 , and is output from the light-emitting element  20  to the outside of the display device  10 . 
     The hole injection layer  22  is a layer for injecting positive holes from the positive electrode  21  into the hole transport layer  23 . The hole transport layer  23  is a layer for transporting positive holes injected from the hole injection layer  22  to the quantum dot layer  24 . Note that, for example, among the hole injection layer  22  and the hole transport layer  23 , only the hole injection layer  22  may be provided between the positive electrode  21  and the quantum dot layer  24 , or the hole injection layer  22  and the hole transport layer  23  may be omitted, and the positive electrode  21  and the quantum dot layer  24  may be directly in contact with each other. 
     The hole injection layer  22  and the hole transport layer  23  can be formed of, for example, an organic material containing a conductive compound such as PEDOT-PSS, TFB, and PVK, or, for example, an inorganic material containing a metal oxide such as NiO, Cr 2 O 3 , MgO, MgZnO, LaNiO 3 , MoO 3 , and WO 3 . 
     The quantum dot layer  24  is provided between the positive electrode  21  and the negative electrode  27 . In the present embodiment, the quantum dot layer  24  is provided between the hole transport layer  23  and the first electron transport layer  25 . The quantum dot layer  24  is a layer including quantum dots  28  that are nano-sized semiconductor particles. The quantum dot layer  24  may be a single layer or a multi-layer. 
     The quantum dots  28  are preferably formed of a material that does not contain cadmium, such as InP, SiC, and SiN. Since the quantum dots  28  do not contain cadmium, the light-emitting element  20  having a small environmental load can be realized. 
     However, in a case where the quantum dots  28  are formed of the material that does not contain cadmium, the electron affinity of the quantum dot layer  24  tends to be reduced as compared with when the quantum dots  28  are formed of a material that contains cadmium. 
     A particle size of the quantum dots  28  is, for example, about from 2 to 15 nm. The light emission wavelength of the quantum dots  28  changes depending on the particle size. Thus, the light emission wavelength of the light-emitting element  20  can be controlled by changing the particle size of the quantum dots  28 . The smaller the particle size of the quantum dots  28 , the shorter the light emission wavelength, and the luminescent color changes from red to green and green to blue. Note that when the display device  10  is configured according to the present embodiment, a red light-emitting element  20 , a green light-emitting element  20 , and a blue light-emitting element  20  are arranged as one set in the light-emitting element layer  15 . In order to increase the depth of the luminescent color by the composite light of the light-emitting elements  20  of three colors, the particle size of the quantum dots  28  included in each light-emitting element  20  is preferably made uniform for each color. 
     For example, the quantum dot layer  24  having quantum dots  28  formed using InP having a particle size that emits green light has an ionization potential of approximately 5.2 eV. The quantum dot layer  24  having quantum dots  28  formed using InP having a particle size that emits red light has an ionization potential of approximately 5.2 eV and an electron affinity of approximately 3.1 eV. The quantum dot layer  24  having quantum dots  28  formed using InP having a particle size that emits blue light has an ionization potential of approximately 5.2 eV and an electron affinity of approximately 2.5 eV. 
     The first electron transport layer  25  is provided in contact with the quantum dot layer  24  between the quantum dot layer  24  and the negative electrode  27 . In the present embodiment, the first electron transport layer  25  is provided between the quantum dot layer  24  and the second electron transport layer  26 . In other words, in the first electron transport layer  25 , one surface is in contact with the quantum dot layer  24 , and the other surface is in contact with the second electron transport layer  26 . The first electron transport layer  25  is a layer for transporting electrons from the second electron transport layer  26  to the quantum dot layer  24 . 
     The first electron transport layer  25  contains a compound formed of, for example, Zn, O, and constituent elements M, and the constituent elements M are elements of at least one of Co, Rh, and Ir. In the present embodiment, for example, a compound having a composition of Zn A M B O C  (A, B, and C are arbitrary values) is used for the first electron transport layer  25 . The constituent elements M contain at least one of Group 9 elements Co, Rh and Ir. 
     The compound Zn A M B O C  used in the first electron transport layer  25  may contain a plurality of compounds, in which the constituent elements M differ from each other. 
     The compounds different from each other that contain the constituent elements M may be, for example, Zn A (Co B1 .Rh B2 )O C , Zn A (Co B1 .Ir B3 )O C , Zn A (Rh B2 .Ir B3 )O C , and Zn A (Co B1 .Rh B2 .Ir B3 )O C , (B1, B2, and B3 are arbitrary values). 
     The compound Zn A M B O C  used in the first electron transport layer  25  can be, for example, A=1, B=2, C=4, in other words compound ZnM 2 O 4 . 
     Accordingly, the content percentage of elements contained in the constituent elements M can be selected such that the electron affinity of the first electron transport layer  25  is within a predetermined range close to the electron affinity of the quantum dot layer  24 . Thus, the electron affinity of the first electron transport layer  25  and the electron affinity of the quantum dot layer  24  can be brought closer to each other to reduce the injection barrier to electrons in a direction from the first electron transport layer  25  toward the quantum dot layer  24 . As a result, electrons transported from the first electron transport layer  25  to the quantum dot layer  24  increase, and the luminous efficiency of the light-emitting element  20  can be increased. Note that ZnM 2 O 4  may have a spinel crystal structure. 
     For example, even if the electron affinity of the quantum dot layer  24  is reduced by forming the quantum dots  28  with a material that does not contain cadmium, the electron affinity of the first electron transport layer  25  can be brought closer to the electron affinity of the quantum dot layer  24  within a predetermined range by using the compound ZnM 2 O 4  as the material of the first electron transport layer  25 . As a result, the quantum dot layer  24  can be configured to be cadmium free (formed of a material that does not contain cadmium), and a reduction in the luminous efficiency of the light-emitting element  20  can be prevented. Details will be described below using examples. 
     The electron affinity of the quantum dot layer  24  is preferably equal to the electron affinity of the first electron transport layer  25  or greater. Alternatively, in the constituent elements M of the first electron transport layer  25 , the kinds of elements included in the constituent elements M and ratio thereof may be appropriately selected such that the electron affinity of the first electron transport layer  25  is greater than the electron affinity of the quantum dot layer  24  and the electron affinity of the first electron transport layer  25  is within the predetermined range close to the electron affinity of the quantum dot layer  24 . 
     Here, the “predetermined range” in which the electron affinity of the first electron transport layer  25  is close to the electron affinity of the quantum dot layer  24  is preferably, for example, 0.5 eV or less which is a value obtained by subtracting the electron affinity of the quantum dot layer  24  from the electron affinity of the first electron transport layer  25 . In the first electron transport layer  25 , since the electron density with respect to energy follows the Boltzmann distribution, electrons are present in a shape with a long tail on the high energy side, and electrons contributing to electrical conduction are present at an unignorable density in a case of 0.5 eV or less. This sufficiently reduces the injection barrier to electrons in the direction from the first electron transport layer  25  toward the quantum dot layer  24 . As a result, electrons transported from the first electron transport layer  25  to the quantum dot layer  24  increases more reliably, and the luminous efficiency of the light-emitting element  20  can be increased. 
     The second electron transport layer  26  is provided between the first electron transport layer  25  and the negative electrode  27 . The second electron transport layer  26  is in contact with each of the first electron transport layer  25  and the negative electrode  27 . The second electron transport layer  26  is formed of a material whose electron affinity and ionization potential are smaller than the electron affinity of the first electron transport layer  25  and smaller than the work function of the negative electrode  27 . As a result, the junction between the second electron transport layer  26  and the negative electrode  27  becomes a tunnel junction, and electrons are easily injected from the negative electrode  27  into the second electron transport layer  26 . Note that, in a case where a combination of materials is such that the barrier between the first electron transport layer  25  and the negative electrode  27  is sufficiently small, the second electron transport layer  26  may be omitted. 
       FIG. 3  is a table exemplifying materials for forming the second electron transport layer  26 . For example, the second electron transport layer  26  can be formed using any of the materials exemplified in  FIG. 3 . The materials exemplified in  FIG. 3  are high-resistance semiconductors having an electron mobility of approximately from 1 to 10 cm 2 /V·sec. Although the materials for forming the second electron transport layer  26  are exemplified in  FIG. 3 , materials having a relationship of the characteristic values with the first electron transport layer  25  and the negative electrode  27  satisfying the above-described condition may be employed as the material for forming the second electron transport layer  26 . In particular, a material whose electrical junction between the second electron transport layer  26  and the negative electrode  27  is a Schottky junction is preferably employed as the material for forming the second electron transport layer  26 . As exemplified in  FIG. 3 , for example, BaO (electron affinity: 1.25 (eV), ionization potential: 1.8 (eV)), SrO (electron affinity: 1.43 (eV), ionization potential: 6.5 (eV)), CaO (electron affinity: 1.78 (eV), ionization potential: 8.8 (eV)), LaB 6  (electron affinity: 2.69 (eV), ionization potential: 1.4 (eV)), TiC (electron affinity: 3.32 (eV), ionization potential: 4.5 (eV)), ZrC (electron affinity: 3.38 (eV), ionization potential: 5.4 (eV)), HfC (electron affinity: 3.40 (eV), ionization potential: 6.6 (eV)), NdC (electron affinity: 3.45 (eV), ionization potential: 6.8 (eV)), TaC (electron affinity: 3.61 (eV), ionization potential: 7.2 (eV)), and the like are included as a material for forming the second electron transport layer  26 . Furthermore, in addition to the materials exemplified in  FIG. 3 , for example, GdB 4 , CeC 2 , YB 4 , CeB 6 , ThO 2 , and the like can also be used as the material for forming the second electron transport layer  26 . 
     The material of the second electron transport layer  26  is preferably a degenerate semiconductor. For example, the material of the second electron transport layer  26  is preferably BaO. BaO has a narrow band gap of 0.5 eV. BaO is a degenerate semiconductor whose Fermi level is higher than the bottom of the conduction band. Thus, in a case where the negative electrode  27  is formed using, for example, Al, the electrical junction between the second electron transport layer  26  and the negative electrode  27  becomes a Schottky junction, and a depletion layer is generated at the junction. Thus, in the direction from the negative electrode  27  toward the second electron transport layer  26 , a barrier to electrons having a very narrow width is generated by the depletion layer, and the tunneling effect increases. As a result, the tunneling current increases to further reduce the contact resistance, and the injection efficiency of electrons can be further improved. As a result, extremely excellent ohmic characteristics can be realized between the negative electrode  27  and the second electron transport layer  26 . 
     Note that in a case where the light-emitting element  20  output light from the negative electrode  27  side to the outside, the first electron transport layer  25 , the second electron transport layer  26 , and the negative electrode  27  are formed of transparent materials. For example, the three layers are preferably formed of materials having the light transmittance of 95% or greater. As a result, attenuation of light emitted from the quantum dot layer  24  to the outside due to the first electron transport layer  25 , the second electron transport layer  26 , and the negative electrode  27  can be suppressed. 
     According to the above-described configuration of the light-emitting element  20 , when a potential difference is applied between the positive electrode  21  and the negative electrode  27 , positive holes are injected from the positive electrode  21  and electrons are injected from the negative electrode  27 , toward the quantum dot layer  24 . 
     In other words, as indicated by arrow h +  in  FIG. 2 , positive holes from the positive electrode  21  reach the quantum dot layer  24  via the hole injection layer  22  and the hole transport layer  23 . Further, as indicated by an arrow e −  in  FIG. 2 , electrons from the negative electrode  27  reach the quantum dot layer  24  via the second electron transport layer  26  and the first electron transport layer  25 . Then, positive holes and electrons that have reached the quantum dot layer  24  recombine with each other at the quantum dots  28 , and light is output from the quantum dot layer  24 . 
     As described above, the light-emitting element  20  emits light. The height of the barrier to positive holes is the difference between the ionization potential or the work function of a layer from which positive holes flow and the ionization potential of a layer into which positive holes flow. The height of the barrier to electrons is the difference between the electron affinity or the work function of a layer from which electrons flow and the electron affinity of a layer into which electrons flow. The smaller the respective barriers, the more easily positive holes and electrons are injected into the quantum dot layer  24 . 
     In the light-emitting element  20  in the present embodiment, for example, the output light of the quantum dot layer  24  reflected by the negative electrode  27  of the metal electrode is transmitted through the light-transmissive positive electrode  21  and is emitted to the outside of the light-emitting element  20 . Alternatively, the positive electrode  21  may be a metal electrode and the negative electrode  27  may be a light-transmissive electrode, and light may be emitted from the negative electrode side to the outside. Alternatively, the order of layering may be reversed, and the negative electrode, the second electron transport layer, the first electron transport layer, the quantum dot layer, the hole transport layer, the hole injection layer, and the positive electrode may be in this order from the bottom. 
     Here, the electron affinity of the quantum dot layer  24  is preferably equal to the electron affinity of the first electron transport layer  25  or greater. Accordingly, the injection barrier to electrons is eliminated in the direction from the first electron transport layer  25  toward the quantum dot layer  24 . As a result, the luminous efficiency of the light-emitting element  20  can be further improved. 
     The ionization potential of the quantum dot layer  24  is preferably equal to the ionization potential of the first electron transport layer  25  or less. According to this configuration, the barrier to positive holes increases in the direction from the quantum dot layer  24  toward the first electron transport layer  25 . Accordingly, the flow of positive holes from the quantum dot layer  24  to the negative electrode  27  via the first electron transport layer  25  can be suppressed. As a result, the number of positive holes remaining in the quantum dot layer  24  increases, and the recombination rate of positive holes and electrons in the quantum dot layer  24  is improved. As a result, the luminous efficiency of the light-emitting element  20  can be further improved. 
     The light-emitting element  20  according to the present embodiment will be specifically described below based on examples 1-1 to 1-11 and a comparative example. 
     Example 1-1 
     As the examples 1-1 to 1-11, light-emitting elements  20  in which the compound ZnM 2 O 4  as the composition of the first electron transport layer  25  was variously changed were prepared and compared with a light-emitting element according to the comparative example. Description will be sequentially made from the example 1-1. 
       FIG. 4  is an energy diagram illustrating an example of the electron affinity and the ionization potential of each layer between the positive electrode  21  and the negative electrode  27  of the light-emitting element  20  according to the example 1-1. Note that in  FIG. 4 , each of the work functions (in eV) of the positive electrode  21  and the negative electrode  27  is expressed by a numerical value. 
       FIG. 5  is a table showing compositions and characteristic values of light-emitting elements according to each of examples and a comparative example. In  FIG. 5 , the proportions (in %) of Group 9 elements contained in the constituent elements M are exemplified for each example. 
     As illustrated in  FIG. 4  and  FIG. 5 , in the example 1-1, the light-emitting element  20  ( FIG. 2 ) in which the first electron transport layer  25  was formed of ZnRh 2 O 4  was prepared. In other words, the example 1-1 is an example in which the composition of the first electron transport layer  25  is such that the constituent elements M of the compound ZnM 2 O 4  is Rh. 
     In  FIG. 4 , a case is illustrated in which from the left side to the right side the positive electrode  21  is formed of ITO, the hole injection layer  22  is formed of PEDOT, the hole transport layer  23  is formed of TFB, the quantum dots  28  included in the quantum dot layer  24  are formed of InP having a particle size that emits green color, the first electron transport layer  25  is formed of ZnRh 2 O 4 , the second electron transport layer  26  is formed of BaO, and the negative electrode  27  is formed of Al. In this case, the hole injection layer  22  had the ionization potential of 5.0 eV and the electron affinity of 3.4 eV. The hole transport layer  23  had the ionization potential of 5.3 eV and the electron affinity of 2.3 eV. The quantum dot layer  24  had the ionization potential of 5.6 eV and the electron affinity of 2.9 eV. The first electron transport layer  25  had the ionization potential of 5.6 eV and the electron affinity of 2.5 eV. The second electron transport layer  26  had the ionization potential of 1.8 eV and the electron affinity of 1.3 eV. The positive electrode  21  had the work function of 4.6 eV and the negative electrode  27  had the work function of 4.3 eV. 
     Here, a state in which positive holes and electrons are transported in each layer of the light-emitting element  20  will be described with reference to  FIG. 2  and  FIG. 4 . In the light-emitting element  20 , when a potential difference is generated between the positive electrode  21  and the negative electrode  27 , positive holes are injected into the hole injection layer  22  from the positive electrode  21 , as indicated by the arrow h +  in  FIG. 2 . Positive holes are then injected from the hole injection layer  22  into the hole transport layer  23 , and positive holes are transported from the hole transport layer  23  to the quantum dot layer  24 . 
     The height of the barrier to positive holes is the difference between the ionization potential or the work function of a layer from which positive holes flow and the ionization potential of a layer into which positive holes flow. 
     In other words, as illustrated in  FIG. 4 , the barrier when positive holes are transported from the positive electrode  21  to the hole injection layer  22  is 0.4 eV expressed by the difference obtained by subtracting the work function of the positive electrode  21  from the ionization potential of the hole injection layer  22 . The barrier when positive holes are transported from the hole injection layer  22  into the hole transport layer  23  is 0.3 eV expressed by the difference obtained by subtracting the ionization potential of the hole transport layer  23  from the ionization potential of the hole injection layer  22 . The barrier when the positive holes are transported from the hole transport layer  23  to the quantum dot layer  24  is −0.1 eV expressed by the difference obtained by subtracting the ionization potential of the quantum dot layer  24  from the ionization potential of the hole transport layer  23 . The barrier when positive holes are transported from the quantum dot layer  24  to the first electron transport layer  25  is 0.4 eV expressed by the difference obtained by subtracting the ionization potential of the first electron transport layer  25  from the ionization potential of the quantum dot layer  24 . 
     On the other hand, as indicated by the arrow e −  in  FIG. 2 , electrons are injected from the negative electrode  27  into the second electron transport layer  26 . Electrons are then injected from the second electron transport layer  26  into the first electron transport layer  25 , and electrons are transported from the first electron transport layer  25  to the quantum dot layer  24 . 
     The height of the barrier to electrons is the difference between the electron affinity or the work function of a layer from which electrons flow and the electron affinity of a layer into which electrons flow. 
     In other words, as illustrated in  FIG. 4 , the barrier when electrons are transported from the negative electrode  27  to the second electron transport layer  26  is 3.0 eV expressed by the difference obtained by subtracting the electron affinity of the second electron transport layer  26  from the work function of the negative electrode  27 . The barrier when electrons are transported from the second electron transport layer  26  to the first electron transport layer  25  is −1.2 eV, which is a difference obtained by subtracting the electron affinity of the first electron transport layer  25  from the electron affinity of the second electron transport layer  26 . The barrier when electrons are transported from the first electron transport layer  25  to the quantum dot layer  24  is −0.4 eV expressed by a difference obtained by subtracting the electron affinity of the quantum dot layer  24  from the electron affinity of the first electron transport layer  25 . 
     As described above, in the example 1-1, since the first electron transport layer  25  was formed of ZnRh 2 O 4 , the barrier when electrons are transported from the first electron transport layer  25  to the quantum dot layer  24  could be −0.4 eV, which is the predetermined range. Note that, as will be described in detail below, the negative electrode  27  and the second electron transport layer  26  form the tunnel junction, and electrons are easily injected into the second electron transport layer  26  from the negative electrode  27 . 
     Here, a comparative example that is a comparison target of the example 1-1 will be described.  FIG. 8  is a cross-sectional view schematically illustrating a light-emitting element  80  according to the comparative example.  FIG. 9  is an energy diagram illustrating an example of the electron affinity and the ionization potential of each layer between a positive electrode  81  and a negative electrode  86  of the light-emitting element  80  according to the comparative example. Note that in  FIG. 9 , each of the work functions (in eV) of the positive electrode  81  and the negative electrode  86  is expressed by a numerical value. 
     As illustrated in  FIG. 8 , the light-emitting element  80  according to the comparative example has a configuration in which the first electron transport layer  25  and the second electron transport layer  26  of the light-emitting element  20  illustrated in  FIG. 2  are changed to an electron transport layer  85  of a single layer. In other words, the light-emitting element  80  is configured to include the positive electrode  81 , a hole injection layer  82 , a hole transport layer  83 , a quantum dot layer  84  including quantum dots  87 , the electron transport layer  85 , and the negative electrode  86 , which are layered in this order. 
     As illustrated in  FIG. 9  and  FIG. 5 , the light-emitting element  80  according to the comparative example was prepared, in which the electron transport layer  85  was formed using ZnO, and other layers were formed of the same materials as the light-emitting element  20  according to the example 1-1. In other words, the light-emitting element  80  illustrates a case in which from the left side to the right side in  FIG. 9 , the positive electrode  81  is formed of ITO, the hole injection layer  82  is formed of PEDOT, the hole transport layer  83  is formed of TFB, the quantum dots  87  included in the quantum dot layer  84  are formed of InP having a particle size that emits green color, the electron transport layer  85  is formed of ZnO, and the negative electrode  86  is formed of Al. 
     In the comparative example, the electron transport layer  85  formed using ZnO had the ionization potential of 7.0 eV and the electron affinity of 3.8 eV. Thus, the barrier when electrons are transported from the electron transport layer  85  to the quantum dot layer  84  is 0.9 eV expressed by a difference obtained by subtracting the electron affinity of the quantum dot layer  84  from the electron affinity of the electron transport layer  85 , and the difference is large. 
     Here, since cadmium has a large environmental load, the quantum dots  87  may be formed of, for example, a material that does not contain cadmium. However, in a case where the quantum dots  87  are formed of the material that does not contain cadmium, the electron affinity of the quantum dot layer  84  tends to be reduced as compared with when the quantum dots  87  are formed of a material that contains cadmium. Thus, in particular, in a case where the quantum dots  87  are formed of the material that does not contain cadmium, the difference between the electron affinity of the quantum dot layer  84  and the electron affinity of the electron transport layer  85  formed using ZnO becomes 0.9 eV, as described above, which is relatively large. As a result, injection of electrons from the electron transport layer  85  into the quantum dots  87  is inhibited, and the luminous efficiency of the light-emitting element  80  is reduced. 
     On the other hand, as described above, as illustrated in  FIG. 4 , according to the light-emitting element  20  according to the example 1-1, since the first electron transport layer  25  was formed of ZnRh 2 O 4 , the barrier when electrons are transported from the first electron transport layer  25  to the quantum dot layer  24  could be −0.4 eV, which is smaller than 0.9 eV according to the comparative example. In other words, the electron affinity of the first electron transport layer  25  and the electron affinity of the quantum dot layer  24  could be brought closer to each other to reduce the injection barrier to electrons in a direction from the first electron transport layer  25  toward the quantum dot layer  24 . As a result, according to the light-emitting element  20  according to the example 1-1, as compared with the light-emitting element  80  according to the comparative example, electrons transported from the first electron transport layer  25  to the quantum dot layer  24  increased, and the luminous efficiency of the light-emitting element  20  could be increased. 
     In other words, in the light-emitting element  20  according to the example 1-1, although the electron affinity of the quantum dot layer  24  was reduced by forming the quantum dots  28  with the material that does not contain cadmium, the electron affinity of the first electron transport layer  25  could be brought closer to the electron affinity of the quantum dot layer  24  by forming the first electron transport layer  25  using ZnM 2 O 4 . As a result, the quantum dot layer  24  could be configured to be cadmium free, and the reduction in the luminous efficiency of the light-emitting element  20  could be prevented. 
     Note that in the present example, setting the barrier when electrons are transported from the first electron transport layer  25  to the quantum dot layer  24  to be within the “predetermined range” can also be expressed as setting an absolute value to be smaller than 0.9 eV, which is the barrier when electrons are transported from the electron transport layer  85  to the quantum dot layer  84  in the comparative example. 
     Then, the luminance and luminous efficiency were measured when each of the light-emitting element  20  according to the example 1-1 and the light-emitting element  80  according to the comparative example was emitted. The light-emitting element  80  according to the comparative example had the luminance of 20000 cd/m 2  and the luminous efficiency of 8%. In contrast, the light-emitting element  20  according to the example 1-1 had the luminance of 45000 cd/m 2  and the luminous efficiency of 15%. As described above, the light-emitting element  20  according to the example 1-1 had higher luminance and luminous efficiency than that of the light-emitting element  80  according to the comparative example. 
     As described above, in the example 1-1, BaO was used for the second electron transport layer  26 . As exemplified in  FIG. 3 , the electron affinity of the second electron transport layer  26  using BaO is 1.3 eV, and the ionization potential is 1.8 eV. As described above, the work function of the negative electrode  27  is 4.3 eV, and the electron affinity of the first electron transport layer  25  is 2.5 eV. Thus, both the electron affinity and the ionization potential of the second electron transport layer  26  are smaller than the electron affinity of the first electron transport layer  25  and smaller than the work function of the negative electrode  27 . As a result, the junction between the second electron transport layer  26  and the negative electrode  27  became a Schottky junction, and a tunneling phenomenon occurred. Thus, in the light-emitting element  20  of the example 1-1, the junction between the second electron transport layer  26  and the negative electrode  27  became the tunnel junction. As a result, the contact resistance between the negative electrode  27  and the second electron transport layer  26  was reduced, and good ohmic characteristics could be realized. Thus, electrons were easily injected from the negative electrode  27  into the second electron transport layer  26 . 
     Since the electron affinity of the second electron transport layer  26  is smaller than the electron affinity of the first electron transport layer  25 , there exists no barrier to electrons when electrons are transported from the second electron transport layer  26  to the first electron transport layer  25 . Thus, electrons injected into the second electron transport layer  26  can be easily transported to the first electron transport layer  25 . As a result, the amount of electrons in the quantum dot layer  24  increased, and the luminous efficiency of the light-emitting element  20  could be further improved. 
     For example, in the light-emitting element in which the second electron transport layer  26  is omitted from that of the example 1-1, and the first electron transport layer  25  and the negative electrode  27  are directly connected to each other, since the electron affinity of the first electron transport layer  25  is 2.50 eV and the ionization potential is 5.6 eV, whereas the work function of the negative electrode  27  is 4.3 eV, a barrier having a height of 1.8 eV is generated to electrons when electrons are transported from the negative electrode  27  to the first electron transport layer  25 . In the comparative example, the electron affinity of the electron transport layer  85  is 3.8 eV and in a case where the negative electrode  27  is Al, the barrier height to electrons is 0.5 eV. Thus, unlike the light-emitting element  20  according to the comparative example, in a case where the second electron transport layer  26  is not provided, the contact resistance between the negative electrode  27  and the first electron transport layer  25  may be greater than that of the comparative example depending on the combination of the materials of the respective layers. In addition, since the ionization potential of the first electron transport layer  25  is greater than the work function of the negative electrode  27 , the tunneling phenomenon does not occur. As a result, the injection efficiency of electrons into the quantum dot layer  24  is reduced, and the luminous efficiency deteriorates. 
     On the other hand, in the example 1-1, as described above, the second electron transport layer  26  was provided between the first electron transport layer  25  and the negative electrode  27 . The ionization potential of the second electron transport layer  26  is smaller than the work function of negative electrode  27 , and the electron affinity of the second electron transport layer  26  is much smaller than the work function of negative electrode  27 . Thus, in the direction from the negative electrode  27  toward the first electron transport layer  25 , the barrier to electrons was very high, and the width of the depletion layer generated at the junction portion was narrowed. As a result, the tunneling effect occurred owing to the narrowing of the barrier, and electrons injected from the negative electrode  27  penetrated the barrier and flowed into the second electron transport layer  26  side. As a result, the tunneling current increased and electrons were easily injected from the negative electrode  27  into the second electron transport layer  26 . 
     Since the electron affinity of the second electron transport layer  26  was smaller than the electron affinity of the first electron transport layer  25 , the barrier to electrons could be substantially zero in the direction from the second electron transport layer  26  toward the first electron transport layer  25 . Thus, electrons flowing into the second electron transport layer  26  from the negative electrode  27  could be efficiently injected into the first electron transport layer  25  side. As a result, the voltage required to transport electrons decreases, and the power consumption of the light-emitting element  20  decreased, and heat generation of the light-emitting element  20  could also be suppressed. 
     Moreover, the rising voltage could be reduced as compared with the comparative example. 
     Example 1-2 
     As shown in  FIG. 5 , in the example 1-2, unlike the example 1-1, the light-emitting element  20  in which the first electron transport layer  25  was formed of ZnIr 2 O 4  was prepared. In other words, the example 1-2 is an example in which the composition of the first electron transport layer  25  is such that the constituent elements M of the compound ZnM 2 O 4  is Ir. Other configuration and materials of the light-emitting element  20  according to the example 1-2 are the same as those of the light-emitting element  20  according to the example 1-1. 
     In the light-emitting element  20  of the example 1-2, the first electron transport layer  25  had the ionization potential of 5.1 eV and the electron affinity of 2.6 eV. Thus, in the light-emitting element  20  of the example 1-2, the barrier when electrons are transported from the first electron transport layer  25  to the quantum dot layer  24  is −0.3 eV expressed by a difference obtained by subtracting the electron affinity of the quantum dot layer  24  from the electron affinity of the first electron transport layer  25 . Thus, even with the light-emitting element  20  of the example 1-2, the barrier (−0.3 eV) when electrons are transported from the first electron transport layer  25  to the quantum dot layer  24  could be made smaller than the barrier (0.9 eV) when electrons are transported from the electron transport layer  85  to the quantum dot layer  84  in the light-emitting element  80  according to the comparative example. In other words, the electron affinity of the first electron transport layer  25  and the electron affinity of the quantum dot layer  24  could be brought closer to each other to reduce the injection barrier to electrons in a direction from the first electron transport layer  25  toward the quantum dot layer  24 . As a result, even with the light-emitting element  20  according to the example 1-2, as compared with the light-emitting element  80  according to the comparative example, electrons transported from the first electron transport layer  25  to the quantum dot layer  24  increased and the luminous efficiency of the light-emitting element  20  could also be increased. 
     The luminance and the luminous efficiency were measured when the light-emitting element  20  according to the example 1-2 was caused to emit light, and the luminance was 43000 cd/m 2 , and the luminous efficiency was 13%. As described above, the light-emitting element  20  according to the example 1-2 had the luminance and the luminous efficiency higher than the luminance (20000 cd/m 2 ) and the luminous efficiency (8%) of the light-emitting element  80  according to the comparative example. 
     The light-emitting element  20  of the example 1-2 was also provided with the second electron transport layer  26  between the first electron transport layer  25  and the negative electrode  27 . As a result, similar to the example 1-1, in the direction from the negative electrode  27  toward the first electron transport layer  25 , the barrier to electrons was significantly reduced owing to the tunnel junction. Thus, according to the first electron transport layer  25  of the example 1-2, the light-emitting element  20  having a higher luminance than that of the comparative example and higher luminous efficiency than that of the comparative example could be realized. 
     Example 1-3 
     As shown in  FIG. 5 , in the example 1-3, unlike the example 1-1, the light-emitting element  20  in which the first electron transport layer  25  was formed of ZnCo 2 O 4  was prepared. In other words, the example 1-3 is an example in which the composition of the first electron transport layer  25  is such that the constituent elements M of the compound ZnM 2 O 4  is Co. Other configuration and materials of the light-emitting element  20  according to the example 1-3 are the same as those of the light-emitting element  20  according to the example 1-1. 
     In the light-emitting element  20  of the example 1-3, the first electron transport layer  25  had the ionization potential of 6.6 eV and the electron affinity of 2.4 eV. Thus, in the light-emitting element  20  of the example 1-3, the barrier when electrons are transported from the first electron transport layer  25  to the quantum dot layer  24  is −0.5 eV expressed by a difference obtained by subtracting the electron affinity of the quantum dot layer  24  from the electron affinity of the first electron transport layer  25 . Thus, even with the light-emitting element  20  of the example 1-3, the barrier (−0.5 eV) when electrons are transported from the first electron transport layer  25  to the quantum dot layer  24  could be made smaller than the barrier (0.9 eV) when electrons were transported from the electron transport layer  85  to the quantum dot layer  84  in the light-emitting element  80  according to the comparative example. In other words, the electron affinity of the first electron transport layer  25  and the electron affinity of the quantum dot layer  24  could be brought closer to each other to reduce the injection barrier to electrons in a direction from the first electron transport layer  25  toward the quantum dot layer  24 . As a result, even with the light-emitting element  20  according to the example 1-3, as compared with the light-emitting element  80  according to the comparative example, electrons transported from the first electron transport layer  25  to the quantum dot layer  24  increased and the luminous efficiency of the light-emitting element  20  could also be increased. 
     The luminance and the luminous efficiency were measured when the light-emitting element  20  according to the example 1-3 was emitted, and the luminance was 50000 cd/m 2 , and the luminous efficiency was 18%. As described above, the light-emitting element  20  according to the example 1-3 had the luminance and the luminous efficiency higher than the luminance (20000 cd/m 2 ) and the luminous efficiency (8%) of the light-emitting element  80  according to the comparative example. 
     The light-emitting element  20  of the example 1-3 was also provided with the second electron transport layer  26  between the first electron transport layer  25  and the negative electrode  27 . As a result, similar to the example 1-1, in the direction from the negative electrode  27  toward the first electron transport layer  25 , the barrier to electrons was significantly reduced owing to the tunnel junction. Thus, according to the first electron transport layer  25  of the example 1-3, the light-emitting element  20  having a higher luminance than that of the comparative example and higher luminous efficiency than that of the comparative example could be realized. 
     Examples 1-4 to 1-11 
     As shown in  FIG. 5 , in each of the examples 1-4 to 1-11, unlike the examples 1-1 to 1-3, the light-emitting element  20  in which the constituent elements M of ZnM 2 O 4  included in the first electron transport layer  25  includes two or more elements of Co, Rh, and Ir was prepared. The example 1-4 is an example in which the composition of the first electron transport layer  25  is such that the constituent elements M of the compound ZnM 2 O 4  is Ir and Rh (Ir:Rh=50:50). The example 1-5 is an example in which the composition of the first electron transport layer  25  is such that the constituent elements M of the compound ZnM 2 O 4  is Ir and Co (Ir:Co=50:50). The example 1-6 is an example in which the composition of the first electron transport layer  25  is such that the constituent elements M of the compound ZnM 2 O 4  is Rh and Co (Rh:Co=50:50). The example 1-7 is an example in which the composition of the first electron transport layer  25  is such that the constituent elements M of the compound ZnM 2 O 4  is Ir, Rh and Co (Ir:Rh:Co=35:30:35). The example 1-8 is an example in which the composition of the first electron transport layer  25  is such that the constituent elements M of the compound ZnM 2 O 4  is Ir and Rh (Ir:Rh=50:50). The example 1-9 is an example in which the composition of the first electron transport layer  25  is such that the constituent elements M of the compound ZnM 2 O 4  is Ir and Co (Ir:Co=50:50). The example 1-10 is an example in which the composition of the first electron transport layer  25  is such that the constituent elements M of the compound ZnM 2 O 4  is Rh and Co (Rh:Co=50:50). The example 1-11 is an example in which the composition of the first electron transport layer  25  is such that the constituent elements M of the compound ZnM 2 O 4  is Ir, Rh and Co (Ir:Rh:Co=35:30:35). Other configuration and materials of the light-emitting element  20  in each of the examples 1-4 to 1-11 are the same as those of the light-emitting element  20  according to the example 1-1. 
     In the examples 1-4 to 1-7, the first electron transport layers  25  formed of a mixture was formed. In the examples 1-8 to 1-11, the first electron transport layers  25  formed of a solid solution was formed. The first electron transport layer  25  formed of the mixture in the examples 1-4 to 1-7 is formed by a three-source sputtering method using ZnIr 2 O 4 , ZnRh 2 O 4  and ZnCo 2 O 4  as separate targets. The content percentage of each element in the constituent elements M is controlled by a ratio of plasma power when the sputtering is performed. For example, in the example 1-7, in order to form a compound in which the constituent elements M contain Ir in a ratio of 35%, Rh in a ratio of 30%, and Co in a ratio of 35%, three targets ZnIr 2 O 4 , ZnRh 2 O 4 , and ZnCo 2 O 4  were prepared, and the plasma power applied to each target was set to a ratio of 35:30:35 when the sputtering was performed. Note that the first electron transport layer  25  in which the constituent elements M contain Ir in a ratio of 35%, Rh in a ratio of 30%, and Co in a ratio of 35% may be formed by sputtering using a compound containing each element mixed in the above-described ratio as one target. 
     The first electron transport layers  25  formed of the solid solutions of the examples 1-8 to 1-11 use Zn or ZnO as well as Ir, Rh, and Co as separate targets. Oxygen gas in addition to Ar gas is fed into the sputtering apparatus to form a ZnM 2 O 4  film by reactive sputtering. In this case, the content percentage of each element was controlled by the ratio of the plasma power applied to each target. 
     As in the examples 1-4 to 1-7 and the examples 1-8 to 1-11, even when the ratio of the elements included in the constituent elements M is the same, the electron affinity and the ionization potential change depending on whether the first electron transport layer  25  is the mixture or the solid solution. In general, the electronic properties differ between the simple mixture and the solid solution. The mixture is a mixture of a plurality of substances, and when each substance is uniformly mixed, for example, in the form of fine particles on the order of the nanoscale or less, the electron affinity and the ionization potential are average values of the mixed substances. On the other hand, the solid solution is in a state in which atoms are bonded to each other, and since a state of valence electron changes due to the bonding, the electron affinity and the ionization potential are not the average value of the solutionized substances and change non-linearly with respect to the composition. 
     Specifically, in the light-emitting element  20  of the example 1-4, the first electron transport layer  25  had the ionization potential of 5.2 eV and the electron affinity of 2.55 eV. In the light-emitting element  20  of the example 1-5, the first electron transport layer  25  had the ionization potential of 6.5 eV and the electron affinity of 2.50 eV. In the light-emitting element  20  of the example 1-6, the first electron transport layer  25  had the ionization potential of 6.6 eV and the electron affinity of 2.45 eV. In the light-emitting element  20  of the example 1-7, the first electron transport layer  25  had the ionization potential of 5.3 eV and the electron affinity of 2.52 eV. In the light-emitting element  20  of the example 1-8, the first electron transport layer  25  had the ionization potential of 5.3 eV and the electron affinity of 2.45 eV. In the light-emitting element  20  of the example 1-9, the first electron transport layer  25  had the ionization potential of 5.8 eV and the electron affinity of 2.30 eV. In the light-emitting element  20  of the example 1-10, the first electron transport layer  25  had the ionization potential of 6.0 eV and the electron affinity of 2.26 eV. In the light-emitting element  20  of the example 1-11, the first electron transport layer  25  had the ionization potential of 6.2 eV and the electron affinity of 2.32 eV. 
     In the light-emitting element  20  of each of the examples 1-4 to 1-11, the barrier when electrons are transported from the first electron transport layer  25  to the quantum dot layer  24  is expressed by a difference obtained by subtracting the electron affinity of the quantum dot layer  24  from the electron affinity of the first electron transport layer  25 , and is −0.35 eV in the example 1-4, −0.40 eV in the example 1-5, −0.45 eV in the example 1-6, −0.38 eV in the example 1-7, −0.45 eV in the example 1-8, −0.60 eV in the example 1-9, −0.64 eV in the example 1-10, and −0.58 eV in the example 1-11. In each of the light-emitting elements  20  according to the examples 1-4 to 1-11, the barrier when electrons are transported from the first electron transport layer  25  to the quantum dot layer  24  could be made smaller than the barrier (0.9 eV) when electrons are transported from the electron transport layer  85  to the quantum dot layer  84  in the light-emitting element  80  according to the comparative example. As a result, according to each of the light-emitting elements  20  according to the examples 1-4 to 1-11, as compared with the light-emitting element  80  according to the comparative example, electrons transported from the first electron transport layer  25  to the quantum dot layer  24  increased and the luminous efficiency of the light-emitting element  20  could be increased. 
     As in the examples 1-4 to 1-11, by using two or more elements as the constituent elements M of the first electron transport layer  25  and adjusting the composition of the two or more elements, the electron affinity and the ionization potential of the first electron transport layer  25  could be more finely adjusted. As a result, the electron affinity and the ionization potential of the first electron transport layer  25  could be readily altered depending on the electron affinity and the ionization potential of each of the adjacent quantum dot layer  24  and second electron transport layer  26 . 
     Examples 1-1 to 1-11 
     For example, in a quantum dot layer using quantum dots  28  of InP emitting green light, the ionization potential is approximately 5.2 eV. In contrast, in the examples 1-1 to 1-11 shown in  FIG. 5 , the range of the ionization potential of the first electron transport layer  25  based on ZnM 2 O 4  is from 5.1 to 6.6 eV. Thus, among the first electron transport layers  25  of these examples, a layer having an ionization potential greater than that of the adjacent quantum dot layer  24  may be selected. As a result, the barrier to the positive holes in a direction from the quantum dot layer  24  toward the first electron transport layer  25  increases, and the positive holes can be prevented from flowing out of the quantum dot layer  24 . 
     In a case where the quantum dots  28  are formed of a material that does not contain cadmium, the electron affinity tends to be reduced as compared with a case where the quantum dots  28  are formed of a material that contains cadmium. In particular, when the particle size of the quantum dots  28  is reduced such that the luminescent color of the light-emitting element  20  is green color or blue color, which is a short wavelength, the energy band at the bottom of the conduction band is displaced, and the electron affinity is further reduced. Due to this effect, there is a high possibility that the electron affinity of the quantum dot layer  24  becomes smaller than that of the first electron transport layer  25 . Accordingly, the barrier to electrons in the direction from the first electron transport layer  25  toward the quantum dot layer  24  increases. As a result, electrons transported from the first electron transport layer  25  to the quantum dot layer  24  decreases. When the number of electrons supplied in the quantum dot layer  24  decreases, the recombination rate between the positive holes and electrons is reduced and the luminous efficiency decreases. 
     For example, similar to the electron transport layer  85  of the comparative example, when ZnO, which is a common material, is used in the first electron transport layer  25 , the electron affinity of the first electron transport layer  25  is 3.8 eV. In contrast, in a case where the light-emitting elements  20  of three primary colors are configured by the quantum dot layer  24  using InP as the quantum dots  28  that do not contain cadmium, the numerical range of the electron affinity of the quantum dot layer  24  is from 2.5 eV to 3.1 eV. Thus, in a case where the material of the first electron transport layer  25  is ZnO, the injection barrier to electrons increases to from 0.7 to 1.3 eV in the direction from the first electron transport layer  25  toward the quantum dot layer  24 , and the luminous efficiency of the quantum dot layer  24  decreases. 
     In contrast, in a case where ZnM 2 O 4  is used as the material of the quantum dots  28  of the first electron transport layer  25 , the numerical range of the electron affinity of the first electron transport layer  25  in the example 1-1 to the example 1-11 is from 2.26 to 2.60 eV as illustrated in  FIG. 4 . Thus, the difference between the electron affinity of the quantum dot layer  24  and the electron affinity of the first electron transport layer  25  is at most 0.1 eV, and the difference is smaller than that in the case where ZnO is used in the first electron transport layer  25 . Thus, the barrier to electrons between the quantum dot layer  24  and the first electron transport layer  25  is reduced, and electrons injected from the negative electrode  27  into the first electron transport layer  25  can be efficiently injected from the first electron transport layer  25  into the quantum dot layer  24 . As a result, the recombination rate between electrons and positive holes in the quantum dots  28  is improved, and the luminous efficiency of the quantum dot layer  24  can be improved. As described above, the first electron transport layer  25  of the present embodiment is suitable for the quantum dots  28  using the material that does not contain cadmium. 
     Second Embodiment 
     A light-emitting element  20  according to the second embodiment differs from the light-emitting element  20  according to the first embodiment in that a composition ratio of the elements included in the constituent elements M of the first electron transport layer  25  changes in the layering direction of the light-emitting element  20 . The structure of the light-emitting element  20  according to the second embodiment is the same as the structure of the light-emitting element  20  according to the first embodiment described with reference to  FIG. 2 . 
     The first electron transport layer  25  included in the light-emitting element  20  according to the second embodiment may be formed of a single layer or may be formed of a plurality of layers, and the composition ratio of Co, Rh, and Ir included in the constituent elements M within the layer of the first electron transport layer  25  differs between the side close to the quantum dot layer  24  and the side close to the negative electrode  27 . Specifically, the ionization potential of the first electron transport layer  25  gradually increases from the side close to the quantum dot layer  24  toward the side close to the negative electrode  27 . For example, in the light-emitting element  20  of the second embodiment, the composition of the first electron transport layer  25  is Zn(Co x Rh y Ir z , 0≤x≤1, 0≤y+z≤1−x) 2 O 4 , and the Co composition x increases from the quantum dot layer  24  toward the negative electrode  27  side. 
     As a result, the ionization potential of the first electron transport layer  25  increases from the quantum dot layer  24  toward the negative electrode  27 . Thus, the positive holes injected into the quantum dot layer  24  can be more reliably prevented from flowing out from the quantum dot layer  24  to the negative electrode  27  side through the first electron transport layer  25 . As a result, a reduction in luminous efficiency in the quantum dot layer  24  can be suppressed. Hereinafter, an example 2, which is an example of the second embodiment, will be described in detail. 
     Example 2 
       FIG. 6  is an energy diagram illustrating an example of the electron affinity and the ionization potential of each layer between the positive electrode  21  and the negative electrode  27  of the light-emitting element  20  according to the example 2. Note that in  FIG. 6 , each of the work functions (in eV) of the positive electrode  21  and the negative electrode  27  is expressed by a numerical value. As illustrated in  FIG. 6 , the light-emitting element  20  in which the first electron transport layer  25  was formed using the compound Zn(Co x Rh 1-x ) 2 O 4  was prepared. Other configuration and materials of the light-emitting element  20  according to the example 2 are the same as those of the light-emitting element  20  according to the example 1-1. 
     In the example 2, the light-emitting element  20  was prepared in which the first electron transport layer  25  was formed by setting the composition of the first electron transport layer  25  such that, of the compound Zn(Co x Rh 1-x ) 2 O 4 , the side in contact with the quantum dot layer  24  is x=0, and the side in contact with the second electron transport layer  26  is x=1. Specifically, the first electron transport layer  25  was formed such that the compound Zn(Co x Rh 1-x ) 2 O 4  in the composition of the first electron transport layer  25  increased from the side close to the quantum dot layer  24 , to the side close to the second electron transport layer  26 . According to this configuration, the compound Zn(Co x Rh 1-x ) 2 O 4  of the first electron transport layer  25  had a composition of ZnRh 2 O 4  on the side in contact with the quantum dot layer  24 , and a composition of ZnCo 2 O 4  on the side in contact with the second electron transport layer  26 . In the middle of them, the composition was Zn(Co.Rh)O 4 . The electron affinity and the ionization potential of the first electron transport layer  25  changed from a value for ZnRh 2 O 4  to a value for ZnCo 2 O 4 . Specifically, the electron affinity of the first electron transport layer  25  was 2.5 eV on the quantum dot layer  24  side, and 2.4 eV on the second electron transport layer  26  side, and in the middle of them, the electron affinity was decreased. The ionization potential of the first electron transport layer  25  was 5.6 eV on the quantum dot layer  24  side, and 6.6 eV on the second electron transport layer  26  side, and in the middle of them, the ionization potential was increased. 
     As described above, in the light-emitting element  20  of the example 2, the ionization potential of the first electron transport layer  25  gradually increases from the quantum dot layer  24  toward the negative electrode  27 . Thus, the positive holes injected into the quantum dot layer  24  could be more reliably prevented from flowing out from the quantum dot layer  24  to the negative electrode  27  side through the first electron transport layer  25 . As a result, the luminous efficiency of the quantum dot layer  24  was further improved. 
     Third Embodiment 
     A light-emitting element  20  according to the third embodiment is the same as the light-emitting element  20  according to the second embodiment in that the composition ratio of the elements included in the constituent elements M of the first electron transport layer  25  changes in the layering direction of the light-emitting element  20 , but differs from the light-emitting element  20  according to the second embodiment in that the first electron transport layer  25  is formed of a plurality of layers, and the ionization potential of the first electron transport layer  25  stepwisely increases from the side close to the quantum dot layer  24  toward the side close to the negative electrode  27 . The structure of the light-emitting element  20  according to the third embodiment is the same as the structure of the light-emitting element  20  according to the first embodiment described with reference to  FIG. 2  except for the first electron transport layer  25 . 
     The first electron transport layer  25  in the light-emitting element  20  according to the third embodiment includes, for example, a first layer, a second layer, and a third layer, which are a plurality of layers sequentially layered from the quantum dot layer  24  toward the negative electrode  27  side, and the first layer, the second layer, and the third layer are different from each other in the composition of Co, Rh, and Ir included in the constituent elements M. As a result, the ionization potential of the first electron transport layer  25  stepwisely increases in the order of the first layer, the second layer, and the third layer. 
     For example, the first electron transport layer  25  including the plurality of layered layers according to the third embodiment is formed of n layers each having a composition different from each other. Of the layers forming the first electron transport layer  25 , a layer in contact with the quantum dot layer  24  is referred to as a first, and a layer furthest away from the quantum dot layer  24  is referred to as an n-th. A composition of each layer forming the n layers is expressed by Zn(Co xn Rh yn Ir zn , 0≤xn≤1, 0≤yn+zn≤1−xn) 2 O 4 . An Ir composition of the first layer of the first electron transport layer  25  including the plurality of layered layers in contact with the quantum dot layer  24 , may be set to be the highest, and a relationship between the Ir composition in each layer may be set to y1≤yn (n≠1). 
     According to the light-emitting element  20  according to the third embodiment, the ionization potential of each of the layers included in the first electron transport layer  25  stepwisely increases in the direction from the side of the quantum dot layer  24  toward the negative electrode  27 . Thus, the positive holes injected from the positive electrode  21  into the quantum dot layer  24  can be more reliably prevented from flowing out from the quantum dot layer  24  to the negative electrode  27  through the first electron transport layer  25 . As a result, a reduction in luminous efficiency in the quantum dot layer  24  can be suppressed. 
     Note that the layers forming the first electron transport layer  25  is not limited to three layers, and may be any number. For example, the first electron transport layer  25  may be two layers or four layers or more. Hereinafter, an example 3, which is an example of the third embodiment, will be described in detail. 
     Example 3 
       FIG. 7  is an energy diagram illustrating an example of the electron affinity and the ionization potential of each layer between the positive electrode  21  and the negative electrode  27  of the light-emitting element  20  according to the example 3. Note that in  FIG. 7 , each of the work functions (in eV) of the positive electrode  21  and the negative electrode  27  is expressed by a numerical value. As illustrated in  FIG. 7 , the first electron transport layer  25  is formed by layering a first layer  251 , a second layer  252 , and a third layer  253 , which are a plurality of layers sequentially layered in order from the quantum dot layer  24  toward the negative electrode  27 , whereby the light-emitting element  20  was prepared. In the light-emitting element  20  according to the example 3, a configuration and materials except for the first electron transport layer  25  are the same as those of the light-emitting element  20  according to the example 1-1. 
     The first electron transport layer  25  was a three-layer structure of the first layer  251 , the second layer  252 , and the third layer  253 , where the first layer  251  was formed using ZnRh 2 O 4 , the second layer  252  was formed using Zn(Co.Rh)O 4 , and the third layer  253  was formed using ZnCo 2 O 4 . As a result, the first layer  251  had the ionization potential of 5.6 eV and the electron affinity of 2.5 eV. The second layer  252  had the ionization potential of 6.6 eV and the electron affinity of 2.45 eV. The third layer  253  had the ionization potential of 6.6 eV and the electron affinity of 2.4 eV. Thus, the ionization potential of each of the first layer  251 , the second layer  252 , and the third layer  253  stepwisely increased in the direction from the side of the quantum dot layer  24  toward the negative electrode  27 . Thus, the positive holes injected from the positive electrode  21  into the quantum dot layer  24  can be more reliably prevented from flowing out from the quantum dot layer  24  to the negative electrode  27  through the first electron transport layer  25 . As a result, the luminous efficiency in the quantum dot layer was further improved. 
     The disclosure is not limited to the embodiments described above. Embodiments obtained by modifying above-described embodiments and embodiments obtained by appropriately combining technical approaches disclosed in above-described embodiments also fall within the scope of the technology of the disclosure.