Patent Publication Number: US-2023143536-A1

Title: Light-emitting element and display device

Description:
TECHNICAL FIELD 
     The present invention relates to a light-emitting element and a display device including the light-emitting element. 
     BACKGROUND ART 
     PTL 1 discloses a method for achieving high efficiency light emission of a light-emitting layer and suppression of deterioration of the light-emitting layer by adding dopants to each organic layer in an organic light-emitting device provided with a plurality of organic layers between electrodes. 
     CITATION LIST 
     Patent Literature 
     
         
         PTL 1: WO 2012/039213 A1 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     Even in the light-emitting device (light-emitting element) disclosed in PTL 1, a decrease in luminous efficiency and a shortening of the lifetime of the light-emitting device (light-emitting element) occur due to an accumulation in each organic layer of carriers injected from each electrode into each organic layer between the electrodes, or in other words, due to a bias of the carrier balance in the light-emitting layer. 
     Solution to Problem 
     In order to solve the problem described above, a light-emitting element according to the present invention is provided with an anode electrode and a cathode electrode, and further includes, between the anode electrode and the cathode electrode in order from the anode electrode side, a first hole transport layer, a second hole transport layer, a light-emitting layer, a first electron transport layer, and a second electron transport layer, wherein at a HOMO level, an energy level difference between the second hole transport layer and the light-emitting layer on the second hole transport layer side is from 0.0 eV to 0.15 eV, at a LUMO level, an energy level difference between the first electron transport layer and the light-emitting layer on the first electron transport layer side is from 0.0 eV to 0.15 eV, and the second electron transport layer is a mixed layer including an organic material having electron transport properties and an electron-accepting material, and containing the electron-accepting material in an amount greater than 50 mass %. 
     Advantageous Effects of Invention 
     According to the configuration described above, a light-emitting element with an extended lifetime and a display device provided with the light-emitting element can be provided in which the carrier injected from each electrode is transported to the light-emitting layer more efficiently, and the luminous efficiency is improved. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a schematic cross-sectional view of a display device according to a first embodiment of the present invention. 
         FIG.  2    is a schematic top view of the display device according to the first embodiment of the present invention. 
         FIG.  3    is an energy diagram illustrating an example of the Fermi level, or the LUMO level and the HOMO level, of each layer in the light-emitting element of the display device according to the first embodiment of the present invention. 
         FIG.  4    is an energy diagram illustrating an example of the Fermi level, or the LUMO level and the HOMO level, of each layer in another light-emitting element of the display device according to the first embodiment of the present invention. 
         FIG.  5    is an energy diagram illustrating an example of the Fermi level, or the LUMO level and the HOMO level, of each layer in the light-emitting elements according to each of Example 1 and Example 2 or the present invention. 
         FIG.  6    is an energy diagram illustrating an example of the Fermi level, or the LUMO level and the HOMO level, of each layer in the light-emitting elements according to each of Example 3 and Example 4 of the present invention. 
         FIG.  7    is a spectrum diagram of a Cole-Cole plot showing the results of impedance measurements carried out on the light-emitting elements according to each of Example 1 and Example 2 of the present invention. 
         FIG.  8    is a spectrum diagram of a Cole-Cole plot showing the results of impedance measurements carried out on the light-emitting elements according to each of Example 3 and Example 4 of the present invention. 
         FIG.  9    is a spectral diagram of a Bode plot showing the results of impedance measurements carried out on the light-emitting elements according to each of Example 1 and Example 2 of the present invention. 
         FIG.  10    is a spectral diagram of a Bode plot showing the results of impedance measurements carried out on the light-emitting elements according to each of Example 3 and Example 4 of the present invention. 
         FIG.  11    is a schematic cross-sectional view of a display device according to a second embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
       FIG.  2    is a schematic top view of a display device  2  according to the present embodiment.  FIG.  1    is a cross-sectional view taken along a line A-A in  FIG.  2   . 
     As illustrated in  FIG.  2   , the display device  2  according to the present embodiment includes a light-emitting region DA from which light emission is extracted and a frame region NA surrounding a periphery of the light-emitting region DA. A terminal T into which is input a signal for driving each light-emitting element of the display device  2  described in detail below is formed in the frame region NA. 
     At a position overlapping with the light-emitting region DA in a plan view, as illustrated in  FIG.  1   , the display device  2  according to the present embodiment includes an array substrate  4  and a light-emitting element layer  6  on the array substrate  4 . In particular, the display device  2  has a structure in which respective layers of the light-emitting element layer  6  are laminated on the array substrate  4 , in which a Thin Film Transistor (TFT; not illustrated) is formed. Note that, in the present specification, a direction from the light-emitting element layer  6  to the array substrate  4  of the display device  2  is referred to as a “downward direction”, and a direction from the light-emitting element layer  6  of the display device  2  to the display surface of the display device  2  is referred to as an “upward direction”. 
     The light-emitting element layer  6  includes, on an anode electrode  8 , a first hole transport layer  10 , a second hole transport layer  12 , a light-emitting layer  14 , a first electron transport layer  16 , a second electron transport layer  18 , and a cathode electrode  20 , sequentially laminated from the lower layer. The anode electrode  8  of the light-emitting element layer  6  formed in an upper layer on the array substrate  4  is electrically connected with TFTs of the array substrate  4 . Note that, in the display device  2 , a sealing layer (not illustrated) is provided to seal the light-emitting element layer  6 . 
     In the present embodiment, the light-emitting element layer  6  includes a light-emitting element  6 R, a light-emitting element  6 G, and a light-emitting element  6 B. The light-emitting element  6 R, the light-emitting element  6 G, and the light-emitting element  6 B may be organic EL elements, that is, OLED elements, in which the light-emitting layer  14  includes an organic fluorescent material or an organic phosphorescent material. In addition to this, the light-emitting element  6 R, the light-emitting element  6 G, and the light-emitting element  6 B may be QLED elements in which the light-emitting layer  14  includes a semiconductor nanoparticle material, that is, a quantum dot material. However, in the present embodiment, various light-emitting elements, without being limited to the OLED elements or the QLED elements, can be used for the light-emitting element  6 R, the light-emitting element  6 G, and the light-emitting element  6 B. The display device  2  has, for example, a plurality of sub-pixels, and each sub-pixel is provided with one light-emitting element  6 R, one light-emitting element  6 G, and one light-emitting element  6 B described above. 
     Here, each of the anode electrode  8 , the second hole transport layer  12 , and light-emitting layer  14  is separated by edge covers  22 . In particular, in the present embodiment, the anode electrode  8  is separated into an anode electrode  8 R for the light-emitting element  6 R, an anode electrode  8 G for the light-emitting element  6 G, and an anode electrode  8 B for the light-emitting element  6 B by the edge covers  22 . The second hole transport layer  12  is separated into a second hole transport layer  12 R for the light-emitting element  6 R, a second hole transport layer  12 G for the light-emitting element  6 G, and a second hole transport layer  12 B for the light-emitting element  6 B by the edge covers  22 . Furthermore, the light-emitting layer  14  is separated into a light-emitting layer  14 R, a light-emitting layer  14 G, and a light-emitting layer  14 B by the edge covers  22 . 
     Furthermore, the light-emitting layer  14 G includes a first light-emitting layer  14 GH and a second light-emitting layer  14 GE laminated from the anode electrode  8  side. The first light-emitting layer  14 GH is a hole transport type light-emitting layer, and the second light-emitting layer  14 GE is an electron transport type light-emitting layer. In other words, the first light-emitting layer  14 GH includes a host material having hole transport properties, and the second light-emitting layer  14 GE includes a host material having electron transport properties. 
     Thus, in the present embodiment, the light-emitting layer  14 G includes at least two or more types of host materials. In particular, in the present embodiment, the light-emitting layer  14 G is provided with only one type of host material in the first light-emitting layer  14 GH and only one type of host material in the second light-emitting layer  14 GE, the host materials thereof being mutually different, and therefore the light-emitting layer  14 G includes only two types of host materials. 
     Note that the first hole transport layer  10 , the first electron transport layer  16 , the second electron transport layer  18 , and the cathode electrode  20  are not separated by the edge covers  22 , and are formed in common. 
     As illustrated in  FIG.  1   , the edge covers  22  may be formed at positions covering the side surfaces and a vicinity of peripheral end portions of the upper faces of the anode electrode  8 . 
     In the present embodiment, the light-emitting element  6 R includes the anode electrode  8 R, the first hole transport layer  10 , the second hole transport layer  12 R, the light-emitting layer  14 R, the first electron transport layer  16 , the second electron transport layer  18 , and the cathode electrode  20 . The light-emitting element  6 G includes the anode electrode  8 G, the first hole transport layer  10 , the second hole transport layer  12 G, the light-emitting layer  14 G, the first electron transport layer  16 , the second electron transport layer  18 , and the cathode electrode  20 . Furthermore, the light-emitting element  6 B includes the anode electrode  8 B, the first hole transport layer  10 , the second hole transport layer  12 B, the light-emitting layer  14 B, the first electron transport layer  16 , the second electron transport layer  18 , and the cathode electrode  20 . 
     In the present embodiment, the light-emitting layer  14 R, the light-emitting layer  14 G, and the light-emitting layer  14 B emit red light, green light, and blue light, respectively. In other words, the light-emitting element  6 R, the light-emitting element  6 G, and the light-emitting element  6 B are light-emitting elements that emit the red light, the green light, and the blue light, respectively. 
     Here, the blue light refers to, for example, light having a light emission central wavelength in a wavelength band of equal to or greater than 400 nm and equal to or less than 500 nm. The green light refers to, for example, light having a light emission central wavelength in a wavelength band of greater than 500 nm and equal to or less than 600 nm. The red light refers to, for example, light having a light emission central wavelength in a wavelength band of greater than 600 nm and equal to or less than 780 nm. 
     In the present embodiment, the light-emitting layer  14 R and the light-emitting layer  14 B are in contact with the second hole transport layer  12  on the anode electrode  8  side and the first electron transport layer  16  on the cathode electrode  20  side. In other words, the single layer light-emitting layer  14 R and the light-emitting layer  14 B are in contact with both the second hole transport layer  12  and the first electron transport layer 
     On the other hand, the first light-emitting layer  14 GH is in contact with the second hole transport layer  12  on the anode electrode  8  side, and is in contact with the second light-emitting layer  14 GE on the cathode electrode  20  side. The second light-emitting layer  14 GE is in contact with the first light-emitting layer  14 GH on the anode electrode  8  side and is in contact with the first electron transport layer  16  on the cathode electrode  20  side. 
     Note that the display device  2  according to the present embodiment is not limited to the configuration described above, and may include another layer between the second hole transport layer  12  and the light-emitting layer  14  or between the light-emitting layer  14  and the first electron transport layer  16 . 
     The anode electrode  8  and the cathode electrode  20  include conductive materials and are electrically connected to the first hole transport layer  10  and the second electron transport layer  18 , respectively. Of the anode electrode  8  and the cathode electrode  20 , the electrode closer to the display surface of the display device  2  is a semitransparent electrode. 
     The anode electrode  8  has a configuration in which ITO (Indium Tin Oxide) is laminated on, for example, an Ag—Pd—Cu alloy. The anode electrode  8  having the above configuration is a reflective electrode that reflects light emitted from the light-emitting layer  14 . Thus, among the light emitted from the light-emitting layer  14 , light directed in the downward direction can be reflected by the anode electrode  8 . 
     On the other hand, the cathode electrode  20  is configured by, for example, a semi-transparent Mg—Ag alloy. In other words, the cathode electrode  20  is a transmissive electrode that transmits light emitted from the light-emitting layer  14 . Thus, among the light emitted from the light-emitting layer  14 , light directed in the upward direction passes through the cathode electrode  20 . In this manner, the display device  2  can emit the light emitted from the light-emitting layer  14  in the upward direction. 
     As described above, in the display device  2 , both the light emitted in the upward direction and the light emitted in the downward direction from the light-emitting layer  14  can be directed toward the cathode electrode  20  (upward direction). That is, the display device  2  is configured as a top-emitting type display device. 
     In the present embodiment, the cathode electrode  20 , which is a semitransparent electrode, may partially reflect the light emitted from the light-emitting layer  14 . In addition, a cavity of the light emitted from the light-emitting layer  14  is formed between the anode electrode  8 , which is a reflective electrode, and the cathode electrode  20 , which is a semitransparent electrode. By forming the cavity between the anode electrode  8  and the cathode electrode  20 , the chromaticity of the light emitted from the light-emitting layer  14  can be improved. 
     Note that the configuration of the anode electrode  8  and the cathode electrode  20  described above is an example, and may be another configuration. 
     The light-emitting layer  14  is a layer that emits light as a result of an occurrence of recombination between the positive holes transported from the anode electrode  8  and the electrons transported from the cathode electrode  20 . Note that in the light-emitting element  6 G, the positive holes transported to the first light-emitting layer  14 GH and the electrons transported to the second light-emitting layer  14 GE are transported to the interface between the first light-emitting layer  14 GH and the second light-emitting layer  14 GE, and recombine in the vicinity of the interface. 
     The first hole transport layer  10  and the second hole transport layer  12  are layers that transport positive holes from the anode electrode  8  to the light-emitting layer  14 . The second hole transport layer  12  has a function of inhibiting the transport of electrons from the cathode electrode  20 . The first electron transport layer  16  and the second electron transport layer  18  are layers that transport electrons from the cathode electrode  20  to the light-emitting layer  14 . The first electron transport layer  16  has a function of inhibiting the transport of positive holes from the anode electrode  8 . 
     In the present embodiment, the second electron transport layer  18  is a mixed layer including an organic material having electron transport properties and an electron-accepting material. In particular, the second electron transport layer  18  contains the electron-accepting material in an amount greater than 50 mass %. 
     The electron-accepting material included in the second electron transport layer  18  has a function of temporarily capturing the electrons transported by the electron-transporting organic material of the second electron transport layer  18  while the electrons are transported to the first electron transport layer  16 . Thus, the electron-accepting material included in the second electron transport layer  18  causes the transport of electrons to the first electron transport layer  16 , and by extension the transport of electrons to the light-emitting layer  14 , to be more stably implemented. Accordingly, an injection of excess electrons in the light-emitting layer  14  is prevented, and electron excess in the light-emitting layer  14  can be prevented. 
     When the vacuum level is referenced, the electron-transporting organic material included in the second electron transport layer  18  has, for example, a HOMO level from −6.60 eV to −6.00 eV and a LUMO level from −2.95 eV to −2.45 eV. In the present embodiment, the electron-transporting organic material included in the second electron transport layer  18  has, for example, an oxadiazole structure or a triazole structure. Specifically, for example, the electron-transporting organic material included in the second electron transport layer  18  is an oxadiazole derivative (OXD-7) represented by the following formula. 
     
       
         
         
             
             
         
       
     
     In addition, the electron-transporting organic material included in the second electron transport layer  18  may be a starburst OXD, an oxadiazole derivative (Bu—PBD), a triazole derivative, or bathocuproine, which are represented by the following formulas, respectively. 
     
       
         
         
             
             
         
       
     
     When the vacuum level is referenced, the electron-accepting material included in the second electron transport layer  18  has, for example, a HOMO level from −5.890 eV to −5.70 eV and a LUMO level from −3.55 eV to −3.35 eV. In the present embodiment, the electron-accepting material included in the second electron transport layer  18  is, for example, a lithium complex or a lithium compound. Specifically, for example, the electron-accepting material included in the second electron transport layer  18  is a lithium quinolate complex (Liq) represented by the following formula. 
     
       
         
         
             
             
         
       
     
     The second electron transport layer  18  includes the lithium quinolate complex as an electron-accepting material, and thereby electrons of the second electron transport layer  18  are more stably transported to the first electron transport layer  16 . 
     In addition, the electron-accepting material included in the second electron transport layer  18  may be trifluoromethanesulfonyl (Li-TFSI), lithium acetoacetate, lithium bis (trimethylsilyl) amide, lithium butoxide, or, alternatively, lithium 1,1,2,2,3,3-hexafluoropropane-1,3-disulfonimide, represented by the following formulas, respectively. 
     
       
         
         
             
             
         
       
     
     The first hole transport layer  10 , the second hole transport layer  12 , the light-emitting layer  14 , the first electron transport layer  16 , and the second electron transport layer  18  may be formed by a conventionally known technique, and may be formed by, for example, vapor deposition using a vapor deposition mask. In particular, the second electron transport layer  18  may be formed by co-evaporation of an organic material having electron transport properties and an electron-accepting material. 
     Note that the display device  2  according to the present embodiment may include, between the anode electrode  8  and the first hole transport layer  10 , a hole injection layer (not illustrated) containing a hole injection material. Similarly, the display device  2  according to the present embodiment may include, between the cathode electrode  20  and the second electron transport layer  18 , an electron injection layer (not illustrated) containing an electron injection material. 
     Each organic layer of each of the light-emitting elements  6 B,  6 G, and  6 R according to the present embodiment has a layer thickness as illustrated in  FIG.  1   . In particular, as illustrated in  FIG.  1   , the first hole transport layer  10 , the second hole transport layer  12 , the light-emitting layer  14 , the first electron transport layer  16 , and the second electron transport layer  18  have layer thicknesses of d 10 , d 12 , d 14 , d 16 , and d 18 , respectively. 
     Here, in the present specification, the layer thickness of a certain layer may be an average value of the layer thickness of the layer, or may be an average value of the layer thickness of the layer at a position at which the layer is formed substantially horizontal to the array substrate  4 . In the light-emitting element  6 G, the layer thickness d 14  is the total layer thickness of the first light-emitting layer  14 GH and the second light-emitting layer  14 GE. Furthermore, the layer thickness d 10 , the layer thickness d 12 , and the layer thickness d 14  may be substantially the same or mutually different between the light-emitting element  6 B, the light-emitting element  6 G, and the light-emitting element  6 R. 
     In each of the light-emitting elements  6 B,  6 G, and  6 R according to the present embodiment, the layer thickness d 14  is greater than both the layer thickness d 12  and the layer thickness d 16 . In other words, in each of the light-emitting elements  6 B,  6 G, and  6 R, the layer thickness of the light-emitting layer  14  is greater than the layer thickness of the second hole transport layer  12 , and is greater than the layer thickness of the first electron transport layer  16 . 
     According to the configuration described above, in each of the light-emitting elements  6 B,  6 G, and  6 R, the layer thickness of the transport layer of carriers, the layer thereof being adjacent to the light-emitting layer  14 , is thinner than the layer thickness of the light-emitting layer  14 . As a result, in each of the light-emitting elements  6 B,  6 G, and  6 R, the carriers are more efficiently transported in the carrier transport layer adjacent to the light-emitting layer  14 , and the carriers are more easily injected by the light-emitting layer  14 . 
     Additionally, in the light-emitting element  6 B, a value obtained by dividing the layer thickness d 12  by the layer thickness d 14  is greater than 0 and equal to or less than 0.25, and a value obtained by dividing the layer thickness d 16  by the layer thickness d 12  is greater than 0 and equal to or less than 0.5. In other words, in the light-emitting element  6 B, the layer thickness of the second hole transport layer  12 B is equal to or less than ¼ the layer thickness of the light-emitting layer  14 B, and the layer thickness of the first electron transport layer  16  is equal to or less than one-half the layer thickness of the second hole transport layer  12 B. However, the layer thickness of the second hole transport layer  12 B and the layer thickness of the first electron transport layer  16  are not 0. 
     On the other hand, in the light-emitting element  6 G, a value obtained by dividing the layer thickness d 12  by the layer thickness d 14  is greater than 0 and equal to or less than 0.75, and a value obtained by dividing the layer thickness d 16  by the layer thickness d 12  is greater than 0 and equal to or less than 0.5. In other words, in the light-emitting element  6 G, the layer thickness of the second hole transport layer  12 G is equal to or less than ¾ the layer thickness of the light-emitting layer  14 G, and the layer thickness of the first electron transport layer  16  is equal to or less than one-half the layer thickness of the second hole transport layer  12 G. However, the layer thickness of the second hole transport layer  12 G and the layer thickness of the first electron transport layer  16  are not 0. 
     According to the configuration described above, in the light-emitting element  6 B and the light-emitting element  6 G, the layer thickness of the second hole transport layer  12  is thinner than the layer thickness of the light-emitting layer  14 , and the layer thickness of the first electron transport layer  16  is even thinner. As a result, in the light-emitting element  6 B and the light-emitting element  6 G, carriers are more efficiently transported in the carrier transport layer adjacent to the light-emitting layer  14 , and the carriers are more easily injected by the light-emitting layer  14 . 
     Furthermore, in each of the light-emitting elements  6 B,  6 G, and  6 R, a value obtained by dividing the layer thickness d 16  by the total layer thickness of the layer thickness d 16  and the layer thickness d 18  is greater than 0 and less than 0.5. In other words, the layer thickness of the first electron transport layer  16  is less than half of the total layer thickness of the first electron transport layer  16  and the second electron transport layer  18 . However, the layer thickness of the first electron transport layer  16  is not 0. 
     Ordinarily, when the total layer thickness of the first electron transport layer  16  and the second electron transport layer  18  is constant, as the film thickness of the first electron transport layer  16  becomes thinner, the amount of electrons injected into the light-emitting layer  14  is reduced. According to the configuration described above, excessive injection of electrons into the light-emitting layer  14  is efficiently reduced, and the lifetime of the light-emitting layer  14  is improved. 
     Next, an energy band in each layer of each light-emitting element included in the light-emitting element layer  6  of the display device  2  according to the present embodiment will be described with reference to  FIG.  3    and  FIG.  4   .  FIG.  3    is an energy band diagram illustrating an example of the Fermi level or the band gap of each layer of the light-emitting element  6 B of the display device  2  according to the present embodiment.  FIG.  4    is an energy band diagram illustrating an example of the Fermi level or the band gap of each layer of the light-emitting element  6 G of the display device  2  according to the present embodiment. 
     Note that the energy band diagram of the present specification illustrates the energy level of each layer on the basis of a vacuum level. Further, the energy band diagram of the present specification illustrates a Fermi level or a band gap of a member corresponding to a provided member number. The Fermi levels are indicated for the anode electrode  8  and the cathode electrode  20 , and the band gaps from the LUMO level to the HOMO level are indicated for the first hole transport layer  10 , the second hole transport layer  12 , the light-emitting layer  14 , the first electron transport layer  16 , and the second electron transport layer  18 . 
     Here, the difference between the HOMO level and the LUMO level between each layer in the light-emitting element layer  6  according to the present embodiment will be described with reference to  FIG.  3    and  FIG.  4   . In the present specification, a value obtained by subtracting the value of the HOMO level of the second layer from the value of the HOMO level of the first layer is referred to as an energy level difference between the HOMO level of the first layer and the HOMO level of the second layer. On the other hand, in the present specification, a value obtained by subtracting the value of the LUMO level of the first layer from the value of the LUMO level of the second layer is referred to as an energy level difference between the LUMO level of the first layer and the LUMO level of the second layer. 
     In  FIG.  3    and  FIG.  4   , H 1  indicates the energy level difference between the HOMO level of the first hole transport layer  10  and the HOMO level of the second hole transport layer  12  in each light-emitting element. H 2  indicates the energy level difference between the HOMO level of the second hole transport layer  12  and the HOMO level of the light-emitting layer  14  in each light-emitting element. H 3  indicates the energy level difference between the HOMO level of the light-emitting layer  14  and the HOMO level of the first electron transport layer  16  in each light-emitting element. H 4  indicates the energy level difference between the HOMO level of the first electron transport layer  16  and the HOMO level of the second electron transport layer  18  in each light-emitting element. 
     Also, in  FIG.  3    and  FIG.  4   , E 1  indicates the energy level difference between the LUMO level of the second electron transport layer  18  and the LUMO level of the first electron transport layer  16  in each light-emitting element. E 2  indicates the energy level difference between the LUMO level of the first electron transport layer  16  and the LUMO level of the light-emitting layer  14  in each light-emitting element. E 3  indicates the energy level difference between the LUMO level of the light-emitting layer  14  and the LUMO level of the second hole transport layer  12  in each light-emitting element. E 4  indicates the energy level difference between the LUMO level of the second hole transport layer  12  and the LUMO level of the first hole transport layer  10  in each light-emitting element. 
     In particular, the energy level difference H 2  in  FIG.  4    indicates the energy level difference between the HOMO level of the second hole transport layer  12 G and the HOMO level of the first light-emitting layer  14 GH in the light-emitting element  6 G. The energy level difference E 2  in  FIG.  4    indicates the energy level difference between the LUMO level of the first electron transport layer  16  and the LUMO level of the second light-emitting layer  14 GE in the light-emitting element  6 G. 
     In  FIG.  4   , H 5  indicates the energy level difference between the HOMO level of the first light-emitting layer  14 GH and the HOMO level of the second light-emitting layer  14 GE in the light-emitting element  6 G. E 5  indicates the energy level difference between the LUMO level of the second light-emitting layer  14 GE and the LUMO level of the first light-emitting layer  14 GH in the light-emitting element  6 G. 
     In each of the light-emitting elements  6 B and  6 G according to the present embodiment, both the energy level difference H 2  and the energy level difference E 2  are from 0.0 eV to 0.15 eV. In other words, in the light-emitting element  6 B, the energy level difference in terms of the HOMO level between the second hole transport layer  12 B and the light-emitting layer  14 B, and the energy level difference in terms of the LUMO level between the first electron transport layer  16  and the light-emitting layer  14 B is from 0.0 eV to 0.15 eV. In addition, in the light-emitting element  6 G, the energy level difference in terms of the HOMO level between the second hole transport layer  12 G and the first light-emitting layer  14 GH, and the energy level difference in terms of the LUMO level between the first electron transport layer  16  and the second light-emitting layer  14 GE is from 0.0 eV to 0.15 eV. 
     According to the configuration described above, in each of the light-emitting elements  6 B and  6 G, barriers of injection of positive holes from the second hole transport layer  12  into the light-emitting layer  14 , and barriers of injection of positive holes from the first electron transport layer  16  into the light-emitting layer  14  are reduced. Thus, the injection efficiency of each carrier into the light-emitting layer  14  is improved in each of the light-emitting elements  6 B and  6 G. 
     The value of the HOMO level of the light-emitting layer  14 B is greater than the value of the HOMO level of the first electron transport layer  16  by 0.25 eV or more, and more preferably by 0.45 eV or more. Furthermore, the value of the LUMO level of the second hole transport layer  12 B is greater than the value of the LUMO level of the light-emitting layer  14 B by 0.25 eV or more, and more preferably by 0.45 eV or more. 
     The value of the HOMO level of the second light-emitting layer  14 GE is greater than the value of the HOMO level of the first electron transport layer  16  by 0.25 eV or more, and more preferably by 0.45 eV or more. Furthermore, the value of the LUMO level of the second hole transport layer  12 G is greater than the value of the LUMO level of the first light-emitting layer  14 GH by 0.25 eV or more, and more preferably by 0.45 eV or more. 
     Also, the value of the HOMO level of each of the light-emitting layer  14 B and the second light-emitting layer  14 GE is greater than the value of the HOMO level of the first electron transport layer  16  and/or the value of the HOMO level of the second electron transport layer  18  by 0.45 eV or more. Furthermore, the value of the LUMO level of the first hole transport layer  10  and/or the value of the LUMO level of the second hole transport layer  12 B and the second hole transport layer  12 G is greater than the value of the LUMO level of each of the light-emitting layer  14 B and the first light-emitting layer  14 GH by 0.45 eV or more. 
     Furthermore, the value of the HOMO level of the first light-emitting layer  14 GH is greater than the value of the HOMO level of the second light-emitting layer  14 GE by 0.25 eV or more. Furthermore, the value of the LUMO level of the first light-emitting layer  14 GH is greater than the value of the LUMO level of the second light-emitting layer  14 GE by 0.25 eV or more. 
     According to these configurations, in each of the light-emitting element  6 B and the light-emitting element  6 G, the outward flow of positive holes injected into the light-emitting layer  14  towards the first electron transport layer  16  side, and the outward flow of electrons injected into the light-emitting layer  14  towards the second hole transport layer  12  side are more effectively reduced. Through this, the electron concentration and the positive hole concentration in the light-emitting layer  14  of each of the light-emitting element  6 B and the light-emitting element  6 G are improved, and the efficiency of recombination of the carriers is enhanced. Furthermore, in each of the light-emitting element  6 B and the light-emitting element  6 G, damage to each organic layer in association with the outward flow of carriers injected into the light-emitting layer  14  is reduced, and thus the lifetime of each of the light-emitting element  6 B and the light-emitting element  6 G is improved. 
     In the light-emitting element  6 B, the recombination of positive holes and electrons occurs in the light-emitting layer  14 B. Thus, light having energy corresponding to the difference between the value of the LUMO level of the light-emitting layer  14 B and the value of the HOMO level of the light-emitting layer  14 B originates from the light-emitting layer  14 B. The difference between the value of a LUMO level of the light-emitting layer  14 B and the value of a HOMO level of the light-emitting layer  14 B is preferably greater than 2.7 eV and equal to or less than 3.1 eV. 
     In the light-emitting element  6 G, the recombination of positive holes and electrons occurs at an interface between the first light-emitting layer  14 GH and the second light-emitting layer  14 GE. Thus, light having energy corresponding to the difference between the value of the LUMO level of the second light-emitting layer  14 GE and the value of the HOMO level of the first light-emitting layer  14 GH is generated from the light-emitting layer  14 G. The difference between the value of the LUMO level of the second light-emitting layer  14 GE and the value of the HOMO level of the first light-emitting layer  14 GH is preferably from 2.4 eV to 2.7 eV. 
     Note that the light-emitting element  6 R according to the present embodiment has the same configuration as the light-emitting element  6 B with the exception that the light from the light-emitting layer  14 R is red light. For example, the relationship between the value of the LUMO level and the value of the HOMO level of each layer of the light-emitting element  6 R and the relationship between the layer thicknesses thereof are the same as the relationship between the value of the LUMO level and the value of the HOMO level of each layer of the light-emitting element  6 B and the relationship between the layer thicknesses thereof. 
     As described above, in each of the light-emitting elements  6 B,  6 G, and  6 R according to the present embodiment, both the energy level difference H 2  and the energy level difference E 2  are from 0.0 eV to 0.15 eV. According to the configuration described above, the injection efficiency of each carrier into the light-emitting layer  14  is improved in each of the light-emitting element  6 B, the light-emitting element  6 G, and the light-emitting element  6 R. 
     In addition, in each of the light-emitting elements  6 B,  6 G, and  6 R according to the present embodiment, the second electron transport layer  18  is a mixed layer that includes an organic material having electron transporting properties and an electron-accepting material, and contains the electron-accepting material in an amount greater than 50 mass %. 
     According to the configuration described above, electrons are more stably transported to the first electron transport layer  16  in each of the light-emitting elements  6 B,  6 G, and  6 R. Accordingly, the light-emitting elements  6 B,  6 G, and  6 R can prevent excessive injection of electrons in the light-emitting layer  14 , and can prevent electron excess in the light-emitting layer  14 . 
     Through the elimination of electron excess in the light-emitting layer  14 , recombination of positive holes and electrons in the light-emitting layer  14  occurs more efficiently. Thus, the luminous efficiency of the light-emitting element  6 B, the light-emitting element  6 G, and the light-emitting element  6 R is improved. 
     In addition, since electron excess in the light-emitting layer  14  is eliminated, recombination of charges that do not contribute to light emission, including a deactivation process such as an Auger process, in the light-emitting layer  14  is unlikely to occur. The outward flow of electrons from the light-emitting layer  14  to each layer of the second hole transport layer  12  side is prevented, and therefore the recombination of charges that do not contribute to light emission is less likely to occur even in each layer further to the second hole transport layer  12  side than the light-emitting layer  14 . Through this, damage to the light-emitting layer  14  and to each layer further to the second hole transport layer  12  side than the light-emitting layer  14  is prevented, and the lifetime of the light-emitting element  6 B, the light-emitting element  6 G, and the light-emitting element  6 R is improved. 
     Accordingly, the light-emitting element  6 B, the light-emitting element  6 G, and the light-emitting element  6 R according to the present embodiment can eliminate electron excess in the light-emitting layer  14  while improving the transport efficiency of each carrier to the light-emitting layer  14 . Therefore, the luminous efficiency and the lifetime of the light-emitting element  6 B, the light-emitting element  6 G, and the light-emitting element  6 R according to the present embodiment are more efficiently improved. 
     Light-emitting elements according to each of Examples 1 to 4 below were prepared with each light-emitting element having the same configuration as each light-emitting element of the display device  2  according to the present embodiment, and the physical properties were measured. 
     Example 1 
     A light-emitting element according to Example 1 was prepared with the same structure as the light-emitting element  6 B of the display device  2  according to the present embodiment. 
     In the manufacturing of the light-emitting element according to the present example, ITO was first formed as an anode electrode  8 . 
     Next, a film of a first hole transport layer  10  (HOMO: −5.50 eV, LUMO:−2.42 eV) containing an aromatic amine-based compound was formed as a hole transport material on the anode electrode  8  through low-temperature chemical vapor deposition (CVD) of the hole transport material. 
     Next, a film of a second hole transport layer  12 B (HOMO: −5.60 eV, LUMO:−2.52 eV) containing a carbazole-based compound was formed as an electron blocking material on an upper layer of the first hole transport layer  10  through low-temperature CVD of the electron blocking material. 
     Next, a light-emitting layer  14 B was formed on the upper layer of the second hole transport layer  12 B. The light-emitting layer  14 B was formed by co-evaporation of an anthracene-adamantane based compound (HOMO: −5.74 eV, LUMO:−2.88 eV) serving as a host material and an anthracene-naphthalene based compound (HOMO: −5.85 eV, LUMO:−2.90 eV) serving as a fluorescence emission dopant. 
     Next, a first electron transport layer  16  (HOMO: −6.00 eV, LUMO:−2.95 eV) containing a triazole-based compound was formed as a hole blocking material on the upper layer of the light-emitting layer  14 B by vapor deposition of the hole blocking material. 
     Next, a second electron transport layer  18  was formed on the upper layer of the first electron transport layer  16 . The second electron transport layer  18  was formed by co-evaporation of an organic material having electron transport properties and an electron-accepting material at a mass ratio of 4:6. An oxadiazole derivative (OXD-7) (HOMO:−6.34 eV, LUMO:−2.92 eV) was used as the organic material having electron transport properties, of the second electron transport layer  18 . A lithium quinolate complex (Liq) (HOMO: −5.78 eV, LUMO:−3.46 eV) was used as the electron-accepting material of the second electron transport layer  18 . 
     In the present example, lithium fluoride was further deposited on the upper layer of the second electron transport layer  18  to form an electron injection layer. 
     An alloy of Mg—Ag was then vapor deposited on the upper layer of the electron injection layer to form a cathode electrode  20 . 
     In the present example, a capping layer made from a compound containing an aromatic amine group was further formed by vapor deposition on the upper layer of the cathode electrode  20 , and the light-emitting element was then sealed using a sealing material containing an inorganic-organic composite material. 
     In the present example, a light-emitting element that emits light of (x, y)=(0.141, 0.045) in CIE chromaticity coordinates was obtained. Note that in the present example, the difference between the value of the LUMO level of the light-emitting layer  14 B and the value of the HOMO level of the light-emitting layer  14 B was 2.95 eV. 
     Also note that in Example 1, the layer thickness d 10 , the layer thickness d 12 , the layer thickness d 14 , the layer thickness d 16 , and the layer thickness d 18  were set to 110 nm, 5 nm, 20 nm, 5 nm, and 25 nm, respectively. 
     Example 2 
     A light-emitting element according to Example 2 was manufactured by the same technique as the light-emitting element according to Example 1 and had the same structure as the light-emitting element according to Example 1 with the exception of the value of the layer thickness d 12 . The layer thickness d 12  of the light-emitting element according to the Example 2 was set to 10 nm. In the present example, a light-emitting element that emits light of (x, y)=(0.140, 0.047) in CIE chromaticity coordinates was obtained. 
     Example 3 
     A light-emitting element according to Example 3 was prepared with the same structure as the light-emitting element  6 G of the display device  2  according to the present embodiment. 
     In the manufacturing of the light-emitting element according to the present example, ITO was first formed as an anode electrode  8 . 
     Next, a film of a first hole transport layer  10  (HOMO: −5.50 eV, LUMO:−2.42 eV) containing an aromatic amine-based compound was formed as a hole transport material on the anode electrode  8  through low-temperature chemical vapor deposition (CVD) of the hole transport material. 
     Next, a film of a second hole transport layer  12 G (HOMO: −5.60 eV, LUMO:−2.47 eV) containing a carbazole-based compound was formed as an electron blocking material on the upper layer of the first hole transport layer  10  through low-temperature CVD of the electron blocking material. 
     Next, the light-emitting layer  14 G was formed by sequentially forming the first light-emitting layer  14 GH and the second light-emitting layer  14 GE on the upper layer of the second hole transport layer  12 G. The light-emitting layer  14 G was formed by co-evaporation of three materials including a rubrene-based compound (HOMO: −5.60 eV, LUMO: −2.34 eV) serving as a hole transport material, Alq3 (tris(8-hydroxyquinolinato) aluminum) (HOMO: −5.96 eV, LUMO: −2.84 eV) serving as an electron-transporting material, and an iridium complex (HOMO: −5.60 eV, LUMO: −2.90 eV) serving as a phosphorescence emission dopant. 
     Next, a first electron transport layer  16  (HOMO: −6.02 eV, LUMO: −2.94 eV) containing a triazole-based compound was formed as a hole blocking material on the upper layer of the light-emitting layer  14 G by vapor deposition of the hole blocking material. 
     Next, a second electron transport layer  18  was formed on the upper layer of the first electron transport layer  16 . The second electron transport layer  18  was formed by co-evaporation of an organic material having electron transport properties and an electron-accepting material at a mass ratio of 4:6. An oxadiazole derivative (OXD-7) (HOMO:−6.34 eV, LUMO: −2.92 eV) was used as the organic material having electron transport properties, of the second electron transport layer  18 . A lithium quinolate complex (Liq) (HOMO: −5.78 eV, LUMO: −3.46 eV) was used as the electron-accepting material of the second electron transport layer  18 . 
     In the present example, lithium fluoride was further deposited on the upper layer of the second electron transport layer  18  to form an electron injection layer. 
     Through the same methods as those in the previous example, the cathode electrode  20  and the capping layer were formed on the upper layer of the electron injection layer, and the light-emitting element was sealed by the sealing material. 
     In the present example, a light-emitting element that emits light of (x, y)=(0.221, 0.720) in CIE chromaticity coordinates was obtained. Note that in the present example, the difference between the value of the LUMO level of the second light-emitting layer  14 GE and the value of the HOMO level of the first light-emitting layer  14 GH was 2.70 eV. 
     Also note that in Example 1, the layer thickness d 10 , the layer thickness d 12 , the layer thickness d 14 , the layer thickness d 16 , and the layer thickness d 18  were set to 110 nm, 25 nm, 40 nm, 5 nm, and 25 nm, respectively. 
     Example 4 
     A light-emitting element according to Example 4 was manufactured by the same technique as the light-emitting element according to Example 3 and had the same structure as the light-emitting element according to Example 3 with the exception of the value of the layer thickness d 12 . The layer thickness d 12  of the light-emitting element according to the Example 4 was set to 35 nm. In the present example, a light-emitting element that emits light of (x, y)=(0.220, 0.720) in CIE chromaticity coordinates was obtained. 
     Next, the physical properties of the light-emitting elements according to each of the examples described above were measured, and the physical properties thereof were compared. 
     First, the values of the HOMO level and the LUMO level of each layer of each light-emitting element were measured, and the HOMO level difference and the LUMO level difference between each layer were measured. Specifically, a Photoemission Yield Spectroscopy (PYS) apparatus (AC-3, available from RIKEN KEIKI Co., Ltd.) was used to determine the value of the HOMO level of each layer of each light-emitting element. Furthermore, the value of the LUMO level was determined by measuring the band gap of each layer of each light-emitting element by the ultraviolet spectrum measurement. 
     The energy diagrams of each layer of the light-emitting elements according to each of the examples are illustrated in  FIGS.  5  and  6    based on the results of the measurements.  FIG.  5    illustrates an energy diagram of each layer of the light-emitting elements according to each of Example 1 and Example 2.  FIG.  6    illustrates an energy diagram of each layer of the light-emitting elements according to each of Example 3 and Example 4. 
     In  FIGS.  5  and  6   , the numeric values of “H 1 ” to “H 5 ” and “E 1 ” to “E 5 ” indicate energy values of the energy level differences H 1  to H 5  and the energy level differences E 1  to E 5 , respectively. Note that in  FIGS.  5  and  6   , the band gap of the second electron transport layer  18  indicates the band gap of the electron-transporting organic material of the second electron transport layer  18 . In other words, the energy level difference H 4  indicates the difference between the HOMO level of the first electron transport layer  16  and the HOMO level of the electron-transporting organic material of the second electron transport layer  18 . Similarly, the energy level difference E 1  indicates the difference between the LUMO level of the first electron transport layer  16  and the LUMO level of the electron-transporting organic material of the second electron transport layer  18 . 
     Also, in each example, the negative numeric value of the “E 1 ” section indicates that the value of the LUMO level of the second electron transport layer  18  is smaller than the value of the LUMO level of the first electron transport layer  16 . In Example 3 and Example 4, the values of the row of “E 3 ” being negative indicates that the value of the LUMO level of the second hole transport layer  12 G is smaller than the value of the LUMO level of the first light-emitting layer  14 GH. 
     Next, under an environmental temperature of 25 degrees Celsius, a voltage generated by a current having a current density of 10 mA/cm 2  was applied between electrodes of each of the light-emitting elements, and the external quantum efficiency and the lifetime were measured. 
     The measured physical properties of the light-emitting elements according to each of the examples and the comparative examples are listed in Table 1 below. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Example 1 
                 Example 2 
                 Example 3 
                 Example 4 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 d10 (nm) 
                 110 
                 110 
                 110 
                 110 
               
               
                 d12 (nm) 
                 5 
                 10 
                 25 
                 35 
               
               
                 d14 (nm) 
                 20 
                 20 
                 40 
                 40 
               
               
                 d16 (nm) 
                 5 
                 5 
                 5 
                 5 
               
               
                 d18 (nm) 
                 25 
                 25 
                 25 
                 25 
               
               
                 Voltage (V) 
                 4.2 
                 4.7 
                 4.2 
                 4.4 
               
               
                 EQE (%) 
                 12.7 
                 12 
                 32.5 
                 29 
               
               
                 Lifetime (h) 
                 2900 
                 2100 
                 3300 
                 1700 
               
               
                   
               
            
           
         
       
     
     In Table 1, the columns of “Example 1” to “Example 4” indicate the physical properties of the light-emitting elements according to the respective examples or comparative examples. 
     In Table 1, the rows of “d 10 ”, “d 12 ”, “d 14 ”, “d 16 ”, and “d 18 ” indicate the values of the layer thicknesses d 10 , d 12 , d 14 , d 16 , and d 18 , respectively, in units of nm. 
     In Table 1, the “voltage” row indicates, in units of V, the magnitude of the voltage required to generate a current with a current density of 10 mA/cm 2  between the electrodes of each light-emitting element. The “EQE” column indicates the percentage of external quantum efficiency of each light-emitting element under the application of the above voltage. The row of “lifetime” indicates the duration until the luminance of each light-emitting element reaches 90 percent of the initial luminance under the application of the above voltage as units of time (h). 
     The layer thickness of the second hole transport layer of the light-emitting element according to Example 1 is thinner than that of the light-emitting element according to Example 2. Thus, with the light-emitting element according to Example 1, positive holes are more easily injected into the light-emitting layer  14 , and electron excess in the light-emitting layer  14  is eliminated. Accordingly, as shown in Table 1, the light-emitting element according to Example 1 exhibits improved external quantum efficiency and lifetime in comparison to the light-emitting element according to Example 2. 
     Similarly, the layer thickness of the second hole transport layer of the light-emitting element according to Example 3 is thinner than that of the light-emitting element according to Example 4. Thus, with the light-emitting element according to Example 3, positive holes are more easily injected into the light-emitting layer  14 , and electron excess in the light-emitting layer  14  is eliminated. Accordingly, as shown in Table 1, the light-emitting element according to Example 3 exhibits improved external quantum efficiency and lifetime in comparison to the light-emitting element according to Example 4. 
     Next, the impedance spectrum of the positive holes of the light-emitting element according to each embodiment was measured, and a comparison was implemented. The impedance spectra of the positive holes of the light-emitting elements according to each embodiment are illustrated in  FIGS.  7  to  10   . 
     In  FIG.  7    and  FIG.  9   , the measurement results for Example 1 are indicated by solid lines, and the measurement results for Example 2 are indicated by dashed lines. In  FIG.  8    and  FIG.  10   , the measurement results for Example 3 are indicated by solid lines, and the measurement results for Example 4 are indicated by dashed lines. 
     The impedance spectrum of the positive holes of the light-emitting element according to each example was measured by measuring the impedance of the positive holes while applying, to the light-emitting element, a voltage in which the AC voltage of a constant amplitude was superimposed on a constant DC voltage. In the measurement of the impedance spectrum of the light-emitting element according to each example, the impedance spectrum was measured for each example while successively increasing, from 0 V to 5 V, the DC voltage component of the voltage applied to the light-emitting element. In  FIG.  7    to  FIG.  10   , the measurement results are shown shifted in the upward direction of the vertical axis for each DC voltage value applied to the light-emitting element. 
       FIG.  7    and  FIG.  8    each present an impedance spectrum based on a Cole-Cole plot of the light-emitting element according to each example. In  FIG.  7    and  FIG.  8   , the horizontal axis indicates a distance of the light-emitting element according to each example from the light-emitting layer  14  in a direction toward the anode electrode  8 . Thus, the left end of each spectrum in  FIGS.  7  and  8    indicates to what distance from the light-emitting layer  14  the positive holes were injected from the anode electrode. Here, in  FIGS.  7  and  9   , a spectrum in which the left end reaches 0 nm on the horizontal axis indicates that the positive holes are injected into the light-emitting layer  14  and are recombined with electrons. 
     As shown in  FIG.  7   , the left end of the impedance spectrum of the light-emitting element according to Example 1 reaches 0 nm on the horizontal axis at a lower DC voltage value compared to the impedance spectrum of the light-emitting element according to Example 2. This indicates that with the light-emitting element according to Example 1, the positive holes are efficiently transported at a lower applied voltage compared to the light-emitting element according to Example 2, and recombination of the positive holes and electrons occurs. This is thought to be because the layer thickness d 12  of the second hole transport layer  12  of the light-emitting element according to Example 1 is thinner than the layer thickness d 12  of the second hole transport layer  12  of the light-emitting element according to Example 2, and thus the positive holes are easily injected into the light-emitting layer. Therefore, in the light-emitting element according to Example 1, a shortage of positive holes in the light-emitting layer  14  is more efficiently eliminated in comparison to the light-emitting element according to Example 2, and the external quantum efficiency and the lifetime are improved. 
     Similarly, as shown in  FIG.  8   , the left end of the impedance spectrum of the light-emitting element according to Example 3 reaches 0 nm on the horizontal axis at a lower DC voltage value compared to the impedance spectrum of the light-emitting element according to Example 4. This is considered to occur for the same reason described in the comparison between the light-emitting element according to Example 1 and the light-emitting element according to Example 2. Therefore, in the light-emitting element according to Example 3, a shortage of positive holes in the light-emitting layer  14  is more efficiently eliminated in comparison to the light-emitting element according to Example 4, and the external quantum efficiency and the lifetime are improved. 
       FIG.  9    and  FIG.  10    each present an impedance spectrum based on a Bode plot of the light-emitting element according to each example. The impedance spectrum based on the Bode plot was measured by measuring the impedance of the carriers while changing the frequency of the AC voltage component of the voltage applied to the light-emitting element of each example from 1 Hz to 10 6  Hz. 
     In  FIG.  9    and  FIG.  10   , the peak of each spectrum indicates the relaxation frequency of each carrier, or in other words, the frequency at which each carrier resonates at the frequency of the AC voltage component of the applied voltage. The frequency of the AC voltage at which the carrier resonates is proportional to the transport rate of the carrier. Thus,  FIG.  9    and  FIG.  10    show that as the position of the peak of each spectrum in the horizontal axis direction becomes a higher frequency, the transport rate of the carrier corresponding to the peak of the spectrum becomes faster. 
     Note that in  FIGS.  9  and  10   , each impedance spectrum may have two peaks, particularly in an impedance spectrum of a low DC voltage value. In this case, a low frequency peak indicates an impedance peak of electrons, and a high frequency peak indicates an impedance peak of positive holes. Accordingly, an overlapping of two peaks forming a single peak indicates that the transport rate of electrons and the transport rate of positive holes are approximately matching. 
     As shown in  FIG.  9   , the impedance spectrum of the light-emitting element according to Example 1 has an impedance peak of positive holes at a higher frequency in terms of the frequency of the AC voltage in comparison to the impedance spectrum of the light-emitting element according to Example 2. This indicates that, in the light-emitting element according to Example 1, the positive holes are transported to the light-emitting layer  14  at a higher speed and at a lower DC voltage value in comparison to the light-emitting element according to Example 2. This is thought to be because the layer thickness d 12  of the second hole transport layer  12  of the light-emitting element according to Example 1 is thinner than the layer thickness d 12  of the second hole transport layer  12  of the light-emitting element according to Example 2, and thus the transportability of the positive holes is higher. Therefore, in the light-emitting element according to Example 1, a shortage of positive holes in the light-emitting layer  14  is more efficiently eliminated in comparison to the light-emitting element according to Example 2, and the external quantum efficiency and the lifetime are improved. 
     Similarly, as shown in  FIG.  10   , the impedance spectrum of the light-emitting element according to Example 3 has an impedance peak of positive holes at a higher frequency in terms of the frequency of the AC voltage in comparison to the impedance spectrum of the light-emitting element according to Example 4. This is considered to occur for the same reason described in the comparison between the light-emitting element according to Example 1 and the light-emitting element according to Example 2. Therefore, in the light-emitting element according to Example 3, a shortage of positive holes in the light-emitting layer  14  is more efficiently eliminated in comparison to the light-emitting element according to Example 4, and the external quantum efficiency and the lifetime are improved. 
     Second Embodiment 
       FIG.  11    is a cross-sectional view of a display device  2  according to a second embodiment, at a position corresponding to  FIG.  1   . As illustrated in  FIG.  11   , in the display device  2  according to the present embodiment, the light-emitting layer  14 R includes, from the anode electrode  8  side, a first light-emitting layer  14 RH and a second light-emitting layer  14 RE, and thereby the display device  2  according to the present embodiment differs in configuration from the display device  2  according to the previous embodiment. With the exception of this point, the display device  2  according to the present embodiment has the same configuration as that of the display device  2  according to the previous embodiment. 
     In the present embodiment, the first light-emitting layer  14 RH is a hole-transporting type red light-emitting layer, and the second light-emitting layer  14 RE is an electron-transporting type red light-emitting layer. The light-emitting element  6 R according to the present embodiment may have the same configuration as the light-emitting element  6 G with the exception that light from the first light-emitting layer  14 RH and the second light-emitting layer RE is red light. For example, the relationship between the value of the LUMO level and the value of the HOMO level of each layer of the light-emitting element  6 R and the relationship between the layer thicknesses thereof are the same as the relationship between the value of the LUMO level and the value of the HOMO level of each layer of the light-emitting element  6 G and the relationship between the layer thicknesses thereof. 
     In the present embodiment as well, for the same reason as in the previous embodiment, the luminous efficiency and the lifetime of the light-emitting element  6 B, the light-emitting element  6 G, and the light-emitting element  6 R are more efficiently improved. 
     A display device  2  having higher luminous efficiency and an improved lifetime can be obtained by providing the display device  2  with the light-emitting element  6 R, the light-emitting element  6 G, and the light-emitting element  6 B according to each embodiment described above. 
     The present invention is not limited to each of the embodiments described above, and various modifications may be made within the scope of the claims. Embodiments obtained by appropriately combining technical approaches disclosed in each of the different embodiments also fall within the technical scope of the present invention. Furthermore, novel technical features can be formed by combining the technical approaches disclosed in each of the embodiments. 
     REFERENCE SIGNS LIST 
     
         
           2  Display device 
           6  Light-emitting element layer 
           6 R,  6 G,  6 B Light-emitting element 
           8  Anode electrode 
           10  First hole transport layer 
           12  Second hole transport layer 
           14  Light-emitting layer 
           14 GH,  14 RH First light-emitting layer 
           14 GE,  14 RE Second light-emitting layer 
           16  First electron transport layer 
           18  Second electron transport layer 
           20  Cathode electrode