Patent Publication Number: US-2023134846-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 degradation of the light-emitting layer by inserting an interlayer film between 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 shortening of the lifetime of the light-emitting device (light-emitting element) occurs 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 of 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 of 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 at an amount greater than 50 mass %. 
     Advantageous Effects of Invention 
     According to the configuration described above, carrier balance in the light-emitting layer of the light-emitting element is improved, and degradation of the organic layer in association with bias of the carrier in the light-emitting layer is suppressed. Therefore, a light-emitting element with a longer lifetime and a display device provided with the light-emitting element can be provided. 
    
    
     
       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 a light-emitting element according to an Example 1 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 a light-emitting element according to an Example 2 of the present invention. 
         FIG.  6    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 B is further provided with a first interlayer organic layer  24  as the interlayer organic layer between the second hole transport layer  12 B and the light-emitting layer  14 B. The first interlayer organic layer  24  is an organic layer having electron transport properties similar to the first electron transport layer  16  and the second electron transport layer  18 . In other words, the first interlayer organic layer  24  is configured from an organic material having electron transport properties, and the electron mobility in the first interlayer organic layer  24  is greater than the hole mobility in the first interlayer organic layer  24 . 
     In particular, the electron mobility of the organic material constituting the first interlayer organic layer  24  is 100 times or more greater than the hole mobility of the organic material thereof. 
     Specifically, the organic material constituting the first interlayer organic layer  24  is a pyrimidine derivative (B3PymPm) represented by the following formula. 
     
       
         
         
             
             
         
       
     
     In addition, the organic material constituting the first interlayer organic layer  24  may be a silole derivative (PyPySPyPy) or a bipyrimidine oxadiazole derivative, which are each represented by the following formulas. 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     In the present embodiment, the first interlayer organic layer  24  has a layer thickness d24. In the present embodiment, the layer thickness d24 is, for example, from 0.1 nm to 2.0 nm, and for a reason described below, the layer thickness d24 is preferably from 0.3 nm to 1.5 nm. 
     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 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 first interlayer organic layer  24 , 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 is a red light-emitting layer that emits red light, the light-emitting layer  14 G is a green light-emitting layer that emits green light, and the light-emitting layer  14 B is a blue light-emitting layer that emits blue light. In other words, the light-emitting element  6 R is a red light-emitting element that emits red light, the light-emitting element  6 G is a green light-emitting element that emits green light, and the light-emitting element  6 B is a blue light-emitting element that emits blue light. 
     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 B is in contact with the first interlayer organic layer  24  at the anode electrode  8  side, and is in contact with the first electron transport layer  16  at the cathode electrode  20  side. In other words, the light-emitting layer  14 R and the light-emitting layer  14 B, which are single layers, are in contact with both the first interlayer organic layer  24  and the first electron transport layer  16 . 
     The light-emitting layer  14 R is in contact with the second hole transport layer  12  at the anode electrode  8  side, and is in contact with the first electron transport layer  16  at the cathode electrode  20  side. In other words, the light-emitting layer  14 R, which is a single layer, is in contact with both the second hole transport layer  12  and the first electron transport layer  16 . 
     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. 
     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 semitransparent 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. 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. 
     In the present embodiment, 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 bathocubroine, which are represented by the following formulas, respectively. 
     
       
         
         
             
             
         
       
     
     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. 
     Next, an energy band of each layer of the light-emitting element  6 B of the display device  2  according to the present embodiment will be described with reference to  FIG.  3   .  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. 
     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 first interlayer organic layer  24 , 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   . 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   , H1 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 B, of the light-emitting element  6 B. H2 indicates the energy level difference between the HOMO level of the second hole transport layer  12 B and the HOMO level of the first interlayer organic layer  24 , of the light-emitting element  6 B. H3 indicates the energy level difference between the HOMO level of the first interlayer organic layer  24  and the HOMO level of the light-emitting layer  14 B, of the light-emitting element  6 B. H4 indicates the energy level difference between the HOMO level of the light-emitting layer  14 B and the HOMO level of the first electron transport layer  16 , of the light-emitting element  6 B. H5 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 , of the light-emitting element  6 B. 
     In  FIG.  3   , E1 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 , of the light-emitting element  6 B. E2 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 B, of the light-emitting element  6 B. E3 indicates the energy level difference between the LUMO level of the light-emitting layer  14  and the LUMO level of the first interlayer organic layer  24 , of the light-emitting element  6 B. E4 indicates the energy level difference between the LUMO level of the first interlayer organic layer  24  and the LUMO level of the second hole transport layer  12 B, of the light-emitting element  6 B. E5 indicates the energy level difference between the LUMO level of the second hole transport layer  12 B and the LUMO level of the first hole transport layer  10 , of the light-emitting element  6 B. 
     In the light-emitting element  6 B according to the present embodiment, the energy level difference H1, the energy level difference E1, and the energy level difference E2 are all from 0.0 eV to 0.20 eV. In other words, in the light-emitting element  6 B, the energy level difference in terms of the HOMO level between the first hole transport layer  10  and second hole transport layer  12 B is from 0.0 eV to 0.20 eV. In addition, the energy level difference in terms of the LUMO level between the first electron transport layer  16  and the second electron transport layer  18 , 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 are both from 0.0 eV to 0.20 eV. 
     According to the configuration described above, in the light-emitting element  6 B, barriers of injection of positive holes from the first hole transport layer  10  to the second hole transport layer  12 B, barriers of injection of electrons from the second electron transport layer  18  to the first electron transport layer  16 , and barriers of injection of electrons from the first electron transport layer  16  to the light-emitting layer  14 B are reduced. Thus, the injection efficiency of each carrier into the light-emitting layer  14 B is improved in the light-emitting element  6 B. 
     Moreover, in the light-emitting element  6 B according to the present embodiment, the energy level difference H2 is from 0.0 eV to 0.20 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 first interlayer organic layer  24  is from 0.0 eV to 0.20 eV. 
     Similarly, in the light-emitting element  6 B according to the present embodiment, the energy level difference H3 is from 0.0 eV to 0.05 eV. In other words, in the light-emitting element  6 B, the energy level difference in terms of the HOMO level between the first interlayer organic layer  24  and the light-emitting layer  14 B is from 0.0 eV to 0.05 eV. 
     According to the configuration described above, in the light-emitting element  6 B, barriers of injection of positive holes from the second hole transport layer  12 B into the first interlayer organic layer  24 , and barriers of injection of positive holes from the first interlayer organic layer  24  into the light-emitting layer  14 B are reduced. Thus, the injection efficiency of the positive holes into the light-emitting layer  14 B is improved in the light-emitting element  6 B, and electron-excess is more efficiently prevented. 
     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 first interlayer organic layer  24  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. 
     According to these configurations, in the light-emitting element  6 B, the outward flow of positive holes injected into the light-emitting layer  14 B towards the first electron transport layer  16  side, and the outward flow of electrons injected into the light-emitting layer  14 B towards the first interlayer organic layer  24  side are more effectively reduced. Through this, the electron concentration and the positive hole concentration in the light-emitting layer  14 B of the light-emitting element  6 B are improved, and the efficiency of recombination of the carriers is enhanced. Furthermore, in the light-emitting element  6 B, damage to each organic layer in association with the outward flow of carriers injected into the light-emitting layer  14 B is reduced, and thus the lifetime of the light-emitting element  6 B is improved. 
     In the light-emitting element  6 B according to the present embodiment, the energy level difference E4 is from 0.0 eV to 0.05 eV. In other words, in the light-emitting element  6 B, the energy level difference in terms of the LUMO level between the second hole transport layer  12 B and the first interlayer organic layer  24  is from 0.0 eV to 0.05 eV. 
     When the value of the LUMO level of the first interlayer organic layer  24  is small compared to the value of the LUMO level of the second hole transport layer  12 B, an outward flow of electrons from the light-emitting layer  14 B to the first interlayer organic layer  24  easily occurs. In association therewith, an outward flow of electrons from the light-emitting layer  14 B to the second hole transport layer  12 B easily occurs via the first interlayer organic layer  24 . 
     According to the configuration described above, the difference between the LUMO levels of the second hole transport layer  12 B and the first interlayer organic layer  24  is sufficiently close. As a result, the value of the LUMO level of the first interlayer organic layer  24  increases compared to the value of the LUMO level of the light-emitting layer  14 B, and the outward flow of electrons from the light-emitting layer  14 B to the second hole transport layer  12 B via the first interlayer organic layer  24  is less likely to occur. Therefore, through the configuration described above, degradation of the second hole transport layer  12 B in association with the outward flow of electrons from the light-emitting layer  14 B to the second hole transport layer  12 B is reduced, and the lifetime of the light-emitting element  6 B is improved. 
     As described above, in the light-emitting element  6 B according to the present embodiment, the energy level difference H1, the energy level difference E1, and the energy level difference E2 are all from 0.0 eV to 0.20 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, the light-emitting element  6 B according to the present embodiment is provided with, between the second hole transport layer  12 B and the light-emitting layer  14 B, a first interlayer organic layer  24  having electron transport properties. Thus, according to the configuration described above, in the light-emitting element  6 B, the first interlayer organic layer  24  can reduce the outward flow of electrons from the light-emitting layer  14 B to the second hole transport layer  12 B. In particular, when the electron mobility of the organic material constituting the first interlayer organic layer  24  is greater than the hole mobility of the organic material thereof by 100-times or more, the outward flow of electrons described above can be more efficiently reduced. 
     By preventing the outward flow of electrons from the light-emitting layer  14  to each layer of the second hole transport layer  12  side, 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 is improved. 
     Accordingly, the light-emitting element  6 B according to the present embodiment can eliminate electron excess in the light-emitting layer  14  while maintaining the transport efficiency of each carrier to the light-emitting layer  14 . Therefore, the light-emitting element  6 B according to the present embodiment more efficiently improves the lifetime. 
     Ordinarily, in a light-emitting element having an organic layer, like the light-emitting elements included in the display device  2  according to the present embodiment, the lifetime of the blue light-emitting element tends to be shorter than that of the green light-emitting element and the red light-emitting element. As described above, the display device  2  according to the present embodiment is provided with a light-emitting element  6 B, which is a blue light-emitting element having an improved lifetime, and thus the lifetime of the light-emitting element  6 B is lengthened in relation to the lifetime of the light-emitting element  6 G and the light-emitting element  6 R. Therefore, the overall lifetime of the display device  2  according to the present embodiment can be further improved. 
     Light-emitting elements according to each of Examples 1 and 2 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 first interlayer organic layer  24  was formed through vapor deposition of the pyrimidine derivative (B3PymPm) (HOMO: −5.77 eV, LUMO: −2.52 eV) described above onto the upper layer of the second hole transport layer  12 B. In the present example, the layer thickness d24 of the first interlayer organic layer  24  was set to 1.0 nm. 
     Next, a light-emitting layer  14 B was formed on the upper layer of the first interlayer organic layer  24 . 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 electron-transporting organic material and a lithium quinolate complex (HOMO: −5.78 eV, LUMO: −3.46 eV) at a mass ratio of 1:1. 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 . 
     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)=(0.140, 0.050) 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. 
     Example 2 
     A light-emitting element according to Example 2 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: −5.49 eV, LUMO: −2.41 eV) containing an aromatic amine-based compound was formed as a hole transport material on the anode electrode  8  through low-temperature CVD of the hole transport material. 
     Next, a film of a second hole transport layer  12 B (HOMO: −5.62 eV, LUMO: −2.53 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, a first interlayer organic layer  24  was formed through vapor deposition of the above-described pyrimidine derivative (B3PymPm) (HOMO: −5.78 eV, LUMO: −2.49 eV) onto the upper layer of the second hole transport layer  12 B. In the present example, the layer thickness d24 of the first interlayer organic layer  24  was set to 1.0 nm. 
     Next, a light-emitting layer  14 B was formed on the upper layer of the first interlayer organic layer  24 . The light-emitting layer  14 B was formed by co-evaporation of an anthracene-phenyl based compound (HOMO: −5.74 eV, LUMO: −2.82 eV) serving as a host material and a rubrene-based compound (HOMO: −5.80 eV, LUMO: −2.86 eV) serving as a fluorescence emission dopant. 
     Next, a first electron transport layer  16  (HOMO: −6.07 eV, LUMO: −2.87 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  having the same configuration as the second electron transport layer  18  according to Example 1 was formed on the upper layer of the first electron transport layer  16  using the same technique as the technique presented in Example 1. 
     In the present example, ytterbium (Yb) was further vapor deposited onto 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)=(0.136, 0.048) 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 2.94 eV. 
     Note that the electron mobility of the first interlayer organic layer  24  of the light-emitting element according to each of Example 1 and Example 2 was 2×10 −2  cm 2 /Vs, and the hole mobility was 7×10.6 cm 2 /Vs. The carrier mobility of the first interlayer organic layer  24  was measured by impedance spectroscopy. 
     Light-emitting elements according to Comparative Example 1 and Comparative Example 2 were prepared in relation to each of the light-emitting elements of Example 1 and Example 2, and the physical properties were measured. 
     The light-emitting element according to Comparative Example 1 was prepared with the same configuration as that of the light-emitting element of Example 1 with the exception that the first interlayer organic layer  24  was not formed. The light-emitting element according to Comparative Example 2 was prepared with the same configuration as that of the light-emitting element of Example 2 with the exception that the first interlayer organic layer  24  was not formed. 
     Next, the physical properties of the light-emitting elements according to the examples and the comparative 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 and comparative examples are illustrated in  FIG.  4    and  FIG.  5    based on the results of the measurements.  FIG.  4    and  FIG.  5    respectively illustrate energy diagrams of each layer of the light-emitting elements according to each of Example 1 and Example 2. 
     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 
               
               
                   
                   
               
               
                   
                   
                   
                 Comparative 
                 Comparative 
               
               
                   
                 Example 1 
                 Example 2 
                 Example 1 
                 Example 2 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 H1(eV) 
                 0.10 
                 0.13 
                 0.10 
                 0.13 
               
               
                 H2(eV) 
                 0.17 
                 0.16 
                 0.14 
                 0.12 
               
               
                 H3(eV) 
                 −0.03 
                 −0.04 
               
               
                 H4(eV) 
                 0.26 
                 0.33 
                 0.26 
                 0.33 
               
               
                 H5(eV) 
                 0.34 
                 0.28 
                 0.34 
                 0.28 
               
               
                 E1(eV) 
                 0.01 
                 0.01 
                 0.01 
                 0.01 
               
               
                 E2(eV) 
                 0.07 
                 0.05 
                 0.07 
                 0.05 
               
               
                 E3(eV) 
                 0.36 
                 0.33 
                 0.36 
                 0.29 
               
               
                 E4(eV) 
                 0.00 
                 −0.04 
               
               
                 E5(eV) 
                 0.10 
                 0.12 
                 0.1 
                 0.12 
               
               
                 Voltage 
                 4.3 
                 4.4 
                 4.2 
                 4.0 
               
               
                 (V) 
               
               
                 EQE(%) 
                 13.0 
                 12.6 
                 13.3 
                 12.9 
               
               
                 Lifetime 
                 5500 
                 6600 
                 3200 
                 3700 
               
               
                 (h) 
               
               
                   
               
            
           
         
       
     
     In Table 1, the columns of “Example 1” and “Example 2” indicate the physical properties of the light-emitting elements according to the respective examples. 
     In Table 1, the rows of “H1” to “H5” and “E1” to “E5” indicate energy values of the energy level differences H1 to H5 and energy level differences E1 to E5, respectively, in units of eV. Note that in each of the examples, the negative numeric values in the row of “H3” indicate that the value of the HOMO level of the first interlayer organic layer  24  was smaller than the value of the HOMO level of the light-emitting layer  14 B. Also, in Example 2, the negative numeric value in the row of “E4” indicates that the value of the LUMO level of the second hole transport layer  12 B was smaller than the value of the LUMO level of the first interlayer organic layer  24 . 
     Note that for each of the comparative examples in Table 1, a level difference between the second hole transport layer  12 B and the light-emitting layer  14 B at the HOMO level is presented in the row “H2”. Similarly, for each of the comparative examples in Table 1, a level difference between the second hole transport layer  12 B and the light-emitting layer  14 B at the LUMO level is presented in the row “E3”. In addition, values are not entered for the comparative examples in the “H3” and “E4” rows of Table 1. 
     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” row indicates the percentage of external quantum efficiency of each light-emitting element under the application of the above voltage. The “lifetime” row indicates, in units of hours (h), the time period until the luminance of each light-emitting element reached 90 percent of the initial luminance at an environmental temperature of 25° C. and under the application of the above voltage. 
     Next, the physical properties of the light-emitting elements according to each of Example 1 and Example 2 were measured for a case in which the layer thickness d24 of the first interlayer organic layer  24  was changed. For the light-emitting element according to Example 1, the layer thickness d24 was changed from 0.3 nm to 2.0 nm, and for the light-emitting element according to Example 2, the layer thickness d24 was changed from 0.1 nm to 2.0 nm, and the physical properties were measured for each case. In addition, cases in which the layer thicknesses d24 of the light-emitting elements of Example 1 and Example 2 were changed to 0 nm, or in other words, cases in which the first interlayer organic layer  24  was not formed in Example 1 and Example 2 were used as Comparative Example 1 and Comparative Example 2, and the physical properties of both were measured. 
     The physical properties of a case in which the layer thickness d24 of the light-emitting element according to Example 1 was changed, and the physical properties of the light-emitting element according to Comparative Example 1 are described in Table 2 below. The physical properties of a case in which the layer thickness d24 of the light-emitting element according to Example 2 was changed, and the physical properties of the light-emitting element according to Comparative Example 2 are described in Table 3 below. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                   
                 Chromaticity 
                   
               
               
                 d24(nm) 
                 Voltage (V) 
                 EQE(%) 
                 (x, y) 
                 Lifetime (h) 
               
               
                   
               
             
            
               
                 0 (Comparative 
                 4.2 
                 13.3 
                 (0.140, 0.050) 
                 3200 
               
               
                 Example 1) 
               
               
                 0.3 
                 4.2 
                 13.2 
                 (0.140, 0.050) 
                 3700 
               
               
                 0.5 
                 4.2 
                 13.1 
                 (0.140, 0.050) 
                 4200 
               
               
                 1.0 
                 4.3 
                 13.0 
                 (0.140, 0.050) 
                 5500 
               
               
                 1.5 
                 4.5 
                 12.6 
                 (0.140, 0.050) 
                 6800 
               
               
                 2.0 
                 4.9 
                 12.0 
                 (0.140, 0.050) 
                 7000 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 3 
               
               
                   
               
               
                   
                   
                   
                 Chromaticity 
                   
               
               
                 d24(nm) 
                 Voltage (V) 
                 EQE(%) 
                 (x, y) 
                 Lifetime (h) 
               
               
                   
               
             
            
               
                 0 (Comparative 
                 4.0 
                 12.9 
                 (0.136, 0.048) 
                 3700 
               
               
                 Example 2) 
               
               
                 0.1 
                 4.0 
                 12.8 
                 (0.136, 0.048) 
                 3800 
               
               
                 0.3 
                 4.2 
                 12.8 
                 (0.137, 0.049) 
                 5000 
               
               
                 0.5 
                 4.3 
                 12.7 
                 (0.136, 0.048) 
                 5700 
               
               
                 1.0 
                 4.4 
                 12.6 
                 (0.136, 0.048) 
                 6600 
               
               
                 1.5 
                 4.7 
                 12.2 
                 (0.137, 0.047) 
                 7200 
               
               
                 2.0 
                 5.1 
                 11.7 
                 (0.137, 0.046) 
                 7300 
               
               
                   
               
            
           
         
       
     
     In Table 2 and Table 3, the “d24” column indicates, in units of nm, the value of the layer thickness d24 of the first interlayer organic layer  24  of each light-emitting element. Here, in the “d24” column in Table 2, the row of 0 nm indicates the physical properties of the light-emitting element according to Comparative Example 1. Additionally, in the “d24” column in Table 3, the row of 0 nm indicates the physical properties of the light-emitting element according to Comparative Example 2. 
     In Table 2 and Table 3, the “voltage” column 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. In the “chromaticity” column, CIE chromaticity coordinates are used to indicate the chromaticity of the light emitted by each light-emitting element. The “lifetime” row indicates, in units of hours (h), the time period until the luminance of each light-emitting element reached 90 percent of the initial luminance at an environmental temperature of 25° C. and under the application of the above voltage. 
     As shown in Table 1 and Table 2, the lifetime of the light-emitting element according to Example 1, which was provided with the first interlayer organic layer  24 , was improved over the lifetime of the light-emitting element according to Comparative Example 1, which did not include the first interlayer organic layer  24 . Similarly, as shown in Table 1 and Table 3, the lifetime of the light-emitting element according to Example 2, which was provided with the first interlayer organic layer  24 , was improved over the lifetime of the light-emitting element according to Comparative Example 2, which did not include the first interlayer organic layer  24 . Also, as shown in Table 1, Table 2, and Table 3, the external quantum efficiency of each of the light-emitting elements according to Example 1 and Example 2 was not significantly reduced compared to the external quantum efficiency of each of the light-emitting elements according to Comparative Example 1 and Comparative Example 2. 
     Thus, in comparison to each of the light-emitting elements according to Comparative Example 1 and Comparative Example 2, the light-emitting elements according to each of Example 1 and Example 2 exhibited an improved lifetime while maintaining the external quantum efficiency. 
     Furthermore, as shown in Table 2 and Table 3, as the layer thickness d24 of the first interlayer organic layer  24  in the light-emitting element according to each example was increased, the lifetime of the light-emitting element tended to become longer. This is thought to be because as the thickness of the first interlayer organic layer  24  increased, the outward flow of electrons from the light-emitting layer  14 B to the second hole transport layer  12 B was reduced, and degradation of the second hole transport layer  12 B was suppressed. 
     On the other hand, as the layer thickness d24 of the first interlayer organic layer  24  in the light-emitting element according to each of the examples was reduced, the external quantum efficiency of the light-emitting element tended to improve. This is thought to be because as the thickness of the first interlayer organic layer  24  was reduced, the efficiency of hole transport from the second hole transport layer  12 B to the light-emitting layer  14 B improved, and the efficiency of recombination of carriers in the light-emitting layer  14 B was enhanced. 
     From the above, and from the perspective of significantly improving the lifetime while maintaining the external quantum efficiency, the layer thickness d24 of the first interlayer organic layer  24  of the light-emitting element according to each example is preferably from 0.3 nm to 1.5 nm. 
     Second Embodiment 
       FIG.  6    is a cross-sectional view of a display device  26  according to a second embodiment at a position corresponding to  FIG.  1   . 
     As illustrated in  FIG.  6   , in comparison to the display device  2  according to the previous embodiment, the display device  26  according to the present embodiment is provided with, between the second electron transport layer  18  and the cathode electrode  20 , a second interlayer organic layer  28  in place of the first interlayer organic layer  24 . In other words, the display device  26  according to the present embodiment is provided with the second interlayer organic layer  28  as the interlayer organic layer, and thereby has a configuration that differs from that of the display device  2  according to the previous embodiment. 
     Also, as illustrated in  FIG.  6   , the display device  26  according to the present embodiment is provided with a hole injection layer  30  between the anode electrode  8  and the first hole transport layer  10 , and with an electron injection layer  32  between the second interlayer organic layer  28  and the cathode electrode  20 . Furthermore, in the present embodiment, the second electron transport layer  18  includes an electron-accepting material and an organic material having electron transport properties, and includes, as the electron-accepting material, the lithium quinolate complex (Liq) described in the previous embodiment. However, in the present embodiment, the electron-transporting organic material included in the second electron transport layer  18  may be the same material as the electron-transporting organic material included in the second electron transport layer  18  of the previous embodiment. 
     With the exception of the abovementioned point, the display device  26  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 second interlayer organic layer  28  is formed commonly for the light-emitting element  6 B, the light-emitting element  6 G, and the light-emitting element  6 R. The second interlayer organic layer  28  is formed from an organic compound having an amino group or a hydroxyl group. In particular, the second interlayer organic layer  28  is formed from an isocyanurate compound having an amino group or a hydroxyl group. Specifically, the second interlayer organic layer  28  is formed from a compound represented by Chemical Formula (1) or Chemical Formula (2) below. 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     Note that, as illustrated in  FIG.  6   , the second interlayer organic layer  28  has a layer thickness d28. 
     The hole injection layer  30  has a function of supporting the injection of positive holes from the anode electrode  8  into the first hole transport layer  10 . For example, the value of the HOMO level of the hole injection layer  30  is a value between the HOMO level value of the anode electrode  8  and the HOMO level value of the first hole transport layer  10 . Therefore, the efficiency of transport of positive holes from the anode electrode  8  to the light-emitting layer  14  is improved by the hole injection layer  30 . 
     The electron injection layer  32  has a function of supporting the injection of electrons from the cathode electrode  20  into the second electron transport layer  18 . For example, the value of the LUMO level of the electron injection layer  32  is a value between the LUMO level value of the cathode electrode  20  and the LUMO level value of the second electron transport layer  18 . Thus, the efficiency of transport of electrons from the cathode electrode  20  to the light-emitting layer  14  is improved by the electron injection layer  32 . Note that by forming the second interlayer organic layer  28  such that the layer thickness d28 is sufficiently thin, electron injection from the cathode electrode  20  to the second electron transport layer  18  through the electron injection layer  32  is achieved by electron tunneling of the second interlayer organic layer  28 . 
     In the present embodiment, the light-emitting element layer  6  includes the hole injection layer  30  as a hole injection layer, the first hole transport layer  10  as a hole transport layer, and the second hole transport layer  12  as an electron blocking layer. Also, in the present embodiment, the light-emitting element layer  6  includes the electron injection layer  32  as an electron injection layer, the second electron transport layer  18  as an electron transport layer, and the first electron transport layer  16  as a hole blocking layer. 
     However, in the present embodiment, the light-emitting element layer  6  is not necessarily required to be provided with the hole injection layer  30 , the second hole transport layer  12 , the first electron transport layer  16 , and the electron injection layer  32 . In the present embodiment, the light-emitting element layer  6  may be provided with, between the anode electrode  8  and the cathode electrode  20  and in order from the anode electrode  8  side, the first hole transport layer  10  as a hole transport layer, the light-emitting layer  14 , the second electron transport layer  18  as an electron transport layer, and the second interlayer organic layer  28  as an interlayer organic layer. 
     Here, in the lithium quinolate complex included in the second electron transport layer  18 , electron exchange may occur within the molecule at high temperatures. The transfer of electrons within the molecule of the lithium quinolate complex occurs in accordance with the following chemical reaction. 
     
       
         
         
             
             
         
       
     
     Specifically, the N atom of the heterocyclic ring of the lithium quinolate complex has an unpaired electron, and therefore at high temperatures, electron transfer occurs between the N atom and the Li atom, and ionization of the lithium quinolate complex proceeds. When the lithium quinolate complex is ionized, the effect of the lithium quinolate complex in stabilizing electron transport from the second electron transport layer  18  to the first electron transport layer  16  is reduced, electron excess occurs in the light-emitting layer  14 , and the lifetime of the light-emitting element overall may be shortened. 
     However, in the present embodiment, as described above, the second interlayer organic layer  28  adjacent to the second electron transport layer  18  is formed from an organic compound having an amino group or a hydroxyl group. Therefore, hydrogen bonding occurs between the amino group or hydroxyl group of the second interlayer organic layer  28  and the N atom of the lithium quinolate complex of the second electron transport layer  18 . 
     Specifically, the second interlayer organic layer  28  is formed from a compound having an amino group, the compound being represented by Chemical Formula (1) above. In this case, as illustrated by the dotted line in the following chemical formula, hydrogen bonding occurs between the amino group of the second interlayer organic layer  28  and the N atom of the lithium quinolate complex of the second electron transport layer  18 . 
     
       
         
         
             
             
         
       
     
     Through this, the N atoms having an unpaired electron in the lithium quinolate complex of the second electron transport layer  18  are reduced, and the stability of the lithium quinolate complex is improved. Therefore, even under high temperature, ionization of the lithium quinolate complex of the second electron transport layer  18  does not easily proceed. Thus, in the present embodiment, the effect of the lithium quinolate complex of the second electron transport layer  18  in stabilizing electron transport from the second electron transport layer  18  to the first electron transport layer  16  is maintained even at high temperatures, and the lifetime of the light-emitting elements overall is improved. 
     When the second interlayer organic layer  28  contains an isocyanurate compound, the above-described ionization of the lithium quinolate complex of the second electron transport layer  18  can be more efficiently suppressed. Furthermore, ionization of the lithium quinolate complex of the second electron transport layer  18  can be more efficiently suppressed by adopting a configuration in which the second interlayer organic layer  28  has a compound represented by Chemical Formula (1) or Chemical Formula (2) described above. 
     In addition, when the compound represented by Chemical Formula (1) or Chemical Formula (2) described above is used as a vapor deposition material, problems such as decomposition are unlikely to occur. Thus, a second interlayer organic layer  28  with suppressed degradation can be formed through vapor deposition by configuring the second interlayer organic layer  28  to have a compound represented by Chemical Formula (1) or Chemical Formula (2) described above. 
     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 more efficiently suppress degradation of the second electron transport layer  18  by having the second interlayer organic layer  28  as described above. This effect is particularly pronounced when each light-emitting element is placed in a high temperature environment. The display device  2  according to the present embodiment is provided with each of the light-emitting elements described above, and thus the lifetime of the display device  2  overall can be improved. 
     Light-emitting elements according to each of Examples 3 to 6 below were prepared with each light-emitting element having the same configuration as each light-emitting element of the display device  26  according to the present embodiment, and the physical properties were measured. 
     Example 3 
     A light-emitting element according to Example 3 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 that the second interlayer organic layer  28  was formed in place of the first interlayer organic layer  24 , and the hole injection layer  30  was formed. 
     The hole injection layer  30  of the light-emitting element according to Example 1 was formed by forming, on the anode electrode  8 , a film of an aromatic compound (HOMO: −7.83 eV, LUMO: −5.60 eV) serving as a hole injection material through low-temperature CVD of the hole injection material. Moreover, the second interlayer organic layer  28  was formed by vapor depositing an organic compound represented by Chemical Formula (1) above on the upper layer of the second electron transport layer  18 . 
     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 that the material of the second interlayer organic layer  28  was excluded. The second interlayer organic layer  28  of the light-emitting element according to Example 4 was formed by vapor depositing an organic compound represented by Chemical Formula (2) above on the upper layer of the second electron transport layer  18 . 
     Example 5 
     A light-emitting element according to Example 5 was prepared with the same structure as the light-emitting element  6 G according to the present embodiment. The light-emitting element according to Example 5 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 that the light-emitting layer  14 G was provided in place of the light-emitting layer  14 B. 
     The light-emitting layer  14 G of the light-emitting element according to Example 5 was formed by sequentially forming a first light-emitting layer  14 GH and a 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. 
     Example 6 
     A light-emitting element according to Example 6 was prepared with the same structure as the light-emitting element  6 R according to the present embodiment. In addition, the light-emitting element according to the present example 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 that the light-emitting layer  14 R was provided in place of the light-emitting layer  14 B. 
     The light-emitting layer  14 R of the light-emitting element according to the present example was formed by co-evaporation of a host material (HOMO: −5.72 eV, LUMO: −2.64 eV) and an iridium complex (HOMO: −4.85 eV, LUMO: −2.90 eV) serving as a phosphorescence emission dopant, onto an upper layer of the second hole transport layer  12 R. 
     Furthermore, light-emitting elements according to Comparative Examples 3 to 6 were prepared in relation to each of the light-emitting elements of Examples 3 to 6, and the physical properties were measured. The light-emitting elements according to each of Comparative Examples 3 to 6 were prepared with the same configurations as the light-emitting elements according to each of Examples 3 to 6 with the exception that the second interlayer organic layer  28  was not formed. 
     Next, the physical properties of the light-emitting elements according to the examples and the comparative examples described above were measured, and the physical properties thereof were compared. Here, the physical properties of the light-emitting elements according to each of Examples 3 to 6 were measured for a case in which the layer thickness d28 of the second interlayer organic layer  28  was changed. The layer thickness d28 of the light-emitting element according to each of the examples was changed from 0.1 nm to 4 nm, and the physical properties were measured. In addition, cases in which the layer thicknesses d28 of the light-emitting elements of each of Examples 3 to 6 were changed to 0 nm, or in other words, cases in which the second interlayer organic layer  28  was not formed in Examples 3 to 6, were used as Comparative Examples 3 to 6, and the physical properties were measured. 
     The physical properties of the light-emitting elements according to each of Examples 3 to 6 when the layer thickness d28 was changed are shown in each of Tables 4 to 7 below. Note that in each of Tables 4 to 7, the physical properties of the light-emitting elements according to each of Comparative Examples 3 to 6 are also described. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 4 
               
               
                   
               
               
                   
                   
                   
                 Normal 
                 High 
               
               
                   
                   
                 Chromaticity 
                 Temperature 
                 Temperature 
               
               
                 d28(nm) 
                 EQE(%) 
                 (x, y) 
                 Lifetime (h) 
                 Lifetime (h) 
               
               
                   
               
             
            
               
                 0 (Comparative 
                 13.3 
                 (0.14, 0.05) 
                 754 
                 114 
               
               
                 Example 3) 
               
               
                 0.1 
                 13.4 
                 (0.14, 0.05) 
                 835 
                 143 
               
               
                 0.3 
                 13.4 
                 (0.14, 0.05) 
                 890 
                 155 
               
               
                 0.5 
                 13.5 
                 (0.14, 0.05) 
                 895 
                 152 
               
               
                 1.0 
                 13.3 
                 (0.14, 0.05) 
                 913 
                 157 
               
               
                 2.0 
                 13.1 
                 (0.14, 0.05) 
                 905 
                 154 
               
               
                 3.0 
                 12.8 
                 (0.14, 0.05) 
                 900 
                 154 
               
               
                 4.0 
                 12.4 
                 (0.14, 0.05) 
                 900 
                 155 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 5 
               
               
                   
               
               
                   
                   
                   
                 Normal 
                 High 
               
               
                   
                   
                 Chromaticity 
                 Temperature 
                 Temperature 
               
               
                 d28(nm) 
                 EQE(%) 
                 (x, y) 
                 Lifetime (h) 
                 Lifetime (h) 
               
               
                   
               
             
            
               
                 0 (Comparative 
                 13.3 
                 (0.14, 0.05) 
                 770 
                 114 
               
               
                 Example 4) 
               
               
                 0.1 
                 13.2 
                 (0.14, 0.05) 
                 810 
                 140 
               
               
                 0.3 
                 13.2 
                 (0.14, 0.05) 
                 875 
                 151 
               
               
                 0.5 
                 13.3 
                 (0.14, 0.05) 
                 870 
                 151 
               
               
                 1.0 
                 13.3 
                 (0.14, 0.05) 
                 875 
                 154 
               
               
                 2.0 
                 13.3 
                 (0.14, 0.05) 
                 880 
                 153 
               
               
                 3.0 
                 13.0 
                 (0.14, 0.05) 
                 875 
                 154 
               
               
                 4.0 
                 12.7 
                 (0.14, 0.05) 
                 875 
                 153 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 6 
               
               
                   
               
               
                   
                   
                   
                 Normal 
                 High 
               
               
                   
                   
                 Chromaticity 
                 Temperature 
                 Temperature 
               
               
                 d28(nm) 
                 EQE(%) 
                 (x, y) 
                 Lifetime (h) 
                 Lifetime (h) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 0 (Comparative 
                 30.3 
                 (0.24, 0.72) 
                 853 
                 95 
               
               
                 Example 5) 
               
               
                 0.1 
                 30.3 
                 (0.24, 0.71) 
                 904 
                 108 
               
               
                 0.3 
                 30.2 
                 (0.24, 0.72) 
                 912 
                 110 
               
               
                 0.5 
                 30.3 
                 (0.24, 0.71) 
                 910 
                 108 
               
               
                 1.0 
                 30.4 
                 (0.24, 0.71) 
                 910 
                 109 
               
               
                 2.0 
                 30.1 
                 (0.24, 0.71) 
                 908 
                 111 
               
               
                 3.0 
                 30.0 
                 (0.24, 0.71) 
                 908 
                 109 
               
               
                 4.0 
                 27.5 
                 (0.24, 0.71) 
                 910 
                 111 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 7 
               
               
                   
               
               
                   
                   
                   
                 Normal 
                 High 
               
               
                   
                   
                 Chromaticity 
                 Temperature 
                 Temperature 
               
               
                 d28(nm) 
                 EQE(%) 
                 (x, y) 
                 Lifetime (h) 
                 Lifetime (h) 
               
               
                   
               
             
            
               
                 0 (Comparative 
                 34.5 
                 (0.70, 0.31) 
                 2150 
                 334 
               
               
                 Example 6) 
               
               
                 0.1 
                 34.8 
                 (0.70, 0.31) 
                 2880 
                 360 
               
               
                 0.3 
                 34.3 
                 (0.71, 0.31) 
                 2950 
                 370 
               
               
                 0.5 
                 34.4 
                 (0.71, 0.31) 
                 3030 
                 373 
               
               
                 1.0 
                 34.4 
                 (0.70, 0.31) 
                 2980 
                 370 
               
               
                 2.0 
                 34.6 
                 (0.70, 0.31) 
                 2970 
                 370 
               
               
                 3.0 
                 33.7 
                 (0.71, 0.31) 
                 3010 
                 372 
               
               
                 4.0 
                 32.0 
                 (0.70, 0.31) 
                 3050 
                 375 
               
               
                   
               
            
           
         
       
     
     In Tables 4 and 7, the “EQE” column indicates the percentage of external quantum efficiency of each light-emitting element under the application of the above voltage. In the “chromaticity” column, CIE chromaticity coordinates are used to indicate the chromaticity of the light emitted by each light-emitting element. The “normal temperature lifetime” column indicates, in units of hours (h), the time period until the luminance of each light-emitting element reached 90 percent of the initial luminance at an environmental temperature of 25° C. and under the application of the above voltage. The “high temperature lifetime” column indicates, in units of hours (h), the time period until the luminance of each light-emitting element reached 90 percent of the initial luminance at an environmental temperature of 70° C. and under the application of the above voltage. 
     As shown in Table 4 to Table 7, the lifetime in a normal temperature environment and the lifetime in a high temperature environment of each of the light-emitting elements according to Examples 3 to 6 were improved in comparison to the lifetimes in normal temperature and high temperature environments of each of the light-emitting elements according to Comparative Examples 3 to 6. Moreover, the lifetime improvement of each of the light-emitting elements according to Examples 3 to 6 was more pronounced in the high-temperature environment. 
     Furthermore, as shown in Table 4 to Table 7, as the layer thickness d28 of the second interlayer organic layer  28  was increased in the light-emitting elements according to each of the examples, the lifetime of the light-emitting elements tended to become longer. This is thought to be because as the second interlayer organic layer  28  became thicker, the progression of ionization of the lithium quinolate complex of the second electron transport layer  18  was more efficiently suppressed, and degradation of the second electron transport layer  18  was suppressed. 
     On the other hand, as shown in Table 4 to Table 7, the external quantum efficiency of the light-emitting elements according to each of Examples 3 to 6 was not significantly reduced when the layer thickness d28 of the second interlayer organic layer  28  was 3 nm or less. This is thought to be because in the injection of electrons from the electron injection layer  32  into the second electron transport layer  18 , electron tunneling of the second interlayer organic layer  28  occurs efficiently due to the second interlayer organic layer  28  being sufficiently thin. 
     From the above, and from the perspective of significantly improving the lifetime while maintaining the external quantum efficiency, the layer thickness d28 of the second interlayer organic layer  28  of the light-emitting element according to each example is preferably from 0.1 nm to 3 nm. 
     A display device  26  having higher luminous efficiency and an improved lifetime can be obtained by providing the display device  26  with the light-emitting element  6 R, the light-emitting element  6 G, and the light-emitting element  6 B according to the present embodiment. 
     Note that in each of the embodiments described above, configurations in which the light-emitting element  6 R was provided with a single light-emitting layer  14 R were described, but no such limitation is intended. For example, the light-emitting element  6 R may be provided with a light-emitting layer  14 R in which a red light-emitting layer of a type having hole transport properties and a red light-emitting layer of a type having electron transport properties are layered, similar to the light-emitting layer  14 G of the light-emitting element  6 G. 
     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 ,  26  Light-emitting 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 
           24  First interlayer organic layer 
           28  Second interlayer organic layer 
           30  Hole injection layer 
           32  Electron injection layer