Patent Publication Number: US-2023165144-A1

Title: Light-Emitting Element, Light-Emitting Device, Electronic Device, Lighting Device, and Organic Compound

Description:
This application is a continuation of copending U.S. application Ser. No. 16/570,062, filed on Sep. 13, 2019 which is a continuation of U.S. application Ser. No. 15/988,152, filed on May 24, 2018 (now U.S. Pat. No. 10,418,565 issued Sep. 17, 2019) which is a continuation of U.S. application Ser. No. 14/807,426, filed on Jul. 23, 2015 (now U.S. Pat. No. 9,985,221 issued May 29, 2018) which are all incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an object, a method, or a manufacturing method. The present invention also relates to a process, a machine, manufacture, or a composition of matter. In particular, one embodiment of the present invention relates to a light-emitting element, a light-emitting device, an electronic device, a lighting device, a driving method thereof, or a manufacturing method thereof. One embodiment of the present invention further relates to an organic compound which can be used in a light-emitting element, a light-emitting device, an electronic device, and a lighting device. 
     2. Description of the Related Art 
     A light-emitting element using an organic compound as a luminous body, which has features such as thinness, lightness, high-speed response, and DC drive at low voltage, is expected to be used in a next-generation flat panel display. In particular, a display device in which light-emitting elements are arranged in a matrix is considered to have advantages in a wide viewing angle and excellent visibility over a conventional liquid crystal display device. 
     The light emission mechanism of a light-emitting element is said to be as follows: when a voltage is applied between a pair of electrodes with an EL layer including a luminous body provided therebetween, electrons injected from a cathode and holes injected from an anode recombine in the light emission center of the EL layer to form molecular excitons, and energy is released and light is emitted when the molecular excitons return to the ground state. Singlet excitation and triplet excitation are known as excited states, and it is thought that light emission can be achieved through either of the excited states. 
     In order to improve element characteristics of such a light-emitting element, improvement of an element structure, development of a material, and the like have been actively carried out (see, for example, Patent Document 1). 
     REFERENCE 
     Patent Document 
     
         
         [Patent Document 1] Japanese Published Patent Application No. 2010-182699 
       
    
     SUMMARY OF THE INVENTION 
     When light-emitting elements are developed, one of important points for commercialization is to improve the reliability of the elements. In order to improve the reliability of the elements, an element structure is needed, which can control the carrier balance and improve the recombination probability of carriers in an EL layer of a light-emitting element. Therefore, it is an object to provide a highly reliable light-emitting element in the following way: an EL layer is made to have a desired structure so that carriers are transferred efficiently in a light-emitting layer. A high current efficiency (or a high quantum efficiency) is also important to decrease the amount of current needed for driving the element and to improve the reliability. 
     In one embodiment of the present invention, a light-emitting element with an improved reliability is provided. In addition, a light-emitting element with a high current efficiency (or a high quantum efficiency) is provided. Furthermore, a novel organic compound that is favorably used in a light-emitting element which is one embodiment of the present invention is provided. In another embodiment of the present invention, a light-emitting element, a light-emitting device, an electronic device, or a lighting device with a high emission efficiency and a high reliability, in which the above organic compound is used as an EL material, is provided. In another embodiment of the present invention, a novel material is provided. In another embodiment of the present invention, a novel light-emitting element and a novel light-emitting device are provided. Note that the descriptions of these objects do not disturb the existence of other objects. In one embodiment of the present invention, there is no need to achieve all the objects. Other objects will be apparent from and can be derived from the descriptions of the specification, the drawings, the claims, and the like. 
     One embodiment of the present invention is a light-emitting element including an EL layer between an anode and a cathode. The EL layer includes a light-emitting layer; the light-emitting layer contains a first organic compound having an electron-transport property and a hole-transport property, a second organic compound having a hole-transport property, and a light-emitting substance; the combination of the first organic compound and the second organic compound forms an exciplex; the HOMO level of the first organic compound is lower than the HOMO level of the second organic compound; and a difference between the HOMO level of the first organic compound and the HOMO level of the second organic compound is less than or equal to 0.4 eV. 
     Another embodiment of the present invention is a light-emitting element including an EL layer between an anode and a cathode. The EL layer includes a light-emitting layer; the light-emitting layer contains a first organic compound having an electron-transport property and a hole-transport property, a second organic compound having a hole-transport property, and a light-emitting substance; the combination of the first organic compound and the second organic compound forms an exciplex; the first organic compound includes a 6-membered nitrogen-containing heteroaromatic ring and a carbazole skeleton and does not include a triarylamine skeleton; and the second organic compound includes a triarylamine skeleton. 
     Another embodiment of the present invention is a light-emitting element including an EL layer between an anode and a cathode. The EL layer includes a light-emitting layer; the light-emitting layer contains a first organic compound having an electron-transport property and a hole-transport property, a second organic compound having a hole-transport property, and a light-emitting substance; the combination of the first organic compound and the second organic compound forms an exciplex; the first organic compound includes a 6-membered nitrogen-containing heteroaromatic ring and a bicarbazole skeleton and does not include a triarylamine skeleton; and the second organic compound includes a triarylamine skeleton. 
     In the above structure, the bicarbazole skeleton is a 3,3′-bicarbazole skeleton or a 2,3′-bicarbazole skeleton. 
     In each of the above structures, the light-emitting substance is a phosphorescent compound. 
     In each of the above structures, the EL layer includes a hole-transport layer; the hole-transport layer is in contact with the light-emitting layer and contains a third organic compound having a hole-transport property; and the HOMO level of the third organic compound is lower than the HOMO level of the second organic compound. 
     In each of the above structures, the first organic compound is represented by the following general formula (G0). 
     
       
         
         
             
             
         
       
     
     In the formula, A represents a dibenzo[f,h]quinoxalinyl group; R 1  to R 15  represent independently hydrogen, a substituted or unsubstituted alkyl group having 1 to 6 carbon atoms, a substituted or unsubstituted cycloalkyl group having 5 to 7 carbon atoms, or a substituted or unsubstituted aryl group having 6 to 13 carbon atoms; and Ar represents a substituted or unsubstituted arylene group having 6 to 25 carbon atoms or a single bond. Preferably, an arylene group in Ar does not include an anthracenylene group. 
     Another embodiment of the present invention is an organic compound represented by the following general formula (G0). 
     
       
         
         
             
             
         
       
     
     In the formula, A represents a dibenzo[f,h]quinoxalinyl group; R 1  to R 15  represent independently hydrogen, a substituted or unsubstituted alkyl group having 1 to 6 carbon atoms, a substituted or unsubstituted cycloalkyl group having 5 to 7 carbon atoms, or a substituted or unsubstituted aryl group having 6 to 13 carbon atoms; and Ar represents a substituted or unsubstituted arylene group having 6 to 25 carbon atoms or a single bond. Preferably, an arylene group in Ar does not include an anthracenylene group. 
     Another embodiment of the present invention is an organic compound represented by the following general formula (G1). 
     
       
         
         
             
             
         
       
     
     In the formula, R 1  to R 24  represent independently hydrogen, a substituted or unsubstituted alkyl group having 1 to 6 carbon atoms, a substituted or unsubstituted cycloalkyl group having 5 to 7 carbon atoms, or a substituted or unsubstituted aryl group having 6 to 13 carbon atoms; and Ar represents a substituted or unsubstituted arylene group having 6 to 25 carbon atoms or a single bond. Preferably, an arylene group in Ar does not include an anthracenylene group. 
     Another embodiment of the present invention is an organic compound represented by the following general formula (G2). 
     
       
         
         
             
             
         
       
     
     In the formula, R 1  to R 24  represent independently hydrogen, a substituted or unsubstituted alkyl group having 1 to 6 carbon atoms, a substituted or unsubstituted cycloalkyl group having 5 to 7 carbon atoms, or a substituted or unsubstituted aryl group having 6 to 13 carbon atoms; and Ar represents a substituted or unsubstituted arylene group having 6 to 25 carbon atoms or a single bond. Preferably, an arylene group in Ar does not include an anthracenylene group. 
     Another embodiment of the present invention is an organic compound represented by the following general formula (G3). 
     
       
         
         
             
             
         
       
     
     In the formula, R 1  to R 24  represent independently hydrogen, a substituted or unsubstituted alkyl group having 1 to 6 carbon atoms, a substituted or unsubstituted cycloalkyl group having 5 to 7 carbon atoms, or a substituted or unsubstituted aryl group having 6 to 13 carbon atoms; and Ar represents a substituted or unsubstituted arylene group having 6 to 25 carbon atoms or a single bond. Preferably, an arylene group in Ar does not include an anthracenylene group. 
     Examples of the alkyl group having 1 to 6 carbon atoms in each of the above general formula (G0), general formula (G2), and general formula (G3) include a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, an isobutyl group, a sec-butyl group, a tert-butyl group, a pentyl group, an isopentyl group, and a hexyl group. Examples of the cycloalkyl group having 5 to 7 carbon atoms include a cyclopentyl group, a cyclohexyl group, and a cycloheptyl group. Examples of the aryl group having 6 to 13 carbon atoms include a phenyl group, a tolyl group, a xylyl group, a biphenyl group, an indenyl group, a naphthyl group, and a fluorenyl group. Examples of the arylene group having 6 to 25 carbon atoms in Ar include a 1,2-, 1,3-, and 1,4-phenylene groups, a 2,6-, 3,5-, and 2,4-toluylene groups, a 4,6-dimethylbenzene-1,3-diyl group, a 2,4,6-trimethylbenzene-1,3-diyl group, a 2,3,5,6-tetramethylbenzene-1,4-diyl group, a 3,3′-, 3,4′-, and 4,4′-biphenylene groups, a 1,1′:3′,1″-terbenzene-3,3″-diyl group, a 1,1′:4′,1″-terbenzene-3,3″-diyl group, a 1,1′:4′,1″-terbenzene-4,4″-diyl group, a 1,1′:3′,1″:3″,1″′-quaterbenzene-3,3′″-diyl group, a 1,1′:3′,1″:4″,1″′-quaterbenzene-3,4′″-diyl group, a 1,1′:4′,1″:4″,1′″-quaterbenzene-4,4″-diyl group, a 1,4-, 1,5-, 2,6-, and 2,7-naphthylene groups, a 2,7-fluorenylene group, a 9,9-dimethyl-2,7-fluorenylene group, a 9,9-diphenyl-2,7-fluorenylene group, a 9,9-dimethyl-1,4-fluorenylene group, a spiro-9,9′-bifluorene-2,7-diyl group, a 9,10-dihydro-2,7-phenanthrenylene group, a 2,7-phenanthrenylene group, a 3,6-phenanthrenylene group, a 9,10-phenanthrenylene group, a 2,7-triphenylenylene group, a 3,6-triphenylenylene group, a 2,8-benzo[a]phenanthrenylene group, a 2,9-benzo[a]phenanthrenylene group, and a 5,8-benzo[c]phenanthrenylene group. 
     The alkyl group having 1 to 6 carbon atoms, the cycloalkyl group having 5 to 7 carbon atoms, the aryl group having 6 to 13 carbon atoms, and the arylene group having 6 to 25 carbon atoms may each have a substituent. Examples of the substituent preferably include alkyl groups each having 1 to 6 carbon atoms, such as a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, an isobutyl group, a sec-butyl group, a tert-butyl group, a pentyl group, an isopentyl group, and a hexyl group; cycloalkyl groups each having 5 to 7 carbon atoms, such as a cyclopentyl group, a cyclohexyl group, and a cycloheptyl group; and aryl groups each having 6 to 13 carbon atoms, which form a ring, such as a phenyl group, a tolyl group, a xylyl group, a biphenyl group, an indenyl group, a naphthyl group, a fluorenyl group, and a 9,9-dimethylfluorenyl group. 
     Another embodiment of the present invention is a light-emitting element containing the organic compound represented by any of the above general formulae (G0) to (G3). 
     Another embodiment of the present invention is a light-emitting device including the light-emitting element having any of the above structures and a housing. 
     One embodiment of the present invention includes, in its category, in addition to a light-emitting device including a light-emitting element, an electronic device including the light-emitting element or the light-emitting device (specifically, an electronic device including the light-emitting element or the light-emitting device and a connection terminal or an operation key) and a lighting device including the light-emitting element or the light-emitting device (specifically, a lighting device including the light-emitting element or the light-emitting device and a housing). A light-emitting device in this specification refers to an image display device or a light source (e.g., a lighting device). A light-emitting device also includes, in its category, all of a module in which a light-emitting device is connected to a connector such as a flexible printed circuit (FPC) or a tape carrier package (TCP), a module in which a printed wiring board is provided on the tip of a TCP, and a module in which an integrated circuit (IC) is directly mounted on a light-emitting element by a chip on glass (COG) method. 
     In accordance with one embodiment of the present invention, a novel dibenzo[f,h]quinoxaline derivative can be provided. In accordance with one embodiment of the present invention, a light-emitting element, a light-emitting device, an electronic device, or a lighting device with a high emission efficiency and a high reliability, in which the above dibenzo[f,h]quinoxaline derivative is used as an EL material, can be provided. In accordance with one embodiment of the present invention, a novel material can be provided. In accordance with another embodiment of the present invention, a novel light-emitting element and a novel light-emitting device can be provided. Note that the descriptions of these effects do not disturb the existence of other effects. One embodiment of the present invention does not necessarily achieve all the effects listed above. Other effects will be apparent from and can be derived from the descriptions of the specification, the drawings, the claims, and the like. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    schematically shows a light-emitting layer of a light-emitting element. 
         FIG.  2    schematically shows a light-emitting layer of a light-emitting element. 
         FIGS.  3 A and  3 B  each illustrate the structure of a light-emitting element. 
         FIGS.  4 A and  4 B  each illustrate the structure of a light-emitting element. 
         FIG.  5    illustrates a light-emitting device. 
         FIGS.  6 A and  6 B  illustrate a light-emitting device. 
         FIGS.  7 A,  7 B,  7 C,  7 D,  7 D ′ 1  and  7 D′ 2  each illustrate an electronic device. 
         FIGS.  8 A to  8 C  illustrate an electronic device. 
         FIG.  9    illustrates lighting devices. 
         FIGS.  10 A and  10 B  are  1 H-NMR charts of a dibenzo[f,h]quinoxaline derivative represented by the structural formula (100). 
         FIG.  11    shows the ultraviolet-visible absorption spectrum and the emission spectrum of the dibenzo[f,h]quinoxaline derivative represented by the structural formula (100). 
         FIG.  12    shows the ultraviolet-visible absorption spectrum and the emission spectrum of the dibenzo[f,h]quinoxaline derivative represented by the structural formula (100). 
         FIGS.  13 A and  13 B  are  1 H-NMR charts of a dibenzo[f,h]quinoxaline derivative represented by the structural formula (101). 
         FIG.  14    shows the ultraviolet-visible absorption spectrum and the emission spectrum of the dibenzo[f,h]quinoxaline derivative represented by the structural formula (101). 
         FIG.  15    shows the ultraviolet-visible absorption spectrum and the emission spectrum of the dibenzo[f,h]quinoxaline derivative represented by the structural formula (101). 
         FIGS.  16 A and  16 B  are  1 H-NMR charts of a dibenzo[f,h]quinoxaline derivative represented by the structural formula (102). 
         FIG.  17    shows the ultraviolet-visible absorption spectrum and the emission spectrum of the dibenzo[f,h]quinoxaline derivative represented by the structural formula (102). 
         FIG.  18    shows the ultraviolet-visible absorption spectrum and the emission spectrum of the dibenzo[f,h]quinoxaline derivative represented by the structural formula (102). 
         FIGS.  19 A and  19 B  are  1 H-NMR charts of a dibenzo[f,h]quinoxaline derivative represented by the structural formula (103). 
         FIG.  20    shows the ultraviolet-visible absorption spectrum and the emission spectrum of the dibenzo[f,h]quinoxaline derivative represented by the structural formula (103). 
         FIG.  21    shows the ultraviolet-visible absorption spectrum and the emission spectrum of the dibenzo[f,h]quinoxaline derivative represented by the structural formula (103). 
         FIG.  22    illustrates the structure of each of a light-emitting element  1 , a light-emitting element  2 , and a comparative light-emitting element  3 . 
         FIG.  23    shows current density-luminance characteristics of the light-emitting element  1 , the light-emitting element  2 , and the comparative light-emitting element  3 . 
         FIG.  24    shows voltage-luminance characteristics of the light-emitting element  1 , the light-emitting element  2 , and the comparative light-emitting element  3 . 
         FIG.  25    shows luminance-current efficiency characteristics of the light-emitting element  1 , the light-emitting element  2 , and the comparative light-emitting element  3 . 
         FIG.  26    shows voltage-current characteristics of the light-emitting element  1 , the light-emitting element  2 , and the comparative light-emitting element  3 . 
         FIG.  27    shows the emission spectra of the light-emitting element  1 , the light-emitting element  2 , and the comparative light-emitting element  3 . 
         FIGS.  28 A and  28 B  show the reliability of each of the light-emitting element  1 , the light-emitting element  2 , and the comparative light-emitting element  3 . 
         FIG.  29    shows current density-luminance characteristics of a light-emitting element  4 . 
         FIG.  30    shows voltage-luminance characteristics of the light-emitting element  4 . 
         FIG.  31    shows luminance-current efficiency characteristics of the light-emitting element  4 . 
         FIG.  32    shows voltage-current characteristics of the light-emitting element  4 . 
         FIG.  33    shows the emission spectrum of the light-emitting element  4 . 
         FIGS.  34 A and  34 B  show the reliability of the light-emitting element  4 . 
         FIGS.  35 A and  35 B  are  1 H-NMR charts of a dibenzo[f,h]quinoxaline derivative represented by the structural formula (122). 
         FIG.  36    shows current density-luminance characteristics of a light-emitting element  5 , a comparative light-emitting element  6 , and a comparative light-emitting element  7 . 
         FIG.  37    shows voltage-luminance characteristics of the light-emitting element  5 , the comparative light-emitting element  6 , and the comparative light-emitting element  7 . 
         FIG.  38    shows luminance-current efficiency characteristics of the light-emitting element  5 , the comparative light-emitting element  6 , and the comparative light-emitting element  7 . 
         FIG.  39    shows voltage-current characteristics of the light-emitting element  5 , the comparative light-emitting element  6 , and the comparative light-emitting element  7 . 
         FIG.  40    shows the emission spectra of the light-emitting element  5 , the comparative light-emitting element  6 , and the comparative light-emitting element  7 . 
         FIGS.  41 A and  41 B  show the reliability of each of the light-emitting element  5 , the comparative light-emitting element  6 , and the comparative light-emitting element  7 . 
         FIG.  42    shows current density-luminance characteristic of a light-emitting element  8 . 
         FIG.  43    shows voltage-luminance characteristic of the light-emitting element  8 . 
         FIG.  44    shows luminance-current efficiency characteristics of the light-emitting element  8 . 
         FIG.  45    shows voltage-current characteristics of the light-emitting element  8 . 
         FIG.  46    shows the emission spectrum of the light-emitting element  8 . 
         FIG.  47    shows voltage-current characteristics of a light-emitting element  1 A after a preservation test. 
         FIG.  48    shows luminance-external quantum efficiency characteristics of the light-emitting element  1 A after the preservation test. 
         FIG.  49    shows voltage-current characteristics of a light-emitting element  2 A after a preservation test. 
         FIG.  50    shows luminance-external quantum efficiency characteristics of the light-emitting element  2 A after the preservation test. 
         FIG.  51    shows voltage-current characteristics of a comparative light-emitting element  3 A after a preservation test. 
         FIG.  52    shows luminance-external quantum efficiency characteristics of the comparative light-emitting element  3 A after the preservation test. 
         FIG.  53    shows voltage-current characteristics of a light-emitting element  4 A after a preservation test. 
         FIG.  54    shows luminance-external quantum efficiency characteristics of the light-emitting element  4 A after the preservation test. 
         FIG.  55    shows voltage-current characteristics of a light-emitting element  8 A after a preservation test. 
         FIG.  56    shows luminance-external quantum efficiency characteristics of the light-emitting element  8 A after the preservation test. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. Note that the present invention is not limited to the following description, and the modes and details thereof can be modified in various ways without departing from the spirit and scope of the present invention. Accordingly, the present invention should not be interpreted as being limited to the content of the embodiments below. 
     Note that the terms “film” and “layer” can be interchanged with each other depending on the case or circumstances. For example, the term “conductive layer” can be changed into the term “conductive film” in some cases. Also, the term “insulating film” can be changed into the term “insulating layer” in some cases. 
     Embodiment 1 
     In this embodiment, a light-emitting element which is one embodiment of the present invention will be described. 
     In a light-emitting element described in this embodiment, an EL layer including a light-emitting layer is provided between a pair of electrodes (a first electrode (anode) and a second electrode (cathode)), and the EL layer includes a hole-injection layer, a hole-transport layer, an electron-transport layer, an electron-injection layer, and the like in addition to the light-emitting layer. 
     When a voltage is applied to the light-emitting element, holes injected from the first electrode side and electrons injected from the second electrode side recombine in the light-emitting layer; with energy generated by the recombination, a light-emitting substance contained in the light-emitting layer emits light. 
     As shown in  FIG.  1   , a light-emitting layer  100  contains a first organic compound (h)  101  having an electron-transport property and a hole-transport property, a second organic compound (a)  102  having a hole-transport property, and a light-emitting substance (not shown in the drawing). Note that the combination of the first organic compound (h)  101  and the second organic compound (a)  102  forms an excited complex (also referred to as an exciplex). That is, the lowest unoccupied molecular orbital (LUMO) level of the first organic compound (h)  101  is at least lower than the LUMO level of the second organic compound (a)  102 , and the highest occupied molecular orbital (HOMO) level of the first organic compound (h)  101  is at least lower than the HOMO level of the second organic compound (a)  102 . Accordingly, as shown in the drawing, the excitation energy of the generated exciplex is influenced by an energy difference (i.e., ΔE e  in the drawing) between the LUMO level of the first organic compound (h)  101  (LUMO (h)) and the HOMO level of the second organic compound (a)  102  (HOMO (a)). 
     Such a light-emitting layer  100  enables energy transfer utilizing an overlap between the emission spectrum of the exciplex and the absorption spectrum of the light-emitting substance (guest material), leading to a high energy transfer efficiency; thus, a light-emitting element with a high external quantum efficiency can be achieved. Furthermore, in order that the exciplex is electrically excited, electric energy (i.e., voltage) corresponding to ΔE e  is needed. This energy ΔE e  is lower than energy ΔE h  needed for electrically exciting the first organic compound (h)  101  and energy ΔE a  needed for electrically exciting the second organic compound (a)  102 . In other words, with such a light-emitting layer  100 , drive voltage (emission start voltage) of the light-emitting element can be reduced. 
     Without using the first organic compound (h)  101  and the second organic compound (a)  102 , one kind of organic compound having a HOMO-LUMO gap corresponding to ΔE e  may be used for a light-emitting layer, whereby drive voltage (emission start voltage) that is low as in the case of the light-emitting layer  100  can be achieved. However, in the case of using one kind of organic compound, triplet excitation energy is significantly decreased as compared to singlet excitation energy; thus, it is difficult to achieve light emission by transfer of triplet excitation energy to a light-emitting substance (guest material). On the other hand, in the exciplex, singlet excitation energy and triplet excitation energy are located at substantially the same level, which makes it possible to transfer both singlet excitation energy and triplet excitation energy to the light-emitting substance. As a result, as well as the above-described effect of reduction in voltage, a higher efficiency can be achieved in the light-emitting element. A detailed mechanism of the higher efficiency will be described below. 
     When the light-emitting substance is a phosphorescent compound, singlet excitation energy and triplet excitation energy of the exciplex are both transferred to the triplet excited state of the phosphorescent compound, and light emission from the triplet excited state (i.e., phosphorescence) is achieved; thus, the light-emitting substance is most preferably a phosphorescent compound in order to achieve a higher efficiency. 
     When the light-emitting substance is a thermally activated delayed fluorescent compound, singlet excitation energy of the exciplex is transferred to the singlet excited state of the light-emitting substance, and light emission from the singlet excited state (i.e., fluorescence) is achieved. In addition, triplet excitation energy of the exciplex is transferred to the triplet excited state of the light-emitting substance; however, reverse intersystem crossing from part of or the entire triplet excited state to the singlet excited state of the light-emitting substance occurs due to thermal activation, from which fluorescence is achieved, leading to a higher efficiency. 
     When the light-emitting substance is a fluorescent compound, singlet excitation energy of the exciplex is transferred to the singlet excited state of the light-emitting substance, and light emission from the singlet excited state (i.e., fluorescence) is achieved. On the other hand, triplet excitation energy of the exciplex is transferred to the triplet excited state of the light-emitting substance and thermally deactivated; thus, it seems that a higher efficiency cannot be achieved. However, the exciplex that is an energy donor has a small difference between singlet excitation energy and triplet excitation energy and thus emits thermally activated delayed fluorescence from itself. In other words, in the exciplex, reverse intersystem crossing from part of or the entire triplet excited state to the singlet excited state occurs, so that the proportion of singlet excitons is higher than that in the normal situation. The proportion of singlet excitons in the exciplex that is an energy donor is high, and singlet excitation energy of the exciplex is transferred to the singlet excited state of the light-emitting substance, whereby the emission efficiency is high even in the case of using a fluorescent compound as a light-emitting substance. This phenomenon is also one feature of the present invention. 
     As described above, a light-emitting element in which an exciplex serves as an energy donor in a light-emitting layer is effective in all the cases where a phosphorescent compound, a thermally activated delayed fluorescent compound, and a fluorescent compound are used as a light-emitting substance; however, there is a problem in controlling a light-emitting region. 
     As already described above, in order to form the exciplex of the first organic compound (h)  101  and the second organic compound (a)  102 , the following condition is needed: the LUMO level of the first organic compound (h)  101  (LUMO(h)) is at least lower than the LUMO level of the second organic compound (a)  102  (LUMO(a)), and the HOMO level of the first organic compound (h)  101  (HOMO(h)) is at least lower than the HOMO level of the second organic compound (a)  102  (HOMO(a)). In particular, in a conventional light-emitting element in which an exciplex serves as an energy donor in the light-emitting layer  100 , an energy difference ΔE HOMO  between the HOMO level of the first organic compound (h)  101  (HOMO(h)) and the HOMO level of the second organic compound (a)  102  (HOMO(a)) is made very large, thereby forming an exciplex. For example, 2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline (abbreviation: 2mDBTBPDBq-II) is used as the first organic compound (h)  101  and N-(1,1′-biphenyl-4-yl)-9,9-dimethyl-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-9H-fluor en-2-amine (abbreviation: PCBBiF) is used as the second organic compound (a)  102 ; in that case, the HOMO level of the first organic compound (h)  101  (HOMO(h)) is −6.22 eV, the HOMO level of the second organic compound (a)  102  (HOMO(a)) is −5.36 eV, and ΔE HOMO  is as large as 0.86 eV. 
     When a difference (ΔE HOMO ) between the HOMO level of the first organic compound (h)  101  (HOMO(h)) and the HOMO level of the second organic compound (a)  102  (HOMO(a)) is large as described above, the carrier balance tends to be significantly changed depending on the amount of the second organic compound (a)  102  in the light-emitting layer  100 . That is, a too small amount of the second organic compound (a)  102  results in excessive electrons, and a light-emitting region exists mainly on the anode side; on the other hand, the amount of the second organic compound (a)  102 , which only slightly exceeds the optimal amount, results in excessive holes, and the holes are transported to the cathode side. In such a device with a narrow margin, even when the first organic compound (h)  101  and the second organic compound (a)  102  are mixed at an optimized ratio, a small change in the carrier balance through long-time driving leads to a reduction in recombination efficiency and a lower luminance. When ΔE HOMO  is large, holes are stored in the second organic compound (a)  102 , so that the recombination region in the light-emitting layer  100  is narrowed. In the case where the recombination region in the light-emitting layer  100  is wide, the entire light-emitting layer  100  can be used and the reliability is high. 
     In the light-emitting element, an electron-injection property is decreased owing to deterioration of an electron-injection electrode (cathode) or the like, which might cause the recombination region to be shifted to the cathode side through long-time driving. In that case, the average distance for transporting holes to the recombination region gets longer; thus, when the light-emitting layer  100  has a poor hole-transport property, the resistance of the light-emitting element is increased. In other words, in the case where the element is driven at constant current, drive voltage is significantly increased over time. When the difference (ΔE HOMO ) between the HOMO level of the first organic compound (h) (HOMO(h)) and the HOMO level of the second organic compound (a) (HOMO(a)) is large, holes are transported with difficulty through the light-emitting layer  100 ; thus, an increase in drive voltage over time is very problematic. 
     One embodiment of the present invention solves a problem in the above-described light-emitting element in which the exciplex serves as an energy donor in the light-emitting layer  100 . That is, a light-emitting element which is one embodiment of the present invention includes an EL layer between an anode and a cathode. The EL layer includes the light-emitting layer  100 ; the light-emitting layer  100  contains the first organic compound (h)  101  having an electron-transport property and a hole-transport property, the second organic compound (a)  102  having a hole-transport property, and the light-emitting substance; the combination of the first organic compound (h)  101  and the second organic compound (a)  102  forms an exciplex; the HOMO level of the first organic compound (h) (HOMO(h)) is lower than the HOMO level of the second organic compound (a) (HOMO(a)); and a difference between the HOMO level of the first organic compound (h) (HOMO(h)) and the HOMO level of the second organic compound (a) (HOMO(a)) is less than or equal to 0.4 eV. 
     With such a structure, holes are injected not only into the second organic compound (a)  102  but also partly into the first organic compound (h)  101 . As a result, holes are unlikely to be stored in the second organic compound (a)  102 ; thus, a light-emitting element can be obtained, in which the carrier balance can be easily maintained and the recombination region in the light-emitting layer  100  is wide. In addition, an increase in voltage through long-time driving (driving at constant current) can be suppressed. In the above case, in part of the light-emitting layer  100 , recombination is caused in the first organic compound (h)  101 , which leads to the formation of an excited state of the first organic compound, but this is rapidly converted into an exciplex; thus, a higher efficiency can be achieved by using the exciplex. Furthermore, since holes are mainly injected into the second organic compound (a)  102 , the effect of reducing drive voltage (emission start voltage) can be maintained. 
     In this way, ΔE HOMO  is set to be less than or equal to 0.4 eV (and greater than 0 eV), and the combination of the first organic compound (h)  101  and the second organic compound (a)  102  forms an exciplex, whereby the above-described problem can be solved. Since holes are injected not only into the second organic compound (a)  102  but also into the first organic compound (h)  101 , ΔE HOMO  is preferably less than or equal to 0.3 eV. 
     Compounds that are suitable to achieve the above concept are as follows. Preferably, the first organic compound (h)  101  includes a 6-membered nitrogen-containing heteroaromatic ring and a carbazole skeleton and does not include a triarylamine skeleton. That is, a compound having an electron-transport property due to including a 6-membered nitrogen-containing heteroaromatic ring and a moderate hole-transport property due to including a carbazole skeleton and not including a triarylamine skeleton is preferably used. The second organic compound (a)  102  has a hole-transport property and preferably includes a triarylamine skeleton so that the HOMO level of the second organic compound (a)  102  is higher than that of the first organic compound (h)  101 . 
     A compound including a triarylamine skeleton has, in many cases, a HOMO level of about −5.5 eV, or higher than or equal to −5.5 eV, on the basis of a cyclic voltammetry (CV) measurement, whereas the HOMO level of a simple 9-phenylcarbazole is −5.88 eV, so that the difference between the HOMO level of the compound including a triarylamine skeleton and that of 9-phenylcarbazole is greater than or equal to 0.4 eV in many cases. Therefore, in one embodiment of the present invention, the first organic compound (h)  101  preferably includes a bicarbazole skeleton as a carbazole skeleton, because the HOMO level of bicarbazole is higher than that of 9-phenylcarbazole. In particular, in one embodiment of the present invention, a 3,3′-bicarbazole skeleton or a 2,3′-bicarbazole skeleton is preferably introduced into the first organic compound (h)  101  because its HOMO level becomes about −5.6 eV to −5.7 eV. 
     Examples of the 6-membered nitrogen-containing heteroaromatic ring include, as well as pyridine, diazine such as pyrazine, pyrimidine, or pyridazine, triazine, and tetrazine. Such a 6-membered nitrogen-containing heteroaromatic ring may further be condensed with a benzene ring or the like. Examples of the 6-membered nitrogen-containing heteroaromatic ring condensed with a benzene ring include quinoline, isoquinoline, and dibenzo[f,h]quinoline. Furthermore, naphthyridine typified by quinoxaline, quinazoline, and phthalazine, dibenzo[f,h]quinoxaline, dibenzo[f,h]quinazoline, and the like can also be used. 
     In one embodiment of the present invention, holes are preferably injected and transported not only to the second organic compound (a)  102  but also to the first organic compound (h)  101 , as described above. Therefore, in order to improve a hole-injection property with respect to both the first organic compound (h)  101  and the second organic compound (a)  102  in the light-emitting layer  100 , preferably, a third organic compound (p)  105  having a hole-transport property is used for a hole-transport layer  104  in contact with the light-emitting layer  100 , and the HOMO level of the third organic compound (p)  105  (HOMO(p)) is set to be lower than the HOMO level of the second organic compound (a)  102  (HOMO(a)), as shown in  FIG.  2   . In particular, the third organic compound (p)  105  is preferably selected so that the HOMO level of the third organic compound (p)  105  (HOMO(p)) can be a level between the HOMO level of the second organic compound (a)  102  (HOMO(a)) and the HOMO level of the first organic compound (h)  101  (HOMO(h)). 
     A specific example of a light-emitting element which is one embodiment of the present invention and has the above structure will be described below with reference to  FIGS.  3 A and  3 B . 
     For a first electrode (anode)  201  and a second electrode (cathode)  203 , a metal, an alloy, an electrically conductive compound, a mixture thereof, and the like can be used. Specifically, indium oxide-tin oxide (indium tin oxide), indium oxide-tin oxide containing silicon or silicon oxide, indium oxide-zinc oxide (indium zinc oxide), indium oxide containing tungsten oxide and zinc oxide, gold (Au), platinum (Pt), nickel (Ni), tungsten (W), chromium (Cr), molybdenum (Mo), iron (Fe), cobalt (Co), copper (Cu), palladium (Pd), and titanium (Ti) can be used. In addition, an element belonging to Group 1 or 2 of the periodic table, for example, an alkali metal such as lithium (Li) or cesium (Cs), an alkaline earth metal such as calcium (Ca) or strontium (Sr), magnesium (Mg), an alloy containing such an element (MgAg, AlLi), a rare earth metal such as europium (Eu) or ytterbium (Yb), an alloy containing such an element, graphene, and the like can be used. The first electrode (anode)  201  and the second electrode (cathode)  203  can be formed by, for example, a sputtering method, an evaporation method (including a vacuum evaporation method), or the like. 
     A hole-injection layer  211  injects holes into a light-emitting layer  213  through a hole-transport layer  212  having a high hole-transport property and contains a substance having a high hole-transport property (also referred as a hole-transport compound) and an acceptor substance. The hole-injection layer  211  contains a substance having a high hole-transport property and an acceptor substance, so that electrons are extracted from the substance having a high hole-transport property by the acceptor substance to generate holes and the holes are injected into the light-emitting layer  213  through the hole-transport layer  212 . The hole-transport layer  212  is formed using a substance having a high hole-transport property. 
     Examples of the substance having a high hole-transport property which is used for the hole-injection layer  211  and the hole-transport layer  212  include aromatic amine compounds such as 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB or α-NPD), N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine (abbreviation: TPD), 4,4′,4″-tris(carbazol-9-yl)triphenylamine (abbreviation: TCTA), 4,4′,4″-tris(N,N′-diphenylamino)triphenylamine (abbreviation: TDATA), 4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine (abbreviation: MTDATA), and 4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl (abbreviation: BSPB); 3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole (abbreviation: PCzPCA1); 3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole (abbreviation: PCzPCA2); and 3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole (abbreviation: PCzPCN1). Alternatively, the following carbazole derivatives and the like can be used: 4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP), 1,3,5-tri s[4-(N-carbazolyl)phenyl]benzene (abbreviation: TCPB), and 9-[4-(10-phenyl-9-anthracenyl)phenyl]-9H-Carbazole (abbreviation: CzPA). The substances described here are mainly substances each having a hole mobility of higher than or equal to 1×10 −6  cm 2 /Vs. Note that other substances may also be used as long as the substances have higher hole-transport properties than electron-transport properties. 
     Alternatively, high molecular compounds such as poly(N-vinylcarbazole) (abbreviation: PVK), poly(4-vinyltriphenylamine) (abbreviation: PVTPA), poly[N-(4-{N′-[4-(4-diphenylamino)phenyl]phenyl-N′-phenylamino}phenyl)methacrylamide] (abbreviation: PTPDMA), and poly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine] (abbreviation: Poly-TPD) can be used. 
     Examples of the acceptor substance that is used for the hole-injection layer  211  include oxides of metals belonging to Groups 4 to 8 of the periodic table. Specifically, molybdenum oxide is particularly preferable. 
     The light-emitting layer  213  is a layer containing a light-emitting substance. When the light-emitting layer  213  has the structure shown in  FIG.  1   , the light-emitting layer  213  contains a first organic compound having an electron-transport property and a hole-transport property, which will be described later, a second organic compound having a hole-transport property as described above, and a light-emitting substance. The combination of the first organic compound and the second organic compound forms an exciplex (also referred to as an excited complex) at the time of recombination of carriers (electrons and holes) in the light-emitting layer. When the exciplex is formed in the light-emitting layer, the fluorescence spectrum of the first organic compound and that of the second organic compound are converted into the emission spectrum of the exciplex which is located on a longer wavelength side. Moreover, when the first organic compound and the second organic compound are selected in such a manner that the emission spectrum of the exciplex largely overlaps with the absorption spectrum of a guest material, energy transfer from the singlet excited state can be maximized. Also in the case of the triplet excited state, energy transfer from the exciplex, not a host material, is assumed to occur. 
     Although the combination of the first organic compound and the second organic compound can be determined such that an exciplex is formed, a combination of a compound which is likely to accept electrons (a compound having an electron-trapping property) and a compound which is likely to accept holes (a compound having a hole-trapping property) is preferably employed. The first organic compound is preferably capable of trapping (or transporting) not only electrons but also holes and thus preferably includes a 6-membered nitrogen-containing heteroaromatic ring and a bicarbazole skeleton and does not include a triarylamine skeleton. For example, a compound represented by the following general formula (G0) is preferably used. 
     
       
         
         
             
             
         
       
     
     In the formula, A represents a dibenzo[f,h]quinoxalinyl group; R 1  to R 15  represent independently hydrogen, a substituted or unsubstituted alkyl group having 1 to 6 carbon atoms, a substituted or unsubstituted cycloalkyl group having 5 to 7 carbon atoms, or a substituted or unsubstituted aryl group having 6 to 13 carbon atoms; and Ar represents a substituted or unsubstituted arylene group having 6 to 25 carbon atoms or a single bond. Preferably, an arylene group in Ar does not include an anthracenylene group. 
     When Ar includes an anthracenylene group, triplet excitation energy of the compound is largely decreased (to an energy of lower than or equal to 1.7 eV), in which case triplet excitation energy of the exciplex might be quenched. Therefore, preferably, an arylene group in Ar does not include an anthracenylene group. 
     Specifically, the compounds represented by the general formulae (G1) to (G3) are preferably used. More specifically, 2-{4-[3-(N-phenyl-9H-carbazol-3-yl)-9H-carbazol-9-yl]phenyl}dibenzo[f,h]quinoxaline (abbreviation: 2PCCzPDBq), 2-{3-[3-(N-phenyl-9H-carbazol-3-yl)-9H-carbazol-9-yl]phenyl}dibenzo[f,h]quinoxaline (abbreviation: 2mPCCzPDBq), 2-{4-[2-(N-phenyl-9H-carbazol-3-yl)-9H-carbazol-9-yl]phenyl}dibenzo[f,h]quinoxaline (abbreviation: 2PCCzPDBq-02), and 2-{3-[2-(N-phenyl-9H-carbazol-3-yl)-9H-carbazol-9-yl]phenyl}dibenzo[f,h]quinoxaline (abbreviation: 2mPCCzPDBq-02) can be used. 
     Examples of the compound which is likely to accept holes include compounds having triarylamine skeletons such as 4-phenyl-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PCBA1BP), 3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole (abbreviation: PCzPCN1), 4,4′,4″-tris[N-(1-naphthyl)-N-phenylamino]triphenylamine (abbreviation: 1′-TNATA), 2,7-bis[N-(4-diphenylaminophenyl)-N-phenylamino]-spiro-9,9′-bifluorene (abbreviation: DPA2SF), N,N′-bis(9-phenylcarbazol-3-yl)-N,N′-diphenylbenzene-1,3-diamine (abbreviation: PCA2B), N-(9,9-dimethyl-2-N,N′-diphenylamino-9H-fluoren-7-yl)diphenylamine (abbreviation: DPNF), N,N′,N″-triphenyl-N,N′,N″-tris(9-phenylcarbazol-3-yl)benzene-1,3,5-tri amine (abbreviation: PCA3B), 2-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]spiro-9,9′-bifluorene (abbreviation: PCASF), 2-[N-(4-diphenylaminophenyl)-N-phenylamino]spiro-9,9′-bifluorene (abbreviation: DPASF), N,N′-bis[4-(carbazol-9-yl)phenyl]-N,N′-diphenyl-9,9-dimethylfluorene-2,7-diamine (abbreviation: YGA2F), 4,4′-bis[N-(3-methylphenyl)-N-phenylamino]biphenyl (abbreviation: TPD), 4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl (abbreviation: DPAB), N-(9,9-dimethyl-9H-fluoren-2-yl)-N-{9,9-dimethyl-2-[N′-phenyl-N-(9,9-dimethyl-9H-fluoren-2-yl)amino]-9H-fluoren-7-yl}phenylamine (abbreviation: DFLADFL), 3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole (abbreviation: PCzPCA1), 3-[N-(4-diphenylaminophenyl)-N-phenylamino]-9-phenylcarbazole (abbreviation: PCzDPA1), 3,6-bis[N-(4-diphenylaminophenyl)-N-phenylamino]-9-phenylcarbazole (abbreviation: PCzDPA2), 4,4′-bis(N-{4-[N-(3-methylphenyl)-N-phenylamino]phenyl}-N-phenylamino)biphenyl (abbreviation: DNTPD), 3,6-bis[N-(4-diphenylaminophenyl)-N-(1-naphthyl)amino]-9-phenylcarbazole (abbreviation: PCzTPN2), and 3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole (abbreviation: PCzPCA2). 
     The first organic compound and the second organic compound are not limited to the above examples as long as the combination of the first organic compound and the second organic compound can form an exciplex, the emission spectrum of the exciplex overlaps with the absorption spectrum of the light-emitting substance, and the peak of the emission spectrum of the exciplex has a longer wavelength than the peak of the absorption spectrum of the light-emitting substance. 
     Note that in the case where the compound which is likely to accept electrons and the compound which is likely to accept holes are used as the first organic compound and the second organic compound, the carrier balance can be controlled by the mixture ratio of the compounds. Specifically, the ratio of the first organic compound to the second organic compound is preferably 1:9 to 9:1. 
     As the materials that can be used as the light-emitting substance and the emission center substance in the light-emitting layer  213 , a light-emitting substance converting singlet excitation energy into light emission, a light-emitting substance converting triplet excitation energy into light emission, and the like can be used alone or in combination. Described below are examples of the light-emitting substance and the emission center substance. 
     As an example of the light-emitting substance converting singlet excitation energy into light emission, a substance which emits fluorescence (a fluorescent compound) can be given. 
     Examples of the substance which emits fluorescence include N,N′-bis[4-(9H-carbazol-9-yl)phenyl]-N,N′-diphenyl stilbene-4,4′-diamine (abbreviation: YGA2S), 4-(9H-carbazol-9-yl)-4′-(10-phenyl-9-anthryl)triphenylamine (abbreviation: YGAPA), 4-(9H-carbazol-9-yl)-4′-(9,10-diphenyl-2-anthryl)triphenylamine (abbreviation: 2YGAPPA), N,9-diphenyl-N-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine (abbreviation: PCAPA), perylene, 2,5,8,11-tetra-tert-butylperylene (abbreviation: TBP), 4-(10-phenyl-9-anthryl)-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PCBAPA), N,N″-(2-tert-butylanthracene-9,10-diyldi-4,1-phenylene)bis[N,N′,N′-triphenyl-1,4-pheny lenediamine] (abbreviation: DPABPA), N,9-diphenyl-N-[4-(9,10-diphenyl-2-anthryl)phenyl]-9H-carbazol-3-amine (abbreviation: 2PCAPPA), N-[4-(9,10-diphenyl-2-anthryl)phenyl]-N,N′,N′-triphenyl-1,4-phenylenediamine (abbreviation: 2DPAPPA), N,N,N′,N′,N″,N″,N′″,N′″-octaphenyldibenzo[g,p]chrysene-2,7,10,15-tetraamine (abbreviation: DBC1), coumarin 30, N-(9,10-diphenyl-2-anthryl)-N, 9-diphenyl-9H-carbazol-3-amine (abbreviation: 2PCAPA), N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,9-diphenyl-9H-carbazol-3-amine (abbreviation: 2PCABPhA), N-(9,10-diphenyl-2-anthryl)-N,N′,N′-triphenyl-1,4-phenylenediamine (abbreviation: 2DPAPA), N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-triphenyl-1,4-phenylenediamine (abbreviation: 2DPABPhA), 9,10-bis(1,1′-biphenyl-2-yl)-N-[4-(9H-carbazol-9-yl)phenyl]-N-phenylanthracen-2-amine (abbreviation: 2YGABPhA), N,N,9-triphenylanthracen-9-amine (abbreviation: DPhAPhA), coumarin 545T, N,N′-diphenylquinacridone (abbreviation: DPQd), rubrene, 5,12-bis(1,1′-biphenyl-4-yl)-6,11-diphenyltetracene (abbreviation: BPT), 2-(2-{2-[4-(dimethylamino)phenyl]ethenyl}-6-methyl-4H-pyran-4-ylidene)propanedinitrile (abbreviation: DCM1), 2-{2-methyl-6-[2-(2,3,6,7-tetrahydro-1H, 5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile (abbreviation: DCM2), N,N,N′,N′-tetrakis(4-methyl phenyl)tetracene-5,11-diamine (abbreviation: p-mPhTD), 7,14-diphenyl-N,N,N′,N′-tetrakis(4-methylphenyl)acenaphtho[1,2-a]fluoranthene-3,10-diamine (abbreviation: p-mPhAFD), {2-isopropyl-6-[2-(1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H, 5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile (abbreviation: DCJTI), {2-tert-butyl-6-[2-(1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H, 5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile (abbreviation: DCJTB), 2-(2,6-bis {2-[4-(dimethylamino)phenyl]ethenyl}-4H-pyran-4-ylidene)propanedinitrile (abbreviation: BisDCM), and 2-{2,6-bis[2-(8-methoxy-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile (abbreviation: BisDCJTM). 
     Examples of the light-emitting substance converting triplet excitation energy into light emission include a substance which emits phosphorescence (a phosphorescent compound) and a thermally activated delayed fluorescent (TADF) material which emits thermally activated delayed fluorescence. Note that “delayed fluorescence” exhibited by the TADF material refers to light emission having the same spectrum as normal fluorescence and an extremely long lifetime. The lifetime is 10 −6  seconds or longer, preferably 10 −3  seconds or longer. 
     Examples of the substance which emits phosphorescence include bis {2-[3′,5′-bis(trifluoromethyl)phenyl]pyridinato-N,C 2′ }iridium(III) picolinate (abbreviation: Ir(CF 3 ppy) 2 (pic)), bis[2-(4′,6′-difluorophenyl)pyridinato-N,C 2′ ]iridium(III) acetylacetonate (abbreviation: FIracac), tris(2-phenylpyridinato)iridium(III) (abbreviation: Ir(ppy) 3 ), bis(2-phenylpyridinato)iridium(III) acetylacetonate (abbreviation: Ir(ppy) 2 (acac)), tris(acetylacetonato) (monophenanthroline)terbium(III) (abbreviation: Tb(acac) 3 (Phen)), bis(benzo[h]quinolinato)iridium(III) acetylacetonate (abbreviation: Ir(bzq) 2 (acac)), bis(2,4-diphenyl-1,3-oxazolato-N,C 2′ )iridium(III) acetylacetonate (abbreviation: Ir(dpo) 2 (acac)), bis {2-[4′-(perfluorophenyl)phenyl]pyridinato-N,C 2′ }iridium(III) acetylacetonate (abbreviation: Ir(p-PF-ph) 2 (acac)), bis(2-phenylbenzothiazolato-N,C 2′ )iridium(III) acetylacetonate (abbreviation: Ir(bt) 2 (acac)), bis[2-(2′-benzo[4,5-a]thienyl)pyridinato-N,C 3′ ]iridium(III) acetylacetonate (abbreviation: Ir(btp) 2 (acac)), bis(1-phenylisoquinolinato-N,C 2′ )iridium(III) acetylacetonate (abbreviation: Ir(piq) 2 (acac)), (acetylacetonato)bis[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(III) (abbreviation: Ir(Fdpq) 2 (acac)), (acetylacetonato)bis(3,5-dimethyl-2-phenylpyrazinato)iridium(III) (abbreviation: [Ir(mppr-Me) 2 (acac)]), (acetylacetonato)bis(5-isopropyl-3-methyl-2-phenylpyrazinato)iridium(III) (abbreviation: [Ir(mppr-iPr) 2 (acac)]), (acetylacetonato)bis(2,3,5-triphenylpyrazinato)iridium(III) (abbreviation: Ir(tppr) 2 (acac)), bis(2,3,5-triphenylpyrazinato) (dipivaloylmethanato)iridium(III) (abbreviation: [Ir(tppr) 2 (dpm)], (acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III) (abbreviation: [Ir(tBuppm) 2 (acac)]), (acetylacetonato)bis(4,6-diphenylpyrimidinato)iridium(III) (abbreviation: [Ir(dppm) 2 (acac)]), 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin platinum(II) (abbreviation: PtOEP), tris(1,3-diphenyl-1,3-propanedionato) (monophenanthroline)europium(III) (abbreviation: Eu(DBM) 3 (Phen)), and tris[1-(2-thenoyl)-3,3,3-trifluoroacetonato] (monophenanthroline)europium(III) (abbreviation: Eu(TTA) 3 (Phen)). 
     Examples of the TADF material include fullerene, a derivative thereof, an acridine derivative such as proflavine, and eosin. Other examples include a metal-containing porphyrin, such as a porphyrin containing magnesium (Mg), zinc (Zn), cadmium (Cd), tin (Sn), platinum (Pt), indium (In), or palladium (Pd). Examples of the metal-containing porphyrin include a protoporphyrin-tin fluoride complex (SnF 2 (Proto IX)), a mesoporphyrin-tin fluoride complex (SnF 2 (Meso IX)), a hematoporphyrin-tin fluoride complex (SnF 2 (Hemato IX)), a coproporphyrin tetramethyl ester-tin fluoride complex (SnF 2 (Copro III-4Me)), an octaethylporphyrin-tin fluoride complex (SnF 2 (OEP)), an etioporphyrin-tin fluoride complex (SnF 2 (Etio I)), and an octaethylporphyrin-platinum chloride complex (PtCl 2 OEP). Alternatively, a heterocyclic compound including a π-electron rich heteroaromatic ring and a π-electron deficient heteroaromatic ring can be used, such as 2-(biphenyl-4-yl)-4,6-bis(12-phenylindolo[2,3-a]carbazol-11-yl)-1,3,5-triazine (PIC-TRZ). Note that a substance in which the π-electron rich heteroaromatic ring is directly bonded to the π-electron deficient heteroaromatic ring is particularly preferably used because both a donor property of the π-electron rich heteroaromatic ring and an acceptor property of the π-electron deficient heteroaromatic ring are increased and the energy difference between the Si level and the Ti level becomes small. 
     The light-emitting layer  213  may have a stacked structure as illustrated in  FIG.  3 B . In that case, each layer in the stacked structure emits light. For example, fluorescence is obtained from a first light-emitting layer  213  (a1), and phosphorescence is obtained from a second light-emitting layer  213  (a2) stacked over the first layer. Note that the stacking order may be reversed. It is preferable that light emission due to energy transfer from an exciplex to a dopant be obtained from the layer that emits phosphorescence. In the case where blue light emission is obtained from one of the first and second layers, orange or yellow light emission can be obtained from the other layer. Each layer may also contain plural kinds of dopants. 
     An electron-transport layer  214  is a layer containing a substance having a high electron-transport property (also referred to as an electron-transport compound). For the electron-transport layer  214 , a metal complex such as tris(8-quinolinolato)aluminum(III) (abbreviation: Alq 3 ), tris(4-methyl-8-quinolinolato)aluminum(III) (abbreviation: Almq 3 ), bis(10-hydroxybenzo[h]quinolinato)beryllium(II) (abbreviation: BeBq 2 ), bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum(III) (abbreviation: BAlq), bis[2-2(hydroxyphenyl)benzoxazolato]zinc(II) (abbreviation: Zn(BOX) 2 ), or bis[2-(2-hydroxyphenyl)benzothiazolato]zinc(II) (abbreviation: Zn(BTZ) 2 ) can be used. Alternatively, it is possible to use a heteroaromatic compound such as 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation: PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene (abbreviation: OXD-7), 3-(4′-tert-butylphenyl)-4-phenyl-5-(4″-biphenyl)-1,2,4-triazole (abbreviation: TAZ), 3-(4-tert-butylphenyl)-4-(4-ethylphenyl)-5-(4-biphenylyl)-1,2,4-triazole (abbreviation: p-EtTAZ), bathophenanthroline (abbreviation: BPhen), bathocuproine (abbreviation: BCP), or 4,4′-bis(5-methylbenzoxazol-2-yl)stilbene (abbreviation: BzOs). Further alternatively, it is possible to use a high molecular compound such as poly(2,5-pyridinediyl) (abbreviation: PPy), poly[(9,9-dihexylfluorene-2,7-diyl)-co-(pyridine-3,5-diyl)] (abbreviation: PF-Py), or poly[(9,9-dioctylfluorene-2,7-diyl)-co-(2,2′-bipyridine-6,6′-diyl)] (abbreviation: PF-BPy). The substances mentioned here are mainly substances each having an electron mobility of higher than or equal to 1×10 −6  cm 2 /Vs. Note that other substances may also be used for the electron-transport layer  214  as long as the substances have higher electron-transport properties than hole-transport properties. 
     The electron-transport layer  214  is not limited to a single layer and may have a stacked structure including two or more layers containing any of the above substances. 
     An electron-injection layer  215  is a layer containing a substance having a high electron-injection property. The electron-injection layer  215  can be formed using an alkali metal, an alkaline earth metal, or a compound thereof, such as lithium fluoride (LiF), cesium fluoride (CsF), calcium fluoride (CaF 2 ), or lithium oxide (LiO x ). Alternatively, a rare earth metal compound such as erbium fluoride (ErF 3 ) can be used. Alternatively, an electrode may be used for the electron-injection layer  215 . Examples of the electrode include a substance in which electrons are added at high concentration to calcium oxide-aluminum oxide. Any of the substances for forming the electron-transport layer  214 , which are given above, can also be used. 
     The electron-injection layer  215  may also be formed using a composite material in which an organic compound and an electron donor are mixed. The composite material is superior in an electron-injection property and an electron-transport property, because electrons are generated in the organic compound by the electron donor. In that case, as the organic compound, a material that can efficiently transport the produced electrons is preferably used; for example, any of the above-described substances that are used to form the electron-transport layer  214  (e.g., a metal complex or a heteroaromatic compound) can be used. As the electron donor, a substance showing an electron-donating property with respect to the organic compound may be used. Specifically, an alkali metal, an alkaline earth metal, and a rare earth metal are preferable, and lithium, cesium, magnesium, calcium, erbium, ytterbium, and the like can be given. Furthermore, an alkali metal oxide or an alkaline earth metal oxide is preferable, and for example, lithium oxide, calcium oxide, barium oxide, and the like can be given. Alternatively, Lewis base such as magnesium oxide can be used. An organic compound such as tetrathiafulvalene (abbreviation: TTF) can also be used. 
     Note that the hole-injection layer  211 , the hole-transport layer  212 , the light-emitting layer  213 , the electron-transport layer  214 , and the electron-injection layer  215 , which are mentioned above, can each be formed by a method, such as an evaporation method (including a vacuum evaporation method), an inkjet method, or a coating method. 
     In the above-described light-emitting element, holes and electrons are recombined in the EL layer  202 , whereby light is emitted. This emitted light is extracted out through the first electrode  201  and/or the second electrode  203 . Therefore, the first electrode  201  and/or the second electrode  203  is an electrode having a light-transmitting property. 
     The light-emitting element described in this embodiment enables energy transfer utilizing an overlap between the emission spectrum of the exciplex and the absorption spectrum of a phosphorescent compound (guest material), leading to a high energy transfer efficiency; thus, a light-emitting element with a high external quantum efficiency can be achieved. 
     Note that the structure described in this embodiment can be combined as appropriate with any of the structures described in the other embodiments. 
     Embodiment 2 
     In this embodiment, a dibenzo[f,h]quinoxaline derivative which can be used in a light-emitting element and is one embodiment of the present invention will be described. 
     A dibenzo[f,h]quinoxaline derivative which is one embodiment of the present invention is represented by the following general formula (G0). 
     
       
         
         
             
             
         
       
     
     In the formula, A represents a dibenzo[f,h]quinoxalinyl group; R 1  to R 15  represent independently hydrogen, a substituted or unsubstituted alkyl group having 1 to 6 carbon atoms, a substituted or unsubstituted cycloalkyl group having 5 to 7 carbon atoms, or a substituted or unsubstituted aryl group having 6 to 13 carbon atoms; and Ar represents a substituted or unsubstituted arylene group having 6 to 25 carbon atoms or a single bond. Preferably, an arylene group in Ar does not include an anthracenylene group. 
     The dibenzo[f,h]quinoxaline derivative represented by the general formula (G0) can be synthesized by the following synthesis method. As shown in the following synthesis scheme (a), the dibenzo[f,h]quinoxaline derivative represented by the general formula (G0) can be obtained by reacting a halogen compound (A1) of a dibenzo[f,h]quinoxaline derivative with an arylboronic acid compound (A2) of a bicarbazole derivative. 
     
       
         
         
             
             
         
       
     
     In the formula, A represents a dibenzo[f,h]quinoxalinyl group; R 1  to R 15  represent independently hydrogen, a substituted or unsubstituted alkyl group having 1 to 6 carbon atoms, a substituted or unsubstituted cycloalkyl group having 5 to 7 carbon atoms, or a substituted or unsubstituted aryl group having 6 to 13 carbon atoms; and Ar represents a substituted or unsubstituted arylene group having 6 to 25 carbon atoms or a single bond. Preferably, an arylene group in Ar does not include an anthracenylene group. Furthermore, X represents a halogen. When Ar is a substituted or unsubstituted arylene group having 6 to 25 carbon atoms, B represents a boronic acid, a boronic ester, a cyclic-triolborate salt, or the like. As the cyclic-triolborate salt, a lithium salt, a potassium salt, or a sodium salt may be used. When Ar is a single bond, B represents hydrogen. 
     In addition, as shown in the following synthesis scheme (b), the dibenzo[f,h]quinoxaline derivative represented by the general formula (G0) can be obtained in such a manner that an intermediate (B2) is obtained through a reaction of a halogen compound (A1) of a dibenzo[f,h]quinoxaline derivative with a halogen-substituted arylboronic acid (B1) and then made to react with a bicarbazole derivative (B3). 
     
       
         
         
             
             
         
       
     
     In the formula, A represents a dibenzo[f,h]quinoxalinyl group; R 1  to R 15  represent independently hydrogen, a substituted or unsubstituted alkyl group having 1 to 6 carbon atoms, a substituted or unsubstituted cycloalkyl group having 5 to 7 carbon atoms, or a substituted or unsubstituted aryl group having 6 to 13 carbon atoms; and Ar represents a substituted or unsubstituted arylene group having 6 to 25 carbon atoms or a single bond. Preferably, an arylene group in Ar does not include an anthracenylene group. Furthermore, X represents a halogen. B represents a boronic acid, a boronic ester, a cyclic-triolborate salt, or the like. 
     When Ar in the general formula (G0) is a single bond, a halogen compound (A1) of a dibenzo[f,h]quinoxaline derivative is directly reacted with a bicarbazole derivative (B3). 
     A dibenzo[f,h]quinoxaline derivative which is one embodiment of the present invention and can be synthesized by any of the above synthesis methods is preferably a compound represented by any of the above general formulae (G1) to (G3). Specific structural formulae of dibenzo[f,h]quinoxaline derivatives which are embodiments of the present invention and represented by the general formulae (G0) to (G3) are shown below (the following structural formulae (100) to (131)). Note that the present invention is not limited thereto. 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     A dibenzo[f,h]quinoxaline derivative which is one embodiment of the present invention is used in a light-emitting element of one embodiment of the present invention, whereby a light-emitting element, a light-emitting device, an electronic device, or a lighting device with a high emission efficiency and a high reliability can be obtained. It is also possible to achieve a light-emitting element, a light-emitting device, an electronic device, or a lighting device with low power consumption. 
     The dibenzo[f,h]quinoxaline derivative represented by any of the general formulae (G0) to (G3) has an electron-transport property and a hole-transport property and thus can be used as a host material of a light-emitting layer or used for an electron-transport layer or a hole-transport layer. Furthermore, the dibenzo[f,h]quinoxaline derivative represented by any of the general formulae (G0) to (G3) emits fluorescence and thus can be used as a light-emitting substance of a light-emitting element. As described above, the dibenzo[f,h]quinoxaline derivative represented by any of the general formulae (G0) to (G3) is a useful novel compound which can be used as various materials in a light-emitting element; thus, a light-emitting element containing the dibenzo[f,h]quinoxaline derivative represented by any of the general formulae (G0) to (G3) is a light-emitting element which is one embodiment of the present invention. 
     Note that the structure described in this embodiment can be combined as appropriate with any of the structures described in the other embodiments. 
     Embodiment 3 
     In this embodiment, as a light-emitting element which is one embodiment of the present invention, a light-emitting element (hereinafter referred to as a tandem light-emitting element) that includes a plurality of EL layers with a charge-generation layer provided therebetween will be described. As illustrated in  FIG.  4 A , the tandem light-emitting element includes a plurality of EL layers (a first EL layer  402 ( 1 ) and a second EL layer  402 ( 2 )) between a pair of electrodes (a first electrode  401  and a second electrode  404 ). 
     In this embodiment, the first electrode  401  functions as an anode, and the second electrode  404  functions as a cathode. The first electrode  401  and the second electrode  404  can have structures similar to those described in Embodiment 1. All or any of the plurality of EL layers (the first EL layer  402 ( 1 ) and the second EL layer  402 ( 2 )) may have structures similar to those described in Embodiment 1. In other words, the structures of the first EL layer  402 ( 1 ) and the second EL layer  402 ( 2 ) may be the same or different from each other and can be similar to those described in Embodiment 1. The dibenzo[f,h]quinoxaline derivative described in Embodiment 2 can be used for any of the plurality of EL layers (the first EL layer  402 ( 1 ) and the second EL layer  402 ( 2 )). 
     A charge-generation layer  405  is provided between the plurality of EL layers (the first EL layer  402 ( 1 ) and the second EL layer  402 ( 2 )). The charge-generation layer  405  has a function of injecting electrons into one of the EL layers and injecting holes into the other of the EL layers when a voltage is applied between the first electrode  401  and the second electrode  404 . In this embodiment, when a voltage is applied such that the potential of the first electrode  401  is higher than that of the second electrode  404 , the charge-generation layer  405  injects electrons into the first EL layer  402 ( 1 ) and injects holes into the second EL layer  402 ( 2 ). 
     In terms of light extraction efficiency, the charge-generation layer  405  preferably has a light-transmitting property with respect to visible light (specifically, the charge-generation layer  405  has a visible light transmittance of higher than or equal to 40%). Furthermore, the charge-generation layer  405  functions even if it has lower conductivity than the first electrode  401  or the second electrode  404 . 
     The charge-generation layer  405  may have either a structure in which an electron acceptor is added to an organic compound having a high hole-transport property or a structure in which an electron donor is added to an organic compound having a high electron-transport property. Alternatively, both of these structures may be stacked. 
     In the case of the structure in which an electron acceptor is added to an organic compound having a high hole-transport property, as the organic compound having a high hole-transport property, for example, an aromatic amine compound such as NPB, TPD, TDATA, MTDATA, or 4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl (abbreviation: BSPB), or the like can be used. The substances described here are mainly substances each having a hole mobility of higher than or equal to 1×10 −6  cm 2 /Vs. Note that other substances may also be used as long as the substances have higher hole-transport properties than electron-transport properties. 
     As the electron acceptor, 7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation: F 4 -TCNQ), chloranil, and the like can be given. In addition, transition metal oxides can be given. Moreover, oxides of metals belonging to Groups 4 to 8 of the periodic table can be given. Specifically, it is preferable to use vanadium oxide, niobium oxide, tantalum oxide, chromium oxide, molybdenum oxide, tungsten oxide, manganese oxide, and rhenium oxide because of their high electron accepting properties. In particular, molybdenum oxide is preferable because of its stability in the air, a low hygroscopic property, and easiness of handling. 
     On the other hand, in the case of the structure in which an electron donor is added to an organic compound having a high electron-transport property, as the organic compound having a high electron-transport property, for example, a metal complex having a quinoline skeleton or a benzoquinoline skeleton, such as Alq 3 , Almq 3 , BeBq 2 , or BAlq, or the like can be used. Alternatively, a metal complex having an oxazole-based ligand or a thiazole-based ligand, such as Zn(BOX) 2  or Zn(BTZ) 2  can be used. Alternatively, in addition to such a metal complex, PBD, OXD-7, TAZ, BPhen, BCP, or the like can be used. The substances described here are mainly substances each having an electron mobility of higher than or equal to 1×10 −6  cm 2 /Vs. Note that other substances may also be used as long as the substances have higher electron-transport properties than hole-transport properties. 
     As the electron donor, it is possible to use an alkali metal, an alkaline earth metal, a rare earth metal, a metal belonging to Group 2 or 13 of the periodic table, or an oxide or carbonate thereof. Specifically, lithium (Li), cesium (Cs), magnesium (Mg), calcium (Ca), ytterbium (Yb), indium (In), lithium oxide, cesium carbonate, or the like is preferably used. Alternatively, an organic compound such as tetrathianaphthacene may be used as the electron donor. 
     Note that forming the charge-generation layer  405  by using any of the above materials can suppress an increase in drive voltage caused by the stack of the EL layers. 
     Although the light-emitting element including two EL layers is described in this embodiment, the present invention can be similarly applied to a light-emitting element in which n EL layers ( 402 ( 1 ) to  402 ( n )) (n is three or more) are stacked as illustrated in  FIG.  4 B . In the case where a plurality of EL layers are included between a pair of electrodes as in the light-emitting element of this embodiment, by providing charge-generation layers ( 405 ( 1 ) to  405 ( n − 1 )) between the EL layers, light emission in a high luminance region can be obtained with current density kept low. Since the current density can be kept low, the element can have a long lifetime. In addition, when the light-emitting element is applied to a light-emitting device, an electronic device, a lighting device, and the like each having a large light-emitting area, voltage drop due to resistance of an electrode material can be reduced, thereby achieving homogeneous light emission in a large area. 
     When the EL layers have different emission colors, a desired emission color can be obtained from the whole light-emitting element. For example, in the light-emitting element including two EL layers, when an emission color of a first EL layer and an emission color of a second EL layer are made to be complementary colors, a light-emitting element emitting white light as a whole light-emitting element can be obtained. Note that “complementary color” means a relation between colors which becomes an achromatic color when they are mixed. In other words, when lights which are complementary to each other are mixed, white light emission can be obtained. Specifically, a combination in which blue light emission is obtained from the first EL layer and yellow (or orange) light emission is obtained from the second EL layer is given as an example. In that case, it is not necessary that blue light emission and yellow (or orange) light emission are both fluorescence or both phosphorescence. For example, a combination in which blue light emission is fluorescence and yellow (or orange) light emission is phosphorescence or a combination in which blue light emission is phosphorescence and yellow (or orange) light emission is fluorescence may be employed. Moreover, a stacked structure suitable for adjustment of an optical path length of the light-emitting element (e.g., a structure in which a first light-emitting layer exhibits yellow light emission and a second light-emitting layer exhibits blue light emission) is preferably employed, in which case the element characteristics can be further improved. 
     Also in a light-emitting element including three EL layers, for example, white light emission can be similarly obtained from a whole light-emitting element when an emission color of a first EL layer is red, an emission color of a second EL layer is green, and an emission color of a third EL layer is blue. 
     Note that the structure described in this embodiment can be combined as appropriate with any of the structures described in the other embodiments. 
     Embodiment 4 
     In this embodiment, one embodiment of a light-emitting device in which the light-emitting element described in Embodiment 1 is combined with a coloring layer (a color filter or the like) will be described. In this embodiment, the structure of a pixel portion of the light-emitting device will be described with reference to  FIG.  5   . 
     In  FIG.  5   , a plurality of FETs (transistors)  502  are formed over a substrate  501 . Each of the FETs  502  is electrically connected to a light-emitting element ( 507 R,  507 G,  507 B, or  507 Y). Specifically, each of the FETs  502  is electrically connected to a first electrode  503  that is a pixel electrode of the light-emitting element. A partition wall  504  is provided to cover end portions of the adjacent first electrodes  503 . 
     The first electrode  503  in this embodiment has a function of a reflective electrode. An EL layer  505  is formed over the first electrode  503 , and a second electrode  510  is formed over the EL layer  505 . The EL layer  505  includes a plurality of light-emitting layers each emitting monochromatic light. The second electrode  510  has a function of a semi-transmissive and semi-reflective electrode. 
     The light-emitting elements ( 507 R,  507 G,  507 B, and  507 Y) emit light of different colors. Specifically, the light-emitting element  507 R is optically adjusted to emit red light, and in a region indicated by  506 R, red light is emitted through a coloring layer  508 R in the direction indicated by an arrow. The light-emitting element  507 G is optically adjusted to emit green light, and in a region indicated by  506 G, green light is emitted through a coloring layer  508 G in the direction indicated by an arrow. The light-emitting element  507 B is optically adjusted to emit blue light, and in a region indicated by  506 B, blue light is emitted through a coloring layer  508 B in the direction indicated by an arrow. The light-emitting element  507 Y is optically adjusted to emit yellow light, and in a region indicated by  506 Y, yellow light is emitted through a coloring layer  508 Y in the direction indicated by an arrow. 
     As illustrated in  FIG.  5   , each of the coloring layers ( 508 R,  508 G,  508 B, and  508 Y) is provided on a transparent sealing substrate  511  that is provided above the substrate  501  over which the light emitting elements ( 507 R,  507 G,  507 B, and  507 Y) are formed. Note that the coloring layers ( 508 R,  508 G,  508 B, and  508 Y) are provided in positions overlapping with the corresponding light-emitting elements ( 507 R,  507 G,  507 B, and  507 Y) which exhibit different emission colors. 
     A black layer (black matrix)  509  is provided to overlap with end portions of the adjacent coloring layers ( 508 R,  508 G,  508 B, and  508 Y). Note that the coloring layers ( 508 R,  508 G,  508 B, and  508 Y) and the black layer  509  may be covered with an overcoat layer that is formed using a transparent material. 
     The above-described light-emitting device has a structure in which light is extracted from the sealing substrate  511  side (a top emission structure), but may have a structure in which light is extracted from the substrate  501  side where the FETs are formed (a bottom emission structure). Note that in the light-emitting device having a top emission structure described in this embodiment, a light-blocking substrate or a light-transmitting substrate can be used as the substrate  501 , whereas in a light-emitting device having a bottom emission structure, a light-transmitting substrate needs to be used as the substrate  501 . 
     In this specification and the like, a transistor or a light-emitting element can be formed using any of a variety of substrates, for example. The type of a substrate is not limited to a certain type. As the substrate, a semiconductor substrate (e.g., a single crystal substrate or a silicon substrate), an SOI substrate, a glass substrate, a quartz substrate, a plastic substrate, a metal substrate, a stainless steel substrate, a substrate including stainless steel foil, a tungsten substrate, a substrate including tungsten foil, a flexible substrate, an attachment film, paper including a fibrous material, a base material film, or the like can be used, for example. Examples of a glass substrate include a barium borosilicate glass substrate, an aluminoborosilicate glass substrate, and a soda lime glass substrate. Examples of the flexible substrate, the attachment film, the base material film, and the like include plastics typified by polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polyether sulfone (PES), and polytetrafluoroethylene (PTFE), a synthetic resin such as acrylic, films formed of polypropylene, polyester, polyvinyl fluoride, and polyvinyl chloride, films formed of polyamide, polyimide, aramid, and epoxy, an inorganic film formed by evaporation, and paper. Specifically, when a transistor is fabricated using a semiconductor substrate, a single crystal substrate, an SOI substrate, or the like, a transistor with few variations in characteristics, size, shape, or the like, high current supply capability, and a small size can be fabricated. A circuit using such a transistor achieves lower power consumption or higher integration. 
     Alternatively, a flexible substrate may be used as the substrate, and the transistor or the light-emitting element may be provided directly over the flexible substrate. Further alternatively, a separation layer may be provided between a substrate and the transistor or the like. The separation layer can be used when part of or the entire semiconductor device formed over the separation layer is separated from the substrate and transferred onto another substrate. In such a case, the transistor or the like can be transferred to a substrate having low heat resistance or a flexible substrate as well. For the above separation layer, a stack including inorganic films, a tungsten film and a silicon oxide film, or an organic resin film of polyimide or the like formed over a substrate can be used, for example. 
     In other words, after the transistor or the light-emitting element is formed using one substrate, the transistor or the light-emitting element may be transferred to another substrate. Examples of a substrate to which the transistor or the light-emitting element is transferred include, in addition to the above-described substrates over which a transistor or the like can be formed, a paper substrate, a cellophane substrate, an aramid film substrate, a polyimide film substrate, a stone substrate, a wood substrate, a cloth substrate (including a natural fiber (e.g., silk, cotton, or hemp), a synthetic fiber (e.g., nylon, polyurethane, or polyester), a regenerated fiber (e.g., acetate, cupra, rayon, or regenerated polyester), or the like), a leather substrate, and a rubber substrate. When such a substrate is used, a transistor or the like with excellent properties or low power consumption can be formed, a device with high durability and high heat resistance can be provided, or a reduction in weight or thickness can be achieved. 
     Note that the structure described in this embodiment can be combined as appropriate with any of the structures described in the other embodiments. 
     Embodiment 5 
     In this embodiment, a light-emitting device including a light-emitting element in which a dibenzo[f,h]quinoxaline derivative which is one embodiment of the present invention is used for an EL layer will be described. 
     The light-emitting device may be either a passive matrix light-emitting device or an active matrix light-emitting device. Note that any of the light-emitting elements described in the other embodiments can be applied to a light-emitting device described in this embodiment. 
     In this embodiment, an active matrix light-emitting device will be described with reference to  FIGS.  6 A and  6 B . 
       FIG.  6 A  is a top view illustrating the light-emitting device, and  FIG.  6 B  is a cross-sectional view taken along the dashed-dotted line A-A′ in  FIG.  6 A . The active matrix light-emitting device of this embodiment includes a pixel portion  602  provided over an element substrate  601 , a driver circuit portion (a source line driver circuit)  603 , and driver circuit portions (gate line driver circuits)  604   a  and  604   b . The pixel portion  602 , the driver circuit portion  603 , and the driver circuit portions  604   a  and  604   b  are sealed with a sealant  605  between the element substrate  601  and a sealing substrate  606 . 
     In addition, over the element substrate  601 , a lead wiring  607  for connecting an external input terminal, through which a signal (e.g., a video signal, a clock signal, a start signal, a reset signal, or the like) or potential is transmitted from the outside to the driver circuit portion  603  and the driver circuit portions  604   a  and  604   b , is provided. Here, an example is described in which a flexible printed circuit (FPC)  608  is provided as the external input terminal. Although only the FPC is illustrated here, the FPC may be provided with a printed wiring board (PWB). The light-emitting device in this specification includes, in its category, not only the light-emitting device itself but also the light-emitting device provided with an FPC or a PWB. 
     Next, a cross-sectional structure will be described with reference to  FIG.  6 B . The driver circuit portions and the pixel portion are formed over the element substrate  601 ; here are illustrated the driver circuit portion  603  which is the source line driver circuit and the pixel portion  602 . 
     In the driver circuit portion  603 , an FET  609  and an FET  610  are combined as an example. Note that the driver circuit portion  603  may be formed with a circuit including transistors having the same conductivity type (either n-channel transistors or p-channel transistors) or a CMOS circuit including an n-channel transistor and a p-channel transistor. In this embodiment, a driver-integrated type in which a driver circuit is formed over a substrate is shown; however, the driver circuit can also be formed outside the substrate. 
     The pixel portion  602  includes a plurality of pixels each including a switching FET  611 , a current control FET  612 , and a first electrode (anode)  613  electrically connected to a wiring (a source electrode or a drain electrode) of the current control FET  612 . In this embodiment, the pixel portion  602  includes two FETs, the switching FET  611  and the current control FET  612 , but one embodiment of the present invention is not limited thereto. The pixel portion  602  may include, for example, three or more FETs and a capacitor in combination. 
     As the FETs  609 ,  610 ,  611 , and  612 , for example, a staggered transistor or an inverted staggered transistor can be used. Examples of a semiconductor material that can be used for the FETs  609 ,  610 ,  611 , and  612  include Group 13 semiconductors (e.g., gallium), Group 14 semiconductors (e.g., silicon), compound semiconductors, oxide semiconductors, and organic semiconductors. In addition, there is no particular limitation on the crystallinity of the semiconductor material, and an amorphous semiconductor film or a crystalline semiconductor film can be used, for example. An oxide semiconductor is preferably used for the FETs  609 ,  610 ,  611 , and  612 . Examples of the oxide semiconductor include an In—Ga oxide and an In—M—Zn oxide (M is Al, Ga, Y, Zr, La, Ce, or Nd). For example, an oxide semiconductor material that has an energy gap of 2 eV or more, preferably 2.5 eV or more, more preferably 3 eV or more is used for the FETs  609 ,  610 ,  611 , and  612 , so that the off-state current of the transistors can be reduced. 
     An insulator  614  is formed to cover an end portion of the first electrode  613 . In this embodiment, the insulator  614  is formed using a positive photosensitive acrylic resin. The first electrode  613  is used as an anode in this embodiment. 
     The insulator  614  preferably has a curved surface with curvature at its upper end portion or lower end portion. This enables the coverage with a film to be formed over the insulator  614  to be favorable. The insulator  614  can be formed using, for example, either a negative photosensitive resin or a positive photosensitive resin. The material of the insulator  614  is not limited to an organic compound, and an inorganic compound such as silicon oxide, silicon oxynitride, or silicon nitride can also be used. 
     An EL layer  615  and a second electrode (cathode)  616  are stacked over the first electrode (anode)  613 . At least a light-emitting layer is provided in the EL layer  615 . Furthermore, in the EL layer  615 , a hole-injection layer, a hole-transport layer, an electron-transport layer, an electron-injection layer, a charge-generation layer, and the like can be provided as appropriate in addition to the light-emitting layer. 
     A light-emitting element  617  is formed of a stacked structure of the first electrode (anode)  613 , the EL layer  615 , and the second electrode (cathode)  616 . For the first electrode (anode)  613 , the EL layer  615 , and the second electrode (cathode)  616 , the materials described in Embodiment 2 can be used. Although not illustrated here, the second electrode (cathode)  616  is electrically connected to the FPC  608  which is an external input terminal. 
     Although the cross-sectional view in  FIG.  6 B  illustrates only one light-emitting element  617 , a plurality of light-emitting elements are arranged in a matrix in the pixel portion  602 . Light-emitting elements which emit light of three kinds of colors (R, G, and B) are selectively formed in the pixel portion  602 , whereby a light-emitting device capable of full color display can be formed. In addition to the light-emitting elements that emit light of three kinds of colors (R, G, and B), for example, light-emitting elements that emit light of white (W), yellow (Y), magenta (M), cyan (C), and the like may be formed. For example, the light-emitting elements that emit light of a plurality of kinds of colors are used in combination with the light-emitting elements that emit light of three kinds of colors (R, G, and B), whereby effects such as an improvement in color purity and a reduction in power consumption can be obtained. Alternatively, a light-emitting device which is capable of full color display may be fabricated by a combination with color filters. Furthermore, the light-emitting device may have an improved emission efficiency and a reduced power consumption by combination with quantum dots. 
     The sealing substrate  606  is attached to the element substrate  601  with the sealant  605 , so that the light-emitting element  617  is provided in a space  618  surrounded by the element substrate  601 , the sealing substrate  606 , and the sealant  605 . The space  618  may be filled with an inert gas (such as nitrogen or argon) or the sealant  605 . 
     An epoxy-based resin or glass frit is preferably used for the sealant  605 . It is preferable that such a material do not transmit moisture or oxygen as much as possible. As the element substrate  601  and the sealing substrate  606 , a glass substrate, a quartz substrate, or a plastic substrate formed of fiber reinforced plastics (FRP), poly(vinyl fluoride) (PVF), polyester, acrylic, or the like can be used. In the case where glass frit is used as the sealant, the element substrate  601  and the sealing substrate  606  are preferably glass substrates for high adhesion. 
     In the above manner, an active matrix light-emitting device can be obtained. 
     Note that the structure described in this embodiment can be combined as appropriate with any of the structures described in the other embodiments. 
     Embodiment 6 
     In this embodiment, various examples of an electronic device manufactured using a light-emitting device which is one embodiment of the present invention will be described with reference to  FIGS.  7 A to  7 D . 
     Examples of an electronic device to which a light-emitting device is applied include television devices (also referred to as TV or television receivers), monitors for computers and the like, cameras such as digital cameras and digital video cameras, digital photo frames, mobile phones (also referred to as cellular phones or portable telephone devices), portable game machines, portable information terminals, audio playback devices, and large game machines such as pin-ball machines. Specific examples of these electronic devices are illustrated in  FIGS.  7 A to  7 D . 
       FIG.  7 A  illustrates an example of a television device. In a television device  7100 , a display portion  7103  is incorporated in a housing  7101 . The display portion  7103  can display images and may be a touch panel (an input/output device) including a touch sensor (an input device). Note that the light-emitting device which is one embodiment of the present invention can be used for the display portion  7103 . In addition, here, the housing  7101  is supported by a stand  7105 . 
     The television device  7100  can be operated by an operation switch of the housing  7101  or a separate remote controller  7110 . With operation keys  7109  of the remote controller  7110 , channels and volume can be controlled and images displayed on the display portion  7103  can be controlled. Furthermore, the remote controller  7110  may be provided with a display portion  7107  for displaying information output from the remote controller  7110 . 
     Note that the television device  7100  is provided with a receiver, a modem, and the like. With the use of the receiver, general television broadcasting can be received. Moreover, when the television device is connected to a communication network with or without wires via the modem, one-way (from a sender to a receiver) or two-way (between a sender and a receiver or between receivers) information communication can be performed. 
       FIG.  7 B  illustrates a computer, which includes a main body  7201 , a housing  7202 , a display portion  7203 , a keyboard  7204 , an external connecting port  7205 , a pointing device  7206 , and the like. Note that this computer can be manufactured using the light-emitting device which is one embodiment of the present invention for the display portion  7203 . The display portion  7203  may be a touch panel (an input/output device) including a touch sensor (an input device). 
       FIG.  7 C  illustrates a smart watch, which includes a housing  7302 , a display panel  7304 , operation buttons  7311  and  7312 , a connection terminal  7313 , a band  7321 , a clasp  7322 , and the like. 
     The display panel  7304  mounted in the housing  7302  serving as a bezel includes a non-rectangular display region. The display panel  7304  can display an icon  7305  indicating time, another icon  7306 , and the like. The display panel  7304  may be a touch panel (an input/output device) including a touch sensor (an input device). 
     The smart watch illustrated in  FIG.  7 C  can have a variety of functions, for example, a function of displaying a variety of information (e.g., a still image, a moving image, and a text image) on a display portion, a touch panel function, a function of displaying a calendar, date, time, and the like, a function of controlling processing with a variety of software (programs), a wireless communication function, a function of being connected to a variety of computer networks with a wireless communication function, a function of transmitting and receiving a variety of data with a wireless communication function, and a function of reading a program or data stored in a recording medium and displaying the program or data on a display portion. 
     The housing  7302  can include a speaker, a sensor (a sensor having a function of measuring force, displacement, position, speed, acceleration, angular velocity, rotational frequency, distance, light, liquid, magnetism, temperature, chemical substance, sound, time, hardness, electric field, current, voltage, electric power, radiation, flow rate, humidity, gradient, oscillation, odor, or infrared rays), a microphone, and the like. Note that the smart watch can be manufactured using the light-emitting device for the display panel  7304 . 
       FIG.  7 D  illustrates an example of a mobile phone (e.g., smartphone). A mobile phone  7400  includes a housing  7401  provided with a display portion  7402 , a microphone  7406 , a speaker  7405 , a camera  7407 , an external connection portion  7404 , an operation button  7403 , and the like. In the case where a light-emitting device is manufactured by forming the light-emitting element of one embodiment of the present invention over a flexible substrate, the light-emitting device can be used for the display portion  7402  with a curved surface as illustrated in  FIG.  7 D . 
     When the display portion  7402  of the mobile phone  7400  illustrated in  FIG.  7 D  is touched with a finger or the like, information can be input into the mobile phone. Furthermore, operations such as making a call and creating an e-mail can be performed by touching the display portion  7402  with a finger or the like. 
     There are mainly three screen modes of the display portion  7402 . The first mode is a display mode mainly for displaying an image. The second mode is an input mode mainly for inputting information such as characters. The third mode is a display-and-input mode in which two modes of the display mode and the input mode are combined. 
     For example, in the case of making a call or creating an e-mail, a character input mode mainly for inputting characters is selected for the display portion  7402  so that characters displayed on the screen can be input. In this case, it is preferable to display a keyboard or number buttons on almost the entire screen of the display portion  7402 . 
     When a detection device such as a gyroscope or an acceleration sensor is provided inside the mobile phone  7400 , display on the screen of the display portion  7402  can be automatically changed by determining the orientation of the mobile phone  7400  (whether the mobile phone is placed horizontally or vertically for a landscape mode or a portrait mode). 
     The screen modes are changed by touch on the display portion  7402  or operation with the operation button  7403  of the housing  7401 . The screen modes can also be changed depending on the kind of images displayed on the display portion  7402 . For example, when a signal of an image displayed on the display portion is a signal of moving image data, the screen mode is changed to the display mode. When the signal is a signal of text data, the screen mode is changed to the input mode. 
     Moreover, in the input mode, if a signal detected by an optical sensor in the display portion  7402  is detected and the input by touch on the display portion  7402  is not performed for a certain period, the screen mode may be controlled so as to be changed from the input mode to the display mode. 
     The display portion  7402  may function as an image sensor. For example, an image of a palm print, a fingerprint, or the like is taken by touch on the display portion  7402  with the palm or the finger, whereby personal authentication can be performed. Furthermore, when a backlight or a sensing light source which emits near-infrared light is provided in the display portion, an image of a finger vein, a palm vein, or the like can be taken. 
     Furthermore, the light-emitting device can also be used for a mobile phone having a structure illustrated in  FIG.  7 D ′ 1  or  FIG.  7 D ′ 2 , which is another structure of the mobile phone (e.g., smartphone). 
     In the case of the structure illustrated in  FIG.  7 D ′ 1  or  FIG.  7 D ′ 2 , character information, image information, or the like can be displayed on second screens  7502 ( 1 ) and  7502 ( 2 ) of housings  7500 ( 1 ) and  7500 ( 2 ) as well as first screens  7501 ( 1 ) and  7501 ( 2 ). Such a structure enables a user to easily see character information, image information, or the like displayed on the second screens  7502 ( 1 ) and  7502 ( 2 ) while the mobile phone is placed in user&#39;s breast pocket. 
       FIGS.  8 A to  8 C  illustrate a foldable portable information terminal  9310 .  FIG.  8 A  illustrates the portable information terminal  9310  that is opened.  FIG.  8 B  illustrates the portable information terminal  9310  that is being opened or being folded.  FIG.  8 C  illustrates the portable information terminal  9310  that is folded. The portable information terminal  9310  is highly portable when folded. When the portable information terminal  9310  is opened, a seamless large display region is highly browsable. 
     A display panel  9311  is supported by three housings  9315  joined together by hinges  9313 . The display panel  9311  may be a touch panel (an input/output device) including a touch sensor (an input device). By bending the display panel  9311  at a connection portion between two housings  9315  with the use of the hinges  9313 , the portable information terminal  9310  can be reversibly changed in shape from an opened state to a folded state. The light-emitting device of one embodiment of the present invention can be used for the display panel  9311 . A display region in the display panel  9311  includes a display region that is positioned at a side surface of the portable information terminal  9310  that is folded. On this display region, information icons, frequently-used applications, file shortcuts to programs, and the like can be displayed, and confirmation of information and start of application can be smoothly performed. 
     As described above, the electronic devices can be obtained by using the light-emitting device of one embodiment of the present invention. Note that the light-emitting device can be used for electronic devices in a variety of fields without being limited to the electronic devices described in this embodiment. 
     Note that the structure described in this embodiment can be combined as appropriate with any of the structures described in the other embodiments. 
     Embodiment 7 
     In this embodiment, examples of a lighting device to which a light-emitting device which is one embodiment of the present invention is applied, will be described with reference to  FIG.  9   . 
       FIG.  9    illustrates an example in which the light-emitting device is used as an indoor lighting device  8001 . Since the area of the light-emitting device can be increased, a lighting device having a large area can be formed. In addition, with the use of a housing with a curved surface, a lighting device  8002  which includes the housing, a cover, or a support and in which a light-emitting region has a curved surface can be obtained. A light-emitting element included in the light-emitting device described in this embodiment is in a thin film form, which allows the housing to be designed more freely. Therefore, the lighting device can be elaborately designed in a variety of ways. Furthermore, a wall of the room may be provided with a large-sized lighting device  8003 . 
     Moreover, when the light-emitting device is used for a table by being used as a surface of the table, a lighting device  8004  which has a function as a table can be obtained. When the light-emitting device is used as part of other furniture, a lighting device which has a function as the furniture can be obtained. 
     In this manner, a variety of lighting devices to which the light-emitting device is applied can be obtained. Note that such lighting devices are also embodiments of the present invention. 
     Note that the structure described in this embodiment can be combined as appropriate with any of the structures described in the other embodiments. 
     Example 1 
     Synthesis Example 1 
     In this example, as a synthesis method of one embodiment of the present invention, a synthesis method of 2-{4-[3-(N-phenyl-9H-carbazol-3-yl)-9H-carbazol-9-yl]phenyl}dibenzo[f,h]quinoxaline (abbreviation: 2PCCzPDBq, represented by the structural formula (100)) will be described. The structure of 2PCCzPDBq is shown below. 
     
       
         
         
             
             
         
       
     
     Synthesis of 2PCCzPDBq 
     First, 1.0 g (2.8 mmol) of 2-(4-chlorophenyl)dibenzo[f,h]quinoxaline, 1.1 g (2.8 mmol) of 3-(9-phenyl-9H-carbazol-3-yl)-9H-carbazole, 0.54 g (5.6 mmol) of sodium-tert-butoxide, and 23 mg (0.10 mmol) of 2-dicyclohexylphosphino-2′,6′-dimethoxybiphenyl (abbreviation: S-Phos) were put in a 200-mL three-neck flask and mixed, and the air in the flask was replaced with nitrogen. To this mixture was added 14 mL of mesitylene, and the resulting mixture was degassed by being stirred while the pressure in the flask was reduced. 
     Then, 16 mg (0.028 mmol) of bis(dibenzylideneacetone)palladium(0) (abbreviation: Pd(dba) 2 ) was added to the mixture. This mixture was stirred at 150° C. for 5 hours under a nitrogen stream, so that a solid was precipitated. The precipitated solid was collected by suction filtration. The collected solid was dissolved in approximately 400 mL of hot toluene, and this solution was suction-filtered through a stack of Celite and alumina. The resulting filtrate was concentrated to give a solid. The solid was recrystallized with toluene to give 1.6 g of a yellow powder, which was the target substance, in a yield of 80%. 
     By a train sublimation method, 1.4 g of the obtained yellow powdered solid, which was the target substance, was purified. The sublimation purification was carried out at 380° C. under a pressure of 3.8 Pa with a flow rate of an argon gas at 10 mL/min. After the sublimation purification, 1.1 g of a yellow glassy solid of 2PCCzPDBq was obtained at a collection rate of 79%. The synthesis scheme of this step is shown in the following scheme (a-1). 
     
       
         
         
             
             
         
       
     
     Analysis results by nuclear magnetic resonance spectroscopy ( 1 H-NMR) of the yellow powdered solid obtained in the above step will be described below.  1 H-NMR charts are shown in  FIGS.  10 A and  10 B .  FIG.  10 B  is a chart in which the range from 7.0 (ppm) to 10 (ppm) on the horizontal axis (δ) in  FIG.  10 A  is enlarged. These results show that 2PCCzPDBq (represented by the structural formula (100)) was obtained in the above step. 
     δ=7.32 (t, J=5.7 Hz, 1H), 7.37 (t, J=8.0 Hz, 1H), 7.43-7.53 (m, 5H), 7.58-7.66 (m, 6H), 7.72-7.89 (m, 8H), 8.25 (d, J=7.4 Hz, 1H), 8.30 (d, J=8.1 Hz, 1H), 8.44 (d, J=7.5 Hz, 1H), 8.50 (d, J=5.4 Hz, 2H), 8.64-8.67 (m, 3H), 9.25 (d, J=8.0 Hz, 1H), 9.37 (d, J=6.3 Hz, 1H), 9.47 (s, 1H). 
     Then,  FIG.  11    shows the absorption spectrum and the emission spectrum of 2PCCzPDBq in a toluene solution of 2PCCzPDBq, and  FIG.  12    shows the absorption spectrum and the emission spectrum of 2PCCzPDBq in a thin film of 2PCCzPDBq. The spectra were measured with a UV-visible spectrophotometer (V550, produced by JASCO Corporation). The spectra of 2PCCzPDBq in the toluene solution of 2PCCzPDBq were measured with the toluene solution of 2PCCzPDBq put in a quartz cell. The spectra of 2PCCzPDBq in the thin film of 2PCCzPDBq were measured with a sample fabricated by deposition of 2PCCzPDBq on a quartz substrate by evaporation. Note that in the case of the absorption spectrum of 2PCCzPDBq in the toluene solution of 2PCCzPDBq, the absorption spectrum obtained by subtraction of the absorption spectra of the quartz cell and toluene from the measured spectrum is shown, and in the case of the absorption spectrum of 2PCCzPDBq in the thin film of 2PCCzPDBq, the absorption spectrum obtained by subtraction of the absorption spectrum of the quartz substrate from the measured spectrum is shown. 
     As shown in  FIG.  11   , in the case of 2PCCzPDBq in the toluene solution of 2PCCzPDBq, the absorption peaks were observed at approximately 305 nm and 385 nm, and the emission wavelength peak was observed at 450 nm (excitation wavelength: 305 nm). As shown in  FIG.  12   , in the case of 2PCCzPDBq in the thin film of 2PCCzPDBq, the absorption peaks were observed at approximately 209 nm, 258 nm, 307 nm, 336 nm, and 396 nm, and the emission wavelength peak was observed at 502 nm (excitation wavelength: 396 nm). 
     Example 2 
     Synthesis Example 2 
     In this example, as a synthesis method of one embodiment of the present invention, a synthesis method of 2-{3-[3-(N-phenyl-9H-carbazol-3-yl)-9H-carbazol-9-yl]phenyl}dibenzo[f,h]quinoxaline (abbreviation: 2mPCCzPDBq, represented by the structural formula (101)) will be described. The structure of 2mPCCzPDBq is shown below. 
     
       
         
         
             
             
         
       
     
     Synthesis of 2mPCCzPDBq 
     First, 1.7 g (5.0 mmol) of 2-(3-chlorophenyl)dibenzo[f,h]quinoxaline, 2.0 g (5.0 mmol) of 3-(9-phenyl-9H-carbazol-3-yl)-9H-carbazole, 0.96 g (10 mmol) of sodium-tert-butoxide, and 41 mg (0.10 mmol) of 2-dicyclohexylphosphino-2′,6′-dimethoxybiphenyl (abbreviation: S-Phos) were put in a 200-mL three-neck flask and mixed, and the air in the flask was replaced with nitrogen. To this mixture was added 25 mL of mesitylene, and the resulting mixture was degassed by being stirred while the pressure in the flask was reduced. 
     Then, 29 mg (0.050 mmol) of bis(dibenzylideneacetone)palladium(0) (abbreviation: Pd(dba) 2 ) was added to the mixture. This mixture was stirred at 150° C. for 5 hours under a nitrogen stream, so that a solid was precipitated. The precipitated solid was collected by suction filtration. The collected solid was dissolved in approximately 400 mL of hot toluene, and this solution was suction-filtered through a stack of Celite and alumina. The resulting filtrate was concentrated to give a solid. The solid was recrystallized with toluene to give 2.8 g of a yellow powder, which was the target substance, in a yield of 79%. 
     By a train sublimation method, 2.2 g of the obtained yellow powdered solid, which was the target substance, was purified. The sublimation purification was carried out at 360° C. under a pressure of 2.5 Pa with a flow rate of an argon gas at 10 mL/min. After the sublimation purification, 1.2 g of a yellow glassy solid of 2mPCCzPDBq was obtained at a collection rate of 55%. The synthesis scheme of this step is shown in the following scheme (b-1). 
     
       
         
         
             
             
         
       
     
     Analysis results by nuclear magnetic resonance spectroscopy ( 1 H-NMR) of the yellow powdered solid obtained in the above step will be described below.  1 H-NMR charts are shown in  FIGS.  13 A and  13 B .  FIG.  13 B  is a chart in which the range from 7.0 (ppm) to 10 (ppm) on the horizontal axis (β) in  FIG.  13 A  is enlarged. These results show that 2-{3[3-(N-phenyl-9H-carbazol-3-yl)-9H-carbazol-9-yl]phenyl}dibenzo[f,h]quinoxaline (abbreviation: 2mPCCzPDBq, represented by the structural formula (101)) was obtained in the above step. 
       1 H-NMR (CDCl 3 , 500 MHz): δ(ppm)=7.31-7.54 (m, 7H), 7.58-7.67 (m, 6H), 7.73-7.90 (m, 8H), 8.25 (d, J=8.0 Hz, 1H), 8.30 (d, J=8.1 Hz, 1H), 8.45 (d, J=6.3 Hz, 1H), 8.49 (d, J=3.7 Hz, 2H), 8.65-8.68 (m, 3H), 9.25 (d, J=2.0 Hz, 1H), 9.37 (d, J=6.9 Hz, 1H), 9.48 (s, 1H). 
     Then,  FIG.  14    shows the absorption spectrum and the emission spectrum of 2mPCCzPDBq in a toluene solution of 2mPCCzPDBq, and  FIG.  15    shows the absorption spectrum and the emission spectrum of 2mPCCzPDBq in a thin film of 2mPCCzPDBq. The spectra were measured with a UV-visible spectrophotometer (V550, produced by JASCO Corporation). The spectra of 2mPCCzPDBq in the toluene solution of 2mPCCzPDBq were measured with the toluene solution of 2mPCCzPDBq put in a quartz cell. The spectra of 2mPCCzPDBq in the thin film of 2mPCCzPDBq were measured with a sample fabricated by deposition of 2mPCCzPDBq on a quartz substrate by evaporation. Note that in the case of the absorption spectrum of 2mPCCzPDBq in the toluene solution of 2mPCCzPDBq, the absorption spectrum obtained by subtraction of the absorption spectra of the quartz cell and toluene from the measured spectrum is shown, and in the case of the absorption spectrum of 2mPCCzPDBq in the thin film of 2mPCCzPDBq, the absorption spectrum obtained by subtraction of the absorption spectrum of the quartz substrate from the measured spectrum is shown. 
     As shown in  FIG.  14   , in the case of 2mPCCzPDBq in the toluene solution of 2mPCCzPDBq, the absorption peaks were observed at approximately 305 nm and 374 nm, and the emission wavelength peak was observed at 480 nm (excitation wavelength: 305 nm). As shown in  FIG.  15   , in the case of 2mPCCzPDBq in the thin film of 2mPCCzPDBq, the absorption peaks were observed at approximately 208 nm, 257 nm, 308 nm, 361 nm, and 379 nm, and the emission wavelength peak was observed at 515 nm (excitation wavelength: 380 nm). 
     Example 3 
     Synthesis Example 3 
     In this example, as a synthesis method of one embodiment of the present invention, a synthesis method of 2-{4-[2-(N-phenyl-9H-carbazol-3-yl)-9H-carbazol-9-yl]phenyl}dibenzo[f,h]quinoxaline (abbreviation: 2PCCzPDBq-02, represented by the structural formula (102)) will be described. The structure of 2PCCzPDBq-02 is shown below. 
     
       
         
         
             
             
         
       
     
     Synthesis of 2PCCzPDBq-02 
     First, 1.4 g (4.2 mmol) of 2-(4-chlorophenyl)dibenzo[f,h]quinoxaline, 1.7 g (4.2 mmol) of 2-(9-phenyl-9H-carbazol-3-yl)-9H-carbazole, 0.81 g (8.4 mmol) of sodium-tert-butoxide, and 34 mg (0.10 mmol) of 2-dicyclohexylphosphino-2′,6′-dimethoxybiphenyl (abbreviation: S-Phos) were put in a 200-mL three-neck flask and mixed, and the air in the flask was replaced with nitrogen. To this mixture was added 21 mL of mesitylene, and the resulting mixture was degassed by being stirred while the pressure in the flask was reduced. 
     Then, 24 mg (0.042 mmol) of bis(dibenzylideneacetone)palladium(0) (abbreviation: Pd(dba) 2 ) was added to the mixture. This mixture was stirred at 150° C. for 5 hours under a nitrogen stream, so that a solid was precipitated. The precipitated solid was collected by suction filtration. The collected solid was dissolved in approximately 400 mL of hot toluene, and this solution was suction-filtered through a stack of Celite and alumina. The resulting filtrate was concentrated to give a solid. The solid was recrystallized with toluene to give 2.5 g of a yellow powder, which was the target substance, in a yield of 84%. 
     By a train sublimation method, 2.0 g of the obtained yellow powdered solid, which was the target substance, was purified. The sublimation purification was carried out at 390° C. under a pressure of 3.7 Pa with a flow rate of an argon gas at 10 mL/min. After the sublimation purification, 1.7 g of a yellow glassy solid of 2PCCzPDBq-02 was obtained at a collection rate of 85%. The synthesis scheme of this step is shown in the following scheme (c-1). 
     
       
         
         
             
             
         
       
     
     Analysis results by nuclear magnetic resonance spectroscopy ( 1 H-NMR) of the yellow powdered solid obtained in the above step will be described below.  1 H-NMR charts are shown in  FIGS.  16 A and  16 B .  FIG.  16 B  is a chart in which the range from 7.0 (ppm) to 10 (ppm) on the horizontal axis (δ) in  FIG.  16 A  is enlarged. These results show that 2-{4-[2-(N-phenyl-9H-carbazol-3-yl)-9H-carbazol-9-yl]phenyl}dibenzo[f,h]quinoxaline (abbreviation: 2PCCzPDBq-02, represented by the structural formula (102)) was obtained in the above step. 
     δ=7.25-7.48 (m, 7H), 7.71-7.75 (m, 2H), 7.71-7.75 (m, 5H), 7.91 (d, J=8.6 Hz, 2H), 8.21 (d, J=7.4 Hz, 1H), 8.26 (d, J=8.0 Hz, 2H), 8.42 (sd, J=1.7 Hz, 2H), 8.63-8.69 (m, 4H), 9.27 (d, J=8.0 Hz, 1H), 9.46 (d, J=6.3 Hz, 1H), 9.51 (s, 1H). 
     Then,  FIG.  17    shows the absorption spectrum and the emission spectrum of 2PCCzPDBq-02 in a toluene solution of 2PCCzPDBq-02, and  FIG.  18    shows the absorption spectrum and the emission spectrum of 2PCCzPDBq-02 in a thin film of 2PCCzPDBq-02. The spectra were measured with a UV-visible spectrophotometer (V550, produced by JASCO Corporation). The spectra of 2PCCzPDBq-02 in the toluene solution of 2PCCzPDBq-02 were measured with the toluene solution of 2PCCzPDBq-02 put in a quartz cell. The spectra of 2PCCzPDBq-02 in the thin film of 2PCCzPDBq-02 were measured with a sample fabricated by deposition of 2PCCzPDBq-02 on a quartz substrate by evaporation. Note that in the case of the absorption spectrum of 2PCCzPDBq-02 in the toluene solution of 2PCCzPDBq-02, the absorption spectrum obtained by subtraction of the absorption spectra of the quartz cell and toluene from the measured spectrum is shown, and in the case of the absorption spectrum of 2PCCzPDBq-02 in the thin film of 2PCCzPDBq-02, the absorption spectrum obtained by subtraction of the absorption spectrum of the quartz substrate from the measured spectrum is shown. 
     As shown in  FIG.  17   , in the case of 2PCCzPDBq-02 in the toluene solution of 2PCCzPDBq-02, the absorption peaks were observed at approximately 323 nm and 381 nm, and the emission wavelength peak was observed at 421 nm (excitation wavelength: 320 nm). As shown in  FIG.  18   , in the case of 2PCCzPDBq-02 in the thin film of 2PCCzPDBq-02, the absorption peaks were observed at approximately 209 nm, 257 nm, 311 nm, 326 nm, 351 nm, and 389 nm, and the emission wavelength peak was observed at 473 nm (excitation wavelength: 396 nm). 
     Example 4 
     Synthesis Example 4 
     In this example, as a synthesis method of one embodiment of the present invention, a synthesis method of 2-{3-[2-(N-phenyl-9H-carbazol-3-yl)-9H-carbazol-9-yl]phenyl}dibenzo[f,h]quinoxaline (abbreviation: 2mPCCzPDBq-02, represented by the structural formula (103)) will be described. The structure of 2mPCCzPDBq-02 is shown below. 
     
       
         
         
             
             
         
       
     
     Synthesis of 2mPCCzPDBq-02 
     First, 1.7 g (5.0 mmol) of 2-(3-chlorophenyl)dibenzo[f,h]quinoxaline, 2.0 g (5.0 mmol) of 2-(9-phenyl-9H-carbazol-3-yl)-9H-carbazole, 0.96 g (10 mmol) of sodium-tert-butoxide, and 41 mg (0.10 mmol) of 2-dicyclohexylphosphino-2′,6′-dimethoxybiphenyl (abbreviation: S-Phos) were put in a 200-mL three-neck flask and mixed, and the air in the flask was replaced with nitrogen. To this mixture was added 25 mL of mesitylene, and the resulting mixture was degassed by being stirred while the pressure in the flask was reduced. 
     Then, 29 mg (0.050 mmol) of bis(dibenzylideneacetone)palladium(0) (abbreviation: Pd(dba) 2 ) was added to the mixture. This mixture was stirred at 150° C. for 4 hours under a nitrogen stream, so that a solid was precipitated. The precipitated solid was collected by suction filtration. The collected solid was dissolved in approximately 400 mL of hot toluene, and this solution was suction-filtered through a stack of Celite, alumina, and Florisil. The resulting filtrate was concentrated to give a solid. The solid was recrystallized with toluene to give 3.1 g of a white powder, which was the target substance, in a yield of 87%. 
     By a train sublimation method, 3.0 g of the obtained white powdered solid, which was the target substance, was purified. The sublimation purification was carried out at 360° C. under a pressure of 10 Pa with a flow rate of an argon gas at 5.0 mL/min. After the sublimation purification, 2.0 g of a yellow glassy solid of 2mPCCzPDBq-02 was obtained at a collection rate of 65%. The synthesis scheme of this step is shown in the following scheme (d-1). 
     
       
         
         
             
             
         
       
     
     Analysis results by nuclear magnetic resonance spectroscopy ( 1 H-NMR) of the white powdered solid obtained in the above step will be described below.  1 H-NMR charts are shown in  FIGS.  19 A and  19 B .  FIG.  19 B  is a chart in which the range from 7.0 (ppm) to 10 (ppm) on the horizontal axis (δ) in  FIG.  19 A  is enlarged. These results show that 2mPCCzPDBq-02 (represented by the structural formula (103)) was obtained in the above step. 
       1 H-NMR (DMSO-P, 500 MHz): δ (ppm)=7.17 (t, J1=7.5 Hz, 1H), 7.31-7.39 (m, 4H), 7.47-7.52 (m, 2H), 7.56-7.57 (m, 3H), 7.63-7.66 (m, 3H), 7.75-7.92 (m, 7H), 7.98 (t, J1=2.5 Hz, 1H), 8.19 (d, J1=7.5 Hz, 1H), 8.30 (d, J1=7.5 Hz, 1H), 8.37 (d, J1=8.0 Hz, 1H), 8.54 (sd, J1=1.5 Hz, 1H), 8.62 (d, J1=8.0 Hz, 1H), 8.79-8.82 (m, 3H), 9.19 (d, J1=8.0 Hz, 1H), 9.25 (d, J1=9.0 Hz, 1H), 9.75 (s, 1H). 
     Then,  FIG.  20    shows the absorption spectrum and the emission spectrum of 2mPCCzPDBq-02 in a toluene solution of 2mPCCzPDBq-02, and  FIG.  21    shows the absorption spectrum and the emission spectrum of 2mPCCzPDBq-02 in a thin film of 2mPCCzPDBq-02. The spectra were measured with a UV-visible spectrophotometer (V550, produced by JASCO Corporation). The spectra of 2mPCCzPDBq-02 in the toluene solution of 2mPCCzPDBq-02 were measured with the toluene solution of 2mPCCzPDBq-02 put in a quartz cell. The spectra of 2mPCCzPDBq-02 in the thin film of 2mPCCzPDBq-02 were measured with a sample fabricated by deposition of 2mPCCzPDBq-02 on a quartz substrate by evaporation. Note that in the case of the absorption spectrum of 2mPCCzPDBq-02 in the toluene solution of 2mPCCzPDBq-02, the absorption spectrum obtained by subtraction of the absorption spectra of the quartz cell and toluene from the measured spectrum is shown, and in the case of the absorption spectrum of 2mPCCzPDBq-02 in the thin film of 2mPCCzPDBq-02, the absorption spectrum obtained by subtraction of the absorption spectrum of the quartz substrate from the measured spectrum is shown. 
     As shown in  FIG.  20   , in the case of 2mPCCzPDBq-02 in the toluene solution of 2mPCCzPDBq-02, the absorption peaks were observed at approximately 281 nm, 305 nm, 319 nm, and 374 nm, and the emission wavelength peaks were observed at 389 nm and 410 nm. As shown in  FIG.  21   , in the case of 2mPCCzPDBq-02 in the thin film of 2mPCCzPDBq-02, the absorption peaks were observed at approximately 209 nm, 257 nm, 309 nm, 327 nm, 354 nm, and 386 nm, and the emission wavelength peak was observed at 484 nm (excitation wavelength: 381 nm). 
     Example 5 
     In this example, a light-emitting element  1  and a light-emitting element  2  each containing a dibenzo[f,h]quinoxaline derivative which is one embodiment of the present invention and a comparative light-emitting element  3  were fabricated. The structure of each light-emitting element will be described with reference to  FIG.  22   . Chemical formulae of materials used in this example are shown below. 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     &lt;&lt;Fabrication of Light-Emitting Element  1 , Light-Emitting Element  2 , and Comparative Light-Emitting Element  3 &gt;&gt; 
     First, indium tin oxide containing silicon oxide (ITSO) was deposited over a glass substrate  1100  by a sputtering method, so that a first electrode  1101  functioning as an anode was formed. Note that the thickness of the first electrode  1101  was set to be 110 nm and that the area of the first electrode  1101  was set to be 2 mm×2 mm. 
     Next, as pretreatment for forming the light-emitting element  1 , the light-emitting element  2 , and the comparative light-emitting element  3  over the substrate  1100 , a surface of the substrate was washed with water, baking was performed at 200° C. for 1 hour, and then UV ozone treatment was performed for 370 seconds. 
     After that, the substrate  1100  was transferred into a vacuum evaporation apparatus in which the pressure had been reduced to approximately 10 −4  Pa, and subjected to vacuum baking at 170° C. for 30 minutes in a heating chamber of the vacuum evaporation apparatus, and then the substrate  1100  was cooled down for about 30 minutes. 
     Next, the substrate  1100  was fixed to a holder provided in the vacuum evaporation apparatus so that a surface of the substrate  1100  over which the first electrode  1101  was formed faced downward. In this example, a case will be described where a hole-injection layer  1111 , a hole-transport layer  1112 , a light-emitting layer  1113 , an electron-transport layer  1114 , and an electron-injection layer  1115  which are included in an EL layer  1102  were sequentially formed by a vacuum evaporation method. 
     After reducing the pressure of the vacuum evaporation apparatus to 10 −4  Pa, 1,3,5-tri(dibenzothiophen-4-yl)benzene (abbreviation: DBT3P-II) and molybdenum oxide were deposited by co-evaporation so that the mass ratio of DBT3P-II to molybdenum oxide was 4:2, whereby the hole-injection layer  1111  was formed on the first electrode  1101 . The thickness of the hole-injection layer  1111  was set to be 20 nm. Note that co-evaporation is an evaporation method in which a plurality of different substances are concurrently vaporized from their respective evaporation sources. 
     Then, 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine (abbreviation: BPAFLP) was deposited by evaporation to a thickness of 20 nm, whereby the hole-transport layer  1112  was formed. 
     Next, the light-emitting layer  1113  was formed on the hole-transport layer  1112 . In the case of the light-emitting element  1 ,  2 -{3-[2-(N-phenyl-9H-carbazol-3-yl)-9H-carbazol-9-yl]phenyl}dibenzo[f,h]quinoxaline (abbreviation: 2mPCCzPDBq-02, represented by the structural formula (103)), N-(1,1′-biphenyl-4-yl)-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-9,9-dimethyl-9H-fluor en-2-amine (abbreviation: PCBBiF), and (acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III) (abbreviation: [Ir(tBuppm) 2 (acac)]) were deposited to a thickness of 20 nm by co-evaporation, so that the mass ratio of 2mPCCzPDBq-02 to PCBBiF and [Ir(tBuppm) 2 (acac)] was 0.7:0.3:0.05. Then, 2mPCCzPDBq-02, PCBBiF, and [Ir(tBuppm) 2 (acac)] were deposited to a thickness of 20 nm by co-evaporation, so that the mass ratio of 2mPCCzPDBq-02 to PCBBiF and [Ir(tBuppm) 2 (acac)] was 0.8:0.2:0.05. In this manner, the light-emitting layer  1113  having a stacked structure and a thickness of 40 nm was formed. 
     In the case of the light-emitting element  2 , 2-{3-[3-(N-phenyl-9H-carbazol-3-yl)-9H-carbazol-9-yl]phenyl}dibenzo[f,h]quinoxaline (abbreviation: 2mPCCzPDBq, represented by the structural formula (101)), PCBBiF, and [Ir(tBuppm) 2 (acac)] were deposited to a thickness of 20 nm by co-evaporation, so that the mass ratio of 2mPCCzPDBq to PCBBiF and [Ir(tBuppm) 2 (acac)] was 0.7:0.3:0.05. Then, 2mPCCzPDBq, PCBBiF, and [Ir(tBuppm) 2 (acac)] were deposited to a thickness of 20 nm by co-evaporation, so that the mass ratio of 2mPCCzPDBq to PCBBiF and [Ir(tBuppm) 2 (acac)] was 0.8:0.2:0.05. In this manner, the light-emitting layer  1113  having a stacked structure and a thickness of 40 nm was formed. 
     In the case of the comparative light-emitting element  3 ,  2 -[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline (abbreviation: 2mDBTBPDBq-II), PCBBiF, and [Ir(tBuppm) 2 (acac)] were deposited to a thickness of 20 nm by co-evaporation, so that the mass ratio of 2mDBTBPDBq-II to PCBBiF and [Ir(tBuppm) 2 (acac)] was 0.7:0.3:0.05. Then, 2mDBTBPDBq-II, PCBBiF, and [Ir(tBuppm) 2 (acac)] were deposited to a thickness of 20 nm by co-evaporation, so that the mass ratio of 2mDBTBPDBq-II to PCBBiF and [Ir(tBuppm) 2 (acac)] was 0.8:0.2:0.05. In this manner, the light-emitting layer  1113  having a stacked structure and a thickness of 40 nm was formed. 
     Then, in the case of the light-emitting element  1 , 2mPCCzPDBq-02 was deposited to a thickness of 20 nm on the light-emitting layer  1113  by evaporation and bathophenanthroline (abbreviation: Bphen) was then deposited to a thickness of 10 nm by evaporation, whereby the electron-transport layer  1114  was formed. In the case of the light-emitting element  2 , 2mPCCzPDBq was deposited to a thickness of 20 nm on the light-emitting layer  1113  by evaporation and bathophenanthroline (abbreviation: Bphen) was then deposited to a thickness of 10 nm by evaporation, whereby the electron-transport layer  1114  was formed. In the case of the comparative light-emitting element  3 , 2mDBTBPDBq-II was deposited to a thickness of 20 nm on the light-emitting layer  1113  by evaporation and bathophenanthroline (abbreviation: Bphen) was then deposited to a thickness of 10 nm by evaporation, whereby the electron-transport layer  1114  was formed. 
     Furthermore, lithium fluoride was deposited to a thickness of 1 nm on the electron-transport layer  1114  by evaporation, whereby the electron-injection layer  1115  was formed. 
     Finally, aluminum was deposited to a thickness of 200 nm on the electron-injection layer  1115  by evaporation to form a second electrode  1103  serving as a cathode; thus, the light-emitting element  1 , the light-emitting element  2 , and the comparative light-emitting element  3  were fabricated. Note that in all the above evaporation steps, evaporation was performed by a resistance-heating method. 
     Table 1 shows the element structures of the light-emitting element  1 , the light-emitting element  2 , and the comparative light-emitting element  3  that were fabricated as described above. 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                 Hole- 
                 Light- 
                   
                 Electron- 
                   
               
               
                   
                 First 
                 Hole-injection 
                 transport 
                 emitting 
                   
                 injection 
                 Second 
               
               
                   
                 Electrode 
                 Layer 
                 Layer 
                 Layer 
                 Electron-transport Layer 
                 Layer 
                 Electrode 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Light- 
                 ITSO 
                 DBT3P-II:MoOx 
                 BPAFLP 
                 * 
                 2mPCCzPDBq-02 
                 Bphen 
                 LiF 
                 Al (200 nm) 
               
               
                 emitting 
                 (110 nm) 
                 (4:2 20 nm) 
                 (20 nm) 
                   
                 (20 nm) 
                 (10 nm) 
                 (1 nm) 
                   
               
               
                 Element 1 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Light- 
                 ITSO 
                 DBT3P-II:MoOx 
                 BPAFLP 
                 ** 
                 2mPCCzPDBq 
                 Bphen 
                 LiF 
                 Al (200 nm) 
               
               
                 emitting 
                 (110 nm) 
                 (4:2 20 nm) 
                 (20 nm) 
                   
                 (20 nm) 
                 (10 nm) 
                 (1 nm) 
                   
               
               
                 Element 2 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Comparative 
                 ITSO 
                 DBT3P-II:MoOx 
                 BPAFLP 
                 *** 
                 2mDBTBPDBq-II 
                 Bphen 
                 LiF 
                 Al (200 nm) 
               
               
                 Light- 
                 (110 nm) 
                 (4:2 20 nm) 
                 (20 nm) 
                   
                 (20 nm) 
                 (10 nm) 
                 (1 nm) 
                   
               
               
                 emitting 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Element 3 
               
               
                   
               
               
                 *2mPCCzPDBq-02:PCBBiF:[Ir(tBuppm) 2 (acac)] (0.7:0.3:0.05 (20 nm)\0.8:0.2:0.05 (20 nm)) 
               
               
                 **2mPCCzPDBq:PCBBiF:[Ir(tBuppm) 2 (acac)] (0.7:0.3:0.05 (20 nm)\0.8:0.2:0.05 (20 nm)) 
               
               
                 ***2mDBTBPDBq-II:PCBBiF:[Ir(tBuppm) 2 (acac)] (0.7:0.3:0.05 (20 nm)\0.8:0.2:0.05 (20 nm)) 
               
            
           
         
       
     
     The fabricated light-emitting element  1 , light-emitting element  2 , and comparative light-emitting element  3  were sealed in a glove box under a nitrogen atmosphere so as not to be exposed to the air (specifically, a sealant was applied to surround the elements, UV treatment was performed, and heat treatment was performed at 80° C. for 1 hour). 
     &lt;&lt;Operation Characteristics of Light-Emitting Element  1 , Light-Emitting Element  2 , and Comparative Light-Emitting Element  3 &gt;&gt; 
     Operation characteristics of the fabricated light-emitting element  1 , light-emitting element  2 , and comparative light-emitting element  3  were measured. Note that the measurements were performed at room temperature (in an atmosphere kept at 25° C.). Results are shown in  FIG.  23   ,  FIG.  24   ,  FIG.  25   , and  FIG.  26   . 
     Table 2 shows initial values of main characteristics of the light-emitting element  1 , the light-emitting element  2 , and the comparative light-emitting element  3  at a luminance of approximately 1000 cd/m 2 . 
     
       
         
           
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 External 
               
               
                   
                   
                   
                   
                   
                   
                 Current 
                 Power 
                 Quantum 
               
               
                   
                 Volt- 
                   
                 Current 
                 Chroma- 
                 Lumi- 
                 Effici- 
                 Effici- 
                 Effici- 
               
               
                   
                 age 
                 Current 
                 Density 
                 ticity 
                 nance 
                 ency 
                 ency 
                 ency 
               
               
                   
                 (V) 
                 (mA) 
                 (mA/cm 2 ) 
                 (x, y) 
                 (cd/m 2 ) 
                 (cd/A) 
                 (Im/W) 
                 (%) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Light- 
                 2.9 
                 0.038 
                 1.0 
                 (0.42, 
                 1100 
                 110 
                 120 
                 29 
               
               
                 emitting 
                   
                   
                   
                 0.57) 
                   
                   
                   
                   
               
               
                 Element 1 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Light- 
                 2.8 
                 0.033 
                 0.8 
                 (0.41, 
                 920 
                 110 
                 120 
                 29 
               
               
                 emitting 
                   
                   
                   
                 0.58) 
                   
                   
                   
                   
               
               
                 Element 2 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Compar- 
                 2.9 
                 0.035 
                 0.88 
                 (0.42, 
                 980 
                 110 
                 120 
                 30 
               
               
                 ative 
                   
                   
                   
                 0.57) 
                   
                   
                   
                   
               
               
                 Light- 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 emitting 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Element 3 
               
               
                   
               
            
           
         
       
     
       FIG.  27    shows the emission spectra of the light-emitting element  1 , the light-emitting element  2 , and the comparative light-emitting element  3 , through which a current flows at a current density of 2.5 mA/cm 2 . As shown in  FIG.  27   , the emission spectra of the light-emitting element  1 , the light-emitting element  2 , and the comparative light-emitting element  3  each have a peak at approximately 546 nm, which is attributed to [Ir(tBuppm) 2 (acac)]. 
       FIG.  28 A  shows results of reliability tests on the light-emitting element  1 , the light-emitting element  2 , and the comparative light-emitting element  3 . In  FIG.  28 A , the vertical axis represents normalized luminance (%) on the assumption that the initial luminance is 100%, and the horizontal axis represents driving time (h) of the elements. Note that in the reliability tests, the light-emitting element  1 , the light-emitting element  2 , and the comparative light-emitting element  3  were driven under the conditions where the initial luminance was set to be 5000 cd/m 2  and the current density was constant. 
     The results show that the light-emitting element  1  fabricated using 2mPCCzPDBq-02 and the light-emitting element  2  fabricated using 2mPCCzPDBq, which are embodiments of the present invention, have a higher reliability and a longer lifetime than the comparative light-emitting element  3  fabricated using 2mDBTBPDBq-II. 
       FIG.  28 B  shows measurement results of the amount of change in voltage at the reliability tests. The vertical axis represents the amount of change in voltage (V), and the horizontal axis represents driving time (h) of the elements. These results show that the amount of increase in voltage in each of the light-emitting element  1  and the light-emitting element  2  which were driven at constant current is smaller than that in the comparative light-emitting element  3 . For example, after the light-emitting elements were driven for approximately 500 hours, the amount of increase in voltage in the comparative light-emitting element  3  is approximately 0.08 V, whereas those in the light-emitting elements  1  and  2  are approximately 0.05 V and 0.02 V, respectively. That is, the amount of increase in voltage in the light-emitting element  1  and that in the light-emitting element  2  are approximately as small as ½ and ¼, respectively, of that in the comparative light-emitting element  3 , which indicates a significant effect of one embodiment of the present invention. 
     Note that the combination of PCBBiF and each of 2mPCCzPDBq-02, 2mPCCzPDBq, and 2mDBTBPDBq-II forms an exciplex (because the mixed film containing PCBBiF and any of these dibenzoquinoxaline compounds exhibits yellow-green light emission having a longer wavelength than the film containing only PCBBiF or the film containing only any of these dibenzoquinoxaline compounds). Furthermore, the HOMO levels of 2mPCCzPDBq-02, 2mPCCzPDBq, 2mDBTBPDBq-II, and PCBBiF are −5.69 eV, −5.63 eV, −6.22 eV, and −5.36 eV, respectively. The HOMO levels were obtained through a cyclic voltammetry (CV) measurement. 
     By using the HOMO levels obtained as described above, ΔE HOMO  in the light-emitting layer of each light-emitting element was calculated. Table 3 shows the results. 
     
       
         
           
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Δ E HOMO  (eV) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 Light-emitting Element 1 
                 0.33 
               
               
                   
                 Light-emitting Element 2 
                 0.27 
               
               
                   
                 Comparative Light-emitting Element 3 
                 0.86 
               
               
                   
                   
               
            
           
         
       
     
     According to the results, it is important that ΔE HOMO  is less than or equal to 0.4 eV, preferably less than or equal to 0.3 eV. 
     Furthermore, the HOMO level of BPAFLP used for the hole-transport layer is −5.51 eV. Therefore, it is found that the HOMO level of the third organic compound used for the hole-transport layer is lower than the HOMO level of PCBBiF that is the second organic compound and is located between the HOMO level of PCBBiF that is the second organic compound and the HOMO level of the first organic compound (2mPCCzPDBq-02 or 2mPCCzPDBq). This is important because holes are injected not only into the second organic compound but also partly into the first organic compound. 
     Example 6 
     In this example, a light-emitting element  4  containing a dibenzo[f,h]quinoxaline derivative which is one embodiment of the present invention was fabricated. The structure of the light-emitting element will be described with reference to  FIG.  22    as in Example 5. Chemical formulae of materials used in this example are shown below. 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     &lt;&lt;Fabrication of Light-Emitting Element  4 &gt;&gt; 
     First, indium tin oxide containing silicon oxide (ITSO) was deposited over a glass substrate  1100  by a sputtering method, so that a first electrode  1101  functioning as an anode was formed. Note that the thickness of the first electrode  1101  was set to be 110 nm and that the area of the first electrode  1101  was set to be 2 mm×2 mm. 
     Next, as pretreatment for forming the light-emitting element  4  over the substrate  1100 , a surface of the substrate was washed with water, baking was performed at 200° C. for 1 hour, and then UV ozone treatment was performed for 370 seconds. 
     After that, the substrate  1100  was transferred into a vacuum evaporation apparatus in which the pressure had been reduced to approximately 10 −4  Pa, and subjected to vacuum baking at 170° C. for 30 minutes in a heating chamber of the vacuum evaporation apparatus, and then the substrate  1100  was cooled down for about 30 minutes. 
     Next, the substrate  1100  was fixed to a holder provided in the vacuum evaporation apparatus so that a surface of the substrate  1100  over which the first electrode  1101  was formed faced downward. In this example, a case will be described where a hole-injection layer  1111 , a hole-transport layer  1112 , a light-emitting layer  1113 , an electron-transport layer  1114 , and an electron-injection layer  1115  which are included in an EL layer  1102  were sequentially formed by a vacuum evaporation method. 
     After reducing the pressure of the vacuum evaporation apparatus to 10 −4  Pa, 1,3,5-tri(dibenzothiophen-4-yl)benzene (abbreviation: DBT3P-II) and molybdenum oxide were deposited by co-evaporation so that the mass ratio of DBT3P-II to molybdenum oxide was 4:2, whereby the hole-injection layer  1111  was formed on the first electrode  1101 . The thickness of the hole-injection layer  1111  was set to be 20 nm. Note that co-evaporation is an evaporation method in which a plurality of different substances are concurrently vaporized from their respective evaporation sources. 
     Then, 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine (abbreviation: BPAFLP) was deposited to a thickness of 20 nm by evaporation, whereby the hole-transport layer  1112  was formed. 
     Next, the light-emitting layer  1113  was formed on the hole-transport layer  1112 . By co-evaporation, 2-{4-[2-(N-phenyl-9H-carbazol-3-yl)-9H-carbazol-9-yl]phenyl}dibenzo[f,h]quinoxaline (abbreviation: 2PCCzPDBq-02, represented by the structural formula (102)), N-(1,1′-biphenyl-4-yl)-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-9,9-dimethyl-9H-fluor en-2-amine (abbreviation: PCBBiF), and (acetylacetonato)bis(4,6-diphenylpyrimidinato)iridium(III) (abbreviation: [Ir(dppm) 2 (acac)]) were deposited to a thickness of 20 nm, so that the mass ratio of 2PCCzPDBq-02 to PCBBiF and [Ir(dppm) 2 (acac)] was 0.7:0.3:0.05. Then, 2PCCzPDBq-02, PCBBiF, and [Ir(dppm) 2 (acac)] were deposited to a thickness of 20 nm by co-evaporation, so that the mass ratio of 2PCCzPDBq-02 to PCBBiF and [Ir(dppm) 2 (acac)] was 0.8:0.2:0.05. In this manner, the light-emitting layer  1113  having a stacked structure and a thickness of 40 nm was formed. 
     Then, 2PCCzPDBq-02 was deposited to a thickness of 20 nm on the light-emitting layer  1113  by evaporation, and then bathophenanthroline (abbreviation: Bphen) was deposited to a thickness of 10 nm by evaporation, whereby the electron-transport layer  1114  was formed. 
     Furthermore, lithium fluoride was deposited to a thickness of 1 nm on the electron-transport layer  1114  by evaporation, whereby the electron-injection layer  1115  was formed. 
     Finally, aluminum was deposited to a thickness of 200 nm on the electron-injection layer  1115  by evaporation to form the second electrode  1103  serving as a cathode; thus, the light-emitting element  4  was fabricated. Note that in all the above evaporation steps, evaporation was performed by a resistance-heating method. 
     Table 4 shows the element structure of the light-emitting element  4  that was fabricated as described above. 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                 TABLE 4 
               
               
                   
               
               
                   
                   
                   
                 Hole- 
                 Light- 
                   
                 Electron- 
                   
               
               
                   
                 First 
                 Hole-injection 
                 transport 
                 emitting 
                 Electron-transport  
                 injection 
                 Second 
               
               
                   
                 Electrode 
                 Layer 
                 Layer 
                 Layer 
                 Layer 
                 Layer 
                 Electrode 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Light-emitting 
                 ITSO 
                 DBT3P-II:MoOx 
                 BPAFLP 
                 * 
                 2PCCzPDBq-02 
                 Bphen 
                 LiF 
                 Al 
               
               
                 Element 4 
                 (110 nm) 
                 (4:2 20 nm) 
                 (20 nm) 
                   
                 (20 nm) 
                 (10 nm) 
                 (1 nm) 
                 (200 nm) 
               
               
                   
               
               
                 *2PCCzPDBq-02:PCBBiF:[Ir(dppm) 2 (acac)] (0.7:0.3:0.05 (20 nm)\0.8:0.2:0.05 (20 nm)) 
               
            
           
         
       
     
     The fabricated light-emitting element  4  was sealed in a glove box under a nitrogen atmosphere so as not to be exposed to the air (specifically, a sealant was applied to surround the element, UV treatment was performed, and heat treatment was performed at 80° C. for 1 hour). 
     &lt;&lt;Operation Characteristics of Light-Emitting Element  4 &gt;&gt; 
     Operation characteristics of the fabricated light-emitting element  4  were measured. Note that the measurements were performed at room temperature (in an atmosphere kept at 25° C.). Results are shown in  FIG.  29   ,  FIG.  30   ,  FIG.  31   , and  FIG.  32   . 
     Table 5 shows initial values of main characteristics of the light-emitting element  4  at a luminance of approximately 1000 cd/m 2 . 
     
       
         
           
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 5 
               
               
                   
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 External 
               
               
                   
                   
                   
                 Current 
                   
                   
                 Current 
                 Power 
                 Quantum 
               
               
                   
                 Voltage 
                 Current 
                 Density 
                 Chromaticity 
                 Luminance 
                 Efficiency 
                 Efficiency 
                 Efficiency 
               
               
                   
                 (V) 
                 (mA) 
                 (mA/cm 2 ) 
                 (x, y) 
                 (cd/m 2 ) 
                 (cd/A) 
                 (Im/W) 
                 (%) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Light-emitting 
                 2.8 
                 0.045 
                 1.1 
                 (0.56, 0.44) 
                 910 
                 81 
                 91 
                 30 
               
               
                 Element 4 
               
               
                   
               
            
           
         
       
     
       FIG.  33    shows the emission spectrum of the light-emitting element  4  through which current flows at a current density of 2.5 mA/cm 2 . As shown in  FIG.  33   , the emission spectrum of the light-emitting element  4  has a peak at approximately 581 nm, which is attributed to [Ir(dppm) 2 (acac)]. 
       FIG.  34 A  shows results of a reliability test on the light-emitting element  4 . In  FIG.  34 A , the vertical axis represents normalized luminance (%) on the assumption that the initial luminance is 100%, and the horizontal axis represents driving time (h) of the element. Note that in the reliability test, the light-emitting element  4  was driven under the conditions where the initial luminance was set to be 5000 cd/m 2  and the current density was constant. 
     The results show that the light-emitting element  4  fabricated using 2PCCzPDBq-02, which is one embodiment of the present invention, has a high reliability and a long lifetime. 
       FIG.  34 B  shows measurement results of the amount of change in voltage at the reliability test. The vertical axis represents the amount of change in voltage (V), and the horizontal axis represents driving time (h) of the element. The results show that the amount of increase in voltage in the light-emitting element  4  which was driven at constant current is small. For example, after the light-emitting element  4  was driven for approximately 500 hours, the amount of increase in voltage is approximately 0.01 V. A comparative light-emitting element  9  was fabricated using 2mDBTBPDBq-II instead of 2PCCzPDBq-02 of the light-emitting element  4  and driven under conditions similar to those of the light-emitting element  4 . In that case, the amount of increase in voltage was approximately 0.06 V after the comparative light-emitting element  9  was driven for approximately 500 hours. That is, the amount of increase in voltage in the light-emitting element  4  is approximately as small as ⅙ of that in the comparative light-emitting element  9 , which indicates a significant effect of one embodiment of the present invention. 
     Note that the combination of 2PCCzPDBq-02 and PCBBiF forms an exciplex (because the mixed film containing this dibenzoquinoxaline compound and PCBBiF exhibits green light emission having a longer wavelength than the film containing only this dibenzoquinoxaline compound or the film containing only PCBBiF). Furthermore, since the HOMO level of 2PCCzPDBq-02 is −5.68 eV, ΔE HOMO  in the light-emitting layer of the light-emitting element  4  is 0.32 eV. Accordingly, it is important that ΔE HOMO  is less than or equal to 0.4 eV. 
     Furthermore, the HOMO level of BPAFLP used for the hole-transport layer is −5.51 eV. Therefore, it is found that the HOMO level of the third organic compound used for the hole-transport layer is lower than the HOMO level of PCBBiF that is the second organic compound and is located between the HOMO level of PCBBiF that is the second organic compound and the HOMO level of the first organic compound (2PCCzPDBq-02). This is important because holes are injected not only into the second organic compound but also partly into the first organic compound. 
     Example 7 
     Synthesis Example 5 
     In this example, as a synthesis method of one embodiment of the present invention, a synthesis method of 2-{3′-[3-(N-phenyl-9H-carbazol-3-yl)-9H-carbazol-9-yl]biphenyl-3-yl}dibenzo[f,h]quin oxaline (abbreviation: 2mPCCzBPDBq, represented by the structural formula (122)) will be described. The structure of 2mPCCzBPDBq is shown below. 
     
       
         
         
             
             
         
       
     
     Synthesis of 2-{3′-[3-(N-phenyl-9H-carbazol-3-yl)-9H-carbazol-9-yl]biphenyl-3-yl}dibenzo[f,h]quinoxaline (abbreviation: 2mPCCzBPDBq) 
     First, 2.0 g (4.3 mmol) of 2-(3′-bromobiphenyl-3-yl)dibenzo[f,h]quinoxaline, 1.8 g (4.3 mmol) of 3-(9-phenyl-9H-carbazol-3-yl)-9H-carbazole, and 0.83 g (8.6 mmol) of sodium-tert-butoxide were put in a 100-mL three-neck flask and mixed, and the air in the flask was replaced with nitrogen. To this mixture was added 22 mL of mesitylene, and the resulting mixture was degassed by being stirred while the pressure in the flask was reduced. 
     Next, 25 mg (0.040 mmol) of bis(dibenzylideneacetone)palladium(0) (abbreviation: Pd(dba) 2 ) and 35 mg (0.09 mmol) of 2-dicyclohexylphosphino-2′,6′-dimethoxybiphenyl (abbreviation: S-Phos) were added to this mixture. This mixture was stirred at 150° C. for 23 hours under a nitrogen stream. After a predetermined time elapsed, water and toluene were added to this mixture, and an aqueous layer of the obtained filtrate was subjected to extraction with toluene. The obtained extract solution and an organic layer were combined, washed with an aqueous solution of sodium hydrogen carbonate and saturated brine, and dried with magnesium sulfate. The obtained mixture was gravity-filtered, and the filtrate was concentrated to give an oily substance. The oily substance was dissolved in toluene, and this solution was suction-filtered through a stack of Celite and alumina. The obtained filtrate was concentrated to give a brown oily substance. This oily substance was purified by high performance liquid chromatography. Column chromatography was performed using chloroform as a developing solvent (the pressure: 4.5 MPa, the flow rate: 100 mL/min, the holding time: 45 minutes, and the injection amount: 0.9 g/30 mL). The obtained fraction was concentrated and recrystallized with hexane to give 0.66 g of a yellow powder, which was the target substance, in a yield of 18%. 
     By a train sublimation method, 0.66 g of the obtained yellow powdered solid, which was the target substance, was purified. The sublimation purification was carried out at 385° C. under a pressure of 2.6 Pa with a flow rate of an argon gas at 5 mL/min. After the sublimation purification, 0.5 g of a yellow glassy solid of 2mPCCzBPDBq was obtained at a collection rate of 83%. The synthesis scheme of this step is shown in the following scheme (e-1). 
     
       
         
         
             
             
         
       
     
     Analysis results by nuclear magnetic resonance spectroscopy ( 1 H-NMR) of the yellow powdered solid obtained in the above step will be described below.  1 H-NMR charts are shown in  FIGS.  35 A and  35 B .  FIG.  35 B  is a chart in which the range from 7.0 (ppm) to 10 (ppm) on the horizontal axis (δ) in  FIG.  35 A  is enlarged. These results show that 2-{3′-[3-(N-phenyl-9H-carbazol-3-yl)-9H-carbazol-9-yl]biphenyl-3-yl}dibenzo[f,h]quin oxaline (abbreviation: 2mPCCzBPDBq, represented by the structural formula (122)) was obtained in the above step. 
       1 H-NMR (CDCl 3 , 500 MHz): δ (ppm)=7.30-7.37 (m, 2H), 7.41-7.53 (m, 5H), 7.59-7.89 (m, 18H), 8.04 (dd, J=1.7 Hz, 1H), 8.23 (d, J=7.5 Hz, 1H), 8.28 (d, 8.0 Hz, 1H), 8.35 (d, J=8.0 Hz, 1H), 8.48 (dd, J=11.4 Hz, J=1.7 Hz, 2H), 8.66 (d, J=8.1 Hz, 1H), 8.70 (s, 1H), 9.25 (dd, J=6.3 Hz, J=1.1 Hz, 1H), 9.43 (dd, J=7.5 Hz, J=1.7 Hz, 1H), 9.47 (s, 1H). 
     Example 8 
     In this example, a light-emitting element  5  containing the dibenzo[f,h]quinoxaline derivative, 2mPCCzPDBq, which is represented by the structural formula (101) and is one embodiment of the present invention, a comparative light-emitting element  6  containing a comparative material, 2mDBTPDBq-II, and a comparative light-emitting element  7  containing a comparative material, 2-[3-(9H-carbazol-9-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation: 2mCzPDBq), were fabricated. A fabrication method of each light-emitting element is basically the same as that in Example 5 and thus omitted. Chemical formulae of materials used in this example are shown below. 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     &lt;&lt;Fabrication of Light-Emitting Element  5 , Comparative Light-Emitting Element  6 , and Comparative Light-Emitting Element  7 &gt;&gt; 
     Table 6 shows the element structures of the light-emitting element  5 , the comparative light-emitting element  6 , and the comparative light-emitting element  7  fabricated in this example. 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                 TABLE 6 
               
               
                   
               
               
                   
                   
                   
                 Hole- 
                 Light- 
                   
                 Electron- 
                   
               
               
                   
                 First 
                 Hole-injection 
                 transport 
                 emitting 
                 Electron-transport  
                 injection 
                 Second 
               
               
                   
                 Electrode 
                 Layer 
                 Layer 
                 Layer 
                 Layer 
                 Layer 
                 Electrode 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Light- 
                 ITO 
                 DBT3P-II:MoOx 
                 BPAFLP 
                 * 
                 2mPCCzPDBq 
                 Bphen 
                 LiF 
                 Al 
               
               
                 emitting 
                 (110 nm) 
                 (4:2 20 nm) 
                 (20 nm) 
                   
                 (20 nm) 
                 (10 nm) 
                 (1 nm) 
                 (200 nm) 
               
               
                 Element 5 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Comparative 
                 ITO 
                 DBT3P-II:MoOx 
                 BPAFLP 
                 ** 
                 2mDBTPDBq-II 
                 Bphen 
                 LiF 
                 Al 
               
               
                 Light- 
                 (110 nm) 
                 (4:2 20 nm) 
                 (20 nm) 
                   
                 (20 nm) 
                 (10 nm) 
                 (1 nm) 
                 (200 nm) 
               
               
                 emitting 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Element 6 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Comparative 
                 ITO 
                 DBT3P-II:MoOx 
                 BPAFLP 
                 *** 
                 2mCzPDBq 
                 Bphen 
                 LiF 
                 Al 
               
               
                 Light- 
                 (110 nm) 
                 (4:2 20 nm) 
                 (20 nm) 
                   
                 (20 nm) 
                 (10 nm) 
                 (1 nm) 
                 (200 nm) 
               
               
                 emitting 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Element 7 
               
               
                   
               
               
                 *2mPCCzPDBq:PCBBiF:[Ir(tBuppm) 2 (acac)] (0.7:0.3:0.05 (20 nm)\0.8:0.2:0.05 (20 nm)) 
               
               
                 **2mDBTPDBq-II:PCBBiF:[Ir(tBuppm) 2 (acac)] (0.7:0.3:0.05 (20 nm)\0.8:0.2:0.05 (20 nm)) 
               
               
                 ***2mCzPDBq:PCBBiF:[Ir(tBuppm) 2 (acac)] (0.7:0.3:0.05 (20 nm)\0.8:0.2:0.05 (20 nm)) 
               
            
           
         
       
     
     The fabricated light-emitting element  5 , comparative light-emitting element  6 , and comparative light-emitting element  7  were sealed in a glove box under a nitrogen atmosphere so as not to be exposed to the air (specifically, a sealant was applied to surround the elements, UV treatment was performed, and heat treatment was performed at 80° C. for 1 hour). 
     &lt;&lt;Operation Characteristics of Light-emitting Element  5 , Comparative Light-emitting Element  6 , and Comparative Light-emitting Element  7 &gt;&gt; 
     Operation characteristics of the fabricated light-emitting element  5 , comparative light-emitting element  6 , and comparative light-emitting element  7  were measured. Note that the measurements were performed at room temperature (in an atmosphere kept at 25° C.). 
       FIG.  36    shows current density-luminance characteristics,  FIG.  37    shows voltage-luminance characteristics,  FIG.  38    shows luminance-current efficiency characteristics, and  FIG.  39    shows voltage-current characteristics of each light-emitting element. 
     Table 7 shows initial values of main characteristics of the light-emitting element  5 , the comparative light-emitting element  6 , and the comparative light-emitting element  7  at a luminance of approximately 1000 cd/m 2 . 
     
       
         
           
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 7 
               
               
                   
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 External 
               
               
                   
                   
                   
                 Current 
                   
                   
                 Current 
                 Power 
                 Quantum 
               
               
                   
                 Voltage 
                 Current 
                 Density 
                 Chromaticity 
                 Luminance 
                 Efficiency 
                 Efficiency 
                 Efficiency 
               
               
                   
                 (V) 
                 (mA) 
                 (mA/cm 2 ) 
                 (x, y) 
                 (cd/m 2 ) 
                 (cd/A) 
                 (Im/W) 
                 (%) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Light- 
                 2.8 
                 0.047 
                 1.2 
                 (0.42, 0.57) 
                 1100 
                 95 
                 110 
                 25 
               
               
                 emitting 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Element 5 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Comparative 
                 3.0 
                 0.044 
                 1.1 
                 (0.43, 0.56) 
                 990 
                 90 
                 94 
                 24 
               
               
                 Light- 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 emitting 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Element 6 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Comparative 
                 2.9 
                 0.060 
                 1.5 
                 (0.41, 0.58) 
                 940 
                 63 
                 68 
                 16 
               
               
                 Light- 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 emitting 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Element 7 
               
               
                   
               
            
           
         
       
     
       FIG.  40    shows the emission spectra of the light-emitting element  5 , the comparative light-emitting element  6 , and the comparative light-emitting element  7 , through which a current flows at a current density of 2.5 mA/cm 2 . As shown in  FIG.  40   , the emission spectra of the light-emitting element  5 , the comparative light-emitting element  6 , and the comparative light-emitting element  7  each have a peak at approximately 544 nm, which is attributed to [Ir(tBuppm) 2 (acac)]. 
       FIG.  41 A  shows results of reliability tests on the light-emitting element  5 , the comparative light-emitting element  6 , and the comparative light-emitting element  7 . In  FIG.  41 A , the vertical axis represents normalized luminance (%) on the assumption that the initial luminance is 100%, and the horizontal axis represents driving time (h) of the elements. Note that in the reliability tests, the light-emitting element  5 , the comparative light-emitting element  6 , and the comparative light-emitting element  7  were driven under the conditions where the initial luminance was set to be 5000 cd/m 2  and the current density was constant. 
     The results show that the light-emitting element  5  fabricated using 2mPCCzPDBq, which is one embodiment of the present invention, has a higher reliability and a longer lifetime than the comparative light-emitting element  6  fabricated using 2mDBTPDBq-II and the comparative light-emitting element  7  fabricated using 2mCzPDBq. 
       FIG.  41 B  shows measurement results of the amount of change in voltage at the reliability tests. The vertical axis represents the amount of change in voltage (V), and the horizontal axis represents the driving time (h) of the elements. These results show that the amount of increase in voltage in the light-emitting element  5  which was driven at constant current is smaller than those in the comparative light-emitting elements  6  and  7 . For example, after the light-emitting elements were driven for approximately 1000 hours, the amount of increase in voltage in the comparative light-emitting element  6  is approximately 0.31 V and that in the comparative light-emitting element  7  is approximately 0.50 V, whereas that in the light-emitting element  5  is approximately 0.04 V. That is, the amount of increase in voltage in the light-emitting element  5  is much smaller than those in the comparative light-emitting elements  6  and  7 , which indicates a significant effect of one embodiment of the present invention. 
     Note that the combination of PCBBiF and each of 2mPCCzPDBq, 2mDBTPDBq-II, and 2mCzPDBq forms an exciplex (because the mixed film containing PCBBiF and any of these dibenzoquinoxaline compounds exhibits yellow-green light emission having a longer wavelength than the film containing only PCBBiF or the film containing only any of these dibenzoquinoxaline compounds). Furthermore, the HOMO levels of 2mPCCzPDBq, 2mDBTPDBq-II, 2mCzPDBq, and PCBBiF are −5.63 eV, −6.22 eV, −5.91 eV, and −5.36 eV, respectively. The HOMO levels were obtained through a cyclic voltammetry (CV) measurement. 
     By using the HOMO levels obtained as described above, ΔE HOMO  in the light-emitting layer of each light-emitting element was calculated. Table 8 shows the results. 
     
       
         
           
               
               
             
               
                   
                 TABLE 8 
               
               
                   
                   
               
               
                   
                 Δ E HOMO  (eV) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 Light-emitting Element 5 
                 0.27 
               
               
                   
                 Comparative Light-emitting Element 6 
                 0.86 
               
               
                   
                 Comparative Light-emitting Element 7 
                 0.55 
               
               
                   
                   
               
            
           
         
       
     
     According to the results, it is important that ΔE HOMO  is less than or equal to 0.4 eV, preferably less than or equal to 0.3 eV. 
     Furthermore, the HOMO level of BPAFLP used for the hole-transport layer is −5.51 eV. Therefore, it is found that the HOMO level of the third organic compound used for the hole-transport layer is lower than the HOMO level of PCBBiF that is the second organic compound and is located between the HOMO level of PCBBiF that is the second organic compound and the HOMO level of the first organic compound (2mPCCzPDBq). This is important because holes are injected not only into the second organic compound but also partly into the first organic compound. 
     Example 9 
     In this example, a light-emitting element  8  containing the dibenzo[f,h]quinoxaline derivative, 2mPCCzBPDBq, which is one embodiment of the present invention, was fabricated. A fabrication method of the light-emitting element  8  is basically the same as that in Example 5 and thus omitted. Chemical formulae of materials used in this example are shown below. 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     &lt;&lt;Fabrication of Light-Emitting Element  8 &gt;&gt; 
     Table 9 shows the element structure of the light-emitting element  8  fabricated in this example. 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                 TABLE 9 
               
               
                   
               
               
                   
                   
                   
                 Hole- 
                 Light- 
                   
                 Electron- 
                   
               
               
                   
                 First 
                 Hole-injection 
                 transport 
                 emitting 
                 Elecron-transport  
                 injection 
                 Second 
               
               
                   
                 Electrode 
                 Layer 
                 Layer 
                 Layer 
                 Layer 
                 Layer 
                 Electrode 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Light- 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 emitting 
                 ITO 
                 DBT3P-II:MoOx 
                 BPAFLP 
                 * 
                 2mPCCzBPDBq 
                 Bphen 
                 LiF 
                 Al 
               
               
                 Element 8 
                 (110 nm) 
                 (4:2 60 nm) 
                 (20 nm) 
                   
                 (20 nm) 
                 (10 nm) 
                 (1 nm) 
                 (200 nm) 
               
               
                   
               
               
                 *2mPCCzBPDBq:PCBBiF:[Ir(dppm)2(acac)] (0.7:0 3:0.05 (20 nm)\0.8:0.2:0.05 (20 nm)) 
               
            
           
         
       
     
     The fabricated light-emitting element  8  was sealed in a glove box under a nitrogen atmosphere so as not to be exposed to the air (specifically, a sealant was applied to surround the element, UV treatment was performed, and heat treatment was performed at 80° C. for 1 hour). 
     &lt;&lt;Operation Characteristics of Light-emitting Element  8 &gt;&gt; 
     Operation characteristics of the fabricated light-emitting element  8  were measured. Note that the measurements were performed at room temperature (in an atmosphere kept at 25° C.). 
       FIG.  42    shows current density-luminance characteristics,  FIG.  43    shows voltage-luminance characteristics,  FIG.  44    shows luminance-current efficiency characteristics, and  FIG.  45    shows voltage-current characteristics of the light-emitting element  8 . 
     Table 10 shows initial values of main characteristics of the light-emitting element  8  at a luminance of approximately 1000 cd/m 2 . 
     
       
         
           
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 10 
               
               
                   
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 External 
               
               
                   
                   
                   
                 Current 
                   
                   
                 Current 
                 Power 
                 Quantum 
               
               
                   
                 Voltage 
                 Current 
                 Density 
                 Chromaticity 
                 Luminance 
                 Efficiency 
                 Efficiency 
                 Efficiency 
               
               
                   
                 (V) 
                 (mA) 
                 (mA/ cm 2 ) 
                 (x, y) 
                 (cd/m 2 ) 
                 (cd/A) 
                 (Im/W) 
                 (%) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Light- 
                 3.0 
                 0.046 
                 1.2 
                 (0.56, 0.44) 
                 900 
                 78 
                 82 
                 30 
               
               
                 emitting 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Element 8 
               
               
                   
               
            
           
         
       
     
       FIG.  46    shows the emission spectrum of the light-emitting element  8 , through which a current flows at a current density of 2.5 mA/cm 2 . As shown in  FIG.  46   , the emission spectrum of the light-emitting element  8  has a peak at approximately 584 nm, which is attributed to [Ir(dppm) 2 (acac)]. 
     Example 10 
     In this example, a light-emitting element which is one embodiment of the present invention was fabricated and subjected to a preservation test. 
     In this example, a light-emitting element  1 A, a light-emitting element  2 A, a comparative light-emitting element  3 A, a light-emitting element  4 A, and a light-emitting element  8 A were fabricated. The structure and fabrication method of the light-emitting element  1 A are the same as those of the light-emitting element  1  in Example 5. The structures and fabrication methods of the light-emitting element  2 A, the comparative light-emitting element  3 A, the light-emitting element  4 A, and the light-emitting element  8 A are the same as those of the light-emitting element  2  in Example 5, the comparative light-emitting element  3  in Example 5, the light-emitting element  4  in Example 6, and the light-emitting element  8  in Example 9, respectively. 
     In preservation tests of this example, the light-emitting elements were each preserved in a thermostatic oven maintained at 100° C. for a predetermined time, and the operation characteristics were measured. Note that the operation characteristics were measured at room temperature (in an atmosphere kept at 25° C.) after the light-emitting elements were taken out of the thermostatic oven. 
       FIG.  47    shows voltage-current characteristics and  FIG.  48    shows luminance-external quantum efficiency characteristics of the light-emitting element  1 A after preservation at 100° C. for a predetermined time. In  FIG.  47   , the horizontal axis represents voltage (V), and the vertical axis represents current (mA). In  FIG.  48   , the horizontal axis represents luminance (cd/m 2 ), and the vertical axis represents external quantum efficiency (%). 
       FIG.  49    shows voltage-current characteristics and  FIG.  50    shows luminance-external quantum efficiency characteristics of the light-emitting element  2 A after preservation at 100° C. for a predetermined time. In  FIG.  49   , the horizontal axis represents voltage (V), and the vertical axis represents current (mA). In  FIG.  50   , the horizontal axis represents luminance (cd/m 2 ), and the vertical axis represents external quantum efficiency (%). 
       FIG.  51    shows voltage-current characteristics and  FIG.  52    shows luminance-external quantum efficiency characteristics of the comparative light-emitting element  3 A after preservation at 100° C. for a predetermined time. In  FIG.  51   , the horizontal axis represents voltage (V), and the vertical axis represents current (mA). In  FIG.  52   , the horizontal axis represents luminance (cd/m 2 ), and the vertical axis represents external quantum efficiency (%). Since light emission could not be observed in the light-emitting element preserved over 20 hours,  FIG.  52    does not show data on luminance-external quantum efficiency characteristics after preservation over 20 hours. 
       FIG.  53    shows voltage-current characteristics and  FIG.  54    shows luminance-external quantum efficiency characteristics of the light-emitting element  4 A after preservation at 100° C. for a predetermined time. In  FIG.  53   , the horizontal axis represents voltage (V), and the vertical axis represents current (mA). In  FIG.  54   , the horizontal axis represents luminance (cd/m 2 ), and the vertical axis represents external quantum efficiency (%). 
       FIG.  55    shows voltage-current characteristics and  FIG.  56    shows luminance-external quantum efficiency characteristics of the light-emitting element  8 A after preservation at 100° C. for a predetermined time. In  FIG.  55   , the horizontal axis represents voltage (V), and the vertical axis represents current (mA). In  FIG.  56   , the horizontal axis represents luminance (cd/m 2 ), and the vertical axis represents external quantum efficiency (%). 
       FIGS.  47  to  50    and  FIGS.  53  to  56    show that the light-emitting element  1 A, the light-emitting element  2 A, the light-emitting element  4 A, and the light-emitting element  8 A suffered only a small change in voltage-current characteristics and luminance-external quantum efficiency characteristics even after being preserved at 100° C. for 500 hours and that the element characteristics hardly deteriorated through preservation at high temperature. In contrast,  FIG.  51    and  FIG.  52    show that the comparative light-emitting element  3 A suffered a considerable change in voltage-current characteristics and luminance-external quantum efficiency characteristics after being preserved at 100° C. and that the element characteristics deteriorated through preservation at high temperature. As shown in  FIG.  51   , in the comparative light-emitting element  3 A, an initial insulating property is not maintained after 100 hours and leakage current is generated, and  FIG.  52    indicates a defect, that is, no light emission of the element. The above results show that heat resistance of a light-emitting element in the case of preservation at high temperature is significantly increased by using a compound which is one embodiment of the present invention. 
     This application is based on Japanese Patent Application serial no. 2014-151493 filed with Japan Patent Office on Jul. 25, 2014, the entire contents of which are hereby incorporated by reference.