Patent Publication Number: US-2019181357-A1

Title: Organometallic Iridium Complex, Light-Emitting Element, Light-Emitting Device, Electronic Device, and Lighting Device

Description:
This application is a continuation of copending U.S. application Ser. No. 14/863,766, filed on Sep. 24, 2015 which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     One embodiment of the present invention relates to an organometallic iridium complex, particularly, to an organometallic iridium complex that is capable of converting triplet excitation energy into light emission. In addition, one embodiment of the present invention relates to a light-emitting element, a light-emitting device, an electronic device, and a lighting device each including the organometallic iridium complex. Note that one embodiment of the present invention is not limited to the above technical field. The technical field of one embodiment of the invention disclosed in this specification and the like relates to an object, a method, or a manufacturing method. In addition, one embodiment of the present invention relates to a process, a machine, manufacture, or a composition of matter. Specifically, examples of the technical field of one embodiment of the present invention disclosed in this specification include a semiconductor device, a display device, a liquid crystal display device, a power storage device, a memory device, an imaging device, a method for driving any of them, and a method for manufacturing any of them. 
     2. Description of the Related Art 
     An organic EL element (light-emitting element) including an EL layer containing a light-emitting substance between a pair of electrodes has a light emission mechanism that is of a carrier injection type: a voltage is applied between the electrodes, electrons and holes injected from the electrodes recombine to put the light-emitting substance into an excited state, and then light is emitted in returning from the excited state to the ground state. The excited state can be a singlet excited state (S*) and a triplet excited state (T*). Light emission from a singlet excited state is referred to as fluorescence, and light emission from a triplet excited state is referred to as phosphorescence. The statistical generation ratio thereof in the light-emitting element is considered to be S*:T*=1:3. 
     Among the above light-emitting substances, a compound capable of converting singlet excitation energy into light emission is called a fluorescent compound (fluorescent material), and a compound capable of converting triplet excitation energy into light emission is called a phosphorescent compound (phosphorescent material). 
     Accordingly, the internal quantum efficiency (the ratio of the number of generated photons to the number of injected carriers) of a light-emitting element including a fluorescent material is thought to have a theoretical limit of 25%, on the basis of S*:T*=1:3, while the internal quantum efficiency of a light-emitting element including a phosphorescent material is thought to have a theoretical limit of 75%. 
     In other words, a light-emitting element including a phosphorescent material has higher efficiency than a light-emitting element including a fluorescent material. Thus, various kinds of phosphorescent materials have been actively developed in recent years. An organometallic complex that contains iridium or the like as a central metal is particularly attracting attention because of its high phosphorescence quantum yield (for example, see Patent Document 1 and Patent Document 2). 
     REFERENCE 
     Patent Document 
     
         
         [Patent Document 1] Japanese Published Patent Application No. 2007-137872 
         [Patent Document 2] Japanese Published Patent Application No. 2008-069221 
       
    
     SUMMARY OF THE INVENTION 
     Although phosphorescent materials exhibiting various emission colors have been actively developed as disclosed in Patent Documents 1 and 2, development of novel materials with higher efficiency has been desired. 
     In view of the above, one embodiment of the present invention provides an organometallic iridium complex having high emission efficiency and high heat resistance and emitting yellow light, as a novel substance. In addition, one embodiment of the present invention provides a light-emitting element with high current efficiency. In addition, one embodiment of the present invention provides a light-emitting device, an electronic device, or a lighting device with low power consumption. 
     One embodiment of the present invention is an organometallic iridium complex including iridium and a ligand. The ligand includes a 5H-indeno[1,2-d]pyrimidine skeleton and an aryl group bonded to the 4-position of the 5H-indeno[1,2-d]pyrimidine skeleton. The 3-position of the 5H-indeno[1,2-d]pyrimidine skeleton and the aryl group are bonded to iridium. 
     Another embodiment of the present invention is an organometallic iridium complex including a first ligand and a second ligand that are bonded to iridium. The first ligand includes a 5H-indeno[1,2-d]pyrimidine skeleton and an aryl group bonded to the 4-position of the 5H-indeno[1,2-d]pyrimidine skeleton. The second ligand is a monoanionic bidentate chelate ligand having a β-diketone structure, a monoanionic bidentate chelate ligand including a carboxyl group, a monoanionic bidentate chelate ligand including a phenolic hydroxyl group, or a monoanionic bidentate chelate ligand in which two coordinating elements are both nitrogen. The 3-position of the 5H-indeno[1,2-d]pyrimidine skeleton and the aryl group are bonded to the iridium. 
     In each of the above structures, the aryl group is a substituted or unsubstituted aryl group having 6 to 13 carbon atoms. 
     One embodiment of the present invention is an organometallic iridium complex including a structure represented by General Formula (G1) below. 
     
       
         
         
             
             
         
       
     
     In General Formula (G1), Ar represents a substituted or unsubstituted aryl group having 6 to 13 carbon atoms, and each of R 1  to R 7  independently represents hydrogen or a substituted or unsubstituted alkyl group having 1 to 6 carbon atoms. 
     Another embodiment of the present invention is an organometallic iridium complex represented by General Formula (G2) below. 
     
       
         
         
             
             
         
       
     
     In General Formula (G2), Ar represents a substituted or unsubstituted aryl group having 6 to 13 carbon atoms, each of R 1  to R 7  independently represents hydrogen or a substituted or unsubstituted alkyl group having 1 to 6 carbon atoms, and L represents a monoanionic ligand. 
     In General Formula (G2), the monoanionic ligand is preferably a monoanionic bidentate chelate ligand having a β-diketone structure, a monoanionic bidentate chelate ligand having a carboxyl group, a monoanionic bidentate chelate ligand having a phenolic hydroxyl group, or a monoanionic bidentate chelate ligand in which two coordinating elements are both nitrogen. A monoanionic bidentate chelate ligand having a β-diketone structure is particularly preferable because the β-diketone structure allows the organometallic complex to have higher solubility in an organic solvent and to be easily purified. The β-diketone structure is preferably included to obtain an organometallic complex with high emission efficiency. Furthermore, the β-diketone structure brings advantages such as a higher sublimation property and excellent evaporativity. 
     The monoanionic ligand is preferably represented by any one of General Formulae (L1) to (L7). These ligands have high coordinative ability and can be obtained at low price, and are thus useful. 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     Note that in the formulae, each of R 71  to R 109  independently represents hydrogen, a substituted or unsubstituted alkyl group having 1 to 6 carbon atoms, a halogen group, a vinyl group, a substituted or unsubstituted haloalkyl group having 1 to 6 carbon atoms, a substituted or unsubstituted alkoxy group having 1 to 6 carbon atoms, or a substituted or unsubstituted alkylthio group having 1 to 6 carbon atoms. Each of A 1  to A 3  independently represents nitrogen, sp 2  hybridized carbon bonded to hydrogen, or sp 2  hybridized carbon with a substituent. The substituent is an alkyl group having 1 to 6 carbon atoms, a halogen group, a haloalkyl group having 1 to 6 carbon atoms, or a phenyl group. 
     Another embodiment of the present invention is an organometallic iridium complex represented by General Formula (G3) below. 
     
       
         
         
             
             
         
       
     
     In General Formula (G3), Ar represents a substituted or unsubstituted aryl group having 6 to 13 carbon atoms, and each of R 1  to R 9  independently represents hydrogen, a substituted or unsubstituted alkyl group having 1 to 6 carbon atoms, or a substituted or unsubstituted phenyl group. 
     Another embodiment of the present invention is an organometallic iridium complex represented by General Formula (G4) below. 
     
       
         
         
             
             
         
       
     
     In General Formula (G4), Ar represents a substituted or unsubstituted aryl group having 6 to 13 carbon atoms, and each of R 1  to R 7  independently represents hydrogen or a substituted or unsubstituted alkyl group having 1 to 6 carbon atoms. 
     Another embodiment of the present invention is an organometallic iridium complex represented by Structural Formula (100) below. 
     
       
         
         
             
             
         
       
     
     The organometallic iridium complex of one embodiment of the present invention is very effective for the following reason: the organometallic iridium complex can emit phosphorescence, that is, it can provide luminescence from a triplet excited state and can exhibit emission, and therefore higher efficiency is possible when the organometallic complex is applied to a light-emitting element. Thus, one embodiment of the present invention also includes a light-emitting element in which the organometallic iridium complex of one embodiment of the present invention is used. 
     The present invention includes, in its scope, not only a light-emitting device including the light-emitting element but also a lighting device including the light-emitting device. The light-emitting device in this specification refers to an image display device and a light source (e.g., a lighting device). In addition, the light-emitting device includes, in its category, all of a module in which a connector such as a flexible printed circuit (FPC) or a tape carrier package (TCP) is connected to a light-emitting device, 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. 
     One embodiment of the present invention can provide an organometallic iridium complex having high emission efficiency and high heat resistance and emitting yellow light (emission wavelength: approximately 555 nm), as a novel substance. Note that the use of the novel organometallic iridium complex enables a light-emitting element that has high current efficiency to be provided. Alternatively, it is possible to provide a light-emitting device, an electronic device, or a lighting device with low power consumption. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  illustrate structures of light-emitting elements. 
         FIGS. 2A and 2B  illustrate structures of light-emitting elements. 
         FIGS. 3A to 3C  illustrate light-emitting devices. 
         FIGS. 4A to 4D, 4D ′- 1 , and  4 D′- 2  illustrate electronic devices. 
         FIGS. 5A to 5C  illustrate an electronic device. 
         FIGS. 6A to 6D  illustrate lighting devices. 
         FIG. 7  illustrates lighting devices. 
         FIGS. 8A and 8B  illustrate an example of a touch panel. 
         FIGS. 9A and 9B  illustrate an example of a touch panel. 
         FIGS. 10A and 10B  illustrate an example of a touch panel. 
         FIGS. 11A and 11B  are a block diagram and a timing chart of a touch sensor. 
         FIG. 12  is a circuit diagram of a touch sensor. 
         FIG. 13  is a  1 H-NMR chart of an organometallic iridium complex represented by Structural Formula (100). 
         FIG. 14  shows an ultraviolet-visible absorption spectrum and an emission spectrum of an organometallic iridium complex represented by Structural Formula (100). 
         FIG. 15  illustrates a light-emitting element. 
         FIG. 16  shows current density-luminance characteristics of Light-emitting Element 1. 
         FIG. 17  shows voltage-luminance characteristics of Light-emitting Element 1. 
         FIG. 18  shows luminance-current efficiency characteristics of Light-emitting Element 1. 
         FIG. 19  shows voltage-current characteristics of Light-emitting Element 1. 
         FIG. 20  shows an emission spectrum of Light-emitting Element 1. 
         FIG. 21  shows LC-MS measurement results of an organometallic iridium complex represented by Structural Formula (100). 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. Note that the present invention is not limited to the following description, and modes and details thereof can be variously modified without departing from the spirit and scope of the present invention. Therefore, the present invention should not be construed as being limited to the description in the following embodiments. 
     Note that the terms “film” and “layer” can be interchanged with each other according to circumstances. For example, in some cases, the term “conductive film” can be used instead of the term “conductive layer,” and the term “insulating layer” can be used instead of the term “insulating film.” 
     Embodiment 1 
     In this embodiment, an organometallic iridium complex of one embodiment of the present invention is described. 
     An organometallic iridium complex of one embodiment of the present invention includes iridium and a ligand. The ligand includes a 5H-indeno[1,2-d]pyrimidine skeleton and an aryl group bonded to the 4-position of the 5H-indeno[1,2-d]pyrimidine skeleton. The 3-position of the 5H-indeno[1,2-d]pyrimidine skeleton and the aryl group are bonded to iridium. One mode of an organometallic iridium complex of one embodiment of the present invention described in this embodiment includes a structure represented by General Formula (G1) below. 
     
       
         
         
             
             
         
       
     
     In General Formula (G1), Ar represents a substituted or unsubstituted aryl group having 6 to 13 carbon atoms, and each of R 1  to R 7  independently represents hydrogen or a substituted or unsubstituted alkyl group having 1 to 6 carbon atoms. 
     One mode of an organometallic iridium complex of one embodiment of the present invention described in this embodiment is represented by General Formula (G2) below. 
     
       
         
         
             
             
         
       
     
     In General Formula (G2), Ar represents a substituted or unsubstituted aryl group having 6 to 13 carbon atoms, each of R 1  to R 7  independently represents hydrogen or a substituted or unsubstituted alkyl group having 1 to 6 carbon atoms, and L represents a monoanionic ligand. 
     The monoanionic ligand in General Formula (G2) is preferably a monoanionic bidentate chelate ligand having a β-diketone structure, a monoanionic bidentate chelate ligand having a carboxyl group, a monoanionic bidentate chelate ligand having a phenolic hydroxyl group, or a monoanionic bidentate chelate ligand in which two coordinating elements are both nitrogen. A monoanionic bidentate chelate ligand having a β-diketone structure is particularly preferable. 
     Specifically, the monoanionic ligand is preferably represented by any one of General Formulae (L1) to (L7). 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     Note that in the formulae, each of R 71  to R 109  independently represents hydrogen, a substituted or unsubstituted alkyl group having 1 to 6 carbon atoms, a halogen group, a vinyl group, a substituted or unsubstituted haloalkyl group having 1 to 6 carbon atoms, a substituted or unsubstituted alkoxy group having 1 to 6 carbon atoms, or a substituted or unsubstituted alkylthio group having 1 to 6 carbon atoms. Each of A 1  to A 3  independently represents nitrogen, sp 2  hybridized carbon bonded to hydrogen, or sp 2  hybridized carbon with a substituent. The substituent is an alkyl group having 1 to 6 carbon atoms, a halogen group, a haloalkyl group having 1 to 6 carbon atoms, or a phenyl group. 
     Note that an organometallic iridium complex of one embodiment of the present invention has a 5H-indeno[1,2-d]pyrimidine skeleton in which the indeno group and the pyrimidine ring are fused. Such a structure in which the indeno group and the pyrimidine ring are fused can improve the heat resistance of the organometallic iridium complex, leading to improved reliability of a light-emitting element using the organometallic iridium complex. Because the pyrimidine ring enhances emission efficiency, the organometallic iridium complex of one embodiment of the present invention offers a yellow-light-emitting material with high emission efficiency. 
     Another embodiment of the present invention is an organometallic iridium complex represented by General Formula (G4) below. 
     
       
         
         
             
             
         
       
     
     In General Formula (G4), Ar represents a substituted or unsubstituted aryl group having 6 to 13 carbon atoms, and each of R 1  to R 7  independently represents hydrogen or a substituted or unsubstituted alkyl group having 1 to 6 carbon atoms. 
     Note that in the case where the above substituted or unsubstituted alkyl group having 1 to 6 carbon atoms or the above substituted or unsubstituted aryl group having 6 to 13 carbon atoms has a substituent, the substituent can be an alkyl group having 1 to 6 carbon atoms (e.g., 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, or a hexyl group) or an aryl group having 6 to 12 carbon atoms (e.g., a phenyl group or a biphenyl group). In General Formulae (G1), (G2), and (G4) above, specific examples of the alkyl group having 1 to 6 carbon atoms which is represented by any of R 1  to R 7  include a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, a sec-butyl group, an isobutyl group, a tert-butyl group, a pentyl group, an isopentyl group, a sec-pentyl group, a tert-pentyl group, a neopentyl group, a hexyl group, an isohexyl group, a sec-hexyl group, a tert-hexyl group, a neohexyl group, a 3-methylpentyl group, a 2-methylpentyl group, a 2-ethylbutyl group, a 1,2-dimethylbutyl group, and a 2,3-dimethylbutyl group. Specific examples of the aryl group having 6 to 13 carbon atoms which is represented by Ar include a phenyl group, a biphenyl group, a fluorenyl group, and a naphthyl group. Note that the above substituents may be bonded to each other and form a ring. In such a case, for example, a spirofluorene skeleton is formed in such a manner that carbon at the 9-position of a fluorenyl group has two phenyl groups as substituents and these phenyl groups are bonded to each other. 
     Next, specific structural formulae of the above-described organometallic iridium complexes, each of which is one embodiment of the present invention, are shown (Structural Formulae (100) to (120) below). Note that the present invention is not limited thereto. 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     Note that the organometallic iridium complexes represented by Structural Formulae (100) to (120) above are novel substances capable of emitting phosphorescence. Note that there can be geometrical isomers and stereoisomers of these substances, as characterized by the type of the ligand. The organometallic iridium complex of one embodiment of the present invention includes all of these isomers. 
     Next, an example of a method for synthesizing the organometallic iridium complex represented by General Formula (G2) above is described. 
     Method for synthesizing 5H-indeno[1,2-d]pyrimidine Derivative Represented by General Formula (G0) 
     First, an example of a method for synthesizing a 5H-indeno[1,2-d]pyrimidine derivative represented by General Formula (G0) below is described. 
     
       
         
         
             
             
         
       
     
     In General Formula (G0), Ar represents a substituted or unsubstituted aryl group having 6 to 13 carbon atoms, and each of R 1  to R 7  independently represents hydrogen or a substituted or unsubstituted alkyl group having 1 to 6 carbon atoms. 
     Synthesis Scheme (A) of the 5H-indeno[1,2-d]pyrimidine derivative represented by General Formula (G0) is shown below. In Synthesis Scheme (A), X represents a halogen. 
     
       
         
         
             
             
         
       
     
     Under Synthesis Scheme (A) above, the 5H-indeno[1,2-d]pyrimidine derivative represented by General Formula (G0) can be synthesized by causing a reaction between aryl lithium or an aryl Grignard reagent (A1) and a 5H-indeno[1,2-d]pyrimidine compound (A2). 
     Since a wide variety of compounds (A1) and (A2) are commercially available or their synthesis is feasible, a great variety of the 5H-indeno[1,2-d]pyrimidine derivatives represented by General Formula (G0) can be synthesized. Thus, a feature of the organometallic complex of one embodiment of the present invention is the abundance of ligand variations. 
     &lt;&lt;Method for Synthesizing Organometallic Iridium Complex of One Embodiment of the Present Invention Represented by General Formula (G2)&gt;&gt; 
     Next, a method for synthesizing the organometallic iridium complex of one embodiment of the present invention represented by General Formula (G2), which is formed using the 5H-indeno[1,2-d]pyrimidine derivative represented by General Formula (G0), is described. 
     
       
         
         
             
             
         
       
     
     In General Formula (G2), Ar represents a substituted or unsubstituted aryl group having 6 to 13 carbon atoms, each of R 1  to R 7  independently represents hydrogen or a substituted or unsubstituted alkyl group having 1 to 6 carbon atoms, and L represents a monoanionic ligand. 
     Synthesis Scheme (B-1) of the organometallic iridium complex represented by General Formula (G2) is shown below. 
     
       
         
         
             
             
         
       
     
     In Synthesis Scheme (B-1), X represents a halogen, Ar represents a substituted or unsubstituted aryl group having 6 to 13 carbon atoms, and each of R 1  to R 7  independently represents hydrogen or a substituted or unsubstituted alkyl group having 1 to 6 carbon atoms. 
     As shown in Synthesis Scheme (B-1) above, the 5H-indeno[1,2-d]pyrimidine derivative represented by General Formula (G0) and a metal compound which contains a halogen (e.g., iridium chloride, iridium bromide, or iridium iodide) are heated in an inert gas atmosphere by using no solvent, an alcohol-based solvent (e.g., glycerol, ethylene glycol, 2-methoxyethanol, or 2-ethoxyethanol) alone, or a mixed solvent of water and one or more of the alcohol-based solvents, whereby a dinuclear complex (P), which is one type of an organometallic complex including a halogen-bridged structure and is a novel substance, can be obtained. 
     There is no particular limitation on a heating means under Synthesis Scheme (B-1), and an oil bath, a sand bath, or an aluminum block may be used. Alternatively, microwaves can be used as a heating means. 
     Furthermore, as shown in Synthesis Scheme (B-2) below, the dinuclear complex (P) obtained in Synthesis Scheme (B-1) above is reacted with HL which is a material for a monoanionic ligand in an inert gas atmosphere, whereby a proton of HL is separated and L coordinates to the central metal. Thus, the organometallic iridium complex of one embodiment of the present invention represented by General Formula (G2) can be obtained. 
     
       
         
         
             
             
         
       
     
     In Synthesis Scheme (B-2), L represents a monoanionic ligand, X represents a halogen, Ar represents a substituted or unsubstituted aryl group having 6 to 13 carbon atoms, and each of R 1  to R 7  independently represents hydrogen or a substituted or unsubstituted alkyl group having 1 to 6 carbon atoms. 
     There is no particular limitation on a heating means under Synthesis Scheme (B-2) either, and an oil bath, a sand bath, or an aluminum block may be used. Alternatively, microwaves can be used as a heating means. 
     The above is the description of the example of a method for synthesizing an organometallic iridium complex of one embodiment of the present invention; however, the present invention is not limited thereto and any other synthesis method may be employed. 
     The above-described organometallic iridium complex of one embodiment of the present invention can emit phosphorescence and thus can be used as a light-emitting material or a light-emitting substance of a light-emitting element. 
     With the use of the organometallic iridium complex of one embodiment of the present invention, a light-emitting element, a light-emitting device, an electronic device, or a lighting device with high emission efficiency can be obtained. Alternatively, it is possible to obtain a light-emitting element, a light-emitting device, an electronic device, or a lighting device with low power consumption. 
     In Embodiment 1, one embodiment of the present invention has been described. Note that one embodiment of the present invention is not limited thereto. 
     The structure described in this embodiment can be combined as appropriate with any of the structures described in other embodiments. 
     Embodiment 2 
     In this embodiment, a light-emitting element in which the organometallic iridium complex described in Embodiment 1 as one embodiment of the present invention is used for a light-emitting layer is described with reference to  FIGS. 1A and 1B . 
     In the light-emitting element described in this embodiment, an EL layer  102  including a light-emitting layer  113  is interposed between a pair of electrodes (a first electrode (anode)  101  and a second electrode (cathode)  103 ), and the EL layer  102  includes a hole-injection layer  111 , a hole-transport layer  112 , an electron-transport layer  114 , an electron-injection layer  115 , a charge-generation layer  116 , and the like in addition to the light-emitting layer  113 . 
     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 such as the organometallic iridium complex that is contained in the light-emitting layer emits light. 
     The hole-injection layer  111  included in the EL layer  102  contains a substance having a high hole-transport property and an acceptor substance. When electrons are extracted from the substance having a high hole-transport property with the acceptor substance, holes are generated. Thus, holes are injected from the hole-injection layer  111  into the light-emitting layer  113  through the hole-transport layer  112 . 
     The charge-generation layer  116  is a layer containing a substance having a high hole-transport property and an acceptor substance. Electrons are extracted from the substance having a high hole-transport property with the acceptor substance, and the extracted electrons are injected from the electron-injection layer  115  having an electron-injection property into the light-emitting layer  113  through the electron-transport layer  114 . 
     A specific example in which the light-emitting element described in this embodiment is fabricated is described below. 
     For the first electrode (anode)  101  and the second electrode (cathode)  103 , a metal, an alloy, an electrically conductive compound, a mixture thereof, and the like can be used. Specific examples are 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). In addition, an element belonging to Group 1 or Group 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 or 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)  101  and the second electrode (cathode)  103  can be formed by, for example, a sputtering method or an evaporation method (including a vacuum evaporation method). 
     Specific examples of the substance having a high hole-transport property, which is used for the hole-injection layer  111 , the hole-transport layer  112 , and the charge-generation layer  116 , 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). Other examples include carbazole derivatives such as 4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP), 1,3,5-tris[4-(N-carbazolyl)phenyl]benzene (abbreviation: TCPB), and 9-[4-(10-phenyl-9-anthracenyl)phenyl]-9H-carbazole (abbreviation: CzPA). The substances listed here are mainly ones that have a hole mobility of 1×10 −6  cm 2 /Vs or higher. Note that any substance other than the substances listed here may be used as long as the hole-transport property is higher than the electron-transport property. 
     A high molecular compound such as poly(N-vinylcarbazole) (abbreviation: PVK), poly(4-vinyltriphenylamine) (abbreviation: PVTPA), poly[N-(4-{N′-4-[4-(4-diphenylamino)phenyl]phenyl-N′-phenylamino}phenyl)methacrylamide] (abbreviation: PTPDMA), or poly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine] (abbreviation: Poly-TPD) can also be used. 
     Examples of the acceptor substance that is used for the hole-injection layer  111  and the charge-generation layer  116  include oxides of metals belonging to Groups 4 to 8 of the periodic table. Specifically, molybdenum oxide is particularly preferable. 
     The light-emitting layer  113  contains a light-emitting substance. Note that the organometallic iridium complex described in Embodiment 1 can be used as the light-emitting substance, and the light-emitting layer  113  may contain, as a host material, a substance having higher triplet excitation energy than the organometallic iridium complex (guest material). In addition to the light-emitting substance, two kinds of organic compounds that can form an exciplex (also called an excited complex) at the time of recombination of carriers (electrons and holes) in the light-emitting layer may be contained. 
     Examples of the organic compounds that can be used as the above two kinds of organic compounds include compounds having an arylamine skeleton, such as 2,3-bis(4-diphenylaminophenyl)quinoxaline (abbreviation: TPAQn) and NPB, carbazole derivatives such as CBP and 4,4′,4″-tris(carbazol-9-yl)triphenylamine (abbreviation: TCTA), and metal complexes such as bis[2-(2-hydroxyphenyl)pyridinato]zinc (abbreviation: Znpp 2 ), bis[2-(2-hydroxyphenyl)benzoxazolato]zinc (abbreviation: Zn(BOX) 2 ), bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum (abbreviation: BAlq), and tris(8-quinolinolato)aluminum (abbreviation: Alq 3 ). Alternatively, a high molecular compound such as PVK can be used. 
     Note that in the case where the light-emitting layer  113  contains the above-described organometallic iridium complex (guest material) and the host material, phosphorescence with high emission efficiency can be obtained from the light-emitting layer  113 . 
     In the light-emitting element, the light-emitting layer  113  does not necessarily have the single-layer structure shown in  FIG. 1A  and may have a stacked-layer structure including two or more layers as shown in  FIG. 1B . In that case, each layer in the stacked-layer structure emits light. For example, fluorescence is obtained from a first light-emitting layer  113 ( a   1 ), and phosphorescence is obtained from a second light-emitting layer  113 ( a   2 ) stacked over the first light-emitting 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 light-emitting layers, orange or yellow light emission can be obtained from the other layer. Each layer may contain various kinds of dopants. 
     Note that in the case where the light-emitting layer  113  has a stacked-layer structure, one or more of the organometallic iridium complex described in Embodiment 1, a light-emitting substance converting singlet excitation energy into light emission, and a light-emitting substance converting triplet excitation energy into light emission can be used alone or in combination, for example. In that case, the following substances can be used. 
     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 emitting fluorescence include N,N′-bis[4-(9H-carbazol-9-yl)phenyl]-N,N′-diphenylstilbene-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-butyl)perylene (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-phenylenediamine] (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]-N,N′,N′-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[ ]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile (abbreviation: DCM2), N,N,N′,N′-tetrakis(4-methylphenyl)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-{2-isopropyl-6-[2-(1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[i]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile (abbreviation: DCJTI), 2-{2-tert-butyl-6-[2-(1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[if]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[j]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 1×10 −6  seconds or longer, preferably 1×10 −3  seconds or longer. 
     Examples of the substance emitting 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 material in which the π-electron rich heteroaromatic ring is directly bonded to the π-electron deficient heteroaromatic ring is particularly preferably used because both the donor property of the π-electron rich heteroaromatic ring and the acceptor property of the π-electron deficient heteroaromatic ring are increased and the energy difference between the S1 level and the T1 level becomes small. 
     The electron-transport layer  114  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  114 , 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 (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, 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) can also be used. 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) can also be used. The substances listed here are mainly ones that have an electron mobility of 1×10 −6  cm 2 /Vs or higher. Note that any substance other than the substances listed here may be used for the electron-transport layer  114  as long as the electron-transport property is higher than the hole-transport property. 
     The electron-transport layer  114  is not limited to a single layer, but may be a stack of two or more layers each containing any of the substances listed above. 
     The electron-injection layer  115  is a layer containing a substance having a high electron-injection property. For the electron-injection layer  115 , 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 ) can be used. A rare earth metal compound like erbium fluoride (ErF 3 ) can also be used. An electride may also be used for the electron-injection layer  115 . Examples of the electride 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  114 , which are given above, can be used. 
     A composite material in which an organic compound and an electron donor (donor) are mixed may also be used for the electron-injection layer  115 . Such a composite material is excellent in an electron-injection property and an electron-transport property because electrons are generated in the organic compound by the electron donor. In this case, the organic compound is preferably a material that is excellent in transporting the generated electrons. Specifically, for example, the substances for forming the electron-transport layer  114  (e.g., a metal complex or a heteroaromatic compound), which are given above, 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, and ytterbium are given. In addition, an alkali metal oxide or an alkaline earth metal oxide is preferable, and lithium oxide, calcium oxide, barium oxide, and the like are given. A Lewis base such as magnesium oxide can also be used. An organic compound such as tetrathiafulvalene (abbreviation: TTF) can also be used. 
     Note that each of the above-described hole-injection layer  111 , hole-transport layer  112 , light-emitting layer  113 , electron-transport layer  114 , electron-injection layer  115 , and charge-generation layer  116  can be formed by a method such as an evaporation method (e.g., a vacuum evaporation method), an ink-jet method, or a coating method. 
     In the above-described light-emitting element, current flows due to a potential difference applied between the first electrode  101  and the second electrode  103  and holes and electrons recombine in the EL layer  102 , whereby light is emitted. Then, the emitted light is extracted outside through one or both of the first electrode  101  and the second electrode  103 . Thus, one or both of the first electrode  101  and the second electrode  103  are electrodes having light-transmitting properties. 
     The above-described light-emitting element can emit phosphorescence originating from the organometallic iridium complex and thus can have higher efficiency than a light-emitting element using only a fluorescent compound. 
     The structure described in this embodiment can be used in appropriate combination with the structure described in any of other embodiments. 
     Embodiment 3 
     Described in this embodiment is a light-emitting element (hereinafter, a tandem light-emitting element) with a structure in which the organometallic iridium complex of one embodiment of the present invention is used as an EL material in an EL layer and a charge-generation layer is provided between a plurality of EL layers. 
     A light-emitting element described in this embodiment is a tandem light-emitting element including a plurality of EL layers (a first EL layer  202 ( 1 ) and a second EL layer  202 ( 2 )) between a pair of electrodes (a first electrode  201  and a second electrode  204 ), as illustrated in  FIG. 2A . 
     In this embodiment, the first electrode  201  functions as an anode, and the second electrode  204  functions as a cathode. Note that the first electrode  201  and the second electrode  204  can have structures similar to those described in Embodiment 2. In addition, either or both of the EL layers (the first EL layer  202 ( 1 ) and the second EL layer  202 ( 2 )) may have structures similar to those described in Embodiment 2. In other words, the structures of the first EL layer  202 ( 1 ) and the second EL layer  202 ( 2 ) may be the same or different from each other and can be similar to those of the EL layers described in Embodiment 2. 
     In addition, a charge-generation layer  205  is provided between the plurality of EL layers (the first EL layer  202 ( 1 ) and the second EL layer  202 ( 2 )). The charge-generation layer  205  has a function of injecting electrons into one of the EL layers and injecting holes into the other of the EL layers when voltage is applied between the first electrode  201  and the second electrode  204 . In this embodiment, when voltage is applied such that the potential of the first electrode  201  is higher than that of the second electrode  204 , the charge-generation layer  205  injects electrons into the first EL layer  202 ( 1 ) and injects holes into the second EL layer  202 ( 2 ). 
     Note that in terms of light extraction efficiency, the charge-generation layer  205  preferably has a property of transmitting visible light (specifically, the charge-generation layer  205  has a visible light transmittance of 40% or more). The charge-generation layer  205  functions even when it has lower conductivity than the first electrode  201  or the second electrode  204 . 
     The charge-generation layer  205  may have either a structure in which an electron acceptor (acceptor) is added to an organic compound having a high hole-transport property or a structure in which an electron donor (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 BSPB, or the like can be used. The substances listed here are mainly ones that have a hole mobility of 1×10 −6  cm 2 /Vs or higher. Note that any organic compound other than the compounds listed here may be used as long as the hole-transport property is higher than the electron-transport property. 
     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. Oxides of metals belonging to Groups 4 to 8 of the periodic table can also be given. Specifically, vanadium oxide, niobium oxide, tantalum oxide, chromium oxide, molybdenum oxide, tungsten oxide, manganese oxide, and rhenium oxide are preferable because of their high electron-accepting properties. Among these, molybdenum oxide is especially preferable because it is stable in the air, has a low hygroscopic property, and is easy to handle. 
     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, 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 listed here are mainly ones that have an electron mobility of 1×10 −6  cm 2 /Vs or higher. Note that any organic compound other than the compounds listed here may be used as long as the electron-transport property is higher than the hole-transport property. 
     As the electron donor, it is possible to use an alkali metal, an alkaline earth metal, a rare earth metal, metals belonging to Groups 2 and 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  205  by using any of the above materials can suppress a drive voltage increase 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 ( 202 ( 1 ) to  202 ( n )) (n is three or more) are stacked as illustrated in  FIG. 2B . In the case where a plurality of EL layers are included between a pair of electrodes as in the light-emitting element according to this embodiment, by providing charge-generation layers ( 205 ( 1 ) to  205 ( 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. When the light-emitting element is applied to light-emitting devices, electronic devices, and lighting devices each having a large light-emitting area, voltage drop due to resistance of an electrode material can be reduced, which results in uniform 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 a light-emitting element having two EL layers, when an emission color of the first EL layer and an emission color of the second EL layer are complementary colors, the light-emitting element can emit white light as a whole. Note that “complementary colors” refer to colors that can produce an achromatic color when mixed. In other words, mixing light of complementary colors allows white emission to be obtained. Specifically, a combination in which blue light emission is obtained from the first EL layer and yellow light emission or orange light emission is obtained from the second EL layer is given as an example. In that case, it is not necessary that both of blue light emission and yellow (or orange) light emission are fluorescence, and the both are not necessarily 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. 
     The same can be applied to a light-emitting element having three EL layers. For example, the light-emitting element as a whole can provide white light emission when the emission color of the first EL layer is red, the emission color of the second EL layer is green, and the emission color of the 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 other embodiments. 
     Embodiment 4 
     Described in this embodiment is a light-emitting device that includes a light-emitting element in which the organometallic iridium complex of one embodiment of the present invention is used for an EL layer. 
     The light-emitting device may be either a passive matrix light-emitting device or an active matrix light-emitting device. Any of the light-emitting elements described in other embodiments can be used for the light-emitting device described in this embodiment. 
     In this embodiment, first, an active matrix light-emitting device is described with reference to  FIGS. 3A to 3C . 
     Note that  FIG. 3A  is a top view illustrating a light-emitting device and  FIG. 3B  is a cross-sectional view taken along the chain line A-A′ in  FIG. 3A . The active matrix light-emitting device according to this embodiment includes a pixel portion  302  provided over an element substrate  301 , a driver circuit portion (a source line driver circuit)  303 , and driver circuit portions (gate line driver circuits)  304   a  and  304   b . The pixel portion  302 , the driver circuit portion  303 , and the driver circuit portions  304   a  and  304   b  are sealed between the element substrate  301  and a sealing substrate  306  with a sealant  305 . 
     In addition, over the element substrate  301 , a lead wiring  307  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 electric potential from the outside is transmitted to the driver circuit portion  303  and the driver circuit portions  304   a  and  304   b , is provided. Here, an example is described in which a flexible printed circuit (FPC)  308  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 the FPC or the PWB. 
     Next, a cross-sectional structure is described with reference to  FIG. 3B . The driver circuit portions and the pixel portion are formed over the element substrate  301 ; the driver circuit portion  303  that is the source line driver circuit and the pixel portion  302  are illustrated here. 
     The driver circuit portion  303  is an example in which an FET  309  and an FET  310  are combined. Note that the driver circuit portion  303  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. Although this embodiment shows a driver integrated type in which the driver circuit is formed over the substrate, the driver circuit is not necessarily formed over the substrate, and may be formed outside the substrate. 
     The pixel portion  302  includes a plurality of pixels each of which includes a switching FET  311 , a current control FET  312 , and a first electrode (anode)  313  which is electrically connected to a wiring (a source electrode or a drain electrode) of the current control FET  312 . Although the pixel portion  302  includes two FETs, the switching FET  311  and the current control FET  312 , in this embodiment, one embodiment of the present invention is not limited thereto. The pixel portion  302  may include, for example, three or more FETs and a capacitor in combination. 
     As the FETs  309 ,  310 ,  311 , and  312 , 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  309 ,  310 ,  311 , and  312  include a Group 13 semiconductor (e.g., gallium), a Group 14 semiconductor (e.g., silicon), a compound semiconductor, an oxide semiconductor, and an organic semiconductor material. 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. In particular, an oxide semiconductor is preferably used for the FETs  309 ,  310 ,  311 , and  312 . 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, further preferably 3 eV or more is used for the FETs  309 ,  310 ,  311 , and  312 , so that the off-state current of the transistors can be reduced. 
     In addition, an insulator  314  is formed to cover end portions of the first electrode (anode)  313 . In this embodiment, the insulator  314  is formed using a positive photosensitive acrylic resin. The first electrode  313  is used as an anode in this embodiment. 
     The insulator  314  preferably has a curved surface with curvature at an upper end portion or a lower end portion thereof. This enables the coverage with a film to be formed over the insulator  314  to be favorable. The insulator  314  can be formed using, for example, either a negative photosensitive resin or a positive photosensitive resin. The material for the insulator  314  is not limited to an organic compound and an inorganic compound such as silicon oxide, silicon oxynitride, or silicon nitride can also be used. 
     The light-emitting element 317 has a stacked-layer structure including the first electrode (anode)  313 , an EL layer  315 , and a second electrode (cathode)  316 , and the EL layer  315  includes at least a light-emitting layer. In the EL layer  315 , 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. 
     For the first electrode (anode)  313 , the EL layer  315 , and the second electrode (cathode)  316 , any of the materials given in Embodiment 2 can be used. Although not illustrated, the second electrode (cathode)  316  is electrically connected to the FPC  308  which is an external input terminal. 
     Although the cross-sectional view in  FIG. 3B  illustrates only one light-emitting element 317, a plurality of light-emitting elements are arranged in a matrix in the pixel portion  302 . Light-emitting elements that emit light of three kinds of colors (R, G, and B) are selectively formed in the pixel portion  302 , whereby a light-emitting device capable of full color display can be obtained. 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 achieved. Alternatively, the light-emitting device may be capable of full color display by combination with color filters. The light-emitting device may have improved emission efficiency and reduced power consumption by combination with quantum dots. 
     Furthermore, the sealing substrate  306  is attached to the element substrate  301  with the sealant  305 , whereby a light-emitting element 317 is provided in a space  318  surrounded by the element substrate  301 , the sealing substrate  306 , and the sealant  305 . Note that the space  318  may be filled with an inert gas (such as nitrogen and argon) or the sealant  305 . In the case where the sealant is applied for attachment of the substrates, one or more of UV treatment, heat treatment, and the like are preferably performed. 
     An epoxy-based resin or glass frit is preferably used for the sealant  305 . The material preferably allows as little moisture and oxygen as possible to penetrate. As the sealing substrate  306 , a glass substrate, a quartz substrate, or a plastic substrate formed of fiber-reinforced plastic (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  301  and the sealing substrate  306  are preferably glass substrates for high adhesion. 
     As described above, an active matrix light-emitting device can be obtained. 
     The light-emitting device including the light-emitting element in which the organometallic iridium complex of one embodiment of the present invention is contained in the EL layer may be of the passive matrix type, instead of the active matrix type described above. 
       FIG. 3C  is a cross-sectional view illustrating a pixel portion of a passive-matrix light-emitting device. 
     As illustrated in  FIG. 3C , a light-emitting element 350 including a first electrode  352 , an EL layer  354 , and a second electrode  353  is formed over a substrate  351 . Note that the first electrode  352  has an island-like shape, and a plurality of the first electrodes  352  are formed in one direction to form a striped pattern. An insulating film  355  is formed over part of the first electrode  352 . 
     A partition  356  formed using an insulating material is provided over the insulating film  355 . The sidewalls of the partition  356  slope so that the distance between one sidewall and the other sidewall gradually decreases toward the surface of the substrate. In other words, a cross section taken along the direction of the short side of the partition  356  is trapezoidal, and the base (a side which is in the same direction as a plane direction of the insulating film  355  and in contact with the insulating film  355 ) is shorter than the upper side (a side which is in the same direction as the plane direction of the insulating film  355  and not in contact with the insulating film  355 ). By providing the partition  356  in such a manner, a defect of the light-emitting element due to static electricity or the like can be prevented. Note that the insulating film  355  has an opening portion over part of the first electrode  352 , and when the EL layer  354  is formed after formation of the partition  356 , the EL layer  354  that is in contact with the first electrode  352  in the opening portion is formed. 
     After formation of the EL layer  354 , the second electrode  353  is formed. Thus, the second electrode  353  is formed over the EL layer  354  and in some cases, is formed over the insulating film  355  without contact with the first electrode  352 . Note that since the EL layer  354  and the second electrode  353  are formed after formation of the partition  356 , the EL layer  354  and the second electrode  353  are also stacked over the partition  356  sequentially. 
     Note that sealing can be performed by a method similar to that used for the active matrix light-emitting device, and description thereof is not made. 
     As described above, a passive matrix light-emitting device can be obtained. 
     Note that 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. As an example of a glass substrate, a barium borosilicate glass substrate, an aluminoborosilicate glass substrate, a soda lime glass substrate, or the like can be given. Examples of the flexible substrate, the attachment film, the base film, and the like are substrates of plastics typified by polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polyether sulfone (PES), and polytetrafluoroethylene (PTFE). Another example is a synthetic resin such as acrylic. Alternatively, polypropylene, polyester, polyvinyl fluoride, polyvinyl chloride, or the like can be used. Alternatively, polyamide, polyimide, aramid, epoxy, an inorganic vapor deposition film, paper, or the like can be used. Specifically, the use of semiconductor substrates, single crystal substrates, SOI substrates, or the like enables the manufacture of small-sized transistors with a small variation in characteristics, size, shape, or the like and with high current supply capability. A circuit using such transistors achieves lower power consumption of the circuit or higher integration of the circuit. 
     Alternatively, a flexible substrate may be used as the substrate, and the transistor or the light-emitting element may be provided directly on the flexible substrate. Still alternatively, a separation layer may be provided between the substrate and the transistor or the light-emitting element. The separation layer can be used when part or the whole of a semiconductor device formed over the separation layer is separated from the substrate and transferred onto another substrate. In such a case, the transistor and the like can be transferred to a substrate having low heat resistance or a flexible substrate. For the separation layer, a stack including inorganic films, which are 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, a transistor or a light-emitting element may be formed using one substrate, and then transferred to another substrate. Examples of a substrate to which a transistor or a light-emitting element is transferred include, in addition to the above-described substrates over which transistors and 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 with excellent characteristics, a transistor with low power consumption, and the like can be formed, a device with high durability or 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 other embodiments. 
     Embodiment 5 
     In this embodiment, examples of an electronic device manufactured using a light-emitting device which is one embodiment of the present invention are described with reference to  FIGS. 4A to 4D, 4D ′- 1 , and  4 D′- 2  and  FIGS. 5A to 5C . 
     Examples of the electronic device including the light-emitting device are 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, cellular phones (also referred to as portable telephone devices), portable game consoles, portable information terminals, audio playback devices, large game machines such as pachinko machines, and the like. Specific examples of the electronic devices are illustrated in  FIGS. 4A to 4D, 4D ′- 1 , and  4 D′- 2 . 
       FIG. 4A  illustrates an example of a television device. In the 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 data 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 broadcasts 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. 4B  illustrates a computer, which includes a main body  7201 , a housing  7202 , a display portion  7203 , a keyboard  7204 , an external connection 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. 4C  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. 4C  can have a variety of functions, such as 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 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 . 
       FIGS. 4D, 4D ′- 1 , and  4 D′- 2  illustrate an example of a cellular phone (e.g., smartphone). A cellular 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 a light-emitting element of one embodiment of the present invention over a flexible substrate, the light-emitting element can be used for the display portion  7402  having a curved surface as illustrated in  FIG. 4D . 
     When the display portion  7402  of the cellular phone  7400  illustrated in  FIG. 4D  is touched with a finger or the like, data can be input to the cellular phone  7400 . In addition, operations such as making a call and composing e-mail can be performed by touch on 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 data 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 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 cellular phone  7400 , display on the screen of the display portion  7402  can be automatically changed by determining the orientation of the cellular phone  7400  (whether the cellular 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 be switched 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 switched to the display mode. When the signal is a signal of text data, the screen mode is switched 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. In addition, by providing a backlight or a sensing light source that emits near-infrared light in the display portion, an image of a finger vein, a palm vein, or the like can be taken. 
     The light-emitting device can be used for a cellular phone having a structure illustrated in  FIG. 4D ′- 1  or  FIG. 4D ′- 2 , which is another structure of the cellular phone (e.g., smartphone). 
     Note that in the case of the structure illustrated in  FIG. 4D ′- 1  or  FIG. 4D ′- 2 , text data, image data, 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 text data, image data, or the like displayed on the second screens  7502 ( 1 ) and  7502 ( 2 ) while the cellular phone is placed in user&#39;s breast pocket. 
       FIGS. 5A to 5C  illustrate a foldable portable information terminal  9310 .  FIG. 5A  illustrates the portable information terminal  9310  which is opened.  FIG. 5B  illustrates the portable information terminal  9310  which is being opened or being folded.  FIG. 5C  illustrates the portable information terminal  9310  that is folded. The portable information terminal  9310  is highly portable when folded. The portable information terminal  9310  is highly browsable when opened because of a seamless large display region. 
     A display panel  9311  is supported by three housings  9315  joined together by hinges  9313 . Note that 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. A light-emitting device of one embodiment of the present invention can be used for the display panel  9311 . A display region  9312  in the display panel  9311  is a display region that is positioned at a side surface of the portable information terminal  9310  that is folded. On the display region  9312 , information icons, file shortcuts of frequently used applications or 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 using the light-emitting device which is 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 other embodiments. 
     Embodiment 6 
     In this embodiment, a structure of a lighting device fabricated using the light-emitting element of one embodiment of the present invention will be described with reference to  FIGS. 6A to 6D . 
       FIGS. 6A to 6D  are examples of cross-sectional views of lighting devices.  FIGS. 6A and 6B  illustrate bottom-emission lighting devices in which light is extracted from the substrate side, and  FIGS. 6C and 6D  illustrate top-emission lighting devices in which light is extracted from the sealing substrate side. 
     A lighting device  4000  illustrated in  FIG. 6A  includes a light-emitting element  4002  over a substrate  4001 . In addition, the lighting device  4000  includes a substrate  4003  with unevenness on the outside of the substrate  4001 . The light-emitting element  4002  includes a first electrode  4004 , an EL layer  4005 , and a second electrode  4006 . 
     The first electrode  4004  is electrically connected to an electrode  4007 , and the second electrode  4006  is electrically connected to an electrode  4008 . In addition, an auxiliary wiring  4009  electrically connected to the first electrode  4004  may be provided. Note that an insulating layer  4010  is formed over the auxiliary wiring  4009 . 
     The substrate  4001  and a sealing substrate  4011  are bonded to each other by a sealant  4012 . A desiccant  4013  is preferably provided between the sealing substrate  4011  and the light-emitting element  4002 . The substrate  4003  has the unevenness illustrated in  FIG. 6A , whereby the extraction efficiency of light emitted from the light-emitting element  4002  can be increased. 
     Instead of the substrate  4003 , a diffusion plate  4015  may be provided on the outside of a substrate  4001  as in a lighting device  4100  illustrated in  FIG. 6B . 
     A lighting device  4200  illustrated in  FIG. 6C  includes a light-emitting element  4202  over a substrate  4201 . The light-emitting element  4202  includes a first electrode  4204 , an EL layer  4205 , and a second electrode  4206 . 
     The first electrode  4204  is electrically connected to an electrode  4207 , and the second electrode  4206  is electrically connected to an electrode  4208 . An auxiliary wiring  4209  electrically connected to the second electrode  4206  may be provided. An insulating layer  4210  may be provided under the auxiliary wiring  4209 . 
     The substrate  4201  and a sealing substrate  4211  with unevenness are bonded to each other by a sealant  4212 . A barrier film  4213  and a planarization film  4214  may be provided between the sealing substrate  4211  and the light-emitting element  4202 . The sealing substrate  4211  has the unevenness illustrated in  FIG. 6C , whereby the extraction efficiency of light emitted from the light-emitting element  4202  can be increased. 
     Instead of the sealing substrate  4211 , a diffusion plate  4215  may be provided over the light-emitting element  4202  as in a lighting device  4300  illustrated in  FIG. 6D . 
     Note that the EL layers  4005  and  4205  in this embodiment can include the organometallic iridium complex of one embodiment of the present invention. In that case, a lighting device with low power consumption can be provided. 
     Note that the structure described in this embodiment can be combined as appropriate with any of the structures described in other embodiments. 
     Embodiment 7 
     In this embodiment, examples of a lighting device that is an application of the light-emitting device in Embodiment 4 are described with reference to  FIG. 7 . 
       FIG. 7  illustrates an example in which the light-emitting device is used as an indoor lighting device  8001 . Since the light-emitting device can have a large area, it can be used for a lighting device having a large area. In addition, with the use of a housing with a curved surface, a lighting device  8002  in which a light-emitting region has a curved surface can also 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. Thus, the lighting device can be elaborately designed in a variety of ways. In addition, a wall of the room may be provided with a large-sized lighting device  8003 . 
     When the light-emitting device is used for a surface of a table, a lighting device  8004  that has a function as a table can be obtained. When the light-emitting device is used as part of other furniture, a lighting device that functions as the furniture can be obtained. 
     As described above, a variety of lighting devices that include the light-emitting device can be obtained. Note that these 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 other embodiments. 
     Embodiment 8 
     In this embodiment, touch panels including a light-emitting element of one embodiment of the present invention or a light-emitting device of one embodiment of the present invention will be described with reference to  FIGS. 8A and 8B ,  FIGS. 9A and 9B ,  FIGS. 10A and 10B ,  FIGS. 11A and 11B , and  FIG. 12 . 
       FIGS. 8A and 8B  are perspective views of a touch panel  2000 . Note that  FIGS. 8A and 8B  illustrate typical components of the touch panel  2000  for simplicity. 
     The touch panel  2000  includes a display panel  2501  and a touch sensor  2595  (see  FIG. 8B ). Furthermore, the touch panel  2000  includes a substrate  2510 , a substrate  2570 , and a substrate  2590 . 
     The display panel  2501  includes a plurality of pixels over the substrate  2510 , and a plurality of wirings  2511  through which signals are supplied to the pixels. The plurality of wirings  2511  are led to a peripheral portion of the substrate  2510 , and part of the plurality of wirings  2511  forms a terminal  2519 . The terminal  2519  is electrically connected to an FPC  2509 ( 1 ). 
     The substrate  2590  includes the touch sensor  2595  and a plurality of wirings  2598  electrically connected to the touch sensor  2595 . The plurality of wirings  2598  are led to a peripheral portion of the substrate  2590 , and part of the plurality of wirings  2598  forms a terminal  2599 . The terminal  2599  is electrically connected to an FPC  2509 ( 2 ). Note that in  FIG. 8B , electrodes, wirings, and the like of the touch sensor  2595  provided on the back side of the substrate  2590  (the side facing the substrate  2510 ) are indicated by solid lines for clarity. 
     As the touch sensor  2595 , a capacitive touch sensor can be used, for example. Examples of the capacitive touch sensor are a surface capacitive touch sensor and a projected capacitive touch sensor. 
     Examples of the projected capacitive touch sensor are a self capacitive touch sensor and a mutual capacitive touch sensor, which differ mainly in the driving method. The use of a mutual capacitive touch sensor is preferable because multiple points can be sensed simultaneously. 
     First, an example of using a projected capacitive touch sensor will be described below with reference to  FIG. 8B . Note that in the case of a projected capacitive touch sensor, a variety of sensors that can sense the closeness or the contact of a sensing target such as a finger can be used. 
     The projected capacitive touch sensor  2595  includes electrodes  2591  and electrodes  2592 . The electrodes  2591  are electrically connected to any of the plurality of wirings  2598 , and the electrodes  2592  are electrically connected to any of the other wirings  2598 . The electrodes  2592  each have a shape of a plurality of quadrangles arranged in one direction with one corner of a quadrangle connected to one corner of another quadrangle with a wiring  2594  in one direction as illustrated in  FIGS. 8A and 8B . In the same manner, the electrodes  2591  each have a shape of a plurality of quadrangles arranged with one corner of a quadrangle connected to one corner of another quadrangle; however, the direction in which the electrodes  2591  are connected is a direction crossing the direction in which the electrodes  2592  are connected. Note that the direction in which the electrodes  2591  are connected and the direction in which the electrodes  2592  are connected are not necessarily perpendicular to each other, and the electrodes  2591  may be arranged to intersect with the electrodes  2592  at an angle greater than 0° and less than 90°. 
     The intersecting area of the wiring  2594  and one of the electrodes  2592  is preferably as small as possible. Such a structure allows a reduction in the area of a region where the electrodes are not provided, reducing unevenness in transmittance. As a result, unevenness in the luminance of light from the touch sensor  2595  can be reduced. 
     Note that the shapes of the electrodes  2591  and the electrodes  2592  are not limited to the above-mentioned shapes and can be any of a variety of shapes. For example, the plurality of electrodes  2591  may be provided so that space between the electrodes  2591  are reduced as much as possible, and the plurality of electrodes  2592  may be provided with an insulating layer sandwiched between the electrodes  2591  and the electrodes  2592 . In that case, between two adjacent electrodes  2592 , a dummy electrode which is electrically insulated from these electrodes is preferably provided, whereby the area of a region having a different transmittance can be reduced. 
     Next, the touch panel  2000  will be described in detail with reference to  FIGS. 9A and 9B .  FIGS. 9A and 9B  are cross-sectional views taken along dashed-dotted line X 1 -X 2  in  FIG. 8A . 
     The touch panel  2000  includes the touch sensor  2595  and the display panel  2501 . 
     The touch sensor  2595  includes the electrodes  2591  and the electrodes  2592  that are provided in a staggered arrangement and in contact with the substrate  2590 , an insulating layer  2593  covering the electrodes  2591  and the electrodes  2592 , and the wiring  2594  that electrically connects the adjacent electrodes  2591  to each other. Between the adjacent electrodes  2591 , the electrode  2592  is provided. 
     The electrodes  2591  and the electrodes  2592  can be formed using a light-transmitting conductive material. As a light-transmitting conductive material, a conductive oxide such as indium oxide, indium tin oxide, indium zinc oxide, zinc oxide, or zinc oxide to which gallium is added can be used. A graphene compound may be used as well. When a graphene compound is used, it can be formed, for example, by reducing a graphene oxide film. As a reducing method, a method with application of heat, a method with laser irradiation, or the like can be employed. 
     For example, the electrodes  2591  and the electrodes  2592  can be formed by depositing a light-transmitting conductive material on the substrate  2590  by a sputtering method and then removing an unneeded portion by any of various patterning techniques such as photolithography. 
     Examples of a material for the insulating layer  2593  are a resin such as acrylic or epoxy resin, a resin having a siloxane bond, and an inorganic insulating material such as silicon oxide, silicon oxynitride, or aluminum oxide. 
     The adjacent electrodes  2591  are electrically connected to each other with a wiring  2594  formed in part of the insulating layer  2593 . Note that a material for the wiring  2594  preferably has higher conductivity than materials for the electrode  2591  and the electrode  2592  to reduce electrical resistance. 
     One wiring  2598  is electrically connected to any of the electrodes  2591  and  2592 . Part of the wiring  2598  serves as a terminal. For the wiring  2598 , a metal material such as aluminum, gold, platinum, silver, nickel, titanium, tungsten, chromium, molybdenum, iron, cobalt, copper, or palladium or an alloy material containing any of these metal materials can be used. 
     Through the terminal  2599 , the wiring  2598  and the FPC  2509 ( 2 ) are electrically connected to each other. The terminal  2599  can be formed using any of various kinds of anisotropic conductive films (ACF), anisotropic conductive pastes (ACP), and the like. 
     An adhesive layer  2597  is provided in contact with the wiring  2594 . That is, the touch sensor  2595  is attached to the display panel  2501  so that they overlap with each other with the adhesive layer  2597  provided therebetween. Note that the substrate  2570  as shown in  FIG. 9A  may be provided over the surface of the display panel  2501  that is adjacent to the adhesive layer  2597 ; however, the substrate  2570  is not always needed. 
     The adhesive layer  2597  has a light-transmitting property. For example, a thermosetting resin or an ultraviolet curable resin can be used; specifically, a resin such as an acrylic-based resin, a urethane-based resin, an epoxy-based resin, or a siloxane-based resin can be used. 
     The display panel  2501  in  FIG. 9A  includes, between the substrate  2510  and the substrate  2570 , a plurality of pixels arranged in a matrix and a driver circuit. Each pixel includes a light-emitting element and a pixel circuit driving the light-emitting element. 
     In  FIG. 9A , a pixel  2502 R is shown as an example of the pixel of the display panel  2501 , and a scan line driver circuit  2503   g  is shown as an example of the driver circuit. 
     The pixel  2502 R includes a light-emitting element  2550 R and a transistor  2502   t  that can supply electric power to the light-emitting element  2550 R. 
     The transistor  2502   t  is covered with the insulating layer  2521 . The insulating layer  2521  covers unevenness caused by the transistor and the like that have been already formed to provide a flat surface. The insulating layer  2521  may serve also as a layer for preventing diffusion of impurities. That is preferable because a reduction in the reliability of the transistor or the like due to diffusion of impurities can be prevented. 
     The light-emitting element  2550 R is electrically connected to the transistor  2502   t  through a wiring. It is one electrode of the light-emitting element  2550 R that is directly connected to the wiring. An end portion of the one electrode of the light-emitting element  2550 R is covered with an insulator  2528 . 
     The light-emitting element  2550 R includes an EL layer between a pair of electrodes. A coloring layer  2567 R is provided to overlap with the light-emitting element  2550 R, and part of light emitted from the light-emitting element  2550 R is transmitted through the coloring layer  2567 R and extracted in the direction indicated by an arrow in the drawing. A light-blocking layer  2567 BM is provided at an end portion of the coloring layer, and a sealing layer  2560  is provided between the light-emitting element  2550 R and the coloring layer  2567 R. 
     Note that when the sealing layer  2560  is provided on the side from which light from the light-emitting element  2550 R is extracted, the sealing layer  2560  preferably has a light-transmitting property. The sealing layer  2560  preferably has a higher refractive index than the air. 
     A scan line driver circuit  2503   g  includes a transistor  2503   t  and a capacitor  2503   c . Note that the driver circuit and the pixel circuits can be formed in the same process over the same substrate. Thus, similarly to the transistor  2502   t  in the pixel circuit, the transistor  2503   t  in the driver circuit (scan line driver circuit  2503   g ) is also covered with the insulating layer  2521 . 
     The wirings  2511  through which a signal can be supplied to the transistor  2503   t  are provided. The terminal  2519  is provided in contact with the wiring  2511 . The terminal  2519  is electrically connected to the FPC  2509 ( 1 ), and the FPC  2509 ( 1 ) has a function of supplying signals such as a pixel signal and a synchronization signal. Note that a printed wiring board (PWB) may be attached to the FPC  2509 ( 1 ). 
     Although the case where the display panel  2501  shown in  FIG. 9A  includes a bottom-gate transistor is described, the structure of the transistor is not limited thereto, and any of transistors with various structures can be used. In each of the transistor  2502   t  and the transistor  2503   t  illustrated in  FIG. 9A , a semiconductor layer including an oxide semiconductor can be used for a channel region. Alternatively, a semiconductor layer containing amorphous silicon or a semiconductor layer containing polycrystalline silicon that is obtained by crystallization process such as laser annealing can be used for a channel region. 
       FIG. 9B  illustrates the structure of the display panel  2501  that includes a top-gate transistor instead of the bottom-gate transistor illustrated in  FIG. 9A . The kind of the semiconductor layer that can be used for the channel region does not depend on the structure of the transistor. 
     In the touch panel  2000  shown in  FIG. 9A , an anti-reflection layer  2567   p  overlapping with at least the pixel is preferably provided on a surface of the touch panel on the side from which light from the pixel is extracted, as shown in  FIG. 9A . As the anti-reflection layer  2567   p , a circular polarizing plate or the like can be used. 
     For the substrate  2510 , the substrate  2570 , and the substrate  2590  in  FIG. 9A , for example, a flexible material having a vapor permeability of 1×10 −5  g/(m 2 ·day) or lower, preferably 1×10 6  g/(m 2 ·day) or lower can be favorably used. Alternatively, it is preferable to use the materials that make these substrates have substantially the same coefficient of thermal expansion. For example, the coefficients of linear expansion of the materials are 1×10 −3 /K or lower, preferably 5×10 −5 /K or lower, and further preferably 1×10 −5 /K or lower. 
     Next, a touch panel  2000 ′ having a structure different from that of the touch panel  2000  shown in  FIGS. 9A and 9B  is described with reference to  FIGS. 10A and 10B . Note that the touch panel  2000 ′ can be used for an application similar to that of the touch panel  2000 . 
       FIGS. 10A and 10B  are cross-sectional views of the touch panel  2000 ′. In the touch panel  2000 ′ illustrated in  FIGS. 10A and 10B , the position of the touch sensor  2595  relative to the display panel  2501  is different from that in the touch panel  2000  illustrated in  FIGS. 9A and 9B . Only different structures will be described below, and the above description of the touch panel  2000  can be referred to for the other similar structures. 
     The coloring layer  2567 R overlaps with the light-emitting element  2550 R. Light from the light-emitting element  2550 R illustrated in  FIG. 10A  is emitted to the side where the transistor  2502   t  is provided. That is, (part of) light emitted from the light-emitting element  2550 R passes through the coloring layer  2567 R and is extracted in the direction indicated by an arrow in  FIG. 10A . Note that the light-blocking layer  2567 BM is provided at an end portion of the coloring layer  2567 R. 
     The touch sensor  2595  is provided on the side of the display panel  2501  that is closer to the transistor  2502   t  than to the light-emitting element  2550 R (see  FIG. 10A ). 
     The adhesive layer  2597  is in contact with the substrate  2510  of the display panel  2501  and attaches the display panel  2501  and the touch sensor  2595  to each other in the structure shown in  FIG. 10A . The substrate  2510  is not necessarily provided between the display panel  2501  and the touch sensor  2595  that are attached to each other by the adhesive layer  2597 . 
     As in the touch panel  2000 , transistors with a variety of structures can be used for the display panel  2501  in the touch panel  2000 ′. Although a bottom-gate transistor is used in  FIG. 10A , a top-gate transistor may be applied as shown in  FIG. 10B . 
     Then, an example of a driving method of the touch panel will be described with reference to  FIGS. 11A and 11B . 
       FIG. 11A  is a block diagram illustrating the structure of a mutual capacitive touch sensor.  FIG. 11A  illustrates a pulse voltage output circuit  2601  and a current sensing circuit  2602 . Note that in the example of  FIG. 11A , six wirings X 1 -X 6  represent electrodes  2621  to which a pulse voltage is supplied, and six wirings Y 1 -Y 6  represent electrodes  2622  that sense a change in current.  FIG. 11A  also illustrates a capacitor  2603  that is formed in a region where the electrodes  2621  and  2622  overlap with each other. Note that functional replacement between the electrodes  2621  and  2622  is possible. 
     The pulse voltage output circuit  2601  is a circuit for sequentially applying a pulse voltage to the wirings X 1  to X 6 . By application of a pulse voltage to the wirings X 1  to X 6 , an electric field is generated between the electrodes  2621  and  2622  of the capacitor  2603 . When the electric field between the electrodes is shielded, for example, a change occurs in the capacitor  2603  (mutual capacitance). The approach or contact of a sensing target can be sensed by utilizing this change. 
     The current sensing circuit  2602  is a circuit for sensing changes in current flowing through the wirings Y 1  to Y 6  that are caused by the change in mutual capacitance in the capacitor  2603 . No change in current value is sensed in the wirings Y 1  to Y 6  when there is no approach or contact of a sensing target, whereas a decrease in current value is sensed when mutual capacitance is decreased owing to the approach or contact of a sensing target. Note that an integrator circuit or the like is used for sensing of current. 
       FIG. 11B  is a timing chart showing input and output waveforms in the mutual capacitive touch sensor illustrated in  FIG. 11A . In  FIG. 11B , sensing of a sensing target is performed in all the rows and columns in one frame period.  FIG. 11B  shows a period when a sensing target is not sensed (not touched) and a period when a sensing target is sensed (touched). Sensed current values of the wirings Y 1  to Y 6  are shown as the waveforms of voltage values. 
     A pulse voltage is sequentially applied to the wirings X 1  to X 6 , and the waveforms of the wirings Y 1  to Y 6  change in accordance with the pulse voltage. When there is no approach or contact of a sensing target, the waveforms of the wirings Y 1  to Y 6  change in accordance with changes in the voltages of the wirings X 1  to X 6 . The current value is decreased at the point of approach or contact of a sensing target and accordingly the waveform of the voltage value changes. By sensing a change in mutual capacitance in this manner, the approach or contact of a sensing target can be sensed. 
     Although  FIG. 11A  illustrates a passive touch sensor in which only the capacitor  2603  is provided at the intersection of wirings as a touch sensor, an active touch sensor including a transistor and a capacitor may be used.  FIG. 12  is a sensor circuit included in an active touch sensor. 
     The sensor circuit illustrated in  FIG. 12  includes the capacitor  2603 , a transistor  2611 , a transistor  2612 , and a transistor  2613 . 
     A signal G2 is input to a gate of the transistor  2613 . A voltage VRES is applied to one of a source and a drain of the transistor  2613 , and one electrode of the capacitor  2603  and a gate of the transistor  2611  are electrically connected to the other of the source and the drain of the transistor  2613 . One of a source and a drain of the transistor  2611  is electrically connected to one of a source and a drain of the transistor  2612 , and a voltage VSS is applied to the other of the source and the drain of the transistor  2611 . A signal G1 is input to a gate of the transistor  2612 , and a wiring ML is electrically connected to the other of the source and the drain of the transistor  2612 . The voltage VSS is applied to the other electrode of the capacitor  2603 . 
     Next, the operation of the sensor circuit illustrated in  FIG. 12  will be described. First, a potential for turning on the transistor  2613  is supplied as the signal G2, and a potential with respect to the voltage VRES is thus applied to the node n connected to the gate of the transistor  2611 . Then, a potential for turning off the transistor  2613  is applied as the signal G2, whereby the potential of the node n is maintained. Then, mutual capacitance of the capacitor  2603  changes owing to the approach or contact of a sensing target such as a finger, and accordingly the potential of the node n is changed from VRES. 
     In reading operation, a potential for turning on the transistor  2612  is supplied as the signal G1. A current flowing through the transistor  2611 , that is, a current flowing through the wiring ML is changed in accordance with the potential of the node n. By sensing this current, the approach or contact of a sensing target can be sensed. 
     In each of the transistors  2611 ,  2612 , and  2613 , an oxide semiconductor layer is preferably used as a semiconductor layer in which a channel region is formed. In particular, such a transistor is preferably used as the transistor  2613  so that the potential of the node n can be held for a long time and the frequency of operation of resupplying VRES to the node n (refresh operation) can be reduced. 
     At least part of this embodiment can be implemented in combination with any of other embodiments described in this specification as appropriate. 
     Example 1 
     Synthesis Example 1 
     In this example, a method for synthesizing bis[2-(5H-indeno[1,2-d]pyrimidin-4-yl-κN3)phenyl-KC](2,4-pentanedionato-κ 2 O,O′)iridium(III) (abbreviation: [Ir(pidpm) 2 (acac)]), which is an organometallic iridium complex of one embodiment of the present invention represented by Structural Formula (100) in Embodiment 1, is described. The structure of [Ir(pidpm) 2 (acac)] is shown below. 
     
       
         
         
             
             
         
       
     
     Step 1: Synthesis of 5H-indeno[1,2-d]pyrimidine 
     First, 5.42 g of 1-indanone, 12.04 g ofN,N,N′-methylidenetrisformamide, 0.44 g of p-toluene sulfonic acid monohydrate, and 9 mL of formamide were put into a 200-mL three-neck flask equipped with a reflux pipe, and the air in the flask was replaced with nitrogen. The mixture was then heated and stirred at 165° C. for 8 hours. The reacted solution was added to a 2N aqueous solution of sodium hydroxide, stirring was performed for 1 hour, and an organic layer was extracted with dichloromethane. The extracted solution was washed with water and saturated brine, and dried with magnesium sulfate. The solution obtained by the drying was filtered. The solvent of this solution was distilled off, and then the obtained residue was purified by silica gel column chromatography using dichloromethane and ethyl acetate as a developing solvent in a ratio of 1:1, so that a pyrimidine derivative, which was the objective substance, was obtained as a yellowish brown powder in a yield of 13%. A synthesis scheme of Step 1 is shown in (a-1). 
     
       
         
         
             
             
         
       
     
     Step 2: Synthesis of 4-phenyl-5H-indeno[1,2-d]pyrimidine (abbreviation: Hpidpm) 
     Next, 3.03 g of 5H-indeno[1,2-d]pyrimidine obtained in Step 1 and 90 mL of dry THF were put into a 300-mL three-neck flask and the air in the flask was replaced with nitrogen. After the flask was cooled with ice, 19 mL of phenyl lithium ( 1 . 9 M solution of phenyl lithium in butyl ether) was added, and the mixture was stirred at room temperature for 24 hours. The reacted solution was added to a saturated aqueous solution of sodium hydrogen carbonate, stirring was performed for 30 minutes, and an organic layer was extracted with dichloromethane. The extracted solution was washed with saturated brine, and dried with magnesium sulfate. The solution obtained by the drying was filtered. The solvent of this solution was distilled off, and then the obtained residue was purified by silica gel column chromatography using dichloromethane and ethyl acetate as a developing solvent in a ratio of 2:1. The obtained fraction was concentrated to give a solid. The solid was purified by flash column chromatography using hexane and ethyl acetate as a developing solvent in a ratio of 2:1, so that a pyrimidine derivative Hpidpm (abbreviation), which was the objective substance, was obtained as a white powder in a yield of 9%. A synthesis scheme of Step 2 is shown in (a-2). 
     
       
         
         
             
             
         
       
     
     Step 3: Synthesis of di-μ-chloro-tetrakis[2-(5H-indeno[1,2-d]pyrimidin-4-yl-κN3)phenyl-κC]diiridium(III) (abbreviation: [Ir(pidpm) 2 Cl] 2 ) 
     Next, into a recovery flask equipped with a reflux pipe were put 15 mL of 2-ethoxyethanol, 5 mL of water, 0.39 g of Hpidpm (abbreviation) obtained in Step 2, and 0.21 g of iridium chloride hydrate (IrCl 3 .H 2 O) (produced by Sigma-Aldrich Corporation), and the air in the flask was replaced with argon. After that, irradiation with microwaves (2.45 GHz, 100 W) was performed for 1 hour to cause a reaction. The solvent was distilled off, and then the obtained residue was suction-filtered and washed with hexane to give [Ir(pidpm) 2 Cl] 2  (abbreviation) that is a dinuclear complex as a brown powder in a yield of 94%. A synthesis scheme of Step 3 is shown in (a-3). 
     
       
         
         
             
             
         
       
     
     Step 4: Synthesis of bis[2-(5H-indeno[1,2-d]pyrimidin-4-yl-κN3)phenyl-κC](2,4-pentanedionato-κ 2 O,O)iridium(III) (abbreviation: [Ir(pidpm) 2 (acac)] 
     In a recovery flask equipped with a reflux pipe were put 20 mL of 2-ethoxyethanol, 0.47 g of the dinuclear complex [Ir(pidpm) 2 Cl] 2  (abbreviation) obtained in Step 3, 0.099 g of acetylacetone (abbreviation: Hacac), and 0.35 g of sodium carbonate, and the air in the flask was replaced with argon. Then, irradiation with microwaves (2.45 GHz, 120 W) was performed for 60 minutes. Here, 0.099 g of Hacac was added, and irradiation with microwaves (2.45 GHz, 120 W) was performed again for 60 minutes so that heating was performed. The solvent was distilled off, and the obtained residue was suction-filtered with ethanol. The obtained solid was washed with water and ethanol. The obtained solid was dissolved in dichloromethane and filtered through a filter aid in which Celite, alumina, and Celite were stacked in this order. The solvent was distilled off, and the resulting solid was recrystallized with a mixed solvent of dichloromethane and hexane; thus, [Ir(pidpm) 2 (acac)] (abbreviation), which is the organometallic complex of one embodiment of the present invention, was obtained as an orange powder in a yield of 37%. A synthesis scheme of Step 4 is shown in (a-4). 
     
       
         
         
             
             
         
       
     
     An analysis result by nuclear magnetic resonance ( 1 H-NMR) spectroscopy of the orange powder obtained in Step 4 is described below.  FIG. 13  shows the  1 H-NMR chart. The result revealed that [Ir(pidpm) 2 (acac)], which is the organometallic iridium complex of one embodiment of the present invention represented by Structural Formula (100) above, was obtained in Synthesis Example 1. 
       1 H-NMR. δ(CDCl 3 ): 1.80 (s, 6H), 4.34 (s, 4H), 5.28 (s, 1H), 6.49 (d, 2H), 6.75 (t, 2H), 6.93 (t, 2H), 7.59-7.61 (m, 4H), 7.76 (d, 2H), 7.93 (d, 2H), 8.20 (d, 2H), 9.21 (s, 2H). 
     Next, an ultraviolet-visible absorption spectrum (hereinafter, simply referred to as an absorption spectrum) and an emission spectrum of a dichloromethane solution of [Ir(pidpm) 2 (acac)] were measured. The measurement of the absorption spectrum was conducted at room temperature, for which an ultraviolet-visible light spectrophotometer (V550 type manufactured by JASCO Corporation) was used and the dichloromethane solution (0.086 mmol/L) was put in a quartz cell. The measurement of the emission spectrum was conducted at room temperature, for which a fluorescence spectrophotometer (FS920, manufactured by Hamamatsu Photonics K.K.) was used and the degassed dichloromethane solution (0.086 mmol/L) was put in a quartz cell.  FIG. 14  shows measurement results of the absorption spectrum and the emission spectrum. The horizontal axis represents wavelength and the vertical axes represent absorption intensity and emission intensity. In  FIG. 14 , two solid lines are shown; a thin line represents the absorption spectrum, and a thick line represents the emission spectrum. The absorption spectrum in  FIG. 14  is a result obtained in such a way that the measured absorption spectrum of only dichloromethane that was in a quartz cell was subtracted from the measured absorption spectrum of the dichloromethane solution (0.090 mmol/L) that was in a quartz cell. 
     As shown in  FIG. 14 , [Ir(pidpm) 2 (acac)], the organometallic iridium complex of one embodiment of the present invention, has an emission peak at 580 nm, and yellow light emission was observed from the dichloromethane solution. 
     Next, [Ir(pidpm) 2 (acac)] (abbreviation) obtained in this example was analyzed by liquid chromatography mass spectrometry (LC-MS). 
     In the analysis by LC-MS, liquid chromatography (LC) separation was carried out with ACQUITY UPLC (manufactured by Waters Corporation) and mass spectrometry (MS) analysis was carried out with Xevo G2 Tof MS (manufactured by Waters Corporation). ACQUITY UPLC BEH C8 (2.1×100 mm, 1.7 μm) was used as a column for the LC separation, and the column temperature was 40° C. Acetonitrile was used for Mobile Phase A and a 0.1% formic acid aqueous solution was used for Mobile Phase B. Further, a sample was prepared in such a manner that [Ir(pidpm) 2 (acac)] (abbreviation) was dissolved in chloroform at a given concentration and the mixture was diluted with acetonitrile. The injection amount was 5.0 μL. 
     In the LC separation, a gradient method in which the composition of mobile phases is changed was employed. The ratio of Mobile Phase A to Mobile Phase B was 50:50 for 0 to 1 minute after the start of the measurement, and then the composition was changed such that the ratio of Mobile Phase A to Mobile Phase B in the 10th minute was 95:5. The ratio was changed linearly. 
     In the MS analysis, ionization was carried out by an electrospray ionization (ESI) method. At this time, the capillary voltage and the sample cone voltage were set to 3.0 kV and 30 V, respectively, and detection was performed in a positive mode. The mass range for the measurement was m/z=100 to 1200. 
     A component with m/z of 779.22 which underwent the separation and the ionization under the above-described conditions was collided with an argon gas in a collision cell to dissociate into product ions. Energy (collision energy) for the collision with argon was 70 eV. The detection results of the dissociated product ions by time-of-flight (TOF) MS are shown in  FIG. 21 . 
       FIG. 21  shows that product ions of [Ir(pidpm) 2 (acac)] (abbreviation), which is the organometallic complex of one embodiment of the present invention represented by Structural Formula (100), are mainly detected around m/z=679.16 and m/z=245.09. The results in  FIG. 21  show characteristics derived from [Ir(pidpm) 2 (acac)](abbreviation) and therefore can be regarded as important data for identifying [Ir(pidpm) 2 (acac)] (abbreviation) contained in a mixture. 
     It is presumed that the product ion around m/z=679.16 is a cation in a state where acetylacetone and a proton were eliminated from the compound represented by Structural Formula (100) and the product ion around m/z=245.09 is a cation in a state where a proton was added to the ligand Hpidpm of the compound represented by Structural Formula (100), and this is characteristic of the organometallic iridium complex of one embodiment of the present invention. 
     Example 2 
     In this example, Light-emitting Element 1 was fabricated and an emission spectrum of the element was measured. For a light-emitting layer of Light-emitting Element 1, [Ir(pidpm) 2 (acac)], which is the organometallic iridium complex of one embodiment of the present invention represented by Structural Formula (100), was used. Note that the fabrication of Light-emitting Element 1 is described with reference to  FIG. 15 . Chemical formulae of materials used in this example are shown below. 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     &lt;&lt;Fabrication of Light-Emitting Element 1&gt;&gt; 
     First, indium tin oxide containing silicon oxide (ITSO) was deposited over a glass substrate  900  by a sputtering method, whereby a first electrode  901  functioning as an anode was formed. Note that the thickness was set to 110 nm and the electrode area was set to 2 mm×2 mm. 
     Next, as pretreatment for fabricating Light-emitting Element 1 over the substrate  900 , UV ozone treatment was performed for 370 seconds after washing of a surface of the substrate with water and baking that was performed at 200° C. for 1 hour. 
     After that, the substrate was transferred into a vacuum evaporation apparatus where the pressure had been reduced to approximately 10 −4  Pa, and subjected to vacuum baking at 170° C. in a heating chamber of the vacuum evaporation apparatus for 30 minutes, and then the substrate  900  was cooled down for approximately 30 minutes. 
     Next, the substrate  900  was fixed to a holder provided in the vacuum evaporation apparatus so that a surface of the substrate over which the first electrode  901  was formed faced downward. In this example, a case is described in which a hole-injection layer  911 , a hole-transport layer  912 , a light-emitting layer  913 , an electron-transport layer  914 , and an electron-injection layer  915 , which are included in an EL layer  902 , are sequentially formed by a vacuum evaporation method. 
     After reducing the pressure in 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  911  was formed over the first electrode  901 . The thickness of the hole-injection layer  911  was set to 20 nm. Note that co-evaporation is an evaporation method in which a plurality of different substances are concurrently vaporized from different evaporation sources. 
     Next, 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  912  was formed. 
     Next, the light-emitting layer  913  was formed over the hole-transport layer  912  by co-evaporation of 2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline (abbreviation: 2mDBTBPDBq-II), 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), and [Ir(pidpm) 2 (acac)] with a mass ratio of 2mDBTBPDBq-II to PCBBiF to [Ir(pidpm) 2 (acac)] being 0.8:0.2:0.01. The thickness of the light-emitting layer  913  was set to 40 nm. 
     Next, the electron-transport layer  914  was formed in such a manner that 2mDBTBPDBq-II was deposited by evaporation over the light-emitting layer  913  to a thickness of 20 nm and then bathophenanthroline (abbreviation: Bphen) was deposited by evaporation to a thickness of 15 nm. Furthermore, lithium fluoride was deposited to a thickness of 1 nm over the electron-transport layer  914  by evaporation, whereby the electron-injection layer  915  was formed. 
     Finally, aluminum was deposited to a thickness of 200 nm over the electron-injection layer  915  by evaporation, whereby a second electrode  903  functioning as a cathode was formed. Through the above-described steps, Light-emitting Element 1 was fabricated. Note that in all the above evaporation steps, evaporation was performed by a resistance-heating method. 
     Table 1 shows the element structure of Light-emitting Element 1 fabricated as described above. 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                   
                 Hole- 
                 Light- 
                   
                 Electron- 
                   
               
               
                   
                 First 
                 Hole-injection 
                 transport 
                 emitting 
                 Electron-transport 
                 injection 
                 Second 
               
               
                   
                 electrode 
                 layer 
                 layer 
                 layer 
                 layer 
                 layer 
                 electrode 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Light- 
                 ITSO 
                 DBT3P-II: MoO x   
                 BPAFLP 
                 * 
                 2mDBTBPDBq-II 
                 Bphen 
                 LiF 
                 Al 
               
               
                 emitting 
                 (110 nm) 
                 (4:2 20 nm) 
                 (20 nm) 
                   
                 (20 nm) 
                 (15 nm) 
                 (1 nm) 
                 (200 nm) 
               
               
                 element 1 
               
               
                   
               
               
                 * 2mDBTBPDBq-II: PCBBiF: [Ir(pidpm) 2 (acac)] (0.8:0.2:0.01 40 nm) 
               
            
           
         
       
     
     Light-emitting Element 1 fabricated was sealed in a glove box under a nitrogen atmosphere so as not to be exposed to the air (a sealant was applied to surround the element, and at the time of sealing, UV treatment was performed and heat treatment was performed at 80° C. for 1 hour). 
     &lt;&lt;Operation Characteristics of Light-Emitting Element 1&gt;&gt; 
     Operation characteristics of Light-emitting Element 1 fabricated were measured. Note that the measurement was carried out at room temperature (in an atmosphere kept at 25° C.). 
       FIG. 16  shows current density-luminance characteristics of Light-emitting Element 1,  FIG. 17  shows voltage-luminance characteristics of Light-emitting Element 1,  FIG. 18  shows luminance-current efficiency characteristics of Light-emitting Element 1, and  FIG. 19  shows voltage-current characteristics of Light-emitting Element 1. 
     These results reveal that Light-emitting Element 1, which is one embodiment of the present invention, has high efficiency. Table 2 shows initial values of main characteristics of Light-emitting Element 1 at a luminance of approximately 1000 cd/m 2 . 
     
       
         
           
               
               
               
               
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 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) 
                 (lm/W) 
                 (%) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Light- 
                 2.9 
                 0.040 
                 1.0 
                 (0.52, 0.48) 
                 900 
                 90 
                 97 
                 29 
               
               
                 emitting 
               
               
                 element 1 
               
               
                   
               
            
           
         
       
     
     The above results show that Light-emitting Element 1 fabricated in this example is a light-emitting element having high luminance and high current efficiency. Moreover, as for color purity, the light-emitting element exhibits yellow light emission with excellent color purity. 
       FIG. 20  shows an emission spectrum of Light-emitting Element 1 to which current was applied at a current density of 25 mA/cm 2 . As shown in  FIG. 20 , the emission spectrum of Light-emitting Element 1 has a peak at around 570 nm and it is suggested that the peak is derived from emission of the organometallic iridium complex of one embodiment of the present invention, [Ir(pidpm) 2 (acac)]. 
     This application is based on Japanese Patent Application serial no. 2014-200271 filed with Japan Patent Office on Sep. 30, 2014, the entire contents of which are hereby incorporated by reference.