Patent Publication Number: US-9406889-B2

Title: Phenanthrene compound, light-emitting element, light-emitting device, electronic device, and lighting device

Description:
This application is a continuation of copending U.S. application Ser. No. 13/282,686, filed on Oct. 27, 2011 which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a phenanthrene compound and a light-emitting element including the phenanthrene compound. The present invention also relates to a light-emitting device, an electronic device, and a lighting device each including the light-emitting element. 
     2. Description of the Related Art 
     In recent years, research and development of light-emitting elements using electroluminescence (EL) have been actively conducted. In a basic structure of such a light-emitting element, a layer containing a light-emitting substance is interposed between a pair of electrodes. By voltage application to this element, light emission can be obtained from the light-emitting substance. 
     Such a light-emitting element is of self-luminous type, and thus has advantages over a liquid crystal display in that visibility of pixels is high, backlight is not needed, and so on. Therefore, such a light-emitting element is regarded as being suitable as a flat panel display element. Besides, such a light-emitting element has advantages in that it can be manufactured to be thin and lightweight, and has very fast response speed. 
     Further, since such a light-emitting element can be formed to have a film shape, plane light emission can be easily obtained. Therefore, a large-area element capable of the plane light emission can be formed. This is a feature that is difficult to obtain with point light sources typified by an incandescent lamp and an LED or linear light sources typified by a fluorescent lamp. Therefore, the light-emitting element is very effective for use as a surface light source applicable to a lighting device and the like. 
     Light-emitting elements utilizing electroluminescence are broadly classified according to whether they use an organic compound or an inorganic compound as a light-emitting substance. In the case where an organic compound is used as a light-emitting substance, by application of voltage to a light-emitting element, electrons and holes are injected into a layer containing the light-emitting organic compound from a pair of electrodes, whereby current flows. Then, these carriers (i.e., electrons and holes) are recombined, whereby the light-emitting organic compound is excited. The light-emitting organic compound returns to the ground state from the excited state, thereby emitting light. Note that the excited state of an organic compound can be a singlet excited state and a triplet excited state, and luminescence from the singlet excited state (S*) is referred to as fluorescence, and luminescence from the triplet excited state (T*) is referred to as phosphorescence. The statistical generation ratio thereof in a light-emitting element is considered to be S*:T*=1:3. 
     At room temperature, a compound that is capable of converting a singlet excited state to luminescence (hereinafter, referred to as a fluorescent compound) generally exhibits only luminescence from the singlet excited state (fluorescence) and does not luminesce from the triplet excited state (phosphorescence). Therefore, the internal quantum efficiency (the ratio of generated photons to injected carriers) in a light-emitting element using a fluorescent compound is assumed to have a theoretical limit of 25% on the basis of S*:T*=1:3. 
     On the other hand, when a compound in which a triplet excited state is converted into luminescence (hereinafter, such a compound is referred to as a “phosphorescent compound”) is used, internal quantum efficiency can be theoretically 75% to 100%. In other words, emission efficiency can be 3 times to 4 times as much as that of the fluorescence compound. For these reasons, a light-emitting element using a phosphorescent compound has been actively developed in recent years in order to achieve a highly efficient light-emitting element (e.g., see Non-Patent Document 1). 
     When a light-emitting layer of a light-emitting element is formed using the above phosphorescent compound, the light-emitting layer is formed so that the phosphorescent compound is dispersed throughout a matrix formed of another material in many cases, for suppression of the concentration quenching of the phosphorescent compound and the quenching due to triplet-triplet annihilation. In this case, the material used for forming the matrix is referred to as a host material, and the material dispersed throughout the matrix is referred to as a guest material. 
     When a phosphorescent compound is used for a guest material, a host material is required to have higher triplet excitation energy (a difference in energy between the ground state and the triplet excited state) than the phosphorescent compound. It is known that CBP, which is used as the host material in Non-Patent Document 1, has higher triplet excitation energy than a phosphorescent compound which emits light of green to red and is widely used as a host material for the phosphorescent compound. 
     However, although CBP has high triplet excitation energy, it has insufficient ability to receive holes or electrons, which results in a problem of an increase in driving voltage. Therefore, a substance which has high triplet excitation energy and also can easily accept and transport both holes and electrons (i.e., a bipolar substance) is needed as a host material for a phosphorescent compound. 
     Furthermore, since singlet excitation energy (an energy difference between a ground state and a singlet excited state) is higher than triplet excitation energy, a substance that has high triplet excitation energy also has high singlet excitation energy. Therefore, a substance which has high triplet excitation energy and a bipolar property as described above is also effective as a host material in a light-emitting element using a fluorescent compound as a light-emitting substance. 
     REFERENCE 
     [Non-Patent Document 1] M. A. Baldo et al., (Jul. 5, 1999),  Applied Physics Letters , vol. 75, No. 1, pp. 4-6. 
     SUMMARY OF THE INVENTION 
     In view of the above, an object of one embodiment of the present invention is to provide a novel compound having high excitation energy, particularly a novel compound having high triplet excitation energy. In addition, an object of one embodiment of the present invention is to provide a novel compound having a bipolar property. 
     One embodiment of the present invention is a phenanthrene compound which has a bipolar property and in which a phenanthryl group having an electron-transport property and a dibenzothiophenyl group or a dibenzofuranyl group having a hole-transport property are bonded to each other through an arylene group. Specifically, one embodiment of the present invention is a phenanthrene compound represented by the general formulae below. 
     One embodiment of the present invention is a phenanthrene compound represented by General Formula (G1). 
     
       
         
         
             
             
         
       
     
     Note that in General Formula (G1), R 11  to R 19  and R 21  to R 27  separately represent any one of hydrogen, an alkyl group having 1 to 4 carbon atoms, and a substituted or unsubstituted aryl group having 6 to 13 carbon atoms. Ar represents a substituted or unsubstituted arylene group having 6 to 13 carbon atoms. Z represents a sulfur atom or an oxygen atom. 
     In General Formula (G1), Ar is preferably a substituted or unsubstituted biphenyldiyl group, more preferably a substituted or unsubstituted phenylene group. 
     One embodiment of the present invention is a phenanthrene compound represented by General Formula (G2). 
     
       
         
         
             
             
         
       
     
     Note that in General Formula (G2), R 11  to R 19  and R 21  to R 27  separately represent any one of hydrogen, an alkyl group having 1 to 4 carbon atoms, and a substituted or unsubstituted aryl group having 6 to 13 carbon atoms. Z represents a sulfur atom or an oxygen atom. 
     One embodiment of the present invention is a light-emitting element including the phenanthrene compound. In the light-emitting element, the phenanthrene compound is preferably contained in a light-emitting layer; more preferably, the phenanthrene compound and a phosphorescent substance are contained in a light-emitting layer. 
     One embodiment of the present invention is a light-emitting device including the light-emitting element. 
     One embodiment of the present invention is an electronic device or a lighting device including the light-emitting device. 
     Note that the light-emitting device in this specification includes, in its category, an image display device, a light-emitting device, and a light source. 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), a tape automated bonding (TAB) tape, or a tape carrier package (TCP) is connected to a panel, a module in which a printed wiring board is provided on the tip of a TAB tape or 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. 
     According to one embodiment of the present invention, a novel phenanthrene compound can be provided. The phenanthrene compound has a bipolar property and is useful as a material of a light-emitting element. Moreover, the phenanthrene compound has high triplet excitation energy, and thus is useful as a host material for a phosphorescent compound. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  each illustrate a light-emitting element of one embodiment of the present invention. 
         FIGS. 2A and 2B  each illustrate a light-emitting element of one embodiment of the present invention. 
         FIGS. 3A and 3B  illustrate a light-emitting device of one embodiment of the present invention. 
         FIGS. 4A and 4B  illustrate a light-emitting device of one embodiment of the present invention. 
         FIGS. 5A to 5E  illustrate electronic devices and a lighting device of one embodiment of the present invention. 
         FIG. 6  illustrates an electronic device of one embodiment of the present invention. 
         FIG. 7  illustrates an electronic device and lighting devices of one embodiment of the present invention. 
         FIGS. 8A and 8B  are NMR charts of DBTPPn-II. 
         FIGS. 9A and 9B  show an absorption spectrum and an emission spectrum of a toluene solution of DBTPPn-II. 
         FIGS. 10A and 10B  show an absorption spectrum and an emission spectrum of a thin film of DBTPPn-II. 
         FIG. 11  illustrates a light-emitting element in Example 2. 
         FIG. 12  shows current density-luminance characteristics of the light emitting element in Example 2. 
         FIG. 13  shows voltage-luminance characteristics of the light-emitting element in Example 2. 
         FIG. 14  shows luminance-current efficiency characteristics of the light-emitting element in Example 2. 
         FIG. 15  shows voltage-current characteristics of the light-emitting element in Example 2. 
         FIG. 16  shows an emission spectrum of the light-emitting element in Example 2. 
         FIGS. 17A and 17B  are NMR charts of mDBTBPPn-II. 
         FIGS. 18A and 18B  show an absorption spectrum and an emission spectrum of a toluene solution of mDBTBPPn-II. 
         FIGS. 19A and 19B  show an absorption spectrum and an emission spectrum of a thin film of mDBTBPPn-II. 
         FIG. 20  illustrates light emitting elements in Example 4. 
         FIG. 21  shows current density-luminance characteristics of the light-emitting elements in Example 4. 
         FIG. 22  shows voltage-luminance characteristics of the light-emitting elements in Example 4. 
         FIG. 23  shows luminance-current efficiency characteristics of the light-emitting elements in Example 4. 
         FIG. 24  shows voltage-current characteristics of the light-emitting elements in Example 4. 
         FIG. 25  shows an emission spectrum of the light-emitting elements in Example 4. 
         FIG. 26  shows results of a reliability test of the light-emitting elements in Example 4. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, embodiments and examples will be described in detail with reference to the accompanying drawings. Note that the present invention is not limited to the following description and it will be easily understood by those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the present invention. Therefore, the present invention should not be construed as being limited to the following description of the embodiments and examples. 
     Embodiment 1 
     In this embodiment, a phenanthrene compound according to one embodiment of the present invention will be described. 
     The phenanthrene compound according to one embodiment of the present invention is a phenanthrene compound represented by General Formula (G1). 
     
       
         
         
             
             
         
       
     
     Note that in General Formula (G1), R 11  to R 19  and R 21  to R 27  separately represent any one of hydrogen, an alkyl group having 1 to 4 carbon atoms, and a substituted or unsubstituted aryl group having 6 to 13 carbon atoms. Ar represents a substituted or unsubstituted arylene group having 6 to 13 carbon atoms. Z represents a sulfur atom or an oxygen atom. 
     The phenanthrene compound represented by General Formula (G1) is a compound having a bipolar property which includes a phenanthryl group having an electron-transport property and a dibenzothiophenyl group or a dibenzofuranyl group having a hole-transport property in the molecule. Further, an arylene group represented by Ar is interposed between the phenanthryl group and the dibenzothiophenyl group or the dibenzofuranyl group, whereby conjugation is not highly expanded from the phenanthryl group to the dibenzothiophenyl group or the dibenzofuranyl group; accordingly, the band gap is considered to be increased. Ar is preferably an arylene group with narrow conjugation, such as a substituted or unsubstituted phenylene group or a substituted or unsubstituted biphenyldiyl group, in order to prevent Ar itself from causing expansion of conjugation of the compound; more preferably, Ar is a substituted or unsubstituted phenylene group. 
     In General Formula (G1), R 11  to R 19  and R 21  to R 27  separately represent any one of hydrogen, an alkyl group having 1 to 4 carbon atoms, and an aryl group having 6 to 13 carbon atoms. In the case where R 11  to R 19  and R 21  to R 27  are each an aryl group having 6 to 13 carbon atoms, a substituent may be incorporated to the aryl group. As examples of the substituent in that case, an alkyl group having 1 to 4 carbon atoms, a phenyl group, a naphthyl group, a fluorenyl group, and the like can be given. Note that in the case where the aryl group has two substituents, these two substituents may be bonded to each other to form a ring and the ring may be a spiro ring. 
     As specific examples of the structures of R 11  to R 19  and R 21  to R 27 , Structural Formulae (R-1) to (R-23) and the like can be given. Note that Structural Formulae (R-16) to (R-23) are specific examples of the structures in the case where at least one of R 11  to R 19  and R 21  to R 27  is an aryl group having 6 to 13 carbon atoms and has a substituent. Further, Structural Formula (R-20) is a specific example in the case where the two substituents on the aryl group are bonded to each other to form a spiro ring system. 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     In General Formula (G1), Ar represents an arylene group having 6 to 13 carbon atoms and may have a substituent. In the case where Ar has a substituent, an alkyl group having 1 to 4 carbon atoms, a phenyl group, a naphthyl group, a fluorenyl group, and the like can be given as examples of the substituent. Note that in the case where Ar has two substituents, the substituents may be bonded to each other to form a ring and the ring may be a spiro ring. 
     As specific examples of the structure of Ar, Structural Formulae (Ar-1) to (Ar-15) and the like can be given. Note that Structural Formulae (Ar-12) to (Ar-15) are specific examples of the structures in the case where Ar has a substituent. Further, Structural Formula (Ar-15) is a specific example in the case where the substituents are bonded to each other to form a spiro ring system. 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     It is more preferable that Ar in the phenanthrene compound represented by General Formula (G1) be a phenylene group in terms of easiness of synthesis and because of its high triplet excitation energy. Therefore, it is more preferable that the phenanthrene compound according to one embodiment of the present invention be a phenanthrene compound represented by General Formula (G2). 
     
       
         
         
             
             
         
       
     
     Note that in General Formula (G2), R 11  to R 19  and R 21  to R 27  separately represent any one of hydrogen, an alkyl group having 1 to 4 carbon atoms, and a substituted or unsubstituted aryl group having 6 to 13 carbon atoms. Z represents a sulfur atom or an oxygen atom. 
     As specific examples of the structures of R 11  to R 19  and R 21  to R 27  in General Formula (G2), Structural Formulae (R-1) to (R-23) and the like can be given. 
     As specific examples of the phenanthrene compound represented by General Formula (G1), phenanthrene compounds represented by Structural Formulae (100) to (171) and Structural Formulae (200) to (271) can be given. However, the present invention is not limited to these. 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     A variety of reactions can be used for a synthesis method of the phenanthrene compound of this embodiment. For example, the phenanthrene compound can be synthesized by a synthesis reaction in Synthesis Method 1 or Synthesis Method 2. 
     &lt;Synthesis Method 1&gt; 
     As shown in Synthesis Scheme (A-1), a halide of a phenanthrene derivative (Compound 1) is coupled with an organoboron compound of a dibenzofuran derivative or a dibenzothiophene derivative (Compound 2) or with boronic acid of a dibenzofuran derivative or a dibenzothiophene derivative (Compound 2) by the Suzuki-Miyaura Reaction, whereby the phenanthrene compound represented by General Formula (G1) can be obtained. 
     
       
         
         
             
             
         
       
     
     Note that in Synthesis Scheme (A-1), R 11  to R 19  and R 21  to R 27  separately represent any one of hydrogen, an alkyl group having 1 to 4 carbon atoms, and a substituted or unsubstituted aryl group having 6 to 13 carbon atoms. Ar represents a substituted or unsubstituted arylene group having 6 to 13 carbon atoms. Z represents a sulfur atom or an oxygen atom. Further, R 50  and R 51  separately represent either hydrogen or an alkyl group having 1 to 6 carbon atoms, and R 50  and R 51  may be bonded to each other to form a ring. X represents a halogen, preferably bromine or iodine. 
     Examples of a palladium catalyst that can be used in Synthesis Scheme (A-1) include palladium(II) acetate, tetrakis(triphenylphosphine)palladium(0), bis(triphenylphosphine)palladium(II) dichloride, and the like. Examples of a ligand of the palladium catalyst that can be used in Synthesis Scheme (A-1) include tri(ortho-tolyl)phosphine, triphenylphosphine, tricyclohexylphosphine, and the like. 
     Examples of a base that can be used in Synthesis Scheme (A-1) include an organic base such as sodium tert-butoxide, an inorganic base such as potassium carbonate or sodium carbonate, and the like. Examples of a solvent that can be used in Synthesis Scheme (A-1) include a mixed solvent of toluene and water; a mixed solvent of toluene, alcohol such as ethanol, and water; a mixed solvent of xylene and water; a mixed solvent of xylene, alcohol such as ethanol, and water; a mixed solvent of benzene and water; a mixed solvent of benzene, alcohol such as ethanol, and water; a mixed solvent of an ether such as ethylene glycol dimethyl ether and water; and the like. Note that a mixed solvent of toluene and water; a mixed solvent of toluene, ethanol, and water; or a mixed solvent of water and ether such as ethylene glycol dimethyl ether is more preferable. 
     As a coupling reaction shown in Synthesis Scheme (A-1), the Suzuki-Miyaura Reaction using the organoboron compound or boronic acid represented by Compound 2 may be replaced with a cross coupling reaction using an organoaluminum compound, an organozirconium compound, an organozinc compound, an organotin compound, or the like. Further, a triflate group or the like may be used other than halogen in the coupling reaction. 
     Further, the phenanthrene compound represented by General Formula (G1) may be synthesized in such a manner that an organoboron compound of a phenanthrene derivative or boronic acid of a phenanthrene derivative, which is used instead of Compound 1, is coupled with a halide of a dibenzofuran derivative or a dibenzothiophene derivative or with a dibenzofuran derivative or a dibenzothiophene derivative having a triflate group as a substituent, which is used instead of Compound 2, in the Suzuki-Miyaura Reaction shown in Synthesis Scheme (A-1). 
     &lt;Synthesis Method 2&gt; 
     As shown in Synthesis Scheme (B-1), a halide of a phenanthrene derivative (Compound 3) is coupled with an organoboron compound of a dibenzofuran derivative or a dibenzothiophene derivative (Compound 4) or with boronic acid of a dibenzofuran derivative or a dibenzothiophene derivative (Compound 4) by the Suzuki-Miyaura Reaction, whereby the phenanthrene compound represented by General Formula (G1) can be obtained. 
     
       
         
         
             
             
         
       
     
     Note that in Synthesis Scheme (B-1), R 11  to R 19  and R 21  to R 27  separately represent any one of hydrogen, an alkyl group having 1 to 4 carbon atoms, and a substituted or unsubstituted aryl group having 6 to 13 carbon atoms. Ar represents a substituted or unsubstituted arylene group having 6 to 13 carbon atoms. Z represents a sulfur atom or an oxygen atom. R 52  and R 53  separately represent either hydrogen or an alkyl group having 1 to 6 carbon atoms, and R 52  and R 53  may be bonded to each other to form a ring. X represents a halogen, preferably bromine or iodine. 
     Examples of a palladium catalyst that can be used in Synthesis Scheme (B-1) include palladium(II) acetate, tetrakis(triphenylphosphine)palladium(0), bis(triphenylphosphine)palladium(II) dichloride, and the like. Examples of a ligand of the palladium catalyst that can be used in Synthesis Scheme (B-1) include tri(ortho-tolyl)phosphine, triphenylphosphine, tricyclohexylphosphine, and the like. 
     Examples of a base that can be used in Synthesis Scheme (B-1) include an organic base such as sodium tert-butoxide, an inorganic base such as potassium carbonate or sodium carbonate, and the like. Examples of a solvent that can be used in Synthesis Scheme (B-1) include a mixed solvent of toluene and water; a mixed solvent of toluene, alcohol such as ethanol, and water; a mixed solvent of xylene and water; a mixed solvent of xylene, alcohol such as ethanol, and water; a mixed solvent of benzene and water; a mixed solvent of benzene, alcohol such as ethanol, and water; a mixed solvent of an ether such as ethylene glycol dimethyl ether and water; and the like. Note that a mixed solvent of toluene and water; a mixed solvent of toluene, ethanol, and water; or a mixed solvent of water and ether such as ethylene glycol dimethyl ether is more preferable. 
     As a coupling reaction shown in Synthesis Scheme (B-1), the Suzuki-Miyaura Reaction using the organoboron compound or boronic acid represented by Compound 4 may be replaced with a cross coupling reaction using an organoaluminum compound, an organozirconium compound, an organozinc compound, an organotin compound, or the like. Further, a triflate group or the like may be used other than halogen in the coupling reaction. 
     Further, the phenanthrene compound represented by General Formula (G1) may be synthesized in such a manner that an organoboron compound of a phenanthrene derivative or boronic acid of a phenanthrene derivative, which is used instead of Compound 3, is coupled with a halide of a dibenzofuran derivative or a dibenzothiophene derivative or with dibenzofuran derivative or a dibenzothiophene derivative having a triflate group as a substituent, which is used instead of Compound 4, in the Suzuki-Miyaura Reaction shown in Synthesis Scheme (B-1). 
     Thus, the phenanthrene compound of this embodiment can be synthesized. 
     The phenanthrene compound of this embodiment has a bipolar property, and thus can be suitably used as a material of a hole-transport layer or an electron-transport layer of a light-emitting element. Further, a composite material in which the phenanthrene compound of this embodiment (an electron donor) and an electron acceptor are mixed can be used for a hole-injection layer of a light-emitting element. Note that the electron acceptor or the electron donor is at least capable of donating and accepting electrons with the assistance of an electric field. 
     The phenanthrene compound of this embodiment is also suitable as a host material in a light-emitting layer of a light-emitting element. In other words, when a light-emitting substance (hereinafter, also referred to as a “dopant”) having a narrower band gap than the phenanthrene compound of this embodiment is added to a layer formed of the phenanthrene compound, light can be emitted from the dopant. At this time, even if a fluorescent dopant which emits light with a relatively short wavelength such as blue light is used, light can be emitted efficiently from the dopant because the phenanthrene compound of this embodiment has a wide band gap. In other words, the phenanthrene compound of this embodiment can be used as a host material for a compound which emits fluorescence in the visible region. In the case where a dopant is a phosphorescent compound, a substance which has a higher T1 level than the dopant is preferably used as a host material. The phenanthrene compound of this embodiment has a high T1 level, and thus can be used as a host material for a compound which emits phosphorescence in the visible region with a wavelength longer than that of at least green light. 
     This embodiment can be implemented in appropriate combination with any of the other embodiments and examples. 
     Embodiment 2 
     In this embodiment, a light-emitting element including the phenanthrene compound described in Embodiment 1 as one embodiment of the present invention will be described with reference to  FIGS. 1A and 1B . 
     In the light-emitting element of this embodiment, an EL layer including at least a light-emitting layer is interposed between a pair of electrodes. The EL layer may have a plurality of layers in addition to the light-emitting layer. The plurality of layers has a structure in which a layer containing a substance having a high carrier-injection property and a layer containing a substance having a high carrier-transport property are combined and stacked so that a light-emitting region is formed in a region away from the electrodes, that is, so that carriers recombine in a region away from the electrodes. The plurality of layers may include, for example, a hole-injection layer, a hole-transport layer, an electron-transport layer, an electron-injection layer, and the like. 
     In the light-emitting element of this embodiment illustrated in  FIG. 1A , an EL layer  102  is provided between a pair of electrodes, a first electrode  101  and a second electrode  103 . The EL layer  102  includes a hole-injection layer  111 , a hole-transport layer  112 , a light-emitting layer  113 , an electron-transport layer  114 , and an electron-injection layer  115 . Note that in the light-emitting element described in this embodiment, the first electrode  101  provided over a substrate  100  functions as an anode and the second electrode  103  functions as a cathode. 
     A substrate  100  is used as a support of the light-emitting element. For example, glass, quartz, plastic, or the like can be used for the substrate  100 . A flexible substrate may be used. A flexible substrate is a substrate that can be bent (is flexible); examples of the flexible substrate include a plastic substrate made of a polycarbonate, a polyarylate, or a polyethersulfone, and the like. A film made of polypropylene, a polyester, poly(vinyl fluoride), poly(vinyl chloride), or the like, an inorganic film formed by evaporation, or the like can be used. Note that materials other than these can be used as long as they can function as a support of the light-emitting element. 
     For the first electrode  101 , a metal, an alloy, an electrically conductive compound, a mixture thereof, or the like which has a high work function (specifically, a work function of 4.0 eV or more) is preferably used. Specifically, for example, indium oxide-tin oxide (ITO: indium tin oxide), indium oxide-tin oxide containing silicon or silicon oxide, indium oxide-zinc oxide (IZO: indium zinc oxide), indium oxide containing tungsten oxide and zinc oxide (IWZO), and the like are given. Although films of these conductive metal oxides are usually formed by sputtering, a sol-gel method or the like may be used. For example, indium zinc oxide (IZO) can be formed by a sputtering method using a target in which zinc oxide is added to indium oxide at 1 wt % to 20 wt %. Indium oxide containing tungsten oxide and zinc oxide (IWZO) can be formed by a sputtering method using a target in which tungsten oxide is added to indium oxide at 0.5 wt % to 5 wt % and zinc oxide is added to indium oxide at 0.1 wt % to 1 wt %. Other examples include gold, platinum, nickel, tungsten, chromium, molybdenum, iron, cobalt, copper, palladium, nitrides of metal materials (e.g., titanium nitride), and the like. 
     Note that, in the EL layer  102 , when a layer in contact with the first electrode  101  is formed using a composite material in which an organic compound and an electron acceptor (acceptor) which are described later are mixed, the first electrode  101  can be formed using any of a variety of metals, alloys, and electrically conductive compounds, a mixture thereof, and the like regardless of the work function. For example, aluminum (Al), silver (Ag), an alloy containing aluminum (e.g., Al—Si), graphene, or the like can be used. 
     In the EL layer  102  formed over the first electrode  101 , at least the light-emitting layer  113  contains the phenanthrene compound of one embodiment of the present invention. The phenanthrene compound of one embodiment of the present invention is a material having a bipolar property, and thus can also be used as a material of a carrier-transport layer (e.g., the hole-transport layer or the electron-transport layer) in the EL layer  102 . For part of the EL layer  102 , a known substance can be used, and either a low molecular compound or a high molecular compound can be used. Note that the substance used for forming the EL layer  102  may have not only a structure formed of only an organic compound but also a structure partly including an inorganic compound. 
     The hole-injection layer  111  is a layer that contains a substance having a high hole-injection property. As the substance having a high hole-injection property, for example, metal oxides such as molybdenum oxide, titanium oxide, vanadium oxide, rhenium oxide, ruthenium oxide, chromium oxide, zirconium oxide, hafnium oxide, tantalum oxide, silver oxide, tungsten oxide, and manganese oxide can be used. A phthalocyanine-based compound such as phthalocyanine (abbreviation: H 2 Pc), or copper(II) phthalocyanine (abbreviation: CuPc) can also be used. 
     The following aromatic amine compounds which are low molecular organic compounds can be used: 4,4′,4″-tris(N,N-diphenylamino)triphenylamine (abbreviation: TDATA), 4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine (abbreviation: MTDATA), 4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl (abbreviation: DPAB), 4,4′-bis(N-{4-[N′-(3-methylphenyl)-N′-phenylamino]phenyl}-N-phenylamino)biphenyl (abbreviation: DNTPD), 1,3,5-tris[N-(4-diphenylaminophenyl)-N-phenylamino]benzene (abbreviation: DPA3B), 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), 3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole (abbreviation: PCzPCN1), or the like. 
     High molecular compounds (such as an oligomer, a dendrimer, or a polymer) can be used. As examples of the high molecular compound, the following are given: poly(N-vinylcarbazole) (abbreviation: PVK), poly(4-vinyltriphenylamine) (abbreviation: PVTPA), poly[N-(4-{N′-[4-(4-diphenylamino)phenyl]phenyl-N-phenylamino}phenyl)methacryl amide] (abbreviation: PTPDMA), and poly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine] (abbreviation: Poly-TPD). A high molecular compound to which an acid is added, such as poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonic acid) (PEDOT/PSS), or polyaniline/poly(styrenesulfonic acid) (PAni/PSS), can also be used. 
     A composite material in which an organic compound and an electron acceptor (acceptor) are mixed may be used for the hole-injection layer  111 . Such a composite material is excellent in a hole-injection property and a hole-transport property because holes are generated in the organic compound by the electron acceptor. In this case, the organic compound is preferably a material excellent in transporting the generated holes (a substance having a high hole-transport property). 
     As the organic compound for the composite material, any of a variety of compounds such as aromatic amine compounds, carbazole derivatives, aromatic hydrocarbons, and high molecular compounds (e.g., oligomer, dendrimer, and polymer) can be used. The organic compound used for the composite material is preferably an organic compound having a high hole-transport property. Specifically, a substance having a hole mobility of 10 −6  cm 2 /Vs or higher is preferably used. Note that any other substances may also be used as long as the hole-transport property thereof is higher than the electron-transport property thereof. The organic compounds that can be used for the composite material will be specifically given below. 
     The phenanthrene compound of one embodiment of the present invention is an organic compound having a high hole-transport property, and thus can be favorably used for the composite material. Besides, examples of the organic compound that can be used for the composite material include aromatic amine compounds such as TDATA, MTDATA, DPAB, DNTPD, DPA3B, PCzPCA1, PCzPCA2, PCzPCN1, 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB or α-NPD), and N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine (abbreviation: TPD) 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine (abbreviation: BPAFLP); and carbazole derivatives such as 4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP), 1,3,5-tris[4-(N-carbazolyl)phenyl]benzene (abbreviation: TCPB), 9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: CzPA), 9-phenyl-3-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: PCzPA), and 1,4-bis[4-(N-carbazolyl)phenyl-2,3,5,6-tetraphenylbenzene. 
     Any of the following aromatic hydrocarbon compounds can be used: 2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation: t-BuDNA), 2-tert-butyl-9,10-di(1-naphthyl)anthracene, 9,10-bis(3,5-diphenylphenyl)anthracene (abbreviation: DPPA), 2-tert-butyl-9,10-bis(4-phenylphenyl)anthracene (abbreviation: t-BuDBA), 9,10-di(2-naphthyl)anthracene (abbreviation: DNA), 9,10-diphenylanthracene (abbreviation: DPAnth), 2-tert-butylanthracene (abbreviation: t-BuAnth), 9,10-bis(4-methyl-1-naphthyl)anthracene (abbreviation: DMNA), 9,10-bis[2-(1-naphthyl)phenyl)-2-tert-butylanthracene, 9,10-bis[2-(1-naphthyl)phenyl]anthracene, 2,3,6,7-tetramethyl-9,10-di(1-naphthyl)anthracene, 2,3,6,7-tetramethyl-9,10-di(2-naphthyl)anthracene, 9,9′-bianthryl, 10,10′-diphenyl-9,9′-bianthryl, 10,10′-bis(2-phenylphenyl)-9,9′-bianthryl, 10,10′-bis[(2,3,4,5,6-pentaphenyl)phenyl]-9,9′-bianthryl, anthracene, tetracene, rubrene, perylene, 2,5,8,11-tetra(tert-butyl)perylene, pentacene, coronene, 4,4′-bis(2,2-diphenylvinyl)biphenyl (abbreviation: DPVBi), 9,10-bis[4-(2,2-diphenylvinyl)phenyl]anthracene (abbreviation: DPVPA), and the like. 
     As examples of the electron acceptor used for the composite material, organic compounds such as 7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation: F 4 -TCNQ) and chloranil; and transition metal oxides can be given. Oxides of metals belonging to Groups 4 to 8 in 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 property. Among these, molybdenum oxide is particularly preferable because it is stable in the air, has a low hygroscopic property, and is easily handled. 
     Note that the composite material may be formed using the above-described electron acceptor and the above high molecular compound such as PVK, PVTPA, PTPDMA, or Poly-TPD and may be used for the hole-injection layer  111 . 
     The hole-transport layer  112  is a layer that contains a substance having a high hole-transport property. Examples of the substance having a high hole-transport property include aromatic amine compounds such as 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB), N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine (abbreviation: TPD), 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), and the like. The substances given here are mainly ones that have a hole mobility of 10 −6  cm 2 /V·s or higher. Note that any other substances may also be used as long as the hole-transport property thereof is higher than the electron-transport property thereof. The phenanthrene compound described in Embodiment 1 can also be used. Note that the layer containing a substance having a high hole-transport property is not limited to a single layer and may be a stack of two or more layers containing any of the above substances. 
     A high molecular compound such as poly(N-vinylcarbazole) (PVK) or poly(4-vinyltriphenylamine) (PVTPA) can also be used for the hole-transport layer  112 . 
     The light-emitting layer  113  is a layer that contains the phenanthrene compound of one embodiment of the present invention (host material) and a light-emitting substance (guest material) dispersed in the phenanthrene compound. For example, a fluorescent compound which emits fluorescence or a phosphorescent compound which emits phosphorescence can be used as the light-emitting substance. Note that it is preferable that a substance which has a lower lowest unoccupied molecular orbital level (LUMO level) and a higher highest occupied molecular orbital level (HOMO level) than the phenanthrene compound of one embodiment of the present invention be used as the light-emitting substance. 
     The phenanthrene compound of one embodiment of the present invention has a wide band gap (the S1 level is high), and thus can be used favorably as a host material in the light-emitting layer  113 . In the case where a light-emitting substance is a phosphorescent compound, a substance which has a higher T1 level than the light-emitting substance is preferably used as a host material for the light-emitting substance. The phenanthrene compound of one embodiment of the present invention has a high T1 level, and thus can also be used favorably as a host material for a phosphorescent substance. 
     As the fluorescent compound that can be used for the light-emitting layer  113 , a material for blue light emission, a material for green light emission, a material for yellow light emission, and a material for red light emission are given. As examples of the material for blue light emission, the following are given: 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-(10-phenyl-9-anthryl)-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PCBAPA), and the like. As examples of the material for green light emission, the following are given: 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), N-[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), and the like. As examples of the material for yellow light emission, rubrene, 5,12-bis(1,1′-biphenyl-4-yl)-6,11-diphenyltetracene (abbreviation: BPT), and the like are given. As examples of the material for red light emission, 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), and the like are given. 
     As the phosphorescent compound that can be used for the light-emitting layer  113 , a material for green light emission, a material for yellow light emission, a material for orange light emission, and a material for red light emission are given. As examples of the material for green light emission, the following are given: tris(2-phenylpyridinato-N,C 2′ )iridium(III) (abbreviation: Ir(ppy) 3 ), bis[2-phenylpyridinato-N,C 2′ ]iridium(III)acetylacetonate (abbreviation: Ir(ppy) 2 (acac)), bis(1,2-diphenyl-1H-benzimidazolato)iridium(III)acetylacetonate (abbreviation: Ir(pbi) 2 (acac)), bis(benzo[h]quinolinato)iridium(III)acetylacetonate (abbreviation: Ir(bzq) 2 (acac)), tris(benzo[h]quinolinato)iridium(III) (abbreviation: Ir(bzq) 3 ), and the like. As examples of the material for yellow light emission, the following are given: bis(2,4-diphenyl-1,3-oxazolato-N,C 2′ )iridium(III)acetylacetonate (abbreviation: Ir(dpo) 2 (acac)), bis[2-(4′-perfluorophenylphenyl)pyridinato]iridium(III)acetylacetonate (abbreviation: Ir(p-PF-ph) 2 (acac)), bis(2-phenylbenzothiazolato-N,C 2′ )iridium(III)acetylacetonate (abbreviation: Ir(bt) 2 (acac)) (acetylacetonato)bis[2,3-bis(4-fluorophenyl)-5-methylpyrazinato]iridium(III) (abbreviation: Ir(Fdppr-Me) 2 (acac)), (acetylacetonato)bis{2-(4-methoxyphenyl)-3,5-dimethylpyrazinato}iridium(III) (abbreviation: Ir(dmmoppr) 2 (acac)), and the like. As examples of the material for orange light emission, the following are given: tris(2-phenylquinolinato-N,C 2′ )iridium(III) (abbreviation: Ir(pq) 3 ), bis(2-phenylquinolinato-N,C 2′ )iridium(III)acetylacetonate (abbreviation: Ir(pq) 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)), and the like. As examples of the material for red light emission, organometallic complexes such as bis[2-(2′-benzo[4,5-α]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(2,3,5-triphenylpyrazinato)iridium(III) (abbreviation: [Ir(tppr) 2 (acac)]), (dipivaloylmethanato)bis(2,3,5-triphenylpyrazinato)iridium(III) (abbreviation: [Ir(tppr) 2 (dpm)]), and (2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin)platinum(II) (abbreviation: PtOEP). In addition, rare-earth metal complexes, such as tris(acetylacetonato)(monophenanthroline)terbium(III) (abbreviation: Tb(acac) 3 (Phen)), 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)), exhibit light emission from rare-earth metal ions (electron transition between different multiplicities), and thus can be used as phosphorescent compounds. 
     A high molecular compound can be used as the light-emitting substance. Specifically, a material for blue light emission, a material for green light emission, and a material for orange to red light emission are given. As examples of the material for blue light emission, the following are given: poly(9,9-dioctylfluorene-2,7-diyl) (abbreviation: PFO), poly[(9,9-dioctylfluorene-2,7-diyl-co-(2,5-dimethoxybenzene-1,4-diyl)] (abbreviation: PF-DMOP), poly{(9,9-dioctylfluorene-2,7-diyl)-co-[N,N′-di-(p-butylphenyl)-1,4-diaminobenzene]} (abbreviation: TAB-PFH), and the like. As examples of the material for green light emission, the following are given: poly(p-phenylenvinylene) (abbreviation: PPV), poly[(9,9-dihexylfluorene-2,7-diyl)-alt-co-(benzo[2,1,3]thiadiazol-4,7-diyl)] (abbreviation: PFBT), poly[(9,9-dioctyl-2,7-divinylenfluorenylene)-alt-co-(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene)], and the like. As examples of the material for orange to red light emission, the following are given: poly[2-methoxy-5-(2′-ethylhexoxy)-1,4-phenylenevinylene] (abbreviation: MEH-PPV), poly(3-butylthiophene-2,5-diyl) (abbreviation: R4-PAT), poly{[9,9-dihexyl-2,7-bis(1-cyanovinylene)fluorenylene]-alt-co-[2,5-bis(N,N′-diphenyl amino)-1,4-phenylene]}, poly{[2-methoxy-5-(2-ethylhexyloxy)-1,4-bis(1-cyanovinylenephenylene)]-alt-co-[2,5-bis(N,N′-diphenylamino)-1,4-phenylene]} (abbreviation: CN-PPV-DPD), and the like. 
     Plural kinds of materials can be used as the host material in the light-emitting layer  113 . For example, in order to suppress crystallization, a substance such as rubrene which suppresses crystallization may be further added to the phenanthrene compound of one embodiment of the present invention. In addition, NPB, Alq, or the like may be further added in order to efficiently transfer energy to the guest material. 
     With a structure in which a guest material is dispersed in a host material, crystallization of the light-emitting layer  113  can be suppressed. In addition, concentration quenching due to an increase in the concentration of the guest material can be prevented. 
     Note that the light-emitting layer  113  may have a structure in which two or more layers are stacked. In that case, at least one of the layers may contain the phenanthrene compound of one embodiment of the present invention. Note that in the case where the light-emitting layer  113  has a structure in which two or more layers are stacked, the stacked layers may emit light of the same color or different colors. Further, a layer containing a fluorescent compound as a light-emitting substance and a layer containing a phosphorescent compound as a light-emitting substance can be stacked. 
     The electron-transport layer  114  is a layer that contains a substance having a high electron-transport property. Examples of the substance having a high electron-transport property include a metal complex having a quinoline skeleton or a benzoquinoline skeleton, such as tris(8-quinolinolato)aluminum (abbreviation: Alq), tris(4-methyl-8-quinolinolato)aluminum (abbreviation: Almq 3 ), bis(10-hydroxybenzo[h]-quinolinato)beryllium (abbreviation: BeBq 2 ), or bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum (abbreviation: BAlq). A metal complex or the like having an oxazole-based or thiazole-based ligand, such as bis[2-(2-hydroxyphenyl)benzoxazolato]zinc (abbreviation: Zn(BOX) 2 ) or bis[2-(2-hydroxyphenyl)benzothiazolato]zinc (abbreviation: Zn(BTZ) 2 ) can also be used. Besides the metal complexes, 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-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole (abbreviation: TAZ), bathophenanthroline (abbreviation: BPhen), bathocuproine (abbreviation: BCP), or the like can also be used. The substances given here are mainly ones that have an electron mobility of 10 −6  cm 2 /V·s or higher. The phenanthrene compound described in Embodiment 1 can also be used. Note that the electron-transport layer is not limited to a single layer and may be a stack of two or more layers containing any of the above substances. 
     The electron-injection layer  115  is a layer that contains 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, cesium, calcium, lithium fluoride, cesium fluoride, calcium fluoride, or lithium oxide, can be used. A rare earth metal compound such as erbium fluoride can also be used. The substances given above for forming the electron-transport layer  114  can also 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 excellent in transporting the generated electrons. Specifically, the above-described materials for forming the electron-transport layer  114  (e.g., a metal complex or a heteroaromatic compound) can be used, for example. As the electron donor, a substance exhibiting an electron-donating property to the organic compound may be used. Specifically, it is preferable to use an alkali metal, an alkaline-earth metal, or a rare earth metal, such as lithium, cesium, magnesium, calcium, erbium, or ytterbium. In addition, it is preferable to use an alkali metal oxide or an alkaline-earth metal oxide, such as lithium oxide, calcium oxide, or barium oxide. Lewis base such as magnesium oxide can also be used. An organic compound such as tetrathiafulvalene (abbreviation: TTF) can also be used. 
     Note that the hole-injection layer  111 , the hole-transport layer  112 , the light-emitting layer  113 , the electron-transport layer  114 , and the electron-injection layer  115  which are described above can each be formed by a method such as an evaporation method (e.g., a vacuum evaporation method), an ink-jet method, or a coating method. 
     When the second electrode  103  functions as a cathode, it can be formed using a metal, an alloy, an electrically-conductive compound, a mixture thereof, or the like having a low work function (preferably, a work function of 3.8 eV or less). Specifically, any of the following can be used: aluminum or silver; an element belonging to Group 1 or Group 2 of the periodic table, that is, an alkali metal such as lithium or cesium or an alkaline earth metal such as magnesium, calcium, or strontium; an alloy of the above metals (e.g., Mg—Ag or Al—Li); a rare earth metal such as europium or ytterbium; an alloy of the above metals; or the like. 
     Note that, in the case where in the EL layer  102 , a layer formed in contact with the second electrode  103  is formed using a composite material in which the organic compound and the electron donor, which are described above, are mixed, a variety of conductive materials such as aluminum, silver, ITO, indium tin oxide containing silicon or silicon oxide, and graphene can be used regardless of the work function. 
     Note that the second electrode  103  can be formed by a vacuum evaporation method or a sputtering method. In the case of using a silver paste or the like, a coating method, an inkjet method, or the like can be used. 
     In the above 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, this emitted light is extracted through one or both of the first electrode  101  and the second electrode  103 . Therefore, one or both of the first electrode  101  and the second electrode  103  is/are an electrode having a property of transmitting visible light. 
     Note that the structure of the layer provided between the first electrode  101  and the second electrode  103  is not limited to the above structure. A structure other than the above may also be employed as long as a light-emitting region in which holes and electrons recombine is provided in a portion away from the first electrode  101  and the second electrode  103  in order to prevent quenching due to proximity of the light-emitting region to a metal. 
     In other words, a stacked structure of the layer is not particularly limited, and a layer formed of a substance having a high electron-transport property, a substance having a high hole-transport property, a substance having a high electron-injection property, a substance having a high hole-injection property, a bipolar substance (a substance having a high electron-transport property and a high hole-transport property), a hole-blocking material, or the like may freely be combined with a light-emitting layer. 
     In a light-emitting element illustrated in  FIG. 1B , the EL layer  102  is provided between the first electrode  101  and the second electrode  103  over the substrate  100 . The EL layer  102  includes a hole-injection layer  111 , a hole-transport layer  112 , the light-emitting layer  113 , an electron-transport layer  114 , and an electron-injection layer  115 . The light-emitting element in  FIG. 1B  includes: the second electrode  103  serving as a cathode over the substrate  100 ; the electron-injection layer  115 , the electron-transport layer  114 , the light-emitting layer  113 , the hole-transport layer  112 , and the hole-injection layer  111  which are stacked over the second electrode  103  in this order; and the first electrode  101  serving as an anode over the hole-injection layer  111 . 
     A specific manufacturing method of a light-emitting element will be described below. 
     The light-emitting element of this embodiment has a structure in which an EL layer is interposed between a pair of electrodes. The electrode (the first electrode or the second electrode) and the EL layer may be formed by a wet process such as a droplet discharging method (an ink-jet method), a spin coating method, or a printing method, or by a dry process such as a vacuum evaporation method, a CVD method, or a sputtering method. The use of a wet process enables formation at atmospheric pressure with a simple device and by a simple process, which gives effects of simplifying the process and improving productivity. In contrast, a dry process does not need dissolution of a material and enables use of a material that has low solubility in a solution, which expands the range of material choices. 
     All the thin films included in the light-emitting element may be formed by a wet method. In this case, the light-emitting element can be manufactured with only facilities needed for a wet process. Alternatively, formation of the stacked layers up to formation of the light-emitting layer may be performed by a wet process whereas functional layers such as the electron-transport layer, the first electrode, and the like which are stacked over the light-emitting layer may be formed by a dry process. Further alternatively, the second electrode and the functional layers may be formed by a dry process before the formation of the light-emitting layer whereas the light-emitting layer, the functional layers stacked thereover, and the first electrode may be formed by a wet process. Needless to say, this embodiment is not limited to this, and the light-emitting element can be formed by appropriate selection from a wet method and a dry method depending on a material to be used, film thickness that is necessary, and the interface state. 
     In the above manner, the light-emitting element can be manufactured using the phenanthrene compound of one embodiment of the present invention. 
     Note that by use of a light-emitting element described in this embodiment, a passive matrix light-emitting device or an active matrix light-emitting device in which driving of the light-emitting element is controlled by a transistor can be manufactured. 
     This embodiment can be implemented in appropriate combination with any of the other embodiments. 
     Embodiment 3 
     In this embodiment, a mode of a light-emitting element having a structure in which a plurality of light-emitting units are stacked (such a light-emitting element is hereinafter referred to as a stacked-type element) will be described with reference to  FIGS. 2A and 2B . This light-emitting element is a light-emitting element including a plurality of light-emitting units between a first electrode and a second electrode. 
     In  FIG. 2A , a first light-emitting unit  311  and a second light-emitting unit  312  are stacked between a first electrode  301  and a second electrode  303 . In this embodiment, the first electrode  301  functions as an anode and the second electrode  303  functions as a cathode. Note that as the materials for the first electrode  301  and the second electrode  303 , those described in Embodiment 2 can be used. The first light-emitting unit  311  and the second light-emitting unit  312  may have the same or different structures. The first light-emitting unit  311  and the second light-emitting unit  312  may have the structure the same as that of the light-emitting layer  113  in Embodiment 2, or either of the units may have a structure different from that of the light-emitting layer  113  in Embodiment 2. 
     Further, a charge generation layer  313  is provided between the first light-emitting unit  311  and the second light-emitting unit  312 . The charge generation layer  313  has a function of injecting electrons into one light-emitting unit and injecting holes into the other light-emitting unit when voltage is applied between the first electrode  301  and the second electrode  303 . In this embodiment, when voltage is applied to the first electrode  301  so that the potential thereof is higher than that of the second electrode  303 , the charge generation layer  313  injects electrons into the first light-emitting unit  311  and injects holes into the second light-emitting unit  312 . 
     Note that the charge generation layer  313  preferably has a property of transmitting visible light in terms of light extraction efficiency. Further, the charge generation layer  313  functions even when it has lower conductivity than the first electrode  301  or the second electrode  303 . 
     The charge generation layer  313  may have either a structure including an organic compound having a high hole-transport property and an electron acceptor or a structure including an organic compound having a high electron-transport property and an electron donor. Alternatively, both of these structures may be stacked. Note that the electron acceptor and the electron donor are at least capable of donating and accepting electrons with the assistance of an electric field. 
     In the case of a 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, the phenanthrene compound of one embodiment of the present invention can be used. Other examples of the organic compound having a high hole-transport property include aromatic amine compounds such as NPB, TPD, TDATA, MTDATA, and 4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl (abbreviation: BSPB), and the like. 
     As examples of the electron acceptor, 7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation: F 4 -TCNQ), chloranil, and the like can be given. In addition, a transition metal oxide can be given. In addition, an oxide of metals that belong to Group 4 to Group 8 of the periodic table can 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 particularly preferable because it is stable in the air, has a low hygroscopic property, and is easily handled. 
     In contrast, in the case of a 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, 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, for example. A metal complex having an oxazole-based ligand or a thiazole-based ligand, such as Zn(BOX) 2  or Zn(BTZ) 2  can also be used. Other than the metal complexes, PBD, OXD-7, TAZ, BPhen, BCP, or the like can be used. The phenanthrene compound of one embodiment of the present invention may also be used. 
     As the electron donor, an alkali metal, an alkaline earth metal, a rare earth metal, a metal belonging to Group 13 of the periodic table, or an oxide or carbonate thereof can be used. Specifically, lithium, cesium, magnesium, calcium, ytterbium, indium, 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 the charge generation layer  313  is formed using any of the above materials, whereby an unnecessary increase in driving voltage caused when the EL layers are stacked can be suppressed. 
     In this embodiment, the light-emitting element having two light-emitting units is described, and one embodiment of the present invention can be similarly applied to a light-emitting element having a stack of three or more light-emitting units as illustrated in  FIG. 2B . For example, in a stacked structure of n layers (n is a natural number greater than or equal to 2), the charge generation layer  313  is interposed between an m-th light-emitting unit and an (m+1)-th light-emitting unit (m is a natural number less than or equal to (n−1)). A plurality of light-emitting units which are partitioned by a charge generation layer are arranged between a pair of electrodes as in the light-emitting element according to this embodiment, whereby it is possible to obtain an element having long lifetime which can emit light with a high luminance while current density is kept low. 
     Further, the light-emitting units emit light having different colors from each other, thereby obtaining light emission of a desired color in the whole light-emitting element. For example, in the light-emitting element having two light-emitting units, when an emission color of the first light-emitting unit and an emission color of the second light-emitting unit are made to be complementary colors, it is possible to obtain a light-emitting element which emits white light as a whole light-emitting element. Note that “complementary colors” refer to colors that can produce an achromatic color when mixed. Examples of the pair of the complementary colors include blue and yellow, blue-green and red, and the like. In the case of a light-emitting element having three light-emitting units in which, for example, a first light-emitting unit emits red light, a second light-emitting unit emits green light, and a third light-emitting unit emits blue light, the whole light-emitting element can emit white light. 
     Note that this embodiment can be freely combined with any of the other embodiments and examples. 
     Embodiment 4 
     In this embodiment, a light-emitting device that is one embodiment of the present invention will be described with reference to  FIGS. 3A and 3B .  FIG. 3A  is a top view illustrating a light-emitting device.  FIG. 3B  is a cross-sectional view taken along lines A-B and C-D in  FIG. 3A . 
     In  FIG. 3A , reference numeral  401  denotes a driver circuit portion (a source side driver circuit), reference numeral  402  denotes a pixel portion, and reference numeral  403  denotes a driver circuit portion (a gate side driver circuit), which are shown by a dotted line. Reference numeral  404  denotes a sealing substrate, reference numeral  405  denotes a sealant, and a portion enclosed by the sealant  405  is a space. 
     Note that a lead wiring  408  is a wiring for transmitting signals that are to be inputted to the source side driver circuit  401  and the gate side driver circuit  403 , and receives a video signal, a clock signal, a start signal, a reset signal, and the like from a flexible printed circuit (FPC)  409  which serves as an external input terminal. Although only the FPC is illustrated here, a printed wiring board (PWB) may be attached to the FPC. 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 will be described with reference to  FIG. 3B . The driver circuit portion and the pixel portion are formed over an element substrate  410 . Here, the source driver circuit  401  which is the driver circuit portion and one pixel in the pixel portion  402  are illustrated. 
     Note that as the source side driver circuit  401 , a CMOS circuit, which is a combination of an n-channel TFT  423  and a p-channel TFT  424 , is formed. The driver circuit may be any of a variety of circuits formed with TFTs, such as a CMOS circuit, a PMOS circuit, or an NMOS circuit. Although a driver-integrated type in which a driver circuit is formed over the substrate is described in this embodiment, the present invention is not limited to this type, and the driver circuit can be formed outside the substrate. 
     The pixel portion  402  includes a plurality of pixels having a switching TFT  411 , a current control TFT  412 , and a first electrode  413  electrically connected to a drain of the current control TFT  412 . An insulator  414  is formed to cover an end portion of the first electrode  413 . Here, the insulator  414  is formed using a positive type photosensitive acrylic resin film. 
     In order to improve the coverage, the insulator  414  is provided such that either an upper end portion or a lower end portion of the insulator  414  has a curved surface with a curvature. For example, when positive type photosensitive acrylic resin is used as a material for the insulator  414 , the insulator  414  preferably has a curved surface with a curvature radius (0.2 μm to 3 μm) only as the upper end. The insulator  414  can be formed using either a negative type resin which becomes insoluble in an etchant by light irradiation or a positive type resin which becomes soluble in an etchant by light irradiation. 
     An EL layer  416  and a second electrode  417  are formed over the first electrode  413 . Each of the first electrode  413 , the EL layer  416 , and the second electrode  417  can be formed using any of the materials described in Embodiment 2. Note that the EL layer  416  includes at least a light-emitting layer that contains the phenanthrene compound described in Embodiment 1. The first electrode  413  functions as an anode and the second electrode  417  functions as a cathode in this embodiment. 
     The sealing substrate  404  is attached to the element substrate  410  with the sealant  405 , whereby a light-emitting element  418  is provided in the space  407  enclosed by the element substrate  410 , the sealing substrate  404 , and the sealant  405 . Note that the space  407  may be filled with filler such as an inert gas (e.g., nitrogen or argon) or with the sealant  405 . 
     Note that an epoxy-based resin is preferably used as the sealant  405 . As a material for the sealing substrate  404 , a glass substrate, a quartz substrate, or a plastic substrate including fiberglass-reinforced plastics (FRP), poly(vinyl fluoride) (PVF), a polyester, an acrylic resin, or the like can be used. It is desirable that materials for the sealing substrate  404 , the sealant  405 , and the element substrate  410  transmit as little moisture or oxygen as possible. 
     In the above manner, the active matrix light-emitting device including the light-emitting element of one embodiment of the present invention can be obtained. 
     Further, the light-emitting element of the present invention can be used for a passive matrix light-emitting device instead of the above active matrix light-emitting device.  FIGS. 4A and 4B  illustrate a perspective view and a cross-sectional view of a passive matrix light-emitting device including the light-emitting element of one embodiment of the present invention.  FIG. 4A  is a perspective view of the light-emitting device.  FIG. 4B  is a cross-sectional view taken along line X-Y in  FIG. 4A . 
     In  FIGS. 4A and 4B , an EL layer  504  is provided between a first electrode  502  and a second electrode  503  over a substrate  501 . An end portion of the first electrode  502  is covered with an insulating layer  505 . In addition, a partition layer  506  is provided over the insulating layer  505 . The sidewalls of the partition layer  506  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 layer  506  is trapezoidal, and the base (side facing in a direction parallel to the plane direction of the insulating layer  505  and being in contact with the insulating layer  505 ) is shorter than the upper side (side facing in the direction parallel to the plane direction of the insulating layer  505  and not being in contact with the insulating layer  505 . The partition layer  506  is provided in such a manner, whereby a defect of a light-emitting element due to static electricity or the like can be prevented. 
     In the above manner, the passive matrix light-emitting device including the light-emitting element of one embodiment of the present invention can be obtained. 
     The light-emitting devices described in this embodiment (the active matrix light-emitting device and the passive matrix light-emitting device) are both formed using the light-emitting element of one embodiment of the present invention. 
     Note that this embodiment can be freely combined with any of the other embodiments and examples. 
     Embodiment 5 
     In this embodiment, examples of a variety of electronic devices and lighting devices which are completed by using a light-emitting device that is one embodiment of the present invention will be described with reference to  FIGS. 5A to 5E ,  FIG. 6 , and  FIG. 7 . 
     Examples of the electronic device to which the light-emitting device is applied include television devices (also referred to as TV or television receivers), monitors for computers and the like, cameras such as digital cameras and digital video cameras, digital photo frames, cellular phones (also referred to as portable telephone devices), portable game machines, portable information terminals, audio reproducing devices, large game machines such as pin-ball machines, and the like.  FIGS. 5A to 5E  illustrate specific examples of these electronic devices and a lighting device. 
       FIG. 5A  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  is capable of displaying images, and the light-emitting device of one embodiment of the present invention can be used for the display portion  7103 . 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 receiver, a general television broadcast can be received. Furthermore, when the television device  7100  is connected to a communication network by wired or wireless connection via the modem, one-way (from a sender to a receiver) or two-way (between a sender and a receiver, between receivers, or the like) data communication can be performed. 
       FIG. 5B  illustrates a computer, which includes a main body  7201 , a housing  7202 , a display portion  7203 , a keyboard  7204 , an external connecting port  7205 , a pointing device  7206 , and the like. Note that this computer is manufactured by using the light-emitting device of one embodiment of the present invention for the display portion  7203 . 
       FIG. 5C  illustrates a portable game machine having two housings, a housing  7301  and a housing  7302 , which are connected with a joint portion  7303  so that the portable game machine can be opened or folded. A display portion  7304  is incorporated in the housing  7301  and a display portion  7305  is incorporated in the housing  7302 . In addition, the portable game machine illustrated in  FIG. 5C  includes a speaker portion  7306 , a recording medium insertion portion  7307 , an LED lamp  7308 , an input means (an operation key  7309 , a connection terminal  7310 , a sensor  7311  (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, smell, or infrared rays), or a microphone  7312 ), and the like. Needless to say, the structure of the portable game machine is not limited to the above as long as the light-emitting device of one embodiment of the present invention is used for at least either the display portion  7304  or the display portion  7305 , or both, and can include other accessories as appropriate. The portable game machine illustrated in  FIG. 5C  has a function of reading out a program or data stored in a storage medium to display it on the display portion, and a function of sharing information with another portable game machine by wireless communication. The portable game machine illustrated in  FIG. 5C  can have a variety of functions without limitation to the above. 
       FIG. 5D  illustrates an example of a mobile phone. The mobile phone  7400  is provided with a display portion  7402  incorporated in a housing  7401 , operation buttons  7403 , an external connection port  7404 , a speaker  7405 , a microphone  7406 , and the like. Note that the mobile phone  7400  is manufactured by using the light-emitting device of one embodiment of the present invention for the display portion  7402 . 
     When the display portion  7402  of the mobile phone  7400  illustrated in  FIG. 5D  is touched with a finger or the like, data can be input into the mobile phone  7400 . Further, operations such as making a call and composing an 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 images. The second mode is an input mode mainly for inputting data such as text. 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 composing an e-mail, a text input mode mainly for inputting text is selected for the display portion  7402  so that text displayed on a screen can be inputted. 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 including a sensor for detecting inclination, such as a gyroscope or an acceleration sensor, is provided inside the mobile phone  7400 , display on the screen of the display portion  7402  can be automatically changed by determining the orientation of the mobile phone  7400  (whether the mobile phone is placed horizontally or vertically for a landscape mode or a portrait mode). 
     The screen modes are switched by touch on the display portion  7402  or operation with the operation buttons  7403  of the housing  7401 . Alternatively, the screen modes can be switched depending on the kind of image 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 switched from the input mode to the display mode. 
     The display portion  7402  can also 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. Moreover, when a backlight or a sensing light source which emits near-infrared light is provided in the display portion, an image of finger veins, palm veins, or the like can be taken. 
       FIG. 5E  illustrates an example of a lighting device. In a lighting device  7500 , light-emitting devices  7503   a  to  7503   d  of one embodiment of the present invention are incorporated in a housing  7501  as light sources. The lighting device  7500  can be attached to a ceiling, a wall, or the like. 
     The light-emitting device of one embodiment of the present invention includes a light-emitting element in a thin film form. Thus, when the light-emitting device is attached to a base with a curved surface, the light-emitting device with a curved surface can be obtained. In addition, when the light-emitting device is located in a housing with a curved surface, an electronic device or a lighting device with a curved surface can be obtained. 
       FIG. 6  illustrates a driver&#39;s seat and the periphery thereof inside a vehicle.  FIG. 6  illustrates an example in which a display device  600  is set on a dashboard and a display device  602  is set on a windshield. In the display device  600  illustrated in  FIG. 6 , a display portion  604  is incorporated in a housing with a curved surface and can display images. In the display device  600 , the light-emitting device of one embodiment of the present invention can be used in the display portion  604 . 
     In the display device  602  illustrated in  FIG. 6 , a display portion  606  is incorporated in a housing with a curved surface and the light-emitting device of one embodiment of the present invention can be used in the display portion  606 . A pair of electrodes and a support of a light-emitting element which are included in the light-emitting device of one embodiment of the present invention are formed using a light-transmitting material, whereby light can be extracted through both a top surface and a bottom surface of the light-emitting device. Thus, the light-emitting device is used in the display portion  606 , whereby a user can see the outside from the display portion  606  through the windshield. Similarly, a user can also see an image displayed on the display portion  606  from the outside through the windshield. 
     Note that the display device  600  or the display device  602  illustrated in  FIG. 6  can also be used as a lighting device. 
       FIG. 7  illustrates an example in which the light-emitting device is used as an indoor lighting device  801 . Since the area of the light-emitting device can be increased, the light-emitting device can be used as a lighting device with a large area. In addition, a lighting device  803  in which a light-emitting region has a curved surface can also be obtained with the use of a housing with a curved surface. A light-emitting element included in the lighting device described in this embodiment is in a thin film form, which allows the housing to be designed more freely. Therefore, the lighting device can be elaborately designed in a variety of ways. 
     A television device  7100   a  that is a television device one example of which is illustrated in  FIG. 5A  can be set in a room provided with the lighting device to which one embodiment of the present invention is applied. The television device  7100   a  may have a three-dimensional display function as well as a normal two-dimensional display function. In  FIG. 7 , a three-dimensional image can be watched with glasses  805  for watching three-dimensional images. 
     As described above, the electronic devices and the lighting devices can be obtained by application of the light-emitting device. The light-emitting device has a remarkably wide application range, and thus can be applied to electronic devices and lighting devices in a variety of fields. 
     Note that the structure described in this embodiment can be combined with any of the structures described in Embodiments 1 to 4 as appropriate. 
     EXAMPLE 1 
     In this example, an example of synthesizing 4-[4-(9-phenanthryl)phenyl]dibenzothiophene (abbreviation: DBTPPn-II) represented by Structural Formula (100) in Embodiment 1 will be described. 
     
       
         
         
             
             
         
       
     
     In a 50-mL three-neck flask were put 1.2 g (3.6 mmol) of 9-(4-bromophenyl)phenanthrene, 0.8 g (3.5 mmol) of dibenzothiophene-4-boronic acid, and 53 mg (0.2 mmol) of tris(2-methylphenyl)phosphine. The air in the flask was replaced with nitrogen. To this mixture were added 3.5 mL of a 2.0 M aqueous potassium carbonate solution, 13 mL of toluene, and 4.0 mL of ethanol. The mixture was degassed by being stirred under reduced pressure. Then, 8.0 mg (36 μmol) of palladium(II) acetate was added to this mixture, and the mixture was stirred at 80° C. for 7 hours under a nitrogen stream. After a certain period of time, the aqueous layer of the obtained mixture was extracted with toluene. 
     The obtained extract solution combined with the organic layer was washed with saturated saline, followed by drying with magnesium sulfate. This mixture was separated by gravity filtration, and the filtrate was concentrated to give an oily substance. This oily substance was purified by silica gel column chromatography. The silica gel column chromatography was conducted using a developing solvent of hexane and toluene (hexane:toluene=20:1). The obtained fractions were concentrated to give an oily substance. To this oily substance was added a mixed solvent of toluene and hexane to allow precipitation of a crystal, giving 0.8 g of white powder that was the substance to be produced, in a yield of 53%. 
     By a train sublimation method, 0.8 g of the obtained white powder was purified. In the purification, the white powder was heated at 240° C. under a pressure of 2.4 Pa with a flow rate of argon gas of 5 mL/min. After the purification, 0.7 g of white powder was obtained in a yield of 88%. The synthesis scheme of the above is shown in (E1) below. 
     
       
         
         
             
             
         
       
     
     A nuclear magnetic resonance (NMR) method identified this compound as 4-[4-(9-phenanthryl)phenyl]dibenzothiophene (abbreviation: DBTPPn-II) that was the substance to be produced. 
       1 H NMR data of the obtained compound is as follows:  1 H NMR (CDCl 3 , 300 MHz): δ (ppm)=7.46-7.53 (m, 2H), 7.58-7.73 (m, 8H), 7.80 (s, 1H), 7.87-7.96 (m, 4H), 8.07 (d, J=8.1 Hz, 1H), 8.18-8.24 (m, 2H), 8.76 (d, J=8.1 Hz, 1H), 8.82 (d, J=7.8 Hz, 1H). 
       FIGS. 8A and 8B  are  1 H NMR charts. Note that  FIG. 8B  is a chart showing an enlarged part of  FIG. 8A  in the range of 7.0 ppm to 9.0 ppm. 
       FIG. 9A  shows an absorption spectrum of a toluene solution of DBTPPn-II, and  FIG. 9B  shows an emission spectrum thereof.  FIG. 10A  shows an absorption spectrum of a thin film of DBTPPn-II, and  FIG. 10B  shows an emission spectrum thereof. The absorption spectrum was measured with an ultraviolet-visible spectrophotometer (V550, produced by JASCO Corporation). The measurements were performed with samples prepared in such a manner that the solution was put in a quartz cell while the thin film was obtained by evaporation onto a quartz substrate. The absorption spectrum of the toluene solution of DBTPPn-II was obtained by subtracting the absorption spectra of the quartz cell and toluene from a raw data, and the absorption spectrum of the thin film of DBTPPn-II was obtained by subtracting the absorption spectrum of the quartz substrate from a raw data. In each of  FIG. 9A  and  FIG. 10A , the horizontal axis represents wavelength (nm) and the vertical axis represents absorption intensity (arbitrary unit). In each of  FIG. 9B  and  FIG. 10B , the horizontal axis represents wavelength (nm) and the vertical axis represents emission intensity (arbitrary unit). In the case of the toluene solution, an absorption peak was observed at 284 nm, shoulders were observed at 303 nm and 331 nm, and emission wavelength peaks were observed at 361 nm and 375 nm (excitation wavelength: 306 nm). In the case of the thin film, an absorption peak and a shoulder were observed at 248 nm and 338 nm, respectively, and an emission wavelength peak was observed at 382 nm (excitation wavelength: 305 nm). 
     The HOMO level and the LUMO level of the thin film of DBTPPn-II were measured. The value of the HOMO level was obtained by conversion of a value of the ionization potential measured with a photoelectron spectrometer (AC-2, manufactured by Riken Keiki Co., Ltd.) in the air into a negative value. The value of the LUMO level was obtained in such a manner that the absorption edge, which was obtained from Tauc plot with an assumption of direct transition using data on the absorption spectrum of the thin film of DBTPPn-II shown in  FIG. 10B , was regarded as an optical energy gap and was added to the value of the HOMO level. According to the results, the HOMO level of DBTPPn-II was −5.86 eV, the energy gap was 3.45 eV, and the LUMO level was −2.41 eV. 
     EXAMPLE 2 
     In this example, a manufacturing method of a light-emitting element of one embodiment of the present invention and measurement results of element characteristics of the light-emitting element will be described with reference to drawings. 
     A manufacturing method of a light-emitting element 1 of this example will be described with reference to  FIG. 11 . In addition, structural formulae of organic compounds used in this example are shown below. 
                         
(Light-Emitting Element 1)
 
     First, indium tin oxide containing silicon oxide (ITSO) was deposited on a glass substrate  1100  by a sputtering method, whereby a first electrode  1101  was formed. Note that the thickness was 110 nm and the electrode area was 2 mm×2 mm. In this example, the first electrode  1101  was used as an anode. 
     Next, the glass substrate  1100  provided with the first electrode  1101  was fixed to a substrate holder in a vacuum evaporation apparatus so that a surface on which the first electrode  1101  was provided faced downward. The pressure in the vacuum evaporation apparatus was reduced to about 10 −4  Pa. After that, 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine (abbreviation: BPAFLP) and molybdenum(VI) oxide were co-evaporated, whereby a hole-injection layer  1111  was formed on the first electrode  1101 . The thickness of the hole-injection layer  1111  was 50 nm. The weight ratio of BPAFLP to molybdenum oxide was adjusted to be 4:2 (=BPAFLP:molybdenum oxide). Note that the co-evaporation method refers to an evaporation method in which evaporation is carried out from a plurality of evaporation sources at the same time in one treatment chamber. 
     Next, BPAFLP was deposited to a thickness of 10 nm on the hole-injection layer  1111 , whereby a hole-transport layer  1112  was formed. 
     Further, 4-[4-(9-phenanthryl)phenyl]dibenzothiophene (abbreviation: DBTPPn-II) synthesized in Example 1 and tris(2-phenylpyridinato-N,C 2′ )iridium(III) (abbreviation: Ir(ppy) 3 ) were co-evaporated, whereby a light-emitting layer  1113  was formed on the hole-transport layer  1112 . Here, the weight ratio of DBTPPn-II to Ir(ppy) 3  was adjusted to be 1:0.08 (=DBTPPn-II:Ir(ppy) 3 ). The thickness of the light-emitting layer  1113  was 40 nm. 
     Next, DBTPPn-II was deposited to a thickness of 15 nm on the light-emitting layer  1113 , whereby a first electron-transport layer  1114   a  was formed. 
     After that, bathophenanthroline (abbreviation: BPhen) was deposited to a thickness of 15 nm on the first electron-transport layer  1114   a , whereby a second electron-transport layer  1114   b  was formed. 
     Furthermore, a lithium fluoride (LiF) film was formed to a thickness of 1 nm on the second electron-transport layer  1114   b  by evaporation, whereby an electron-injection layer  1115  was formed. 
     Lastly, a 200-nm-thick film of aluminum was formed by evaporation, whereby a second electrode  1103  functioning as a cathode was formed. Thus, the light-emitting element 1 of this example was manufactured. 
     Note that a resistance heating method was used in all of the above evaporation steps. 
     Table 1 shows an element structure of the light-emitting element 1 obtained as described above. 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 First 
                   
                   
                   
                 Electron-transport 
                 Electron- 
                 Second 
               
               
                   
                 electrode 
                 Hole-injection 
                 Hole-transport 
                 Light-emitting 
                 layer 
                 injection layer 
                 electrode 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                 1101 
                 layer 1111 
                 layer 1112 
                 layer 1113 
                 1114a 
                 1114b 
                 1115 
                 1103 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Light-emitting 
                 ITSO 
                 BPAFLP:MoO x   
                 BPAFLP 
                 DBTPPn-II:Ir(ppy) 3   
                 DBTPPn-II 
                 BPhen 
                 LiF 
                 Al 
               
               
                 element 1 
                 110 nm 
                 (=4:2) 
                 10 nm 
                 (=1:0.08) 
                 15 nm 
                 15 nm 
                 1 nm 
                 200 nm 
               
               
                   
                   
                 50 nm 
                   
                 40 nm 
               
               
                   
               
               
                 *The mixture ratios are represented in weight ratios. 
               
            
           
         
       
     
     In a glove box containing a nitrogen atmosphere, the light-emitting element 1 was sealed so as not to be exposed to the air. Then, operation characteristics of the light-emitting element were measured. Note that the measurement was carried out at room temperature (in an atmosphere kept at 25° C.). 
       FIG. 12  shows current density-luminance characteristics of the light-emitting element 1.  FIG. 13  shows voltage-luminance characteristics thereof.  FIG. 14  shows luminance-current efficiency characteristics thereof.  FIG. 15  shows voltage-current characteristics thereof. In  FIG. 12 , the horizontal axis represents current density (mA/cm 2 ) and the vertical axis represents luminance (cd/m 2 ). In  FIG. 13 , the horizontal axis represents voltage (V) and the vertical axis represents luminance (cd/m 2 ). In  FIG. 14 , the horizontal axis represents luminance (cd/m 2 ) and the vertical axis represents current efficiency (cd/A). In  FIG. 15 , the horizontal axis represents voltage (V) and the vertical axis represents current (mA).  FIG. 16  shows an emission spectrum of the light-emitting element 1. In  FIG. 16 , the horizontal axis represents wavelength (nm) and the vertical axis represents emission intensity (arbitrary unit). 
     In the light-emitting element 1, the voltage necessary for a luminance of 800 cd/m 2  was 4.8 V, and the current flowed at that time was 0.070 mA (current density was 1.76 mA/cm 2 ). The current efficiency at that time was 45.7 cd/A. 
       FIG. 12 ,  FIG. 13 ,  FIG. 14 ,  FIG. 15 , and  FIG. 16  confirm that the light-emitting element of this example has characteristics as a light-emitting element and functions well. 
     According to  FIG. 16 , the emission spectrum of the light-emitting element 1 has a peak at around 520 nm, and green emission originating from Ir(ppy) 3  that is a guest material was observed. This shows that the phenanthrene compound described in Embodiment 1 (DBTPPn-II in this example) functions as a bipolar host material in the light-emitting layer of the light-emitting element. 
     Thus, the phenanthrene compound of one embodiment of the present invention is used in the light-emitting layer and the electron-transport layer, whereby the light-emitting element can have excellent characteristics. Further, the phenanthrene compound according to one embodiment of the present invention is used as a host material for a green phosphorescent compound in the light-emitting element of this example, and the T1 level of the phenanthrene compound according to one embodiment of the present invention was confirmed to be sufficiently high (higher than the T1 level of at least a green light-emitting material.) 
     EXAMPLE 3 
     In this example, an example of synthesizing 4-[3′-(9-phenanthryl)biphenyl-3-yl]dibenzothiophene (abbreviation: mDBTBPPn-II) represented by Structural Formula (109) in Embodiment 1 will be described. 
     
       
         
         
             
             
         
       
     
     In a 50-mL three-neck flask were put 1.1 g (2.9 mmol) of 3-[3-(dibenzothiophen-4-yl)phenyl]phenylboronic acid, 0.75 g (2.9 mmol) of 9-bromophenanthrene, and 45 mg (0.15 mmol) of tris(2-methylphenyl)phosphine. The air in the flask was replaced with nitrogen. To the mixture were added 2.9 mL of a 2M aqueous potassium carbonate solution, 12 mL of toluene, and 3.5 mL of ethanol. The mixture was degassed by being stirred under reduced pressure. Then, 6.6 mg (0.029 mmol) of palladium(II) acetate was added to this mixture, and the mixture was stirred at 90° C. for 6 hours under a nitrogen stream. After a certain period of time, the aqueous layer of this mixture was extracted with toluene. 
     The obtained extract solution combined with the organic layer was washed with water and saturated saline, followed by drying with magnesium sulfate. This mixture was separated by gravity filtration, and the filtrate was concentrated to give an oily light brown substance. This oily substance was purified by silica gel column chromatography (developing solvent: toluene). The obtained fractions were concentrated to give an oily colorless substance. To this oily substance was added a mixed solvent of toluene and hexane to allow precipitation of a crystal, giving 1.2 g of a white solid that was the substance to be produced, in a yield of 80%. 
     By a train sublimation method, 1.2 g of the obtained white solid was purified. In the purification, the white solid was heated at 250° C. under a pressure of 3.6 Pa with a flow rate of argon gas of 5.0 mL/min. After the purification, 1.0 g of a white solid was obtained in a yield of 83%. The synthesis scheme of the above is shown in (F1) below. 
     
       
         
         
             
             
         
       
     
     A nuclear magnetic resonance (NMR) method identified this compound as 4-[3′-(9-phenanthryl)biphenyl-3-yl]dibenzothiophene (abbreviation: mDBTBPPn-II) that was the substance to be produced. 
       1 H NMR data of the obtained compound is as follows:  1 H NMR (CDCl 3 , 300 MHz): δ (ppm)=7.44-7.48 (m, 2H), 7.52-7.84 (m, 14H), 7.89-7.92 (m, 2H), 8.01 (dd, 1H), 8.07-8.08 (m, 1H), 8.14-8.21 (m, 2H), 8.74 (d, 1H), 8.79 (d, 1H). 
       FIGS. 17A and 17B  are  1 H NMR charts. Note that  FIG. 17B  is a chart showing an enlarged part of  FIG. 17A  in the range of 7.0 ppm to 9.0 ppm. 
       FIG. 18A  shows an absorption spectrum of a toluene solution of mDBTBPPn-II, and  FIG. 18B  shows an emission spectrum thereof.  FIG. 19A  shows an absorption spectrum of a thin film of mDBTBPPn-II, and  FIG. 19B  shows an emission spectrum thereof. The absorption spectrum was measured with an ultraviolet-visible spectrophotometer (V550, produced by JASCO Corporation). The measurements were performed with samples prepared in such a manner that the solution was put in a quartz cell while the thin film was obtained by evaporation onto a quartz substrate. The absorption spectrum of the toluene solution of mDBTBPPn-II was obtained by subtracting the absorption spectra of the quartz cell and toluene from a raw data, and the absorption spectrum of the thin film of mDBTBPPn-II was obtained by subtracting the absorption spectrum of the quartz substrate from a raw data. In each of  FIG. 18A  and  FIG. 19A , the horizontal axis represents wavelength (nm) and the vertical axis represents absorption intensity (arbitrary unit). In each of  FIG. 18B  and  FIG. 19B , the horizontal axis represents wavelength (nm) and the vertical axis represents emission intensity (arbitrary unit). In the case of the toluene solution, absorption peaks were observed at 282 nm and 331 nm, an absorption shoulder is observed at 298 nm, and emission wavelength peaks were observed at 356 nm, 365 nm, 374 nm, and 393 nm (excitation wavelength: 330 nm). In the case of the thin film, absorption peaks were observed at around 212 nm, 249 nm, and 334 nm, absorption shoulders are observed at 261 nm, 287 nm, and 305 nm, and emission wavelength peaks were observed at 361 nm and 377 nm, with a shoulder at 395 nm (excitation wavelength: 335 nm). 
     Further, the HOMO level and the LUMO level of the thin film of mDBTBPPn-II were measured. The value of the HOMO level was obtained by conversion of a value of the ionization potential measured with a photoelectron spectrometer (AC-2, manufactured by Riken Keiki Co., Ltd.) in the air into a negative value. The value of the LUMO level was obtained in such a manner that the absorption edge, which was obtained from Tauc plot with an assumption of direct transition using data on the absorption spectrum of the thin film of mDBTBPPn-II shown in  FIG. 19B , was regarded as an optical energy gap and was added to the value of the HOMO level. According to the results, the HOMO level of mDBTBPPn-II was −5.90 eV, the energy gap was 3.47 eV, and the LUMO level was −2.43 eV. 
     EXAMPLE 4 
     In this example, manufacturing methods of light-emitting elements of one embodiment of the present invention and measurement results of element characteristics of the light-emitting elements will be described with reference to drawings. 
     Manufacturing methods of a light-emitting element 2 and a light-emitting element 3 of this example will be described with reference to  FIG. 20 . An organic compound used in this example is shown below. Note that description of organic compounds the same as those described in Example 2 is omitted. 
                         
(Light-Emitting Element 2)
 
     First, indium tin oxide containing silicon oxide (ITSO) was deposited on a glass substrate  1100  by a sputtering method, whereby a first electrode  1101  was formed. Note that the thickness was 110 nm and the electrode area was 2 mm×2 mm. In this example, the first electrode  1101  was used as an anode. 
     Next, the glass substrate  1100  provided with the first electrode  1101  was fixed to a substrate holder in a vacuum evaporation apparatus so that a surface on which the first electrode  1101  was provided faced downward. The pressure in the vacuum evaporation apparatus was reduced to about 10 −4  Pa. After that, 1,3,5-tri(dibenzothiophen-4-yl)-benzene (abbreviation: DBT3P-II) and molybdenum(VI) oxide were co-evaporated, whereby a hole-injection layer  1111  was formed on the first electrode  1101 . The thickness of the hole-injection layer  1111  was 40 nm. The weight ratio of DBT3P-II to molybdenum oxide was adjusted to be 4:2 (=DBT3P-II:molybdenum oxide). Note that the co-evaporation method refers to an evaporation method in which evaporation is carried out from a plurality of evaporation sources at the same time in one treatment chamber. 
     Next, BPAFLP was deposited to a thickness of 20 nm on the hole-injection layer  1111 , whereby a hole-transport layer  1112  was formed. 
     Further, 4-[4-(9-phenanthryl)phenyl]dibenzothiophene (abbreviation: DBTPPn-II) synthesized in Example 1 and tris(2-phenylpyridinato-N,C 2′ )iridium(III) (abbreviation: Ir(ppy) 3 ) were co-evaporated, whereby a light-emitting layer was formed on the hole-transport layer  1112 . The light-emitting layer here had a stacked structure of a first light-emitting layer  1113   a  with a thickness of 10 nm and a second light-emitting layer  1113   b  with a thickness of 20 nm. The first light-emitting layer  1113   a  was formed in such a manner that the weight ratio of DBTPPn-II to Ir(ppy) 3  was adjusted to be 1:0.08 (=DBTPPn-II:Ir(ppy) 3 ). The second light-emitting layer  1113   b  was formed in such a manner that the weight ratio of DBTPPn-II to Ir(ppy) 3  was adjusted to be 1:0.04 (=DBTPPn-II:Ir(ppy) 3 ). 
     Next, DBTPPn-II was deposited to a thickness of 10 nm on the second light-emitting layer  1113   b , whereby a first electron-transport layer  1114   a  was formed. 
     Then, bathophenanthroline (abbreviation: BPhen) was deposited to a thickness of 20 nm on the first electron-transport layer  1114   a , whereby a second electron-transport layer  1114   b  was formed. 
     Furthermore, a lithium fluoride (LiF) film was formed to a thickness of 1 nm on the second electron-transport layer  1114   b  by evaporation, whereby an electron-injection layer  1115  was formed. 
     Lastly, a 200-nm-thick film of aluminum was formed by evaporation, whereby a second electrode  1103  functioning as a cathode was formed. Thus, the light-emitting element 2 of this example was manufactured. 
     Note that a resistance heating method was used in all of the above evaporation steps. 
     Next, a manufacturing method of the light-emitting element 3 will be described. 
     (Light-Emitting Element 3) 
     The light-emitting element 3 was manufactured in a manner similar to that of the light-emitting element 2 by using the same substrate and using 4-[3′-(9-phenanthryl)biphenyl-3-yl]dibenzothiophene (abbreviation: mDBTBPPn-II) described in Example 3 instead of DBTPPn-II in a light-emitting layer. In other words, the light-emitting layer of the light-emitting element 3 was formed by co-evaporation of mDBTBPPn-II and tris(2-phenylpyridinato-N,C 2′ )iridium(III) (abbreviation: Ir(ppy) 3 ). The light-emitting layer here had a stacked structure of a first light-emitting layer  1113   a  with a thickness of 10 nm and a second light-emitting layer  1113   b  with a thickness of 20 nm. The first light-emitting layer  1113   a  was formed in such a manner that the weight ratio of mDBTBPPn-II to Ir(ppy) 3  was adjusted to be 1:0.08 (=mDBTBPPn-II:Ir(ppy) 3 ). The second light-emitting layer  1113   b  was formed in such a manner that the weight ratio of mDBTBPPn-II to Ir(ppy) 3  was adjusted to be 1:0.04 (=mDBTBPPn-II:Ir(ppy) 3 ). 
     Next, mDBTBPPn-II was deposited to a thickness of 10 nm on the second light-emitting layer  1113   b , whereby a first electron-transport layer  1114   a  was formed. 
     Note that the light-emitting element 3 was formed in a manner similar to that of the light-emitting element 2 except for the light-emitting layer and the first electron-transport layer  1114   a.    
     Table 2 shows element structures of the light-emitting element 2 and the light-emitting element 3. 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
             
            
               
                   
                   
               
               
                   
                   
                   
                 Hole- 
                   
                   
                 Electron- 
                   
               
               
                   
                 First 
                 Hole- 
                 transport 
                   
                 Electron-transport 
                 injection 
                 Second 
               
               
                   
                 electrode 
                 injection 
                 layer 
                 Light-emitting layer 
                 layer 
                 layer 
                 electrode 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 1101 
                 layer 1111 
                 1112 
                 1113a 
                 1113b 
                 1114a 
                 1114b 
                 1115 
                 1103 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 Light-emitting 
                 ITSO 
                 DBT3P-II:MoO x   
                 BPAFLP 
                 DBTPPn-II:Ir(ppy) 3   
                 DBTPPn-II:Ir(ppy) 3   
                 DBTPPn- 
                 BPhen 
                 LiF 
                 Al 
               
               
                 element 2 
                 110 nm 
                 (=4:2) 
                 20 nm 
                 (=1:0.08) 
                 (=1:0.04) 
                 II 
                 20 nm 
                 1 nm 
                 200 nm 
               
               
                   
                   
                 40 nm 
                   
                 10 nm 
                 20 nm 
                 10 nm 
               
               
                 Light-emitting 
                 ITSO 
                 DBT3P-II:MoO x   
                 BPAFLP 
                 mDBTBPPn- 
                 mDBTBPPn- 
                 DBTPPn- 
                 BPhen 
                 LiF 
                 Al 
               
               
                 element 3 
                 110 nm 
                 (=4:2) 
                 20 nm 
                 II:Ir(ppy) 3   
                 II:Ir(ppy) 3   
                 II 
                 20 nm 
                 1 nm 
                 200 nm 
               
               
                   
                   
                 40 nm 
                   
                 (=1:0.08) 
                 (=1:0.04) 
                 10 nm 
               
               
                   
                   
                   
                   
                 10 nm 
                 20 nm 
               
               
                   
               
               
                 *The mixture ratios are represented in weight ratios. 
               
            
           
         
       
     
     In a glove box containing a nitrogen atmosphere, the thus obtained light-emitting element 2 and light-emitting element 3 were sealed so as not to be exposed to the air. Then, operation characteristics of each of the light-emitting elements were measured. Note that the measurement was carried out at room temperature (in an atmosphere kept at 25° C.). 
       FIG. 21  shows current density-luminance characteristics of the light-emitting element 2 and the light-emitting element 3.  FIG. 22  shows voltage-luminance characteristics thereof.  FIG. 23  shows luminance-current efficiency characteristics thereof.  FIG. 24  shows voltage-current characteristics thereof. In  FIG. 21 , the horizontal axis represents current density (mA/cm 2 ) and the vertical axis represents luminance (cd/m 2 ). In  FIG. 22 , the horizontal axis represents voltage (V) and the vertical axis represents luminance (cd/m 2 ). In  FIG. 23 , the horizontal axis represents luminance (cd/m 2 ) and the vertical axis represents current efficiency (cd/A). In  FIG. 24 , the horizontal axis represents voltage (V) and the vertical axis represents current (mA).  FIG. 25  shows emission spectra of the light-emitting element 2 and the light-emitting element 3. In  FIG. 25 , the horizontal axis represents wavelength (nm) and the vertical axis represents emission intensity (arbitrary unit). 
     According to  FIG. 21 ,  FIG. 22 ,  FIG. 23 , and  FIG. 24 , the luminance of the light-emitting element 2 at an applied voltage of 4.8 V was 1130 cd/m 2 , and the current flowed at that time was 0.13 mA (the current density was 3.2 mA/cm 2 ). The current efficiency at that time was 35.5 cd/A. These results showed that light emission was able to be obtained efficiently from the light-emitting element 2. 
     In contrast, the luminance of the light-emitting element 3 at an applied voltage of 4.4 V was 820 cd/m 2 , and the current flowed at that time was 0.05 mA (the current density was 1.2 mA/cm 2 ). The current efficiency at that time was 68.8 cd/A. These results showed that light emission was able to be obtained from the light-emitting element 3 more efficiently. 
     According to  FIG. 25 , the emission spectra of the light-emitting element 2 and the light-emitting element 3 gave peaks at 515 to 520 nm, and green emission originating from Ir(ppy) 3  that is a guest material was observed therefrom. The above means that the phenanthrene compounds described in Embodiment 1 (DBTPPn-II and mDBTBPPn-II in this example) function as bipolar host materials in the light-emitting layers of the light-emitting elements. 
     Next, the light-emitting element 2 and the light-emitting element 3 were subjected to a reliability test. In the reliability test, the initial luminance was set at 5000 cd/m 2 , the light-emitting element 2 and the light-emitting element 3 of this example were driven at a constant current density, and the luminance was measured at regular intervals.  FIG. 26  shows results of the reliability test. In  FIG. 26 , the vertical axis represents the percentage of luminance at each time point on the assumption that the initial luminance is 100%, that is, normalized luminance (%), and the horizontal axis represents driving time (h) of the elements. 
     According to  FIG. 26 , the light-emitting element 2 and the light-emitting element 3 respectively maintained 52% and 79% of the initial luminance even after being driven for 140 hours. The results show that the light-emitting element in which mDBTBPPn-II is used has long lifetime. Therefore, the light-emitting element in which the phenanthrene compound of the present invention is used has a long lifetime. 
     REFERENCE EXAMPLE 
     In this reference example, an example of synthesizing 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine (abbreviation: BPAFLP) that was used as a material of the light-emitting element 1, the light-emitting element 2, and the light-emitting element 3 will be described. 
     Step 1: Synthesis Method of 9-(4-bromophenyl)-9-phenylfluorene 
     In a 100-mL three-neck flask, 1.2 g (50 mmol) of magnesium was heated and stirred under reduced pressure for 30 minutes to be activated. After the flask was cooled to room temperature and was made to have a nitrogen atmosphere, several drops of dibromoethane were added, so that bubble release and heat generation were confirmed. After 12 g (50 mmol) of 2-bromobiphenyl dissolved in 10 mL of dehydrated diethyl ether was slowly added into this mixture, the mixture was stirred and heated under reflux for 2.5 hours, whereby a Grignard reagent was prepared. 
     In a 500-mL three-neck flask were put 10 g (40 mmol) of 4-bromobenzophenone and 100 mL of dehydrated diethyl ether. After the Grignard reagent synthesized in advance was slowly added into this mixture, the mixture was heated and stirred under reflux for 9 hours. 
     After reaction, the mixture was filtered to give a residue. The residue was dissolved in 150 mL of ethyl acetate, and 1N hydrochloric acid was added to acidify the solution which was then stirred for 2 hours. The organic layer of this solution was washed with water. Then, magnesium sulfate was added to remove moisture. This suspension was filtered, and the resulting filtrate was concentrated to give an oily substance. In a 500-mL recovery flask were put this oily substance, 50 mL of glacial acetic acid, and 1.0 mL of hydrochloric acid. The mixture was reacted by heating at 130° C. for 1.5 hours under a nitrogen atmosphere with stirring. 
     After reaction, this reaction mixture was filtered to give a residue. The obtained residue was washed with water, a sodium hydroxide aqueous solution, water, and methanol in this order, and then dried to give 11 g of white powder that was the substance to be produced, in a yield of 69%. The synthesis scheme of Step 1 is shown in (J-1) below. 
     
       
         
         
             
             
         
       
     
     Step 2: Synthesis Method of 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine (Abbreviation: BPAFLP) 
     In a 100-mL three-neck flask were put 3.2 g (8.0 mmol) of 9-(4-bromophenyl)-9-phenylfluorene, 2.0 g (8.0 mmol) of 4-phenyldiphenylamine, 1.0 g (10 mmol) of sodium tert-butoxide, and 23 mg (0.04 mmol) of bis(dibenzylideneacetone)palladium(0). The atmosphere in the flask was replaced with nitrogen. Then, 20 mL of dehydrated xylene was added to this mixture. After the mixture was deaerated while being stirred under reduced pressure, 0.2 mL (0.1 mmol) of tri(tert-butyl)phosphine (10 wt % hexane solution) was added thereto. This mixture was allowed to be reacted by heating at 110° C. for 2 hours under a nitrogen atmosphere with stirring. 
     After reaction, 200 mL of toluene was added to the reaction mixture, and the resulting suspension was filtered through Florisil (produced by Wako Pure Chemical Industries, Ltd., Catalog No. 540-00135) and Celite (produced by Wako Pure Chemical Industries, Ltd., Catalog No. 531-16855). The obtained filtrate was concentrated, and the resulting substance was purified by silica gel column chromatography (with a developing solvent containing toluene and hexane in a 1:4 ratio). The obtained fractions were concentrated, and acetone and methanol were added thereto. The mixture was irradiated with ultrasonic waves and allowed a crystal to precipitate, giving 4.1 g of white powder that was the substance to be produced, in a yield of 92%. The synthesis scheme of Step 2 is shown in (J-2) 
     
       
         
         
             
             
         
       
     
     The Rf values of the substance to be produced, 9-(4-bromophenyl)-9-phenylfluorene, and 4-phenyl-diphenylamine were respectively 0.41, 0.51, and 0.27, which were obtained by silica gel thin layer chromatography (TLC) (with a developing solvent containing ethyl acetate and hexane in a 1:10 ratio). 
     A nuclear magnetic resonance (NMR) method identified this compound as 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine (abbreviation: BPAFLP) that was the substance to be produced. 
       1 H NMR data of the obtained compound is as follows:  1 H NMR (CDCl 3 , 300 MHz): δ (ppm)=6.63-7.02 (m, 3H), 7.06-7.11 (m, 6H), 7.19-7.45 (m, 18H), 7.53-7.55 (m, 2H), 7.75 (d, J=6.9, 2H). 
     This application is based on Japanese Patent Application serial no. 2010-243323 filed with Japan Patent Office on Oct. 29, 2010, the entire contents of which are hereby incorporated by reference.