Patent Publication Number: US-11393997-B2

Title: Light-emitting element, light-emitting device, electronic device, and lighting device

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     One embodiment of the present invention relates to a light-emitting element in which an organic compound capable of providing light emission by application of an electric field is provided between a pair of electrodes, and also relates to a light-emitting device, an electronic device, and a lighting device including such a light-emitting element. 
     2. Description of the Related Art 
     Light-emitting elements including an organic compound as a luminous body, which have features such as thinness, lightness, high-speed response, and DC driving at low voltage, are expected to be applied to next-generation flat panel displays. In particular, display devices in which light-emitting elements are arranged in a matrix are considered to have advantages of a wide viewing angle and high visibility over conventional liquid crystal display devices. 
     It is said that the light emission mechanism of a light-emitting element is as follows: when a voltage is applied between a pair of electrodes with an EL layer including a luminous body provided therebetween, electrons injected from the cathode and holes injected from the anode recombine in the light emission center of the EL layer to form molecular excitons, and energy is released and light is emitted when the molecular excitons relax to the ground state. A singlet excited state and a triplet excited state are known as the excited states, and it is thought that light emission can be obtained through either of the excited states. 
     In order to improve element characteristics of such light-emitting elements, improvement of an element structure, development of a material, and the like have been actively carried out (see, for example, Patent Document 1). 
     REFERENCE 
     [Patent Document 1] Japanese Published Patent Application No. 2010-182699 
     SUMMARY OF THE INVENTION 
     However, it is said that the light extraction efficiency of a light-emitting element at present is approximately 20% to 30%. Even considering light absorption by a reflective electrode and a transparent electrode, the external quantum efficiency of a light-emitting element including a phosphorescent compound has a limit of approximately 25% at most. 
     In one embodiment of the present invention, a light-emitting element with high external quantum efficiency is provided. In another embodiment of the present invention, a light-emitting element having a long lifetime is provided. 
     One embodiment of the present invention is a light-emitting element including a light-emitting layer between a pair of electrodes (anode and cathode). The light-emitting layer has a stacked-layer structure including a first light-emitting layer and a second light-emitting layer. The first light-emitting layer is formed on the anode side and contains at least a first light-emitting substance (guest material) converting triplet excitation energy into light emission, a first organic compound (host material) having an electron-transport property, and a second organic compound (assist material) having a hole-transport property. The second light-emitting layer contains at least a second light-emitting substance (guest material) converting triplet excitation energy into light emission, the first organic compound (host material) having an electron-transport property, and a third organic compound (assist material) having a hole-transport property. In the first light-emitting layer, a combination of the first organic compound (host material) and the second organic compound (assist material) forms an exciplex, and in the second light-emitting layer, a combination of the first organic compound (host material) and the third organic compound (assist material) forms an exciplex. 
     Another embodiment of the present invention is a light-emitting element including a light-emitting layer between an anode and a cathode, a hole-transport layer between the anode and the light-emitting layer, and an electron-transport layer between the cathode and the light-emitting layer. The light-emitting layer is a stack of a first light-emitting layer and a second light-emitting layer. The first light-emitting layer is formed in contact with the hole-transport layer and contains at least a first light-emitting substance converting triplet excitation energy into light emission, a first organic compound having an electron-transport property, and a second organic compound having a hole-transport property. The second light-emitting layer is formed in contact with the electron-transport layer and contains at least a second light-emitting substance converting triplet excitation energy into light emission, the first organic compound having an electron-transport property, and a third organic compound having a hole-transport property. In the first light-emitting layer, a combination of the first organic compound (host material) and the second organic compound (assist material) forms an exciplex, and in the second light-emitting layer, a combination of the first organic compound (host material) and the third organic compound (assist material) forms an exciplex. 
     Note that in each of the above structures, the emission wavelength of the exciplex formed by the first organic compound (host material) and the second organic compound (assist material) in the first light-emitting layer is located on the longer wavelength side with respect to the emission wavelength (fluorescent wavelength) of each of the first and second organic compounds (host and assist materials). Therefore, by formation of the exciplex, the fluorescence spectrum of the first organic compound (host material) and the fluorescence spectrum of the second organic compound (assist material) can be converted into an emission spectrum which is located on the longer wavelength side. Further, the emission wavelength of the exciplex formed by the first organic compound (host material) and the third organic compound (assist material) in the second light-emitting layer is located on the longer wavelength side with respect to the emission wavelength (fluorescent wavelength) of each of the first and third organic compounds (host and assist materials). Therefore, by formation of the exciplex, the fluorescence spectrum of the first organic compound (host material) and the fluorescence spectrum of the third organic compound (assist material) can be converted into an emission spectrum which is located on the longer wavelength side. 
     Note that in each of the above structures, in the first light-emitting layer, an exciplex is formed from an anion of the first organic compound and a cation of the second organic compound, and in the second light-emitting layer, an exciplex is formed from an anion of the first organic compound and a cation of the third organic compound. 
     In the above structures, the first light-emitting layer and the second light-emitting layer contain the same first organic compound (host material), and the highest occupied molecular orbital level (HOMO level) of the second organic compound (assist material) contained in the first light-emitting layer is lower than that of the third organic compound (assist material) contained in the second light-emitting layer. 
     In addition, in the above structures, the first light-emitting substance contained in the first light-emitting layer is a substance which emits light with a wavelength shorter than that of light emitted from the second light-emitting substance contained in the second light-emitting layer. 
     In the above structures, the first light-emitting substance and the second light-emitting substance are light-emitting substances which convert triplet excitation energy into light emission, and phosphorescent compounds such as an organometallic complex, or a material emitting thermally activated delayed fluorescence, i.e., a thermally activated delayed fluorescence (TADF) material, can be used. Further, the first organic compound is mainly an electron-transport material with an electron mobility of 10 −6  cm 2 /Vs or more, specifically, a π-electron deficient heteroaromatic compound. The second organic compound and the third organic compound are mainly hole-transport materials with a hole mobility of 10 −6  cm 2 /Vs or more, specifically, π-electron rich heteroaromatic compounds or aromatic amine compounds. 
     Further, the present invention includes, in its scope, electronic devices and lighting devices including light-emitting devices, as well as light-emitting devices including light-emitting elements. The light-emitting device in this specification refers to an image display device and a light source (e.g., a lighting device). In addition, the light-emitting device includes all the following modules: a module in which a connector, such as a flexible printed circuit (FPC) or a tape carrier package (TCP), is attached to a light-emitting device; a module in which a printed wiring board is provided at the end of a TCP; and a module in which an integrated circuit (IC) is directly mounted on a light-emitting element by a chip-on-glass (COG) method. 
     Note that in a light-emitting element that is one embodiment of the present invention, an exciplex can be formed in each of the first light-emitting layer and the second light-emitting layer which are included in the light-emitting layer; thus, the light-emitting element can have high energy transfer efficiency and high external quantum efficiency. 
     Further, in a light-emitting element that is one embodiment of the present invention, owing to the above-described element structure, the exciplex formed in the first light-emitting layer has higher excitation energy than the exciplex formed in the second light-emitting layer; therefore, when a substance emitting light with a wavelength shorter than that of light emitted from the second light-emitting substance (guest material) which is contained in the second light-emitting layer and converts triplet excitation energy into light emission is used as the first light-emitting substance (guest material) which is contained in the first light-emitting layer and converts triplet excitation energy into light emission, the first light-emitting layer and the second light-emitting layer can provide light emission at the same time. Further, part of excitation energy of the exciplex formed in the first light-emitting layer which does not contribute to light emission can be utilized as excitation energy for the second light-emitting substance (guest material) which converts triplet excitation energy into light emission in the second light-emitting layer. Accordingly, emission efficiency in the light-emitting element can be further increased. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  illustrate a concept of one embodiment of the present invention. 
         FIG. 2  illustrates a concept of one embodiment of the present invention. 
         FIG. 3  show calculation results according to one embodiment of the present invention. 
       FIGS.  4 A 1 ,  4 A 2 ,  4 B 1 ,  4 B 2 ,  4 C 1 , and  4 C 2  show calculation results according to one embodiment of the present invention. 
         FIG. 5  illustrates a structure of a light-emitting element. 
         FIGS. 6A and 6B  each illustrate a structure of a light-emitting element. 
         FIG. 7  illustrates a light-emitting device. 
         FIGS. 8A and 8B  illustrate a light-emitting device. 
         FIGS. 9A to 9D  illustrate electronic devices. 
         FIGS. 10A to 10C  illustrate an electronic device. 
         FIG. 11  illustrates lighting devices. 
         FIG. 12  illustrates a light-emitting element. 
         FIG. 13  shows current density-luminance characteristics of a light-emitting element  1 . 
         FIG. 14  shows voltage-luminance characteristics of a light-emitting element  1 . 
         FIG. 15  shows luminance-current efficiency characteristics of a light-emitting element  1 . 
         FIG. 16  shows voltage-current characteristics of a light-emitting element  1 . 
         FIG. 17  shows an emission spectrum of a light-emitting element  1 . 
         FIG. 18  shows emission spectra of substances used in a light-emitting element  1 . 
         FIG. 19  shows emission spectra of substances used in a light-emitting element  1 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the present invention will be described below with reference to the drawings. Note that the present invention is not limited to the following description, and various changes and modifications can be made without departing from the spirit and scope of the invention. Therefore, the present invention should not be construed as being limited to the description in the following embodiments. 
     Elementary Process of Light Emission in Light-Emitting Element 
     First, a description is given of general elementary processes of light emission in a light-emitting element which uses a light-emitting substance converting triplet excitation energy into light emission (e.g., a phosphorescent compound or a thermally activated delayed fluorescence (TADF) material) as a guest material. Note that a molecule providing excitation energy is referred to as a host molecule, while a molecule receiving the excitation energy is referred to as a guest molecule. 
     (1) The case where an electron and a hole recombine in a guest molecule, and the guest molecule is put in an excited state (direct recombination process). 
     (1-1) When the excited state of the guest molecule is a triplet excited state, the guest molecule emits phosphorescence. 
     (1-2) When the excited state of the guest molecule is a singlet excited state, the guest molecule in the singlet excited state undergoes intersystem crossing to a triplet excited state and emits phosphorescence. 
     In other words, in the direct recombination process in (1), as long as the efficiency of intersystem crossing and the phosphorescence quantum yield of the guest molecule are high, high emission efficiency can be obtained. Note that the T 1  level of the host molecule is preferably higher than the T 1  level of the guest molecule. 
     (2) The case where an electron and a hole recombine in a host molecule and the host molecule is put in an excited state (energy transfer process). 
     (2-1) When the excited state of the host molecule is a triplet excited state and the T 1  level of the host molecule is higher than the T 1  level of the guest molecule, excitation energy is transferred from the host molecule to the guest molecule, and thus the guest molecule is put in a triplet excited state. The guest molecule in the triplet excited state emits phosphorescence. Note that energy transfer from the T 1  level of the host molecule to a singlet excitation energy level (S 1  level) of the guest molecule is forbidden unless the host molecule emits phosphorescence, and is unlikely to be a main energy transfer process; therefore, a description thereof is omitted here. In other words, energy transfer from the host molecule in the triplet excited state (3H*) to the guest molecule in the triplet excited state (3G*) is important as represented by Formula (2-1) below (where 1G represents the singlet ground state of the guest molecule and 1H represents the singlet ground state of the host molecule).
 
3 H*+ 1 G→ 1 H+ 3 G*   (2-1)
 
     (2-2) When the excited state of the host molecule is a singlet excited state and the S 1  level of the host molecule is higher than the S 1  level and T 1  level of the guest molecule, excitation energy is transferred from the host molecule to the guest molecule, and thus, the guest molecule is put in a singlet excited state or a triplet excited state. The guest molecule in the triplet excited state emits phosphorescence. In addition, the guest molecule in the singlet excited state undergoes intersystem crossing to a triplet excited state, and emits phosphorescence. 
     In other words, there can be a process where energy is transferred from the host molecule in the singlet excited state (1H*) to the guest molecule in the singlet excited state (1G*) and then the guest molecule is put in the triplet excited state (3G*) by intersystem crossing, as represented by Formula (2-2A) below, and a process where energy is directly transferred from the host molecule in the singlet excited state (1H*) to the guest molecule in the triplet excited state (3G*), as represented by Formula (2-2B) below.
 
1 H*+ 1 G→ 1 H+ 1 G *→(Intersystem crossing)→1 H+ 3 G*   (2-2A)
 
1 H*+ 1 G→ 1 H+ 3 G*   (2-2B)
 
     When all the energy transfer processes described above in (2) occur efficiently, both the triplet excitation energy and the singlet excitation energy of the host molecule are efficiently converted into the triplet excited state (3G*) of the guest molecule. Thus, high-efficiency light emission is possible. In contrast, before the excitation energy of the host molecule is transferred to the guest molecule, when the host molecule itself is deactivated by emitting the excitation energy as light or heat, the emission efficiency decreases. 
     Next, factors controlling the above-described processes of intermolecular energy transfer between the host molecule and the guest molecule are described. As mechanisms of the intermolecular energy transfer, the following two mechanisms are proposed. 
     One mechanism is Forster mechanism (dipole-dipole interaction) in which energy transfer does not require direct contact between molecules and energy is transferred through a resonant phenomenon of dipolar oscillation between a host molecule and a guest molecule. By the resonant phenomenon of dipolar oscillation, the host molecule provides energy to the guest molecule, and thus, the host molecule is put in a ground state and the guest molecule is put in an excited state. Note that the rate constant k h*→g  of Forster mechanism is expressed by Formula (1). 
     
       
         
           
             
               
                 
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                     Formula 
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     In Formula (1), v denotes a frequency, f′ h (v) denotes a normalized emission spectrum of a host molecule (a fluorescence spectrum in energy transfer from a singlet excited state, and a phosphorescence spectrum in energy transfer from a triplet excited state), ε g (v) denotes a molar absorption coefficient of a guest molecule, N denotes Avogadro&#39;s number, n denotes a refractive index of a medium, R denotes an intermolecular distance between the host molecule and the guest molecule, τ denotes a measured lifetime of an excited state (fluorescence lifetime or phosphorescence lifetime), c denotes the speed of light, ϕ denotes a luminescence quantum yield (a fluorescence quantum yield in energy transfer from a singlet excited state, and a phosphorescence quantum yield in energy transfer from a triplet excited state), and K 2  denotes a coefficient (0 to 4) of orientation of a transition dipole moment between the host molecule and the guest molecule. Note that K 2 =⅔ in random orientation. 
     The other is Dexter mechanism (electron exchange interaction) in which a host molecule and a guest molecule are close to a contact effective range where their orbitals overlap, and the host molecule in an excited state and the guest molecule in a ground state exchange their electrons, which leads to energy transfer. Note that the rate constant k h*→g  of Dexter mechanism is expressed by Formula (2). 
     
       
         
           
             
               
                 
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     In Formula (2), h denotes a Planck constant, K′ denotes a constant having an energy dimension, v denotes a frequency, f′ h (v) denotes a normalized emission spectrum of a host molecule (a fluorescence spectrum in energy transfer from a singlet excited state, and a phosphorescence spectrum in energy transfer from a triplet excited state), ε′ g (v) denotes a normalized absorption spectrum of a guest molecule, L denotes an effective molecular radius, and R denotes an intermolecular distance between the host molecule and the guest molecule. 
     Here, the efficiency of energy transfer from the host molecule to the guest molecule (energy transfer efficiency Φ ET ) is thought to be expressed by Formula (3). In the formula, k r  denotes a rate constant of a light-emission process (fluorescence in energy transfer from a singlet excited state, and phosphorescence in energy transfer from a triplet excited state) of a host molecule, k n  denotes a rate constant of a non-light-emission process (thermal deactivation or intersystem crossing) of a host molecule, and τ denotes a measured lifetime of an excited state of a host molecule. 
     
       
         
           
             
               
                 
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     According to Formula (3), it is found that the energy transfer efficiency Φ ET  can be increased by increasing the rate constant k h*→g  of energy transfer so that another competing rate constant k r +k n (=1/τ) becomes relatively small. 
     Energy Transfer Efficiency in (2-1) 
     Here, the energy transfer process in (2-1) is considered first. Since Forster mechanism (Formula (1)) is forbidden in this case, only Dexter mechanism (Formula (2)) should be considered. According to Formula (2), in order to increase the rate constant k h*→g , it is preferable that an emission spectrum of a host molecule (here, a phosphorescence spectrum because energy transfer from a triplet excited state is discussed) largely overlap with an absorption spectrum of a guest molecule (absorption corresponding to direct transition from a singlet ground state to a triplet excited state). 
     In one embodiment of the present invention, a light-emitting substance converting triplet excitation energy into light emission (e.g., a phosphorescent compound or a thermally activated delayed fluorescence (TADF) material) is used as a guest material. In an absorption spectrum of the phosphorescent compound, absorption corresponding to direct transition from a singlet ground state to a triplet excited state is observed in some cases, which is an absorption band on the longest wavelength side. In particular, light-emitting iridium complexes have a broad absorption band at around 500 nm to 600 nm as the absorption band on the longest wavelength side (as a matter of fact, the broad absorption band can be on a shorter or longer wavelength side depending on emission wavelengths). This absorption band is mainly based on a triplet MLCT (metal to ligand charge transfer) transition. Note that it is considered that the absorption band also includes absorptions based on a triplet π-π* transition and a singlet MLCT transition, and that these absorptions overlap each other to form a broad absorption band on the longest wavelength side in the absorption spectrum. In other words, the difference between the lowest singlet excited state and the lowest triplet excited state is small, and absorptions based on these states overlap each other to form a broad absorption band on the longest wavelength side in the absorption spectrum. Therefore, when an organometallic complex (especially iridium complex) is used as the guest material, the broad absorption band on the longest wavelength side largely overlaps with the phosphorescence spectrum of the host material as described above, whereby the rate constant k h*→g  can be increased and energy transfer efficiency can be increased. 
     Furthermore, a fluorescent compound is generally used as the host material; thus, a phosphorescence lifetime (τ) is a millisecond or longer which is extremely long (i.e., k r +k n  is low). This is because the transition from the triplet excited state to the ground state (singlet) is a forbidden transition. Formula (3) shows that this is favorable to energy transfer efficiency Φ ET . 
     The above description also suggests that energy transfer from the host material in the triplet excited state to the guest material in the triplet excited state, i.e., the process in Formula (2-1), generally tends to occur as long as the phosphorescence spectrum of the host material overlaps with the absorption spectrum corresponding to the direct transition of the guest material from the singlet ground state to the triplet excited state. 
     Energy Transfer Efficiency in (2-2) 
     Next, the energy transfer process in (2-2) is considered. The process in Formula (2-2A) is affected by the efficiency of intersystem crossing of the guest material. Therefore, in order to maximize emission efficiency, the process in Formula (2-2B) is considered to be important. Since Dexter mechanism (Formula (2)) is forbidden in this case, only Forster mechanism (Formula (1)) should be considered. 
     When τ is eliminated from Formula (1) and Formula (3), it can be said that the energy transfer efficiency Φ ET  is higher when the quantum yield ϕ (here, a fluorescent quantum yield because energy transfer from a singlet excited state is discussed) is higher. However, in practice, a more important factor is that the emission spectrum of the host molecule (here, a fluorescence spectrum because energy transfer from a singlet excited state is discussed) largely overlaps with the absorption spectrum of the guest molecule (absorption corresponding to the direct transition from the singlet ground state to the triplet excited state) (note that it is preferable that the molar absorption coefficient of the guest molecule be also high). This means that the fluorescence spectrum of the host material overlaps with the absorption band of the phosphorescent compound used as the guest material which is on the longest wavelength side. 
     However, this has conventionally been very difficult to achieve. The reason is that, in order to enable both of the above-described processes (2-1) and (2-2) to occur efficiently, it is clear from the above discussion that the host material should be designed so as to have not only its phosphorescence spectrum but also its fluorescence spectrum overlapping with the absorption band of the guest material which is on the longest wavelength side. In other words, the host material should be designed so as to have its fluorescence spectrum in a position similar to that of the phosphorescence spectrum. 
     However, in general, the S 1  level differs greatly from the T 1  level (S 1  level&gt;T 1  level); therefore, the fluorescence emission wavelength also differs greatly from the phosphorescence emission wavelength (fluorescence emission wavelength&lt;phosphorescence emission wavelength). For example, 4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP), which is commonly used as a host material in a light-emitting element including a phosphorescent compound, has a phosphorescence spectrum at around 500 nm and has a fluorescence spectrum at around 400 nm, which are largely different by about 100 nm. This example also shows that it is extremely difficult to design a host material so as to have its fluorescence spectrum in a position similar to that of its phosphorescence spectrum. Therefore, it is very important to improve efficiency in energy transfer from the host material in the singlet excited state to the guest material. 
     Therefore, one embodiment of the present invention provides a useful technique which can overcome such a problem of the efficiency of the energy transfer from the host material in the singlet excited state to the guest material. Specific embodiments thereof will be described below. 
     Embodiment 1 
     In this embodiment, a structural concept of a light-emitting element in one embodiment of the present invention and a specific structure of the light-emitting element will be described. Note that a light-emitting element in one embodiment of the present invention includes, between a pair of electrodes (anode and cathode), an EL layer which includes a light-emitting layer. The light-emitting layer has a stacked-layer structure including a first light-emitting layer and a second light-emitting layer. The first light-emitting layer is formed on the anode side and contains at least a first light-emitting substance (guest material) converting triplet excitation energy into light emission, a first organic compound (host material) having an electron-transport property, and a second organic compound (assist material) having a hole-transport property. The second light-emitting layer contains at least a second light-emitting substance (guest material) converting triplet excitation energy into light emission, the first organic compound (host material) having an electron-transport property, and a third organic compound (assist material) having a hole-transport property. 
     Note that the highest occupied molecular orbital level (HOMO level) of the second organic compound (assist material) contained in the first light-emitting layer is lower than that of the third organic compound (assist material) contained in the second light-emitting layer. Therefore, excitation energy (E A ) of an exciplex formed in the first light-emitting layer can be designed to be higher than excitation energy (E B ) of an exciplex formed in the second light-emitting layer. 
     Further, the first light-emitting substance contained in the first light-emitting layer is a substance which emits light with a wavelength shorter than that of light emitted from the second light-emitting substance contained in the second light-emitting layer. 
     First, an element structure of a light-emitting element which is an example of the present invention is described with reference to  FIG. 1A . 
     In the element structure illustrated in  FIG. 1A , an EL layer  103  including a light-emitting layer  106  is provided between a pair of electrodes (an anode  101  and a cathode  102 ), and the EL layer  103  has a structure in which a hole-injection layer  104 , a hole-transport layer  105 , the light-emitting layer  106  ( 106   a  and  106   b ), an electron-transport layer  107 , an electron-injection layer  108 , and the like are sequentially stacked over the anode  101 . 
     As illustrated in  FIG. 1A , the light-emitting layer  106  in one embodiment of the present invention has a stacked-layer structure including the first light-emitting layer  106   a  and the second light-emitting layer  106   b . The first light-emitting layer  106   a  is formed on the anode side and contains at least a first light-emitting substance (guest material)  109   a  converting triplet excitation energy into light emission, a first organic compound (host material)  110  having an electron-transport property, and a second organic compound (assist material)  111  having a hole-transport property. The second light-emitting layer  106   b  contains at least a second light-emitting substance (guest material)  109   b  converting triplet excitation energy into light emission, the first organic compound (host material)  110  having an electron-transport property, and a third organic compound (assist material)  112  having a hole-transport property. An electron-transport material having an electron mobility of 10 −6  cm 2 /Vs or more is mainly used as the first organic compound  110 , and hole-transport materials having a hole mobility of 10 −6  cm 2 /Vs or more are mainly used as the second organic compound  111  and the third organic compound  112 . In this specification, the first organic compound  110  is referred to as a host material, and the second organic compound  111  and the third organic compound  112  are referred to as assist materials. 
     A combination of the first organic compound (host material)  110  and the second organic compound (assist material)  111  in the first light-emitting layer  106   a  forms an exciplex (also referred to as an excited complex). Further, the emission wavelength of the exciplex formed by the first organic compound (host material)  110  and the second organic compound (assist material)  111  is located on the longer wavelength side with respect to the emission wavelength (fluorescent wavelength) of each of the first and second organic compounds (host and assist materials)  110  and  111 . Therefore, the fluorescence spectrum of the first organic compound (host material)  110  and the fluorescence spectrum of the second organic compound (assist material)  111  can be converted into an emission spectrum which is located on the longer wavelength side. 
     The same applies to the second light-emitting layer  106   b . Thus, a combination of the first organic compound (host material)  110  and the third organic compound (assist material)  112  in the second light-emitting layer  106   b  forms an exciplex (also referred to as an excited complex). Further, the emission wavelength of the exciplex formed by the first organic compound (host material)  110  and the third organic compound (assist material)  112  is located on the longer wavelength side with respect to the emission wavelength (fluorescent wavelength) of each of the first and third organic compounds (host and assist materials)  110  and  112 . Therefore, the fluorescence spectrum of the first organic compound (host material)  110  and the fluorescence spectrum of the third organic compound (assist material)  112  can be converted into an emission spectrum which is located on the longer wavelength side. 
     Note that in the above structure, it is preferable that the level of a triplet excitation energy (T 1  level) of each of the first and second organic compounds (host and assist materials)  110  and  111  be higher than the T 1  level of the first light-emitting substance (guest material)  109   a  converting triplet excitation energy into light emission. This is because, when the T 1  level of the first organic compound  110  (or the second organic compound  111 ) is lower than the T 1  level of the first light-emitting substance (guest material)  109   a , the triplet excitation energy of the first light-emitting substance (guest material)  109   a , which contributes to light emission, is quenched by the first organic compound  110  (or the second organic compound  111 ) and accordingly the emission efficiency decreases. 
     In a similar manner, it is preferable that the level of a triplet excitation energy (T 1  level) of each of the first and third organic compounds (host and assist materials)  110  and  112  be higher than the T 1  level of the second light-emitting substance (guest material)  109   b  converting triplet excitation energy into light emission. This is because, when the T 1  level of the first organic compound  110  (or the third organic compound  112 ) is lower than the T 1  level of the second light-emitting substance (guest material)  109   b , the triplet excitation energy of the second light-emitting substance (guest material)  109   b , which contributes to light emission, is quenched by the first organic compound  110  (or the third organic compound  112 ) and accordingly the emission efficiency decreases. 
     Furthermore, in the first light-emitting layer  106   a  included in the light-emitting layer  106 , either the first organic compound (host material)  110  or the second organic compound (assist material)  111  may be contained in a higher proportion, and in the second light-emitting layer  106   b  included in the light-emitting layer  106 , either the first organic compound (host material)  110  or the third organic compound (assist material)  112  may be contained in a higher proportion; the present invention includes all the cases in its scope. 
       FIG. 1B  is a band diagram which explains energy relation between the first organic compound (host material)  110 , the second organic compound (assist material)  111 , and the third organic compound (assist material)  112  in the light-emitting layer  106  (the first light-emitting layer  106   a  and the second light-emitting layer  106   b ) having the above-described structure. 
     As illustrated in  FIG. 1B , in the first light-emitting layer  106   a , the excitation energy (E A ) of the exciplex formed by the first organic compound (host material)  110  and the second organic compound (assist material)  111  is dependent on the HOMO level of the second organic compound (assist material)  111  and the LUMO level of the first organic compound (host material)  110 . Further, in the second light-emitting layer  106   b , the excitation energy (E B ) of the exciplex formed by the first organic compound (host material)  110  and the third organic compound (assist material)  112  is dependent on the HOMO level of the third organic compound (assist material)  112  and the LUMO level of the first organic compound (host material)  110 . Note that the second organic compound (assist material)  111  contained in the first light-emitting layer  106   a  has a lower HOMO level than the third organic compound (assist material)  112  contained in the second light-emitting layer  106   b , which means that the excitation energy (E A ) of the exciplex formed in the first light-emitting layer  106   a  can be designed to be higher than the excitation energy (E B ) of the exciplex formed in the second light-emitting layer  106   b.    
     Note that owing to the above-described element structure, the exciplex formed in the first light-emitting layer  106   a  has higher excitation energy than the exciplex formed in the second light-emitting layer  106   b ; therefore, when a substance emitting light with a wavelength shorter than that of light emitted from the second light-emitting substance (guest material)  109   b  which is contained in the second light-emitting layer  106   b  and converts triplet excitation energy into light emission is used as the first light-emitting substance (guest material)  109   a  which is contained in the first light-emitting layer  106   a  and converts triplet excitation energy into light emission, the first light-emitting layer  106   a  and the second light-emitting layer  106   b  can provide light emission at the same time. Further, part of excitation energy of the exciplex formed in the first light-emitting layer  106   a  which does not contribute to light emission can be utilized as excitation energy for the second light-emitting substance (guest material)  109   b  which converts triplet excitation energy into light emission in the second light-emitting layer  106   b . Accordingly, emission efficiency in the light-emitting element can be further increased. 
     In the light-emitting element described in this embodiment, exciplexes are formed in the light-emitting layer  106  (the first light-emitting layer  106   a  and the second light-emitting layer  106   b ), and the fluorescence spectrum of the first organic compound (host material)  110 , the fluorescence spectrum of the second organic compound (assist material)  111 , or the fluorescence spectrum of the third organic compound (assist material)  112  can be converted into an emission spectrum which is located on the longer wavelength side. This means that even when the fluorescence spectrum of the first organic compound  110 , the second organic compound  111 , or the third organic compound  112  is located on the shorter wavelength side with respect to the absorption band on the longest wavelength side of the light-emitting substance  109  (the first light-emitting substance (guest material)  109   a  and the second light-emitting substance (guest material)  109   b ) converting triplet excitation energy into light emission as illustrated in  FIG. 2 , or does not have an overlap with the absorption band on the longest wavelength side of the light-emitting substance  109  (the first light-emitting substance (guest material)  109   a  and the second light-emitting substance (guest material)  109   b ) converting triplet excitation energy into light emission, the emission spectrum of the exciplex and the absorption band can have an increased overlap. Accordingly, the energy transfer efficiency in Formula (2-2B) above can be increased. 
     Furthermore, the exciplex is considered to have an extremely small difference between singlet excited energy and triplet excited energy. In other words, the emission spectrum of the exciplex from the singlet state and the emission spectrum thereof from the triplet state are highly close to each other. Accordingly, in the case where design is implemented such that the emission spectrum of the exciplex (generally the emission spectrum of the exciplex from the singlet state) overlaps with the absorption band on the longest wavelength side of the light-emitting substance  109  converting triplet excitation energy into light emission as described above, the emission spectrum of the exciplex from the triplet state (which is not observed at room temperature and also not observed in many cases at low temperature) also overlaps with the absorption band on the longest wavelength side of the light-emitting substance  109  converting triplet excitation energy into light emission. In other words, not only the efficiency of the energy transfer from the singlet excited state ((2-2)) but also the efficiency of the energy transfer from the triplet excited state ((2-1)) can be increased, and as a result, energy from both the singlet and triplet excited states can be efficiently converted into light emission. 
     Thus, molecular orbital calculations were performed as described below to verify whether or not an exciplex actually has such characteristics. In general, a combination of a heteroaromatic compound and an aromatic amine often forms an exciplex under the influence of the lowest unoccupied molecular orbital (LUMO) level of the heteroaromatic compound which is deeper than the LUMO level of the aromatic amine (the property of easily accepting electrons) and the highest occupied molecular orbital (HOMO) level of the aromatic amine which is shallower than the HOMO level of the heteroaromatic compound (the property of easily accepting holes). Thus, calculations were performed using a combination of dibenzo[f,h]quinoxaline (abbreviation: DBq), which is a typical skeleton forming the LUMO level of a heteroaromatic compound and is a model of the first organic compound  110  in one embodiment of the present invention, and triphenylamine (abbreviation: TPA), which is a typical skeleton forming the HOMO level of an aromatic amine and is a model of the second organic compound  111  (or the third organic compound  112 ) in one embodiment of the present invention. 
     First, the optimal molecular structures and the excitation energies of one molecule of DBq (abbreviation) and one molecule of TPA (abbreviation) in the lowest singlet excited state (S 1 ) and the lowest triplet excited state (T 1 ) were calculated using the time-dependent density functional theory (TD-DFT). Furthermore, the excitation energy of a dimer of DBq (abbreviation) and TPA (abbreviation) was also calculated. 
     In the DFT (density functional theory), the total energy is represented as the sum of potential energy, electrostatic energy between electrons, electronic kinetic energy, and exchange-correlation energy including all the complicated interactions between electrons. Also in the DFT, an exchange-correlation interaction is approximated by a functional (a function of another function) of one electron potential represented in terms of electron density to enable high-speed and high-accuracy calculations. Here, B3LYP which was a hybrid functional was used to specify the weight of each parameter related to exchange-correlation energy. 
     In addition, as a basis function, 6-311 (a basis function of a triple-split valence basis set using three contraction functions for each valence orbital) was applied to all the atoms. 
     By the above basis function, for example, 1s to 3s orbitals are considered in the case of hydrogen atoms, while 1s to 4s and 2p to 4p orbitals are considered in the case of carbon atoms. Furthermore, to improve calculation accuracy, the p function and the d function as polarization basis sets were added to hydrogen atoms and atoms other than hydrogen atoms, respectively. 
     Note that Gaussian 09 was used as a quantum chemistry computational program. A high performance computer (Altix 4700, manufactured by SGI Japan, Ltd.) was used for the calculations. 
     First, the HOMO levels and the LUMO levels of one molecule of DBq (abbreviation), one molecule of TPA (abbreviation), and a dimer of DBq (abbreviation) and TPA (abbreviation) were calculated.  FIG. 3  shows the HOMO levels and the LUMO levels, and FIGS.  4 A 1 ,  4 A 2 ,  4 B 1 ,  4 B 2 ,  4 C 1 , and  4 C 2  show HOMO and LUMO distributions. 
     FIG.  4 A 1  shows the LUMO distribution of one molecule of DBq (abbreviation); FIG.  4 A 2 , the HOMO distribution of one molecule of DBq (abbreviation); FIG.  4 B 1 , the LUMO distribution of one molecule of TPA (abbreviation); FIG.  4 B 2 , the HOMO distribution of one molecule of TPA (abbreviation); FIG.  4 C 1 , the LUMO distribution of the dimer of DBq (abbreviation) and TPA (abbreviation); and FIG.  4 C 2 , the HOMO distribution of the dimer of DBq (abbreviation) and TPA (abbreviation). 
     As shown in  FIG. 3 , it is suggested that the dimer of DBq (abbreviation) and TPA (abbreviation) forms an exciplex of DBq (abbreviation) and TPA (abbreviation) under the influence of the LUMO level (−1.99 eV) of DBq (abbreviation) which is deeper (lower) than the LUMO level of TPA (abbreviation) and the HOMO level (−5.21 eV) of TPA (abbreviation) which is shallower (higher) than the HOMO level of DBq (abbreviation). In fact, as is clear from FIGS.  4 C 1  and  4 C 2 , the LUMO of the dimer of DBq (abbreviation) and TPA (abbreviation) is distributed on the DBq (abbreviation) side, and the HOMO thereof is distributed on the TPA (abbreviation) side. 
     Next, excitation energies obtained from the optimal molecular structures of one molecule of DBq (abbreviation) at S 1  and T 1  levels will be shown. Here, the excitation energies at the S 1  and T 1  levels correspond to fluorescence and phosphorescence wavelengths, respectively, obtained from one molecule of DBq (abbreviation). The excitation energy at the S 1  level of one molecule of DBq (abbreviation) is 3.294 eV, and the fluorescence wavelength is 376.4 nm. The excitation energy at the T 1  level of one molecule of DBq (abbreviation) is 2.460 eV, and the phosphorescence wavelength is 504.1 nm. 
     In addition, excitation energies obtained from the optimal molecular structures of one molecule of TPA (abbreviation) at S 1  and T 1  levels will be shown. Here, the excitation energies at the S 1  and T 1  levels correspond to fluorescence and phosphorescence wavelengths, respectively, obtained from one molecule of TPA (abbreviation). The excitation energy at the S 1  level of one molecule of TPA (abbreviation) is 3.508 eV, and the fluorescence wavelength is 353.4 nm. The excitation energy at the T 1  level of one molecule of TPA (abbreviation) is 2.610 eV, and the phosphorescence wavelength is 474.7 nm. 
     Furthermore, excitation energies obtained from the optimal molecular structures of the dimer of DBq (abbreviation) and TPA (abbreviation) at S 1  and T 1  levels will be shown. The excitation energies at the S 1  and T 1  levels correspond to fluorescence and phosphorescence wavelengths, respectively, obtained from the dimer of DBq (abbreviation) and TPA (abbreviation). The excitation energy at the S 1  level of the dimer of DBq (abbreviation) and TPA (abbreviation) is 2.036 eV, and the fluorescence wavelength is 609.1 nm. The excitation energy at the T 1  level of the dimer of DBq (abbreviation) and TPA (abbreviation) is 2.030 eV, and the phosphorescence wavelength is 610.0 nm. 
     It is found from the above that each of the phosphorescence wavelengths of one molecule of DBq (abbreviation) and one molecule of TPA (abbreviation) is shifted to the longer wavelength side by about 100 nm. This result shows a tendency similar to that of CBP (abbreviation) (measured values) described above and supports the validity of the calculations. 
     On the other hand, it is found that the fluorescence wavelength of the dimer of DBq (abbreviation) and TPA (abbreviation) is located on the longer wavelength side with respect to the fluorescence wavelengths of one molecule of DBq (abbreviation) and one molecule of TPA (abbreviation). It is also found that the difference between the fluorescence wavelength and the phosphorescence wavelength of the dimer of DBq (abbreviation) and TPA (abbreviation) is only 0.9 nm and that these wavelengths are substantially the same. 
     These results indicate that the singlet excitation energy and the triplet excitation energy of the exciplex are substantially equivalent. Therefore, it is indicated as described above that the exciplex can efficiently transfer energy from both the singlet state and the triplet state to the light-emitting substance (the guest materials including the first light-emitting substance and the second light-emitting substance) which converts triplet excitation energy into light emission. 
     In the above manner, the light-emitting element in one embodiment of the present invention transfers energy by utilizing an overlap between the emission spectrum of the exciplex formed in the light-emitting layer and the absorption spectrum of the light-emitting substance (the guest materials including the first light-emitting substance and the second light-emitting substance) which converts triplet excitation energy into light emission and thus has high energy transfer efficiency. Therefore, the light-emitting element can achieve high external quantum efficiency. 
     In addition, the exciplex exists only in an excited state and thus has no ground state capable of absorbing energy. Therefore, a phenomenon in which the light-emitting substance (guest material) converting triplet excitation energy into light emission is deactivated by energy transfer from the light-emitting substance (guest material) converting triplet excitation energy into light emission in the singlet excited state and triplet excited state to the exciplex before light emission (i.e., emission efficiency is lowered) is not considered to occur in principle. This also contributes to improvement of external quantum efficiency. 
     Note that the above-described exciplex is formed by an interaction between dissimilar molecules in excited states. The exciplex is generally known to be easily formed between a material having a relatively deep LUMO level and a material having a relatively shallow HOMO level. 
     An emission wavelength of the exciplex depends on a difference in energy between the HOMO level and the LUMO level. As a general tendency, when the energy difference is large, the emission wavelength is short, and when the energy difference is small, the emission wavelength is long. 
     Therefore, the HOMO levels and LUMO levels of the first organic compound (host material)  110 , the second organic compound (assist material)  111 , and the third organic compound (assist material)  112  in this embodiment are different from each other as illustrated in  FIG. 1B . Specifically, the energy levels vary in the following order: the HOMO level of the first organic compound  110 &lt;the HOMO levels of the second organic compound  111  and the third organic compound  112 &lt;the LUMO level of the first organic compound  110 &lt;the LUMO levels of the second organic compound  111  and the third organic compound  112 . 
     In each of the light-emitting layers, when the exciplex is formed by the two organic compounds (the first organic compound  110  and the second organic compound  111  in the first light-emitting layer  106   a , and the first organic compound  110  and the third organic compound  112  in the second light-emitting layer  106   b ), the LUMO levels of the exciplexes in the first light-emitting layer  106   a  and the second light-emitting layer  106   b  depend on the first organic compound (host material)  110 , the HOMO level of the exciplex in the first light-emitting layer  106   a  depends on the second organic compound (assist material)  111 , and the HOMO level of the exciplex in the second light-emitting layer  106   b  depends on the HOMO level of the third organic compound (assist material)  112 . Therefore, the excitation energy (E A ) of the exciplex in the first light-emitting layer  106   a  is higher than the excitation energy (E B ) of the exciplex in the second light-emitting layer  106   b . In other words, when a substance emitting light with a wavelength shorter than that of light emitted from the second light-emitting substance  109   b  which is contained in the second light-emitting layer and converts triplet excitation energy into light emission is used as the first light-emitting substance  109   a  which is contained in the first light-emitting layer and converts triplet excitation energy into light emission, a light-emitting element with high emission efficiency can be formed. Moreover, light-emitting materials with different emission wavelengths can efficiently emit light at the same time. 
     Note that the process of the exciplex formation in one embodiment of the present invention can be either of the following two processes. 
     One formation process is that an exciplex is formed from the second and third organic compounds (assist materials) having carriers (specifically, cation). 
     In general, when an electron and a hole recombine in a host material, excitation energy is transferred from the host material in an excited state to a guest material, whereby the guest material is brought into an excited state to emit light. Before the excitation energy is transferred from the host material to the guest material, the host material itself emits light or the excitation energy turns into thermal energy, which leads to partial deactivation of the excitation energy. In particular, when the host material is in a singlet excited state, energy transfer does not readily occur as described in (2-2). Such deactivation of excitation energy is one of causes for a decrease in lifetime of a light-emitting element. 
     However, in one embodiment of the present invention, an exciplex is formed from the first organic compound (host material) and the second organic compound (or the third organic compound) (assist material) having carriers (cation or anion); therefore, formation of a singlet exciton of the first organic compound (host material) can be suppressed. In other words, there can be a process where an exciplex is directly formed without formation of a singlet exciton. Thus, deactivation of the singlet excitation energy can be inhibited. Accordingly, a light-emitting element having a long lifetime can be obtained. 
     For example, in the case where the first organic compound is an electron-trapping compound having the property of easily capturing electrons (carrier) (having a deep LUMO level) among electron-transport materials and the second organic compound (or the third organic compound) is a hole-trapping compound having the property of easily capturing holes (carrier) (having a shallow HOMO level) among hole-transport materials, an exciplex is formed directly from an anion of the first organic compound and a cation of the second organic compound (or the third organic compound). An exciplex formed through such a process is particularly referred to as an electroplex. A light-emitting element having high emission efficiency can be obtained by suppressing the generation of the singlet excited state of the first organic compound (host material) and transferring energy from an electroplex to the light-emitting substance (guest material) which converts triplet excitation energy into light emission, in the above-described manner. Note that in this case, the generation of the triplet excited state of the first organic compound (host material) is similarly suppressed and an exciplex is directly formed; therefore, energy transfer is considered to occur from the exciplex to the light-emitting substance (guest material) which converts triplet excitation energy into light emission. 
     The other formation process is an elementary process where one of the host material (the first organic compound) and the assist material (the second organic compound or the third organic compound) forms a singlet exciton and then interacts with the other in the ground state to form an exciplex. Unlike an electroplex, a singlet excited state of the first organic compound (host material), the second organic compound (assist material), or the third organic compound (assist material) is temporarily generated in this case, but this is rapidly converted into an exciplex, and thus, deactivation of singlet excitation energy can be inhibited. Thus, it is possible to inhibit deactivation of excitation energy of the first organic compound (host material), the second organic compound (assist material), or the third organic compound (assist material). Note that in this case, it is considered that the triplet excited state of the host material is similarly rapidly converted into an exciplex and energy is transferred from the exciplex to the light-emitting substance (guest material) which converts triplet excitation energy into light emission. 
     Note that in the case where the first organic compound (host material) is an electron-trapping compound, the second organic compound (or the third organic compound) (assist material) is a hole-trapping compound, and the difference between the HOMO levels and the difference between the LUMO levels of these compounds are large (specifically, 0.3 eV or more), electrons are selectively injected into the first organic compound (host material) and holes are selectively injected into the second organic compound (or the third organic compound) (assist material). In this case, it is thought that the process where an electroplex is formed takes precedence over the process where an exciplex is formed through a singlet exciton. 
     To make the emission spectrum of the exciplex and the absorption spectrum of the light-emitting substance (guest material) converting triplet excitation energy into light emission sufficiently overlap each other, the difference between the energy of a peak of the emission spectrum and the energy of a peak of the absorption band on the lowest energy side in the absorption spectrum is preferably 0.3 eV or less. The difference is more preferably 0.2 eV or less, even more preferably 0.1 eV or less. 
     In the light-emitting element in one embodiment of the present invention, it is also preferable that the excitation energy of the exciplex be sufficiently transferred to the light-emitting substance (guest material) which converts triplet excitation energy into light emission, and that light emission from the exciplex be not substantially observed. Therefore, it is preferable that energy be transferred to the light-emitting substance which converts triplet excitation energy into light emission through an exciplex so that the light-emitting substance which converts triplet excitation energy into light emission emits light. Note that as the light-emitting substance which converts triplet excitation energy into light emission, a phosphorescent compound (e.g., an organometallic complex), a thermally activated delayed fluorescence (TADF) material, or the like is preferably used. 
     In the case where a light-emitting substance which converts triplet excitation energy into light emission is used as the first organic compound (host material) in the first light emitting layer  106   a  of the light-emitting element in one embodiment of the present invention, the first organic compound (host material) itself readily emits light and does not readily allow energy to be transferred to the light-emitting substance (guest material) which converts triplet excitation energy into light emission. In this case, it is favorable if the first organic compound could emit light efficiently, but it is difficult to achieve high emission efficiency because a high concentration of the host material may cause a problem of concentration quenching. Therefore, the case where at least one of the first organic compound (host material) and the second organic compound (or the third organic compound) (assist material) is a fluorescent compound (i.e., a compound which readily undergoes light emission or thermal deactivation from the singlet excited state) is effective. For this reason, it is preferable that at least one of the first organic compound (host material) and the second organic compound (or the third organic compound) (assist material) be a fluorescent compound and an exciplex be used as a medium for energy transfer. 
     Note that the structure described in this embodiment can be combined as appropriate with any of the structures described in the other embodiments. 
     Embodiment 2 
     In this embodiment, an example of a light-emitting element in one embodiment of the present invention is described with reference to  FIG. 5 . 
     In the light-emitting element described in this embodiment, as illustrated in  FIG. 5 , an EL layer  203  including a light-emitting layer  206  is provided between a pair of electrodes (a first electrode (anode)  201  and a second electrode (cathode)  202 ), and the EL layer  203  includes a hole-injection layer  204 , a hole-transport layer  205 , an electron-transport layer  207 , an electron-injection layer  208 , and the like in addition to the light-emitting layer  206  having a stacked-layer structure including a first light-emitting layer  206   a  and a second light-emitting layer  206   b.    
     Note that the light-emitting layer  206  in this embodiment has a stacked-layer structure including the first light-emitting layer  206   a  and the second light-emitting layer  206   b . In the light-emitting layer  206 , the first light-emitting layer  206   a  contains a first light-emitting substance (guest material)  209   a  converting triplet excitation energy into light emission, a first organic compound (host material)  210  having an electron-transport property, and a second organic compound (assist material)  211  having a hole-transport property; and the second light-emitting layer  206   b  contains a second light-emitting substance (guest material)  209   b  converting triplet excitation energy into light emission, the first organic compound (host material)  210  having an electron-transport property, and the third organic compound (assist material)  212  having a hole-transport property. 
     Note that the HOMO level of the second organic compound (assist material) contained in the first light-emitting layer is lower than that of the third organic compound (assist material) contained in the second light-emitting layer. Therefore, excitation energy (E A ) of an exciplex formed in the first light-emitting layer can be designed to be higher than excitation energy (E B ) of an exciplex formed in the second light-emitting layer. 
     Note that crystallization of the light-emitting layer  206  (the first light-emitting layer  206   a  and the second light-emitting layer  206   b ) can be suppressed by employing a structure in which the first light-emitting substance  209   a  converting triplet excitation energy into light emission is dispersed in the first organic compound (host material)  210  and the second organic compound (assist material)  211  in the first light-emitting layer  206   a , and the second light-emitting substance  209   b  converting triplet excitation energy into light emission is dispersed in the first organic compound (host material)  210  and the third organic compound (assist material)  212  in the second light-emitting layer  206   b . Further, it is possible to suppress concentration quenching due to high concentration of the light-emitting substance  209  ( 209   a  and  209   b ), and thus the light-emitting element can have higher emission efficiency. 
     It is preferable that the level of a triplet excitation energy (T 1  level) of each of the first organic compound  210 , the second organic compound  211 , and the third organic compound  212  be higher than the T 1  level of the light-emitting substance  209  ( 209   a  and  209   b ) converting triplet excitation energy into light emission. This is because, when the T 1  level of each of the first organic compound  210 , the second organic compound  211 , and the third organic compound  212  is lower than the T 1  level of the light-emitting substance  209  ( 209   a  and  209   b ) converting triplet excitation energy into light emission, the triplet excitation energy of the light-emitting substance  209  ( 209   a  and  209   b ) converting triplet excitation energy into light emission, which contributes to light emission, is quenched by the first organic compound  210  (or the second organic compound  211 , or the third organic compound  212 ) and accordingly the emission efficiency decreases. 
     In the light-emitting layer  206  in this embodiment, at the time of recombination of carriers (electrons and holes) injected from the respective electrodes, the first organic compound  210  and the second organic compound  211  form an exciplex in the first light-emitting layer  206   a , and the first organic compound  210  and the third organic compound  212  form an exciplex in the second light-emitting layer  206   b . Thus, a fluorescence spectrum of the first organic compound  210  and that of the second organic compound  211  in the first light-emitting layer  206   a  can be converted into an emission spectrum of the exciplex which is located on a longer wavelength side, and a fluorescence spectrum of the first organic compound  210  and that of the third organic compound  212  in the second light-emitting layer  206   b  can be converted into an emission spectrum of the exciplex which is located on a longer wavelength side. In consideration of this, the first organic compound  210  and the second organic compound  211  in the first light-emitting layer  206   a  and the first organic compound  210  and the third organic compound  212  in the second light-emitting layer  206   b  are selected to make an overlap between the emission spectrum of the exciplex and the absorption spectrum of the light-emitting substance (guest material)  209  converting triplet excitation energy into light emission large and thereby to maximize energy transfer from a singlet excited state. That is, it is assumed here that energy transfer from the exciplex, not the host material, occurs also in the case of a triplet excited state. 
     Note that as the light-emitting substance  209  (the first light-emitting substance  209   a  and the second light-emitting substance  209   b ) which converts triplet excitation energy into light emission, a phosphorescent compound (e.g., an organometallic complex), a thermally activated delayed fluorescence (TADF) material, or the like is preferably used. An electron-transport material is preferably used as the first organic compound (host material)  210 . Hole-transport materials are preferably used as the second organic compound (assist material)  211  and the third organic compound (assist material)  212 . 
     Note that examples of the organometallic complex include bis[2-(4′,6′-difluorophenyl)pyridinato-N,C 2 ′]iridium(III) tetrakis(1-pyrazolyl)borate (abbreviation: FIr6), bis[2-(4′,6′-difluorophenyl)pyridinato-N,C 2′ ]iridium(III) picolinate (abbreviation: FIrpic), bis[2-(3∝,5′-bistrifluoromethylphenyl)pyridinato-N,C 2 ′]iridium(III) picolinate (abbreviation: Ir(CF 3 ppy) 2 (pic)), bis[2-(4′,6′-difluorophenyl)pyridinato-N,C 2 ′]iridium(III) acetylacetonate (abbreviation: FIracac), tris(2-phenylpyridinato)iridium(III) (abbreviation: Ir(ppy) 3 ), bis(2-phenylpyridinato)iridium(III) acetylacetonate (abbreviation: Ir(ppy) 2 (acac)), bis(benzo[h]quinolinato)iridium(III) acetylacetonate (abbreviation: Ir(bzq) 2 (acac)), bis(2,4-diphenyl-1,3-oxazolato-N,C 2 ′)iridium(III) acetylacetonate (abbreviation: Ir(dpo) 2 (acac)), bis{2-[4′-(perfluorophenyl)phenyl]pyridinato-N,C 2′ }iridium(III) acetylacetonate (abbreviation: Ir(p-PF-ph) 2 (acac)), bis(2-phenylbenzothiazolato-N,C 2 ′)iridium(III) acetylacetonate (abbreviation: Ir(bt) 2 (acac)), bis[2-(2′-benzo[4,5-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)), 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin platinum(II) (abbreviation: PtOEP), 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)). 
     As the electron-transport material, a π-electron deficient heteroaromatic compound such as a nitrogen-containing heteroaromatic compound is preferable, examples of which include quinoxaline derivatives and dibenzoquinoxaline derivatives such as 2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation: 2mDBTPDBq-II), 2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline (abbreviation: 2mDBTBPDBq-II), 2-[4-(3,6-diphenyl-9H-carbazol-9-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation: 2CzPDBq-III), 7-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation: 7mDBTPDBq-II), and 6-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation: 6mDBTPDBq-II). 
     As the hole-transport material, a π-electron rich heteroaromatic compound (e.g., a carbazole derivative or an indole derivative) or an aromatic amine compound is preferable, examples of which include 4-phenyl-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PCBA1BP), 4,4′-di(1-naphthyl)-4″-(9-phenyl-9H-carbazol-3-yOtriphenylamine (abbreviation: PCBNBB), 3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole (abbreviation: PCzPCN1), 4,4′,4″-tris[N-(1-naphthyl)-N-phenylamino]triphenylamine (abbreviation: 1′-TNATA), 2,7-bis[N-(4-diphenylaminophenyl)-N-phenylamino]-spiro-9,9′-bifluorene (abbreviation: DPA2 SF), N,N′-bis(9-phenylcarbazol-3-yl)-N,N′-diphenylbenzene-1,3-diamine (abbreviation: PCA2B), N-(9,9-dimethyl-2-diphenylamino-9H-fluoren-7-yl)diphenylamine (abbreviation: DPNF), N,N′,N″-triphenyl-N,N′,N″-tris(9-phenylcarbazol-3-yl)benzene-1,3,5-triamine (abbreviation: PCA3B), 2-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]spiro-9,9′-bifluorene (abbreviation: PCASF), 2-[N-(4-diphenylaminophenyl)-N-phenylamino]spiro-9,9′-bifluorene (abbreviation: DPASF), N,N′-bis[4-(carbazol-9-yl)phenyl]-N,N′-diphenyl-9,9-dimethylfluorene-2,7-diamine (abbreviation: YGA2F), 4,4′-bis[N-(3-methylphenyl)-N-phenylamino]biphenyl (abbreviation: TPD), 4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl (abbreviation: DPAB), N-(9,9-dimethyl-9H-fluoren-2-yl)-N-{9,9-dimethyl-2-[N′-phenyl-N′-(9,9-dimethyl-9H-fluoren-2-yl)amino]-9H-fluoren-7-yl}phenylamine (abbreviation: DFLADFL), 3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole (abbreviation: PCzPCA1), 3-[N-(4-diphenylaminophenyl)-N-phenylamino]-9-phenylcarbazole (abbreviation: PCzDPA1), 3,6-bis[N-(4-diphenylaminophenyl)-N-phenylamino]-9-phenylcarbazole (abbreviation: PCzDPA2), 4,4′-bis(N-{4-[N′-(3-methylphenyl)-N′-phenylamino]phenyl}-N-phenylamino)biphenyl (abbreviation: DNTPD), 3,6-bis[N-(4-diphenylaminophenyl)-N-(1-naphthyl)amino]-9-phenylcarbazole (abbreviation: PCzTPN2), and 3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole (abbreviation: PCzPCA2). 
     Note that materials which can be used for the light-emitting substance (guest material)  209  (the first light-emitting substance  209   a  and the second light-emitting substance  209   b ) converting triplet excitation energy into light emission, the first organic compound (host material)  210 , the second organic compound (assist material)  211 , and the third organic compound (assist material)  212  are not limited to the above examples. The combination is determined so that an exciplex can be formed, the emission spectrum of the exciplex overlaps with the absorption spectrum of the light-emitting substance (guest material)  209  (the first light-emitting substance  209   a  or the second light-emitting substance  209   b ) converting triplet excitation energy into light emission, and the peak of the emission spectrum of the exciplex has a longer wavelength than the peak of the absorption spectrum of the light-emitting substance (guest material)  209  (the first light-emitting substance  209   a  or the second light-emitting substance  209   b ) converting triplet excitation energy into light emission. 
     In the case where an electron-transport material is used as the first organic compound  210  and a hole-transport material is used as the second organic compound  211 , carrier balance can be controlled by the mixture ratio of the compounds. Specifically, the ratio of the first organic compound  210  to the second organic compound  211  is preferably 1:9 to 9:1. 
     A specific example in which the light-emitting element described in this embodiment is manufactured is described below. 
     For the first electrode (anode)  201  and the second electrode (cathode)  202 , a metal, an alloy, an electrically conductive compound, a mixture thereof, or the like can be used. Specifically, indium oxide-tin oxide (ITO: indium tin oxide), indium oxide-tin oxide containing silicon or silicon oxide, indium oxide-zinc oxide (indium zinc oxide), indium oxide containing tungsten oxide and zinc oxide, gold (Au), platinum (Pt), nickel (Ni), tungsten (W), chromium (Cr), molybdenum (Mo), iron (Fe), cobalt (Co), copper (Cu), palladium (Pd), or titanium (Ti) can be used. In addition, an element belonging to Group 1 or Group 2 of the periodic table, for example, an alkali metal such as lithium (Li) or cesium (Cs), an alkaline earth metal such as magnesium (Mg), calcium (Ca), or strontium (Sr), an alloy containing such an element (e.g., MgAg or AlLi), a rare earth metal such as europium (Eu) or ytterbium (Yb), an alloy containing such an element, graphene, or the like can be used. The first electrode (anode)  201  and the second electrode (cathode)  202  can be formed by, for example, a sputtering method, an evaporation method (including a vacuum evaporation method), or the like. 
     Examples of a substance having a high hole-transport property which is used for the hole-injection layer  204  and the hole-transport layer  205  include aromatic amine compounds such as 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB or α-NPD), N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine (abbreviation: TPD), 4,4′,4″-tris(carbazol-9-yl)triphenylamine (abbreviation: TCTA), 4,4′,4″-tris(N,N-diphenylamino)triphenylamine (abbreviation: TDATA), 4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine (abbreviation: MTDATA), and 4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl (abbreviation: BSPB), 3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole (abbreviation: PCzPCA1), 3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole (abbreviation: PCzPCA2), and 3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole (abbreviation: PCzPCN1). Other examples include carbazole derivatives such as 4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP), 1,3,5-tris[4-(N-carbazolyl)phenyl]benzene (abbreviation: TCPB), and 9-[4-(10-phenyl-9-anthracenyl)phenyl]-9H-carbazole (abbreviation: CzPA). The substances mentioned here are mainly substances that have a hole mobility of 10 −6  cm 2 /Vs or more. Note that other than these substances, any substance that has a property of transporting more holes than electrons may be used. 
     Still other examples include high molecular compounds such as poly(N-vinylcarbazole) (abbreviation: PVK), poly(4-vinyltriphenylamine) (abbreviation: PVTPA), poly[N-(4-{N′-[4-(4-diphenylamino)phenyl]phenyl-N′-phenylamino}phenyl)methacrylamide] (abbreviation: PTPDMA), and poly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine] (abbreviation: Poly-TPD). 
     Further, examples of an acceptor substance which can be used for the hole-injection layer  204  include oxides of transition metals, oxides of metals belonging to Groups 4 to 8 of the periodic table, and the like. Specifically, molybdenum oxide is particularly preferable. 
     The light-emitting layer  206  ( 206   a  and  206   b ) is formed such that, as described above, the first light-emitting layer  206   a  contains at least the first light-emitting substance  209   a , the first organic compound (host material)  210 , and the second organic compound (assist material)  211 , and the second light-emitting layer  206   b  contains at least the second light-emitting substance  209   b , the first organic compound (host material)  210 , and the third organic compound (assist material)  212 . 
     The electron-transport layer  207  is a layer that contains a substance having a high electron-transport property. For the electron-transport layer  207 , it is possible to use a metal complex such as Alq 3 , tris(4-methyl-8-quinolinolato)aluminum (abbreviation: Almq 3 ), bis(10-hydroxybenzo[h]quinolinato)beryllium (abbreviation: BeBq 2 ), BAlq, Zn(BOX) 2 , or bis[2-(2-hydroxyphenyl)benzothiazolato]zinc (abbreviation: Zn(BTZ) 2 ). Alternatively, it is possible to use a heteroaromatic compound such as 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation: PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene (abbreviation: OXD-7), 3-(4-tert-butylphenyl)-4-phenyl-5-(4-biphenylyl)-1,2,4-triazole (abbreviation: TAZ), 3-(4-tert-butylphenyl)-4-(4-ethylphenyl)-5-(4-biphenylyl)-1,2,4-triazole (abbreviation: p-EtTAZ), bathophenanthroline (abbreviation: BPhen), bathocuproine (abbreviation: BCP), or 4,4′-bis(5-methylbenzoxazol-2-yl)stilbene (abbreviation: BzOs). Further alternatively, it is possible to use a high molecular compound such as poly(2,5-pyridinediyl) (abbreviation: PPy), poly[(9,9-dihexylfluorene-2,7-diyl)-co-(pyridine-3,5-diyl)] (abbreviation: PF-Py), or poly[(9,9-dioctylfluorene-2,7-diyl)-co-(2,2′-bipyridine-6,6′-diyl)] (abbreviation: PF-BPy). The substances mentioned here are mainly substances that have an electron mobility of 10 −6  cm 2 /Vs or more. Note that other than these substances, any substance that has a property of transporting more electrons than holes may be used for the electron-transport layer  207 . 
     The electron-transport layer  207  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  208  is a layer that contains a substance having a high electron-injection property. Examples of the substance that can be used for the electron-injection layer  208  include alkali metals, alkaline earth metals, and compounds thereof, such as lithium fluoride (LiF), cesium fluoride (CsF), calcium fluoride (CaF 2 ), and lithium oxide (LiO x ), and rare earth metal compounds, such as erbium fluoride (ErF 3 ). Alternatively, the above-mentioned substances for forming the electron-transport layer  207  can be used. 
     Alternatively, a composite material in which an organic compound and an electron donor (a donor) are mixed may be used for the electron-injection layer  208 . Such a composite material, in which electrons are generated in the organic compound by the electron donor, has high electron-injection and electron-transport properties. The organic compound here is preferably a material excellent in transporting the generated electrons, and specifically any of the above substances (such as metal complexes and heteroaromatic compounds) for the electron-transport layer  207  can be used. As the electron donor, a substance showing an electron-donating property with respect to the organic compound may be used. Specifically, alkali metals, alkaline earth metals, and rare earth metals are preferable, and lithium, cesium, magnesium, calcium, erbium, ytterbium, and the like can be given. Any of alkali metal oxides and alkaline earth metal oxides is preferable, examples of which are lithium oxide, calcium oxide, barium oxide, and the like, and a Lewis base such as magnesium oxide or an organic compound such as tetrathiafulvalene (abbreviation: TTF) can be used. 
     Note that the hole-injection layer  204 , the hole-transport layer  205 , the light-emitting layer  206  ( 206   a  and  206   b ), the electron-transport layer  207 , and the electron-injection layer  208  which are mentioned above can each be formed by a method such as an evaporation method (including a vacuum evaporation method), an inkjet method, or a coating method. 
     Light emission obtained in the light-emitting layer  206  of the above-described light-emitting element is extracted to the outside through either the first electrode  201  or the second electrode  202  or both. Therefore, either the first electrode  201  or the second electrode  202  in this embodiment, or both, is an electrode having a light-transmitting property. 
     In the light-emitting element described in this embodiment, energy transfer efficiency can be improved owing to energy transfer utilizing an overlap between an emission spectrum of an exciplex and an absorption spectrum of a light-emitting substance (guest material) which converts triplet excitation energy into light emission; accordingly, the light-emitting element can achieve high external quantum efficiency. 
     Note that the light-emitting element described in this embodiment is one embodiment of the present invention and is particularly characterized by the structure of the light-emitting layer. Therefore, when the structure described in this embodiment is employed, a passive matrix light-emitting device, an active matrix light-emitting device, and the like can be manufactured. Each of these light-emitting devices is included in the present invention. 
     Note that there is no particular limitation on the structure of a TFT in the case of manufacturing the active matrix light-emitting device. For example, a staggered TFT or an inverted staggered TFT can be used as appropriate. Further, a driver circuit formed over a TFT substrate may be formed using both an n-type TFT and a p-type TFT or either an n-type TFT or a p-type TFT. Furthermore, there is also no particular limitation on the crystallinity of a semiconductor film used for the TFT. For example, an amorphous semiconductor film, a crystalline semiconductor film, an oxide semiconductor film, or the like can be used. 
     Note that the structure described in this embodiment can be combined as appropriate with any of the structures described in the other embodiments. 
     Embodiment 3 
     In this embodiment, as one embodiment of the present invention, a light-emitting element (hereinafter referred to as tandem light-emitting element) in which a charge generation layer is provided between a plurality of EL layers will be described. 
     The light-emitting element described in this embodiment is a tandem light-emitting element including a plurality of EL layers (a first EL layer  302 ( 1 ) and a second EL layer  302 ( 2 )) between a pair of electrodes (a first electrode  301  and a second electrode  304 ) as illustrated in  FIG. 6A . 
     In this embodiment, the first electrode  301  functions as an anode, and the second electrode  304  functions as a cathode. Note that the first electrode  301  and the second electrode  304  can have structures similar to those described in Embodiment 1. In addition, all or any of the plurality of EL layers (the first EL layer  302 ( 1 ) and the second EL layer  302 ( 2 )) may have structures similar to those described in Embodiment 1 or 2. In other words, the structures of the first EL layer  302 ( 1 ) and the second EL layer  302 ( 2 ) may be the same or different from each other and can be similar to those of the EL layers described in Embodiment 1 or 2. 
     Further, a charge generation layer (I)  305  is provided between the plurality of EL layers (the first EL layer  302 ( 1 ) and the second EL layer  302 ( 2 )). The charge generation layer (I)  305  has a function of injecting electrons into one of the EL layers and injecting holes into the other of the EL layers when a voltage is applied between the first electrode  301  and the second electrode  304 . In this embodiment, when a voltage is applied such that the potential of the first electrode  301  is higher than that of the second electrode  304 , the charge generation layer (I)  305  injects electrons into the first EL layer  302 ( 1 ) and injects holes into the second EL layer  302 ( 2 ). 
     Note that in terms of light extraction efficiency, the charge generation layer (I)  305  preferably has a light-transmitting property with respect to visible light (specifically, the charge generation layer (I)  305  preferably has a visible light transmittance of 40% or more). Further, the charge generation layer (I)  305  functions even if it has lower conductivity than the first electrode  301  or the second electrode  304 . 
     The charge generation layer (I)  305  may have either a structure in which an electron acceptor (acceptor) is added to an organic compound having a high hole-transport property or a structure in which an electron donor (donor) is added to an organic compound having a high electron-transport property. Alternatively, both of these structures may be stacked. 
     In the case where the electron acceptor is added to the organic compound having a high hole-transport property, 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. The substances mentioned here are mainly substances that have a hole mobility of 10 −6  cm 2 /Vs or more. Note that other than these substances, any organic compound that has a property of transporting more holes than electrons may be used. 
     Examples of the electron acceptor include 7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation: F 4 -TCNQ), chloranil, oxides of transition metals, and oxides of metals that belong to Groups 4 to 8 of the periodic table. 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 especially preferable since it is stable in the air, has a low hygroscopic property, and is easy to handle. 
     On the other hand, in the case where the electron donor is added to the organic compound having a high electron-transport property, examples of the organic compound having a high electron-transport property which can be used are metal complexes having a quinoline skeleton or a benzoquinoline skeleton, such as Alq, Almq 3 , BeBq 2 , and BAlq, and the like. Other examples are metal complexes having an oxazole-based or thiazole-based ligand, such as Zn(BOX) 2  and Zn(BTZ) 2 . Other than metal complexes, PBD, OXD-7, TAZ, BPhen, BCP, or the like can be used. The substances mentioned here are mainly substances that have an electron mobility of 10 −6  cm 2 /Vs or more. Note that other than these substances, any organic compound that has a property of transporting more electrons than holes may be used. 
     Examples of the electron donor which can be used are alkali metals, alkaline earth metals, rare earth metals, metals that belong to Group 13 of the periodic table, and oxides or carbonates thereof. Specifically, lithium (Li), cesium (Cs), magnesium (Mg), calcium (Ca), ytterbium (Yb), indium (In), lithium oxide, cesium carbonate, and the like are preferable. An organic compound, such as tetrathianaphthacene, may be used as the electron donor. 
     Note that forming the charge generation layer (I)  305  by using any of the above materials can suppress a drive voltage increase caused by the stack of the EL layers. 
     Although this embodiment shows the light-emitting element having two EL layers, the present invention can be similarly applied to a light-emitting element in which n EL layers (n is 3 or more) are stacked as illustrated in  FIG. 6B . In the case where a plurality of EL layers is included between a pair of electrodes as in the light-emitting element according to this embodiment, by provision of the charge generation layers (I) between the EL layers, light emission in a high luminance region can be obtained with current density kept low. Since the current density can be kept low, the element can have a long lifetime. When the light-emitting element is applied to lighting, voltage drop due to resistance of an electrode material can be reduced, thereby achieving homogeneous light emission in a large area. Moreover, it is possible to achieve a light-emitting device which can be driven at a low voltage and has low power consumption. 
     Furthermore, by making emission colors of EL layers different, light of a desired color can be obtained from the light-emitting element as a whole. For example, the emission colors of first and second EL layers are complementary in a light-emitting element having the two EL layers, so that the light-emitting element can be made to emit white light as a whole. Note that the term “complementary” means color relationship in which an achromatic color is obtained when colors are mixed. That is, emission of white light can be obtained by mixture of light emitted from substances whose emission colors are complementary colors. 
     Further, the same applies to a light-emitting element having three EL layers. For example, the light-emitting element as a whole can emit white light when the emission color of the first EL layer is red, the emission color of the second EL layer is green, and the emission color of the third EL layer is blue. 
     Note that the structure described in this embodiment can be combined as appropriate with any of the structures described in the other embodiments. 
     Embodiment 4 
     In this embodiment, a light-emitting device which is one embodiment of the present invention will be described. 
     A light-emitting device described in this embodiment has a micro optical resonator (microcavity) structure in which a light resonant effect between a pair of electrodes is utilized. The light-emitting device includes a plurality of light-emitting elements each of which has at least an EL layer  405  between a pair of electrodes (a reflective electrode  401  and a semi-transmissive and semi-reflective electrode  402 ) as illustrated in  FIG. 7 . Further, the EL layer  405  includes at least light-emitting layers  404  ( 404 R,  404 G, and  404 B) each serving as a light-emitting region and may further include a hole-injection layer, a hole-transport layer, an electron-transport layer, an electron-injection layer, a charge generation layer (E), and the like. Note that the light-emitting layers  404  ( 404 R,  404 G, and  404 B) may have the structure of the light-emitting layer according to one embodiment of the present invention which is described in Embodiment 1 or 2. 
     In this embodiment, a light-emitting device is described which includes light-emitting elements (a first light-emitting element (R)  410 R, a second light-emitting element (G)  410 G, and a third light-emitting element (B)  410 B) having different structures as illustrated in  FIG. 7 . 
     The first light-emitting element (R)  410 R has a structure in which a first transparent conductive layer  403   a ; an EL layer  405  including a first light-emitting layer (B)  404 B, a second light-emitting layer (G)  404 G, and a third light-emitting layer (R)  404 R in part; and a semi-transmissive and semi-reflective electrode  402  are sequentially stacked over a reflective electrode  401 . The second light-emitting element (G)  410 G has a structure in which a second transparent conductive layer  403   b , the EL layer  405 , and the semi-transmissive and semi-reflective electrode  402  are sequentially stacked over the reflective electrode  401 . The third light-emitting element (B)  410 B has a structure in which the EL layer  405  and the semi-transmissive and semi-reflective electrode  402  are sequentially stacked over the reflective electrode  401 . 
     Note that the reflective electrode  401 , the EL layer  405 , and the semi-transmissive and semi-reflective electrode  402  are common to the light-emitting elements (the first light-emitting element (R)  410 R, the second light-emitting element (G)  410 G, and the third light-emitting element (B)  410 B). The first light-emitting layer (B)  404 B emits light (λ B ) having a peak in a wavelength region from 420 nm to 480 nm. The second light-emitting layer (G)  404 G emits light (λ G ) having a peak in a wavelength region from 500 nm to 550 nm. The third light-emitting layer (R)  404 R emits light (λ R ) having a peak in a wavelength region from 600 nm to 760 nm. Thus, in each of the light-emitting elements (the first light-emitting element (R)  410 R, the second light-emitting element (G)  410 G, and the third light-emitting element (B)  410 B), light emitted from the first light-emitting layer (B)  404 B, light emitted from the second light-emitting layer (G)  404 G, and light emitted from the third light-emitting layer (R)  404 R overlap with each other; accordingly, light having a broad emission spectrum that covers a visible light region can be emitted. Note that the above wavelengths satisfy the relation of λ B &lt;λ G &lt;λ R . 
     Each of the light-emitting elements described in this embodiment has a structure in which the EL layer  405  is provided between the reflective electrode  401  and the semi-transmissive and semi-reflective electrode  402 . Light emitted in all directions from the light-emitting layers included in the EL layer  405  is resonated by the reflective electrode  401  and the semi-transmissive and semi-reflective electrode  402  which function as a micro optical resonator (microcavity). Note that the reflective electrode  401  is formed using a conductive material having reflectivity, and a film whose visible light reflectivity is 40% to 100%, preferably 70% to 100%, and whose resistivity is 1×10 −2  Ωcm or lower is used. In addition, the semi-transmissive and semi-reflective electrode  402  is formed using a conductive material having reflectivity and a conductive material having a light-transmitting property, and a film whose visible light reflectivity is 20% to 80%, preferably 40% to 70%, and whose resistivity is 1×10 −2  Ωcm or lower is used. 
     In this embodiment, the thicknesses of the transparent conductive layers (the first transparent conductive layer  403   a  and the second transparent conductive layer  403   b ) provided in the first light-emitting element (R)  410 R and the second light-emitting element (G)  410 G, respectively, are varied between the light-emitting elements, whereby the light-emitting elements differ from each other in the optical path length from the reflective electrode  401  to the semi-transmissive and semi-reflective electrode  402 . In other words, in light having a broad emission spectrum, which is emitted from the light-emitting layers of each of the light-emitting elements, light with a wavelength that is resonated between the reflective electrode  401  and the semi-transmissive and semi-reflective electrode  402  can be intensified while light with a wavelength that is not resonated therebetween can be attenuated. Thus, when the elements differ from each other in the optical path length from the reflective electrode  401  to the semi-transmissive and semi-reflective electrode  402 , light with different wavelengths can be extracted. 
     Note that the optical path length (also referred to as optical distance) is expressed as a product of an actual distance and a refractive index, and in this embodiment, is a product of an actual thickness and n (refractive index). That is, an optical path length=actual thickness×n. 
     Further, the total thickness from the reflective electrode  401  to the semi-transmissive and semi-reflective electrode  402  is set to mλ R /2 (m is a natural number) in the first light-emitting element (R)  410 R; the total thickness from the reflective electrode  401  to the semi-transmissive and semi-reflective electrode  402  is set to mλ G /2 (m is a natural number) in the second light-emitting element (G)  410 G; and the total thickness from the reflective electrode  401  to the semi-transmissive and semi-reflective electrode  402  is set to mλ B /2 (m is a natural number) in the third light-emitting element (B)  410 B. 
     In this manner, the light (λ R ) emitted from the third light-emitting layer (R)  404 R included in the EL layer  405  is mainly extracted from the first light-emitting element (R)  410 R, the light (λ G ) emitted from the second light-emitting layer (G)  404 G included in the EL layer  405  is mainly extracted from the second light-emitting element (G)  410 G, and the light (λ B ) emitted from the first light-emitting layer (B)  404 B included in the EL layer  405  is mainly extracted from the third light-emitting element (B)  410 B. Note that the light extracted from each of the light-emitting elements is emitted from the semi-transmissive and semi-reflective electrode  402  side. 
     Further, strictly speaking, the total thickness from the reflective electrode  401  to the semi-transmissive and semi-reflective electrode  402  can be the total thickness from a reflection region in the reflective electrode  401  to a reflection region in the semi-transmissive and semi-reflective electrode  402 . However, it is difficult to precisely determine the positions of the reflection regions in the reflective electrode  401  and the semi-transmissive and semi-reflective electrode  402 ; therefore, it is presumed that the above effect can be sufficiently obtained wherever the reflection regions may be set in the reflective electrode  401  and the semi-transmissive and semi-reflective electrode  402 . 
     Next, in the first light-emitting element (R)  410 R, the optical path length from the reflective electrode  401  to the third light-emitting layer (R)  404 R is adjusted to a desired thickness ((2m′+1)λ R /4, where m′ is a natural number); thus, light emitted from the third light-emitting layer (R)  404 R can be amplified. Light (first reflected light) that is reflected by the reflective electrode  401  of the light emitted from the third light-emitting layer (R)  404 R interferes with light (first incident light) that directly enters the semi-transmissive and semi-reflective electrode  402  from the third light-emitting layer (R)  404 R. Therefore, by adjusting the optical path length from the reflective electrode  401  to the third light-emitting layer (R)  404 R to the desired value ((2m′+1)λ R /4, where m′ is a natural number), the phases of the first reflected light and the first incident light can be aligned with each other and the light emitted from the third light-emitting layer (R)  404 R can be amplified. 
     Note that strictly speaking, the optical path length from the reflective electrode  401  to the third light-emitting layer (R)  404 R can be the optical path length from a reflection region in the reflective electrode  401  to a light-emitting region in the third light-emitting layer (R)  404 R. However, it is difficult to precisely determine the positions of the reflection region in the reflective electrode  401  and the light-emitting region in the third light-emitting layer (R)  404 R; therefore, it is presumed that the above effect can be sufficiently obtained wherever the reflection region and the light-emitting region may be set in the reflective electrode  401  and the third light-emitting layer (R)  404 R, respectively. 
     Next, in the second light-emitting element (G)  410 G, the optical path length from the reflective electrode  401  to the second light-emitting layer (G)  404 G is adjusted to a desired thickness ((2m″+1)λ G /4, where m″ is a natural number); thus, light emitted from the second light-emitting layer (G)  404 G can be amplified. Light (second reflected light) that is reflected by the reflective electrode  401  of the light emitted from the second light-emitting layer (G)  404 G interferes with light (second incident light) that directly enters the semi-transmissive and semi-reflective electrode  402  from the second light-emitting layer (G)  404 G. Therefore, by adjusting the optical path length from the reflective electrode  401  to the second light-emitting layer (G)  404 G to the desired value ((2m″+1)λ G /4, where m″ is a natural number), the phases of the second reflected light and the second incident light can be aligned with each other and the light emitted from the second light-emitting layer (G)  404 G can be amplified. 
     Note that strictly speaking, the optical path length from the reflective electrode  401  to the second light-emitting layer (G)  404 G can be the optical path length from a reflection region in the reflective electrode  401  to a light-emitting region in the second light-emitting layer (G)  404 G. However, it is difficult to precisely determine the positions of the reflection region in the reflective electrode  401  and the light-emitting region in the second light-emitting layer (G)  404 G; therefore, it is presumed that the above effect can be sufficiently obtained wherever the reflection region and the light-emitting region may be set in the reflective electrode  401  and the second light-emitting layer (G)  404 G, respectively. 
     Next, in the third light-emitting element (B)  410 B, the optical path length from the reflective electrode  401  to the first light-emitting layer (B)  404 B is adjusted to a desired thickness ((2m″′+1)λ B /4, where m″′ is a natural number); thus, light emitted from the first light-emitting layer (B)  404 B can be amplified. Light (third reflected light) that is reflected by the reflective electrode  401  of the light emitted from the first light-emitting layer (B)  404 B interferes with light (third incident light) that directly enters the semi-transmissive and semi-reflective electrode  402  from the first light-emitting layer (B)  404 B. Therefore, by adjusting the optical path length from the reflective electrode  401  to the first light-emitting layer (B)  404 B to the desired value ((2m″′+1)λ B /4, where m″′ is a natural number), the phases of the third reflected light and the third incident light can be aligned with each other and the light emitted from the first light-emitting layer (B)  404 B can be amplified. 
     Note that strictly speaking, the optical path length from the reflective electrode  401  to the first light-emitting layer (B)  404 B in the third light-emitting element can be the optical path length from a reflection region in the reflective electrode  401  to a light-emitting region in the first light-emitting layer (B)  404 B. However, it is difficult to precisely determine the positions of the reflection region in the reflective electrode  401  and the light-emitting region in the first light-emitting layer (B)  404 B; therefore, it is presumed that the above effect can be sufficiently obtained wherever the reflection region and the light-emitting region may be set in the reflective electrode  401  and the first light-emitting layer (B)  404 B, respectively. 
     Note that although each of the light-emitting elements in the above-described structure includes a plurality of light-emitting layers in the EL layer, the present invention is not limited thereto; for example, the structure of the tandem light-emitting element which is described in Embodiment 3 can be combined, in which case a plurality of EL layers and a charge generation layer interposed therebetween are provided in one light-emitting element and one or more light-emitting layers are formed in each of the EL layers. 
     The light-emitting device described in this embodiment has a microcavity structure, in which light with wavelengths which differ depending on the light-emitting elements can be extracted even when they include the same EL layer, so that it is not needed to form light-emitting elements for the colors of R, G, and B. Therefore, the above structure is advantageous for full color display owing to easiness in achieving higher resolution display or the like. Note that a combination with coloring layers (color filters) is also possible. In addition, emission intensity with a predetermined wavelength in the front direction can be increased, whereby power consumption can be reduced. The above structure is particularly useful in the case of being applied to a color display (image display device) including pixels of three or more colors but may also be applied to lighting or the like. 
     Embodiment 5 
     In this embodiment, a light-emitting device including a light-emitting element which is one embodiment of the present invention will be described. 
     The light-emitting device can be either a passive matrix light-emitting device or an active matrix light-emitting device. Note that any of the light-emitting elements described in the other embodiments can be applied to the light-emitting device described in this embodiment. 
     In this embodiment, an active matrix light-emitting device is described with reference to  FIGS. 8A and 8B . 
     Note that  FIG. 8A  is a top view illustrating a light-emitting device and  FIG. 8B  is a cross-sectional view taken along the chain line A-A′ in  FIG. 8A . The active matrix light-emitting device according to this embodiment includes a pixel portion  502  provided over an element substrate  501 , a driver circuit portion (a source line driver circuit)  503 , and driver circuit portions (gate line driver circuits)  504  ( 504   a  and  504   b ). The pixel portion  502 , the driver circuit portion  503 , and the driver circuit portions  504  are sealed between the element substrate  501  and a sealing substrate  506  with a sealant  505 . 
     In addition, there is provided a lead wiring  507  over the element substrate  501 . The lead wiring  507  is provided for connecting an external input terminal through which a signal (e.g., a video signal, a clock signal, a start signal, or a reset signal) or a potential from the outside is transmitted to the driver circuit portion  503  and the driver circuit portions  504 . Here is shown an example in which a flexible printed circuit (FPC)  508  is provided as the external input terminal. Although only the FPC is illustrated, this FPC may be provided with a printed wiring board (PWB). The light-emitting device in this specification includes, in its category, not only the light-emitting device itself but also the light-emitting device provided with the FPC or the PWB. 
     Next, a cross-sectional structure is described with reference to  FIG. 8B . The driver circuit portions and the pixel portion are formed over the element substrate  501 ; here are illustrated the driver circuit portion  503  which is the source line driver circuit and the pixel portion  502 . 
     The driver circuit portion  503  is an example where a CMOS circuit is formed, which is a combination of an n-channel TFT  509  and a p-channel TFT  510 . Note that a circuit included in the driver circuit portion may be formed using any of various circuits, 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  502  includes a plurality of pixels each of which includes a switching TFT  511 , a current control TFT  512 , and a first electrode (anode)  513  which is electrically connected to a wiring (a source electrode or a drain electrode) of the current control TFT  512 . Note that an insulator  514  is formed to cover end portions of the first electrode (anode)  513 . In this embodiment, the insulator  514  is formed using a positive photosensitive acrylic resin. 
     The insulator  514  preferably has a curved surface with curvature at an upper end portion or a lower end portion thereof in order to obtain favorable coverage by a film which is to be stacked over the insulator  514 . For example, in the case of using a positive photosensitive acrylic resin as a material for the insulator  514 , the insulator  514  preferably has a curved surface with a curvature radius (0.2 μm to 3 μm) at the upper end portion. The insulator  514  can be formed using either a negative photosensitive resin or a positive photosensitive resin. It is possible to use, without limitation to an organic compound, either an organic compound or an inorganic compound such as silicon oxide or silicon oxynitride. 
     An EL layer  515  and a second electrode (cathode)  516  are stacked over the first electrode (anode)  513 . In the EL layer  515 , at least a light-emitting layer is provided. The light-emitting layer has such a stacked-layer structure as described in Embodiment 1. Further, in the EL layer  515 , a hole-injection layer, a hole-transport layer, an electron-transport layer, an electron-injection layer, a charge generation layer, and the like can be provided as appropriate in addition to the light-emitting layer. 
     The stacked-layer structure including the first electrode (anode)  513 , the EL layer  515 , and the second electrode (cathode)  516  forms a light-emitting element  517 . For the first electrode (anode)  513 , the EL layer  515 , and the second electrode (cathode)  516 , the materials described in Embodiment 2 can be used. Although not illustrated, the second electrode (cathode)  516  is electrically connected to the FPC  508  which is an external input terminal. 
     Although the cross-sectional view in  FIG. 8B  illustrates only one light-emitting element  517 , a plurality of light-emitting elements is arranged in a matrix in the pixel portion  502 . Light-emitting elements which provide three kinds of light emission (R, G, and B) are selectively formed in the pixel portion  502 , whereby a light-emitting device capable of full color display can be fabricated. Alternatively, a light-emitting device capable of full color display may be fabricated by a combination with coloring layers (color filters). 
     Further, the sealing substrate  506  is attached to the element substrate  501  with the sealant  505 , whereby the light-emitting element  517  is provided in a space  518  surrounded by the element substrate  501 , the sealing substrate  506 , and the sealant  505 . The space  518  may be filled with an inert gas (such as nitrogen or argon) or the sealant  505 . 
     An epoxy-based resin, low-melting-point glass, or the like is preferably used for the sealant  505 . It is preferable that such a material do not transmit moisture or oxygen as much as possible. As the sealing substrate  506 , a glass substrate, a quartz substrate, or a plastic substrate formed of fiberglass reinforced plastic (FRP), polyvinyl fluoride) (PVF), polyester, acrylic, or the like can be used. 
     As described above, an active matrix light-emitting device can be obtained. 
     Note that the structure described in this embodiment can be combined as appropriate with any of the structures described in the other embodiments. 
     Embodiment 6 
     In this embodiment, examples of a variety of electronic devices which are completed using a light-emitting device will be described with reference to  FIGS. 9A to 9D  and  FIGS. 10A to 10C . The light-emitting device is fabricated using a light-emitting element which is one embodiment of the present invention. 
     Examples of electronic devices to which the light-emitting device is applied are television devices (also referred to as TV or television receivers), monitors for computers and the like, cameras such as digital cameras and digital video cameras, digital photo frames, cellular phones (also referred to as portable telephone devices), portable game machines, portable information terminals, audio playback devices, large game machines such as pin-ball machines, and the like. Specific examples of these electronic devices are illustrated in  FIGS. 9A to 9D . 
       FIG. 9A  illustrates an example of a television device. In a television device  7100 , a display portion  7103  is incorporated in a housing  7101 . The display portion  7103  is capable of displaying images, and a light-emitting device can be used for the display portion  7103 . In addition, here, the housing  7101  is supported by a stand  7105 . 
     The television device  7100  can be operated with an operation switch provided in 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, general television broadcasting 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 transmitter to a receiver) or two-way (between a transmitter and a receiver, between receivers, or the like) data communication can be performed. 
       FIG. 9B  illustrates a computer, which includes a main body  7201 , a housing  7202 , a display portion  7203 , a keyboard  7204 , an external connection port  7205 , a pointing device  7206 , and the like. Note that this computer is manufactured by using a light-emitting device for the display portion  7203 . 
       FIG. 9C  illustrates a portable game machine, which includes two housings, i.e., a housing  7301  and a housing  7302 , connected to each other via a joint portion  7303  so that the portable game machine can be opened or closed. 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. 9C  includes a speaker portion  7306 , a recording medium insertion portion  7307 , an LED lamp  7308 , 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, electric current, voltage, electric power, radiation, flow rate, humidity, gradient, oscillation, odor, or infrared rays), and a microphone  7312 ), and the like. It is needless to say that the structure of the portable game machine is not limited to the above structure as long as a light-emitting device is used for at least either the display portion  7304  or the display portion  7305 , or both, and may include other accessories as appropriate. The portable game machine illustrated in  FIG. 9C  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. Note that the portable game machine illustrated in  FIG. 9C  can have a variety of functions without limitation to those above. 
       FIG. 9D  illustrates an example of a cellular phone. A cellular 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 cellular phone  7400  is manufactured using a light-emitting device for the display portion  7402 . 
     When the display portion  7402  of the cellular phone  7400  illustrated in  FIG. 9D  is touched with a finger or the like, data can be input to the cellular phone  7400 . Further, operations such as making a call and creating e-mail can be performed by touch on the display portion  7402  with a finger or the like. 
     There are mainly three screen modes for the display portion  7402 . The first mode is a display mode mainly for displaying an image. The second mode is an input mode mainly for inputting information such as characters. The third mode is a display-and-input mode in which two modes of the display mode and the input mode are mixed. 
     For example, in the case of making a call or creating e-mail, a character input mode mainly for inputting characters is selected for the display portion  7402  so that characters displayed on the screen can be input. In this case, it is preferable to display a keyboard or number buttons on almost the entire screen of the display portion  7402 . 
     When a detection device including a sensor for detecting inclination, such as a gyroscope or an acceleration sensor, is provided inside the cellular phone  7400 , display on the screen of the display portion  7402  can be automatically changed by determining the orientation of the cellular phone  7400  (whether the cellular phone is placed horizontally or vertically for a landscape mode or a portrait mode). 
     The screen modes are changed by touch on the display portion  7402  or operation with the operation buttons  7403  of the housing  7401 . Alternatively, the screen modes can be changed depending on the kind of image displayed on the display portion  7402 . For example, when a signal for an image to be displayed on the display portion is data of moving images, the screen mode is changed to the display mode. When the signal is text data, the screen mode is changed to the input mode. 
     Moreover, in the input mode, if a signal detected by an optical sensor in the display portion  7402  is detected and the input by touch on the display portion  7402  is not performed for a certain period, the screen mode may be controlled so as to be changed from the input mode to the display mode. 
     The display portion  7402  may function as an image sensor. For example, an image of a palm print, a fingerprint, or the like is taken by touch on the display portion  7402  with the palm or the finger, whereby personal identification can be performed. Furthermore, when a backlight or a sensing light source which emits near-infrared light is provided for the display portion, an image of a finger vein, a palm vein, or the like can also be taken. 
       FIGS. 10A and 10B  illustrate a foldable tablet terminal. The tablet terminal is opened in  FIG. 10A . The tablet terminal includes a housing  9630 , a display portion  9631   a , a display portion  9631   b , a display mode switch  9034 , a power switch  9035 , a power saver switch  9036 , a clasp  9033 , and an operation switch  9038 . The tablet terminal is manufactured using the light-emitting device for either the display portion  9631   a  or the display portion  9631   b  or both. 
     Part of the display portion  9631   a  can be a touch panel region  9632   a  and data can be input when a displayed operation key  9637  is touched. Although a structure in which a half region in the display portion  9631   a  has only a display function and the other half region also has a touch panel function is shown as an example, the display portion  9631   a  is not limited to the structure. The whole region in the display portion  9631   a  may have a touch panel function. For example, the display portion  9631   a  can display keyboard buttons in the whole region to be a touch panel, and the display portion  9631   b  can be used as a display screen. 
     As in the display portion  9631   a , part of the display portion  9631   b  can be a touch panel region  9632   b . When a keyboard display switching button  9639  displayed on the touch panel is touched with a finger, a stylus, or the like, a keyboard can be displayed on the display portion  9631   b.    
     Touch input can be performed in the touch panel region  9632   a  and the touch panel region  9632   b  at the same time. 
     The display mode switch  9034  can switch the display between portrait mode, landscape mode, and the like, and between monochrome display and color display, for example. The power saver switch  9036  can control display luminance in accordance with the amount of external light in use of the tablet terminal detected by an optical sensor incorporated in the tablet terminal. In addition to the optical sensor, another detection device including a sensor for detecting inclination, such as a gyroscope or an acceleration sensor, may be incorporated in the tablet terminal. 
       FIG. 10A  shows an example in which the display portion  9631   a  and the display portion  9631   b  have the same display area; however, without limitation thereon, one of the display portions may be different from the other display portion in size and display quality. For example, one display panel may be capable of higher-definition display than the other display panel. 
     The tablet terminal is closed in  FIG. 10B . The tablet terminal includes the housing  9630 , a solar cell  9633 , a charge and discharge control circuit  9634 , a battery  9635 , and a DCDC converter  9636 . In  FIG. 10B , a structure including the battery  9635  and the DCDC converter  9636  is illustrated as an example of the charge and discharge control circuit  9634 . 
     Since the tablet terminal is foldable, the housing  9630  can be closed when the tablet terminal is not used. As a result, the display portion  9631   a  and the display portion  9631   b  can be protected; thus, a tablet terminal which has excellent durability and excellent reliability in terms of long-term use can be provided. 
     In addition, the tablet terminal illustrated in  FIGS. 10A and 10B  can have a function of displaying a variety of kinds of data (e.g., a still image, a moving image, and a text image), a function of displaying a calendar, a date, the time, or the like on the display portion, a touch-input function of operating or editing the data displayed on the display portion by touch input, a function of controlling processing by a variety of kinds of software (programs), and the like. 
     The solar cell  9633  provided on a surface of the tablet terminal can supply power to the touch panel, the display portion, a video signal processing portion, or the like. Note that the solar cell  9633  can be provided on one or both surfaces of the housing  9630  and the battery  9635  can be charged efficiently. The use of a lithium ion battery as the battery  9635  is advantageous in downsizing or the like. 
     The structure and the operation of the charge and discharge control circuit  9634  illustrated in  FIG. 10B  will be described with reference to a block diagram in  FIG. 10C . The solar cell  9633 , the battery  9635 , the DCDC converter  9636 , a converter  9638 , switches SW 1  to SW 3 , and a display portion  9631  are illustrated in  FIG. 10C , and the battery  9635 , the DCDC converter  9636 , the converter  9638 , and the switches SW 1  to SW 3  correspond to the charge and discharge control circuit  9634  illustrated in  FIG. 10B . 
     First, an example of the operation in the case where power is generated by the solar cell  9633  using external light is described. The voltage of power generated by the solar cell  9633  is stepped up or down by the DCDC converter  9636  so that the power has a voltage for charging the battery  9635 . Then, when the power from the solar cell  9633  is used for the operation of the display portion  9631 , the switch SW 1  is turned on and the voltage of the power is stepped up or down by the converter  9638  so as to be a voltage needed for the display portion  9631 . In addition, when display on the display portion  9631  is not performed, the switch SW 1  is turned off and the switch SW 2  is turned on so that the battery  9635  may be charged. 
     Note that the solar cell  9633  is described as an example of a power generation means; however, without limitation thereon, the battery  9635  may be charged using another power generation means such as a piezoelectric element or a thermoelectric conversion element (Peltier element). For example, a non-contact electric power transmission module which transmits and receives power wirelessly (without contact) to charge the battery  9635 , or a combination of the solar cell  9633  and another means for charge may be used. 
     It is needless to say that an embodiment of the present invention is not limited to the electronic device illustrated in  FIGS. 10A to 10C  as long as the display portion described in the above embodiment is included. 
     As described above, the electronic devices can be obtained by the use of the light-emitting device which is one embodiment of the present invention. The light-emitting device has a remarkably wide application range, and can be applied to electronic devices in a variety of fields. 
     Note that the structure described in this embodiment can be combined as appropriate with any of the structures described in the other embodiments. 
     Embodiment 7 
     In this embodiment, examples of lighting devices will be described with reference to  FIG. 11 . A light-emitting device including a light-emitting element which is one embodiment of the present invention is applied to the lighting devices. 
       FIG. 11  illustrates an example in which a light-emitting device is used for an interior lighting device  8001 . Since the light-emitting device can have a larger area, a lighting device having a large area can also be formed. In addition, a lighting device  8002  in which a light-emitting region has a curved surface can also be formed with the use of a housing with a curved surface. A light-emitting element included in the light-emitting device described in this embodiment is in the form of a thin film, which allows the housing to be designed more freely. Therefore, the lighting device can be elaborately designed in a variety of ways. Further, a wall of the room may be provided with a large-sized lighting device  8003 . 
     Moreover, when the light-emitting device is used for a table by being used as a surface of a table, a lighting device  8004  which has a function as a table can be obtained. When the light-emitting device is used as part of other furniture, a lighting device which has a function as the furniture can be obtained. 
     In this manner, a variety of lighting devices to which the light-emitting device is applied can be obtained. Note that such lighting devices are also embodiments of the present invention. 
     The structure described in this embodiment can be combined as appropriate with any of the structures described in the other embodiments. 
     Example 1 
     In this example, a light-emitting element  1  which is one embodiment of the present invention is described with reference to  FIG. 12 . Chemical formulae of materials used in this example are shown below. 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     Fabrication of Light-Emitting Element  1   
     First, a film of indium tin oxide containing silicon oxide (ITSO) was formed over a glass substrate  1100  by a sputtering method, so that a first electrode  1101  functioning as an anode was formed. Note that the thickness was set to 110 nm and the electrode area was set to 2 mm×2 mm. 
     Next, as pretreatment for forming the light-emitting element  1  over the substrate  1100 , UV ozone treatment was performed for 370 seconds after washing of a surface of the substrate with water and baking that was performed at 200° C. for one hour. 
     After that, the substrate was transferred into a vacuum evaporation apparatus where the pressure had been reduced to approximately 10 −4  Pa, and was subjected to vacuum baking at 170° C. for 30 minutes in a heating chamber of the vacuum evaporation apparatus, and then the substrate  1100  was cooled down for about 30 minutes. 
     Next, the substrate  1100  was fixed to a holder in the vacuum evaporation apparatus so that a surface on which the first electrode  1101  was provided faced downward. In this example, a case is described in which a hole-injection layer  1111 , a hole-transport layer  1112 , a light-emitting layer  1113 , an electron-transport layer  1114 , and an electron-injection layer  1115  which are included in an EL layer  1102  are sequentially formed by a vacuum evaporation method. 
     The pressure in the vacuum evaporation apparatus was reduced to about 10 −4  Pa. Then, 1,3,5-tri(dibenzothiophen-4-yl)benzene (abbreviation: DBT3P-II) and molybdenum(VI) oxide were co-evaporated with a mass ratio of DBT3P-II (abbreviation) to molybdenum oxide being 4:2, whereby the hole-injection layer  1111  was formed over the first electrode  1101 . The thickness of the hole-injection layer  1111  was set to 20 nm. Note that co-evaporation is an evaporation method by which a plurality of different substances is concurrently vaporized from respective different evaporation sources. 
     Next, the hole-transport layer  1112  was formed by evaporation of 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine (abbreviation: BPAFLP) to a thickness of 20 nm. 
     Next, the light-emitting layer  1113  was formed over the hole-transport layer  1112 . The light-emitting layer  1113  having a stacked-layer structure was formed by forming a first light-emitting layer  1113   a  with a thickness of 20 nm by co-evaporation of 2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline (abbreviation: 2mDBTBPDBq-II), N-(1,1′-biphenyl-4-yl)-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-9,9-dimethyl-9H-fluoren-2-amine (abbreviation: PCBBiF), and (acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III) (abbreviation: [Ir(tBuppm) 2 (acac)]) with a mass ratio of 2mDBTBPDBq-II (abbreviation) to PCBBiF (abbreviation) and [Ir(tBuppm) 2 (acac)] (abbreviation) being 0.7:0.3:0.05, and then forming a second light-emitting layer  1113   b  with a thickness of 20 nm by co-evaporation of 2mDBTBPDBq-II (abbreviation), N,N′-bis(9,9-dimethylfluoren-2-yl)-N,N′-di(biphenyl-4-yl)-1,4-phenylenediamine (abbreviation: FBi2P), and bis{4,6-dimethyl-2-[5-(2,6-dimethylphenyl)-3-(3,5-dimethylphenyl)-2-pyrazinyl-κN]phenyl-κC}(2,8-dimethyl-4,6-nonanedionato-κ 2 O,O′)iridium(III) (abbreviation: [Ir(dmdppr-dmp) 2 (divm)]) with a mass ratio of 2mDBTBPDBq-II (abbreviation) to FBi2P (abbreviation) and [Ir(dmdppr-dmp) 2 (divm)] (abbreviation) being 0.8:0.2:0.05. 
     Next, over the light-emitting layer  1113 , the electron-transport layer  1114  was formed in such a manner that a film of 2mDBTBPDBq-II (abbreviation) was formed by evaporation to a thickness of 15 nm and then a film of bathophenanthroline (abbreviation: Bphen) was formed by evaporation to a thickness of 10 nm. Further, over the electron-transport layer  1114 , a film of lithium fluoride was formed by evaporation to a thickness of 1 nm to form the electron-injection layer  1115 . 
     Lastly, over the electron-injection layer  1115 , an aluminum film was formed by evaporation to a thickness of 200 nm as a second electrode  1103  functioning as a cathode. Thus, the light-emitting element  1  was fabricated. Note that in all the above evaporation steps, evaporation was performed by a resistance-heating method. 
     Table 1 shows an element structure of the light-emitting element  1  obtained as described above. 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 First 
                 Hole-injection 
                 Hole-transport 
                 Light-emitting 
                   
                 Electron- 
                 Second 
               
               
                   
                 Electrode 
                 Layer 
                 Layer 
                 Layer 
                 Electron-transport Layer 
                 injection Layer 
                 Electrode 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 Light- 
                 ITSO 
                 DBT3P-II: 
                 BPAFLP 
                 * 
                 ** 
                 2mDBTBPDBq-II 
                 Bphen 
                 LiF 
                 Al 
               
               
                 emitting 
                 (110 nm) 
                 MoO x   
                 (20 nm) 
                   
                   
                 (15 nm) 
                 (10 nm) 
                 (1 nm) 
                 (200 nm) 
               
               
                 Element 1 
                   
                 (4:2 20 nm) 
               
               
                   
               
               
                 * 2mDBTBPDBq-II:PCBBiF:[Ir(tBuppm) 2 (acac)] (0.7:0.3:0.05 20 nm) 
               
               
                 ** 2mDBTBPDBq-II:FBi2P:[Ir(dmdppr-dmp) 2 (divm)] (0.8:0.2:0.05 20 nm) 
               
            
           
         
       
     
     The fabricated light-emitting element  1  was sealed in a glove box containing a nitrogen atmosphere so as not to be exposed to air (specifically, a sealant was applied to an outer edge of the element and heat treatment was performed at 80° C. for 1 hour at the time of sealing). 
     Operation Characteristics of Light-Emitting Element  1   
     Operation characteristics of the fabricated light-emitting element  1  were measured. Note that the measurements were carried out at room temperature (in an atmosphere kept at 25° C.). 
     First,  FIG. 13  shows current density-luminance characteristics of the light-emitting element  1 . In  FIG. 13 , the vertical axis represents luminance (cd/m 2 ), and the horizontal axis represents current density (mA/cm 2 ).  FIG. 14  shows voltage-luminance characteristics of the light-emitting element  1 . In  FIG. 14 , the vertical axis represents luminance (cd/m 2 ), and the horizontal axis represents voltage (V).  FIG. 15  shows luminance-current efficiency characteristics of the light-emitting element  1 . In  FIG. 15 , the vertical axis represents current efficiency (cd/A), and the horizontal axis represents luminance (cd/m 2 ).  FIG. 16  shows voltage-current characteristics of the light-emitting element  1 . In  FIG. 16 , the vertical axis represents current (mA), and the horizontal axis represents voltage (V). 
       FIG. 15  reveals high efficiency of the light-emitting element  1  that is one embodiment of the present invention. Table 2 below shows initial values of main characteristics of the light-emitting element  1  at a luminance of about 1000 cd/m 2 . 
     
       
         
           
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 External 
               
               
                   
                   
                   
                 Current 
                   
                   
                 Current 
                 Power 
                 Quantum 
               
               
                   
                 Voltage 
                 Current 
                 Density 
                 Chromaticity 
                 Luminance 
                 Efficiency 
                 Efficiency 
                 Efficiency 
               
               
                   
                 (V) 
                 (mA) 
                 (mA/cm 2 ) 
                 (x, y) 
                 (cd/m 2 ) 
                 (cd/A) 
                 (lm/W) 
                 (%) 
               
               
                   
               
             
            
               
                 Light- 
                 3 
                 0.064 
                 1.6 
                 (0.51, 0.48) 
                 1000 
                 62 
                 64 
                 22 
               
               
                 emitting 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Element 1 
               
               
                   
               
            
           
         
       
     
     The above results show that the light-emitting element  1  fabricated in this example has high external quantum efficiency, which means its high emission efficiency. 
       FIG. 17  shows an emission spectrum when a current at a current density of 25 mA/cm 2  was supplied to the light-emitting element  1 . As shown in  FIG. 17 , the emission spectrum of the light-emitting element  1  has two peaks at around 546 nm and 615 nm, which suggests that the two peaks are respectively derived from emission from the phosphorescent organometallic iridium complexes [Ir(tBuppm) 2 (acac)] and [Ir(dmdppr-dmp) 2 (divm)]. 
     Note that by cyclic voltammetry, the HOMO level of PCBBiF, which is the second organic compound with a hole-transport property, was estimated to be −5.36 eV, and the HOMO level of FBi2P, which is the third organic compound with a hole-transport property, was estimated to be −5.13 eV. Thus, the second organic compound has a lower HOMO level than the third organic compound. 
     Here,  FIG. 18  shows an emission spectrum of a thin film of 2mDBTBPDBq-II, which is the first organic compound with an electron-transport property, an emission spectrum of a thin film of PCBBiF, which is the second organic compound with a hole-transport property, and an emission spectrum of a mixed film (co-evaporation film) of 2mDBTBPDBq-II and PCBBiF. Further,  FIG. 19  shows an emission spectrum of a thin film of 2mDBTBPDBq-II, which is the first organic compound with an electron-transport property, an emission spectrum of a thin film of FBi2P, which is the third organic compound with a hole-transport property, and an emission spectrum of a mixed film (co-evaporation film) of 2mDBTBPDBq-II and FBi2P. 
     In each of  FIG. 18  and  FIG. 19 , the emission wavelength of the mixed film is located on a longer wavelength side with respect to the emission wavelengths of the single films of the materials, which indicates that the first organic compound (2mDBTBPDBq-II) and the second organic compound (PCBBiF) formed an exciplex and the first organic compound (2mDBTBPDBq-II) and the third organic compound (FBi2P) formed an exciplex. In addition, while the exciplex formed by the first organic compound (2mDBTBPDBq-II) and the third organic compound (FBi2P) has an emission wavelength of 553 nm, the exciplex formed by the first organic compound (2mDBTBPDBq-II) and the second organic compound (PCBBiF) has an emission wavelength of 511 nm to show that it has higher energy. 
     This application is based on Japanese Patent Application serial no. 2012-091533 filed with Japan Patent Office on Apr. 13, 2012, Japanese Patent Application serial no. 2012-225061 filed with Japan Patent Office on Oct. 10, 2012, and Japanese Patent Application serial no. 2013-047830 filed with Japan Patent Office on Mar. 11, 2013, the entire contents of which are hereby incorporated by reference.