Patent Publication Number: US-2022238810-A1

Title: Light-emitting element, display 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, or a display device, an electronic device, and a lighting device each including the light-emitting element. 
     Note that one embodiment of the present invention is not limited to the above technical field. The technical field of one embodiment of the invention disclosed in this specification and the like relates to an object, a method, or a manufacturing method. In addition, one embodiment of the present invention relates to a process, a machine, manufacture, or a composition of matter. Specifically, examples of the technical field of one embodiment of the present invention disclosed in this specification include a semiconductor device, a display device, a liquid crystal display device, a light-emitting device, a lighting device, a power storage device, a storage device, a method of driving any of them, and a method of manufacturing any of them. 
     2. Description of the Related Art 
     In recent years, research and development have been extensively conducted on light-emitting elements using electroluminescence (EL). In a basic structure of such a light-emitting element, a layer containing a light-emitting substance (an EL layer) is interposed between a pair of electrodes. By application of a voltage between the electrodes of this element, light emission from the light-emitting substance can be obtained. 
     Since the above light-emitting element is a self-luminous type, a display device using this light-emitting element has advantages such as high visibility, no necessity of a backlight, and low power consumption. Furthermore, such a light-emitting element also has advantages in that the element can be manufactured to be thin and lightweight, and has high response speed. 
     In a light-emitting element whose EL layer contains an organic compound as a light-emitting substance and is provided between a pair of electrodes (e.g., an organic EL element), application of a voltage between the pair of electrodes causes injection of electrons from a cathode and holes from an anode into the EL layer having a light-emitting property and thus a current flows. By recombination of the injected electrons and holes, the light-emitting organic compound is brought into an excited state to provide light emission. 
     Note that an excited state formed by an organic compound can be a singlet excited state (S*) or a triplet excited state (T*). Light emission from the singlet-excited state is referred to as fluorescence, and light emission from the triplet excited state is referred to as phosphorescence. The formation ratio of S* to T* in the light-emitting element is 1:3. In other words, a light-emitting element containing a compound emitting phosphorescence (phosphorescent compound) has higher emission efficiency than a light-emitting element containing a compound emitting fluorescence (fluorescent compound). Therefore, light-emitting elements containing phosphorescent compounds capable of converting a triplet excited state into light emission has been actively developed in recent years. 
     Among light-emitting elements containing phosphorescent compounds, a light-emitting element that emits blue light in particular has yet been put into practical use because it is difficult to develop a stable compound having a high triplet excited energy level. For this reason, the development of a light-emitting element containing a more stable fluorescent compound has been conducted and a technique for increasing the emission efficiency of a light-emitting element containing a fluorescent compound (fluorescent element) has been searched. 
     As one of materials capable of partly converting the triplet excited state into light emission, a thermally activated delayed fluorescent (TADF) emitter has been known. In a thermally activated delayed fluorescent emitter, a singlet excited state is generated from a triplet excited state by reverse intersystem crossing, and the singlet excited state is converted into light emission. 
     In order to increase emission efficiency of a light-emitting element using a thermally activated delayed fluorescent emitter, not only efficient generation of a singlet excited state from a triplet excited state but also efficient emission from a singlet excited state, that is, high fluorescence quantum yield are important in a thermally activated delayed fluorescent emitter. It is, however, difficult to design a light-emitting material that meets these two. 
     Patent Document 1 discloses a method: in a light-emitting element containing a thermally activated delayed fluorescent emitter and a fluorescent compound, singlet excitation energy of the thermally activated delayed fluorescent emitter is transferred to the fluorescent compound and light emission is obtained from the fluorescent compound. 
     REFERENCE 
     Patent Document 
     
         
         [Patent Document 1] Japanese Published Patent Application No. 2014-45179 
       
    
     SUMMARY OF THE INVENTION 
     In order to increase emission efficiency of a light-emitting element containing a thermally activated delayed fluorescent emitter and a fluorescent compound, efficient generation of a singlet excited state from a triplet excited state is preferable. In addition, efficient energy transfer from a singlet excited state of the thermally activated delayed fluorescent emitter to a singlet excited state of the fluorescent compound is preferable. Moreover, energy transfer from a triplet excited state of the thermally activated delayed fluorescent emitter to a triplet excited state of the fluorescent compound is preferably inhibited. 
     In view of the above, an object of one embodiment of the present invention is to provide a light-emitting element that contains a fluorescent compound and has high emission efficiency. Another object of one embodiment of the present invention is to provide a light-emitting element with low power consumption. Another object of one embodiment of the present invention is to provide a novel light-emitting element. Another object of one embodiment of the present invention is to provide a novel light-emitting device. Another object of one embodiment of the present invention is to provide a novel display device. 
     Note that the description of the above object does not preclude the existence of other objects. In one embodiment of the present invention, there is no need to achieve all the objects. Objects other than the above objects will be apparent from and can be derived from the description of the specification and the like. 
     In one embodiment of the present invention, a light-emitting element includes a light-emitting layer in which an exciplex is formed, whereby triplet excitons can be converted into singlet excitons and light can be emitted from the singlet excitons. The light-emitting element can emit light from a fluorescent compound by utilizing energy transfer of the singlet excitons. 
     Thus, one embodiment of the present invention is a light-emitting element including a fluorescent material and a host material. The host material contains a first organic compound and a second organic compound. The first organic compound and the second organic compound can form an exciplex. The proportion of a delayed fluorescence component in light emitted from the exciplex is higher than or equal to 5%, and the delayed fluorescence component contains a delayed fluorescence component whose fluorescence lifetime is 10 ns or longer and 50 μs or shorter. 
     In the above structure, the exciplex is preferably capable of transferring excited energy to the fluorescent material. 
     In each of the above structures, light emitted from the exciplex preferably has a region overlapping with an absorption band of the fluorescent material on the lowest energy side. 
     In each of the above structures, one of the first organic compound and the second organic compound preferably has an electron transporting property, and the other of the first organic compound and the second organic compound preferably has a hole transporting property. Alternatively, one of the first organic compound and the second organic compound preferably includes a π-electron deficient heteroaromatic ring skeleton, and the other of the first organic compound and the second organic compound preferably includes a π-electron rich heteroaromatic ring skeleton or an aromatic amine skeleton. 
     Another embodiment of the present invention is a display device including the light-emitting element having any of the above-described structures, and at least one of a color filter and a transistor. Another embodiment of the present invention is an electronic device including the display device, and at least one of a housing and a touch sensor. Another embodiment of the present invention is a lighting device including the light-emitting element having any of the above-described structures, and at least one of a housing and a touch sensor. The category of one embodiment of the present invention includes not only the light-emitting device including the light-emitting element but also an electronic device including the light-emitting device. Accordingly, the light-emitting device in this specification refers to an image display device and a light source (e.g., a lighting device). The light-emitting device may be included in a display module in which a connector such as a flexible printed circuit (FPC) or a tape carrier package (TCP) is connected to a light-emitting device, a display module in which a printed wiring board is provided on the tip of a TCP, or a display module in which an integrated circuit (IC) is directly mounted on a light-emitting element by a chip on glass (COG) method. 
     According to one embodiment of the present invention, a light-emitting element that has high emission efficiency and contains a fluorescent compound can be provided. According to another embodiment of the present invention, a light-emitting element with low power consumption can be provided. According to another embodiment of the present invention, a novel light-emitting element can be provided. According to another embodiment of the present invention, a novel light-emitting device can be provided. According to another embodiment of the present invention, a novel display device can be provided. 
     Note that the description of these effects does not disturb the existence of other effects. One embodiment of the present invention does not necessarily have all the effects described above. Other effects will be apparent from and can be derived from the description of the specification, the drawings, the claims, and the like. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIGS. 1A and 1B  are schematic cross-sectional views of a light-emitting element of one embodiment of the present invention and  FIG. 1C  is a diagram illustrating the correlation of energy levels in a light-emitting layer; 
         FIGS. 2A and 2B  are schematic cross-sectional views of a light-emitting element of one embodiment of the present invention and  FIG. 2C  is a diagram illustrating the correlation of energy levels in a light-emitting layer; 
         FIGS. 3A and 3B  are schematic cross-sectional views of a light-emitting element of one embodiment of the present invention and  FIG. 3C  is a diagram illustrating the correlation of energy levels in a light-emitting layer; 
         FIGS. 4A and 4B  are each a schematic cross-sectional view of a light-emitting element of one embodiment of the present invention; 
         FIGS. 5A and 5B  are each a schematic cross-sectional view of a light-emitting element of one embodiment of the present invention; 
         FIGS. 6A to 6C  are schematic cross-sectional views illustrating a method for manufacturing a light-emitting element of one embodiment of the present invention; 
         FIGS. 7A to 7C  are schematic cross-sectional views illustrating a method for manufacturing a light-emitting element of one embodiment of the present invention; 
         FIGS. 8A and 8B  are a top view and a schematic cross-sectional view illustrating a display device of one embodiment of the present invention; 
         FIGS. 9A and 9B  are schematic cross-sectional views each illustrating a display device of one embodiment of the present invention; 
         FIG. 10  is a schematic cross-sectional view illustrating a display device of one embodiment of the present invention; 
         FIGS. 11A and 11B  are schematic cross-sectional views each illustrating a display device of one embodiment of the present invention; 
         FIGS. 12A and 12B  are schematic cross-sectional views each illustrating a display device of one embodiment of the present invention; 
         FIG. 13  is a schematic cross-sectional view illustrating a display device of one embodiment of the present invention; 
         FIGS. 14A and 14B  are schematic cross-sectional views each illustrating a display device of one embodiment of the present invention; 
         FIGS. 15A and 15B  are a block diagram and a circuit diagram illustrating a display device of one embodiment of the present invention; 
         FIGS. 16A and 16B  are circuit diagrams each illustrating a pixel circuit of a display device of one embodiment of the present invention; 
         FIGS. 17A and 17B  are circuit diagrams each illustrating a pixel circuit of a display device of one embodiment of the present invention; 
         FIGS. 18A and 18B  are perspective views of an example of a touch panel of one embodiment of the present invention; 
         FIGS. 19A to 19C  are cross-sectional views of examples of a display device and a touch sensor of one embodiment of the present invention; 
         FIGS. 20A and 20B  are cross-sectional views each illustrating an example of a touch panel of one embodiment of the present invention; 
         FIGS. 21A and 21B  are a block diagram and a timing chart of a touch sensor of one embodiment of the present invention; 
         FIG. 22  is a circuit diagram of a touch sensor of one embodiment of the present invention; 
         FIG. 23  is a perspective view of a display module of one embodiment of the present invention; 
         FIGS. 24A to 24G  illustrate electronic devices of one embodiment of the present invention; 
         FIGS. 25A to 25C  are a perspective view and cross-sectional views illustrating a light-emitting device of one embodiment of the present invention; 
         FIGS. 26A to 26D  are cross-sectional views each illustrating a light-emitting device of one embodiment of the present invention; 
         FIGS. 27A to 27C  illustrate a lighting device and an electronic device of one embodiment of the present invention; 
         FIG. 28  illustrates lighting devices of one embodiment of the present invention; 
         FIG. 29  shows current efficiency-luminance characteristics of light-emitting elements in Example; 
         FIG. 30  shows external quantum efficiency-luminance characteristics of light-emitting elements in Example; 
         FIG. 31  shows luminance-voltage characteristics of light-emitting elements in Example; 
         FIG. 32  shows electroluminescence spectra of light-emitting elements in Example; 
         FIG. 33  shows current efficiency-luminance characteristics of light-emitting elements in Example; 
         FIG. 34  shows external quantum efficiency-luminance characteristics of light-emitting elements in Example; 
         FIG. 35  shows luminance-voltage characteristics of light-emitting elements in Example; 
         FIG. 36  shows electroluminescence spectra of light-emitting elements in Example; 
         FIG. 37  shows emission spectra of thin films in Example; 
         FIG. 38  shows an absorption spectrum of a solution in Example; and 
         FIGS. 39A and 39B  show results of time-resolved fluorescence measurement of thin films in Example. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the present invention will be described below with reference to the drawings. However, the present invention is not limited to description to be given below, and it is to be easily understood that modes and details thereof can be variously modified without departing from the purpose and the scope of the present invention. Accordingly, the present invention should not be interpreted as being limited to the content of the embodiments below. 
     Note that the position, the size, the range, or the like of each structure illustrated in drawings and the like is not accurately represented in some cases for simplification. Therefore, the disclosed invention is not necessarily limited to the position, the size, the range, or the like disclosed in the drawings and the like. 
     Note that the ordinal numbers such as “first”, “second”, and the like in this specification and the like are used for convenience and do not denote the order of steps or the stacking order of layers. Therefore, for example, description can be made even when “first” is replaced with “second” or “third”, as appropriate. In addition, the ordinal numbers in this specification and the like are not necessarily the same as those which specify one embodiment of the present invention. 
     In the description of modes of the present invention in this specification and the like with reference to the drawings, the same components in different diagrams are commonly denoted by the same reference numeral in some cases. 
     In this specification and the like, the terms “film” and “layer” can be interchanged with each other depending on the case or circumstances. For example, the term “conductive layer” can be changed into the term “conductive film” in some cases. Also, the term “insulating film” can be changed into the term “insulating layer” in some cases. 
     In this specification and the like, a singlet excited state (S*) refers to a singlet state having excited energy. The lowest level of the singlet excited energy level (S1 level) refers to the excited energy level of the lowest singlet excited state. A triplet excited state (T*) refers to a triplet state having excited energy. The lowest level of the triplet excited energy level (T1 level) refers to the excited energy level of the lowest triplet excited state. 
     In this specification and the like, a fluorescent material or a fluorescent compound refers to a material or a compound that emits light in the visible light region when the relaxation from the singlet excited state to the ground state occurs. A phosphorescent material or a phosphorescent compound refers to a material or a compound that emits light in the visible light region at room temperature when the relaxation from the triplet excited state to the ground state occurs. That is, a phosphorescent material or a phosphorescent compound refers to a material or a compound that can convert triplet excited energy into visible light. 
     Note that in this specification and the like, “room temperature” refers to a temperature higher than or equal to 0° C. and lower than or equal to 40° C. 
     In this specification and the like, a wavelength range of blue refers to a wavelength range of greater than or equal to 400 nm and less than 490 nm, and blue light has at least one peak in that wavelength range in an emission spectrum. A wavelength range of green refers to a wavelength range of greater than or equal to 490 nm and less than 580 nm, and green light has at least one peak in that wavelength range in an emission spectrum. A wavelength range of red refers to a wavelength range of greater than or equal to 580 nm and less than or equal to 680 nm, and red light has at least one peak in that wavelength range in an emission spectrum. 
     Embodiment 1 
     In this embodiment, a light-emitting element of one embodiment of the present invention will be described below with reference to  FIGS. 1A to 1C . 
     &lt;Structure Example of Light-Emitting Element&gt; 
     First, a structure of the light-emitting element of one embodiment of the present invention will be described with reference to  FIGS. 1A to 1C . 
       FIG. 1A  is a schematic cross-sectional view of a light-emitting element  250  of one embodiment of the present invention. 
     The light-emitting element  250  includes a pair of electrodes (an electrode  101  and an electrode  102 ) and an EL layer  100  between the pair of electrodes. The EL layer  100  includes at least a light-emitting layer  130 . 
     The EL layer  100  illustrated in  FIG. 1A  includes functional layers such as a hole-injection layer  111 , a hole-transport layer  112 , an electron-transport layer  118 , and an electron-injection layer  119 , in addition to the light-emitting layer  130 . 
     In this embodiment, although description is given assuming that the electrode  101  and the electrode  102  of the pair of electrodes serve as an anode and a cathode, respectively, they are not limited thereto for the structure of the light-emitting element  250 . That is, the electrode  101  may be a cathode, the electrode  102  may be an anode, and the stacking order of the layers between the electrodes may be reversed. In other words, the hole-injection layer  111 , the hole-transport layer  112 , the light-emitting layer  130 , the electron-transport layer  118 , and the electron-injection layer  119  may be stacked in this order from the anode side. 
     The structure of the EL layer  100  is not limited to the structure illustrated in  FIG. 1A , and a structure including at least one layer selected from the hole-injection layer  111 , the hole-transport layer  112 , the electron-transport layer  118 , and the electron-injection layer  119  may be employed. Alternatively, the EL layer  100  may include a functional layer which is capable of lowering a hole- or electron-injection barrier, improving a hole- or electron-transport property, inhibiting a hole- or electron-transport property, or suppressing a quenching phenomenon by an electrode, for example. Note that the functional layers may each be a single layer or stacked layers. 
       FIG. 1B  is a schematic cross-sectional view illustrating an example of the light-emitting layer  130  in  FIG. 1A . The light-emitting layer  130  in  FIG. 11B  includes a host material  131  and a guest material  132 . The host material  131  includes an organic compound  131 _ 1  and an organic compound  131 _ 2 . 
     The guest material  132  may be a light-emitting organic compound, and the light-emitting organic compound is preferably a substance capable of emitting fluorescence (hereinafter also referred to as a fluorescent compound). A structure in which a fluorescent compound is used as the guest material  132  will be described below. The guest material  132  may be referred to as the fluorescent material or the fluorescent compound. 
     In the light-emitting element  250  of one embodiment of the present invention, voltage application between the pair of electrodes (the electrodes  101  and  102 ) allows electrons and holes to be injected from the cathode and the anode, respectively, into the EL layer  100  and thus a current flows. By recombination of the injected carriers (electrons and holes), excitons are formed. The ratio of singlet excitons to triplet excitons (hereinafter referred to as exciton generation probability) which are generated by carrier (electrons and holes) recombination is approximately 1:3 according to the statistically obtained probability. Accordingly, in a light-emitting element that uses a fluorescent material, the probability of generation of singlet excitons, which contribute to light emission, is 25% and the probability of generation of triplet excitons, which do not contribute to light emission, is 75%. Therefore, converting the triplet excitons, which do not contribute to light emission, into singlet excitons, which contribute to light emission, is important in increasing the emission efficiency of the light-emitting element 
     &lt;Light Emission Mechanism of Light-Emitting Element&gt; 
     Next, the light emission mechanism of the light-emitting layer  130  is described below. 
     The organic compound  131 _ 1  and the organic compound  131 _ 2  included in the host material  131  in the light-emitting layer  130  form an exciplex. 
     Although it is acceptable as long as the combination of the organic compound  131 _ 1  and the organic compound  131 _ 2  can form an exciplex, it is preferable that one of them be a compound having a function of transporting holes (a hole-transport property) and the other be a compound having a function of transporting electrons (an electron-transport property). In that case, a donor-acceptor exciplex is formed easily; thus, efficient formation of an exciplex is possible. In the case where the combination of the organic compounds  131 _ 1  and  131 _ 2  is a combination of a compound having a hole-transport property and a compound having an electron-transport property, the carrier balance can be easily controlled depending on the mixture ratio. Specifically, the weight ratio of the compound having a hole-transport property to the compound having an electron-transport property is preferably within a range of 1:9 to 9:1. Since the carrier balance can be easily controlled with the structure, a carrier recombination region can also be controlled easily. 
     In order to efficiently form an exciplex, the combination of the host materials preferably satisfies the follows the highest occupied molecular orbital (also referred to as HOMO) level of one of the organic compound  131 _ 1  and the organic compound  131 _ 2  is higher than the HOMO level of the other of the organic compounds, and the lowest unoccupied molecular orbital (also referred to as LUMO) level of the one of the organic compounds is higher than the LUMO level of the other of the organic compounds. For example, when one of the organic compounds has a hole-transport property and the other of the organic compounds has an electron-transport property, it is preferable that the HOMO level of the one of the organic compounds be higher than the HOMO level of the other of the organic compounds and the LUMO level of the one of the organic compounds be higher than the LUMO level of the other of the organic compounds. Specifically, a difference in HOMO level between the organic compounds is preferably greater than or equal to 0.05 eV, further preferably greater than or equal to 0.1 eV, and still further preferably greater than or equal to 0.2 eV A difference in LUMO level between the organic compounds is preferably greater than or equal to 0.05 eV, further preferably greater than or equal to 0.1 eV, and still further preferably greater than or equal to 0.2 eV. 
       FIG. 1C  shows a correlation of energy levels of the organic compound  131 _ 1 , the organic compound  131 _ 2 , and the guest material  132  in the light-emitting layer  130 . The following explains what terms and signs in  FIG. 1C  represent: 
     Host ( 131 _ 1 ): the organic compound  131 _ 1 ; 
     Host ( 131 _ 2 ): the organic compound  131 _ 2 ; 
     Guest ( 132 ): the guest material  132  (the fluorescent compound); 
     S H : the S1 level of the organic compound  131 _ 1  (the host material); 
     T H : the T1 level of the organic compound  131 _ 1  (the host material); 
     S G : the S1 level of the guest material  132  (the fluorescent compound); 
     T G : the T1 level of the guest material  132  (the fluorescent compound); 
     S E : the S1 level of the exciplex; and 
     T E : the T1 level of the exciplex. 
     In the light-emitting element of one embodiment of the present invention, the organic compounds  131 _ 1  and  131 _ 2  included in the light-emitting layer  130  form an exciplex. The lowest singlet excited energy level of the exciplex (S E ) and the lowest triplet excited energy level of the exciplex (T E ) are adjacent to each other (see Route E 3  in  FIG. 1C ). 
     An exciplex is an excited state formed from two kinds of substances. In photoexcitation, the exciplex is formed by interaction between one substance in an excited state and the other substance in a ground state. The two kinds of substances that have formed the exciplex return to a ground state by emitting light and then serve as the original two kinds of substances. In electrical excitation, when one substance is brought into an excited state, the one immediately interacts with the other substance to form an exciplex. Alternatively, one substance receives a hole and the other substance receives an electron to readily form an exciplex. In this case, any of the substances can form an exciplex without forming an excited state and; accordingly, most excitons in the light-emitting layer  130  can exist as exciplexes. Because the excited energy levels of the exciplex (S E  and T E ) are lower than the singlet excited energy level of the host materials (S H ) (the organic compound  131 _ 1  and the organic compound  131 _ 2 ) that form the exciplex, the excited state of the host material  131  can be formed with lower excited energy. Accordingly, the driving voltage of the light-emitting element  250  can be reduced. 
     Since the singlet excited energy level (S E ) and the triplet excited energy level (T E ) of the exciplex are adjacent to each other, the exciplex has a function of exhibiting thermally activated delayed fluorescence. In other words, the exciplex has a function of converting triplet excited energy to singlet excited energy by reverse intersystem crossing (upconversion) (see Route E 4  in  FIG. 1C ). Thus, the triplet excited energy generated in the light-emitting layer  130  is partly converted into singlet excited energy by the exciplex. In order to cause this conversion, the energy difference between the singlet excited energy level (S E ) and the triplet excited energy level (T E ) of the exciplex is preferably greater than 0 eV and less than or equal to 0.2 eV. Note that in order to efficiently make reverse intersystem crossing occur, the triplet excited energy level of the exciplex (T E ) is preferably lower than the triplet excited energy levels of the organic compounds (the organic compound  131 _ 1  and the organic compound  131 _ 2 ) in the host material which form the exciplex. Thus, quenching of the triplet excited energy of the exciplex due to the organic compounds is less likely to occur, which causes reverse intersystem crossing efficiently. 
     Furthermore, the singlet excited energy level of the exciplex (S E ) is preferably higher than the singlet excited energy level of the guest material  132  (S G ). In this way, the singlet excited energy of the formed exciplex can be transferred from the singlet excited energy level of the exciplex (S E ) to the singlet excited energy level of the guest material  132  (S G ), so that the guest material  132  is brought into the singlet excited state, causing light emission (see Route E 5  in  FIG. 1C ). 
     To obtain efficient light emission from the singlet excited state of the guest material  132 , the fluorescence quantum yield of the guest material  132  is preferably high, and specifically, 50% or higher, further preferably 70% or higher, still further preferably 90% or higher. 
     Note that since direct transition from a singlet ground state to a triplet excited state in the guest material  132  is forbidden, energy transfer from the singlet excited energy level of the exciplex (S E ) to the triplet excited energy level of the guest material  132  (T G ) is unlikely to be a main energy transfer process. 
     When transfer of the triplet excited energy from the triplet excited energy level of the exciplex (T E ) to the triplet excited energy level of the guest material  132  T G ) occurs, the triplet excited energy is deactivated (see Route E 6  in  FIG. 1C ). Thus, it is preferable that the energy transfer of Route E 6  be less likely to occur because the efficiency of generating the triplet excited state of the guest material  132  can be decreased and thermal deactivation can be reduced. In order to make this condition, the weight ratio of the guest material  132  to the host material  131  is preferably low, specifically, preferably greater than or equal to 0.001 and less than or equal to 0.05, further preferably greater than or equal to 0.001 and less than or equal to 0.01. 
     Note that when the direct carrier recombination process in the guest material  132  is dominant, a large number of triplet excitons are generated in the light-emitting layer  130 , resulting in decreased emission efficiency due to thermal deactivation. Thus, it is preferable that the probability of the energy transfer process through the exciplex formation process (Routes E 4  and E 5  in  FIG. 1C ) be higher than the probability of the direct carrier recombination process in the guest material  132  because the efficiency of generating the triplet excited state of the guest material  132  can be decreased and thermal deactivation can be reduced. Therefore, as described above, the weight ratio of the guest material  132  to the host material  131  is preferably low, specifically, preferably greater than or equal to 0.001 and less than or equal to 0.05, further preferably greater than or equal to 0.001 and less than or equal to 0.01. 
     By making all the energy transfer processes of Routes E 4  and E 5  efficiently occur in the above-described manner, both the singlet excited energy and the triplet excited energy of the host material  131  can be efficiently converted into the singlet excited energy of the guest material  132 , whereby the light-emitting element  250  can emit light with high emission efficiency. 
     The above-described processes through Routes E 3 , E 4 , and E 5  may be referred to as exciplex-singlet energy transfer (ExSET) or exciplex-enhanced fluorescence (ExEF) in this specification and the like. In other words, in the light-emitting layer  130 , excited energy is transferred from the exciplex to the guest material  132 . 
     When the light-emitting layer  130  has the above-described structure, light emission from the guest material  132  of the light-emitting layer  130  can be obtained efficiently. 
     &lt;Energy Transfer Mechanism&gt; 
     Next, factors controlling the processes of intermolecular energy transfer between the host material  131  and the guest material  132  will be described. As mechanisms of the intermolecular energy transfer, two mechanisms, i.e., Forster mechanism (dipole-dipole interaction) and Dexter mechanism (electron exchange interaction), have been proposed. Although the intermolecular energy transfer process between the host material  131  and the guest material  132  is described here, the same can apply to a case where the host material  131  is an exciplex. 
     &lt;&lt;Förster Mechanism&gt;&gt; 
     In Förster mechanism, energy transfer does not require direct contact between molecules and energy is transferred through a resonant phenomenon of dipolar oscillation between the host material  131  and the guest material  132 . By the resonant phenomenon of dipolar oscillation, the host material  131  provides energy to the guest material  132 , and thus, the host material  131  in an excited state is brought to a ground state and the guest material  132  in a ground state is brought to an excited state. Note that the rate constant k h*→g  of Förster mechanism is expressed by Formula (1). 
     
       
         
           
             
               
                 
                   
                     
                       
                         k 
                       
                       
                         h 
                         * 
                       
                     
                     → 
                     g 
                   
                   = 
                   
                     
                       
                         9000 
                         ⁢ 
                         
                           c 
                           4 
                         
                         ⁢ 
                         
                           K 
                           2 
                         
                         ⁢ 
                         ϕ 
                         ⁢ 
                         ln 
                         ⁢ 
                         10 
                       
                       
                         128 
                         ⁢ 
                         
                           π 
                           5 
                         
                         ⁢ 
                         
                           n 
                           4 
                         
                         ⁢ 
                         N 
                         ⁢ 
                         τ 
                         ⁢ 
                         
                           R 
                           6 
                         
                       
                     
                     ⁢ 
                     
                       ∫ 
                       
                         
                           
                             
                               
                                 f 
                                 h 
                                 ′ 
                               
                               ( 
                               v 
                               ) 
                             
                             ⁢ 
                             
                               
                                 ε 
                                 g 
                               
                               ( 
                               v 
                               ) 
                             
                           
                           
                             v 
                             4 
                           
                         
                         ⁢ 
                         dv 
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     In Formula (1), ν denotes a frequency, f′ h (ν) denotes a normalized emission spectrum of the host material  131  (a fluorescent spectrum in energy transfer from a singlet excited state, and a phosphorescent spectrum in energy transfer from a triplet excited state), ε g (ν) denotes a molar absorption coefficient of the guest material  132 , N denotes Avogadro&#39;s number, n denotes a refractive index of a medium, R denotes an intermolecular distance between the host material  131  and the guest material  132 , 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 material  131  and the guest material  132 . Note that K 2  is ⅔ in random orientation. 
     &lt;&lt;Dexter Mechanism&gt;&gt; 
     In Dexter mechanism, the host material  131  and the guest material  132  are close to a contact effective range where their orbitals overlap, and the host material  131  in an excited state and the guest material  132  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). 
     
       
         
           
             
               
                 
                   
                     
                       
                         k 
                       
                       
                         h 
                         * 
                       
                     
                     → 
                     g 
                   
                   = 
                   
                     
                       ( 
                       
                         
                           2 
                           ⁢ 
                           π 
                         
                         h 
                       
                       ) 
                     
                     ⁢ 
                     
                       K 
                       2 
                     
                     ⁢ 
                     exp 
                     ⁢ 
                     
                       ( 
                       
                         - 
                         
                           
                             2 
                             ⁢ 
                             R 
                           
                           L 
                         
                       
                       ) 
                     
                     ⁢ 
                     
                       ∫ 
                       
                         
                           
                             f 
                             h 
                             ′ 
                           
                           ( 
                           v 
                           ) 
                         
                         ⁢ 
                         
                           
                             ε 
                             g 
                             ′ 
                           
                           ( 
                           v 
                           ) 
                         
                         ⁢ 
                         dv 
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     In Formula (2), h denotes a Planck constant, K denotes a constant having an energy dimension, ν denotes a frequency, f′ h (ν) denotes a normalized emission spectrum of the host material  131  (a fluorescent spectrum in energy transfer from a singlet excited state, and a phosphorescent spectrum in energy transfer from a triplet excited state), ε′ g (ν) denotes a normalized absorption spectrum of the guest material  132 , L denotes an effective molecular radius, and R denotes an intermolecular distance between the host material  131  and the guest material  132 . 
     Here, the efficiency of energy transfer from the host material  131  to the guest material  132  (energy transfer efficiency ϕ ET ) is 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 the host material  131 , k n  denotes a rate constant of a non-light-emission process (thermal deactivation or intersystem crossing) of the host material  131 , and τ denotes a measured lifetime of an excited state of the host material  131 . 
     
       
         
           
             
               
                 
                   
                     
                       ϕ 
                       ET 
                     
                     ⁢ 
                     
                       
                         
                           
                             k 
                           
                           
                             h 
                             * 
                           
                         
                         → 
                         g 
                       
                       
                         
                           
                             k 
                             r 
                           
                           + 
                           
                             k 
                             n 
                           
                           + 
                           
                             
                               k 
                             
                             
                               h 
                               * 
                             
                           
                         
                         → 
                         g 
                       
                     
                   
                   = 
                   
                     
                       
                         
                           k 
                         
                         
                           h 
                           * 
                         
                       
                       → 
                       g 
                     
                     
                       
                         
                           ( 
                           
                             1 
                             τ 
                           
                           ) 
                         
                         + 
                         
                           
                             k 
                           
                           
                             h 
                             * 
                           
                         
                       
                       → 
                       g 
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     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. 
     &lt;&lt;Concept for Promoting Energy Transfer&gt;&gt; 
     First, an energy transfer by Förster mechanism is considered. When Formula (1) is substituted into Formula (3), τ can be eliminated. Thus, in Förster mechanism, the energy transfer efficiency ϕ ET  does not depend on the lifetime τ of the excited state of the host material  131 . In addition, it can be said that the energy transfer efficiency ϕ ET  is higher when the luminescence quantum yield ϕ (here, the fluorescence quantum yield because energy transfer from a singlet excited state is discussed) is higher. In general, the luminescence quantum yield of an organic compound in a triplet excited state is extremely low at room temperature. Thus, in the case where the host material  131  is in a triplet excited state, a process of energy transfer by Förster mechanism can be ignored, and a process of energy transfer by Förster mechanism is considered only in the case where the host material  131  is in a singlet excited state. 
     Furthermore, it is preferable that the emission spectrum (the fluorescent spectrum in the case where energy transfer from a singlet excited state is discussed) of the host material  131  largely overlap with the absorption spectrum (absorption corresponding to the transition from the singlet ground state to the singlet excited state) of the guest material  132 . Moreover, it is preferable that the molar absorption coefficient of the guest material  132  be also high. This means that the emission spectrum of the host material  131  overlaps with the absorption band of the guest material  132  which is on the longest wavelength side. Since direct transition from the singlet ground state to the triplet excited state of the guest material  132  is forbidden, the molar absorption coefficient of the guest material  132  in the triplet excited state can be ignored. Thus, a process of energy transfer to a triplet excited state of the guest material  132  by Förster mechanism can be ignored, and only a process of energy transfer to a singlet excited state of the guest material  132  is considered. That is, in Förster mechanism, a process of energy transfer from the singlet excited state of the host material  131  to the singlet excited state of the guest material  132  is considered. 
     Next, an energy transfer by Dexter mechanism is considered. According to Formula (2), in order to increase the rate constant k h*→g , it is preferable that an emission spectrum of the host material  131  (a fluorescent spectrum in the case where energy transfer from a singlet excited state is discussed) largely overlap with an absorption spectrum of the guest material  132  (absorption corresponding to transition from a singlet ground state to a singlet excited state). Therefore, the energy transfer efficiency can be optimized by making the emission spectrum of the host material  131  overlap with the absorption band of the guest material  132  which is on the longest wavelength side. 
     When Formula (2) is substituted into Formula (3), it is found that the energy transfer efficiency ϕ ET  in Dexter mechanism depends on τ. In Dexter mechanism, which is a process of energy transfer based on the electron exchange, as well as the energy transfer from the singlet excited state of the host material  131  to the singlet excited state of the guest material  132 , energy transfer from the triplet excited state of the host material  131  to the triplet excited state of the guest material  132  occurs. 
     In the light-emitting element of one embodiment of the present invention in which the guest material  132  is a fluorescent material, the efficiency of energy transfer to the triplet excited state of the guest material  132  is preferably low. That is, the energy transfer efficiency based on Dexter mechanism from the host material  131  to the guest material  132  is preferably low and the energy transfer efficiency based on Förster mechanism from the host material  131  to the guest material  132  is preferably high. 
     As described above, the energy transfer efficiency in Förster mechanism does not depend on the lifetime τ of the excited state of the host material  131 . In contrast, the energy transfer efficiency in Dexter mechanism depends on the excitation lifetime τ of the host material  131 . To reduce the energy transfer efficiency in Dexter mechanism, the excitation lifetime τ of the host material  131  is preferably short. 
     In a manner similar to that of the energy transfer from the host material  131  to the guest material  132 , the energy transfer by both Förster mechanism and Dexter mechanism also occurs in the energy transfer process from the exciplex to the guest material  132 . 
     Accordingly, one embodiment of the present invention provides a light-emitting element including, as the host material  131 , the organic compound  131 _ 1  and the organic compound  131 _ 2  which are a combination for forming an exciplex which functions as an energy donor capable of efficiently transferring energy to the guest material  132 . The exciplex formed by the organic compound  131 _ 1  and the organic compound  131 _ 2  has a singlet excited energy level and a triplet excited energy level which are adjacent to each other; accordingly, transition from a triplet exciton generated in the light-emitting layer  130  to a singlet exciton (reverse intersystem crossing) is likely to occur. This can increase the efficiency of generating singlet excitons in the light-emitting layer  130 . Furthermore, in order to facilitate energy transfer from the singlet excited state of the exciplex to the singlet excited state of the guest material  132  having a function as an energy acceptor, it is preferable that the emission spectrum of the exciplex overlap with the absorption band of the guest material  132  which is on the longest wavelength side (lowest energy side). Thus, the efficiency of generating the singlet excited state of the guest material  132  can be increased. 
     In addition, fluorescence lifetime of a thermally activated delayed fluorescence component in light emitted from the exciplex is preferably short, and specifically, preferably 10 ns or longer and 50 μs or shorter, further preferably 10 ns or longer and 40 μs or shorter, still further preferably 10 ns or longer and 30 μs or shorter. 
     The proportion of a thermally activated delayed fluorescence component in the light emitted from the exciplex is preferably high. Specifically, the proportion of a thermally activated delayed fluorescence component in the light emitted from the exciplex is preferably higher than or equal to 5%, further preferably higher than or equal to 8%, still further preferably higher than or equal to 10%. 
     &lt;Material&gt; 
     Next, components of a light-emitting element of one embodiment of the present invention are described in detail below. 
     &lt;&lt;Light-Emitting Layer&gt;&gt; 
     Next, materials that can be used for the light-emitting layer  130  will be described below. 
     In the light-emitting layer  130 , the host material  131  is present in the largest proportion by weight, and the guest material  132  (the fluorescent material) is dispersed in the host material  131 . The S1 level of the host material  131  (the organic compound  131 _ 1  and the organic compound  131 _ 2 ) in the light-emitting layer  130  is preferably higher than the S1 level of the guest material  132  (the fluorescent material) in the light-emitting layer  130 . The T1 level of the host material  131  (the organic compound  131 _ 1  and the organic compound  131 _ 2 ) in the light-emitting layer  130  is preferably higher than the T1 level of the guest material  132  (the fluorescent material) in the light-emitting layer  130 . 
     In the light-emitting layer  130 , the guest material  132  is preferably, but not particularly limited to, an anthracene derivative, a tetracene derivative, a chrysene derivative, a phenanthrene derivative, a pyrene derivative, a perylene derivative, a stilbene derivative, an acridone derivative, a coumarin derivative, a phenoxazine derivative, a phenothiazine derivative, or the like, and for example, any of the following materials can be used. 
     The examples include 5,6-bis[4-(10-phenyl-9-anthryl)phenyl]-2,2′-bipyridine (abbreviation: PAP2BPy), 5,6-bis[4′-(10-phenyl-9-anthryl)biphenyl-4-yl]-2,2′-bipyridine (abbreviation: PAPP2BPy), N,N′-diphenyl-N,N′-bis[4-(9-phenyl-9H-fluoren-9-yl)phenyl]pyrene-1,6-diamine (abbreviation: 1,6FLPAPrn), N,N′-bis(3-methylphenyl)-N,N′-bis[3-(9-phenyl-9H-fluoren-9-yl)phenyl]pyrene-1,6-dia mine (abbreviation: 1,6mMemFLPAPrn), N,N′-bis[4-(9H-carbazol-9-yl)phenyl]-N,N′-diphenylstilbene-4,4′-diamine (abbreviation: YGA2S), 4-(9H-carbazol-9-yl)-4′-(10-phenyl-9-anthryl)triphenylamine (abbreviation: YGAPA), 4-(9H-carbazol-9-yl)-4′-(9,10-diphenyl-2-anthryl)triphenylamine (abbreviation: 2YGAPPA), N,9-diphenyl-N-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine (abbreviation: PCAPA), perylene, 2,5,8,11-tetra(tert-butyl)perylene (abbreviation: TBP), 4-(10-phenyl-9-anthryl)-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PCBAPA), N,N″-(2-tert-butylanthracene-9,10-diyldi-4,1-phenylene)bis[N,N′N′-triphenyl-1,4-phenylenediamine] (abbreviation: DPABPA), N,9-diphenyl-N-[4-(9,10-diphenyl-2-anthryl)phenyl]-9H-carbazol-3-amine (abbreviation: 2PCAPPA), N-[4-(9,10-diphenyl-2-anthryl)phenyl]-N,N′,N′-triphenyl-1,4-phenylenediamine (abbreviation: 2DPAPPA), N,N,N′,N′,N″,N″,N′″,N′″-octaphenyidibenzo[g,p]chrysene-2,7,10,15-tetraamine (abbreviation: DBC1), coumarin 30, N-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine (abbreviation: 2PCAPA), N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,9-diphenyl-9H-carbazol-3-amine (abbreviation: 2PCABPhA), N-(9,10-diphenyl-2-anthryl)-N,N′,′-triphenyl-1,4-phenylenediamine (abbreviation: 2DPAPA), N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,N′,N′-triphenyl-1,4-phenylenediamine (abbreviation: 2DPABPhA), 9,10-bis(1,1′-biphenyl-2-yl)-N-[4-(9H-carbazol-9-yl)phenyl]-N-phenylanthracen-2-amine (abbreviation: 2YGABPhA), N,N,9-triphenylanthracen-9-amine (abbreviation: DPhAPhA), coumarin 6, coumarin 545T, N,N′-diphenylquinacridone (abbreviation: DPQd), rubrene, 5,12-bis(1,1′-biphenyl-4-yl)-6,11-diphenyltetracene (abbreviation: BPT), 2-(2-{2-[4-(dimethylamino)phenyl]ethenyl}-6-methyl-4H-pyran-4-ylidene)propanedinitrile (abbreviation: DCM1), 2-{2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrite (abbreviation: DCM2), N,N,N′,N′-tetrakis(4-methylphenyl)tetracene-5,11-diamine (abbreviation: p-mPhTD), 7,14-diphenyl-N,N,N′,N′-tetrakis(4-methylphenyl)acenaphtho[1,2-a]fluoranthene-3,10-diamine (abbreviation: p-mPhAFD), 2-{2-isopropyl-6-[2-(1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile (abbreviation: DCJTI), 2-{2-tert-butyl-6-[2-(1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile (abbreviation: DCJTB), 2-(2,6-bis{2-[4-(dimethylamino)phenyl]ethenyl}-4H-pyran-4-ylidene)propanedinitrile (abbreviation: BisDCM), 2-{2,6-bis[2-(8-methoxy-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile (abbreviation: BisDCJTM), and 5,10,15,20-tetraphenylbisbenzo[5,6]indeno[1,2,3-cd:1′,2′,3′-lm]perylene. 
     Examples of the organic compound  131 _ 1  include a zinc- or aluminum-based metal complex, an oxadiazole derivative, a triazole derivative, a benzimidazole derivative, a quinoxaline derivative, a dibenzoquinoxaline derivative, a dibenzothiophene derivative, a dibenzofuran derivative, a pyrimidine derivative, a triazine derivative, a pyridine derivative, a bipyridine derivative, a phenanthroline derivative, and the like. Other examples are an aromatic amine, a carbazole derivative, and the like. 
     Any of the following hole-transport materials and electron-transport materials can be used. 
     A material having a property of transporting more holes than electrons can be used as the hole-transport material, and a material having a hole mobility of 1×10 −6  cm 2 /Vs or higher is preferable. Specifically, an aromatic amine, a carbazole derivative, an aromatic hydrocarbon, a stilbene derivative, or the like can be used. Furthermore, the hole-transport material may be a high molecular compound. 
     Examples of the material having a high hole-transport property are N,N′-di(p-tolyl)-N,N′-diphenyl-p-phenylenediamine (abbreviation: DTDPPA), 4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl (abbreviation: DPAB), N,N′-bis{4-[bis(3-methylphenyl)amino]phenyl}-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-dia mine (abbreviation: DNTPD), 1,3,5-tris[N-(4-diphenylaminophenyl)-N-phenylamino]benzene (abbreviation: DPA3B), and the like. 
     Specific examples of the carbazole derivative are 3-[N-(4-diphenylaminophenyl)-N-phenylamino]-9-phenylcarbazole (abbreviation: PCzDPA1), 3,6-bis[N-(4-diphenylaminophenyl)-N-phenylamino]-9-phenylcarbazole (abbreviation: PCzDPA2), 3,6-bis[N-(4-diphenylaminophenyl)-N-(1-naphthyl)amino]-9-phenylcarbazole (abbreviation: PCzTPN2), 3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole (abbreviation: PCzPCA1), 3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole (abbreviation: PCzPCA2), 3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole (abbreviation: PCzPCN1), and the like. 
     Other examples of the carbazole derivative are 4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP), 1,3,5-tris[4-(N-carbazolyl)phenyl]benzene (abbreviation: TCPB), 9-[4-(10-phenyl-9-anthracenyl)phenyl]-9H-carbazole (abbreviation: CzPA), 1,4-bis[4-(N-carbazolyl)phenyl]-2,3,5,6-tetraphenylbenzene, and the like. 
     Examples of the aromatic hydrocarbon are 2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation: t-BuDNA), 2-tert-butyl-9,10-di(1-naphthyl)anthracene, 9,10-bis(3,5-diphenylphenyl)anthracene (abbreviation: DPPA), 2-tert-butyl-9,10-bis(4-phenylphenyl)anthracene (abbreviation: t-BuDBA), 9,10-di(2-naphthyl)anthracene (abbreviation: DNA), 9,10-diphenylanthracene (abbreviation: DPAnth), 2-tert-butylanthracene (abbreviation: t-BuAnth), 9,10-bis(4-methyl-1-naphthyl)anthracene (abbreviation: DMNA), 2-tert-butyl-9,10-bis[2-(1-naphthyl)phenyl]anthracene, 9,10-bis[2-(1-naphthyl)phenyl]anthracene, 2,3,6,7-tetramethyl-9,10-di(1-naphthyl)anthracene, 2,3,6,7-tetramethyl-9,10-di(2-naphthyl)anthracene, 9,9′-bianthryl, 10,10′-diphenyl-9,9′-bianthryl, 10,10′-bis(2-phenylphenyl)-9,9′-bianthryl, 10,10′-bis[(2,3,4,5,6-pentaphenyl)phenyl]-9,9′-bianthryl, anthracene, tetracene, rubrene, perylene, 2,5,8,11-tetra(tert-butyl)perylene, and the like. Other examples are pentacene, coronene, and the like. The aromatic hydrocarbon having a hole mobility of 1×10 −6  cm 2 /Vs or higher and having 14 to 42 carbon atoms is particularly preferable. 
     The aromatic hydrocarbon may have a vinyl skeleton. Examples of the aromatic hydrocarbon having a vinyl group are 4,4′-bis(2,2-diphenylvinyl)biphenyl (abbreviation: DPVBi), 9,10-bis[4-(2,2-diphenylvinyl)phenyl]anthracene (abbreviation: DPVPA), and the like. 
     Other examples are high molecular compounds such as poly(N-vinylcarbazole) (abbreviation: PVK), poly(4-vinyltriphenylamine) (abbreviation: PVTPA), poly[N,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). 
     Examples of the material having a high hole-transport property are 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-(1-naphthyl)-N-phenylamino]triphenylamine (abbreviation: 1′-TNATA), 4,4′,4″-tris(N,N-diphenylamino)triphenylamine (abbreviation: TDATA), 4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine (abbreviation: MTDATA), 4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl (abbreviation: BSPB), 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine (abbreviation: BPAFLP), 4-phenyl-3′-(9-phenylfluoren-9-yl)triphenylamine (abbreviation: mBPAFLP), 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), N-(9,9-dimethyl-2-diphenylamino-9H-fluoren-7-yl)diphenylamine (abbreviation: DPNF), 2-[N-(4-diphenylaminophenyl)-NV-phenylamino]spiro-9,9′-bifluorene (abbreviation: DPASF), 4-phenyl-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PCBA1BP), 4,4′-diphenyl-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PCBBi1BP), 4-(1-naphthyl)-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PCBANB), 4,4′-di(1-naphthyl)-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PCBNRB), 4-phenyldiphenyl-(9-phenyl-9H-carbazol-3-yl)amine (abbreviation: PCA1BP), N,N′-bis(9-phenylcarbazol-3-yl)-N,N′-diphenylbenzene-1,3-diamine (abbreviation: PCA2B), N,N′,N″-triphenyl-N,N′,N″-tris(9-phenylcarbazol-3-yl)benzene-1,3,5-triamine (abbreviation: PCA3B), N-(4-biphenyl)-N-(9,9-dimethyl-9H-fluoren-2-yl)-9-phenyl-9H-carbazol-3-amine (abbreviation: PCBiF), N-(1,1′-biphenyl-4-yl)-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-9,9-dimethyl-9H-fluor en-2-amine (abbreviation: PCBBiF), 9,9-dimethyl-N-phenyl-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]fluoren-2-amine (abbreviation: PCBAF), N-phenyl-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]spiro-9,9′-bifluoren-2-amine (abbreviation: PCBASF), 2-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]spiro-9,9′-bifluorene (abbreviation: PCASF), 2,7-bis[N-(4-diphenylaminophenyl)-N-phenylamino]-spiro-9,9′-bifluorene (abbreviation: DPA2SF), N-[4-(9H-carbazol-9-yl)phenyl]-N-(4-phenyl)phenylaniline (abbreviation: YGA1BP), and N,N-bis[4-(carbazol-9-yl)phenyl]-N,N-diphenyl-9,9-dimethylfluorene-2,7-diamine (abbreviation: YGA2F). Other examples are amine compounds, carbazole compounds, thiophene compounds, furan compounds, fluorene compounds; triphenylene compounds; phenanthrene compounds, and the like such as 3-[4-(1-naphthyl)-phenyl]-9-phenyl-9H-carbazole (abbreviation: PCPN), 3-[4-(9-phenanthryl)-phenyl]-9-phenyl-9H-carbazole (abbreviation: PCPPn), 3,3′-bis(9-phenyl-9H-carbazole) (abbreviation: PCCP), 1,3-bis(N-carbazolyl)benzene (abbreviation: mCP), 3,6-bis(3,5-diphenylphenyl)-9-phenylcarbazole (abbreviation: CzTP), 4-(3-[3-(9-phenyl-9H-fluoren-9-yl)phenyl]phenyl)dibenzofuran (abbreviation: mmDBFFLBi-II), 4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzofuran) (abbreviation: DBF3P-III), 1,3,5-tri(dibenzothiophen-4-yl)-benzene (abbreviated as DBT3P-II), 2,8-diphenyl-4-[4-(9-phenyl-9H-fluoren-9-vl)phenyl]dibenzothiophene (abbreviation: DBTFLP-II), 4-[4-(9-phenyl-9H-fluoren-9-yl)phenyl]-6-phenyldibenzothiophene (abbreviation: DBTFLP-IV), and 4-[3-(triphenylene-2-yl)phenyl]dibenzothiophene (abbreviation: mDBTPTp-II). The substances described here are mainly substances having a hole mobility of 1×10 −6  cm 2 /Vs or higher. Note that other than these substances, any substance that has a property of transporting more holes than electrons may be used. 
     As the electron-transport material, a material having a property of transporting more electrons than holes can be used, and a material having an electron mobility of 1×10 −6  cm 2 /Vs or higher is preferable. A π-electron deficient heteroaromatic compound such as a nitrogen-containing heteroaromatic compound, a metal complex, or the like can be used as the material which easily accepts electrons (the material having an electron-transport property). Specific examples include a metal complex having a quinoline ligand, a benzoquinoline ligand, an oxazole ligand, and a thiazole ligand. Other examples include an oxadiazole derivative, a triazole derivative, a phenanthroline derivative, a pyridine derivative, a bipyridine derivative, a pyrimidine derivative, and the like. 
     Examples include metal complexes having a quinoline or benzoquinoline skeleton, such as tris(8-quinolinolato)aluminum(II) (abbreviation: Alq), tris(4-methyl-8-quinolinolato)aluminum(III) (abbreviation: Almq 3 ), bis(10-hydroxybenzo[h]quinolinato)beryllium(II) (abbreviation: BeBq 2 ), bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum(III) (abbreviation: BAlq) and bis(8-quinolinolato)zinc(II) (abbreviation: Znq), and the like. Alternatively, a metal complex having an oxazole-based or thiazole-based ligand, such as bis[2-(2-benzoxazolyl)phenolate]zinc(II) (abbreviation: ZnPBO) or bis[2-(2-benzothiazolyl)phenolato]zinc(II) (abbreviation: ZnBTZ) can be used. Other than such metal complexes, any of the following can be used: heterocyclic compounds 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), 9-[4-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl]-9H-carbazole (abbreviation: COl1), 3-(biphenyl-4-vl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole (abbreviation: TAZ), 2,2′,2″-(1,3,5-benzenetriyl)tris(1-phenyl-1H-benzimidazole) (abbreviation: TPBI3), 2-[3-(dibenzothiophen-4-yl)phenyl]-1-phenyl-1H-benzimidazole (abbreviation: mDBTB1m-II), bathophenanthroline (abbreviation: BPhen), bathocuproine (abbreviation: BCP), and 2,9-bis(naphthalen-2-yl)-4,7-diphenyl-1,10-phenanthroline (abbreviation: NBPhen); heterocyclic compounds having a diazine skeleton 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-[3′-(9H-carbazol-9-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline (abbreviation: 2mCzBIPDBq), 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), 6-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation: 6mDBTPDBq-II), 4,6-bis[3-(phenanthren-9-yl)phenyl]pyrimidine (abbreviation: 4,6mPnP2Pm), 4,6-bis[3-(4-dibenzothienyl)phenyl]pyrimidine (abbreviation: 4,6mDBTP2Pm-II), and 4,6-bis[3-(9H-carbazol-9-yl)phenyl]pyrimidine (abbreviation: 4,6mCzP2Pm); heterocyclic compounds having a triazine skeleton such as 2-{4-[3-(N-phenyl-9H-carbazol-3-yl)-9H-carbazol-9-yl]phenyl}-4,6-diphenyl-1,3,5-triazine (abbreviation: PCCzPTzn); heterocyclic compounds having a pyridine skeleton such as 3,5-bis[3-(9H-carbazol-9-yl)phenyl]pyridine (abbreviation: 35DCzPPy); and heteroaromatic compounds such as 4,4′-bis(5-methylbenzoxazol-2-yl)stilbene (abbreviation: BzOs). Further alternatively, a high molecular compound such as poly(2,5-pyridinediyl) (abbreviation: PPy), poly[(9,9-dihexylfluorene-2,7-diyl)-co-(pyridine-3,5-diyl)] (abbreviation: PF-Py), or poly[(9,9-dioctylfluorene-2,7-diyl)-co-(2,2′-bipyridine-6,6′-diyl)] (abbreviation: PF-BPy) can be used. The substances described here are mainly substances having an electron mobility of 1×10 −6  cm 2 /Vs or higher. Note that other substances may also be used as long as their electron-transport properties are higher than their hole-transport properties. 
     As the organic compound  131 _ 2 , a substance which can form an exciplex together with the organic compound  131 _ 1  is used. Specifically, any of the hole-transport materials and the electron-transport materials described above can be used. In that case, it is preferable that the organic compound  131 _ 1 , the organic compound  131 _ 2 , and the guest material  132  (the fluorescent material) be selected such that the emission peak of the exciplex formed by the organic compound  131 _ 1  and the organic compound  131 _ 2  overlaps with an absorption band on the longest wavelength side (lowest energy side) of the guest material  132  (the fluorescent material). This makes it possible to provide a light-emitting element with drastically improved emission efficiency. 
     As the host material  131  (the organic compound  131 _ 1  and the organic compound  131 _ 2 ) included in the light-emitting layer  130 , a material having a function of converting triplet excited energy into singlet excited energy is preferable. As the material having a function of converting triplet excited energy into singlet excited energy, a thermally activated delayed fluorescent (TADF) material can be given in addition to the exciplex. Therefore, the term “exciplex” in the description can be replaced with the term “thermally activated delayed fluorescence material”. Note that the thermally activated delayed fluorescence material is a material having a small difference between the triplet excited energy level and the singlet excited energy level and a function of converting triplet excited energy into singlet excited energy by reverse intersystem crossing. Thus, the thermally activated delayed fluorescence material can up-convert a triplet excited state into a singlet excited state (i.e., reverse intersystem crossing) using a little thermal energy and efficiently exhibit light emission (fluorescence) from the singlet excited state. Thermally activated delayed fluorescence is efficiently obtained under the condition where the difference between the triplet excited energy level and the singlet excited energy level is more than 0 eV and less than or equal to 0.2 eV, preferably more than 0 eV and less than or equal to 0.1 eV. 
     The material that exhibits thermally activated delayed fluorescence may be a material that can form a singlet excited state by itself from a triplet excited state by reverse intersystem crossing. In the case where the thermally activated delayed fluorescence material is composed of one kind of material, any of the following materials can be used, for example. 
     First, a fullerene, a derivative thereof, an acridine derivative such as proflavine, eosin, and the like can be given. Furthermore, a metal-containing porphyrin, such as a porphyrin containing magnesium (Mg), zinc (Zn), cadmium (Cd), tin (Sn), platinum (Pt), indium (In), or palladium (Pd), can be given. Examples of the metal-containing porphyrin include a protoporphyrin-tin fluoride complex (SnF 2 (Proto IX)), a mesoporphyrin-tin fluoride complex (SnF 2 (Meso IX)), a hematoporphyrin-tin fluoride complex (SnF 2 (Hemato IX)), a coproporphyrin tetramethyl ester-tin fluoride complex (SnF 2 (Copro III-4Me)), an octaethylporphyrin-tin fluoride complex (SnF 2 (OEP)), an etioporphyrin-tin fluoride complex (SnF 2 (Etio I)), and an octaethylporphyrin-platinum chloride complex (PtCl 2 (OEP)). 
     As the thermally activated delayed fluorescence material composed of one kind of material, a heterocyclic compound having a π-electron rich heteroaromatic ring and a π-electron deficient heteroaromatic ring can be used. Specifically, 2-(biphenyl-4-yl)-4,6-bis(12-phenylindolo[2,3-d]carbazol-1-yl)-1,3,5-triazine (abbreviation: PIC-TRZ), 2-{4-[3-(N-phenyl-9H-carbazol-3-yl)-9H-carbazol-9-yl]phenyl}-4,6-diphenyl-1,3,5-triazine (abbreviation: PCCzPTTn), 2-[4-(10H-phenoxazin-10-yl)phenyl]-1,6-diphenyl-1,3,5-triazine (abbreviation: PXZ-TRZ), 3-[4-(5-phenyl-5,10-dihydrophenazine-10-yl)phenyl]-4,5-diphenyl-1,2,4-triazole (abbreviation: PPZ-3TPT), 3-(9,9-dimethyl-9H-acridin-10-yl)-9H-xanthen-9-one (abbreviation: ACRXTN), bis[4-(9,9-dimethyl-9,10-dihydroacridine)phenyl]sulfone (abbreviation: DMAC-DPS), or 10-phenyl-10H,10′H-spiro[acridin-9,9′-anthracen]-10′-one (abbreviation: ACRSA), can be used. The heterocyclic compound is preferable because of having the π-electron rich heteroaromatic ring and the π-electron deficient heteroaromatic ring, for which the electron-transport property and the hole-transport property are high. Note that a substance in which the π-electron rich heteroaromatic ring is directly bonded to the π-electron deficient heteroaromatic ring is particularly preferable because the donor property of the π-electron rich heteroaromatic ring and the acceptor property of the π-electron deficient heteroaromatic ring are both increased and the difference between the singlet excited level and the triplet excited level becomes small. 
     The light-emitting layer  130  can have a structure in which two or more layers are stacked. For example, in the case where the light-emitting layer  130  is formed by stacking a first light-emitting layer and a second light-emitting layer in this order from the hole-transport layer side, the first light-emitting layer is formed using a substance having a hole-transport property as the host material and the second light-emitting layer is formed using a substance having an electron-transport property as the host material. 
     The light-emitting layer  130  may include a material other than the host material  131  and the guest material  132 . 
     &lt;&lt;Pair of Electrodes&gt;&gt; 
     The electrode  101  and the electrode  102  have functions of injecting holes and electrons into the light-emitting layer  130 . The electrode  101  and the electrode  102  can be formed using a metal, an alloy, or a conductive compound, a mixture or a stack thereof, or the like. A typical example of the metal is aluminum (Al); besides, a transition metal such as silver (Ag), tungsten, chromium, molybdenum, copper, or titanium, an alkali metal such as lithium (Li) or cesium, or a Group 2 metal such as calcium or magnesium (Mg) can be used. As the transition metal, a rare earth metal such as ytterbium (Yb) may be used. An alloy containing any of the above metals can be used as the alloy, and MgAg and AlLi can be given as examples. Examples of the conductive compound include metal oxides such as indium tin oxide (hereinafter, referred to as ITO), indium tin oxide containing silicon or silicon oxide (ITSO), indium zinc oxide, indium oxide containing tungsten and zinc, and the like. It is also possible to use an inorganic carbon-based material such as graphene as the conductive compound. As described above, the electrode  101  and/or the electrode  102  may be formed by stacking two or more of these materials. 
     Light emitted from the light-emitting layer  130  is extracted through the electrode  101  and/or the electrode  102 . Therefore, at least one of the electrodes  101  and  102  transmits visible light. As the conductive material transmitting light, a conductive material having a visible light transmittance higher than or equal to 40% and lower than or equal to 100%, preferably higher than or equal to 60% and lower than or equal to 100%, and a resistivity lower than or equal to 1×10 −2  Ω·cm can be used. The electrode on the light extraction side may be formed using a conductive material having functions of transmitting light and reflecting light. As the conductive material, a conductive material having a visible light reflectivity higher than or equal to 20% and lower than or equal to 80%, preferably higher than or equal to 40% and lower than or equal to 70%, and a resistivity lower than or equal to 1×10 −2  Ω·cm can be used. In the case where the electrode through which light is extracted is formed using a material with low light transmittance, such as metal or alloy, the electrode  101  and/or the electrode  102  is formed to a thickness that is thin enough to transmit visible light (e.g., a thickness of 1 nm to 10 nm). 
     In this specification and the like, as the electrode transmitting light, a material that transmits visible light and has conductivity is used. Examples of the material include, in addition to the above-described oxide conductor layer typified by an ITO, an oxide semiconductor layer and an organic conductor layer containing an organic substance Examples of the organic conductive layer containing an organic substance include a layer containing a composite material in which an organic compound and an electron donor (donor material) are mixed and a layer containing a composite material in which an organic compound and an electron acceptor (acceptor material) are mixed. The resistivity of the transparent conductive layer is preferably lower than or equal to 1×10 5  Ω·cm, further preferably lower than or equal to 1×10 4  Ω·cm. 
     As the method for forming the electrode  101  and the electrode  102 , a sputtering method, an evaporation method, a printing method, a coating method, a molecular beam epitaxy (MBE) method, a CVD method, a pulsed laser deposition method, an atomic layer deposition (ALD) method, or the like can be used as appropriate. 
     &lt;&lt;Hole-Injection Layer&gt;&gt; 
     The hole-injection layer  111  has a function of reducing a barrier for hole injection from one of the pair of electrodes (the electrode  101  or the electrode  102 ) to promote hole injection and is formed using a transition metal oxide, a phthalocyanine derivative, or an aromatic amine, for example. As the transition metal oxide, molybdenum oxide, vanadium oxide, ruthenium oxide, tungsten oxide, manganese oxide, or the like can be given. As the phthalocyanine derivative, phthalocyanine, metal phthalocyanine, or the like can be given. As the aromatic amine, a benzidine derivative, a phenylenediamine derivative, or the like can be given. It is also possible to use a high molecular compound such as polythiophene or polyaniline; a typical example thereof is poly(ethylenedioxythiophene)/poly(styrenesulfonic acid), which is self-doped polythiophene. 
     As the hole-injection layer  111 , a layer containing a composite material of a hole-transport material and a material having a property of accepting electrons from the hole-transport material can also be used. Alternatively, a stack of a layer containing a material having an electron accepting property and a layer containing a hole-transport material may also be used. In a steady state or in the presence of an electric field, electric charge can be transferred between these materials. As examples of the material having an electron-accepting property, organic acceptors such as a quinodimethane derivative, a chloranil derivative, and a hexaazatriphenylene derivative can be given. A specific example is a compound having an electron-withdrawing group (a halogen group or a cyano group), such as 7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation: Fa-TCNQ), chloranil, or 2,3,6,7,10,11-hexacyano-1,4,5,8,9,12-hexaazatriphenylene (abbreviation: HAT-CN). Alternatively, a transition metal oxide such as an oxide of a metal from Group 4 to Group 8 can also be used. Specifically, vanadium oxide, niobium oxide, tantalum oxide, chromium oxide, molybdenum oxide, tungsten oxide, manganese oxide, rhenium oxide, or the like can be used. In particular, molybdenum oxide is preferable because it is stable in the air, has a low hygroscopic property, and is easily handled. 
     A material having a property of transporting more holes than electrons can be used as the hole-transport material, and a material having a hole mobility of 1×10 −6  cm 2 /Vs or higher is preferable. Specifically, any of the aromatic amine, carbazole derivative, aromatic hydrocarbon, stilbene derivative, and the like described as examples of the hole-transport material that can be used in the light-emitting layer  130  can be used. Furthermore, the hole-transport material may be a high molecular compound. 
     &lt;&lt;Hole-Transport Layer&gt;&gt; 
     The hole-transport layer  112  is a layer containing a hole-transport material and can be formed using any of the materials given as examples of the material of the hole-injection layer  111 . In order that the hole-transport layer  112  has a function of transporting holes injected into the hole-injection layer  111  to the light-emitting layer  130 , the HOMO level of the hole-transport layer  112  is preferably equal or close to the HOMO level of the hole-injection layer  111 . 
     As the hole-transport material, any of the materials given as examples of the material of the hole-injection layer  111  can be used. As the hole-transport material, a substance having a hole mobility of 1×10 −6  cm 2 /Vs or higher is preferably used. Note that any substance other than the above substances may be used as long as the hole-transport property is higher than the electron-transport property. The layer including a substance having a high hole-transport property is not limited to a single layer, and two or more layers containing the aforementioned substances may be stacked. 
     &lt;&lt;Electron-Transport Layer&gt;&gt; 
     The electron-transport layer  118  has a function of transporting, to the light-emitting layer  130 , electrons injected from the other of the pair of electrodes (the electrode  101  or the electrode  102 ) through the electron-injection layer  119 . A material having a property of transporting more electrons than holes can be used as the electron-transport material, and a material having an electron mobility of 1×10 −6  cm 2 /Vs or higher is preferable. As the compound which easily accepts electrons (the material having an electron-transport property), a π-electron deficient heteroaromatic compound such as a nitrogen-containing heteroaromatic compound, a metal complex, or the like can be used, for example. Specifically, a metal complex having a quinoline ligand, a benzoquinoline ligand, an oxazole ligand, or a thiazole ligand, which are described as the electron-transport materials that can be used in the light-emitting layer  130 , can be given. Further, an oxadiazole derivative; a triazole derivative, a phenanthroline derivative, a pyridine derivative, a bipyridine derivative, a pyrimidine derivative, and the like can be given. A substance having an electron mobility of higher than or equal to 1×10 −6  cm 2 /Vs is preferable. 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. The electron-transport layer  118  is not limited to a single layer, and may include stacked two or more layers containing the aforementioned substances. 
     Between the electron-transport layer  118  and the light-emitting layer  130 , a layer that controls transfer of electron carriers may be provided. This is a layer formed by addition of a small amount of a substance having a high electron-trapping property to a material having a high electron-transport property described above, and the layer is capable of adjusting carrier balance by suppressing transfer of electron carriers. Such a structure is very effective in preventing a problem (such as a reduction in element lifetime) caused when electrons pass through the light-emitting layer. 
     &lt;&lt;Electron-Injection Layer&gt;&gt; 
     The electron-injection layer  119  has a function of reducing a barrier for electron injection from the electrode  102  to promote electron injection and can be formed using a Group 1 metal or a Group 2 metal, or an oxide, a halide, or a carbonate of any of the metals, for example. Alternatively, a composite material containing an electron-transport material (described above) and a material having a property of donating electrons to the electron-transport material can also be used. As the material having an electron-donating property, a Group 1 metal, a Group 2 metal, an oxide of any of the metals, or the like can be given. Specifically, an alkali metal, an alkaline earth metal, or a compound thereof, such as lithium fluoride (LiF), cesium fluoride (CsF), calcium fluoride (CaF 2 ), or lithium oxide (LiO x ), can be used. Alternatively, a rare earth metal compound like erbium fluoride (ErF 3 ) can be used. Electride may also be used for the electron-injection layer  119 . Examples of the electride include a substance in which elections are added at high concentration to calcium oxide-aluminum oxide. The electron-injection layer  119  can be formed using the substance that can be used for the electron-transport layer  118 . 
     A composite material in which an organic compound and an electron donor (donor) are mixed may also be used for the electron-injection layer  119 . Such a composite material is excellent in an electron-injection property and an electron-transport property because electrons are generated in the organic compound by the electron donor. In this case, the organic compound is preferably a material that is excellent in transporting the generated electrons. Specifically, the above-listed substances for forming the electron-transport layer  118  (e.g., the metal complexes and heteroaromatic compounds) can be used, for example. As the electron donor, a substance showing an electron-donating property with respect to the organic compound may be used. Specifically, an alkali metal, an alkaline earth metal, and a rare earth metal are preferable, and lithium, cesium, magnesium, calcium, erbium, and ytterbium are given. In addition, an alkali metal oxide or an alkaline earth metal oxide is preferable, and lithium oxide, calcium oxide, barium oxide, and the like are given. A Lewis base such as magnesium oxide can also be used. An organic compound such as tetrathiafulvalene (abbreviation: TTF) can also be used. 
     Note that the light-emitting layer, the hole-injection layer, the hole-transport layer, the electron-transport layer, and the electron-injection layer described above can each be formed by an evaporation method (including a vacuum evaporation method), an inkjet method, a coating method, a gravure printing method, or the like. Besides the above-mentioned materials, an inorganic compound such as a quantum dot or a high molecular compound (e.g., an oligomer, a dendrimer, and a polymer) may be used in the light-emitting layer, the hole-injection layer, the hole-transport layer, the electron-transport layer, and the electron-injection layer. 
     The quantum dot may be a colloidal quantum dot, an alloyed quantum dot, a core-shell quantum dot, or a core quantum dot, for example. The quantum dot containing elements belonging to Groups 2 and 16, elements belonging to Groups 13 and 15, elements belonging to Groups 13 and 17, elements belonging to Groups 11 and 17, or elements belonging to Groups 14 and 15 may be used. Alternatively, the quantum dot containing an element such as cadmium (Cd), selenium (Se), zinc (Zn), sulfur (S), phosphorus (P), indium (in), tellurium (Te), lead (Pb), gallium (Ga), arsenic (As), or aluminum (Al) may be used 
     &lt;&lt;Substrate&gt;&gt; 
     A light-emitting element in one embodiment of the present invention may be formed over a substrate of glass, plastic, or the like. As the way of stacking layers over the substrate, layers may be sequentially stacked from the electrode  101  side or sequentially stacked from the electrode  102  side. 
     For the substrate over which the light-emitting element of one embodiment of the present invention can be formed, glass, quartz, plastic, or the like can be used, for example. Alternatively, a flexible substrate can be used. The flexible substrate means a substrate that can be bent, such as a plastic substrate made of polycarbonate or polyarylate, for example. Alternatively, a film, an inorganic vapor deposition film, or the like can be used. Another material may be used as long as the substrate functions as a support in a manufacturing process of the light-emitting element or an optical element or as long as it has a function of protecting the light-emitting element or an optical element. 
     In this specification and the like, a light-emitting element can be formed using any of a variety of substrates, for example. The type of a substrate is not limited particularly. Examples of the substrate include a semiconductor substrate (e.g., a single crystal substrate or a silicon substrate), an SOI substrate, a glass substrate, a quartz substrate, a plastic substrate, a metal substrate, a stainless steel substrate, a substrate including stainless steel foil, a tungsten substrate, a substrate including tungsten foil, a flexible substrate, an attachment film, paper including a fibrous material, a base material film, and the like. As an example of a glass substrate, a barium borosilicate glass substrate, an aluminoborosilicate glass substrate, a soda lime glass substrate, and the like can be given. Examples of the flexible substrate, the attachment film, the base material film, and the like are substrates of plastics typified by polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polyether sulfone (PES), and polytetrafluoroethylene (PTFE). Another example is a resin such as acrylic. Furthermore, polypropylene, polyester, polyvinyl fluoride, and polyvinyl chloride can be given as examples. Other examples are polyamide, polyimide, aramid, epoxy, an inorganic vapor deposition film, paper, and the like. 
     Alternatively, a flexible substrate may be used as the substrate such that the light-emitting element is provided directly on the flexible substrate. Further alternatively, a separation layer may be provided between the substrate and the light-emitting element. The separation layer can be used when part or the whole of a light-emitting element formed over the separation layer is separated from the substrate and transferred onto another substrate. In such a case, the light-emitting element can be transferred to a substrate having low heat resistance or a flexible substrate as well. For the above separation layer, a stack including inorganic films, which are a tungsten film and a silicon oxide film, and a structure in which a resin film of polyimide or the like is formed over a substrate can be used, for example. 
     In other words, after the light-emitting element is formed using a substrate, the light-emitting element may be transferred to another substrate. Example of the substrate to which the light-emitting element is transferred are, in addition to the above substrates, a cellophane substrate, a stone substrate, a wood substrate, a cloth substrate (including a natural fiber (e.g., silk, cotton, and hemp), a synthetic fiber (e.g., nylon, polyurethane, and polyester), a regenerated fiber (e.g., acetate, cupra, rayon, and regenerated polyester), and the like), a leather substrate, a rubber substrate, and the like. When such a substrate is used, a light-emitting element with high durability, high heat resistance, reduced weight, or reduced thickness can be formed. 
     The light-emitting element  250  may be formed over an electrode electrically connected to a field-effect transistor (FET), for example, which is formed over any of the above-described substrates. Accordingly, an active matrix display device in which the FET controls the driving of the light-emitting element can be manufactured. 
     In this embodiment, one embodiment of the present invention has been described. Other embodiments of the present invention are described in the other embodiments. One embodiment of the present invention is not limited to the above-described examples. In one embodiment of the present invention, an example where a light-emitting element contains a fluorescent material and a host material and the host material contains a first organic compound and a second organic compound has been described; however, one embodiment of the present invention is not limited to this example. Depending on circumstances or conditions, in one embodiment of the present invention, the host material does not necessarily contain the first organic compound or the second organic compound. In addition, in one embodiment of the present invention, an example where an exciplex contains a delayed fluorescence material whose fluorescence lifetime is 10 ns or longer and 50 μs or shorter has been described; however, one embodiment of the present invention is not limited to this example. Depending on circumstances or conditions, in one embodiment of the present invention, the exciplex may contain a delayed fluorescence material whose fluorescence lifetime is shorter than 10 ns. Alternatively, the exciplex may contain a delayed fluorescence material whose fluorescence lifetime is longer than 50 μs. In addition, in one embodiment of the present invention, an example where a delayed fluorescence component accounts for 5% or more of emission of an exciplex is shown; however, one embodiment of the present invention is not limited thereto. Depending on circumstances or conditions, in one embodiment of the present invention, a delayed fluorescence component may account for less than 5% of emission of the exciplex. 
     The structure described above in this embodiment can be combined with any of the structures described in the other embodiments as appropriate. 
     Embodiment 2 
     In this embodiment, light-emitting elements having structures different from that described in Embodiment 1 and light emission mechanisms of the light-emitting elements are described below with reference to  FIGS. 2A to 2C  and  FIGS. 3A to 3C . In  FIGS. 2A to 2C  and  FIGS. 3A to 3C , a portion having a function similar to that in  FIG. 1A  is represented by the same hatch pattern as in  FIG. 1A  and not especially denoted by a reference numeral in some cases. In addition, common reference numerals are used for portions having similar functions, and a detailed description of the portions is omitted in some cases. 
     &lt;Structure Example 1 of Light-Emitting Element&gt; 
       FIG. 2A  is a schematic cross-sectional view of a light-emitting element  260 . 
     The light-emitting element  260  illustrated in  FIG. 2A  includes a plurality of light-emitting units (a light-emitting unit  106  and a light-emitting unit  108  in  FIG. 2A ) between a pair of electrodes (the electrode  101  and the electrode  102 ). One light-emitting unit has the same structure as the EL layer  100  illustrated in  FIG. 1A . That is, the light-emitting element  250  in  FIG. 1A  includes one light-emitting unit, while the light-emitting element  260  includes a plurality of light-emitting units. Note that the electrode  101  functions as an anode and the electrode  102  functions as a cathode in the following description of the light-emitting element  260 ; however, the functions may be interchanged in the light-emitting element  260 . 
     In the light-emitting element  260  illustrated in  FIG. 2A , the light-emitting unit  106  and the light-emitting unit  108  are stacked, and a charge-generation layer  115  is provided between the light-emitting unit  106  and the light-emitting unit  108 . Note that the light-emitting unit  106  and the light-emitting unit  108  may have the same structure or different structures. For example, it is preferable that the EL layer  100  illustrated in  FIG. 1A  be used in the light-emitting unit  108 . 
     The light-emitting element  260  includes a light-emitting layer  120  and the light-emitting layer  130 . The light-emitting unit  106  includes the hole-injection layer  111 , the hole-transport layer  112 , an electron-transport layer  113 , and an electron-injection layer  114  in addition to the light-emitting layer  120 . The light-emitting unit  108  includes a hole-injection layer  116 , a hole-transport layer  117 , an electron-transport layer  118 , and an electron-injection layer  119  in addition to the light-emitting layer  130 . 
     The charge-generation layer  115  may have either a structure in which an acceptor substance that is an electron acceptor is added to a hole-transport material or a structure in which a donor substance that is an electron donor is added to an electron-transport material. Alternatively, both of these structures may be stacked. 
     In the case where the charge-generation layer  115  contains a composite material of an organic compound and an acceptor substance, the composite material that can be used for the hole-injection layer  111  described in Embodiment 1 may be used for the composite material. As the organic compound, a variety of compounds such as an aromatic amine compound, a carbazole compound, an aromatic hydrocarbon, and a high molecular compound (such as an oligomer, a dendrimer, or a polymer) can be used. A substance having a hole mobility of 1×10 −6  cm 2 /Vs or higher is preferably used as the organic compound. Note that any other material may be used as long as it has a property of transporting more holes than electrons. Since the composite material of an organic compound and an acceptor substance has excellent carrier-injection and carrier-transport properties, low-voltage driving or low-current driving can be realized. Note that when a surface of a light-emitting unit on the anode side is in contact with the charge-generation layer  115  like the light-emitting unit  108 , the charge-generation layer  115  can also serve as a hole-injection layer or a hole-transport layer of the light-emitting unit; thus, a hole-injection layer or a hole-transport layer need not be included in the light-emitting unit. 
     The charge-generation layer  115  may have a stacked structure of a layer containing the composite material of an organic compound and an acceptor substance and a layer containing another material. For example, the charge-generation layer  115  may be formed using a combination of a layer containing the composite material of an organic compound and an acceptor substance with a layer containing one compound selected from among electron-donating materials and a compound having a high electron-transport property. Furthermore, the charge-generation layer  115  may be formed using a combination of a layer containing the composite material of an organic compound and an acceptor substance with a layer containing a transparent conductive material. 
     The charge-generation layer  115  provided between the light-emitting unit  106  and the light-emitting unit  108  may have any structure as long as electrons can be injected to the light-emitting unit on one side and holes can be injected into the light-emitting unit on the other side when a voltage is applied between the electrode  101  and the electrode  102 . For example, in  FIG. 2A , the charge-generation layer  115  injects electrons into the light-emitting unit  106  and holes into the light-emitting unit  108  when a voltage is applied such that the potential of the electrode  101  is higher than that of the electrode  102 . 
     Note that in terms of light extraction efficiency, the charge-generation layer  115  preferably has a visible light transmittance (specifically, a visible light transmittance of higher than or equal to 40%). The charge-generation layer  115  functions even if it has lower conductivity than the pair of electrodes (the electrodes  101  and  102 ). In the case where the conductivity of the charge-generation layer  115  is as high as those of the pair of electrodes, carriers generated in the charge-generation layer  115  flow toward the film surface direction, so that light is emitted in a region where the electrode  101  and the electrode  102  do not overlap, in some cases. To suppress such a defect, the charge-generation layer  115  is preferably formed using a material whose conductivity is lower than those of the pair of electrodes. 
     Note that forming the charge-generation layer  115  by using any of the above materials can suppress an increase in drive voltage caused by the stack of the light-emitting layers. 
     The light-emitting element having two light-emitting units is described with reference to  FIG. 2A ; however, a similar structure can be applied to a light-emitting element in which three or more light-emitting units are stacked. With a plurality of light-emitting units partitioned by the charge-generation layer between a pair of electrodes as in the light-emitting element  260 , it is possible to provide a light-emitting element which can emit light with high luminance with the current density kept low and has a long lifetime. A light-emitting element with low power consumption can be provided. 
     When the structure of the EL layer  100  illustrated in  FIG. 1A  is used for at least one of the plurality of units, a light-emitting element with high emission efficiency can be provided. 
     It is preferable that the light-emitting layer  130  included in the light-emitting unit  108  have the structure described in Embodiment 1. Thus, the light-emitting element  260  contains a fluorescent material as a light-emitting material and has high luminous efficiency, which is preferable. 
     Furthermore, the light-emitting layer  120  included in the light-emitting unit  108  contains a host material  121  and a guest material  122  as illustrated in  FIG. 2B . Note that the guest material  122  is described below as a fluorescent material. 
     &lt;Light Emission Mechanism of Light-Emitting Layer  120 &gt; 
     The light emission mechanism of the light-emitting layer  120  is described below. 
     By recombination of the electrons and holes injected from the pair of electrodes (the electrode  101  and the electrode  102 ) or the charge-generation layer in the light-emitting layer  120 , excitons are formed. Because the amount of the host material  121  is larger than that of the guest material  122 , the host material  121  is brought into an excited state by the exciton generation. 
     Note that the term “exciton” refers to a carrier (electron and hole) pair. Since excitons have energy, a material where excitons are generated is brought into an excited state. 
     In the case where the formed excited state of the host material  121  is a singlet excited state, singlet excited energy transfers from the S1 level of the host material  121  to the S1 level of the guest material  122 , thereby forming the singlet excited state of the guest material  122 . 
     Since the guest material  122  is a fluorescent material, when a singlet excited state is formed in the guest material  122 , the guest material  122  readily emits light. To obtain high emission efficiency in this case, the fluorescence quantum yield of the guest material  122  is preferably high. The same can apply to a case where a singlet excited state is formed by recombination of carriers in the guest material  122 . 
     Next, a case where recombination of carriers forms a triplet excited state of the host material  121  is described. The correlation of energy levels of the host material  121  and the guest material  122  in this case is shown in  FIG. 2C . The following explains what terms and signs in  FIG. 2C  represent. Note that because it is preferable that the T1 level of the host material  121  be lower than the T1 level of the guest material  122 ,  FIG. 2C  shows this preferable case. However, the T1 level of the host material  121  may be higher than the T1 level of the guest material  122 . 
     Host ( 121 ): the host material  121 ; 
     Guest ( 122 ): the guest material  122  (the fluorescent material); 
     S FH : the S1 level the host material  121 : 
     T FH : the T1 level of the host material  121 ; 
     S FG : the S1 level of the guest material  122  (the fluorescent material); and 
     T FG : the T1 level of the guest material  122  (the fluorescent material). 
     As illustrated in  FIG. 2C , triplet excitons formed by carrier recombination become adjacent to each other by triplet-triplet annihilation (TTA), a reaction in which one of the triplet excitons is converted into a singlet exciton having energy of the S1 level of the host material  121  (S FH ) is caused (see TTA in  FIG. 2C ). The singlet excited energy of the host material  121  is transferred from S FH  to the S1 level of the guest material  122  (S FG ) having a lower energy than S FH  (see Route E 1  in  FIG. 2C ), and a singlet excited state of the guest material  122  is formed, whereby the guest material  122  emits light. 
     Note that in the case where the density of triplet excitons in the light-emitting layer  120  is sufficiently high (e.g., 1×10 −12  cm −3  or more), only the reaction of two triplet excitons close to each other can be considered whereas deactivation of a single triplet exciton can be ignored. 
     In the case where a triplet excited state of the guest material  122  is formed by carrier recombination, the triplet excited state of the guest material  122  is thermally deactivated and is difficult to use for light emission. However, in the case where the T1 level of the host material  121  (T FH ) is lower than the T1 level of the guest material  122  (T FG ), the triplet excited energy of the guest material  122  can be transferred from the T1 level of the guest material  122  (T FG ) to the T1 level of the host material  121  (T FH ) (see Route E 2  in  FIG. 2C ) and then is utilized for TTA. 
     In other words, the host material  121  preferably has a function of converting triplet excited energy into singlet excited energy by causing TTA, so that the triplet excited energy generated in the light-emitting layer  120  can be partly converted into singlet excited energy by TTA in the host material  121 . The singlet excited energy can be transferred to the guest material  122  and extracted as fluorescence. In order to achieve this, the S1 level of the host material  121  (S FH ) is preferably higher than the S1 level of the guest material  122  (S FG ). In addition, the T1 level of the host material  121  (T FH ) is preferably lower than the T1 level of the guest material  122  (T FG ). 
     Note that particularly in the case where the T1 level of the guest material  122  (T GF ) is lower than the T1 level of the host material  121  (T FH ), the weight ratio of the guest material  122  to the host material  121  is preferably low. Specifically, the weight ratio of the guest material  122  to the host material  121  is preferably greater than 0 and less than or equal to 0.05, in which case, the probability of carrier recombination in the guest material  122  can be reduced. In addition, the probability of energy transfer from the T1 level of the host material  121  (T FH ) to the T1 level of the guest material  122  (T FG ) can be reduced. 
     Note that the host material  121  may be composed of a single compound or a plurality of compounds. 
     Note that in each of the above-described structures, the guest materials (fluorescent materials) used in the light-emitting unit  106  and the light-emitting unit  108  may be the same or different. In the case where the same guest material is used for the light-emitting unit  106  and the light-emitting unit  108 , the light-emitting element  260  can exhibit high emission luminance at a small current value, which is preferable. In the case where different guest materials are used for the light-emitting unit  106  and the light-emitting unit  108 , the light-emitting element  260  can exhibit multi-color light emission, which is preferable. It is particularly favorable to select the guest materials so that white light emission with high color rendering properties or light emission of at least red, green, and blue can be obtained 
     &lt;Structure Example 2 of Light-Emitting Element&gt; 
       FIG. 3A  is a schematic cross-sectional view of a light-emitting element  262 . 
     The light-emitting element  262  illustrated in  FIG. 3A  includes, like the light-emitting element  260  described above, a plurality of light-emitting units (a light-emitting unit  106  and a light-emitting unit  108  in  FIG. 3A ) between a pair of electrodes (the electrode  101  and the electrode  102 ). One light-emitting unit has the same structure as the EL layer  100  illustrated in  FIG. 1A . Note that the light-emitting unit  106  and the light-emitting unit  108  may have the same structure or different structures. 
     In the light-emitting element  262  illustrated in  FIG. 3A , the light-emitting unit  106  and the light-emitting unit  108  are stacked, and a charge-generation layer  115  is provided between the light-emitting unit  106  and the light-emitting unit  108 . For example, it is preferable that the EL layer  100  illustrated in  FIG. 1A  be used in the light-emitting unit  106 . 
     The light-emitting element  262  includes the light-emitting layer  130  and a light-emitting layer  140 . The light-emitting unit  106  includes the hole-injection layer  111 , the hole-transport layer  112 , the electron-transport layer  113 , and the electron-injection layer  114  in addition to the light-emitting layer  130 . The light-emitting unit  108  includes the hole-injection layer  116 , the hole-transport layer  117 , the electron-transport layer  118 , and the electron-injection layer  119  in addition to the light-emitting layer  140 . 
     In addition, the light-emitting layer of the light-emitting unit  108  preferably contains a phosphorescent material. That is, it is preferable that the light-emitting layer  130  included in the light-emitting unit  106  have the structure described in Embodiment 1 and the light-emitting layer  140  included in the light-emitting unit  108  contain a phosphorescent material. A structure example of the light-emitting element  262  in this case is described below. 
     Furthermore, the light-emitting layer  140  included in the light-emitting unit  108  contains a host material  141  and a guest material  142  as illustrated in  FIG. 3B . The host material  141  contains an organic compound  141 _ 1  and an organic compound  141 _ 2 . Note that the guest material  142  included in the light-emitting layer  140  is described below as a phosphorescent material. 
     &lt;Light Emission Mechanism of Light-Emitting Layer  140 &gt; 
     Next, the light emission mechanism of the light-emitting layer  140  is described. The organic compound  141 _ 1  and the organic compound  141 _ 2  which are included in the light-emitting layer  140  form an exciplex. 
     It is acceptable as long as the combination of the organic compound  141 _ 1  and the organic compound  141 _ 2  can form an exciplex in the light-emitting layer  140 , and it is preferred that one organic compound have a hole-transport property and the other organic compound have an electron-transport property. 
       FIG. 3C  illustrates the correlation of energy levels of the organic compound  141 _ 1 , the organic compound  141 _ 2 , and the guest material  142  in the light-emitting layer  140 . The following explains what terms and signs in  FIG. 3C  represent: 
     Host ( 141 _ 1 ): the organic compound  141 _ 1  (host material); 
     Host ( 141   2 ): the organic compound  141   2  (host material); 
     Guest ( 142 ): the guest material  142  (phosphorescent material); 
     S PH : the S1 level of the organic compound  141 _ 1  (host material); 
     T PH : the T1 level of the organic compound  141   1  (host material); 
     T PG : the T1 level of the guest material  142  (phosphorescent material); 
     S PF : the S1 level of the exciplex; and 
     T PE : the T1 level of the exciplex. 
     The level (S PE ) of the lowest singlet excited state of the exciplex formed by the organic compounds  141 _ 1  and  141 _ 2  and the level (T PE ) of the lowest triplet excited state of the exciplex are close to each other (see Route C in  FIG. 3C ). 
     Both energies of S PE  and T PE  of the exciplex are then transferred to the level of the lowest triplet excited state of the guest material  142  (phosphorescent material); thus, light emission is obtained (see Route D in  FIG. 3C ). 
     The above-described processes through Route C and Route D may be referred to as exciplex-triplet energy transfer (ExTET) in this specification and the like. 
     When one of the organic compounds  141 _ 1  and  141 _ 2  receives holes and the other receives electrons, the exciplex is formed. Alternatively, when one compound is brought into an excited state, the one interacts with the other compound to form the exciplex. Therefore, most excitons in the light-emitting layer  140  exist as exciplexes. The band gap of the exciplex is narrower than that of each of the organic compounds  141 _ 1  and  141 _ 2 ; therefore, an excited state can be formed with lower excitation energy. Thus, the formation of the exciplex can lower the drive voltage of the light-emitting element. 
     When the light-emitting layer  140  has the above structure, light emission from the guest material  142  (phosphorescent material) of the light-emitting layer  140  can be efficiently obtained. 
     Note that light emitted from the light-emitting layer  130  preferably has a peak on the shorter wavelength side than light emitted from the light-emitting layer  140 . Since the luminance of a light-emitting element using a phosphorescent material emitting light with a short wavelength tends to be degraded quickly, fluorescence with a short wavelength is employed so that a light-emitting element with less degradation of luminance can be provided. 
     Furthermore, the light-emitting layer  130  and the light-emitting layer  140  may be made to emit light with different emission wavelengths, so that the light-emitting element can be a multicolor light-emitting element. In that case, the emission spectrum of the light-emitting element is formed by combining light having different emission peaks, and thus has at least two peaks. 
     The above structure is also suitable for obtaining white light emission. When the light-emitting layer  130  and the light-emitting layer  140  emit light of complementary colors, white light emission can be obtained. 
     In addition, white light emission with a high color rendering property that is formed of three primary colors or four or more colors can be obtained by using a plurality of light-emitting materials emitting light with different wavelengths for one of the light-emitting layers  130  and  140  or both. In that case, one of the light-emitting layers  130  and  140  or both may be divided into layers and each of the divided layers may contain a light-emitting material different from the others. 
     &lt;Material that can be Used in Light-Emitting Layers&gt; 
     Next, materials that can be used in the light-emitting layers  120 ,  130 , and  140  are described. 
     &lt;&lt;Material that can be Used in Light-Emitting Layer  130 &gt;&gt; 
     As a material that can be used in the light-emitting layer  130 , a material that can be used in the light-emitting layer  130  in Embodiment 1 may be used. Thus, a light-emitting element with high generation efficiency of a singlet excited state and high emission efficiency can be fabricated. 
     &lt;&lt;Material that can be Used in Light-Emitting Layer  120 &gt;&gt; 
     In the light-emitting layer  120 , the host material  121  is present in the largest proportion by weight, and the guest material  122  (the fluorescent material) is dispersed in the host material  121 . The S1 level of the host material  121  is preferably higher than the S1 level of the guest material  122  (the fluorescent material) while the T1 level of the host material  121  is preferably lower than the T1 level of the guest material  122  (the fluorescent material). 
     In the light-emitting layer  120 , although the guest material  122  is not particularly limited, for example, any of materials which are described as examples of the guest material  132  in Embodiment 1 can be used. 
     Although there is no particular limitation on a material that can be used as the host material  121  in the light-emitting layer  120 , any of the following materials can be used, for example: metal complexes such as tris(8-quinolinolato)aluminum(III) (abbreviation: Alq), tris(4-methyl-8-quinolinolato)aluminum(III) (abbreviation: Almq 3 ), bis(10-hydroxybenzo[h]quinolinato)beryllium(II) (abbreviation: BeBq 2 ), bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum(III) (abbreviation: BA1q), bis(8-quinolinolato)zinc(II) (abbreviation: Znq), bis[2-(2-benzoxazolyl)phenolato]zinc(II) (abbreviation: ZnPBO), and bis[2-(2-benzothiazolyl)phenolato]zinc(II) (abbreviation: ZnBTZ); heterocyclic compounds 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-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole (abbreviation: IAZ), 2,2′,2″-(1,3,5-benzenetriyl)-tris(1-phenyl-1H-benzimidazole) (abbreviation: TPBI), bathophenanthroline (abbreviation: BPhen), bathocuproine (abbreviation: BCP), 2,9-bis(naphtthalen-2-yl)-4,7-diphenyl-1,10-phenanthroline (abbreviation: NBphen), and 9-[4-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl]-9H-carbazole (abbreviation: COl1); and 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), and 4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl (abbreviation: BSPB). In addition, condensed polycyclic aromatic compounds such as anthracene derivatives, phenanthrene derivatives, pyrene derivatives, chrysene derivatives, and dibenzo[g,p]chrysene derivatives can be given, and specific examples are 9,10-diphenylanthracene (abbreviation: DPAnth), N,N-diphenyl-9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine (abbreviation: CzA1PA), 4-(10-phenyl-9-anthryl)triphenylamine (abbreviation: DPhPA), 4-(9H-carbazol-9-yl)-4′-(10-phenyl-9-anthryl)triphenylamine (abbreviation: YGAPA), N,N-diphenyl-N-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine (abbreviation: PCAPA), N,9-diphenyl-N-{4-[4-(10-phenyl-9-anthryl)phenyl]phenyl}-9H-carbazol-3-amine (abbreviation: PCAPBA), N,9-diphenyl-N-(9,10-diphenyl-2-anthryl)-9H-carbazol-3-amine (abbreviation: 2PCAPA), 6,12-dimethoxy-5,11-diphenylchrysene, N,N,N′,N′,N″,N″,N′″,N′″-octaphenyldibenzo[g,p]chrysene-2,7,10,15-tetramine (abbreviation: DBC1), 9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: CzPA), 3,6-diphenyl-9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: DPCzPA), 9,10-bis(3,5-diphenylphenyl)anthracene (abbreviation: DPPA), 9,10-di(2-naphthyl)anthracene (abbreviation: DNA), 2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation: t-RuDNA), 9,9′-bianthryl (abbreviation: BANT), 9,9′-(stilbene-3,3′-diyl)diphenanthrene (abbreviation: DPNS), 9,9′-(stilbene-4,4′-diyl)diphenanthrene (abbreviation: DPNS2), 1,3,5-tri(1-pyrenyl)benzene (abbreviation: TPB3), and the like. One or more substances having a wider energy gap than the guest material  122  is preferably selected from these substances and known substances. 
     The light-emitting layer  120  can have a structure in which two or more layers are stacked. For example, in the case where the light-emitting layer  120  is formed by stacking a first light-emitting layer and a second light-emitting layer in this order from the hole-transport layer side, the first light-emitting layer is formed using a substance having a hole-transport property as the host material and the second light-emitting layer is formed using a substance having an electron-transport property as the host material. 
     In the light-emitting layer  120 , the host material  121  may be composed of one kind of compound or a plurality of compounds. Alternatively, the light-emitting layer  120  may contain a material other than the host material  121  and the guest material  122 . 
     &lt;&lt;Material that can be Used in Light-Emitting Layer  140 &gt;&gt; 
     In the light-emitting layer  140 , the host material  141  exists in the largest proportion in weight ratio, and the guest material  142  (phosphorescent material) is dispersed in the host material  141 . The T1 levels of the host materials  141  (organic compounds  141 _ 1  and  141 _ 2 ) of the light-emitting layer  140  are preferably higher than the T1 level of the guest material (guest material  142 ) of the light-emitting layer  140 . 
     Examples of the organic compound  141   1  include a zinc- or aluminum-based metal complex, an oxadiazole derivative, a triazole derivative, a benzimidazole derivative, a quinoxaline derivative, a dibenzoquinoxaline derivative, a dibenzothiophene derivative, a dibenzofuran derivative, a pyrimidine derivative, a triazine derivative, a pyridine derivative, a bipyridine derivative, a phenanthroline derivative, and the like Other examples are an aromatic amine, a carbazole derivative, and the like. Specifically, the electron-transport material and the hole-transport material described in Embodiment 1 can be used. 
     As the organic compound  141 _ 2 , a substance which can form an exciplex together with the organic compound  141 _ 1  is preferably used. Specifically, the electron-transport material and the hole-transport material described in Embodiment 1 can be used. In that case, it is preferable that the organic compound  141 _ 1 , the organic compound  141 _ 2 , and the guest material  142  (phosphorescent material) be selected such that the emission peak of the exciplex formed by the organic compound  141 _ 1  and the organic compound  141 _ 2  overlaps with an absorption band, specifically an absorption band on the longest wavelength side, of a triplet metal to ligand charge transfer (MLCT) transition of the guest material  142  (phosphorescent material). This makes it possible to provide a light-emitting element with drastically improved emission efficiency. Note that in the case where a thermally activated delayed fluorescent material is used instead of the phosphorescent material, it is preferable that the absorption band on the longest wavelength side be a singlet absorption band. 
     As the guest material  142  (phosphorescent material), an iridium-, rhodium-, or platinum-based organometallic complex or metal complex can be used; in particular, an organoiridium complex such as an iridium-based ortho-metalated complex is preferable. As an ortho-metalated ligand, a 4H-triazole ligand, a 1H-triazole ligand, an imidazole ligand, a pyridine ligand, a pyrimidine ligand, a pyrazine ligand, an isoquinoline ligand, and the like can be given. As the metal complex, a platinum complex having a porphyrin ligand and the like can be given. 
     Examples of the substance that has an emission peak in the blue or green wavelength range include organometallic iridium complexes having a 4H-triazole skeleton, such as tris{2-[5-(2-methylphenyl)-4-(2,6-dimethylphenyl)-4H-1,2,4-triazol-3-yl-κN2]phenyl-κC}iridium(III) (abbreviation: Ir(mpptz-dmp) 3 ), tris(5-methyl-3,4-diphenyl-4H-1,2,4-triazolato)iridium(III) (abbreviation: Ir(Mptz) 3 ), tris[4-(3-biphenyl)-5-isopropyl-3-phenyl-4H-1,2,4-triazolato]iridium(III) (abbreviation: Ir(iPrptz-3b) 3 ), and tris[3-(5-biphenyl)-5-isopropyl-4-phenyl-4H-1,2,4-triazolato]iridium(III) (abbreviation: Ir(iPr5btz) 3 ); organometallic iridium complexes having a 1H-triazole skeleton, such as tris[3-methyl-1-(2-methylphenyl)-5-phenyl-1H-1,2,4-triazolato]iridium(III) (abbreviation: Ir(Mptz1-mp) 3 ) and tris(1-methyl-5-phenyl-3-propyl-1H-1,2,4-triazolato)iridium(III) (abbreviation: Ir(Prptz1-Me) 3 ); organometallic iridium complexes having an imidazole skeleton, such as fac-tris[1-(2,6-diisopropylphenyl)-2-phenyl-1H-imidazole]iridium(III) (abbreviation: Ir(iPrpmi) 3 ) and tris[3-(2,6-dimethylphenyl)-7-methylimidazo[1,2-f]phenanthridinato]iridium(III) (abbreviation: Ir(dmpimpt-Me) 3 ); and organometallic iridium complexes in which a phenylpyridine derivative having an electron-withdrawing group is a ligand, such as 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′-bis(trifluoromethyl)phenyl]pyridinato-N,C 2′ }iridium(III)picolinate (abbreviation: Ir(CF 3 ppy) 2 (pic)), and bis[2-(4′,6′-difluorophenyl)pyridinato-N,C 2′ ]iridium(III) acetylacetonate (abbreviation: FIr(acac)). Among the materials given above, the organometallic iridium complexes having a 4H-triazole skeleton have high reliability and high emission efficiency and are thus especially preferable. 
     Examples of the substance that has an emission peak in the green or yellow wavelength range include organometallic iridium complexes having a pyrimidine skeleton, such as tris(4-methyl-6-phenylpyrimidinato)iridium(III) (abbreviation: Ir(mppm) 3 ), tris(4-t-butyl-6-phenylpyrimidinato)iridium(III) (abbreviation: Ir(tBuppm) 3 ), (acetylacetonato)bis(6-methyl-4-phenylpyrimidinato)iridium(III) (abbreviation: Ir(mppm)-(acac)), (acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III) (abbreviation: Ir(tBuppm) 2 (acac)), (acetylacetonato)bis[4-(2-norbornyl)-6-phenylpyrimidinato]iridium(III) (abbreviation: Ir(nbppm) 2 (acac)), (acetylacetonato)bis[5-methyl-6-(2-methylphenyl)-4-phenylpyrimidinato]iridium(III) (abbreviation: Ir(mpmppm) 2 (acac)), (acetylacetonato)bis{4,6-dinmethyl-2-[6-(2,6-diminethylphenyl)-4-pyrimidinyl-κN3]pheny l-κC}iridium(III) (abbreviation: Ir(dmppm-dmp) 2 (acac)), (acetylacetonato)bis(4,6-diphenylpyrimidinato)iridium(III) (abbreviation: Ir(dppm) 2 (acac)); organometallic iridium complexes having a pyrazine skeleton, such as (acetylacetonato)bis(3,5-dimethyl-2-phenylpyrazinato)iridium(III) (abbreviation: Ir(mppr-Me) 2 (acac)) and (acetylacetonato)bis(5-isopropyl-3-methyl-2-phenylpyrazinato)iridium(III) (abbreviation: Ir(mppr-iPr) 2 (acac)); organometallic iridium complexes having a pyridine skeleton, such as tris(2-phenylpyridinato-N,C 2′ )iridium(III) (abbreviation: Ir(ppy) 3 ), bis(2-phenylpyridinato-N,C 2′ )iridium(III) acetylacetonate (abbreviation: Ir(ppy) 2 (acac)), bis(benzo[h]quinolinato)iridium(III) acetylacetonate (abbreviation: Ir(bzq) 2 (acac)), tris(benzo[h]quinolinato)iridium(III) (abbreviation: Ir(bzq) 3 ), tris(2-phenylquinolinato-N,C 2′ )iridium(III) (abbreviation Ir(pq) 3 ), and bis(2-phenylquinolinato-N,C 2′ )iridium(III) acetylacetonate (abbreviation: Ir(pq) 2 (acac)); organometallic iridium complexes such as 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)), and bis(2-phenylbenzothiazolato-N,C 2′ )iridium(III)acetylacetonate (abbreviation: Ir(bt) 2 (acac)); and a rare earth metal complex such as tris(acetylacetonato)(monophenanthroline)terbium(III) (abbreviation: Tb(acac) 3 (Phen)). Among the materials given above, the organometallic iridium complexes having a pyrimidine skeleton have distinctively high reliability and emission efficiency and are thus particularly preferable. 
     Examples of the substance that has an emission peak in the yellow or red wavelength range include organometallic iridium complexes having a pyrimidine skeleton, such as (diisobutyrylmethanato)bis[4,6-bis(3-methylphenyl)pyrimidinato]iridium(III) (abbreviation: Ir(5mdppm) 2 (dibm)), bis[4,6-bis(3-methlylphenyl)pyrimidinato](dipivaloylmethanto)iridium(III) (abbreviation: Ir(5mdppm) 2 (dpm)), and bis[4,6-di(naphthalen-1-yl)pyrimidinato](dipivaloylmethanato)iridium(III) (abbreviation: Ir(d1npm) 2 (dpm)); organometallic iridium complexes having a pyrazine skeleton, such as (acetylacetonato)bis(2,3,5-triphenylpyrazinato)iridium(III) (abbreviation: Ir(tppr) 2 (acac)), bis(2,3,5-triphenylpyrazinato) (dipivaloylmethanato)iridium(III) (abbreviation: Ir(tppr) 2 (dpm)), and (acetylacetonato)bis[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(III) (abbreviation: Ir(Fdpq) 2 (acac)); organometallic iridium complexes having a pyridine skeleton, such as tris(1-phenylisoquinolinato-N,C 2′ )iridium(III) (abbreviation: Ir(piq) 3 ) and bis(1-phenylisoquinolinato-N,C 2′ )iridium(III)acetylacetonate (abbreviation: Ir(piq) 2 (acac)); a platinum complex such as 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin platinum(II) (abbreviation: PtOEP); and rare earth metal complexes such as tris(1,3-diphenyl-1,3-propanedionato)(monophenanthroline)europium(III) (abbreviation: Eu(DBM) 3 (Phen)) and tris[1-(2-thenoyl)-3,3,3-trifluoroacctonato](monophenanthroline)europium(III) (abbreviation: Eu(TTA) 3 (Phen)). Among the materials given above, the organometallic iridium complexes having a pyrimidine skeleton have distinctively high reliability and emission efficiency and are thus particularly preferable. Further, the organometallic iridium complexes having a pyrazine skeleton can provide red light emission with favorable chromaticity. 
     As the light-emitting material included in the light-emitting layer  140 , any material can be used as long as the material can convert the triplet excitation energy into light emission. As an example of the material that can convert the triplet excitation energy into light emission, a thermally activated delayed fluorescent (TADF) material can be given in addition to a phosphorescent material. Therefore, it is acceptable that the “phosphorescent material” in the description is replaced with the “thermally activated delayed fluorescence material” Note that the thermally activated delayed fluorescence material is a material having a small difference between the triplet excited energy level and the singlet excited energy level and a function of converting triplet excitcd energy into singlet excitcd energy by reverse intersystem crossing. Thus, the TADF material can up-convert a triplet excited state into a singlet excited state (i.e., reverse intersystem crossing is possible) using a little thermal energy and efficiently exhibit light emission (fluorescence) from the singlet excited state. The TADF is efficiently obtained under the condition where the difference in energy between the triplet excited energy level and the singlet excited energy level is preferably greater than 0 eV and less than or equal to 0.2 eV, further preferably greater than 0 eV and less than or equal to 0.1 eV. 
     The material that exhibits thermally activated delayed fluorescence may be a material that can form a singlet excited state by itself from a triplet excited state by reverse intersystem crossing or may be a combination of a plurality of materials which form an exciplex. 
     In the case where the material exhibiting thermally activated delayed fluorescence is formed of one kind of material, any of the thermally activated delayed fluorescent materials described in Embodiment 1 can be specifically used. 
     In the case where the thermally activated delayed fluorescent material is used as the host material, it is preferable to use a combination of two kinds of compounds which form an exciplex. In this case, it is particularly preferable to use the above-described combination of a compound which easily accepts electrons and a compound which easily accepts holes, which form an exciplex. 
     There is no limitation on the emission colors of the light-emitting materials contained in the light-emitting layers  120 ,  130 , and  140 , and they may be the same or different. Light emitted from the light-emitting materials is mixed and extracted out of the element; therefore, for example, in the case where their emission colors are complementary colors, the light-emitting element can emit white light. In consideration of the reliability of the light-emitting element, the emission peak wavelength of the light-emitting material included in the light-emitting layer  120  is preferably shorter than that of the light-emitting material included in the light-emitting layer  140 . 
     Note that the light-emitting units  106  and  108  and the charge-generation layer  115  can be formed by an evaporation method (including a vacuum evaporation method), an ink-jet method, a coating method, gravure printing, or the like. 
     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, examples of light-emitting elements having structures different from those described in Embodiments 1 and 2 are described below with reference to  FIGS. 4A and 4B ,  FIGS. 5A and 5B ,  FIGS. 6A to 6C , and  FIGS. 7A to 7C . 
     &lt;Structure Example 1 of Light-Emitting Element&gt; 
       FIGS. 4A and 4B  are cross-sectional views each illustrating a light-emitting element of one embodiment of the present invention. In  FIGS. 4A and 4B , a portion having a function similar to that in  FIG. 1A  is represented by the same hatch pattern as in  FIG. 1A  and not especially denoted by a reference numeral in some cases. In addition, common reference numerals are used for portions having similar functions, and a detailed description of the portions is omitted in some cases. 
     Light-emitting elements  270   a  and  270   b  in  FIGS. 4A and 4B  may have a bottom-emission structure in which light is extracted through the substrate  200  or may have a top-emission structure in which light emitted from the light-emitting element is extracted in the direction opposite to the substrate  200 . However, one embodiment of the present invention is not limited to this structure, and a light-emitting element having a dual-emission structure in which light emitted from the light-emitting element is extracted in both top and bottom directions of the substrate  200  may be used. 
     In the case where the light-emitting elements  270   a  and  270   b  each have a bottom emission structure, the electrode  101  preferably has a function of transmitting light and the electrode  102  preferably has a function of reflecting light. Alternatively, in the case where the light-emitting elements  270   a  and  270   b  each have a top emission structure, the electrode  101  preferably has a function of reflecting light and the electrode  102  preferably has a function of transmitting light. 
     The light-emitting elements  270   a  and  270   b  each include the electrode  101  and the electrode  102  over the substrate  200 . Between the electrodes  101  and  102 , a light-emitting layer  123 B, a light-emitting layer  123 G, and a light-emitting layer  123 R are provided. The hole-injection layer  111 , the hole-transport layer  112 , the electron-transport layer  118 , and the electron-injection layer  119  are also provided. 
     The light-emitting element  270   b  includes, as part of the electrode  101 , a conductive layer  101   a , a conductive layer  101   b  over the conductive layer  101   a , and a conductive layer  101   c  under the conductive layer  101   a . In other words, the light-emitting element  270   b  includes the electrode  101  having a structure in which the conductive layer  101   a  is sandwiched between the conductive layer  101   b  and the conductive layer  101   c.    
     In the light-emitting element  270   b , the conductive layer  101   b  and the conductive layer  101   c  may be formed with different materials or the same material. The electrode  101  preferably has a structure in which the conductive layer  101   a  is sandwiched by the layers formed of the same conductive material, in which case patterning by etching can be performed easily. 
     In the light-emitting element  270   b , the electrode  101  may include one of the conductive layer  101   b  and the conductive layer  101   c.    
     For each of the conductive layers  101   a ,  101   b , and  101   c , which are included in the electrode  101 , the structure and materials of the electrode  101  or  102  described in Embodiment 1 can be used. 
     In  FIGS. 4A and 4B , a partition wall  145  is provided between a region  221 B, a region  221 G, and a region  221 R, which are sandwiched between the electrode  101  and the electrode  102 . The partition wall  145  has an insulating property. The partition wall  145  covers end portions of the electrode  101  and has openings overlapping with the electrode. With the partition wall  145 , the electrode  101  provided over the substrate  200  in the regions can be divided into island shapes. 
     Note that the light-emitting layer  123 B and the light-emitting layer  123 G may overlap with each other in a region where they overlap with the partition wall  145 . The light-emitting layer  123 G and the light-emitting layer  123 R may overlap with each other in a region where they overlap with the partition wall  145 . The light-emitting layer  123 R and the light-emitting layer  123 B may overlap with each other in a region where they overlap with the partition wall  145 . 
     The partition wall  145  has an insulating property and is formed using an inorganic or organic material. Examples of the inorganic material include silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, aluminum oxide, and aluminum nitride. Examples of the organic material include photosensitive resin materials such as an acrylic resin and a polyimide resin. 
     The light-emitting layers  123 R,  123 G, and  123 B preferably contain light-emitting materials having functions of emitting light of different colors. For example, when the light-emitting layer  123 R contains a light-emitting material having a function of emitting red, the region  221 R emits red light. When the light-emitting layer  123 G contains a light-emitting material having a function of emitting green, the region  221 G emits green light. When the light-emitting layer  123 B contains a light-emitting material having a function of emitting blue, the region  221 B emits blue light. The light-emitting element  270   a  or  270   b  having such a structure is used in a pixel of a display device, whereby a full-color display device can be fabricated. The thicknesses of the light-emitting layers may be the same or different. 
     Any one or more of the light-emitting layers  123 B,  123 G, and  123 R preferably include the light-emitting layer  130  described in Embodiment 1, in which case a light-emitting element with high emission efficiency can be fabricated. 
     One or more of the light-emitting layers  123 B,  123 G, and  123 R may include two or more stacked layers. 
     When at least one light-emitting layer includes the light-emitting layer described in Embodiment 1 as described above and the light-emitting element  270   a  or  270   b  including the light-emitting layer is used in pixels in a display device, a display device with high emission efficiency can be fabricated. The display device including the light-emitting element  270   a  or  270   b  can thus have reduced power consumption. 
     By providing a color filter over the electrode through which light is extracted, the color purity of each of the light-emitting elements  270   a  and  270   b  can be improved. Therefore, the color purity of a display device including the light-emitting element  270   a  or  270   b  can be improved. 
     By providing a polarizing plate over the electrode through which light is extracted, the reflection of external light by each of the light-emitting elements  270   a  and  270   b  can be reduced. Therefore, the contrast ratio of a display device including the light-emitting element  270   a  or  270   b  can be improved. 
     For the other components of the light-emitting elements  270   a  and  270   b , the components of the light-emitting element in Embodiment 1 may be referred to. 
     &lt;Structure Example 2 of Light-Emitting Element&gt; 
     Next, structure examples different from the light-emitting elements illustrated in  FIGS. 4A and 4B  will be described below with reference to  FIGS. 5A and 5B . 
       FIGS. 5A and 5B  are cross-sectional views of a light-emitting element of one embodiment of the present invention. In  FIGS. 5A and 5B , a portion having a function similar to that in  FIGS. 4A and 4B  is represented by the same hatch pattern as in  FIGS. 4A and 4B  and not especially denoted by a reference numeral in some cases. In addition, common reference numerals are used for portions having similar functions, and a detailed description of such portions is not repeated in some cases. 
       FIGS. 5A and 5B  illustrate structure examples of a light-emitting element including the light-emitting layer between a pair of electrodes. A light-emitting element  272   a  illustrated in  FIG. 5A  has a top-emission structure in which light is extracted in a direction opposite to the substrate  200 , and a light-emitting element  272   b  illustrated in  FIG. 5B  has a bottom-emission structure in which light is extracted to the substrate  200  side. However, one embodiment of the present invention is not limited to these structures and may have a dual-emission structure in which light emitted from the light-emitting element is extracted in both top and bottom directions with respect to the substrate  200  over which the light-emitting element is formed. 
     The light-emitting elements  272   a  and  272   b  each include the electrode  101 , the electrode  102 , an electrode  103 , and an electrode  104  over the substrate  200 . At least a light-emitting layer  130  and the charge-generation layer  115  are provided between the electrode  101  and the electrode  102 , between the electrode  102  and the electrode  103 , and between the electrode  102  and the electrode  104 . The hole-injection layer  111 , the hole-transport layer  112 , a light-emitting layer  150 , the electron-transport layer  113 , the electron-injection layer  114 , the hole-injection layer  116 , the hole-transport layer  117 , the electron-transport layer  118 , and the electron-injection layer  119  are further provided. 
     The electrode  101  includes a conductive layer  101   a  and a conductive layer  101   b  over and in contact with the conductive layer  101   a  The electrode  103  includes a conductive layer  103   a  and a conductive layer  103   b  over and in contact with the conductive layer  103   a . The electrode  104  includes a conductive layer  104   a  and a conductive layer  104   b  over and in contact with the conductive layer  104   a.    
     The light-emitting element  272   a  illustrated in  FIG. 5A  and the light-emitting element  272   b  illustrated in  FIG. 5B  each include a partition wall  145  between a region  222 B sandwiched between the electrode  101  and the electrode  102 , a region  222 G sandwiched between the electrode  102  and the electrode  103 , and a region  222 R sandwiched between the electrode  102  and the electrode  104 . The partition wall  145  has an insulating property. The partition wall  145  covers end portions of the electrodes  101 ,  103 , and  104  and has openings overlapping with the electrodes With the partition wall  145 , the electrodes provided over the substrate  200  in the regions can be separated into island shapes. 
     The light-emitting elements  272   a  and  272   b  each include a substrate  220  provided with an optical element  224 B, an optical element  224 G, and an optical element  224 R in the direction in which light emitted from the region  222 B, light emitted from the region  222 G, and light emitted from the region  222 R are extracted. The light emitted from each region is emitted outside the light-emitting element through each optical element. In other words, the light from the region  222 B, the light from the region  222 G, and the light from the region  222 R are emitted through the optical element  224 B, the optical element  224 G, and the optical element  224 R, respectively. 
     The optical elements  224 B,  224 G, and  224 R each have a function of selectively transmitting light of a particular color out of incident light. For example, the light emitted from the region  222 B through the optical element  224 B is blue light, the light emitted from the region  222 G through the optical element  224 G is green light, and the light emitted from the region  222 R through the optical element  224 R is red light. 
     For example, a coloring layer (also referred to as color filter), a hand pass filter, a multilayer filter, or the like can be used for the optical elements  224 R,  224 G, and  224 B. Alternatively, color conversion elements can be used as the optical elements. A color conversion element is an optical element that converts incident light into light having a longer wavelength than the incident light. As the color conversion elements, quantum-dot elements can be favorably used. The usage of the quantum-dot type can increase color reproducibility of the display device. 
     A plurality of optical elements may also be stacked over each of the optical elements  224 R,  224 G, and  224 B. As another optical element, a circularly polarizing plate, an anti-reflective film, or the like can be provided, for example. A circularly polarizing plate provided on the side where light emitted from the light-emitting element of the display device is extracted can prevent a phenomenon in which light entering from the outside of the display device is reflected inside the display device and returned to the outside. An anti-reflective film can weaken external light reflected by a surface of the display device. This leads to clear observation of light emitted from the display device. 
     Note that in  FIGS. 5A and 5B , blue light (B), green light (G), and red light (R) emitted from the regions through the optical elements are schematically illustrated by arrows of dashed lines. 
     A light-blocking layer  223  is provided between the optical elements. The light-blocking layer  223  has a function of blocking light emitted from the adjacent regions. Note that a structure without the light-blocking layer  223  may also be employed. 
     The light-blocking layer  223  has a function of reducing the reflection of external light. The light-blocking layer  223  has a function of preventing mixture of light emitted from an adjacent light-emitting element. As the light-blocking layer  223 , a metal, a resin containing black pigment, carbon black, a metal oxide, a composite oxide containing a solid solution of a plurality of metal oxides, or the like can be used. 
     For the substrate  200  and the substrate  220  provided with the optical elements, the substrate in Embodiment 1 may be referred to. 
     Furthermore, the light-emitting elements  272   a  and  272   b  have a microcavity structure. 
     Light emitted from the light-emitting layer  130  and the light-emitting layer  150  resonates between a pair of electrodes (e.g., the electrode  101  and the electrode  102 ). The light-emitting layer  130  and the light-emitting layer  150  are formed at such a position as to intensify the light of a desired wavelength among light to be emitted. For example, by adjusting the optical length from a reflective region of the electrode  101  to the light-emitting region of the light-emitting layer  130  and the optical length from a reflective region of the electrode  102  to the light-emitting region of the light-emitting layer  130 , the light of a desired wavelength among light emitted from the light-emitting layer  130  can be intensified. By adjusting the optical length from the reflective region of the electrode  101  to the light-emitting region of the light-emitting layer  150  and the optical length from the reflective region of the electrode  102  to the light-emitting region of the light-emitting layer  150 , the light of a desired wavelength among light emitted from the light-emitting layer  150  can be intensified. In the case of a light-emitting element in which a plurality of light-emitting layers (here, the light-emitting layers  130  and  150 ) are stacked, the optical lengths of the light-emitting layers  130  and  150  are preferably optimized. 
     In each of the light-emitting elements  272   a  and  272   b , by adjusting the thicknesses of the conductive layers (the conductive layer  101   b , the conductive layer  103   b , and the conductive layer  104   b ) in each region, the light of a desired wavelength among light emitted from the light-emitting layers  130  and  150  can be increased. Note that the thickness of at least one of the hole-injection layer  111  and the hole-transport layer  112  may differ between the regions to increase the light emitted from the light-emitting layers  130  and  150 . 
     For example, in the case where the refractive index of the conductive material having a function of reflecting light in the electrodes  101  to  104  is lower than the refractive index of the light-emitting layer  130  or  150 , the thickness of the conductive layer  101   b  of the electrode  101  is adjusted so that the optical length between the electrode  101  and the electrode  102  is m B λ B /2 (m B  is a natural number and λ B  is the wavelength of light intensified in the region  2223 ). Similarly, the thickness of the conductive layer  103   b  of the electrode  103  is adjusted so that the optical length between the electrode  103  and the electrode  102  is m G λ G /2 (m G  is a natural number and λ G  is the wavelength of light intensified in the region  222 G). Furthermore, the thickness of the conductive layer  104   b  of the electrode  104  is adjusted so that the optical length between the electrode  104  and the electrode  102  is m R λ R /2 (m R  is a natural number and λ R  is the wavelength of light intensified in the region  222 R). 
     In the above manner, with the microcavity structure, in which the optical length between the pair of electrodes in the respective regions is adjusted, scattering and absorption of light in the vicinity of the electrodes can be suppressed, resulting in high light extraction efficiency. In the above structure, the conductive layers  101   b ,  103   b , and  104   b  preferably have a function of transmitting light. The materials of the conductive layers  101   b ,  103   b , and  104   b  may be the same or different. Each of the conductive layers  101   b ,  103   b , and  104   b  may have a stacked structure of two or more layers. 
     Since the light-emitting element  272   a  illustrated in  FIG. 5A  has a top-emission structure, it is preferable that the conductive layer  101   a , the conductive layer  103   a , and the conductive layer  104   a  have a function of reflecting light. In addition, it is preferable that the electrode  102  have functions of transmitting light and reflecting light. 
     Since the light-emitting element  272   b  illustrated in  FIG. 5B  has a bottom-emission structure, it is preferable that the conductive layer  101   a , the conductive layer  103   a , and the conductive layer  104   a  have functions of transmitting light and reflecting light. In addition, it is preferable that the electrode  102  have a function of reflecting light. 
     In each of the light-emitting elements  272   a  and  272   b , the conductive layers  101   a ,  103   a , and  104   a  may be formed of different materials or the same material. When the conductive layers  101   a .  103   a , and  104   a  are formed of the same material, manufacturing cost of the light-emitting elements  272   a  and  272   b  can be reduced. Note that each of the conductive layers  101   a ,  103   a , and  104   a  may have a stacked structure including two or more layers. 
     The light-emitting layer  130  in the light-emitting elements  272   a  and  272   b  preferably has the structure described in Embodiment 1, in which case light-emitting elements with high emission efficiency can be fabricated. 
     Either or both of the light-emitting layers  130  and  150  may have a stacked structure of two layers, like a light-emitting layer  150   a  and a light-emitting layer  150   b . The two light-emitting layers including two kinds of light-emitting materials (a first light-emitting material and a second light-emitting material) for emitting different colors of light enable light emission of a plurality of colors. It is particularly preferable to select the light-emitting materials of the light-emitting layers so that white light can be obtained by combining light emissions from the light-emitting layers  130  and  150 . 
     Either or both of the light-emitting layers  130  and  150  may have a stacked structure of three or more layers, in which a layer not including a light-emitting material may be included. 
     In the above-described manner, the light-emitting element  272   a  or  272   b  including the light-emitting laver which has the structure described in Embodiment 1 is used in pixels in a display device, whereby a display device with high emission efficiency can be fabricated. Accordingly, the display device including the light-emitting element  272   a  or  272   b  can have low power consumption. 
     For the other components of the light-emitting elements  272   a  and  272   b , the components of the light-emitting element  270   a  or  270   b  or the light-emitting element in Embodiment 1 or 2 may be referred to. 
     &lt;Fabrication Method of Light-Emitting Element&gt; 
     Next, a method for fabricating a light-emitting element of one embodiment of the present invention is described below with reference to  FIGS. 6A to 6C  and  FIGS. 7A to 7C . Here, a method for fabricating the light-emitting element  272   a  illustrated in  FIG. 5A  is described. 
       FIGS. 6A to 6C  and  FIGS. 7A to 7C  are cross-sectional views illustrating a method for fabricating the light-emitting element of one embodiment of the present invention. 
     The method for manufacturing the light-emitting element  272   a  described below includes first to seventh steps. 
     &lt;&lt;First Step&gt;&gt; 
     In the first step, the electrodes (specifically the conductive layer  101   a  of the electrode  101 , the conductive layer  103   a  of the electrode  103 , and the conductive layer  104   a  of the electrode  104 ) of the light-emitting elements are formed over the substrate  200  (see  FIG. 6A ). 
     In this embodiment, a conductive layer having a function of reflecting light is formed over the substrate  200  and processed into a desired shape; whereby the conductive layers  101   a ,  103   a , and  104   a  are formed. As the conductive layer having a function of reflecting light, an alloy film of silver, palladium, and copper (also referred to as an Ag—Pd—Cu film and APC) is used. The conductive layers  101   a ,  103   a , and  104   a  are preferably formed through a step of processing the same conductive layer, because the manufacturing cost can be reduced. 
     Note that a plurality of transistors may be formed over the substrate  200  before the first step. The plurality of transistors may be electrically connected to the conductive layers  101   a ,  103   a , and  104   a.    
     &lt;&lt;Second Step&gt;&gt; 
     In the second step, the transparent conductive layer  101   b  having a function of transmitting light is formed over the conductive layer  101   a  of the electrode  101 , the transparent conductive layer  103   b  having a function of transmitting light is formed over the conductive layer  103   a  of the electrode  103 , and the transparent conductive layer  104   b  having a function of transmitting light is formed over the conductive layer  104   a  of the electrode  104  (see  FIG. 61B ). 
     In this embodiment, the conductive layers  101   b ,  103   b , and  104   b  each having a function of transmitting light are formed over the conductive layers  101   a ,  103   a , and  104   a  each having a function of reflecting light, respectively, whereby the electrode  101 , the electrode  103 , and the electrode  104  are formed. As the conductive layers  101   b ,  103   b , and  104   b , ITSO films are used. 
     The conductive layers  101   b ,  103   b , and  104   b  having a function of transmitting light may be formed through a plurality of steps. When the conductive layers  101   b ,  103   b , and  104   b  having a function of transmitting light are formed through a plurality of steps, they can be formed to have thicknesses which enable microcavity structures appropriate in the respective regions. 
     &lt;&lt;Third Step&gt;&gt; 
     In the third step, the partition wall  145  that covers end portions of the electrodes of the light-emitting element is formed (see  FIG. 6C ). 
     The partition wall  145  includes an opening overlapping with the electrode. The conductive film exposed by the opening functions as the anode of the light-emitting element. As the partition wall  145 , a polyimide-based resin is used in this embodiment. 
     In the first to third steps, since there is no possibility of damaging the EL layer (a layer containing an organic compound), a variety of film formation methods and fine processing technologies can be employed. In this embodiment, a reflective conductive layer is formed by a sputtering method, a pattern is formed over the conductive layer by a lithography method, and then the conductive layer is processed into an island shape by a dry etching method or a wet etching method to form the conductive layer  101   a  of the electrode  101 , the conductive layer  103   a  of the electrode  103 , and the conductive layer  104   a  of the electrode  104 . Then, a transparent conductive film is formed by a sputtering method, a pattern is formed over the transparent conductive film by a lithography method, and then the transparent conductive film is processed into island shapes by a wet etching method to form the electrodes  101 ,  103 , and  104 . 
     &lt;&lt;Fourth Step&gt;&gt; 
     In the fourth step, the hole-injection layer  111 , the hole-transport layer  112 , the light-emitting layer  150 , the electron-transport layer  113 , the electron-injection layer  114 , and the charge-generation layer  115  are formed (see  FIG. 7A ). 
     The hole-injection layer  111  can be formed by co-evaporating a hole-transport material and a material containing an acceptor substance. Note that a co-evaporation method is an evaporation method in which a plurality of different substances is concurrently vaporized from respective different evaporation sources. The hole-transport layer  112  can be formed by evaporating a hole-transport material. 
     The light-emitting layer  150  can be formed by evaporating the guest material that emits light of at least one of green, yellow green, yellow, orange, and red. As the guest material, a fluorescent or phosphorescent organic compound can be used. In addition, the light-emitting layer having any of the structures described in Embodiments 1 and 2 is preferably used. The light-emitting layer  150  may have a two-layer structure. In that case, the two light-emitting layers preferably contain light-emitting substances that emit light of different colors. 
     The electron-transport layer  113  can be formed by evaporating a substance having a high electron-transport property. The electron-injection layer  114  can be formed by evaporating a substance having a high electron-injection property. 
     The charge-generation layer  115  can be formed by evaporating a material obtained by adding an electron acceptor (acceptor) to a hole-transport material or a material obtained by adding an electron donor (donor) to an electron-transport material. 
     &lt;&lt;Fifth Step&gt;&gt; 
     In the fifth step, the hole-injection layer  116 , the hole-transport layer  117 , the light-emitting layer  130 , the electron-transport layer  118 , the electron-injection layer  119 , and the electrode  102  are formed (see  FIG. 7B ). 
     The hole-injection layer  116  can be formed by using a material and a method which are similar to those of the hole-injection layer  111 . The hole-transport layer  117  can be formed by using a material and a method which are similar to those of the hole-transport layer  112 . 
     The light-emitting layer  130  can be formed by evaporating the guest material that emits light of at least one color selected from violet, blue, and blue green. As the guest material, a fluorescent organic compound can be used. The fluorescent organic compound may be evaporated alone or the fluorescent organic compound mixed with another material may be evaporated. For example, the fluorescent organic compound may be used as a guest material, and the guest material may be dispersed into a host material having higher excitation energy than the guest material. 
     The electron-transport layer  118  can be formed by using a material and a method which are similar to those of the electron-transport layer  113 . The electron-injection layer  119  can be formed by using a material and a method which are similar to those of the electron-injection layer  114 . 
     The electrode  102  can be formed by stacking a reflective conductive film and a light-transmitting conductive film. The electrode  102  may have a single-layer structure or a stacked-layer structure. 
     Through the above-described steps, the light-emitting element including the region  222 B, the region  222 G, and the region  222 R over the electrode  101 , the electrode  103 , and the electrode  104 , respectively, are formed over the substrate  200 . 
     &lt;&lt;Sixth Step&gt;&gt; 
     In the sixth step, the light-blocking layer  223 , the optical element  224 B, the optical element  224 G, and the optical element  224 R are formed over the substrate  220  (see  FIG. 7C ). 
     As the light-blocking layer  223 , a resin film containing black pigment is formed in a desired region. Then, the optical element  224 B, the optical element  224 G, and the optical element  224 R are formed over the substrate  220  and the light-blocking layer  223 . As the optical element  224 B, a resin film containing blue pigment is formed in a desired region. As the optical element  224 G, a resin film containing green pigment is formed in a desired region. As the optical element  224 R, a resin film containing red pigment is formed in a desired region. 
     &lt;&lt;Seventh Step&gt;&gt; 
     In the seventh step, the light-emitting element formed over the substrate  200  is attached to the light-blocking layer  223 , the optical element  2241 , the optical element  224 G, and the optical element  224 R formed over the substrate  220 , and sealed with a sealant (not illustrated). 
     Through the above-described steps, the light-emitting element  272   a  illustrated in  FIG. 5A  can be formed. 
     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 display device of one embodiment of the present invention will be described below with reference to  FIGS. 8A and 8B ,  FIGS. 9A and 9B ,  FIG. 10 ,  FIGS. 11A and 11B ,  FIGS. 12A and 12B ,  FIG. 13 , and  FIGS. 14A and 14B . 
     &lt;Structure Example 1 of Display Device&gt; 
       FIG. 8A  is a top view illustrating a display device  600  and  FIG. 8B  is a cross-sectional view taken along the dashed-dotted line A-B and the dashed-dotted line C-D in  FIG. 8A . The display device  600  includes driver circuit portions (a signal line driver circuit portion  601  and a scan line driver circuit portion  603 ) and a pixel portion  602 . Note that the signal line driver circuit portion  601 , the scan line driver circuit portion  603 , and the pixel portion  602  have a function of controlling light emission of a light-emitting element. 
     The display device  600  also includes an element substrate  610 , a sealing substrate  604 , a sealant  605 , a region  607  surrounded by the sealant  605 , a lead wiring  608 , and an FPC  609 . 
     Note that the lead wiring  608  is a wiring for transmitting signals to be input to the signal line driver circuit portion  601  and the scan line driver circuit portion  603  and for receiving a video signal, a clock signal, a start signal, a reset signal, and the like from the FPC  609  serving as an external input terminal. Although only the FPC  609  is illustrated here, the FPC  609  may be provided with a printed wiring board (PWB). 
     As the signal line driver circuit portion  601 , a CMOS circuit in which an n-channel transistor  623  and a p-channel transistor  624  are combined is formed. As the signal line driver circuit portion  601  or the scan line driver circuit portion  603 , various types of circuits such as a CMOS circuit, a PMOS circuit, or an NMOS circuit can be used. Although a driver in which a driver circuit portion is formed and a pixel are formed over the same surface of a substrate in the display device of this embodiment, the driver circuit portion is not necessarily formed over the substrate and can be formed outside the substrate. 
     The pixel portion  602  includes a switching transistor  611 , a current control transistor  612 , and a lower electrode  613  electrically connected to a drain of the current control transistor  612 . Note that a partition wall  614  is formed to cover end portions of the lower electrode  613 . As the partition wall  614 , for example, a positive type photosensitive acrylic resin film can be used. 
     In order to obtain favorable coverage by a film which is formed over the partition wall  614 , the partition wall  614  is formed to have a curved surface with curvature at its upper or lower end portion. For example, in the case of using a positive photosensitive acrylic as a material of the partition wall  614 , it is preferable that only the upper end portion of the partition wall  614  have a curved surface with curvature (the radius of the curvature being 0.2 μm to 3 μm). As the partition wall  614 , either a negative photosensitive resin or a positive photosensitive resin can be used. 
     Note that there is no particular limitation on a structure of each of the transistors (the transistors  611 ,  612 ,  623 , and  624 ). For example, a staggered transistor can be used. In addition, there is no particular limitation on the polarity of these transistors. For these transistors, n-channel and p-channel transistors may be used, or either n-channel transistors or p-channel transistors may be used, for example. Furthermore, there is no particular limitation on the crystallinity of a semiconductor film used for these transistors. For example, an amorphous semiconductor film or a crystalline semiconductor film may be used. Examples of a semiconductor material include Group 14 semiconductors (e.g., a semiconductor including silicon), compound semiconductors (including oxide semiconductors), organic semiconductors, and the like. For example, it is preferable to use an oxide semiconductor that has an energy gap of 2 eV or more, preferably 2.5 eV or more and further preferably 3 eV or more, for the transistors, so that the off-state current of the transistors can be reduced. Examples of the oxide semiconductor include an In—Ga oxide and an In-M-Zn oxide (M is aluminum (Al), gallium (Ga), yttrium (Y), zirconium (Zr), lanthanum (La), cerium (Ce), tin (Sn), hafnium (Hf), or neodymium (Nd)). 
     An EL layer  616  and an upper electrode  617  are formed over the lower electrode  613 . Here, the lower electrode  613  functions as an anode and the upper electrode  617  functions as a cathode. 
     In addition, the EL layer  616  is formed by various methods such as an evaporation method with an evaporation mask, an ink-jet method, or a spin coating method. As another material included in the EL layer  616 , a low molecular compound or a high molecular compound (including an oligomer or a dendrimer) may be used. 
     Note that a light-emitting element  618  is formed with the lower electrode  613 , the EL layer  616 , and the upper electrode  617 . The light-emitting element  618  has any of the structures described in Embodiments 1 to 3. In the case where the pixel portion includes a plurality of light-emitting elements, the pixel portion may include both any of the light-emitting elements described in Embodiments 1 to 3 and a light-emitting element having a different structure. 
     When the sealing substrate  604  and the element substrate  610  are attached to each other with the sealant  605 , the light-emitting element  618  is provided in the region  607  surrounded by the element substrate  610 , the sealing substrate  604 , and the sealant  605 . The region  607  is filled with a filler. In some cases, the region  607  is filled with an inert gas (nitrogen, argon, or the like) or filled with an ultraviolet curable resin or a thermosetting resin which can be used for the sealant  605 . For example, a polyvinyl chloride (PVC)-based resin, an acrylic-based resin, a polyimide-based resin, an epoxy-based resin, a silicone-based resin, a polyvinyl butyral (PVB)-based resin, or an ethylene vinyl acetate (EVA)-based resin can be used. It is preferable that the sealing substrate be provided with a recessed portion and the desiccant be provided in the recessed portion, in which case deterioration due to influence of moisture can be inhibited. 
     An optical element  621  is provided below the sealing substrate  604  to overlap with the light-emitting element  618 . A light-blocking layer  622  is provided below the sealing substrate  604 . The structures of the optical element  621  and the light-blocking layer  622  can be the same as those of the optical element and the light-blocking layer in Embodiment 3, respectively. 
     An epoxy-based resin or glass frit is preferably used for the sealant  605 . It is preferable that such a material do not transmit moisture or oxygen as much as possible. As the sealing substrate  604 , a glass substrate, a quartz substrate, or a plastic substrate formed of fiber reinforced plastic (FRP), poly(vinyl fluoride) (PVF), polyester, acrylic, or the like can be used. 
     In the above-described manner, the display device including any of the light-emitting elements and the optical elements which are described in Embodiments 1 to 3 can be obtained 
     &lt;Structure Example 2 of Display Device&gt; 
     Next, another example of the display device is described with reference to  FIGS. 9A and 9B  and  FIG. 10 . Note that  FIGS. 9A and 9B  and  FIG. 10  are each a cross-sectional view of a display device of one embodiment of the present invention. 
     In  FIG. 9A , a substrate  1001 , a base insulating film  1002 , a gate insulating film  1003 , gate electrodes  1006 ,  1007 , and  1008 , a first interlayer insulating film  1020 , a second interlayer insulating film  1021 , a peripheral portion  1042 , a pixel portion  1040 , a driver circuit portion  1041 , lower electrodes  1024 R,  1024 G, and  1024 B of light-emitting elements, a partition wall  1025 , an EL layer  1028 , an upper electrode  1026  of the light-emitting elements, a sealing layer  1029 , a sealing substrate  1031 , a sealant  1032 , and the like are illustrated. 
     In  FIG. 9A , examples of the optical elements, coloring layers (a red coloring layer  1034 R, a green coloring layer  1034 G, and a blue coloring layer  1034 B) are provided on a transparent base material  1033 . Further, a light-blocking layer  1035  may be provided. The transparent base material  1033  provided with the coloring layers and the light-blocking layer is positioned and fixed to the substrate  1001 . Note that the coloring layers and the light-blocking layer are covered with an overcoat layer  1036 . In the structure in  FIG. 9A , red light, green light, and blue light transmit the coloring layers, and thus an image can be displayed with the use of pixels of three colors. 
       FIG. 9B  illustrates an example in which, as examples of the optical elements, the coloring layers (the red coloring layer  1034 R, the green coloring layer  1034 G, and the blue coloring layer  1034 B) are provided between the gate insulating film  1003  and the first interlayer insulating film  1020 . As in this structure, the coloring layers may be provided between the substrate  1001  and the sealing substrate  1031 . 
       FIG. 10  illustrates an example in which, as examples of the optical elements, the coloring layers (the red coloring layer  1034 R, the green coloring layer  1034 G, and the blue coloring layer  1034 R) are provided between the first interlayer insulating film  1020  and the second interlayer insulating film  1021 . As in this structure, the coloring layers may be provided between the substrate  1001  and the sealing substrate  1031 . 
     The above-described display device has a structure in which light is extracted from the substrate  1001  side where the transistors are formed (a bottom-emission structure), but may have a structure in which light is extracted from the sealing substrate  1031  side (a top-emission structure). 
     &lt;Structure Example 3 of Display Device&gt; 
       FIGS. 11A and 11D  are each an example of a cross-sectional view of a display device having a top emission structure. Note that  FIGS. 11A and 11B  are each a cross-sectional view illustrating the display device of one embodiment of the present invention, and the driver circuit portion  1041 , the peripheral portion  1042 , and the like, which are illustrated in  FIGS. 9A and 9B  and  FIG. 10 , are not illustrated therein. 
     In this case, as the substrate  1001 , a substrate that does not transmit light can be used. The process up to the step of forming a connection electrode which connects the transistor and the anode of the light-emitting element is performed in a manner similar to that of the display device having a bottom-emission structure. Then, a third interlayer insulating film  1037  is formed to cover an electrode  1022 . This insulating film may have a planarization function. The third interlayer insulating film  1037  can be formed by using a material similar to that of the second interlayer insulating film, or can be formed by using any other known materials. 
     The lower electrodes  1024 R,  1024 G, and  1024 B of the light-emitting elements each function as an anode here, but may function as a cathode. Further, in the case of a display device having a top-emission structure as illustrated in  FIGS. 11A and 11B , the lower electrodes  1024 R,  1024 G, and  1024 B preferably have a function of reflecting light. The upper electrode  1026  is provided over the EL layer  1028 . It is preferable that the upper electrode  1026  have a function of reflecting light and a function of transmitting light and that a microcavity structure be used between the upper electrode  1026  and the lower electrodes  1024 R,  1024 G, and  1024 B, in which case the intensity of light having a specific wavelength is increased. 
     In the case of a top-emission structure as illustrated in  FIG. 11A , sealing can be performed with the sealing substrate  1031  on which the coloring layers (the red coloring layer  1034 R, the green coloring layer  1034 G, and the blue coloring layer  1034 B) are provided. The sealing substrate  1031  may be provided with the light-blocking layer  1035  which is positioned between pixels. Note that a light-transmitting substrate is favorably used as the sealing substrate  1031 . 
       FIG. 11A  illustrates the structure provided with the light-emitting elements and the coloring layers for the light-emitting elements as an example; however, the structure is not limited thereto. For example, as shown in  FIG. 11B , a structure including the red coloring layer  1034 R and the blue coloring layer  1034 B but not including a green coloring layer may be employed to achieve full color display with the three colors of red, green, and blue. The structure as illustrated in  FIG. 11A  where the light-emitting elements are provided with the coloring layers is effective to suppress reflection of external light. In contrast, the structure as illustrated in  FIG. 11B  where the light-emitting elements are provided with the red coloring layer and the blue coloring layer and without the green coloring layer is effective to reduce power consumption because of small energy loss of light emitted from the green light-emitting element. 
     &lt;Structure Example 4 of Display Device&gt; 
     Although a display device including sub-pixels of three colors (red, green, and blue) is described above, the number of colors of sub-pixels may be four (red, green, blue, and yellow, or red, green, blue, and white).  FIGS. 12A and 12B ,  FIG. 13 , and  FIGS. 14A and 14B  illustrate structures of display devices each including the lower electrodes  1024 R,  1024 G,  1024 B, and  1024 Y.  FIGS. 12A and 12B  and  FIG. 13  each illustrate a display device having a structure in which light is extracted from the substrata  1001  side on which transistors are formed (bottom-emission structure), and  FIGS. 14A and 14B  each illustrate a display device having a structure in which light is extracted from the sealing substrate  1031  side (top-emission structure). 
       FIG. 12A  illustrates an example of a display device in which optical elements (the coloring layer  1034 R, the coloring layer  1034 G, the coloring layer  1034 B, and a coloring layer  1034 Y) are provided on the transparent base material  1033 .  FIG. 12B  illustrates an example of a display device in which optical elements (the coloring layer  1034 R, the coloring layer  1034 G, and the coloring layer  1034 B) are provided between the gate insulating film  1003  and the first interlayer insulating film  1020 .  FIG. 13  illustrates an example of a display device in which optical elements (the coloring layer  1034 R, the coloring layer  1034 G, the coloring layer  1034 B, and the coloring layer  1034 Y) are provided between the first interlayer insulating film  1020  and the second interlayer insulating film  1021 . 
     The coloring layer  1034 R transmits red light, the coloring layer  1034 G transmits green light, and the coloring layer  1034 B transmits blue light. The coloring layer  1034 Y transmits yellow light or transmits light of a plurality of colors selected from blue, green, yellow, and red. When the coloring layer  1034 Y can transmit light of a plurality of colors selected from blue, green, yellow, and red, light released from the coloring layer  1034 Y may be white light. Since the light-emitting element which transmits yellow or white light has high emission efficiency, the display device including the coloring layer  1034 Y can have lower power consumption. 
     In the top-emission display devices illustrated in  FIGS. 14A and 14B , a light-emitting element including the lower electrode  1024 Y preferably has a microcavity structure between the upper electrode  1026  and the lower electrodes  1024 R,  1024 G,  1024 B, and  1024 Y as in the display device illustrated in  FIG. 11A . In the display device illustrated in  FIG. 14A , sealing can be performed with the sealing substrate  1031  on which the coloring layers (the red coloring layer  1034 R, the green coloring layer  1034 G, the blue coloring layer  1034 B, and the yellow coloring layer  1034 Y) are provided. 
     Light emitted through the microcavity and the yellow coloring layer  1034 Y has an emission spectrum in a yellow region. Since yellow is a color with a high luminosity factor, a light-emitting element emitting yellow light has high emission efficiency. Therefore, the display device of  FIG. 14A  can reduce power consumption. 
       FIG. 14A  illustrates the structure provided with the light-emitting elements and the coloring layers for the light-emitting elements as an example; however, the structure is not limited thereto. For example, as shown in  FIG. 14B , a structure including the red coloring layer  1034 R, the green coloring layer  1034 G, and the blue coloring layer  1034 B but not including a yellow coloring layer may be employed to achieve full color display with the four colors of red, green, blue, and yellow or of red, green, blue, and white. The structure as illustrated in  FIG. 14A  where the light-emitting elements are provided with the coloring layers is effective to suppress reflection of external light. In contrast, the structure as illustrated in  FIG. 14B  where the light-emitting elements are provided with the red coloring layer, the green coloring layer, and the blue coloring layer and without the yellow coloring layer is effective to reduce power consumption because of small energy loss of light emitted from the yellow or white light-emitting element. 
     The structure described in this embodiment can be combined with any of the structures in this embodiment and the other embodiments. 
     Embodiment 5 
     In this embodiment, a display device including a light-emitting element of one embodiment of the present invention will be described with reference to  FIGS. 15A and 15B ,  FIGS. 16A and 16B , and  FIGS. 17A and 17B . 
       FIG. 15A  is a block diagram illustrating the display device of one embodiment of the present invention, and  FIG. 15B  is a circuit diagram illustrating a pixel circuit of the display device of one embodiment of the present invention. 
     &lt;Description of Display Device&gt; 
     The display device illustrated in  FIG. 15A  includes a region including pixels of display elements (the region is hereinafter referred to as a pixel portion  802 ), a circuit portion provided outside the pixel portion  802  and including circuits for driving the pixels (the portion is hereinafter referred to as a driver circuit portion  804 ), circuits having a function of protecting elements (the circuits are hereinafter referred to as protection circuits  806 ), and a terminal portion  807 . Note that the protection circuits  806  are not necessarily provided. 
     A part or the whole of the driver circuit portion  804  is preferably formed over a substrate over which the pixel portion  802  is formed, in which case the number of components and the number of terminals can be reduced. When a part or the whole of the driver circuit portion  804  is not formed over the substrate over which the pixel portion  802  is formed, the part or the whole of the driver circuit portion  804  can be mounted by COG or tape automated bonding (TAB). 
     The pixel portion  802  includes a plurality of circuits for driving display elements arranged in X rows (X is a natural number of 2 or more) and Y columns (Y is a natural number of 2 or more) (such circuits are hereinafter referred to as pixel circuits  801 ). The driver circuit portion  804  includes driver circuits such as a circuit for supplying a signal (scan signal) to select a pixel (the circuit is hereinafter referred to as a scan line driver circuit  804   a ) and a circuit for supplying a signal (data signal) to drive a display element in a pixel (the circuit is hereinafter referred to as a signal line driver circuit  804   b ). 
     The scan line driver circuit  804   a  includes a shift register or the like. Through the terminal portion  807 , the scan line driver circuit  804   a  receives a signal for driving the shift register and outputs a signal. For example, the scan line driver circuit  804   a  receives a start pulse signal, a clock signal, or the like and outputs a pulse signal. The scan line driver circuit  804   a  has a function of controlling the potentials of wirings supplied with scan signals (such wirings are hereinafter referred to as scan lines GL_ 1  to GL_X). Note that a plurality of scan line driver circuits  804   a  may be provided to control the scan lines GL_ 1  to GL_X separately. Alternatively, the scan line driver circuit  804   a  has a function of supplying an initialization signal. Without being limited thereto, the scan line driver circuit  804   a  can supply another signal. 
     The signal line driver circuit  804   b  includes a shift register or the like. The signal line driver circuit  804   b  receives a signal (image signal) from which a data signal is derived, as well as a signal for driving the shift register, through the terminal portion  807 . The signal line driver circuit  804   b  has a function of generating a data signal to be written to the pixel circuit  801  which is based on the image signal. In addition, the signal line driver circuit  804   b  has a function of controlling output of a data signal in response to a pulse signal produced by input of a start pulse signal, a clock signal, or the like. Furthermore, the signal line driver circuit  804   b  has a function of controlling the potentials of wirings supplied with data signals (such wirings are hereinafter referred to as data lines DL_ 1  to DL_Y). Alternatively, the signal line driver circuit  804   b  has a function of supplying an initialization signal. Without being limited thereto, the signal line driver circuit  804   b  can supply another signal. 
     The signal line driver circuit  804   b  includes a plurality of analog switches or the like, for example. The signal line driver circuit  804   b  can output, as the data signals, signals obtained by time-dividing the image signal by sequentially turning on the plurality of analog switches. The signal line driver circuit  804   b  may include a shift register or the like. 
     A pulse signal and a data signal are input to each of the plurality of pixel circuits  801  through one of the plurality of scan lines GL supplied with scan signals and one of the plurality of data lines DL supplied with data signals, respectively. Writing and holding of the data signal to and in each of the plurality of pixel circuits  801  are controlled by the scan line driver circuit  804   a . For example, to the pixel circuit  801  in the m-th row and the n-th column (m is a natural number of less than or equal to X, and n is a natural number of less than or equal to Y), a pulse signal is input from the scan line driver circuit  804   a  through the scan line GL_m, and a data signal is input from the signal line driver circuit  804   b  through the data line DL n in accordance with the potential of the scan line GL_m. 
     The protection circuit  806  shown in  FIG. 15A  is connected to, for example, the scan line GL between the scan line driver circuit  804   a  and the pixel circuit  801 . Alternatively, the protection circuit  806  is connected to the data line DL between the signal line driver circuit  804   b  and the pixel circuit  801 . Alternatively, the protection circuit  806  can be connected to a wiring between the scan line driver circuit  804   a  and the terminal portion  807 . Alternatively, the protection circuit  806  can be connected to a wiring between the signal line driver circuit  804   b  and the terminal portion  807 . Note that the terminal portion  807  means a portion having terminals for inputting power, control signals, and image signals to the display device from external circuits. 
     The protection circuit  806  is a circuit that electrically connects a wiring connected to the protection circuit to another wiring when a potential out of a certain range is applied to the wiring connected to the protection circuit. 
     As illustrated in  FIG. 15A , the protection circuits  806  are provided for the pixel portion  802  and the driver circuit portion  804 , so that the resistance of the display device to overcurrent generated by electrostatic discharge (ESD) or the like can be improved. Note that the configuration of the protection circuits  806  is not limited to that, and for example, a configuration in which the protection circuits  806  are connected to the scan line driver circuit  804   a  or a configuration in which the protection circuits  806  are connected to the signal line driver circuit  804   b  may be employed. Alternatively, the protection circuits  806  may be configured to be connected to the terminal portion  807 . 
     In  FIG. 15A , an example in which the driver circuit portion  804  includes the scan line driver circuit  804   a  and the signal line driver circuit  804   b  is shown; however, the structure is not limited thereto. For example, only the scan line driver circuit  804   a  may be formed and a separately prepared substrate where a signal line driver circuit is formed (e.g., a driver circuit substrate formed with a single crystal semiconductor film or a polycrystalline semiconductor film) may be mounted. 
     &lt;Structural Example of Pixel Circuit&gt; 
     Each of the plurality of pixel circuits  801  in  FIG. 15A  can have a structure illustrated in  FIG. 15B , for example. 
     The pixel circuit  801  illustrated in  FIG. 15B  includes transistors  852  and  854 , a capacitor  862 , and a light-emitting element  872 . 
     One of a source electrode and a drain electrode of the transistor  852  is electrically connected to a wiring to which a data signal is supplied (a data line DL_n). A gate electrode of the transistor  852  is electrically connected to a wiring to which a gate signal is supplied (a scan line GL_m). 
     The transistor  852  has a function of controlling whether to write a data signal. 
     One of a pair of electrodes of the capacitor  862  is electrically connected to a wiring to which a potential is supplied (hereinafter referred to as a potential supply line VL_a), and the other is electrically connected to the other of the source electrode and the drain electrode of the transistor  852 . 
     The capacitor  862  functions as a storage capacitor for storing written data. 
     One of a source electrode and a drain electrode of the transistor  854  is electrically connected to the potential supply line VL_a. Furthermore, a gate electrode of the transistor  854  is electrically connected to the other of the source electrode and the drain electrode of the transistor  852 . 
     One of an anode and a cathode of the light-emitting element  872  is electrically connected to a potential supply line VL_b, and the other is electrically connected to the other of the source electrode and the drain electrode of the transistor  854 . 
     As the light-emitting element  872 , any of the light-emitting elements described in Embodiments 1 to 3 can be used. 
     Note that a high power supply potential VDD is supplied to one of the potential supply line VL_a and the potential supply line VL_b, and a low power supply potential VSS is supplied to the other. 
     In the display device including the pixel circuits  801  in  FIG. 15B , the pixel circuits  801  are sequentially selected row by row by the scan line driver circuit  804   a  in  FIG. 15A , for example, whereby the transistors  852  are turned on and a data signal is written. 
     When the transistors  852  are turned off, the pixel circuits  801  in which the data has been written are brought into a holding state. Furthermore, the amount of current flowing between the source electrode and the drain electrode of the transistor  854  is controlled in accordance with the potential of the written data signal. The light-emitting element  872  emits light with a luminance corresponding to the amount of flowing current. This operation is sequentially performed row by row; thus, an image is displayed. 
     Alternatively, the pixel circuit can have a function of compensating variation in threshold voltages or the like of a transistor.  FIGS. 16A and 163  and  FIGS. 17A and 17B  illustrate examples of the pixel circuit. 
     The pixel circuit illustrated in  FIG. 16A  includes six transistors (transistors  303 _ 1  to  303 _ 6 ), a capacitor  304 , and a light-emitting element  305 . The pixel circuit illustrated in  FIG. 16A  is electrically connected to wirings  301 _ 1  to  301 _ 5  and wirings  302 _ 1  and  302 _ 2 . Note that as the transistors  303 _ 1  to  3036 , for example, p-channel transistors can be used. 
     The pixel circuit shown in  FIG. 16B  has a configuration in which a transistor  303 _ 7  is added to the pixel circuit shown in  FIG. 16A . The pixel circuit illustrated in  FIG. 16B  is electrically connected to wirings  301 _ 6  and  301 _ 7 . The wirings  301 _ 5  and  301 _ 6  may be electrically connected to each other. Note that as the transistor  303   7 , for example, a p-channel transistor can be used. 
     The pixel circuit shown in  FIG. 17A  includes six transistors (transistors  308 _ 1  to  308   6 ), the capacitor  304 , and the light-emitting element  305 . The pixel circuit illustrated in  FIG. 17A  is electrically connected to wirings  306 _ 1  to  306 _ 3  and wirings  307 _ 1  to  307 _ 3 . The wirings  306 _ 1  and  306 _ 3  may be electrically connected to each other. Note that as the transistors  308 _ 1  to  308 _ 6 , for example, p-channel transistors can be used. 
     The pixel circuit illustrated in  FIG. 17B  includes two transistors (transistors  309 _ 1  and  309 _ 2 ), two capacitors (capacitors  304 _ 1  and  304 _ 2 ), and the light-emitting element  305 . The pixel circuit illustrated in  FIG. 17B  is electrically connected to wirings  311 _ 1  to  311 _ 3  and wirings  312 _ 1  and  312 _ 2 . With the configuration of the pixel circuit illustrated in  FIG. 17B , the pixel circuit can be driven by a voltage inputting current driving method (also referred to as CVCC). Note that as the transistors  309 _ 1  and  309 _ 2 , for example, p-channel transistors can be used. 
     A light-emitting element of one embodiment of the present invention can be used for an active matrix method in which an active element is included in a pixel of a display device or a passive matrix method in which an active element is not included in a pixel of a display device. 
     In the active matrix method, as an active element (a non-linear element), not only a transistor but also a variety of active elements (non-linear elements) can be used. For example, a metal insulator metal (MIM), a thin film diode (TFD), or the like can also be used. Since these elements can be formed with a smaller number of manufacturing steps, manufacturing cost can be reduced to yield can be improved. Alternatively, since the size of these elements is small, the aperture ratio can be improved, so that power consumption can be reduced and higher luminance can be achieved. 
     As a method other than the active matrix method, the passive matrix method in which an active element (a non-linear element) is not used can also be used. Since an active element (a non-linear element) is not used, the number of manufacturing steps is small, so that manufacturing cost can be reduced or yield can be improved. Alternatively, since an active element (a non-linear element) is not used, the aperture ratio can be improved, so that power consumption can be reduced or higher luminance can be achieved, for example. 
     The structure described in this embodiment can be used in appropriate combination with the structure described in any of the other embodiments. 
     Embodiment 6 
     In this embodiment, a display device including a light-emitting element of one embodiment of the present invention and an electronic device in which the display device is provided with an input device will be described with reference to  FIGS. 18A and 18B ,  FIGS. 19A to 19C ,  FIGS. 20A and 20B ,  FIGS. 21A and 21B , and  FIG. 22 . 
     &lt;Description 1 of Touch Panel&gt; 
     In this embodiment, a touch panel  2000  including a display device and an input device will be described as an example of an electronic device. In addition, an example in which a touch sensor is used as an input device will be described. 
       FIGS. 18A and 18B  are perspective views of the touch panel  2000 . Note that  FIGS. 18A and 18B  illustrate only main components of the touch panel  2000  for simplicity. 
     The touch panel  2000  includes a display device  2501  and a touch sensor  2595  (see  FIG. 18B ). The touch panel  2000  also includes a substrata  2510 , a substrata  2570 , and a substrate  2590 . The substrate  2510 , the substrate  2570 , and the substrate  2590  each have flexibility. Note that one or all of the substrates  2510 ,  2570 , and  2590  may be inflexible. 
     The display device  2501  includes a plurality of pixels over the substrate  2510  and a plurality of wirings  2511  through which signals are supplied to the pixels. The plurality of wirings  2511  are led to a peripheral portion of the substrate  2510 , and parts of the plurality of wirings  2511  form a terminal  2519 . The terminal  2519  is electrically connected to an FPC  2509 ( 1 ). The plurality of wirings  2511  can supply signals from a signal line driver circuit  2503   s ( 1 ) to the plurality of pixels. 
     The substrate  2590  includes the touch sensor  2595  and a plurality of wirings  2598  electrically connected to the touch sensor  2595 . The plurality of wirings  2598  are led to a peripheral portion of the substrate  2590 , and parts of the plurality of wirings  2598  form a terminal. The terminal is electrically connected to an FPC  2509 ( 2 ). Note that in  FIG. 18B , electrodes, wirings, and the like of the touch sensor  2595  provided on the back side of the substrate  2590  (the side facing the substrate  2510 ) are indicated by solid lines for clarity. 
     As the touch sensor  2595 , a capacitive touch sensor can be used. Examples of the capacitive touch sensor are a surface capacitive touch sensor and a projected capacitive touch sensor. 
     Examples of the projected capacitive touch sensor are a self capacitive touch sensor and a mutual capacitive touch sensor, which differ mainly in the driving method. The use of a mutual capacitive type is preferable because multiple points can be sensed simultaneously. 
     Note that the touch sensor  2595  illustrated in  FIG. 18B  is an example of using a projected capacitive touch sensor. 
     Note that a variety of sensors that can sense approach or contact of a sensing target such as a finger can be used as the touch sensor  2595 . 
     The projected capacitive touch sensor  2595  includes electrodes  2591  and electrodes  2592 . The electrodes  2591  are electrically connected to any of the plurality of wirings  2598 , and the electrodes  2592  are electrically connected to any of the other wirings  2598 . 
     The electrodes  2592  each have a shape of a plurality of quadrangles arranged in one direction with one corner of a quadrangle connected to one corner of another quadrangle as illustrated in  FIGS. 18A and 18B . 
     The electrodes  2591  each have a quadrangular shape and are arranged in a direction intersecting with the direction in which the electrodes  2592  extend. 
     A wiring  2594  electrically connects two electrodes  2591  between which the electrode  2592  is positioned. The intersecting area of the electrode  2592  and the wiring  2594  is preferably as small as possible. Such a structure allows a reduction in the area of a region where the electrodes are not provided, reducing variation in transmittance. As a result, variation in luminance of light passing through the touch sensor  2595  can be reduced. 
     Note that the shapes of the electrodes  2591  and the electrodes  2592  are not limited thereto and can be any of a variety of shapes. For example, a structure may be employed in which the plurality of electrodes  2591  are arranged so that gaps between the electrodes  2591  are reduced as much as possible, and the electrodes  2592  are spaced apart from the electrodes  2591  with an insulating layer interposed therebetween to have regions not overlapping with the electrodes  2591 . In this case, it is preferable to provide, between two adjacent electrodes  2592 , a dummy electrode electrically insulated from these electrodes because the area of regions having different transmittances can be reduced. 
     &lt;Description of Display Device&gt; 
     Next, the display device  2501  will be described in detail with reference to  FIG. 19A .  FIG. 19A  corresponds to a cross-sectional view taken along dashed-dotted line X 1 -X 2  in  FIG. 18B . 
     The display device  2501  includes a plurality of pixels arranged in a matrix. Each of the pixels includes a display element and a pixel circuit for driving the display element. 
     In the following description, an example of using a light-emitting element that emits white light as a display element will be described; however, the display element is not limited to such an element. For example, light-emitting elements that emit light of different colors may be included so that the light of different colors can be emitted from adjacent pixels. 
     For the substrate  2510  and the substrate  2570 , for example, a flexible material with a vapor permeability of lower than or equal to 1×10 −5  g·m −2 ·day −1 , preferably lower than or equal to 1×10 −6  g·m −2 ·day −1  can be favorably used. Alternatively, materials whose thermal expansion coefficients are substantially equal to each other are preferably used for the substrate  2510  and the substrate  2570 . For example, the coefficients of linear expansion of the materials are preferably lower than or equal to 1×10 −3 /K, further preferably lower than or equal to 5×10 −5 /K, and still further preferably lower than or equal to 1×10 −5 /K. 
     Note that the substrate  2510  is a stacked body including an insulating layer  2510   a  for preventing impurity diffusion into the light-emitting element, a flexible substrate  2510   b , and an adhesive layer  2510   c  for attaching the insulating layer  2510   a  and the flexible substrate  2510   b  to each other. The substrate  2570  is a stacked body including an insulating layer  2570   a  for preventing impurity diffusion into the light-emitting element, a flexible substrate  2570   b , and an adhesive layer  2570   c  for attaching the insulating layer  2570   a  and the flexible substrate  2570   b  to each other. 
     For the adhesive layer  2510   c  and the adhesive layer  2570   c , for example, polyester, polyolefin, polyamide (e.g., nylon, aramid), polyimide, polycarbonate, or acrylic, urethane, or epoxy can be used. Alternatively, a material that includes a resin having a siloxane bond can be used. 
     A sealing layer  2560  is provided between the substrate  2510  and the substrate  2570 . The sealing layer  2560  preferably has a refractive index higher than that of air. In the case where light is extracted to the sealing layer  2560  side as illustrated in  FIG. 19A , the sealing layer  2560  can also serve as an optical adhesive layer. 
     A sealant may be formed in the peripheral portion of the sealing layer  2560 . With the use of the sealant, a light-emitting element  2550 R can be provided in a region surrounded by the substrate  2510 , the substrate  2570 , the sealing layer  2560 , and the sealant. Note that an inert gas (such as nitrogen and argon) may be used instead of the sealing layer  2560 . A drying agent may be provided in the inert gas so as to adsorb moisture or the like. An epoxy-based resin or a glass frit is preferably used as the sealant. As a material used for the sealant, a material which is impermeable to moisture and oxygen is preferably used. 
     The display device  2501  includes a pixel  2502 R. The pixel  2502 R includes a light-emitting module  2580 R. 
     The pixel  2502 R includes the light-emitting element  2550 R and a transistor  25021  that can supply electric power to the light-emitting element  2550 R. Note that the transistor  2502   t  functions as part of the pixel circuit. The light-emitting module  2580 R includes the light-emitting element  2550 R and a coloring layer  2567 R. 
     The light-emitting element  2550 R includes a lower electrode, an upper electrode, and an EL layer between the lower electrode and the upper electrode. As the light-emitting element  2550 R, any of the light-emitting elements described in Embodiments 1 to 3 can be used. 
     A microcavity structure may be employed between the lower electrode and the upper electrode so as to increase the intensity of light having a specific wavelength. 
     In the case where the sealing layer  2560  is provided on the light extraction side, the sealing layer  2560  is in contact with the light-emitting element  2550 R and the coloring layer  2567 R. 
     The coloring layer  2567 R is positioned in a region overlapping with the light-emitting element  2550 R. Accordingly, part of light emitted from the light-emitting element  2550 R passes through the coloring layer  2567 R and is emitted to the outside of the light-emitting module  2580 R as indicated by an arrow in  FIG. 19A . 
     The display device  2501  includes a light-blocking layer  2567 BM on the light extraction side. The light-blocking layer  2567 BM is provided so as to surround the coloring layer  2567 R. 
     The coloring layer  2567 R is a coloring layer having a function of transmitting light in a particular wavelength range. For example, a color filter for transmitting light in a red wavelength range, a color filter for transmitting light in a green wavelength range, a color filter for transmitting light in a blue wavelength range, a color filter for transmitting light in a yellow wavelength range, or the like can be used. Each color filter can be formed with any of various materials by a printing method, an inkjet method, an etching method using a photolithography technique, or the like. 
     An insulating layer  2521  is provided in the display device  2501 . The insulating layer  2521  covers the transistor  2502   t . Note that the insulating layer  2521  has a function of covering unevenness caused by the pixel circuit. The insulating layer  2521  may have a function of suppressing impurity diffusion. This can prevent the reliability of the transistor  2502   t  or the like from being lowered by impurity diffusion. 
     The light-emitting element  2550 R is formed over the insulating layer  2521 . A partition  2528  is provided so as to overlap with an end portion of the lower electrode of the light-emitting element  2550 R. Note that a spacer for controlling the distance between the substrate  2510  and the substrate  2570  may be formed over the partition  2528 . 
     A scan line driver circuit  2503   g ( 1 ) includes a transistor  2503   t  and a capacitor  2503   c . Note that the driver circuit can be formed in the same process and over the same substrate as those of the pixel circuits. 
     The wirings  2511  through which signals can be supplied are provided over the substrate  2510 . The terminal  2519  is provided over the wirings  2511 . The FPC  2509 ( 1 ) is electrically connected to the terminal  2519 . The FPC  2509 ( 1 ) has a function of supplying a video signal, a clock signal, a start signal, a reset signal, or the like. Note that the FPC  2509 ( 1 ) may be provided with a PWB. 
     In the display device  2501 , transistors with any of a variety of structures can be used.  FIG. 19A  illustrates an example of using bottom-gate transistors; however, the present invention is not limited to this example, and top-gate transistors may be used in the display device  2501  as illustrated in  FIG. 19B . 
     In addition, there is no particular limitation on the polarity of the transistor  2502   t  and the transistor  2503   t . For these transistors, n-channel and p-channel transistors may be used, or either n-channel transistors or p-channel transistors may be used, for example. Furthermore, there is no particular limitation on the crystallinity of a semiconductor film used for the transistors  2502   t  and  2503   t . For example, an amorphous semiconductor film or a crystalline semiconductor film may be used. Examples of semiconductor materials include Group 14 semiconductors (e.g., a semiconductor including silicon), compound semiconductors (including oxide semiconductors), organic semiconductors, and the like. An oxide semiconductor that has an energy gap of 2 eV or more, preferably 2.5 eV or more, further preferably 3 eV or more is preferably used for one of the transistors  2502   t  and  2503   t  or both, so that the off-state currant of the transistors can be reduced. Examples of the oxide semiconductors include an In—Ga oxide, an In—M—Zn oxide (M represents Al, Ga, Y, Zr, La, Ce, Sn, Hf, or Nd), and the like. 
     &lt;Description of Touch Sensor&gt; 
     Next, the touch sensor  2595  will be described in detail with reference to  FIG. 19C .  FIG. 19C  corresponds to a cross-sectional view taken along dashed-dotted line X 3 -X 4  in  FIG. 18B . 
     The touch sensor  2595  includes the electrodes  2591  and the electrodes  2592  provided in a staggered arrangement on the substrate  2590 , an insulating layer  2593  covering the electrodes  2591  and the electrodes  2592 , and the wiring  2594  that electrically connects the adjacent electrodes  2591  to each other. 
     The electrodes  2591  and the electrodes  2592  are formed using a light-transmitting conductive material. As a light-transmitting conductive material, a conductive oxide such as indium oxide, indium tin oxide, indium zinc oxide, zinc oxide, or zinc oxide to which gallium is added can be used. Note that a film including graphene may be used as well. The film including graphene can be formed, for example, by reducing a film containing graphene oxide. As a reducing method, a method with application of heat or the like can be employed. 
     The electrodes  2591  and the electrodes  2592  may be formed by, for example, depositing a light-transmitting conductive material on the substrate  2590  by a sputtering method and then removing an unnecessary portion by any of various pattern forming techniques such as photolithography. 
     Examples of a material for the insulating layer  2593  are a resin such as an acrylic resin or an epoxy resin, a resin having a siloxane bond, and an inorganic insulating material such as silicon oxide, silicon oxynitride, or aluminum oxide. 
     Openings reaching the electrodes  2591  are formed in the insulating layer  2593 , and the wiring  2594  electrically connects the adjacent electrodes  2591 . A light-transmitting conductive material can be favorably used as the wiring  2594  because the aperture ratio of the touch panel can be increased. Moreover, a material with higher conductivity than the conductivities of the electrodes  2591  and  2592  can be favorably used for the wiring  2594  because electric resistance can be reduced. 
     One electrode  2592  extends in one direction, and a plurality of electrodes  2592  are provided in the form of stripes. The wiring  2594  intersects with the electrode  2592 . 
     Adjacent electrodes  2591  are provided with one electrode  2592  provided therebetween. The wiring  2594  electrically connects the adjacent electrodes  2591 . 
     Note that the plurality of electrodes  2591  are not necessarily arranged in the direction orthogonal to one electrode  2592  and may be arranged to intersect with one electrode  2592  at an angle of more than 0 degrees and less than 90 degrees. 
     The wiring  2598  is electrically connected to any of the electrodes  2591  and  2592 . Part of the wiring  2598  functions as a terminal. For the wiring  2598 , a metal material such as aluminum, gold, platinum, silver, nickel, titanium, tungsten, chromium, molybdenum, iron, cobalt, copper, or palladium or an alloy material containing any of these metal materials can be used. 
     Note that an insulating layer that covers the insulating layer  2593  and the wiring  2594  may be provided to protect the touch sensor  2595 . 
     A connection layer  2599  electrically connects the wiring  2598  to the FPC  2509 ( 2 ). 
     As the connection layer  2599 , any of various anisotropic conductive films (ACF), anisotropic conductive pastes (ACP), and the like can be used. 
     &lt;Description 2 of Touch Panel&gt; 
     Next, the touch panel  2000  will be described in detail with reference to  FIG. 20A .  FIG. 20A  corresponds to a cross-sectional view taken along dashed-dotted line X 5 -X 6  in  FIG. 18A . 
     In the touch panel  2000  illustrated in  FIG. 20A , the display device  2501  described with reference to  FIG. 19A  and the touch sensor  2595  described with reference to  FIG. 19C  are attached to each other. 
     The touch panel  2000  illustrated in  FIG. 20A  includes an adhesive layer  2597  and an anti-reflective layer  2567   p  in addition to the components described with reference to  FIGS. 19A and 19C . 
     The adhesive layer  2597  is provided in contact with the wiring  2594 . Note that the adhesive layer  2597  attaches the substrate  2590  to the substrate  2570  so that the touch sensor  2595  overlaps with the display device  2501 . The adhesive layer  2597  preferably has a light-transmitting property. A heat curable resin or an ultraviolet curable resin can be used for the adhesive layer  2597 . For example, an acrylic resin, a urethane-based resin, an epoxy-based resin, or a siloxane-based resin can be used. 
     The anti-reflective layer  2567   p  is positioned in a region overlapping with pixels. As the anti-reflective layer  2567   p , a circularly polarizing plate can be used, for example. 
     Next, a touch panel having a structure different from that illustrated in  FIG. 20A  will be described with reference to  FIG. 20B . 
       FIG. 20B  is a cross-sectional view of a touch panel  2001 . The touch panel  2001  illustrated in  FIG. 20B  differs from the touch panel  2000  illustrated in  FIG. 20A  in the position of the touch sensor  2595  relative to the display device  2501 . Different parts are described in detail below, and the above description of the touch panel  2000  is referred to for the other similar parts. 
     The coloring layer  2567 R is positioned in a region overlapping with the light-emitting element  2550 R. The light-emitting element  2550 R illustrated in  FIG. 20B  emits light to the side where the transistor  2502   t  is provided. Accordingly, part of light emitted from the light-emitting element  2550 R passes through the coloring layer  2567 R and is emitted to the outside of the light-emitting module  2580 R as indicated by an arrow in  FIG. 20B . 
     The touch sensor  2595  is provided on the substrate  2510  side of the display device  2501 . 
     The adhesive layer  2597  is provided between the substrate  2510  and the substrate  2590  and attaches the touch sensor  2595  to the display device  2501 . 
     As illustrated in  FIG. 20A or 20B , light may be emitted from the light-emitting element to one or both of upper and lower sides of the substrate  2510 . 
     &lt;Description of Method for Driving Touch Panel&gt; 
     Next, an example of a method for driving a touch panel will be described with reference to  FIGS. 21A and 21B . 
       FIG. 21A  is a block diagram illustrating the structure of a mutual capacitive touch sensor  FIG. 21A  illustrates a pulse voltage output circuit  2601  and a current sensing circuit  2602 . Note that in  FIG. 21A , six wirings X 1  to X 6  represent the electrodes  2621  to which a pulse voltage is applied, and six wirings Y 1  to Y 6  represent the electrodes  2622  that detect changes in current  FIG. 21A  also illustrates capacitors  2603  that are each formed in a region where the electrodes  2621  and  2622  overlap with each other. Note that functional replacement between the electrodes  2621  and  2622  is possible. 
     The pulse voltage output circuit  2601  is a circuit for sequentially applying a pulse voltage to the wirings X 1  to X 6 . By application of a pulse voltage to the wirings X 1  to X 6 , an electric field is generated between the electrodes  2621  and  2622  of the capacitor  2603 . When the electric field between the electrodes is shielded, for example, a change occurs in the capacitor  2603  (mutual capacitance). The approach or contact of a sensing target can be sensed by utilizing this change. 
     The current sensing circuit  2602  is a circuit for detecting changes in current flowing through the wirings Y 1  to Y 6  that are caused by the change in mutual capacitance in the capacitor  2603 . No change in current value is detected in the wirings Y 1  to Y 6  when there is no approach or contact of a sensing target, whereas a decrease in current value is detected when mutual capacitance is decreased owing to the approach or contact of a sensing target. Note that an integrator circuit or the like is used for sensing of current values. 
       FIG. 21B  is a timing chart showing input and output waveforms in the mutual capacitive touch sensor illustrated in  FIG. 21A . In  FIG. 21B , sensing of a sensing target is performed in all the rows and columns in one frame period.  FIG. 21B  shows a period when a sensing target is not sensed (not touched) and a period when a sensing target is sensed (touched). In  FIG. 21B , sensed current values of the wirings Y 1  to Y 6  are shown as the waveforms of voltage values. 
     A pulse voltage is sequentially applied to the wirings X 1  to X 6 , and the waveforms of the wirings Y 1  to Y 6  change in accordance with the pulse voltage. When there is no approach or contact of a sensing target, the waveforms of the wirings Y 1  to Y 6  change in accordance with changes in the voltages of the wirings X 1  to X 6 . The current value is decreased at the point of approach or contact of a sensing target and accordingly the waveform of the voltage value changes. 
     By detecting a change in mutual capacitance in this manner, the approach or contact of a sensing target can be sensed. 
     &lt;Description of Sensor Circuit&gt; 
     Although  FIG. 21A  illustrates a passive matrix type touch sensor in which only the capacitor  2603  is provided at the intersection of wirings as a touch sensor, an active matrix type touch sensor including a transistor and a capacitor may be used.  FIG. 22  illustrates an example of a sensor circuit included in an active matrix type touch sensor. 
     The sensor circuit in  FIG. 22  includes the capacitor  2603  and transistors  2611 ,  2612 , and  2613 . 
     A signal G 2  is input to a gate of the transistor  2613 . A voltage VRES is applied to one of a source and a drain of the transistor  2613 , and one electrode of the capacitor  2603  and a gate of the transistor  2611  are electrically connected to the other of the source and the drain of the transistor  2613 . One of a source and a drain of the transistor  2611  is electrically connected to one of a source and a drain of the transistor  2612 , and a voltage VSS is applied to the other of the source and the drain of the transistor  2611 . A signal G 1  is input to a gate of the transistor  2612 , and a wiring ML is electrically connected to the other of the source and the drain of the transistor  2612 . The voltage VSS is applied to the other electrode of the capacitor  2603 . 
     Next, the operation of the sensor circuit in  FIG. 22  will be described. First, a potential for turning on the transistor  2613  is supplied as the signal G 2 , and a potential with respect to the voltage VRES is thus applied to the node n connected to the gate of the transistor  2611 . Then, a potential for turning off the transistor  2613  is applied as the signal G 2 , whereby the potential of the node n is maintained. 
     Then, mutual capacitance of the capacitor  2603  changes owing to the approach or contact of a sensing target such as a finger, and accordingly the potential of the node n is changed from VRES. 
     In reading operation, a potential for turning on the transistor  2612  is supplied as the signal G 1 . A current flowing through the transistor  2611 , that is, a current flowing through the wiring ML is changed in accordance with the potential of the node n. By sensing this current, the approach or contact of a sensing target can be sensed. 
     In each of the transistors  2611 ,  2612 , and  2613 , an oxide semiconductor layer is preferably used as a semiconductor layer in which a channel region is formed. In particular, such a transistor is preferably used as the transistor  2613  so that the potential of the node n can be held for a long time and the frequency of operation of resupplying VRES to the node n (refresh operation) can be reduced. 
     The structure described in this embodiment can be used in appropriate combination with the structure described in any of the other embodiments. 
     Embodiment 7 
     In this embodiment, a display module and electronic devices including a light-emitting element of one embodiment of the present invention will be described with reference to  FIG. 23  and  FIGS. 24A to 24G . 
     &lt;Description of Display Module&gt; 
     In a display module  8000  in  FIG. 23 , a touch sensor  8004  connected to an FPC  8003 , a display device  8006  connected to an FPC  8005 , a frame  8009 , a printed board  8010 , and a battery  8011  are provided between an upper cover  8001  and a lower cover  8002 . 
     The light-emitting element of one embodiment of the present invention can be used for the display device  8006 , for example. 
     The shapes and sizes of the upper cover  8001  and the lower cover  8002  can be changed as appropriate in accordance with the sizes of the touch sensor  8004  and the display device  8006 . 
     The touch sensor  8004  can be a resistive touch sensor or a capacitive touch sensor and may be formed to overlap with the display device  8006 . A counter substrate (sealing substrate) of the display device  8006  can have a touch sensor function. A photosensor may be provided in each pixel of the display device  8006  so that an optical touch sensor is obtained. 
     The frame  8009  protects the display device  8006  and also serves as an electromagnetic shield for blocking electromagnetic waves generated by the operation of the printed board  8010 . The frame  8009  may serve as a radiator plate. 
     The printed board  8010  has a power supply circuit and a signal processing circuit for outputting a video signal and a clock signal. As a power source for supplying power to the power supply circuit, an external commercial power source or the battery  8011  provided separately may be used. The battery  8011  can be omitted in the case of using a commercial power source. 
     The display module  8000  can be additionally provided with a member such as a polarizing plate, a retardation plate, or a prism sheet. 
     &lt;Description of Electronic Device&gt; 
       FIGS. 24A to 24G  illustrate electronic devices. These electronic devices can include a housing  9000 , a display portion  9001 , a speaker  9003 , operation keys  9005  (including a power switch or an operation switch), a connection terminal  9006 , a sensor  9007  (a sensor having a function of measuring or sensing force, displacement, position, speed, acceleration, angular velocity, rotational frequency, distance, light, liquid, magnetism, temperature, chemical substance, sound, time, hardness, electric field, current, voltage, electric power, radiation, flow rate, humidity, gradient, oscillation, odor, or infrared ray), a microphone  9008 , and the like. 
     The electronic devices illustrated in  FIGS. 24A to 24G  can have a variety of functions, for example, a function of displaying a variety of data (a still image, a moving image, a text image, and the like) on the display portion, a touch sensor function, a function of displaying a calendar, date, time, and the like, a function of controlling a process with a variety of software (programs), a wireless communication function, a function of being connected to a variety of computer networks with a wireless communication function, a function of transmitting and receiving a variety of data with a wireless communication function, a function of reading a program or data stored in a memory medium and displaying the program or data on the display portion, and the like. Note that functions that can be provided for the electronic devices illustrated in  FIGS. 24A to 24G  are not limited to those described above, and the electronic devices can have a variety of functions. Although not illustrated in  FIGS. 24A to 24G , the electronic devices may include a plurality of display portions. The electronic devices may have a camera or the like and a function of taking a still image, a function of taking a moving image, a function of storing the taken image in a memory medium (an external memory medium or a memory medium incorporated in the camera), a function of displaying the taken image on the display portion, or the like. 
     The electronic devices illustrated in  FIGS. 24A to 24G  will be described in detail below. 
       FIG. 24A  is a perspective view of a portable information terminal  9100 . The display portion  9001  of the portable information terminal  9100  is flexible. Therefore, the display portion  9001  can be incorporated along a bent surface of a bent housing  9000 . In addition, the display portion  9001  includes a touch sensor, and operation can be performed by touching the screen with a finger, a stylus, or the like. For example, when an icon displayed on the display portion  9001  is touched, an application can be started. 
       FIG. 24B  is a perspective view of a portable information terminal  9101 . The portable information terminal  9101  functions as, for example, one or more of a telephone set, a notebook, and an information browsing system. Specifically, the portable information terminal can be used as a smartphone. Note that the speaker  9003 , the connection terminal  9006 , the sensor  9007 , and the like, which are not shown in  FIG. 24B , can be positioned in the portable information terminal  9101  as in the portable information terminal  9100  shown in  FIG. 24A . The portable information terminal  9101  can display characters and image information on its plurality of surfaces. For example, three operation buttons  9050  (also referred to as operation icons, or simply, icons) can be displayed on one surface of the display portion  9001 . Furthermore, information  9051  indicated by dashed rectangles can be displayed on another surface of the display portion  9001 . Examples of the information  9051  include display indicating reception of an incoming email, social networking service (SNS) message, call, and the like; the title and sender of an email and SNS message; the date; the time; remaining battery; and the reception strength of an antenna Instead of the information  9051 , the operation buttons  9050  or the like may be displayed on the position where the information  9051  is displayed. 
       FIG. 24C  is a perspective view of a portable information terminal  9102 . The portable information terminal  9102  has a function of displaying information on three or more surfaces of the display portion  9001 . Here, information  9052 , information  9053 , and information  9054  are displayed on different surfaces. For example, a user of the portable information terminal  9102  can see the display (here, the information  9053 ) with the portable information terminal  9102  put in a breast pocket of his/her clothes. Specifically, a caller&#39;s phone number, name, or the like of an incoming call is displayed in a position that can be seen from above the portable information terminal  9102 . Thus, the user can see the display without taking out the portable information terminal  9102  from the pocket and decide whether to answer the call. 
       FIG. 24D  is a perspective view of a watch-type portable information terminal  9200 . The portable information terminal  9200  is capable of executing a variety of applications such as mobile phone calls, e-mailing, viewing and editing texts, music reproduction, Internet communication, and computer games. The display surface of the display portion  9001  is bent, and images can be displayed on the bent display surface. The portable information terminal  9200  can employ near field communication that is a communication method based on an existing communication standard. In that case, for example, mutual communication between the portable information terminal  9200  and a headset capable of wireless communication can be performed, and thus hands-free calling is possible. The portable information terminal  9200  includes the connection terminal  9006 , and data can be directly transmitted to and received from another information terminal via a connector. Power charging through the connection terminal  9006  is possible. Note that the charging operation may be performed by wireless power feeding without using the connection terminal  9006 . 
       FIGS. 24E, 24F, and 24G  are perspective views of a foldable portable information terminal  9201 .  FIG. 24E  is a perspective view illustrating the portable information terminal  9201  that is opened.  FIG. 24F  is a perspective view illustrating the portable information terminal  9201  that is being opened or being folded  FIG. 24G  is a perspective view illustrating the portable information terminal  9201  that is folded. The portable information terminal  9201  is highly portable when folded. When the portable information terminal  9201  is opened, a seamless large display region is highly browsable. The display portion  9001  of the portable information terminal  9201  is supported by three housings  9000  joined together by hinges  9055 . By folding the portable information terminal  9201  at a connection portion between two housings  9000  with the hinges  9055 , the portable information terminal  9201  can be reversibly changed in shape from an opened state to a folded state. For example, the portable information terminal  9201  can be bent with a radius of curvature of greater than or equal to 1 mm and less than or equal to 150 mm. 
     The electronic devices described in this embodiment each include the display portion for displaying some sort of data. Note that the light-emitting element of one embodiment of the present invention can also be used for an electronic device which does not have a display portion. The structure in which the display portion of the electronic device described in this embodiment is flexible and display can be performed on the bent display surface or the structure in which the display portion of the electronic device is foldable is described as an example; however, the structure is not limited thereto and a structure in which the display portion of the electronic device is not flexible and display is performed on a plane portion may be employed. 
     The structure described in this embodiment can be used in appropriate combination with the structure described in any of the other embodiments. 
     Embodiment 8 
     In this embodiment, a light-emitting device including the light-emitting element of one embodiment of the present invention will be described with reference to  FIGS. 25A to 25C  and  FIGS. 26A to 26D . 
       FIG. 25A  is a perspective view of a light-emitting device  3000  shown in this embodiment, and  FIG. 25B  is a cross-sectional view along dashed-dotted line F-F in  FIG. 25A . Note that in  FIG. 25A , some components are illustrated by broken lines in order to avoid complexity of the drawing. 
     The light-emitting device  3000  illustrated in  FIGS. 25A and 25B  includes a substrate  3001 , a light-emitting element  3005  over the substrate  3001 , a first sealing region  3007  provided around the light-emitting element  3005 , and a second sealing region  3009  provided around the first sealing region  3007 . 
     Light is emitted from the light-emitting element  3005  through one or both of the substrate  3001  and a substrate  3003 . In  FIGS. 25A and 25B , a structure in which light is emitted from the light-emitting element  3005  to the lower side (the substrate  3001  side) is illustrated. 
     As illustrated in  FIGS. 25A and 25B , the light-emitting device  3000  has a double sealing structure in which the light-emitting element  3005  is surrounded by the first sealing region  3007  and the second sealing region  3009 . With the double sealing structure, entry of impurities (e.g., water, oxygen, and the like) from the outside into the light-emitting element  3005  can be favorably suppressed. Note that it is not necessary to provide both the first sealing region  3007  and the second sealing region  3009 . For example, only the first sealing region  3007  may be provided. 
     Note that in  FIG. 25B , the first sealing region  3007  and the second sealing region  3009  are each provided in contact with the substrate  3001  and the substrate  3003 . However, without limitation to such a structure, for example, one or both of the first sealing region  3007  and the second sealing region  3009  may be provided in contact with an insulating film or a conductive film provided on the substrate  3001 . Alternatively, one or both of the first sealing region  3007  and the second sealing region  3009  may be provided in contact with an insulating film or a conductive film provided on the substrate  3003 . 
     The substrate  3001  and the substrate  3003  can have structures similar to those of the substrate  200  and the substrate  220  described in Embodiment 3, respectively. The light-emitting element  3005  can have a structure similar to that of any of the light-emitting elements described in Embodiments 1 to 3. 
     For the first sealing region  3007 , a material containing glass (e.g., a glass frit, a glass ribbon, and the like) can be used. For the second sealing region  3009 , a material containing a resin can be used. With the use of the material containing glass for the first sealing region  3007 , productivity and a sealing property can be improved. Moreover, with the use of the material containing a resin for the second sealing region  3009 , impact resistance and heat resistance can be improved. However, the materials used for the first sealing region  3007  and the second sealing region  3009  are not limited to such, and the first sealing region  3007  may be formed using the material containing a resin and the second sealing region  3009  may be formed using the material containing glass. 
     The glass fit may contain, for example, magnesium oxide, calcium oxide, strontium oxide, barium oxide, cesium oxide, sodium oxide, potassium oxide, boron oxide, vanadium oxide, zinc oxide, tellurium oxide, aluminum oxide, silicon dioxide, lead oxide, tin oxide, phosphorus oxide, ruthenium oxide, rhodium oxide, iron oxide, copper oxide, manganese dioxide, molybdenum oxide, niobium oxide, titanium oxide, tungsten oxide, bismuth oxide, zirconium oxide, lithium oxide, antimony oxide, lead borate glass, tin phosphate glass, vanadate glass, or borosilicate glass. The glass frit preferably contains at least one kind of transition metal to absorb infrared light. 
     As the above glass frits, for example, a frit paste is applied to a substrate and is subjected to heat treatment, laser light irradiation, or the like. The frit paste contains the glass frit and a resin (also referred to as a binder) diluted by an organic solvent. Note that an absorber which absorbs light having the wavelength of laser light may be added to the glass frit. For example, an Nd:YAG laser or a semiconductor laser is preferably used as the laser. The shape of laser light may be circular or quadrangular. 
     As the above material containing a resin, for example, materials that include polyester, polyolefin, polyamide (e.g., nylon, aramid), polyimide, polycarbonate, an acrylic resin, urethane, an epoxy resin, or a resin having a siloxane bond can be used. 
     Note that in the case where the material containing glass is used for one or both of the first sealing region  3007  and the second sealing region  3009 , the material containing glass preferably has a thermal expansion coefficient close to that of the substrate  3001 . With the above structure, generation of a crack in the material containing glass or the substrate  3001  due to thermal stress can be suppressed. 
     For example, the following advantageous effect can be obtained in the case where the material containing glass is used for the first sealing region  3007  and the material containing a resin is used for the second sealing region  3009 . 
     The second sealing region  3009  is provided closer to an outer portion of the light-emitting device  3000  than the first sealing region  3007  is. In the light-emitting device  3000 , distortion due to external force or the like increases toward the outer portion. Thus, the outer portion of the light-emitting device  3000  where a larger amount of distortion is generated, that is, the second sealing region  3009  is sealed using the material containing a resin and the first sealing region  3007  provided on an inner side of the second sealing region  3009  is sealed using the material containing glass, whereby the light-emitting device  3000  is less likely to be damaged even when distortion due to external force or the like is generated. 
     Furthermore, as illustrated in  FIG. 25B , a first region  3011  corresponds to the region surrounded by the substrate  3001 , the substrate  3003 , the first sealing region  3007 , and the second sealing region  3009 . A second region  3013  corresponds to the region surrounded by the substrate  3001 , the substrate  3003 , the light-emitting element  3005 , and the first sealing region  3007 . 
     The first region  3011  and the second region  3013  are preferably filled with, for example, an inert gas such as a rare gas or a nitrogen gas. Note that for the first region  3011  and the second region  3013 , a reduced pressure state is preferred to an atmospheric pressure state. 
       FIG. 25C  illustrates a modification example of the structure in  FIG. 25B .  FIG. 25C  is a cross-sectional view illustrating the modification example of the light-emitting device  3000 . 
       FIG. 25C  illustrates a structure in which a desiccant  3018  is provided in a recessed portion provided in part of the substrate  3003 . The other components are the same as those of the structure illustrated in  FIG. 25B . 
     As the desiccant  3018 , a substance which adsorbs moisture and the like by chemical adsorption or a substance which adsorbs moisture and the like by physical adsorption can be used. Examples of the substance that can be used as the desiccant  3018  include alkali metal oxides, alkaline earth metal oxide (e.g., calcium oxide, barium oxide, and the like), sulfate, metal halides, perchlorate, zeolite, silica gel, and the like. 
     Next, modification examples of the light-emitting device  3000  which is illustrated in  FIG. 25B  are described with reference to  FIGS. 26A to 26D . Note that  FIGS. 26A to 26D  are cross-sectional views illustrating the modification examples of the light-emitting device  3000  illustrated in  FIG. 25B . 
     In each of the light-emitting devices illustrated in  FIGS. 26A to 26D , the second sealing region  3009  is not provided but only the first sealing region  3007  is provided. Moreover, in each of the light-emitting devices illustrated in  FIGS. 26A to 26D , a region  3014  is provided instead of the second region  3013  illustrated in  FIG. 25B . 
     For the region  3014 , for example, materials that include polyester, polyolefin, polyamide (e.g., nylon or aramid), polyimide, polycarbonate, an acrylic resin, an epoxy resin, urethane, an epoxy resin, or a resin having a siloxane bond can be used. 
     When the above-described material is used for the region  3014 , what is called a solid-sealing light-emitting device can be obtained. 
     In the light-emitting device illustrated in  FIG. 26B , a substrate  3015  is provided on the substrate  3001  side of the light-emitting device illustrated in  FIG. 26A . 
     The substrate  3015  has unevenness as illustrated in  FIG. 26B . With a structure in which the substrate  3015  having unevenness is provided on the side through which light emitted from the light-emitting element  3005  is extracted, the efficiency of extraction of light from the light-emitting element  3005  can be improved. Note that instead of the structure having unevenness and illustrated in  FIG. 26B , a substrate having a function as a diffusion plate may be provided. 
     In the light-emitting device illustrated in  FIG. 26C , light is extracted through the substrate  3003  side, unlike in the light-emitting device illustrated in  FIG. 26A , in which light is extracted through the substrate  3001  side. 
     The light-emitting device illustrated in  FIG. 26C  includes the substrate  3015  on the substrate  3003  side. The other components are the same as those of the light-emitting device illustrated in  FIG. 26B . 
     In the light-emitting device illustrated in  FIG. 26D , the substrate  3003  and the substrate  3015  included in the light-emitting device illustrated in  FIG. 26C  are not provided but a substrate  3016  is provided. 
     The substrate  3016  includes first unevenness positioned closer to the light-emitting element  3005  and second unevenness positioned farther from the light-emitting element  3005 . With the structure illustrated in  FIG. 26D , the efficiency of extraction of light from the light-emitting element  3005  can be further improved. 
     Thus, the use of the structure described in this embodiment can provide a light-emitting device in which deterioration of a light-emitting element due to impurities such as moisture and oxygen is suppressed. Alternatively, with the structure described in this embodiment, a light-emitting device having high light extraction efficiency can be obtained. 
     Note that the structure described in this embodiment can be combined with the structure described in any of the other embodiments as appropriate. 
     Embodiment 9 
     In this embodiment, examples in which the light-emitting element of one embodiment of the present invention is used for various lighting devices and electronic devices will be described with reference to  FIGS. 27A to 27C  and  FIG. 28 . 
     An electronic device or a lighting device that has a light-emitting region with a curved surface can be obtained with the use of the light-emitting element of one embodiment of the present invention which is manufactured over a substrate having flexibility. 
     Furthermore, a light-emitting device to which one embodiment of the present invention is applied can also be used for lighting for motor vehicles, examples of which are lighting for a dashboard, a windshield, a ceiling, and the like. 
       FIG. 27A  is a perspective view illustrating one surface of a multifunction terminal  3500 , and  FIG. 27B  is a perspective view illustrating the other surface of the multifunction terminal  3500 . In a housing  3502  of the multifunction terminal  3500 , a display portion  3504 , a camera  3506 , lighting  3508 , and the like are incorporated. The light-emitting device of one embodiment of the present invention can be used for the lighting  3508 . 
     The lighting  3508  that includes the light-emitting device of one embodiment of the present invention functions as a planar light source. Thus, unlike a point light source typified by an LED, the lighting  3508  can provide light emission with low directivity. When the lighting  3508  and the camera  3506  are used in combination, for example, imaging can be performed by the camera  3506  with the lighting  3508  lighting or flashing. Because the lighting  3508  functions as a planar light source, a photograph as if taken under natural light can be taken. 
     Note that the multifunction terminal  3500  illustrated in  FIGS. 27A and 27B  can have a variety of functions as in the electronic devices illustrated in  FIGS. 24A to 24G . 
     The housing  3502  can include a speaker, a sensor (a sensor having a function of measuring force, displacement, position, speed, acceleration, angular velocity, rotational frequency, distance, light, liquid, magnetism, temperature, chemical substance, sound, time, hardness, electric field, current, voltage, electric power, radiation, flow rate, humidity, gradient, oscillation, odor, or infrared rays), a microphone, and the like. When a detection device including a sensor for detecting inclination, such as a gyroscope or an acceleration sensor, is provided inside the multifunction terminal  3500 , display on the screen of the display portion  3504  can be automatically switched by determining the orientation of the multifunction terminal  3500  (whether the multifunction terminal is placed horizontally or vertically for a landscape mode or a portrait mode). 
     The display portion  3504  may function as an image sensor. For example, an image of a palm print, a fingerprint, or the like is taken when the display portion  3504  is touched with the palm or the finger, whereby personal authentication can be performed. Furthermore, by providing a backlight or a sensing light source which emits near-infrared light in the display portion  3504 , an image of a finger vein, a palm vein, or the like can be taken. Note that the light-emitting device of one embodiment of the present invention may be used for the display portion  3504 . 
       FIG. 27C  is a perspective view of a security light  3600 . The security light  3600  includes lighting  3608  on the outside of the housing  3602 , and a speaker  3610  and the like are incorporated in the housing  3602 . The light-emitting device of one embodiment of the present invention can be used for the lighting  3608 . 
     The security light  3600  emits light when the lighting  3608  is gripped or held, for example. An electronic circuit that can control the manner of light emission from the security light  3600  may be provided in the housing  3602 . The electronic circuit may be a circuit that enables light emission once or intermittently plural times or may be a circuit that can adjust the amount of emitted light by controlling the current value for light emission. A circuit with which a loud audible alarm is output from the speaker  3610  at the same time as light emission from the lighting  3608  may be incorporated. 
     The security light  3600  can emit light in various directions; therefore, it is possible to intimidate a thug or the like with light, or light and sound. Moreover, the security light  3600  may include a camera such as a digital still camera to have a photography function. 
       FIG. 28  illustrates an example in which the light-emitting element is used for an indoor lighting device  8501 . Since the light-emitting element can have a larger area, a lighting device having a large area can also be formed. In addition, a lighting device  8502  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 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. Furthermore, a wall of the room may be provided with a large-sized lighting device  8503 . Touch sensors may be provided in the lighting devices  8501 ,  8502 , and  8503  to control the power on/off of the lighting devices. 
     Moreover, when the light-emitting element is used on the surface side of a table, a lighting device  8504  which has a function as a table can be obtained. When the light-emitting element is used as part of other furniture, a lighting device which has a function as the furniture can be obtained. 
     As described above, lighting devices and electronic devices can be obtained by application of the light-emitting device of one embodiment of the present invention. Note that the light-emitting device can be used for lighting devices and electronic devices in a variety of fields without being limited to the lighting devices and the electronic devices described in this embodiment. 
     Note that the structure described in this embodiment can be combined as appropriate with any of the structures described in the other embodiments. 
     Example 
     In this example, examples of fabricating light-emitting elements of embodiments of the present invention will be described. Schematic cross-sectional views of light-emitting elements fabricated in this example are similar to that of the light-emitting element  250  in  FIG. 1A . Table 1 shows the detailed structures of the elements. In addition, structures and abbreviations of compounds used in this example are given below. 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 reference 
                 thickness 
                   
                   
               
               
                   
                 layer 
                 numeral 
                 (nm) 
                 materials 
                 weight ratio 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Light-emitting 
                 electrode 
                 102 
                 200 
                 Al 
                 — 
               
               
                 element 1 
                 electron-injection layer 
                 119 
                 1 
                 LiF 
                 — 
               
               
                   
                 electron-transport layer 
                 118(2) 
                 10 
                 BPhen 
                 — 
               
               
                   
                   
                 118(1) 
                 20 
                 4,6mCzP2Pm 
                 — 
               
               
                   
                 light-emitting layer 
                 130 
                 40 
                 4,6mCzP2Pm:PCCzTp:Rubrene 
                 0.8:0.2:0 01 
               
               
                   
                 hole-transport layer 
                 112 
                 20 
                 BPAFLP 
                 — 
               
               
                   
                 hole-injection layer 
                 111 
                 20 
                 DBT3P-II:MoO 3   
                 1:0.5 
               
               
                   
                 electrode 
                 101 
                 110 
                 ITSO 
                 — 
               
               
                 Light-emitting 
                 electrode 
                 102 
                 200 
                 Al 
                 — 
               
               
                 element 2 
                 electron-injection layer 
                 119 
                 1 
                 LiF 
                 — 
               
               
                   
                 electron-transport layer 
                 118(2) 
                 10 
                 BPhen 
                 — 
               
               
                   
                   
                 118(1) 
                 20 
                 4,6mCzP2Pm 
                 — 
               
               
                   
                 light emitting layer 
                 130 
                 40 
                 4,6mCzP2Pm:FrBBiF II:Rubrene 
                 0.8:0.2:0.01 
               
               
                   
                 hole-transport layer 
                 112 
                 20 
                 BPAFLP 
                 — 
               
               
                   
                 hole-injection layer 
                 111 
                 20 
                 DBT3P-II:MoO 3   
                 1:0.5 
               
               
                   
                 electrode 
                 101 
                 110 
                 ITSO 
                 — 
               
               
                 Light-emitting 
                 electrode 
                 102 
                 200 
                 Al 
                 — 
               
               
                 element 3 
                 electron-injection layer 
                 119 
                 1 
                 LiF 
                 — 
               
               
                   
                 electron-transport layer 
                 118(2) 
                 10 
                 BPhen 
                 — 
               
               
                   
                   
                 118(1) 
                 20 
                 4,6mCzP2Pm 
                 — 
               
               
                   
                 light-emitting layer 
                 130 
                 40 
                 4,6mCzP2Pm:PCBBiF:Rubrene 
                 0.8:0.2:0.01 
               
               
                   
                 hole-transport layer 
                 112 
                 20 
                 BPAFLP 
                 — 
               
               
                   
                 hole-injection layer 
                 111 
                 20 
                 DBT3P-II:MoO 3   
                 1:0.5 
               
               
                   
                 electrode 
                 101 
                 110 
                 ITSO 
                 — 
               
               
                 Light-emitting 
                 electrode 
                 102 
                 200 
                 Al 
                 — 
               
               
                 element 4 
                 electron-injection layer 
                 119 
                 1 
                 LiF 
                 — 
               
               
                   
                 electron-transport layer 
                 118(2) 
                 10 
                 BPhen 
                 — 
               
               
                   
                   
                 118(1) 
                 20 
                 4,6mCzP2Pm 
                 — 
               
               
                   
                 light-emitting layer 
                 130 
                 40 
                 4,6mCzP2Pm:PCBiF:Rubrene 
                 0.8:0.2:0.01 
               
               
                   
                 hole-transport layer 
                 112 
                 20 
                 BPAFLP 
                 — 
               
               
                   
                 hole-injection layer 
                 111 
                 20 
                 DBT3P-II:MoO 3   
                 1:0.5 
               
               
                   
                 electrode 
                 101 
                 110 
                 ITSO 
                 — 
               
               
                   
               
            
           
         
       
     
     &lt;Fabrication of Light-Emitting Element 1&gt; 
     As the electrode  101 , an ITSO film was formed to a thickness of 110 nm over a substrate. The area of the electrode  101  was set to 4 mm 2  (2 mm 2 mm). 
     Next, the EL layer  100  was formed over the electrode  101 . As the hole-injection layer  111 , 1,3,5-tri(dibenzothiophen-4-yl)benzene (abbreviation: DBT3P-II) and molybdenum oxide (MOO 3 ) were deposited by co-evaporation such that the deposited layer has a weight ratio of DBT3P-II to MoO 3  of 1:0.5 and a thickness of nm. Then, as the hole-transport layer  112 , 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine (abbreviation: BPAFLP) was deposited by evaporation to a thickness of 20 nm. 
     Next, as the light-emitting layer  130 , 4,6-bis[3-(9H-carbazol-9-yl)phenyl]pyrimidine (abbreviation: 4,6mC7P2Pm), 9-phenyl-9′-(triphenylen-2-yl)-3,3′-bi-9H-carbazole (abbreviation: PCCzTp), and Rubrene were deposited by co-evaporation such that the deposited layer has a weight ratio of 4,6mCzP2Pm and PCCzTp to Rubrene of 0.8:0.2:0.01 and a thickness of 40 nm. In the light-emitting layer  130 , 4,6mCzP2Pm and PCCzTp serve as the host material  131  and Rubrene serves as the guest material  132  (fluorescent material). 
     As the elcctron-transport layer  118 , 4,6mCzP2Pm and Bphen were sequentially deposited by evaporation to thicknesses of 20 nm and 10 nm, respectively, over the light-emitting layer  130 . Next, as the electron-injection layer  119 , lithium fluoride (LiF) was deposited by evaporation to a thickness of 1 nm. 
     As the electrode  102 , aluminum (Al) was deposited to a thickness of 200 nm. 
     Next, in a glove box containing a nitrogen atmosphere, Light-emitting element 1 was sealed by fixing a sealing substrate to the substrate provided with the EL layer  100  using a sealant for an organic EL device. Specifically, after the sealant was applied to surround the EL layer  100  over the substrate and the substrate was bonded to the sealing substrate, irradiation with ultraviolet light having a wavelength of 365 nm at 6 J/cm 2  and heat treatment at 80° C. for one hour were performed. Through the above process, Light-emitting element 1 was obtained. 
     &lt;Fabrication of Light-Emitting Elements 2 to 4&gt; 
     Light-emitting elements 2 to 4 are different from the above-described Light-emitting element 1 in only the host material of the light-emitting layer  130 , and steps for the other components are the same as those in a method for fabricating Light-emitting element 1. 
     As the light-emitting layer  130  of Light-emitting element 2, 4,6mCzP2Pm, N-(1,1′-biphenyl-4-yl)-N-[4-(dibenzofuran-4-yl)phenyl]-9,9-dimethyl-9H-fluoren-2-amine (abbreviation: FrBBiF-II), and Rubrene were deposited by co-evaporation such that the deposited layer has a weight ratio of 4,6mCzP2Pm to FrBBiF-II and Rubrene of 0.8:0.2:0.01 and a thickness of 40 nm. In the light-emitting layer  130 , 4,6mCzP2Pm and FrBBiF-II serve as the host material  131  and Rubrene serves as the guest material  132  (fluorescent material). 
     As the light-emitting layer  130  of Light-emitting element 3, 4,6mCzP2Pm, N-(1,1′-biphenyl-4-yl)-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-9,9-dimethyl-9H-fluor en-2-amine (abbreviation: PCBBiF), and Rubrene were deposited by co-evaporation such that the deposited layer has a weight ratio of 4,6mCzP2Pm to PCBBiF and Rubrene of 0.8:0.2:0.01 and a thickness of 40 nm. In the light-emitting layer  130 , 4,6mCzP2Pm and PCBBiF serve as the host material  131  and Rubrene serves as the guest material  132  (fluorescent material). 
     As the light-emitting layer  130  of Light-emitting element 4, 4,6mCzP2Pm, N-(4-biphenyl)-N-(9,9-dimethyl-9H-fluoren-2-yl)-9-phenyl-9H-carbazol-3-amine (abbreviation: PCBiF), and Rubrene were deposited by co-evaporation such that the deposited layer has a weight ratio of 4,6mCzP2Pm to PCBiF and Rubrene of 0.8:0.2:0.01 and a thickness of 40 nm. In the light-emitting layer  130 , 4,6mCzP2Pm and PCBiF serve as the host material  131  and Rubrene serves as the guest material  132  (fluorescent material). 
     &lt;Operation Characteristics 1 of Light-Emitting Elements&gt; 
     Next, emission characteristics of the fabricated Light-emitting elements 1 to 4 were measured. Note that the measurement was performed at room temperature (in an atmosphere kept at 23° C.). 
     The emission characteristics of the light-emitting elements at a luminance around 1000 cd/m 2  are shown below in Table 2. The current efficiency-luminance characteristics, external quantum efficiency-luminance characteristics, and luminance-voltage characteristics of the light-emitting elements are shown in  FIG. 29 ,  FIG. 30 , and  FIG. 31 , respectively.  FIG. 32  shows electroluminescence spectra at the time when a current was made to flow in the light-emitting elements at a current density of 2.5 mA/cm 2 . 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                 TABLE 
               
               
                   
               
               
                   
                   
                 Current 
                 CIE 
                   
                 Current 
                 Power 
                 External 
               
               
                   
                 Voltage 
                 density 
                 chromaticity 
                 Luminance 
                 Efficiency 
                 Efficiency 
                 Quantrum 
               
               
                   
                 (V) 
                 (mA/cm 2 ) 
                 (x, y) 
                 (cd/m 2 ) 
                 (cd/A) 
                 (lm/W) 
                 Efficiency (%) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Light-emitting 
                 4.4 
                 6.1 
                 (0.49, 0.49) 
                 1000 
                 16 
                 12 
                 5.1 
               
               
                 element 1 
                   
                   
                   
                   
                   
                   
                   
               
               
                 Light-emitting 
                 4.0 
                 5.3 
                 (0.49, 0.50) 
                 860 
                 16 
                 13 
                 5.0 
               
               
                 element 2 
                   
                   
                   
                   
                   
                   
                   
               
               
                 Light-emitting 
                 4.0 
                 5.7 
                 (0.48, 0.51) 
                 1200 
                 20 
                 16 
                 6.1 
               
               
                 element 3 
                   
                   
                   
                   
                   
                   
                   
               
               
                 Light-emitting 
                 3.8 
                 3.8 
                 (0.48, 0.51) 
                 1100 
                 29 
                 24 
                 8.5 
               
               
                 element 4 
                   
                   
                   
                   
                   
                   
                   
               
               
                   
               
            
           
         
       
     
     As shown by the peaks in the electroluminescence spectra in  FIG. 32 , only yellow light emission derived from Rubrene, which is a fluorescent material, was observed from Light-emitting elements 1 to 4. 
     In addition, as shown in  FIG. 29 ,  FIG. 30 , and Table 2, Light-emitting elements 1 to 4 have high current efficiency and high external quantum efficiency. In particular, Light-emitting element 4 has external quantum efficiency of higher than 8% at a luminance around 1000 cd/m 2 . 
     Since the probability of formation of singlet excitons which are generated by recombination of carriers (holes and electrons) injected from the pair of electrodes is at most 25%, the external quantum efficiency in the case where the light extraction efficiency to the outside is 20% is at most 5%. Light-emitting elements 1 to 4 can have external quantum efficiency of higher than 5%. This is because Light-emitting elements 1 to 4 emit, in addition to light originating from singlet excitons generated by recombination of carriers (holes and electrons) injected from the pair of electrodes, light originating from singlet excitons generated from triplet excitons by ExEF. 
     Moreover, as shown in  FIG. 31  and Table 2, Light-emitting elements 1 to 4 drive at a low voltage. That is, by including a light-emitting layer using ExEF, a light-emitting element that drives at a low voltage can be fabricated. Furthermore, a light-emitting element with reduced power consumption can be fabricated. 
     Next, to examine whether Light-emitting elements 1 to 4 emit light by ExEF, light-emitting elements in which the guest material  132  is not contained were fabricated and measured. Table 3 shows the detailed structures of the elements. 
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 3 
               
               
                   
               
               
                   
                   
                 reference 
                 thickness 
                   
                   
               
               
                   
                 layer 
                 numeral 
                 (nm) 
                 materials 
                 weight ratio 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Light-emitting 
                 electrode 
                 102 
                 200 
                 Al 
                 — 
               
               
                 element 5 
                 electron-injection layer 
                 119 
                 1 
                 LiF 
                 — 
               
               
                   
                 electron-transport layer 
                 118(2) 
                 10 
                 BPhen 
                 — 
               
               
                   
                   
                 118(1) 
                 20 
                 4,6mCzP2Pm 
                 — 
               
               
                   
                 light-emitting layer 
                 130 
                 40 
                 4,6mCzP2Pm:PCCzTp 
                 0.8:0.2 
               
               
                   
                 hole-transport layer 
                 112 
                 20 
                 BPAFLP 
                 — 
               
               
                   
                 hole-injection layer 
                 111 
                 20 
                 DBT3P-II:MoO 3   
                 1:0.5 
               
               
                   
                 electrode 
                 101 
                 110 
                 ITSO 
                 — 
               
               
                 Light-emitting 
                 electrode 
                 102 
                 200 
                 Al 
                 — 
               
               
                 element 6 
                 electron-injection layer 
                 119 
                 1 
                 LiF 
                 — 
               
               
                   
                 electron-transport layer 
                 118(2) 
                 10 
                 BPhen 
                 — 
               
               
                   
                   
                 118(1) 
                 20 
                 4,6mCzP2Pm 
                 — 
               
               
                   
                 light-emitting layer 
                 130 
                 40 
                 4,6mCzP2Pm:FrBBiF-II 
                 0.8:0.2 
               
               
                   
                 hole-transport layer 
                 112 
                 20 
                 BPAFLP 
                 — 
               
               
                   
                 hole-injection layer 
                 111 
                 20 
                 DBT3P-II:MoO 3   
                 1:0.5 
               
               
                   
                 electrode 
                 101 
                 110 
                 ITSO 
                 — 
               
               
                 Light-emitting 
                 electrode 
                 102 
                 200 
                 Al 
                 — 
               
               
                 element 7 
                 electron-injection layer 
                 119 
                 1 
                 LiF 
                 — 
               
               
                   
                 electron-transport layer 
                 118(2) 
                 10 
                 BPhen 
                 — 
               
               
                   
                   
                 118(1) 
                 20 
                 4,6mCzP2Pm 
                 — 
               
               
                   
                 light-emitting layer 
                 130 
                 40 
                 4,6mCzP2Pm:PCBBiF 
                 0.8:0.2 
               
               
                   
                 hole-transport layer 
                 112 
                 20 
                 BPAFLP 
                 — 
               
               
                   
                 hole-injection layer 
                 111 
                 20 
                 DBT3P-II:MoO 3   
                 1:0.5 
               
               
                   
                 electrode 
                 101 
                 110 
                 ITSO 
                 — 
               
               
                 Light-emitting 
                 electrode 
                 102 
                 200 
                 Al 
                 — 
               
               
                 element 8 
                 electron-injection layer 
                 119 
                 1 
                 LiF 
                 — 
               
               
                   
                 electron-transport layer 
                 118(2) 
                 10 
                 BPhen 
                 — 
               
               
                   
                   
                 118(1) 
                 20 
                 4,6mCzP2Pm 
                 — 
               
               
                   
                 light-emitting layer 
                 130 
                 40 
                 4,6mCzP2Pm:PCBiF 
                 0.8:0.2 
               
               
                   
                 hole-transport layer 
                 112 
                 20 
                 BPAFLP 
                 — 
               
               
                   
                 hole-injection layer 
                 111 
                 20 
                 DBT3P-II:MoO 3   
                 1:0.5 
               
               
                   
                 electrode 
                 101 
                 110 
                 ITSO 
                 — 
               
               
                   
               
            
           
         
       
     
     &lt;Fabrication of Light-Emitting Elements 5 to 8&gt; 
     Light-emitting elements 5 to 8 are different from the above-described Light-emitting elements 1 to 4 in that the guest material  132  is not contained in the light-emitting layer  130 , and steps for the other components are the same as those in methods for fabricating Light-emitting elements 1 to 4. 
     As the light-emitting layer  130  of Light-emitting element 5, 4,6mCzP2Pm and PCCzTp were deposited by co-evaporation such that the deposited layer has a weight ratio of 4,6mCzP2Pm to PCCzTp of 0.8:0.2 and a thickness of 40 nm. In the light-emitting layer  130 , 4,6mCzP2Pm and PCCzTp correspond to the host material  131  and the guest material  132  is not contained. 
     As the light-emitting layer  130  of Light-emitting element 6, 4,6mCzP2Pm and FrBBiF-II were deposited by co-evaporation such that the deposited layer has a weight ratio of 4,6mCzP2Pm to FrBBiF-II of 0.8:0.2 and a thickness of 40 nm. In the light-emitting layer  130 , 4,6mCzP2Pm and FrBBiF-II correspond to the host material  131  and the guest material  132  is not contained. 
     As the light-emitting layer  130  of Light-emitting element 7, 4,6mCzP2Pm and PCBBiF were deposited by co-evaporation such that the deposited layer has a weight ratio of 4,6mCzP2Pm to PCBBiF of 0.8:0.2 and a thickness of 40 nm. In the light-emitting layer  130 , 4,6mCzP2Pm and PCBBiF correspond to the host material  131  and the guest material  132  is not contained. 
     As the light-emitting layer  130  of Light-emitting element 8, 4,6mCzP2Pm and PCBiF were deposited by co-evaporation such that the deposited layer has a weight ratio of 4,6mCzP2Pm to PCBiF of 0.8:0.2 and a thickness of 40 nm. In the light-emitting layer  130 , 4,6mCzP2Pm and PCBiF correspond to the host material  131  and the guest material  132  is not contained. 
     &lt;Operation Characteristics 2 of Light-Emitting Elements&gt; 
     Next, emission characteristics of the fabricated Light-emitting elements 5 to 8 were measured. Note that the measurement was performed at room temperature (in an atmosphere kept at 23° C.). 
     The emission characteristics of the light-emitting elements at a luminance around 1000 cd/m 2  are shown below in Table 4. The current efficiency-luminance characteristics, external quantum efficiency-luminance characteristics, and luminance-voltage characteristics of the light-emitting elements are shown in  FIG. 33 ,  FIG. 34 , and  FIG. 35 , respectively.  FIG. 36  shows electroluminescence spectra at the time when a current was made to flow in the light-emitting elements at a current density of 2.5 mA/cm 2 . 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                 TABLE 4 
               
               
                   
               
               
                   
                   
                 Current 
                 CIE 
                   
                 Current 
                 Power 
                 External 
               
               
                   
                 Voltage 
                 density 
                 chromaticity 
                 Luminance 
                 Efficiency 
                 Efficiency 
                 Quantrum 
               
               
                   
                 (V) 
                 (mA/cm 2 ) 
                 (x, y) 
                 (cd/m 2 ) 
                 (cd/A) 
                 (lm/W) 
                 Efficiency (%) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Light-emitting 
                 4.4 
                 20 
                 (0.22, 0.36) 
                 950 
                 4.8 
                 3.4 
                 2.0 
               
               
                 element 5 
                   
                   
                   
                   
                   
                   
                   
               
               
                 Light-emitting 
                 4.4 
                 19 
                 (0.26, 0.47) 
                 970 
                 5.1 
                 3.7 
                 1.7 
               
               
                 element 6 
                   
                   
                   
                   
                   
                   
                   
               
               
                 Light-emitting 
                 3.7 
                 5.2 
                 (0.35, 0.56) 
                 1000 
                 20 
                 17 
                 6.1 
               
               
                 element 7 
                   
                   
                   
                   
                   
                   
                   
               
               
                 Light-emitting 
                 3.5 
                 2.9 
                 (0.41, 0.56) 
                 1100 
                 38 
                 34 
                 11 
               
               
                 element 8 
                   
                   
                   
                   
                   
                   
                   
               
               
                   
               
            
           
         
       
     
       FIG. 37  shows emission spectra of thin films of 4,6mCzP2Pm, PCCzTp, FrBBiF-II, PCBBiF, and PCBiF, which are used as host materials. Note that the emission spectra of these thin films were measured with a PL-FL measurement apparatus (manufactured by Hamamatsu Photonics K.K.). 
     As shown in  FIG. 37 , from 4,6mCzP2Pm, PCCzTp, FrBBiF-II, PCBBiF, and PCBiF, which were used as host materials, blue light emissions having peak wavelengths of 493 nm, 418 nm, 428 nm, 436 nm, and 430 nm were observed, respectively. 
     In contrast, as shown in  FIG. 36 , from the electroluminescence spectrum peaks of Light-emitting element 5, Light-emitting element 6, Light-emitting element 7, and Light-emitting element 8, green to yellow light emissions having peak wavelengths of 499 nm, 513 nm, 536 nm, and 552 nm were observed, respectively, and the full width at half maximum of any of the emission spectra is larger than those of the emission spectra of individual compounds. The wavelengths of emission spectra of Light-emitting element 5, Light-emitting element 6, Light-emitting element 7, and Light-emitting element 8 correlate with the energy differences between the LUMO level of 4,6mCzP2Pm and the HOMO level of PCCzTp, between the LUMO level of 4,6mCzP2Pm and the HOMO level of PrBBiF-II, between the LUMO level of 4,6mCzP2Pm and the HOMO level of PCBBiF, and between the LUMO level of 4,6mCzP2Pm and the HOMO level of PCBiF, respectively. Thus, Light-emitting element 5, Light-emitting element 6. Light-emitting element 7, and Light-emitting element 8 emit light emitted from exciplexes formed of 4,6mCzP2Pm and PCCzTp, 4,6mCzP2Pm and FrBBiF-II, 4,6mCzP2Pm and PCBBiF, and 4,6mCzP2Pm and PCBiF, respectively. 
     In addition,  FIG. 38  shows a measurement result of an absorption spectrum of Rubrene in a toluene solution, which was used as a guest material of Light-emitting elements 1 to 4. 
     As shown in  FIG. 38 , the absorption spectrum of Rubrene has an absorption band which has a high molar absorption coefficient at around 450 nm to 550 nm. This wavelength range substantially corresponds to the wavelength ranges of the electroluminescence spectra of the exciplexes shown in  FIG. 36 . Therefore, it is found that the compounds which form an exciplex is preferably used as the host material of Light-emitting elements 1 to 4 because the efficiency of energy transfer to the guest material is increased. 
     In addition, as shown in  FIG. 33 ,  FIG. 34 , and Table 4, Light-emitting elements 7 and 8 have high emission efficiency (current efficiency and external quantum efficiency). In particular, in Light-emitting element 8, a drop (roll-off) in emission efficiency is small even on the high luminance side. 
     At a luminance around 1000 cd/m 2 , the emission efficiency of Light-emitting element 5 and the emission efficiency of Light-emitting element 6 substantially correspond to each other, the emission efficiency of Light-emitting element 7 is higher than those of Light-emitting elements 5 and 6, and the emission efficiency of Light-emitting element 8 is higher than that of Light-emitting element 7. In addition, as for the above-described Light-emitting elements 1 to 4, at a luminance around 1000 cd/m 2 , the emission efficiency of Light-emitting element 1 and the emission efficiency of Light-emitting element 2 substantially correspond to each other, the emission efficiency of Light-emitting element 3 is higher than those of Light-emitting elements 1 and 2, and the emission efficiency of Light-emitting element 4 is higher than that of Light-emitting element 3. 
     That is, the emission efficiency of light emitted from the exciplex correlates with the emission efficiency of light emitted from the guest material. The high emission efficiency of the exciplex means a small rate constant of non-radiative deactivation in the exciplex, which indicates a large rate constant of reverse intersystem crossing. 
     Then, the time-resolved fluorescence measurement was performed on thin films similar to light-emitting layers of Light-emitting elements 5 to 8. 
     &lt;Fabrication of Thin-Film Samples&gt; 
     For the time-resolved fluorescence measurement of light-emitting layers of light-emitting elements, thin-film samples were fabricated on a quartz substrate by a vacuum evaporation method. 
     As Thin film 1, 4,6mCzP2Pm and FrBBiF-II were deposited by co-evaporation such that the deposited film has a weight ratio of 4,6mCzP2Pm to FrBBiF-II of 0.8:0.2 and a thickness of 50 nm. 
     As Thin film 2, 4,6mCzP2Pm and PCBBiF were deposited by co-evaporation such that the deposited film has a weight ratio of 4,6mCzP2Pm to PCBBiF of 0.8:0.2 and a thickness of 50 nm. 
     As Thin film 3, 4,6mCzP2Pm and PCBiF were deposited by co-evaporation such that the deposited film has a weight ratio of 4,6mCzP2Pm to PCBiF of 0.8:0.2 and a thickness of 50 nm. 
     Thin films 1 to 3 were each sealed by fixing a sealing substrate to the quartz substrate over which the thin-film sample was deposited using a sealant for an organic EL device in a glove box under a nitrogen atmosphere. Specifically, after a sealant was applied to surround each of the thin films over the quartz substrate and the quartz substrate was bonded to the sealing substrate, irradiation with ultraviolet light having a wavelength of 365 nm at 6 J/cm 2  and heat treatment at 80° C. for one hour were performed. 
     &lt;Time-Resolved Fluorescence Measurement of Thin-Film Samples&gt; 
     A picosecond fluorescence lifetime measurement system (manufactured by Hamamatsu Photonics K.K.) was used for the measurement. In this measurement, the thin film was irradiated with pulsed laser, and emission of the thin film which was attenuated from the laser irradiation underwent time-resolved measurement using a streak camera to measure the lifetime of fluorescent emission of the thin film. A nitrogen gas laser with a wavelength of 337 nm was used as the pulsed laser. The thin film was irradiated with pulsed laser with a pulse width of 500 ps at a repetition rate of 10 Hz. By integrating data obtained by the repeated measurement, data with a high S/N ratio was obtained. The measurement was performed at room temperature (in an atmosphere kept at 23° C.) 
     From each of Thin films 1 to 3, light emitted from an exciplex formed of two compounds was observed. The attenuation curves obtained by the measurement are shown in  FIGS. 39A and 39B . 
     The attenuation curves shown in  FIGS. 39A and 39B  were fitted with Formula (4). 
     
       
         
           
             
               
                 
                   L 
                   = 
                   
                     
                       ∑ 
                       
                         n 
                         = 
                         1 
                       
                     
                     
                       
                         A 
                         n 
                       
                       ⁢ 
                       
                         exp 
                         ⁡ 
                         ( 
                         
                           - 
                           
                             t 
                             
                               a 
                               n 
                             
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     In Formula (4), L and t represent normalized emission intensity and elapsed time, respectively. The attenuation curve was able to be fitted when n was 1 and 2. This fitting results show that: the emission component of Thin film 1 contains a prompt fluorescent component having a fluorescence lifetime of 0.60 μs and a delayed fluorescence component having a fluorescence lifetime of 96 μs; the emission component of Thin film 2 contains a prompt fluorescent component having a fluorescence lifetime of 0.72 μs and a delayed fluorescence component having a fluorescence lifetime of 55 μs; and the emission component of Thin film 3 contains a prompt fluorescent component having a fluorescence lifetime of 0.67 μs and a delayed fluorescence component having a fluorescence lifetime of 23 μs. That is, it is found that delayed fluorescence lifetimes of the exciplexes used in Light-emitting elements 4 and 8 which have high efficiency are 50 μs or less, which is shorter than those of the exciplexes used in Light-emitting elements 2, 3, 6, and 7. In addition, the percentages of the delayed fluorescence component in light emitted from Thin film 1, Thin film 2, and Thin film 3 were calculated to 0.084%, 3.8%, and 9.4%, respectively. 
     As described above, Thin film 3 contains a delayed fluorescence component having a relatively short fluorescence lifetime. The short lifetime of the delayed fluorescence component indicates a large rate constant of reverse intersystem crossing, which supports the results obtained from Light-emitting element 8. That is, triplet excitons in the exciplex are converted into singlet excitons in a relatively short time, whereby there is no saturation of exciton density even in a high-luminance region (a region where exciton density is high); thus, the emission efficiency of Light-emitting element 8 is less likely to be reduced in the high-luminance region. The comparison between Light-emitting elements 1 to 4 shows that the emission efficiency of Light-emitting element 3 (the delayed fluorescence lifetime measured using the time-resolved fluorescence measurement of the exciplex is 55 μs) is increased compared with Light-emitting element 1 and 2 and the emission efficiency of Light-emitting element 4 (the delayed fluorescence lifetime measured using the time-resolved fluorescence measurement of the exciplex is 23 μs) is significantly increased. Thus, in one embodiment of the present invention, the delayed fluorescence lifetime measured using the time-resolved fluorescence measurement of the exciplex is preferably 50 μs or less, further preferably 40 μs or less, still further preferably 30 μs or less. 
     In addition, the time-resolved fluorescence measurement shows that Thin film 3 contains the delayed fluorescence component at 5% or more of light emitted from Thin film 3; thus, it indicates that the energy transfer between the singlet excited state and the triplet excited state occurs at a relatively high probability. 
     Furthermore, Light-emitting element 4 which contains an exciplex containing a short-life delayed fluorescence component as a host material has high emission efficiency. This is because the triplet excitons in the exciplex are converted into the singlet excitons in a relatively short time, whereby the efficiency of energy transfer from the triplet excited state of the exciplex to the triplet excited state of the guest material can be reduced and the generation efficiency of the singlet excitons in the exciplex and the guest material can be increased. 
     As described above, compounds which form an exciplex are contained as a host material and the exciplex contains a delayed fluorescence component having a relatively short fluorescence lifetime of 50 μs or less at 5% or more, so that a light-emitting element having high emission efficiency like Light-emitting element 4 can be fabricated. 
     The structure described in this example can be combined as appropriate with any of the embodiments. 
     This application is based on Japanese Patent Application serial no. 2015-046376 filed with Japan Patent Office on Mar. 9, 2015, the entire contents of which are hereby incorporated by reference.