Patent Publication Number: US-2023157042-A1

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

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 16/923,526, filed Jul. 8, 2020, now allowed, which is a continuation of U.S. application Ser. No. 16/513,869, filed Jul. 17, 2019, now U.S. Pat. No. 10,714,700, which is a continuation of U.S. application Ser. No. 15/874,950, filed Jan. 19, 2018, now U.S. Pat. No. 10,693,095, which is a continuation of U.S. application Ser. No. 14/837,083, filed Aug. 27, 2015, now U.S. Pat. No. 9,911,936, which claims the benefit of foreign priority applications filed in Japan as Serial No. 2014-175163 on Aug. 29, 2014, Serial No. 2014-240985 on Nov. 28, 2014, and Serial No. 2015-108786 on May 28, 2015, all of which are incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     One embodiment of the present invention relates to a light-emitting element in which a light-emitting layer capable of providing light emission by application of an electric field is provided between a pair of electrodes, and also relates to a display device, an electronic device, and a lighting device 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. 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 memory device, a method for driving any of them, and a method for manufacturing any of them. 
     2. Description of the Related Art 
     In recent years, research and development have been extensively conducted on light-emitting elements utilizing electroluminescence (EL). In a basic structure of these light-emitting elements, a layer containing a light-emitting substance (an EL layer) is provided 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. The display device using the light-emitting element also has advantages in that it can be manufactured to be thin and lightweight and has high response speed. 
     A variety of studies have been conducted to improve the emission efficiency of light-emitting elements. For example, a method has been proposed for a light-emitting element including a thermally activated delayed fluorescence (TADF) material and a material which emits fluorescence (hereinafter also referred to as a fluorescent material) to transfer energy from the S 1  of the TADF material to the S 1  of the fluorescent material (see Patent Document 1). 
     REFERENCE 
     
         
         [Patent Document 1] Japanese Published Patent Application No. 2014-45179 
       
    
     SUMMARY OF THE INVENTION 
     In order to increase the emission efficiency of a light-emitting element including a fluorescent material as a light-emitting substance, it is important not only to generate a singlet excited state from a triplet excited state but also to obtain light emission efficiently from the singlet excited state, that is, to increase fluorescence quantum efficiency. 
     An object of one embodiment of the present invention is to provide a light-emitting element having high emission efficiency which includes a fluorescent material as a light-emitting substance. 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 element with high emission efficiency and low power consumption. Another object is to provide a novel display device. 
     Note that the descriptions of the above objects do not disturb the existence of other objects. In one embodiment of the present invention, there is no need to achieve all the objects. Other objects will be apparent from and can be derived from the descriptions of the specification and the like. 
     One embodiment of the present invention is a light-emitting element which includes a pair of electrodes and an EL layer between the pair of electrodes. The EL layer includes a light-emitting layer. The light-emitting layer includes a host material and a guest material. The host material has a difference of more than 0 eV and less than or equal to 0.2 eV between a singlet excitation energy level and a triplet excitation energy level. The guest material is capable of emitting fluorescence. The triplet excitation energy level of the host material is higher than a triplet excitation energy level of the guest material. 
     Another embodiment of the present invention is a light-emitting element which includes a pair of electrodes and an EL layer between the pair of electrodes. The EL layer includes a light-emitting layer. The light-emitting layer includes a host material and a guest material. The host material is capable of exhibiting thermally activated delayed fluorescence at room temperature. The guest material is capable of emitting fluorescence. A thermally activated delayed fluorescence emission energy of the host material is higher than a phosphorescence emission energy of the guest material. 
     Another embodiment of the present invention is a light-emitting element which includes a pair of electrodes and an EL layer between the pair of electrodes. The EL layer includes a light-emitting layer. The light-emitting layer includes a host material and a guest material. The host material includes a first organic compound and a second organic compound. A combination of the first organic compound and the second organic compound forms an exciplex. The exciplex has a difference of more than 0 eV and less than or equal to 0.2 eV between a singlet excitation energy level and a triplet excitation energy level. The guest material is capable of emitting fluorescence. The triplet excitation energy level of the exciplex is higher than a triplet excitation energy level of the guest material. 
     Another embodiment of the present invention is a light-emitting element which includes a pair of electrodes and an EL layer between the pair of electrodes. The EL layer includes a light-emitting layer. The light-emitting layer includes a host material and a guest material. The host material includes a first organic compound and a second organic compound. A combination of the first organic compound and the second organic compound forms an exciplex. The exciplex is capable of exhibiting thermally activated delayed fluorescence at room temperature. The guest material is capable of emitting fluorescence. A thermally activated delayed fluorescence emission energy of the exciplex is higher than a phosphorescence emission energy of the guest material. 
     Another embodiment of the present invention is a light-emitting element which includes a pair of electrodes and an EL layer between the pair of electrodes. The EL layer includes a light-emitting layer. The light-emitting layer includes a host material and a guest material. The host material includes a first organic compound and a second organic compound. A combination of the first organic compound and the second organic compound forms an exciplex. The exciplex has a difference of more than 0 eV and less than or equal to 0.2 eV between a singlet excitation energy level and a triplet excitation energy level. The guest material is capable of emitting fluorescence. The triplet excitation energy level of each of the first organic compound and the second organic compound is higher than the triplet excitation energy level of the exciplex. 
     Another embodiment of the present invention is a light-emitting element which includes a pair of electrodes and an EL layer between the pair of electrodes. The EL layer includes a light-emitting layer. The light-emitting layer includes a host material and a guest material. The host material includes a first organic compound and a second organic compound. A combination of the first organic compound and the second organic compound forms an exciplex. The exciplex is capable of exhibiting thermally activated delayed fluorescence. The guest material is capable of emitting fluorescence. A phosphorescence emission energy of each of the first organic compound and the second organic compound is higher than a thermally activated delayed fluorescence emission energy of the exciplex. 
     Another embodiment of the present invention is a light-emitting element which includes a pair of electrodes and an EL layer between the pair of electrodes. The EL layer includes a light-emitting layer. The light-emitting layer includes a host material and a guest material. The host material includes a first organic compound and a second organic compound. A combination of the first organic compound and the second organic compound forms an exciplex. The exciplex has a difference of more than 0 eV and less than or equal to 0.2 eV between a singlet excitation energy level and a triplet excitation energy level. The guest material is capable of emitting fluorescence. The triplet excitation energy level of the exciplex is higher than a triplet excitation energy level of the guest material. The triplet excitation energy level of each of the first organic compound and the second organic compound is higher than the triplet excitation energy level of the exciplex. 
     Another embodiment of the present invention is a light-emitting element which includes a pair of electrodes and an EL layer between the pair of electrodes. The EL layer includes a light-emitting layer. The light-emitting layer includes a host material and a guest material. The host material includes a first organic compound and a second organic compound. A combination of the first organic compound and the second organic compound forms an exciplex. The exciplex is capable of exhibiting thermally activated delayed fluorescence. The guest material is capable of emitting fluorescence. A thermally activated delayed fluorescence emission energy of the exciplex is higher than a phosphorescence emission energy of the guest material. The phosphorescence emission energy of each of the first organic compound and the second organic compound is higher than the thermally activated delayed fluorescence emission energy of the exciplex. 
     In the above embodiment, it is preferable that the weight ratio of the guest material to the host material be more than 0 and less than or equal to 0.05. 
     In the above embodiment, it is preferable that the difference between the triplet excitation energy level of the first organic compound and the triplet excitation energy level of the second organic compound be less than 0.4 eV. 
     In the above embodiment, it is preferable that one of the first organic compound and the second organic compound have a condensed heterocyclic skeleton and that the condensed heterocyclic skeleton have a diazine skeleton. Alternatively, it is preferable that one of the first organic compound and the second organic compound have a carbazole skeleton and a condensed heterocyclic skeleton and that the condensed heterocyclic skeleton have a diazine skeleton. Alternatively, it is preferable that one of the first organic compound and the second organic compound have a carbazole skeleton and a condensed heterocyclic skeleton, that the condensed heterocyclic skeleton have a diazine skeleton, and that the carbazole skeleton and the condensed heterocyclic skeleton be bonded to each other through an arylene group. It is more preferable that the 9-position of the carbazole skeleton and the condensed heterocyclic skeleton be bonded to each other through an arylene group. Alternatively, it is preferable that one of the first organic compound and the second organic compound have a carbazole skeleton and a benzofuropyrimidine skeleton and that the 9-position of the carbazole skeleton and the benzofuropyrimidine skeleton be bonded to each other through an arylene group. 
     In the above embodiment, it is preferable that one of the first organic compound and the second organic compound have a carbazole skeleton and an aromatic amine skeleton and that the 9-position of the carbazole skeleton and the aromatic amine skeleton be bonded to each other or be bonded to each other through an arylene group. 
     In the above embodiment, it is preferable that the EL layer further include one selected from a hole-injection layer, a hole-transport layer, an electron-transport layer, and an electron-injection layer. 
     Another embodiment of the present invention is a display device which includes the light-emitting element in the above embodiment and a color filter. Another embodiment of the present invention is an electronic device which includes the display device and a housing or a touch sensor. Another embodiment of the present invention is a lighting device which includes the light-emitting element in the above embodiment and a housing or a touch sensor. 
     In one embodiment of the present invention, a light-emitting element having high emission efficiency which includes a fluorescent material as a light-emitting substance can be provided. In another embodiment of the present invention, a novel light-emitting element can be provided. In another embodiment of the present invention, a novel light-emitting element with high emission efficiency and low power consumption can be provided. In another embodiment of the present invention, a novel display device can be provided. 
     Note that the descriptions of these effects do not disturb the existence of other effects. In one embodiment of the present invention, there is no need to achieve all the above effects. Other effects will be apparent from and can be derived from the descriptions of the specification, the drawings, the claims, and the like. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS.  1 A to  1 C  are a schematic cross-sectional view of a light-emitting element, a schematic cross-sectional view of a light-emitting layer, and a schematic diagram illustrating the correlation of energy levels. 
         FIGS.  2 A and  2 B  are a schematic cross-sectional view of a light-emitting layer and a schematic diagram illustrating the correlation of energy levels. 
         FIGS.  3 A and  3 B  illustrate emission spectra according to one embodiment. 
         FIGS.  4 A and  4 B  illustrate phosphorescence spectra according to one embodiment. 
         FIGS.  5 A and  5 B  are a schematic cross-sectional view of a light-emitting element and a diagram illustrating the correlation of energy levels in a light-emitting layer. 
         FIGS.  6 A and  6 B  are a schematic cross-sectional view of a light-emitting element and a diagram illustrating the correlation of energy levels in a light-emitting layer. 
         FIGS.  7 A and  7 B  are a block diagram and a circuit diagram illustrating a display device. 
         FIGS.  8 A and  8 B  are perspective views of an example of a touch panel. 
         FIGS.  9 A to  9 C  are cross-sectional views of examples of a display device and a touch sensor. 
         FIGS.  10 A and  10 B  are each a cross-sectional view of an example of a touch panel. 
         FIGS.  11 A and  11 B  are a block diagram and a timing chart of a touch sensor. 
         FIG.  12    is a circuit diagram of a touch sensor. 
         FIG.  13    is a perspective view of a display module. 
         FIGS.  14 A to  14 G  illustrate electronic devices. 
         FIG.  15    illustrates lighting devices. 
         FIG.  16    is a cross-sectional view of a structure of a light-emitting element in an example. 
         FIG.  17    shows luminance-current density characteristics of light-emitting elements 1 to 4 in an example. 
         FIG.  18    shows luminance-current density characteristics of light-emitting elements 5 to 7 in an example. 
         FIG.  19    shows luminance-voltage characteristics of the light-emitting elements 1 to 4 in the example. 
         FIG.  20    shows luminance-voltage characteristics of the light-emitting elements 5 to 7 in the example. 
         FIG.  21    shows current efficiency-luminance characteristics of the light-emitting elements 1 to 4 in the example. 
         FIG.  22    shows current efficiency-luminance characteristics of the light-emitting elements 5 to 7 in the example. 
         FIG.  23    shows current-voltage characteristics of the light-emitting elements 1 to 4 in the example. 
         FIG.  24    shows current-voltage characteristics of the light-emitting elements 5 to 7 in the example. 
         FIG.  25    shows external quantum efficiency-luminance characteristics of the light-emitting elements 1 to 4 in the example. 
         FIG.  26    shows external quantum efficiency-luminance characteristics of the light-emitting elements 5 to 7 in the example. 
         FIG.  27    shows electroluminescence spectra of the light-emitting elements 1 to 4 in the example. 
         FIG.  28    shows electroluminescence spectra of the light-emitting elements 5 to 7 in the example. 
         FIG.  29    shows transient EL characteristics of the light-emitting elements 1, 6, and 7 in the example. 
         FIG.  30    shows a transient EL spectrum of the light-emitting element 6 in the example. 
         FIG.  31    shows luminance-current density characteristics of light-emitting elements 8 and 9 in an example. 
         FIG.  32    shows luminance-voltage characteristics of the light-emitting elements 8 and 9 in the example. 
         FIG.  33    shows current efficiency-luminance characteristics of the light-emitting elements 8 and 9 in the example. 
         FIG.  34    shows current-voltage characteristics of the light-emitting elements 8 and 9 in the example. 
         FIG.  35    shows external quantum efficiency-luminance characteristics of the light-emitting elements 8 and 9 in the example. 
         FIG.  36    shows electroluminescence spectra of the light-emitting element 8 and 9 in the example. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the present invention will be described below with reference to the drawings. Note that the present invention is not limited to the following description, and various changes and modifications can be made without departing from the spirit and scope of the present invention. Therefore, the present invention should not be construed as being limited to the description in the following embodiments. 
     Note that the position, size, range, or the like of each component illustrated in drawings and the like is not accurately represented in some cases for easy understanding. Therefore, the disclosed invention is not necessarily limited to the position, size, range, or the like disclosed in the drawings and the like. 
     Ordinal numbers such as “first” and “second” in this specification and the like are used for convenience and do not denote the order of steps or the stacking order of layers in some cases. Therefore, for example, the term “first” can be replaced with the term “second”, “third”, or the like as appropriate. In addition, the ordinal numbers in this specification and the like are not necessarily the same as the ordinal numbers used to 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. For example, the term “conductive layer” can be changed into the term “conductive film” in some cases, and the term “insulating film” can be changed into the term “insulating layer” in some cases. 
     In this specification and the like, a fluorescent material refers to a material that emits light in the visible light region when the level of the lowest singlet excited state (S 1  level) relaxes to the ground state. A phosphorescent material refers to a material that emits light in the visible light region at room temperature when the level of the lowest triplet excited state (T 1  level) relaxes to the ground state. That is, a phosphorescent material refers to a material that can convert triplet excitation energy into visible light. 
     In this specification and the like, a thermally activated delayed fluorescence emission energy refers to an emission peak (including a shoulder) on the shortest wavelength side of thermally activated delayed fluorescence. In this specification and the like, a phosphorescence emission energy or a triplet excitation energy refers to a phosphorescence emission peak (including a shoulder) on the shortest wavelength side of phosphorescence emission. Note that the phosphorescence emission can be observed by time-resolved photoluminescence in a low-temperature (e.g., 10 K) environment. 
     Note that in this specification and the like, room temperature refers to a temperature in the range from 0° C. to 40° C. 
     Embodiment 1 
     In this embodiment, a light-emitting element according to one embodiment of the present invention will be described below with reference to  FIGS.  1 A to  1 C ,  FIGS.  2 A and  2 B ,  FIGS.  3 A and  3 B , and  FIGS.  4 A and  4 B . 
     &lt;1. Structure Example 1 of Light-Emitting Element&gt; 
     First, a structure of a light-emitting element of one embodiment of the present invention will be described below with reference to  FIGS.  1 A to  1 C . 
       FIG.  1 A  is a schematic cross-sectional view of a light-emitting element  150  of one embodiment of the present invention. 
     The light-emitting element  150  includes an EL layer  100  between a pair of electrodes (an electrode  101  and an electrode  102 ). The EL layer  100  includes at least a light-emitting layer  113 . Although description is made in this embodiment assuming that the electrode  101  is an anode and the electrode  102  is a cathode, the electrodes may be interchanged in the light-emitting element  150 . 
     The EL layer  100  illustrated in  FIG.  1 A  includes a hole-injection layer  111 , a hole-transport layer  112 , an electron-transport layer  115 , and an electron-injection layer  116  in addition to the light-emitting layer  113 . Note that the structure of the EL layer  100  is not limited to the structure illustrated in  FIG.  1 A , and at least one selected from the hole-injection layer  111 , the hole-transport layer  112 , the electron-transport layer  115 , and the electron-injection layer  116  is included. Alternatively, the EL layer  100  may include a functional layer which is capable of lowering a carrier injection barrier, improving a carrier-transport property, or suppressing the occurrence of quenching due to the electrode. 
       FIG.  1 B  is a schematic cross-sectional view of an example of the light-emitting layer  113  in  FIG.  1 A . The light-emitting layer  113  in  FIG.  1 B  includes a host material  121  and a guest material  122 . 
     It is preferable that the host material  121  have a difference of more than 0 eV and less than or equal to 0.2 eV between a singlet excitation energy level and a triplet excitation energy level. It is particularly preferable that the host material  121  be a substance which exhibits thermally activated delayed fluorescence at room temperature. Note that the host material  121  may be composed of a single material or may include a plurality of materials. The guest material  122  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 material). 
     An example in which a fluorescent material is used as the guest material  122  will be described below. Note that the guest material  122  may be read as the fluorescent material. 
     &lt;&lt;1-1. Emission Mechanism of Light-Emitting Element&gt;&gt; 
     First, an emission mechanism of the light-emitting element  150  will be described below. 
     In the light-emitting element  150  of one embodiment of the present invention, voltage application between a pair of electrodes (the electrodes  101  and  102 ) causes electrons and holes to be injected from the cathode and the anode, respectively, into the EL layer  100  and thus current flows. By recombination of the injected electrons and holes, the guest material  122  in the light-emitting layer  113  of the EL layer  100  is brought into an excited state to provide light emission. 
     Note that light emission from the guest material  122  can be obtained through the following two processes:
         (α) direct recombination process; and   (β) energy transfer process.       

     &lt;&lt;1-2. (α) Direct Recombination Process&gt;&gt; 
     Carriers (electrons and holes) are recombined in the guest material  122 , and the guest material  122  is brought into an excited state. In the case where the excited state of the guest material  122  is a singlet excited state, fluorescence is obtained. In contrast, in the case where the excited state of the guest material  122  is a triplet excited state, thermal deactivation occurs. 
     In (α) direct recombination process, high emission efficiency can be obtained when the fluorescence quantum efficiency of the guest material  122  is high. 
     &lt;&lt;1-3. (β) Energy Transfer Process&gt;&gt; 
     Carriers are recombined in the host material  121 , and the host material  121  is brought into an excited state. In the case where the excited state of the host material  121  is a singlet excited state and the singlet excitation energy level of the host material  121  is higher than the singlet excitation energy level of the guest material  122 , excitation energy is transferred from the host material  121  to the guest material  122 , and thus the guest material  122  is brought into a singlet excited state. Fluorescence is obtained from the guest material  122  in the singlet excited state. Therefore, the singlet excitation energy level of the host material  121  is preferably higher than the singlet excitation energy level of the guest material  122 . 
     Note that since direct transition of the guest material  122  from a singlet ground state to a triplet excited state is forbidden, energy transfer from the host material  121  in the singlet excited state to the guest material  122  in the triplet excited state is unlikely to be a main energy transfer process; therefore, a description thereof is omitted here. In other words, energy transfer from the host material  121  in the singlet excited state to the guest material  122  in the singlet excited state is important as represented by the following general formula (G1). 
         1 H*+ 1 G→ 1 H+ 1 G*  (G1)
 
     Note that in the general formula (G1),  1 H* represents the singlet excited state of the host material  121 ;  1 G represents the singlet ground state of the guest material  122 ;  1 H represents the singlet ground state of the host material  121 ; and  1 G* represents the singlet excited state of the guest material  122 . 
     Next, in order to describe the energy transfer process of the host material  121  and the guest material  122 , a schematic diagram illustrating the correlation of energy levels is shown in  FIG.  1 C . The following explains what terms and signs in  FIG.  1 C  represent: 
     Host ( 121 ): the host material  121 ; 
     Guest ( 122 ): the guest material  122  (fluorescent material); 
     S H : the level of the lowest singlet excitation energy of the host material  121 ; 
     T H : the level of the lowest triplet excitation energy of the host material  121 ; 
     S G : the level of the lowest singlet excitation energy of the guest material  122  (fluorescent material); and 
     T G : the level of the lowest triplet excitation energy of the guest material  122  (fluorescent material). 
     Even in the case where the exited state of the host material  121  is the triplet excited state, when the S H  of the host material  121  is higher than the S G  of the guest material  122 , fluorescence is obtained through the following two processes. 
     The host material  121  has a small energy difference of less than or equal to 0.2 eV between the singlet excitation energy level and the triplet excitation energy level. Therefore, as for a first process, excitation energy is transferred from the T H  to the S H  of the host material  121  by reverse intersystem crossing (upconversion) as shown by a route A 1  in  FIG.  1 C . 
     As for a subsequent second process, excitation energy is transferred from the S H  of the host material  121  to the S G  of the guest material  122  as shown by a route E 1  in  FIG.  1 C , whereby the guest material  122  is brought into the singlet excited state. Fluorescence is obtained from the guest material  122  in the singlet excited state. 
     The above-described first and second processes are represented by the following general formula (G2). 
         3 H*+ 1 G→(reverse intersystem crossing)→ 1 H*+ 1 G→ 1 H+ 1 G*  (G2)
 
     Note that in the general formula (G2),  3 H* represents the triplet excited state of the host material  121 ;  1 G represents the singlet ground state of the guest material  122 ;  1 H* represents the singlet excited state of the host material  121 ;  1 H represents the singlet ground state of the host material  121 ; and  1 G* represents the singlet excited state of the guest material  122 . 
     As represented by the general formula (G2), the singlet excited state ( 1 H*) of the host material  121  is generated from the triplet excited state ( 3 H*) of the host material  121  by reverse intersystem crossing, and then energy is transferred to the guest material  122  in the singlet excited state ( 1 G*). 
     When all the energy transfer processes described above in (β) energy transfer process occur efficiently, both the triplet excitation energy and the singlet excitation energy of the host material  121  are efficiently converted into the singlet excited state ( 1 G*) of the guest material  122 . Thus, high-efficiency light emission is possible. 
     However, when the host material  121  is deactivated by emitting excitation energy as light or heat before the excitation energy is transferred from the singlet excited state and the triplet excited state of the host material  121  to the singlet excited state of the guest material  122 , the emission efficiency of the light-emitting element  150  is decreased. For example, in the case where the level of the lowest triplet excitation energy of the host material  121  is lower than the T G  of the guest material  122  as indicated by broken line B 1  in  FIG.  1 C  (shown as “T H ′” in  FIG.  1 C ), the excitation energy of the guest material  122  is transferred from the T G  of the guest material  122  to the T H ′ of the host material  121  as shown by a route E 3  in  FIG.  1 C . In that case, since there is a large energy difference between T H ′ and S H , reverse intersystem crossing shown by the route A 1 ′ in  FIG.  1 C  and the subsequent energy transfer process shown by the route E 1 , are unlikely to occur, resulting in thermal deactivation of the excitation energy of the guest material  122 . Therefore, the generation efficiency of the singlet excited state of the guest material  122  is decreased. Thus, it is preferable that the T H  of the host material  121  be higher than the T G  of the guest material  122 . That is, in the case where the host material  121  is a substance which exhibits thermally activated delayed fluorescence, it is preferable that the thermally activated delayed fluorescence emission energy of the host material  121  be higher than the phosphorescence emission energy of the guest material  122 . 
     In the case where excitation energy is transferred from the T H  of the host material  121  to the T G  of the guest material  122  as shown by a route E 2  in  FIG.  1 C , the excitation energy is also thermally deactivated. Therefore, it is preferable that the energy transfer process shown by the route E 2  in  FIG.  1 C  be less likely to occur because the generation efficiency of the triplet excited state of the guest material  122  can be decreased and the occurrence of thermal deactivation can be reduced. To achieve this, it is preferable that the weight ratio of the guest material  122  to the host material  121  be low. Specifically, the weight ratio of the guest material  122  to the host material  121  is preferably more than 0 and less than or equal to 0.05, more preferably more than 0 and less than or equal to 0.03. 
     Note that when the direct recombination process in the guest material  122  is dominant, a large number of triplet excited states of the guest material  122  are generated in the light-emitting layer, resulting in a decreased emission efficiency due to thermal deactivation. That is, it is preferable that the probability of (β) energy transfer process be higher than that of (α) direct recombination process because the occurrence of thermal deactivation when the excited state of the guest material  122  is a triplet excited state can be reduced. To achieve this, it is again preferable that the weight ratio of the guest material  122  to the host material  121  be low. Specifically, the weight ratio of the guest material  122  to the host material  121  is preferably more than 0 and less than or equal to 0.05, more preferably more than 0 and less than or equal to 0.03. 
     Next, factors controlling the above-described processes of intermolecular energy transfer between the host material  121  and the guest material  122  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. 
     &lt;&lt;1-4. 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  121  and the guest material  122 . By the resonant phenomenon of dipolar oscillation, the host material  121  provides energy to the guest material  122 , and thus, the host material  121  in an excited state is put in a ground state and the guest material  122  in a ground state is put in an excited state. Note that the rate constant k h*→g  of Förster mechanism is expressed by Formula (1). 
     
       
         
           
             
               
                 
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                   1 
                   ) 
                 
               
             
           
         
       
     
     In Formula (1), v denotes a frequency, f′ h (v) denotes a normalized emission spectrum of the host material  121  (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 (v) denotes a molar absorption coefficient of the guest material  122 , N denotes Avogadro&#39;s number, n denotes a refractive index of a medium, R denotes an intermolecular distance between the host material  121  and the guest material  122 , 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  121  and the guest material  122 . Note that K 2 =⅔ in random orientation. 
     &lt;&lt;1-5. Dexter Mechanism&gt;&gt; 
     In Dexter mechanism, the host material  121  and the guest material  122  are close to a contact effective range where their orbitals overlap, and the host material  121  in an excited state and the guest material  122  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). 
     
       
         
           
             
               
                 
                   [ 
                   
                     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, v denotes a frequency, f′ h (v) denotes a normalized emission spectrum of the host material  121  (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 (v) denotes a normalized absorption spectrum of the guest material  122 , L denotes an effective molecular radius, and R denotes an intermolecular distance between the host material  121  and the guest material  122 . 
     Here, the efficiency of energy transfer from the host material  121  to the guest material  122  (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  121 , k n  denotes a rate constant of a non-light-emission process (thermal deactivation or intersystem crossing) of the host material  121 , and ti denotes a measured lifetime of an excited state of the host material  121 . 
     
       
         
           
             
               
                 
                   [ 
                   
                     Formula 
                     ⁢ 
                         
                     3 
                   
                   ] 
                 
               
               
                  
               
             
             
               
                 
                   
                     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;1-6. Concept for Promoting Energy Transfer&gt;&gt; 
     In both the energy transfer processes of the general formulae (G1) and (G2), since energy is transferred from the singlet excited state ( 1 H*) of the host material  121  to the singlet excited state ( 1 G*) of the guest material  122 , energy transfers by both Forster mechanism (Formula (1)) and Dexter mechanism (Formula (2)) occur. 
     First, an energy transfer by Forster mechanism is considered. When τ is eliminated from Formula (1) and Formula (3), it can be said that the energy transfer efficiency ϕ ET  is higher when the quantum yield ϕ (here, a fluorescence quantum yield because energy transfer from a singlet excited state is discussed) is higher. However, in practice, a more important factor is that the emission spectrum of the host material  121  (here, a fluorescent spectrum because energy transfer from a singlet excited state is discussed) largely overlaps with the absorption spectrum of the guest material  122  (absorption corresponding to the transition from the singlet ground state to the singlet excited state). Note that it is preferable that the molar absorption coefficient of the guest material  122  be also high. This means that the emission spectrum of the host material  121  overlaps with the absorption band of the guest material  122  which is on the longest wavelength side. 
     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  121  (here, a fluorescent spectrum because energy transfer from a singlet excited state is discussed) largely overlap with an absorption spectrum of the guest material  122  (absorption corresponding to transition from a singlet ground state to a singlet excited state). 
     The above description suggests that the energy transfer efficiency can be optimized by making the emission spectrum of the host material  121  overlap with the absorption band of the guest material  122  which is on the longest wavelength side. 
     In view of this, one embodiment of the present invention provides a light-emitting element which includes the host material  121  having a function as an energy donor capable of efficiently transferring energy to the guest material  122 . A feature of the host material  121  is that the singlet excitation energy level and the triplet excitation energy level are close to each other. Specifically, it is preferable that the host material  121  have a difference of more than 0 eV and less than or equal to 0.2 eV between the singlet excitation energy level and the triplet excitation energy level. This enables transition (reverse intersystem crossing) of the host material  121  from the triplet excited state to the singlet excited state to be likely to occur. Therefore, the generation efficiency of the singlet excited state of the host material  121  can be increased. Furthermore, in order to facilitate energy transfer from the singlet excited state of the host material  121  to the singlet excited state of the guest material  122  having a function as an energy acceptor, it is preferable that the emission spectrum of the host material  121  (here, the emission spectrum of a substance having a function of exhibiting thermally activated delayed fluorescence) overlap with the absorption band of the guest material  122  which is on the longest wavelength side. Thus, the generation efficiency of the singlet excited state of the guest material  122  can be increased. 
     Since the triplet excitation energy level of the host material  121  is higher than the triplet excitation energy level of the guest material  122  in the light-emitting element  150  of one embodiment of the present invention, transition of the host material  121  from the triplet excited state to the singlet excited state and energy transfer from the singlet excited state of the host material  121  to the singlet excited state of the guest material  122  are likely to occur. For this reason, thermal deactivation is less likely to occur in the light-emitting element  150 ; thus, the emission efficiency of the light-emitting element  150  can be increased. In the case where the host material  121  is a substance which exhibits thermally activated delayed fluorescence at room temperature, since the thermally activated delayed fluorescence emission energy is higher than the phosphorescence emission energy of the guest material  122 , transition of the host material  121  from the triplet excited state to the singlet excited state and energy transfer from the host material  121  in the singlet excited state to the guest material  122  in the singlet excited state occur efficiently. For this reason, thermal deactivation is less likely to occur in the light-emitting element  150 ; thus, the emission efficiency of the light-emitting element  150  can be increased. 
     &lt;&lt;1-7. Material&gt;&gt; 
     In the light-emitting layer  113 , the host material  121  may be composed of one kind of material or may include a plurality of materials. For example, in the case where the host material  121  is composed of one kind of material, any of the following materials can be used. 
     First, a fullerene, a derivative thereof, an acridine derivative such as proflavine, eosin, or 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)), which are shown in the following structural formulae. 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     Alternatively, a heterocyclic compound having a π-electron rich heteroaromatic ring and a π-electron deficient heteroaromatic ring, such as 2-(biphenyl-4-yl)-4,6-bis(12-phenylindolo[2,3-a]carbazol-11-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: PCCzPTzn), 2-[4-(10H-phenoxazin-10-yl)phenyl]-4,6-diphenyl-1,3,5-triazine (abbreviation: PXZ-TRZ), 3-[4-(5-phenyl-5,10-dihydrophenazin-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-dihydro acridine)phenyl]sulfone (abbreviation: DMAC-DPS), or 10-phenyl-10H,10′H-spiro[acridin-9,9′-anthracen]-10′-one (abbreviation: ACRSA) shown in the following structural formulae, can be used as the host material  121  composed of one kind of material. The heterocyclic compound is preferably used because of 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 preferably used 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 level of the singlet excited state and the level of the triplet excited state becomes small. 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     In the light-emitting layer  113 , the guest material  122  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′-bis[4-(9-phenyl-9H-fluoren-9-yl)phenyl]-N,N′-diphenyl-pyrene-1,6-diamine (abbreviation: 1,6FLPAPrn), N-bis(3-methylphenyl)-N,N′-bis[3-(9-phenyl-9H-fluoren-9-yl)phenyl]pyrene-1,6-diamine (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)triphenyl amine (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]-triphenyl-1,4-phenylenediamine (abbreviation: 2DPAPPA), N,N,N′,N′,N″,N″,N′″,N′″-octaphenyldibenzo[g,p]chrysene-2,7,10,15-tetraamine (abbreviation: DBC1), coumarin 30, N-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine (abbreviation: 2PCAPA), N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,9-diphenyl-9H-carbazol-3-amine (abbreviation: 2PCABPhA), N-(9,10-diphenyl-2-anthryl)-N,N′,N′-triphenyl-1,4-phenylenediamine (abbreviation: 2DPAPA), N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,N′-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[i,j]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile (abbreviation: DCM2), N,N,N′,N′-tetrakis(4-methylphenyl)tetracene-5,11-diamine (abbreviation: p-mPhTD), 7,14-diphenyl-N,N,N′,N′-tetrakis(4-methylphenyl)acenaphtho[1,2-α]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[y]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[i,j]quinolizin 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-{12,6-bis[2-(8-methoxy-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[i,j]quinolizin 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. 
     Note that the light-emitting layer  113  can be formed by an evaporation method (including a vacuum evaporation method), an inkjet method, a coating method, gravure printing, or the like. 
     &lt;2. Structure Example 2 of Light-Emitting Element&gt; 
     Next, a structure different from the structure illustrated in  FIGS.  1 B and  1 C  will be described below with reference to  FIGS.  2 A and  2 B . 
       FIG.  2 A  is a schematic cross-sectional view of an example of the light-emitting layer  113  in  FIG.  1 A . The light-emitting layer  113  in  FIG.  2 A  includes the host material  121  and the guest material  122 . The host material  121  includes an organic compound  121 _ 1  and an organic compound  121 _ 2 . 
     It is preferable that a combination of the organic compound  121 _ 1  and the organic compound  121 _ 2  form an exciplex (also referred to as an excited complex). An exciplex tends to have a very small difference between the singlet excitation energy level and the triplet excitation energy level, and thus transition (reverse intersystem crossing) from the triplet excited state to the singlet excited state is likely to occur. One of the organic compounds  121 _ 1  and  121 _ 2  serves as a host material for the light-emitting layer  113 , and the other of the organic compounds  121 _ 1  and  121 _ 2  serves as an assist material for the light-emitting layer  113 . Note that the organic compound  121 _ 1  serves as the host material and the organic compound  121 _ 2  serves as the assist material in the following description. 
     Note that also in the case of using a host material which allows a combination of the organic compound  121 _ 1  and the organic compound  121 _ 2  to form an exciplex, light emission from the guest material  122  can be obtained through the following two processes: 
     (α) direct recombination process; and 
     (β) energy transfer process. 
     Note that (α) direct recombination process is not described here because it is similar to the process described above in the subsection 1-2. 
     &lt;&lt;2-1. Emission Mechanism Through (β) Energy Transfer Process&gt;&gt; 
     Although there is no limitation on the combination of the organic compound  121 _ 1  and the organic compound  121 _ 2  in the light-emitting layer  113  as long as an exciplex can be formed, it is preferred that one organic compound be a material having a hole-transport property and the other organic compound be a material having an electron-transport property. In that case, a donor-acceptor excited state is formed easily, which allows an exciplex to be formed efficiently. In the case where the combination of the organic compound  121 _ 1  and the organic compound  121 _ 2  is a combination of the material having a hole-transport property and the material having an electron-transport property, the carrier balance can be easily controlled depending on the mixture ratio. Specifically, the weight ratio of the material having a hole-transport property to the material having an electron-transport property is preferably within the range of 1:9 to 9:1. Since the carrier balance can be easily controlled in the structure, a recombination region can also be easily adjusted. 
     It is preferable that the exciplex formed by the organic compound  121 _ 1  and the organic compound  121 _ 2  have a difference of more than 0 eV and less than or equal to 0.2 eV between the singlet excitation energy level and the triplet excitation energy level. This enables transition (reverse intersystem crossing) of the exciplex from the triplet excitation energy level to the singlet excitation energy level to be likely to occur. Therefore, the generation efficiency of the singlet excited state of the exciplex, i.e., the host material  121 , can be increased. 
     Furthermore, it is preferable that the emission spectrum of the host material  121  (here, the emission spectrum of the exciplex formed by the organic compound  121 _ 1  and the organic compound  121 _ 2 ) overlap with the absorption band of the guest material  122  which is on the longest wavelength side. This facilitates energy transfer from the singlet excited state of the host material  121  to the singlet excited state of the guest material  122 . Therefore, the generation efficiency of the singlet excited state of the guest material  122  can be increased; thus, emission efficiency can be increased. 
     Here, in order to describe the energy transfer process of the exciplex, a schematic diagram illustrating the correlation of energy levels is shown in  FIG.  2 B . The following explains what terms and signs in  FIG.  2 B  represent: 
     Host ( 121 ): the host material (organic compound  121 _ 1 ); 
     Guest ( 122 ): the guest material  122  (fluorescent material); 
     Assist: the host material (organic compound  121 _ 2 ); 
     S H : the level of the lowest singlet excitation energy of the host material (organic compound  121 _ 1 ); 
     T H : the level of the lowest triplet excitation energy of the host material (organic compound  121 _ 1 ); 
     S E : the level of the lowest singlet excitation energy of the exciplex; 
     T E : the level of the lowest triplet excitation energy of the exciplex; 
     S G : the level of the lowest singlet excitation energy of the guest material  122  (fluorescent material); and 
     T G : the level of the lowest triplet excitation energy of the guest material  122  (fluorescent material). 
     When carriers (holes and electrons) are transported to the light-emitting layer  113  and one of the organic compounds  121 _ 1  and  121 _ 2  receiving holes and the other receiving electrons come close to each other, the exciplex is formed at once. Alternatively, when one compound is brought into an excited state, the one immediately interacts with the other compound to form the exciplex. Therefore, most excitons in the light-emitting layer  113  exist as the exciplexes. The band gap of the exciplex is narrower than that of each of the organic compounds  121 _ 1  and  121 _ 2 ; therefore, the driving voltage can be lowered when the exciplex is formed. 
     Since the organic compounds  121 _ 1  and  121 _ 2  can form a donor-acceptor excited state, the S E  and the T E  of the exciplex are close to each other. 
     In the case where the excited state of the exciplex is a single excited state, excitation energy is transferred from the S E  of the exciplex to the S G  of the guest material  122  as shown by a route E 4  in  FIG.  2 B , whereby the guest material  122  is brought into the singlet excited state. Fluorescence is obtained from the guest material  122  in the singlet excited state. In other words, energy transfer occurs from the exciplex in the singlet excited state to the guest material  122  in the singlet excited state as represented by the following general formula (G3). 
         1 [H-A]*+ 1 G→ 1 H+ 1 A+ 1 G*  (G3)
 
     Note that in the general formula (G3),  1 [H-A]* represents the singlet excited state of the exciplex formed by the organic compound  121 _ 1  and the organic compound  121 _ 2 ;  1 G represents the singlet ground state of the guest material  122 ;  1 H represents the singlet ground state of the organic compound  121 _ 1 ;  1 A represents the singlet ground state of the organic compound  121 _ 2 ; and  1 G* represents the singlet excited state of the guest material  122 . 
     Even in the case where the exited state of the exciplex is the triplet excited state, when the S E  of the exciplex is higher than the S G  of the guest material  122 , fluorescence is obtained through the following two processes. 
     As for a first process, excitation energy is transferred from the T E  to the S E  of the exciplex by reverse intersystem crossing (upconversion) as shown by a route A 2  in  FIG.  2 B . 
     As for a subsequent second process, excitation energy is transferred from the S E  of the exciplex to the S G  of the guest material  122  as shown by a route E 4  in  FIG.  2 B , whereby the guest material  122  is brought into the singlet excited state. Fluorescence is obtained from the guest material  122  in the singlet excited state. 
     The above-described processes through the route A 2  and the route E 4  may be referred to as exciplex-singlet energy transfer (ExSET) or exciplex-enhanced fluorescence (ExEF) in this specification and the like. 
     The above-described first and second processes are represented by the following general formula (G4). 
         3 [H-A]*+ 1 G→(reverse intersystem crossing)→ 1 [H-A]*+ 1 G→ 1 H+ 1 A+ 1 G*  (G4)
 
     Note that in the general formula (G4),  3 [H-A]* represents the triplet excited state of the exciplex formed by the organic compound  121 _ 1  and the organic compound  121 _ 2 ;  1 G represents the singlet ground state of the guest material  122 ;  1 [H-A]* represents the singlet excited state of the exciplex formed by the organic compound  121 _ 1  and the organic compound  121 _ 2 ; represents the singlet ground state of the organic compound  121 _ 1 ;  1 A represents the singlet ground state of the organic compound  121 _ 2 ; and  1 G* represents the singlet excited state of the guest material  122 . 
     As represented by the general formula (G4), the singlet excited state ( 1 [H-A]*) of the exciplex is generated from the triplet excited state ( 3 [H-A]*) of the exciplex by reverse intersystem crossing, and then energy is transferred to the guest material  122  in the singlet excited state ( 1 G*). 
     When the host material  121  has the above structure, (β) energy transfer process occurs efficiently, and both the singlet excitation energy and the triplet excitation energy of the exciplex are efficiently converted into the singlet excited state of the guest material  122 . Thus, light emission can be efficiently obtained from the guest material  122  (fluorescent material) of the light-emitting layer  113 . 
     However, when the exciplex is deactivated by emitting the excitation energy as light or heat before excitation energy is transferred from the exciplex to the guest material  122 , the emission efficiency may be decreased. For example, in the case where excitation energy is transferred from the T E  of the exciplex to the T G  of the guest material  122  as shown by a route E 5  in  FIG.  2 B , the excitation energy is thermally deactivated. Therefore, the weight ratio of the guest material  122  to the host material  121  is preferably more than 0 and less than or equal to 0.05, more preferably more than 0 and less than or equal to 0.03. 
     In the case where the T H  of the host material  121 , i.e., the triplet excitation energy level of the organic compound  121 _ 1  or  121 _ 2 , is lower than the T E  of the exciplex as indicated by broken line B 2  in  FIG.  2 B  (shown as “T H ′” in  FIG.  2 C ), the excitation energy is transferred from the T E  of the exciplex to the T H ′ of the host material  121  as shown by a route E 6  in  FIG.  2 B  and is then thermally deactivated. Thus, it is preferable that the triplet excitation energy level of each of the organic compounds  121 _ 1  and  121 _ 2  be higher than the T E  of the exciplex. In that case, reverse intersystem crossing occurs efficiently in the exciplex. 
     Since the S E  and T E  of the exciplex are close to each other, in the case where the T E  is lower than the T G  of the guest material  122 , the energy level of S E  is significantly lowered to the vicinity of T G  or lower than T G . As a result, energy transfer from S E  to the S G  of the guest material  122  (route E 4 ) is unlikely to occur, and fluorescence is not easily obtained from the guest material  122 . Thus, it is preferable that the T E  of the exciplex be higher than the T G  of the guest material  122 . 
     Accordingly, in the case where the exciplex exhibits thermally activated delayed fluorescence at room temperature, it is preferable that the phosphorescence emission energy of each of the organic compounds  121 _ 1  and  121 _ 2  be higher than the thermally activated delayed fluorescence emission energy of the exciplex. It is also preferable that the thermally activated delayed fluorescence emission energy of the exciplex be higher than the phosphorescence emission energy of the guest material  122 . 
     &lt;&lt;2-2. Material&gt;&gt; 
     In the case where the host material  121  in the light-emitting layer  113  includes the organic compounds  121 _ 1  and  121 _ 2 , i.e., two kinds of materials, any of the following materials can be used. 
     Note that as the organic compounds  121 _ 1  and  121 _ 2 , a combination of two kinds of organic compounds which form an exciplex is preferably used. In this case, a variety of carrier-transport materials can be used as appropriate. In order to form an exciplex efficiently, it is particularly preferable to combine a material which easily accepts electrons (a material having an electron-transport property) and a material which easily accepts holes (a material having a hole-transport property). 
     This is because in the case where the combination of a material having an electron-transport property and a material having a hole-transport property which form an exciplex is used as a host material, the carrier balance between holes and electrons in the light-emitting layer can be easily optimized by adjustment of the mixture ratio of the material having an electron-transport property and the material having a hole-transport property. The optimization of the carrier balance between holes and electrons in the light-emitting layer can prevent a region in which electrons and holes are recombined from existing on one side in the light-emitting layer. By preventing the region in which electrons and holes are recombined from existing on one side, the reliability of the light-emitting element can be improved. 
     As the material which easily accepts electrons (the material having an electron-transport property), a π-electron deficient heteroaromatic compound, a metal complex, or the like can be used. Specific examples include a metal complex such as bis(10-hydroxybenzo[h]quinolinato)beryllium(II) (abbreviation: BeBq 2 ), bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum(III) (abbreviation: BAlq), bis(8-quinolinolato)zinc(II) (abbreviation: Znq), bis[2-(2-benzoxazolyl)phenolato]zinc(II) (abbreviation: ZnPBO), or bis[2-(2-benzothiazolyl)phenolato]zinc(II) (abbreviation: ZnBTZ); a heterocyclic compound having an azole skeleton such as 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation: PBD), 3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole (abbreviation: TAZ), 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]carbazole (abbreviation: CO11), 2,2′,2″-(1,3,5-benzenetriyl)tris(1-phenyl-1H-benzimidazole) (abbreviation: TPBI), or 2-[3-(dibenzothiophen-4-yl)phenyl]-1-phenyl-1H-benzimidazole (abbreviation: mDBTBIm-II); a heterocyclic compound having a diazine skeleton such as 2-[3-(dibenzothiophen 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: 2mCzBPDBq), 4,6-bis[3-(phenanthren-9-yl)phenyl]pyrimidine (abbreviation: 4,6mPnP2Pm), or 4,6-bis[3-(4-dibenzothienyl)phenyl]pyrimidine (abbreviation: 4,6mDBTP2Pm-II); a heterocyclic compound 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); and a heterocyclic compound having a pyridine skeleton such as 3,5-bis[3-(9H-carbazol-9-yl)phenyl]pyridine (abbreviation: 35DCzPPy) or 1,3,5-tri[3-(3-pyridyl)phenyl]benzene (abbreviation: TmPyPB). Among the above materials, heterocyclic compounds having diazine skeletons and triazine skeletons and heterocyclic compounds having pyridine skeletons have high reliability and are thus preferable. Heterocyclic compounds having diazine (pyrimidine or pyrazine) skeletons and triazine skeletons have a high electron-transport property and contribute to a decrease in drive voltage. In order to obtain high reliability and a high triplet excitation energy level, it is preferable that a condensed heterocyclic skeleton be included and that the condensed heterocyclic skeleton have a diazine skeleton. Alternatively, it is preferable that a carbazole skeleton and a condensed heterocyclic skeleton be included and that the condensed heterocyclic skeleton have a diazine skeleton. Alternatively, it is preferable that a carbazole skeleton and a condensed heterocyclic skeleton be included, that the condensed heterocyclic skeleton have a diazine skeleton, and that the carbazole skeleton and the condensed heterocyclic skeleton can be bonded to each other through an arylene group, and it is particularly preferable that the 9-position of the carbazole skeleton and the condensed heterocyclic skeleton can be bonded to each other through an arylene group. The condensed heterocyclic skeleton having a diazine skeleton is preferably a quinoxaline skeleton, a quinazoline skeleton, a benzoquinoxaline skeleton, a benzoquinazoline skeleton, a dibenzoquinoxaline skeleton, a dibenzoquinazoline skeleton, a benzofuropyrimidine skeleton, or the like. 
     As the material which easily accepts holes (the material having a hole-transport property), a π-electron rich heteroaromatic compound, an aromatic amine compound, or the like can be favorably used. Specific examples include a compound having an aromatic amine skeleton such as 2-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]spiro-9,9′-bifluorene (abbreviation: PCASF), 4,4′-bis[N-(1-naphthyl)-N-phenyl amino]biphenyl (abbreviation: NPB), N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine (abbreviation: TPD), 4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl (abbreviation: BSPB), 4-phenyl-4′-(9-phenylfluoren yl)triphenylamine (abbreviation: BPAFLP), 4-phenyl-3′-(9-phenylfluoren-9-yl)triphenylamine (abbreviation: mBPAFLP), 4-phenyl-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PCBA1BP), 4,4′-diphenyl-4″-(9-phenyl-9H-carbazol-3-yl)triphenyl amine (abbreviation: PCBBi1BP), 4-(1-naphthyl)-4′-(9-phenyl-9H-carbazol-3-yl)triphenyl amine (abbreviation: PCBANB), 4,4′-di(1-naphthyl)-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PCBNBB), 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), N-(1,1′-biphenyl-4-yl)-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-9,9-dimethyl-9H-fluoren-2-amine (abbreviation: PCBBiF); a compound having a carbazole skeleton such as 1,3-bis(N-carbazolyl)benzene (abbreviation: mCP), 4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP), 3,6-bis(3,5-diphenylphenyl)-9-phenylcarbazole (abbreviation: CzTP), or 9-phenyl-9H-3-(9-phenyl-9H-carbazol-3-yl)carbazole (abbreviation: PCCP); a compound having a thiophene skeleton such as 4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzothiophene) (abbreviation: DBT3P-II), 2,8-diphenyl-4-[4-(9-phenyl-9H-fluoren-9-yl)phenyl]dibenzothiophene (abbreviation: DBTFLP-III), or 4-[4-(9-phenyl-9H-fluoren-9-yl)phenyl]-6-phenyldibenzothiophene (abbreviation: DBTFLP-IV); and a compound having a furan skeleton such as 4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzofuran) (abbreviation: DBF3P-II) or 4-{3-[3-(9-phenyl-9H-fluoren-9-yl)phenyl]phenyl}dibenzofuran (abbreviation: mmDBFFLBi-II). 
     Among the above materials, compounds having aromatic amine skeletons and compounds having carbazole skeletons are preferable because these compounds are highly reliable and have high hole-transport properties to contribute to a reduction in drive voltage. In order to obtain a high triplet excitation energy level, it is preferable that a carbazole skeleton be included. Alternatively, it is preferable that one aromatic amine skeleton be included. Alternatively, it is preferable that a carbazole skeleton and an aromatic amine skeleton be included and that the 9-position of the carbazole skeleton and the aromatic amine skeleton can be bonded to each other, and it is particularly preferable that the 9-position of the carbazole skeleton and the aromatic amine skeleton can be bonded to each other through an arylene group. 
     The organic compounds  121 _ 1  and  121 _ 2  are not limited to the above-described compounds, as long as they can transport carriers, the combination can form an exciplex, and light emission of the exciplex overlaps with an absorption band on the longest wavelength side in an absorption spectrum of a light-emitting substance (an absorption corresponding to the transition of the light-emitting substance from the singlet ground state to the singlet excited state), and other materials may be used. 
     Note that a material which can be used as the guest material  122  in the light-emitting layer  113  illustrated in  FIG.  2 A  is similar to the guest material  122  described in the subsection 1-7 and is therefore not described here. 
     &lt;&lt;2-3. Relationship Between Emission Energy of Exciplex and Triplet Energy Level of Host Material&gt;&gt; 
     Combinations of organic compounds which can form an exciplex of one embodiment of the present invention and the relationship between the emission energy of the exciplex and the triplet energy level of a host material will be described in detail below. 
       FIGS.  3 A and  3 B  show the emission spectra of thin films of individual organic compounds and the emission spectra of mixed films each including a combination of two different kinds of organic compounds. Note that  FIG.  3 A  shows the emission spectra of compounds 1 to 4 and exciplexes 1 to 3, and  FIG.  3 B  shows the emission spectra of compounds 5 to 7 and exciplexes 4 to 6. 
       FIGS.  3 A and  3 B  show the emission spectra of thin films of 2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation: 2mDBTPDBq-II) as the compound 1,4,6-bis[3-(4-dibenzothienyl)phenyl]pyrimidine (abbreviation: 4,6mDBTP2Pm-II) as the compound 2,2,7-bis[N-(4-diphenylaminophenyl)-N-phenylamino]-spiro-9,9′-bifluorene (abbreviation: DPA2SF) as the compound 3,2-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]spiro-9,9′-bifluorene (abbreviation: PCASF) as the compound 4, 4,6-bis[3-(9H-carbazol-9-yl)phenyl]pyrimidine (abbreviation: 4,6mCzP2Pm) as the compound 5, N-(4-biphenyl)-N-(9,9-dimethyl-9H-fluoren-2-yl)-9-phenyl-9H-carabzol-3-amine (abbreviation: PCBiF) as the compound 6, 9-phenyl-9H-3-(9-phenyl-9H-carbazol-3-yl)carbazole (abbreviation: PCCP) as the compound 7, and 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB) as a compound 8. Structural formulae and abbreviations of the compounds 1 to 8 are shown below. 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     The emission spectrum of the exciplex 1 in  FIG.  3 A  is a result of measuring light emission from a mixed film of the compounds 1 and 3. That is, one of the compounds 1 and 3 corresponds to the organic compound  121 _ 1 , and the other corresponds to the organic compound  121 _ 2 . 
     Similarly, the emission spectra of the exciplexes 2 and 3 are results of measuring light emissions from a mixed film of the compounds 1 and 4 and a mixed film of the compounds 2 and 4, respectively. Note that the emission spectra correspond to measured light emissions obtained by ultraviolet light irradiation of thin films of individual compounds and mixed films each including two compounds. Note that the emission spectra were measured with a PL-EL measurement apparatus (manufactured by Hamamatsu Photonics K.K.). The thin films were formed by vacuum evaporation of the individual compounds to a thickness of 50 nm onto a quartz substrate. Note that the mixed films were formed by an evaporation method (co-evaporation) where two different compounds are concurrently evaporated from respective evaporation sources. 
     For each of the exciplexes 1 and 2, the compound 1 is used, and the other compound for exciplex formation differs between the exciplexes 1 and 2. Therefore, although there is little difference between the emission spectrum peaks of the compounds 3 and 4, the wavelength at the emission spectrum peak of the exciplex 1 is 579 nm and the wavelength at the emission spectrum peak of the exciplex 2 is 543 nm; there is a difference of 30 nm or more between these peaks. This is because the exciplexes 1 and 2 are donor-acceptor exciplexes. That is, the singlet excitation energy of an exciplex corresponds to the energy difference between a higher highest occupied molecular orbital (also referred to as HOMO) of HOMOs of two organic compounds and a lower lowest unoccupied molecular orbital (also referred to as LUMO) of LUMOs of the two organic compounds. Therefore, when one of the compounds is different, the energy level of the HOMO or LUMO for exciplex formation is changed and thus the emission wavelength of an exciplex can also be changed. 
     For each of the exciplexes 2 and 3, the compound 4 is used, and the other compound for exciplex formation differs between the exciplexes 2 and 3. The wavelength at the emission spectrum peak of the exciplex 3 is 528 nm; there is a difference of 15 nm between the emission spectrum peaks of the exciplexes 2 and 3. In this manner, the emission wavelength of an exciplex can be easily changed simply by changing one of compounds which form the exciplex. 
     Note that since the wavelength at the emission spectrum peak of the exciplex 1 is 579 nm, the exciplex 1 is preferably mixed with a light-emitting material having a function of emitting yellow to red light as the guest material  122  for use in a light-emitting element. Furthermore, since the wavelengths at the emission spectrum peaks of the exciplexes 2 and 3 are 543 nm and 528 nm, respectively, the exciplex 2 or 3 is preferably mixed with a light-emitting material having a function of emitting green to red light as the guest material  122  for use in a light-emitting element. 
     The emission spectrum of the exciplex 4 in  FIG.  3 B  is a result of measuring light emission from a mixed film of the compounds 5 and 6. That is, one of the compounds 5 and 6 corresponds to the organic compound  121 _ 1 , and the other corresponds to the organic compound  121 _ 2 . Similarly, the emission spectra of the exciplexes 5 and 6 are results of measuring light emissions from a mixed film of the compounds 5 and 7 and a mixed film of the compounds 5 and 8, respectively. Note that the emission spectra correspond to measured light emissions obtained by ultraviolet light irradiation of thin films of individual compounds and mixed films each including two compounds. 
     For each of the exciplexes 4 to 6, the compound 5 is used, and the other compound for exciplex formation differs between the exciplexes 4 to 6. Therefore, there are differences between the emission spectrum peaks of the exciplexes 4 to 6. In this manner, the emission wavelength of an exciplex can be easily changed simply by changing one of compounds which form the exciplex. 
     Note that since the wavelengths at the emission spectrum peaks of the exciplexes 4 and 6 are 545 nm and 516 nm, respectively, the exciplex 4 or 6 is preferably mixed with a light-emitting material having a function of emitting green to red light as the guest material  122  for use in a light-emitting element. Furthermore, since the wavelength at the emission spectrum peak of the exciplex 5 is 488 nm, the exciplex 5 is preferably mixed with a light-emitting material having a function of emitting blue to red light as the guest material  122  for use in a light-emitting element. 
     Next, phosphorescence spectra of the compounds 1 to 8 are shown in  FIGS.  4 A and  4 B . The phosphorescence spectra shown in  FIGS.  4 A and  4 B  are obtained as a result of irradiation of thin films of the individual compounds using an ultraviolet laser (He—Cd laser) with an emission wavelength of 325 nm and measurement by time-resolved photoluminescence in a low-temperature (10 K) environment. Note that the measurement was performed by using a PL microscope, LabRAM HR-PL, produced by HORIBA, Ltd., and a CCD detector. The thin films were formed by vacuum evaporation of the individual compounds to a thickness of 50 nm onto a quartz substrate. 
     Tables 1 and 2 show phosphorescence emission energies obtained from phosphorescence emission peaks (including shoulders) on the shortest wavelength side of phosphorescence spectra. Note that a triplet excitation energy level and a singlet excitation energy level are close to each other in an exciplex; therefore, it is difficult to separate a phosphorescence spectrum and a fluorescence spectrum (thermally activated delayed fluorescence spectrum) from each other. Accordingly, the phosphorescence emission energies of the exciplexes in Tables 1 and 2 were obtained from the peaks of emission spectra assuming that the emission spectrum peaks (thermally activated delayed fluorescence spectrum peaks) of the exciplexes are the same as the phosphorescence spectrum peaks. That is, it is assumed that the thermally activated delayed fluorescence emission energy and the phosphorescence emission energy of the exciplex are substantially equal. 
     
       
         
           
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Phosphorescence 
               
               
                   
                 emission energy (eV) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 Compound 1 
                 2.40 
               
               
                   
                 Compound 2 
                 2.62 
               
               
                   
                 Compound 3 
                 2.32 
               
               
                   
                 Compound 4 
                 2.48 
               
               
                   
                 Exciplex 1 (Compounds 1 &amp; 3) 
                 2.14 
               
               
                   
                 Exciplex 2 (Compounds 1 &amp; 4) 
                 2.28 
               
               
                   
                 Exciplex 3 (Compounds 2 &amp; 4) 
                 2.35 
               
               
                   
                   
               
            
           
         
       
     
     
       
         
           
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Phosphorescence 
               
               
                   
                 emission energy (eV) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 Compound 5 
                 2.70 
               
               
                   
                 Compound 6 
                 2.45 
               
               
                   
                 Compound 7 
                 2.65 
               
               
                   
                 Compound 8 
                 2.30 
               
               
                   
                 Exciplex 4 (Compounds 5 &amp; 6) 
                 2.27 
               
               
                   
                 Exciplex 5 (Compounds 5 &amp; 7) 
                 2.54 
               
               
                   
                 Exciplex 6 (Compounds 5 &amp; 8) 
                 2.40 
               
               
                   
                   
               
            
           
         
       
     
     The phosphorescence emission energy (thermally activated delayed fluorescence emission energy) of each of the exciplexes 1 to 5 in Tables 1 and 2 is lower than the phosphorescence emission energy of each of the compounds which form the exciplexes. That is, the phosphorescence emission energy of each of the compounds 1 and 3 is higher than the phosphorescence emission energy (thermally activated delayed fluorescence emission energy) of the exciplex 1. The phosphorescence emission energy of each of the compounds 1 and 4 is higher than the phosphorescence emission energy (thermally activated delayed fluorescence emission energy) of the exciplex 2. The phosphorescence emission energy of each of the compounds 2 and 4 is higher than the phosphorescence emission energy (thermally activated delayed fluorescence emission energy) of the exciplex 3. The phosphorescence emission energy of each of the compounds 5 and 6 is higher than the phosphorescence emission energy (thermally activated delayed fluorescence emission energy) of the exciplex 4. The phosphorescence emission energy of each of the compounds 5 and 7 is higher than the phosphorescence emission energy (thermally activated delayed fluorescence emission energy) of the exciplex 5. 
     In other words, the triplet energy level of each of the compounds which form the exciplexes 1 to 5 is higher than the triplet energy level of the corresponding exciplex. Therefore, no energy transfer from the triplet excitation energy level of the exciplex to the triplet excitation energy levels of the organic compounds  121 _ 1  and  121 _ 2  which form the exciplex occurs through the energy transfer process indicated by E 6  in  FIG.  2 B ; thermal deactivation of the triplet excited state of the exciplex can be suppressed. 
     Therefore, the excitation energy of the triplet excited state of the exciplex can be efficiently transferred to the singlet excited state of the guest material  122  through the singlet excited state of the exciplex generated by reverse intersystem crossing. For this reason, light emission can be efficiently obtained from the guest material  122 . 
     On the other hand, the phosphorescence emission energy of the exciplex 6 shown in Table 2 is higher than the phosphorescence emission energy of the compound 8, which is one of the compounds which form the exciplex. That is, the triplet energy level of the compound 8, which is one of the compounds which form the exciplex is lower than the triplet energy level of the exciplex 6. Thus, the energy transfer from the triplet excitation energy level of the exciplex to the triplet excitation energy level of one of the organic compounds  121 _ 1  and  121 _ 2  which form the exciplex through the energy transfer process indicated by E 6  in  FIG.  2 B  might occur, resulting in thermal deactivation of the triplet excited state of the exciplex. In order to prevent this, it is preferable that the triplet excitation energy level of each of the organic compounds  121 _ 1  and  121 _ 2  which form the exciplex be higher than the triplet excitation energy of the exciplex. Therefore, it is preferable that the difference between the triplet excitation energy levels of the organic compounds  121 _ 1  and  121 _ 2  which form the exciplex be less than 0.4 eV. 
     Table 3 shows the level of the lowest triplet excitation energy of the guest material  122 . 
     
       
         
           
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Triplet excitation 
               
               
                   
                 energy (eV) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 1,6mMemFLPAPrn 
                 1.73 
               
               
                   
                 TBP 
                 1.50 
               
               
                   
                 DCM1 
                 1.65 
               
               
                   
                 DCM2 
                 1.61 
               
               
                   
                 Coumarin 6 
                 1.88 
               
               
                   
                 DPQd 
                 1.93 
               
               
                   
                 Rubrene 
                 0.95 
               
               
                   
                   
               
            
           
         
       
     
     In order to obtain the level of the lowest triplet excitation energy in Table 3, the most stable structure in the lowest triplet excited state of the guest material  122  and the most stable structure in the singlet ground state were calculated using density functional theory (DFT). In addition, vibration analysis was conducted on each of the most stable structures, and the level of the lowest triplet excitation energy was obtained by calculation of the difference between the lowest vibrational energies in the lowest triplet excited state and the singlet ground state. Note that Gaussian 09 was used as a quantum chemical calculation program. As a basis function, 6-311G(d,p) was used, and as a functional, B3LYP was used. A high performance computer (ICE X manufactured by SGI Japan, Ltd.) was used for the calculation. 
     Each of the triplet excitation energy levels of the compounds in Table 3 is lower than each of the triplet excitation energy levels of the exciplexes 1 to 5 in Tables 1 and 2. Therefore, when the compound in Table 3 is used as the guest material  122  and the above-described compounds which form the exciplexes 1 to 5 are used as the host material  121 , the energy transfer indicated by the routes A 2  and E 4  in  FIG.  2 B  is likely to occur, and excitation energy can be efficiently transferred from the exciplex to the guest material  122 . 
     Therefore, it is preferable that the triplet excitation energy level of each of compounds which form an exciplex be higher than the triplet excitation energy level of the exciplex. That is, it is preferable that the triplet excitation energy level of each of the organic compounds  121 _ 1  and  121 _ 2  be higher than the triplet excitation energy level of the exciplex formed by the organic compounds  121 _ 1  and  121 _ 2 . It is also preferable that the triplet excitation energy level of the exciplex be higher than the triplet excitation energy level of the guest material  122 . In the case where the exciplex exhibits thermally activated delayed fluorescence at room temperature, it is preferable that the phosphorescence emission energy of each of the organic compounds  121 _ 1  and  121 _ 2  be higher than the thermally activated delayed fluorescence emission energy of the exciplex formed by the organic compounds  121 _ 1  and  121 _ 2 . It is also preferable that the thermally activated delayed fluorescence emission energy of the exciplex be higher than the phosphorescence emission energy of the guest material  122 . 
     As described above, when the singlet excitation energy levels and the triplet excitation energy levels of the host material  121  and the guest material  122  in the light-emitting layer are set as described above in one embodiment of the present invention, a light-emitting element with high emission efficiency can be provided. 
     &lt;3. Components of Light-Emitting Element&gt; 
     Next, details of other components of the light-emitting element  150  in  FIG.  1 A  will be described below. 
     &lt;&lt;3-1. Pair of Electrodes&gt;&gt; 
     The electrode  101  and the electrode  102  have functions of injecting holes and electrons into the light-emitting layer  113 . The electrodes  101  and  102  can be formed using a metal, an alloy, or a conductive compound, or a mixture or a stack thereof, for example A typical example of the metal is aluminum (Al); besides, a transition metal such as silver (Ag), tungsten (W), chromium (Cr), molybdenum (Mo), copper (Cu), or titanium (Ti), an alkali metal such as lithium (Li), sodium (Na), or cesium (Cs), or a Group 2 metal such as calcium (Ca) 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. As the conductive compound, a metal oxide such as indium oxide-tin oxide (indium tin oxide) can be given. 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  113  is extracted through the electrode  101  and/or the electrode  102 . Therefore, at least one of the electrodes  101  and  102  transmits visible light. 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). 
     &lt;&lt;3-2. Hole-Injection Layer&gt;&gt; 
     The hole-injection layer  111  has a function of reducing a barrier for hole injection from the electrode  101  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: F 4 -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, 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. 
     &lt;&lt;3-3. 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  113 , the HOMO level of the hole-transport layer  112  is preferably equal or close to the HOMO level of the hole-injection layer  111 . 
     &lt;&lt;3-4. Electron-Transport Layer&gt;&gt; 
     The electron-transport layer  115  has a function of transporting, to the light-emitting layer  113 , electrons injected from the electrode  102  through the electron-injection layer  116 . A material having a property of transporting more electrons than holes can be used as an electron-transport material, and a material having an electron mobility of 1×10 −6  cm 2 /Vs or higher is preferable. Specific examples include a metal complex having a quinoline ligand, a benzoquinoline ligand, an oxazole ligand, or a thiazole ligand; an oxadiazole derivative; a triazole derivative; a phenanthroline derivative; a pyridine derivative; and a bipyridine derivative. 
     &lt;&lt;3-5. Electron-Injection Layer&gt;&gt; 
     The electron-injection layer  116  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. 
     Note that the hole-injection layer  111 , the hole-transport layer  112 , the electron-transport layer  115 , and the electron-injection layer  116  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 or a high molecular compound (e.g., an oligomer, a dendrimer, or a polymer) may be used for the hole-injection layer  111 , the hole-transport layer  112 , the light-emitting layer  113 , the electron-transport layer  115 , and the electron-injection layer  116 . 
     &lt;&lt;3-6. Substrate&gt;&gt; 
     The light-emitting element  150  is fabricated 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. 
     Note that, for example, glass, quartz, plastic, or the like can be used for the substrate over which the light-emitting element  150  can be formed. Alternatively, a flexible substrate can be used. The flexible substrate is a substrate that can be bent, such as a plastic substrate made of polycarbonate or polyarylate, for example A film, an inorganic film formed by evaporation, or the like can also be used. Note that materials other than these can be used as long as they can function as a support in a manufacturing process of the light-emitting element and an optical element or as long as they have a function of protecting the light-emitting element and the optical element. 
     The light-emitting element  150  can be formed using a variety of substrates, for example. The type of substrate is not limited to a certain type. As the substrate, a semiconductor substrate (e.g., a single crystal substrate or a silicon substrate), an SOI substrate, a glass substrate, a quartz substrate, a plastic substrate, a metal substrate, a stainless steel substrate, a substrate including stainless steel foil, a tungsten substrate, a substrate including tungsten foil, a flexible substrate, an attachment film, paper including a fibrous material, a base material film, or the like can be used, for example. Examples of the glass substrate include a barium borosilicate glass substrate, an aluminoborosilicate glass substrate, and a soda lime glass substrate. Examples of the flexible substrate, the attachment film, the base 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. Other examples are polypropylene, polyester, polyvinyl fluoride, polyvinyl chloride, and the like. Other examples are polyamide, polyimide, aramid, epoxy, an inorganic film formed by evaporation, paper, and the like. 
     Alternatively, a flexible substrate may be used as the substrate, and the light-emitting element may be provided directly on the flexible substrate. 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 the light-emitting element formed over the separation layer is completed, separated from the substrate, and transferred to 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, or a resin film of polyimide or the like 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. Examples of a substrate to which the light-emitting element is transferred include, in addition to the above-described substrates, a cellophane substrate, a stone substrate, a wood substrate, a cloth substrate (including a natural fiber (e.g., silk, cotton, or hemp), a synthetic fiber (e.g., nylon, polyurethane, or polyester), a regenerated fiber (e.g., acetate, cupra, rayon, or regenerated polyester, or the like), a leather substrate, and a rubber substrate. By using such a substrate, a light-emitting element with high durability, a light-emitting element with high heat resistance, a lightweight light-emitting element, or a thin light-emitting element can be obtained. 
     The light-emitting element  150  may be formed over an electrode electrically connected to a field-effect transistor (FET), for example, which is formed over the above-mentioned substrate, so that an active matrix display device in which the FET controls the drive of the light-emitting element  150  can be manufactured. 
     In this embodiment, one embodiment of the present invention has been described. Embodiments of the present invention are described in other embodiments. Note that one embodiment of the present invention is not limited to the above examples. For example, one embodiment of the present invention is not limited to the above example in which the host material has a difference of more than 0 eV and less than or equal to 0.2 eV between the singlet excitation energy level and the triplet excitation energy level. Depending on circumstances or conditions, the host material in one embodiment of the present invention does not necessarily have a difference of more than 0 eV and less than or equal to 0.2 eV between the singlet excitation energy level and the triplet excitation energy level, for example. Alternatively, one embodiment of the present invention is not limited to the above example in which the host material is a substance which exhibits thermally activated delayed fluorescence at room temperature. Depending on circumstances or conditions, the host material in one embodiment of the present invention may include a substance other than the substance which exhibits thermally activated delayed fluorescence at room temperature, for example. Alternatively, depending on circumstances or conditions, the host material in one embodiment of the present invention does not necessarily include the substance which exhibits thermally activated delayed fluorescence at room temperature, for example. Alternatively, one embodiment of the present invention is not limited to the above example in which the triplet excitation energy level of the host material is higher than the triplet excitation energy level of the guest material. Depending on circumstances or conditions, the triplet excitation energy level of the host material in one embodiment of the present invention is not necessarily higher than the triplet excitation energy level of the guest material, for example. 
     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, a light-emitting element having a structure different from that described in Embodiment 1 and an emission mechanism of the light-emitting element will be described below with reference to  FIGS.  5 A and  5 B . 
     &lt;Structure Example of Light-Emitting Element&gt; 
       FIG.  5 A  is a schematic cross-sectional view of a light-emitting element  450 . 
     The light-emitting element  450  illustrated in  FIG.  5 A  includes a plurality of light-emitting units (in  FIG.  5 A , a light-emitting unit  441  and a light-emitting unit  442 ) between a pair of electrodes (an electrode  401  and an electrode  402 ). One light-emitting unit has the same structure as the EL layer  100  illustrated in  FIG.  1 A . That is, the light-emitting element  150  in  FIG.  1 A  includes one light-emitting unit, while the light-emitting element  450  includes the plurality of light-emitting units. Note that the electrode  401  functions as an anode and the electrode  402  functions as a cathode in the following description of the light-emitting element  450 ; however, the functions may be interchanged in the light-emitting element  450 . 
     In the light-emitting element  450  illustrated in  FIG.  5 A , the light-emitting unit  441  and the light-emitting unit  442  are stacked, and a charge-generation layer  445  is provided between the light-emitting unit  441  and the light-emitting unit  442 . Note that the light-emitting unit  441  and the light-emitting unit  442  may have the same structure or different structures. For example, it is preferable that the EL layer  100  illustrated in  FIG.  1 A  be used in the light-emitting unit  441  and that a light-emitting layer containing a phosphorescent material as a light-emitting material be used in the light-emitting unit  442 . 
     That is, the light-emitting element  450  includes a light-emitting layer  443  and a light-emitting layer  444 . The light-emitting unit  441  includes a hole-injection layer  411 , a hole-transport layer  412 , an electron-transport layer  413 , and an electron-injection layer  414  in addition to the light-emitting layer  443 . The light-emitting unit  442  includes a hole-injection layer  415 , a hole-transport layer  416 , an electron-transport layer  417 , and an electron-injection layer  418  in addition to the light-emitting layer  444 . 
     The charge-generation layer  445  contains a composite material of an organic compound and an acceptor substance. For the composite material, the composite material that can be used for the hole-injection layer  111  described in Embodiment 1 may be used. 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. An organic compound having a hole mobility of 1×10 −6  cm 2 /Vs or higher is preferably used. Note that any other substance may be used as long as the substance 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  445  as that of the light-emitting unit  442 , the charge-generation layer  445  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 does not need to be included in the light-emitting unit. 
     The charge-generation layer  445  may have a stacked-layer 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  445  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 substances and a compound having a high electron-transport property. Furthermore, the charge-generation layer  445  may be formed using a combination of a layer containing the composite material of an organic compound and an acceptor substance with a layer including a transparent conductive film. 
     The charge-generation layer  445  can be formed by an evaporation method (including a vacuum evaporation method), an inkjet method, a coating method, a gravure printing method, or the like. 
     The charge-generation layer  445  provided between the light-emitting unit  441  and the light-emitting unit  442  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  401  and the electrode  402 . For example, in  FIG.  5 A , the charge-generation layer  445  injects electrons into the light-emitting unit  441  and holes into the light-emitting unit  442  when a voltage is applied such that the potential of the electrode  401  is higher than that of the electrode  402 . 
     The light-emitting element having two light-emitting units is described with reference to  FIG.  5 A ; 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  450 , 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  is applied to at least one of the plurality of units, a light-emitting element with high emission efficiency can be provided. 
     The light-emitting layer  443  contains a host material  421  and a guest material  422 . The light-emitting layer  444  contains a host material  431  and a guest material  432 . The host material  421  includes an organic compound  421 _ 1  and an organic compound  421 _ 2 . The host material  431  includes an organic compound  431 _ 1  and an organic compound  431 _ 2 . 
     In this embodiment, the light-emitting layer  443  has a structure similar to that of the light-emitting layer  113  in  FIG.  2 A . That is, the host material  421  (the organic compound  421 _ 1  and the organic compound  421 _ 2 ) and the guest material  422  in the light-emitting layer  443  correspond to the host material  121  (the organic compound  121 _ 1  and the organic compound  121 _ 2 ) and the guest material  122  in the light-emitting layer  113 , respectively. In the following description, the guest material  432  contained in the light-emitting layer  444  is a phosphorescent material. Note that the electrode  401 , the electrode  402 , the hole-injection layers  411  and  415 , the hole-transport layers  412  and  416 , the electron-transport layers  413  and  417 , and the electron-injection layers  414  and  418  correspond to the electrode  101 , the electrode  102 , the hole-injection layer  111 , the hole-transport layer  112 , the electron-transport layer  115 , and the electron-injection layer  116  in Embodiment 1, respectively. Therefore, detailed description thereof is omitted in this embodiment. 
     &lt;Emission Mechanism of Light-Emitting Layer  443 &gt; 
     An emission mechanism of the light-emitting layer  443  is similar to that of the light-emitting layer  113  in  FIG.  2 B . 
     &lt;Emission Mechanism of Light-Emitting Layer  444 &gt; 
     Next, an emission mechanism of the light-emitting layer  444  will be described below. 
     The organic compound  431 _ 1  and the organic compound  431 _ 2  which are contained in the light-emitting layer  444  form an exciplex. The organic compound  431 _ 1  serves as a host material and the organic compound  431 _ 2  serves as an assist material in the description here. 
     Although it is acceptable as long as the combination of the organic compound  431 _ 1  and the organic compound  431 _ 2  in the light-emitting layer  444  can form an exciplex, it is preferred that one organic compound be a material having a hole-transport property and the other organic compound be a material having an electron-transport property. Note that the combination of the organic compound  431 _ 1  and the organic compound  431 _ 2  may be similar to the combination of the organic compound  421 _ 1  and the organic compound  421 _ 2  which form an exciplex in the light-emitting layer  443 . 
       FIG.  5 B  illustrates the correlation of energy levels of the organic compound  431 _ 1 , the organic compound  431 _ 2 , and the guest material  432  in the light-emitting layer  444 . The following explains what terms and signs in  FIG.  5 B  represent: 
     Host ( 431 _ 1 ): the host material (organic compound  431 _ 1 ); 
     Assist ( 431 _ 2 ): the assist material (organic compound  431 _ 2 ); 
     Guest ( 432 ): the guest material  432  (phosphorescent material); 
     S PH : the level of the lowest singlet excited state of the host material (organic compound  431 _ 1 ); 
     T PH : the level of the lowest triplet excited state of the host material (organic compound  431 _ 1 ); 
     T PG : the level of the lowest triplet excited state of the guest material  432  (the phosphorescent material); 
     S PE : the level of the lowest singlet excited state of the exciplex; and 
     T PE : the level of the lowest triplet excited state of the exciplex. 
     The level (S PE ) of the lowest singlet excited state of the exciplex, which is formed by the organic compound  431 _ 1  and the organic compound  431 _ 2 , and the level (T PE ) of the lowest triplet excited state of the exciplex are close to each other (see E 7  in  FIG.  5 B ). 
     Both energies of S PE  and T PE  of the exciplex are then transferred to the level (T PG ) of the lowest triplet excited state of the guest material  432  (the phosphorescent material); thus, light emission is obtained (see E 8  in  FIG.  5 B ). 
     The above-described processes through the route E 7  and the route E 8  may be referred to as exciplex-triplet energy transfer (ExTET) in this specification and the like. 
     When one of the organic compounds  431 _ 1  and  431 _ 2  receiving holes and the other receiving electrons come close to each other, the exciplex is formed at once. Alternatively, when one compound is brought into an excited state, the one immediately takes in the other compound to form the exciplex. Therefore, most excitons in the light-emitting layer  444  exist as the exciplexes. The band gap of the exciplex is narrower than that of each of the organic compounds  431 _ 1  and  431 _ 2 ; therefore, the driving voltage of the light-emitting element can be lowered when the exciplex is formed. 
     When the light-emitting layer  444  has the above structure, light emission from the guest material  432  (the phosphorescent material) of the light-emitting layer  444  can be efficiently obtained. 
     Note that light emitted from the light-emitting layer  443  preferably has a peak on the shorter wavelength side than light emitted from the light-emitting layer  444 . The luminance of a light-emitting element using the phosphorescent material emitting light with a short wavelength tends to degrade quickly. In view of the above, fluorescence is used for light emission with a short wavelength, so that a light-emitting element with less degradation of luminance can be provided. 
     Furthermore, the light-emitting layer  443  and the light-emitting layer  444  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 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  443  and the light-emitting layer  444  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 substances emitting light with different wavelengths for one of the light-emitting layers  443  and  444  or both. In that case, one of the light-emitting layers  443  and  444  or both may be divided into layers and each of the divided layers may contain a different light-emitting material from the others. 
     Next, materials that can be used for the light-emitting layers  443  and  444  will be described. 
     &lt;Material that can be Used for Light-Emitting Layer  443 &gt; 
     A material that can be used for the light-emitting layer  113  described in the above embodiment may be used as a material that can be used for the light-emitting layer  443 . 
     &lt;Material that can be Used for Light-Emitting Layer  444 &gt; 
     In the light-emitting layer  444 , the host material (the organic compound  431 _ 1  and the organic compound  431 _ 2 ) is present in the highest proportion in weight ratio, and the guest material  432  (the phosphorescent material) is dispersed in the host material (the organic compound  431 _ 1  and the organic compound  431 _ 2 ). 
     Examples of the organic compound  431 _ 1  (the host material) 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. 
     As the organic compound  431 _ 2  (the assist material), a substance which can form an exciplex together with the organic compound  431 _ 1  is used. In that case, it is preferable that the organic compound  431 _ 1 , the organic compound  431 _ 2 , and the guest material  432  (the phosphorescent material) be selected such that the emission peak of the exciplex overlaps with an adsorption band, specifically an adsorption band on the longest wavelength side, of a triplet metal to ligand charge transfer (MLCT) transition of the 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 fluorescence material is used instead of the phosphorescent material, it is preferable that the adsorption band on the longest wavelength side be a singlet absorption band. 
     As the guest material  432  (the 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 or the like can be given. 
     As the guest material  432  included in the light-emitting layer  444 , any material can be used as long as the material can convert triplet excitation energy into light emission. As an example of the material that can convert triplet excitation energy into light emission, a thermally activated delayed fluorescence material can be given in addition to the phosphorescent material. Therefore, the term “phosphorescent material” 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 that 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 exhibits light emission (fluorescence) from the singlet excited state. Thermally activated delayed fluorescence is efficiently obtained under the condition where the difference between the triplet excitation energy level and the singlet excitation 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. 
     There is no limitation on the emission colors of the light-emitting material included in the light-emitting layer  443  and the light-emitting material included in the light-emitting layer  444 , 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 (the guest material  422 ) included in the light-emitting layer  443  is preferably shorter than that of the light-emitting material (the guest material  432 ) included in the light-emitting layer  444 . 
     Note that the light-emitting layers  443  and  444  can be formed by an evaporation method (including a vacuum evaporation method), an inkjet method, a coating method, gravure printing, or the like. 
     Note that the structure described above in this embodiment can be combined with any of the structures described in the other embodiments as appropriate. 
     Embodiment 3 
     In this embodiment, a light-emitting element having a structure different from those described in Embodiment 1 and Embodiment 2 will be described below with reference to  FIGS.  6 A and  6 B . 
     &lt;Structure Example of Light-Emitting Element&gt; 
       FIG.  6 A  is a schematic cross-sectional view of a light-emitting element  452  of one embodiment of the present invention. 
     The light-emitting element  452  includes a plurality of light-emitting units (in  FIG.  6 A , a light-emitting unit  446  and a light-emitting unit  447 ) between an electrode  401  and an electrode  402 . One light-emitting unit has the same structure as the EL layer  100  illustrated in  FIG.  1 A . That is, the light-emitting element  150  in  FIG.  1 A  includes one light-emitting unit, while the light-emitting element  452  includes the plurality of light-emitting units. Note that the electrode  401  functions as an anode and the electrode  402  functions as a cathode in the following description of this embodiment; however, the functions may be interchanged in the light-emitting element  452 . 
     In the light-emitting element  452  illustrated in  FIG.  6 A , the light-emitting unit  446  and the light-emitting unit  447  are stacked, and a charge-generation layer  445  is provided between the light-emitting unit  446  and the light-emitting unit  447 . Note that the light-emitting unit  446  and the light-emitting unit  447  may have the same structure or different structures. For example, it is preferable that a light-emitting layer containing a fluorescent material as a light-emitting material be used in the light-emitting unit  446  and that the EL layer  100  illustrated in  FIG.  1 A  be used in the light-emitting unit  447 . 
     That is, the light-emitting element  452  includes a light-emitting layer  448  and a light-emitting layer  449 . The light-emitting unit  446  includes a hole-injection layer  411 , a hole-transport layer  412 , an electron-transport layer  413 , and an electron-injection layer  414  in addition to the light-emitting layer  448 . The light-emitting unit  447  includes a hole-injection layer  415 , a hole-transport layer  416 , an electron-transport layer  417 , and an electron-injection layer  418  in addition to the light-emitting layer  449 . 
     The light-emitting element having two light-emitting units is described with reference to  FIG.  6 A ; 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  452 , 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 display device with low power consumption can be provided. 
     When the structure of the EL layer  100  is applied to at least one of the plurality of units, a light-emitting element with high emission efficiency can be provided. 
     The light-emitting layer  448  contains a host material  461  and a guest material  462 . The light-emitting layer  449  contains a host material  471  and a guest material  472 . The host material  471  includes an organic compound  471 _ 1  and an organic compound  471 _ 2 . 
     In this embodiment, the light-emitting layer  449  has a structure similar to that of the light-emitting layer  113  in  FIG.  2 A . That is, the host material  471  (the organic compound  471 _ 1  and the organic compound  471 _ 2 ) and the guest material  472  in the light-emitting layer  449  correspond to the host material  121  (the organic compound  121 _ 1  and the organic compound  121 _ 2 ) and the guest material  122  in the light-emitting layer  113 , respectively. In the following description, the guest material  462  contained in the light-emitting layer  448  is a fluorescent material. 
     &lt;Emission Mechanism of Light-Emitting Layer  448 &gt; 
     First, an emission mechanism of the light-emitting layer  448  will be described below. 
     In the light-emitting layer  448 , excitons are generated by recombination of carriers. Because the amount of the host material  461  is large as compared to the guest material  462 , the host material  461  is brought into an excited state by the exciton generation. The ratio of singlet excitons to triplet excitons generated by carrier recombination (hereinafter referred to as exciton generation probability) is approximately 1:3. 
     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. 
     First, a case where the T 1  level of the host material  461  is higher than the T 1  level of the guest material  462  will be described below. 
     The triplet excitation energy of the host material  461  is transferred from the T 1  level of the host material  461  to the T 1  level of the guest material  462  (triplet energy transfer). However, the triplet excitation energy does not provide light emission in a visible light region because the guest material  462  is the fluorescent material. Thus, it is difficult to use the triplet excitation energy of the host material  461  for light emission. Therefore, when the T 1  level of the host material  461  is higher than the T 1  level of the guest material  462 , it is difficult to use more than approximately 25% of injected carriers for light emission. 
       FIG.  6 B  illustrates the correlation of energy levels of the host material  461  and the guest material  462  in the light-emitting layer  448  of one embodiment of the present invention. The following explains what terms and signs in  FIG.  6 B  represent: 
     Host: the host material  461 ; 
     Guest: the guest material  462  (fluorescent material); 
     S FH : the level of the lowest singlet excited state of the host material  461  (S 1  level); 
     T FH : the level of the lowest triplet excited state of the host material  461  (T 1  level); 
     S FG : the level of the lowest singlet excited state of the guest material  462  (fluorescent material) (S 1  level); and 
     T FG : the level of the lowest triplet excited state of the guest material  462  (fluorescent material) (T 1  level). 
     As illustrated in  FIG.  6 B , the T 1  level of the guest material  462  (T FG  in  FIG.  6 B ) is higher than the T 1  level of the host material  461  (T FH  in  FIG.  6 B ). 
     In addition, as illustrated in  FIG.  6 B , triplet excitons collide with each other by triplet-triplet annihilation (TTA) (see E 9  in  FIG.  6 B ), and some of them are converted into singlet excitons having an energy at the level of the lowest singlet excited state of the host material  461  (S FH ). The singlet excitation energy of the host material  461  is transferred from the level of the lowest singlet excited state of the host material  461  (S FH ) to the level of the lowest singlet excited state of the guest material  462  (the fluorescent material) (SF G ) that is a level lower than S FH  (see E 10  in  FIG.  6 B ). Thus, the guest material  462  (the fluorescent material) is brought into the singlet excited state and accordingly emits light. 
     Because the T 1  level of the host material is lower than the T 1  level of the guest material, excitation energy at T FG  is transferred to T FH  without deactivation (see E 11  in  FIG.  6 B ), which is utilized for TTA. 
     When the light-emitting layer  448  has the above structure, light emission from the guest material  462  of the light-emitting layer  448  can be efficiently obtained. 
     Note that the light-emitting layer  448  and the light-emitting layer  449  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 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  448  and the light-emitting layer  449  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 substances emitting light with different wavelengths for one of the light-emitting layers  448  and  449  or both. In that case, one of the light-emitting layers  448  and  449  or both may be divided into layers and each of the divided layers may contain a different light-emitting material from the others. 
     &lt;Emission Mechanism of Light-Emitting Layer  449 &gt; 
     An emission mechanism of the light-emitting layer  449  is similar to that of the light-emitting layer  113  in  FIG.  2 B . 
     Next, materials that can be used for the light-emitting layers  448  and  449  will be described. 
     &lt;Material that can be Used for Light-Emitting Layer  448 &gt; 
     In the light-emitting layer  448 , the host material  461  is present in the highest proportion in weight ratio, and the guest material  462  (the fluorescent material) is dispersed in the host material  461 . The S 1  level of the host material  461  is preferably higher than the S 1  level of the guest material  462  (the fluorescent material), while the T 1  level of the host material  461  is preferably lower than the T 1  level of the guest material  462  (the fluorescent material). 
     An anthracene derivative or a tetracene derivative is preferably used as the host material  461 . This is because these derivatives each have a high S 1  level and a low T 1  level. Specific examples include 9-phenyl-3-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: PCzPA), 3-[4-(1-naphthyl)-phenyl]-9-phenyl-9H-carbazole (abbreviation: PCPN), 9-[4-(10-phenyl-9-anthracenyl)phenyl]-9H-carbazole (abbreviation: CzPA), 7-[4-(10-phenyl-9-anthryl)phenyl]-7H-dibenzo[c,g]carbazole (abbreviation: cgDBCzPA), 6-[3-(9,10-diphenyl-2-anthryl)phenyl]-benzo[b]naphtho[1,2-d]furan (abbreviation: 2mBnfPPA), and 9-phenyl-10-{4-(9-phenyl-9H-fluoren-9-yl)biphenyl-4′-yl}anthracene (abbreviation: FLPPA). Besides, 5,12-diphenyltetracene, 5,12-bis(biphenyl-2-yl)tetracene, and the like can be given. 
     Examples of the guest material  462  (the fluorescent material) include a pyrene derivative, an anthracene derivative, a triphenylene derivative, a fluorene derivative, a carbazole derivative, a dibenzothiophene derivative, a dibenzofuran derivative, a dibenzoquinoxaline derivative, a quinoxaline derivative, a pyridine derivative, a pyrimidine derivative, a phenanthrene derivative, a naphthalene derivative, and the like. A pyrene derivative is particularly preferable because it has a high emission quantum yield. Specific examples of the pyrene derivative include N,N′-bis(3-methylphenyl)-N,N′-bis[3-(9-phenyl-9H-fluoren-9-yl)phenyl]pyrene-1,6-diamine (abbreviation: 1,6mMemFLPAPrn), N,N′-bis[4-(9-phenyl-9H-fluoren-9-yl)phenyl]-N,N′-diphenylpyrene-1,6-diamine (abbreviation: 1,6FLPAPrn), N,N′-bis(dibenzofuran-2-yl)-N,N′-diphenylpyrene-1,6-diamine (abbreviation: 1,6FrAPrn), N,N′-bis(dibenzothiophen-2-yl)-N,N′-diphenylpyrene-1,6-diamine (abbreviation: 1,6ThAPrn), and the like. 
     &lt;Material that can be Used for Light-Emitting Layer  449 &gt; 
     A material that can be used for the light-emitting layer  113  described in the above embodiment may be used as a material that can be used for the light-emitting layer  449 . 
     There is no limitation on the emission colors of the light-emitting material included in the light-emitting layer  448  and the light-emitting material included in the light-emitting layer  449 , 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 (the guest material  462 ) included in the light-emitting layer  448  is preferably shorter than that of the light-emitting material (the guest material  472 ) included in the light-emitting layer  449 . 
     Note that the light-emitting layers  448  and  449  can be formed by an evaporation method (including a vacuum evaporation method), an inkjet method, a coating method, gravure printing, or the like. 
     Note that the above-described structure can be combined with any of the structures in this embodiment and the other embodiments. 
     Embodiment 4 
     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.  7 A and  7 B . 
       FIG.  7 A  is a block diagram illustrating the display device of one embodiment of the present invention, and  FIG.  7 B  is a circuit diagram illustrating a pixel circuit of the display device of one embodiment of the present invention. 
     &lt;Display Device&gt; 
     The display device illustrated in  FIG.  7 A  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 chip-on-glass (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 (video 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 video 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 video 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 (in 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.  7 A  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 video 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.  7 A , 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.  7 A , 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.  7 A  can have a structure illustrated in  FIG.  7 B , for example. 
     The pixel circuit  801  illustrated in  FIG.  7 B  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 (hereinafter referred to as a signal line DL_n). A gate electrode of the transistor  852  is electrically connected to a wiring to which a gate signal is supplied (hereinafter referred to as 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.  7 B , the pixel circuits  801  are sequentially selected row by row by the scan line driver circuit  804   a  in  FIG.  7 A , 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. 
     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 or 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 or 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 combined with any of the structures described in the other embodiments as appropriate. 
     Embodiment 5 
     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.  8 A and  8 B ,  FIGS.  9 A to  9 C ,  FIGS.  10 A and  10 B ,  FIGS.  11 A and  11 B , and  FIG.  12   . 
     &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.  8 A and  8 B  are perspective views of the touch panel  2000 . Note that  FIGS.  8 A and  8 B  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.  8 B ). The touch panel  2000  also includes a substrate  2510 , a substrate  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.  8 B , 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.  8 B  is an example of using a projected capacitive touch sensor. 
     Note that a variety of sensors that can sense proximity or touch 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.  8 A and  8 B . 
     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;Display Device&gt; 
     Next, the display device  2501  will be described in detail with reference to  FIG.  9 A .  FIG.  9 A  corresponds to a cross-sectional view taken along dashed-dotted line X 1 -X 2  in  FIG.  8 B . 
     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.  9 A , the sealing layer  2560  can also serve as an 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 or 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 ultraviolet curable resin or a heat curable resin may be used; for example, a polyvinyl chloride (PVC) based resin, an acrylic 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. For example, 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 or 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  2502   t  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, for example. 
     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.  9 A . 
     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 region. 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 planarizing 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 printed wiring board (PWB). 
     In the display device  2501 , transistors with any of a variety of structures can be used.  FIG.  9 A  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.  9 B . 
     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 13 semiconductors (e.g., a semiconductor including gallium), 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 current of the transistors can be reduced. Examples of the oxide semiconductors include an In—Ga oxide, an In-M-Zn oxide (M represents Al, gallium (Ga), yttrium (Y), zirconium (Zr), lanthanum (La), cerium (Ce), tin (Sn), hafnium (Hf), or neodymium (Nd)), and the like. 
     &lt;Touch Sensor&gt; 
     Next, the touch sensor  2595  will be described in detail with reference to  FIG.  9 C .  FIG.  9 C  corresponds to a cross-sectional view taken along dashed-dotted line X 3 -X 4  in  FIG.  8 B . 
     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), or 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.  10 A .  FIG.  10 A  corresponds to a cross-sectional view taken along dashed-dotted line X 5 -X 6  in  FIG.  8 A . 
     In the touch panel  2000  illustrated in  FIG.  10 A , the display device  2501  described with reference to  FIG.  9 A  and the touch sensor  2595  described with reference to  FIG.  9 C  are attached to each other. 
     The touch panel  2000  illustrated in  FIG.  10 A  includes an adhesive layer  2597  and an anti-reflective layer  2567   p  in addition to the components described with reference to  FIGS.  9 A and  9 C . 
     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, an 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.  10 A  will be described with reference to  FIG.  10 B . 
       FIG.  10 B  is a cross-sectional view of a touch panel  2001 . The touch panel  2001  illustrated in  FIG.  10 B  differs from the touch panel  2000  illustrated in  FIG.  10 A  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.  10 B  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.  10 B . 
     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.  10 A or  10 B , light may be emitted from the light-emitting element to one of upper and lower sides, or both, of the substrate. 
     &lt;Method for Driving Touch Panel&gt; 
     Next, an example of a method for driving a touch panel will be described with reference to  FIGS.  11 A and  11 B . 
       FIG.  11 A  is a block diagram illustrating the structure of a mutual capacitive touch sensor.  FIG.  11 A  illustrates a pulse voltage output circuit  2601  and a current sensing circuit  2602 . Note that in  FIG.  11 A , 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.  11 A  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.  11 B  is a timing chart showing input and output waveforms in the mutual capacitive touch sensor illustrated in  FIG.  11 A . In  FIG.  11 B , sensing of a sensing target is performed in all the rows and columns in one frame period.  FIG.  11 B  shows a period when a sensing target is not sensed (not touched) and a period when a sensing target is sensed (touched). Sensed current values of the wirings Y 1  to Y 6  are shown as the waveforms of voltage values. 
     A pulse voltage is sequentially applied to the wirings X 1  to X 6 , and the waveforms of the wirings Y 1  to Y 6  change in accordance with the pulse voltage. When there is no approach or contact of a sensing target, the waveforms of the wirings Y 1  to Y 6  change in accordance with changes in the voltages of the wirings X 1  to X 6 . The current value is decreased at the point of approach or contact of a sensing target and accordingly the waveform of the voltage value changes. 
     By detecting a change in mutual capacitance in this manner, the approach or contact of a sensing target can be sensed. 
     &lt;Sensor Circuit&gt; 
     Although  FIG.  11 A  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.  12    illustrates an example of a sensor circuit included in an active matrix type touch sensor. 
     The sensor circuit in  FIG.  12    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.  12    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 combined with any of the structures described in the other embodiments as appropriate. 
     Embodiment 6 
     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.  13    and  FIGS.  14 A to  14 G . 
     &lt;Display Module&gt; 
     In a display module  8000  in  FIG.  13   , 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;Electronic Device&gt; 
       FIGS.  14 A to  14 G  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.  14 A to  14 G  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.  14 A to  14 G  are not limited to those described above, and the electronic devices can have a variety of functions. Although not illustrated in  FIGS.  14 A to  14 G , 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.  14 A to  14 G  will be described in detail below. 
       FIG.  14 A  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.  14 B  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.  14 B , can be positioned in the portable information terminal  9101  as in the portable information terminal  9100  shown in  FIG.  14 A . 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.  14 C  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.  14 D  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.  14 E,  14 F, and  14 G  are perspective views of a foldable portable information terminal  9201 .  FIG.  14 E  is a perspective view illustrating the portable information terminal  9201  that is opened.  FIG.  14 F  is a perspective view illustrating the portable information terminal  9201  that is being opened or being folded.  FIG.  14 G  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 combined with any of the structures described in the other embodiments as appropriate. 
     Embodiment 7 
     In this embodiment, examples of lighting devices in which the light-emitting element of one embodiment of the present invention is used will be described with reference to  FIG.  15   . 
       FIG.  15    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. 
     In this manner, a variety of lighting devices to which the light-emitting element is applied can be obtained. Note that such lighting devices are also embodiments of the present invention. 
     The structure described in this embodiment can be combined with any of the structures described in the other embodiments as appropriate. 
     Example 1 
     In this example, an example of fabricating light-emitting elements of one embodiment of the present invention and comparative light-emitting elements will be described. Light-emitting elements 1 to 7 were fabricated in this example. 
     Note that the light-emitting elements 1, 2, 5, and 7 are comparative light-emitting elements, and the light-emitting elements 3, 4, and 6 are light-emitting elements of one embodiment of the present invention. The light-emitting element 1 is a host-guest light-emitting element. The light-emitting elements 2, 5, and 7 are light-emitting elements which utilize exciplex emission. The light-emitting elements 3, 4, and 6 are light-emitting elements which utilize ExSET. 
       FIG.  16    is a schematic cross-sectional view of each of the light-emitting elements 1 to 7, and Tables 4 and 5 show details of the element structure. In addition, structures and abbreviations of compounds used here are given below. 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                   
                 Reference 
                 Thickness 
                   
                 Weight 
               
               
                   
                 Layer 
                 numeral 
                 (nm) 
                 Material 
                 ratio 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Light- 
                 Electrode 
                 1102 
                 200 
                 Al 
                 — 
               
               
                 emitting 
                 Electron- 
                 1116 
                 1 
                 LiF 
                 — 
               
               
                 element 1 
                 injection 
               
               
                   
                 layer 
               
               
                   
                 Electron- 
                 1115 
                 15 
                 Bphen 
                 — 
               
               
                   
                 transport 
                   
                 10 
                 4,6mCzP2Pm 
                 — 
               
               
                   
                 layer 
               
               
                   
                 Light- 
                 1113 
                 40 
                 4,6mCzP2Pm:Rubrene 
                    1:0.005 
               
               
                   
                 emitting 
               
               
                   
                 layer 
               
               
                   
                 Hole- 
                 1112 
                 20 
                 BPAFLP 
                 — 
               
               
                   
                 transport 
               
               
                   
                 layer 
               
               
                   
                 Hole- 
                 1111 
                 20 
                 DBT3P-II:MoO 3   
                 2:1 
               
               
                   
                 injection 
               
               
                   
                 layer 
               
               
                   
                 Electrode 
                 1101 
                 110 
                 ITSO 
                 — 
               
               
                 Light- 
                 Electrode 
                 1102 
                 200 
                 Al 
                 — 
               
               
                 emitting 
                 Electron- 
                 1116 
                 1 
                 LiF 
                 — 
               
               
                 element 2 
                 injection 
               
               
                   
                 layer 
               
               
                   
                 Electron- 
                 1115 
                 15 
                 Bphen 
                 — 
               
               
                   
                 transport 
                   
                 10 
                 4,6mCzP2Pm 
                 — 
               
               
                   
                 layer 
               
               
                   
                 Light- 
                 1113 
                 40 
                 4,6mCzP2Pm:PCBBiF 
                 0.8:0.2 
               
               
                   
                 emitting 
               
               
                   
                 layer 
               
               
                   
                 Hole- 
                 1112 
                 20 
                 BPAFLP 
                 — 
               
               
                   
                 transport 
               
               
                   
                 layer 
               
               
                   
                 Hole- 
                 1111 
                 20 
                 DBT3P-II:MoOx 
                 2:1 
               
               
                   
                 injection 
               
               
                   
                 layer 
               
               
                   
                 Electrode 
                 1101 
                 110 
                 ITSO 
                 — 
               
               
                 Light- 
                 Electrode 
                 1102 
                 200 
                 Al 
                 — 
               
               
                 emitting 
                 Electron- 
                 1116 
                 1 
                 LiF 
                 — 
               
               
                 element 3 
                 injection 
               
               
                   
                 layer 
               
               
                   
                 Electron- 
                 1115 
                 15 
                 Bphen 
                 — 
               
               
                   
                 transport 
                   
                 10 
                 4,6mCzP2Pm 
                 — 
               
               
                   
                 layer 
               
               
                   
                 Light- 
                 1113 
                 40 
                 4,6mCzP2Pm:PCBBiF:Rubrene 
                 0.8:0.2:0.005 
               
               
                   
                 emitting 
               
               
                   
                 layer 
               
               
                   
                 Hole- 
                 1112 
                 20 
                 BPAFLP 
                 — 
               
               
                   
                 transport 
               
               
                   
                 layer 
               
               
                   
                 Hole- 
                 1111 
                 20 
                 DBT3P-II:MoO 3   
                 2:1 
               
               
                   
                 injection 
               
               
                   
                 layer 
               
               
                   
                 Electrode 
                 1101 
                 110 
                 ITSO 
                 — 
               
               
                 Light- 
                 Electrode 
                 1102 
                 200 
                 Al 
                 — 
               
               
                 emitting 
                 Electron- 
                 1116 
                 1 
                 LiF 
                 — 
               
               
                 element 4 
                 injection 
               
               
                   
                 layer 
               
               
                   
                 Electron- 
                 1115 
                 15 
                 Bphen 
                 — 
               
               
                   
                 transport 
                   
                 10 
                 4,6mCzP2Pm 
                 — 
               
               
                   
                 layer 
               
               
                   
                 Light- 
                 1113 
                 40 
                 4,6mCzP2Pm:PCBBiF:Rubrene 
                 0.8:0.2:0.01 
               
               
                   
                 emitting 
               
               
                   
                 layer 
               
               
                   
                 Hole- 
                 1112 
                 20 
                 BPAFLP 
                 — 
               
               
                   
                 transport 
               
               
                   
                 layer 
               
               
                   
                 Hole- 
                 1111 
                 20 
                 DBT3P-II:MoO 3   
                 2:1 
               
               
                   
                 injection 
               
               
                   
                 layer 
               
               
                   
                 Electrode 
                 1101 
                 110 
                 ITSO 
                 — 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 5 
               
               
                   
                   
               
               
                   
                   
                 Reference 
                 Thickness 
                   
                 Weight 
               
               
                   
                 Layer 
                 numeral 
                 (nm) 
                 Material 
                 ratio 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Light- 
                 Electrode 
                 1102 
                 200 
                 Al 
                 — 
               
               
                 emitting 
                 Electron- 
                 1116 
                 1 
                 LiF 
                 — 
               
               
                 element 5 
                 injection 
               
               
                   
                 layer 
               
               
                   
                 Electron- 
                 1115 
                 10 
                 Bphen 
                 — 
               
               
                   
                 transport 
                   
                 20 
                 4mCzBPBfPm 
                 — 
               
               
                   
                 layer 
               
               
                   
                 Light- 
                 1113 
                 40 
                 4mCzBPBfPm:PCzPCA1 
                 0.8:0.2 
               
               
                   
                 emitting 
               
               
                   
                 layer 
               
               
                   
                 Hole- 
                 1112 
                 20 
                 BPAFLP 
                 — 
               
               
                   
                 transport 
               
               
                   
                 layer 
               
               
                   
                 Hole- 
                 1111 
                 20 
                 DBT3P-II:MoO 3   
                 2:1 
               
               
                   
                 injection 
               
               
                   
                 layer 
               
               
                   
                 Electrode 
                 1101 
                 110 
                 ITSO 
                 — 
               
               
                 Light- 
                 Electrode 
                 1102 
                 200 
                 Al 
                 — 
               
               
                 emitting 
                 Electron- 
                 1116 
                 1 
                 LiF 
                 — 
               
               
                 element 6 
                 injection 
               
               
                   
                 layer 
               
               
                   
                 Electron- 
                 1115 
                 10 
                 Bphen 
                 — 
               
               
                   
                 transport 
                   
                 20 
                 4,6mCzP2Pm 
                 — 
               
               
                   
                 layer 
               
               
                   
                 Light- 
                 1113 
                 40 
                 4,6mCzP2Pm:PCzPCA1:Rubrene 
                 0.8:0.2:0.01 
               
               
                   
                 emitting 
               
               
                   
                 layer 
               
               
                   
                 Hole- 
                 1112 
                 20 
                 BPAFLP 
                 — 
               
               
                   
                 transport 
               
               
                   
                 layer 
               
               
                   
                 Hole- 
                 1111 
                 20 
                 DBT3P-II:MoO 3   
                 2:1 
               
               
                   
                 injection 
               
               
                   
                 layer 
               
               
                   
                 Electrode 
                 1101 
                 110 
                 ITSO 
                 — 
               
               
                 Light- 
                 Electrode 
                 1102 
                 200 
                 Al 
                 — 
               
               
                 emitting 
                 Electron- 
                 1116 
                 1 
                 LiF 
                 — 
               
               
                 element 7 
                 injection 
               
               
                   
                 layer 
               
               
                   
                 Electron- 
                 1115 
                 10 
                 Bphen 
                 — 
               
               
                   
                 transport 
                   
                 20 
                 4,6mCzP2Pm 
                 — 
               
               
                   
                 layer 
               
               
                   
                 Light- 
                 1113 
                 40 
                 4,6mCzP2Pm:PCzPCA1 
                 0.8:0.2 
               
               
                   
                 emitting 
               
               
                   
                 layer 
               
               
                   
                 Hole- 
                 1112 
                 20 
                 BPAFLP 
                 — 
               
               
                   
                 transport 
               
               
                   
                 layer 
               
               
                   
                 Hole- 
                 1111 
                 20 
                 DBT3P-II:MoO 3   
                 2:1 
               
               
                   
                 injection 
               
               
                   
                 layer 
               
               
                   
                 Electrode 
                 1101 
                 110 
                 ITSO 
                 — 
               
               
                   
               
            
           
         
       
     
     &lt;1-1. Method for Fabricating Light-Emitting Element  1 &gt; 
     First, a film of indium tin oxide containing silicon oxide (abbreviation: ITSO) was formed as an electrode  1101  over a substrate  1100  by a sputtering method. Note that the thickness of the electrode  1101  was 110 nm, and the area of the electrode  1101  was 4 mm 2  (2 mm×2 mm). 
     Next, as pretreatment of evaporation of an organic compound layer, the electrode  1101  side of the substrate  1100  was washed with water, baking was performed at 200° C. for one hour, and then UV ozone treatment was performed on a surface of the electrode  1101  for 370 seconds. 
     After that, the substrate  1100  was transferred into a vacuum evaporation apparatus where the pressure had been reduced to approximately 1×10 −4  Pa, and was subjected to vacuum baking at 170° C. for 30 minutes in a heating chamber of the vacuum evaporation apparatus, and then the substrate  1100  was cooled down for about 30 minutes. 
     Next, the substrate  1100  was fixed to a holder in the vacuum evaporation apparatus so that a side on which the electrode  1101  was provided faced downward. In this example, a hole-injection layer  1111 , a hole-transport layer  1112 , a light-emitting layer  1113 , an electron-transport layer  1115 , an electron-injection layer  1116 , and an electrode  1102  were sequentially formed by a vacuum evaporation method. The fabrication method will be described in detail below. 
     First, DBT3P-II and molybdenum oxide (abbreviation: MoO 3 ) were co-evaporated as the hole-injection layer  1111  over the electrode  1101  so that the weight ratio of DBT3P-II to molybdenum oxide was 2:1. Note that the thickness of the hole-injection layer  1111  was 20 nm. 
     Next, the hole-transport layer  1112  was formed over the hole-injection layer  1111 . As the hole-transport layer  1112 , BPAFLP was evaporated. Note that the thickness of the hole-transport layer  1112  was 20 nm. 
     Next, the light-emitting layer  1113  was formed over the hole-transport layer  1112 . As the light-emitting layer  1113 , 4,6mCzP2Pm and rubrene were co-evaporated so that the weight ratio of 4,6mCzP2Pm to rubrene was 1:0.005. Note that the thickness of the light-emitting layer  1113  was 40 nm. In the light-emitting layer  1113 , 4,6mCzP2Pm is a host material and rubrene is a guest material. 
     Next, 4,6mCzP2Pm and bathophenanthroline (abbreviation: Bphen) were sequentially evaporated to thicknesses of 10 nm and 15 nm, respectively, as the electron-transport layer  1115  over the light-emitting layer  1113 . Then, lithium fluoride (abbreviation: LiF) was evaporated to a thickness of 1 nm as the electron-injection layer  1116  over the electron-transport layer  1115 . 
     Next, aluminum (Al) was evaporated as the electrode  1102  over the electron-injection layer  1116 . Note that the thickness of the electrode  1102  was 200 nm. 
     The light-emitting element fabricated over the substrate  1100  as described above was sealed by being attached to a sealing substrate  1150  in a glove box in a nitrogen atmosphere so as not to be exposed to the air. Specifically, after a sealant was applied to surround the light-emitting element over the substrate  1100  and the substrate  1100  was attached to the sealing substrate  1150 , 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, the light-emitting element 1 was fabricated. 
     &lt;1-2. Method for Fabricating Light-Emitting Element  2 &gt; 
     The light-emitting element 2 is different from the above-described light-emitting element 1 in the structure of the light-emitting layer  1113 . The other components are the same as those of the light-emitting element 1. 
     As the light-emitting layer  1113  of the light-emitting element 2, 4,6mCzP2Pm and PCBBiF were co-evaporated so that the weight ratio of 4,6mCzP2Pm to PCBBiF was 0.8:0.2. Note that the thickness of the light-emitting layer  1113  was 40 nm. In the light-emitting layer  1113 , 4,6mCzP2Pm is a host material and PCBBiF is an assist material. 
     &lt;1-3. Method for Fabricating Light-Emitting Element  3 &gt; 
     The light-emitting element 3 is different from the above-described light-emitting element 1 in the structure of the light-emitting layer  1113 . The other components are the same as those of the light-emitting element 1. 
     As the light-emitting layer  1113  of the light-emitting element 3, 4,6mCzP2Pm, PCBBiF, and rubrene were co-evaporated so that the weight ratio of 4,6mCzP2Pm to PCBBiF and rubrene was 0.8:0.2:0.005. Note that the thickness of the light-emitting layer  1113  was 40 nm. In the light-emitting layer  1113 , 4,6mCzP2Pm is a host material, PCBBiF is an assist material, and rubrene is a guest material. 
     &lt;1-4. Method for Fabricating Light-Emitting Element  4 &gt; 
     The light-emitting element 4 is different from the above-described light-emitting element  1  in the structure of the light-emitting layer  1113 . The other components are the same as those of the light-emitting element 1. 
     As the light-emitting layer  1113  of the light-emitting element 4, 4,6mCzP2Pm, PCBBiF, and rubrene were co-evaporated so that the weight ratio of 4,6mCzP2Pm to PCBBiF and rubrene was 0.8:0.2:0.01. Note that the thickness of the light-emitting layer  1113  was 40 nm. In the light-emitting layer  1113 , 4,6mCzP2Pm is a host material, PCBBiF is an assist material, and rubrene is a guest material. Note that the concentration of the guest material differs between the light-emitting element 3 and the light-emitting element 4. 
     &lt;1-5. Method for Fabricating Light-Emitting Element  5 &gt; 
     The light-emitting element 5 is different from the above-described light-emitting element 1 in the structures of the light-emitting layer  1113  and the electron-transport layer  1115 . The other components are the same as those of the light-emitting element 1. 
     As the light-emitting layer  1113  of the light-emitting element 5, 4-{3-[3′-(9H-carbazol-9-yl)]biphenyl-3-yl}benzofuro[3,2-d]pyrimidine (abbreviation: 4mCzBPBfPm) and 3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole (abbreviation: PCzPCA1) were co-evaporated so that the weight ratio of 4mCzBPBfPm to PCzPCA1 was 0.8:0.2. Note that the thickness of the light-emitting layer  1113  was 40 nm. In the light-emitting layer  1113 , 4mCzBPBfPm is a host material, and PCzPCA1 is an assist material. 
     As the electron-transport layer  1115  of the light-emitting element 5, 4mCzBPBfPm and Bphen were evaporated to thicknesses of 20 nm and 10 nm, respectively. 
     &lt;1-6. Method for Fabricating Light-Emitting Element  6 &gt; 
     The light-emitting element 6 is different from the above-described light-emitting element 1 in the structures of the light-emitting layer  1113  and the electron-transport layer  1115 . The other components are the same as those of the light-emitting element 1. 
     As the light-emitting layer  1113  of the light-emitting element 6, 4,6mCzP2Pm, PCzPCA1, and rubrene were co-evaporated so that the weight ratio of 4,6mCzP2Pm to PCzPCA1 and rubrene was 0.8:0.2:0.01. Note that the thickness of the light-emitting layer  1113  was 40 nm. In the light-emitting layer  1113 , 4,6mCzP2Pm is a host material, PCzPCA1 is an assist material, and rubrene is a guest material. 
     As the electron-transport layer  1115  of the light-emitting element 6, 4,6mCzP2Pm and Bphen were evaporated to thicknesses of 20 nm and 10 nm, respectively. 
     &lt;1-7. Method for Fabricating Light-Emitting Element  7 &gt; 
     The light-emitting element 7 is different from the above-described light-emitting element 1 in the structures of the light-emitting layer  1113  and the electron-transport layer  1115 . The other components are the same as those of the light-emitting element 1. 
     As the light-emitting layer  1113  of the light-emitting element 7, 4,6mCzP2Pm and PCzPCA1 were co-evaporated so that the weight ratio of 4,6mCzP2Pm to PCzPCA1 was 0.8:0.2. Note that the thickness of the light-emitting layer  1113  was 40 nm. In the light-emitting layer  1113 , 4,6mCzP2Pm is a host material and PCzPCA1 is an assist material. 
     As the electron-transport layer  1115  of the light-emitting element 7, 4,6mCzP2Pm and Bphen were evaporated to thicknesses of 20 nm and 10 nm, respectively. 
     Note that in all the above evaporation steps for the light-emitting elements 1 to 7, a resistive heating method was used as an evaporation method. 
     &lt;1-8. Characteristics of Light-Emitting Elements 1 to 7&gt; 
       FIGS.  17 ,  19 ,  21 ,  23 , and  25    show luminance-current density characteristics, luminance-voltage characteristics, current efficiency-luminance characteristics, current-voltage characteristics, and external quantum efficiency-luminance characteristics, respectively, of the light-emitting elements 1 to 4.  FIGS.  18 ,  20 ,  22 ,  24 , and  26    show luminance-current density characteristics, luminance-voltage characteristics, current efficiency-luminance characteristics, current-voltage characteristics, and external quantum efficiency-luminance characteristics, respectively, of the light-emitting elements 5 to 7. Note that the measurements of the light-emitting elements were carried out at room temperature (in an atmosphere kept at 25° C.). 
     Table 6 shows element characteristics of the light-emitting elements 1 to 7 at maximum external quantum efficiencies. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                   
                 TABLE 6 
               
               
                   
                   
               
               
                   
                   
                   
                   
                   
                   
                 External 
               
               
                   
                   
                   
                   
                   
                 Current 
                 quantum 
               
               
                   
                 Volt- 
                 Current 
                 Chroma- 
                 Lumi- 
                 effi- 
                 effi- 
               
               
                   
                 age 
                 density 
                 ticity 
                 nance 
                 ciency 
                 ciency 
               
               
                   
                 (V) 
                 (mA/cm 2 ) 
                 (x, y) 
                 (cd/m 2 ) 
                 (cd/A) 
                 (%) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Light- 
                 2.6 
                 0.014 
                 (0.48, 0.51) 
                 2.1 
                 15 
                 4.6 
               
               
                 emitting 
               
               
                 element 
               
               
                 1 
               
               
                 Light- 
                 2.4 
                 0.0077 
                 (0.35, 0.58) 
                 4.1 
                 53 
                 17 
               
               
                 emitting 
               
               
                 element 
               
               
                 2 
               
               
                 Light- 
                 2.5 
                 0.039 
                 (0.44, 0.54) 
                 12 
                 31 
                 8.9 
               
               
                 emitting 
               
               
                 element 
               
               
                 3 
               
               
                 Light- 
                 2.5 
                 0.043 
                 (0.48, 0.51) 
                 9.9 
                 23 
                 6.8 
               
               
                 emitting 
               
               
                 element 
               
               
                 4 
               
               
                 Light- 
                 2.9 
                 0.89 
                 (0.48, 0.51) 
                 290 
                 32 
                 11 
               
               
                 emitting 
               
               
                 element 
               
               
                 5 
               
               
                 Light- 
                 3.2 
                 0.65 
                 (0.49, 0.50) 
                 200 
                 31 
                 9.4 
               
               
                 emitting 
               
               
                 element 
               
               
                 6 
               
               
                 Light- 
                 2.8 
                 0.20 
                 (0.47, 0.52) 
                 69 
                 34 
                 11 
               
               
                 emitting 
               
               
                 element 
               
               
                 7 
               
               
                   
               
            
           
         
       
     
       FIG.  27    shows the electroluminescence spectra of the light-emitting elements 1 to 4 through which current flows at a current density of 2.5 mA/cm 2 .  FIG.  28    shows the electroluminescence spectra of the light-emitting elements 5 to 7 through which current flows at a current density of 2.5 mA/cm 2 . 
     Note that the emission spectra of a thin film of 4,6mCzP2Pm alone and a thin film of PCBBiF alone were measured and found to have peaks at wavelengths of 439 nm and 436 nm, respectively. Meanwhile, as shown in  FIG.  27   , the electroluminescence spectrum of the light-emitting element 2 has a peak at a wavelength of 527 nm. That is, the electroluminescence spectrum of the light-emitting element 2 differs from the emission spectra of 4,6mCzP2Pm and PCBBiF, and corresponds to light emission which is exhibited by an exciplex formed by 4,6mCzP2Pm and PCBBiF. 
     The emission spectra of a thin film of 4mCzBPBfPm alone and a thin film of PCzPCA1 alone were measured and found to have peaks at wavelengths of 440 nm and 443 nm, respectively. 
     Meanwhile, as shown in  FIG.  28   , the electroluminescence spectrum of the light-emitting element 5 has a peak at a wavelength of 567 nm. That is, the electroluminescence spectrum of the light-emitting element 5 differs from the emission spectra of 4mCzBPBfPm and PCzPCA1, and corresponds to light emission which is exhibited by an exciplex formed by 4mCzBPBfPm and PCzPCA1. As shown in  FIG.  28   , the electroluminescence spectrum of the light-emitting element 7 has a peak at a wavelength of 558 nm. That is, the electroluminescence spectrum of the light-emitting element 7 differs from the emission spectra of 4,6mCzP2Pm and PCzPCA1, and corresponds to light emission which is exhibited by an exciplex formed by 4,6mCzP2Pm and PCzPCA1. 
     As shown in  FIGS.  27  and  28   , the light-emitting elements 1, 3, 4, and 6 show emission spectra exhibited by rubrene that is a guest material and have electroluminescence spectrum peaks at wavelengths of 557 nm, 553 nm, 559 nm, and 557 nm, respectively. Therefore, it is found that the light-emitting element 2 has high emission energy owing to its emission peak at a wavelength shorter than those of the light-emitting elements 3 and 4 and that the exciplex formed by 4,6mCzP2Pm and PCBBiF can be used as a host material in a light-emitting element which contains rubrene as a guest material. It can be said that the light-emitting elements 6 and 7 have comparable emission energy because they show emission peaks at substantially the same wavelengths; the electroluminescence spectra of the light-emitting elements 6 and 7 suggest that the exciplex formed by 4,6mCzP2Pm and PCzPCA1 can be used as a host material in a light-emitting element which contains rubrene as a guest material. 
     Phosphorescence spectra of 4,6mCzP2Pm, PCBBiF, and PCzPCA1 were measured and found to have peaks on the shortest wavelength side at wavelengths of 459 nm, 509 nm, and 495 nm, respectively, and phosphorescence emission energies calculated from the peak wavelengths were 2.70 eV, 2.44 eV, and 2.51 eV, respectively. Note that the phosphorescence spectra were measured using the method described in Embodiment 1. Note that an exciplex has a feature of having a singlet excitation energy level and a triplet excitation energy level that are close to each other; thus, the phosphorescence spectrum of an exciplex is regarded as being the same as the fluorescence spectrum (thermally activated delayed fluorescence spectrum). 
     As described in Embodiment 1, rubrene has a calculated triplet excitation energy level of 0.95 eV. 
     According to the above results, the phosphorescence spectra of 4,6mCzP2Pm and PCBBiF have peaks at shorter wavelengths than the phosphorescence spectrum of the exciplex formed by these materials, and the triplet excitation energy levels of 4,6mCzP2Pm and PCBBiF are higher than the triplet excitation energy level of the exciplex formed by these materials. The triplet excitation energy level of the exciplex is higher than the triplet excitation energy level of rubrene that is a guest material. Furthermore, the phosphorescence spectra of 4,6mCzP2Pm and PCzPCA1 have peaks at shorter wavelengths than the emission spectrum of the exciplex formed by these materials, and the triplet excitation energy levels of 4,6mCzP2Pm and PCBBiF are higher than the triplet excitation energy level of the exciplex formed by these materials. The triplet excitation energy level of the exciplex is higher than the triplet excitation energy level of rubrene that is a guest material. Therefore, 4,6mCzP2Pm, PCBBiF, and PCzPCA1 are suitable for a host material of a light-emitting element of one embodiment of the present invention. 
     As shown in  FIGS.  17  to  26    and Table 6, the maximum external quantum efficiency of the light-emitting element 1 is 4.6%; the maximum external quantum efficiency of the light-emitting element 2 is 17%; the maximum external quantum efficiency of the light-emitting element 3 is 8.9%; the maximum external quantum efficiency of the light-emitting element 4 is 6.8%; the maximum external quantum efficiency of the light-emitting element 5 is 11%; the maximum external quantum efficiency of the light-emitting element 6 is 9.4%; and the maximum external quantum efficiency of the light-emitting element 7 is 11%. 
     Although the light-emitting elements 2, 5, and 7 have high maximum external quantum efficiencies, a phenomenon in which efficiency drastically drops in a high-luminance region (also referred to as roll-off) is caused. In contrast, the light-emitting elements 3, 4, and 6 of one embodiment of the present invention have higher maximum external quantum efficiencies than the light-emitting element 1, and show suppressed roll-off unlike in the light-emitting elements 2, 5, and 7. This is an excellent advantageous effect that can be achieved only by ExSET of one embodiment of the present invention. 
     As shown in  FIGS.  17  to  26    and Table 6, a comparison between the light-emitting elements 3, 4, and 6 of one embodiment of the present invention reveals that the light-emitting element 6 exhibits high emission efficiency, especially a high maximum external quantum efficiency of 9.4%, and has excellent characteristics. Then, transient EL measurement was performed on the light-emitting elements 1, 6, and 7 to examine whether the light-emitting element 6 utilizes ExSET.  FIG.  29    shows the transient EL characteristics of the light-emitting elements 1, 6, and 7.  FIG.  30    shows the transient EL spectrum of the light-emitting element 6. 
     &lt;1-9. Measurement of Transient EL Spectra of Light-Emitting Elements 1, 6, and 7&gt; 
     A picosecond fluorescence lifetime measurement system (manufactured by Hamamatsu Photonics K.K.) was used for the measurement. In this measurement, the transient EL characteristics of the light-emitting elements were measured as follows. A square wave pulse voltage was applied to the light-emitting element, and light which was attenuated from the falling of the voltage underwent time-resolved measurement using a streak camera. The measurement was performed at room temperature (25° C.). 
     In  FIG.  29   , the vertical axis represents emission intensity normalized to that in a state where carriers were steadily injected (when the pulse voltage is ON). The horizontal axis represents time elapsed after the folling of the pulse voltage. 
     It is found from the transient EL characteristics shown in  FIG.  29    that the light-emitting elements 6 and 7 each have a longer lifetime than the light-emitting element 1 and exhibit delayed fluorescence based on reverse intersystem crossing. 
     In  FIG.  30   , prompt EL refers to a prompt component of the measured transient EL spectrum, and delayed EL refers to a delayed component. Note that the prompt component is an emission component during voltage application, and the delayed component is an emission component at 8 us to 45 us after voltage application. 
     As shown in  FIG.  30   , both the prompt and delayed EL spectra are roughly in accordance with the emission spectrum of rubrene that is a guest material. Thus, it is suggested that the energies of prompt excitons of S E  (singlet excitons directly generated by recombination of injected carriers) and delayed excitons of S E  generated by reverse intersystem crossing (singlet excitons generated by reverse intersystem crossing of triplet exitons) are both transferred to the S G  of rubrene and contribute to light emission. 
     As described above, it has been confirmed that a light-emitting element of one embodiment of the present invention utilizes ExSET and therefore has high emission efficiency. 
     The structure described above in this example can be combined with any of the structures described in the other examples and embodiments as appropriate. 
     Example 2 
     In this example, an example of fabricating a light-emitting element of one embodiment of the present invention and a comparative light-emitting element will be described. Note that light-emitting elements 8 and 9 were fabricated in this example. 
     The light-emitting element 8 is a light-emitting element of one embodiment of the present invention which utilizes ExSET, and the light-emitting element 9 is a reference light-emitting element which utilizes exciplex emission.  FIG.  16    is a schematic cross-sectional view of each of the light-emitting elements 8 and 9, and Table 7 shows details of the element structure. In addition, structures and abbreviations of compounds used here are given below. Except the compounds given below, compounds similar to those described in Example 1 were used. 
     
       
         
         
             
             
         
       
     
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 7 
               
               
                   
                   
               
               
                   
                   
                 Reference 
                 Thickness 
                   
                 Weight 
               
               
                   
                 Layer 
                 numeral 
                 (nm) 
                 Material 
                 ratio 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Light- 
                 Electrode 
                 1102 
                 200 
                 Al 
                 — 
               
               
                 emitting 
                 Electron- 
                 1116 
                 1 
                 LiF 
                 — 
               
               
                 element 8 
                 injection 
               
               
                   
                 layer 
               
               
                   
                 Electron- 
                 1115 
                 15 
                 Bphen 
                 — 
               
               
                   
                 transport 
                   
                 10 
                 4mCzBPBfPm 
                 — 
               
               
                   
                 layer 
               
               
                   
                 Light- 
                 1113 
                 30 
                 4mCzBPBfPm:DPhAmCP:1,6mMemFLPAPrn 
                 0.8:0.2:0.005 
               
               
                   
                 emitting 
               
               
                   
                 layer 
               
               
                   
                 Hole- 
                 1112 
                 20 
                 DPhAmCP 
                 — 
               
               
                   
                 transport 
               
               
                   
                 layer 
               
               
                   
                 Hole- 
                 1111 
                 15 
                 DBT3P-II:MoO 3   
                 2:1 
               
               
                   
                 injection 
               
               
                   
                 layer 
               
               
                   
                 Electrode 
                 1101 
                 70 
                 ITSO 
                 — 
               
               
                 Light- 
                 Electrode 
                 1102 
                 200 
                 Al 
                 — 
               
               
                 emitting 
                 Electron- 
                 1116 
                 1 
                 LiF 
                 — 
               
               
                 element 9 
                 injection 
               
               
                   
                 layer 
               
               
                   
                 Electron- 
                 1115 
                 15 
                 Bphen 
                 — 
               
               
                   
                 transport 
                   
                 10 
                 4mCzBPBfPm 
                 — 
               
               
                   
                 layer 
               
               
                   
                 Light- 
                 1113 
                 30 
                 4mCzBPBfPm:DPhAmCP 
                 0.8:0.2 
               
               
                   
                 emitting 
               
               
                   
                 layer 
               
               
                   
                 Hole- 
                 1112 
                 20 
                 DPhAmCP 
                 — 
               
               
                   
                 transport 
               
               
                   
                 layer 
               
               
                   
                 Hole- 
                 1111 
                 15 
                 DBT3P-II:MoO 3   
                 2:1 
               
               
                   
                 injection 
               
               
                   
                 layer 
               
               
                   
                 Electrode 
                 1101 
                 70 
                 ITSO 
                 — 
               
               
                   
               
            
           
         
       
     
     &lt;2-1. Method for Fabricating Light-Emitting Element  8 &gt; 
     First, a film of ITSO was formed as an electrode  1101  over a substrate  1100  by a sputtering method. Note that the thickness of the electrode  1101  was 70 nm, and the area of the electrode  1101  was 4 mm 2  (2 mm×2 mm). 
     Next, as pretreatment of evaporation of an organic compound layer, the electrode  1101  side of the substrate  1100  was washed with water, baking was performed at 200° C. for one hour, and then UV ozone treatment was performed on a surface of the electrode  1101  for 370 seconds. 
     After that, the substrate  1100  was transferred into a vacuum evaporation apparatus where the pressure had been reduced to approximately 1×10 −4  Pa, and was subjected to vacuum baking at 170° C. for 30 minutes in a heating chamber of the vacuum evaporation apparatus, and then the substrate  1100  was cooled down for about 30 minutes. 
     Next, the substrate  1100  was fixed to a holder in the vacuum evaporation apparatus so that a side on which the electrode  1101  was provided faced downward. In this example, a hole-injection layer  1111 , a hole-transport layer  1112 , a light-emitting layer  1113 , an electron-transport layer  1115 , an electron-injection layer  1116 , and an electrode  1102  were sequentially formed by a vacuum evaporation method. The fabrication method will be described in detail below. 
     First, DBT3P-II and molybdenum oxide were co-evaporated as the hole-injection layer  1111  over the electrode  1101  so that the weight ratio of DBT3P-II to molybdenum oxide was 2:1. Note that the thickness of the hole-injection layer  1111  was 15 nm. 
     Next, the hole-transport layer  1112  was formed over the hole-injection layer  1111 . As the hole-transport layer  1112 , 3,5-di(carbazol-9-yl)-N,N-diphenylaniline (abbreviation: DPhAmCP) was evaporated. Note that the thickness of the hole-transport layer  1112  was 20 nm. 
     Next, the light-emitting layer  1113  was formed over the hole-transport layer  1112 . As the light-emitting layer  1113 , 4mCzBPBfPm, DPhAmCP, and 1,6mMemFLPAPrn were co-evaporated so that the weight ratio of 4mCzBPBfPm to DPhAmCP and 1,6mMemFLPAPrn was 0.8:0.2:0.005. Note that the thickness of the light-emitting layer  1113  was 30 nm. In the light-emitting layer  1113 , 4mCzBPBfPm is a host material, DPhAmCP is an assist material, and 1,6mMemFLPAPrn is a guest material. 
     Next, 4mCzBPBfPm and Bphen were sequentially evaporated to thicknesses of 10 nm and 15 nm, respectively, as the electron-transport layer  1115  over the light-emitting layer  1113 . Then, LiF was evaporated to a thickness of 1 nm as the electron-injection layer  1116  over the electron-transport layer  1115 . 
     Next, Al was evaporated as the electrode  1102  over the electron-injection layer  1116 . Note that the thickness of the electrode  1102  was 200 nm. 
     The light-emitting element fabricated over the substrate  1100  as described above was sealed by being attached to a sealing substrate  1150  in a glove box in a nitrogen atmosphere so as not to be exposed to the air. Note that the sealing method is similar to that for the light-emitting element 1 described in Example 1. 
     Through the above process, the light-emitting element 8 was fabricated. 
     &lt;2-2. Method for Fabricating Light-Emitting Element  9 &gt; 
     The light-emitting element 9 is different from the above-described light-emitting element 8 in the structure of the light-emitting layer  1113 . The other components are the same as those of the light-emitting element 8. 
     As the light-emitting layer  1113  of the light-emitting element 9, 4mCzBPBfPm and DPhAmCP were co-evaporated so that the weight ratio of 4mCzBPBfPm to DPhAmCP was 0.8:0.2. Note that the thickness of the light-emitting layer  1113  was 40 nm. In the light-emitting layer  1113 , 4mCzBPBfPm is a host material and DPhAmCP is an assist material. 
     Note that in all the above evaporation steps for the light-emitting elements 8 and 9, a resistive heating method was used as an evaporation method. 
     &lt;2-3. Characteristics of Light-Emitting Elements 8 and 9&gt; 
       FIGS.  31 ,  32 ,  33 ,  34 , and  35    show luminance-current density characteristics, luminance-voltage characteristics, current efficiency-luminance characteristics, current-voltage characteristics, and external quantum efficiency-luminance characteristics, respectively, of the light-emitting elements 8 and 9. Note that the measurements of the light-emitting elements were carried out at room temperature (in an atmosphere kept at 25° C.). 
     Table 8 shows element characteristics of the light-emitting elements 8 and 9 at maximum external quantum efficiencies. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                   
                 TABLE 8 
               
               
                   
                   
               
               
                   
                   
                   
                   
                   
                   
                 External 
               
               
                   
                   
                   
                   
                   
                 Current 
                 quantum 
               
               
                   
                 Volt- 
                 Current 
                 Chroma- 
                 Lumi- 
                 effi- 
                 effi- 
               
               
                   
                 age 
                 density 
                 ticity 
                 nance 
                 ciency 
                 ciency 
               
               
                   
                 (V) 
                 (mA/cm 2 ) 
                 (x, y) 
                 (cd/m 2 ) 
                 (cd/A) 
                 (%) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Light- 
                 2.9 
                 0.92 
                 (0.14, 0.17) 
                 77 
                 8.3 
                 6.9 
               
               
                 emitting 
               
               
                 element 
               
               
                 8 
               
               
                 Light- 
                 3.0 
                 1.6 
                 (0.15, 0.14) 
                 32 
                 2.0 
                 1.8 
               
               
                 emitting 
               
               
                 element 
               
               
                 9 
               
               
                   
               
            
           
         
       
     
       FIG.  36    shows the electroluminescence spectra of the light-emitting elements 8 and 9 through which current flows at a current density of 2.5 mA/cm 2 . 
     As shown in  FIG.  36   , the light-emitting elements 8 and 9 have electroluminescence spectra in a blue wavelength range. Note that emission spectra of a thin film of 4mCzBPBfPm used as a host material and a thin film of DPhAmCP used as an assist material in the light-emitting elements 8 and 9 were measured and found to have peaks at wavelengths of 440 nm and 375 nm, respectively. Meanwhile, the electroluminescence spectrum of the light-emitting element 9 has a peak at a wavelength of 457 nm. That is, the electroluminescence spectrum of the light-emitting element 9 differs from the emission spectra of 4mCzBPBfPm and DPhAmCP, and corresponds to light emission which is exhibited by an exciplex formed by 4mCzBPBfPm and DPhAmCP. 
     The electroluminescence spectrum of the light-emitting element 8 has a peak at a wavelength of 465 nm and corresponds to the emission spectrum exhibited by 1,6mMemFLPAPrn that is a guest material. No light emission exhibited by the exciplex formed by 4mCzBPBfPm and DPhAmCP is observed. It is found that the light-emitting element 9 has high emission energy owing to its emission peak at a wavelength shorter than that of the light-emitting element 8. Accordingly, it can be said that excitation energy of the exciplex formed in the light-emitting element 8 is efficiently transferred to the guest material. 
     That is, a compound which has a condensed heterocyclic skeleton having a diazine skeleton, such as 4mCzBPBfPm, can be suitably used in a blue light-emitting element. In addition, a compound in which an aromatic amine skeleton is bonded to the 9-position of a carbazole skeleton, such as DPhAmCP, can be suitably used in a blue light-emitting element. Furthermore, an exciplex formed by the compound which has a condensed heterocyclic skeleton having a diazine skeleton, such as 4mCzBPBfPm, and the compound in which an aromatic amine skeleton is bonded to the 9-position of a carbazole skeleton, such as DPhAmCP, can be suitably used as a host material in a blue light-emitting element. 
     As shown in  FIGS.  31  to  35    and Table 8, the light-emitting element 8 of one embodiment of the present invention exhibits high emission efficiency, especially a high maximum external quantum efficiency of 6.9%, as a blue light-emitting element. In addition, the light-emitting element 8 shows suppressed roll-off in a high-luminance region. This is an excellent advantageous effect that can be achieved only by ExSET of one embodiment of the present invention. 
     As described above, it has been confirmed that a light-emitting element of one embodiment of the present invention utilizes ExSET and therefore has high emission efficiency. 
     The structure described above in this example can be combined with any of the structures described in the other examples and embodiments as appropriate. 
     This application is based on Japanese Patent Application serial no. 2014-175163 filed with Japan Patent Office on Aug. 29, 2014, Japanese Patent Application serial no. 2014-240985 filed with Japan Patent Office on Nov. 28, 2014, and Japanese Patent Application serial no. 2015-108786 filed with Japan Patent Office on May 28, 2015, the entire contents of which are hereby incorporated by reference.