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

Abstract

The luminous efficiency of fluorescent light-emitting devices (LEDs) is improved by effectively utilizing triplet excitons generated in the emissive layer. The emissive layer of an LED comprises at least a host material and a guest material. Triplet-triplet annihilation (TTA) is used to convert triplet excitons generated by the host material in the emissive layer into singlet excitons. The energy transfer from these singlet excitons causes the guest material (fluorescent dopant) to emit light, thereby improving the luminous efficiency of the LED.

Description

Light-emitting element, light-emitting device, electronic device, and lighting equipment One aspect of the present invention relates to a light-emitting element, a light-emitting device having the light-emitting element, an electronic device, and a lighting equipment. Since a light-emitting element having a structure in which a light-emitting layer containing an organic compound as a light-emitting substance is provided between a pair of electrodes has characteristics such as being thin and lightweight, having a high response speed, and being able to be driven at a low DC voltage, it has attracted attention as a next-generation flat panel display element. In addition, a display including the light-emitting element also has characteristics of excellent contrast, high image quality, and wide viewing angle. In the light-emitting mechanism of the light-emitting element, a voltage is applied between a pair of electrodes to inject electrons and holes, and the electrons and holes recombine to form excitons to emit light. Both electrons and holes have a spin of 1 / 2. When electrons and holes recombine to form excitons, according to spin statistics, the generation ratio of singlet excitons with spin 0 is 25%, and the generation ratio of triplet excitons with spin 1 is 75%. In addition, since fluorescence is light emission from singlet excitons, the theoretical limit of the internal quantum efficiency of fluorescence is 25%. On the other hand, triplet excitons with spin 1 generated at a ratio of 75% usually do not contribute to light emission and therefore do not emit light and return to the ground state in many cases. It is known that triplet-triplet annihilation (TTA) caused by multiple triplet excitons also occurs depending on the situation. TTA refers to a process in which energy exchange and transfer and spin angular momentum exchange and transfer occur due to the collision of two triplet excitons, and as a result, singlet excitons are generated. In order to effectively utilize the triplet excitons generated in the light-emitting mechanism, a phosphorescent light-emitting element having an internal quantum efficiency exceeding the theoretical value and the development of a compound (hereinafter referred to as a phosphorescent compound) capable of converting triplet excitation energy into light emission have been proposed (for example, refer to Patent Document 1). Specifically, in addition to the above light-emitting substance, a phosphorescent compound is also used, and thus phosphorescent light emission generated by transitions between energy states of different multiplicities is obtained. [Patent Document 1] Pamphlet of International Publication No. WO2005 / 105746 In one aspect of the present invention, the light emission efficiency of the fluorescent light-emitting element is improved by effectively utilizing the triplet excitons generated in the light-emitting layer. In one embodiment of the present invention, the light-emitting layer of the light-emitting element contains at least a host material and a guest material, and by efficiently generating TTA (triplet-triplet annihilation) in the light-emitting layer, triplet excitons that do not contribute to light emission are converted into singlet excitons, and the guest material (fluorescent dopant) is caused to emit light by energy transfer from the singlet excitons, thereby improving the light-emitting efficiency of the light-emitting element. Note that based on considerations of spin statistics, the light-emitting efficiency of TTA (triplet-triplet annihilation) is estimated to increase by about 15%. When added to the internal quantum efficiency of 25% when TTA does not occur, an internal quantum efficiency of 40% can be theoretically obtained. In addition, in order to efficiently generate TTA due to triplet excitons generated by the host material in the light-emitting layer, it is important to increase the collision probability between triplet excitons present in the light-emitting layer. Therefore, the T1 energy level (lowest triplet exciton energy level) of the host material in the light-emitting layer is preferably lower than the T1 energy level of the guest material. Generally, the proportion of the host material in the light-emitting layer is much higher than that of the guest material in the light-emitting layer. By forming the light-emitting element in such a way that the T1 energy level of the host material is lower than the T1 energy level of the guest material, it is possible to prevent a decrease in the collision probability between triplet excitons due to triplet excitons generated in the light-emitting layer being captured and localized by only a small amount of the guest material in the light-emitting layer, thereby improving the occurrence probability of TTA. In addition, in order not to allow triplet excitons generated in the light-emitting layer to diffuse outside the light-emitting layer, as the material for the hole transport layer (HTL) formed in contact with the light-emitting layer on the anode side of the light-emitting element, a material having a T1 energy level higher than the T1 energy level of the host material of the light-emitting layer is preferably used. By adopting the above structure, it is possible to make triplet excitons generated in the light-emitting layer less likely to transfer from the light-emitting layer to the HTL. In addition, when TTA occurs, light emission (delayed fluorescence) having a longer lifetime than the fluorescence lifetime of the fluorescent material obtained when TTA does not occur is generated. By observing the light emission decay after stopping the continuous injection of carriers at a certain point in time, this delayed fluorescence can be confirmed. Note that delayed fluorescence refers to light emission that continues for 1×10 -6 sec or more, and this light emission has a light emission intensity of 0.01 or more of the light emission intensity when carriers are continuously injected. In this case, the shape of the spectrum of the delayed fluorescence is consistent with the shape of the light emission spectrum when carriers are continuously injected. Therefore, one aspect of the present invention is a light-emitting element including an EL layer between a pair of electrodes, wherein the EL layer includes at least a light-emitting layer and a hole transport layer, the light-emitting layer contains a guest material and a host material whose T1 energy level is lower than that of the guest material, the hole transport layer contains a hole transporting material, and the T1 energy level of the hole transporting material is higher than the T1 energy level of the host material. Other aspects of the present invention are a light-emitting element including an EL layer between a pair of electrodes, wherein the EL layer includes at least a light-emitting layer and a hole transport layer, the light-emitting layer contains a guest material and a host material whose T1 energy level is lower than that of the guest material, the hole transport layer contains a hole transporting material, and the T1 energy level of the hole transporting material is higher than the T1 energy levels of the guest material and the host material. Other aspects of the present invention are a light-emitting element including an EL layer between a pair of electrodes, wherein the EL layer includes at least a light-emitting layer and a hole transport layer, the light-emitting layer contains a guest material and a host material whose T1 energy level is lower than that of the guest material, the hole transport layer contains a hole transporting material whose T1 energy level is higher than the T1 energy level of the host material, the shape of the delayed fluorescence spectrum due to TTA is the same as the shape of the fluorescence spectrum of the guest material, and the period during which the intensity of the delayed fluorescence with respect to the light emission during continuous carrier injection is 0.01 or more is 1×10 -6 sec or more. Other aspects of the present invention are a light-emitting element including an EL layer between a pair of electrodes, wherein the EL layer includes at least a light-emitting layer and a hole transport layer, the light-emitting layer contains a guest material and a host material whose T1 energy level is lower than that of the guest material, the hole transport layer contains a hole transporting material whose T1 energy level is higher than the T1 energy levels of the guest material and the host material, the shape of the spectrum of the delayed fluorescence due to TTA is the same as the shape of the fluorescence spectrum of the guest material, and the period during which the intensity of the delayed fluorescence with respect to the light emission during continuous carrier injection is 0.01 or more is 1×10 -6 sec or more. In addition, one aspect of the present invention includes not only a light-emitting device having a light-emitting element within its scope, but also an electronic device and a lighting device having the light-emitting device. Therefore, the light-emitting device in this specification refers to an image display device or a light source (including a lighting device). In addition, the light-emitting device further includes the following modules: a module in which a connector such as an FPC (Flexible Printed Circuit) or a TCP (Tape Carrier Package) is installed in the light-emitting device; a module in which a printed circuit board is provided at the end of the TCP; or a module in which an IC (Integrated Circuit) is directly mounted on the light-emitting element by the COG (Chip On Glass) method. According to one aspect of the present invention, a highly efficient fluorescent light-emitting element can be provided. It is also possible to provide a light-emitting device, an electronic device, or a lighting device with low power consumption. 101: Light-emitting layer 102: Host material 103: Guest material 201: Hole transport layer 202: Light-emitting layer 211: First electrode (anode) 212: Second electrode (cathode) 213: Hole injection layer 214: Electron transport layer 215: Electron injection layer 301: First electrode (anode) 302: Second electrode (cathode) 303: EL layer 304: Hole injection layer 305: Hole transport layer 306: Light-emitting layer 307: Electron transport layer 308: Electron injection layer 311: Host material 312: Guest material 313: Hole-transporting material 401: First electrode 402(1): First EL layer 402(2): Second EL layer 402(n - 1): (n - 1)th EL layer 402(n): nth EL layer 404: Second electrode 405: Charge generation layer (I) 405(1): First charge generation layer (I) 405(2): Second Charge Generation Layer (I) 405(n - 2): (n - 2)th Charge Generation Layer (I) 405(n - 1): (n - 1)th Charge Generation Layer (I) 501: Element Substrate 502: Pixel Section 503: Driving Circuit Section (Source Line Driving Circuit) 504a, 504b: Driving Circuit Section (Gate Line Driving Circuit) 505: Sealing Material 506: Sealing Substrate 507: Wiring 508: FPC (Flexible Printed Circuit) 509: n - Channel TFT 510: p - Channel TFT 511: Switching TFT 512: Current Control TFT 513: First Electrode (Anode) 514: Insulator 515: EL Layer 516: Second Electrode (Cathode) 517: Light - Emitting Element 518: Space 900: Substrate 901: First Electrode 902: EL Layer 903: Second Electrode 911: Hole Injection Layer 912: Hole Transport Layer 913: Light - Emitting Layer 914: Electron Transport Layer 915: Electron Injection Layer 7100: Television Set 7101: Outer Shell 7103: Display Section 7105: Bracket 7107: Display Section 7109: Operation Keys 7110: Remote Controller 7201: Main Body 7202: Outer Shell 7203: Display Section 7204: Keyboard 7205: External Connection Port 7206: Pointing Device 7302: Housing 7304: Display panel 7305: Icon indicating time 7306: Other icons 7311, 7312: Operation buttons 7313: Connection terminal 7321: Wristband 7322: Watchband buckle 7400: Mobile phone 7401: Housing 7402: Display unit 7403: Operation button 7404: External connection port 7405: Speaker 7406: Microphone 8001: Lighting device 8002: Lighting device 8003: Lighting device 8004: Lighting device 9033: Clip 9034: Display mode switch 9035: Power switch 9036: Power saving mode switch 9038: Operation switch 9630: Housing 9631: Display unit 9631a: Display unit 9631b: Display unit 9632a: Touch panel area 9632b: Touch panel area 9633: Solar cell 9634: Charge and discharge control circuit 9635: Battery 9636: DCDC converter 9637: Operation key 9638: Converter 9639: Button In the drawings: Figure 1 is a diagram illustrating the mechanism of using TTA for light emission; Figure 2 is a diagram illustrating the structure of the light-emitting element of one embodiment of the present invention; Figures 3A and 3B are diagrams illustrating the structure of the light-emitting element; Figures 4A and 4B are diagrams illustrating the light-emitting element; Figures 5A and 5B are diagrams illustrating the light-emitting device; FIGS. 6A to 6D are diagrams for explaining an electronic device; FIGS. 7A to 7C are diagrams for explaining an electronic device; FIG. 8 is a diagram for explaining a lighting device; FIG. 9 is a diagram for explaining the structure of a light-emitting element; FIG. 10 is a diagram showing the fluorescence lifetimes of the light-emitting element 1 and the comparative light-emitting element 2; FIG. 11 is a diagram showing the fluorescence spectrum of the light-emitting element 1; FIG. 12 is a diagram showing the fluorescence lifetimes of the light-emitting element 1 and the comparative light-emitting element 2; FIG. 13 is a diagram showing the fluorescence lifetimes of the light-emitting element 3 and the comparative light-emitting element 4. Next, embodiments of the present invention will be described in detail with reference to the drawings. However, the present invention is not limited to the content described below, and its mode and details can be changed into various forms without departing from the spirit and scope of the present invention. Therefore, the present invention should not be construed as being limited only to the content described in the embodiments shown below. Embodiment 1 In this embodiment, the mechanism of a light-emitting element according to one mode of the present invention will be described. A light-emitting element according to one mode of the present invention includes at least a light-emitting layer between a pair of electrodes (an anode and a cathode), and further includes a hole transport layer in contact with the light-emitting layer between the light-emitting layer and the anode. In addition, the light-emitting layer contains a host material and a guest material. In the light-emitting element, when holes and electrons are respectively injected from the anode and the cathode, singlet excitons and triplet excitons are generated in the light-emitting layer at a ratio of 1:3. At this time, as shown in FIG. 1, since a light-emitting element according to one mode of the present invention is designed such that the T1 energy level (T1(h)) of the host material 102 contained in the light-emitting layer 101 is lower than the T1 energy level (T1(g)) of the guest material 103 in terms of energy, triplet excitons generally gather on the host material 102 contained in a large amount in the light-emitting layer and are not captured by the guest material 103 which is only a trace amount (about 5 wt%) in the host material 102. Thus, in order to increase the occurrence probability of the triplet-triplet annihilation (TTA) described later, it is important to satisfy the relationship of T1(h) < T1(g). In addition, singlet excitons emit fluorescence, and triplet excitons generally do not contribute to light emission and disappear. However, when the so-called triplet-triplet annihilation (TTA) occurs as shown in FIG. 1, singlet excitons can be further generated to contribute to light emission. In this TTA, two triplet excitons collide to exchange and transfer energy and exchange and transfer spin angular momentum. As described above, when the condition of T1(h) < T1(g) is satisfied, the singlet excitons generated by TTA are located at the S1 energy level (S1(h)) of the host material. However, since the S1 energy level of the host material is set to be higher than the S1 energy level of the guest material, unlike the energy transfer of triplet excitons, the energy of singlet excitons is transferred from S1 of the host material to S1 of the guest material. Thus, luminescence from the guest material can be obtained. Therefore, when the structure shown in FIG. 1 is adopted, fluorescence from the guest material utilizing singlet excitons and fluorescence from the guest material due to TTA utilizing triplet excitons can be obtained, so that a highly efficient fluorescence emitting element can be formed. Moreover, in order not to allow the triplet excitons generated in the light emitting layer to diffuse outside the light emitting layer, the light emitting element according to one embodiment of the present invention adopts the structure shown in FIG. 2. That is, as shown in FIG. 2, in the hole transport layer 201 and the light emitting layer 202 formed between the first electrode (anode) 211 and the second electrode (cathode) 212, the T1 energy level of the hole transporting material (HT) 203 included in the hole transport layer 201 is set to be higher than the T1 energy level of the host material (h) 204 included in the light emitting layer 202. Thus, the transfer of triplet excitons from T1(h) to T1(HT) can be suppressed, and the triplet excitons generated in the light emitting layer 202 can be retained in the light emitting layer 202. Further, by setting the T1 energy level of the hole transporting material (HT) 203 to be higher than the T1 energy level of the guest material (g) 205 included in the light emitting layer 202, the transfer of triplet excitons from T1(g) to T1(HT) can also be suppressed, which is more preferable. In addition, the hole transport layer 201 having such a high T1 energy level is particularly preferable when the recombination region is located on the anode side in the light emitting layer 202. For the same reason, when a material having a T1 energy level higher than the T1 energy levels of the host material and the guest material of the light emitting layer 202 is used as the material for the layer (such as an electron transport layer (ETL)) formed in contact with the light emitting layer 202 on the cathode side of the light emitting element, the triplet excitons generated in the light emitting layer 202 can be retained in the light emitting layer, which is preferable. In addition, the electron transport layer having such a high T1 energy level is particularly preferable when the recombination region is located on the cathode side in the light emitting layer. In addition, in FIG. 2, a hole injection layer 213 having a function of improving the hole injectability from the first electrode 211 may be provided between the first electrode (anode) 211 and the hole transport layer 201. Further, an electron injection layer 215 having a function of improving the electron injectability from the second electrode (cathode) 212 and an electron transport layer 214 having high electron transportability may be provided between the second electrode (cathode) 212 and the light emitting layer 202. As described above, by using the light-emitting element of one aspect of the present invention, a light-emitting device, an electronic device, or a lighting device with high luminous efficiency can be realized. In addition, a light-emitting device, an electronic device, or a lighting device with low power consumption can be realized. The structure shown in this embodiment can be implemented by appropriately combining with the structures shown in other embodiments. Embodiment 2 In this embodiment, a light-emitting element of one aspect of the present invention will be described with reference to FIGS. 3A and 3B. As shown in FIG. 3A, in the light-emitting element shown in this embodiment, an EL layer 303 including a light-emitting layer 306 and a hole transport layer 305 is sandwiched between a pair of electrodes (a first electrode (anode) 301 and a second electrode (cathode) 302). In addition, the light-emitting layer 306 includes a host material 311 and a guest material 312, and the hole transport layer 305 includes a hole transport material 313. Further, as shown in FIG. 3B, the EL layer 303 further includes a hole injection layer 304, an electron transport layer 307, an electron injection layer 308, etc. in addition to the light-emitting layer 306 and the hole transport layer 305. By applying a voltage to such a light-emitting element, holes injected from the first electrode 301 side and electrons injected from the second electrode 302 side recombine in the light-emitting layer 306 to form excitons. Through the conversion from triplet excitons to singlet excitons due to TTA and the energy transfer of excitons, the guest material 312 included in the light-emitting layer 306 emits light. In addition, the hole injection layer 304 in the EL layer 303 is a layer including a substance with high hole transportability and a receptor substance. Since the receptor substance extracts electrons from the substance with high hole transportability, holes are generated thereby. Therefore, holes are injected from the hole injection layer 304 through the hole transport layer 305 into the light-emitting layer 306. In addition, in the layer including a substance with high hole transportability and a receptor substance, electrons are extracted from the substance with high hole transportability by the receptor substance. Therefore, this layer can be said to be a layer in which electrons are generated by extraction. That is, by disposing this layer on the cathode side, electrons can be injected into the light-emitting layer 306 through the electron transport layer 307. Thus, a so-called charge generation layer having a function of generating charges such as holes or electrons can be appropriately disposed in the light-emitting element of one aspect of the present invention as a functional layer of the light-emitting element. Next, a specific example when manufacturing the light-emitting element shown in this embodiment will be described. As the first electrode (anode) 301 and the second electrode (cathode) 302, metals, alloys, conductive compounds, and mixtures of these substances can be used. Specifically, in addition to indium tin oxide (ITO), indium tin oxide containing silicon or silicon oxide, indium zinc oxide, indium containing tungsten oxide and zinc oxide, gold (Au), platinum (Pt), nickel (Ni), tungsten (W), chromium (Cr), molybdenum (Mo), iron (Fe), cobalt (Co), copper (Cu), palladium (Pd), titanium (Ti), elements belonging to Group 1 or Group 2 of the periodic table can also be used, that is, alkali metals such as lithium (Li) or cesium (Cs), alkaline earth metals such as calcium (Ca) or strontium (Sr), magnesium (Mg), and alloys containing these metals (MgAg, AlLi), rare earth metals such as europium (Eu) or ytterbium (Yb) and alloys containing these metals. In addition, graphene and the like can also be used. Further, the first electrode (anode) 301 and the second electrode (cathode) 302 can be formed, for example, by a sputtering method or an evaporation method (including a vacuum evaporation method). As a substance with high hole transport properties for the hole injection layer 304 and the hole transport layer 305 (also including the above-mentioned charge generation layer), for example, 3-[4-(1-naphthyl)phenyl]-9-phenyl-9H-carbazole (abbreviation: PCPN), 3-[4-(9-phenanthryl)phenyl]-9-phenyl-9H-carbazole (abbreviation: PCPPn), 4-phenyl-4'-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PCBA1BP), 4,4'-bis(1-naphthyl)-4''-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PCBNBB), 4-phenyldiphenyl-(9-phenyl-9H-carbazol-3-yl)amine (abbreviation: PCA1BP), 3,3'-bis(9-phenyl-9H-carbazole) (abbreviation: PCCP), N-[4-(9H-carbazol-9-yl)phenyl]-N-(4-phenyl)phenylaniline (abbreviation: YGA1BP), 1,3,5-tris(dibenzothiophen-4-yl)benzene (abbreviation: DBT3P-II), 4,4',4''-(benzene-1,3,5-triyl)tris(dibenzofuran) (abbreviation: DBF3P-II), 4-phenyl-4'-(9-phenylinden-9-yl)triphenylamine (abbreviation: BPAFLP), 4-[3-(terphenyl-2-yl)phenyl]dibenzothiophene (abbreviation: mDBTPTp-II), 4,4'-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB or α-NPD), N,N'-bis(3-methylphenyl)-N,N'-diphenyl-[1,1'-biphenyl]-4,4'-diamine (abbreviation: TPD), 4,4',4''-tris(carbazol-9-yl)triphenylamine (abbreviation: TCTA), 4,4',4''-tris(N,N-diphenylamino)triphenylamine (abbreviation: TDATA), 4,4',4''-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine (abbreviation: MTDATA), 4,4'-bis[N-(spiro-9,9'-binden-2-yl)-N-phenylamino]biphenyl (abbreviation: BSPB), 3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole (abbreviation: PCzPCA1), 3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole (abbreviation: PCzPCA2), 3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole (abbreviation: PCzPCN1), etc. In addition, carbazole compounds such as 4,4'-bis(N-carbazolyl)biphenyl (abbreviation: CBP), 1,3,5-tris[4-(N-carbazolyl)phenyl]benzene (abbreviation: TCPB), amine compounds, dibenzothiophene compounds, dibenzofuran compounds, indene compounds, terphenyl compounds, or phenanthrene compounds can also be used. The substances described herein mainly have a hole mobility of 1×10 -6 cm 2 A substance with an ionization potential of 5.0 eV or more. However, as long as the hole transport property is higher than the electron transport property, substances other than the above substances can be used. Note that in the light-emitting element according to one embodiment of the present invention, it is preferable to select the hole-transporting substance from these substances in such a way that the energy relationship of the above-mentioned T1 energy level is satisfied among the hole-transporting substance in the hole transport layer 305, the host material of the light-emitting layer 306, and the guest material of the light-emitting layer 306. Furthermore, polymer compounds such as poly(N-vinylcarbazole) (abbreviation: PVK), poly(4-vinyltriphenylamine) (abbreviation: PVTPA), poly[N-(4-{N’-[4-(4-diphenylamino)phenyl]phenyl-N’-phenylamino}phenyl)methacrylamide] (abbreviation: PTPDMA), and poly[N,N’-bis(4-butylphenyl)-N,N’-bis(phenyl)benzidine] (abbreviation: Poly-TPD) can also be used. In addition, as the acceptor substance for the hole injection layer 304 (including the above-mentioned charge generation layer), transition metal oxides or oxides of metals belonging to Groups 4 to 8 of the periodic table can be cited. Specifically, molybdenum oxide is particularly preferable. As shown in Embodiment 1, the light-emitting layer 306 contains a host material and a guest material, and the T1 energy level of the host material is lower than the T1 energy level of the guest material. Preferable examples of the host material include 3-[4-(1-naphthyl)phenyl]-9-phenyl-9H-carbazole (abbreviation: PCPN) and anthracene compounds such as 9-phenyl-3-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: PCzPA), 9-[4-(10-phenyl-9-anthryl)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), 9-phenyl-10-{4-(9-phenyl-9H-fluoren-9-yl)biphenyl-4’-yl}anthracene (abbreviation: FLPPA), etc. Among the anthracene compounds, the S1 energy level is high and the T1 energy level is low, so they are preferable. In addition, as preferred examples of the guest material, pyrene compounds such as 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(dibenzothiophene-2-yl)-N,N'-diphenylpyrene-1,6-diamine (abbreviation: 1,6ThAPrn), anthracene compounds, terphenyl compounds, fluorene compounds, carbazole compounds, dibenzothiophene compounds, dibenzofuran compounds, dibenzoquin oline compounds, quin oline compounds, pyridine compounds, pyrimidine compounds, phenanthrene compounds, naphthalene compounds, etc. In particular, pyrene compounds have a high luminescence quantum yield, so they are preferred. Note that in the light-emitting element of one embodiment of the present invention, it is preferable to select the host material and the guest material from these substances in such a way that the energy relationship of the above-mentioned T1 energy level is satisfied between the host material and the guest material. The electron transport layer 307 is a layer containing a substance with high electron transport properties. As the electron transport layer 307, metal complexes such as Alq 3 , tris(4-methyl-8-hydroxyquinoline)aluminum(III) (abbreviation: Almq 3 ), bis(10-hydroxybenzo[h]quinoline)beryllium(II) (abbreviation: BeBq 2 ), BAlq, Zn(BOX) 2 , bis[2-(2-hydroxyphenyl)benzothiazole]zinc(II) (abbreviation: Zn(BTZ) 2) etc. In addition, heteroaromatic compounds such as 2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation: PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene (abbreviation: OXD-7), 3-(4'-tert-butylphenyl)-4-phenyl-5-(4''-biphenyl)-1,2,4-triazole (abbreviation: TAZ), 3-(4-tert-butylphenyl)-4-(4-ethylphenyl)-5-(4-biphenyl)-1,2,4-triazole (abbreviation: p-EtTAZ), bathocuproine (abbreviation: Bphen), bathophenanthroline (abbreviation: BCP), 4,4'-bis(5-methylbenzoxazol-2-yl)stilbene (abbreviation: BzOs), etc. can also be used. Additionally, polymer compounds such as poly(2,5-pyridinediyl) (abbreviation: PPy), poly[(9,9-dihexylfluorene-2,7-diyl)-co-(pyridine-3,5-diyl)] (abbreviation: PF-Py), poly[(9,9-dioctylfluorene-2,7-diyl)-co-(2,2'-bipyridine-6,6'-diyl)] (abbreviation: PF-BPy) can also be used. The substances described herein mainly have an electron mobility of 1×10 -6 cm 2 / Vs or higher. Additionally, as long as the substance has higher electron transport property than hole transport property, substances other than the above-mentioned substances can be used for the electron transport layer 307. Note that as the substance for the hole transport layer in the light-emitting element as one aspect of the present invention, it is preferably selected from these substances in such a way that the energy relationship of the above-mentioned T1 energy level is satisfied among the host material of the electron transport layer 307, the host material of the light-emitting layer 306, and the guest material of the light-emitting layer 306. In addition, as the electron transport layer 307, not only a single layer can be adopted, but also a laminate of two or more layers composed of the above-mentioned substances can be adopted. The electron injection layer 308 is a layer containing a substance with high electron injection property. As the electron injection layer 308, lithium fluoride (LiF), cesium fluoride (CsF), calcium fluoride (CaF 2 ), and lithium oxide (LiO x ), etc., alkali metals, alkaline earth metals, magnesium (Mg), or compounds of these metals can be used. In addition, rare earth metal compounds such as erbium fluoride (ErF 3 ) can be used. Additionally, the substances constituting the electron transport layer 307 as described above can also be used. In addition, a composite material obtained by mixing an organic compound and an electron donor can be used for the electron injection layer 308. In such a composite material, since the electron donor generates electrons in the organic compound, the electron injection property and the electron transport property are high. In this case, the organic compound is preferably a material having excellent performance in transporting the generated electrons. Specifically, substances (such as metal complexes and heteroaromatic compounds) constituting the electron transport layer 307 as described above can be used. As the electron donor, any substance that exhibits electron-donating properties to the organic compound can be used. For example, alkali metals, alkaline earth metals, and rare earth metals are preferably used. Specifically, examples include lithium, cesium, calcium, erbium, ytterbium, magnesium, etc. In addition, alkali metal oxides or alkaline earth metal oxides are preferably used, and examples include lithium oxide, calcium oxide, barium oxide, etc. Furthermore, Lewis bases such as magnesium oxide can be used. In addition, organic compounds such as tetrathiafulvalene (abbreviation: TTF) can also be used. In addition, the above-mentioned hole injection layer 304, hole transport layer 305, light-emitting layer 306, electron transport layer 307, and electron injection layer 308 (including the above-mentioned charge generation layer) can all be formed by evaporation methods (including vacuum evaporation methods), inkjet methods, coating methods, etc. In addition, in the above-mentioned light-emitting element, the light obtained in the light-emitting layer 306 is taken out to the outside through either or both of the first electrode 301 and the second electrode 302. Therefore, either or both of the first electrode 301 and the second electrode 302 are electrodes having light-transmitting properties. The above-mentioned light-emitting element is a fluorescent light-emitting element that utilizes singlet excitons generated from triplet excitons that are usually not helpful for light emission due to triplet-triplet annihilation (TTA) for light emission. Therefore, compared with conventional light-emitting elements using fluorescent compounds, this light-emitting element can have higher efficiency. Note that the light-emitting element shown in this embodiment is an example of a fluorescent light-emitting element that utilizes energy transfer due to TTA for light emission. In addition, as the structure of a light-emitting device including the above-mentioned light-emitting element, a passive matrix type light-emitting device and an active matrix type light-emitting device can be manufactured. A light-emitting device having a microcavity structure including a light-emitting element having other structures can also be manufactured. The above-mentioned light-emitting devices are all included in the scope of the present invention. In addition, in the case of adopting an active matrix light-emitting device, there is no particular limitation on the structure of the TFT. For example, a staggered TFT or an anti-staggered TFT can be appropriately used. In addition, the driving circuit formed on the TFT substrate can be formed of one or both of an N-type TFT and a P-type TFT. Further, there is no particular limitation on the crystallinity of the semiconductor film used for the TFT. For example, an amorphous semiconductor film or a crystalline semiconductor film can be used. As the semiconductor material, in addition to group-IV (silicon, germanium, etc.) semiconductors and compound semiconductors (including oxide semiconductors), organic semiconductors and the like can also be used. In addition, the structure shown in this embodiment can be implemented in appropriate combination with the structures shown in other embodiments. Embodiment 3 In this embodiment, as one aspect of the present invention, a light-emitting element (hereinafter, referred to as a tandem light-emitting element) having a structure including a plurality of EL layers sandwiching a charge generation layer will be described. The light-emitting element shown in this embodiment is a tandem light-emitting element having a plurality of EL layers (a first EL layer 402(1) and a second EL layer 402(2)) between a pair of electrodes (a first electrode 401 and a second electrode 404) as shown in FIG. 4A. In this embodiment, the first electrode 401 is an electrode used as an anode, and the second electrode 404 is an electrode used as a cathode. In addition, the first electrode 401 and the second electrode 404 can adopt the same structure as that in Embodiment 2. Further, at least one of the plurality of EL layers (the first EL layer 402(1) and the second EL layer 402(2)) preferably has the same structure as the structure of the EL layer shown in Embodiment 1, and the hole transport layer and the light-emitting layer in contact therewith preferably have the same structure as the structure shown in Embodiment 1. In addition, a charge generation layer (I) 405 is provided between the plurality of EL layers (the first EL layer 402(1) and the second EL layer 402(2)). The charge generation layer (I) 405 has the following functions: when a voltage is applied to the first electrode 401 and the second electrode 404, injecting electrons into one of the EL layers and injecting holes into the other EL layer. In this embodiment, when a voltage is applied such that the potential of the first electrode 401 is higher than that of the second electrode 404, electrons are injected from the charge generation layer (I) 405 into the first EL layer 402(1), and holes are injected into the second EL layer 402(2). In addition, from the viewpoint of light extraction efficiency, the charge generation layer (I) 405 preferably has a function of transmitting visible light (specifically, the charge generation layer (I) 405 preferably has a visible light transmittance of 40% or more). In addition, the charge generation layer (I) 405 functions even if its conductivity is lower than that of the first electrode 401 or the second electrode 404. The charge generation layer (I) 405 may have a structure in which an electron acceptor (acceptor) is added to an organic compound having high hole transportability, or may have a structure in which an electron donor (donor) is added to an organic compound having high electron transportability. Alternatively, these two structures may be stacked. When adopting the structure in which an electron acceptor is added to an organic compound having high hole transportability, as the organic compound having high hole transportability, for example, aromatic amine compounds such as NPB, TPD, TDATA, MTDATA, or 4,4'-bis[N-(spiro-9,9'-bifluorene-2-yl)-N-phenylamino]biphenyl (abbreviation: BSPB) etc. can be used. The substances described herein mainly have a hole mobility of 1×10 -6 cm 2 / Vs or more. However, as long as it is an organic compound having higher hole transportability than electron transportability, substances other than the above substances can be used. In addition, as the electron acceptor, 7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinoxaline dimethane (abbreviation: F4TCNQ), chloranil, etc. can be cited. In addition, transition metal oxides can be cited. In addition, oxides of metals belonging to Groups 4 to 8 in the periodic table can be cited. Specifically, it is preferably to use vanadium oxide, niobium oxide, tantalum oxide, chromium oxide, molybdenum oxide, tungsten oxide, manganese oxide, and rhenium oxide, because they have high electron acceptability. Among them, it is particularly preferably to use molybdenum oxide, because molybdenum oxide is stable in the atmosphere, has low hygroscopicity, and is easy to handle. On the other hand, when adopting the structure in which an electron donor is added to an organic compound having high electron transportability, as the organic compound having high electron transportability, for example, metal complexes having a quinoline skeleton or a benzoquinoline skeleton such as Alq, Almq 3 , BeBq 2 or BAlq etc. can be used. In addition to these, metal complexes having an oxazolyl ligand or a thiazolyl ligand such as Zn(BOX) 2 or Zn(BTZ) 2 etc. can also be used. Furthermore, in addition to metal complexes, PBD, OXD-7, TAZ, Bphen, BCP, etc. can be used. The substances described herein mainly have an electron mobility of 1×10 -6 cm 2Substances with a voltage of / Vs or higher. In addition, as long as the organic compound has a higher electron-transporting property than a hole-transporting property, substances other than the above-mentioned substances can be used. In addition, as the electron donor, an alkali metal, an alkaline earth metal, a rare earth metal, a metal belonging to Group 2 or Group 13 of the periodic table of elements, or an oxide or carbonate of these metals can be used. Specifically, it is preferably to use lithium (Li), cesium (Cs), magnesium (Mg), calcium (Ca), ytterbium (Yb), indium (In), lithium oxide, cesium carbonate, etc. In addition, an organic compound such as tetrathianaphthacene can also be used as the electron donor. In addition, by using the above materials to form the charge generation layer (I) 405, an increase in the driving voltage caused when the EL layer is stacked can be suppressed. Although a light-emitting element having two EL layers has been described in this embodiment, as shown in FIG. 4B, the present invention can be similarly applied to a light-emitting element in which n (note that n is 3 or more) EL layers (402(1) to 402(n)) are stacked. As in the light-emitting element according to this embodiment, when there are a plurality of EL layers between a pair of electrodes, by respectively providing charge generation layers (I) (405(1) to 405(n - 1)) between the EL layers, light can be emitted in a high-brightness region while maintaining a low current density. Since a low current density can be maintained, a long-life element can be realized. In addition, when the above light-emitting element is used for lighting, since a voltage drop due to the resistance of the electrode material can be reduced, uniform light emission in a large area can be realized. In addition, a light-emitting device capable of low-voltage driving and low power consumption can be realized. In addition, by causing each EL layer to emit light of a different color, the light-emitting element as a whole can emit light of a desired color. For example, in a light-emitting element having two EL layers, when the emission color of the first EL layer and the emission color of the second EL layer are in a complementary color relationship, a light-emitting element that emits white light as a whole can be obtained. Note that the term "complementary color relationship" means a color relationship in which achromatic color is obtained when colors are mixed. In other words, when colors in a complementary color relationship are mixed, white light emission can be obtained. Similarly, for a light-emitting element having three EL layers, for example, when the emission color of the first EL layer is red, the emission color of the second EL layer is green, and the emission color of the third EL layer is blue, the light-emitting element can obtain white light emission as a whole. In addition, the structure shown in this embodiment can be implemented in appropriate combination with the structures shown in other embodiments. Embodiment 4 In this embodiment, a light-emitting device manufactured using a light-emitting element according to one mode of the present invention will be described. In addition, the above-mentioned light-emitting device can be either a passive matrix light-emitting device or an active matrix light-emitting device. In addition, the light-emitting device shown in this embodiment can use the light-emitting elements described in other embodiments. In this embodiment, the active matrix light-emitting device will be described with reference to FIGS. 5A and 5B. In addition, FIG. 5A is a plan view showing the light-emitting device, and FIG. 5B is a cross-sectional view taken along the dotted line A-A' in FIG. 5A. The active matrix light-emitting device according to this embodiment includes a pixel portion 502, a driving circuit portion (source line driving circuit) 503, and a driving circuit portion (gate line driving circuit) 504 (504a and 504b) provided on an element substrate 501. The pixel portion 502, the driving circuit portion 503, and the driving circuit portion 504 are sealed between the element substrate 501 and a sealing substrate 506 with a sealing material 505. In addition, a guiding wiring 507 is provided on the element substrate 501, and the guiding wiring 507 is used to connect an external input terminal that conveys signals (such as video signals, clock signals, start signals, or reset signals, etc.) or potentials from the outside to the driving circuit portion 503 and the driving circuit portion 504. Here, an example of providing an FPC (flexible printed circuit) 508 as an external input terminal is shown. In addition, although only the FPC is illustrated here, a printed wiring board (PWB) may also be mounted on the FPC. The light-emitting device in this specification includes not only the light-emitting device main body but also a light-emitting device on which an FPC or a PWB is mounted. Next, the cross-sectional structure will be described with reference to FIG. 5B. A driving circuit portion and a pixel portion are formed on the element substrate 501, and the driving circuit portion 503 of the source line driving circuit and the pixel portion 502 are shown here. Here, an example of forming a CMOS circuit combining an n-channel type TFT 509 and a p-channel type TFT 510 as the driving circuit portion 503 is shown. In addition, the driving circuit portion can also be formed using various CMOS circuits, PMOS circuits, or NMOS circuits. In addition, in this embodiment, although an example of an integrated driver type in which the driving circuit is formed on the substrate is shown, it is not necessarily required to be so, and the driving circuit can also be formed outside and not on the substrate. In addition, the pixel portion 502 is formed of a plurality of pixels including a switching TFT 511, a current control TFT 512, and a first electrode (anode) 513 that is electrically connected to a wiring (source electrode or drain electrode) of the current control TFT 512. In addition, an insulator 514 is formed so as to cover the end portion of the first electrode (anode) 513. Here, the insulator 514 is formed using a positive-type photosensitive acrylic resin. In addition, in order to improve the coverage of the film stacked on the insulator 514, it is preferable to form the upper end portion or the lower end portion of the insulator 514 into a curved surface with curvature. For example, as the material of the insulator 514, a negative photosensitive resin or a positive photosensitive resin can be used, and it is not limited to organic compounds, and inorganic compounds such as silicon oxide and silicon oxynitride can also be used. An EL layer 515 and a second electrode (cathode) 516 are stacked on the first electrode (anode) 513. In the EL layer 515, at least a light-emitting layer is provided, and in addition to the light-emitting layer, a hole injection layer, a hole transport layer, an electron transport layer, an electron injection layer, a charge generation layer, etc. can be appropriately provided. In addition, a light-emitting element 517 is formed by the stacked structure of the first electrode (anode) 513, the EL layer 515, and the second electrode (cathode) 516. As the materials for the first electrode (anode) 513, the EL layer 515, and the second electrode (cathode) 516, the materials shown in Embodiment 2 can be used. In addition, although not shown here, the second electrode (cathode) 516 is electrically connected to the FPC 508 as an external input terminal. In addition, although only one light-emitting element 517 is shown in the cross-sectional view shown in FIG. 5B, a plurality of light-emitting elements are arranged in a matrix in the pixel portion 502. Light-emitting elements capable of obtaining three kinds of (R, G, B) light emissions are selectively formed in the pixel portion 502, respectively, so that a light-emitting device capable of performing full-color display can be formed. In addition, a light-emitting device capable of performing full-color display can also be realized by combining with a color filter. Furthermore, by bonding the sealing substrate 506 and the element substrate 501 together with the sealing material 505, a structure is obtained in which the light-emitting element 517 is provided in the space 518 surrounded by the element substrate 501, the sealing substrate 506, and the sealing material 505. In addition, the space 518 can be filled with an inert gas (nitrogen, argon, etc.) in addition to being filled with the sealing material 505. In addition, as the sealing material 505, it is preferable to use an epoxy resin or glass powder. In addition, these materials are preferably materials that do not allow moisture and oxygen to pass through as much as possible. In addition, as the material for the sealing substrate 506, in addition to a glass substrate and a quartz substrate, a plastic substrate made of FRP (Fiber-Reinforced Plastics), PVF (polyvinyl fluoride), polyester, acrylic resin, etc. can also be used. When glass powder is used as the sealing material, from the viewpoint of adhesiveness, it is preferable to use a glass substrate as the element substrate 501 and the sealing substrate 506. By the above steps, an active matrix light-emitting device can be obtained. In addition, the structure shown in this embodiment can be implemented by appropriately combining with the structures shown in other embodiments. Embodiment 5 In this embodiment, an example of various electronic devices of a light-emitting device manufactured using a light-emitting element according to one aspect of the present invention will be described with reference to FIGS. 6A to 7C. Examples of electronic devices to which the light-emitting device is applied include, for example, a television set (also referred to as a TV or a television receiver), a display for a computer or the like, a digital camera, a digital video camera, a digital photo frame, a mobile phone (also referred to as a mobile phone or a mobile phone device), a portable game machine, a portable information terminal, an audio reproduction device, a pachinko machine or other large game machines. FIGS. 6A to 6D show specific examples of these electronic devices. FIG. 6A shows an example of a television set. In the television set 7100, a display unit 7103 is assembled in a housing 7101. An image can be displayed by the display unit 7103, and the light-emitting device can be used for the display unit 7103. In addition, a structure in which the housing 7101 is supported by a bracket 7105 is shown here. The television set 7100 can be operated by using an operation switch provided in the housing 7101 or a separately provided remote controller 7110. By using the operation keys 7109 provided in the remote controller 7110, the channels and volume can be operated, and the image displayed on the display unit 7103 can be operated. In addition, a display unit 7107 for displaying information output from the remote controller 7110 can also be provided in the remote controller 7110. In addition, the television set 7100 has a structure including a receiver and a modem or the like. General television broadcasts can be received by using the receiver. Furthermore, by connecting the television set 7100 to a communication network in a wired or wireless manner via the modem, one-way (from a sender to a receiver) or two-way (between a sender and a receiver or between receivers, etc.) information communication can be performed. FIG. 6B shows a computer, including a main body 7201, a housing 7202, a display unit 7203, a keyboard 7204, an external connection port 7205, a pointing device 7206, and the like. In addition, this computer can be manufactured by using the light-emitting device for its display unit 7203. FIG. 6C is a smart watch, including a housing 7302, a display panel 7304, operation buttons 7311, 7312, connection terminals 7313, a wristband 7321, a watchband buckle 7322, and the like. The display panel 7304 installed in the housing 7302 that also serves as a bezel has a non-rectangular display area. The display panel 7304 can display an icon 7305 indicating time and other icons 7306. In addition, the smartwatch shown in FIG. 6C can have various functions. For example, it can have the following functions: the function of displaying various information (static images, dynamic images, text images, etc.) on the display unit; the touch panel function; the function of displaying a calendar, date, or time, etc.; the function of controlling processing by using various software (programs); the wireless communication function; the function of connecting to various computer networks by means of the wireless communication function; the function of sending or receiving various data by means of the wireless communication function; the function of reading a program or data stored in a storage medium and displaying it on the display unit, etc. In addition, inside the housing 7302, there can be a speaker, sensors (including the function of measuring the following factors: force, displacement, position, speed, acceleration, angular velocity, rotation number, distance, light, liquid, magnetism, temperature, chemical substance, sound, time, hardness, electric field, current, voltage, power, radiation, flow rate, humidity, slope, vibration, odor, or infrared rays), a microphone, etc. Additionally, the smartwatch can be manufactured by using a light-emitting device for its display panel 7304. FIG. 6D shows an example of a mobile phone. The mobile phone 7400 includes operation buttons 7403, an external connection port 7404, a speaker 7405, a microphone 7406, etc. in addition to the display unit 7402 assembled in the housing 7401. Additionally, the mobile phone 7400 is manufactured by using a light-emitting device for the display unit 7402. For the mobile phone 7400 shown in FIG. 6D, information can be input by touching the display unit 7402 with a finger or the like. In addition, operations such as making a call or creating an email can be performed by touching the display unit 7402 with a finger or the like. The display unit 7402 mainly has the following three screen modes: the first is a display mode mainly for image display; the second is an input mode mainly for information input such as text; the third is a display and input mode that combines the two modes of the display mode and the input mode. For example, in the case of making a call or creating an email, the display unit 7402 can be set to a text input mode mainly for text input, and the input operation of the text displayed on the screen can be performed. In this case, preferably, a keyboard or number buttons are displayed on most of the screen of the display unit 7402. In addition, by providing a detection device inside the mobile phone 7400 that has sensors for detecting the inclination such as a gyroscope and an acceleration sensor, the direction (vertical or horizontal) of the mobile phone 7400 is determined, so that the screen display of the display unit 7402 can be automatically switched. In addition, by operating the touch display unit 7402 or the operation button 7403 on the housing 7401, the screen mode is switched. The screen mode can also be switched according to the type of image displayed on the display unit 7402. For example, when the image signal displayed on the display unit is data of a moving image, the screen mode is switched to the display mode, and when the image signal displayed on the display unit is text data, the screen mode is switched to the input mode. In addition, when it is known from the signal detected by the light sensor of the display unit 7402 that there is no touch operation input to the display unit 7402 within a certain period in the input mode, it can also be controlled to switch the screen mode from the input mode to the display mode. The display unit 7402 can also be used as an image sensor. For example, by touching the display unit 7402 with a palm or a finger to capture a palm print, a fingerprint, etc., identity recognition can be performed. In addition, by using a backlight that emits near-infrared light or a sensing light source that emits near-infrared light in the display unit, finger veins, palm veins, etc. can also be captured. FIG. 7A and FIG. 7B are foldable tablet terminals. FIG. 7A is a tablet terminal in an open state, and the tablet terminal includes a housing 9630, a display unit 9631a, a display unit 9631b, a display mode switch 9034, a power switch 9035, a power saving mode switch 9036, a clip 9033, and an operation switch 9038. In addition, the tablet terminal is manufactured by using a light-emitting device for one or both of the display unit 9631a and the display unit 9631b. In the display unit 9631a, a part of it can be used as a touch panel area 9632a, and data can be input by touching the displayed operation keys 9637. In addition, as an example, a structure is shown in which half of the display unit 9631a only has a display function and the other half has a touch panel function, but it is not limited to this structure. A structure in which all areas of the display unit 9631a have a touch panel function can also be adopted. For example, the entire surface of the display unit 9631a can be used as a touch panel for displaying keyboard buttons, and the display unit 9631b can be used as a display screen. In addition, similar to the display unit 9631a, a part of the display unit 9631b can also be used as a touch panel area 9632b. In addition, by touching the position of the keyboard display switch 9639 on the touch panel with a finger or a stylus, etc., keyboard buttons can be displayed on the display unit 9631b. In addition, touch input can also be performed on the touch panel area 9632a and the touch panel area 9632b simultaneously. In addition, the display mode switching switch 9034 can switch the display direction such as portrait display and landscape display, as well as black and white display and color display, etc. The power saving mode switching switch 9036 can set the display brightness to the most appropriate brightness according to the amount of external light detected by the light sensor built in the tablet terminal during use. In addition to the light sensor, the tablet terminal can also be built in with other detection devices such as a gyroscope and an acceleration sensor for detecting the inclination. In addition, FIG. 7A shows an example where the display area of the display unit 9631b is the same as that of the display unit 9631a, but it is not limited thereto. The size of one can be different from that of the other, and there can also be a difference in their display quality. For example, a structure can be adopted in which one of the display units 9631a and 9631b can perform high-definition display compared to the other. FIG. 7B shows the tablet terminal in the closed state, and the tablet terminal includes a housing 9630, a solar cell 9633, a charge and discharge control circuit 9634, a battery 9635, and a DCDC converter 9636. In addition, in FIG. 7B, a structure having a battery 9635 and a DCDC converter 9636 is shown as an example of the charge and discharge control circuit 9634. In addition, since the tablet terminal can be folded, the housing 9630 can be closed when not in use. Therefore, the display units 9631a and 9631b can be protected, and thus a tablet terminal with good durability and good reliability from the perspective of long-term use can be provided. In addition, the tablet terminal shown in FIGS. 7A and 7B can also have the following functions: a function of displaying various kinds of information (static images, moving images, text images, etc.); a function of displaying a calendar, date, or time on the display unit; a touch input function of operating or editing the information displayed on the display unit; a function of controlling processing by various software (programs), etc. By using the solar cell 9633 installed on the surface of the tablet terminal, electric power can be supplied to the touch panel, the display unit, the video signal processing unit, etc. In addition, since the solar cell 9633 can be provided on one or both sides of the housing 9630, the battery 9635 can be charged efficiently. In addition, when a lithium ion battery is used as the battery 9635, there are advantages such as miniaturization. In addition, the structure and operation of the charge and discharge control circuit 9634 shown in FIG. 7B are described with reference to the block diagram shown in FIG. 7C. FIG. 7C shows the solar cell 9633, the battery 9635, the DCDC converter 9636, the converter 9638, the switches SW1 to SW3, and the display unit 9631. The battery 9635, the DCDC converter 9636, the converter 9638, and the switches SW1 to SW3 correspond to the charge and discharge control circuit 9634 shown in FIG. 7B. First, an example of the operation when generating electricity using external light in a solar cell 9633 will be described. A DCDC converter 9636 is used to step up or step down the electricity generated by the solar cell 9633 to a voltage for charging the battery 9635. Also, when operating the display unit 9631 using the electricity from the solar cell 9633, the switch SW1 is turned on, and the converter 9638 steps up or steps down this electricity to the voltage required for the display unit 9631. Additionally, when not displaying on the display unit 9631, the switch SW1 is turned off and the switch SW2 is turned on to charge the battery 9635. Note that the solar cell 9633 is shown as an example of a power generation unit, but it is not limited thereto. Other power generation units such as a piezoelectric element or a thermoelectric conversion element (Peltier element) can also be used to charge the battery 9635. For example, a wireless power transmission module capable of wirelessly (non-contact) transmitting and receiving power for charging or a combination of other charging units can also be used for charging. As described above, an electronic device can be obtained by applying a light-emitting device according to one aspect of the present invention. The application range of the light-emitting device is extremely wide and can be applied to electronic devices in various fields. In addition, the structure shown in the present embodiment can be implemented in appropriate combination with the structures shown in other embodiments. Embodiment 6 In the present embodiment, an example of a lighting device applying a light-emitting device including a light-emitting element according to one aspect of the present invention will be described with reference to FIG. 8. FIG. 8 shows an example of using the light-emitting device for an indoor lighting device 8001. Also, since the light-emitting device can be made large in area, a large-area lighting device can also be formed. In addition, a lighting device 8002 having a curved light-emitting area can be formed by using a housing having a curved surface. The light-emitting element included in the light-emitting device shown in the present embodiment is in a film shape, so the degree of freedom in the design of the housing is high. Therefore, lighting devices corresponding to various designs can be formed. Furthermore, a large lighting device 8003 can also be provided on the wall surface of the room. In addition, by using the light-emitting device on the surface of a table, a lighting device 8004 having the function of a table can be provided. In addition, by using the light-emitting device on a part of other furniture, a lighting device having the function of the furniture can be provided. As described above, various lighting devices applying the light-emitting device can be obtained. In addition, such lighting devices are included in the scope of one aspect of the present invention. In addition, the structure shown in the present embodiment can be implemented in appropriate combination with the structures shown in other embodiments. Example 1 In this example, the T1 energy level of the material of the light-emitting element for one mode of the present invention was calculated and the S1 energy level was measured. The light-emitting element for one mode of the present invention is a fluorescent light-emitting element. In a fluorescent material, intersystem crossing hardly occurs and the light emission from the triplet excited state is very weak, so it is difficult to measure the T1 energy level of the fluorescent material. Therefore, the T1 energy level is calculated by quantum chemical calculation. On the other hand, the S1 energy level of the fluorescent material is estimated by measuring the absorption spectrum. The calculation method is shown below. Gaussian09 was used as the quantum chemical calculation program, and a high-performance computer (manufactured by SGI, Altix4700) was used for the calculation. First, the most stable structure in the singlet state was calculated using density functional theory. 6-311 (a basis function of the triple split valence basis type using three contracted functions for each atomic valence orbital) as the basis function was applied to all atoms. By using the above basis function, for example, regarding the hydrogen atom, the orbitals from 1s to 3s were considered, and regarding the carbon atom, the orbitals from 1s to 4s and 2p to 4p were considered. Furthermore, in order to improve the calculation accuracy, as the polarization basis type, a p function was added to the hydrogen atom and a d function was added to atoms other than the hydrogen atom. B3LYP was used as the functional. Next, the most stable structure in the triplet state was calculated. The energy of the T1 energy level was calculated from the energy difference between the most stable structures of the singlet state and the triplet state. 6-311G(d,p) was used as the basis function. B3LYP was used as the functional. The method for estimating the S1 energy level is shown below. First, a thin film (about 500 nm) was formed on a quartz substrate by vacuum evaporation and used as a thin film sample, and the absorption spectrum was measured. When measuring the absorption spectrum, a UV-visible spectrophotometer (manufactured by JASCO Corporation, model V550) was used. The absorption spectrum of quartz was subtracted from the absorption spectrum of the measured sample. The absorption edge of the absorption spectrum of the thin film was calculated, and this absorption edge energy was taken as the S1 energy level. In addition, as the fluorescent materials for measurement, N,N'-bis(3-methylphenyl)-N,N'-bis[3-(9-phenyl-9H-fluoren-9-yl)phenyl]-pyrene-1,6-diamine (abbreviation: 1,6mMemFLPAPrn), 9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: CzPA), 3-[4-(1-naphthyl)phenyl]-9-phenyl-9H-carbazole (abbreviation: PCPN), 9-phenyl-3-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: PCzPA) were used. Table 1 below shows the results. Next, in order to confirm the results of the quantum chemical calculations, the phosphorescence emission of the material was measured in this example. Since the fluorescence quantum efficiency of the substance of the light-emitting element for one mode of the present invention is extremely high, it is very difficult to directly observe phosphorescence in a thin film sample using only this substance by low-temperature PL measurement. Therefore, the phosphorescence emission was measured by the method using a triplet photosensitizer described below, and the T1 energy level was estimated. As this method, the fluorescent material to be measured and Ir(ppy) as a triplet photosensitizer 3 were co-evaporated to fabricate a thin film, which was measured by low-temperature PL measurement, and the T1 energy level was estimated from the measured phosphorescence spectrum. In this measurement, a microscopic PL device LabRAM HR-PL (manufactured by Horiba, Japan) was used, a He-Cd laser (325 nm) was used as the excitation light, a CCD detector was used as the detector, and the measurement temperature was set to 10K. Co-evaporating Ir(ppy) 3 with the fluorescent material increases the occurrence probability of intersystem crossing of the fluorescent material to be measured, so that the phosphorescence emission from the fluorescent material, which is difficult to measure when not co-evaporated, can be measured. In addition, a 30-nm-thick thin film was formed on a quartz substrate, and after another quartz substrate was bonded to the evaporation surface in a nitrogen atmosphere, it was used for measurement. Table 2 shows the measurement results. From the results, it can be seen that the value of the T1 energy level measured in this example is close to the value of the T1 energy level calculated by quantum chemical calculations. Therefore, the value of the T1 energy level measured in this example can be cited as a parameter when manufacturing a light-emitting element for one mode of the present invention. Example 2 This example shows the measurement results of the characteristics of the light-emitting element 1 as a light-emitting element for one mode of the present invention. In this light-emitting element 1, a hole-transporting material (abbreviation: PCPN) whose T1 energy level is higher than that of the host material of the light-emitting layer is used for the hole-transporting layer. This example also shows the measurement results of the characteristics of the comparative light-emitting element 2. In this comparative light-emitting element 2, a hole-transporting material (abbreviation: PCzPA) whose T1 energy level is equal to or lower than that of the host material of the light-emitting layer is used for the hole-transporting layer. In addition, the light-emitting element 1 and the comparative light-emitting element 2 in this example will be described with reference to FIG. 9. In addition, the chemical formulas of the materials used in this example are shown below. 《Manufacture of the Light-Emitting Element 1 and the Comparative Light-Emitting Element 2》 First, a film of indium oxide - tin oxide (ITSO) containing silicon oxide is formed on a glass substrate 900 by sputtering, thereby forming a first electrode 901 serving as an anode. Additionally, the thickness is set to 110 nm, and the electrode area is set to 2 mm × 2 mm. Next, as a pretreatment for forming a light - emitting element on the substrate 900, the surface of the substrate is washed with water, baked at 200 °C for 1 hour, and then subjected to a UV - ozone treatment for 370 seconds. Then, the substrate is placed into a vacuum evaporation apparatus whose internal pressure is reduced to about 10 -4 Pa, and in the heating chamber of the vacuum evaporation apparatus, vacuum baking is performed at 170 °C for 30 minutes, and then the substrate 900 is cooled for about 30 minutes. Next, the substrate 900 is fixed to a bracket provided in the vacuum evaporation apparatus with the surface on which the first electrode 901 is formed facing downwards. In this embodiment, a case where a hole injection layer 911, a hole transport layer 912, a light - emitting layer 913, an electron transport layer 914, and an electron injection layer 915 constituting the EL layer 902 are sequentially formed by vacuum evaporation is described. After reducing the internal pressure of the vacuum evaporation apparatus to 10 -4 Pa, the hole injection layer 911 is formed on the first electrode 901 by co - evaporating 9 - phenyl - 3 - [4 - (10 - phenyl - 9 - anthryl)phenyl] - 9H - carbazole (abbreviation: PCzPA) and molybdenum(VI) oxide in a ratio of PCzPA:molybdenum(VI) oxide = 4:2 (mass ratio). The thickness is set to 50 nm. Note that co - evaporation is an evaporation method in which multiple different substances are evaporated simultaneously from different evaporation sources. Next, 3 - [4 - (1 - naphthyl)phenyl] - 9 - phenyl - 9H - carbazole (abbreviation: PCPN) is evaporated to a thickness of 30 nm to form the hole transport layer 912 of the light - emitting element 1. Additionally, in the comparative light - emitting element 2, PCzPA is evaporated to a thickness of 30 nm to form the hole transport layer 912 on the hole injection layer 911. Next, a light - emitting layer 913 is formed on the hole transport layer 912. First, 9 - [4 - (10 - phenyl - 9 - anthryl)phenyl] - 9H - carbazole (abbreviation: CzPA) and N,N’ - bis(3 - methylphenyl) - N,N’ - bis[3 - (9 - phenyl - 9H - fluoren - 9 - yl)phenyl] - pyrene - 1,6 - diamine (abbreviation: 1,6mMemFLPAPrn) are co - evaporated in a ratio of CzPA:1,6mMemFLPAPrn = 1:0.05 (mass ratio). Additionally, the thickness is set to 25 nm. Thus, the light - emitting layer 913 is formed. Next, bathophenanthroline (abbreviation: Bphen) was vapor-deposited on the light-emitting layer 913 to a thickness of 25 nm to form an electron transport layer 914. Further, lithium fluoride was vapor-deposited on the electron transport layer 914 to a thickness of 1 nm to form an electron injection layer 915. Finally, aluminum was vapor-deposited on the electron injection layer 915 to a thickness of 200 nm to form a second electrode 903 serving as a cathode, thereby obtaining a light-emitting element 1. Note that in the above vapor deposition process, vapor deposition was carried out by a resistance heating method. Table 3 shows the element structures of the light-emitting element 1 and the comparative light-emitting element 2 obtained by the above steps. In addition, the manufactured light-emitting element 1 and the comparative light-emitting element 2 were sealed in a glove box in a nitrogen atmosphere in such a manner that the light-emitting element was not exposed to the atmosphere (a sealing material was applied around the element, and heat treatment was performed at 80 °C for 1 hour during sealing). "Measurement of Fluorescence Lifetime of Light-Emitting Element 1 and Comparative Light-Emitting Element 2" The fluorescence lifetimes of the manufactured light-emitting element 1 and the comparative light-emitting element 2 were measured. In the measurement, a picosecond fluorescence lifetime measurement system (manufactured by Hamamatsu Photonics K.K., Japan) was used. In this measurement, in order to measure the lifetime of fluorescence emission in the light-emitting element, a rectangular pulse voltage was applied to the light-emitting element, and time-resolved measurement of the emission that decayed after the voltage was turned off was performed using a streak camera. The pulse voltage was applied at a frequency of 10 Hz, and data with a high S / N ratio was obtained by accumulating the data of repeated measurements. In addition, this measurement was performed at room temperature (300 K) under the following conditions: the pulse voltage was 3.5 V, the pulse time width was 10 μsec, and the measurement time was 20 μsec. FIG. 10 shows the measurement results. Note that in FIG. 10, the vertical axis represents the normalized intensity when the emission intensity in the state where carriers were continuously injected (when the pulse voltage was ON) was taken as 1. The horizontal axis represents the elapsed time after the pulse voltage was turned off. Regarding the decay curve of the light-emitting element 1 shown in FIG. 10, fitting was performed using an exponential function. As a result, the fluorescence lifetime τ of the light-emitting element 1 could be estimated to be 1.50 μsec. The fluorescence lifetime is generally several nanoseconds, so it can be considered that fluorescence emission including a delayed fluorescence component was measured in the light-emitting element 1. In addition, the fluorescence lifetime τ of the comparative light-emitting element 2 could also be estimated to be 1.52 μsec. Therefore, both the light-emitting element 1 and the comparative light-emitting element 2 emit fluorescence emission including a delayed fluorescence component. Note that, as factors causing delayed fluorescence in the fluorescence measurement shown in Fig. 10, in addition to the generation of singlet excitons due to triplet-triplet annihilation (TTA), the generation of singlet excitons due to the recombination of carriers remaining in the light-emitting element when the pulse voltage is OFF can also be considered as a cause. Therefore, in order to suppress the recombination of the remaining carriers, the light-emitting element 1 and the comparative light-emitting element 2 in Fig. 10 were measured under the same conditions as the above measurement except for the condition of continuously applying a negative bias voltage (-5V) and using the same device. Fig. 12 shows the measurement results. Compared with the measurement results in Fig. 10, a fluorescence component including a delayed fluorescence component was also measured in the measurement results shown in Fig. 12 under the condition of applying a negative bias voltage and suppressing the recombination of the remaining carriers. Therefore, it was confirmed that the delayed fluorescence component shown in the measurement results of Fig. 10 and Fig. 12 is due to TTA. However, as can be seen from Fig. 10 and Fig. 12, the normalized emission intensity of the delayed fluorescence component of the comparative light-emitting element 2 is smaller than that of the delayed fluorescence component of the light-emitting element 1. This is considered to be because, in the comparative light-emitting element 2, the T1 energy level of the hole-transporting material (PCzPA) in the hole-transporting layer is equal to or lower than the T1 energy level of the host material (CzPA) in the light-emitting layer, while in the light-emitting element 1, the T1 energy level of the hole-transporting material (PCPN) in the hole-transporting layer is higher than the T1 energy level of the host material (CzPA) in the light-emitting layer. In addition, the S1 energy level of PCPN is also higher than the S1 energy level of CzPA, so it can be considered that the excitation energy of CzPA is not easily transferred to PCPN. That is, it can be considered that by adopting the structure of the light-emitting element 1, the triplet excitons generated in the light-emitting layer can be confined in the light-emitting layer to increase the occurrence probability of TTA. In Fig. 11, the emission spectrum of the delayed fluorescence component of the light-emitting element 1 shown in Fig. 10 is represented by a solid line. In addition, as a comparative example, the fluorescence emission spectrum in the state where carriers are continuously injected into the light-emitting element 1 (when the pulse voltage is ON) is represented by a dashed line. In addition, the vertical axis represents the normalized intensity with the maximum emission intensity taken as 1. As can be seen from Fig. 11, the shape of the emission spectrum of the delayed fluorescence component is大致 the same as the shape of the fluorescence emission spectrum in the state where carriers are continuously injected (when the pulse voltage is ON), and both are emissions from the guest material (1,6mMemFLPAPrn). Therefore, it can be considered that the delayed fluorescence corresponding to the emission from the singlet excitons generated by TTA does not originate from the host material or the like, but from the guest material. 《Operating Characteristics of Light-Emitting Element 1 and Comparative Light-Emitting Element 2》 Next, the operating characteristics of the fabricated light-emitting element 1 and the comparative light-emitting element 2 were measured. In addition, the measurement was carried out at room temperature (in an atmosphere maintained at 25°C). Here, Table 4 below shows 1000 cd / m 2 Main initial characteristic values of the nearby light-emitting element 1 and the comparative light-emitting element 2. From the above results, it can be seen that: the current efficiency and external quantum efficiency of the light-emitting element 1 manufactured in this embodiment are both higher than those of the comparative light-emitting element 2. That is, it can be considered that: by using a substance whose T1 energy level is higher than that of the host material used in the light-emitting layer for the hole transport layer, the triplet excitons generated in the light-emitting layer can be suppressed from diffusing outside the light-emitting layer, and thus TTA can occur efficiently within the light-emitting layer. And it can be considered that: the intensity of the delayed fluorescence component due to TTA becomes larger, and as a result, the light-emitting characteristics of the light-emitting element 1 are improved. Example 3 In this embodiment, the characteristic measurement results of the light-emitting element 3 and the comparative light-emitting element 4 are respectively shown. The light-emitting element 3 was manufactured as a light-emitting element according to one aspect of the present invention. In this light-emitting element 3, 1,6mMemFLPAPrn as the guest material (fluorescent dopant) and CzPA as the host material are used to form the light-emitting layer, where the T1 energy level of CzPA is lower than that of 1,6mMemFLPAPrn. In addition, as a comparative example, the comparative light-emitting element 4 was manufactured. In this comparative light-emitting element 4, 1,6mMemFLPAPrn as the guest material (fluorescent dopant) and 35DCzPPy as the host material are used to form the light-emitting layer, where the T1 energy level of 35DCzPPy is higher than that of 1,6mMemFLPAPrn. Note that the T1 energy level of 35DCzPPy is higher than that of PCPPn used in the hole transport layer, and the T1 energy level of this PCPPn is higher than that of 1,6mMemFLPAPrn. In addition, the S1 energy levels of both 35DCzPPy and PCPPn are higher than the S1 energy level of 1,6mMemFLPAPrn. In this embodiment, the light-emitting element 3 and the comparative light-emitting element 4 are described with reference to FIG. 9 in the same manner as in Example 2. The chemical formulas of the materials used in this embodiment are shown below. 《Manufacture of the Light-Emitting Element 3 and the Comparative Light-Emitting Element 4》 First, a film of indium tin oxide (ITSO) containing silicon oxide is formed on the glass substrate 900 by sputtering to form the first electrode 901 serving as the anode. In addition, the thickness is set to 110 nm, and the electrode area is set to 2 mm × 2 mm. Next, as a pretreatment for forming the light-emitting element on the substrate 900, the surface of the substrate is washed with water, baked at 200 °C for 1 hour, and then subjected to a UV ozone treatment for 370 seconds. Then, the substrate is placed inside which is depressurized to 10 -4 In a vacuum evaporation apparatus at about Pa, and in a heating chamber inside the vacuum evaporation apparatus, vacuum baking is performed at 170 °C for 30 minutes, and then the substrate 900 is cooled for about 30 minutes. Next, the substrate 900 is fixed to a bracket provided inside the vacuum evaporation apparatus with the surface on which the first electrode 901 is formed facing downward. In this embodiment, a case where a hole injection layer 911, a hole transport layer 912, a light-emitting layer 913, an electron transport layer 914, and an electron injection layer 915 that constitute the EL layer 902 are sequentially formed by a vacuum evaporation method is described. After reducing the inside of the vacuum evaporation apparatus to 10 -4 Pa, the hole injection layer 911 is formed on the first electrode 901 by co-evaporating 9-{4-(9-H-9-phenylcarbazol-3-yl)phenyl}phenanthrene (abbreviation: PCPPn) and molybdenum(VI) oxide in a ratio of PCPPn:molybdenum(VI) oxide = 1:0.5 (mass ratio). The thickness is set to 20 nm. Note that co-evaporation is an evaporation method in which multiple different substances are evaporated simultaneously from different evaporation sources. Next, the hole transport layer 912 of the light-emitting element 3 and the comparative light-emitting element 4 is formed by evaporating PCPPn to a thickness of 20 nm. Next, the light-emitting layer 913 is formed on the hole transport layer 912. First, in the light-emitting element 3, 9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: CzPA) and N,N'-bis(3-methylphenyl)-N,N'-bis[3-(9-phenyl-9H-fluoren-9-yl)phenyl]pyrene-1,6-diamine (abbreviation: 1,6mMemFLPAPrn) are co-evaporated in a ratio of CzPA:1,6mMemFLPAPrn = 1:0.05 (mass ratio). Note that the thickness is set to 25 nm. In addition, in the comparative light-emitting element 4, 3,5-bis[3-(9H-carbazol-9-yl)phenyl]pyridine (abbreviation: 35DCzPPy) and 1,6mMemFLPAPrn are co-evaporated in a ratio of 35DCzPPy:1,6mMemFLPAPrn = 1:0.05 (mass ratio). Note that the thickness is set to 25 nm. Thus, the light-emitting layer 913 is formed. Next, an electron transport layer 914 is formed on the light-emitting layer 913. In the light-emitting element 3, after evaporating CzPA in a thickness of 10 nm, bathophenanthroline (abbreviation: Bphen) is evaporated in a thickness of 15 nm to form it. In addition, in the comparative light-emitting element 4, after evaporating 35DCzPPy in a thickness of 10 nm, Bphen is evaporated in a thickness of 15 nm to form it. And, lithium fluoride is evaporated in a thickness of 1 nm on the electron transport layer 914 to form an electron injection layer 915. Finally, aluminum is evaporated in a thickness of 200 nm on the electron injection layer 915 to form a second electrode 903 serving as a cathode, thereby obtaining the light-emitting element 3 and the comparative light-emitting element 4. Note that in the above evaporation process, evaporation is carried out by the resistance heating method. Table 5 shows the element structures of the light-emitting element 3 and the comparative light-emitting element 4 obtained by the above steps. In addition, the manufactured light-emitting element 3 and the comparative light-emitting element 4 are sealed in a glove box in a nitrogen atmosphere in such a manner that the light-emitting element is not exposed to the atmosphere (a sealing material is applied around the element, and heat treatment is carried out at 80 °C for 1 hour during sealing). "Measurement of Fluorescence Lifetime of Light-Emitting Element 3 and Comparative Light-Emitting Element 4" The fluorescence lifetimes of the manufactured light-emitting element 3 and the comparative light-emitting element 4 were measured. In the measurement, a picosecond fluorescence lifetime measurement system (manufactured by Hamamatsu Photonics K.K., Japan) was used. In this measurement, in order to measure the lifetime of fluorescence emission in the light-emitting element, a rectangular pulse voltage was applied to the light-emitting element, and time-resolved measurement was carried out on the emission that decays after the voltage is turned off using a streak camera. The pulse voltage was applied at a frequency of 10 Hz, and data with a high S / N ratio was obtained by accumulating the data of repeated measurements. In addition, this measurement was carried out at room temperature (300 K) under the following conditions: the applied pulse voltage was 3.5 V (for the light-emitting element 3), 5.2 V (for the comparative light-emitting element 4), the pulse width was 100 μsec, a negative bias voltage of -10 V was applied, and the measurement time was 50 μsec. Among them, the pulse voltage was adjusted in the light-emitting element 3 and the comparative light-emitting element 4 so that the current values flowing through the respective elements were the same. FIG. 13 shows the measurement results. In addition, in FIG. 13, the vertical axis represents the normalized intensity with the emission intensity in the state where carriers are continuously injected (when the pulse voltage is ON) being 1. The horizontal axis represents the elapsed time after the pulse voltage is turned off. Regarding the attenuation curve of the light-emitting element 3 shown in Fig. 13, an exponential function is used for fitting. As a result, it can be seen that the proportion of delayed fluorescence of the light-emitting element 3 is higher than that of the comparative light-emitting element 4. Therefore, it is confirmed that: compared with the comparative light-emitting element 4 in which 35DCzPPy having a T1 energy level higher than that of the guest material (fluorescent dopant) 1,6mMemFLPAPrn is used as the host material for the light-emitting layer, in the light-emitting element 3 in which CzPA having a T1 energy level lower than that of 1,6mMemFLPAPrn is used as the host material for the light-emitting layer, the occurrence probability of triplet-triplet annihilation (TTA) is higher. 《Operating Characteristics of Light-Emitting Element 3 and Comparative Light-Emitting Element 4》 Next, the operating characteristics of the manufactured light-emitting element 3 and comparative light-emitting element 4 are measured. In addition, the measurement is performed at room temperature (in an atmosphere maintained at 25°C). Here, Table 6 below shows the main initial characteristic values of the light-emitting element 3 and comparative light-emitting element 4 near 1000 cd / m 2 of the light-emitting element 3 and comparative light-emitting element 4. From the above results, it can be seen that the current efficiency and external quantum efficiency of the light-emitting element 3 manufactured in this embodiment are both higher than those of the comparative light-emitting element 4. That is, the measurement results of the fluorescence lifetimes and operating characteristics of the light-emitting element 3 and comparative light-emitting element 4 indicate that: compared with the case where the T1 energy level of the host material is higher than that of the guest material, in the case where the T1 energy level of the guest material is higher than that of the host material, TTA is likely to occur, and as a result, the operating characteristics such as the external quantum efficiency and current efficiency of the light-emitting element are improved. This is because: the decrease in the collision probability between triplet excitons caused by the capture and localization of triplet excitons by only a small amount of guest material in the light-emitting layer can be prevented, and the occurrence probability of TTA can be increased. 101: Light-emitting layer 102: Host material 103: Guest material

Claims

1. A light-emitting element, comprising: A hole injection layer, a hole transport layer, a light-emitting layer, and an electron transport layer are provided between a pair of electrodes. The light-emitting layer comprises a guest material and a host material with a T1 energy level lower than that of the guest material. The electron transport layer comprises an electron transport material with a T1 energy level higher than that of the host material. The hole transport layer comprises a first hole transport material with a T1 energy level higher than that of the host material. The hole injection layer comprises a second hole transport material and a compound containing at least one of cyano and fluorine. The first hole transport material and the second hole transport material are each independently a carbazole compound, a dibenzothiophene compound, a dibenzofuran compound, a fentanyl compound, a triphenylene compound, or a phenanthrene compound, and the host material is a compound having an anthracene skeleton.

2. A light-emitting element, comprising: A hole injection layer, a hole transport layer, a light-emitting layer, and an electron transport layer are provided between a pair of electrodes. The light-emitting layer comprises a guest material and a host material with a T1 energy level lower than the guest material. The electron transport layer comprises an electron transport material with a T1 energy level higher than the host material. The hole transport layer comprises a first hole transport material with a T1 energy level higher than both the guest material and the host material. The hole injection layer comprises a second hole transport material and a compound containing at least one of cyano and fluorine. The first and second hole transport materials are each independently a carbazole compound, a dibenzothiophene compound, a dibenzofuran compound, a fentanyl compound, a triphenylene compound, or a phenanthrene compound, and the host material is a compound with an anthracene skeleton.

3. A light-emitting element, comprising: A hole injection layer, a hole transport layer, and a light-emitting layer are disposed between a pair of electrodes. The hole transport layer is located between the hole injection layer and the light-emitting layer. The light-emitting layer comprises a guest material and a host material with a T1 energy level lower than that of the guest material. The hole transport layer comprises a first hole transport material with a T1 energy level higher than that of the host material. The hole injection layer comprises a second hole transport material and a compound containing at least one of cyano and fluorine. The first hole transport material and the second hole transport material are each independently a carbazole compound, a dibenzothiophene compound, a dibenzofuran compound, a fentanyl compound, a triphenylene compound, or a phenanthrene compound. The host material is a compound with an anthracene skeleton. The shape of the emission spectrum of the delayed fluorescence component caused by triplet-triplet state annihilation is the same as the shape of the fluorescence spectrum in the state of continuous carrier injection.

4. A light-emitting element, comprising: A hole injection layer, a hole transport layer, and a light-emitting layer are provided between a pair of electrodes. The hole transport layer is located between the hole injection layer and the light-emitting layer. The light-emitting layer comprises a guest material and a host material with a T1 energy level lower than that of the guest material. The hole transport layer comprises a first hole-transporting material with a T1 energy level higher than that of the guest material and the host material. The hole injection layer comprises a second hole-transporting material and a compound containing at least one of cyano and fluorine. The first hole-transporting material and the second hole-transporting material are each independently a carbazole compound, a dibenzothiophene compound, a dibenzofuran compound, a fentanyl compound, a triphenylene compound, or a phenanthrene compound. The host material is a compound with an anthracene skeleton. The shape of the emission spectrum of the delayed fluorescence component caused by triplet-triplet state annihilation is the same as the shape of the fluorescence spectrum in the state of continuous carrier injection.

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