Display device

Abstract

Display device comprising: a substrate (501); a transistor comprising an oxide semiconductor film over the substrate; and a tandem light-emitting element (517) comprising a plurality of EL layers between a pair of electrodes, the tandem light-emitting element overlying and electrically connected to the transistor, wherein a charge generation layer is provided between the plurality of EL layers, wherein one of the pair of electrodes serves as an anode and comprises ITO, where the emission colors of the EL layers differ from each other, wherein white light is obtained from the tandem light-emitting element as a whole, wherein one of the plurality of EL layers emits blue light and comprises an electron transport layer comprising a polyazole compound, wherein another of the plurality of EL layers comprises a first light-emitting layer comprising a first organic compound, a second organic compound and a first iridium complex, and a second light-emitting layer in contact with the first light-emitting layer, and wherein the first organic compound and the second organic compound form an exciplex.

Description

Background of the invention 1. Field of the invention

[0001] The present invention relates to a tandem light-emitting element in which an organic compound capable of emitting light by applying an electric field between a pair of electrodes is provided. 2. Description of the state of the art

[0002] Light-emitting elements containing an organic compound as a luminous body, which exhibit characteristics such as thinness, lightness, fast response, and low-voltage DC operation, are expected to be applied to next-generation flat-panel displays. In particular, display devices in which light-emitting elements are arranged in a matrix are considered to offer advantages over conventional liquid crystal displays, such as a wide viewing angle and high visibility.

[0003] The light-emitting mechanism of a light-emitting element is said to be as follows: When a voltage is applied between a pair of electrodes with an EL layer containing a luminous element sandwiched between them, electrons injected from the cathode and holes injected from the anode recombine in the light-emitting center of the EL layer to form molecular excitons. Energy is released, and light is emitted when the molecular excitons decay back to the ground state. A singlet excitation state and a triplet excitation state are known as the excited states, and it is thought that light emission can be obtained from either of the two excited states.

[0004] In order to improve element properties of such light-emitting elements, improvement of an element structure, development of a material and the like are actively carried out (see, for example, Patent Document 1). [Reference] Patent Document 1: Japanese Patent Laid-Open No. JP 2010 - 182 699 A Patent document 2: US 2012 / 0 302 762 A1 Summary of the invention

[0005] However, it is said that the light extraction efficiency of a light-emitting element is currently approximately 20% to 30%. Even when light absorption by a reflective electrode and a transparent electrode is taken into account, the external quantum efficiency of a light-emitting element containing a phosphorescent compound has a maximum limit of approximately 25%.

[0006] The present invention is a display device as defined by the claims.

[0007] Further embodiments include a light-emitting element including a light-emitting layer between a pair of electrodes (anode and cathode). The light-emitting layer has a layer-stack structure including a first light-emitting layer and a second light-emitting layer. The first light-emitting layer is formed on the anode side and contains at least a first light-emitting substance (guest material) that converts triplet excitation energy into light emission, a first organic compound (host material) having an electron-transport property, and a second organic compound (auxiliary material) having a hole-transport property.The second light-emitting layer contains at least a second light-emitting substance (guest material) that converts triplet excitation energy into light emission, the first organic compound (host material) with an electron-transport property, and a third organic compound (auxiliary material) with a hole-transport property. In the first light-emitting layer, a combination of the first organic compound (host material) and the second organic compound (auxiliary material) forms an exciplex. In the second light-emitting layer, a combination of the first organic compound (host material) and the third organic compound (auxiliary material) forms an exciplex.

[0008] Further embodiments include a light-emitting element including a light-emitting layer between an anode and a cathode, a hole-transport layer between the anode and the light-emitting layer, and an electron-transport layer between the cathode and the light-emitting layer. The light-emitting layer is a stack of a first light-emitting layer and a second light-emitting layer. The first light-emitting layer is in contact with the hole-transport layer and contains at least a first light-emitting substance that converts triplet excitation energy into light emission, a first organic compound having an electron-transport property, and a second organic compound having a hole-transport property.The second light-emitting layer is in contact with the electron-transport layer and contains at least one second light-emitting substance that converts triplet excitation energy into light emission, the first organic compound with an electron-transport property, and a third organic compound with a hole-transport property. In the first light-emitting layer, a combination of the first organic compound (host material) and the second organic compound (auxiliary material) forms an exciplex. In the second light-emitting layer, a combination of the first organic compound (host material) and the third organic compound (auxiliary material) forms an exciplex.

[0009] Note that in each of the above structures, the emission wavelength of the exciplex formed by the first organic compound (host material) and the second organic compound (auxiliary material) in the first light-emitting layer is on the longer wavelength side than the emission wavelength (fluorescence wavelength) of each of the first and second organic compounds (host and auxiliary materials). Therefore, by forming the exciplex, the fluorescence spectrum of the first organic compound (host material) and the fluorescence spectrum of the second organic compound (auxiliary material) can be converted into an emission spectrum that is on the longer wavelength side.Furthermore, the emission wavelength of the exciplex formed by the first organic compound (host material) and the third organic compound (auxiliary material) in the second light-emitting layer is on the longer wavelength side than the emission wavelength (fluorescence wavelength) of each of the first and third organic compounds (host and auxiliary materials). Therefore, by forming the exciplex, the fluorescence spectrum of the first organic compound (host material) and the fluorescence spectrum of the third organic compound (auxiliary material) can be converted into an emission spectrum that is on the longer wavelength side.

[0010] It should be noted that in each of the above structures, an exciplex of an anion of the first organic compound and a cation of the second organic compound is formed in the first light-emitting layer, and an exciplex of an anion of the first organic compound and a cation of the third organic compound is formed in the second light-emitting layer.

[0011] In the above structures, the first light-emitting layer and the second light-emitting layer contain the same first organic compound (host material), and the highest occupied molecular orbital (HOMO) level of the second organic compound (auxiliary material) in the first light-emitting layer is lower than that of the third organic compound (auxiliary material) in the second light-emitting layer.

[0012] In addition, in the above structures, the first light-emitting substance contained in the first light-emitting layer is a substance that emits light having a wavelength shorter than that of light emitted from the second light-emitting substance contained in the second light-emitting layer.

[0013] In the above structures, the first light-emitting substance and the second light-emitting substance are light-emitting substances that convert triplet excitation energy into light emission, and phosphorescent compounds such as an organometallic complex or a material that emits thermally stimulated delayed fluorescence, i.e., a thermally stimulated delayed fluorescence (TADF) material, can be used. Furthermore, the first organic compound is mainly an electron-transport material with an electron mobility of 10-6 cm 2 / Vs or more, especially a π-electron-deficient heteroaromatic compound. The second organic compound and the third organic compound are mainly hole-transport materials with a hole mobility of 10 -6 cm 2 / Vs or more, especially π-electron-rich heteroaromatic compounds or aromatic amine compounds.

[0014] Furthermore, the present invention encompasses electronic devices and lighting devices that include light-emitting devices, in addition to light-emitting devices that include light-emitting elements. The light-emitting device in this specification refers to an image display device and a light source (e.g., a lighting device). In addition, the light-emitting device includes any of the following modules: a module in which a connector such as a flexible printed circuit (FPC) or a tape carrier package (TCP) is attached to a light-emitting device; a module in which a printed circuit board is provided at the end of a TCP; and a module in which an integrated circuit (IC) is directly mounted on a light-emitting element by a chip-on-glass (COG) method.

[0015] Note that in a light-emitting element embodying the present invention, an exciplex can be formed in both the first light-emitting layer and the second light-emitting layer included in the light-emitting layer. Therefore, the light-emitting element can exhibit high energy transfer efficiency and high external quantum efficiency.

[0016] Furthermore, in a light-emitting element embodying the present invention, thanks to the element structure described above, the exciplex formed in the first light-emitting layer has higher excitation energy than the exciplex formed in the second light-emitting layer. Therefore, when a substance that emits light with a wavelength shorter than that emitted by the second light-emitting substance (guest material) contained in the first light-emitting layer and converts triplet excitation energy into light emission is used as the first light-emitting substance (guest material) contained in the first light-emitting layer and converts triplet excitation energy into light emission, the first light-emitting layer and the second light-emitting layer can emit light simultaneously.In addition, a portion of the excitation energy of the exciplex formed in the first light-emitting layer that does not contribute to light emission can be used as excitation energy for the second light-emitting substance (guest material), which converts triplet excitation energy into light emission in the second light-emitting layer. Consequently, the emission efficiency of the light-emitting element can be further increased. Short description of the drawings Fig. 1A and Fig. 1B show a concept of an embodiment of the present invention. Fig. 2 shows a concept of an embodiment of the present invention. Fig. 3 shows calculation results according to an embodiment of the present invention. Fig. 4A1, Fig. 4A2, Fig. 4B1, Fig. 4B2, Fig. 4C1 and Fig.4C2 show calculation results according to an embodiment of the present invention. Fig. 5 shows a structure of a light-emitting element. Fig. 6A and Fig. 6B each show a structure of a light-emitting element. Fig. 7 shows a light-emitting device. Fig. 8A and Fig. 8B show a light-emitting device. Fig. 9A to Fig. 9D show electronic devices. Fig. 10A to Fig. 10C shows an electronic device. Fig. 11 shows lighting devices. Fig. 12 shows a light-emitting element. Fig. 13 shows current density-luminance characteristics of a light-emitting element 1. Fig. Figure 14 shows voltage-luminance characteristics of a light-emitting element 1. Fig.Figure 15 shows luminance-current efficiency characteristics of a light-emitting element 1. Fig. 16 shows voltage-current characteristics of a light-emitting element 1. Fig. 17 shows an emission spectrum of a light-emitting element 1. Fig. 18 shows emission spectra of substances used in a light-emitting element 1. Fig. 19 shows emission spectra of substances used in a light-emitting element 1. Detailed Description of the Invention (Basic Process of Light Emission in a Light-Emitting Element)

[0017] First, general basic processes of light emission in a light-emitting element are described. This element uses a light-emitting substance that converts triplet excitation energy into light emission (e.g., a phosphorescent compound or a thermally stimulated delayed fluorescence (TADF) material) as a guest material. Note that a molecule that releases excitation energy is called a host molecule, while a molecule that receives the excitation energy is called a guest molecule. (1) The case where an electron and a hole recombine in a guest molecule and where the guest molecule is brought into an excited state (direct recombination process). (1-1) If the excited state of the guest molecule is a triplet excited state, the guest molecule emits phosphorescence. (1-2) If the excited state of the guest molecule is a singlet excited state, the guest molecule in the singlet excited state undergoes intersystem crossing into a triplet excited state and emits phosphorescence.

[0018] In other words, in the direct recombination process in (1), high emission efficiency can be achieved as long as the intersystem transition efficiency and the phosphorescence quantum yield of the guest molecule are high. Note that the T1 level of the host molecule is preferably higher than the T1 level of the guest molecule.

[0019] (2) The case where an electron and a hole recombine in a host molecule and the host molecule is brought into an excited state (energy transfer process).

[0020] (2-1) When the excited state of the host molecule is a triplet excited state and the T1 level of the host molecule is higher than the T1 level of the guest molecule, excitation energy is transferred from the host molecule to the guest molecule, thereby putting the guest molecule into a triplet excited state. The guest molecule in the triplet excited state emits phosphorescence. Note that energy transfer from the T1 level of the host molecule to a singlet excitation energy level (S1 level) of the guest molecule is forbidden unless the host molecule emits phosphorescence, and it is unlikely to be a major energy transfer process. Therefore, a description of such energy transfer is omitted here.In other words, energy transfer from the host molecule in the triplet excited state (3H*) to the guest molecule in the triplet excited state (3G*) is important, as shown by the following formula (2-1) (where 1G represents the singlet ground state of the guest molecule, and 1H represents the singlet ground state of the host molecule). 3H*+1G→1H+3G* (2-1)

[0021] (2-2) When the excited state of the host molecule is a singlet excited state and the S1 level of the host molecule is higher than the S1 level and the T1 level of the guest molecule, excitation energy is transferred from the host molecule to the guest molecule, thereby placing the guest molecule in a singlet excited state or a triplet excited state. The guest molecule in the triplet excited state emits phosphorescence. In addition, the guest molecule in the singlet excited state undergoes intersystem crossing to a triplet excited state and emits phosphorescence.

[0022] In other words, there may be a process in which energy is transferred from the host molecule in the singlet excited state (1H*) to the guest molecule in the singlet excited state (1G*) and then the guest molecule is put into the triplet excited state (3G*) through an inter-system transition, as shown by formula (2-2A) below, and a process in which energy is directly transferred from the host molecule in the singlet excited state (1H*) to the guest molecule in the triplet excited state (3G*) as shown by formula (2-2B) below. 1H* + 1G → 1H + 1G* → (intersystem crossing) → 1H + 3G* (2-2A) (2-2A) 1H* + 1G → 1H + 3G* (2-2B)

[0023] If all of the energy transfer processes described above in (2) occur efficiently, both the triplet excitation energy and the singlet excitation energy of the host molecule are efficiently converted into the triplet excited state (3G*) of the guest molecule. Therefore, highly efficient light emission is possible. In contrast, the emission efficiency decreases if the host molecule itself is deactivated by emitting the excitation energy as light or heat before the excitation energy of the host molecule is transferred to the guest molecule.

[0024] Next, we describe the factors that control the processes of intermolecular energy transfer between the host molecule and the guest molecule described above. The following two mechanisms are presented as the mechanisms of intermolecular energy transfer.

[0025] One mechanism is the Förster mechanism (dipole-dipole interaction), in which energy transfer does not require direct contact between molecules and is transferred through a resonance phenomenon of dipole vibration between a host molecule and a guest molecule. Through the resonance phenomenon of dipole vibration, the host molecule transfers energy to the guest molecule, thus placing the host molecule in a ground state and the guest molecule in an excited state. Note that the rate constant k h*→g of the Förster mechanism is represented by formula (1). [Formula 1] kh*→g=9000c4K2ϕln 10128π5n4N τR6∫f'h(v)εg(v)v4dv

[0026] In formula (1) v represents a frequency, f' h(ν) represents a normalized emission spectrum of a host molecule (a fluorescence spectrum upon energy transfer from a singlet excited state, and a phosphorescence spectrum upon energy transfer from a triplet excited state), ε g (ν) represents a molar absorption coefficient of a guest molecule, N represents the Avogadro number, n represents a refractive index of a medium, R represents an intermolecular distance between the host molecule and the guest molecule, τ represents a measured lifetime of an excited state (fluorescence lifetime or phosphorescence lifetime), c represents the speed of light, ϕ represents a luminescence quantum yield (a fluorescence quantum yield upon energy transfer from a singlet excited state, and a phosphorescence quantum yield upon energy transfer from a triplet excited state), and K 2represents a coefficient (0 to 4) for the orientation of a transition dipole moment between the host molecule and the guest molecule. It should be noted that in the case of random orientation K 2 = 2 / 3 is fulfilled.

[0027] The other is the Dexter mechanism (electron exchange interaction), in which a host molecule and a guest molecule are close to a contact-causing region where their orbitals overlap, and the host molecule in an excited state and the guest molecule in a ground state exchange their electrons, resulting in energy transfer. Note that the rate constant k h* → g of the Dexter mechanism is represented by formula (2). [Formula 2] kh*→g=(2πh)K'2 exp(−2RL)∫f'h(v)ε'g(v)dv

[0028] In formula (2), h represents a Planck constant, K' represents a constant with an energy dimension, v represents a frequency, f'h (ν) represents a normalized emission spectrum of a host molecule (a fluorescence spectrum upon energy transfer from a singlet excited state, and a phosphorescence spectrum upon energy transfer from a triplet excited state), ε' g (ν) represents a normalized absorption spectrum of a guest molecule, L represents an effective molecular radius, and R represents an intermolecular distance between the host molecule and the guest molecule.

[0029] It is assumed that the efficiency of energy transfer from the host molecule to the guest molecule (energy transfer efficiency Φ ET ) is represented by formula (3). In the formula, k r a rate constant of a light emission process (fluorescence upon energy transfer from a singlet excited state, and phosphorescence upon energy transfer from a triplet excited state) of a host molecule, k nrepresents a rate constant of a process without light emission (thermal deactivation or intersystem transition) of a host molecule, and τ represents a measured lifetime of an excited state of a host molecule. [Formula 3] ΦET=kh*→gkr+kn+kh*→g=kh*→g(1τ)+kh*→g

[0030] According to formula (3) we find that the energy transfer efficiency Φ ET by increasing the rate constant k h*→g for energy transfer can be increased so that another competing rate constant k r + k n (= 1 / τ) becomes relatively small. (Energy transfer efficiency in (2-1))

[0031] Here, the energy transfer process in (2-1) is considered first. Since the Förster mechanism (formula (1)) is forbidden in this case, only the Dexter mechanism (formula (2)) will be considered. To determine the rate constant k h*→gAccording to formula (2), in order to increase the energy transfer efficiency, it is preferred that an emission spectrum of a host molecule (a phosphorescence spectrum here because it is energy transfer from a triplet excited state) largely overlaps with an absorption spectrum of a guest molecule (absorption corresponding to direct transition from a singlet ground state to a triplet excited state).

[0032] In one embodiment of the present invention, a light-emitting substance that converts triplet excitation energy into light emission (e.g., a phosphorescent compound or a thermally stimulated delayed fluorescence (TADF) material) is used as a guest material. In an absorption spectrum of the phosphorescent compound, absorption corresponding to direct transition from a singlet ground state to a triplet excited state is sometimes observed, which is an absorption band on the longest wavelength side. In particular, light-emitting iridium complexes have a broad absorption band at approximately 500 nm to 600 nm as the absorption band on the longest wavelength side (in fact, the broad absorption band may be on a shorter or longer wavelength side depending on the emission wavelength).This absorption band is primarily based on a triplet MLCT (metal-to-ligand charge transfer) transition. Note that the absorption band also includes absorptions based on a triplet π-π* transition and a singlet MLCT transition, and these absorptions overlap with each other, forming a broad absorption band at the longest wavelength side in the absorption spectrum. In other words, the difference between the lowest singlet excited state and the lowest triplet excited state is small, and the absorptions based on these states overlap with each other, forming a broad absorption band at the longest wavelength side in the absorption spectrum.Therefore, when an organometallic complex (especially an iridium complex) is used as the guest material, the broad absorption band at the longest wavelength largely overlaps with the phosphorescence spectrum of the host material as described above. This allows the rate constant k to be reduced. h*→g can be enlarged and the energy transfer efficiency can be increased.

[0033] In addition, a fluorescent compound is generally used as the host material. Therefore, a phosphorescence lifetime (τ) is one millisecond or longer, which is extremely long (i.e., k r + k n is low). This is because the transition from the triplet excited state to the ground state (singlet) is a forbidden transition. Formula (3) shows that this is important for the energy transfer efficiency Φ ET is cheap.

[0034] The above description also suggests that energy transfer from the host material in the triplet excited state to the guest material in the triplet excited state, i.e., the process in formula (2-1), is generally promising to occur as long as the phosphorescence spectrum of the host material overlaps with the absorption spectrum corresponding to the direct transition of the guest material from the singlet ground state to the triplet excited state. (Energy transfer efficiency in (2-2))

[0035] Next, the energy transfer process in (2-2) is considered. The process in formula (2-2A) is influenced by the intersystem crossing efficiency of the guest material. Therefore, to maximize emission efficiency, the process in formula (2-2B) is considered important. Since the Dexter mechanism (formula (2)) is forbidden in this case, only the Förster mechanism (formula (1)) will be considered.

[0036] If τ is eliminated from formula (1) and formula (3), it can be said that the energy transfer efficiency Φ ETis higher when the quantum yield ϕ (a fluorescence quantum yield here, because it involves energy transfer from a singlet excited state) is higher. In reality, however, a more important factor is as follows: the emission spectrum of the host molecule (a fluorescence spectrum here, because it involves energy transfer from a singlet excited state) largely overlaps with the absorption spectrum of the guest molecule (absorption corresponding to the direct transition from the singlet ground state to the triplet excited state) (note that the molar absorption coefficient of the guest molecule is also preferably high). This means that the fluorescence spectrum of the host material overlaps with the absorption band located on the longest wavelength side of the phosphorescent compound used as the guest material.

[0037] However, this is usually very difficult to achieve. The reason for this is as follows: to efficiently enable both of the processes (2-1) and (2-2) described above, it is clear from the above discussion that the host material must be designed such that not only its phosphorescence spectrum but also its fluorescence spectrum overlaps with the absorption band of the guest material located on the longest wavelength side. In other words, the host material must be designed such that its fluorescence spectrum lies in a position close to that of the phosphorescence spectrum.

[0038] In general, however, the S1 level differs greatly from the T1 level (S1 level > T1 level). Accordingly, the fluorescence emission wavelength also differs greatly from the phosphorescence emission wavelength (fluorescence emission wavelength < phosphorescence emission wavelength). For example, 4,4'-di(N-carbazolyl)biphenyl (abbreviation: CBP), which is commonly used as a host material in a light-emitting element containing a phosphorescent compound, has a phosphorescence spectrum at approximately 500 nm and a fluorescence spectrum at approximately 400 nm, with a large difference of approximately 100 nm between them. This example further demonstrates that it is extremely difficult to create a host material such that its fluorescence spectrum lies in a position close to that of its phosphorescence spectrum.Therefore, it is very important to improve the efficiency of energy transfer from the host material in the singlet excited state to the guest material.

[0039] Accordingly, an embodiment of the present invention provides a useful technique that can overcome such a problem of the efficiency of energy transfer from the host material in the singlet excited state to the guest material. Specific embodiments thereof are described below. (Embodiment 1)

[0040] In this embodiment, a structural concept of a light-emitting element in one embodiment of the present invention and a concrete structure of the light-emitting element are described. Note that a light-emitting element in one embodiment of the present invention includes an EL layer including a light-emitting layer between a pair of electrodes (anode and cathode). The light-emitting layer has a layer-stack structure including a first light-emitting layer and a second light-emitting layer. The first light-emitting layer is formed on the anode side and includes at least a first light-emitting substance (guest material) that converts triplet excitation energy into light emission, a first organic compound (host material) having an electron-transport property, and a second organic compound (auxiliary material) having a hole-transport property.The second light-emitting layer contains at least a second light-emitting substance (guest material) that converts triplet excitation energy into light emission, the first organic compound (host material) with an electron transport property, and a third organic compound (auxiliary material) with a hole transport property.

[0041] It should be noted that the highest occupied molecular orbital (HOMO) level of the second organic compound (auxiliary material) contained in the first light-emitting layer is lower than that of the third organic compound (auxiliary material) contained in the second light-emitting layer. Therefore, the excitation energy (E A ) of an exciplex formed in the first light-emitting layer can be adjusted to be higher than the excitation energy (E B) of an exciplex formed in the second light-emitting layer.

[0042] Furthermore, the first light-emitting substance contained in the first light-emitting layer is a substance that emits light having a wavelength shorter than that of light emitted by the second light-emitting substance contained in the second light-emitting layer.

[0043] First, an element structure of a light-emitting element which is an example of the present invention will be described with reference to Fig. 1A described.

[0044] The element structure, which is Fig.1A, an EL layer 103 including a light-emitting layer 106 is provided between a pair of electrodes (an anode 101 and a cathode 102), and the EL layer 103 has a structure in which a hole-injection layer 104, a hole-transport layer 105, the light-emitting layer 106 (106a and 106b), an electron-transport layer 107, an electron-injection layer 108, and the like are sequentially stacked over the anode 101.

[0045] As in Fig.As shown in Figure 1A, in one embodiment of the present invention, the light-emitting layer 106 has a layer-stack structure including the first light-emitting layer 106a and the second light-emitting layer 106b. The first light-emitting layer 106a is formed on the anode side and includes at least a first light-emitting substance (guest material) 109a that converts triplet excitation energy into light emission, a first organic compound (host material) 110 having an electron-transport property, and a second organic compound (auxiliary material) 111 having a hole-transport property.The second light-emitting layer 106b contains at least a second light-emitting substance (guest material) 109b that converts triplet excitation energy into light emission, the first organic compound (host material) 110 with an electron-transport property, and a third organic compound (auxiliary material) 112 with a hole-transport property. An electron-transport material with an electron mobility of 10. -6 cm 2 / Vs or more is mainly used as the first organic compound 110, and hole transport materials with a hole mobility of 10 -6 cm 2 / Vs or more are mainly used as the second organic compound 111 and the third organic compound 112. In this specification, the first organic compound 110 is referred to as a host material, and the second organic compound 111 and the third organic compound 112 are referred to as auxiliary materials.

[0046] A combination of the first organic compound (host material) 110 and the second organic compound (auxiliary material) 111 in the first light-emitting layer 106a forms an exciplex (also referred to as an excited complex). Furthermore, the emission wavelength of the exciplex formed by the first organic compound (host material) 110 and the second organic compound (auxiliary material) 111 is on the longer wavelength side than the emission wavelength (fluorescence wavelength) of each of the first and second organic compounds (host and auxiliary materials) 110 and 111. Therefore, the fluorescence spectrum of the first organic compound (host material) 110 and the fluorescence spectrum of the second organic compound (auxiliary material) 111 can be converted into an emission spectrum that is on the longer wavelength side.

[0047] The same applies to the second light-emitting layer 106b. Therefore, a combination of the first organic compound (host material) 110 and the third organic compound (auxiliary material) 112 in the second light-emitting layer 106b forms an exciplex (also referred to as an excited complex). Furthermore, the emission wavelength of the exciplex formed by the first organic compound (host material) 110 and the third organic compound (auxiliary material) 112 is on the longer wavelength side than the emission wavelength (fluorescence wavelength) of each of the first and third organic compounds (host and auxiliary materials) 110 and 112. Therefore, the fluorescence spectrum of the first organic compound (host material) 110 and the fluorescence spectrum of the third organic compound (auxiliary material) 112 can be converted into an emission spectrum that is on the longer wavelength side.

[0048] Note that in the above structure, it is preferable that the triplet excitation energy level (T1 level) of each of the first and second organic compounds (host and auxiliary materials) 110 and 111 be higher than the T1 level of the first light-emitting substance (guest material) 109a, which converts the triplet excitation energy into light emission. This is because, when the T1 level of the first organic compound 110 (or the second organic compound 111) is lower than the T1 level of the first light-emitting substance (guest material) 109a, the triplet excitation energy of the first light-emitting substance (guest material) 109a, which contributes to light emission, is quenched by the first organic compound 110 (or the second organic compound 111), and thus the emission efficiency decreases.

[0049] Similarly, it is preferable that the triplet excitation energy level (T1 level) of each of the first and third organic compounds (host and auxiliary materials) 110 and 112 be higher than the T1 level of the second light-emitting substance (guest material) 109b, which converts the triplet excitation energy into light emission. This is because when the T1 level of the first organic compound 110 (or the third organic compound 112) is lower than the T1 level of the second light-emitting substance (guest material) 109b, the triplet excitation energy of the second light-emitting substance (guest material) 109b, which contributes to light emission, is quenched by the first organic compound 110 (or the third organic compound 112), and thus the emission efficiency decreases.

[0050] In addition, in the first light-emitting layer 106a in the light-emitting layer 106, either the first organic compound (host material) 110 or the second organic compound (auxiliary material) 111 may be contained at a higher proportion, and in the second light-emitting layer 106b in the light-emitting layer 106, either the first organic compound (host material) 110 or the third organic compound (auxiliary material) 112 may be contained at a higher proportion.

[0051] Fig. 1B is a band diagram illustrating the energy relationship between the first organic compound (host material) 110, the second organic compound (auxiliary material) 111, and the third organic compound (auxiliary material) 112 in the light-emitting layer 106 (the first light-emitting layer 106a and the second light-emitting layer 106b) having the structure described above.

[0052] As in Fig. 1B, in the first light-emitting layer 106a, the excitation energy (E A ) of the exciplex formed by the first organic compound (host material) 110 and the second organic compound (auxiliary material) 111 depends on the HOMO level of the second organic compound (auxiliary material) 111 and the LUMO level of the first organic compound (host material) 110. Furthermore, in the second light-emitting layer 106b, the excitation energy (E B) of the exciplex formed by the first organic compound (host material) 110 and the third organic compound (auxiliary material) 112 depends on the HOMO level of the third organic compound (auxiliary material) 112 and the LUMO level of the first organic compound (host material) 110. It should be noted that the second organic compound (auxiliary material) 111 contained in the first light-emitting layer 106a has a lower HOMO level than the third organic compound (auxiliary material) 112 contained in the second light-emitting layer 106b, which means that the excitation energy (E A ) of the exciplex formed in the first light-emitting layer 106a can be set to be higher than the excitation energy (E B ) of the exciplex formed in the second light-emitting layer 106b.

[0053] Note that, thanks to the element structure described above, the exciplex formed in the first light-emitting layer 106a has higher excitation energy than the exciplex formed in the second light-emitting layer 106b. Therefore, when a substance that emits light with a wavelength shorter than that emitted by the second light-emitting substance (guest material) 109b contained in the first light-emitting layer 106a and converts triplet excitation energy into light emission is used as the first light-emitting substance (guest material) 109a contained in the first light-emitting layer 106a and converts triplet excitation energy into light emission, the first light-emitting layer 106a and the second light-emitting layer 106b can emit light simultaneously.In addition, a portion of the excitation energy of the exciplex formed in the first light-emitting layer 106a that does not contribute to light emission can be used as excitation energy for the second light-emitting substance (guest material) 109b, which converts triplet excitation energy into light emission in the second light-emitting layer 106b. Consequently, the emission efficiency of the light-emitting element can be further increased.

[0054] In the light-emitting element described in this embodiment, exciplexes are formed in the light-emitting layer 106 (the first light-emitting layer 106a and the second light-emitting layer 106b), and the fluorescence spectrum of the first organic compound (host material) 110, the fluorescence spectrum of the second organic compound (auxiliary material) 111, or the fluorescence spectrum of the third organic compound (auxiliary material) 112 can be converted into an emission spectrum that is on the longer wavelength side. This means that, as shown in Fig.2, even if the fluorescence spectrum of the first organic compound 110, the second organic compound 111, or the third organic compound 112 is on the shorter wavelength side compared to the absorption band on the longest wavelength side of the light-emitting substance 109 (the first light-emitting substance (guest material) 109a and the second light-emitting substance (guest material) 109b) that converts triplet excitation energy into light emission, or has no overlap with the absorption band on the longest wavelength side of the light-emitting substance 109 (the first light-emitting substance (guest material) 109a and the second light-emitting substance (guest material) 109b) that converts triplet excitation energy into light emission, the emission spectrum of the exciplex and the absorption band may have a larger overlap.Consequently, the above energy transfer efficiency in formula (2-2B) can be increased.

[0055] Furthermore, it is considered that the exciplex exhibits a very small difference between the singlet excitation energy and the triplet excitation energy. In other words, the emission spectrum of the exciplex due to the singlet state and its emission spectrum due to the triplet state are very close to each other.Accordingly, in the case where the emission spectrum of the exciplex (generally, the emission spectrum of the exciplex due to the singlet state) is set to overlap with the absorption band on the longest wavelength side of the light-emitting substance 109, which converts triplet excitation energy into light emission as described above, the emission spectrum of the exciplex due to the triplet state (which is not observed at room temperature and in many cases is not observed at low temperature) also overlaps with the absorption band on the longest wavelength side of the light-emitting substance 109, which converts triplet excitation energy into light emission.In other words, not only the efficiency of energy transfer from the singlet excited state ((2-2)), but also the efficiency of energy transfer from the triplet excited state ((2-1)) can be increased, and as a result, energy from both the singlet and triplet excited states can be efficiently converted into light emission.

[0056] Therefore, molecular orbital calculations were performed as follows to determine whether an exciplex actually exhibits such properties. In general, a combination of a heteroaromatic compound and an aromatic amine often forms an exciplex through the action of the lowest unoccupied molecular orbital (LUMO) level of the heteroaromatic compound, which is deeper than the LUMO level of the aromatic amine (the property of easily accepting electrons), and the highest occupied molecular orbital (HOMO) level of the aromatic amine, which is shallower than the HOMO level of the heteroaromatic compound (the property of easily accepting holes).Therefore, calculations were performed using a combination of dibenzo[f,h]quinoxaline (abbreviation: DBq), which is a typical framework forming the LUMO level of a heteroaromatic compound and is a model of the first organic compound 110 in one embodiment of the present invention, and triphenylamine (abbreviation: TPA), which is a typical framework forming the HOMO level of an aromatic amine and is a model of the second organic compound 111 (or the third organic compound 112) in one embodiment of the present invention.

[0057] First, the optimal molecular structures and excitation energies of a molecule of DBq (abbreviation) and a molecule of TPA (abbreviation) in the lowest singlet excited state (S1) and the lowest triplet excited state (T1) were calculated using time-dependent density functional theory (TD-DFT). Furthermore, the excitation energy of a dimer of DBq (abbreviation) and TPA (abbreviation) was also calculated.

[0058] In DFT (density functional theory), the total energy is represented as the sum of potential energy, electrostatic energy between electrons, electronic kinetic energy, and exchange-correlation energy, including all complex interactions between electrons. In DFT, an exchange-correlation interaction is also approximated by a functional (a function of another function) of a one-electron potential, represented as an electron density, to enable fast and highly accurate calculations. B3LYP, a hybrid functional, was used to determine the weight of each parameter with respect to exchange-correlation energy.

[0059] In addition, a basis function 6-311 (a basis function of a triple split valence basis set using three contraction functions for each valence orbital) was applied to all atoms.

[0060] According to the basis function above, for example, in the case of hydrogen atoms, 1s to 3s orbitals are considered, while in the case of carbon atoms, 1s to 4s and 2p to 4p orbitals are considered. Furthermore, the p-function and the d-function were added as polarization basis sets to hydrogen atoms and non-hydrogen atoms, respectively, to improve computational accuracy.

[0061] It should be noted that Gaussian 09 was used as a quantum chemistry computing program. A high-performance computer (Altix 4700, manufactured by SGI Japan, Ltd.) was used for the calculations.

[0062] First, the HOMO levels and the LUMO levels of a molecule of DBq (abbreviation), a molecule of TPA (abbreviation) and a dimer of DBq (abbreviation) and TPA (abbreviation) were calculated. Fig. 3 shows the HOMO levels and the LUMO levels, and Fig. 4A1, Fig. 4A2, Fig. 4B1, Fig. 4B2, Fig. 4C1 and Fig. 4C2 show HOMO and LUMO distributions.

[0063] Fig. Figure 4A1 shows the LUMO distribution of a molecule of DBq (abbreviation); Fig. 4A2, the HOMO distribution of a molecule of DBq (abbreviation); Fig. 4B1, the LUMO distribution of a molecule of TPA (abbreviation); Fig. 4B2, the HOMO distribution of a molecule of TPA (abbreviation); Fig. 4C1, the LUMO distribution of the dimer of DBq (abbreviation) and TPA (abbreviation); and Fig. 4C2, the HOMO distribution of the dimer of DBq (abbreviation) and TPA (abbreviation).

[0064] As in Fig.3, it is suggested that the dimer of DBq (abbreviation) and TPA (abbreviation) forms an exciplex of DBq (abbreviation) and TPA (abbreviation) under the influence of the LUMO level (-1.99 eV) of DBq (abbreviation), which is deeper (lower) than the LUMO level of TPA (abbreviation), and the HOMO level (-5.21 eV) of TPA (abbreviation), which is less deep (higher) than the HOMO level of DBq (abbreviation). It is actually from Fig. 4C1 and Fig. 4C2 it is clear that the LUMO of the dimer of DBq (abbreviation) and TPA (abbreviation) is distributed on the DBq (abbreviation) side, and its HOMO is distributed on the TPA (abbreviation) side.

[0065] Next, excitation energies obtained from the optimal molecular structures of a molecule of DBq (abbreviation) at the S1 and T1 levels are shown. Here, the excitation energies at the S1 and T1 levels correspond to fluorescence and phosphorescence wavelengths obtained from a molecule of DBq (abbreviation), respectively. The excitation energy at the S1 level of a molecule of DBq (abbreviation) is 3.294 eV, and the fluorescence wavelength is 376.4 nm. The excitation energy at the T1 level of a molecule of DBq (abbreviation) is 2.460 eV, and the phosphorescence wavelength is 504.1 nm.

[0066] Additionally, excitation energies obtained from the optimal molecular structures of a molecule of TPA (abbreviation) at the S1 and T1 levels are shown. The excitation energies at the S1 and T1 levels correspond to fluorescence and phosphorescence wavelengths obtained from a molecule of TPA (abbreviation), respectively. The excitation energy at the S1 level of a molecule of TPA (abbreviation) is 3.508 eV, and the fluorescence wavelength is 353.4 nm. The excitation energy at the T1 level of a molecule of TPA (abbreviation) is 2.610 eV, and the phosphorescence wavelength is 474.7 nm.

[0067] Additionally, excitation energies obtained from the optimal molecular structures of the dimer of DBq (abbreviation) and TPA (abbreviation) at the S1 and T1 levels are shown. The excitation energies at the S1 and T1 levels correspond to fluorescence and phosphorescence wavelengths obtained from the dimer of DBq (abbreviation) and TPA (abbreviation), respectively. The excitation energy at the S1 level of the dimer of DBq (abbreviation) and TPA (abbreviation) is 2.036 eV, and the fluorescence wavelength is 609.1 nm. The excitation energy at the T1 level of the dimer of DBq (abbreviation) and TPA (abbreviation) is 2.030 eV, and the phosphorescence wavelength is 610.0 nm.

[0068] From the above description, it can be found that each of the phosphorescence wavelengths of a molecule of DBq (abbreviation) and a molecule of TPA (abbreviation) shifts toward the longer wavelength side by approximately 100 nm. This result shows a trend similar to that described above for CBP (abbreviation) (measured values) and reinforces the validity of the calculations.

[0069] On the other hand, it is found that the fluorescence wavelength of the dimer of DBq (abbreviation) and TPA (abbreviation) is on the longer wavelength side compared to the fluorescence wavelengths of one molecule of DBq (abbreviation) and one molecule of TPA (abbreviation). It is also found that the difference between the fluorescence wavelength and the phosphorescence wavelength of the dimer of DBq (abbreviation) and TPA (abbreviation) is only 0.9 nm, and that these wavelengths are essentially the same.

[0070] These results indicate that the singlet excitation energy and the triplet excitation energy of the exciplex are essentially equivalent. Therefore, it is suggested that, as described above, the exciplex can efficiently transfer energy from both the singlet state and the triplet state to the light-emitting substance (the guest materials including the first light-emitting substance and the second light-emitting substance), which converts the triplet excitation energy into light emission.

[0071] In the above manner, the light-emitting element in one embodiment of the present invention transfers energy by utilizing an overlap between the emission spectrum of the exciplex formed in the light-emitting layer and the absorption spectrum of the light-emitting substance (the guest materials including the first light-emitting substance and the second light-emitting substance), converting triplet excitation energy into light emission, and therefore exhibits high energy transfer efficiency. Consequently, the light-emitting element can achieve high external quantum efficiency.

[0072] In addition, the exciplex exists in only one excited state and thus has no ground state in which energy can be absorbed. Therefore, it is assumed that a phenomenon in which the light-emitting substance (guest material) converts triplet excitation energy into light emission is deactivated by energy transfer from the light-emitting substance (guest material) to the exciplex before light emission (i.e., the emission efficiency decreases), whereby the light-emitting substance converts the triplet excitation energy into light emission in both the singlet excited state and the triplet excited state, does not occur in principle. This also contributes to improving the external quantum efficiency.

[0073] It should be noted that the exciplex described above is formed by an interaction between dissimilar molecules in excited states. It is well known that the exciplex is easily formed between a material with a relatively deep LUMO level and a material with a relatively shallow HOMO level.

[0074] The emission wavelength of an exciplex depends on the energy difference between the HOMO and LUMO levels. A general trend is that the emission wavelength is short when the energy difference is large and that the emission wavelength is long when the energy difference is small.

[0075] Therefore, the HOMO levels and LUMO levels of the first organic compound (host material) 110, the second organic compound (auxiliary material) 111 and the third organic compound (auxiliary material) 112 differ from each other in this embodiment, as shown in Fig.1B. Specifically, the energy levels vary in the following order: the HOMO level of the first organic compound 110 < the HOMO levels of the second organic compound 111 and the third organic compound 112 < the LUMO level of the first organic compound 110 < the LUMO levels of the second organic compound 111 and the third organic compound 112.

[0076] In each of the light-emitting layers, when the exciplex is formed by the two organic compounds (the first organic compound 110 and the second organic compound 111 in the first light-emitting layer 106a, and the first organic compound 110 and the third organic compound 112 in the second light-emitting layer 106b), the LUMO levels of the exciplexes in the first light-emitting layer 106a and the second light-emitting layer 106b depend on the first organic compound (host material) 110, the HOMO level of the exciplex in the first light-emitting layer 106a depends on the second organic compound (auxiliary material) 111, and the HOMO level of the exciplex in the second light-emitting layer 106b depends on the HOMO level of the third organic compound (auxiliary material) 112. Therefore, the excitation energy (E A) of the exciplex in the first light-emitting layer 106a is higher than the excitation energy (E B ) of the exciplex in the second light-emitting layer 106b. In other words, when a substance that emits light with a wavelength shorter than that emitted by the second light-emitting substance 109b contained in the second light-emitting layer and converts triplet excitation energy into light emission is used as the first light-emitting substance 109a contained in the first light-emitting layer and converts triplet excitation energy into light emission, a light-emitting element with high emission efficiency can be manufactured. Furthermore, light-emitting materials with different emission wavelengths can efficiently emit light simultaneously.

[0077] It should be noted that the process of forming an exciplex in one embodiment of the present invention may be any of the following two processes.

[0078] A formation process is as follows: an exciplex is formed from the second and third organic compounds (auxiliary materials) with charge carriers (especially cation).

[0079] Generally, when an electron and a hole recombine in a host material, excitation energy is transferred from the host material in an excited state to the guest material, causing the guest material to enter an excited state capable of emitting light. Before the excitation energy is transferred from the host material to the guest material, the host material itself emits light, or the excitation energy is converted into heat energy, resulting in the deactivation of part of the excitation energy. In particular, when the host material is in a singlet excited state, energy transfer does not occur easily, as described in (2-2). Such deactivation of excitation energy is one of the causes of a decrease in the lifetime of a light-emitting element.

[0080] However, in one embodiment of the present invention, an exciplex is formed by the first organic compound (host material) and the second organic compound (or the third organic compound) (auxiliary material) containing charge carriers (cation or anion). Therefore, the formation of a singlet exciton of the first organic compound (host material) can be prevented. In other words, a process is possible in which an exciplex is directly formed without forming a singlet exciton. Therefore, deactivation of the singlet excitation energy can be prevented. Consequently, a light-emitting element with a long lifetime can be obtained.

[0081] For example, in the case where the first organic compound is an electron-capturing compound with the property of easily capturing electrons (charge carriers) (with a deep LUMO level) among electron-transport materials, and the second organic compound (or the third organic compound) is a hole-capturing compound with the property of easily capturing holes (charge carriers) (with a shallow HOMO level) among hole-transport materials, an exciplex is formed directly from an anion of the first organic compound and a cation of the second organic compound (or the third organic compound). An exciplex formed in such a process is specifically called an electroplex.A light-emitting element with high emission efficiency can be obtained by inhibiting the generation of the singlet excited state of the first organic compound (host material) and transferring energy from an electroplex to the light-emitting substance (guest material), which converts triplet excitation energy into light emission, in the manner described above. Note that in this case, the generation of the triplet excited state of the first organic compound (host material) is similarly inhibited, and an exciplex is directly formed. Therefore, it is assumed that energy transfer occurs from the exciplex to the light-emitting substance (guest material), which converts triplet excitation energy into light emission.

[0082] The other formation process is a fundamental process in which either the host material (the first organic compound) or the auxiliary material (the second organic compound or the third organic compound) forms a singlet exciton and then interacts with the other in the ground state to form an exciplex. Unlike an electroplex, a singlet excited state of the first organic compound (host material), the second organic compound (auxiliary material), or the third organic compound (auxiliary material) is temporarily generated in this case, but it is quickly converted into an exciplex. This can prevent deactivation of singlet excitation energy. Consequently, it is possible to prevent deactivation of excitation energy of the first organic compound (host material), the second organic compound (auxiliary material), or the third organic compound (auxiliary material).It should be noted that in this case, it is assumed that the triplet excitation state of the host material is converted into an exciplex at a similar rate, and energy is transferred from the exciplex to the light-emitting substance (guest material), which converts triplet excitation energy into light emission.

[0083] It should be noted that in the case where the first organic compound (host material) is an electron-trapping compound, and the second organic compound (or the third organic compound) (auxiliary material) is a hole-trapping compound, and where the difference between the HOMO levels and the LUMO levels of these compounds is large (specifically, 0.3 eV or more), electrons are selectively injected into the first organic compound (host material) and holes are selectively injected into the second organic compound (or the third organic compound) (auxiliary material). In this case, the process of forming an electroplex is considered to take precedence over the process of forming an exciplex by a singlet exciton.

[0084] To ensure sufficient overlap between the emission spectrum of the exciplex and the absorption spectrum of the light-emitting substance (guest material) that converts triplet excitation energy into light emission, the difference between the energy of a peak in the emission spectrum and the energy of a peak in the absorption band at the lowest energy side in the absorption spectrum is preferably 0.3 eV or less. The difference is more preferably 0.2 eV or less, and even more preferably 0.1 eV or less.

[0085] In the light-emitting element in one embodiment of the present invention, it is also preferable that the excitation energy of the exciplex is sufficiently transferred to the light-emitting substance (guest material) that converts triplet excitation energy into light emission, and that substantially no light emission is observed from the exciplex. Therefore, it is preferable that energy is transferred to the light-emitting substance that converts triplet excitation energy into light emission through an exciplex, so that the light-emitting substance that converts triplet excitation energy into light emission emits light. Note that, as the light-emitting substance that converts triplet excitation energy into light emission, a phosphorescent compound (e.g.,an organometallic complex), a thermally stimulated delayed fluorescence (TADF) material, or the like.

[0086] In the case where a light-emitting substance that converts triplet excitation energy into light emission is used as the first organic compound (host material) in the first light-emitting layer 106a of the light-emitting element in one embodiment of the present invention, the first organic compound (host material) itself readily emits light and does not easily transfer energy to the light-emitting substance (guest material) that converts triplet excitation energy into light emission. In this case, it would be advantageous if the first organic compound could efficiently emit light, but it is difficult to achieve high emission efficiency because a high concentration of the host material can cause a concentration quenching problem.Therefore, the case where the first organic compound (host material) and / or the second organic compound (or the third organic compound) (auxiliary material) are / is a fluorescent compound (i.e., a compound that readily performs light emission or thermal deactivation from the singlet excited state) is effective. For this reason, it is preferable that the first organic compound (host material) and / or the second organic compound (or the third organic compound) (auxiliary material) are / is a fluorescent compound, and that an exciplex is used as an energy transfer medium.

[0087] It should be noted that the structure described in this embodiment may be combined with any of the structures described in the other embodiments as appropriate. (Embodiment 2)

[0088] In this embodiment, an example of a light-emitting element in an embodiment of the present invention is explained with reference to Fig. 5 described.

[0089] In the light-emitting element described in this embodiment, as shown in Fig. 5, an EL layer 203 including a light-emitting layer 206 is provided between a pair of electrodes (a first electrode (anode) 201 and a second electrode (cathode) 202), and the EL layer 203 includes a hole injection layer 204, a hole transport layer 205, an electron transport layer 207, an electron injection layer 208, and the like in addition to the light-emitting layer 206 having a layer-stack structure including a first light-emitting layer 206a and a second light-emitting layer 206b.

[0090] Note that the light-emitting layer 206 in this embodiment has a layer-stack structure including the first light-emitting layer 206a and the second light-emitting layer 206b. In the light-emitting layer 206, the first light-emitting layer 206a includes a first light-emitting substance (guest material) 209a that converts triplet excitation energy into light emission, a first organic compound (host material) 210 having an electron-transport property, and a second organic compound (auxiliary material) 211 having a hole-transport property. and the second light-emitting layer 206b contains a second light-emitting substance (guest material) 209b that converts triplet excitation energy into light emission, the first organic compound (host material) 210 having an electron transport property, and the third organic compound (auxiliary material) 212 having a hole transport property.

[0091] It should be noted that the HOMO level of the second organic compound (auxiliary material) contained in the first light-emitting layer is lower than that of the third organic compound (auxiliary material) contained in the second light-emitting layer. Therefore, the excitation energy (E A ) of an exciplex formed in the first light-emitting layer can be adjusted to be higher than the excitation energy (E B ) of an exciplex formed in the second light-emitting layer.

[0092] It should be noted that crystallization of the light-emitting layer 206 (the first light-emitting layer 206a and the second light-emitting layer 206b) can be suppressed by using a structure in which the first light-emitting substance 209a, which converts triplet excitation energy into light emission, is dispersed in the first organic compound (host material) 210 and the second organic compound (auxiliary material) 211 in the first light-emitting layer 206a, and in which the second light-emitting substance 209b, which converts triplet excitation energy into light emission, is dispersed in the first organic compound (host material) 210 and the third organic compound (auxiliary material) 212 in the second light-emitting layer 206b.Furthermore, it is possible to suppress concentration quenching due to a high concentration of the light-emitting substance 209 (209a and 209b), and therefore the light-emitting element can have higher emission efficiency.

[0093] It is preferable that the level of a triplet excitation energy (T1 level) of each of the first organic compound 210, the second organic compound 211, and the third organic compound 212 is higher than the T1 level of the light-emitting substance 209 (209a and 209b) that converts triplet excitation energy into light emission.This is because when the T1 level of each of the first organic compound 210, the second organic compound 211, and the third organic compound 212 is lower than the T1 level of the light-emitting substance 209 (209a and 209b) that converts triplet excitation energy into light emission, the triplet excitation energy of the light-emitting substance 209 (209a and 209b) that converts triplet excitation energy into light emission, which contributes to light emission, is quenched by the first organic compound 210 (or the second organic compound 211 or the third organic compound 212), and thus the emission efficiency decreases.

[0094] In the light-emitting layer 206 in this embodiment, at the time of recombination of carriers (electrons and holes) injected from the respective electrodes, the first organic compound 210 and the second organic compound 211 form an exciplex in the first light-emitting layer 206a, and the first organic compound 210 and the third organic compound 212 form an exciplex in the second light-emitting layer 206b.Accordingly, a fluorescence spectrum of the first organic compound 210 and that of the second organic compound 211 in the first light-emitting layer 206a can be converted into an emission spectrum of the exciplex located on a longer wavelength side, and a fluorescence spectrum of the first organic compound 210 and that of the third organic compound 212 in the second light-emitting layer 206b can be converted into an emission spectrum of the exciplex located on a longer wavelength side.With this in mind, the first organic compound 210 and the second organic compound 211 in the first light-emitting layer 206a, and the first organic compound 210 and the third organic compound 212 in the second light-emitting layer 206b, are selected to create a large overlap between the emission spectrum of the exciplex and the absorption spectrum of the light-emitting substance (guest material) 209, which converts triplet excitation energy into light emission, thereby maximizing energy transfer from a singlet excited state. That is, it is assumed here that energy transfer occurs from the exciplex, not from the host material, even in the case of a triplet excited state.

[0095] Note that, as the light-emitting substance 209 (the first light-emitting substance 209a and the second light-emitting substance 209b) that converts triplet excitation energy into light emission, a phosphorescent compound (e.g., an organometallic complex), a thermally stimulated delayed fluorescence (TADF) material, or the like is preferably used. An electron-transport material is preferably used as the first organic compound (host material) 210. Hole-transport materials are preferably used as the second organic compound (auxiliary material) 211 and the third organic compound (auxiliary material) 212.

[0096] It should be noted that examples of the organometallic complex include: Bis[2-(4',6'-difluorophenyl)pyridinato-N,C 2']iridium(III)tetrakis(1-pyrazolyl)borate (abbreviation: FIr6), bis[2-(4',6'-difluorophenyl)pyridinato-N,C 2 ']iridium(III)picolinate (abbreviation: Flrpic), bis[2-(3',5'-bistrifluoromethylphenyl)pyridinato-N, C 2 ']iridium(III)picolinate (abbreviation: Ir(CF3ppy)2(pic)), bis[2-(4',6'-difluorophenyl)pyridinato-N, C 2 ']iridium(III)acetylacetonate (abbreviation: Flracac), tris(2-phenylpyridinato)iridium(III) (abbreviation: Ir(ppy)3), bis(2-phenylpyridinato)iridium(III)acetylacetonate (abbreviation: Ir(ppy)2(acac)), bis(benzo[h]quinolinato)iridium(III)acetylacetonate (abbreviation: Ir(bzq)2(acac)), bis(2,4-diphenyl-1,3-oxazolato-N, C 2 ')iridium(III)acetylacetonate (abbreviation: Ir(dpo)2(acac)), bis{2-[4'-(perfluorophenyl)phenyl]pyridinato-N,C 2}iridium(III)acetylacetonate (abbreviation: Ir(p-PF-ph)2(acac)), bis(2-phenylbenzothiazolato-N, C 2 ')iridium(III)acetylacetonate (abbreviation: Ir(bt)2(acac)), bis[2-(2'-benzo[4,5-α]thienyl)pyridinato-N, C 3']iridium(III)acetylacetonate (abbreviation: Ir(btp)2(acac)), bis(1-phenylisoquinolinato-N, C 2 ')iridium(III)acetylacetonate (abbreviation: Ir(piq)2(acac)), (acetylacetonato)bis[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(III) (abbreviation: Ir(Fdpq)2(acac)), (acetylacetonato)bis(2,3,5-triphenylpyrazinato)iridium(III) (abbreviation: Ir(tppr)2(acac)), (2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin)platinum(II) (abbreviation: PtOEP), tris(acetylacetonato)(monophenanthroline)terbium(III) (abbreviation: Tb(acac)3(Phen)), Tris(1,3-diphenyl-1,3-propanedionato)(monophenanthroline)europium(III) (abbreviation: Eu(DBM)3(Phen)) and Tris[1-(2-thenoyl)-3,3,3-trifluoroacetonato](monophenanthroline)europium(III) (abbreviation: Eu(TTA)3(Phen)).

[0097] As the electron transport material, a π-electron-deficient heteroaromatic compound is preferred, such as a nitrogen-containing heteroaromatic compound, and examples thereof include quinoxaline derivatives and dibenzoquinoxaline derivatives such as quinoxaline derivatives. B. 2-[3-(Dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation: 2mDBTPDBq-II), 2-[3'-(Dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline (abbreviation: 2mDBTPDBq-II), 2-[4-(3,6-Diphenyl-9H-carbazol-9-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation: 2CzPDBq-III), 7-[3-(Dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation: 7mDBTPDBq-II) and 6-[3-(Dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation: 6mDBTPDBq-II).

[0098] As the hole-transporting material, a π-electron-rich heteroaromatic compound (e.g., a carbazole derivative or an indole derivative) or an aromatic amine compound is preferred, and examples thereof include 4-phenyl-4'-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PCBA1BP), 4,4'-di(1-naphthyl)-4"-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PCBNBB), 3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole (abbreviation: PCzPCN1), 4,4',4''-tris[N-(1-naphthyl)-N-phenylamino]triphenylamine (abbreviation: 1'-TNATA), 2,7-Bis[N-(4-diphenylaminophenyl)-N-phenylamino]-spiro-9,9'-bifluorene (abbreviation: DPA2SF), N,N'-Bis(9-phenylcarbazol-3-yl)-N,N'-diphenylbenzene-1,3-diamine (abbreviation: PCA2B), N-(9,9-Dimethyl-2-diphenylamino-9H-fluoren-7-yl)diphenylamine (abbreviation: DPNF), N,N',N''-Triphenyl-N,N',N''-tris(9-phenylcarbazol-3-yl)benzene-1,3,5-triamine (abbreviation: PCA3B), 2-[N-(9-Phenylcarbazol-3-yl)-N-phenylamino]spiro-9,9'-bifluorene (abbreviation: PCASF),2-[N-(4-Diphenylaminophenyl)-N-phenylamino]spiro-9,9'-bifluorene (abbreviation: DPASF), N,N'-bis[4-(carbazol-9-yl)phenyl]-N,N'-diphenyl-9,9-dimethylfluorene-2,7-diamine, (Abbreviation: YGA2F), 4,4'-Bis[N-(3-methylphenyl)-N-phenylamino]biphenyl (Abbreviation: TPD), 4,4'-Bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl (Abbreviation: DPAB), N-(9,9-Dimethyl-9H-fluoren-2-yl)-N-{9,9-dimethyl-2-[N'-phenyl-N'-(9,9-dimethyl-9H-fluoren-2-yl)amino]-9H-fluoren-7-yl}phenylamine (Abbreviation: DFLADFL), 3-[N-(9-Phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole (Abbreviation: PCzPCA1), 3-[N-(4-Diphenylaminophenyl)-N-phenylamino]-9-phenylcarbazole (Abbreviation: PCzDPA1), 3,6-Bis[N-(4-diphenylaminophenyl)-N-phenylamino]-9-phenylcarbazole (abbreviation: PCzDPA2), 4,4'-bis(N-{4-[N'-(3-methylphenyl)-N'-phenylamino]phenyl}-N-phenylamino)biphenyl (abbreviation: DNTPD), 3,6-bis[N-(4-diphenylaminophenyl)-N-(1-naphthyl)amino]-9-phenylcarbazole (abbreviation: PCzTPN2) and 3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole (abbreviation: PCzPCA2).

[0099] It should be noted that materials that can be used for the light-emitting substance (guest material) 209 (the first light-emitting substance 209a and the second light-emitting substance 209b) that converts triplet excitation energy into light emission, the first organic compound (host material) 210, the second organic compound (auxiliary material) 211, and the third organic compound (auxiliary material) 212 are not limited to the above examples.The combination is determined such that an exciplex can be formed such that the emission spectrum of the exciplex overlaps with the absorption spectrum of the light-emitting substance (guest material) 209 (the first light-emitting substance 209a or the second light-emitting substance 209b) that converts triplet excitation energy into light emission, and such that the peak of the emission spectrum of the exciplex has a longer wavelength than the peak of the absorption spectrum of the light-emitting substance (guest material) 209 (the first light-emitting substance 209a or the second light-emitting substance 209b) that converts triplet excitation energy into light emission.

[0100] In the case where an electron-transport material is used as the first organic compound 210 and a hole-transport material is used as the second organic compound 211, charge carrier balance can be controlled by the mixing ratio of the compounds. Specifically, the ratio of the first organic compound 210 to the second organic compound 211 is preferably 1:9 to 9:1.

[0101] A concrete example in which the light-emitting element described in this embodiment is manufactured will be described below.

[0102] For the first electrode (anode) 201 and the second electrode (cathode) 202, a metal, an alloy, an electrically conductive compound, a mixture thereof, or the like can be used. Specifically, indium oxide-tin oxide (indium tin oxide (ITO)), indium oxide-tin oxide containing silicon or silicon oxide, indium oxide-zinc oxide (indium zinc oxide), indium oxide containing tungsten oxide and zinc oxide, gold (Au), platinum (Pt), nickel (Ni), tungsten (W), chromium (Cr), molybdenum (Mo), iron (Fe), cobalt (Co), copper (Cu), palladium (Pd), or titanium (Ti) can be used. In addition, an element belonging to Group 1 or Group 2 of the periodic table, for example, an alkali metal such as lithium (Li) or cesium (Cs), or an alkaline earth metal such as Ni, Ni, or Ni(II) can be used. B. Magnesium (Mg), Calcium (Ca) or Strontium (Sr), an alloy containing such an element (e.g. MgAg or AlLi), a rare earth metal such asEuropium (Eu) or ytterbium (Yb), an alloy containing such an element, graphene, or the like may be used. The first electrode (anode) 201 and the second electrode (cathode) 202 can be formed, for example, by a sputtering method, an evaporation method (including a vacuum evaporation method), or the like.

[0103] Examples of a substance having a high hole transport property used for the hole injection layer 204 and the hole transport layer 205 include aromatic amine compounds such as: B. 4,4'-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB or α-NPD), N,N'-bis(3-methylphenyl)-N,N'-diphenyl-[1,1'-biphenyl]-4,4'-diamine (abbreviation: TPD), 4,4',4''-tris(carbazol-9-yl)triphenylamine (abbreviation: TCTA), 4,4',4''-tris(N,N-diphenylamino)triphenylamine (abbreviation: TDATA), 4,4',4''-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine (abbreviation: MTDATA) and 4,4'-bis[N-(spiro-9,9'-bifluoren-2-yl)-N-phenylamino]biphenyl (abbreviation: BSPB), 3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole (abbreviation: PCzPCA1), 3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole (abbreviation: PCzPCA2), and 3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole (abbreviation: PCzPCN1). Other examples include carbazole derivatives such as:4,4'-Di(N-carbazolyl)biphenyl (abbreviation: CBP), 1,3,5-Tris[4-(N-carbazolyl)phenyl]benzene (abbreviation: TCPB), and 9-[4-(10-phenyl-9-anthracenyl)phenyl]-9H-carbazole (abbreviation: CzPA). The substances listed here are primarily those with a hole mobility of 10. -6 cm 2 / Vs or more. It should be noted that, in addition to these substances, any substance that has the property of transporting more holes than electrons can be used.

[0104] Further examples include high molecular weight 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).

[0105] Further, examples of an acceptor substance that can be used for the hole-injection layer 204 include oxides of transition metals, oxides of metals belonging to groups 4 to 8 of the periodic table, and the like. Specifically, molybdenum oxide is particularly preferred.

[0106] The light-emitting layer 206 (206a and 206b) is formed such that, as described above, the first light-emitting layer 206a contains at least the first light-emitting substance 209a, the first organic compound (host material) 210, and the second organic compound (auxiliary material) 211, and the second light-emitting layer 206b contains at least the second light-emitting substance 209b, the first organic compound (host material) 210, and the third organic compound (auxiliary material) 212.

[0107] The electron-transport layer 207 is a layer containing a substance with high electron-transport properties. It is possible to use a metal complex for the electron-transport layer 207, such as Alq3, tris(4-methyl-8-quinolinolato)aluminum (abbreviation: Almq3), bis(10-hydroxybenzo[h]quinolinato)beryllium (abbreviation: BeBq2), BAlq, Zn(BOX)2, or bis[2-(2-hydroxyphenyl)benzothiazolato]zinc (abbreviation: Zn(BTZ)2).Alternatively, it is possible to use a heteroaromatic compound, such as 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation: PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene (abbreviation: OXD-7), 3-(4-tert-butylphenyl)-4-phenyl-5-(4-biphenylyl)-1,2,4-triazole (abbreviation: TAZ), 3-(4-tert-butylphenyl)-4-(4-ethylphenyl)-5-(4-biphenylyl)-1,2,4-triazole (abbreviation: p-EtTAZ), bathophenanthroline (abbreviation: BPhen), bathocuproine (abbreviation: BCP) or 4,4'-Bis(5-methylbenzoxazol-2-yl)stilbene (abbreviation: BzOs). Alternatively, it is possible to use a high-molecular-weight compound, such as poly(2,5-pyridinediyl) (abbreviation: PPy), poly[(9,9-dihexylfluorene-2,7-diyl)-co-(pyridine-3,5-diyl)] (abbreviation: PF-Py), or poly[(9,9-dioctylfluorene-2,7-diyl)-co-(2,2'-bipyridine-6,6'-diyl)] (abbreviation: PF-BPy). The substances listed here are primarily those with an electron mobility of 10. -6 cm 2 / Vs or more. Note that, in addition to these substances, any substance having a property of transporting more electrons than holes can be used for the electron-transport layer 207.

[0108] The electron transport layer 207 is not limited to a single layer and may be a stack of two or more layers containing any of the above substances.

[0109] The electron injection layer 208 is a layer containing a substance with a high electron injection property. Examples of the substance that can be used for the electron injection layer 208 include alkali metals, alkaline earth metals, and compounds thereof, such as lithium fluoride (LiF), cesium fluoride (CsF), calcium fluoride (CaF2), and lithium oxide (LiO). x), and rare earth metal compounds such as erbium fluoride (ErF3). Alternatively, the substances listed above can be used to form the electron-transport layer 207.

[0110] Alternatively, a composite material in which an organic compound and an electron donor (a donor) are mixed can be used for the electron-injection layer 208. Such a composite material, in which electrons are generated in the organic compound by the electron donor, has high electron-injection and electron-transport properties. Here, the organic compound is preferably a material that can transport the generated electrons very well, and in particular, one of the above substances (such as metal complexes and heteroaromatic compounds) can be used for the electron-transport layer 207. As the electron donor, a substance that exhibits an electron-donating property with respect to the organic compound can be used.In particular, alkali metals, alkaline earth metals, and rare earth metals are preferred, and lithium, cesium, magnesium, calcium, erbium, ytterbium, and the like can be cited. Preferred is one of alkali metal oxides and alkaline earth metal oxides, and examples include lithium oxide, calcium oxide, barium oxide, and the like. A Lewis base such as magnesium oxide or an organic compound such as tetrathiafulvalene (abbreviation: TTF) can be used.

[0111] Note that the hole injection layer 204, the hole transport layer 205, the light-emitting layer 206 (206a and 206b), the electron transport layer 207, and the electron injection layer 208 described above can each be formed by a method such as an evaporation method (including a vacuum evaporation method), an inkjet method, or a coating method.

[0112] Light emission obtained in the light-emitting layer 206 of the above-described light-emitting element is taken out through the first electrode 201 and / or the second electrode 202. Therefore, in this embodiment, the first electrode 201 and / or the second electrode 202 is / are an electrode with light transmittance.

[0113] In the light-emitting element described in this embodiment, energy transfer efficiency can be improved thanks to energy transfer that utilizes an overlap between the emission spectrum of an exciplex and the absorption spectrum of a light-emitting substance (guest material), which converts triplet excitation energy into light emission. Consequently, the light-emitting element can achieve high external quantum efficiency.

[0114] It should be noted that the light-emitting element described in this embodiment is one embodiment of the present invention and is particularly characterized by the structure of the light-emitting layer. Therefore, when the structure described in this embodiment is used, a passive matrix light-emitting device, an active matrix light-emitting device, and the like can be manufactured. Each of these light-emitting devices is included in the present invention.

[0115] Note that in the case of manufacturing the active matrix light-emitting device, there is no particular limitation on the structure of a TFT. For example, a gradated TFT or an inverted gradated TFT can be used as needed. In addition, a driving circuit formed over a TFT substrate can be formed using both an N-type TFT and a P-type TFT, or either an N-type TFT or a P-type TFT. Furthermore, there is also no particular limitation on the crystallinity of a semiconductor film used for the TFT. For example, an amorphous semiconductor film, a crystalline semiconductor film, an oxide semiconductor film, or the like can be used.

[0116] It should be noted that the structure described in this embodiment may be combined with any of the structures described in the other embodiments as appropriate. (Embodiment 3)

[0117] In this embodiment, as one embodiment of the present invention, a light-emitting element (hereinafter referred to as a tandem light-emitting element) in which a charge generation layer is provided between a plurality of EL layers is described.

[0118] The light-emitting element described in this embodiment is a tandem light-emitting element including a plurality of EL layers (a first EL layer 302(1) and a second EL layer 302(2)) between a pair of electrodes (a first electrode 301 and a second electrode 304) as shown in Fig. 6A is shown.

[0119] In this embodiment, the first electrode 301 serves as an anode, and the second electrode 304 serves as a cathode. Note that the first electrode 301 and the second electrode 304 may have structures similar to those described in Embodiment 1. In addition, all or any of the plurality of EL layers (the first EL layer 302(1) and the second EL layer 302(2)) may have structures similar to those described in Embodiment 1 or 2. In other words, the structures of the first EL layer 302(1) and the second EL layer 302(2) may be the same as or different from each other, and may be similar to those of the EL layers described in Embodiment 1 or 2.

[0120] Furthermore, a charge generation layer (I) 305 is provided between the plurality of EL layers (the first EL layer 302(1) and the second EL layer 302(2)). The charge generation layer (I) 305 has a function of injecting electrons into one of the EL layers and injecting holes into the other of the EL layers when a voltage is applied between the first electrode 301 and the second electrode 304. In this embodiment, when a voltage is applied such that the potential of the first electrode 301 is higher than that of the second electrode 304, the charge generation layer (I) 305 injects electrons into the first EL layer 302(1) and injects holes into the second EL layer 302(2).

[0121] Note that, in terms of light extraction efficiency, the charge generation layer (I) 305 preferably has a visible light transmittance (specifically, the charge generation layer (I) 305 preferably has a visible light transmittance of 40% or more). Furthermore, the charge generation layer (I) 305 functions even if it has a lower conductivity than the first electrode 301 or the second electrode 304.

[0122] The charge generation layer (I) 305 may have a structure in which an electron acceptor (Acceptor) of an organic compound with a high hole-transport property is added, or a structure in which an electron donor (Donor) of an organic compound with a high electron-transport property is added. Alternatively, both of these structures may be stacked.

[0123] In the case where the electron acceptor is added to the organic compound having a high hole-transport property, examples of the organic compound having a high hole-transport property include aromatic amine compounds such as NPB, TPD, TDATA, MTDATA, and 4,4'-bis[N-(spiro-9,9'-bifluoren-2-yl)-N-phenylamino]biphenyl (abbreviation: BSPB), and the like. The substances mentioned here are mainly substances having a hole mobility of 10 -6 cm 2 / Vs or more. It should be noted that, in addition to these substances, any organic compound that has the property of transporting more holes than electrons can be used.

[0124] Examples of the electron acceptor include 7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation: F4-TCNQ), chloranil, transition metal oxides, and oxides of metals belonging to groups 4 to 8 of the periodic table. In particular, vanadium oxide, niobium oxide, tantalum oxide, chromium oxide, molybdenum oxide, tungsten oxide, manganese oxide, and rhenium oxide are preferred due to their high electron-accepting properties. Among these, molybdenum oxide is particularly preferred because of its stability in air, low hygroscopicity, and ease of handling.

[0125] On the other hand, in the case where the electron donor is added to the organic compound having a high electron transport property, examples of the usable organic compound having a high electron transport property are metal complexes having a quinoline skeleton or a benzoquinoline skeleton, such as Alq, Almq3, BeBq2, and BAlq, and the like. Other examples are metal complexes having an oxazole-based or thiazole-based ligand, such as Zn(BOX)2 and Zn(BTZ)2. Apart from metal complexes, PBD, OXD-7, TAZ, BPhen, BCP, or the like can be used. The substances specified here are mainly substances having an electron mobility of 10 -6 cm 2 / Vs or more. It should be noted that, in addition to these substances, any organic compound that has the property of transporting more electrons than holes can be used.

[0126] Examples of usable electron donors include alkali metals, alkaline earth metals, rare earth metals, metals belonging to Group 13 of the Periodic Table, and oxides or carbonates thereof. Particularly preferred are lithium (Li), cesium (Cs), magnesium (Mg), calcium (Ca), ytterbium (Yb), indium (In), lithium oxide, cesium carbonate, and the like. An organic compound such as tetrathianaphthacene can be used as the electron donor.

[0127] Note that forming the charge generation layer (I) 305 using any of the above materials can suppress an increase in operating voltage caused by the stacking of EL layers.

[0128] Although this embodiment shows the light-emitting element having two EL layers, the present invention can similarly be applied to a light-emitting element having n EL layers (where n is 3 or more) as shown in Fig.6B. In the case where a plurality of EL layers are included between a pair of electrodes as in the light-emitting element according to this embodiment, by providing the charge generation layers (I) between the EL layers, light emission in a high luminance range can be achieved while maintaining a low current density. Since a low current density can be maintained, the element can have a long lifetime. When the light-emitting element is used for lighting, voltage drop due to a resistance of an electrode material can be reduced, resulting in uniform light emission in a large area. In addition, it is possible to obtain a light-emitting device that can be operated at a low voltage and has low power consumption.

[0129] Furthermore, when the emission colors of the EL layers are different, light of a desired color can be obtained from the light-emitting element as a whole. For example, in a light-emitting element having two EL layers, the emission colors of a first and a second EL layer are complementary, so that the light-emitting element as a whole can emit white light. Note that the term "complementary" refers to a color relationship in which an achromatic color is obtained when colors are mixed. That is, white light emission can be obtained when light emitted by substances whose emission colors are complementary colors is mixed.

[0130] Furthermore, the same applies to a light-emitting element with three EL layers. The light-emitting element can emit white light as a whole if, for example, 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.

[0131] It should be noted that the structure described in this embodiment may be combined with any of the structures described in the other embodiments as appropriate. (Embodiment 4)

[0132] In this embodiment, a light-emitting device which is an embodiment of the present invention will be described.

[0133] A light-emitting device described in this embodiment has an optical microresonator (microcavity) structure that utilizes a light resonance effect between a pair of electrodes. The light-emitting device includes a plurality of light-emitting elements, each having at least one EL layer 405 between a pair of electrodes (a reflective electrode 401 and a semi-transparent and semi-reflective electrode 402), as shown in Fig.7. Furthermore, the EL layer 405 includes at least light-emitting layers 404 (404R, 404G, and 404B), each serving as a light-emitting region, and may further include a hole-injection layer, a hole-transport layer, an electron-transport layer, an electron-injection layer, a charge-generation layer (E), and the like. Note that the light-emitting layers 404 (404R, 404G, and 404B) have the structure of the light-emitting layer according to one embodiment of the present invention, which is described in Embodiment 1 or 2.

[0134] In this embodiment, a light-emitting device is described which includes light-emitting elements (a first light-emitting element (R) 410R, a second light-emitting element (G) 410G and a third light-emitting element (B) 410B) having different structures as shown in Fig. 7 includes.

[0135] The first light-emitting element (R) 410R has a structure in which a first transparent conductive layer 403a; an EL layer 405 including a first light-emitting layer (B) 404B, a second light-emitting layer (G) 404G, and a third light-emitting layer (R) 404R as parts; and a semi-transparent and semi-reflective electrode 402 are sequentially stacked above a reflective electrode 401. The second light-emitting element (G) 410G has a structure in which a second transparent conductive layer 403b, the EL layer 405, and the semi-transparent and semi-reflective electrode 402 are sequentially stacked above the reflective electrode 401. The third light-emitting element (B) 410B has a structure in which the EL layer 405 and the semi-transparent and semi-reflective electrode 402 are sequentially stacked over the reflective electrode 401.

[0136] Note that the reflective electrode 401, the EL layer 405, and the semi-transparent and semi-reflective electrode 402 are common to the light-emitting elements (the first light-emitting element (R) 410R, the second light-emitting element (G) 410G, and the third light-emitting element (B) 410B). The first light-emitting layer (B) 404B emits light (λ B ) with a peak in a wavelength range of 420 nm to 480 nm. The second light-emitting layer (G) 404G emits light (λ G ) with a peak in a wavelength range of 500 nm to 550 nm. The third light-emitting layer (R) 404R emits light (λ R) with a peak in a wavelength range of 600 nm to 760 nm. Therefore, in each of the light-emitting elements (the first light-emitting element (R) 410R, the second light-emitting element (G) 410G, and the third light-emitting element (B) 410B), light emitted from the first light-emitting layer (B) 404B, light emitted from the second light-emitting layer (G) 404G, and light emitted from the third light-emitting layer (R) 404R overlap with each other. Accordingly, light with a broad emission spectrum including a visible light range can be emitted. Note that the above wavelengths represent the relationship of λ B < λ G < λ R fulfill.

[0137] Each of the light-emitting elements described in this embodiment has a structure in which the EL layer 405 is provided between the reflective electrode 401 and the semi-transparent and semi-reflective electrode 402. Light emitted from the light-emitting layers in the EL layer 405 in all directions resonates with the reflective electrode 401 and the semi-transparent and semi-reflective electrode 402, which serve as an optical micro-resonator (micro-cavity). Note that the reflective electrode 401 is formed using a conductive material with reflective ability, and a film is used whose visible light reflectivity is 40% to 100%, preferably 70% to 100%, and whose specific resistance is 1 × 10 -2Ωcm or lower. In addition, the semi-transparent and semi-reflective electrode 402 is formed using a conductive material having reflectivity and a conductive material having light transmittance, and a film is used whose visible light reflectivity is 20% to 80%, preferably 40% to 70%, and whose specific resistance is 1 × 10 -2 Ωcm or lower.

[0138] In this embodiment, the thicknesses of the transparent conductive layers (the first transparent conductive layer 403a and the second transparent conductive layer 403b) provided in the first light-emitting element (R) 410R and the second light-emitting element (G) 410G, respectively, differ between the light-emitting elements, so that the light-emitting elements differ from each other in the optical path length from the reflective electrode 401 to the semi-transparent and semi-reflective electrode 402.In other words, in light with a broad emission spectrum emitted from the light-emitting layers of each of the light-emitting elements, light with a wavelength that resonates between the reflective electrode 401 and the semi-transparent and semi-reflective electrode 402 can be amplified, while light with a wavelength that does not resonate between them can be attenuated. Therefore, when the elements differ in the optical path length from the reflective electrode 401 to the semi-transparent and semi-reflective electrode 402, light with different wavelengths can be extracted.

[0139] Note that the optical path length (also referred to as optical distance) is represented as a product of an actual distance and a refractive index, and in this embodiment, it is a product of an actual thickness and n (refractive index). That is, an optical path length = actual thickness × n.

[0140] Furthermore, the total thickness from the reflective electrode 401 to the semi-transparent and semi-reflective electrode 402 is mλ R / 2 (m is a natural number) at the first light-emitting element (R) 410R. The total thickness from the reflective electrode 401 to the semi-transparent and semi-reflective electrode 402 is set to mλ G / 2 (m is a natural number) at the second light-emitting element (G) 410G. The total thickness from the reflective electrode 401 to the semi-transparent and semi-reflective electrode 402 is set to mλ B / 2 (m is a natural number) at the third light-emitting element (B) 410B.

[0141] In this way, the light (λ R ) emitted from the third light-emitting layer (R) 404R in the EL layer 405 is mainly taken from the first light-emitting element (R) 410R, which emits light (λ G ) emitted from the second light-emitting layer (G) 404G in the EL layer 405 is mainly taken from the second light-emitting element (G) 410G, and the light (λ B ) emitted from the first light-emitting layer (B) 404B in the EL layer 405 is mainly taken from the third light-emitting element (B) 410B. Note that the light taken from each of the light-emitting elements is emitted from the side of the semi-transparent and semi-reflective electrode 402.

[0142] Furthermore, strictly speaking, the total thickness from the reflective electrode 401 to the semi-transparent and semi-reflective electrode 402 may be the total thickness from a reflection region in the reflective electrode 401 to a reflection region in the semi-transparent and semi-reflective electrode 402. However, it is difficult to determine precise positions of the reflection regions in the reflective electrode 401 and the semi-transparent and semi-reflective electrode 402. Therefore, it is believed that the above effect can be sufficiently obtained wherever the reflection regions in the reflective electrode 401 and the semi-transparent and semi-reflective electrode 402 are located.

[0143] Furthermore, in the first light-emitting element (R) 410R, the optical path length from the reflective electrode 401 to the third light-emitting layer (R) 404R is set to a desired thickness ((2m'+1)λ R / 4, where m' is a natural number). Thus, light emitted from the third light-emitting layer (R) 404R can be amplified. Light (first reflected light) reflected by the reflective electrode 401 from the light emitted by the third light-emitting layer (R) 404R interferes with light (first incident light) directly entering the semi-transparent and semi-reflective electrode 402 from the third light-emitting layer (R) 404R. Therefore, by setting the optical path length from the reflective electrode 401 to the third light-emitting layer (R) 404R to the desired value ((2m'+1)λ R / 4, where m' is a natural number), the phases of the first reflected light and the first incident light can be adjusted to each other, and the light emitted from the third light-emitting layer (R) 404R can be amplified.

[0144] Note that, strictly speaking, the optical path length from the reflective electrode 401 to the third light-emitting layer (R) 404R may be the optical path length from a reflection region in the reflective electrode 401 to a light-emitting region in the third light-emitting layer (R) 404R. However, it is difficult to determine precise positions of the reflection region in the reflective electrode 401 and the light-emitting region in the third light-emitting layer (R) 404R. Therefore, it is believed that the above effect can be sufficiently obtained wherever the reflection region and the light-emitting region are located in the reflective electrode 401 and the third light-emitting layer (R) 404R, respectively.

[0145] Furthermore, in the second light-emitting element (G) 410G, the optical path length from the reflective electrode 401 to the second light-emitting layer (G) 404G is set to a desired thickness ((2m''+1)λ G / 4, where m'' is a natural number). Thus, light emitted from the second light-emitting layer (G) 404G can be amplified. Light (second reflected light) reflected by the reflective electrode 401 from the light emitted by the second light-emitting layer (G) 404G interferes with light (second incident light) directly entering the semi-transparent and semi-reflective electrode 402 from the second light-emitting layer (G) 404G. Therefore, by setting the optical path length from the reflective electrode 401 to the second light-emitting layer (G) 404G to the desired value ((2m'+1)λ G / 4, where m'' is a natural number), the phases of the second reflected light and the second incident light can be adjusted to each other, and the light emitted from the second light-emitting layer (G) 404G can be amplified.

[0146] Note that, strictly speaking, the optical path length from the reflective electrode 401 to the second light-emitting layer (G) 404G may be the optical path length from a reflection region in the reflective electrode 401 to a light-emitting region in the second light-emitting layer (G) 404G. However, it is difficult to determine precise positions of the reflection region in the reflective electrode 401 and the light-emitting region in the second light-emitting layer (G) 404G. Therefore, it is believed that the above effect can be sufficiently obtained wherever the reflection region and the light-emitting region are located in the reflective electrode 401 and the second light-emitting layer (G) 404G, respectively.

[0147] Furthermore, in the third light-emitting element (B) 410B, the optical path length from the reflective electrode 401 to the first light-emitting layer (B) 404B is set to a desired thickness ((2m'''+1)λ B / 4, where m''' is a natural number). Thus, light emitted from the first light-emitting layer (B) 404B can be amplified. Light (third reflected light) reflected by the reflective electrode 401 from the light emitted by the first light-emitting layer (B) 404B interferes with light (third incident light) directly entering the semi-transparent and semi-reflective electrode 402 from the first light-emitting layer (B) 404B. Therefore, by setting the optical path length from the reflective electrode 401 to the first light-emitting layer (B) 404B to the desired value ((2m'''+1)λ B / 4, where m''' is a natural number), the phases of the third reflected light and the third incident light are adjusted to each other, and the light emitted from the first light-emitting layer (B) 404B can be amplified.

[0148] Note that, strictly speaking, the optical path length from the reflective electrode 401 to the first light-emitting layer (B) 404B in the third light-emitting element may be the optical path length from a reflection region in the reflective electrode 401 to a light-emitting region in the first light-emitting layer (B) 404B. However, it is difficult to determine precise positions of the reflection region in the reflective electrode 401 and the light-emitting region in the first light-emitting layer (B) 404B. Therefore, it is believed that the above effect can be sufficiently obtained wherever the reflection region and the light-emitting region are located in the reflective electrode 401 and the first light-emitting layer (B) 404B, respectively.

[0149] Note that although each of the light-emitting elements in the above-described structure includes a plurality of light-emitting layers in the EL layer, the present invention is not limited thereto. For example, the structure of the tandem light-emitting element described in Embodiment 3 may be combined, in which case a plurality of EL layers and an intermediate charge generation layer are provided in one light-emitting element, and one or more light-emitting layers are formed in each of the EL layers.

[0150] The light-emitting device described in this embodiment has a microcavity structure in which light with wavelengths different depending on the light-emitting elements can be extracted even though they include the same EL layer. This eliminates the need to manufacture light-emitting elements for colors of R, G, and B. The above structure is therefore advantageous for color display due to the ease with which a higher-resolution display or the like can be achieved. Note that combination with color layers (color filters) is also possible. In addition, the intensity of emission with a predetermined wavelength can be increased in the forward direction, thereby reducing power consumption.The above structure is useful particularly in the case where it is applied to a color display (image display device) including pixels of three or more colors, but it can also be applied to lighting or the like. (Embodiment 5)

[0151] In this embodiment, a light-emitting device including a light-emitting element which is an embodiment of the present invention will be described.

[0152] The light-emitting device may be a passive matrix light-emitting device or an active matrix light-emitting device. Note that any of the light-emitting elements described in the other embodiments may be applied to the light-emitting device described in this embodiment.

[0153] In this embodiment, an active matrix light-emitting device is manufactured using Fig. 8A and Fig. 8B.

[0154] It should be noted that Fig. 8A is a plan view showing a light-emitting device, and Fig. 8B is a cross-sectional view along the dashed line AA' in Fig. 8A. The active matrix light-emitting device according to this embodiment includes a pixel section 502 above an element substrate 501, a drive circuit section (a source line drive circuit) 503, and drive circuit sections (gate line drive circuits) 504 (504a and 504b). The pixel section 502, the drive circuit section 503, and the drive circuit sections 504 are sealed between the element substrate 501 and a sealing substrate 506 by means of a sealant 505.

[0155] Additionally, a lead wire 507 is provided above the element substrate 501. The lead wire 507 is provided to connect an external input terminal through which a signal (e.g., an image signal, a clock signal, a start signal, or a reset signal) or a potential from the outside is sent to the drive circuit section 503 and the drive circuit sections 504. Here, an example is shown in which a flexible printed circuit (FPC) 508 is provided as the external input terminal. Although only the FPC is shown, this FPC may be provided with a printed wiring board (PWB). The light-emitting device in this specification includes in its category not only the light-emitting device itself but also the light-emitting device provided with the FPC or the PWB.

[0156] Next, a cross-sectional structure is created using Fig. 8B. The driver circuit sections and the pixel section are formed above the element substrate 501. Shown here are the driver circuit section 503, which is the source line driver circuit, and the pixel section 502.

[0157] The driver circuit section 503 is an example in which a CMOS circuit is formed, which is a combination of an N-channel TFT 509 and a P-channel TFT 510. Note that a circuit included in the driver circuit section may be formed using any of various circuits, such as a CMOS circuit, a PMOS circuit, or an NMOS circuit. In this embodiment, a driver-integrated type in which a driver circuit is formed above the substrate is described; however, the present invention is not limited to this type, and the driver circuit may also be formed outside the substrate.

[0158] The pixel section 502 includes a plurality of pixels, each including a TFT 511 for switching, a TFT 512 for current control, and a first electrode (anode) 513 electrically connected to a line (a source electrode or a drain electrode) of the TFT 512 for current control. Note that an insulator 514 is formed to cover end portions of the first electrode (anode) 513. In this embodiment, the insulator 514 is formed using a positive photosensitive acrylic resin.

[0159] The insulator 514 preferably has a curved surface with a curvature in its upper end region or its lower end region to obtain good coverage with a film that is subsequently stacked over the insulator 514. For example, in the case of using a positive photosensitive acrylic resin as a material for the insulator 514, the insulator 514 preferably has a curved surface with a radius of curvature (0.2 µm to 3 µm) in the upper end region. The insulator 514 can be formed using a negative photosensitive resin or a positive photosensitive resin. It is possible to use either an organic compound or an inorganic compound, such as silicon oxide or silicon oxynitride, without being limited to an organic compound.

[0160] An EL layer 515 and a second electrode (cathode) 516 are stacked above the first electrode (anode) 513. At least one light-emitting layer is provided in the EL layer 515. The light-emitting layer has a stacked structure as described in Embodiment 1. In the EL layer 515, a hole-injection layer, a hole-transport layer, an electron-transport layer, an electron-injection layer, a charge-generation layer, and the like may also be provided as needed in addition to the light-emitting layer.

[0161] The layer stack structure including the first electrode (anode) 513, the EL layer 515, and the second electrode (cathode) 516 forms a light-emitting element 517. The materials described in Embodiment 2 can be used for the first electrode (anode) 513, the EL layer 515, and the second electrode (cathode) 516. Although not shown, the second electrode (cathode) 516 is electrically connected to the FPC 508, which is an external input terminal.

[0162] The cross-sectional view in Fig.Although FIG. 8B shows only one light-emitting element 517, a plurality of light-emitting elements are arranged in a matrix in the pixel section 502. Light-emitting elements exhibiting three types of light emission (R, G, and B) are selectively formed in the pixel section 502, whereby a light-emitting device capable of color display can be manufactured. Alternatively, a light-emitting device capable of color display can be manufactured by combining it with color layers (color filters).

[0163] Furthermore, the sealing substrate 506 is attached to the element substrate 501 through the sealant 505, whereby the light-emitting element 517 is located in a space 518 surrounded by the element substrate 501, the sealing substrate 506, and the sealant 505. The space 518 may be filled with an inert gas (such as nitrogen or argon) or the sealant 505.

[0164] An epoxy-based resin, low-melting glass, or the like is preferably used for the sealant 505. It is preferable that such a material permits as little moisture or oxygen permeation as possible. A glass substrate, a quartz substrate, or a plastic substrate formed of fiberglass reinforced plastic (FRP), poly(vinyl fluoride) (PVF), polyester, acrylic, or the like can be used as the sealant substrate 506.

[0165] In the manner described above, an active matrix light-emitting device can be obtained.

[0166] It should be noted that the structure described in this embodiment may be combined with any of the structures described in the other embodiments as appropriate. (Embodiment 6)

[0167] In this embodiment, examples of various electronic devices completed using a light-emitting device are described with reference to Fig. 9A to Fig. 9D and Fig. 10A to Fig. 10C. The light-emitting device is manufactured using a light-emitting element that is an embodiment of the present invention.

[0168] Examples of electronic devices in which the light-emitting device is used are television sets (also referred to as TV or television receivers), monitors for computers and the like, cameras such as digital cameras and digital video cameras, digital photo frames, mobile phones (also referred to as portable telephone devices), portable game devices, portable information terminals, audio playback devices, large game devices such as pinball machines and the like. Specific examples of these electronic devices are given in Fig.9A to Fig. 9D shown.

[0169] Fig. 9A shows an example of a television set. In a television set 7100, a display section 7103 is installed in a housing 7101. The display section 7103 can display images, and a light-emitting device can be used for the display section 7103. In addition, the housing 7101 is supported by a stand 7105.

[0170] The television set 7100 can be operated using an operating switch in the housing 7101 or a separate remote control 7110. Operation buttons 7109 of the remote control 7110 can control the television channels and volume, and images displayed on the display section 7103. Furthermore, the remote control 7110 can include a display section 7107 for displaying data output from the remote control 7110.

[0171] It should be noted that the television set 7100 is equipped with a receiver, a modem, and the like. The receiver can receive general television broadcasts. Furthermore, when the television set 7100 is connected to a communications network wired or wirelessly via the modem, unidirectional (from a transmitter to a receiver) or bidirectional (between a transmitter and a receiver, between receivers, or the like) data communication can be performed.

[0172] Fig. 9B shows a computer including a main body 7201, a casing 7202, a display section 7203, a keyboard 7204, an external connection port 7205, a pointing device 7206, and the like. Note that this computer is manufactured using a light-emitting device for the display section 7203.

[0173] Fig.Figure 9C shows a portable game device including two housings, i.e., a housing 7301 and a housing 7302, which are connected to each other via a hinge part 7303 so that the portable game device can be opened or closed. A display section 7304 is installed in the housing 7301, and a display section 7305 is installed in the housing 7302. In addition, the portable game device includes Fig.9C, a speaker section 7306, a recording medium insertion section 7307, an LED lamp 7308, input means (an operation button 7309, a connection terminal 7310, a sensor 7311 (a sensor with a function for measuring force, displacement, position, speed, acceleration, angular velocity, rotational speed, distance, light, liquid, magnetism, temperature, chemical substance, sound, time, hardness, electric field, electric current, voltage, electric power, radiation, flow rate, humidity, gradient, vibration, odor, or infrared rays), and a microphone 7312), and the like. Of course, the structure of the portable game device is not limited to the above structure as long as a light-emitting device is used for the display section 7304 and / or the display section 7305, and may include other accessories as necessary. The portable game device in Fig.9C has a function for reading a program or data stored in a storage medium to display it on the display section, and a function for sharing information with another portable game device via wireless communication. Note that the portable game device in Fig. 9C can have various functions without limitation to the above functions.

[0174] Fig. 9D shows an example of a mobile phone. A mobile phone 7400 includes a display portion 7402 installed in a housing 7401, operation buttons 7403, an external connection terminal 7404, a speaker 7405, a microphone 7406, and the like. Note that the mobile phone 7400 is manufactured using a light-emitting device for the display portion 7402.

[0175] When the display section 7402 of the mobile phone 7400 in Fig.9D is touched with a finger or the like, data can be input into the mobile phone 7400. Furthermore, operations such as making a call and writing an email can be performed by touching the display section 7402 with a finger or the like.

[0176] There are mainly three screen modes for the display section 7402. The first mode is a display mode, which mainly displays an image. The second mode is an input mode, which mainly inputs information such as letters. The third mode is a display and input mode, which combines two modes: display mode and input mode.

[0177] For example, in the case of making a call or writing an email, a letter input mode in which mainly letters are input is selected for the display section 7402 so that letters displayed on the screen can be input. In this case, it is preferable to display a keyboard or number buttons on almost the entire screen of the display section 7402.

[0178] When a detector device having a sensor for detecting inclination, such as a gyroscope or an acceleration sensor, is provided within the mobile phone 7400, the display on the screen of the display section 7402 can be automatically changed by determining the direction of the mobile phone 7400 (whether the mobile phone is placed horizontally or vertically for a landscape or portrait orientation).

[0179] The screen modes are changed by touching the display section 7402 or operating the operation buttons 7403 of the body 7401. Alternatively, the screen modes can be changed depending on the type of image displayed on the display section 7402. For example, if a signal for an image to be displayed on the display section is moving image data, the screen mode is changed to the display mode. If the signal is text data, the screen mode is changed to the input mode.

[0180] Furthermore, in the input mode, when a signal detected by an optical sensor in the display section 7402 is detected and input by touching the display section 7402 is not performed for a certain period of time, the screen mode can be controlled to change from the input mode to the display mode.

[0181] The display section 7402 can serve as an image sensor. By touching the display section 7402 with a palm or finger, for example, an image of a palm print, a fingerprint, or the like is captured, thereby enabling authentication. Furthermore, if a backlight or a scanning light source that emits near-infrared light is provided in the display section, an image of a finger vein, a palm vein, or the like can also be captured.

[0182] Fig. 10A and Fig. 10B show a foldable tablet computer. The tablet computer is in Fig.10A opened. The tablet computer includes a housing 9630, a display section 9631a, a display section 9631b, a display mode switch 9034, a power switch 9035, a power saver switch 9036, a clasp 9033, and an operation switch 9038. The tablet computer is manufactured using the light-emitting device for the display section 9631a and / or the display section 9631b.

[0183] Part of the display section 9631a may be a touchscreen area 9632a, and data can be input when a displayed operation button 9637 is touched. Although a structure is shown as an example in which one half of the display section 9631a has only a display function and the other half also has a touchscreen function, the display section 9631a is not limited to this structure. The entire area of ​​the display section 9631a may have a touchscreen function. For example, the display section 9631a may display keyboard buttons in the entire area intended to be a touchscreen, and the display section 9631b may be used as a display screen.

[0184] As with the display section 9631a, a part of the display section 9631b may be a touchscreen area 9632b. When a button 9639 for switching a keyboard display displayed on the touchscreen is touched with a finger, a stylus, or the like, a keyboard may be displayed on the display section 9631b.

[0185] An input operation by touch can be performed in the touch screen area 9632a and the touch screen area 9632b at the same time.

[0186] For example, the display mode switch 9034 can switch the display between portrait mode, landscape mode, and the like, and between monochrome display and color display. The power saving switch 9036 can control the display luminance according to the amount of external light when using the tablet computer, which is detected by an optical sensor in the tablet computer. In addition to the optical sensor, another detection device with a sensor for detecting inclination, such as a gyroscope or an acceleration sensor, can be installed in the tablet computer.

[0187] Fig.10A shows an example in which display section 9631a and display section 9631b have the same display area. However, without limitation, one of the display sections may be different from the other in terms of size and display quality. For example, one display section may display an image with a higher resolution than the other display section.

[0188] The tablet computer is in Fig. 10B closed. The tablet computer includes the housing 9630, a solar cell 9633, a charge and discharge control circuit 9634, a battery 9635 and a DC-DC converter 9636. In Fig. 10B shows a structure including the battery 9635 and the DC-DC converter 9636 as an example of the charge and discharge control circuit 9634.

[0189] Since the tablet computer is foldable, the case 9630 can be closed when the tablet computer is not in use. Consequently, the display portion 9631a and the display portion 9631b can be protected. Therefore, a tablet computer with excellent durability and reliability for long-term use can be provided.

[0190] Furthermore, the tablet computer can be used in Fig. 10A and Fig. 10B may have a function of displaying various types of data (e.g., a still image, a moving image, and a text image), a function of displaying a calendar, a date, a time, or the like on the display portion, a touch input function for operation or editing of the data displayed on the display portion, a function of controlling processing by various types of software (programs), and the like.

[0191] The solar cell 9633 on a surface of the tablet computer can supply power to the touchscreen, the display section, an image signal processing section, or the like. Note that the solar cell 9633 can be provided on one or both surfaces of the case 9630 so that the battery 9635 can be charged efficiently. Using a lithium-ion battery as the battery 9635 is advantageous for miniaturization and the like.

[0192] The structure and operation of the charge and discharge control circuit 9634 shown in Fig. 10B are shown using a block diagram in Fig. 10C. The solar cell 9633, the battery 9635, the DC-DC converter 9636, a converter 9638, switches SW1 to SW3 and a display section 9631 are shown in Fig.10C, and the battery 9635, the DC-DC converter 9636, the converter 9638 and the switches SW1 to SW3 correspond to the charge and discharge control circuit 9634 in Fig. 10B.

[0193] First, an example of operation in the case where power is generated by the solar cell 9633 using external light will be described. The voltage corresponding to the power generated by the solar cell 9633 is increased or decreased by the DC-DC converter 9636 so that it corresponds to a voltage required for charging the battery 9635. Then, when the power from the solar cell 9633 is used to operate the display section 9631, the switch SW1 is turned on, and the voltage corresponding to the power is increased or decreased by the converter 9638 so that it corresponds to a voltage required for the display section 9631. Further, when there is no display on the display section 9631, the switch SW1 is turned off, and the switch SW2 is turned on so that the battery 9635 can be charged.

[0194] It should be noted that while the solar cell 9633 is described as an example of a power generation means, the battery 9635 may also be charged using, but is not limited to, another power generation means, such as a piezoelectric element or a thermoelectric conversion element (Peltier element). For example, a non-contact power transfer module that wirelessly (without contact) transmits and receives power to charge the battery 9635, or a combination of the solar cell 9633 and another charging means may be used.

[0195] Of course, an embodiment of the present invention is not limited to the electronic device used in Fig. 10A to Fig. 10C, as long as the display section described in the above embodiment is included therein.

[0196] As described above, electronic devices can be obtained by using the light-emitting device, which is an embodiment of the present invention. The light-emitting device has a very wide range of applications and can be applied to electronic devices in various fields.

[0197] It should be noted that the structure described in this embodiment may be combined with any of the structures described in the other embodiments as appropriate. (Embodiment 7)

[0198] In this embodiment, examples of lighting devices are described using Fig. 11. A light-emitting device including a light-emitting element, which is an embodiment of the present invention, finds application in lighting devices.

[0199] Fig.11 shows an example in which a light-emitting device is used for an indoor lighting device 8001. Since the light-emitting device can have a larger area, a lighting device with a large area can also be manufactured. In addition, a lighting device 8002 in which a light-emitting region has a curved surface can also be manufactured using a housing with a curved surface. A light-emitting element included in the light-emitting device described in this embodiment is in the form of a thin film, which allows the housing to be designed more freely. Thus, the lighting device can be designed in various artistic ways. In addition, a large lighting device 8003 can be mounted on a wall of the room.

[0200] Furthermore, when the light-emitting device is used as a surface of a table, a lighting device 8004 having a function as a table can be obtained. When the light-emitting device is used as a part of other furniture, a lighting device having a function of the furniture can be obtained.

[0201] In this way, various lighting devices in which the light-emitting device is used can be obtained. It should be noted that such lighting devices are also embodiments of the present invention.

[0202] The structure described in this embodiment may be combined with any of the structures described in the other embodiments as appropriate. [Example 1]

[0203] In this example, a light-emitting element 1 which is an embodiment of the present invention is described with reference to Fig. 12. Chemical formulas of the materials used in this example are given below. ((Production of the light-emitting element 1〉〉

[0204] First, a film of indium tin oxide containing silicon oxide (ITSO) was formed over a glass substrate 1100 by a sputtering method, forming a first electrode 1101 serving as an anode. Note that the thickness was 110 nm and the electrode area was 2 mm × 2 mm.

[0205] Next, as a pretreatment for manufacturing the light-emitting element 1 over the substrate 1100, UV ozone treatment was performed for 370 seconds after a surface of the substrate was washed with water and heated at 200 °C for one hour.

[0206] The substrate was then transferred to a vacuum evaporation device, where the pressure was increased to approximately 10 -4 Pa, and was heated in a heating chamber of the vacuum evaporation device at 170 °C for 30 minutes in vacuum, and then the substrate 1100 was cooled for about 30 minutes.

[0207] Then, the substrate 1100 was fixed to a holder in the vacuum evaporation device such that a surface on which the first electrode 1101 was deposited was provided facing downward. In this example, a case will be described in which a hole injection layer 1111, a hole transport layer 1112, a light-emitting layer 1113, an electron transport layer 1114, and an electron injection layer 1115 included in an EL layer 1102 were sequentially formed by a vacuum evaporation method.

[0208] The pressure in the vacuum evaporation device was set to about 10 -4Pa. Then, 1,3,5-tri(dibenzothiophen-4-yl)benzene (abbreviation: DBT3P-II) and molybdenum(VI) oxide were co-evaporated, with the mass ratio of DBT3P-II (abbreviation) to molybdenum oxide being 4:2. This formed the hole-injection layer 1111 over the first electrode 1101. The thickness of the hole-injection layer 1111 was 20 nm. It should be noted that co-evaporation is an evaporation process in which a plurality of different substances are simultaneously evaporated from different evaporation sources.

[0209] Next, the hole transport layer 1112 with a thickness of 20 nm was formed by evaporation of 4-phenyl-4'-(9-phenylfluoren-9-yl)triphenylamine (abbreviation: BPAFLP).

[0210] Thereafter, the light-emitting layer 1113 was formed over the hole-transport layer 1112. The light-emitting layer 1113 having a layer-stack structure was formed as follows.A first light-emitting layer 1113a with a thickness of 20 nm was formed by co-evaporation of 2-[3'-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline (abbreviation: 2mDBTBPDBq-II), N-(1,1'-biphenyl-4-yl)-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-9,9-dimethyl-9H-fluoren-2-amine (abbreviation: PCBBiF) and (acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III) (abbreviation: [Ir(tBuppm)2(acac)]), wherein the mass ratio of 2mDBTBPDBq-II (abbreviation) to PCBBiF (abbreviation) and [Ir(tBuppm)2(acac)] (abbreviation) was 0.7:0.3:0.05, and then a second light-emitting layer 1113b with a thickness of 20 nm was formed by co-evaporation of 2mDBTBPDBq-II (abbreviation), N,N'-bis(9,9-dimethylfluoren-2-yl)-N,N'-di(biphenyl-4-yl)-1,4-phenylenediamine (abbreviation: FBi2P) and bis{4,6-dimethyl-2-[5-(2,6-dimethylphenyl)-3-(3,5-dimethylphenyl)-2-pyrazinyl--κN]phenyl-κC}(2,8-dimethyl-4,6-nonanedionato-κ. 2O,O')iridium (III) (abbreviation: [Ir(dmdppr-dmp)2(divm)]) was formed, with the mass ratio of 2mDBTBPDBq-II (abbreviation) to FBi2P (abbreviation) and [Ir(dmdppr-dmp)2(divm)] (abbreviation) being 0.8:0.2:0.05.

[0211] Next, the electron-transport layer 1114 was formed over the light-emitting layer 1113 in such a manner that a 15 nm thick film of 2mDBTBPDBq-II (abbreviation) was formed by evaporation, and then a 10 nm thick film of bathophenanthroline (abbreviation: Bphen) was formed by evaporation. A 1 nm thick lithium fluoride film was then formed over the electron-transport layer 1114 by evaporation to form the electron-injection layer 1115.

[0212] Finally, an aluminum film with a thickness of 200 nm was formed over the electron injection layer 1115 as a second electrode 1103 serving as a cathode by evaporation. Thus, the light-emitting element 1 was fabricated. Note that in the previously described evaporation steps, evaporation was performed by a resistance heating method.

[0213] Table 1 shows an element structure of the light-emitting element 1 obtained as described above. [Table 1] FirstElectrode Hole injection layer Hole transport layer Light-emitting layer Electron transport layer Electron injection layer Second electrode Light-emitting element 1 ITSO(110 nm) DBT3P-II:MoOx(4:2 20 nm) BPAFLP(20 nm) * ** 2mDBTBPDBq-II (15 nm) Bphen(10 nm) LiF(1 nm) Al(200 nm) 2mDBTBPDBq-II:PCBBiF:[Ir(tBuppm)2(acac)] (0.7:0.3:0.05 20 nm) ** 2mDBTBPDBq-II:FBi2P:[Ir(dmdppr-dmp)2(divm)] (0.8:0.2:0.05 20 nm)

[0214] Sealing of the fabricated light-emitting element 1 was performed in a glove box containing a nitrogen atmosphere so that it was not exposed to the air (specifically, a sealant was painted on an outer edge of the element and heat treatment was performed at 80 °C for one hour at the time of sealing). ((Operating characteristics of the light-emitting element 1〉〉

[0215] The operating characteristics of the fabricated light-emitting element were measured. Note that the measurements were conducted at room temperature (in an atmosphere of 25 °C).

[0216] First shows Fig. 13 the current density-luminance characteristics of the light-emitting element 1. In Fig. 13 the vertical axis indicates the luminance (cd / m 2 ), and the horizontal axis indicates the current density (mA / cm 2 ). Fig. 14 shows the voltage-luminance characteristics of the light-emitting element 1. In Fig. 14 the vertical axis indicates the luminance (cd / m 2 ) and the horizontal axis indicates the voltage (V). Fig. 15 shows the luminance-current efficiency characteristics of the light-emitting element 1. In Fig.15, the vertical axis indicates the current efficiency (cd / A), and the horizontal axis indicates the luminance (cd / m 2 ). Fig. 16 shows the voltage-current characteristics of the light-emitting element 1. In Fig. 16 the vertical axis indicates current (mA) and the horizontal axis indicates voltage (V).

[0217] Fig. Figure 15 shows the high efficiency of the light-emitting element 1, which is an embodiment of the present invention. Table 2 below shows initial values ​​of the main characteristics of the light-emitting element 1 at a luminance of about 1000 cd / m 2 . [Table 2] Voltage (V) Current (mA) Current density (mA / cm2) Chromaticity(x, y) Luminance (cd / m 2 ) Power efficiency (cd / A) Power efficiency (Im / W) External quantum yield (%) Light-emitting element 1 3 0,064 1,6 (0,51, 0,48) 1000 62 64 22

[0218] The above results show that the light-emitting element 1 fabricated in this example has high external quantum yield, which means that its emission efficiency is high.

[0219] Fig.Figure 17 shows an emission spectrum at the time of applying a current with a current density of 25 mA / cm 2 to the light-emitting element 1. As in Fig. As shown in Figure 17, the emission spectrum of the light-emitting element 1 exhibits two peaks at approximately 546 nm and 615 nm, indicating that the two peaks originate from the emission of the phosphorescent organometallic iridium complexes [Ir(tBuppm)2(acac)] and [Ir(dmdppr-dmp)2(divm)], respectively.

[0220] It should be noted that by cyclic voltammetry, the HOMO level of PCBBiF, which is the second organic compound with hole-transport properties, was estimated to be -5.36 eV, and the HOMO level of FBi2P, which is the third organic compound with hole-transport properties, was estimated to be -5.13 eV. Thus, the second organic compound has a lower HOMO level than the third organic compound.

[0221] Fig.Figure 18 shows an emission spectrum of a thin film of 2mDBTBPDBq-II, which is the first organic compound with electron transport properties, an emission spectrum of a thin film of PCBBiF, which is the second organic compound with hole transport properties, and an emission spectrum of a mixed film (co-evaporation film) of 2mDBTBPDBq-II and PCBBiF. Furthermore, Fig. 19 shows an emission spectrum of a thin film of 2mDBTBPDBq-II, which is the first organic compound with an electron transport property, an emission spectrum of a thin film of FBi2P, which is the third organic compound with a hole transport property, and an emission spectrum of a mixed film (co-evaporation film) of 2mDBTBPDBq-II and FBi2P.

[0222] Both in Fig. 18 as well as Fig.19, the emission wavelength of the mixed film is on a longer wavelength side relative to the emission wavelengths of the individual films of the materials. This indicates that the first organic compound (2mDBTBPDBq-II) and the second organic compound (PCBBiF) formed an exciplex, and that the first organic compound (2mDBTBPDBq-II) and the third organic compound (FBi2P) formed an exciplex. In addition, while the exciplex formed by the first organic compound (2mDBTBPDBq-II) and the third organic compound (FBi2P) has an emission wavelength of 553 nm, the exciplex formed by the first organic compound (2mDBTBPDBq-II) and the second organic compound (PCBBiF) has an emission wavelength of 511 nm. This indicates that the latter exciplex has higher energy.

Claims

[1] Display device comprising: a substrate (501); a transistor comprising an oxide semiconductor film over the substrate; and a tandem light-emitting element (517) comprising a plurality of EL layers between a pair of electrodes, the tandem light-emitting element overlying and electrically connected to the transistor, wherein a charge generation layer is provided between the plurality of EL layers, wherein one of the pair of electrodes serves as an anode and comprises ITO, where the emission colors of the EL layers differ from each other, wherein white light is obtained from the tandem light-emitting element as a whole, wherein one of the plurality of EL layers emits blue light and comprises an electron transport layer comprising a polyazole compound, wherein another of the plurality of EL layers comprises a first light-emitting layer comprising a first organic compound, a second organic compound and a first iridium complex, and a second light-emitting layer in contact with the first light-emitting layer, and wherein the first organic compound and the second organic compound form an exciplex. [2] The display device according to claim 1, wherein the other of the plurality of EL layers comprises a second light-emitting layer comprising a second iridium complex. [3] The display device according to claim 1, wherein an emission spectrum of the exciplex overlaps with an absorption band of the first iridium complex on the longest wavelength side. [4] The display device according to claim 2, further comprising: a second electron transport layer comprising a polyazole compound. [5] Display device comprising: a substrate (501), a transistor comprising an oxide semiconductor film over the substrate; and a tandem light-emitting element (517) comprising a first EL layer and a second EL layer between an anode and a cathode, the tandem light-emitting element being located above and electrically connected to the transistor, wherein a charge generation layer is provided between the first EL layer and the second EL layer, wherein the anode comprises ITO, wherein the emission colors of the first EL layer and the second EL layer are complementary, so that white light is obtained from the tandem light-emitting element as a whole, where the first EL layer emits blue light, wherein the second EL layer comprises a first light-emitting layer comprising a first organic compound, a second organic compound and a first iridium complex, and a second light-emitting layer in contact with the first light-emitting layer, wherein an electron transport layer comprising a polyazole compound is provided on a third light-emitting layer of the first EL layer, and wherein the first organic compound and the second organic compound form an exciplex. [6] The display device according to claim 5, wherein the second EL layer comprises the second light-emitting layer comprising a second iridium complex. [7] The display device according to claim 5, wherein an emission spectrum of the exciplex overlaps with an absorption band on the longest wavelength side of the first iridium complex. [8] A display device according to claim 6, further comprising: a second electron transport layer comprising a polyazole compound over the first light-emitting layer. [9] Display device comprising: a substrate (501); a transistor comprising an oxide semiconductor film over the substrate; and a tandem light-emitting element (517) comprising a first EL layer, a second EL layer and a third EL layer between a pair of electrodes, wherein a first charge generation layer is provided between the first EL layer and the second EL layer, wherein a second charge generation layer is provided between the second EL layer and the third EL layer, wherein one of the pair of electrodes serves as an anode and comprises ITO, wherein the emission colors of the first EL layer and the second EL layer differ from each other, wherein white light is obtained from the tandem light-emitting element as a whole, wherein one of the first EL layer and the third EL layer emits blue light, wherein the second EL layer comprises a first light-emitting layer comprising a first organic compound, a second organic compound and a first iridium complex, and wherein the first organic compound and the second organic compound form an exciplex. [10] The display device according to claim 9, wherein an electron transport layer comprising a polyazole compound is provided between a third light-emitting layer of the first EL layer and the first light-emitting layer, and wherein the second EL layer comprises a second light-emitting layer comprising a second iridium complex. [11] The display device according to claim 9, wherein an emission spectrum of the exciplex overlaps with an absorption band on the longest wavelength side of the first iridium complex. [12] The display device according to claim 10, wherein the first light-emitting layer is in contact with the second light-emitting layer. [13] A display device according to claim 9, wherein the first EL layer and the third EL layer emit the same color. [14] A display device according to claim 10, further comprising: a second electron transport layer comprising a polyazole compound between a fourth light-emitting layer of the third EL layer and the first light-emitting layer, wherein at least one organic compound in the first EL layer and the third EL layer is the same. [15] The display device according to claim 1 or 5, wherein the emission colors of the first light-emitting layer and the second light-emitting layer are different. [16] A display device according to claim 1 or 5, wherein the charge generation layer comprises an electron acceptor or an electron donor. [17] A display device according to claim 1 or 5, wherein the charge generation layer comprises lithium or lithium oxide.

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