Light-emitting device

Abstract

Light-emitting device comprising: a hole transport layer and a light-emitting layer between a pair of electrodes, wherein the hole transport layer is in contact with the light-emitting layer, wherein the light-emitting layer comprises a first organic compound, a second organic compound and a third organic compound which converts the triplet excitation energy into light emission, wherein a combination of the first organic compound and the second organic compound forms an exciplex, wherein a HOMO level of the first organic compound is lower than a HOMO level of the second organic compound, wherein a LUMO level of the first organic compound is lower than a LUMO level of the second organic compound, wherein the hole transport layer comprises a fourth organic compound whose HOMO level is higher than the HOMO level of the second organic compound, wherein the second organic compound comprises a carbazol-3-yl group, and wherein a difference between the energy value of a peak of an emission spectrum of the exciplex and the energy value of a peak of an absorption band on the lowest energy side of an absorption spectrum of the third organic compound is 0.2 eV or less.

Description

TECHNICAL FIELD

[0001] The present invention relates to a light-emitting element in which an organic compound that emits light upon application of an electric field is provided between a pair of electrodes. STATE OF THE ART

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

[0003] A light-emitting element is said to have the following light-emitting mechanism: When voltage is applied between a pair of electrodes between which a light-emitting layer comprising a luminous body is provided, electrons injected from the cathode and holes injected from the anode recombine at a light-emitting center of the light-emitting layer to form molecular excitons, and energy is released and light is emitted when the molecular excitons relax to the ground state. An excited singlet state and an excited triplet state are known as excited states, and light emission can presumably be achieved from either state. Light emission from the excited singlet state (S*) is called fluorescence, and light emission from the excited triplet state (T*) is called phosphorescence.

[0004] In order to improve the element properties of such light-emitting elements, improvements of an element structure, development of a material and the like have been actively carried out (see, for example, Patent Document 1). [Reference]

[0005] [Patent Document 1] Japanese Patent Laid-Open No. JP 2010-182699 A

[0006] Further relevant state of the art are KONDAKOVA, Marina E. [et al.]: High-efficiency, low-voltage phosphorescent organic light-emitting diode devices with mixed host. In: Journal of Applied Physics, Vol. 104, 2008, No. 9, pp. 094501-1 - 094501-17. ISSN 0021-8979 and GONG, Xiong [et al.]: Phosphorescence from iridium complexes doped into polymer blends. In: Journal of Applied Physics, Vol. 95, 2004, No. 3, pp. 948-953. ISSN 0021-8979. DISCLOSURE OF THE INVENTION

[0007] As disclosed in Patent Document 1, an improved element structure or the like has been developed; however, light-emitting elements still need to be improved in emission efficiency, reliability, and emission characteristics, and light-emitting elements with better characteristics are to be developed.

[0008] In view of the above, it is an object of one embodiment of the present invention to provide a light-emitting element which operates at a low voltage and has a high emission efficiency.

[0009] One embodiment of the present invention is a light-emitting element comprising a hole-transport layer between a pair of electrodes and a light-emitting layer on the hole-transport layer. The light-emitting layer contains a first organic compound having an electron-transport property, a second organic compound having a hole-transport property, and a light-emitting third organic compound that converts triplet excitation energy into light emission. A combination of the first organic compound and the second organic compound forms an exciplex. The hole-transport layer contains at least a fourth organic compound whose HOMO level is lower than or equal to that of the second organic compound, and a fifth organic compound whose HOMO level is higher than that of the second organic compound.

[0010] One embodiment of the present invention is a light-emitting element comprising, between a pair of electrodes, a hole-injection layer, a hole-transport layer on the hole-injection layer, a light-emitting layer on the hole-transport layer, an electron-transport layer on the light-emitting layer, and an electron-injection layer on the electron-transport layer. The light-emitting layer contains a first organic compound having an electron-transport property, a second organic compound having a hole-transport property, and a light-emitting third organic compound that converts the triplet excitation energy into light emission. A combination of the first organic compound and the second organic compound forms an exciplex.The hole transport layer contains at least a fourth organic compound whose HOMO level is lower than or equal to that of the second organic layer, and a fifth organic compound whose HOMO level is higher than that of the second organic compound.

[0011] It should be noted that in each of the structures described above, it is preferable that the first organic compound be used as a host material, the second organic compound be used as an assist material, and the third organic compound be used as a guest material. In other words, the mass fraction (or volume fraction) of the third organic compound in the light-emitting layer is preferably lower than the mass fraction of each of the first organic compound and the second organic compound.

[0012] Furthermore, in each of the structures described above, an emission wavelength of an exciplex formed of the first organic compound (host material) and the second organic compound (assist material) is longer than an emission wavelength (fluorescence wavelength) of each of the first organic compound (host material) and the second organic compound (assist material); therefore, a fluorescence spectrum of the first organic compound (host material) and a fluorescence spectrum of the second organic compound (assist material) can be converted into an emission spectrum located on the longer wavelength side.

[0013] Therefore, when an exciplex is formed in the light-emitting layer of the light-emitting element of one embodiment of the present invention, energy transfer can be performed by utilizing an overlap between an absorption spectrum of the third organic compound (guest material) and an emission spectrum of the exciplex located on the longer wavelength side than an emission wavelength (fluorescence wavelength) of each of the first organic compound (host material) and the second organic compound (assist material), thereby increasing energy transfer efficiency, so that the light-emitting element can have high emission efficiency.

[0014] Furthermore, the hole-transport layer contains at least the fourth organic compound whose HOMO level is lower than or equal to that of the second organic compound (assist material), and the fifth organic compound whose HOMO level is higher than that of the second organic compound (assist material). The fourth organic compound of the hole-transport layer has a function of injecting holes into the light-emitting layer (mainly the second organic compound (assist material) of the light-emitting layer) to facilitate the entry of holes into the light-emitting layer, resulting in an increase in emission efficiency. Since the HOMO level of the fifth organic compound in the hole-transport layer is higher than that of the second organic compound (assist material), the driving voltage (simply voltage), especially the turn-on voltage, can be reduced.This effect can only be achieved if the hole transport layer contains two or more types of organic compounds and the HOMO level relationship described above exists.

[0015] Furthermore, in each of the above-described structures, it is preferable that the lowest triplet excitation energy level (T1 level) of the fourth organic compound be higher than that of the first organic compound and that of the second organic compound. Such a structure makes it possible to prevent the triplet excitation energy of the light-emitting layer from diffusing into the hole-transport layer, thus increasing the emission efficiency of the element. It should be noted that the mass fraction (or volume fraction) of the fourth organic compound in the hole-transport layer is preferably 20% or more (less than 100%) to prevent the triplet excitation energy of the light-emitting layer from diffusing to the hole-transport layer.

[0016] It is further preferred in each of the structures described above that the first organic compound (host material) is a π-electron-poor heteroaromatic compound, the second organic compound (assist material) is a π-electron-rich heteroaromatic compound or an aromatic amine compound, and the third organic compound (guest material) is a phosphorescent compound.

[0017] Furthermore, an embodiment of the present invention includes in its category light-emitting devices comprising light-emitting elements and electronic devices and lighting devices containing the light-emitting devices. Therefore, the light-emitting device of this specification relates to an image display device and a light source (e.g., a lighting device). In addition, the light-emitting device in the category includes any module in which a light-emitting device is connected to a terminal, such as a flexible printed circuit board (FPC), a tape-automated bonding (TAB) tape, or a tape carrier package (TCP), a module in which a printed circuit board is provided on the tip of a TAB tape or 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.

[0018] When an exciplex is formed in the light-emitting layer of the light-emitting element of one embodiment of the present invention, energy transfer including an overlap between an absorption spectrum of the third organic compound (guest material) and an emission spectrum of the exciplex located on the longer wavelength side than an emission wavelength (fluorescence wavelength) of each of the first organic compound (host material) and the second organic compound (assist material) can be performed, whereby the energy transfer efficiency becomes high, so that the light-emitting element can have a high emission efficiency. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows a light-emitting element of an embodiment of the present invention. Fig. 2A and Fig. 2B show a concept of an embodiment of the present invention. Fig. Figure 3 shows the energy levels of a molecule of DBq (abbreviation), a molecule of TPA (abbreviation) and a dimer of DBq (abbreviation) and TPA (abbreviation). Fig. Figures 4A to 4F show the HOMO and LUMO distribution of a molecule of DBq (abbreviation), a molecule of TPA (abbreviation), and a dimer of DBq (abbreviation) and TPA (abbreviation). Fig. 5A to 5C show concepts of the energy levels of the hole transport layers and a light emitting layer of an embodiment of the present invention. Fig. 6 shows a light-emitting element of an embodiment of the present invention. Fig. 7A and Fig. 7B show structures of light-emitting elements. Fig. 8 shows structures of light-emitting elements. Fig. 9A and Fig. 9B show a light-emitting device. Fig.10A to 10D show electronic devices. Fig. 11A to 11D show electronic devices. Fig. 12A to 12C show lighting devices. Fig. 13 shows a structure of a light-emitting element of the examples. Fig. Figure 14 shows the luminance-current density characteristics of a light-emitting element described in Example 1. Fig. Figure 15 shows the luminance-voltage characteristics of the light-emitting element described in Example 1. Fig. 16 shows the current efficiency-luminance characteristics of the light-emitting element described in Example 1. Fig. Figure 17 shows the current-voltage characteristics of the light-emitting element described in Example 1. Fig. Figure 18 shows the emission spectra of the light-emitting elements described in Example 1. Fig. Figure 19 shows the current efficiency-luminance characteristics of the light-emitting elements in Example 1. Fig. Figure 20 shows a phosphorescence spectrum of 2mDBTPDBq-II (abbreviation). Fig. Figure 21 shows a phosphorescence spectrum of PCBA1BP (abbreviation). Fig. Figure 22 shows a phosphorescence spectrum of BPAFLP (abbreviation). BEST MODE FOR CARRYING OUT THE INVENTION

[0019] Embodiments of the present invention will be described in detail below with reference to the accompanying drawings. It should be noted that the present invention is not limited to the following description; the forms and details may be modified in various ways without departing from the spirit and scope of the present invention. Therefore, the invention should not be construed as limited by the description of the following embodiments.

[0020] It should be noted that the position, size, range, or the like of each component illustrated in the drawings and the like is not precisely illustrated in some cases for ease of understanding. Therefore, the disclosed invention is not necessarily limited to the position, size, range, or the like illustrated in the drawings and the like.

[0021] In this specification and the like, ordinal numbers such as "first," "second," and "third" are used to avoid confusion among the components and not to numerically limit the components. (Embodiment 1)

[0022] In this embodiment, a structural concept of a light-emitting element, which is an embodiment of the present invention, and a specific structure of the light-emitting element will be described. First, an element structure of the light-emitting element, which is an embodiment of the present invention, will be described with reference to Fig. 1 described.

[0023] The element structure, which in Fig.1, comprises, between a pair of electrodes (a first electrode 101 and a second electrode 103), a hole-transport layer 112, and a light-emitting layer 113 on the hole-transport layer 112. The light-emitting layer 113 contains a first organic compound 120 having an electron-transport property, a second organic compound 122 having a hole-transport property, and a light-emitting third organic compound 124 that converts triplet excitation energy into light emission. A combination of the first organic compound 120 and the second organic compound 122 forms an exciplex. The hole-transport layer 112 contains at least a fourth organic compound 126 whose HOMO level is lower than or equal to that of the second organic compound 122, and a fifth organic compound 128 whose HOMO level is higher than that of the second organic compound 122.

[0024] It should be noted that in Fig. 1, a hole injection layer or a hole transport layer can be formed in a region between the first electrode 101 and the hole transport layer 112. Furthermore, in Fig. 1 an electron injection layer or an electron transport layer may be formed in a region between the second electrode 103 and the light emitting layer 113, if necessary.

[0025] It should be noted that it is preferable that the first organic compound 120 be used as a host material, the second organic compound 122 be used as an assist material, and the third organic compound 124 be used as a guest material. In other words, the mass fraction (or volume fraction) of the third organic compound in the light-emitting layer is preferably less than the mass fraction of each of the first organic compound and the second organic compound. In the following description, in some cases, the first organic compound 120, the second organic compound 122, and the third organic compound 124 are referred to as a host material, an assist material, and a guest material.

[0026] An electron transport material with an electron mobility of e.g. 10 -6 cm 2 / Vs or more can be used as the first organic compound 120 (host material). In addition, a hole transport material with a hole mobility of, for example, 10 -6 cm 2 / Vs or more than the second organic compound 122 (assist material) may be used.

[0027] It should be noted that in the above structure, the lowest triplet excitation energy level (T1 level) of each of the first organic compound 120 (host material) and the second organic compound 122 (assist material) is preferably higher than the T1 level of the third organic compound 124 (guest material). The reason for this is as follows: When the T1 level of each of the first organic compound 120 (host material) and the second organic compound 122 (assist material) is lower than the T1 level of the third organic compound 124 (guest material), the triplet excitation energy of the third organic compound 124 (guest material), which contributes to light emission, is quenched by the first organic compound 120 (host material) and the second organic compound 122 (assist material), resulting in a decrease in emission efficiency.

[0028] Furthermore, to improve the efficiency of energy transfer from a host material to a guest material, the Förster mechanism (dipole-dipole interaction) and the Dexter mechanism (electron exchange interaction), known as energy transfer mechanisms between molecules, are considered. According to these mechanisms, it is preferable that the emission spectrum of a host material (fluorescence spectrum in the case of energy transfer from an excited singlet state, phosphorescence spectrum in the case of energy transfer from an excited triplet state) significantly overlaps with the absorption spectrum of a guest material (especially the spectrum in an absorption band on the longest wavelength (lowest energy) side).

[0029] However, in the case where, for example, a phosphorescent compound is used as a guest material, it is difficult to achieve an overlap between a fluorescence spectrum of a host material and an absorption spectrum in an absorption band on the longest wavelength (lowest energy) side of the guest material. The reason for this is as follows: When the fluorescence spectrum of the host material overlaps with the absorption spectrum in the absorption band on the longest wavelength (lowest energy) side of the guest material, the T1 level of the host material is lower than the T1 level of the phosphorescent compound, and the quenching problem described above occurs because a phosphorescence spectrum of the host material is located on a longer wavelength (lower energy) side than the fluorescence spectrum.However, if the host material is designed such that the T1 level of the host material is higher than the T1 level of the phosphorescent compound used as the guest material, to avoid the problem of quenching, the fluorescence spectrum of the host material shifts toward the shorter wavelength (higher energy) side, and therefore, the fluorescence spectrum shows no overlap with the absorption spectrum in the absorption band on the longest wavelength (lowest energy) side of the guest material. For this reason, it is generally difficult to achieve an overlap between a fluorescence spectrum of a host material and an absorption spectrum in an absorption band on the longest wavelength (lowest energy) side of a guest material, thus maximizing energy transfer from an excited singlet state of a host material.

[0030] Therefore, in one embodiment of the present invention, the combination of the first organic compound 120 and the second organic compound 122 forms an exciplex (also referred to as an excited complex). An exciplex is described below with reference to Fig. 2A and Fig. 2B.

[0031] Fig. 2A shows a schematic view illustrating the concept of an exciplex; a fluorescence spectrum of the first organic compound 120 (or the second organic compound 122), a phosphorescence spectrum of the first organic compound 120 (or the second organic compound 122), an absorption spectrum of the third organic compound 124, and an emission spectrum of the exciplex are shown.

[0032] For example, in the light-emitting layer 113, the fluorescence spectrum of the first organic compound 120 (host material) and the fluorescence spectrum of the second organic compound 122 (assist material) are converted into an emission spectrum of an exciplex located on the longer wavelength side. Furthermore, selecting the first organic compound 120 (host material) and the second organic compound 122 (assist material) so that the emission spectrum of the exciplex significantly overlaps with the absorption spectrum of the third organic compound 124 (guest material) allows for maximizing the energy transfer from the excited singlet state (see Fig. 2A).

[0033] It should be noted that even in the case of an excited triplet state, energy transfer is assumed to occur from the exciplex, not the host material.

[0034] Since the emission wavelength of the formed exciplex is not longer than the emission wavelength (fluorescence wavelength) of each of the first organic compound 120 (host material) and the second organic compound 122 (assist material), the fluorescence spectrum of the first organic compound 120 (host material) or the fluorescence spectrum of the second organic compound 122 (assist material) can become an emission spectrum located on the longer wavelength side.

[0035] Furthermore, the exciplex presumably exhibits an extremely small difference between the singlet excitation energy and the triplet excitation energy. In other words, the emission spectrum of the singlet state exciplex and the emission spectrum of the triplet state are very close to each other.Therefore, in the case where a design is implemented in which the emission spectrum of the exciplex (generally, the emission spectrum of the singlet state exciplex) overlaps with the absorption band of the third organic compound 124 (guest material) located on the longest wavelength side as described above, the emission spectrum of the triplet state exciplex (which is not observed at room temperature and in many cases is not observed at low temperature either) also overlaps with the absorption band of the third organic compound 124 (guest material) located on the longest wavelength side.In other words, the efficiency of energy transfer from the excited triplet state as well as the efficiency of energy transfer from the excited singlet state can be increased and as a result, light emission from both the singlet and triplet excited states can be effectively obtained.

[0036] As described above, in the light-emitting element of one embodiment of the present invention, energy is transferred by utilizing the overlap between the emission spectrum of the exciplex formed in the light-emitting layer 113 and the absorption spectrum of the third organic compound 124 (guest material); therefore, the energy transfer efficiency is high.

[0037] In addition, the exciplex exists only in an excited state and therefore lacks a ground state to absorb energy. Therefore, a phenomenon in which the third organic compound (guest material) is deactivated prior to light emission (i.e., the light emission efficiency is reduced) by energy transfer from the excited singlet state and the excited triplet state of the third compound 124 (guest material) to the exciplex is unlikely to occur in principle.

[0038] It should be noted that the exciplex described above is formed by an interaction between dissimilar molecules in excited states. The exciplex is generally known to be formed simply between a material exhibiting a relatively low LUMO level (LUMO: lowest unoccupied orbital of a molecule) and a material exhibiting a relatively high HOMO level (HOMO: highest occupied orbital of a molecule).

[0039] Here, the concepts of the energy levels of the first organic compound 120, the second organic compound 122 and the exciplex are explained with reference to Fig. 2B. It should be noted that Fig. Figure 2B schematically illustrates the energy levels of the first organic compound 120, the second organic compound 122 and the exciplex.

[0040] The HOMO levels and the LUMO levels of the first organic compound 120 (host material) and the second organic compound 122 (assist material) are different from each other. Specifically, the energy levels change in the following order: the HOMO level of the first organic compound 120 < the HOMO level of the second organic compound 122 < the LUMO level of the first organic compound 120 < the LUMO level of the second organic compound 122. When the exciplex is formed by these two organic compounds, the LUMO level and the HOMO level of the exciplex originate from the first organic compound 120 (host material) and the second organic compound 122 (assist material), respectively (see Fig. 2B).

[0041] The emission wavelength depends on the energy difference between the HOMO and LUMO levels. As a general trend, the emission wavelength is short when the energy difference is large, and the emission wavelength is long when the energy difference is small.

[0042] Therefore, the energy difference of the exciplex is smaller than the energy difference between the first organic compound 120 (host material) and the energy difference between the second organic compound 122 (assist material). In other words, the emission wavelength of the exciplex is longer than the emission wavelength of each of the first organic compound 120 and the second organic compound 122.

[0043] Molecular orbital calculations were performed as described below to confirm whether an exciplex actually exhibits such properties. In general, a combination of a heteroaromatic compound and an aromatic amine often forms an exciplex under the influence of the LUMO level of the heteroaromatic compound, which is lower than the LUMO level of the aromatic amine (the property of readily accepting electrons), and the HOMO level of the aromatic amine, which is higher than the HOMO level of the heteroaromatic compound (the property of readily accepting holes).Therefore, the calculations were performed using a combination of dibenzene[f,h]quinoxaline (abbreviation: DBq), which is a typical scaffold that forms the LUMO of a heteroaromatic compound and is a model of the first organic compound 120 in an embodiment of the present invention, and triphenylamine (abbreviation: TPA), which is a typical scaffold that forms the HOMO level of an aromatic amine and is a model of the second organic compound 122 in an embodiment of the present invention.

[0044] First, the optimal molecular structures and excitation energies of a molecule of DBq (abbreviation) and a molecule of TPA (abbreviation) in the lowest excited singlet state (S1) and the lowest excited triplet 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.

[0045] In density functional theory (DFT), the total energy is represented as the sum of the potential energy, the electrostatic energy between electrons, the electrokinetic energy, and the exchange-correlation energy, including all the complex interactions between electrons. Furthermore, in DFT, an exchange-correlation interaction is approximated by a functional (a function of another function) of an electron potential, which is represented with respect to an electron density, to enable high-speed and high-accuracy calculations. B3LYP, a hybrid functional, was used to specify the weight of each parameter with respect to the exchange-correlation energy.

[0046] 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.

[0047] For example, the above basis function considers 1s to 3s orbitals in the case of hydrogen atoms, while 1s to 4s and 2p to 4p orbitals are considered in the case of carbon atoms. To further improve calculation accuracy, the p-function and the d-function were added as polarization basis sets to hydrogen atoms and atoms other than hydrogen atoms.

[0048] It should be noted that Gaussian 09 was used as a quantum chemistry software package. A high-performance computer (Altix 4700, manufactured by SGI Japan, Ltd.) was used for the calculations.

[0049] 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. Figure 3 shows the HOMO levels and the LUMO levels.

[0050] As in Fig. 3, it is proposed 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 lower than the LUMO level of TPA (abbreviation), and the HOMO level (-5.21 eV) of TPA (abbreviation), which is higher than the HOMO level of DBq (abbreviation).

[0051] Fig. Figures 4A to 4F show HOMO level and LUMO level distributions of a molecule of DBq (abbreviation), a molecule of TPA (abbreviation), and a dimer of DBq (abbreviation) and TPA (abbreviation).

[0052] It should be noted that Fig. 4A shows the LUMO distribution of a molecule of DBq (abbreviation), Fig. Figure 4B shows the HOMO distribution of a molecule of DBq, Fig. Figure 4C shows the LUMO distribution of a molecule of TPA (abbreviation), Fig.Figure 4D shows the HOMO distribution of a molecule of TPA (abbreviation), Fig. Figure 4E shows the LUMO distribution of the dimer of DBq (abbreviation) and TPA (abbreviation) and Fig. Figure 4F shows the HOMO distribution of the dimer of DBq (abbreviation) and TPA (abbreviation).

[0053] As in the Fig. 4E and Fig. As shown in Figure 4F, the LUMO of the dimer of DBq (abbreviation) and TPA (abbreviation) is distributed on the side of DBq (abbreviation), and the HOMO of which is distributed on the side of TPA (abbreviation), which is consistent with the Fig. 3 results shown.

[0054] The method of exciplex formation in one embodiment of the present invention may be one of the methods described below.

[0055] One method of exciplex formation is that an exciplex is formed from the first organic compound 120 (host material) and the second organic compound 122 (assist material) with carriers (cations or anions).

[0056] 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 a guest material, causing the guest material to enter an excited state and emit light. Before the excitation energy is transferred from the host material to the guest material, the host material itself emits light or the excitation energy is converted into thermal energy, resulting in partial deactivation of the excitation energy.

[0057] However, in one embodiment of the present invention, an exciplex is formed from the first organic compound 120 (host material) and the second organic compound 122 (assist material) with carriers (cations or anions); therefore, the formation of a singlet exciton of the first organic compound 120 (host material) can be suppressed. In other words, a method may be provided in which an exciplex is directly formed without the formation of a singlet exciton. Therefore, the deactivation of the singlet excitation energy can be inhibited. In this way, a light-emitting element with a long lifetime can be obtained.

[0058] For example, in the case where the first organic compound 120 is an electron-capturing compound having the property of easily capturing electrons (carriers) (with a low LUMO level) among electron-transport materials, and the second organic compound 122 is a hole-capturing compound having the property of easily capturing holes (carriers) (with a high HOMO level) among hole-transport materials, an exciplex can be directly formed from an anion of the first organic compound 120 and a cation of the second organic compound 122. An exciplex formed by such a method is specifically referred to as an electroplex.

[0059] A light-emitting element with high emission efficiency can be obtained by suppressing the generation of the excited singlet state of the first organic compound 120 (host material) and transferring energy from an electroplex to the third organic compound 124 (guest material) in the manner described above. It should be noted that in this case, the generation of the excited triplet state of the first organic compound 120 (host material) is similarly suppressed and an exciplex is formed directly; therefore, energy transfer presumably occurs from the exciplex to the third organic compound 124 (guest material).

[0060] The other method of exciplex formation is a basic method in which the first organic compound 120 (host material) and the second organic compound 122 (assist material) form a singlet exciton and then interact with each other in the ground state to form an exciplex. Unlike an electroplex, an excited singlet state of the first organic compound 120 (host material) or the second organic compound 122 (assist material) is temporarily generated in this case but is quickly converted into an exciplex; therefore, the deactivation of the singlet excitation energy can be suppressed. Therefore, it is possible to suppress the deactivation of the excitation energy of the first organic compound 120 or the second organic compound 122.It should be noted that in this case, the excited triplet state of the first organic compound 120 (host material) is converted into an exciplex at a similar rate and the energy is transferred from the exciplex to the third organic compound 124 (host material).

[0061] It should be noted that in the case where the first organic compound 120 (host material) is an electron-trapping compound, the second organic compound 122 (assist material) is a hole-trapping compound, and the difference between the HOMO level and the LUMO level of these compounds is large (specifically, 0.3 eV or more), electrons are selectively injected into the first organic compound 120 (host material) and holes are selectively injected into the second organic compound 122 (assist material). In this case, the method in which an electroplex is formed is believed to prevail over the method in which an exciplex is formed by singlet excitation.

[0062] The excitation energy calculation results are shown below. The S1 excitation energy of a molecule of DBq (abbreviation) is 3.294 eV, and the fluorescence wavelength is 376.4 nm. The T1 excitation energy of a molecule of DBq (abbreviation) is 2.460 eV, and the phosphorescence wavelength is 504.1 nm. In contrast, the S1 excitation energy of a molecule of TPA (abbreviation) is 3.508 eV, and the fluorescence wavelength is 353.4 nm. The T1 excitation energy of a molecule of TPA (abbreviation) is 2.610 eV, and the phosphorescence wavelength is 474.7 nm.

[0063] The following shows the excitation energies obtained from the optimal molecular structures of the dimer of DBq (abbreviation) and TPA (abbreviation) at the S1 and T1 levels. The S1 excitation energy of the dimer of DBq (abbreviation) and TPA (abbreviation) is 2.036 eV, and the fluorescence wavelength is 609.1 nm. The T1 excitation energy of the dimer of DBq (abbreviation) and TPA (abbreviation) is 2.030 eV, and the phosphorescence wavelength is 610.0 nm.

[0064] The above shows that the fluorescence wavelength of the dimer of DBq (abbreviation) and TPA (abbreviation) is longer than the fluorescence wavelength of one molecule of DBq (abbreviation) and the fluorescence wavelength of one molecule of TPA (abbreviation). The above also shows 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.

[0065] These results demonstrate that the exciplex can integrate the singlet excitation energy and the triplet excitation energy into essentially the same energy. Therefore, as described above, it is demonstrated that the exciplex can efficiently transfer energy to the phosphorescent compound from both the singlet and triplet states.

[0066] Such an effect is specific to the use of an exciplex as an energy transfer medium. Generally, energy transfer from the excited singlet state or the excited triplet state of a host material to a phosphorescent compound is considered. On the other hand, one embodiment of the present invention differs significantly from a conventional method in that an exciplex of a host material and another material is first formed and energy transfer from the exciplex is utilized. In addition, this difference provides unprecedentedly high emission efficiency.

[0067] It should be noted that the use of an exciplex for a light-emitting layer of a light-emitting element generally has value in that it is possible to control the emission color, but usually causes a significant reduction in emission efficiency. Therefore, the use of an exciplex was considered unsuitable for obtaining a highly efficient light-emitting element. However, the present inventors have found that, in contrast, the use of an exciplex as an energy transfer medium makes it possible to maximize emission efficiency, as shown in an embodiment of the present invention. This technical idea is contrary to the conventional established idea.

[0068] In order for the emission spectrum of the exciplex and the absorption spectrum of the third organic compound 124 (guest material) to sufficiently overlap with each other, the difference between the energy of a peak in the emission spectrum and the energy of a peak in the absorption band on the lowest energy side of the absorption spectrum is preferably 0.3 eV or less. This difference is preferably 0.2 eV or less, and even more preferably 0.1 eV or less.

[0069] It is preferable that, in the light-emitting element embodying the present invention, the excitation energy of the exciplex is sufficiently transferred to the third organic compound 124 (guest material), and that light emission from the exciplex is substantially not observed. Therefore, the energy is preferably transferred to the third organic compound 124 (guest material) through the exciplex, so that the third organic compound 124 emits phosphorescence. It should be noted that the third organic compound 124 is preferably a light-emitting material that converts the triplet excitation energy into light emission, and is particularly preferably a phosphorescent compound.

[0070] Hereinafter, the concepts of the energy levels of the hole transport layer and the light-emitting layer 113, which are an embodiment of the present invention, will be explained with reference to the Fig. 5A to 5C. It should be noted that the Fig. 5A to 5C schematically illustrate the energy levels of the hole transport layer 112 (hole transport layers 112a, 112b and 112c) and the light-emitting layer 113.

[0071] It should be noted that in the light-emitting layer 113, which is in the Fig. 5A to 5C, the above-described combination of the first organic compound 120 and the second organic compound 122 forms an exciplex.

[0072] Fig. 5A shows, as an example, the energy levels of the hole transport layer 112a formed of a type of organic compound whose T1 level is higher than that of the second organic compound 122 (assist material) and whose HOMO level is lower than that of the second organic compound 122 (assist material).

[0073] In the case of Fig.In the structure shown in Figure 5A, the HOMO level of the hole transport layer 112a is lower than the HOMO level of the light-emitting layer 113 in which an exciplex is formed; therefore, the entry of holes into deeper regions of the light-emitting layer 113 from the hole transport layer 112a is facilitated, thereby enabling the emission efficiency of the light-emitting layer 113 to be increased. Meanwhile, the turn-on voltage is determined by the energy of E1 (energy difference between the LUMO and HOMO levels of the light-emitting layer 113 in which an exciplex is formed) in Fig. 5A and cannot be lower than the voltage corresponding to the energy. For example, if E1 is 2 eV, the turn-on voltage cannot be lower than 2 V. This is because in the case shown in Fig. 5A the energy of E 2a(Energy difference between the LUMO level of the light-emitting layer 113, in which an exciplex is formed, and the HOMO level of the hole transport layer 112a) is higher than the energy of E1.

[0074] Below you can see the Fig. 5B shows, as an example, the energy levels of the hole transport layer 112b, which is formed by a type of organic compound whose T1 level is higher than that of the second organic compound 122 (assist material) and whose HOMO level is higher than that of the second organic compound 122 (assist material).

[0075] In the case of Fig. In the structure shown in Figure 5B, the HOMO level of the hole transport layer 112b is higher than the HOMO level of the light-emitting layer 113, in which an exciplex is formed; therefore, the drive voltage (in particular, the turn-on voltage) depends on the HOMO level of the hole transport layer 112b. In particular, the turn-on voltage is determined by the energy E2b (Energy difference between the LUMO level of the light-emitting layer 113, in which an exciplex is formed, and the HOMO level of the hole transport layer 112b) is determined, which in Fig. 5B. Therefore, in this case, the Fig. 5B the energy of E 2b than the energy of E1 (energy difference between the LUMO and HOMO levels of the light-emitting layer 113, in which an exciplex is formed); consequently, the turn-on voltage can be reduced. However, holes are not easily transported from the hole-transport layer 112b to the light-emitting layer 113, resulting in a reduction in emission efficiency, especially in a region with a high luminance (practical luminance).

[0076] Below you will find Fig.5C shows, as an example, the energy levels of the hole transport layer 112c, which contains at least the fourth organic compound 126, whose HOMO level is lower than or corresponds to that of the second organic compound 122 (assist material), and the fifth organic compound 128, whose HOMO level is higher than that of the second organic compound 122 (assist material). The energy levels of the hole transport layer 112c, which in Fig. 5C are a concept of an embodiment of the present invention.

[0077] In the case of Fig.In the structure shown in Figure 5C, the HOMO level of the hole-transport layer 112c may be lower than or equal to the HOMO level of the second organic compound 122 (assist material) due to the fourth organic compound 126. Therefore, the entry of holes into deeper regions of the light-emitting layer 113 from the hole-transport layer 112c is facilitated, which makes it possible to increase the emission efficiency of the light-emitting layer 113. Furthermore, the drive voltage (in particular, the turn-on voltage) depends on the HOMO level of the hole-transport layer 112c. The HOMO level of the hole-transport layer 112c is influenced by the fifth organic compound 128. The LUMO level of the hole-transport layer 112c is influenced by the fourth organic compound 126. That is, the turn-on voltage is determined by the energy E 2c(Energy difference between the LUMO level of the light-emitting layer 113, in which an exciplex is formed, and the HOMO level of the fifth organic compound 128), as shown in Fig. 5C. Therefore, in the case of the Fig. 5C, the turn-on voltage can be reduced because the energy of E 2c is lower than the energy of E1 (energy difference between the LUMO and HOMO levels of the light-emitting layer 113 in which an exciplex is formed): At the Fig.In the structure illustrated in Figure 5C, the recombination current can flow with lower electrical energy than the emission energy from the light-emitting layer 113. Generally, it is assumed that the turn-on voltage is the voltage corresponding to the emission energy of the light-emitting layer 113, and that operation does not start at voltages lower than this voltage. However, the turn-on voltage in one embodiment of the present invention is influenced by the fifth organic compound 128 included in the hole-transport layer 112c and can therefore be lower than the previously generally accepted turn-on voltage.Furthermore, although the emission efficiency is likely to be low at the start of propulsion due to the contribution of the fifth organic compound 128 to generating excitation energy, the luminance is low enough to allow optical detection at the start of propulsion; therefore, no practical problems are caused. In fact, if the luminance is practical (especially higher than or equivalent to 1 cd / m), the luminance is sufficient. 2 ), the hole injection is mainly carried out from the fourth organic compound 126 to the light-emitting layer 113, and the fifth organic compound 128 is hardly responsible for the hole injection into the light-emitting layer 113 or an excited state; therefore, a high light emission can be obtained.

[0078] As described above, in the light-emitting element described in this embodiment, the hole-transport layer 112c contains at least the fourth organic compound 126 whose HOMO level is lower than or equal to that of the second organic compound 122 (assist material), and the fifth organic compound 128 whose HOMO level is higher than that of the second organic compound 122 (assist material). The fourth organic compound 126 of the hole-transport layer 112c has a function of injecting holes into the light-emitting layer 113 (mainly the second organic compound 122 (assist material) into the light-emitting layer 113), so that the emission efficiency can be increased.Furthermore, the HOMO level of the fifth organic compound 128 of the hole-transport layer 112c is higher than that of the second organic compound 122 (assist material); therefore, the drive voltage, especially the turn-on voltage for the light-emitting layer, can be reduced. This effect can only be achieved if the hole-transport layer contains two or more types of organic compounds and the HOMO level relationship described above exists.

[0079] When a phosphorescent compound is used as the first organic compound 120 (host material) in the light-emitting element of one embodiment of the present invention, the first organic compound 120 itself tends to emit light and will not tend to transfer energy to the third organic compound 124 (guest material). In this case, it is preferable if the first organic compound 120 can effectively emit light, but it is difficult to achieve high emission efficiency because the first organic compound 120 (host material) causes a problem of concentration quenching. Therefore, the case where at least one of the first organic compound 120 (host material) and the second organic compound 122 (assist material) is a fluorescent compound (ie,a compound that is prone to light emission or thermal deactivation from the excited singlet state) is more effective. Therefore, it is preferred that at least one of the first organic compound 120 and the second organic compound 1232 be a fluorescent compound.

[0080] As described above, in the light-emitting element described in this embodiment, the energy transfer efficiency can be improved due to the energy transfer, which is an overlap between an emission spectrum of an exciplex and an absorption spectrum of the third organic compound (guest material); accordingly, the light-emitting element can have high emission efficiency.

[0081] Furthermore, in the light-emitting element described in this embodiment, the fourth organic compound, whose HOMO level is lower than or equal to that of the second organic compound, having a light-transporting property is used in the light-emitting layer, and the fifth organic compound, whose HOMO level is higher than that of the second organic compound, is used in the transport layer. Such a structure makes it possible to increase the current efficiency of the light-emitting element and reduce the voltage, particularly the turn-on voltage, of the light-emitting element.

[0082] It should be noted that the structure described in this embodiment can be suitably combined with any of the structures described in other embodiments. (Embodiment 2)

[0083] In this embodiment, a modified example of the light-emitting element of Embodiment 1 will be described with reference to Fig. 6. It should be noted that parts having similar functions to the parts in the light-emitting element of Embodiment 1 are designated by the same reference numerals, and the description of these parts will be omitted.

[0084] As in Fig.As shown in Figure 6, the light-emitting element described in this embodiment includes a hole-injection layer 111 between a pair of electrodes (the first electrode 101 and the second electrode 103), the hole-transport layer 112 on the hole-injection layer 111, the light-emitting layer 113 above the hole-transport layer 112, an electron-transport layer 114 on the light-emitting layer 113, and an electron-injection layer 115 on the electron-transport layer 114. The light-emitting layer 113 contains a first organic compound 120 having an electron-transport property, a second organic compound 122 having a hole-transport property, and a light-emitting third organic compound 124 that converts triplet excitation energy into light emission. A combination of the first organic compound 120 and the second organic compound 122 forms an exciplex.The hole transport layer 112 contains at least a fourth organic compound 126 whose HOMO level is lower than or equal to that of the second organic compound 122 and a fifth organic compound 128 whose HOMO level is higher than that of the second organic compound 122.

[0085] It should be noted that in the light-emitting layer 113, the third organic compound 124 (guest material) is dispersed in the first organic compound 120 (host material) and the second organic compound 122 (assist material), so that concentration quenching due to high concentration in the light-emitting layer 113 can be suppressed; therefore, the emission efficiency of the light-emitting element can be increased.

[0086] The combination of the first organic compound 120 (host material) and the second organic compound 122 (assist material) forms an exciplex.

[0087] A specific example in which the light-emitting element described in this embodiment is manufactured will be described below.

[0088] A substrate 100 is used as a support of the light-emitting element. For example, glass, quartz, plastic, or the like can be used for the substrate 100. Furthermore, a flexible substrate can be used. The flexible substrate is a substrate that can be bent, such as a plastic substrate made of, for example, polycarbonate, polyarylate, or polyethersulfone. Alternatively, a film (made of polypropylene, polyester, polyvinyl fluoride, polyvinyl chloride, or the like), an inorganic film formed by evaporation, or the like can be used.

[0089] It should be noted that another substrate may be used as long as it can be used as a support in a process for manufacturing the light-emitting element.

[0090] As the first electrode 101 and the second electrode 103, a metal, an alloy, an electrically conductive compound, a mixture of these, or the like can be used. Specifically, indium oxide-tin oxide (ITO: indium tin oxide), indium oxide-tin oxide containing silicon or silicon oxide, indium oxide-zinc oxide, indium oxide containing tungsten 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, e.g., can be used. B. an alkali metal such as lithium (Li) or cesium (Cs), an alkaline earth metal such as magnesium (Mg), calcium (Ca) or strontium (Sr), an alloy containing such an element (e.g. MgAg or AlLi), a rare earth metal such as europium (Eu) or ytterbium (Yb), an alloy containing such an element, graphene or the like.It should be noted that the first electrode 101 and the second electrode 103 can be formed, for example, by a sputtering method, an evaporation method (including a vacuum evaporation method), or the like. It should be noted that in this embodiment, the first electrode 101 is used as an anode, and the second electrode 103 is used as a cathode.

[0091] As the substance having a high hole transport property used for the hole injection layer 111 and the hole transport layer 112, the following can be given, for example, 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); 3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole (abbreviation: PCzPCN1); and the like.The following carbazole derivatives can also be used: 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-anthryl)phenyl]-9H-carbazole (abbreviation: CzPA). The hole-transport materials listed below can also be used. The substances listed here are primarily those with a hole mobility of 10. -6 cm 2 / Vs or more. It should be noted that any substance other than the above substances can be used as long as its hole-transporting property is higher than its electron-transporting property.

[0092] A high molecular compound 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) or poly[N,N'-bis(4-butylphenyl)-N,N'-bis(phenyl)benzidine] (abbreviation: poly-TPD) can also be used for hole injection layer 111 and the hole transport layer 112.

[0093] Examples of the acceptor substance that can be used for the hole-injection layer 111 include transition metal oxides and oxides of metals belonging to groups 4 to 8 of the Periodic Table. In particular, molybdenum oxide is particularly preferred.

[0094] For the hole-transport layer 112, the substances used as the fourth organic compound 126 whose HOMO level is lower than or equal to that of the second organic compound 122 (assist material) and the fifth organic compound 128 whose HOMO level is higher than that of the second organic compound 122 (assist material) can be selected from the substances listed above. It should be noted that the substances used for the fourth organic compound 126 and the fifth organic compound 128 are not limited to the substances listed above, and the substances can be appropriately selected by those skilled in the art depending on the substance used as the second organic compound 122 (assist material).

[0095] The light-emitting layer 113 contains the first organic compound 120 (host material), the second organic compound 122 (assist material) and the third organic compound 124 (guest material).

[0096] An electron-transport material is preferably used as the first organic compound 120 (host material). A hole-transport material is preferably used as the second organic compound 122 (assist material). A light-emitting material that converts the triplet excitation energy into light emission is preferably used as the third organic compound 124 (guest material).

[0097] As the guest material, a π-electron-deficient heteroaromatic compound, such as a nitrogen-containing heteroaromatic compound, is preferred; e.g. For example, the following can be mentioned: heterocyclic compounds (e.g., an oxadiazole derivative, an imidazole derivative, and a triazole derivative) with polyazole skeletons, such as 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation: PBD), 3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole (abbreviation: TAZ), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene (abbreviation: OXD-7), 9-[4-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl]-9H-carbazole (abbreviation: CO11), 2,2',2"-(1,3,5-benzenetriyl)tris(1-phenyl-1H-benzimidazole) (abbreviation: TPBI) and 2-[3-(dibenzothiophen-4-yl)phenyl]-1-phenyl-1H-benzimidazole (abbreviation: mDBTBIm-II); heterocyclic compounds (e.g.a pyrazine derivative, a pyrimidine derivative, a pyridazine derivative, a quinoxaline derivative and a dibenzoquinoxaline derivative) with diazine skeletons, such as 2-[3-(Eibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation: 2mDBTPDBq-II), 2-[3'-(Dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline (abbreviation: 2mDBTBPDBq-II), 2-[3'-(9H-Carbazol-9-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline (abbreviation: 2mCzBPDBq), 4,6-Bis[3-(phenanthren-9-yl)phenyl]pyrimidine (abbreviation: 4,6mPnP2Pm) and 4,6-bis[3-(4-dibenzothienyl)phenyl]pyrimidine (abbreviation: 4,6mDBTP2Pm-II); and heterocyclic compounds (e.g., a pyridine derivative, a quinoline derivative, and a dibenzoquinoline derivative) with pyridine skeletons, such as 3,5-bis[3-(9H-carbazol-9-yl)phenyl]pyridine (abbreviation: 35DCzPPy) and 1,3,5-tri[3-(3-pyridyl)phenyl]benzene (abbreviation: TmPyPB).Among the materials mentioned here, the heterocyclic compound with a diazine skeleton and the heterocyclic compound with a pyridine skeleton exhibit high reliability and are therefore preferred. In particular, the heterocyclic compound with a diazine (pyrimidine or pyrazine) skeleton exhibits high electron transport properties and contributes to a reduction in the drive voltage.

[0098] As the hole transport material, a π-electron-rich heteroaromatic compound (e.g., a carbazole derivative or an indole derivative) or an aromatic amine compound is preferred;In particular, the following can be mentioned: compounds with aromatic amine skeletons, such as 4,4'-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB), N,N'-bis(3-methylphenyl)-N,N'-diphenyl-[1,1'-biphenyl]-4,4'-diamine (abbreviation: TPD), 4,4'-bis[N-(spiro-9,9'-bifluoren-2-yl)-N-phenylamino]biphenyl (abbreviation: BSPB), 4-phenyl-4'-(9-phenylfluoren-9-yl)triphenylamine (abbreviation: BPAFLP), 4-phenyl-3'-(9-phenylfluoren-9-yl)triphenylamine (abbreviation: mBPAFLP), 4-phenyl-4'-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PCBA1BP), 4,4'-diphenyl-4"-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PCBBi1BP), 4-(1-naphthyl)-4'-(9-phenyl-9H-carbazol-3-yl)-triphenylamine (abbreviation: PCBANB), 4,4'-di(1-naphthyl)-4"-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PCBNBB), 9,9-dimethyl-N-phenyl-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-fluoren-2-amine (abbreviation: PCBAF) and N-phenyl-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]spiro-9,9'-bifluoren-2-amine (abbreviation: PCBASF);Compounds with carbazole skeletons, such as 1,3-bis(N-carbazolyl)benzene (abbreviation: mCP), 4,4'-di(N-carbazolyl)biphenyl (abbreviation: CBP), 3,6-bis(3,5-diphenylphenyl)-9-phenylcarbazole (abbreviation: CzTP) and 3,3'-bis(9-phenyl-9H-carbazole) (abbreviation: PCCP); Compounds with thiophene skeletons, such as 4,4',4"-(benzene-1,3,5-triyl)tri(dibenzothiophene) (abbreviation: DBT3P-II), 2,8-diphenyl-4-[4-(9-phenyl-9H-fluoren-9-yl)phenyl]dibenzothiophene (abbreviation: DBTFLP-III) and 4-[4-(9-phenyl-9H-fluoren-9-yl)phenyl]-6-phenyldibenzothiophene (abbreviation: DBTFLP-IV);and compounds with furan skeletons, such as 4,4',4"-(benzene-1,3,5-triyl)tri(dibenzofuran) (abbreviation: DBF3P-II) and 4-{3-[3-(9-phenyl-9H-fluoren-9-yl)phenyl]phenyl}dibenzofuran (abbreviation: mmDBFFLBi-II). Among the above materials, the compounds with an aromatic amine skeleton and the compound with a carbazole skeleton are preferred because these compounds exhibit high reliability and hole-transport properties to contribute to a reduction in the driving voltage.

[0099] It should be noted that these electron-transport materials and hole-transport materials are preferred to have no absorption spectrum in the blue wavelength range. In particular, an absorption edge of the absorption spectrum is preferably located at 440 nm or less.

[0100] Examples of the light-emitting material that converts triplet excitation energy into light emission include a fluorescent material and a thermally excited delayed fluorescence (TADF) material.

[0101] As the phosphorescent material, for example, a phosphorescent material is mentioned having an emission peak at 440 nm to 520 nm, examples of which include organometallic iridium complexes with 4H-triazole skeletons, such as Tris{2-[5-(2-methylphenyl)-4-(2,6-dimethylphenyl)-4H-1,2,4-triazol-3-yl-ĸN 2]phenyl-ĸC}iridium(III) (abbreviation: Ir(mpptz-dmp)3), tris(5-methyl-3,4-diphenyl-4H-1,2,4-triazolato)iridium(III) (abbreviation: [Ir(Mptz)3] and tris[4-(3-biphenyl)-5-isopropyl-3-phenyl-4H-1,2,4-triazolato]iridium(III) (abbreviation: Ir(iPrptz-3b)3); organometallic iridium complexes with 1H-triazole skeletons, such as tris[3-methyl-1-(2-methylphenyl)-5-phenyl-1H-1,2,4-triazolato]iridium(III) (abbreviation: [Ir(Mptz1-mp)3]) and Tris(1-methyl-5-phenyl-3-propyl-1H-1,2,4-triazolato)iridium(III) (abbreviation: Ir(Prptz1-Me)3); organometallic iridium complexes with imidazole frameworks, such as fac-tris[1-(2,6-diisopropylphenyl)-2-phenyl-1H-imidazole]iridium(III) (abbreviation: Ir(iPrpmi)3) and tris[3-(2,6-dimethylphenyl)-7-methylimidazo[1,2-f]phenanthridinato]iridium(III) (abbreviation: Ir(dmpimpt-Me)3); and organometallic iridium complexes in which a phenylpyridine derivative with an electron-withdrawing group is a ligand, such as bis[2-(4',6'-difluorophenyl)pyridinato-N,C 2']iridium(III)tetrakis(1-pyrazolyl)borate (abbreviation: FIr6), bis[2-(4',6'-difluorophenyl)pyridinato-N,C 2' ]iridium(III)picolinate (abbreviation: Flrpic), bis{2-[3',5'-bis(trifluoromethyl)phenyl]pyridinato-N,C 2'}iridium(III)picolinate (abbreviation: Ir(CF3ppy)2(pic)) and bis[2-(4',6'-difluorophenyl)pyridinato-N,C 2' ]iridium(III) acetylacetonate (abbreviation: Fir(acac)). Among the above-mentioned materials, the organometallic iridium complex with a 4H-triazole framework exhibits high reliability and high emission efficiency and is therefore particularly preferred.

[0102] Examples of the phosphorescent material with an emission peak at 520 nm to 600 nm include organometallic iridium complexes with pyrimidine skeletons, such as tris(4-methyl-6-phenylpyrimidinato)iridium(III) (abbreviation: Ir(mppm)3), tris(4-t-butyl-6-phenylpyrimidinato)iridium(III) (abbreviation: Ir(tBuppm)3), (acetylacetonato)bis(6-methyl-4-phenylpyrimidinato)iridium(III) (abbreviation: Ir(mppm)2(acac)), (acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III) (abbreviation: Ir(tBuppm)2(acac)), (acetylacetonato)bis[4-(2-norbornyl)-6-phenylpyrimidinato]iridium(III) (Endo and exo mixture) (abbreviation: Ir(nbppm)2(acac)), (acetylacetonato)bis[5-methyl-6-(2-methylphenyl)-4-phenylpyrimidinato]iridium(III) (abbreviation: Ir(mpmppm)2(acac)) and (acetylacetonato)bis(4,6-diphenylpyrimidinato)iridium(III) (abbreviation: Ir(dppm)2(acac));organometallic iridium complexes with pyrazine frameworks, such as (acetylacetonato)bis(3,5-dimethyl-2-phenylpyrazinato)iridium(III) (abbreviation: Ir(mppr-Me)2(acac)) and (acetylacetonato)bis(5-isopropyl-3-methyl-2-phenylpyrazinato)iridium(III) (abbreviation: Ir(mppr-iPr)2(acac)); organometallic iridium complexes with pyridine frameworks, such as tris(2-phenylpyridinato-N,C; 2' )iridium(III) (abbreviation: Ir(ppy)3), bis(2-phenylpyridinato-N,C 2' )iridium(III)acetylacetonate (abbreviation: Ir(ppy)2acac), bis(benzo[h]quinolinato)iridium(III)acetylacetonate (abbreviation: Ir(bzq)2(acac)), tris(benzo[h]quinolinato)iridium(III) (abbreviation: Ir(bzq)3), tris(2-phenylquinolinato-N,C 2' )iridium(III) (abbreviation: Ir(pq)3 and bis(2-phenylquinolinato-N,C 2')iridium(III) acetylacetonate (abbreviation: Ir(pq)2(acac)); and a rare earth metal complex, such as tris(acetylacetonato)(monophenanthroline)terbium(III) (abbreviation: Tb(acac)3(Phen)). Among the above-mentioned materials, the organometallic iridium complex with a pyrimidine framework exhibits significantly higher reliability and emission efficiency and is therefore particularly preferred.

[0103] Examples of the phosphorescent material having an emission peak at 600 nm to 700 nm include organometallic iridium complexes with pyrimidine skeletons, such as bis[4,6-bis(3-methylphenyl)pyrimidinato](diisobutylylmethano)iridium(III) (abbreviation: Ir(5mdppm)2(dpm)), bis[4,6-bis(3-methylphenyl)pyrimidinato](dipivaloylmethanato)iridium(III) (abbreviation: Ir(5mdppm)2(dpm)) and bis[4,6-di(naphthalen-1-yl)pyrimidinato](dipivaloylmethanato)iridium(III) (abbreviation: Ir(d1npm)2(dpm)); organometallic iridium complexes with pyrazine frameworks, such as (acetylacetonato)bis(2,3,5-triphenylpyrazinato)iridium(III) (abbreviation: Ir(tppr)2(acac)), bis(2,3,5-triphenylpyrazinato)(dipivaloylmethanato)iridium(III) (abbreviation: Ir(tppr)2(dpm)) or (acetylacetonato)bis[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(III) (abbreviation: Ir(Fdpq)2(acac)); organometallic iridium complexes with pyridine frameworks, such as tris(1-phenylisoquinolinato-N,C 2' )iridium(III) (abbreviation: Ir(piq)3) and bis(1-phenylisoquinolinato-N,C2' iridium(III) acetylacetonate (abbreviation: Ir(piq)2acac); a platinum complex such as 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrinplatinum(II) (abbreviation: PtOEP); and rare earth metal complexes such as tris(1,3-diphenyl-1,3-propanedionato)(monophenanthroline)europium(III) (abbreviation: Eu(DBM)3(Phen)) and tris[1-(2-thenoyl)-3,3,3-trifluoroacetonato](monophenanthroline)europium(III) (abbreviation: Eu(TTA)3(Phen)). Among the above materials, the organometallic iridium complex with a pyrimidine framework exhibits significantly higher reliability and emission efficiency and is therefore particularly preferred. Furthermore, the organometallic iridium complex with a pyrazine framework can provide red light emission with preferential chromacity.

[0104] It should be noted that the “delayed fluorescence” exhibited by TADF materials refers to light emission, has the same spectrum as normal fluorescence, and has an extremely long lifetime. The lifetime is 10 -6 seconds or more, preferably 10 -3Seconds or more. Specific examples of TADF materials include fullerene, its derivative, and acridine derivatives such as proflavine and eosin. A metal-containing porphyrin, such as a porphyrin containing magnesium (Mg), zinc (Zn), cadmium (Cd), tin (Sn), platinum (Pt), indium (In), or palladium (Pd), can also be cited. Examples of the metal-containing porphyrin include a protoporphyrin-tin fluoride complex (SnF2(Proto IX)), a mesoporphyrin-tin fluoride complex (SnF2(Meso IX)), a hematoporphyrin-tin fluoride complex (SnF2(Hemato IX)), a coproporphyrin tetramethyl ester-tin fluoride complex (SnF2(Copro III-4Me)), an octaethylporphyrin-tin fluoride complex (SnF2(OEP)), an etioporphyrin-tin fluoride complex (SnF2(Etio I)) and an octaethylporphyrin-platinum chloride complex (PtCl2(OEP)).Alternatively, a heterocyclic compound including a π-electron-rich heteroaromatic ring and a π-electron-poor heteroaromatic ring can be used, such as 2-(biphenyl-4-yl)-4,6-bis(12-phenylindolo[2,3-α]carbazol-11-yl)-1,3,5-triazine (abbreviation: PIC-TRZ). It should be noted that a material in which the π-electron-rich heteroaromatic ring is directly bonded to the π-electron-poor heteroaromatic ring is particularly preferred, since in this case, the donor property of the π-electron-rich heteroaromatic ring and the acceptor property of the π-electron-poor heteroaromatic ring are both enhanced, or the energy difference between the S1 level and the T1 level is small.

[0105] It should be noted that the materials used as the first organic compound 120 (host material), the second organic compound 122 (assist material), and the third organic compound 124 (guest material) are not limited to those mentioned above. The combination is determined so that an exciplex can be formed, wherein the emission spectrum of the exciplex overlaps with the absorption spectrum of the third organic compound 124 (guest material), and the peak of the emission spectrum of the exciplex has a longer wavelength than the peak of the absorption spectrum of the third organic compound 124 (guest material).

[0106] In the case where an electron-transport material is used as the first organic compound 120 (host material) and a hole-transport material is used as the second organic compound 122 (assist material), the carrier balance can be controlled by the mixing ratio of the compounds. Specifically, the ratio of the first organic compound 120 to the second organic compound 122 is preferably 1:9 to 9:1.

[0107] The electron-transport layer 114 is a layer containing a substance with high electron-transport properties. In addition to the above-mentioned electron-transport materials, a metal complex such as tris(8-quinolinolato)aluminum (abbreviation: 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) can be used as the electron-transport layer 114.A heteroaromatic compound such as 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation: PBD), 1,3-Bs[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) can also be used. 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) can also be used. The substances listed here are mainly those with an electron mobility of 10. -6 cm 2 / Vs or more. It should be noted that any substance other than the above substances can be used for the electron-transport layer 114 as long as its electron-transport property is higher than its hole-transport property.

[0108] The electron transport layer 114 is not limited to a single layer, but may be a stack of two or more layers containing one of the above substances.

[0109] The electron injection layer 115 is a layer containing a substance with a high electron injection property. For the electron injection layer 115, a compound of an alkali metal or an alkaline earth metal, such as lithium fluoride (LiF), cesium fluoride (CsF), calcium fluoride (CaF2), or lithium oxide (LiO) may be used. x) may be used. A rare earth metal compound such as erbium fluoride (ErF3) may also be used. Any of the substances forming the electron-transport layer 114 listed above may also be used.

[0110] A composite material in which an organic compound and an electron donor (donor) are mixed can also be used as the electron-injection layer 115. Such a composite material has excellent electron-injection properties and electron-transport properties because the electron donor causes electron generation in the organic compound. In this case, the organic compound is preferably a material that is excellent at transporting the generated electrons. Specifically, for example, the substances for forming the electron-transport layer 114 (e.g., a metal complex and a heteroaromatic compound) described above can be used. As the electron donor, a substance that has an electron-donating property with respect to the organic compound can be used.In particular, an alkali metal, an alkaline earth metal, and a rare earth metal are preferred, and lithium, cesium, magnesium, calcium, erbium, ytterbium, and the like can be cited. Alkali metal oxides or alkaline earth metal oxides are also preferred, and examples of these include lithium oxide, calcium oxide, and barium oxide. A Lewis base such as magnesium oxide can also be used. An organic compound such as tetrathiafulvalene (abbreviation: TTF) can also be used.

[0111] It should be noted that each of the above-described hole injection layer 111, hole transport layer 112, light emitting layer 113, electron transport layer 114 and electron injection layer 115 can be formed by a method such as an evaporation method (e.g., a vacuum evaporation method), an inkjet method or a coating method.

[0112] The light emission obtained in the light-emitting layer 113 of the above-described light-emitting element is extracted to the outside through one or both of the first electrode 101 and the second electrode 103. Therefore, in this embodiment, one or both of the first electrode and the second electrode 103 is an electrode with a light-transmitting property.

[0113] As described above, in the light-emitting element described in this embodiment, since energy transfer utilizes an overlap between an emission spectrum of an exciplex and an absorption spectrum of the third organic compound (guest material), the energy transfer efficiency can be improved; accordingly, the light-emitting element can have high emission efficiency.

[0114] Furthermore, in the light-emitting element described in this embodiment, the fourth organic compound, whose HOMO level is lower than or equal to that of the second organic compound, having a hole-transporting property is used in the light-emitting layer, and the fifth organic compound, whose HOMO level is higher than that of the second organic compound, is used in the hole-transporting layer. Such a structure makes it possible to increase the current efficiency of the light-emitting element and reduce the voltage, especially the turn-on voltage.

[0115] Furthermore, the light-emitting element described in this embodiment is an embodiment of the present invention and is particularly characterized by the structures of the hole-transport layer and the light-emitting layer. Therefore, when the structure described in this embodiment is employed, a passive matrix light-emitting device, an active matrix light-emitting device, and the like can be manufactured. Each of these light-emitting devices is included in the present invention.

[0116] It should be noted that there is no particular limitation on the structure of the TFT in the case of the active matrix light-emitting device. For example, a staggered TFT or an inverted staggered TFT can be used as appropriate. Furthermore, a driving circuit formed on a TFT substrate can be formed using both an n-type TFT and a p-type TFT, or only an n-type TFT or a p-type TFT. Furthermore, there is no particular limitation on the crystallinity of a semiconductor film used as a TFT. For example, an amorphous semiconductor film, a crystalline semiconductor film, an oxide semiconductor film, or the like can be used.

[0117] It should be noted that the structures used in this embodiment can be suitably combined with the structures described in the other embodiments. (Embodiment 3)

[0118] In this embodiment, as one embodiment of the present invention, a light-emitting element (referred to as a tandem light-emitting element) in which a charge generation layer is provided between a plurality of light-emitting layers will be described.

[0119] A light-emitting element described in this embodiment is a tandem light-emitting element including a plurality of light-emitting layers (a first light-emitting layer 311 and a second light-emitting layer 312) between a pair of electrodes (a first electrode 301 and a second electrode 303) as shown in Fig. 7A.

[0120] In this embodiment, the first electrode 301 serves as an anode, and the second electrode 303 serves as a cathode. It should be noted that the first electrode 301 and the second electrode 303 have structures similar to those described in Embodiment 2. In addition, although the plurality of light-emitting layers (the first light-emitting layer 311 and the second light-emitting layer 312) may have structures similar to those described in Embodiment 1 or 2, each of the light-emitting layers may have a structure similar to those described in Embodiments 1 or 2. In other words, the structure of the first light-emitting layer 311 and the second light-emitting layer 312 may be the same as or different from each other, and may be similar to the structure described in Embodiments 1 or 2.It should be noted that one or both of the first light-emitting layer 311 and the second light-emitting layer 312 includes the hole transport layer described in Embodiment 1 or 2.

[0121] Furthermore, a charge generation layer 313 is provided between the plurality of light-emitting layers (the first light-emitting layer 311 and the second light-emitting layer 312). The charge generation layer 313 has a function of injecting electrons into one of the light-emitting layers and a function of injecting holes into one of the other light-emitting layers when a voltage is applied between the first electrode 301 and the second electrode 303. In this embodiment, the charge generation layer 313 injects electrons into the first light-emitting layer 311 and injects holes into the second light-emitting layer 312 when a voltage is applied, so that the potential of the first electrode 301 is higher than that of the second electrode 303.

[0122] It should be noted that, in terms of light extraction efficiency, the charge generation layer 313 preferably has a visible light transmitting property (specifically, the charge generation layer 313 has a visible light transmittance of 40% or more). Furthermore, the charge generation layer 313 functions even if it has a lower conductivity than the first electrode 301 or the second electrode 303.

[0123] The charge generation layer 313 may have either a structure in which an electron acceptor (acceptor) is added to an organic compound with a high hole-transport property or a structure in which an electron donor (donor) is added to an organic compound with a high electron-transport property. Alternatively, both of these structures may be stacked.

[0124] In the case of the structure in which an electron acceptor is added to an organic compound having a high hole-transport property, such as the organic compound having a high hole-transport property, for example, an aromatic amine compound such as NPB, TPD, TDATA, MTDATA, or 4,4'-bis[N-(spiro-9,9'-bifluoren-2-yl)-N-phenylamino]biphenyl (abbreviation: BSPB) or the like can be used. The substances indicated here are mainly those having a hole mobility of 10 -6 cm 2 / Vs or more. It should be noted that any substance other than the above substances can be used as long as its hole-transporting property is higher than its electron-transporting property.

[0125] Further examples of the electron acceptor include 7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation: F4-TCNQ), chloranil, and the like. Other examples include transition metal oxides. Other examples include oxides of metals belonging to Group 4 to Group 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 metal oxides, molybdenum oxide is particularly preferred because of its stability in air, low hygroscopicity, and ease of handling.

[0126] On the other hand, in the case of the structure in which an electron donor is added to an organic compound with a high electron-transport property, a metal complex having a quinoline skeleton or a benzoquinoline skeleton, such as Alq, Almq3, BeBq2, or BAlq, or the like, can be used as the organic compound with a high electron-transport property. Alternatively, a metal complex having an oxazole-based ligand or a thiazole-based ligand, such as Zn(BOX)2 or Zn(BTZ)2, can be used. Besides such a metal complex, PBD, OXD-7, TAZ, BPhen, BCP, or the like can be used. The substances mentioned here are mainly those having an electron mobility of 10 -6 cm 2 / Vs or more. It should be noted that any substance other than the above substances can be used as long as its electron transport property is higher than its hole transport property.

[0127] Furthermore, an alkali metal, an alkaline earth metal, a rare earth metal, a metal belonging to Group 13 of the Periodic Table, or an oxide or carbonate thereof can be used as an electron donor. In particular, lithium (Li), cesium (Cs), magnesium (Mg), calcium (Ca), ytterbium (Yb), indium (In), lithium oxide, cesium carbonate, or the like are preferably used. An organic compound such as tetrathianaphthacene can also be used as the electron donor.

[0128] By forming the charge generation layer 313 with any of the above materials, it is possible to suppress an increase in the driving voltage caused when the light-emitting layers are stacked.

[0129] Although Fig. 7A illustrates the light-emitting element containing two light-emitting layers, the present invention can be similarly applied to a light-emitting element in which n light-emitting layers (n is 3 or more) are stacked as shown in Fig.7B. In the case where a plurality of light-emitting layers are provided between a pair of electrodes, as in the light-emitting element in this embodiment, by providing a charge generation layer 313 between the light-emitting layers, the light-emitting element can emit light in a high luminance range while keeping the current density low. Since the current density can be kept low, the element can have a long lifetime. When the light-emitting element is used for illumination, a voltage loss due to the resistance of an electrode material can be reduced, thereby achieving homogeneous light emission in a wide area. Furthermore, it is possible to obtain a light-emitting device that operates at a low voltage and has low power consumption.

[0130] Furthermore, by making the emission colors of the light-emitting layers different, light of a desired color can be obtained from the light-emitting element as a whole. For example, the emission colors of the first and second light-emitting layers are complementary in a light-emitting element having two light-emitting layers, so that the light-emitting element can be manufactured to emit white light as a whole. It should be noted that the term "complementary" refers to the color relationship in which an achromatic color is obtained when colors are mixed. In other words, the emission of white light can be obtained by mixing lights emitted from substances whose emission colors are complementary colors.

[0131] Furthermore, the same applies to a light-emitting element with three light-emitting layers. For example, the light-emitting element can emit white light as a whole if the emission color of the first light-emitting layer is red, the emission color of the second light-emitting layer is green, and the emission color of the third light-emitting layer is blue.

[0132] It should be noted that the structure described in this embodiment can be suitably combined with any of the structures described in the other embodiments. (Example 4)

[0133] In this embodiment, a light-emitting device which is an embodiment of the present invention will be described.

[0134] A light-emitting device described in this embodiment has a micro-optical resonator (microcavity) structure in which a light resonance effect is used between a pair of electrodes. The light-emitting device comprises, as shown in Fig. 8, a plurality of light-emitting elements, at least one of which has an EL layer 455 between a pair of electrodes (a reflective electrode 451 and a semi-transparent and semi-reflective electrode 452). Furthermore, the EL layer 455 includes at least one hole-transport layer (not shown) and light-emitting layers 454 (454R, 454G, and 454B), and may further include a hole-injection layer, an electron-transport layer, an electron-injection layer, a charge-generation layer, and the like.

[0135] A first light-emitting element 450R has a structure in which a first transparent conductive layer 453A, an EL layer 455 including a first light-emitting layer 454B, a second light-emitting layer 454G, and a third light-emitting layer 454R, and the semi-transmissive and semi-reflective electrode 452 are sequentially stacked on the reflective electrode 451. A second light-emitting element 450G has a structure in which a transparent conductive layer 453B, the EL layer 455, and the semi-transmissive and semi-reflective electrode 452 are sequentially stacked on the reflective electrode 451. A third light-emitting element 450B has a structure in which the EL layer 455 and the semi-transmissive and semi-reflective electrode 452 are sequentially stacked on the reflective electrode 451.

[0136] It should be noted that the reflective electrode 451, the EL layer 455, and the semi-transparent and semi-reflective electrode 452 are common to the light-emitting elements (the first light-emitting element 450E, the second light-emitting element 450G, and the third light-emitting element 450B). The first light-emitting layer 450B emits light (λ B ) with a peak in a wavelength range of 420 nm to 480 nm. The second light-emitting layer 454G emits light (λ G ) with a peak in a wavelength range of 500 nm to 550 nm. The third light-emitting layer 454R 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 450R, the second light-emitting element 450G, and the third light-emitting element 450B), the lights emitted from the first light-emitting layer 454B, the second light-emitting layer 454G, and the third light-emitting layer 454R overlap with each other; consequently, light with a broad emission spectrum covering a range of visible light can be emitted. It should be noted that the above wavelength satisfies the relationship λ B < λ G < λ R fulfill.

[0137] Each of the light-emitting elements described in this embodiment has a structure in which an EL layer 455 is disposed between the reflective electrode 451 and the semi-transparent and semi-reflective electrode 452. The light emitted in all directions from the light-emitting layers, including the EL layer 455, is resonated by the reflective electrode 451 and the semi-transparent and semi-reflective electrode 452, which serve as a micro-optical resonator (microcavity). It should be noted that the reflective electrode 451 is formed using a conductive material with reflectivity, and a film whose visible light reflectivity is 40% to 100%, preferably 70% to 100%, and whose resistance is 1 × 10 -2Ωcm or less is used. In addition, the semi-transparent and semi-reflective electrode 452 is formed using a conductive material having a reflectivity and a conductive material having a light transmittance property, and a film whose visible light reflectivity is 20% to 80%, preferably 40% to 70%, and whose resistance is 1 × 10 -2 Ωcm or less is used.

[0138] In this embodiment, the thickness of the transparent conductive layers (the first transparent conductive layer 453A and the second transparent conductive layer 453B) provided in the first light-emitting element 450R and the second light-emitting element 450G are varied between the light-emitting elements, whereby the light-emitting elements differ in the optical path length from the reflective electrodes 451 to the semi-transparent and semi-reflective electrode 452. In other words, for light with a broad emission spectrum emitted from the light-emitting layers of each of the light-emitting elements, light with a wavelength that oscillates between the reflective electrode 451 and the semi-transparent and semi-reflective electrode 452 can be amplified, while light with a wavelength that does not oscillate between them is attenuated.Therefore, if the elements differ in the optical path length from the reflective electrode 451 to the semi-transparent and semi-reflective electrode 452, light with different wavelengths can be extracted.

[0139] It should be noted that the optical path length (also referred to as optical distance) is expressed as a product of the actual distance and the refractive index. In this embodiment, the optical path length is a product of the actual thickness and n (refractive index); ie: optical path length = actual thickness x n.

[0140] Furthermore, the optical path length from the reflective electrode 451 to the semi-transparent and semi-reflective electrode 452 is set to mλ R / 2 (m is a natural number of 1 or more) in the first light-emitting element 450R; the optical path length from the reflective electrode 451 to the semi-transparent and semi-reflective electrode 452 is set to mλ G / 2 (m is a natural number of 1 or more) in the second light-emitting element 450G; and the optical path length from the reflective electrode 451 to the semi-transparent and semi-reflective electrode 452 is set to mλ B / 2 (m is a natural number of 1 or more) in the third light-emitting element 450B.

[0141] In this way, the light (λ R ), which extracts from the third light-emitting layer 454R contained in the EL layer 455, mainly from the first light-emitting element 450R, the light (λ G) emitted from the second light-emitting layer 454G contained in the EL layer 455 is mainly extracted by the second light-emitting element 450G, and the light (λ B ) emitted from the first light-emitting layer 454B included in the EL layer 455 is mainly extracted by the third light-emitting element 450B. It should be noted that the light extracted from each of the light-emitting elements is emitted through the side of the semi-transparent and semi-reflective electrode 452.

[0142] Furthermore, strictly speaking, the optical path length from the reflective electrode 451 to the semi-transparent and semi-reflective electrode 452 is the distance from a reflection region in the reflective electrode 451 to a reflection region in the semi-transparent and semi-reflective electrode 452. However, it is difficult to precisely determine the position of the reflection regions in the reflective electrode 451 and the semi-transparent and semi-reflective electrode 452; therefore, it is believed that the above effect can be sufficiently achieved by adjusting the reflection regions in the reflective electrode 451 and the semi-transparent and semi-reflective electrode 452.

[0143] Subsequently, the optical path length from the reflective electrode 451 to the third light-emitting layer 454R is set to (2n R -1)λ R / 4 (n Ris a natural number of 1 or more) because, in the first light-emitting element 450R, light (first reflected light) reflected by the reflective electrode 451 of the light emitted from the third light-emitting layer 454R interferes with light (first incident light) directly entering the semi-transmissive and semi-reflective electrode 452 from the third light-emitting layer 454R. By adjusting the optical path length, the colors of the first reflected light and the first incident light can be aligned with each other, and the light emitted from the third light-emitting layer 454R can be amplified.

[0144] It should be noted that the optical path length from the reflective electrode 451 to the third light-emitting layer 454R may, strictly speaking, be the optical path length from a reflection region in the reflective electrode 451 to a light-emitting region in the third light-emitting layer 454R. However, it is difficult to precisely determine the positions of the reflection regions in the reflective electrode 451 and the light-emitting region in the third light-emitting layer 454R; therefore, it is considered that the above effect is sufficiently achieved wherever the reflection region and the light-emitting region are arranged in the reflective electrode 451 and the third light-emitting layer 454R.

[0145] Subsequently, the optical path length from the second reflective electrode 451 to the second light-emitting layer 454G is set to (2n G -1)λ G / 4 (n Gis a natural number of 1 or more) because, in the second light-emitting element 450G, light (second reflected light) reflected by the reflective electrode 451 of the light emitted from the second light-emitting layer 454G interferes with light (second incident light) directly entering the semi-transmissive and semi-reflective electrode 452 from the second light-emitting layer 454G. By adjusting the optical path length, the phases of the second reflected light and the second incident light can be aligned with each other, and the light emitted from the second light-emitting layer 454G can be amplified.

[0146] It should be noted that the optical path length from the reflective electrode 451 to the second light-emitting layer 454G may, strictly speaking, be the optical path length from a reflection region in the reflective electrode 451 to a light-emitting region in the second light-emitting layer 454G. However, it is difficult to determine the positions of the reflection regions in the reflective electrode 451 and the light-emitting region in the second light-emitting layer 454G; therefore, it is believed that the above effect can be sufficiently achieved if the reflection region and the light-emitting region are arranged on the reflective electrode 451 and the second light-emitting layer 454G.

[0147] Subsequently, the optical path length from the reflective electrode 451 to the first light-emitting layer 454B is set to (2n B -1)λ B / 4 (n Bis a natural number of 1 or more) because, in the third light-emitting element 450B, light (third reflected light) emitted from the reflective electrode 451 of the light emitted from the first light-emitting layer 454B interferes with light (third incident light) directly entering the semi-transmissive and semi-reflective electrode 452 from the first light-emitting layer 454B. By adjusting the optical path length, the phases of the third reflected light and the third incident light can be aligned with each other, and the light emitted from the first light-emitting layer 454B can be amplified.

[0148] It should be noted that the optical path length from the reflective electrode 451 to the first light-emitting layer 454B may be the optical path length from a reflection region in the reflective electrode 451 to a light-emitting region in the first light-emitting layer 454B. However, it is difficult to precisely determine the position of the reflection region in the reflective electrode 451 and the light-emitting region in the first light-emitting layer 454B; therefore, it is believed that the above effect can be sufficiently achieved if the reflection region and the light-emitting region are arranged on the reflective electrode 451 and the first light-emitting layer 454B.

[0149] It should be noted that although each of the light-emitting elements in the above-described structures includes a plurality of light-emitting layers in the EL layer, the present invention is not limited thereto; for example, the structure of a tandem (stacked type) light-emitting element described in Embodiment 3 may be combined, in which case a plurality of light-emitting layers are provided so that a charge generation layer is interposed between each light-emitting element.

[0150] The light-emitting device described in this embodiment has a microcavity structure in which light with wavelengths that vary depending on the light-emitting elements can be extracted even when they comprise EL layers with the same structure, so that it is not necessary to form the light-emitting elements for the R, G, and B colors. Therefore, the above structure is advantageous for a full-color display because a display with higher resolution or the like can be easily achieved. In addition, the emission intensity with a predetermined wavelength in the front direction can be increased, thereby reducing power consumption. The above structure is particularly suitable when applied to a color display (image display device) including pixels of three or more colors, but can also be applied to an illumination or the like.

[0151] It should be noted that the structure described in this embodiment can be suitably combined with any of the structures described in the other embodiments. (Embodiment 5)

[0152] In this embodiment, a light-emitting device including a light-emitting element, which is an embodiment of the present invention, will be described.

[0153] The light-emitting device can be either a passive matrix light-emitting device or an active matrix light-emitting device. It should be noted that any of the light-emitting elements described in the other embodiments can be used in the light-emitting device described in this embodiment.

[0154] In this embodiment, an active matrix light emitting device is described with reference to the Fig.9A and Fig. 9B.

[0155] Fig. Fig. 9A is a plan view showing a light-emitting device and Fig. Figure 9B shows a cross section along the dashed line AB in Fig. 9A. The active matrix light-emitting device of this embodiment includes a pixel region 502 provided on an element substrate 501, a drive circuit region (a source line drive circuit) 503, and drive circuit regions (gate line drive circuit) 504 (504a and 504b). The pixel region 502, the drive circuit region 503, and the drive circuit region 504 are sealed with a sealant 505 between the element substrate 501 and a sealing substrate 506.

[0156] In addition, on the element substrate 501, a lead wiring 507 is provided to connect an external input terminal through which a signal (e.g., a video signal, a timing signal, a start signal, a reset signal, or the like) or an electric potential is transmitted from the outside to the drive circuit portion 503 and the drive circuit portion 504. Here, an example will be described in which a flexible printed circuit (FPC) 508 is provided as the external input terminal. Although only the FPC is illustrated here, a printed circuit board (PWB) may also be attached to the FPC. The light-emitting device in this specification includes in the category not only the light-emitting device itself but also the light-emitting device provided with the FPC or the PWB.

[0157] Below, a cross-sectional structure is presented with reference to Fig.9B. The driver circuit region and the pixel region are formed on the element substrate 501; here, the driver circuit region 503, which is the source line driver circuit, and the pixel region 502 are shown.

[0158] An example is shown in which a CMOS circuit comprising a combination of an n-channel TFT 509 and a p-channel TFT 510 is formed as the driver circuit portion 503. It should be noted that a circuit included in the driver circuit portion can be formed using any of a variety of circuits, such as a CMOS circuit, a PMOS circuit, or an NMOS circuit. In this embodiment, although a driver-integrated structure in which a driver circuit is formed on a substrate is described, a driver circuit is not necessarily formed on a substrate but may be formed outside a substrate.

[0159] The pixel region 502 is formed by a plurality of pixels, each including a switching TFT 511, a current control TFT 512, and a first electrode 513 electrically connected to a wiring (a source electrode or a drain electrode) of the current control TFT 512. An insulator 514 is formed so as to cover an edge portion of the first electrode 513. In this embodiment, the insulator 514 is formed using a positive photosensitive acrylic resin. It should be noted that the first electrode 513 is used as an anode in this embodiment.

[0160] In addition, in order to achieve preferential coverage by a film to be applied or stacked on the insulator 514, the insulator 514 is preferably formed to have a curved surface with a curvature at the upper edge portion or a lower edge portion. For example, in the case where a positive photosensitive acrylic resin is used as a material for the insulator 514, the insulator 514 is preferably formed to have a curved surface with a radius of curvature (0.2 μm to 3 μm) at the upper edge portion. The insulator 514 can be formed using either 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.

[0161] An EL layer 515 and a second electrode 516 are stacked on the first electrode 513. At least a hole-transporting layer and a light-emitting layer are provided in the EL layer 515. The structure described in Embodiment 1 or 2 can be used for the hole-transporting layer and the light-emitting layer. It should be noted that the second electrode 516 is used as a cathode in this embodiment.

[0162] A light-emitting element 517 is formed on a stacked structure of the first electrode 513, the EL layer 515, and the second electrode 516. Any of the materials described in Embodiment 2 can be used for each of the first electrode 513, the EL layer 515, and the second electrode 516. Although not described, the second electrode 516 is electrically connected to an FPC 508, which is an external input terminal.

[0163] Although the cross-section in Fig. While FIG. 9B illustrates only one light-emitting element 517, a plurality of light-emitting elements are arranged in a matrix in the pixel region 502. Light-emitting elements emitting three types of colors (R, G, and B) are selectively formed in the pixel region 502, whereby a light-emitting device suitable for a full-color display can be obtained. Alternatively, a light-emitting device suitable for a full-color display can be manufactured by combining it with color filters.

[0164] Furthermore, the sealing substrate 506 is attached to the element substrate 501 via the sealant 505, thereby providing a light-emitting element 517 in a space 518 surrounded by the element substrate 501, the sealing substrate 506, and the sealant 505. It should be noted that the space 518 may be filled with an inert gas (such as nitrogen and argon) or the sealant 505.

[0165] An epoxy-based resin is preferably used as the sealant 505. Such a material ensures that as little moisture and oxygen as possible can penetrate. As the sealing substrate 506, a plastic substrate formed from fiberglass reinforced plastic (FRP), polyvinyl fluoride (PVF), polyester, acrylic, or the like can be used, in addition to a glass substrate or a cross-layer substrate.

[0166] As described above, an active matrix light-emitting device can be obtained.

[0167] It should be noted that the structure described in this embodiment can be appropriately combined with any structures described in the other embodiments. (Embodiment 6)

[0168] In this embodiment, electronic devices are described, each including the light-emitting device 3 of an embodiment of the present invention described in the above embodiment. Examples of the electronic device include cameras such as video cameras and digital cameras, Google-type displays, navigation systems, audio playback devices (e.g., automotive audio systems and audio systems), computers, game consoles, portable information terminals (e.g., mobile computers, mobile phones, smartphones, portable game consoles, e-book readers, and tablets), and image playback devices in which a recording medium is provided (in particular, devices capable of playing recorded media, such as DVDs, and equipped with a display device capable of displaying an image). Specific examples of these electronic devices will be described with reference to the Fig. 10 to 10D and Fig. 11A to 11D.

[0169] Fig. 10A shows a television of one embodiment of the present invention, including a cabinet 611, a base plate 612, a display portion 613, speaker portions 614, video input terminals 615, and the like. In this television, the light-emitting device of one embodiment of the present invention can be incorporated into the display portion 613. Since the light-emitting device of one embodiment of the present invention operates at a low voltage and has high power efficiency, a television with reduced power consumption can be obtained by incorporating the light-emitting device of one embodiment of the present invention.

[0170] Fig.10B shows a computer according to an embodiment of the present invention, including a main body 621, a casing 622, a display portion 623, a keyboard 624, an external connection terminal 625, a pointing device 626, and the like. In this computer, the light-emitting device according to an embodiment of the present invention can be incorporated into the display portion 623. Since the light-emitting device according to an embodiment of the present invention operates at a low voltage and has a high current efficiency, a computer with reduced power consumption can be obtained by employing the light-emitting device according to an embodiment of the present invention.

[0171] Fig.10C shows a mobile phone of one embodiment of the present invention, including a main body 631, a casing 632, a display device 633, an audio input portion 634, an audio output portion 635, operation keys 636, an external connection terminal 637, an antenna 638, and the like. In this mobile phone, the light-emitting device of one embodiment of the present invention can be employed in the display portion 633. Since the light-emitting device of one embodiment of the present invention operates at a low voltage and has a high current efficiency, a mobile phone with reduced power consumption can be achieved by employing the light-emitting device of one embodiment of the present invention.

[0172] Fig.10D shows a camera of one embodiment of the present invention, including a main body 641, a display portion 642, a housing 643, an external connection terminal 644, a remote control input portion 645, an image receiving portion 646, a battery 647, an audio input portion 648, operation buttons 649, an eye portion 650, and the like. In this camera, the light-emitting device of one embodiment of the present invention can be employed in the display portion 642. Since the light-emitting device of one embodiment of the present invention operates at a low voltage and has a high current efficiency, a camera with reduced power consumption can be obtained by employing the light-emitting device of one embodiment of the present invention.

[0173] Fig. 11A to 11D show examples of portable terminals of an embodiment of the present invention. Fig. 11A to 11C show a portable terminal 5000 and Fig. 11D shows a portable Terminal 6000.

[0174] Fig. 11A, Fig. 11B and Fig. 11C show a front view, side view, and rear view of the Portable Terminal 5000. Fig. Figure 11D shows a front view of the portable terminal 6000.

[0175] The portable terminal 5000 includes a housing 5001, a display portion 5003, a power button 5005, a front camera 5007, a rear camera 5009, a first external connection port 5011, a second external connection port 5013, and the like.

[0176] In addition, the display portion 5003 is installed in the housing 5001 and can be used as a touch panel. For example, emailing or schedule management can be performed by touching an icon 5015 and the like on the display portion 5003. Furthermore, the front camera 5007 is installed on the front side of the housing 5001, allowing an image to be captured from the user's side. The rear camera 5009 is installed on the rear side of the housing 5001, allowing an image to be captured from the opposite side of the user. Furthermore, the housing 5001 includes the first external connection port 5011 and the second external connection port 5013. Sound can be output to a headset or the like through the first external connection port 5011, and data can be input through the second external connection port 5013.

[0177] The portable Terminal 6000 in Fig. 11D includes a first housing 6001, a second housing 6003, a hinge portion 6005, a first display portion 6007, a second display portion 6009, an on / off button 6011, a first camera 6013, a second camera 6015, and the like.

[0178] The first display portion 6007 is installed in the first housing 6001. The second display portion 6009 is installed in the second housing 6003. For example, the first display portion 6001 and the second display portion 6009 are used as a display panel and a touch panel, respectively. A user can select images and input characters by touching an icon 6019 displayed on the second display portion 6009 or a keyboard 6021 (actually, a keyboard image displayed on the second display portion 6009) while looking at a text icon 6017 displayed on the first display portion 6007. Alternatively, the first display portion 6007 and the second display portion 6009 may be a touch panel and a display panel, or the first display portion 6007 and the second display portion 6009 may be touch panels.

[0179] The first housing 6001 and the second housing 6003 are connected to each other and open and close at the hinge portion 6005. With such a structure, the first display portion 6007, which is installed in the first housing 6001, and the second display portion 6009, which is installed in the second housing 6003, are preferably arranged to face each other when the portable terminal 6000 is carried, so that the surfaces of the first display portion 6007 and the second display portion 6009 (e.g., plastic substrates) can be protected.

[0180] Alternatively, the first housing 6001 and the second housing 6003 can be separated by the hinge portion 6005 (referred to as a convertible type). This allows the application range of the portable terminal 6000 to be expanded, and the first housing 6001 can be used, for example, in a vertical orientation and the second housing 6003 in a horizontal orientation.

[0181] Furthermore, the first camera 6013 and the second camera 6015 can capture 3D images.

[0182] The Portable Terminal 5000 and Portable Terminal 6000 can send and receive data wirelessly. For example, desired data can be purchased and downloaded via a wireless internet connection.

[0183] The portable terminal 5000 and the portable terminal 6000 may include functions such as displaying various types of data (e.g., a still image, a moving image, and a text image), displaying a calendar, date, time, or the like on the display area, touch input for operating or editing the data displayed on the display area through touch input, and controlling processing by various types of software (programs). A detector such as a photodetector capable of optimizing the luminance of the display in accordance with external illumination or light, or a sensor for detecting inclination, such as a gyroscope or accelerometer, may be included.

[0184] The light-emitting device of one embodiment of the present invention can be used in the display area 5003 of the portable terminal 5000 and the first display area 6007 of the portable terminal 6000 and / or the second display area 6009 of the portable terminal 6000. Since the light-emitting device of one embodiment of the present invention operates at a low voltage and has a high current efficiency, a portable terminal with reduced power consumption can be obtained by using the light-emitting device of one embodiment of the present invention.

[0185] The light-emitting device of one embodiment of the present invention can also be used as a lighting device. Specific examples of the lighting device will be described with reference to Fig. 12A to 12C.

[0186] Fig.12A shows an example of a liquid crystal display device using the light-emitting device of one embodiment of the present invention as a backlight. The liquid crystal display device shown in Fig.12A includes a package 701, a liquid crystal panel 702, a backlight 703, and a case 704. The liquid crystal panel 702 is connected to a drive IC 705. The light-emitting device of one embodiment of the present invention is used as the backlight 703, and power is supplied via a terminal 706. By using the light-emitting device of one embodiment of the present invention as a backlight of a liquid crystal display device as described above, a backlight with low power consumption can be obtained. Furthermore, since the light-emitting device of one embodiment of the present invention is an illumination device for surface light emission and the scaling of the light-emitting device is possible, the backlight can be made larger.Therefore, a liquid crystal display device with a larger area can be obtained with low power consumption.

[0187] Fig. 12B shows an example in which the light-emitting device of one embodiment of the present invention is used as a table lamp, which is a lighting device. Fig. The table lamp shown in Figure 12B includes a housing 801 and a light source 802, and the light-emitting device of one embodiment of the present invention is used as a light source 802. Since the light-emitting device of one embodiment of the present invention operates at a low voltage and has a high current efficiency, a table lamp with reduced power consumption can be obtained by using the light-emitting device of one embodiment of the present invention.

[0188] Fig.12C shows an example in which the light-emitting device of one embodiment of the present invention is used for an indoor lighting device 901. Since the light-emitting device of one embodiment of the present invention can have a large area, the light-emitting device of one embodiment of the present invention can be used as a large-area lighting device. Since the light-emitting device of one embodiment of the present invention operates at a low voltage and has a high current efficiency, a lighting device with reduced power consumption can be obtained by using the light-emitting device of one embodiment of the present invention.In a room in which the light-emitting device of an embodiment of the present invention is used as the indoor lighting device 901 as described above, a television 902 of an embodiment of the present invention described with reference to FIG. 4 may be used. Fig. 10A, so that public television and films can be viewed.

[0189] It should be noted that this embodiment may be freely combined with any of the other embodiments, where appropriate. [Example 1]

[0190] In this example, a light-emitting element of an embodiment of the present invention (a light-emitting element 1) and a light-emitting element for comparison purposes (a comparative light-emitting element 2) are described with reference to Fig.13. Chemical formulas of the materials used in this example are shown below.

[0191] Manufacturing processes of the light-emitting element 1 and the comparative light-emitting element 2 are described below. (Light-emitting element 1)

[0192] First, an indium tin oxide compound containing silicon or silicon oxide (ITO-SiO2, hereinafter referred to as ITSO) was deposited over a substrate 1100 by a sputtering method, forming the first electrode 1101. It should be noted that the composition ratio of In2O3 to SnO2 and SiO2 in the target used was 85:10:5 wt%. The thickness of the first electrode 1101 was set to 110 nm, and the electrode area was 2 mm × 2 mm. Here, the first electrode 1101 is an electrode serving as an anode of the light-emitting element.

[0193] Subsequently, as a pretreatment for fabricating the light-emitting element on the substrate 1100, UV ozone treatment was performed for 370 seconds after a surface of the substrate was washed with water and the substrate was heated at 200 °C for one hour.

[0194] The substrate 1100 was then transferred to a vacuum deposition device in which pressure was increased to approximately 10 -4 Pa and heated in a heating chamber of the vacuum evaporation device for 30 minutes in vacuum at 170 °C and then the substrate 1100 was cooled for about 30 minutes.

[0195] Thereafter, the substrate 1100 on which the first electrode 1101 was formed was fixed to a holder in the vacuum evaporation apparatus such that a surface of the substrate 1100 on which the first electrode 1101 was formed was provided facing downward. The pressure in the vacuum evaporation apparatus was set to approximately 10 -4Pa. Subsequently, by a vapor deposition method using resistance heating, 4,4',4"-(benzene-1,3,5-triyl)tri(dibenzothiophene) (abbreviation: DBT3P-II) and molybdenum oxide were co-evaporated, thereby forming a hole-injection layer 1111. The thickness of the hole-injection layer 1111 was set to 40 nm, and the weight ratio of DBT3P-II (abbreviation) to molybdenum oxide was set to 4:2 (= DBT3P-II:molybdenum oxide).

[0196] Subsequently, 4-phenyl-4'-(9-phenylfluoren-9-yl)triphenylamine (abbreviation: BPAFLP) and 3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole (abbreviation: PCzPCA1) were co-deposited onto the hole-injection layer 1111, forming a hole-transport layer 1112. The thickness of the hole-transport layer 1112 was set to 20 nm. The weight ratio of BPAFLP (abbreviation) to PCzPCA1 (abbreviation) was set to 0.5 to 0.5 (= BPAFLP:PCzPCA1).

[0197] It should be noted that BPAFLP (abbreviation) is a fourth organic compound and PCzPCA1 (abbreviation) is a fifth organic compound in the hole transport layer 1112.

[0198] Subsequently, 2mDBTPDBq-II (abbreviation), 4-phenyl-4'-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PCBA1BP), and (acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III) (abbreviation: [Ir(tBuppm)2(acac)]) were co-evaporated onto the hole-transport layer 1112, forming a first light-emitting layer 1113. The weight ratio of 2mDBTPDBq-II (abbreviation) to PCBA1BP (abbreviation) and [Ir(tBuppm)2(acac)] (abbreviation) was set to 0.8:0.2:0.06 (= 2mDBTPDBq-II:PCBA1BP:[Ir(tBuppm)2(acac)]). The thickness of the first light-emitting layer 1113a was set to 20 nm.

[0199] It should be noted that in the first light-emitting layer 1113a, 2mDBTPDBq-II (abbreviation) is a first organic compound (host material), PCBA1BP (abbreviation) is a second organic compound (assist material), and [Ir(tBuppm)2(acac)] (abbreviation) is a third organic compound (guest material).

[0200] Subsequently, 2mDBTPDBq-II (abbreviation), PCBA1BP (abbreviation), and [Ir(tBuppm)2(acac)] (abbreviation) were co-deposited on the first light-emitting layer 1113a, forming a second light-emitting layer 1113b. The weight ratio of 2mDBTPDBq-II (abbreviation) to PCBA1BP (abbreviation) and [Ir(tBuppm)2(acac)] (abbreviation) was set to 0.8:0.2:0.05 (= 2mDBTPDBq-II:PCBA1BP:[Ir(tBuppm)2(acac)]). The thickness of the second light-emitting layer 1113b was set to 20 nm.

[0201] It should be noted that in the second light-emitting layer 1113b, 2mDBTPDBq-II (abbreviation) is a first organic compound (host material), PCBA1BP (abbreviation) is a second organic compound (assist material), and [Ir(tBuppm)2(acac)] (abbreviation) is a third organic compound (guest material).

[0202] Subsequently, 2mDBTPDBq-II (abbreviation) was deposited on the second light-emitting layer 1113b with a thickness of 10 nm, thereby forming a first electron transport layer 1114a.

[0203] Subsequently, a bathophenanthroline (abbreviation: BPhen) with a thickness of 20 nm was deposited on the first electron transport layer 1114a, forming a second electron transport layer 1114b.

[0204] Then, lithium fluoride (LiF) was deposited to a thickness of 1 nm on the second electron transport layer 1114b by evaporation, thereby forming an electron injection layer 1115.

[0205] Finally, aluminum (Al) was deposited by evaporation to a thickness of 200 nm as a second electrode 1103, which served as a cathode. In this way, the light-emitting element of this example was fabricated.

[0206] It should be noted that in all the above deposition steps, the deposition was carried out by a resistance heating method. (Light-emitting reference element 2)

[0207] The structure of the comparative light-emitting element 2 is the same as that of the light-emitting element 1, except for a structure of the hole transport layer 1112. Only the different structure will be described below.

[0208] BPAFLP (abbreviation) was deposited on the hole-injection layer 1111 by evaporation, forming the hole-transport layer 1112. The thickness of the hole-transport layer 1112 was set to 20 nm.

[0209] Table 1 shows the element structures of the light-emitting element 1 and the comparative light-emitting element 2 obtained as described above. Subsequently, the electrochemical properties (HOMO levels and LUMO levels) of the thin films of BPAFLP (abbreviation) and PCzPCA1 (abbreviation) used in the hole-transport layer of light-emitting element 1 and comparative light-emitting element 2 in this example, and a thin film of PCBA1BP (abbreviation) used in the first light-emitting layer and the second light-emitting layer of light-emitting element 1 and comparative light-emitting element 2 in this example, were measured (measuring instrument: AC-2, manufactured by Riken Keiki Co., Ltd.). It should be noted that the electrochemical properties of the thin films were measured as described below.

[0210] The HOMO level value was obtained by converting the ionization potential value measured with a photoelectron spectrometer (AC-2, manufactured by Riken Keiki Co., Ltd.) in air into a negative value. The LUMO level value was obtained by measuring the absorption spectrum values ​​of each film, and the absorption edge obtained from the Tauc plot assuming a direct transition using the absorption spectrum data was considered as an optical energy gap and added to the HOMO level value.

[0211] Table 2 shows the measurement results of the electrochemical properties of the thin films. [Table 2] [eV] HOMO level LUMO level Bg(ΔE) BPAFLP -5,63 -2,29 3,34 PCzPCA1 -5,17 -2,26 2,91 PCBA1BP -5.42 -2.21 3.21

[0212] According to Table 2, the HOMO level, LUMO level, and band gap (Bg) of BPAFLP (abbreviation) were -5.63 eV, -2.29 eV, and 3.34 eV, the HOMO level, LUMO level, and band gap (Bg) of PCzPCA1 (abbreviation) were -5.17 eV, -2.26 eV, and 2.91 eV, and the HOMO level, LUMO level, and band gap (Bg) of PCBA1BP (abbreviation) were -5.42 eV, -2.21 eV, and 3.21 eV.

[0213] Table 3 shows the materials and HOMO levels of the hole transport layer and the light-emitting layer (second organic compound) of the light-emitting element 1 and the comparative light-emitting element 2 in this example according to Table 1 and Table 2. [Table 3] Compound name / HOMO level Hole transport layer Light-emitting layer (second organic compound) Light-emitting element 1 Connection name (abbreviation) BPAFLP PCzPCA1 PCBA1BP HOMO level [eV] -5,63 -5,17 -5,42 Light-emitting reference element 2 Connection name (abbreviation) BPAFLP - PCBA1BP HOMO level [eV] -5,63 - -5,42

[0214] As shown in Table 3, in the hole-transport layer of the light-emitting element 1 of this example, BPAFLP (abbreviation) was used as the fourth organic compound with a HOMO level equal to or lower than that of PCBA1BP (abbreviation), which is the second organic compound (assist material), and PCzPCA1 (abbreviation) was used as the fifth organic compound with a HOMO level higher than that of PCBA1BP (abbreviation), which is the second organic compound (assist material). In contrast, in the comparative light-emitting element 2, a type of organic compound (BPAFLP (abbreviation)) with a HOMO level lower than or equal to that of PCBA1BP (abbreviation), which is the second organic compound (assist material), was used.

[0215] In a glove box containing a nitrogen atmosphere, light-emitting element 1 and comparative light-emitting element 2 were each sealed with a glass substrate so that they were not exposed to air (i.e., a sealant was applied to an outer edge of the element and a heat treatment was performed at 80°C for 1 hour at the time of sealing). Subsequently, the operating characteristics of the light-emitting elements were measured. It should be noted that the measurement was conducted at room temperature (in an atmosphere maintained at 25°C).

[0216] Fig. 14 shows the luminance-current density characteristics of the light-emitting element 1 and the comparative light-emitting element 2. In Fig. 14 the horizontal axis represents the current density (mA / cm 2 ) and the vertical axis represents the luminance (cd / m 2 ). Fig.15 shows the luminance-voltage characteristics of the light-emitting element 1 and the comparison light-emitting element 2. In Fig. 15, the horizontal axis represents the voltage (V) and the vertical axis represents the luminance (cd / m 2 ). Fig. 16 shows the current efficiency-luminance characteristics of the light-emitting element 1 and the comparative light-emitting element 2. In Fig. 16 the horizontal axis represents the luminance (cd / m 2 ) and the vertical axis represents the current efficiency (cd / A). Fig. 17 shows the current-voltage characteristics of the light-emitting element 1 and the comparison light-emitting element 2. In Fig. 17, the horizontal axis represents voltage (V) and the vertical axis represents current (mA). Fig. 18 shows the emission spectrum of the light-emitting element 1 and the comparison light-emitting element 2. In Fig.18, the horizontal axis represents the wavelength (nm) and the vertical axis represents the light emission intensity (arbitrary unit). It should be noted that in Fig. 18 the data of the light-emitting elements substantially overlap with each other. Fig. Figure 19 shows the power efficiency (lm / W) luminance characteristics of the light-emitting element 1 and the comparison light-emitting element 2.

[0217] As in Fig. 19, the power efficiency of the light-emitting element 1 at a luminance of approximately 1000 cd / m 2 105 (lm / W) and the power efficiency of the reference light-emitting element 2 at a luminance of approximately 1000 cd / m 2 93 (Im / W). Therefore, the power efficiency of the light-emitting element of one embodiment of the present invention is higher than that of the comparative light-emitting element 2 by 12 (Im / W).

[0218] Furthermore, Table 4 shows the voltage (V), current density (mA / cm 2 ), CIE chromaticity coordinates (x, y), current efficiency (cd / A) and external quantum efficiency (%) of each light-emitting element at a luminance of approximately 1000 cd / m 2 . [Table 4] Voltage (V) Current density (mA / cm 2 ) CIE chromaticity coordinates Luminance (cd / m 2 ) Current efficiency (cd / A) External quantum efficiency (%) x y Light-emitting element 1 2,6 1,3 0,43 0,56 1112 87 24 Light-emitting reference element 2 2,9 1,2 0,43 0,56 1037 85 24

[0219] As shown in Table 4, the CIE chromaticity coordinates (x, y) of the light-emitting element 1 at a luminance of 1112 cd / m 2 (0.43, 0.56) and the CIE chromaticity coordinates (x, y) of the light-emitting reference element 2 at a luminance of 1037 cd / m 2 (0.43, 0.56).

[0220] The voltage and current efficiency of the light-emitting element 1 are at a luminance of 1112 cd / m 2 2.6 V and 87 cd / A. The voltage and current efficiency of the light-emitting reference element 2 are at a luminance of 1037 cd / m 22.9 V and 85 cd / A.

[0221] Therefore, the voltage of the light-emitting element 1, which is an embodiment of the present invention, is 0.3 V lower than that of the comparative light-emitting element 2, and the current efficiency of the light-emitting element 1 is equal to or higher than that of the comparative light-emitting element 2. Furthermore, as shown in Fig. 17, the turn-on voltage of the light-emitting element 1 is lower than that of the comparison light-emitting element 2.

[0222] As described above, in the light-emitting element of one embodiment of the present invention, the hole-transport layer contains the fourth organic compound whose HOMO level is lower than or equal to that of the second organic compound having a hole-transport property used in the light-emitting layer, and the fifth organic compound whose HOMO level is higher than that of the second organic compound. With such a structure, the driving voltage of the light-emitting element can be reduced and the current efficiency can be increased.

[0223] It should be noted that the PL peak wavelength of the thin film of the first organic compound (2mDBTPDBq-II) used in the light-emitting layer (of the light-emitting element 1) in this example is 426 nm, the PL peak wavelength of the second organic compound (PCBA1BP) is 416 nm, and the PL peak wavelength of a film of a mixture of these organic compounds is 519 nm, indicating that the wavelength is shifted toward the longer wavelength side. Therefore, the combination of these two types of organic compounds forms an exciplex.

[0224] As described above, in the light-emitting element 1, the fifth organic compound (PCzPCA1), whose HOMO level is higher than that of the second organic compound (PCBA1BP), which contributes to the formation of an exciplex in the light-emitting layer, is added to the hole-transport layer; therefore, the turn-on voltage of the light-emitting element 1 can be lower than that of the comparative light-emitting element 2 to which the fifth organic compound is not added.

[0225] It should be noted that when PCzPCA1 is added to the light-emitting layer instead of PCBA1BP as the second organic compound, the turn-on voltage can be reduced, as in the case of the light-emitting element 1. However, PCzPCA1 has a HOMO level higher than that of PCBA1BP; therefore, an emission wavelength of an exciplex formed by the first organic compound (2mDBTPDBq-II) and PCzPCA1 is 571 nm, which is located on the longer wavelength side compared to the case where PCBA1BP is used (519 nm). Therefore, in the case where a green phosphorescent material is used in the light-emitting layer as in this example, the green phosphorescent material is not easily excited, resulting in a reduction in the emission efficiency (external quantum efficiency). In contrast, the present invention (the light-emitting element 1) does not have such a problem.In other words, the structure of one embodiment of the present invention is particularly effective in reducing the voltage of a light-emitting element emitting green or blue light where the energy gap is large (ie, the turn-on voltage is high in principle) without reducing the emission efficiency. (Reference example 1)

[0226] A synthesis example of (acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III) (abbreviation: [Ir(tBuppm)2(acac)]), used in the above example, is described in detail. The structure of [Ir(tBuppm)2(acac)] is shown below. <Schritt 1: Herstellung von 4-tert-Butyl-6-phenylpyrimidin (Abkürzung: HtBuppm)>

[0227] First, 22.5 g of 4,4-dimethyl-1-phenylpentane-1,3-dione and 50 g of formamide were added to a round-bottomed flask equipped with a reflux tube, and the air in the flask was replaced with nitrogen. This reaction vessel was heated so that the reacted solution was refluxed for 5 hours. Subsequently, this solution was poured into an aqueous sodium hydroxide solution, and an organic layer was extracted with dichloromethane. The obtained organic layer was washed with water and saturated saline and dried with magnesium sulfate. The dried solution was filtered. The solvent of this solution was distilled off, and the resulting residue was purified by silica gel column chromatography using hexane and ethyl acetate as the eluent in a volume ratio of 10:1 to obtain a pyrimidine derivative HtBuppm (colorless oily substance, yield 14%).The synthesis scheme of step 1 is shown below in (a-1). <Schritt 2: Herstellung von Di-µ-chlor-bis[bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III)] (Abkürzung: [Ir(tBuppm)2Cl]2)>

[0228] Subsequently, 15 mL of 2-ethoxyethanol, 5 mL of water, 1.49 g of HtBuppm obtained in step 1, and 1.04 g of iridium chloride hydrate (IrCl3 × H2O) were introduced into a round-bottomed flask equipped with a reflux tube, and the air in the flask was replaced with argon. Subsequently, microwave irradiation (2.45 GHz, 100 W) was carried out for 1 hour to initiate a reaction. The solvent was distilled off, and the resulting residue was filtered off with suction and washed with ethanol to obtain a dinuclear complex [Ir(tBuppm)2Cl]2 (yellow-green powder, yield 73%). A synthesis scheme of step 2 is shown below in (a-2). <Schritt 3: Herstellung von (Acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III) (Abkürzung: [Ir(tBuppm)2(acac)])>

[0229] Furthermore, 40 mL of 2-ethoxyethanol, 1.61 g of the dinuclear complex [Ir(tBuppm)2Cl]2 obtained in Step 2, 0.36 g of acetylacetone, and 1.27 g of sodium carbonate were introduced into a round-bottomed flask equipped with a reflux tube, and the air in the flask was replaced with argon. Subsequently, microwave irradiation (2.45 GHz, 100 W) was carried out for 1 hour to initiate a reaction. The solvent was distilled off, and the resulting residue was filtered with ethanol and washed with water and ethanol. This solid was dissolved in dichloromethane, and the mixture was filtered through a filter aid in which Celite (manufactured by Wako Pure Chemical Industries, Ltd., Catalog No. 531-16855), alumina, and Celite were stacked in that order.The solvent was distilled off, and the resulting solid was recrystallized with a mixed solvent of dichloromethane and hexane to obtain the desired substance as a yellow powder (yield 68%). A synthesis scheme of step 3 is shown below in (a-3).

[0230] It should be noted that the compound obtained by the above synthesis procedure was characterized by a nuclear magnetic resonance spectroscopy ( 1 H-NMR method. The measurement results show that [Ir(tBuppm)2(acac)] (abbreviation) was obtained.

[0231] The 1 H-NMR values ​​of the obtained substance were as follows: 1 H NMR. δ (CDCl3): 1.50 (s, 18H), 1.79 (s, 6H), 5.26 (s, 1H), 6.33 (d, 2H), 6.77 (t, 2H), 6.85 (t, 2H), 7.70 (d, 2H), 7.76 (s, 2H), 9.02 (s, 2H). (Reference example 2)

[0232] The T1 levels of 2mDBTPDBq-II (abbreviation), PCBA1BP (abbreviation), and BPAFLP (abbreviation), which were used in the light-emitting elements of the above examples, were measured. It is noted that the T1 level was measured by measuring the phosphorescence emission of each substance and converting the phosphorescence emission wavelength into electron volts. During the measurement, each substance was irradiated with excitation light having a wavelength of 325 nm, and the measurement temperature was 10 K. It should be noted that when measuring an energy level, calculating the absorption wavelength is more accurate than calculating the emission wavelength. Here, the absorption of each of the T lHowever, the T1 levels are extremely low and difficult to measure, so each of the T1 levels was measured by measuring a peak wavelength located on the shortest wavelength side of a phosphorescence spectrum. For this reason, some errors may exist in the measured values.

[0233] Fig. 20, Fig. 21 and Fig.Figure 22 shows the measured phosphorescence of 2mDBTPDBq-II (abbreviation), measured phosphorescence of PCBA1BP (abbreviation), and measured phosphorescence of BPAFLP (abbreviation). Table 5 shows the measured results. From these results, it is clear that the T1 level of BPAFLP (abbreviation), which was used in the hole-transport layer and corresponds to the fourth organic compound, is higher than the T1 levels of 2mDBTPDBq-II (abbreviation), which was used as the first organic compound in the above example, and of PCBA1BP (abbreviation), which was used as the second organic compound in the above example. [Table 5] Connection name Phosphorescence emission wavelength (nm) T1 level (eV) 2mDBTPDBq-II (abbreviation) 516 2,40 PCBA1BP (abbreviation) 504 2,46 BPAFLP (abbreviation) 497 2,49 REFERENCE SYMBOL

[0234] 100: substrate, 101: first electrode, 103: second electrode, 111: hole injection layer, 112: hole transport layer, 112a: hole transport layer, 112b: hole transport layer, 112c: hole transport layer, 113: light-emitting layer, 114: electron transport layer, 115: electron injection layer, 120: first organic compound, 122: second organic compound, 124: third organic compound, 126: fourth organic compound, 128: fifth organic compound, 301: first electrode, 303: second electrode, 311: first light-emitting layer, 312: second light-emitting layer, 313: charge generation layer, 450R: first light-emitting element, 450G: second light-emitting element, 450B: third light-emitting element, 451: reflective electrode, 452: semi-transparent and semi-reflective electrode, 453a: first transparent conductive layer, 453b: second transparent conductive layer, 454: light-emitting layer, 454B: first light-emitting layer,454G: second light-emitting layer, 454R: third light-emitting layer, 455: EL layer, 501: element substrate, 502: pixel region, 503: driving circuit region, 504: driving circuit region, 505: sealant, 506: sealing substrate, 507: wiring, 508: FPC, 509: n-channel TFT, 510: p-channel TFT, 511: switching TFT, 512: current control TFT, 513: first electrode, 514: insulator, 515: EL layer, 516: second electrode, 517: light-emitting element, 518: space, 611: package, 612: support, 613: display region, 614: speaker region, 615: Video input connector, 621: Main body, 622: Housing, 623: Display area, 624: Keyboard, 625: External connection connector, 626: Pointing device, 631: Main body, 632: Housing, 633: Display area, 634: Audio input area, 635: Audio output area, 636: Operation button, 637: External connection connector, 638: Antenna, 641: Main body, 642: Display area, 643: Housing, 644: External connection connector,645: Remote control receiving area, 646: Image receiving area, 647: Battery, 648: Audio input area, 649: Operation button, 650: Eye area, 701: Housing, 702: Liquid crystal panel, 703: Backlight, 704: Housing, 705: Drive IC, 706: Terminal, 801: Housing, 802: Light source, 901: Lighting device, 902 TV, 1100: Substrate, 1101: First electrode, 1103: Second electrode, 1111: Hole injection layer, 1112: Hole transport layer, 1113a: First light-emitting layer, 1113b: Second light-emitting layer, 1114a: First electron transport layer, 1114b: Second electron transport layer, 1115: Electron injection layer, 5000: portable terminal, 5001: housing, 5003: display area, 5005: power switch, 5007: front camera, 5009: rear camera, 5011: external connection port, 5013: external connection port, 5015: icon, 6000: portable terminal, 6001: housing, 6003: housing, 6005: hinge area, 6007: display area, 6009: display area,6011: Power button, 6013: Camera, 6015: Camera, 6017: Text icon, 6019: Icon and 6021: Keyboard.

Claims

[1] A light-emitting device comprising: a hole transport layer and a light-emitting layer between a pair of electrodes, wherein the hole transport layer is in contact with the light-emitting layer, wherein the light-emitting layer comprises a first organic compound, a second organic compound and a third organic compound which converts the triplet excitation energy into light emission, wherein a combination of the first organic compound and the second organic compound forms an exciplex, wherein a HOMO level of the first organic compound is lower than a HOMO level of the second organic compound, wherein a LUMO level of the first organic compound is lower than a LUMO level of the second organic compound, wherein the hole transport layer comprises a fourth organic compound whose HOMO level is higher than the HOMO level of the second organic compound, wherein the second organic compound comprises a carbazol-3-yl group, and wherein a difference between the energy value of a peak of an emission spectrum of the exciplex and the energy value of a peak of an absorption band on the lowest energy side of an absorption spectrum of the third organic compound is 0.2 eV or less. [2] A light-emitting device comprising: a hole injection layer, a hole transport layer and a light-emitting layer between an anode and a cathode, wherein the hole injection layer is in contact with the anode, wherein the hole transport layer is in contact with the light-emitting layer, wherein the hole injection layer comprises a compound with fluorine, a compound with a cyano group and / or a transition metal oxide, wherein the light-emitting layer comprises a first organic compound, a second organic compound and a third organic compound which converts the triplet excitation energy into light emission, wherein a combination of the first organic compound and the second organic compound forms an exciplex, wherein a HOMO level of the first organic compound is lower than a HOMO level of the second organic compound, wherein a LUMO level of the first organic compound is lower than a LUMO level of the second organic compound, wherein the hole transport layer comprises a fourth organic compound whose HOMO level is higher than the HOMO level of the second organic compound, wherein the second organic compound comprises a carbazol-3-yl group, and wherein a difference between the energy value of a peak of an emission spectrum of the exciplex and the energy value of a peak of an absorption band on the lowest energy side of an absorption spectrum of the third organic compound is 0.2 eV or less. [3] A light-emitting device comprising: a hole transport layer and a light-emitting layer between a pair of electrodes, wherein the hole transport layer is in contact with the light-emitting layer, wherein the light-emitting layer comprises a first organic compound, a second organic compound and a third organic compound which converts the triplet excitation energy into light emission, wherein a combination of the first organic compound and the second organic compound forms an exciplex, wherein a HOMO level of the first organic compound is lower than a HOMO level of the second organic compound, wherein a LUMO level of the first organic compound is lower than a LUMO level of the second organic compound, wherein the hole transport layer comprises a fourth organic compound whose HOMO level is higher than the HOMO level of the second organic compound, wherein the fourth organic compound is an aromatic amine compound or a carbazole derivative, and wherein a difference between the energy value of a peak of an emission spectrum of the exciplex and the energy value of a peak of an absorption band on the lowest energy side of an absorption spectrum of the third organic compound is 0.2 eV or less. [4] A light-emitting device comprising: a hole injection layer, a hole transport layer and a light-emitting layer between an anode and a cathode, wherein the hole injection layer is in contact with the anode, wherein the hole transport layer is in contact with the light-emitting layer, wherein the hole injection layer comprises a compound with fluorine, a compound with a cyano group and / or a transition metal oxide, wherein the light-emitting layer comprises a first organic compound, a second organic compound and a third organic compound which converts the triplet excitation energy into light emission, wherein a combination of the first organic compound and the second organic compound forms an exciplex, wherein a HOMO level of the first organic compound is lower than a HOMO level of the second organic compound, wherein a LUMO level of the first organic compound is lower than a LUMO level of the second organic compound, wherein the hole transport layer comprises a fourth organic compound whose HOMO level is higher than the HOMO level of the second organic compound, wherein the fourth organic compound is an aromatic amine compound or a carbazole derivative, and wherein a difference between the energy value of a peak of an emission spectrum of the exciplex and the energy value of a peak of an absorption band on the lowest energy side of an absorption spectrum of the third organic compound is 0.2 eV or less. [5] The light-emitting device according to any one of claims 1 to 4, wherein the third organic compound is an iridium complex. [6] The light-emitting device according to any one of claims 1 to 4, wherein the difference between the energy value of the peak of the emission spectrum of the exciplex and the energy value of the peak of the absorption band on the lowest energy side of the absorption spectrum of the third organic compound is 0.1 eV or less. [7] Light-emitting device according to one of claims 1 to 4, wherein the first organic compound is a π-electron-deficient heteroaromatic compound, and wherein the second organic compound is a π-electron-rich heteroaromatic compound. [8] Light-emitting device according to one of claims 1 to 4, wherein a T1 level of the first organic compound is higher than a T1 level of the third organic compound, and wherein a T1 level of the second organic compound is higher than the T1 level of the third organic compound. [9] Light-emitting device according to one of claims 1 to 4, wherein a difference between the HOMO level of the first organic compound and the HOMO level of the second organic compound is 0.3 eV or more, and wherein a difference between the LUMO level of the first organic compound and the LUMO level of the second organic compound is 0.3 eV or more.

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