Light-emitting element

Abstract

Light-emitting element comprising: a light-emitting layer comprising a first organic compound, a second organic compound and a guest material, wherein the first organic compound is a heteroaromatic compound and the second organic compound is a carbazole compound, wherein the first compound and the second compound may form an exciplex, wherein the emission peak wavelength of the exciplex is longer than or corresponds to the peak wavelength of the absorption band located on the longest wavelength side of an absorption spectrum of the guest material, wherein this peak wavelength of the absorption band on the longest wavelength side is an absorption wavelength corresponding to the direct transition from the singlet ground state to the lowest excited triplet state, and wherein the combinations in which the phosphorescent compound is [Ir(dppm)2(acac)], [Ir(mppr-Me)2(dpm)] or [Ir(mppm)2(acac)], the first organic compound is 2mDBTPDBq-II, and the second organic compound is PCBA1BP are excluded.

Description

Technical area

[0001] The present invention relates to light-emitting elements utilizing an organic electroluminescence (EL) phenomenon (hereinafter, such light-emitting elements are also referred to as organic EL elements). State of the art

[0002] An organic EL element has been actively researched and developed. In the basic structure of the organic EL element, a layer containing a luminescent organic compound (hereinafter also referred to as a light-emitting layer) is sandwiched between a pair of electrodes. The organic EL element has attracted attention as a next-generation flat panel display element due to its thinner and lighter fabrication capabilities, fast response times for signal input, and low-voltage DC operation. In addition, a display using such a light-emitting element has the features of excellent contrast and image quality, and a wide viewing angle.Furthermore, since it is a planar light source, attempts have been made to use the organic EL element as a light source, such as a backlight of a liquid crystal display and as a lighting device.

[0003] The emission mechanism of the organic EL element is a carrier-injection type. This means that by applying a voltage through a light-emitting layer arranged between electrodes, electrons and holes injected from the electrodes are recombined, so that a light-emitting substance is excited, and light is emitted when the excited state relaxes to the ground state. There are two types of excited states: a singlet excited state (S*) and a triplet excited state (T*). The ratio of the statistical generation of excited states in a light-emitting element is given as S*:T* = 1:3.

[0004] Generally, the ground state of a light-emitting organic compound is a singlet state. Therefore, light emission from the excited singlet state (S*) is called fluorescence, as it occurs through electron transfer between multiples of the same spin. On the other hand, light emission from the excited triplet state (T*) is called phosphorescence, in which electron transfer occurs between multiples with different spins. In this case, a compound that emits fluorescence (hereinafter referred to as a fluorescent compound) generally does not exhibit phosphorescence at room temperature, and only fluorescence is observed.Consequently, the internal quantum efficiency (the ratio of generated photons to injected carriers) in a light-emitting element containing a fluorescent compound is assumed to have a theoretical limit of 25%, based on S*:T* = 1:3.

[0005] On the other hand, when a compound that emits phosphorescence (hereinafter referred to as a phosphorescent compound) is used, a theoretical internal quantum efficiency of 100% can be achieved. That is, a higher emission yield can be achieved than when a fluorescent compound is used. For these reasons, a light-emitting element containing a phosphorescent compound has been actively developed in recent years to obtain a highly efficient light-emitting element. As the phosphorescent compound, an organometallic complex containing iridium or the like as the central metal has received particular attention due to its high phosphorescence quantum yield. For example, an organometallic compound containing iridium as a central metal is disclosed in Patent Document 1 as a phosphorescent material.

[0006] When a light-emitting layer of a light-emitting element is formed using a phosphorescent compound described above, the light-emitting layer is often formed such that the phosphorescent compound is dispersed in a matrix of another compound to suppress concentration quenching or quenching due to triplet-triplet annihilation in the phosphorescent compound. Here, the compound serving as the matrix is called the host material, and the compound dispersed in the matrix, such as a phosphorescent compound, is called the guest material.

[0007] Die Dokumente M. E. Kondakova et al. „High-efficiency, low-voltage phosphorescent organic lightemitting diode devices with mixed host“, Journal of Applied Physics 104, 094501 (2008); X. Gong et al. „Phosphorescence from iridium complexes doped into polymer blends“, J. Appl. Phys. 95, 2004; Y. Hino et al. „Red Phosphorescent Organic Light-Emitting Diodes Using Mixture System of Small-Molecule and Polymer Host“, Jpn. J. Appl. Phys. 2005, 44, 2790; XIA, Hong, et al. Efficient electrophosphorescence from low-cost copper (I) complex. Optical Materials, 2007, 29. Jg., Nr. 6, S. 667-671 offenbaren PHOLEDs (phosphoreszierende OLEDs). ReferenzPatentdokument

[0008] Patentdokument 1: PCT internationale Veröffentlichung Nr. WO 00 / 70 655 A3 Offenbarung der Erfindung

[0009] However, the light extraction efficiency of an organic EL element is said to be approximately 20% to 30%. Therefore, the external quantum efficiency of a light-emitting element containing a phosphorescent compound has a limit of approximately 25% at most, considering light absorption by a reflective electrode and a transparent electrode.

[0010] Furthermore, as described above, the use of organic EL elements in displays and lighting has been considered. One of the objectives to be achieved here is a reduction in energy consumption. To reduce energy consumption, it is necessary to reduce the drive voltage of the organic EL element.

[0011] One embodiment of the present invention provides a light-emitting element with high external quantum efficiency. Another embodiment of the present invention provides a light-emitting element with a low drive voltage.

[0012] It should be noted that the invention to be disclosed below aims at achieving at least one of the objects described above.

[0013] One embodiment of the present invention is a light-emitting element having a light-emitting layer comprising a first organic compound, a second organic compound, and a guest material, wherein the first organic compound is a heteroaromatic compound and the second organic compound is a carbazole compound, wherein the first compound and the second compound can form an exciplex, wherein the emission peak wavelength of the exciplex is longer than or corresponds to the peak wavelength of the absorption band located on the longest wavelength side of an absorption spectrum of the guest material, wherein this peak wavelength of the absorption band on the longest wavelength side is an absorption wavelength corresponding to the direct transition from the singlet ground state to the lowest excited triplet state, and wherein the combinations,where the phosphorescent compound is [Ir(dppm)2(acac)], [Ir(mppr-Me)2(dpm)] or [Ir(mppm)2(acac)], the first organic compound is 2mDBTPDBq-II, and the second organic compound is PCBA1BP, are excluded.

[0014] Another embodiment of the present invention is a light-emitting element having a light-emitting layer comprising a first organic compound, a second organic compound, and a guest material, wherein the first organic compound is a heteroaromatic compound and the second organic compound is a carbazole compound, wherein the first compound and the second compound can form an exciplex, wherein the emission peak wavelength of the exciplex is longer than or corresponds to the peak wavelength of the absorption band located on the longest wavelength side of an absorption spectrum of the guest material, wherein this peak wavelength of the absorption band on the longest wavelength side is an absorption wavelength corresponding to the direct transition from the singlet ground state to the lowest excited triplet state,and is shorter than or equal to the emission peak wavelength of the guest material, and wherein the combinations in which the phosphorescent compound is [Ir(dppm)2(acac)], [Ir(mppr-Me)2(dpm)] or [Ir(mppm)2(acac)], the first organic compound is 2mDBTPDBq-II, and the second organic compound is PCBA1BP are excluded.

[0015] Another embodiment of the present invention is a light-emitting element having a light-emitting layer comprising a first organic compound, a second organic compound, and a guest material, wherein the first organic compound is a heteroaromatic compound and the second organic compound is a carbazole compound, wherein the first compound and the second compound can form an exciplex, wherein the emission spectrum of the exciplex overlaps the absorption band located on the longest wavelength side of the absorption spectrum of the guest material, wherein this peak wavelength of the absorption band on the longest wavelength side is an absorption wavelength corresponding to the direct transition from the singlet ground state to the lowest excited triplet state,wherein the difference between the peak wavelength of the emission spectrum of the exciplex and the peak wavelength of the emission spectrum of the guest material is 30 nm or less, and wherein the combinations in which the phosphorescent compound is [Ir(dppm)2(acac)], [Ir(mppr-Me)2(dpm)] or [Ir(mppm)2(acac)], the first organic compound is 2mDBTPDBq-II, and the second organic compound is PCBA1BP are excluded.

[0016] Furthermore, an embodiment of the present invention is the aforementioned light-emitting element in which the exciplex is formed by a singlet exciton of the first organic compound.

[0017] Furthermore, an embodiment of the present invention is the aforementioned light-emitting element in which the exciplex is formed by an anion of the first organic compound and a cation of the second organic compound.

[0018] In the aforementioned light-emitting element, it is preferable that the excitation energy of the exciplex is transferred to the phosphorescent compound so that the phosphorescent compound emits phosphorescence.

[0019] In the above-mentioned light-emitting element, it is preferable that at least one of the first organic compound and the second organic compound is a fluorescent compound.

[0020] In the aforementioned light-emitting element, it is preferable that the phosphorescent compound is an organometallic complex.

[0021] The light-emitting element of one embodiment of the present invention can be used in a light-emitting device, an electronic device, and a lighting device.

[0022] According to one embodiment of the present invention, a light-emitting element with a high external quantum efficiency can be provided. According to another embodiment of the present invention, a light-emitting element with a low drive voltage can be provided. Short description of the drawings Fig. 1 shows an absorption spectrum and an emission spectrum according to Example 1. Fig. Figure 2 shows an absorption spectrum and an emission spectrum according to Example 1. Fig. Figure 3 shows an absorption spectrum and an emission spectrum according to Example 1. Fig. Figure 4 shows an absorption spectrum and an emission spectrum according to Example 1. Fig. 5 shows a concept of an embodiment of the present invention. Fig. Figure 6 shows the energy levels of an exciplex applied to an embodiment of the present invention. Fig. 7A to 7C each show a light-emitting element of an embodiment of the present invention. Fig. 8 shows a structure of a light-emitting element according to Example 2. Fig. Figure 9 shows the voltage-luminance characteristic of the light-emitting element of Example 2. Fig. Figure 10 shows the voltage-current characteristics of the light-emitting element of Example 2. Fig. Figure 11 shows the luminance-power efficiency characteristic of the light-emitting element of Example 2. Fig. Figure 12 shows the luminance-external quantum efficiency characteristic of the light-emitting element of Example 2. Fig. 13 shows an emission spectrum of the light-emitting element of Example 2. Fig. Figure 14 shows the results of the reliability tests of the light-emitting element of Example 2. Fig. Figure 15 shows a relationship between a peak wavelength of an emission spectrum of an exciplex and a HOMO level of a substance X according to Example 3. Fig. 16 shows a relationship between a peak wavelength of an emission spectrum of an exciplex and the external quantum efficiency of a light-emitting element according to Example 3. Fig. 17 shows the calculation results according to an embodiment of the invention. Fig. 18A1, Fig. 18A2, Fig. 18B1, Fig. 18B2, Fig. 18C1 and Fig. 18C2 show the calculation results according to an embodiment of the present invention. BEST MODE FOR CARRYING OUT THE INVENTION

[0023] Embodiments will be described with reference to the drawings. It should be noted that the invention is not limited to the following description, and it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the scope and spirit of the invention. Therefore, the invention should not be construed as being limited to the description in the following embodiments. It should be noted that in the structures of the invention described below, the same parts or parts having similar functions are denoted by the same reference numerals in different drawings, and the description of these parts will not be repeated. Examples that do not have the claimed combination of heteroaromatic compound and carbazole compound as the first and second organic compounds, respectively, are comparative examples not according to the invention. (Embodiment 1)

[0024] In this embodiment, a light-emitting element of an embodiment of the present invention will be described.

[0025] The light-emitting element of this embodiment comprises a light-emitting layer containing a guest material as a light-emitting substance, a first organic compound, and a second organic compound. Specifically, a phosphorescent compound is used as the guest material. It should be noted that one of the first and second organic compounds, whose proportion is greater than the proportion of the others in the light-emitting layers, is referred to as a host material.

[0026] The structure in which the guest material is dispersed in the host material can prevent the light-emitting layer from crystallizing. Furthermore, it is possible to suppress concentration quenching due to the high concentration of the guest material, and thus the light-emitting element can exhibit higher emission efficiency.

[0027] It should be noted that in this embodiment, it is preferable that the triplet excitation energy level (T1 level) of each of the first and second organic compounds be higher than that of the guest material. This is because if the T1 level of the first organic compound (or the second organic compound) is lower than that of the guest material, the triplet excitation energy of the guest material, which contributes to light emission, is quenched by the first organic compound (or the second organic compound), and consequently, the emission efficiency is reduced. <Elementare Verfahren der Lichtemission>

[0028] First, a description is given of the general elementary processes of light emission in a light-emitting element using a phosphorescent compound as a guest material. (1) The case where an electron and a hole recombine in a guest molecule and the guest molecule is excited (direct recombination process) (1-1) If the excited state of the guest molecule is a triplet excited state, the guest molecule emits phosphorescence. (1-2) If the excited state of the guest molecule is an excited singlet state, the guest molecule in the excited singlet state undergoes intersystem crossing to an excited triplet state and emits phosphorescence.

[0029] In other words, in the direct recombination process in (1), as long as the intersystem crossing efficiency and the phosphorescence quantum yield of the guest molecule are high, high emission efficiency can be achieved. It should be noted, as described above, that the T1 level of the host molecule is preferably higher than the T1 level of the guest molecule.

[0030] (2) The case where an electron and a hole recombine in a host molecule and the host molecule is brought into an excited state (energy transfer process).

[0031] (2-1) If the excited state of the host molecule is a triplet excited state and the T1 level of the host molecule is higher than that of the guest molecule, the excitation energy is transferred from the host molecule to the guest molecule, and thus the guest molecule is converted into a triplet excited state. The guest molecule in the triplet excited state emits phosphorescence. It should be noted that energy transfer to a singlet excitation energy level (S1 level) of the guest molecule can theoretically occur, but this is unlikely to be a major energy transfer process because, in many cases, the S1 level of the guest molecule has a higher energy than the T1 level of the host molecule; therefore, no description of this process is given here.

[0032] (2-2) If the excited state of the host molecule is a singlet excited state and the S1 level of the host molecule is higher than the S1 level and the T1 level of the guest molecule, the excitation energy is transferred from the host molecule to the guest molecule, and thus the guest molecule is converted into an excited singlet state or a triplet excited state. The guest molecule in the triplet excited state emits phosphorescence. In addition, the guest molecule in the singlet excited state undergoes intersystem crossing to an excited triplet state and emits phosphorescence.

[0033] In other words, in the energy transfer process in (2), it is important how efficiently both the triplet excitation energy and the singlet excitation energy of the host molecule can be transferred to the guest molecule.

[0034] Regarding the energy transfer methods described above, the emission efficiency is reduced if the host molecule itself is deactivated by emitting the excitation energy as light or heat before the excitation energy of the host molecule is transferred to the guest molecule. The inventors found that when the host molecule is in a singlet excited state (the above (2-2)), the energy is unlikely to be transferred to the guest molecule, i.e., the phosphorescent compound, and the emission efficiency tends to be reduced compared with the case where the host molecule is in a triplet excited state (the above (2-1)). Therefore, the inventors focused on this fact as a subject. The reason for this follows from the consideration of a more detailed transfer method. <Energieübertragungsverfahren>

[0035] The following describes the energy transfer processes between molecules in detail.

[0036] First, the following two mechanisms are proposed as a mechanism for energy transfer between molecules. A molecule that provides excitation energy is called a host molecule, while a molecule that receives the excitation energy is called a guest molecule. <<Förster-Mechanismus (Dipol-Dipol-Interaktion)> >

[0037] In the Förster mechanism, direct intermolecular contact is not necessary for energy transfer. Energy transfer occurs through a resonance phenomenon of dipolar oscillation between a host molecule and a guest molecule. Through the resonance phenomenon of dipolar oscillation, the host molecule provides energy to the guest molecule, thus placing the host molecule in a ground state and the guest molecule in an excited state. The rate constant k h*→g of the Förster mechanism is expressed by formula (1). [Formula 1)] kh*→g=9000c4K2ϕln10128π5n4N τR6∫f'h(v)εg(v)v4dv

[0038] In formula (1) v denotes a frequency, f' h(v) denotes a normalized emission spectrum of a host molecule (a fluorescence spectrum upon energy transfer from an excited singlet state and a phosphorescence spectrum upon energy transfer from an excited triplet state), ε g (v) denotes a molar absorption coefficient of a guest molecule, N denotes Avogadro's number, n denotes a refractive index of a medium, R denotes an intermolecular distance between the host molecule and the guest molecule, τ denotes a measured lifetime of an excited state (fluorescence lifetime or phosphorescence lifetime), c denotes the speed of light, ϕ denotes a luminescence quantum yield (a fluorescence quantum yield in the energy transfer from an excited singlet state and a phosphorescence quantum yield in the energy transfer from an excited triplet state), and K 2denotes a coefficient (0 to 4) of the orientation of a transition dipole moment between the host molecule and the guest molecule. It should be noted that K 2 = 2 / 3 in statistical orientation. << Dexter mechanism (electron exchange interaction)>>

[0039] In the Dexter mechanism, a host molecule and a guest molecule are near a contact-effective region where their orbitals overlap, and the host molecule, which is in an excited state, and the guest molecule, which is in a ground state, exchange their electrons, resulting in energy transfer. The rate constant k n*→g of the Dexter mechanism is expressed by formula (2). [Formula (2)] kh*→g=(2πh)K2exp(−2RL)∫f'h(v)ε'g(v)dv

[0040] In formula (2), h denotes a Planck constant, K denotes a constant with an energy dimension, v denotes a frequency, f' h (v) denotes a normalized emission spectrum of a host molecule (a fluorescence spectrum upon energy transfer from an excited singlet state and a phosphorescence spectrum upon energy transfer from an excited triplet state), ε' g (v) denotes a normalized absorption spectrum of a guest molecule, L denotes an effective molecular radius, and R denotes an intermolecular distance between the host molecule and the guest molecule.

[0041] The aim is to determine the efficiency of energy transfer from the host molecule to the guest molecule (energy transfer efficiency Φ ET ) can be expressed by formula (3). In the formula, k denotes ra rate constant of a light emission process (fluorescence in energy transfer from an excited singlet state and phosphorescence in energy transfer from an excited triplet state) of a host molecule, k n denotes a rate constant of a non-light emission process (thermal deactivation or intersystem crossing) of a host molecule and τ denotes a measured lifetime of an excited state of a host molecule. [Formula (3)] ΦET=kh*→gkr+kn+kh*→g=kh*→g(1τ)+kh*→g

[0042] First, according to formula (3), the energy transfer efficiency Φ ET can be increased by increasing the rate constant k h*→g of energy transfer is further increased compared to another competing rate constant kr + kn (= 1 / τ). To determine the rate constant k h*→gof energy transfer based on formulas (1) and (2) in the Förster mechanism and the Dexter mechanism, it is preferable that an emission spectrum of a host molecule (a fluorescence spectrum in energy transfer from a singlet excited state and a phosphorescence spectrum in energy transfer from a triplet excited state) significantly overlaps with an absorption spectrum of a guest molecule.

[0043] Here, the present inventors considered that the absorption band on the longest wavelength side (lowest energy side) in the absorption spectrum of the guest molecule is important in considering the overlap between the emission spectrum of the host molecule and the absorption spectrum of the guest molecule.

[0044] In this embodiment, a phosphorescent compound is used as the guest material. In an absorption spectrum of the phosphorescent compound, an absorption band believed to contribute significantly to light emission is an absorption wavelength corresponding to a direct transition from a singlet ground state to a triplet excited state, and a vicinity of the absorption wavelength is located on the longest wavelength side. Therefore, it is believed that it is preferable that the emission spectrum (a fluorescence spectrum and a phosphorescence spectrum) of the host material overlap with the absorption band on the longest wavelength side in the absorption spectrum of the phosphorescent compound.

[0045] For example, most organometallic complexes, especially light-emitting iridium complexes, exhibit a broad absorption band at approximately 500 nm to 600 nm as the absorption band on the longest wavelength side (actually, the broad absorption band can be on the shorter or longer wavelength side, depending on the emission wavelength). This absorption band is mainly based on a triplet MLCT (metal-ligand charge transfer) transition. It should be noted that the absorption band is believed to also include absorptions based on a triplet π-π* transition and a singlet MLCT transition, and that these absorptions overlap to form a broad absorption band on the longest wavelength side in the absorption spectrum.In other words, the difference between the lowest excited singlet state and the lowest excited triplet state is small, and the absorption based on these states overlaps to form a broad absorption band on the longest wavelength side in the absorption spectrum. Therefore, as described above, it is preferable that the broad absorption band on the longest wavelength side significantly overlaps with the emission spectrum of the host material when using an organometallic complex (especially an iridium complex) as the guest material.

[0046] Here, the energy transfer from a host material in an excited triplet state is first considered. Based on the discussion above, it is preferable that the phosphorescence spectrum of the host material and the absorption band on the longest wavelength side of the guest material overlap significantly during energy transfer from an excited triplet state.

[0047] It should be noted that generally a fluorescent compound is used as the host material; therefore, the phosphorescence lifetime (τ) is 1 ms or longer, which is extremely long (i.e., k r + k n is low). This is because the transition from the excited triplet state to the ground state (singlet) is a forbidden transition. Formula (3) shows that this is compared to the energy transfer efficiency Φ ETis preferred. This also suggests that energy is more likely to be transferred from the host material in the excited triplet state to the guest material in the excited triplet state.

[0048] One issue here concerns energy transfer from the host material in the excited singlet state. To achieve energy transfer not only from the excited triplet state but also from the excited singlet state, it is clear from the above discussion that the host material must be designed so that not only its phosphorescence spectrum but also its fluorescence spectrum overlaps with the absorption band on the longest wavelength side of the guest material. In other words, if the host material is not designed to exhibit its fluorescence spectrum in a position similar to that of the phosphorescence spectrum, it is not possible to achieve effective energy transfer from the host material in both the excited singlet state and the excited triplet state.

[0049] However, the S1 level differs significantly from the T1 level (S1 level > T1 level); therefore, the fluorescence emission wavelength also differs significantly from the phosphorescence emission wavelength (fluorescence emission wavelength < phosphorescence emission wavelength). For example, 4,4'-di(N-carbazolyl)biphenyl (abbreviation: CBP), which is conventionally used as a host material in a light-emitting element containing a phosphorescent compound, exhibits a phosphorescence spectrum at approximately 500 nm and a fluorescence spectrum at approximately 400 nm, which differ significantly by approximately 100 nm. This example demonstrates that it is extremely difficult to form a host material whose fluorescence spectrum is in a similar position to the phosphorescence spectrum.Therefore, the present inventors have found it a major challenge to improve the efficiency of energy transfer from the host material in the excited singlet state to the guest material.

[0050] It should be noted that the fluorescence lifetime (τ) of a fluorescent compound used as the host material is on the order of nanoseconds, which is extremely short (i.e., k r + k n is high). This is because the transition from the excited singlet state to the ground state (singlet) is a allowed transition. Formula (3) shows that the energy transfer efficiency Φ ET is not preferred. This also suggests that energy is unlikely to be transferred from the host material in the excited singlet state to the guest material.

[0051] An embodiment of the present invention is a suitable method which overcomes such a problem of the efficiency of energy transfer from the host material in the excited singlet state to the guest material.

[0052] It should be noted that it was previously assumed that a light-emitting element containing a phosphorescent compound could theoretically achieve an internal quantum efficiency of 100% because intersystem crossing allows both the singlet excited state and the triplet excited state to be converted into light emission (refer to "(1) Direct Recombination Method" described above). Additionally, it has been discussed that a light-emitting element with an external quantum efficiency of as much as 20% achieves an internal quantum efficiency of essentially 100%, assuming a light extraction efficiency of 20%. However, it is believed that these conventional light-emitting elements do not achieve an internal quantum efficiency of 100% because the energy transfer from the singlet excited state of the host material described above was overlooked.This is based on the fact that the present inventors have achieved an external quantum efficiency of 27% or more by carrying out an embodiment of the present invention described below (see . Fig. 12 and Example 2). The value can be said to equal or exceed a conventional theoretical limit of the external quantum efficiency. In other words, an external quantum efficiency of at least 27% or more corresponds to an internal quantum efficiency of 100%, and an embodiment of the present invention is a suitable method for achieving this. It should be noted that this shows that a conventional external quantum efficiency of 20% can correspond to an internal quantum efficiency of 75% or less.

[0053] As described above, by using an embodiment of the present invention, a light-emitting element with high external quantum efficiency can be provided. <Eine Ausführungsform der vorliegenden Erfindung>

[0054] An embodiment of the present invention is a light-emitting element comprising a light-emitting layer having a phosphorescent compound, a first organic compound, and a second organic compound between a pair of electrodes, wherein a combination of the first organic compound and the second organic compound forms an exciplex, wherein an emission spectrum of the exciplex overlaps an absorption band located on the longest wavelength side of an absorption spectrum of the phosphorescent compound, and wherein a peak wavelength of the emission spectrum of the exciplex is longer than or equal to a peak wavelength of the absorption band located on the longest wavelength side of the phosphorescent compound.

[0055] The first organic compound and the second organic compound form an exciplex (also called an excited complex) through carrier recombination (i.e., electrons and holes) (or from a singlet exciton). When the formed exciplex emits light, its emission wavelength is located on the longer wavelength side relative to the emission wavelength (fluorescence wavelength) of each of the first and second organic compounds. In other words, by forming the exciplex, the fluorescence spectrum of the first organic compound and the fluorescence spectrum of the second organic compound can be converted into an emission spectrum located on the longer wavelength side.

[0056] Therefore, as in Fig.As shown in Figure 5, even when the fluorescence spectrum of the first organic compound (or the second organic compound) is located on the shorter wavelength side compared with the absorption band of the phosphorescent compound located on the longest wavelength side and has no overlap with the absorption band, an emission spectrum with a long wavelength can be obtained by forming an exciplex so as to obtain a large overlap with the absorption band. The light-emitting element of one embodiment of the present invention transfers energy by utilizing the overlap between the emission spectrum of the exciplex and the absorption spectrum of the phosphorescent compound and therefore has high energy transfer efficiency. Therefore, in one embodiment of the present invention, a light-emitting element with high external quantum efficiency can be obtained.

[0057] In addition, the exciplex exists only in an excited state and lacks a ground state capable of absorbing energy. Therefore, it is not expected that a phenomenon can occur in which the phosphorescent compound can be deactivated prior to light emission (i.e., the emission efficiency is reduced) by reverse energy transfer from the excited singlet state and the excited triplet state of the phosphorescent compound to the exciplex. This also contributes to improving the external quantum efficiency.

[0058] In addition, it is assumed that the exciplex exhibits an extremely small difference between the excited singlet energy and the excited triplet energy. In other words, the emission spectrum of the exciplex in the singlet state and its emission spectrum in the triplet state are close to each other. Therefore, in the case of a design in which the emission spectrum of the exciplex (generally, the emission spectrum of the exciplex in the singlet state) overlaps with the absorption band of the phosphorescent compound on the longest wavelength side, as described above, the emission spectrum of the exciplex in the triplet state (which is not observed at room temperature and in many cases is not observed at low temperature) also overlaps with the absorption band of the phosphorescent compound located on the longest wavelength side.Furthermore, this means that the energy can be efficiently transferred to the phosphorescent compound from the exciplex in both the singlet and triplet states.

[0059] Molecular orbital calculations were performed as described below to confirm whether an exciplex actually possesses such properties. In general, a combination of a heteroaromatic compound and an aromatic amine often forms an exciplex under the influence of the lowest unoccupied orbital (LUMO) level of the heteroaromatic compound, which is deeper than the LUMO level of the aromatic amine (the property of easily accepting electrons), and the highest occupied orbital (HOMO) level of the aromatic amine, which is shallower than the HOMO level of the heteroaromatic compound (the property of easily accepting holes).Therefore, calculations were performed using a combination of dibenzo[f,h]quinoxaline (abbreviation: DBq), which is a typical scaffold forming the LUMO of a heteroaromatic compound, and triphenylamine (abbreviation: TPA), which is a typical scaffold forming the HOMO of an aromatic amine.

[0060] First, the optimal molecular structures and excitation energies of DBq alone and TPA alone in the lowest excited singlet state (S1) and the lowest excited triplet state (T1) were calculated using TD-DFT (time-dependent density functional theory). Furthermore, the excitation energy of a dimer of DBq and TPA was also calculated. In DFT, the total energy is represented as the sum of the potential energy, electrostatic energy between electrons, electrokinetic energy, and exchange-correlation energy, including all 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, represented in terms of the electron density, to enable high-speed, high-accuracy calculations.Here, B3LYP, a hybrid functional, was used to specify the significance of each parameter in terms of exchange-correlation energy. Additionally, 6-311 (a triple-split valence basis set basis function using three contraction functions for each valence orbital) was applied to all atoms as a basis function. 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 the accuracy of the calculation, the p function and the d function were added as polarization basis sets to the hydrogen atoms and the atoms other than hydrogen atoms.

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

[0062] First, the HOMO levels and LUMO levels of DBq alone, TPA alone, and a dimer of DBq and TPA were calculated. Fig. 17 shows the HOMO levels and the LUMO levels and Fig. 18A1, Fig. 18A2, Fig. 18B1, Fig. 18B2, Fig. 18C1 and Fig. 18C2 shows the HOMO and LUMO distributions.

[0063] Fig. Figure 18A1 shows the LUMO distribution of DBq alone; Fig. 18A2 the HOMO distribution of DBq alone; Fig. 18B1 the LUMO distribution of TPA alone; Fig. 18B2 the HOMO distribution of TPA alone; Fig. 18C1 the LUMO distribution of the dimer of DBq and TPA; and Fig. 18C2 the HOMO distribution of the dimer of DBq and TPA.

[0064] As in Fig. 17, it is suggested that the dimer of DBq and TPA forms an exciplex of DBq and TPA under the influence of the LUMO level (-1.99 eV) of DBq, which is deeper (lower) than the LUMO level of TPA, and the HOMO level (-5.21 eV) of TPA, which is shallower (higher) than the HOMO level of DBq. In fact, as can be seen from Fig. 18C1 and Fig. 18C2, the LUMO of the dimer of DBq and TPA is distributed on the DBq side and the HOMO is distributed on the TPA side.

[0065] The excitation energies obtained by the optimal molecular structures of DBq alone in S1 and T1 are shown below. The S1 and T1 excitation energies correspond to the fluorescence and phosphorescence wavelengths obtained by DBq alone. The S1 excitation energy of DBq alone is 3.294 eV and the fluorescence wavelength is 376.4 nm. The T1 excitation energy of DBq alone is 2.460 eV and the phosphorescence wavelength is 504.1 nm.

[0066] Additionally, the excitation energies obtained by the optimal molecular structures of TPA alone in S1 and T1 are shown. The S1 and T1 excitation energies correspond to the fluorescence and phosphorescence wavelengths obtained by TPA alone. The S1 excitation energy of TPA alone is 3.508 eV and the fluorescence wavelength is 353.4 nm. The T1 excitation energy of TPA alone is 2.610 eV and the phosphorescence wavelength is 474.7 nm.

[0067] Furthermore, the excitation energies obtained by the optimal molecular structures of the DBq and TPA dimer in S1 and T1 are shown. The S1 and T1 excitation energies correspond to the fluorescence and phosphorescence wavelengths obtained by the DBq and TPA dimer. The S1 excitation energy of the DBq and TPA dimer is 2.036 eV, and the fluorescence wavelength is 609.1 nm. The T1 excitation energy of the DBq and TPA dimer is 2.030 eV, and the phosphorescence wavelength is 610.0 nm.

[0068] From the above, it was found that each of the phosphorescence wavelengths of DBq alone and TPA alone is shifted toward the longer wavelength side by approximately 100 nm with respect to the fluorescence wavelength. This result shows a similar trend to that of the CBP (measured values) described above and supports the validity of the calculations.

[0069] On the other hand, it was found that the fluorescence wavelength of the dimer of DBq and TPA is on the longer wavelength side compared to the fluorescence wavelengths of DBq alone and TPA alone. This result shows a similar trend to the examples described below (measured values) and supports the validity of the calculations. It was also found that the difference between the fluorescence wavelength and the phosphorescence wavelength of the dimer of DBq and TPA is only 0.9 nm, and that these wavelengths are essentially the same.

[0070] These results demonstrate that the exciplex can combine the singlet excitation energy and the triplet excitation energy into essentially the same energy. Therefore, as shown above, the exciplex can efficiently transfer energy to the phosphorescent compound from both the singlet and triplet states.

[0071] 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, an embodiment of the present invention differs significantly from a conventional method because an exciplex is first formed from a host material and another material (an exciplex of a first organic compound and a second organic compound), and energy transfer from the exciplex is utilized. In addition, this difference provides unprecedentedly high emission efficiency.

[0072] It should be noted that the use of an exciplex for a light-emitting layer of a light-emitting element generally has value in controlling the emission color, but it 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 the use of an exciplex as a medium for energy transfer to a phosphorescent compound, on the contrary, makes it possible to maximize emission efficiency, as shown in one embodiment of the invention. This technical idea is contrary to the conventional wisdom.

[0073] Furthermore, in the light-emitting element of one embodiment of the present invention, the threshold voltage at which an exciplex is formed by carrier recombination (or from a singlet exciton) depends on the energy of a peak in the exciplex's emission spectrum. Therefore, if the exciplex's emission spectrum has a peak at 620 nm (2.0 eV), the threshold voltage required to form an exciplex with electrical energy is approximately 2.0 V.

[0074] If the energy of the exciplex's emission spectrum peak is too high (i.e., if the wavelength is too short), the threshold voltage at which an exciplex is formed increases. This is not preferable because a higher voltage is required for the phosphorescent compound to emit light through energy transfer from the exciplex to the phosphorescent compound, and therefore additional energy is consumed.

[0075] In view of this, it is preferable that the energy of the peak of the emission spectrum of the exciplex be lower (the wavelength is longer), because in this case, the threshold voltage is lower. Accordingly, the light-emitting element of one embodiment of the present invention, in which the peak wavelength of the emission spectrum of the exciplex is longer than or equal to a peak wavelength of the absorption band located on the longest wavelength side of the absorption spectrum of the phosphorescent compound, can be operated at a low drive voltage.Additionally, in the light-emitting element of one embodiment of the present invention, even when the peak wavelength of the emission spectrum of the exciplex is longer than the peak wavelength of the absorption spectrum of the phosphorescent compound, energy can be transferred by utilizing the overlap between the emission spectrum of the exciplex and the absorption band located on the longest wavelength side of the absorption spectrum of the phosphorescent compound, resulting in high emission efficiency of the light-emitting element. As described above, high emission efficiency is maintained at a reduced drive voltage, thus achieving high power efficiency.

[0076] Since the peak wavelength of the exciplex emission spectrum is particularly long in the light-emitting element described above, the drive voltage can be lower. This can be explained as follows.

[0077] One embodiment of the present invention includes a light-emitting element in which the peak wavelength of the emission spectrum of the exciplex is longer than the peak wavelength of the absorption band located on the longest wavelength side of the absorption spectrum of the phosphorescent compound (i.e., the energy of the emission peak of the exciplex is lower than or equal to the energy of the absorption peak of the phosphorescent compound). Therefore, in the light-emitting element, a voltage value at which an exciplex is formed by carrier recombination is smaller than a voltage value at which the phosphorescent compound starts to emit light by carrier recombination.

[0078] In other words, even if a voltage lower than the voltage at which the phosphorescent compound begins to emit light is applied to the light-emitting element, carrier recombination occurs and an exciplex is formed, thus causing a recombination current to flow into the light-emitting element. In this way, a light-emitting element with a lower drive voltage (with more favorable voltage-current characteristics) can be provided.

[0079] Accordingly, by the time the voltage reaches a value at which the phosphorescent compound begins to emit light, a sufficient number of carriers exist in the light-emitting layer, and carrier recombination, which can contribute to the light emission of the phosphorescent compound, can easily occur many times. Therefore, the luminance becomes very high at a voltage near the threshold voltage (emission onset voltage) of the phosphorescent compound. In other words, a curve representing the voltage-luminance characteristic can be very steep in an increasing region near the onset of emission, so the drive voltage required to achieve the desired luminance can be very low.Furthermore, in order to obtain practical luminance, the operation is carried out at a voltage which is higher than or equal to the threshold voltage (voltage of onset of emission) of the phosphorescent compound, in which case the emitted light comes mainly from the phosphorescent compound and the light-emitting element therefore has a high current efficiency.

[0080] It should be noted that the phosphorescent compound used in one embodiment of the present invention has a singlet absorption spectrum and a triplet absorption spectrum located close to each other. Furthermore, in an exciplex formed in one embodiment of the present invention, a peak of an emission spectrum of the exciplex in the singlet state and a peak of an emission spectrum of the exciplex in the triplet state can be considered to be located close to each other. Even if the peak of the emission spectrum of the exciplex (generally, the emission spectrum of the exciplex in the singlet state) is located near the peak of the emission spectrum of the phosphorescent compound, quenching of the triplet excitation energy of the phosphorescent compound due to the exciplex in the triplet state can be suppressed.First, the exciplex does not exhibit an absorption spectrum, so a phenomenon in which the triplet excitation energy of the phosphorescent compound is transferred to the exciplex to be quenched is unlikely to occur. This suggests that the light-emitting element of the present invention has a high external quantum efficiency. The above is also an advantage of using an exciplex.

[0081] An embodiment of the present invention is a light-emitting element comprising a light-emitting layer containing a phosphorescent compound, a first organic compound, and a second organic compound between a pair of electrodes, wherein a combination of the first organic compound and the second organic compound forms an exciplex, wherein an emission spectrum of the exciplex overlaps an absorption band located on the longest wavelength side of an absorption spectrum of the phosphorescent compound, and wherein a difference between a peak wavelength of the emission spectrum of the exciplex and a peak wavelength of an emission spectrum of the phosphorescent compound is 30 nm or less.

[0082] The light-emitting element can be operated at a low drive voltage and exhibits sufficiently high emission efficiency when the peak of the emission spectrum of the exciplex (generally, the emission spectrum of the singlet state exciplex) is located near the peak of the emission spectrum of the phosphorescent compound, as described above. The effect of reducing the drive voltage is particularly enhanced when the peak of the emission spectrum of the exciplex is located in a range extending from the peak of the emission spectrum of the phosphorescent compound to a wavelength 30 nm longer than the peak of the emission spectrum of the phosphorescent compound.Furthermore, a relatively high emission efficiency can be maintained when the peak of the emission spectrum of the exciplex is located in a range extending from the peak of the emission spectrum of the phosphorescent compound to a wavelength 30 nm shorter than the peak of the emission spectrum of the phosphorescent compound.

[0083] When the peak of the exciplex's emission spectrum is located on the longer wavelength side compared to the peak of the phosphorescent compound's emission spectrum, the external quantum efficiency of the light-emitting element is reduced in some cases. This is because the overlap between the exciplex's emission spectrum and the absorption band located on the longest wavelength side of the phosphorescent compound's absorption spectrum is reduced under these conditions. Therefore, excitation energy transfer from the exciplex to the phosphorescent compound hardly occurs, and the exciplex itself is easily deactivated by the emission of excitation energy, such as light or heat.

[0084] Therefore, in order to achieve extremely high emission efficiency, another embodiment of the present invention is a light-emitting element comprising a light-emitting layer containing a phosphorescent compound, a first organic compound, and a second organic compound between a pair of electrodes, wherein a combination of the first organic compound and the second organic compound forms an exciplex, wherein an emission spectrum of the exciplex overlaps an absorption band located on the longest wavelength side of an absorption spectrum of the phosphorescent compound, and wherein a peak wavelength of the emission spectrum of the exciplex is longer than a peak wavelength of the absorption band located on the longest wavelength side of the absorption spectrum of the phosphorescent compound,or this corresponds to and is shorter than a peak wavelength of an emission spectrum of the phosphorescent compound or corresponds to this.,

[0085] In the light-emitting element, the peak wavelength of the emission spectrum of the exciplex is shorter than or equal to the peak wavelength of the emission spectrum of the phosphorescent compound; and therefore, the overlap between the emission spectrum of the exciplex and the absorption band located on the longest wavelength side of the absorption spectrum of the phosphorescent compound is increased. Therefore, the excitation energy can be efficiently transferred from the exciplex to the phosphorescent compound. Therefore, energy deactivation can be suppressed. Consequently, a light-emitting element with a low driving voltage and high external quantum efficiency can be fabricated.

[0086] In particular, it is preferable that a difference between the peak wavelength of the emission spectrum of the exciplex and the peak wavelength of the emission spectrum of the phosphorescent compound is 30 nm or less.

[0087] Additionally, in one embodiment of the present invention, an exciplex is formed from a singlet exciton of the first organic compound or the second organic compound.

[0088] In a light-emitting element of one embodiment of the present invention, one possible elementary formation method of an exciplex is one in which one of the first and second organic compounds forms a singlet exciton and then interacts with the other in the ground state. As described above, the emission spectrum of the exciplex and the absorption spectrum of the phosphorescent compound can significantly overlap; therefore, the energy transfer efficiency can be increased. Consequently, a light-emitting element with high quantum efficiency can be effectively achieved.

[0089] The singlet exciton has a short excitation lifetime (small τ), as described above. Therefore, there is a problem that part of the excitation energy is deactivated (by light emission or thermal deactivation) before the excitation energy is transferred from the singlet exciton to a gas material (Φ ETtends to be small in formula (3). In one embodiment of the present invention, such deactivation of the excitation energy can be suppressed because the singlet exciton quickly forms an exciplex. Furthermore, the exciplex has a relatively long excitation lifetime, which is considered preferable for the energy transfer efficiency Φ ET Accordingly, deactivation of the singlet excitation energy of the host material can not only affect the efficiency of an element, but also its lifetime can be suppressed by applying an embodiment of the present invention, so that a light-emitting element with a long lifetime can be obtained.

[0090] In one embodiment of the present invention, it is also preferred that the excitation energy of the exciplex be sufficient to be transferred to the phosphorescent compound, and that light emission from the exciplex is substantially not observed. Therefore, the energy is preferably transferred through the exciplex to the phosphorescent compound, so that the phosphorescent compound emits phosphorescence.

[0091] According to the energy transfer concept described above, one embodiment of the present invention is effective in the case where at least one of the first and second organic compounds is a fluorescent compound (ie, a compound prone to light emission or thermal deactivation from the excited singlet state). Therefore, it is preferable that at least one of the first and second organic compounds is a fluorescent compound.

[0092] It should be noted that in the case where a phosphorescent compound is used as an organic compound serving as a host material, the organic compound itself tends to emit light and does not transfer energy to a guest material. In this case, it is preferable that the organic compound emits light efficiently, but it is difficult to achieve high light emission efficiency because the organic compound serving as the host material causes the problem of concentration quenching. For this reason, it is preferable that the organic compound be a fluorescent compound and that energy transfer is achieved with the composition described above.

[0093] Additionally, in one embodiment of the present invention, it is preferred that the phosphorescent compound is an organometallic complex.

[0094] The exciplex used in one embodiment of the present invention is described in detail below. <exciplex>

[0095] The exciplex (excited complex) is formed by an interaction between dissimilar molecules in the excited state. The exciplex is known to be easily formed between a material with a relatively deep LUMO level and a material with a relatively shallow HOMO level.

[0096] An emission wavelength depends on the energy difference between the HOMO and LUMO levels. If the energy difference is large, the wavelength is short.

[0097] If the energy difference is small, the emission wavelength is long.

[0098] Here, the HOMO levels and LUMO levels of the first organic compound and the second organic compound used in an embodiment of the present invention are different from each other. Specifically, the energy levels are higher in the following order: the HOMO level of the first organic compound < the HOMO level of the second organic compound < the LUMO level of the first organic compound < the LUMO level of the second organic compound (see Fig. 6).

[0099] 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 and the second organic compound, respectively (see Fig. 6). Therefore, the energy difference of the exciplex is smaller than the energy difference between the first organic compound and the second organic compound. In other words, the emission wavelength of the exciplex is longer than the emission wavelengths of the first organic compound and the second organic compound.

[0100] The preparation process of the exciplex used in one embodiment of the present invention can be roughly divided into two processes. < <elektroplex>>

[0101] In this specification, the term "electroplex" means that the first ground-state organic compound and the second ground-state organic compound directly form an exciplex.

[0102] As described above, in general, when an electron and a hole recombine in a host material, the excitation energy is transferred from the host material in an excited state to a guest material, thereby causing the guest material to enter an excited state to emit light.

[0103] At this time, 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. In particular, when the host material is in a singlet excited state, the excitation lifetime is shorter than when it is in a triplet excited state, which easily leads to deactivation of the singlet excitation energy. Deactivation of the excitation energy is one of the reasons for a reduction in the lifetime of the light-emitting element.

[0104] However, in one embodiment of the present invention, an electroplex is formed by the first organic compound and the second organic compound with carriers (cation or anion); therefore, the formation of a singlet exciton with a short excitation lifetime can be suppressed. In other words, there is a method in which an exciplex is directly formed without the formation of a singlet exciton. Therefore, the deactivation of the singlet excitation energy can be prevented. Consequently, a light-emitting element with a long lifetime can be obtained.

[0105] For example, in the case where the first organic compound is a compound with an electron-trapping property and the second organic compound is a compound with a hole-trapping property, an electroplex is directly formed by an anion of the first organic compound and a cation of the second organic compound. It is a novel concept to obtain a light-emitting element with high emission efficiency by suppressing the generation of the singlet excited state of a host material and transferring energy from an electroplex to a guest material in the manner described above. It should be noted that the generation of the triplet excited state of the host material is similarly suppressed, and an electroplex is directly formed; therefore, energy transfer is believed to occur from the electroplex to the guest material. This mechanism is also novel.

[0106] The emission spectrum of the formed electroplex is located on the longer wavelength side with respect to the emission wavelength of each of the first and second organic compounds.

[0107] The overlap between the emission spectrum of the electroplex and the absorption spectrum of the phosphorescent compound is greater than the overlap between the emission spectrum of the first organic compound (or the second organic compound) and the absorption spectrum of the phosphorescent compound. The light-emitting element of one embodiment of the present invention transfers energy using the overlap between the emission spectrum of the electroplex and the absorption spectrum of the phosphorescent compound and therefore has a higher energy transfer efficiency. Accordingly, in one embodiment of the present invention, a light-emitting element with a high external quantum efficiency can be obtained. <<Bildung eines Exciplex durch ein Exziton> >

[0108] As another method, there is an elementary method in which one of the first and second organic compounds forms a singlet exciton and then interacts with the other in the ground state to form an exciplex. Unlike an electroplex, an excited singlet state of the first organic compound or the second organic compound in this case is temporarily generated but is quickly converted into an exciplex, and therefore, deactivation of the singlet excitation energy can be prevented. Therefore, it is possible to prevent deactivation of the excitation energy of the first organic compound or the second organic compound. Accordingly, in one embodiment of the present invention, a light-emitting element with a long lifetime can be obtained.It should be noted that the excited triplet state of the host material is also assumed to be rapidly converted into an exciplex and energy is transferred from the exciplex to the guest material.

[0109] The emission spectrum of the formed exciplex is located on the longer wavelength side compared with the emission wavelength of each of the first and second organic compounds.

[0110] The overlap between the emission spectrum of the exciplex and the absorption spectrum of the phosphorescent compound is greater than the overlap between the emission spectrum of the first organic compound (or the second organic compound) and the absorption spectrum of the phosphorescent compound. The light-emitting element of one embodiment of the present invention transfers energy using the overlap between the emission spectrum of the exciplex and the absorption spectrum of the phosphorescent compound and therefore has a higher energy transfer efficiency. Accordingly, in one embodiment of the present invention, a light-emitting element with a high external quantum efficiency can be obtained.

[0111] For example, in the case where the first organic compound is a compound with electron-trapping properties, the second organic compound is a compound with hole-trapping properties, and the difference between the HOMO levels and the LUMO levels of these compounds is large (specifically, 0.3 eV or more), electrons are selectively injected into the first organic compound, and holes are selectively injected into the second organic compound. In this case, the method of forming an electroplex is considered to be superior to the method of forming an exciplex by a singlet exciton.

[0112] It should be noted that this embodiment may be combined with any other embodiment where appropriate. (Embodiment 2)

[0113] In this embodiment, a light-emitting element of an embodiment of the present invention will be described with reference to Fig. 7A to 7C.

[0114] Fig. Figure 7A shows a light-emitting element comprising an EL layer 102 between a first electrode 103 and a second electrode 108. The light-emitting element in Fig. 7A includes a hole injection layer 701, a hole transport layer 702, a light emitting layer 703, an electron transport layer 704, and an electron injection layer 705 stacked in this order on the first electrode, and the second electrode 108 is provided thereover.

[0115] The first electrode 103 is preferably formed using metals, alloys, conductive compounds, mixtures thereof, and the like that have a high work function (particularly 4.0 eV or more). Specific examples include indium oxide-tin oxide (ITO: indium tin oxide), indium oxide-tin oxide containing silicon or silicon oxide, indium oxide-zinc oxide (indium zinc oxide), indium oxide containing tungsten oxide and zinc oxide (IWZO), and the like. Films of these conductive metal oxides are generally formed by a sputtering method, but can also be formed by using a sol-gel method or the like. For example, an indium oxide-tin oxide film can be formed by a sputtering method using a target in which zinc oxide is added to indium oxide at 1 wt% to 20 wt%.Furthermore, an IWZO film can be formed by a sputtering method using a target in which tungsten oxide is added to indium oxide at 0.5 wt% to 5 wt% and zinc oxide is added to indium oxide at 0.1 wt% to 1 wt%. Other examples include graphene, gold, platinum, nickel, tungsten, chromium, molybdenum, iron, cobalt, copper, palladium, nitrides of metal materials (e.g., titanium nitride), and the like.

[0116] It should be noted that when a layer included in the EL layer 102 and formed in contact with the first electrode 103 is formed using a composite material described below, which is formed by combining an organic compound and an electron acceptor (an acceptor), as a substance used for the first electrode 103, any of a variety of metals, alloys, electrically conductive compounds, mixtures thereof, and the like can be used regardless of the work function; for example, aluminum, silver, an aluminum-containing alloy (e.g., Al-Si), or the like can also be used.

[0117] The first electrode 103 can be formed by, for example, a sputtering method, an evaporation method (including a vacuum evaporation method) or the like.

[0118] The second electrode 108 is preferably formed using metals, alloys, electrically conductive compounds, mixtures thereof, and the like, which have a low work function (preferably 3.8 eV or less). Specific examples of these include elements belonging to Groups 1 and 2 of the Periodic Table, i.e., alkali metals such as lithium and cesium, alkaline earth metals such as calcium and strontium, magnesium, their alloys (e.g., Mg-Ag and Al-Li), rare earth metals such as europium and ytterbium, their alloys, aluminum, silver, and the like.

[0119] When a layer included in the EL layer 102 and formed in contact with the second electrode 108 is formed using a later-described composite material formed by combining an organic compound and an electron donor (a donor), a variety of conductive materials such as Al, Ag, ITO, and indium oxide-tin oxide containing silicon or silicon oxide can be used regardless of the work function.

[0120] It should be noted that when forming the second electrode 108, a vacuum evaporation method or a sputtering method may be used. When using a silver paste or the like, a coating method, an inkjet method, or the like may be used.

[0121] The EL layer 102 includes at least the light-emitting layer 703. A known substance can be used for a portion of the EL layer 102, and either a low-molecular compound or a high-molecular compound can be used. It should be noted that the substances constituting the EL layer 102 may be organic compounds or may contain an organic compound as a portion.

[0122] Furthermore, the EL layer 102 comprises, as in Fig. 7A, not only the light-emitting layer 703, but also a suitable combination of the following layers: the hole-injection layer 701 containing a substance having a high hole-transport property, the hole-transport layer 702 containing a substance having a high hole-transport property, the electron-transport layer 704 containing a substance having a high electron-transport property, the electron-injection layer 705 containing a substance having a high electron-injection property, and the like.

[0123] The hole-injection layer 701 is a layer containing a substance with a high hole-injection property. A metal oxide such as molybdenum oxide, titanium oxide, vanadium oxide, rhenium oxide, ruthenium oxide, chromium oxide, zirconium oxide, hafnium oxide, tantalum oxide, silver oxide, tungsten oxide, or manganese oxide can be used as the substance with a high hole-injection property. Alternatively, a phthalocyanine-based compound such as phthalocyanine (abbreviation: H2Pc) or copper(II) phthalocyanine (abbreviation: CuPc) can be used.

[0124] Other examples of the substance that can be used are aromatic compounds and the like, which are low molecular weight compounds, such as 4,4',4''-tris(N,N-diphenylamino)triphenylamine (abbreviation: TDATA), 4,4',4''-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine (abbreviation: MTDATA), 4,4'-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl (abbreviation: DPAB), 4,4'-bis(N-{4-[N'-(3-methylphenyl)-N'phenylamino]phenyl}-N-phenylamino)biphenyl (abbreviation: DNTPD), 1,3,5-tris[N-(4-diphenylaminophenyl)-N-phenylamino]benzene (abbreviation: DPA3B), 3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole (abbreviation: PCzPCA1), 3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole (abbreviation: PCzPCA2) and 3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole (abbreviation: PCzPCN1).

[0125] Still other examples of the substance that can be used are high molecular weight compounds (e.g. oligomers, dendrimers and polymers), such as poly(N-vinylcarbazole) (abbreviation: PVK), poly(4-vinyltriphenylamine) (abbreviation: PVTPA), poly[N-(4-{N'-[4-(4-diphenylamino)phenyl]phenyl-N'-phenylamino}phenyl)methacrylamide] (abbreviation: PTPDMA) and poly[N,N'-bis(4-butylphenyl)-N,N'-bis(phenyl)benzidine] (abbreviation: poly-TPD) and high molecular weight compounds to which acid is added, such as poly(3,4-ethylenedioxythiophene) / poly(styrenesulfonic acid) (PEDOT / PSS) and polyaniline / poly(styrenesulfonic acid) (PAni / PSS).

[0126] For the hole-injection layer 701, a composite material formed by combining an organic compound and an electron acceptor (acceptor) can be used. Such a composite material, in which holes are formed in the organic compound by the electron acceptor, exhibits high hole-injection and hole-transport properties. In this case, the organic compound is preferably a material that excels at transporting generated holes (a substance with a high hole-transport property).

[0127] Examples of the organic compound used for the composite material can be a variety of compounds, such as aromatic amine compounds, carbazole derivatives, aromatic hydrocarbons, and high-molecular compounds (e.g., oligomers, dendrimers, and polymers). The organic compound used for the composite material is preferably an organic compound with a high hole-transport property, and is particularly preferably a substance with a hole mobility of 10 -6 cm 2 / Vs or more. It should be noted that in addition to these substances, any substance that has the property of transporting more holes than electrons can be used. Organic compounds that can be used are specifically described below.

[0128] Examples of the organic compound that can be used as the composite material are aromatic amine compounds such as TDATA, MTDATA, DPAB, DNTPD, DPA3B, PCzPCA1, PCzPCA2, PCzPCN1, 4,4'-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB or α-NPD), N,N'-bis(3-methylphenyl)-N,N'-diphenyl-[1,1'-biphenyl]-4,4'-diamine (abbreviation: TPD) and 4-phenyl-4'-(9-phenylfluoren-9-yl)triphenylamine (abbreviation: BPAFLP), and carbazole derivatives such as 4,4'-di(N-carbazolyl)biphenyl (abbreviation: CBP), 1,3,5-tris[4-(N-carbazolyl)phenyl]benzene (abbreviation: TCPB), 9-[4-(N-carbazolyl)phenyl]-10-phenylanthracene (abbreviation: CzPA), 9-phenyl-3-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: PczPA) and 1,4-bis[4-(N-carbazolyl)phenyl]-2,3,5,6-tetraphenylbenzene.

[0129] Other examples of the organic compound that can be used are aromatic hydrocarbon compounds such as 2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation: t-BuDNA), 2-tert-butyl-9,10-di(1-naphthyl)anthracene, 9,10-bis(3,5-diphenylphenyl)anthracene (abbreviation: DPPA), 2-tert-butyl-9,10-bis(4-phenylphenyl)anthracene (abbreviation: t-BuDBA), 9,10-di(2-naphthyl)anthracene (abbreviation: DNA), 9,10-diphenylanthracene (abbreviation: DPAnth), 2-tert-butylanthracene (abbreviation: t-BuAnth), 9,10-bis(4-methyl-1-naphthyl)anthracene (abbreviation: DMNA), 9,10-bis[2-(1-naphthyl)phenyl]-2-tert-butylanthracene, 9,10-bis[2-(1-naphthyl)phenyl]anthracene and 2,3,6,7-tetramethyl-9,10-di(1-naphthyl)anthracene.

[0130] Other examples of organic compounds that can be used are aromatic hydrocarbons such as 2,3,6,7-tetramethyl-9,10-di(2-naphthyl)anthracene, 9,9'-bianthryl, 10,10'-diphenyl-9,9'-bianthryl, 10,10'-bis(2-phenylphenyl)-9,9'-bianthryl, 10,10'-bis[(2,3,4,5,6-pentaphenyl)phenyl]-9,9'-bianthryl, anthracene, tetracene, rubrene, perylene, 2,5,8,11-tetra(tert-butyl)perylene, pentacene, coronene, 4,4'-bis(2,2-diphenylvinyl)biphenyl (abbreviation: DPVBi) and 9,10-Bis[4-(2,2-diphenylvinyl)phenyl]anthracene (abbreviation: DPVPA).

[0131] Further examples of the electron acceptor include organic compounds such as 7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation: F4-TCNQ) and chloranil, oxides of transition metals, oxides of metals belonging to Groups 4 to 8 of the Periodic Table, and the like. 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 property. Among these, molybdenum oxide is particularly preferred because of its stability in air, low hygroscopicity, and ease of handling.

[0132] The composite material can be formed using the above-described electron acceptor and the above-described high-molecular compound, such as PVK, PVTPA, PTPDMA, or poly-TPD, and can be used for the hole injection layer 701.

[0133] The hole-transport layer 702 is a layer containing a substance with a high hole-transport property. Examples of the substance with a high hole-transport property are aromatic amine compounds such as NPB, TPD, BPAFLP, 4,4'-bis[N-(9,9-dimethylfluoren-2-yl)-N-phenylamino]biphenyl (abbreviation: DFLDPBi), and 4,4'-bis[N-(spiro-9,9'-bifluoren-2-yl)-N-phenylamino]biphenyl (abbreviation: BSPB). The substances mentioned here are mainly substances that have a hole mobility of 10 -6 cm 2 / Vs or more. It should be noted that any substance that has the property of transporting more holes than electrons can be used. It should be noted that the layer containing a substance with a high hole-transport property is not limited to a single layer and can be a stack of two or more layers containing any of the above substances.

[0134] For the hole transport layer 702, a carbazole derivative such as CBP, CzPA or PczPA or an anthracene derivative such as t-BuDNA, DNA or DPAnth can be used.

[0135] For the hole transport layer 702, a high molecular compound such as PVK, PVTPA, PTPDMA or Poly-TPD can also be used.

[0136] The light-emitting layer 703 is a layer containing a light-emitting substance. The light-emitting layer 703 of this embodiment contains a phosphorescent compound, a first organic compound, and a second organic compound. The phosphorescent compound is a light-emitting substance (guest material). One of the first and second organic compounds, whose content in the light-emitting layer 703 is higher than that of the others, is a host material. For details, refer to Embodiment 1.

[0137] As the phosphorescent compound, an organometallic complex is preferred, and in particular, an iridium complex is preferred. Considering the energy transfer due to the Förster mechanism described above, the molar absorption coefficient of the absorption band of the phosphorescent compound located on the longest wavelength side is preferably 2000 M -1 ·cm -1 or more, more preferably 5000 M -1 ·cm -1 or more. Examples of the compound with such a high molar absorption coefficient are bis(3,5-dimethyl-2-phenylpyrazinato)(dipivaloylmethanato)iridium(III) (abbreviation: [Ir(mppr-Me)2(dpm)]), (acetylacetonato)bis(4,6-diphenylpyrimidinato)iridium(III) (abbreviation: [Ir(dppm)2(acac)]), and the like.

[0138] As the first organic compound and the second organic compound, a combination of a compound that tends to accept electrons (a compound with electron trapping property) and a compound that tends to accept holes (a compound with hole trapping property) is preferably used. With such a composition, it is possible to achieve the effect of improving emission efficiency and lifetime not only by energy transfer from an exciplex, but also by adjusting the carrier balance between hole transport and electron transport in the light-emitting layer.

[0139] Heteroaromatic compounds are typical examples of compounds that tend to accept electrons. Examples include 2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation: 2mDBTPDBq-II), 2-[4-(3,6-diphenyl-9H-carbazol-9-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation: 2CzPDBq-III), 7-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation: 7mDBTPDBq-II), and 6-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation: 6mDBTPDBq-II).

[0140] Typical examples of compounds that tend to accept holes are aromatic amine compounds and carbazole compounds. For example, the following can be mentioned: 4-phenyl-4'-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PCBA1BP), 3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole (abbreviation: PCzPCN1), 4,4',4''-tris[N-(1-naphthyl)-N-phenylamino]triphenylamine (abbreviation: 1'-TNATA or 1-TNATA), 2,7-bis[N-(4-diphenylaminophenyl)-N-phenylamino]-spiro-9,9'-bifluorene (abbreviation: DPA2SF), N,N'-bis(9-phenylcarbazol-3-yl)-N,N'-diphenylbenzene-1,3-diamine (abbreviation: PCA2B), N-(9,9-Dimethyl-2-N',N'-diphenylamino-9H-fluoren-7-yl)diphenylamine (abbreviation: DPNF), N,N',N''-triphenyl-N,N',N''-tris(9-phenylcarbazol-3-yl)benzene-1,3,5-triamine (abbreviation: PCA3B), 2-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]spiro-9,9'-bifluorene (abbreviation: PCASF), 2-[N-(4-Diphenylaminophenyl)-N-phenylamino]spiro-9,9'-bifluorene (abbreviation: DPASF), N,N'-Bis[4-(carbazol-9-yl)phenyl]-N,N'-diphenyl-9,9-dimethylfluorene-2,7-diamine (abbreviation: YGA2F), N,N'-Bis(3-methylphenyl)-N,N'-diphenyl-[1,1'-biphenyl]-4,4'-diamine (abbreviation: TPD), 4,4'-Bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl (abbreviation: DPAB), N-(9,9-Dimethyl-9H-fluoren-2-yl)-N-{9,9-dimethyl-2-[N'-phenyl-N'-(9,9-dimethyl-9H-fluoren-2-yl)amino]-9H-fluoren-7-yl}phenylamine (abbreviation: DFLADFL), 3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole (abbreviation: PCzPCA1), 3-[N-(4-diphenylaminophenyl)-N-phenylamino]-9-phenylcarbazole (abbreviation: PCzDPA1), 3,6-bis[N-(4-diphenylaminophenyl)-N-phenylamino]-9-phenylcarbazole (abbreviation: PCzDPA2), 4,4'-bis(N-{4-[N'-(3-methylphenyl)-N'phenylamino]phenyl}-N-phenylamino)biphenyl (abbreviation: DNTPD), 3,6-bis[N-(4-diphenylaminophenyl)-N-(1-naphthyl)amino]-9-phenylcarbazole (abbreviation: PCzTPN2) and 3,6-Bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole (abbreviation: PCzPCA2).

[0141] It should be noted that the present invention is not limited to the above examples with respect to the first organic compound and the second organic compound. The combination is determined so that an exciplex can be formed, wherein the emission spectrum of the exciplex overlaps the absorption spectrum of the phosphorescent compound, and the peak of the emission spectrum of the exciplex has a longer wavelength than the peak of the absorption spectrum of the phosphorescent compound.

[0142] It should be noted that in the case where a compound tending to accept electrons and a compound tending to accept holes are used as the first organic compound and the second organic compound, the carrier balance can be controlled by a mixing ratio of the compounds. That is, another feature of an embodiment of the present invention is that the optimal carrier balance, in which the probability of recombination of holes and electrons in the light-emitting layer and the emission efficiency are increased, can be formed by adjusting the mixing ratio. In view of the carrier balance and the formation of an exciplex, it is preferable that the amount of the first organic compound and that of the second organic compound do not differ significantly from each other.In particular, the ratio of the first organic compound to the second organic compound is preferably 1:9 to 9:1.

[0143] Furthermore, the exciplex can be formed at the interface between two layers. For example, when a layer containing the second organic compound and a layer containing the first organic compound are stacked on top of each other, the exciplex is formed near the interface between them. These two layers can be used as the light-emitting layer in one embodiment of the present invention. In this case, the phosphorescent compound is added near the interface. The phosphorescent compound can be added to one of the layers or to both.

[0144] The electron-transport layer 704 is a layer containing a substance with high electron-transport properties. Examples of substances with high electron-transport properties include metal complexes such as Alq3, tris(4-methyl-8-quinolinolato)aluminum (abbreviation: Almq3), bis(10-hydroxybenzo[h]quinolinato)beryllium (abbreviation: BeBq2), Balq, Zn(BOX)2, and bis[2-(2-hydroxyphenyl)benzothiazolato]zinc (abbreviation: Zn(BTZ)2). Other examples of these are heteroaromatic compounds such as 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation: PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene (abbreviation: OXD-7), 3-(4-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), 4,4'-bis(5-methylbenzoxazol-2-yl)stilbene (Abbreviation: BzOs).Other examples include high-molecular-weight compounds such as poly(2,5-pyridine-diyl) (abbreviation: PPy), poly[(9,9-dihexylfluorene-2,7-diyl)-co-(pyridine-3,5-diyl)] (abbreviation: PF-Py), and poly[(9,9-dioctylfluorene-2,7-diyl)-co-(2,2'-bipyridine-6,6'-diyl)] (abbreviation: PF-BPy). The substances mentioned here are mainly those with an electron mobility of 10. -6 cm 2 / Vs or more. It should be noted that in addition to these substances, any substance that has the property of conducting more holes than electrons can be used for the electron-transport layer.

[0145] Furthermore, the electron transport layer is not limited to a single layer and can be a stack of two or more layers containing any of the above substances.

[0146] The electron injection layer 705 is a layer containing a substance with high electron injection properties. Examples of substances that can be used for the electron injection layer 705 include alkali metals, alkaline earth metals, and their compounds, such as lithium, cesium, calcium, lithium fluoride, cesium fluoride, calcium fluoride, and lithium oxide; rare earth metal compounds such as erbium fluoride; and the aforementioned substances used for the electron transport layer 704.

[0147] Alternatively, a composite material formed by combining an organic compound and an electron donor (donor) can be used for the electron-injection layer 705. Such a composite material, in which electrons are generated in the organic compound by the electron donor, has high electron-injection and electron-transport properties. Here, the organic compound is preferably a material excellent in conducting generated electrons, and in particular, any of the above substances (such as metal complexes and heteroaromatic compounds) can be used for the electron-transport layer 704. As the electron donor, a substance exhibiting an electron-donating property with respect to the organic compound can be used.Preferred specific examples of the electron donor are alkali metals, alkaline earth metals, and rare earth metals, such as lithium, cesium, magnesium, calcium, erbium, and ytterbium. Any alkali metal oxide and alkaline earth oxide is preferred, such as lithium oxide, calcium oxide, barium oxide, and the like. A Lewis base such as magnesium oxide or an organic compound such as tetrathiafulvalene (abbreviation: TTF) can also be used.

[0148] It should be noted that the hole injection layer 701, the hole transport layer 702, the light emitting layer 703, the electron transport layer 704 and the electron injection layer 705 mentioned above can each be formed by a method such as an evaporation method (including a vacuum evaporation method), an inkjet method or a coating method.

[0149] A plurality of EL layers may be stacked between the first electrode 103 and the second electrode 108, as shown in Fig. 7B. In this case, a charge generation layer 803 is preferably provided between a first EL layer 800 and a second EL layer 801, which are stacked. The charge generation layer 803 can be formed using the composite materials described above. Furthermore, the charge generation layer 803 can have a stacked structure including a layer containing the composite material and a layer containing another material. In this case, as the layer containing another material, a layer containing an electron donor substance and a substance with a high electron-transport property, a layer formed on a transparent conductive film, or the like can be used.In a light-emitting element with such a structure, problems such as energy transfer and quenching are minimal, and a light-emitting element with both high emission efficiency and a long lifetime can be easily obtained due to the wide range of materials available. Furthermore, a light-emitting element that provides phosphorescence from one of the EL layers and fluorescence from the other of the EL layers can be easily fabricated. This structure can be combined with any of the EL layer structures described above.

[0150] Furthermore, by making the emission colors of the EL layers different, light of a desired color can be obtained from the light-emitting element as a whole. For example, in a light-emitting element with two EL layers, the emission colors of the first and second EL layers are complementary, so that the light-emitting element as a whole emits white light. The same applies to a light-emitting element with three or more layers.

[0151] As in Fig. 7C, the EL layer 102 may include the hole injection layer 701, the hole transport layer 702, the light emitting layer 703, the electron transport layer 704, an electron injection buffer layer 706, an electron relay layer 707, and a composite material layer 708 in contact with the second electrode 108 between the first electrode 103 and the second electrode 108.

[0152] It is preferable to provide the composite material layer 708 in contact with the second electrode 108, since in this case, damage to the EL layer 102 can be reduced, especially when the second electrode 108 is formed by a sputtering method. The composite material layer 708 can be formed using the above-described composite material in which an organic compound with a high hole-transport property contains an acceptor substance.

[0153] Further, by providing the electron injection buffer layer 706, an injection barrier between the composite material layer 708 and the electron transport layer 704 can be reduced, therefore, electrons generated in the composite material layer 708 can be easily injected into the electron transport layer 704.

[0154] As the electron injection buffer layer 706, a substance having a high electron injection property can be used, such as an alkali metal, an alkaline earth metal, a rare earth metal, a compound of the above metal (e.g., an alkali metal compound (including an oxide such as lithium oxide, a halide, and a carbonate such as lithium carbonate or cesium carbonate), an alkaline earth metal compound (including an oxide, a halide, and a carbonate), or a rare earth metal compound (including an oxide, a halide, and a carbonate).

[0155] Furthermore, in a case where the electron injection buffer layer 706 contains a substance having a high electron transport property and a donor substance, the donor substance is preferably added so that the mass ratio of the donor substance to the substance having a high electron transport property is in the range of 0.001:1 to 0.1:1. It should be noted that an organic compound such as tetrathianaphthacene (abbreviation: TTN), nickelocene, or decamethylnickelocene, as well as an alkali metal, alkaline earth metal, rare earth metal, a compound of the above metal (e.g.,an alkali metal compound (including an oxide such as lithium oxide, a halide, and a carbonate such as lithium carbonate or cesium carbonate), an alkaline earth metal compound (including an oxide, a halide, and a carbonate), and a rare earth metal compound (including an oxide, a halide, and a carbonate). It should be noted that as the substance having a high electron-transport property, a material similar to the material described above with respect to the electron-transport layer 704 can be used.

[0156] Furthermore, it is preferable that the electron relay layer 707 be formed between the electron injection buffer layer 706 and the composite material layer 708. The electron relay layer 707 is not necessarily provided; by providing the electron relay layer 707 with a high electron transport property, electrons can be quickly conducted into the electron injection buffer layer 706.

[0157] The structure in which the electron relay layer 707 is disposed between the composite material layer 708 and the electron injection buffer layer 706 is a structure in which the acceptor substance contained in the composite material layer 708 and the donor substance contained in the electron injection buffer layer 706 are less likely to interact with each other, and therefore their functions hardly interfere with each other. Therefore, an increase in the drive voltage can be prevented.

[0158] The electron relay layer 707 contains a substance with a high electron transport property and is formed such that the LUMO level of the substance with a high electron transport property lies between the LUMO level of the acceptor substance contained in the composite material layer 708 and the LUMO level of the substance with a high electron transport property contained in the electron transport layer 704. In the case where the electron relay layer 707 contains a donor substance, the donor level of the donor substance is also controlled to lie between the LUMO level of the acceptor substance contained in the composite material layer 708 and the LUMO level of the substance with a high electron transport property contained in the electron transport layer 704.As a specific value of the energy level, the LUMO level of the substance having a high electron transport property contained in the electron relay layer 707 is preferably -5.0 eV or more, more preferably -5.0 eV or more and -3.0 eV or less.

[0159] As the substance having a high electron transport property contained in the electron relay layer 707, a phthalocyanine-based material or a metal complex having a metal-oxygen bond and an aromatic ligand can be preferably used.

[0160] As the phthalocyanine-based material contained in the electron relay layer 707, any of CuPc, a phthalocyanine tin(II) complex (SnPc), a phthalocyanine zinc complex (ZnPc), cobalt(II) phthalocyanine, β-form (CoPc), phthalocyanine iron (FePc), and vanadyl 2,9,16,23-tetraphenoxy-29H,31 H-phthalocyanine (PhO-VOPc) is preferably used.

[0161] As the metal complex containing a metal-oxygen bond and an aromatic ligand contained in the electron relay layer 707, a metal complex containing a metal-oxygen double bond is preferably used. The metal-oxygen double bond has an acceptor property (a property of easily accepting electrons); therefore, electrons can be more easily conducted (donated and accepted). Furthermore, the metal complex containing a metal-oxygen double bond is considered stable. Therefore, the use of the metal complex containing the metal-oxygen double bond enables the light-emitting element to operate more stably at low voltages.

[0162] As a metal complex with a metal-oxygen bond and an aromatic ligand, a phthalocyanine-based material is preferred. In particular, any of vanadyl phthalocyanine (VOPc), a phthalocyanine tin(IV) oxide complex (SnOPc), and a phthalocyanine titanium oxide complex (TiOPc) are preferred because a metal-oxygen double bond is more likely to act on another molecule in terms of molecular structure and its acceptor property is high.

[0163] It should be noted that, among the above-mentioned phthalocyanine-based materials, a phthalocyanine-based material having a phenoxy group is preferred. In particular, a phthalocyanine derivative having a phenoxy group, such as PhO-VOPc, is preferred. The phthalocyanine derivative having a phenoxy group is soluble in a solvent and therefore has the advantage of being easy to handle during the formation of a light-emitting element and the advantage of simplifying the maintenance of an apparatus used for film production.

[0164] The electron relay layer 707 may further contain a donor substance. An organic compound such as tetrathianaphthacene (abbreviation: TTN), nickelocene, or decamethylnickelocene may be used as the donor substance, as well as an alkali metal, an alkaline earth metal, a rare earth metal, and a compound of the above metals (e.g., an alkali metal compound (including an oxide such as lithium oxide, a halide, and a carbonate such as lithium carbonate or cesium carbonate), an alkaline earth metal compound (including an oxide, a halide, and a carbonate), and a rare earth metal compound (including an oxide, a halide, and a carbonate)). When such a donor substance is contained in the electron relay layer 707, electrons can be easily conducted, and the light-emitting element can be operated at low voltage.

[0165] In the case where a donor substance is contained in the electron relay layer 707, in addition to the materials mentioned above as substances with a high electron-transport property, a substance having a higher LUMO level than the acceptor level of the acceptor substance contained in the composite material layer 708 can be used. Specifically, it is preferable to use a substance having a LUMO level of -5.0 eV or more, preferably -5.0 eV or more and -3.0 eV or less. Examples of such a substance include a perylene derivative, a nitrogen-containing condensed aromatic compound, and the like. It should be noted that a nitrogen-containing condensed compound is preferably used for the electron relay layer 707 because of its stability.

[0166] Specific examples of the perylene derivative are 3,4,9,10-perylenetetracarboxylic dianhydride (abbreviation: PTCDA), 3,4,9,10-perylenetetracarboxyl-bis-benzimidazole (abbreviation: PTCBI), N,N'-dioctyl-3,4,9,10-perylenetetracarboxylic diimide (abbreviation: PTCDI-C8H), N,N'-dihexyl-3,4,9,10-perylenetetracarboxylic diimide (abbreviation: Hex PTC) and the like.

[0167] Specific examples of the nitrogen-containing condensed aromatic compound are pirazino[2,3-f][1,10]phenanthroline-2,3-dicarbonitrile (abbreviation: PPDN), 2,3,6,7,10,11-hexacyano-1,4,5,8,9,12-hexaazatriphenylene (abbreviation: HAT(CN)6), 2,3-diphenylpyrido[2,3-b]pyrazine (abbreviation: 2PYPR), 2,3-bis(4-fluorophenyl)pyrido[2,3-b]pyrazine (abbreviation: F2PYPR) and the like.

[0168] In addition, 7,7,8,8-tetracyanoquinodimethane (abbreviation: TCNQ), 1,4,5,8-naphthalenetetracarboxylic dianhydride (abbreviation: NTCDA), perfluoropentacene, copper hexadecafluorophthalocyanine (abbreviation: F 16 CuPc), N,N'-bis(2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-pentadecafluorooctyl)-1,4,5,8-naphthalenetetracarboxylic acid diimide (abbreviation: NTCDI-C8F), 3',4'-dibutyl-5,5''-bis(dicyanomethylene)-5,5''-dihydro-2,2':5',2''-terthiophene (abbreviation: DCMT), methanofullerenes (e.g. [6,6]-phenyl C 61 Butyric acid methyl ester) or the like may be used.

[0169] It should be noted that in the case where a donor substance is included in the electron relay layer 707, the electron relay layer 707 can be formed by such a method as co-evaporation of the substance having a high electron transport property and the donor substance.

[0170] The hole injection layer 701, the hole transport layer 702, the light emitting layer 703 and the electron transport layer 704 can each be formed using the materials described above.

[0171] As described above, the EL layer 102 of this embodiment can be formed.

[0172] In the light-emitting element described above, a current flows due to a potential difference between the first electrode 103 and the second electrode 108, and holes and electrons recombine in the EL layer 102, thus emitting light. This light emission is then emitted to the outside through either the first electrode 103 or the second electrode 108, or both. Therefore, either the first electrode 103 or the second electrode 108, or both, is an electrode with a visible light transmitting property.

[0173] It should be noted that the structure of the layers provided between the first electrode 103 and the second electrode 108 is not limited to the structure described above. A structure other than the above can alternatively be employed, as long as a light-emitting region in which holes and electrons recombine is provided in a region remote from the first electrode 103 and the second electrode 108 to prevent quenching due to the proximity of the light-emitting region to the metal.

[0174] In other words, there is no particular limitation on the stacked structure of the layers. A layer containing a substance with a high electron-transport property, a substance with a high hole-transport property, a substance with a high electron-injection property, a substance with a high hole-injection property, a bipolar substance (a substance with a high electron-transport property and a high hole-transport property), a lock-blocking material, or the like can be freely combined with a light-emitting layer.

[0175] In the manner described above, the light-emitting element of one embodiment of the present invention can be manufactured.

[0176] Using the light-emitting element described in this embodiment, a passive matrix light-emitting device or an active matrix light-emitting device in which the driving of the light-emitting element is controlled by a transistor can be manufactured. Furthermore, the light-emitting device can be used in an electronic device, a lighting device, or the like.

[0177] It should be noted that this embodiment may be combined with any of the other embodiments, where appropriate. (Example 1)

[0178] In this example, an example of a combination of a first organic compound, a second organic compound and a phosphorescent compound is described with reference to the Fig. 1, Fig. 2, Fig. 3 and Fig. 4B, which can be used as a light-emitting element of an embodiment of the present invention.

[0179] The phosphorescent compound used in Structural Examples 1 to 4 of this example is (acetylacetonato)bis(4,6-diphenylpyrimidinato)iridium(III) (abbreviation: [Ir(dppm)2(acac)]). The first organic compound used in Structural Examples 1 to 4 of this example is 2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation: 2mDBTPDBq-II). As the second organic compound of this example, 4-phenyl-4'-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PCBA1BP) is used in Structural Example 1; 3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole (abbreviation: PCzPCN1) is used in Structural Example 2; 4,4',4''-tris[N-(1-naphthyl)-N-phenylamino]triphenylamine (abbreviation: 1'-TNATA or 1-TNATA) was used in structural example 3 and 2,7-bis[N-(4-diphenylaminophenyl)-N-phenylamino]-spiro-9,9'-bifluorene (abbreviation: DPA2SF) was used in structural example 4.

[0180] The chemical formulas of the materials used in this example are shown below. (Structural example 1)

[0181] Fig. Figure 1 shows an emission spectrum of a thin film of 2mDBTPDBq-II, which is the first organic compound (an emission spectrum 1a), an emission spectrum of a thin film of PCBA1BP, which is the second organic compound (an emission spectrum 2a), and an emission spectrum of a thin film made of a mixed material of 2mDBTPDBq-II and PCBA1BP (an emission spectrum 3a). Furthermore, a UV absorption spectrum (hereinafter referred to simply as an absorption spectrum) and an emission spectrum (an emission spectrum 4a) of [Ir(dppm)2(acac)], which is the phosphorescent compound of [Ir(dppm)2(acac)], in a dichloromethane solution are also shown.

[0182] It should be noted that in this example, the absorption spectrum of [Ir(dppm)2(acac)] was measured using a UV light spectrophotometer (V-550, manufactured by JASCO Corporation) in a state where the dichloromethane solution (0.093 mmol / L) was introduced into a quartz cell at room temperature.

[0183] In Fig. 1, the horizontal axis represents the wavelength (nm) and the vertical axis represents the molar absorption coefficient ε (M -1 ·cm -1 ) and the emission intensity (arbitrary unit).

[0184] From the absorption spectrum in Fig. 1 shows that [Ir(dppm)2(acac)] exhibits a broad absorption band at approximately 510 nm. This absorption band is believed to contribute significantly to the light emission.

[0185] Emission spectrum 3a exhibits a peak at a longer wavelength than emission spectra 1a and 2a. Additionally, the peak of emission spectrum 3a is closer to the absorption band than the peaks of emission spectra 1a and 2a. Fig. Figure 1 shows that the emission spectrum 3a has the largest overlap with the absorption band in the absorption spectrum, which contributes significantly to the light emission.

[0186] It was found that the emission spectrum of the mixed material of 2mDBTPDBq-II and PCBA1BP exhibits a peak at a longer wavelength than the emission spectrum of each organic compound alone. This indicates that an exciplex is formed by mixing 2mDBTPDBq-II with PCBA1BP.

[0187] It was found that the peak of emission spectrum 3a exhibits a large overlap with the absorption band in the absorption spectrum of [Ir(dppm)2(acac)], which is believed to contribute significantly to light emission. Therefore, the light-emitting element containing [Ir(dppm)2(acac)] and the mixed material of 2mDBTPDBq-II and PCBA1BP is shown to exhibit particularly high energy transfer efficiency because it transfers energy using the overlap between the emission spectrum of the mixed material and the absorption spectrum of the phosphorescent compound. Consequently, it is demonstrated that a light-emitting element with particularly high external quantum efficiency can be obtained.

[0188] Furthermore, the peak of the emission spectrum 3a is located on the longer wavelength side compared to the peak of the absorption spectrum, and is located on a shorter wavelength side compared to the peak of the emission spectrum 4a.

[0189] From the emission spectrum of the mixed material, whose peak is located on the longer wavelength side, it is found that a light-emitting material with a low driving voltage can be obtained using the mixed material. (Structural example 2)

[0190] Fig. Figure 2 shows an emission spectrum of a thin film of 2mDBTPDBq-II, which is the first organic compound (an emission spectrum 1b), an emission spectrum of a thin film of PCzPCN1, which is the second organic compound (an emission spectrum 2b), and an emission spectrum of a thin film made of a mixed material of 2mDBTPDBq-II and PCzPCN1 (an emission spectrum 3b). Furthermore, an absorption spectrum and an emission spectrum (an emission spectrum 4b) of [Ir(dppm)2(acac)], which is the phosphorescent compound of [Ir(dppm)2(acac)], in a dichloromethane solution are also shown.

[0191] In Fig. 2, the horizontal axis represents the wavelength (nm) and the vertical axis represents the molar absorption coefficient ε (M -1 ·cm -1 ) and the emission intensity (arbitrary unit).

[0192] From the absorption spectrum in Fig. 2 shows that [Ir(dppm)2(acac)] exhibits a broad absorption band at approximately 510 nm. This absorption band is believed to contribute significantly to the light emission.

[0193] Emission spectrum 3b exhibits a peak at a longer wavelength than emission spectra 1b and 2b. It was found that the emission spectrum of the mixed material of 2mDBTPDBq-II and PCzPCN1 exhibits a peak at a longer wavelength than the emission spectrum of each organic compound alone. This indicates that an exciplex is formed by mixing 2mDBTPDBq-II with PCzPCN1.

[0194] Furthermore, the peak of the emission spectrum 3b shows an overlap with the absorption spectrum of [Ir(dppm)2(acac)]. Therefore, it is demonstrated that the light-emitting element containing [Ir(dppm)2(acac)] and the mixed material of 2mDBTPDBq-II and PCzPCN1 exhibits particularly high energy transfer efficiency, as it transfers energy using the overlap between the emission spectrum of the mixed material and the absorption spectrum of the phosphorescent compound. Consequently, it is demonstrated that a light-emitting element with particularly high external quantum efficiency can be obtained.

[0195] Furthermore, the peak of the emission spectrum 3b is located on the longer wavelength side compared to the peak of the absorption spectrum, and is located on the shorter wavelength side compared to the peak of the emission spectrum 4b. In addition, the difference between the peak of the emission spectrum 3b and the emission spectrum 4b is 21 nm, which is very small.

[0196] From the emission spectrum of the mixed material whose peak is located on the particularly longer wavelength side, it is found that a light-emitting material with a particularly low driving voltage can be obtained by using the mixed material. (Structural example 3)

[0197] Fig. Figure 3 shows an emission spectrum of a thin film of 2mDBTPDBq-II, which is the first organic compound (an emission spectrum 1c), an emission spectrum of a thin film of 1'-TNATA, which is the second organic compound (an emission spectrum 2c), and an emission spectrum of a thin film made of a mixed material of 2mDBTPDBq-II and 1'-TNATA (an emission spectrum 3c). Furthermore, an absorption spectrum and an emission spectrum (an emission spectrum 4c) of [Ir(dppm)2(acac)], which is the phosphorescent compound of [Ir(dppm)2(acac)], in a dichloromethane solution are also shown.

[0198] In Fig. 3, the horizontal axis represents the wavelength (nm) and the vertical axis represents the molar absorption coefficient ε (M -1 ·cm -1 ) and the emission intensity (arbitrary unit).

[0199] From the absorption spectrum in Fig. 3 shows that [Ir(dppm)2(acac)] exhibits a broad absorption band at approximately 510 nm. This absorption band is believed to contribute significantly to the light emission.

[0200] The emission spectrum 3c exhibits a peak at a longer wavelength than the emission spectra 1c and 2c. It was found that the emission spectrum of the mixed material of 2mDBTPDBq-II and 1'-TNATA exhibits a peak at a longer wavelength than the emission spectrum of each organic compound alone. This indicates that an exciplex is formed by mixing 2mDBTPDBq-II with 1'-TNATA.

[0201] Furthermore, it was shown that the peak of the emission spectrum 3c overlaps with the absorption spectrum of [Ir(dppm)2(acac)]. Therefore, the light-emitting element containing [Ir(dppm)2(acac)] and the mixed material of 2mDBTPDBq-II and 1'-TNATA exhibits particularly high energy transfer efficiency, as it transfers energy using the overlap between the emission spectrum of the mixed material and the absorption spectrum of the phosphorescent compound. Consequently, it is demonstrated that a light-emitting element with high external quantum efficiency can be obtained.

[0202] Furthermore, the peak of the emission spectrum 3c is located on the longer wavelength side compared to the peak of the absorption spectrum. In addition, the difference between the peak of the emission spectrum 3c and the emission spectrum 4c is 24 nm, which is very small.

[0203] It can be assumed that in the light-emitting element using the mixed material, a voltage value at which an exciplex is formed by carrier recombination is lower than a voltage value at which light emission of the phosphorescent compound starts through carrier recombination. In other words, even when a voltage lower than the voltage value at which light emission of the phosphorescent compound starts is applied to the light-emitting element, carrier recombination occurs to form an exciplex, and therefore, current begins to flow into the light-emitting element. Therefore, it is demonstrated that a light-emitting element with a particularly low drive voltage can be obtained. (Structural example 4)

[0204] Fig. Figure 4 shows an emission spectrum of a thin film of 2mDBTPDBq-II, which is the first organic compound (an emission spectrum 1d), an emission spectrum of a thin film of DPA2SF, which is the second organic compound (an emission spectrum 2d), and an emission spectrum of a thin film made of a mixed material of 2mDBTPDBq-II and DPA2SF (an emission spectrum 3d). Furthermore, an absorption spectrum and an emission spectrum (an emission spectrum 4d) of [Ir(dppm)2(acac)], which is the phosphorescent compound of [Ir(dppm)2(acac)], in a dichloromethane solution are also shown.

[0205] In Fig. 4, the horizontal axis represents the wavelength (nm) and the vertical axis represents the molar absorption coefficient ε (M -1 ·cm -1 ) and the emission intensity (arbitrary unit).

[0206] From the absorption spectrum in Fig. 4 shows that [Ir(dppm)2(acac)] exhibits a broad absorption band at approximately 510 nm. This absorption band is believed to contribute significantly to the light emission.

[0207] The emission spectrum 3d exhibits a peak at a longer wavelength than the emission spectra 1d and 2d. It was found that the emission spectrum of the mixed material of 2mDBTPDBq-II and DPA2SF exhibits a peak at a longer wavelength than the emission spectrum of each organic compound alone. This indicates that an exciplex is formed by mixing 2mDBTPDBq-II with DPA2SF.

[0208] Furthermore, the peak of the emission spectrum 3d overlaps with the absorption spectrum of [Ir(dppm)2(acac)]. Therefore, it is demonstrated that the light-emitting element containing [Ir(dppm)2(acac)] and the mixed material of 2mDBTPDBq-II and DPA2SF exhibits high energy transfer efficiency, as it transfers energy using the overlap between the emission spectrum of the mixed material and the absorption spectrum of the phosphorescent compound. Consequently, it is demonstrated that a light-emitting element with high external quantum efficiency can be obtained.

[0209] Furthermore, the peak of the 3d emission spectrum is located on the longer wavelength side compared to the peak of the absorption spectrum. In addition, the difference between the peak of the 3d emission spectrum and the 4d emission spectrum is 13 nm, which is very small.

[0210] From the emission spectrum of the mixed material, whose peak is located on the extra-long wavelength side, it is found that a light-emitting material with an extra-low driving voltage can be obtained by using the mixed material. (Example 2)

[0211] In this example, a light-emitting element of an embodiment of the present invention is described with reference to Fig. 8. The chemical formulas of the materials used in this embodiment are shown below. The chemical formulas of the materials used in the above examples are omitted here.

[0212] Methods for manufacturing the light-emitting elements 1 to 4 of this example are described below. (Light-emitting element 1)

[0213] First, a film of indium tin oxide containing silicon oxide (ITSO) was formed on a glass substrate 1100 by a sputtering method, forming a first electrode 1101 serving as an anode. Note that the thickness was set to 110 nm and the electrode area was set to 2 mm × 2 mm.

[0214] Subsequently, as a pretreatment for forming the light-emitting element on the substrate 1100, a UV-ozone treatment was performed for 370 seconds after a surface of the substrate was washed with water and it was baked at 200 °C for one hour.

[0215] The substrate was then transferred to a vacuum evaporation device in which the pressure was increased to approximately 10 -4 Pa, and was subjected to vacuum baking at 170 °C for 30 minutes in a heating chamber of the vacuum evaporation apparatus, and then the substrate 1100 was cooled for about 30 minutes.

[0216] Thereafter, the substrate 1100 provided with the first electrode 1101 was attached to a substrate holder in the vacuum evaporation apparatus so that a surface on which the first electrode 1101 was provided faced downward. The pressure in the vacuum evaporation apparatus was set to approximately 10 -4 Pa. Subsequently, 4,4',4''-(1,3,5-benzenetriyl)tri(dibenzothiophene) (abbreviation: DBT3P-II) and molybdenum(VI) oxide were co-evaporated to form a hole-injection layer 1111 on the first electrode 1101. The thickness of the hole-injection layer 1111 was set to 40 nm, and the weight ratio of DBT3P-II to molybdenum oxide was set to 1:05 (=DBT3P-II:molybdenum oxide).

[0217] Thereafter, a film of PCBA1BP with a thickness of 20 nm was formed on the hole injection layer 1111 to form a hole transport layer 1112.

[0218] Furthermore, 2mDBTPDBq-II, PCBA1BP, and [Ir(dppm)2(acac)] were co-evaporated to form a light-emitting layer 1113 on the hole-transport layer 1112. The weight ratio of 2mDBTPDBq-II to PCBA1BP and [Ir(dppm)2(acac)] was set to 0.7:0.3:0.05 (= 2mDBTPDBq-II:PCBA1BP:[Ir(dppm)2(acac)]). The thickness of the light-emitting layer 1113 was set to 40 nm.

[0219] Furthermore, a film of 2mDBTPDBq-II with a thickness of 10 nm was formed on the light-emitting layer 1113 to form a first electron transport layer 1114a.

[0220] Then, a film of bathophenanthroline (abbreviation: BPhen) with a thickness of 20 nm was formed on the first electron transport layer 1114a to form a second electron transport layer 1114b.

[0221] Furthermore, a film of lithium fluoride (LiF) was formed on the second electron transport layer 1114b by evaporation with a thickness of 1 nm to form an electron injection layer 1115.

[0222] Finally, an aluminum film with a thickness of 200 nm was formed by evaporation as a second electrode 1103, which served as a cathode. Thus, the light-emitting element 1 of this example was fabricated. (Light-emitting element 2)

[0223] The hole transport layer 1112 of the light-emitting element 2 was formed by forming a film of PCzPCN1 with a thickness of 20 nm.

[0224] The light-emitting layer 1113 of the light-emitting element 2 was formed by co-evaporation of 2mDBTPDBq-II, PCzPCN1, and [Ir(dppm)2(acac)]. The weight ratio of 2mDBTPDBq-II to PCzPCN1 and [Ir(dppm)2(acac)] was set to 0.7:0.3:0.05 (= 2mDBTPDBq-II: PCzPCN1: [Ir(dppm)2(acac)]). The thickness of the light-emitting layer 1113 was set to 40 nm. The other components except the light-emitting layer 1113 were prepared in a similar manner to the light-emitting element 1. (Light-emitting element 3)

[0225] The hole transport layer 1112 of the light-emitting element 3 was formed by forming a film of 1'-TNATA with a thickness of 20 nm.

[0226] The light-emitting layer 1113 of the light-emitting element 3 was formed by co-evaporation of 2mDBTPDBq-II, 1'-TNATA, and [Ir(dppm)2(acac)]. The weight ratio of 2mDBTPDBq-II to 1'-TNATA and [Ir(dppm)2(acac)] was set to 0.7:0.3:0.05 (= 2mDBTPDBq-II: 1'-TNATA: [Ir(dppm)2(acac)]). The thickness of the light-emitting layer 1113 was set to 40 nm. The other components except the light-emitting layer 1113 were prepared in a similar manner to the light-emitting element 1. (Light-emitting element 4)

[0227] The hole transport layer 1112 of the light-emitting element 4 was formed by forming a film of DPA2SF with a thickness of 20 nm.

[0228] The light-emitting layer 1113 of the light-emitting element 4 was formed by co-evaporation of 2mDBTPDBq-II, DPA2SF, and [Ir(dppm)2(acac)]. The weight ratio of 2mDBTPDBq-II to DPA2SF and [Ir(dppm)2(acac)] was set to 0.7:0.3:0.05 (= 2mDBTPDBq-II: DPA2SF: [Ir(dppm)2(acac)]). The thickness of the light-emitting layer 1113 was set to 40 nm. The other components except the light-emitting layer 1113 were prepared in a similar manner to the light-emitting element 1.

[0229] It should be noted that in all the above evaporation steps, the evaporation was carried out by a resistance heating method.

[0230] Table 1 shows element structures of the light-emitting elements 1 to 4 obtained as described above. (Table 1) First electrode Hole injection layer Hole transport layer Light-emitting layer First electron transport layer Second electron transport layer Electron injection layer Second electrode Light-emitting element 1 ITSO 110 nm DBT3P-II :MoOx (=1:0.5) 40nm PCBA1BP 20nm 2mDBTPDBq-II : PCBA1BP: [Ir(dppm)2(acac)] (= 0.7:0.3:0.05) 40 nm 2mDBTPDBq-II 10 nm BPhen 20 nm LiF 1 nm Al 200 nm Light-emitting element 2 ITSO 110 nm DBT3P-II :MoOx (=1:0.5) 40nm PCzPCN1 20nm 2mDBTPDBq-II : PCzPCN1 : [Ir(dppm)2(acac)] (= 0.7:0.3:0.05) 40 nm 2mDBTPDBq-II 10 nm BPhen 20 nm LiF 1 nm Al 200 nm Light-emitting element 3 ITSO 110 nm DBT3P-II :MoOx (=1:0.5) 40nm 1'-TNATA 20nm 2mDBTPDBq-II : 1'-TNATA : [Ir(dppm)2(acac)] (= 0.7:0.3:0.05) 40 nm 2mDBTPDBq-II 10 nm BPhen 20 nm LiF 1 nm Al 200 nm Light-emitting element 4 ITSO 110 nm DBT3P-II :MoOx (=1:0.5) 40nm DPA2SF 20nm 2mDBTPDBq-II : DPA2SF : [Ir(dppm)2(acac)] (= 0.7:0.3:0.05) 40 nm 2mDBTPDBq-II 10 nm BPhen 20 nm LiF 1 nm Al 200 nm

[0231] These light-emitting elements were sealed in a glove box containing a nitrogen atmosphere to prevent exposure to air. The operating characteristics of these light-emitting elements were then measured. It should be noted that the measurements were conducted at room temperature (in an atmosphere maintained at 25 °C).

[0232] Fig. 9 shows the voltage-luminance characteristics of the light-emitting elements 1 to 4. In Fig. 9, the horizontal axis represents the voltage (V) and the vertical axis represents the luminance (cd / m 2 ). Furthermore, Fig. 10 the voltage-current characteristic. In Fig. 10 the horizontal axis represents the voltage (V) and the vertical axis represents the current (mA). Fig. 11 shows the luminance-power efficiency characteristics of these. In Fig. 11 the horizontal axis represents the luminance (cd / m 2 ) and the vertical axis represents the power efficiency (IM / W). Fig. 12 shows the luminance-external quantum efficiency characteristics of these. In Fig. 12 the horizontal axis represents the luminance (cd / m 2 )) and the vertical axis represents the external quantum efficiency (%).

[0233] Furthermore, Table 2 shows the voltage (V), the current density (mA / cm 2 ), the CIE chromaticity coordinates (x,y), the current efficiency (cd / A), the power efficiency (Im / W) and the external quantum efficiency (%) of each of the light-emitting elements 1 to 4 at a luminance of approximately 1000 cd / m 2 . (Table 2) Voltage (V) Current density (mA / cm 2 ) Color value components (x, y) Luminance (cd / m 2 ) Current efficiency (cd / A) Power efficiency (Im / W) External quantum efficiency (%) Light-emitting element 1 2,7 1,5 (0,56, 0,44) 1000 68 79 27 Light-emitting element 2 2,5 1,6 (0,56, 0,43) 1000 67 84 26 Light-emitting element 3 2,7 2,3 (0,56, 0,43) 780 34 39 14 Light-emitting element 3 2,4 1,3 (0,56, 0,43) 800 62 81 25

[0234] Fig. Figure 13 shows the emission spectra of the light-emitting elements 1 to 4, which were obtained by applying a current of 0.1 mA. Fig. 13, the horizontal axis represents the wavelength (nm) and the vertical axis represents the emission intensity (arbitrary unit). As shown in Table 2, the CIE chromaticity coordinates of the light-emitting element 1 at a luminance of 1000 cd / m 2 (x, y) = (0.56, 0.44); the CIE chromaticity coordinates of the light-emitting element 2 at a luminance of 1000 cd / m 2 (x, y) = (0.56, 0.43); the CIE chromaticity coordinates of the light-emitting element 3 at a luminance of 780 cd / m 2 (x, y) = (0.56, 0.43); and the CIE chromaticity coordinates of the light-emitting reference element 4 at a luminance of 800 cd / m 2 were (x, y) = (0.56, 0.43). These results indicate that the orange light emission originating from [Ir(dppm)2(acac)] was obtained from light-emitting elements 1 to 4.

[0235] As Table 82, Fig. 11 and Fig. As can be seen from Figure 12, the light-emitting elements 1 to 4 exhibit high current efficiency, high power efficiency and high external quantum efficiency.

[0236] In the light-emitting elements of this example, the first organic compound, the second organic compound, and the guest material described in Example 1 were used as the light-emitting layer. As described in Example 1, the emission spectrum of the mixed material of 2mDBTPDBq-II and the second organic compound (the emission spectrum of an exciplex) overlaps the absorption spectrum of [Ir(dppm)2(acac)]. It is assumed that the light-emitting elements of this example exhibit high energy transfer efficiency by utilizing the overlap and therefore have high external quantum efficiency.

[0237] In this example, the light-emitting elements 1, 2 and 4 have a higher external quantum efficiency than the light-emitting element 3 (see Fig. 12). It is assumed that the reason for this is the larger overlap between the emission spectrum of the exciplex and the absorption spectrum of [Ir(dppm)2(acac)] of the light-emitting elements 1, 2 and 4, compared to the light-emitting element 3 (reference is made to the Fig. 1, Fig. 2, Fig. 3, Fig. 4 and Example 1 taken).

[0238] Out of Fig. 9 and Fig. 10 shows that the light-emitting elements 1 to 4 have a low emission start voltage. The theoretical value of the emission start voltage of an orange-emitting organic EL element is said to be approximately 2.1 V, which is extremely close to the emission start voltage of the light-emitting element of one embodiment of the present invention.

[0239] In this example, the light-emitting elements 2 to 4 have a lower emission starting voltage than the light-emitting element 1 (see Fig. 10 and Fig. 11) It is believed that this is because the emission spectrum of the exciplex of each of the light-emitting elements 2 to 4 has a peak at a higher wavelength than the emission spectrum of the exciplex of the light-emitting element 1 (refer to the Fig. 1, Fig. 2, Fig. 3 Fig. 4 and Example 1).

[0240] The above results demonstrate that an element with a high external quantum efficiency can be obtained using an embodiment of the present invention. Furthermore, it is demonstrated that an element with a low drive voltage can be obtained using an embodiment of the present invention.

[0241] The light-emitting element was then subjected to four reliability tests. The results of the reliability tests are shown in Fig. 14. In Fig. 14, the vertical axis represents the normalized luminance (%) with an initial luminance of 100% and the horizontal axis represents the operating time (h) of the element.

[0242] During the reliability tests, the light-emitting element 4 was operated under conditions where the initial luminance was set to 5000 cd / m 2 was set and the current density was constant.

[0243] The luminance of the light-emitting element 4 was 93% of the initial luminance after 260 hours. These results indicate that the light-emitting element 4 has a long service life.

[0244] The above results show that an element with a low driving voltage and high reliability can be achieved by using an embodiment of the present invention. (Example 3)

[0245] This example describes a light-emitting element of one embodiment of the present invention. Chemical formulas of the materials used in this example are shown below. The chemical formulas of the materials used in the above examples are omitted here.

[0246] Light-emitting elements formed in this example are structural examples a to s. An element structure of structural examples a to s is shown in Table 3. It should be noted that structural examples a to s differ from each other in a substance X used in hole-transport layers and light-emitting layers. The names of the substances X used in the structural examples are shown below. Furthermore, Table 4 shows the HOMO levels (eV) of the substances X used in the structural examples and the emission peak wavelengths (nm) of the exciplexes formed in the structural examples. It should be noted that in this example, a photoelectron spectrometer (AC-2, product of Riken Keiki Co., Ltd.) was used to measure the HOMO level. (Table 3) First electrode Hole injection layer Hole transport layer Light-emitting layer First electron transport layer Second electron transport layer Electron injection layer Second electrode Light-emitting element 1 ITSO 110 nm DBT3P-II:MoOx (=1:0.5) 40nm Substance X 20nm 2mDBTPDBq-II : Substance X : [Ir(dppm)2(acac)] (= 0.7:0.3:0.05) 40 nm 2mDBTPDBq-II 10 nm BPhen 20 nm LiF 1 nm Al 200 nm (Table 4) Structural example Substance X HOMO level of substance X (eV) Peakwellenlänge des Emissionsspektrums des Exciplexes a PCBA1BP -5,42 519 b PCA2B -5,40 546 c DFNF -5,35 555 d PCA3B -5,31 553 e PCASF -5,30 543 f DPASF -5,30, 571 g YGA2F -5,27 540 h TPD -5,25 537 i DPAB -5,23 573 j DFLADFL -5,20 557 k PCzPCA1 -5,17 571 l PCzDPA1 -5,16 581 m PCzDPA2 -5,16 586 n PCzPCN1 -5,15 571 o DNTPD -5,14 573 p PCzTPN2 -5,13 582 q DPA2SF -5,09 579 r 1'-TNATA -5,09 616 s PCzPCA2 -5,08 575 (Structural example a)

[0247] Structural example a is the light-emitting element 1 described in Example 2. PCBA1BP was used as substance X. (Structural example b)

[0248] N,N'-Bis(9-phenylcarbazol-3-yl)-N,N'-diphenylbenzene-1,3-diamine (abbreviation: PCA2B) was used as substance X. (Structural example c)

[0249] N-(9,9-Dimethyl-2-N',N'-diphenylamino-9H-fluoren-7-yl)diphenylamine (abbreviation: DPNF) was used as substance X. (Structural example d)

[0250] N,N',N''-Triphenyl-N,N',N''-tris(9-phenylcarbazol-3-yl)benzene-1,3,5-triamine (abbreviation: PCA3B) was used as substance X. (Structural example e)

[0251] 2-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]spiro-9,9'-bifluorene (abbreviation: PCASF) was used as substance X. (Structural example f)

[0252] As substance X, 2-[N-(4-diphenylaminophenyl)-N-phenylamino]spiro-9,9'-bifluorene (abbreviation: DPASF) was used. (Structural example g)

[0253] N,N'-Bis[4-(carbazol-9-yl)phenyl]-N,N'-diphenyl-9,9-dimethylfluorene-2,7-diamine (abbreviation: YGA2F) was used as substance X. (Structural example h)

[0254] N,N'-bis(3-methylphenyl)-N,N'-diphenyl-[1,1'-biphenyl]-4,4'-diamine (abbreviation: TPD) was used as substance X. (Structural example i)

[0255] As substance X, 4,4'-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl was used (abbreviation: DPAB). (Structural example j)

[0256] N-(9,9-Dimethyl-9H-fluoren-2-yl)-N-{9,9-dimethyl-2-[N'-phenyl-N'-(9,9-dimethyl-9H-fluoren-2-yl)amino]-9H-fluoren-7-yl}phenylamine (abbreviation: DFLADFL) was used as substance X. (Structural example k)

[0257] As substance X, 3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole was used (abbreviation: PCzPCA1). (Structural example I)

[0258] As substance X, 3-[N-(4-diphenylaminophenyl)-N-phenylamino]-9-phenylcarbazole was used (abbreviation: PCzDPA1). (Structural example m)

[0259] As substance X, 3,6-bis[N-(4-diphenylaminophenyl)-N-phenylamino]-9-phenylcarbazole was used (abbreviation: PCzDPA2). (Structural example n)

[0260] The structural example n is the light-emitting element 2 described in Example 2. PCzPCN1 was used as the substance X. (Structural example o)

[0261] As substance X, 4,4'-bis(N-{4-[N'-(3-methylphenyl)-N'-phenylamino]phenyl}-N-phenylamino)biphenyl was used (abbreviation: DNTPD). (Structural example p)

[0262] As substance X, 3,6-bis[N-(4-diphenylaminophenyl)-N-(1-naphthyl)amino]-9-phenylcarbazole was used (abbreviation: PCzTPN2). (Structural example q)

[0263] The structural example q is the light-emitting element 4 described in Example 2. DPA2SF was used as the substance X. (Structural example r)

[0264] The structural example r is the light-emitting element 3, which is described in Example 2. 1'-TNATA was used as the substance X. (Structural example s)

[0265] As substance X, 3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole was used (abbreviation: PCzPCA2).

[0266] Fig. Figure 15 shows a relationship between the peak wavelength of the emission spectrum of the exciplex and the HOMO level of substance X in each structural example. Fig. 15, the horizontal axis represents the peak wavelength (nm) and the vertical axis represents the HOMO level (eV). Furthermore, Fig. 16 shows a relationship between the peak wavelength of the exciplex emission spectrum and the relative external quantum efficiency in each structural example. Fig. 16, the horizontal axis represents the peak wavelength (nm) and the vertical axis represents the relative external quantum efficiency (arbitrary unit). It should be noted that the relative external quantum efficiency in Fig. 16 is shown as a value related to the external quantum efficiency of the light-emitting element of structural example a, which is assumed to be 1. In Fig. 16, the relative external quantum efficiency of structural example e and structural example h is not shown.

[0267] Out of Fig. Figure 15 shows that when the HOMO level of substance X is higher, the emission spectrum of the exciplex formed by 2mDBTPDBq-II and substance X exhibits a peak at a longer wavelength. When the HOMO level of substance X is higher and the emission spectrum of the exciplex exhibits a peak at a longer wavelength, the emission bias voltage can be lower. Consequently, the light-emitting element can exhibit a lower drive voltage.

[0268] Out of Fig. Figure 16 clearly shows that the external quantum efficiency of the light-emitting element is low when the peak wavelength of the exciplex emission spectrum is too long. This suggests that the peak wavelength of the exciplex emission spectrum represents a preferred range of high external quantum efficiency and low drive voltage for a light-emitting element.In particular, it is indicated that in order to achieve the low driving voltage and high external quantum efficiency, the peak wavelength of the emission spectrum of the exciplex is preferably longer than or equal to the peak wavelength (in this example, approximately 510 nm in the absorption spectrum of the phosphorescent compound in a solution) of the absorption band located on the side of the longest wavelength of the absorption spectrum of the phosphorescent compound, and preferably shorter than or equal to the peak wavelength (in this example, approximately 580 nm in an emission spectrum of electroluminescence) of the emission spectrum of the phosphorescent compound. (Reference example 1)

[0269] A synthesis example of an organometallic complex (acetylacetonato)bis(4,6-diphenylpyrimidinato)iridium(III) (other name: bis[2-(6-phenyl-4-pyrimidinyl-κN3)phenyl-κC](2,4-pentanedionato-κ 2 O,O')iridium(III) (abbreviation: [Ir(dppm)2(acac)]), used in the above examples, is described. The structure of [Ir(dppm)2(acac)] is shown below. 〈Step 1: Synthesis of 4,6-diphenylpyrimidine (abbreviation: Hdppm)〉

[0270] First, 5.02 g of 4,6-dichloropyrimidine, 8.29 g of phenylboronic acid, 7.19 g of sodium carbonate, 0.29 g of bis(triphenylphosphine)palladium(II) dichloride (abbreviation: Pd(PPh3)2Cl2), 20 ml of water, and 20 ml of acetonitrile were introduced into a recovery flask equipped with a reflux tube, and the air in the flask was replaced with argon. This reaction vessel was heated by microwave irradiation (2.45 GHz, 100 W) for 60 minutes. Further, 2.08 g of phenylboronic acid, 1.79 g of sodium carbonate, 0.070 g of Pd(PPh3)2Cl2, 5 ml of water, and 5 ml of acetonitrile were added to the flask, and the mixture was heated again by microwave irradiation (2.45 GHz, 100 W) for 60 minutes. Subsequently, water was added to the solution, and an organic layer was extracted with dichloromethane. The resulting extract solution was washed with water and dried with magnesium sulfate. The solution was filtered after drying.The solvent was distilled off, and the resulting residue was purified by silica gel column chromatography using dichloromethane as the eluent to obtain a pyrimidine derivative Hdppm (yellow-white powder, yield 38%). It should be noted that a microwave synthesis system (Discover, manufactured by CEM Corporation) was used for microwave irradiation. A synthesis scheme (a-1) of step 1 is shown below. 〈Step 2: Synthesis of di-m-chloro-bis[bis(4,6-diphenylpyrimidinato)iridium(III)] (abbreviation: [Ir(dppm)2Cl]2)〉

[0271] Subsequently, 15 ml of 2-ethoxyethanol, 5 ml of water, 1.10 g of Hdppm obtained in step 1, and 0.69 g of iridium chloride hydrate (IrCl3×H 2O ) was introduced into a recovery flask equipped with a reflux tube, and the air in the recovery 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 and washed with ethanol to obtain a dinuclear complex [Ir(dppm)2Cl]2 (reddish-brown powder, yield 88%). A synthesis scheme (a-2) of step 2 is shown below. 〈Step 3: Synthesis of (acetylacetonato)bis(4,6-diphenylpyrimidinato)iridium(III) (abbreviation: [Ir(dppm)2(acac)])〉

[0272] Furthermore, 40 ml of 2-ethoxyethanol, 1.44 g of [Ir(dppm)2Cl]2 obtained in Step 2, 0.30 g of acetylacetone, and 1.07 g of sodium carbonate were introduced into a recovery flask equipped with a reflux tube, and the air in the recovery flask was replaced with argon. Subsequently, microwave irradiation (2.45 GHz, 120 W) was carried out for 60 minutes to cause a reaction. The solvent was filtered off, and the resulting residue was dissolved in dichloromethane, and filtration was performed to remove the insoluble matter. The resulting filtrate was washed with water and then with brine and dried with magnesium sulfate. The solution was filtered after drying.The solvent of this solution was distilled off, and the resulting residue was purified by silica gel column chromatography using dichloromethane and ethyl acetate as the eluent in a volume ratio of 50:1. Subsequently, recrystallization was performed using a mixed solvent of dichloromethane and hexane to obtain the target orange powder (yield 32%). A synthesis scheme (a-3) of step 3 is shown below.

[0273] An analysis result by nuclear magnetic resonance spectroscopy ( 1 H NMR) of the orange powder obtained in step 3 is described below. These results showed that the organometallic complex [Ir(dppm)2(acac)] was obtained.

[0274] 1 H NMR. δ (CDCl3): 1.83 (s, 6H), 5.29 (s, 1H), 6.48 (d, 2H), 6.80 (t, 2H), 6.90 (t, 2H), 7.55-7.63 (m, 6H), 7.77 (d, 2H), 8.17 (s, 2H), 8.24 (d, 4H), 9.17 (s, 2H). (Reference example 2)

[0275] A method for the synthesis of 2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation: 2mDBTPDBq-II), which is used in the above examples, is described. Synthesis of 2-[3-(Dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation: 2mDBTPDBq-II)

[0276] A synthesis scheme (b-1) of 2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation: 2mDBTPDBq-II) is shown below.

[0277] First, 5.3 g (20 mmol) of 2-chlorodibenzo[f,h]quinoxaline, 6.1 g (20 mmol) of 3-(dibenzothiophen-4-yl)phenylboronic acid, 460 mg (0.4 mmol) of tetrakis(triphenylphosphine)palladium(0), 300 mL of toluene, 20 mL of ethanol, and 20 mL of a 2M aqueous solution of potassium carbonate were introduced into a 2-L three-necked flask. The mixture was degassed by stirring under reduced pressure, and the air in the three-necked flask was replaced with nitrogen. This mixture was stirred under a nitrogen stream at 100 °C for 7.5 hours. After cooling to room temperature, the resulting mixture was filtered to obtain a white residue. The resulting residue was washed with water and ethanol in that order and then dried. The obtained dried solid was dissolved in approximately 600 ml of hot toluene, followed by suction filtration through Celite (manufactured by Wako Pure Chemical Industries, Ltd.(catalog number 531-16855) and Florisil (manufactured by Wako Pure Chemical Industries, Ltd., catalog number 540-00135), to obtain a clear, colorless filtrate. The obtained filtrate was concentrated and purified by silica gel column chromatography using approximately 700 ml of silica gel. Chromatography was carried out using hot toluene as the eluent. Acetone and ethanol were added to the resulting solid, followed by irradiation with ultrasonic waves. Subsequently, the generated suspended solid was collected by filtration, and the resulting solid was dried to obtain 7.85 g of white powder in 80% yield.

[0278] The above target substance was relatively soluble in hot toluene, but was a material that tended to precipitate upon cooling. Furthermore, the substance was poorly soluble in other solvents, such as acetone and ethanol. Therefore, the use of these different degrees of solubility led to a high-yield synthesis by a simple procedure as above. In particular, after the reaction was completed, the mixture was returned to room temperature, and the precipitate was collected by filtration, which allowed most impurities to be easily removed. Furthermore, the target substance, which precipitates easily, was easily purified by column chromatography with hot toluene.

[0279] 4.0 g of the resulting white powder was purified using a train sublimation process. During purification, the white powder was heated to 300 °C under a pressure of 5.0 Pa and an argon flow rate of 5.0 ml / min. After purification, the target substance was obtained as 3.5 g of white powder with a yield of 88%.

[0280] A nuclear magnetic resonance spectroscopy ( 1 H NMR) identified this compound as the target 2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation: 2mDBTPDBq-II).

[0281] The 1 H NMR values of the obtained substance are shown below.

[0282] 1 H NMR (CDCl3, 300 MHz): δ (ppm) = 7.45-7.52 (m, 2H), 7.59-7.65 (m, 2H), 7.71-7.91 (m, 7H), 8.20-8.25 (m, 2H), 8.41 (d, J = 7.8 Hz, 1H), 8.65 (d, J = 7.5 Hz, 2H), 8.77-8.78 (m, 1H), 9.23 (dd, J = 7.2 Hz, 1.5 Hz, 1H), 9.42 (dd, J = 7.8 Hz, 1.5 Hz, 1H), 9.48 (s, 1H). Explanation of reference symbols

[0283] 102: EL layer, 103: first electrode, 108: second electrode, 701: hole injection layer, 702: hole transport layer, 703: light-emitting layer, 704: electron transport layer, 705: electron injection layer, 706: electron injection buffer layer, 707: electron relay layer, 708: composite material layer, 800: first EL layer, 801: second EL layer, 803: charge generation layer, 1100: substrate, 1101: first electrode, 1103: second electrode, 1111: hole injection layer, 1112: hole transport layer, 1113: light-emitting layer, 1114a: first electron transport layer, 1114b: second electron transport layer, and 1115: electron injection layer.< / elektroplex> < / exciplex>

Claims

[1] Light-emitting element comprising: a light-emitting layer comprising a first organic compound, a second organic compound and a guest material, wherein the first organic compound is a heteroaromatic compound and the second organic compound is a carbazole compound, wherein the first compound and the second compound may form an exciplex, wherein the emission peak wavelength of the exciplex is longer than or corresponds to the peak wavelength of the absorption band located on the longest wavelength side of an absorption spectrum of the guest material, wherein this peak wavelength of the absorption band on the longest wavelength side is an absorption wavelength corresponding to the direct transition from the singlet ground state to the lowest excited triplet state, and wherein the combinations in which the phosphorescent compound is [Ir(dppm)2(acac)], [Ir(mppr-Me)2(dpm)] or [Ir(mppm)2(acac)], the first organic compound is 2mDBTPDBq-II, and the second organic compound is PCBA1BP are excluded. [2] Light-emitting element comprising: a light-emitting layer comprising a first organic compound, a second organic compound and a guest material, wherein the first organic compound is a heteroaromatic compound and the second organic compound is a carbazole compound, wherein the first compound and the second compound may form an exciplex, wherein the emission peak wavelength of the exciplex is longer than or corresponds to the peak wavelength of the absorption band located on the longest wavelength side of an absorption spectrum of the guest material, wherein this peak wavelength of the absorption band on the longest wavelength side is an absorption wavelength corresponding to the direct transition from the singlet ground state to the lowest excited triplet state, and is shorter than or corresponds to the emission peak wavelength of the guest material, and wherein the combinations in which the phosphorescent compound is [Ir(dppm)2(acac)], [Ir(mppr-Me)2(dpm)] or [Ir(mppm)2(acac)], the first organic compound is 2mDBTPDBq-II, and the second organic compound is PCBA1BP are excluded. [3] Light-emitting element comprising: a light-emitting layer comprising a first organic compound, a second organic compound and a guest material, wherein the first organic compound is a heteroaromatic compound and the second organic compound is a carbazole compound, wherein the first compound and the second compound may form an exciplex, wherein the emission spectrum of the exciplex overlaps the absorption band located on the longest wavelength side of the absorption spectrum of the guest material, said peak wavelength of the absorption band on the longest wavelength side being an absorption wavelength corresponding to the direct transition from the singlet ground state to the lowest excited triplet state, wherein the difference between the peak wavelength of the emission spectrum of the exciplex and the peak wavelength of the emission spectrum of the guest material is 30 nm or less, and wherein the combinations in which the phosphorescent compound is [Ir(dppm)2(acac)], [Ir(mppr-Me)2(dpm)] or [Ir(mppm)2(acac)], the first organic compound is 2mDBTPDBq-II, and the second organic compound is PCBA1BP are excluded. [4] The light-emitting element according to any one of claims 1, 2 or 3, wherein the guest material is a light-emitting substance. [5] A light-emitting element according to any one of claims 1, 2 or 3, wherein the guest material is a phosphorescent compound. [6] A light-emitting element according to any one of claims 1, 2 or 3, wherein an excitation energy of the exciplex is transferable to the guest material so that the guest material emits phosphorescent light. [7] The light-emitting element according to any one of claims 1, 2 or 3, wherein the first compound has an electron-trapping property, and wherein the second compound has a hole-trapping property. [8] A light-emitting device comprising the light-emitting element according to any one of claims 1, 2 or 3.

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