Light-emitting element

Abstract

Light-emitting element comprising: an anode; a cathode; and a light-emitting layer comprising a guest material and a host material between the anode and the cathode, wherein the guest material is an iridium compound, wherein the host material comprises a heterocyclic compound having a six-membered ring with two nitrogen atoms, and wherein an emission spectrum of the host material overlaps the absorption band at the longest wavelength side in the absorption spectrum of the guest material.

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). Background of the technology

[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. Due to properties such as the ability to be thinner and lighter, high-speed response to input signals, and the ability to operate at low voltage with direct current, the organic EL element has attracted attention as a next-generation flat panel display element. Furthermore, a feature of a display using such a light-emitting element is that it has excellent contrast and image quality and a wide viewing angle.Since it is a planar light source, further attempts have been made to use the organic EL element as a light source such as a backlight of a liquid crystal display and a lighting device.

[0003] The emission mechanism of the organic EL element is a charge injection type. That is, by applying a voltage with a light-emitting layer sandwiched between electrodes, electrons and holes injected from the electrodes recombine to excite a light-emitting substance. Light is emitted when the substance in the excited state relaxes to the ground state. Two types of excited states are possible: a singlet excited state (S*) and a triplet excited state (T*). The statistical generation ratio of excited states in a light-emitting element is assumed to be 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 is caused by an electron transfer between the same spin multiplicities. In contrast, light emission from the excited triplet state (T*) is called phosphorescence, where an electron transfer between different spin multiplicities occurs. In this case, a compound that emits fluorescence (hereinafter referred to as a fluorescent compound) generally does not exhibit phosphorescence at room temperature; only fluorescence is observed.Accordingly, the internal quantum efficiency (the ratio of generated photons to injected charge 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] In contrast, when a phosphorescence-emitting compound (hereinafter referred to as a phosphorescent compound) is used, an internal quantum efficiency of 100% can theoretically be achieved. That is, higher emission efficiency can be achieved than when a fluorescent compound is used. For these reasons, in recent years, a light-emitting element incorporating a phosphorescent compound has been actively developed to achieve a highly efficient light-emitting element. As a phosphorescent compound, an organometallic complex having iridium or the like as a central metal has attracted particular attention due to its high phosphorescence quantum yield. For example, an organometallic complex having iridium as a central metal is disclosed in Patent Document 1 and Publications 1 to 4 as a phosphorescent material.

[0006] When a light-emitting layer of a light-emitting element is formed using a phosphorescent compound described above, in order to suppress concentration quenching or quenching due to triplet-triplet annihilation in the phosphorescent compound, the light-emitting layer is often formed by dispersing the phosphorescent compound in a matrix of another compound. Here, the compound serving as a matrix is called a host material, and the compound dispersed in the matrix, such as a phosphorescent compound, is called a guest material. [Reference][Patent specification] [Patent Document 1] International Publication No. WO 00 / 70655 A3 [Publication 1] ME Kondakova et al. "High-efficiency, low-voltage phosphorescent organic light-emitting diode devices with mixed host," Journal of Applied Physics 104, 094501, 2008. [Publication 2] X. Gong et al. “Phosphorescence from iridium complexes doped into polymer blends,” J. Appl. Phys. 95, 2004. [Publication 3]: Y. Hino et al. “Red Phosphorescent Organic Light-Emitting Diodes Using Mixture System of Small-Molecule and Polymer Host,” Jpn. J. Appl. Phys., 44, 2790, 2005. [Publication 4] AB Tamayo et al. “Synthesis and Characterization of Facial and Meridional Triscyclometalated Iridium(111) Complexes,” JACS, 2003. Disclosure of the invention

[0007] However, in general, the light extraction efficiency of an organic EL element is approximately 20% to 30%. Considering the light absorption by a reflective electrode and a transparent electrode, the external quantum efficiency of a light-emitting element containing a phosphorescent compound has a limit of approximately 25% at most.

[0008] An object of one embodiment of the present invention is to provide a light-emitting element with a high external quantum efficiency. Another object of one embodiment of the present invention is to provide a light-emitting element with a long lifetime.

[0009] One embodiment of the present invention is a light-emitting element including a light-emitting layer including a guest material and a host material between a pair of electrodes, in which an emission spectrum of the host material overlaps an absorption spectrum of the guest material and phosphorescence is emitted by converting an excitation energy of the host material into an excitation energy of the guest material.

[0010] Another embodiment of the present invention is a light-emitting element including, between a pair of electrodes, a light-emitting layer including a guest material and a host material, in which an emission spectrum of the host material overlaps an absorption band on the longest wavelength side (the lowest energy side) in an absorption spectrum of the guest material, and phosphorescence is emitted by converting an excitation energy of the host material into an excitation energy of the guest material.

[0011] In the above light-emitting element, the absorption band on the longest wavelength side preferably includes absorption based on a triplet MLCT (metal to ligand charge transfer) transition.

[0012] In the above light-emitting element, the emission spectrum of the host material is preferably a fluorescence spectrum.

[0013] In the above light-emitting element, the guest material is preferably an organometallic complex, more preferably an iridium complex.

[0014] In the above light-emitting element, the difference between the energy value of a peak of the emission spectrum and the energy value of a peak of the absorption band on the lowest energy side in the absorption spectrum is preferably 0.3 eV or less.

[0015] In the above light-emitting element, the molar absorption coefficient of the absorption band on the longest wavelength side in the absorption spectrum is preferably 5,000 M -1 ·cm -1 or more.

[0016] 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 long lifetime can be provided. Short description of the drawings

[0017] The following applies to the attached drawings: Fig. 1A and Fig. 1B shows absorption spectra and emission spectra according to Example 1; Fig. 2 illustrates the structure of light-emitting elements of examples; Fig. 3 shows a current density-luminance characteristic of light-emitting elements of Example 2; Fig. 4 shows a voltage-luminance characteristic of the light-emitting elements of Example 2; Fig.5 shows a luminance-current efficiency characteristic of the light-emitting elements of Example 2; Fig. Figure 6 shows a luminance versus external quantum efficiency curve of the light-emitting elements of Example 2; Fig. Figure 7 shows emission spectra of the light-emitting elements of Example 2; Fig. 8 shows results of reliability tests of the light-emitting elements of Example 2; Fig. 9 shows a current density-luminance characteristic of a light-emitting element of Example 3; Fig. 10 shows a voltage-luminance characteristic of the light-emitting element of Example 3; Fig. 11 shows a luminance-current efficiency characteristic of the light-emitting element of Example 3; Fig.12 shows a luminance versus external quantum efficiency characteristic of the light-emitting element of Example 3; Fig. 13 shows an emission spectrum of the light-emitting element of Example 3; Fig. 14 shows results of reliability tests of the light-emitting element of Example 3; Fig. 15A to 15C illustrate light-emitting elements of embodiments of the present invention; Fig. Figure 16 shows absorption spectra and an emission spectrum according to Example 1; Fig. 17A and Fig. 17B shows absorption spectra and emission spectra according to Example 4; Fig. 18 shows a current density-luminance characteristic of a light-emitting element of Example 5; Fig. 19 shows a voltage-luminance characteristic of the light-emitting element of Example 5; Fig. 20 shows a luminance-current efficiency characteristic of the light-emitting element of Example 5; Fig. 21 shows a luminance versus external quantum efficiency characteristic curve of the light-emitting element of Example 5; Fig. 22 shows an emission spectrum of the light-emitting element of Example 5; Fig. 23A and Fig. 23B shows absorption spectra and emission spectra according to Example 6; Fig. 24 shows a current density-luminance characteristic of a light-emitting element of Example 7; Fig. 25 shows a voltage-luminance characteristic of the light-emitting element of Example 7; Fig. 26 shows a luminance-current efficiency characteristic of the light-emitting element of Example 7; Fig.Figure 27 shows a luminance versus external quantum efficiency characteristic of the light-emitting element of Example 7; Fig. 28 shows an emission spectrum of the light-emitting element of Example 7; Fig. 29 shows results of reliability tests of the light-emitting element of Example 7; Fig. 30A and Fig. 30B shows absorption spectra and emission spectra according to Example 8; Fig. 31 shows a current density-luminance characteristic of light-emitting elements of Example 9; Fig. 32 shows a voltage-luminance characteristic of the light-emitting elements of Example 9; Fig. 33 shows a luminance-current efficiency characteristic of the light-emitting elements of Example 9; Fig.34 shows a luminance versus external quantum efficiency characteristic of the light-emitting elements of Example 9; Fig. 35 shows emission spectra of the light-emitting elements of Example 9; Fig. 36 shows results of reliability tests of the light-emitting elements of Example 9; Fig. 37A and Fig. 37B shows absorption spectra and emission spectra according to Example 10; Fig. 38 shows a current density-luminance characteristic of light-emitting elements of Example 11; Fig. 39 shows a voltage-luminance characteristic of the light-emitting elements of Example 11; Fig. 40 shows a luminance-current efficiency characteristic of the light-emitting elements of Example 11; Fig.41 shows a luminance versus external quantum efficiency characteristic of the light-emitting elements of Example 11; Fig. 42 shows emission spectra of the light-emitting elements of Example 11; Fig. 43A and Fig. 43B shows absorption spectra and emission spectra according to Example 12; Fig. 44 shows a current density-luminance characteristic of light-emitting elements of Example 13; Fig. 45 shows a voltage-luminance characteristic of the light-emitting elements of Example 13; Fig. 46 shows a luminance-current efficiency characteristic of the light-emitting elements of Example 13; Fig. Figure 47 shows a luminance versus external quantum efficiency characteristic of the light-emitting elements of Example 13; Fig.48 shows emission spectra of the light-emitting elements of Example 13; Fig. 49A and Fig. 49B shows absorption spectra and emission spectra according to Example 14; Fig. 50 shows a current density-luminance characteristic of light-emitting elements of Example 15; Fig. 51 shows a voltage-luminance characteristic of the light-emitting elements of Example 15; Fig. 52 shows a luminance-current efficiency characteristic of the light-emitting elements of Example 15; Fig. Figure 53 shows a luminance versus external quantum efficiency curve of the light-emitting elements of Example 15; and Fig. Figure 54 shows emission spectra of the light-emitting elements of Example 15. The best mode for carrying out the invention

[0018] Embodiments will be described with reference to drawings. It should be noted that the invention is not limited to the following description, and it will be readily apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope 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 portions or portions having similar functions are denoted by the same reference numerals in different drawings, and the description of such portions will not be repeated. (Embodiment 1)

[0019] Embodiment 1 illustrates a light-emitting element of one embodiment of the present invention.

[0020] The light-emitting element of this embodiment includes a light-emitting layer containing a guest material as the light-emitting substance and a host material in which the guest material is dispersed. A phosphorescent compound is used as the guest material. One or more types of organic compounds can be used as the host material.

[0021] The structure in which the guest material is dispersed in the host material can prevent the light-emitting layer from crystallizing. Furthermore, concentration quenching due to a high concentration of the guest material can be suppressed, and as a result, the light-emitting element can exhibit higher emission efficiency.

[0022] According to this embodiment, it is desirable that the triplet excitation energy level (T1 level) of the organic compound used as the host material be higher than that of the guest material. This is because if the T1 level of the host material is lower than that of the guest material, the triplet excitation energy of the guest material, which is intended to contribute to light emission, is quenched by the host material, and the emission efficiency is accordingly reduced. (Elementary processes of light emission)

[0023] First, general elementary processes of light emission in a light-emitting element using a phosphorescent compound as guest material are described.

[0024] (1) The case where an electron and a hole are recombined in a guest molecule and the guest molecule is excited (direct recombination process).

[0025] (1-1) If the excited state of the guest molecule is a triplet excited state, the guest molecule emits phosphorescence.

[0026] (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.

[0027] In other words, in the direct recombination process in (1), high emission efficiency can be achieved as long as the intersystem crossing efficiency and the phosphorescence quantum yield of the guest molecule are high. As described above, it is desirable that the T1 level of the host molecule be higher than that of the guest molecule.

[0028] (2) The case where an electron and a hole are recombined in a host molecule and the host molecule is brought into an excited state (energy transfer process).

[0029] (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, excitation energy is transferred from the host molecule to the guest molecule, and the guest molecule is consequently converted into a triplet excited state. The guest molecule in the triplet excited state emits phosphorescence. Note that, theoretically, the energy can be transferred to a singlet excitation energy level (S1 level) of the guest molecule. However, since the S1 level of the guest molecule is often higher in energy than the T1 level of the host molecule, energy transfer to the S1 level of the guest molecule is unlikely to be a major energy transfer process; therefore, this is not described here.

[0030] (2-2) If the excited state of the host molecule is an excited singlet state and the S1 level of the host molecule is higher than the S1 level and the T1 level of the guest molecule, excitation energy is transferred from the host molecule to the guest molecule, and the guest molecule is consequently converted into an excited singlet state or an excited triplet state. The guest molecule in the excited triplet state emits phosphorescence. Furthermore, the guest molecule in the excited singlet state undergoes intersystem crossing to an excited triplet state and emits phosphorescence.

[0031] 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. (Energy transfer process)

[0032] Energy transfer processes between molecules are described in detail below.

[0033] First, the following two mechanisms are proposed for energy transfer between molecules. A molecule that releases excitation energy is called a host molecule, whereas a molecule that absorbs the excitation energy is called a guest molecule. (Förster mechanism (dipole-dipole interaction))

[0034] In the Förster mechanism (also known as Förster resonance energy transfer), no direct intermolecular contact is required for energy transfer. Energy transfer occurs through a resonance phenomenon of a dipole vibration between a host molecule and a guest molecule. The resonance phenomenon of the dipole vibration causes the host molecule to transfer energy to the guest molecule; as a result, the host molecule relaxes to a ground state, and the guest molecule is excited. The rate constant k h*→g of the Förster mechanism is expressed by a formula (1). [Formula 1)] kh*→g=9000c4K2ϕln10128π5n4NτR6∫f'h(v)εg(v)v4dv

[0035] In formula (1) means a frequency, f' h(v) represents 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 (a fluorescence lifetime or phosphorescence lifetime), c denotes the speed of light, ϕ denotes a luminescence quantum yield (a fluorescence quantum yield upon energy transfer from an excited singlet state and a phosphorescence quantum yield upon energy transfer from an excited triplet state), and K 2represents 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 for arbitrary orientation, K 2 = 2 / 3 applies. (Dexter mechanism (electron exchange interaction))

[0036] In the Dexter mechanism (also called Dexter electron transfer), a host molecule and a guest molecule are close to a contact region where their orbitals overlap, and the host molecule in an excited state and the guest molecule in a ground state exchange their electrons, resulting in energy transfer. The rate constant k h*→g of the Dexter mechanism is expressed by a formula (2). [Formula (2)] kh*→g=(2πh)K2exp(−2RL)∫f'h(v)ε'g(v)dv

[0037] In formula (2), h means a Planck constant, K means a constant having an energy dimension, v means a frequency, f' h (v) represents 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) represents a normalized absorption spectrum of a guest molecule, L represents an effective molecular radius, and R represents an intermolecular distance between the host molecule and the guest molecule.

[0038] Here it is assumed that the efficiency of energy transfer from the host molecule to the guest molecule (energy transfer efficiency Φ ET ) is expressed by a formula (3). In the formula, k ra rate constant of a light emission process (fluorescence during energy transfer from an excited singlet state and phosphorescence during energy transfer from an excited triplet state) of the host molecule, k n represents a rate constant of a non-light emission process (thermal deactivation or intersystem crossing) of the host molecule, and τ represents a measured lifetime of the excited state of the host molecule. [Formula (3)] ΦET=kh*→gkr+kn+kh*→g=kh*→g(1τ)+kh*→g

[0039] According to formula (3), in order to determine the energy transfer efficiency Φ ET to increase the rate constant k h*→g of energy transfer initially in comparison with another competing rate constant k r + k n (= 1 / τ) can be further increased. To determine the rate constant k h*→gIn order to increase the energy transfer, an emission spectrum of a host molecule (a fluorescence spectrum upon energy transfer from an excited singlet state largely overlaps a phosphorescence spectrum upon energy transfer from an excited triplet state) and an absorption spectrum of a guest molecule are then preferably determined on the basis of formulas (1) and (2) in the Förster mechanism and the Dexter mechanism.

[0040] Here, one embodiment of the present invention is a light-emitting element including, between a pair of electrodes, a light-emitting layer including a guest material and a host material, in which an emission spectrum of the host material overlaps an absorption spectrum of the guest material and phosphorescence is emitted by converting an excitation energy of the host material into an excitation energy of the guest material.

[0041] According to one embodiment of the present invention, by utilizing the overlap of the emission spectrum of the host material and the absorption spectrum of the guest material, energy is smoothly transferred from the host material to the guest material, resulting in high energy transfer efficiency. Thus, according to one embodiment of the present invention, a light-emitting element with high external quantum efficiency can be achieved.

[0042] Given the energy transfer processes described above, if the host molecule itself is deactivated by emitting the excitation energy as light or heat before the excitation energy is transferred to the guest molecule, the emission efficiency will be reduced and the lifetime will be shortened. However, according to one embodiment of the present invention, the energy is transferred smoothly, so that the deactivation of the excitation energy can be suppressed. In this way, a light-emitting element with a long lifetime can be achieved.

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

[0044] According to this embodiment, a phosphorescent compound is used as the guest material. In an absorption spectrum of the phosphorescent compound, an absorption band that is considered to contribute most to light emission is an absorption wavelength corresponding to a direct transition from a singlet excited state to a triplet excited state and a proximity to the absorption wavelength located on the longest wavelength side. Therefore, it is considered desirable 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 of approximately 500 nm to 600 nm as the absorption band on the longest wavelength side (actually, the broad absorption band can be located on a shorter or longer wavelength side depending on the emission wavelengths). This absorption band is primarily based on a triplet MLCT (metal-ligand charge transfer) transition. Note that the absorption band also includes absorptions based on a triplet π-π* transition and a singlet MLCT transition, and these absorptions overlap with each other to form a broad absorption band on the longest wavelength side in the absorption spectrum.Therefore, as described above, it is desirable that the broad absorption band at the longest wavelength side largely overlaps the emission spectrum of the host material when an organometallic complex (especially an iridium complex) is used as a guest material.

[0046] Accordingly, another embodiment of the present invention is a light-emitting element including, between a pair of electrodes, a light-emitting layer including a guest material and a host material, in which an emission spectrum of the host material overlaps an absorption band on the longest wavelength side in an absorption spectrum of the guest material, and phosphorescence is emitted by converting an excitation energy of the host material into an excitation energy of the guest material.

[0047] In the above light-emitting element, the absorption band preferably includes absorption based on the triplet MLCT transition. A triplet MLCT excited state is the lowest triplet excited state of the phosphorescent compound, which is the guest material, and thus the phosphorescent compound emits phosphorescence therefrom. That is, phosphorescence from the triplet MLCT excited state is associated with few deactivation processes other than light emission, and thus, it is expected that high emission efficiency can be achieved by making the rate of presence of this excited state as high as possible.For these reasons, numerous energy transfer processes are preferred in which energies are transferred directly from the host material to the excited triplet MLCT state using absorption based on the triplet MLCT transition. In the above light-emitting element, the guest material is preferably an organometallic complex, more preferably an iridium complex.

[0048] The inventors found that when the host molecule is in an excited singlet 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 is likely to be reduced compared to when the host molecule is in an excited triplet state (the above (2-1)). Therefore, the inventors focused on this condition as their objective.

[0049] A fluorescent compound is generally used as the host material, but its fluorescence lifetime (τ) is extremely short and is in the nanosecond range (k r +k n is high). The reason for this is that a transition from the excited singlet state to the ground state (singlet) is a allowed transition. From formula (3) it follows that this is unfavorable for the energy transfer efficiency Φ ET Taking this into account, it is generally unlikely that energy is transferred from the host material in an excited singlet state to the guest material.

[0050] However, an embodiment of the present invention can solve such a problem of the efficiency of energy transfer from the host material in the excited singlet state to the guest material. That is, according to one embodiment of the present invention, a light-emitting element includes a light-emitting layer including a guest material and a host material between a pair of electrodes. A fluorescence spectrum of the host material preferentially overlaps an absorption band on the longest wavelength side in an absorption spectrum of the guest material, and phosphorescence is emitted by converting an excitation energy of the host material into an excitation energy of the guest material.

[0051] That is, in a light-emitting element according to an embodiment of the present invention, the fluorescence spectrum of the host material overlaps the absorption band on the longest wavelength side in the absorption spectrum of the guest material, and phosphorescence is emitted by converting an excitation energy of the host material into an excitation energy of the guest material by means of the overlap. Such a structure can suppress the deactivation of the singlet excitation energy. Accordingly, application of an embodiment of the present invention can suppress the deactivation of the singlet excitation energy of the host material, which can affect not only the efficiency of an element but also its lifetime, so that a light-emitting element with a long lifetime can be achieved.Here, it is desirable that sufficient excitation energy of the host material is transferred to the phosphorescent compound and that essentially no fluorescence is observed from the excited singlet state.

[0052] To ensure that the emission spectrum of the host material sufficiently overlaps with the absorption spectrum of the guest material, the difference between the energy value of a peak in the emission spectrum and the energy value of a peak in the absorption band on the lowest energy side of the absorption spectrum is preferably 0.3 eV or less. The difference is more preferably 0.2 eV or less, and even more preferably 0.1 eV or less.

[0053] Furthermore, the Förster mechanism is considered important for the energy transfer from the host material in the excited singlet state. Taking this into account, it follows from formula (1) that the molar absorption coefficient of the absorption band at the longest wavelength side of the guest material is preferably 2,000 M -1 ·cm -1 or higher, preferably 5,000 M -1 ·cm -1 or higher.

[0054] It should be noted that this embodiment may be combined with the other embodiment if necessary. (Embodiment 2)

[0055] This embodiment presents a light-emitting element of an embodiment of the present invention with reference to Fig. 15A to 15C.

[0056] Fig.Figure 15A illustrates a light-emitting element including an EL layer 102 between a first electrode 103 and a second electrode 108. The light-emitting element in Fig. 15A 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 over the first electrode 103, and the second electrode 108 provided thereover.

[0057] The first electrode 103 is preferably formed using any metals, alloys, conductive compounds, mixtures thereof, and the like that have a high work function (particularly 4.0 eV or higher). 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 usually formed by a sputtering method, but may be formed by applying a sol-gel method or the like. For example, an indium oxide-zinc oxide film may 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.

[0058] Note 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 later, which is formed by combining an organic compound and an electron acceptor (an acceptor), the first electrode 103 can be formed using any variety of metals, alloys, electrically conductive compounds, mixtures thereof, and the like, regardless of the work function; for example, aluminum, silver, an aluminum-containing alloy (e.g., Al-Si), or the like can also be used.

[0059] The first electrode 103 can be formed, for example, by a sputtering method, a vapor deposition method (such as a vacuum vapor deposition method), or the like.

[0060] The second electrode 108 is preferably formed using any metals, alloys, electrically conductive compounds, mixtures thereof, and the like that have a low work function (particularly 3.8 eV or less). Specific examples include elements belonging to Groups 1 and 2 in the Periodic Table, that is, alkali metals such as lithium and cesium, alkaline earth metals such as calcium and strontium, magnesium, alloys thereof (e.g., Mg-Ag and Al-Li), rare earth metals such as europium and ytterbium, alloys thereof, aluminum, silver, and the like.

[0061] 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), the second electrode 108 can be formed using any of a variety of conductive materials such as Al, Ag, ITO, and indium oxide-tin oxide containing silicon or silicon oxide, regardless of the work function.

[0062] Note that when forming the second electrode 108, a vacuum evaporation method or a sputtering method may be used. If a silver paste or the like is used, a coating method, an inkjet method, or the like may be used.

[0063] The EL layer 102 includes at least the light-emitting layer 703. A known substance can be used for a part of the EL layer 102, and either a low-molecular compound or a high-molecular compound can be used. Note that the substances constituting the EL layer 102 may be organic compounds or may include an organic compound as a part.

[0064] As in Fig.15A, the EL layer 102 further includes not only the light-emitting layer 703 but also the following layers in appropriate combination: the hole-injection layer 701 including a substance having a high hole-injection ability, the hole-transport layer 702 including a substance having a high hole-transport ability, the electron-transport layer 704 including a substance having a high electron-transport ability, the electron-injection layer 705 including a substance having a high electron-injection ability, and the like.

[0065] The hole-injection layer 701 is a layer containing a substance with high hole-injection capability. Examples of the substance with high hole-injection capability include metal oxides such as molybdenum oxide, titanium oxide, vanadium oxide, rhenium oxide, ruthenium oxide, chromium oxide, zirconium oxide, hafnium oxide, tantalum oxide, silver oxide, tungsten oxide, and manganese oxide. Alternatively, a phthalocyanine-based compound such as phthalocyanine (abbreviation: H2Pc) or copper(II) phthalocyanine (abbreviation: CuPc) can be used.

[0066] Other examples include aromatic amine compounds and the like, which are low molecular weight organic 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).

[0067] Further examples include 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 an acid is added, such as poly(3,4-ethylenedioxythiophene) / poly(styrenesulfonic acid) (PEDOT / PSS) and polyaniline / poly(styrenesulfonic acid) (PAni / PSS).

[0068] The hole-injection layer 701 can be formed using a composite material formed by combining an organic compound and an electron acceptor (an acceptor). Such a composite material has high hole-injection and hole-transport capabilities because holes are generated in the organic compound by the electron acceptor. In this case, the organic compound is preferably a material that excellently transports the generated holes (a substance that has a high hole-transport capability).

[0069] Examples of the organic compound used for the composite material may include 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-transporting capacity, and particularly preferably a substance having a hole mobility of 10 -6 cm 2 / V s or higher. Note that, in addition to these substances, any substance capable of transporting more holes than electrons can be used. Organic compounds that can be used for the composite material are described in detail below.

[0070] Examples of the organic compound that can be used for 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.

[0071] Other examples 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.

[0072] Further examples are aromatic hydrocarbon compounds 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).

[0073] 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 in 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 capacity. Of these, molybdenum oxide is particularly preferred because of its air resistance, low hygroscopicity, and ease of handling.

[0074] 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.

[0075] The hole-transport layer 702 is a layer containing a substance with a high hole-transport capacity. Examples of the substance with a high hole-transport capacity include 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 ? / V s or higher. Note that, in addition to these substances, any substance capable of transporting more holes than electrons can be used. Note that the layer containing a substance with a high hole-transport capacity is not limited to a single layer and can be a stack of two or more layers containing any of the above substances.

[0076] Alternatively, the hole transport layer 702 may be formed using a carbazole derivative such as CBP, CzPA or PCzPA or an anthracene derivative such as t-BuDNA, DNA or DPAnth.

[0077] Furthermore, the hole transport layer 702 may alternatively be formed using a high molecular compound such as PVK, PVTPA, PTPDMA or Poly-TPD.

[0078] The light-emitting layer 703 is a layer containing a light-emitting substance. The light-emitting layer 703 of this embodiment includes a guest material and a host material. Several types of materials can be used as the host material. For details, refer to Embodiment 1.

[0079] As the phosphorescent compound, an organometallic complex is preferred, and in particular, an iridium complex is preferred. Taking into account energy transfer due to the Förster mechanism described above, the molar absorption coefficient of the absorption band at the longest wavelength side of the phosphorescent compound is preferably 2,000 M -1 ·cm -1 or higher, preferably 5,000 M -1 ·cm -1or higher. Specific examples of a compound exhibiting such a high molar absorption coefficient include 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)]), bis(2,3,5-triphenylpyrazinato)(dipivaloylmethanato)iridium(III) (abbreviation: [Ir(tppr)2(dpm)]), (acetylacetonato)bis(6-methyl-4-phenylpyrimidinato)iridium(III) (abbreviation: [Ir(mppm)2(acac)]), (Acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III) (abbreviation: [Ir(tBuppm)2(acac)]) and the like. In particular, a material having a molar absorption coefficient of 5,000 M -1 ·cm -1 or higher, such as [Ir(dppm)2(acac)], can provide a highly efficient light-emitting element with an external quantum efficiency of about 30%.

[0080] A preferred host material is a mixture of a compound that is likely to accept electrons (usually a heterocycle) and a compound that is likely to accept holes (usually an aromatic amine compound or a carbazole compound). With such a structure, a light-emitting layer can exhibit an excellent charge carrier balance between hole transport and electron transport, thereby increasing emission efficiency and lifetime.Specific examples of the host material include a material mixture of 2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation: 2mDBTPDBq-II) and 4-phenyl-4'-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PCBA1BP), a composite material of 2mDBTPDBq-II and 4,4'-di(1-naphthyl)-4"-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PCBNBB), a composite material of 2-[4-(3,6-diphenyl-9H-carbazol-9-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation: 2CzPDBq-III) and PCBNBB, a composite material of 2-[4-(Dibenzothiophen-4-yl)phenyl]-1-phenyl-1H-benzimidazole (abbreviation: DBTBIm-II) and 4,4',4"-tris[N-(1-naphthyl)-N-phenylamino]-triphenylamine (abbreviation: 1'-TNATA) and the like.Alternatively, a mixture of 2mDBTPDBq-II and any of the following can be used as the host material: 4,4'-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB), 4-(1-naphthyl)-4'-phenyltriphenylamine (abbreviation: αNBA1BP), 2,7-bis[N-(diphenylaminophenyl)-N-phenylamino]-spiro-9,9'-bifluorene (abbreviation: DPA2SF), 9-phenyl-9H-3-(9-phenyl-9H-carbazol-3-yl)carbazole (abbreviation: PCCP), and 1'-TNATA. It should be noted that any other known host material can be used without limitation to the above substances.

[0081] Furthermore, by providing a plurality of layers and making their emission colors different, light emission of a desired color can be achieved from the light-emitting element as a whole. For example, in a light-emitting element comprising the two light-emitting layers, the emission colors of the first and second light-emitting layers are complementary, so that the light-emitting element as a whole can be made to emit white light. Note that the word "complementary" means a color ratio in which an achromatic color is achieved when colors are mixed. That is, white light emission can be achieved by mixing light from substances whose emission colors are complementary colors. This can be applied to a light-emitting element having three or more light-emitting layers.

[0082] The electron-transport layer 704 is a layer containing a substance with a high electron-transport capacity. Examples of the substance with a high electron-transport capacity 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 include 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).Further 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 / V s or higher. Note that, in addition to these substances, any substance capable of transporting more holes than electrons can be used for the electron-transport layer.

[0083] Furthermore, the electron transport layer is not limited to a single layer, and it can be a stack of two or more layers containing any of the above substances.

[0084] The electron injection layer 705 is a layer containing a substance with high electron injection capability. Examples of the substance that can be used for the electron injection layer 705 include alkali metals, alkaline earth metals, and compounds thereof, 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.

[0085] Alternatively, a composite material formed by combining an organic compound and an electron donor (a donor) can be used for the electron-injection layer 705. Such a composite material has high electron injection and electron transport capabilities because electrons are generated in the organic compound by the electron donor. Here, the organic compound is preferably a material that excels at transporting the 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 that exhibits an ability to donate electrons with respect to the organic compound can be used.Preferred specific examples of the electron donor include alkali metals, alkaline earth metals, and rare earth metals such as lithium, cesium, magnesium, calcium, erbium, and ytterbium, as well as alkali metal oxides and alkaline earth metal oxides such as lithium oxide, calcium oxide, and barium oxide. Furthermore, a Lewis base such as magnesium oxide or an organic compound such as tetrathiafulvalene (TTF) can be used.

[0086] Note that each of the above-mentioned hole injection layer 701, hole transport layer 702, light emitting layer 703, electron transport layer 704 and electron injection layer 705 can be formed by a method such as a vapor deposition method (e.g., a vacuum vapor deposition method), an inkjet method or a coating method.

[0087] A plurality of EL layers may be stacked between the first electrode 103 and the second electrode 108, as shown in Fig.15B. 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 material described above. Furthermore, the charge generation layer 803 can have a stacked structure of a layer including the composite material and a layer including another material. In this case, as the layer including another material, a layer including an electron-donating substance and a substance with a high electron-transporting ability, a layer formed of a transparent conductive film, or the like can be used.A light-emitting element with such a structure hardly encounters energy transfer and extinction problems, and due to a wider range of materials, a light-emitting element with both high emission efficiency and a long lifetime can be easily achieved. Furthermore, a light-emitting element that provides phosphorescence from one of the EL layers and fluorescence from the other of the EL layers can easily be achieved. This structure can be combined with any of the EL layer structures described above.

[0088] Furthermore, by designing the emission colors of EL layers differently, light emission in a desired color can be achieved from the light-emitting element as a whole. For example, the emission colors of the first and second EL layers in a light-emitting element comprising both EL layers are complementary, so that the light-emitting element can emit white light as a whole. The same applies to a light-emitting element with three or more EL layers.

[0089] As in Fig.15C, 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.

[0090] Preferably, the composite material layer 708 is provided in contact with the second electrode 108, in which case, damage caused to the EL layer 102 can be reduced, particularly 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 having a high hole-transport capacity contains an acceptor substance.

[0091] Furthermore, 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; thus, electrons generated in the composite material layer 708 can be easily injected into the electron transport layer 704.

[0092] The electron injection buffer layer 706 can be formed using a substance having a high electron injection capability, such as an alkali metal, an alkaline earth metal, a rare earth metal, a compound of the above metals (e.g., an alkali metal compound (e.g., an oxide such as lithium oxide, a halide, and a carbonate such as lithium carbonate or cesium carbonate), an alkaline earth metal compound (e.g., an oxide, a halide, and a carbonate), or a rare earth metal compound (e.g., an oxide, a halide, and a carbonate).

[0093] If the electron injection buffer layer 706 further contains a substance with a high electron transport capacity and a donor substance, the donor substance is preferably added such that the mass ratio of the donor substance to the substance with a high electron transport capacity is in the range of 0.001:1 to 0.1:1. Note that an organic compound such as tetrathianaphthacene (abbreviation: TTN), nickelocene, or decamethylnickelocene, as well as an alkali metal, an alkaline earth metal, a rare earth metal, a compound of the above metals (e.g., an alkali metal compound (e.g., an oxide such as lithium oxide, a halide, and a carbonate such as lithium carbonate or cesium carbonate), an alkaline earth metal compound (e.g., an oxide, a halide, and a carbonate), and a rare earth metal compound (e.g., an oxide, a halide, and a carbonate)) can be used as the donor substance.Note that a material similar to the above-described material for the electron transport layer 704 can be used as a substance having a high electron transport capacity.

[0094] Furthermore, the electron relay layer 707 is preferably formed between the electron injection buffer layer 706 and the composite material layer 708. The electron relay layer 707 is not necessarily provided; however, by providing the electron relay layer 707 with a high electron transport capability, electrons can be quickly transported to the electron injection buffer layer 706.

[0095] In the structure where the electron relay layer 707 is disposed between the composite material layer 708 and the electron injection buffer layer 706, 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 thus their functions hardly interfere with each other. Therefore, a rise in the control voltage can be prevented.

[0096] The electron relay layer 707 includes a substance with a high electron transport capacity and is formed such that the LUMO level of the substance with a high electron transport capacity is located between the LUMO level of the acceptor substance included in the composite material layer 708 and the LUMO level of the substance with a high electron transport capacity included in the electron transport layer 704. In the case where the electron relay layer 707 includes a donor substance, the donor level of the donor substance is also controlled to be located between the LUMO level of the acceptor substance included in the composite material layer 708 and the LUMO level of the substance with a high electron transport capacity included 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 ability included in the electron relay layer 707 is preferably higher than or equal to -5.0 eV, more preferably higher than or equal to -5.0 eV and lower than or equal to -3.0 eV.

[0097] As a substance having a high electron transport capacity included in the electron relay layer 707, a phthalocyanine-based material or a metal complex having a metal-oxygen bond and an aromatic ligand is preferably used.

[0098] As the phthalocyanine-based material included in the electron relay layer 707, it is particularly preferable to alternatively use CuPc, a phthalocyanine-tin(II) complex (SnPc), a phthalocyanine-zinc complex (ZnPc), cobalt(II) phthalocyanine, β-form (CoPc), phthalocyanine-iron (FePc), or vanadyl 2,9,16,23-tetraphenoxy-29H,31H-phthalocyanine (PhO-VOPc).

[0099] As a metal complex having a metal-oxygen bond and an aromatic ligand included in the electron relay layer 707, a metal complex having a metal-oxygen double bond is preferably used. The metal-oxygen double bond has an acceptor property (the ability to easily accept electrons); thus, electrons can be transferred (donated and accepted) more easily. Furthermore, the metal complex having a metal-oxygen double bond is considered stable. Therefore, the use of the metal complex having the metal-oxygen double bond enables the light-emitting element to operate more stably at low voltage.

[0100] As a metal complex containing a metal-oxygen bond and an aromatic ligand, a phthalocyanine-based material is preferred. In particular, vanadyl phthalocyanine (VOPc), a phthalocyanine-tin(IV) oxide complex (SnOPc), or a phthalocyanine-titanium oxide complex (TiOPc) are preferred alternatives, as a metal-oxygen double bond is likely to act on another molecule in terms of molecular structure and exhibits high acceptor capacity.

[0101] Note 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 formation.

[0102] The electron relay layer 707 may further include a donor substance. An organic compound such as tetrathianaphthacene (abbreviation: TTN), nickelocene, or decamethylnickelocene, as well as an alkali metal, an alkaline earth metal, a rare earth metal, a compound of the above metals (e.g., an alkali metal compound (e.g., an oxide such as lithium oxide, a halide, and a carbonate such as lithium carbonate or cesium carbonate), an alkaline earth metal compound (e.g., an oxide, a halide, and a carbonate), and a rare earth metal compound (e.g., an oxide, a halide, and a carbonate)) may be used as the donor substance. When such a donor substance is included in the electron relay layer 707, electrons can be easily transferred, and the light-emitting element can be operated at a lower voltage.

[0103] If a donor substance other than the materials specified above as substances with high electron transport capacity is included in the electron relay layer 707, a substance having a higher LUMO level than the acceptor level of the acceptor substance included in the composite material layer 708 may be used. Specifically, a substance having a LUMO level higher than or equal to -5.0 eV is preferably used, preferably higher than or equal to -5.0 eV and lower than or equal to -3.0 eV is preferably used. Examples of such a substance include a perylene derivative, a nitrogen-containing condensed aromatic compound, and the like. Note that a nitrogen-containing condensed aromatic compound is preferably used for the electron relay layer 707 because of its high stability.

[0104] Specific examples of the perylene derivative include 3,4,9,10-perylenetetracarboxylic dianhydride (abbreviation: PTCDA), 3,4,9,10-perylenetetracarboxylic 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.

[0105] Specific examples of the nitrogen-containing condensed aromatic compound include pyrazino[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.

[0106] In addition, 7,7,8,8-tetracyanoquinodimethane (abbreviation: TCNQ), 1,4,5,8-naphthalenetetracarboxylic dianhydride (abbreviation: NTCDA), perfluoropentacene, copper hexadecafluorophthalocyanine (abbreviation: F 16CuPc), N,N'-bis(2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-penta-decafluorooctyl)-1,4,5,8-naphthalenetetracarboxylic diimide (abbreviation: NTCDI-C8F), 3',4'-dibutyl-5,5"-bis(dicyanomethylene)-5,5"-dihydro-2,2':5',2"-terthiophene (abbreviation: DCMT), methanefullerenes (e.g. [6,6]-phenyl-C 61 -butanoic acid methyl ester) or the like.

[0107] Note that, if a donor substance is included in the electron relay layer 707, the electron relay layer 707 may be formed by a method such as co-evaporation of the substance having a high electron transport ability and the donor substance.

[0108] 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.

[0109] The EL layer 102 of this embodiment can be formed as described above.

[0110] In the light-emitting element described above, a current flows due to a potential difference generated 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 extracted 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 capable of transmitting visible light.

[0111] Note that the structure of layers provided between the first electrode 103 and the second electrode 108 is not limited to the structure described above. Alternatively, a structure other than the above may be adopted, provided that a light-emitting region where holes and electrons recombine is provided in a portion remote from the first electrode 103 and the second electrode 108 to prevent extinction due to proximity of the light-emitting region to metal.

[0112] In other words, there is no particular restriction on the stacking structure of the layers. A layer containing a substance with a high electron transport capacity, a substance with a high hole transport capacity, a substance with a high electron injection capacity, a substance with a high hole injection capacity, a bipolar substance (a substance with both high electron transport capacity and high hole transport capacity), a hole-blocking material, or the like can be combined with a light-emitting layer as desired.

[0113] By using the light-emitting element described in this embodiment, a passive matrix light-emitting device or an active matrix light-emitting device in which driving of the light-emitting element is controlled by a transistor can be fabricated. Furthermore, the light-emitting device can be applied to an electronic device, a lighting device, or the like.

[0114] In the manner described above, the light-emitting element of one embodiment of the present invention can be manufactured.

[0115] It should be noted that this embodiment may be combined with the other embodiment if necessary. [Example 1]

[0116] Example 1 presents guest materials and a host material that can be included in the light-emitting element of an embodiment of the present invention, with reference to Fig. 1A and Fig. 1B and Fig. 16.

[0117] The guest materials used in this example are the following three types: (acetylacetonato)bis(4,6-diphenylpyrimidinato)iridium(III) (abbreviation: [Ir(dppm)2(acac)]), bis(3,5-dimethyl-2-phenylpyrazinato)(dipivaloylmethanato)iridium(III) (abbreviation: [Ir(mppr-Me)2(dpm)]) and (acetylacetonato)bis(6-methyl-4-phenylpyrimidinato)iridium(III) (abbreviation: [Ir(mppm)2(acac)]). The host material also used in this example is a mixture of 2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation: 2mDBTPDBq-II) and 4-phenyl-4'-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PCBA1BP). Chemical formulas of the materials used in this example are shown below. (Measurement results of absorption spectra of guest materials and an emission spectrum of a host material)(Absorption spectra)

[0118] Fig. 1A and Fig.16 show an ultraviolet-visible absorption spectrum (hereinafter referred to simply as absorption spectrum) of [Ir(dppm)2(acac)] in a dichloromethane solution of [Ir(dppm)2(acac)] as absorption spectrum 1. Similarly, Fig. 1A and Fig. 16 also shows an absorption spectrum of [Ir(mppr-Me)2(dpm)] in a dichloromethane solution of [Ir(mppr-Me)2(dpm)] as absorption spectrum 2 and an absorption spectrum of [Ir(mppm)2(acac)] in a dichloromethane solution of [Ir(mppm)2(acac)] as absorption spectrum 3.

[0119] Measurements of the respective absorption spectra were performed at room temperature using a UV / Vis spectrophotometer (Type V550, manufactured by JASCO Corporation) with the dichloromethane solutions in quartz cuvettes. (emission spectrum)

[0120] Fig. 1A and Fig. 16 also show an emission spectrum of a thin film of a material mixture of 2mDBTPDBq-II and PCBA1BP. In Fig. 1A, the horizontal axis represents the wavelength (nm), and the vertical axes represent the molar absorption coefficient ε (M -1 ·cm -1 ) and the emission intensity (arbitrary unit). In Fig. 16, the horizontal axis represents the energy (eV), and the vertical axes represent the molar absorption coefficient ε (M -1 ·cm -1 ) and the emission intensity (arbitrary unit).

[0121] Fig. 1A and Fig.16 shows that absorption spectra 1 to 3 each overlap the emission spectrum. Thus, it is demonstrated that a light-emitting element including a light-emitting layer using any of the guest materials of this example and the host material of this example together has high energy transfer efficiency because energy is transferred using the overlap of the emission spectrum of the host material and the absorption spectrum of the guest material.

[0122] Here in Fig. 1A and Fig.16 absorption band maxima on the longest wavelength side (the lowest energy side) (absorption bands expected to contribute significantly to light emission) in the absorption spectra and one emission spectrum peak are considered. Of the peaks of absorption spectra 1 to 3, the peak of absorption spectrum 1 is closest to the emission spectrum peak, and the peak of absorption spectrum 3 is farthest from the emission spectrum peak.

[0123] In detail, Fig. 16 the difference between the maximum of absorption spectrum 1 and the maximum of emission spectrum is 0.02 eV, the difference between the maximum of absorption spectrum 2 and the maximum of emission spectrum is 0.12 eV, and the difference between the maximum of absorption spectrum 3 and the maximum of emission spectrum is 0.23 eV.

[0124] Next, the molar absorption coefficients at the maxima of the absorption bands at the longest wavelength side (the side of lowest energy) in the absorption spectra are calculated in Fig. 1A. Of the absorption spectra 1 to 3, absorption spectrum 1 has the highest molar absorption coefficient, and absorption spectrum 2 has the lowest molar absorption coefficient.

[0125] That is, of the absorption spectra 1 to 3, the absorption spectrum 1 has the maximum of the absorption band at the longest wavelength side (the side of lowest energy) closest to the maximum of the emission spectrum and the highest molar absorption coefficient at the maximum.

[0126] From the above, it can be seen that the absorption spectrum 1 overlaps the emission spectrum particularly strongly. Therefore, a light-emitting element containing a material mixture of 2mDBTPDBq-II and PCBA1BP as the host material and [Ir(dppm)2(acac)] as the guest material exhibits a particularly high energy transfer efficiency, as energy is transferred using the overlap of the emission spectrum of the material mixture and the absorption spectrum of [Ir(dppm)2(acac)]. (Calculation results of absorption spectra of guest materials)

[0127] Next, a reproduction of the absorption spectra of [Ir(dppm)2(acac)] and [Ir(mppr-Me)2(dpm)] (absorption spectra 1 and 2 in Fig. 1A), which had been obtained by the above measurements, were attempted by calculation.

[0128] To obtain the absorption spectra of [Ir(dppm)2(acac)] and [Ir(mppr-Me)2(dpm)], excitation energies and oscillator intensities were calculated using the most stable structures of the respective molecules in their ground states. The absorption spectra were obtained based on the calculated oscillator intensities. The specific calculation methods are presented below.

[0129] The most stable ground-state structures of [Ir(dppm)2(acac)] and [Ir(mppr-Me)2(dpm)] were calculated using density functional theory (DFT). Next, the excitation energies and oscillator intensities of [Ir(dppm)2(acac)] and [Ir(mppr-Me)2(dpm)] were obtained using time-dependent density functional theory (TD-DFT), and their absorption spectra were calculated using the results. In DFT, the total energy is represented as the sum of the potential energy, the electrostatic energy between electrons, the electronic kinetic energy, and the exchange-correlation energy, which includes all the complicated interactions between electrons.Furthermore, in DFT, an exchange-correlation interaction is approximated by a functional (i.e., a function of another function) of an electron potential represented by an electron density to enable highly accurate calculations. Here, B3PW91, a hybrid functional, was used to specify the weight of each parameter with respect to the exchange-correlation energy. In addition, LanL2DZ was applied as a basis function to Ir atoms, and 6-311 (the basis function of a triple-split valence basis set using three contraction functions for each valence orbital) was applied to atoms other than Ir atoms. For example, the above basis function includes the 1s to 3s orbitals for hydrogen atoms, and the 1s to 4s orbitals and the 2p to 4p orbitals for carbon atoms.To improve the computational accuracy, the p-function and the d-function were further added as polarization basis sets to hydrogen atoms and atoms other than hydrogen atoms, respectively.

[0130] It should be noted that Gaussian 09 was used as the quantum chemistry computational program. A high-performance computer (Altix 4700, manufactured by SGI Japan, Ltd.) was used for the calculations.

[0131] Fig. Figure 1B shows the absorption spectra obtained by the calculations. For comparison, Fig.1B also shows the absorption spectra obtained by the measurements described above. Specifically, the absorption spectrum of [Ir(dppm)2(acac)] obtained by the measurements is represented as Absorption Spectrum 1, and the absorption spectrum thereof obtained by the calculations is represented as Absorption Spectrum 1'. Furthermore, the absorption spectrum of [Ir(mppr-Me)2(dpm)] obtained by the measurements is represented as Absorption Spectrum 2, and the absorption spectrum thereof obtained by the calculations is represented as Absorption Spectrum 2'. In Fig. 1B, the horizontal axis represents the wavelength (nm), and the vertical axes represent the molar absorption coefficient ε (M -1 ·cm -1 ) and the emission intensity (arbitrary unit).

[0132] As in Fig.As shown in Figure 1B, the absorption spectrum 1 and absorption spectrum 2 obtained by measurements have essentially the same shapes as the absorption spectrum 1' and absorption spectrum 2' obtained by calculations, respectively. In particular, the following two tendencies of absorption spectra 1 and 2 are also evident in absorption spectra 1' and 2'.

[0133] Trend 1: The maximum wavelength of absorption spectrum 1 (1') is closer to the maximum wavelength of emission spectrum than the maximum wavelength of absorption spectrum 2 (2').

[0134] Trend 2: Absorption spectrum 1 (1') has a higher molar absorption coefficient at the maximum wavelength of the absorption band on the longest wavelength side of it than absorption spectrum 2 (2'). [Example 2]

[0135] Example 2 illustrates a light-emitting element of an embodiment of the present invention with reference to Fig. 2. Chemical formulas of materials used in this example are shown below. Note that chemical formulas of materials used in the above examples are omitted here.

[0136] The following describes a method for manufacturing light-emitting elements 1 to 3 of this example. (Light-emitting element 1)

[0137] First, a film of indium tin oxide containing silicon oxide (ITSO) was sputtered over a glass substrate 1100 to form a first electrode 1101 serving as the anode. The thickness was 110 nm, and the electrode area was 2 mm × 2 mm.

[0138] Next, as a pretreatment for forming the light-emitting element over the substrate 1100, the surface of the substrate was washed with water, heated at 200 °C for one hour, and subjected to UV ozone treatment for 370 seconds.

[0139] Subsequently, the substrate 1100 was transferred to a vacuum deposition device in which the pressure was increased to about 10 -4 Pa, and subjected to vacuum heating 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.

[0140] Next, the substrate 1100 provided with the first electrode 1101 was attached to a substrate holder in a vacuum evaporation apparatus so that a surface on which the first electrode 1101 was provided was directed downward. The pressure in the vacuum evaporation apparatus was set to about 10 -4Pa. Then, 4-phenyl-4'-(9-phenylfluoren-9-yl)triphenylamine (abbreviation: BPAFLP) and molybdenum(VI) oxide were co-evaporated over the first electrode 1101 to form a hole-injection layer 1111. The thickness of the hole-injection layer 1111 was 40 nm, and the weight ratio of BPAFLP to molybdenum oxide was set to 4:2 (= BPAFLP:molybdenum oxide).

[0141] Next, a BPAFLP film was formed to a thickness of 20 nm over the hole injection layer 1111, so that a hole transport layer 1112 was formed.

[0142] Furthermore, 2mDBTPDBq-II, PCBA1BP, and [Ir(dppm)2(acac)] were co-evaporated to form a light-emitting layer 1113 over the hole-transport layer 1112. The weight ratio of 2mDBTPDBq-II to PCBA1BP and [Ir(dppm)2(acac)] was set to 0.8:0.2:0.05 (= 2mDBTPDBq-II:PCBA1BP:[Ir(dppm)2(acac)]). The thickness of the light-emitting layer 1113 was 40 nm.

[0143] Next, a 2mDBTPDBq-II film was formed to a thickness of 10 nm over the light-emitting layer 1113, so that a first electron-transport layer 1114a was formed.

[0144] Next, a bathophenanthroline (abbreviation: BPhen) film was formed in a thickness of 20 nm over the first electron transport layer 1114a, so that a second electron transport layer 1114b was formed.

[0145] Furthermore, a lithium fluoride (LiF) film was formed in a thickness of 1 nm by evaporation over the second electron transport layer 1114b, so that an electron injection layer 1115 was formed.

[0146] Finally, an aluminum foil with a thickness of 200 nm was formed by evaporation to form a second electrode 1103 serving as a cathode. Thus, the light-emitting element 1 of this example was fabricated. (Light-emitting element 2)

[0147] The light-emitting layer 1113 of the light-emitting element 2 was formed by co-evaporation of 2mDBTPDBq-II, PCBA1BP, and [Ir(mppr-Me)2(dpm)]. The weight ratio of 2mDBTPDBq-II to PCBA1BP and [Ir(mppr-Me)2(dpm)] was set to 0.8:0.2:0.05 (= 2mDBTPDBq-II:PCBA1BP:[Ir(mppr-Me)2(dpm)]). The thickness of the light-emitting layer 1113 was 40 nm. The layers other than the light-emitting layer 1113 were formed in the same manner as those of the light-emitting element 1. (Light-emitting element 3)

[0148] The light-emitting layer 1113 of the light-emitting element 3 was formed by co-evaporation of 2mDBTPDBq-II, PCBA1BP, and [Ir(mppm)2(acac)]. The weight ratio of 2mDBTPDBq-II to PCBA1BP and [Ir(mppm)2(acac)] was set to 0.8:0.2:0.05 (= 2mDBTPDBq-II:PCBA1BP:[Ir(mppm)2(acac)]). The thickness of the light-emitting layer 1113 was 40 nm. The layers other than the light-emitting layer 1113 were formed in the same manner as those of the light-emitting element 1.

[0149] It should be noted that in the above evaporation process, all evaporation was carried out by a resistance heating method.

[0150] Table 1 shows element structures of the light-emitting elements 1 to 3 obtained in this way.

[0151] [Table 1] 1. Electrode Hole injection layer Lochtran sports layer Light-emitting layer 1. Electron transport layer 2. Electron transport layer Electron injection layer 2. Electrode Light-emitting element 1 ITSO 110 nm BPAFLP: MoOx (= 4:2) 40 nm BPAFLP 20 nm 2mDBTPDBq-ll: PCBA1BP: [Ir(dppm)2(acac)] (= 0.8:0.2:0.05) 40 nm 2mDBTPDBq-II 10 nm BPhen 20 nm LiF 1 nm Al 200 nm Light-emitting element 2 ITSO 110 nm BPAFLP: MoOx (= 4:2) 40 nm BPAFLP 20 nm 2mDBTPDBq-II: PCBA1BP: [Ir(mppr-Me)2(dpm)] (= 0.8:0.2:0.05) 40 nm 2mDBTPDBq-II 10 nm BPhen 20 nm LiF 1 nm Al 200 nm Light-emitting element 3 ITSO 110 nm BPAFLP: MoOx (= 4:2) 40 nm BPAFLP 20 nm 2mDBTPDBq-ll: PCBA1BP: [Ir(mppm)2(acac)] (= 0.8:0.2:0.05) 40 nm 2mDBTPDBq-II 10 nm BPhen 20 nm LiF 1 nm Al 200 nm

[0151] These light-emitting elements were enclosed in a glove box containing a nitrogen atmosphere to prevent exposure to air. The operating characteristics of these light-emitting elements were then measured. Note that the measurements were performed at room temperature (in an atmosphere maintained at 25°C).

[0152] Fig. 3 shows a current density-luminance characteristic curve of the light-emitting elements 1 to 3. In Fig. 3 the horizontal axis represents the current density (mA / cm 2 ), and the vertical axis represents the luminance (cd / m 2 ). Fig. 4 represents a voltage-luminance characteristic curve. In Fig. 4, the horizontal axis represents the voltage (V) and the vertical axis represents the luminance (cd / m 2 ). Fig. 5 shows a luminance-current efficiency curve. In Fig.5 the horizontal axis represents the luminance (cd / m 2 ) and the vertical axis represents the current efficiency (cd / A). Fig. 6 shows a characteristic curve of luminance versus external quantum efficiency. In Fig. 6 the horizontal axis represents the luminance (cd / m 2 ), and the vertical axis represents the external quantum efficiency (%).

[0153] 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 efficiency (Im / W) and the external quantum efficiency (%) of the light-emitting elements 1 to 3 at a luminance of about 1,000 cd / m 2 represents. [Table 2] Voltage (V) Current density (mA / cm 2 ) Chromaticity (x, y) Luminance (cd / m 2 ) Current efficiency (cd / A) Efficiency (Im / W) External quantum efficiency (%) Light-emitting element 1 3,0 1,3 (0,56, 0,44) 840 65 68 26 Light-emitting element 2 3,0 1,6 (0,55, 0,45) 1.000 63 66 24 Light-emitting element 3 3,0 1,3 (0,44, 0,55) 940 77 76 20

[0154] Fig. Figure 7 shows emission spectra of the light-emitting elements 1 to 3 obtained by applying a current of 0.1 mA. Fig.7, 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 were (x, y) = (0.56, 0.44) at a luminance of 840 cd / m 2 , the CIE chromaticity coordinates of the light-emitting element 2 were (x, y) = (0.55, 0.45) at a luminance of 1,000 cd / m 2 , and the CIE chromaticity coordinates of the light-emitting element 3 were (x, y) = (0.44, 0.55) at a luminance of 940 cd / m 2 . The results show that light-emitting element 1 emits light originating from [Ir(dppm)2(acac)], light-emitting element 2 emits light originating from [Ir(mppr-Me)2(dpm)], and light-emitting element 3 emits light originating from [Ir(mppm)2(acac)].

[0155] As can be seen from Table 2 and Fig.3 to 6, each of the light-emitting elements 1 to 3 has a high current efficiency, a high efficiency and a high external quantum efficiency.

[0156] In the light-emitting elements of this example, the light-emitting layers include the host material and the guest materials described in Example 1. As described in Example 1, the respective absorption spectra of the guest materials included in the light-emitting elements 1 to 3 overlap the emission spectrum of the host material. The light-emitting elements of this example are considered to have high energy transfer efficiency and external quantum efficiency because energies are transferred using the overlaps.

[0157] Furthermore, the light-emitting element 1 has a higher external quantum efficiency than the light-emitting elements 2 and 3. According to the results in Example 1, the absorption band on the lowest energy side in the absorption spectrum of the guest material included in the light-emitting element 1 has the maximum closest to the maximum of the emission spectrum (the difference between the maxima is 0.02 eV), and the molar absorption coefficient at the maximum wavelength is the highest (> 5,000 M -1 ·cm -1 ). Consequently, it is shown that the particularly high energy transfer efficiency of the light-emitting element 1 results in the high external quantum efficiency.

[0158] Furthermore, the light-emitting element 2 has a higher external quantum efficiency than the light-emitting element 3. According to the results in Example 1, the maximum wavelength of the absorption spectrum 2 is closer to the maximum wavelength of the emission spectrum than the maximum wavelength of the absorption spectrum 3. Consequently, it is shown that this is the reason why the external quantum efficiency characteristics of the light-emitting elements 2 and 3 are different from each other.

[0159] The above results show that the application of an embodiment of the present invention makes it possible to obtain a light-emitting element with a high external quantum efficiency.

[0160] Next, reliability tests of the light-emitting elements 1 to 3 were performed. Fig. Figure 8 shows the results of the reliability tests. In Fig.8, the vertical axis represents the normalized luminance (%) assuming an initial luminance of 100%, and the horizontal axis represents the driving time (h) of the elements.

[0161] In the reliability tests, the light-emitting elements 1 to 3 were each driven under conditions where the initial luminance was set to 5,000 cd / m 2 was set and the current density was constant.

[0162] After driving for 470 hours, light-emitting element 1 retained 85% of its initial luminance. After driving for 470 hours, light-emitting element 2 retained 72% of its initial luminance. After driving for 280 hours, light-emitting element 3 retained 72% of its initial luminance.

[0163] The above results show that the application of an embodiment of the present invention makes it possible to obtain a light-emitting element with a long lifetime. [Example 3]

[0164] Example 3 presents a light-emitting element of an embodiment of the present invention with reference to Fig. 2. The materials used in this example are used in the above examples, and therefore their chemical formulas are omitted here.

[0165] The following describes a method for manufacturing a light-emitting element 4 of this example. (Light-emitting element 4)

[0166] First, a film of indium tin oxide containing silicon oxide (ITSO) was sputtered over the glass substrate 1100 to form the first electrode 1101, which served as the anode. The thickness was 110 nm, and the electrode area was 2 mm × 2 mm.

[0167] Next, as a pretreatment for forming the light-emitting element over the substrate 1100, the surface of the substrate was washed with water, heated at 200 °C for one hour, and subjected to UV ozone treatment for 370 seconds.

[0168] Subsequently, the substrate 1100 was transferred to a vacuum deposition device in which the pressure was increased to about 10 -4 Pa, and subjected to vacuum heating 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.

[0169] Next, the substrate 1100 provided with the first electrode 1101 was attached to a substrate holder in a vacuum evaporation apparatus so that a surface on which the first electrode 1101 was provided was directed downward. The pressure in the vacuum evaporation apparatus was set to about 10 -4 Pa. Then, BPAFLP and molybdenum(VI) oxide were co-evaporated over the first electrode 1101 to form the hole-injection layer 1111. The thickness of the hole-injection layer 1111 was 40 nm, and the weight ratio of BPAFLP to molybdenum oxide was set to 4:2 (= BPAFLP:molybdenum oxide).

[0170] Next, a BPAFLP film was formed to a thickness of 20 nm over the hole injection layer 1111, so that the hole transport layer 1112 was formed.

[0171] Furthermore, 2mDBTPDBq-II, PCBA1BP, and [Ir(dppm)2(acac)] were co-evaporated to form the light-emitting layer 1113 over the hole-transport layer 1112. The weight ratio of 2mDBTPDBq-II to PCBA1BP and [Ir(dppm)2(acac)] was set to 0.8:0.2:0.1 (= 2mDBTPDBq-II:PCBA1BP:[Ir(dppm)2(acac)]). The thickness of the light-emitting layer 1113 was 40 nm.

[0172] Next, a 2mDBTPDBq-II film was formed to a thickness of 15 nm over the light-emitting layer 1113, so that the first electron-transport layer 1114a was formed.

[0173] Next, a BPhen film was formed to a thickness of 15 nm over the first electron transport layer 1114a, so that the second electron transport layer 1114b was formed.

[0174] Furthermore, a LiF film with a thickness of 1 nm was formed by evaporation over the second electron transport layer 1114b, so that the electron injection layer 1115 was formed.

[0175] Finally, an aluminum foil with a thickness of 200 nm was formed by evaporation to form the second electrode 1103, which served as the cathode. Thus, the light-emitting element 4 of this example was fabricated.

[0176] It should be noted that in the above evaporation process, all evaporation was carried out by a resistance heating method.

[0177] Table 3 shows element structures of the light-emitting element 4 obtained in this way. [Table 3] 1. Electrode Hole injection layer Hole transport layer Light-emitting layer 1. Electron transport layer 2. Electron transport layer Electron injection layer 2. Electrode Light-emitting element 4 ITSO 110 nm BPAFLP: MoOx (= 4:2) 40 nm BPAFLP 20 nm 2mDBTPDBq-II: PCBA1BP: [Ir(dppm)2(acac)] (= 0.8:0.2:0.1) 40 nm 2mDBTP DBq-II 15 nm Bphen 15 nm LiF 1 nm Al 200 nm

[0178] The light-emitting element 4 was enclosed in a glove box containing a nitrogen atmosphere to prevent exposure to air. The operating characteristics of the light-emitting element were then measured. Note that the measurements were performed at room temperature (in an atmosphere maintained at 25°C).

[0179] Fig. 9 shows a current density-luminance characteristic curve of the light-emitting element 4. In Fig. 9 the horizontal axis represents the current density (mA / cm 2 ), and the vertical axis represents the luminance (cd / m 2 ). Fig. 10 represents a voltage-luminance characteristic curve. In Fig. 10, the horizontal axis represents the voltage (V), and the vertical axis represents the luminance (cd / m 2 ). Fig. 11 shows a luminance-current efficiency curve. In Fig.11 the horizontal axis represents the luminance (cd / m 2 ) and the vertical axis represents the current efficiency (cd / A). Fig. 12 shows a characteristic curve of luminance versus external quantum efficiency. In Fig. 12 the horizontal axis represents the luminance (cd / m 2 ), and the vertical axis represents the external quantum efficiency (%).

[0180] Furthermore, Table 4 shows the voltage (V), the current density (mA / cm 2 ), the CIE chromaticity coordinates (x, y), the current efficiency (cd / A), the efficiency (Im / W) and the external quantum efficiency (%) of the light-emitting element 4 at a luminance of about 1,100 cd / m 2 represents. [Table 4] Voltage (V) Current density (mA / cm 2 ) Chromaticity (x, y) Current efficiency (cd / A) Efficiency (Im / W) External quantum efficiency (%) Light-emitting element 4 3,0 1,4 (0,57, 0,43) 76 70 31

[0181] Fig. Figure 13 shows an emission spectrum of the light-emitting element 4 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 4, the CIE chromaticity coordinates of the light-emitting element 4 were (x, y) = (0.57, 0.43) at a luminance of 1,100 cd / m 2 The results show that the light-emitting element 4 emits light originating from [Ir(dppm)2(acac)].

[0182] As can be seen from Table 4 and Fig. As can be seen from Figures 9 to 12, the light-emitting element 4 has a high current yield, a high efficiency, and a high external quantum efficiency. In particular, the light-emitting element 4 has an extremely high external quantum efficiency at a luminance of 1,100 cd / m 2 which is 31%. As described above, the limit for external quantum efficiency is approximately 25%. However, the result lies beyond this limit.

[0183] In the light-emitting element of this example, the light-emitting layer includes the host material and the guest material described in Example 1. As described in Example 1, the absorption spectrum of the guest material included in the light-emitting element 4 overlaps the emission spectrum of the host material. The light-emitting element of this example is considered to have high energy transfer efficiency and external quantum efficiency because energy is transferred using the overlap.

[0184] Furthermore, according to the results in Example 1, in the absorption band on the longest wavelength side in the absorption spectrum of the guest material included in the light-emitting element 4, the maximum wavelength thereof is close to the maximum wavelength of the emission spectrum (the difference between the maxima is 0.02 eV), and the molar absorption coefficient at the maximum wavelength is high (> 5,000 M -1 ·cm -1 ). Consequently, it is shown that the particularly high energy transfer efficiency of the light-emitting element 4 results in the new high external quantum efficiency.

[0185] The above results show that the application of an embodiment of the present invention makes it possible to obtain a light-emitting element with a high external quantum efficiency.

[0186] Next, reliability tests of the light-emitting element 4 were performed. Fig. 14 shows the results of the reliability tests. In Fig. 14, the vertical axis represents the normalized luminance (%) assuming an initial luminance of 100%, and the horizontal axis represents the driving time (h) of the element.

[0187] In the reliability tests, the light-emitting element 4 was driven under conditions where the initial luminance was set to 5,000 cd / m 2 was set and the current density was constant.

[0188] After driving for 170 hours, the light-emitting element 4 retained 95% of the initial luminance.

[0189] The above results show that the application of an embodiment of the present invention makes it possible to obtain a light-emitting element with high reliability. [Example 4]

[0190] Example 4 presents examples of a guest material and a host material that can be included in the light-emitting element of an embodiment of the present invention, with reference to Fig. 17A and Fig. 17B.

[0191] The guest material used in this example is [Ir(dppm)2(acac)]. The host material also used in this example is a mixture of 2mDBTPDBq-II and 4,4'-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB). The following shows a chemical formula of a material used in this example. Note that the chemical formulas of the materials used in the above examples are omitted here. (Absorption spectrum)

[0192] Fig. 17A and Fig.Figure 17B shows an ultraviolet-visible absorption spectrum (absorption spectrum a) of [Ir(dppm)2(acac)] in a dichloromethane solution of [Ir(dppm)2(acac)]. Absorption spectrum measurements were performed at room temperature using a UV / Vis spectrophotometer (Type V550, manufactured by JASCO Corporation) with the dichloromethane solution (0.093 mmol / L) in a quartz cuvette. (emission spectrum)

[0193] Fig. 17A and Fig. 17B also shows an emission spectrum (emission spectrum a) of a thin film of a material mixture of 2mDBTPDBq-II and NPB. In Fig. 17A, the horizontal axis represents the wavelength (nm), and the vertical axes represent the molar absorption coefficient ε (M -1 ·cm -1 ) and the emission intensity (arbitrary unit). In Fig. 17B, the horizontal axis represents the energy (eV), and the vertical axes represent the molar absorption coefficient ε (M-1 ·cm -1 ) and the emission intensity (arbitrary unit).

[0194] The absorption spectrum a in Fig. Figure 17A shows that [Ir(dppm)2(acac)] exhibits a broad absorption band around 520 nm. This absorption band is considered to contribute significantly to light emission.

[0195] It is found that the maximum of the emission spectrum a and the absorption band in the absorption spectrum a, which is expected to contribute significantly to light emission, have a strong overlap. Specifically, the difference between the maximum (515 nm) of the absorption band in the absorption spectrum a and the maximum of the emission spectrum a is 0.09 eV. Thus, it is demonstrated that a light-emitting element including a light-emitting layer using the guest material and the host material of this example together has high energy transfer efficiency because energy is transferred using the overlap of the emission spectrum of the host material and the absorption spectrum of the guest material. Therefore, it is demonstrated that a light-emitting element with high external quantum efficiency can be achieved. [Example 5]

[0196] Example 5 illustrates a light-emitting element of an embodiment of the present invention with reference to Fig. 2. The following shows a chemical formula of a material used in this example. Note that chemical formulas of the materials used in the above examples are omitted here.

[0197] The following describes a method for manufacturing a light-emitting element 5 of this example. (Light-emitting element 5)

[0198] First, an ITSO film was sputtered over the glass substrate 1100 to form the first electrode 1101, which served as the anode. The thickness was 110 nm, and the electrode area was 2 mm × 2 mm.

[0199] Next, as a pretreatment for forming the light-emitting element over the substrate 1100, the surface of the substrate was washed with water, heated at 200 °C for one hour, and subjected to UV ozone treatment for 370 seconds.

[0200] Subsequently, the substrate 1100 was transferred to a vacuum deposition device in which the pressure was increased to about 10 -4 Pa, and subjected to vacuum heating 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.

[0201] Next, the substrate 1100 provided with the first electrode 1101 was attached to a substrate holder in a vacuum evaporation apparatus so that a surface on which the first electrode 1101 was provided was directed downward. The pressure in the vacuum evaporation apparatus was set to about 10 -4Pa. Then, 4,4',4"-(1,3,5-benzenetriyl)tri(dibenzothiophene) (abbreviation: DBT3P-II) and molybdenum(VI) oxide were co-evaporated over the first electrode 1101 to form the hole-injection layer 1111. The thickness of the hole-injection layer 1111 was 40 nm, and the weight ratio of DBT3P-II to molybdenum oxide was set to 4:2 (= DBT3P-II:molybdenum oxide).

[0202] Next, a BPAFLP film was formed to a thickness of 20 nm over the hole injection layer 1111, so that the hole transport layer 1112 was formed.

[0203] Furthermore, 2mDBTPDBq-II, NPB, and [Ir(dppm)2(acac)] were co-evaporated to form the light-emitting layer 1113 over the hole-transport layer 1112. The weight ratio of 2mDBTPDBq-II to NPB and [Ir(dppm)2(acac)] was set to 0.8:0.2:0.05 (= 2mDBTPDBq-II:NPB:[Ir(dppm)2(acac)]). The thickness of the light-emitting layer 1113 was 40 nm.

[0204] Next, a 2mDBTPDBq-II film was formed to a thickness of 10 nm over the light-emitting layer 1113, so that the first electron-transport layer 1114a was formed.

[0205] Next, a BPhen film was formed to a thickness of 20 nm over the first electron transport layer 1114a, so that the second electron transport layer 1114b was formed.

[0206] Furthermore, a LiF film with a thickness of 1 nm was formed by evaporation over the second electron transport layer 1114b, so that the electron injection layer 1115 was formed.

[0207] Finally, an aluminum foil with a thickness of 200 nm was formed by vapor deposition to form the second electrode 1103, which served as the cathode. Thus, the light-emitting element 5 of this example was fabricated.

[0208] It should be noted that in the above evaporation process, all evaporation was carried out by a resistance heating method.

[0209] Table 5 shows element structures of the light-emitting element 5 obtained in this way. [Table 5] 1. Electrode Hole injection layer Lochtran sports layer Light-emitting layer 1. Electron transport layer 2. Electron transport layer Electron injection layer 2. Electrode Light-emitting element 5 ITSO 110 nm DBT3P-II: MoOx (= 4:2) 40 nm BPAFLP 20 nm 2mDBTPDBq-II: NPB: [Ir(dppm)2(acac)] (= 0.8:0.2:0.05) 40 nm 2mDBTPDBq-II 10 nm BPhen 20 nm LiF 1 nm Al 200 nm

[0210] The light-emitting element 5 was enclosed in a glove box containing a nitrogen atmosphere to prevent exposure to air. The operating characteristics of the light-emitting element were then measured. Note that the measurements were performed at room temperature (in an atmosphere maintained at 25°C).

[0211] Fig. 18 shows a current density-luminance characteristic curve of the light-emitting element 5. In Fig. 18 the horizontal axis represents the current density (mA / cm 2 ), and the vertical axis represents the luminance (cd / m 2). Fig. 19 shows a voltage-luminance characteristic curve. In Fig. 19, the horizontal axis represents the voltage (V) and the vertical axis represents the luminance (cd / m 2 ). Fig. 20 shows a luminance-current efficiency curve. In Fig. 20 the horizontal axis represents the luminance (cd / m 2 ) and the vertical axis represents the current efficiency (cd / A). Fig. 21 shows a characteristic curve of luminance versus external quantum efficiency. In Fig. 21 the horizontal axis represents the luminance (cd / m 2 ), and the vertical axis represents the external quantum efficiency (%).

[0212] Furthermore, Table 6 shows the voltage (V), the current density (mA / cm 2 ), the CIE chromaticity coordinates (x, y), the current efficiency (cd / A), the efficiency (Im / W) and the external quantum efficiency (%) of the light-emitting element 5 at a luminance of about 1,100 cd / m2 represents. [Table 6] Voltage (V) Current density (mA / cm 2 ) Chromaticity (x, y) Current efficiency (cd / A) Efficiency (Im / W) External quantum efficiency (%) Light-emitting element 5 2,9 1,5 (0,57, 0,43) 75 81 29

[0213] Fig. Figure 22 shows an emission spectrum of the light-emitting element 5 obtained by applying a current of 0.1 mA. Fig. 22, the horizontal axis represents the wavelength (nm), and the vertical axis represents the emission intensity (arbitrary unit). As shown in Table 6, the CIE chromaticity coordinates of the light-emitting element 5 were (x, y) = (0.57, 0.43) at a luminance of 1,100 cd / m 2 The results show that the light-emitting element 5 emits light originating from [Ir(dppm)2(acac)].

[0214] As can be seen from Table 6 and Fig. As can be seen from Figures 18 to 21, the light-emitting element 5 has a high current yield, a high efficiency and a high external quantum efficiency.

[0215] In the light-emitting element 5, the light-emitting layer includes 2mDBTPDBq-II, NPB, and [Ir(dppm)2(acac)] described in Example 4. As described in Example 4, the emission spectrum of the material mixture of 2mDBTPDBq-II and NPB and the absorption band largely overlap, which is expected to significantly contribute to light emission in the absorption spectrum of [Ir(dppm)2(acac)]. It is considered that the light-emitting element 5 has high energy transfer efficiency and external quantum efficiency because energy is transferred using the overlap.

[0216] Furthermore, according to the results in Example 4, in the absorption band on the longest wavelength side in the absorption spectrum of the guest material included in the light-emitting element 5, the maximum thereof is close to the maximum of the emission spectrum, and the molar absorption coefficient at the maximum is high (> 5,000 M -1 ·cm -1 ). Consequently, it is shown that the particularly high energy transfer efficiency of the light-emitting element 5 results in the new high external quantum efficiency.

[0217] The above results show that the application of an embodiment of the present invention makes it possible to obtain a light-emitting element with a high external quantum efficiency. [Example 6]

[0218] Example 6 presents examples of a guest material and a host material that can be included in the light-emitting element of an embodiment of the present invention, with reference to Fig. 23A and Fig. 23B.

[0219] The guest material used in this example is bis(2,3,5-triphenylpyrazinato)(dipivaloylmethanato)iridium(III) (abbreviation: [Ir(tppr)2(dpm)]). The host material also used in this example is a mixture of 2mDBTPDBq-II and NPB. The following shows a chemical formula of a material used in this example. Note that the chemical formulas of the materials used in the above examples are omitted here. (Absorption spectrum)

[0220] Fig. 23A and Fig.Figure 23B shows an ultraviolet-visible absorption spectrum (absorption spectrum b) of [Ir(tppr)2(dpm)] in a dichloromethane solution of [Ir(tppr)2(dpm)]. Absorption spectrum measurements were performed at room temperature using a UV / Vis spectrophotometer (Type V550, manufactured by JASCO Corporation) with the dichloromethane solution (0.094 mmol / L) in a quartz cuvette. (emission spectrum)

[0221] Fig. 23A and Fig. 23B also show an emission spectrum (emission spectrum b) of a thin film of a material mixture of 2mDBTPDBq-II and NPB. In Fig. 23A, the horizontal axis represents the wavelength (nm), and the vertical axes represent the molar absorption coefficient ε (M -1 ·cm -1 ) and the emission intensity (arbitrary unit). In Fig. 23 B, the horizontal axis represents the energy (eV), and the vertical axes represent the molar absorption coefficient ε (M-1 ·cm -1 ) and the emission intensity (arbitrary unit).

[0222] The absorption spectrum b in Fig. Figure 23A shows that [Ir(tppr)2(dpm)] exhibits a broad absorption band around 530 nm. This absorption band is considered to contribute significantly to light emission.

[0223] It is found that the maximum of the emission spectrum b and the absorption band in the absorption spectrum b, which is expected to contribute significantly to light emission, have a strong overlap. Specifically, the difference between the maximum (peak at about 530 nm) of the absorption band in the absorption spectrum b and the maximum of the emission spectrum b is 0.01 eV. Thus, it is demonstrated that a light-emitting element including a light-emitting layer using the guest material and the host material of this example together has high energy transfer efficiency because energy is transferred using the overlap of the emission spectrum of the host material and the absorption spectrum of the guest material. Therefore, it is demonstrated that a light-emitting element with high external quantum efficiency can be achieved. [Example 7]

[0224] Example 7 illustrates a light-emitting element of an embodiment of the present invention with reference to Fig. 2. The materials used in this example are used in the above examples, and therefore their chemical formulas are omitted here.

[0225] A method for manufacturing a light-emitting element 6 of this example will be described below. (Light-emitting element 6)

[0226] The light-emitting layer 1113 of the light-emitting element 6 was formed by co-evaporation of 2mDBTPDBq-II, NPB, and [Ir(tppr)2(dpm)]. The weight ratio of 2mDBTPDBq-II to NPB and [Ir(tppr)2(dpm)] was set to 0.8:0.2:0.05 (= 2mDBTPDBq-II:NPB:[Ir(tppr)2(dpm)]). The thickness of the light-emitting layer 1113 was 40 nm. Except for the light-emitting layer 1113, the light-emitting element 6 was fabricated in a manner similar to the light-emitting element 5 described in Example 5.

[0227] Table 7 shows element structures of the light-emitting element 6 obtained in this way. [Table 7] 1. Electrode Hole injection layer Hole transport layer Light-emitting layer 1. Electron transport layer 2. Electron transport layer Electron injection layer 2. Electrode Light-emitting element 6 ITSO 110 nm DBT3P-II:MoOx (= 4:2) 40 nm BPAFL P 20 nm 2mDBTPDBq-lI: NPB: [Ir(tppr)2(dpm)] (= 0.8:0.2:0.05) 40 nm 2mDBTPDBq-II 10 nm BPhen 20 nm LiF 1 nm Al 200 nm

[0228] The light-emitting element 6 was enclosed in a glove box containing a nitrogen atmosphere to prevent exposure to air. The operating characteristics of the light-emitting element were then measured. Note that the measurements were performed at room temperature (in an atmosphere maintained at 25°C).

[0229] Fig. 24 shows a current density-luminance characteristic curve of the light-emitting element 6. In Fig. 24 the horizontal axis represents the current density (mA / cm 2 ), and the vertical axis represents the luminance (cd / m 2 ). Fig. 25 represents a voltage-luminance characteristic curve. In Fig. 25, the horizontal axis represents the voltage (V) and the vertical axis represents the luminance (cd / m 2 ). Fig. 26 shows a luminance-current efficiency curve. In Fig.26 the horizontal axis represents the luminance (cd / m 2 ) and the vertical axis represents the current efficiency (cd / A). Fig. 27 shows a characteristic curve of luminance versus external quantum efficiency. In Fig. 27 the horizontal axis represents the luminance (cd / m 2 ), and the vertical axis represents the external quantum efficiency (%).

[0230] Furthermore, Table 8 shows the voltage (V), the current density (mA / cm 2 ), the CIE chromaticity coordinates (x, y), the current efficiency (cd / A), the efficiency (Im / W) and the external quantum efficiency (%) of the light-emitting element 6 at a luminance of about 1,100 cd / m 2 represents. [Table 8] Voltage (V) Current density (mA / cm 2 ) Chromaticity (x, y) Current efficiency (cd / A) Efficiency (Im / W) External quantum efficiency (%) Light-emitting element 6 3,3 4,1 (0,66, 0,34) 26 25 22

[0231] Fig. Figure 28 shows an emission spectrum of the light-emitting element 6 obtained by applying a current of 0.1 mA. Fig.28, the horizontal axis represents the wavelength (nm), and the vertical axis represents the emission intensity (arbitrary unit). As shown in Table 8, the CIE chromaticity coordinates of the light-emitting element 6 were (x, y) = (0.66, 0.34) at a luminance of 1,100 cd / m 2 The results show that the light-emitting element 6 emits light originating from [Ir(tppr)2(dpm)].

[0232] As can be seen from Table 8 and Fig. As can be seen from Figures 24 to 27, the light-emitting element 6 has a high current yield, a high efficiency and a high external quantum efficiency.

[0233] In the light-emitting element 6, the light-emitting layer includes 2mDBTPDBq-II, NPB, and [Ir(tppr)2(dpm)] described in Example 6. As described in Example 6, the emission spectrum of the material mixture of 2mDBTPDBq-II and NPB and the absorption band largely overlap, which is expected to significantly contribute to light emission in the absorption spectrum of [Ir(tppr)2(dpm)]. It is considered that the light-emitting element 6 has high energy transfer efficiency and external quantum efficiency because energy is transferred using the overlap.

[0234] The above results show that the application of an embodiment of the present invention makes it possible to obtain a light-emitting element with a high external quantum efficiency.

[0235] Next, reliability tests of the light-emitting element 6 were performed. Fig.29 presents the results of the reliability tests. In Fig. 29, the vertical axis represents the normalized luminance (%) assuming an initial luminance of 100%, and the horizontal axis represents the driving time (h) of the element.

[0236] In the reliability tests, the light-emitting element 6 was driven under conditions where the initial luminance was set to 5,000 cd / m 2 was set and the current density was constant.

[0237] After 98 hours of operation, light-emitting element 6 retained 87% of its initial luminance. These results demonstrate that light-emitting element 6 has a long service life.

[0238] The above results show that the application of an embodiment of the present invention makes it possible to obtain a light-emitting element with high reliability. [Example 8]

[0239] Example 8 provides examples of a guest material and host materials that can be included in the light-emitting element of an embodiment of the present invention, with reference to Fig. 30A and Fig. 30B.

[0240] The guest material used in this example is [Ir(mppm)2(acac)]. The host materials also used in this example are the following two types: a mixture of 2mDBTPDBq-II and PCBA1BP, and a mixture of 2mDBTPDBq-II and 4-(1-naphthyl)-4'-phenyltriphenylamine (abbreviation: αNBA1BP). The following shows a chemical formula of a material used in this example. Note that the chemical formulas of the materials used in the above examples are omitted here. (Absorption spectrum)

[0241] Fig. 30A and Fig.Figure 30B shows an ultraviolet-visible absorption spectrum (absorption spectrum c) of [Ir(mppm)2(acac)] in a dichloromethane solution of [Ir(mppm)2(acac)]. Absorption spectrum measurements were performed at room temperature using a UV / Vis spectrophotometer (Type V550, manufactured by JASCO Corporation) with the dichloromethane solution (0.10 mmol / L) in a quartz cuvette. (emission spectra)

[0242] Fig. 30A and Fig. 30B also shows an emission spectrum (emission spectrum c-1) of a thin film of a material mixture of 2mDBTPDBq-II and PCBA1BP and an emission spectrum (emission spectrum c-2) of a thin film of a material mixture of 2mDBTPDBq-II and αNBA1BP. In Fig. 30A, the horizontal axis represents the wavelength (nm), and the vertical axes represent the molar absorption coefficient ε (M -1 ·cm -1 ) and the emission intensity (arbitrary unit). In Fig.30 B, the horizontal axis represents the energy (eV), and the vertical axes represent the molar absorption coefficient ε (M -1 ·cm -1 ) and the emission intensity (arbitrary unit).

[0243] The absorption spectrum c in Fig. Figure 30A shows that [Ir(mppm)2(acac)] exhibits a broad absorption band around 490 nm. This absorption band is considered to contribute significantly to light emission.

[0244] It is found that each of the peaks of the emission spectra c-1 and c-2 and the absorption band in the absorption spectrum c, which is expected to contribute significantly to light emission, have a strong overlap. Thus, it is demonstrated that a light-emitting element including a light-emitting layer using the guest material and one of the host materials of this example together has high energy transfer efficiency because energy is transferred using the overlap of the emission spectrum of the host material and the absorption spectrum of the guest material. Therefore, it is demonstrated that a light-emitting element with high external quantum efficiency can be achieved.

[0245] Here, the emission spectrum c-2 has a peak on a shorter wavelength side (higher energy side) than the emission spectrum c-1. The peak of the emission spectrum c-2 is closer to the above absorption band than the peak of the emission spectrum c-1. Specifically, the difference between the peak of the absorption band in the absorption spectrum c (peaking at about 490 nm) and the peak of the emission spectrum c-1 is 0.15 eV, and the difference between the peak of the absorption band in the absorption spectrum c (peaking at about 490 nm) and the peak of the emission spectrum c-2 is 0.01 eV.

[0246] The difference between the maxima of the emission spectra c-1 and c-2 is believed to be due to the difference between the HOMO levels of PCBA1BP and αNBA1BP. Specifically, the HOMO level of PCBA1BP is -5.43 eV, whereas the HOMO level of αNBA1BP is -5.52 eV (both values were calculated by cyclic voltammetry (CV) measurements). Since αNBA1BP has a lower HOMO level than PCBA1BP, the peak of the emission spectrum c-2 is believed to be located at a shorter wavelength (higher energy) side than the peak of the emission spectrum c-1. [Example 9]

[0247] Example 9 presents a light-emitting element of an embodiment of the present invention with reference to Fig. 2. The materials used in this example are used in the above examples, and therefore their chemical formulas are omitted here.

[0248] The following describes a method for manufacturing light-emitting elements 7 and 8 of this example. (Light-emitting element 7)

[0249] First, an ITSO film was sputtered over the glass substrate 1100 to form the first electrode 1101, which served as the anode. The thickness was 110 nm, and the electrode area was 2 mm × 2 mm.

[0250] Next, as a pretreatment for forming the light-emitting element over the substrate 1100, the surface of the substrate was washed with water, heated at 200 °C for one hour, and subjected to UV ozone treatment for 370 seconds.

[0251] Subsequently, the substrate 1100 was transferred to a vacuum deposition device in which the pressure was increased to about 10 -4Pa, and subjected to vacuum heating 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.

[0252] Next, the substrate 1100 provided with the first electrode 1101 was attached to a substrate holder in a vacuum evaporation apparatus so that a surface on which the first electrode 1101 was provided was directed downward. The pressure in the vacuum evaporation apparatus was set to about 10 -4 Pa. Then, BPAFLP and molybdenum(VI) oxide were co-evaporated over the first electrode 1101 to form the hole-injection layer 1111. The thickness of the hole-injection layer 1111 was 40 nm, and the weight ratio of BPAFLP to molybdenum oxide was set to 4:2 (= BPAFLP:molybdenum oxide).

[0253] Next, a BPAFLP film was formed to a thickness of 20 nm over the hole injection layer 1111, so that the hole transport layer 1112 was formed.

[0254] Furthermore, 2mDBTPDBq-II, PCBA1BP, and [Ir(mppm)2(acac)] were co-evaporated to form the light-emitting layer 1113 over the hole-transport layer 1112. The weight ratio of 2mDBTPDBq-II to PCBA1BP and [Ir(mppm)2(acac)] was set to 0.8:0.2:0.05 (= 2mDBTPDBq-II:PCBA1BP:[Ir(mppm)2(acac)]). The thickness of the light-emitting layer 1113 was 40 nm.

[0255] Next, a 2mDBTPDBq-II film was formed to a thickness of 10 nm over the light-emitting layer 1113, so that the first electron-transport layer 1114a was formed.

[0256] Next, a BPhen film was formed to a thickness of 20 nm over the first electron transport layer 1114a, so that the second electron transport layer 1114b was formed.

[0257] Furthermore, a LiF film with a thickness of 1 nm was formed by evaporation over the second electron transport layer 1114b, so that the electron injection layer 1115 was formed.

[0258] Finally, an aluminum foil with a thickness of 200 nm was formed by evaporation to form the second electrode 1103, which served as the cathode. Thus, the light-emitting element 7 of this example was fabricated. (Light-emitting element 8)

[0259] The light-emitting layer 1113 of the light-emitting element 8 was formed by co-evaporating 2mDBTPDBq-II, αNBA1BP, and [Ir(mppm)2(acac)]. The weight ratio of 2mDBTPDBq-II to αNBA1BP and [Ir(mppm)2(acac)] was set to 0.8:0.2:0.05 (= 2mDBTPDBq-II:αNBA1BP:[Ir(mppm)2(acac)]). The thickness of the light-emitting layer 1113 was 40 nm. The layers other than the light-emitting layer 1113 were formed in the same manner as those of the light-emitting element 7.

[0260] It should be noted that in the above evaporation process, all evaporation was carried out by a resistance heating method.

[0261] Table 9 shows element structures of the light-emitting elements 7 and 8 obtained in this way. [Table 9] 1. Electrode Hole injection layer Lochtran sports layer Light-emitting layer 1. Electron transport layer 2. Electron transport layer Electron injection layer 2. Electrode Light-emitting element 7 ITSO 110 nm BPAFLP: MoOx (= 4:2) 40 nm BPAFLP 20 nm 2mDBTPDBq-II: PCBA1BP: [Ir(mppm)2(acac)] (= 0.8:0.2:0.05) 40 nm 2mDBTPDBq-II 10 nm BPhen 20 nm LiF 1 nm Al 200 nm Light-emitting element 8 ITSO 110 nm BPAFLP: MoOx (= 4:2) 40 nm BPAFLP 20 nm 2mDBTPDBq-II: áNBA1BP: [Ir(mppm)2(acac)] (= 0.8:0.2:0.05) 40 nm 2mDBTPDBq-II 10 nm BPhen 20 nm LiF 1 nm Al 200 nm

[0262] These light-emitting elements were enclosed in a glove box containing a nitrogen atmosphere to prevent exposure to air. The operating characteristics of these light-emitting elements were then measured. Note that the measurements were performed at room temperature (in an atmosphere maintained at 25°C).

[0263] Fig. 31 shows a current density-luminance characteristic curve of the light-emitting elements 7 and 8. In Fig. 31 the horizontal axis represents the current density (mA / cm 2 ), and the vertical axis represents the luminance (cd / m 2 ). Fig. 32 represents a voltage-luminance characteristic curve. In Fig. 32, the horizontal axis represents the voltage (V) and the vertical axis represents the luminance (cd / m 2 ). Fig. 33 shows a luminance-current efficiency curve. In Fig.33 the horizontal axis represents the luminance (cd / m 2 ) and the vertical axis represents the current efficiency (cd / A). Fig. 34 shows a characteristic curve of luminance versus external quantum efficiency. In Fig. 34 the horizontal axis represents the luminance (cd / m 2 ), and the vertical axis represents the external quantum efficiency (%).

[0264] Furthermore, Table 10 shows the voltage (V), the current density (mA / cm 2 ), the CIE chromaticity coordinates (x, y), the current efficiency (cd / A), the efficiency (Im / W) and the external quantum efficiency (%) of the light-emitting elements 7 and 8 at a luminance of about 1,000 cd / m 2 represents. [Table 10] Voltage (V) Current density (mA / cm 2 ) Chromaticity (x, y) Luminance (cd / m 2 ) Current efficiency (cd / A) Efficiency (Im / W) External quantum efficiency (%) Light-emitting element 7 3,2 1,6 (0,43, 0,56) 1.100 69 68 20 Light-emitting element 8 3,0 1,1 (0,43, 0,56) 860 75 79 21

[0265] Fig. Figure 35 shows emission spectra of the light-emitting elements 7 and 8 obtained by applying a current of 0.1 mA. Fig.35, the horizontal axis represents the wavelength (nm), and the vertical axis represents the emission intensity (arbitrary unit). As shown in Table 10, the CIE chromaticity coordinates of the light-emitting element 7 were (x, y) = (0.43, 0.56) at a luminance of 1,100 cd / m 2 , and the CIE chromaticity coordinates of the light-emitting element 8 were (x, y) = (0.43, 0.56) at a luminance of 860 cd / m 2 The results show that the light-emitting elements 7 and 8 emit a yellow-green light originating from [Ir(mppm)2(acac)].

[0266] As can be seen from Table 10 and Fig. 31 to 34, each of the light-emitting elements 7 and 8 has a high current efficiency, a high efficiency and a high external quantum efficiency.

[0267] The light-emitting layer of the light-emitting element 7 includes PCBA1BP, 2mDBTPDBq-II, and [Ir(mppm)2(acac)] described in Example 8, and the light-emitting layer of the light-emitting element 8 includes αNBA1BP, 2mDBTPDBq-II, and [Ir(mppm)2(acac)] described in Example 8. As described in Example 8, the emission spectra of the material mixture of 2mDBTPDBq-II and PCBA1BP and the material mixture of 2mDBTPDBq-II and αNBA1BP have strong overlaps with the absorption band in the absorption spectrum of [Ir(mppm)2(acac)], which is considered to contribute significantly to light emission. It is considered that the light-emitting elements 7 and 8 have high energy transfer efficiency and external quantum efficiency because energies are transferred using the overlaps.

[0268] The above results show that the application of an embodiment of the present invention makes it possible to obtain a light-emitting element with a high external quantum efficiency.

[0269] Next, reliability tests of the light-emitting elements 7 and 8 were performed. Fig. 36 presents the results of the reliability tests. In Fig. 36, the vertical axis represents the normalized luminance (%) assuming an initial luminance of 100%, and the horizontal axis represents the driving time (h) of the elements.

[0270] In the reliability tests, the light-emitting elements 7 and 8 were each driven under the conditions where the initial luminance was set to 5,000 cd / m 2 was set and the current density was constant.

[0271] After driving for 260 hours, light-emitting element 7 retained 74% of its initial luminance. After driving for 260 hours, light-emitting element 8 retained 75% of its initial luminance. The results demonstrate that light-emitting elements 7 and 8 have a long lifetime.

[0272] The above results show that the application of an embodiment of the present invention makes it possible to obtain a light-emitting element with high reliability. [Example 10]

[0273] Example 10 provides examples of a guest material and host materials that can be included in the light-emitting element of an embodiment of the present invention, with reference to Fig. 37A and Fig. 37B.

[0274] The guest material used in this example is (acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III) (abbreviation: [Ir(tBuppm)2(acac)]). The host materials also used in this example are the following two types: a material mixture of 2mDBTPDBq-II and NPB, and a material mixture of 2mDBTPDBq-II and 2,7-bis[N-(diphenylaminophenyl)-N-phenylamino]-spiro-9,9'-bifluorene (abbreviation: DPA2SF). Chemical formulas of materials used in this example are shown below. Note that chemical formulas of the materials used in the above examples are omitted here. (Absorption spectrum)

[0275] Fig. 37A and Fig.Figure 37B shows an ultraviolet-visible absorption spectrum (absorption spectrum d) of [Ir(tBuppm)2(acac)] in a dichloromethane solution of [Ir(tBuppm)2(acac)]. Absorption spectrum measurements were performed at room temperature using a UV / Vis spectrophotometer (Type V550, manufactured by JASCO Corporation) with the dichloromethane solution (0.093 mmol / L) in a quartz cuvette. (emission spectra)

[0276] Fig. 37A and Fig. 37B also show an emission spectrum (emission spectrum d-1) of a thin film of a material mixture of 2mDBTPDBq-II and DPA2SF and an emission spectrum (emission spectrum d-2) of a thin film of a material mixture of 2mDBTPDBq-II and NPB. In Fig. 37A, the horizontal axis represents the wavelength (nm), and the vertical axes represent the molar absorption coefficient ε (M -1 ·cm -1 ) and the emission intensity (arbitrary unit). In Fig.37 B, the horizontal axis represents the energy (eV), and the vertical axes represent the molar absorption coefficient ε (M -1 ·cm -1 ) and the emission intensity (arbitrary unit).

[0277] The absorption spectrum d in Fig. Figure 37A shows that [Ir(tBuppm)2(acac)] exhibits a broad absorption band around 490 nm. This absorption band is considered to contribute significantly to light emission.

[0278] It is found that each of the peaks of the emission spectra d-1 and d-2 and the absorption band in the absorption spectrum d, which is expected to contribute significantly to light emission, have a strong overlap. Thus, it is demonstrated that a light-emitting element including a light-emitting layer using the guest material and one of the host materials of this example together has high energy transfer efficiency because energy is transferred using the overlap of the emission spectrum of the host material and the absorption spectrum of the guest material. Therefore, it is demonstrated that a light-emitting element with high external quantum efficiency can be achieved.

[0279] Here, the emission spectrum d-2 has a maximum at a shorter wavelength side (higher energy side) than the emission spectrum d-1. The maximum of the emission spectrum d-2 is closer to the above absorption band than the maximum of the emission spectrum d-1. From the above in Fig. 37A and Fig. 37B shows that the emission spectrum that has the strongest overlap with the absorption band in the absorption spectrum d, which contributes significantly to light emission, is the emission spectrum d-2. Specifically, the difference between the maximum of the absorption band in the absorption spectrum d and the maximum of the emission spectrum d-1 is 0.39 eV, and the difference between the maximum of the absorption band in the absorption spectrum d and the maximum of the emission spectrum d-2 is 0.19 eV.

[0280] The difference between the maxima of the emission spectra d-1 and d-2 is believed to be due to the difference between the HOMO levels of DPA2SF and NPB. Specifically, the HOMO level of DPA2SF is -5.09 eV, whereas the HOMO level of NPB is -5.38 eV (both values were calculated by CV measurements). It is believed that since NPB has a lower HOMO level than DPA2SF, the maximum of the emission spectrum d-2 is located at a shorter wavelength (higher energy) side than the maximum of the emission spectrum d-1.

[0281] From the above, it can be seen that a light-emitting element including a light-emitting layer using a material mixture of 2mDBTPDBq-II and NPB and [Ir(tBuppm)2(acac)] exhibits high energy transfer efficiency compared to a light-emitting element including a light-emitting layer using a material mixture of 2mDBTPDBq-II and DPA2SF and [Ir(tBuppm)2(acac)], because energy is transferred using the strong overlap of the emission spectrum of the host material and the absorption spectrum of the guest material. Therefore, it is demonstrated that a light-emitting element with a higher external quantum efficiency can be achieved. [Example 11]

[0282] Example 11 presents a light-emitting element of an embodiment of the present invention with reference to Fig.2. The materials used in this example are used in the above examples, and therefore their chemical formulas are omitted here.

[0283] The following describes a method for manufacturing light-emitting elements 9 and 10 of this example. (Light-emitting element 9)

[0284] First, an ITSO film was sputtered over the glass substrate 1100 to form the first electrode 1101, which served as the anode. The thickness was 110 nm, and the electrode area was 2 mm × 2 mm.

[0285] Next, as a pretreatment for forming the light-emitting element over the substrate 1100, the surface of the substrate was washed with water, heated at 200 °C for one hour, and subjected to UV ozone treatment for 370 seconds.

[0286] Subsequently, the substrate 1100 was transferred to a vacuum deposition device in which the pressure was increased to about 10 -4 Pa, and subjected to vacuum heating 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.

[0287] Next, the substrate 1100 provided with the first electrode 1101 was attached to a substrate holder in a vacuum evaporation apparatus so that a surface on which the first electrode 1101 was provided was directed downward. The pressure in the vacuum evaporation apparatus was set to about 10 -4Pa. Then, DBT3P-II and molybdenum(VI) oxide were co-evaporated over the first electrode 1101 to form the hole-injection layer 1111. The thickness of the hole-injection layer 1111 was 40 nm, and the weight ratio of DBT3P-II to molybdenum oxide was set to 4:2 (= DBT3P-II:molybdenum oxide).

[0288] Next, a BPAFLP film was formed to a thickness of 20 nm over the hole injection layer 1111, so that the hole transport layer 1112 was formed.

[0289] Furthermore, 2mDBTPDBq-II, DPA2SF, and [Ir(tBuppm)2(acac)] were co-evaporated to form the light-emitting layer 1113 over the hole-transport layer 1112. The weight ratio of 2mDBTPDBq-II to DPA2SF and [Ir(tBuppm)2(acac)] was set to 0.8:0.2:0.05 (= 2mDBTPDBq-II:DPA2SF:[Ir(tBuppm)2(acac)]). The thickness of the light-emitting layer 1113 was 40 nm.

[0290] Next, a 2mDBTPDBq-II film was formed to a thickness of 10 nm over the light-emitting layer 1113, so that the first electron-transport layer 1114a was formed.

[0291] Next, a BPhen film was formed to a thickness of 20 nm over the first electron transport layer 1114a, so that the second electron transport layer 1114b was formed.

[0292] Furthermore, a LiF film with a thickness of 1 nm was formed by evaporation over the second electron transport layer 1114b, so that the electron injection layer 1115 was formed.

[0293] Finally, an aluminum foil with a thickness of 200 nm was formed by vapor deposition to form the second electrode 1103, which served as the cathode. Thus, the light-emitting element 9 of this example was fabricated. (Light-emitting element 10)

[0294] The light-emitting layer 1113 of the light-emitting element 10 was formed by co-evaporating 2mDBTPDBq-II, NPB, and [Ir(tBuppm)2(acac)]. The weight ratio of 2mDBTPDBq-II to NPB and [Ir(tBuppm)2(acac)] was set to 0.8:0.2:0.05 (= 2mDBTPDBq-II:NPB:[Ir(tBuppm)2(acac)]). The thickness of the light-emitting layer 1113 was 40 nm. The layers other than the light-emitting layer 1113 were formed in the same manner as those of the light-emitting element 9.

[0295] It should be noted that in the above evaporation process, all evaporation was carried out by a resistance heating method.

[0296] Table 11 shows element structures of the light-emitting elements 9 and 10 obtained in this way. [Table 11] 1. Electrode Hole injection layer Lochtran sports layer Light-emitting layer 1. Electron transport layer 2. Electron transport layer Electron injection layer 2. Electrode Light-emitting element 9 ITSO 110 nm DBT3P-II: MoOx (= 4:2) 40 nm BPAFLP 20 nm 2mDBTPDBq-II: DPA2SF: [Ir(tBuppm)2(acac)] (= 0.8:0.2:0.05) 40 nm 2mDBTPDBq-II 10 nm BPhen 20 nm LiF 1 nm Al 200 nm Light-emitting element 10 ITSO 110 nm DBT3P-II: MoOx (= 4:2) 40 nm BPAFLP 20 nm 2mDBTPDBq-II: NPB: [Ir(tBuppm)2(acac)] (= 0.8:0.2:0.05) 40 nm 2mDBTPDBq-II 10 nm BPhen 20 nm LiF 1 nm Al 200 nm

[0297] These light-emitting elements were enclosed in a glove box containing a nitrogen atmosphere to prevent exposure to air. The operating characteristics of these light-emitting elements were then measured. Note that the measurements were performed at room temperature (in an atmosphere maintained at 25°C).

[0298] Fig. 38 shows a current density-luminance characteristic curve of the light-emitting elements 9 and 10. In Fig. 38 the horizontal axis represents the current density (mA / cm 2 ), and the vertical axis represents the luminance (cd / m 2 ). Fig. 39 represents a voltage-luminance characteristic curve. In Fig. 39, the horizontal axis represents the voltage (V) and the vertical axis represents the luminance (cd / m 2 ). Fig. 40 represents a luminance-current efficiency curve. In Fig.40 the horizontal axis represents the luminance (cd / m 2 ) and the vertical axis represents the current efficiency (cd / A). Fig. 41 shows a characteristic curve of luminance versus external quantum efficiency. In Fig. 41 the horizontal axis represents the luminance (cd / m 2 ), and the vertical axis represents the external quantum efficiency (%).

[0299] Furthermore, Table 12 shows the voltage (V), the current density (mA / cm 2 ), the CIE chromaticity coordinates (x, y), the current efficiency (cd / A), the efficiency (Im / W) and the external quantum efficiency (%) of the light-emitting elements 9 and 10 at a luminance of about 1,000 cd / m 2 represents. [Table 12] Voltage (V) Current density (mA / cm 2 ) Chromaticity (x, y) Luminance (cd / m 2 ) Current efficiency (cd / A) Efficiency (Im / W) External quantum efficiency (%) Light-emitting element 9 2,9 1,7 (0,43, 0,56) 890 52 56 15 Light-emitting element 10 2,8 1,0 (0,42, 0,57) 820 84 95 23

[0300] Fig. Figure 42 shows emission spectra of the light-emitting elements 9 and 10 obtained by applying a current of 0.1 mA. Fig.42, the horizontal axis represents the wavelength (nm), and the vertical axis represents the emission intensity (arbitrary unit). As shown in Table 12, the CIE chromaticity coordinates of the light-emitting element 9 were (x, y) = (0.43, 0.56) at a luminance of 890 cd / m 2 , and the CIE chromaticity coordinates of the light-emitting element 10 were (x, y) = (0.42, 0.57) at a luminance of 820 cd / m 2 The results show that the light-emitting elements 9 and 10 emit a yellow-green light originating from [Ir(tBuppm)2(acac)].

[0301] As can be seen from Table 12 and Fig. 38 to 41, each of the light-emitting elements 9 and 10 has a high current efficiency, a high efficiency and a high external quantum efficiency.

[0302] The light-emitting layer of the light-emitting element 9 includes DPA2SF, 2mDBTPDBq-II, and [Ir(tBuppm)2(acac)] described in Example 10, and the light-emitting layer of the light-emitting element 10 includes NPB, 2mDBTPDBq-II, and [Ir(tBuppm)2(acac)] described in Example 10. As described in Example 10, the emission spectra of the material mixture of 2mDBTPDBq-II and DPA2SF and the material mixture of 2mDBTPDBq-II and NPB have strong overlaps with the absorption band in the absorption spectrum of [Ir(tBuppm)2(acac)], which is considered to contribute significantly to light emission. It is considered that the light-emitting elements 9 and 10 have high energy transfer efficiency and external quantum efficiency because energies are transferred using the overlaps.In particular, the emission spectrum of the mixed material of 2mDBTPDBq-II and NPB exhibits a stronger overlap with the absorption band than the emission spectrum of the mixed material of 2mDBTPDBq-II and DPA2SF. Therefore, the light-emitting element 10 is considered to have higher energy transfer efficiency and external quantum efficiency than the light-emitting element 9 because energy is transferred using the stronger overlap. Referring to the results of Example 10, the difference between the energy value of a peak in the emission spectrum of the host material and the energy value of a peak in the absorption band on the lowest energy side in the absorption spectrum of the guest material is preferably 0.3 eV or less.

[0303] The above results show that the application of an embodiment of the present invention makes it possible to obtain a light-emitting element with a high external quantum efficiency. [Example 12]

[0304] Example 12 provides examples of a guest material and host materials that can be included in the light-emitting element of an embodiment of the present invention, with reference to Fig. 43A and Fig. 43B.

[0305] The guest material used in this example is [Ir(mppr-Me)2(dpm)]. The host materials also used in this example are the following two types: a mixed material of 2mDBTPDBq-II and 4,4',4"-tris[N-(1-naphthyl)-N-phenylamino]triphenylamine (abbreviation: 1'-TNATA) and a mixed material of 2-[4-(dibenzothiophen-4-yl)phenyl]-1-phenyl-1H-benzimidazole (abbreviation: DBTBIm-II) and 1'-TNATA. The following shows a chemical formula of a material used in this example. Note that chemical formulas of the materials used in the above examples are omitted here. (Absorption spectrum)

[0306] Fig. 43A and Fig.Figure 43B shows an ultraviolet-visible absorption spectrum (absorption spectrum e) of [Ir(mppr-Me)2(dpm)] in a dichloromethane solution of [Ir(mppr-Me)2(dpm)]. Absorption spectrum measurements were performed at room temperature using a UV / Vis spectrophotometer (Type V550, manufactured by JASCO Corporation) with the dichloromethane solution (0.093 mmol / L) in a quartz cuvette. (emission spectra)

[0307] Fig. 43A and Fig. 43B also show an emission spectrum (emission spectrum e-1) of a thin film of a material mixture of 2mDBTPDBq-II and 1'-TNATA and an emission spectrum (emission spectrum e-2) of a thin film of a material mixture of DBTBlm-II and 1'-TNATA. In Fig. 43A, the horizontal axis represents the wavelength (nm), and the vertical axes represent the molar absorption coefficient ε (M -1 ·cm -1 ) and the emission intensity (arbitrary unit). In Fig.43 B, the horizontal axis represents the energy (eV), and the vertical axes represent the molar absorption coefficient ε (M -1 ·cm -1 ) and the emission intensity (arbitrary unit).

[0308] The absorption spectrum e in Fig. Figure 43A shows that [Ir(mppr-Me)2(dpm)] exhibits a broad absorption band around 520 nm. This absorption band is considered to contribute significantly to light emission.

[0309] It is found that each of the peaks of the emission spectra e-1 and e-2 and the absorption band in the absorption spectrum e, which is expected to contribute significantly to light emission, have a strong overlap. Thus, it is demonstrated that a light-emitting element including a light-emitting layer using the guest material and one of the host materials of this example together has high energy transfer efficiency because energy is transferred using the overlap of the emission spectrum of the host material and the absorption spectrum of the guest material. Therefore, it is demonstrated that a light-emitting element with high external quantum efficiency can be achieved.

[0310] Here, the emission spectrum e-2 has a maximum at a shorter wavelength side (higher energy side) than the emission spectrum e-1. The maximum of the emission spectrum e-2 is closer to the above absorption band than the maximum of the emission spectrum e-1. From the above in Fig. 43A and Fig. 43B shows that the emission spectrum that has the strongest overlap with the absorption band in the absorption spectrum e, which contributes significantly to light emission, is the emission spectrum e-2. Specifically, the difference between the maximum of the absorption band in the absorption spectrum e (top-off at about 520 nm) and the maximum of the emission spectrum e-1 is 0.35 eV, and the difference between the maximum of the absorption band in the absorption spectrum e (top-off at about 520 nm) and the maximum of the emission spectrum e-2 is 0.01 eV.

[0311] The difference between the maxima of the emission spectra e-1 and e-2 is believed to be due to the difference between the LUMO levels of 2mDBTPDBq-II and DBTBIm-II. Specifically, the LUMO level of 2mDBTPDBq-II is -2.95 eV, whereas the LUMO level of DBTBIm-II is -2.52 eV (both values were calculated by CV measurements). It is considered that since DBTBIm-II has a higher (shallower) LUMO level than 2mDBTPDBq-II, even when DBTBIm-II is mixed with 1'-TNATA, which has a high HOMO level, the wavelength side where the maximum of the emission spectrum of the composite material is located is not too long (that is, the maximum of the emission spectrum e-2 is located at a shorter wavelength side than the maximum of the emission spectrum e-1).

[0312] From the above, it can be seen that a light-emitting element comprising a material mixture of DBTBIm-II and 1'-TNATA and [Ir(mppr-Me)2(dpm)] exhibits a higher energy transfer efficiency than a light-emitting element comprising a material mixture of 2mDBTPDBq-II and 1'-TNATA and [Ir(mppr-Me)2(dpm)], because energy is transferred using the strong overlap between the emission spectrum of the material mixture and the absorption spectrum of the phosphorescent compound. Therefore, it is demonstrated that a light-emitting element with a higher external quantum efficiency can be achieved. [Example 13]

[0313] Example 13 illustrates a light-emitting element of an embodiment of the present invention with reference to Fig. 2. The materials used in this example are used in the above examples, and therefore their chemical formulas are omitted here.

[0314] The following describes a method for manufacturing light-emitting elements 11 and 12 of this example. (Light-emitting element 11)

[0315] First, an ITSO film was sputtered over the glass substrate 1100 to form the first electrode 1101, which served as the anode. The thickness was 110 nm, and the electrode area was 2 mm × 2 mm.

[0316] Next, as a pretreatment for forming the light-emitting element over the substrate 1100, the surface of the substrate was washed with water, heated at 200 °C for one hour, and subjected to UV ozone treatment for 370 seconds.

[0317] Subsequently, the substrate 1100 was transferred to a vacuum deposition device in which the pressure was increased to about 10 -4Pa, and subjected to vacuum heating 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.

[0318] Next, the substrate 1100 provided with the first electrode 1101 was attached to a substrate holder in a vacuum evaporation apparatus so that a surface on which the first electrode 1101 was provided was directed downward. The pressure in the vacuum evaporation apparatus was set to about 10 -4 Pa. Then, BPAFLP and molybdenum(VI) oxide were co-evaporated over the first electrode 1101 to form the hole-injection layer 1111. The thickness of the hole-injection layer 1111 was 40 nm, and the weight ratio of BPAFLP to molybdenum oxide was set to 4:2 (= BPAFLP:molybdenum oxide).

[0319] Next, a BPAFLP film was formed to a thickness of 20 nm over the hole injection layer 1111, so that the hole transport layer 1112 was formed.

[0320] Furthermore, 2mDBTPDBq-II, 1'-TNATA, and [Ir(mppr-Me)2(dpm)] were co-evaporated to form the light-emitting layer 1113 over the hole-transport layer 1112. The weight ratio of 2mDBTPDBq-II to 1'-TNATA and [Ir(mppr-Me)2(dpm)] was set to 0.8:0.2:0.05 (= 2mDBTPDBq-II:1'-TNATA:[Ir(mppr-Me)2(dpm)]). The thickness of the light-emitting layer 1113 was 20 nm.

[0321] Next, a 2mDBTPDBq-II film was formed to a thickness of 30 nm over the light-emitting layer 1113, so that the first electron-transport layer 1114a was formed.

[0322] Next, a BPhen film was formed to a thickness of 20 nm over the first electron transport layer 1114a, so that the second electron transport layer 1114b was formed.

[0323] Furthermore, a LiF film with a thickness of 1 nm was formed by evaporation over the second electron transport layer 1114b, so that the electron injection layer 1115 was formed.

[0324] Finally, an aluminum foil with a thickness of 200 nm was formed by vapor deposition to form the second electrode 1103, which served as the cathode. Thus, the light-emitting element 11 of this example was fabricated. (Light-emitting element 12)

[0325] The light-emitting layer 1113 of the light-emitting element 12 was formed by co-evaporation of DBTBIm-II, 1'-TNATA, and [Ir(mppr-Me)2(dpm)]. The weight ratio of DBTBIm-II to 1'-TNATA and [Ir(mppr-Me)2(dpm)] was set to 0.8:0.2:0.05 (= DBTBIm-II:1'-TNATA:[Ir(mppr-Me)2(dpm)]). The thickness of the light-emitting layer 1113 was 20 nm.

[0326] The first electron-transport layer 1114a of the light-emitting element 12 was formed by forming a DBTBIm-II film with a thickness of 30 nm. The layers other than the light-emitting layer 1113 and the first electron-transport layer 1114a were formed in the same manner as those of the light-emitting element 11.

[0327] It should be noted that in the above evaporation process, all evaporation was carried out by a resistance heating method.

[0328] Table 13 shows element structures of the light-emitting elements 11 and 12 obtained in this way. [Table 13] 1. Electrode Hole injection layer Lochtran sports layer Light-emitting layer 1. Electron transport layer 2. Electron transport layer Electron injection layer 2. Electrode Light-emitting element 11 ITSO 110 nm BPAFLP : MoOx (= 4:2) 40 nm BPAFLP 20 nm 2mDBTPDBq-II: 1'-TNATA: [Ir(mppr-Me)2(dpm)] (= 0.8:0.2:0.05) 20 nm 2mDBTPDBq-II 30 nm BPhen 20 nm LiF 1 nm Al 200 nm Light-emitting element 12 ITSO 110 nm BPAFLP : MoOx (= 4:2) 40 nm BPAFLP 20 nm DBTBIm-II: 1'-TNATA: [Ir(mppr-Me)2(dpm)] (= 0.8:0.2:0.05) 20 nm DBTBIm-II 30 nm BPhen 20 nm LiF 1 nm Al 200 nm

[0329] These light-emitting elements were enclosed in a glove box containing a nitrogen atmosphere to prevent exposure to air. The operating characteristics of these light-emitting elements were then measured. Note that the measurements were performed at room temperature (in an atmosphere maintained at 25°C).

[0330] Fig. 44 shows a current density-luminance characteristic curve of the light-emitting elements 11 and 12. In Fig. 44 the horizontal axis represents the current density (mA / cm 2 ), and the vertical axis represents the luminance (cd / m 2 ). Fig. 45 represents a voltage-luminance characteristic curve. In Fig.45, the horizontal axis represents the voltage (V) and the vertical axis represents the luminance (cd / m 2 ). Fig. 46 shows a luminance-current efficiency curve. In Fig. 46 the horizontal axis represents the luminance (cd / m 2 ) and the vertical axis represents the current efficiency (cd / A). Fig. 47 shows a characteristic curve of luminance versus external quantum efficiency. In Fig. 47 the horizontal axis represents the luminance (cd / m 2 ), and the vertical axis represents the external quantum efficiency (%).

[0331] Furthermore, Table 14 shows the voltage (V), the current density (mA / cm 2 ), the CIE chromaticity coordinates (x, y), the current efficiency (cd / A), the efficiency (Im / W) and the external quantum efficiency (%) of the light-emitting elements 11 and 12 at a luminance of about 860 cd / m 2 represents. [Table 14] Voltage (V) Current density (mA / cm 2 ) Chromaticity (x, y) Luminance (cd / m 2 ) Current efficiency (cd / A) Efficiency (Im / W) External quantum efficiency (%) Light-emitting element 11 3,1 2,9 (0,53, 0,46) 860 29 30 11 Light-emitting element 12 3,8 1,5 (0,53, 0,46) 860 58 48 21

[0332] Fig. Figure 48 shows emission spectra of the light-emitting elements 11 and 12 obtained by applying a current of 0.1 mA. Fig. 48, the horizontal axis represents the wavelength (nm), and the vertical axis represents the emission intensity (arbitrary unit). As shown in Table 14, the CIE chromaticity coordinates of the light-emitting element 11 and the light-emitting element 12 were (x, y) = (0.53, 0.46) at a luminance of 860 cd / m 2 The results show that the light-emitting elements 11 and 12 emit an orange light originating from [Ir(mppr-Me)2(dpm)].

[0333] As can be seen from Table 14 and Fig. 44 to 47, each of the light-emitting elements 11 and 12 has a high current efficiency, a high efficiency and a high external quantum efficiency.

[0334] The light-emitting layer of the light-emitting element 11 includes 2mDBTPDBq-II, 1'-TNATA, and [Ir(mppr-Me)2(dpm)] described in Example 12, and the light-emitting layer of the light-emitting element 12 includes DBTBIm-II, 1'-TNATA, and [Ir(mppr-Me)2(dpm)] described in Example 12. As described in Example 12, the emission spectra of the material mixture of 2mDBTPDBq-II and 1'-TNATA and the material mixture of DBTBIm-II and 1'-TNATA have strong overlaps with the absorption band in the absorption spectrum of [Ir(mppr-Me)2(dpm)], which is considered to contribute significantly to light emission. It is considered that the light-emitting elements 11 and 12 have high energy transfer efficiency and external quantum efficiency because energies are transferred using the overlaps.In particular, the emission spectrum of the mixed material of DBTBIm-II and 1'-TNATA exhibits a stronger overlap with the absorption band than the emission spectrum of the mixed material of 2mDBTPDBq-II and 1'-TNATA. Therefore, the light-emitting element 12 is considered to have higher energy transfer efficiency and external quantum efficiency than the light-emitting element 11 because energy is transferred using the stronger overlap. Referring to the results of Example 12, the difference between the energy value of a peak in the emission spectrum of the host material and the energy value of a peak in the absorption band on the lowest energy side in the absorption spectrum of the guest material is preferably 0.3 eV or less.

[0335] The above results show that the application of an embodiment of the present invention makes it possible to obtain a light-emitting element with a high external quantum efficiency. [Example 14]

[0336] Example 14 provides examples of a guest material and host materials that can be included in the light-emitting element of an embodiment of the present invention, with reference to Fig. 49A and Fig. 49B.

[0337] The guest material used in this example is [Ir(mppr-Me)2(dpm)]. The host materials also used in this example are the following two types: a mixture of 2mDBTPDBq-II and PCBNBB, and a mixture of 2mDBTPDBq-II and 9-phenyl-9H-3-(9-phenyl-9H-carbazol-3-yl)carbazole (abbreviation: PCCP). Chemical formulas of materials used in this example are shown below. Note that chemical formulas of the materials used in the above examples are omitted here. (Absorption spectrum)

[0338] Fig. 49A and Fig.Figure 49B shows an ultraviolet-visible absorption spectrum (absorption spectrum f) of [Ir(mppr-Me)2(dpm)] in a dichloromethane solution of [Ir(mppr-Me)2(dpm)]. Absorption spectrum measurements were performed at room temperature using a UV / Vis spectrophotometer (Type V550, manufactured by JASCO Corporation) with the dichloromethane solution (0.093 mmol / L) in a quartz cuvette. (emission spectra)

[0339] Fig. 49A and Fig. 49B also show an emission spectrum (emission spectrum f-1) of a thin film of a material mixture of 2mDBTPDBq-II and PCBNBB and an emission spectrum (emission spectrum f-2) of a thin film of a material mixture of 2mDBTPDBq-II and PCCP. In Fig. 49A, the horizontal axis represents the wavelength (nm), and the vertical axes represent the molar absorption coefficient ε (M -1 ·cm -1 ) and the emission intensity (arbitrary unit). In Fig.49 B, the horizontal axis represents the energy (eV), and the vertical axes represent the molar absorption coefficient ε (M -1 ·cm -1 ) and the emission intensity (arbitrary unit).

[0340] The absorption spectrum f in Fig. Figure 49A shows that [Ir(mppr-Me)2(dpm)] exhibits a broad absorption band around 500 nm. This absorption band is considered to contribute significantly to light emission.

[0341] It is found that each of the peaks of the emission spectra f-1 and f-2 and the absorption band in the absorption spectrum f, which is expected to contribute significantly to light emission, have a strong overlap. Thus, it is demonstrated that a light-emitting element including a light-emitting layer using the guest material and one of the host materials of this example together has high energy transfer efficiency because energy is transferred using the overlap of the emission spectrum of the host material and the absorption spectrum of the guest material. Therefore, it is demonstrated that a light-emitting element with high external quantum efficiency can be achieved.

[0342] Example 14 shows that either the material mixture containing the carbazole compound or the composite material containing the aromatic amine compound can be used as the host material. [Example 15]

[0343] Example 15 presents a light-emitting element of an embodiment of the present invention with reference to Fig. 2. The materials used in this example are used in the above examples, and therefore their chemical formulas are omitted here.

[0344] The following describes a method for manufacturing light-emitting elements 13 and 14 of this example. (Light-emitting element 13)

[0345] First, an ITSO film was sputtered over the glass substrate 1100 to form the first electrode 1101, which served as the anode. The thickness was 110 nm, and the electrode area was 2 mm × 2 mm.

[0346] Next, as a pretreatment for forming the light-emitting element over the substrate 1100, the surface of the substrate was washed with water, heated at 200 °C for one hour, and subjected to UV ozone treatment for 370 seconds.

[0347] Subsequently, the substrate 1100 was transferred to a vacuum deposition device in which the pressure was increased to about 10 -4 Pa, and subjected to vacuum heating 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.

[0348] Next, the substrate 1100 provided with the first electrode 1101 was attached to a substrate holder in a vacuum evaporation apparatus so that a surface on which the first electrode 1101 was provided was directed downward. The pressure in the vacuum evaporation apparatus was set to about 10 -4 Pa. Then, BPAFLP and molybdenum(VI) oxide were co-evaporated over the first electrode 1101 to form the hole-injection layer 1111. The thickness of the hole-injection layer 1111 was 40 nm, and the weight ratio of BPAFLP to molybdenum oxide was set to 4:2 (= BPAFLP:molybdenum oxide).

[0349] Next, a BPAFLP film was formed to a thickness of 20 nm over the hole injection layer 1111, so that the hole transport layer 1112 was formed.

[0350] Furthermore, 2mDBTPDBq-II, PCBNBB, and [Ir(mppr-Me)2(dpm)] were co-evaporated to form the light-emitting layer 1113 over the hole-transport layer 1112. The weight ratio of 2mDBTPDBq-II to PCBNBB and [Ir(mppr-Me)2(dpm)] was set to 0.8:0.2:0.05 (= 2mDBTPDBq-II:PCBNBB:[Ir(mppr-Me)2(dpm)]). The thickness of the light-emitting layer 1113 was 20 nm.

[0351] Next, 2mDBTPDBq-II, PCBNBB, and [Ir(mppr-Me)2(dpm)] were co-evaporated over the light-emitting layer 1113 to form the first electron-transport layer 1114a over the light-emitting layer 1113. The weight ratio of 2mDBTPDBq-II to PCBNBB and [Ir(mppr-Me)2(dpm)] was set to 0.8:0.2:0.05 (= 2mDBTPDBq-II:PCBNBB:[Ir(mppr-Me)2(dpm)]). The thickness of the first electron-transport layer 1114a was 40 nm.

[0352] Next, a BPhen film was formed to a thickness of 10 nm over the first electron transport layer 1114a, so that the second electron transport layer 1114b was formed.

[0353] Furthermore, a LiF film with a thickness of 1 nm was formed by evaporation over the second electron transport layer 1114b, so that the electron injection layer 1115 was formed.

[0354] Finally, an aluminum foil with a thickness of 200 nm was formed by vapor deposition to form the second electrode 1103, which served as the cathode. Thus, the light-emitting element 13 of this example was fabricated. (Light-emitting element 14)

[0355] The light-emitting layer 1113 of the light-emitting element 14 was formed by co-evaporation of 2mDBTPDBq-II, 9-phenyl-9H-3-(9-phenyl-9H-carbazol-3-yl)carbazole (abbreviation: PCCP), and [Ir(mppr-Me)2(dpm)]. The weight ratio of 2mDBTPDBq-II to PCCP and [Ir(mppr-Me)2(dpm)] was set to 0.8:0.2:0.05 (= 2mDBTPDBq-II:PCCP:[Ir(mppr-Me)2(dpm)]). The thickness of the light-emitting layer 1113 was 20 nm. The layers other than the light-emitting layer 1113 were formed in the same manner as those of the light-emitting element 13.

[0356] It should be noted that in the above evaporation process, all evaporation was carried out by a resistance heating method.

[0357] Table 15 shows element structures of the light-emitting elements 13 and 14 obtained in this way. [Table 15] 1. Electrode Hole injection layer Lochtran sports layer Light-emitting layer 1. Electron transport layer 2. Electron transport layer Electron injection layer 2. Electrode Light-emitting element 13 ITSO 110 nm BPAFLP: MoOx (= 4:2) 40 nm BPAFLP 20 nm 2mDBTPDBq-II: PCBNBB: [Ir(mppr-Me)2(dpm)] (= 0.8:0.2:0.05) 20 nm 2mDBTPDBq-II: PCBNBB: [Ir(mppr-Me)2(dpm)] (= 0.8:0.2:0.05) 40 nm BPhen 10 nm LiF 1 nm Al 200 nm Light-emitting element 14 ITSO 110 nm BPAFLP: MoOx (= 4:2) 40 nm BPAFLP 20 nm 2mDBTPDBq-II: PCCP: [Ir(mppr-Me)2(dpm)] (= 0.8:0.2:0.05) 20 nm 2mDBTPDBq-II: PCBNBB: [Ir(mppr-Me)2(dpm)] (= 0.8:0.2:0.05) 40 nm BPhen 10 nm LiF 1 nm Al 200 nm

[0358] These light-emitting elements were enclosed in a glove box containing a nitrogen atmosphere to prevent exposure to air. The operating characteristics of these light-emitting elements were then measured. Note that the measurements were performed at room temperature (in an atmosphere maintained at 25°C).

[0359] Fig. 50 shows a current density-luminance characteristic curve of the light-emitting elements 13 and 14. In Fig. 50 the horizontal axis represents the current density (mA / cm 2 ), and the vertical axis represents the luminance (cd / m 2 ). Fig. 51 represents a voltage-luminance characteristic curve. In Fig. 51, the horizontal axis represents the voltage (V) and the vertical axis represents the luminance (cd / m 2 ). Fig. 52 shows a luminance-current efficiency characteristic curve. In Fig.52 the horizontal axis represents the luminance (cd / m 2 ) and the vertical axis represents the current efficiency (cd / A). Fig. 53 shows a characteristic curve of luminance versus external quantum efficiency. In Fig. 53 the horizontal axis represents the luminance (cd / m 2 ), and the vertical axis represents the external quantum efficiency (%).

[0360] Furthermore, Table 16 shows the voltage (V), the current density (mA / cm 2 ), the CIE chromaticity coordinates (x, y), the current efficiency (cd / A), the efficiency (Im / W) and the external quantum efficiency (%) of the light-emitting elements 13 and 14 at a luminance of about 1,200 cd / m 2 represents. [Table 16] Voltage (V) Current density (mA / cm 2 ) Chromaticity (x, y) Luminance (cd / m 2 ) Current efficiency (cd / A) Efficiency (Im / W) External quantum efficiency (%) Light-emitting element 13 3,0 1,7 (0,54, 0,45) 1.200 67 70 24 Light-emitting element 14 3,0 1,7 (0,54, 0,46) 1.200 69 72 24

[0361] Fig. Figure 54 shows emission spectra of the light-emitting elements 13 and 14 obtained by applying a current of 0.1 mA. Fig.54, the horizontal axis represents the wavelength (nm), and the vertical axis represents the emission intensity (arbitrary unit). As shown in Table 16, the CIE chromaticity coordinates of the light-emitting element 13 were (x, y) = (0.54, 0.45) at a luminance of 1,200 cd / m 2 , and the CIE chromaticity coordinates of the light-emitting element 14 were (x, y) = (0.54, 0.46) at a luminance of 1,200 cd / m 2 The results show that the light-emitting elements 13 and 14 emit an orange light originating from [Ir(mppr-Me)2(dpm)].

[0362] As can be seen from Table 16 and Fig. 50 to 53, each of the light-emitting elements 13 and 14 has a high current efficiency, a high efficiency and a high external quantum efficiency.

[0363] The light-emitting layer of the light-emitting element 13 includes 2mDBTPDBq-II, PCBNBB, and [Ir(mppr-Me)2(dpm)] described in Example 14, and the light-emitting layer of the light-emitting element 14 includes 2mDBTPDBq-II, PCCP, and [Ir(mppr-Me)2(dpm)] described in Example 14. As described in Example 14, the emission spectra of the material mixture of 2mDBTPDBq-II and PCBNBB and the material mixture of 2mDBTPDBq-II and PCCP have strong overlaps with the absorption band in the absorption spectrum of [Ir(mppr-Me)2(dpm)], which is considered to contribute significantly to light emission. It is considered that the light-emitting elements 13 and 14 have high energy transfer efficiency and external quantum efficiency because energies are transferred using the overlaps.

[0364] Example 15 indicates that when the material mixture containing the carbazole compound (PCCP) is used as the host material in the light-emitting layer instead of the material mixture containing the aromatic amine compound (PCBNBB), a light-emitting element with a high external quantum efficiency can also be obtained.

[0365] The above results show that the application of an embodiment of the present invention makes it possible to obtain a light-emitting element with a high external quantum efficiency. (Reference example 1)

[0366] The following is a synthesis example of an organometallic complex (acetylacetonato)bis(4,6-diphenylpyrimidinato)iridium(III) (other name: bis[2-(6-phenyl-4-pyrimidinyl-ĸN3)phenyl-ĸC](2,4-pentanedionato-ĸ 2O,O')iridium(III)) (abbreviation: [Ir(dppm)2(acac)]), which is used in the above examples. The structure of [Ir(dppm)2(acac)] is shown below. 〈Step 1: Synthesis of 4,6-diphenylpyrimidine (abbreviation: Hdppm)〉

[0367] 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 placed in a round-bottomed flask equipped with a reflux tube, and the air in the flask was replaced with argon. This reaction vessel was heated for 60 minutes by microwave irradiation (2.45 GHz, 100 W). Then, 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 further added to the flask, and the mixture was heated again for 60 minutes by microwave irradiation (2.45 GHz, 100 W). Water was then added to this solution, and an organic layer was extracted with dichloromethane. The resulting extract solution was washed with water and dried with magnesium sulfate. After drying, the solution was filtered.The solvent of this solution was distilled off, and the residue was purified by silica gel column chromatography using dichloromethane as the eluent to obtain a pyrimidine derivative Hdppm (yellow-white powder, yield 38%). Note 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-µ-chloro-bis[bis(4,6-diphenylpyrimidinato)iridium(III)] (abbreviation: [Ir(dppm)2Cl]2)〉

[0368] Next, 15 mL of 2-ethoxyethanol, 5 mL of water, 1.10 g of the Hdppm obtained in step 1, and 0.69 g of iridium chloride hydrate (IrCl3·H2O) were added to a round-bottomed flask equipped with a reflux tube, and the air in the flask was replaced with argon. Microwave irradiation (2.45 GHz, 100 W) was then carried out for 1 hour to cause a reaction. The solvent was distilled off, and then the obtained residue was filtered and washed with ethanol to produce a binuclear 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)]}〉

[0369] Furthermore, 40 ml of 2-ethoxyethanol, 1.44 g of the [Ir(dppm)2Cl]2 obtained in step 2, 0.30 g of acetylacetone, and 1.07 g of sodium carbonate were placed in a round-bottomed flask equipped with a reflux tube, and the air in the flask was replaced with argon. Subsequently, microwave irradiation (2.45 GHz, 120 W) was carried out for 60 minutes to cause a reaction. The solvent was distilled off, the obtained residue was dissolved in dichloromethane, and filtration was carried out to remove insoluble substances. The obtained filtrate was washed with water and then with saturated saline solution and dried with magnesium sulfate. After drying, the solution was filtered. The solvent of this solution was distilled off, and then the obtained residue was purified by silica gel column chromatography using dichloromethane and ethyl acetate as the eluent in a volume ratio of 50:1.Recrystallization was then performed using a mixed solvent of dichloromethane and hexane, yielding the target compound as an orange powder (yield 32%). A synthesis scheme (a-3) of step 3 is shown below.

[0370] The following is an analysis result by nuclear magnetic resonance spectrometry ( 1 H NMR) of the orange powder obtained in step 3. These results showed that the organometallic complex [Ir(dppm)2(acac)] was obtained.

[0371] 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)

[0372] The following is a synthesis example of an organometallic complex (acetylacetonato)bis(6-methyl-4-phenylpyrimidinato)iridium(III) (other name: bis[2-(6-methyl-4-pyrimidinyl-ĸN3)phenyl-ĸC](2,4-pentanedionato-ĸ 2 O,O')iridium(III)) (abbreviation: [Ir(mppm)2(acac)]), which is used in the above examples. The structure of [Ir(mppm)2(acac)] is shown below. (Step 1: Synthesis of 4-methyl-6-phenylpyrimidine (abbreviation: Hmppm))

[0373] First, 4.90 g of 4-chloro-6-methylpyrimidine, 4.80 g of phenylboronic acid, 4.03 g of sodium carbonate, 0.16 g of bis(triphenylphosphine)palladium(II) dichloride (abbreviation: Pd(PPh3)2Cl2), 20 ml of water, and 10 ml of acetonitrile were placed in a round-bottomed flask equipped with a reflux tube, and the air in the flask was replaced with argon. This reaction vessel was heated for 60 minutes by microwave irradiation (2.45 GHz, 100 W). Then, 2.28 g of phenylboronic acid, 2.02 g of sodium carbonate, 0.082 g of Pd(PPh3)2Cl2, 5 ml of water, and 10 ml of acetonitrile were further added to the flask, and the mixture was again heated for 60 minutes by microwave irradiation (2.45 GHz, 100 W). Water was then added to this solution, and extraction with dichloromethane was performed.The resulting extract solution was washed with a saturated aqueous sodium carbonate solution, water, and then with a saturated saline solution, and dried with magnesium sulfate. After drying, the solution was filtered. 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 9:1 to obtain the pyrimidine derivative Hmppm as the target substance (an orange oily substance, yield 46%). Note that microwave irradiation was performed using a microwave synthesis system (Discover, manufactured by CEM Corporation). A synthesis scheme (b-1) of step 1 is shown below. 〈Step 2: Synthesis of di-µ-chloro-bis[bis(6-methyl-4-phenylpyrimidinato)iridium(III)] (abbreviation: [Ir(mppm)2Cl]2)〉

[0374] Next, 15 mL of 2-ethoxyethanol, 5 mL of water, 1.51 g of the Hmppm obtained in step 1, and 1.26 g of iridium chloride hydrate (IrCl3·H2O) were placed in a round-bottomed flask equipped with a reflux tube, and the air in the round-bottomed flask was replaced with argon. Microwave irradiation (2.45 GHz, 100 W) was then carried out for 1 hour to cause a reaction. The solvent was distilled off, and then the obtained residue was washed with ethanol and filtered to obtain a binuclear complex [Ir(mppm)2Cl]2 (dark green powder, yield 77%). A synthesis scheme (b-2) of step 2 is shown below. 〈Step 3: Synthesis of (acetylacetonato)bis(6-methyl-4-phenylpyrimidinato)iridium(III) (abbreviation: [Ir(mppm)2(acac)])〉

[0375] Furthermore, 40 ml of 2-ethoxyethanol, 1.84 g of the dinuclear complex [Ir(mppm)2Cl]2 obtained in step 2, 0.48 g of acetylacetone, and 1.73 g of sodium carbonate were placed in a round-bottomed flask equipped with a reflux tube, and the air in the flask was replaced with argon. Subsequently, microwave irradiation (2.45 GHz, 120 W) was carried out for 60 minutes to cause a reaction. The solvent was distilled off, the obtained residue was dissolved in dichloromethane, and filtered to remove insoluble substances. The obtained filtrate was washed with water and then with saturated saline and dried with magnesium sulfate. After drying, the solution was filtered. The solvent of this solution was distilled off, and then the obtained residue was purified by silica gel column chromatography using dichloromethane and ethyl acetate as the eluent in a volume ratio of 4:1.Recrystallization was then performed using a solvent mixture of dichloromethane and hexane, yielding the target compound as a yellow powder (yield 22%). A synthesis scheme (b-3) of step 3 is shown below.

[0376] The following is an analysis result by nuclear magnetic resonance spectrometry ( 1 H NMR) of the yellow powder obtained in step 3. These results showed that the organometallic complex [Ir(mppm)2(acac)] was obtained.

[0377] 1 H NMR. δ (CDCl3): 1.78 (s, 6H), 2.81 (s, 6H), 5.24 (s, 1H), 6.37 (d, 2H), 6.77 (t, 2H), 6.85 (t, 2H), 7.61-7.63 (m, 4H), 8.97 (s, 2H). (Reference example 3)

[0378] The following is a synthesis example of an organometallic complex (acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III) (other name: bis[2-(6-tert-butyl-4-pyrimidinyl-ĸN3)phenyl-ĸC](2,4-pentanedionato-ĸ2 O,O')iridium(III)) (abbreviation: [Ir(tBuppm)2(acac)]), which is used in the above examples. The structure of [Ir(tBuppm)2(acac)] is shown below. 〈Step 1: Synthesis of 4-tert-butyl-6-phenylpyrimidine (abbreviation: HtBuppm)〉

[0379] First, 22.5 g of 4,4-dimethyl-1-phenylpentane-1,3-dione and 50 g of formamide were placed in a round-bottomed flask equipped with a reflux tube, and the air in the flask was replaced with nitrogen. This reaction vessel was heated so that the reacted solution was refluxed for 5 hours. Afterward, this solution was poured into an aqueous sodium hydroxide solution, and an organic layer was extracted with dichloromethane. The obtained organic layer was washed with water and saturated saline solution and dried with magnesium sulfate. After drying, the solution was filtered. The solvent of this solution was distilled off, and then the obtained residue was purified by silica gel column chromatography using hexane and ethyl acetate as the eluent in a volume ratio of 10:1 to obtain a pyrimidine derivative HtBuppm (colorless, oily substance, yield 14%).A synthesis scheme (c-1) of step 1 is shown below. 〈Step 2: Synthesis of di-µ-chloro-bis[bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III)] (abbreviation: [Ir(tBuppm)2Cl]2)〉

[0380] Next, 15 mL of 2-ethoxyethanol, 5 mL of water, 1.49 g of the HtBuppm obtained in step 1, and 1.04 g of iridium chloride hydrate (IrCl3·H2O) were added to a round-bottomed flask equipped with a reflux tube, and the air in the flask was replaced with argon. Microwave irradiation (2.45 GHz, 100 W) was then carried out for 1 hour to cause a reaction. The solvent was distilled off, and then the obtained residue was filtered off with suction and washed with ethanol to obtain a binuclear complex [Ir(tBuppm)2Cl]2 (yellow-green powder, yield 73%). A synthesis scheme (c-2) of step 2 is shown below. 〈Step 3: Synthesis of (acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III) (abbreviation: [Ir(tBuppm)2(acac)])〉

[0381] Furthermore, 40 mL of 2-ethoxyethanol, 1.61 g of the dinuclear complex [Ir(tBuppm)2Cl]2 obtained in step 2, 0.36 g of acetylacetone, and 1.27 g of sodium carbonate were placed in a round-bottomed flask equipped with a reflux tube, and the air in the flask was replaced with argon. Subsequently, microwave irradiation (2.45 GHz, 120 W) was carried out for 60 minutes to cause a reaction. The solvent was distilled off, and the obtained residue was filtered with ethanol under suction and washed with water and ethanol. This solid was dissolved in dichloromethane, and the mixture was filtered through a filter aid in which Celite (manufactured by Wako Pure Chemical Industries, Ltd., Catalog No. 531-16855), alumina, and Celite were stacked in that order.The solvent was distilled off, and the resulting solid was recrystallized with a mixed solvent of dichloromethane and hexane to obtain the target compound as a yellow powder (yield 68%). A synthesis scheme (c-3) of step 3 is shown below.

[0382] The following is an analysis result by nuclear magnetic resonance spectrometry ( 1 H NMR) of the yellow powder obtained in step 3. These results showed that the organometallic complex [Ir(tBuppm)2(acac)] was obtained.

[0383] 1 H NMR. δ (CDCl3): 1.50 (s, 18H), 1.79 (s, 6H), 5.26 (s, 1H), 6.33 (d, 2H), 6.77 (t, 2H), 6.85 (t, 2H), 7.70 (d, 2H), 7.76 (s, 2H), 9.02 (s, 2H). (Reference example 4)

[0384] The following describes a method for synthesizing 2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation: 2mDBTPDBq-II), which is used in examples. (Synthesis of 2-[3-(Dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation: 2mDBTPDBq-II)〉

[0385] A synthesis scheme (d-1) of 2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation: 2mDBTPDBq-II) is presented below.

[0386] To a 2-liter three-necked flask, 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 potassium carbonate solution were added. 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 for 7.5 hours under a nitrogen stream at 100 °C. After cooling to room temperature, the obtained mixture was filtered to produce a white residue. The obtained residue was washed with water and ethanol in that order and then dried. The recovered solid was dissolved in approximately 600 ml of hot toluene, followed by suction filtration through Celite and Florisil to obtain a clear, colorless filtrate.The resulting filtrate was concentrated and purified by silica gel column chromatography using approximately 700 ml of silica gel. The chromatography was performed using hot toluene as the eluent. Acetone and ethanol were added to the resulting solid, followed by ultrasonic irradiation. The resulting suspended solid was then collected by filtration, and the resulting solid was dried to yield 7.85 g of a white powder with a yield of 80%.

[0387] The target substance above was relatively soluble in hot toluene, but was a material that would likely precipitate upon cooling. Furthermore, the substance was poorly soluble in other organic solvents such as acetone and ethanol. Consequently, utilizing these varying degrees of solubility resulted in a high-yield synthesis by a simple procedure such as the one above. Specifically, after the reaction was complete, the mixture was returned to room temperature, and the precipitated solid was collected by filtration, which easily removed most impurities. Furthermore, the target substance, which was likely to precipitate, could be readily purified by column chromatography using hot toluene as the eluent.

[0388] 4.0 g of the resulting white powder was purified using a train sublimation process. During purification, the white powder was heated at 300 °C under a pressure of 5.0 Pa and an argon gas flow rate of 5 ml / min. After purification, the target substance was obtained as 3.5 g of a white powder with a yield of 88%.

[0389] A nuclear magnetic resonance spectrometry ( 1 H NMR) identified this compound as the target substance 2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[fh]quinoxaline (abbreviation: 2mDBTPDBq-II).

[0390] 1 H-NMR data of the obtained substance are presented below.

[0391] 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 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, 1115: Electron injection layer.

[0392] This application is based on Japanese patent application No. 2011-031.426 filed with the Japan Intellectual Property Office on February 16, 2011, the entire contents of which are hereby incorporated by reference.

Claims

[1] Light-emitting element comprising: an anode; a cathode; and a light-emitting layer comprising a guest material and a host material between the anode and the cathode, wherein the guest material is an iridium compound, wherein the host material comprises a heterocyclic compound having a six-membered ring with two nitrogen atoms, and wherein an emission spectrum of the host material overlaps the absorption band at the longest wavelength side in the absorption spectrum of the guest material. [2] Light-emitting element comprising: an anode; a cathode; and a light-emitting layer comprising a guest material and a host material between the anode and the cathode, wherein the guest material is an iridium compound, wherein the host material comprises a heterocyclic compound having a six-membered ring with two nitrogen atoms, wherein an emission spectrum of the host material overlaps the absorption band at the longest wavelength side in the absorption spectrum of the guest material, and wherein a difference between the energy value of the maximum of the emission spectrum of the host material and the energy value of the maximum of the absorption band on the longest wavelength side in the absorption spectrum of the guest material is 0.3 eV or less. [3] A light-emitting element according to claim 1 or 2, further comprising: a first layer between the anode and the light-emitting layer, wherein the first layer comprises an aromatic amine compound and an electron acceptor. [4] A light-emitting element according to claim 3, further comprising: a second layer between the first layer and the light-emitting layer, wherein the second layer comprises an aromatic amine compound, and wherein the aromatic amine compound has a fluoroenyl group. [5] A light-emitting element according to claim 1 or 2, further comprising: a third layer between the light-emitting layer and the cathode, wherein the third layer comprises a metal complex with a quinoline-based ligand. [6] The light-emitting element according to claim 1 or 2, wherein one of the anode and the cathode comprises ytterbium. [7] A light-emitting element according to claim 5, further comprising: a fourth layer above the third layer, where the fourth layer comprises ytterbium. [8] The light-emitting element according to claim 1 or 2, wherein the absorption band on the longest wavelength side is located at an absorption wavelength corresponding to the direct transition from a singlet ground state to a triplet excited state. [9] The light-emitting element according to claim 1 or 2, wherein a level of triplet excitation energy of the host material is higher than a level of triplet excitation energy of the guest material. [10] The light-emitting element according to claim 1 or 2, wherein a fluorescence spectrum of the host material and a phosphorescence spectrum of the host material overlap the absorption band on the longest wavelength side in the absorption spectrum of the guest material. [11] The light-emitting element according to claim 2, wherein the difference between the energy value of the maximum of the emission spectrum of the host material and the energy value of the maximum of the absorption band on the longest wavelength side in the absorption spectrum of the guest material is 0.2 eV or less. [12] The light-emitting element according to claim 2, wherein the difference between the energy value of the maximum of the emission spectrum of the host material and the energy value of the maximum of the absorption band on the longest wavelength side in the absorption spectrum of the guest material is 0.1 eV or less. [13] The light-emitting element according to claim 1 or 2, wherein a maximum of the emission spectrum of the host material overlaps the absorption band on the longest wavelength side in the absorption spectrum of the guest material.

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