Light emitting element

Abstract

Provided is a light-emitting element with high external quantum efficiency. Provided is a light-emitting element with long lifetime. Provided is a light-emitting element including a light-emitting layer containing a phosphorescent compound, a first organic compound, and a second organic compound between a pair of electrodes, wherein a combination of the first organic compound and the second organic compound forms an exciplex. The light-emitting element has high energy transfer efficiency because of energy transfer by overlap of an emission spectrum of the exciplex and an absorption spectrum of the phosphorescent compound. Thus, a light-emitting element with high external quantum efficiency can be realized.

Description

[0001] This application is a divisional application of the divisional application 201911264494.3. The divisional application 201911264494.3 is a divisional application of the patent application with the application number "201280002425.5" and the application date of February 6, 2012, and the invention name of "Light-emitting element". TECHNICAL FIELD

[0002] The present application relates to a light-emitting element (hereinafter also referred to as an organic EL element) utilizing an organic electroluminescence (EL) phenomenon. BACKGROUND

[0003] An organic EL element is actively researched and developed. The basic structure of an organic EL element is a structure in which a layer containing a light-emitting organic compound (hereinafter also referred to as a light-emitting layer) is interposed between a pair of electrodes. An organic EL element is attracting attention as a next-generation flat panel display element because of its characteristics such as thinness and lightweight, high-speed response to an input signal, and direct-current low-voltage driving. In addition, a display using such a light-emitting element has features of excellent contrast and image quality and a wide viewing angle. Furthermore, since an organic EL element is a surface light source, it is expected to be applied to a light source of a backlight of a liquid crystal display and an illuminating device, for example.

[0004] The light-emitting mechanism of an organic EL element belongs to a carrier injection type. In other words, by applying a voltage between electrodes interposing a light-emitting layer, electrons and holes injected from the electrodes recombine to make a light-emitting substance into an excited state, and light is emitted when the excited state returns to a ground state. There are two types of excited states: a singlet excited state (S * ) and a triplet excited state (T * ). In a light-emitting element, the statistical generation ratio of the singlet excited state and the triplet excited state is considered to be S * :T * = 1:3.

[0005] The ground state of a light-emitting organic compound is usually a singlet excited state. Therefore, light emission from a singlet excited state (S * ) is called fluorescence because it is an electronic transition between the same spin multiplicity. On the other hand, light emission from a triplet excited state (T * ) is called phosphorescence because it is an electronic transition between different spin multiplicities. Here, in a compound that emits fluorescence (hereinafter, referred to as a fluorescent compound), phosphorescence is usually not observed at room temperature, and only fluorescence is observed. Therefore, based on S * :T *= 1:3, the internal quantum efficiency (the ratio of generated photons to injected carriers) in a light-emitting element using a fluorescent compound is considered to be theoretically limited to 25%.

[0006] On the other hand, if a compound that emits phosphorescence (hereinafter referred to as a phosphorescent compound) is used, the internal quantum efficiency can theoretically reach 100 %. In other words, a higher emission efficiency can be obtained than in the case of using a fluorescent compound. For the above reasons, in recent years, light-emitting elements using a phosphorescent compound have been actively developed in order to achieve a high-efficiency light-emitting element. In particular, an organometallic complex in which iridium or the like is used as a central metal has attracted attention as a phosphorescent compound because of its high phosphorescence quantum yield. For example, Patent Document 1 discloses an organometallic complex in which iridium is used as a central metal as a phosphorescent material.

[0007] When a phosphorescent compound is used to form a light-emitting layer of a light-emitting element, in order to suppress concentration quenching of the phosphorescent compound or quenching due to triplet-triplet annihilation, the light-emitting layer is usually formed in such a manner that the phosphorescent compound is dispersed in a matrix of another compound. Here, the compound used as the matrix is referred to as a host material, and the compound such as a phosphorescent compound dispersed in the matrix is referred to as a guest material.

[0008] [REFERENCE]

[0009] [Patent Document]

[0010] [Patent Document 1] International Publication No. WO00 / 70655 SUMMARY

[0011] However, it is generally considered that the light extraction efficiency in an organic EL element is about 20 % to 30 %. Thus, when the absorption of light by a reflective electrode and a transparent electrode is taken into account, the external quantum efficiency of a light-emitting element using a phosphorescent compound is limited to at most about 25 %.

[0012] An object of one embodiment of the present application is to provide a light-emitting element with high external quantum efficiency. Another object of one embodiment of the present application is to provide a light-emitting element with a long lifetime.

[0013] One embodiment of the present application is a light-emitting element including a light-emitting layer containing a phosphorescent compound, a first organic compound, and a second organic compound between a pair of electrodes, in which a combination of the first organic compound and the second organic compound forms an exciplex.

[0014] Another embodiment of the present application is a light-emitting element including a light-emitting layer containing a phosphorescent compound, a first organic compound, and a second organic compound between a pair of electrodes, in which a combination of the first organic compound and the second organic compound forms an exciplex, and the exciplex functions on the phosphorescent compound to cause the phosphorescent compound to emit phosphorescence.

[0015] Another embodiment of the present application is a light-emitting element including a light-emitting layer containing a phosphorescent compound, a first organic compound, and a second organic compound between a pair of electrodes, in which a singlet excitation energy of the first organic compound forms an exciplex.

[0016] Another embodiment of the present application is a light-emitting element including a light-emitting layer containing a phosphorescent compound, a first organic compound, and a second organic compound between a pair of electrodes, in which an anion of the first organic compound and a cation of the second organic compound form an exciplex.

[0017] In the above light-emitting element, excitation energy of the exciplex is preferably transferred to the phosphorescent compound to cause the phosphorescent compound to emit phosphorescence.

[0018] In the above light-emitting element, at least one of the first organic compound and the second organic compound is preferably a fluorescent compound.

[0019] In the above light-emitting element, the phosphorescent compound is preferably an organometallic complex.

[0020] The light-emitting element of one embodiment of the present application can be applied to a light-emitting device, an electronic device, and a lighting device.

[0021] One embodiment of the present application can provide a light-emitting element with high external quantum efficiency. Another embodiment of the present application can provide a light-emitting element with long lifetime. BRIEF DESCRIPTION OF DRAWINGS

[0022] FIG. 1A and FIG. 1B is a graph showing absorption and emission spectra relating to Example 1;

[0023] FIG. 2A and FIG. 2B is a graph showing absorption and emission spectra relating to Example 2;

[0024] FIG. 3 is a graph showing current density-luminance characteristics of the light-emitting element of Example 3;

[0025] FIG. 4 is a graph showing voltage-luminance characteristics of the light-emitting element of Example 3;

[0026] FIG. 5FIG. 3 is a graph showing luminance-current efficiency characteristics of the light-emitting element of Example 3;

[0027] FIG. 6 FIG. 4 is a graph showing luminance-external quantum efficiency characteristics of the light-emitting element of Example 3;

[0028] FIG. 7 FIG. 5 is a graph showing emission spectrum of the light-emitting element of Example 3;

[0029] FIG. 8 FIG. 6 is a graph showing results of reliability test of the light-emitting element of Example 3;

[0030] FIG. 9 FIG. 9 is a graph showing current density-luminance characteristics of the light-emitting element of Example 4;

[0031] FIG. 10 FIG. 10 is a graph showing voltage-luminance characteristics of the light-emitting element of Example 4;

[0032] FIG. 11 FIG. 11 is a graph showing luminance-current efficiency characteristics of the light-emitting element of Example 4;

[0033] FIG. 12 FIG. 12 is a graph showing luminance-external quantum efficiency characteristics of the light-emitting element of Example 4;

[0034] FIG. 13 FIG. 13 is a graph showing emission spectrum of the light-emitting element of Example 4;

[0035] FIG. 14 FIG. 14 is a graph showing results of reliability test of the light-emitting element of Example 4;

[0036] FIG. 15 FIG. 17 is a graph showing a structure of a light-emitting element in an embodiment;

[0037] FIG. 16A to FIG. 16C FIG. 18 is a graph showing a light-emitting element of one embodiment of the present application;

[0038] FIG. 17 FIG. 19 is a graph showing energy levels of an exciplex applied in one embodiment of the present application;

[0039] FIG. 18A and FIG. 18B FIG. 22 is a graph showing absorption spectrum and emission spectrum of Example 5;

[0040] FIG. 19 FIG. 25 is a graph showing current density-luminance characteristics of the light-emitting element of Example 6;

[0041] FIG. 20 FIG. 26 is a graph showing voltage-luminance characteristics of the light-emitting element of Example 6;

[0042] FIG. 21 FIG. 6 is a graph showing luminance-current efficiency characteristics of the light-emitting element of Example 6;

[0043] FIG. 22 FIG. 7 is a graph showing luminance-external quantum efficiency characteristics of the light-emitting element of Example 6;

[0044] FIG. 23 FIG. 8 is a graph showing emission spectrum of the light-emitting element of Example 6;

[0045] FIG. 24 FIG. 9 is a graph showing results of reliability test of the light-emitting element of Example 6;

[0046] FIG. 25 FIG. 10 is a graph showing the concept of one embodiment of the present application;

[0047] FIG. 26A and FIG. 26B FIG. 15 is a graph showing absorption spectrum and emission spectrum relating to Example 7;

[0048] FIG. 27 FIG. 20 is a graph showing current density-luminance characteristics of the light-emitting element of Example 8;

[0049] FIG. 28 FIG. 21 is a graph showing voltage-luminance characteristics of the light-emitting element of Example 8;

[0050] FIG. 29 FIG. 22 is a graph showing luminance-current efficiency characteristics of the light-emitting element of Example 8;

[0051] FIG. 30 FIG. 23 is a graph showing luminance-external quantum efficiency characteristics of the light-emitting element of Example 8;

[0052] FIG. 31 FIG. 24 is a graph showing emission spectrum of the light-emitting element of Example 8;

[0053] FIG. 32A and FIG. 32B FIG. 29 is a graph showing absorption spectrum and emission spectrum relating to Example 9;

[0054] FIG. 33 FIG. 34 is a graph showing current density-luminance characteristics of the light-emitting element of Example 10;

[0055] FIG. 34 FIG. 35 is a graph showing voltage-luminance characteristics of the light-emitting element of Example 10;

[0056] FIG. 35 FIG. 36 is a graph showing luminance-current efficiency characteristics of the light-emitting element of Example 10;

[0057] FIG. 36FIG. 10 is a graph showing luminance-external quantum efficiency characteristics of the light-emitting element of Example 10;

[0058] FIG. 37 FIG. 11 is a graph showing emission spectra of the light-emitting element of Example 10;

[0059] FIG. 38 FIG. 12 is a graph showing results of reliability tests of the light-emitting element of Example 10;

[0060] FIG. 39A and FIG. 39B FIG. 15 is a graph showing absorption spectra and emission spectra relating to Example 11;

[0061] FIG. 40 FIG. 16 is a graph showing current density-luminance characteristics of the light-emitting element of Example 12;

[0062] FIG. 41 FIG. 17 is a graph showing voltage-luminance characteristics of the light-emitting element of Example 12;

[0063] FIG. 42 FIG. 18 is a graph showing luminance-current efficiency characteristics of the light-emitting element of Example 12;

[0064] FIG. 43 FIG. 19 is a graph showing luminance-external quantum efficiency characteristics of the light-emitting element of Example 12;

[0065] FIG. 44 FIG. 20 is a graph showing emission spectra of the light-emitting element of Example 12;

[0066] FIG. 45 FIG. 21 is a graph showing results of reliability tests of the light-emitting element of Example 12;

[0067] FIG. 46A and FIG. 46B FIG. 24 is a graph showing absorption spectra and emission spectra relating to Example 13;

[0068] FIG. 47 FIG. 25 is a graph showing current density-luminance characteristics of the light-emitting element of Example 14;

[0069] FIG. 48 FIG. 26 is a graph showing voltage-luminance characteristics of the light-emitting element of Example 14;

[0070] FIG. 49 FIG. 27 is a graph showing luminance-current efficiency characteristics of the light-emitting element of Example 14;

[0071] FIG. 50 FIG. 28 is a graph showing luminance-external quantum efficiency characteristics of the light-emitting element of Example 14;

[0072] FIG. 51FIG. 7 is a graph showing the emission spectrum of the light-emitting element of Example 14;

[0073] FIG. 52A and FIG. 52B FIG. 8 is a graph showing the absorption spectrum and the emission spectrum of Example 15;

[0074] FIG. 53 FIG. 9 is a graph showing the current density-luminance characteristics of the light-emitting element of Example 16;

[0075] FIG. 54 FIG. 10 is a graph showing the voltage-luminance characteristics of the light-emitting element of Example 16;

[0076] FIG. 55 FIG. 11 is a graph showing the luminance-current efficiency characteristics of the light-emitting element of Example 16;

[0077] FIG. 56 FIG. 12 is a graph showing the luminance-external quantum efficiency characteristics of the light-emitting element of Example 16;

[0078] FIG. 57 FIG. 13 is a graph showing the emission spectrum of the light-emitting element of Example 16;

[0079] FIG. 58A and FIG. 58B FIG. 14 is a graph showing the absorption spectrum and the emission spectrum of Example 17;

[0080] FIG. 59 FIG. 15 is a graph showing the current density-luminance characteristics of the light-emitting element of Example 18;

[0081] FIG. 60 FIG. 16 is a graph showing the voltage-luminance characteristics of the light-emitting element of Example 18;

[0082] FIG. 61 FIG. 17 is a graph showing the luminance-current efficiency characteristics of the light-emitting element of Example 18;

[0083] FIG. 62 FIG. 18 is a graph showing the luminance-external quantum efficiency characteristics of the light-emitting element of Example 18;

[0084] FIG. 63 FIG. 19 is a graph showing the emission spectrum of the light-emitting element of Example 18.

[0085] FIG. 64 FIG. 20 is a graph showing the calculation results of one embodiment of the present application;

[0086] FIG. 65A1 , FIG. 65A2 , FIG. 65B1 , FIG. 65B2 , FIG. 65C1 , FIG. 65C2FIG. 1 is a graph showing the results of calculation relating to one embodiment of the present application. DETAILED DESCRIPTION

[0087] Embodiments are described in detail with reference to the accompanying drawings. However, the application is not limited to the following description, and it will be readily appreciated by those skilled in the art that the application can be changed in various ways without departing from the spirit and scope of the present application. Therefore, the application should not be interpreted as being limited to the content of the following description merely illustrating the embodiments. Note that in the following description of the application structures, the same portions or portions having the same function are denoted with the same reference numerals throughout the different drawings for describing the embodiments, and repeated explanation is omitted.

[0088] (Embodiment 1)

[0089] This embodiment describes a light-emitting element of one embodiment of the present application.

[0090] The light-emitting element of this embodiment includes a light-emitting layer including a guest material as a light-emitting substance, a first organic compound, and a second organic compound. Specifically, a phosphorescent compound is used as the guest material. Further, in this specification, a material included in a light-emitting layer in a larger amount is referred to as a host material between the first organic compound and the second organic compound.

[0091] By employing a structure in which a guest material is dispersed in a host material, crystallization of a light-emitting layer can be suppressed. Further, by suppressing concentration quenching due to a high concentration of a guest material, the emission efficiency of a light-emitting element can be increased.

[0092] Further, in this embodiment, each of the first organic compound and the second organic compound preferably has a higher T1 level than the guest material. This is because if the T1 level of the first organic compound (or the second organic compound) is lower than the T1 level of the guest material, the first organic compound (or the second organic compound) quenches the T1 level of the guest material which contributes to emission, which leads to a decrease in emission efficiency.

[0093] <Basic process of emission>

[0094] First, a general basic process of emission in a light-emitting element in which a phosphorescent compound is used as a guest material is described.

[0095] (1) A case where a guest molecule is in an excited state when an electron and a hole are recombined in the guest molecule (direct recombination process).

[0096] (1-1) When the excited state of the guest molecule is a triplet excited state, the guest molecule emits phosphorescence.

[0097] (1-2) When the excited state of the guest molecule is a singlet excited state, the guest molecule in the singlet excited state undergoes intersystem crossing to a triplet excited state and emits phosphorescence.

[0098] In other words, in the direct recombination process of the above (1), as long as the intersystem crossing efficiency and the phosphorescence quantum yield of the guest molecule are high, high emission efficiency can be obtained. Further, as described above, the T1 level of the host molecule is preferably higher than the T1 level of the guest molecule.

[0099] (2) When an electron and a hole are recombined in the host molecule, the host molecule is in an excited state (energy transfer process).

[0100] (2-1) When the excited state of the host molecule is a triplet excited state, and the T1 level of the host molecule is higher than the T1 level of the guest molecule, the excited energy is transferred from the host molecule to the guest molecule, and the guest molecule is in a triplet excited state. The guest molecule in the triplet excited state emits phosphorescence. Further, it is theoretically possible that the energy is transferred to the singlet excited level (S1 level) of the guest molecule, but in many cases, the S1 level of the guest molecule is higher in energy than the T1 level of the host molecule, and it is not easy to become a main energy transfer process, and thus the description is omitted here.

[0101] (2-2) When the excited state of the host molecule is a singlet excited state, and the S1 level of the host molecule is higher than the S1 level and the T1 level of the guest molecule, the excited energy is transferred from the host molecule to the guest molecule, and thus the guest molecule is in a singlet excited state or a triplet excited state. The guest molecule in the triplet excited state emits phosphorescence. Further, the guest molecule in the singlet excited state undergoes intersystem crossing to a triplet excited state and emits phosphorescence.

[0102] In other words, in the energy transfer process of the above (2), it is important to efficiently transfer both the triplet excited energy and the singlet excited energy of the host molecule to the guest molecule as much as possible.

[0103] In view of the above energy transfer process, if the host molecule itself releases the excited energy as light or heat before the excited energy is transferred from the host molecule to the guest molecule, deactivation occurs, and the emission efficiency decreases. Here, the present inventors have found that when the host molecule is in a singlet excited state (the above (2-2)), compared to when the host molecule is in a triplet excited state (the above (2-1)), the energy is not easily transferred to the guest molecule of the phosphorescent compound, and the emission efficiency easily decreases. And, the present inventors have taken this as a problem. The reason can be found from the energy transfer process described in more detail as follows.

[0104] <Energy transfer process>

[0105] Hereinafter, the energy transfer process between molecules will be described in detail.

[0106] First, the following two mechanisms have been proposed as mechanisms of energy transfer between molecules. Here, the molecule that provides excitation energy is referred to as the host molecule, and the molecule that receives excitation energy is referred to as the guest molecule.

[0107] Forster Mechanism (Dipole-Dipole Interaction)

[0108] In the Förster mechanism (also known as Förster resonance energy transfer ( In resonance energy transfer, energy transfer does not require direct contact between molecules. Energy transfer occurs through the resonance phenomenon of dipole oscillations between the host molecule and the guest molecule. Through the resonance phenomenon of dipole oscillations, the host molecule supplies energy to the guest molecule, the host molecule is in the ground state, and the guest molecule is in the excited state. Formula (1) shows the rate constant of the Förster mechanism:

[0109] [Formula 1]

[0110]

[0111] In formula (1), ν represents frequency, f' h (ν) represents the normalized emission spectrum of the host molecule (fluorescence spectrum of singlet excited state energy transfer, phosphorescence spectrum of triplet excited state energy transfer), ε g (ν) represents the molar absorption coefficient of the guest molecule, N represents the Avogadro number, n represents the refractive index of the medium, R represents the intermolecular distance between the host molecule and the guest molecule, τ represents the lifetime of the measured excited state (fluorescence lifetime or phosphorescence lifetime), c represents the speed of light, φ represents the luminescence quantum yield (fluorescence quantum yield of singlet excited state energy transfer, phosphorescence quantum yield of triplet excited state energy transfer), K 2 A coefficient (0 to 4) representing the orientation of the transition dipole moment of the host molecule and the guest molecule. In addition, in the random orientation, K 2 =2 / 3.

[0112] Dexter Mechanism (Electron Exchange Interaction)

[0113] In the Dexter mechanism (also known as Dexter electron transfer), the host molecule and the guest molecule are close to the contact effective distance that produces orbital overlap, and the excited host molecule and the ground state guest molecule exchange electrons, resulting in energy transfer. Formula 2 shows the rate constant of the Dexter mechanism.

[0114] [Formula 2]

[0115]

[0116] In formula (2), h represents Planck's constant, K represents a constant having an energy dimension, v represents a frequency, f h (ν) represents a normalized emission spectrum of a host molecule (fluorescence spectrum from energy transfer of a singlet excited state, phosphorescence spectrum from energy transfer of a triplet excited state), ε g (ν) represents a normalized absorption spectrum of a guest molecule, L represents an effective molecular radius, and R represents an intermolecular distance between the host molecule and the guest molecule.

[0117] Here, it can be considered that the energy transfer efficiency Φ ET from the host molecule to the guest molecule is represented by formula (3). r In formula (3), k n represents a rate constant of a luminescence process (fluorescence from energy transfer of a singlet excited state, phosphorescence from energy transfer of a triplet excited state) of the host molecule, k ET represents a rate constant of a non-luminescence process (thermal deactivation or intersystem crossing) of the host molecule, and τ represents a lifetime of an excited state of the host molecule detected.

[0118] [Formula 3]

[0119]

[0120] First, from formula (3), in order to increase the energy transfer efficiency Φ ET , it is sufficient to further increase the rate constant k of energy transfer compared to other competing rate constants k r + k n (= 1 / τ). Furthermore, in order to increase the rate constant k of energy transfer, it is preferable that the emission spectrum of the host molecule (fluorescence spectrum from energy transfer of a singlet excited state, phosphorescence spectrum from energy transfer of a triplet excited state) and the absorption spectrum of the guest molecule overlap.

[0121] Here, the present inventors have considered that, in terms of the overlap of the emission spectrum of the host molecule and the absorption spectrum of the guest molecule, the absorption band on the longest wavelength (low energy) side of the absorption spectrum of the guest molecule is important.

[0122] In the present embodiment, a phosphorescent compound is used as the guest material. In the absorption spectrum of the phosphorescent compound, the absorption band considered to contribute most to luminescence is located at and around the absorption wavelength corresponding to the direct transition from the singlet ground state to the triplet excited state, which is the absorption band present on the longest wavelength side. Thus, it can be considered preferable that the emission spectrum (fluorescence spectrum and phosphorescence spectrum) of the host material overlaps with the absorption band on the longest wavelength side of the absorption spectrum of the phosphorescent compound.

[0123] For example, most organometallic complexes, especially luminescent iridium complexes, have a wide absorption band on the longest wavelength side in the vicinity of 500 nm to 600 nm (of course, the wide absorption band can be present on the shorter wavelength side or the longer wavelength side depending on the luminescent wavelength). This absorption band is mainly derived from the triplet MLCT (Metal to Ligand Charge Transfer) transition. However, it can be considered that this absorption band also includes absorption derived from the triplet π-π * transition and the singlet MLCT transition, which overlap with each other, forming a wide absorption band on the longest wavelength side of the absorption spectrum. In other words, the difference between the lowest singlet excited state and the lowest triplet excited state is small, and the absorptions derived from these excited states overlap with each other, forming a wide absorption band on the longest wavelength side of the absorption spectrum. Thus, when an organometallic complex (especially an iridium complex) is used as the guest material, it is preferable that the wide absorption band present on the longest wavelength side as described above overlaps with the emission spectrum of the host material more.

[0124] Here, first, the energy transfer from the triplet excited state of the host material is considered. As described above, in the energy transfer from the triplet excited state, it is preferable that the phosphorescence spectrum of the host material and the absorption band on the longest wavelength side of the guest material overlap with each other more.

[0125] Further, in general, since a fluorescent compound is used as the host material, the phosphorescence lifetime (τ) is very long, i.e., more than a millisecond (k r + n s). This is because the transition from the triplet excited state to the ground state (singlet) is a forbidden transition. As is clear from Equation (3), this is advantageous for the energy transfer efficiency Φ ET . This also means that the energy transfer from the triplet excited state of the host material to the triplet excited state of the guest material easily occurs.

[0126] However, at this time, energy transfer from the singlet excited state of the host material becomes a problem. When energy transfer from the singlet excited state is also efficiently performed in addition to energy transfer from the triplet excited state, it is necessary to design so that the fluorescence spectrum overlaps with the absorption band on the longest wavelength side of the guest material in addition to the phosphorescence spectrum of the host material, as is apparent from the above description. In other words, if the host material is not designed so that the fluorescence spectrum of the host material is located at substantially the same position as the phosphorescence spectrum, energy transfer from both the singlet excited state and the triplet excited state of the host material cannot be efficiently performed.

[0127] However, in general, the emission wavelength of fluorescence and the emission wavelength of phosphorescence are greatly different because the S1 level and the T1 level are greatly different (S1 level > T1 level) (emission wavelength of fluorescence < emission wavelength of phosphorescence). For example, in a light emitting element using a phosphorescent compound, 4,4'-bis(N-carbazolyl)biphenyl (abbreviation: CBP), which is generally used as a host material, has a phosphorescence spectrum around 500 nm and a fluorescence spectrum around 400 nm, and there is a gap of 100 nm between them. From the above example, it is extremely difficult to design a host material so that the fluorescence spectrum of the host material is located at substantially the same position as the phosphorescence spectrum. Thus, the present inventors and the like have considered that improvement in the energy transfer efficiency from the singlet excited state of the host material to the guest material is an important issue.

[0128] Further, the fluorescence lifetime (τ) of a fluorescent compound used as a host material is very short, i.e., on the order of nanoseconds (k r +k n seconds). This is because the transition from the singlet excited state to the ground state (singlet) is an allowed transition. As is apparent from Formula (3), this is not favorable for the energy transfer efficiency Φ ET This also means that energy transfer from the singlet excited state of the host material to the guest material does not easily occur.

[0129] One embodiment of the present application is a useful method that can overcome the problem of the energy transfer efficiency from the singlet excited state of the host material to the guest material.

[0130] Further, in the light-emitting element using the phosphorescent compound, the internal quantum efficiency can be theoretically 100 % because the singlet excitation state and the triplet excitation state can both be converted into light by utilizing intersystem crossing (see (1) Direct recombination process above). Moreover, it has been discussed that a light-emitting element whose external quantum efficiency is 20 % can have an internal quantum efficiency of almost 100 % (see (2) External quantum efficiency above). However, it is considered that the internal quantum efficiency of 100 % has not been achieved in the light-emitting element in the related art because of the energy transfer from the singlet excitation state of the host material, which has not been taken into account. This is because the present inventors have achieved an external quantum efficiency of 30 % by implementing one embodiment of the present application described below. In other words, an external quantum efficiency of at least 30 % corresponds to an internal quantum efficiency of 100 %, and one embodiment of the present application is a useful method for achieving the same. Note that it is estimated from this that an external quantum efficiency of 20 % in the related art corresponds to an internal quantum efficiency of less than or equal to 70 %.

[0131] <One embodiment of the present application>

[0132] One embodiment of the present application is a light-emitting element including a light-emitting layer containing a phosphorescent compound, a first organic compound, and a second organic compound between a pair of electrodes, in which a combination of the first organic compound and the second organic compound forms an exciplex.

[0133] The first organic compound and the second organic compound form an exciplex (also referred to as an excimer) by recombination of carriers (or singlet excitation state). When the formed exciplex emits light, the wavelength of the light is on the longer wavelength side with respect to each of the wavelengths of light (fluorescent wavelengths) of the first organic compound and the second organic compound. In other words, by forming an exciplex, the fluorescent spectrum of the first organic compound or the fluorescent spectrum of the second organic compound can be converted into an emission spectrum on the longer wavelength side.

[0134] Thus, as FIG. 25 is shown, the fluorescent spectrum of the first organic compound (or the second organic compound) is on the shorter wavelength side even when compared with an absorption band on the longest wavelength side of the phosphorescent compound and there is no overlap with the absorption band, and by forming an exciplex, an emission spectrum on the longer wavelength side can be obtained, so that the overlap with the absorption band can be increased. The light-emitting element of one embodiment of the present application can have high energy transfer efficiency because energy transfer is performed using the overlap between the emission spectrum of the exciplex and the absorption spectrum of the phosphorescent compound. Thus, one embodiment of the present application can achieve a light-emitting element with high external quantum efficiency.

[0135] Further, since the exciplex exists only in the excited state, there is no ground state that absorbs energy. Therefore, it can be considered that the phenomenon in which the energy of the singlet excited state and the energy of the triplet excited state of the phosphorescent compound are reversely transferred to the exciplex, and the phosphorescent compound is deactivated (that is, the luminous efficiency is reduced) before emitting light does not occur in principle. This also contributes to the improvement of the external quantum efficiency.

[0136] Further, it can be considered that the difference between the singlet excited energy and the triplet excited energy of the exciplex is extremely small. In other words, the emission spectrum of the singlet state of the exciplex is extremely close to the emission spectrum of the triplet excited state thereof. Thus, as described above, when the emission spectrum of the exciplex (generally, the emission spectrum of the singlet state of the exciplex) is designed to overlap with the absorption band on the longest wavelength side of the phosphorescent compound, the emission spectrum of the triplet state of the exciplex (which is not observed at ordinary temperature, and in many cases, is not observed even at low temperature) also overlaps with the absorption band on the longest wavelength side of the phosphorescent compound. More specifically, this means that the energy is efficiently transferred from the singlet state and the triplet state of the exciplex to the phosphorescent compound.

[0137] As to whether the exciplex actually has the above-described characteristics, the following examination is made using molecular orbital calculation. In general, the combination of a heteroaromatic compound and an aromatic amine forms an exciplex in many cases under the influence of the LUMO level (the property of easily accepting an electron) of the heteroaromatic compound which is deeper than the LUMO energy level of the aromatic amine (Lowest Unoccupied Molecular Orbital) and the HOMO level (the property of easily accepting a hole) of the aromatic amine which is shallower than the HOMO energy level of the heteroaromatic compound (Hightest Occupied Molecular Orbital). Thus, calculation is made using the combination of dibenzo[f,h]quinoxaline (abbreviation: DBq) which is a typical skeleton constituting the LUMO of the heteroaromatic compound and triphenylamine (abbreviation: TPA) which is a typical skeleton constituting the HOMO of the aromatic amine.

[0138] First, the optimal molecular structure and excitation energy in the lowest excited singlet state (S1) and the lowest excited triplet state (T1) of the DBq monomer and the TPA monomer were calculated using the time-dependent density functional method (TD-DFT). Further, the excitation energy of the dimer of DBq and TPA was calculated. The total energy of DFT is represented by the sum of the potential energy, the electrostatic energy between electrons, the kinetic energy of electrons, and the exchange correlation energy including all other complex interactions between electrons. In DFT, since the exchange correlation is approximately expressed using a functional (another function of a function) of a single-electron potential expressed in terms of electron density, the calculation speed is fast and the accuracy is high. Here, B3LYP, which is a hybrid functional, was used to define the weight of each parameter related to the exchange-correlation energy. Further, as a basis function, 6-311 (a basis function of a triple split valence basis class using three contraction functions for each atomic valence orbital) was applied to all atoms. With the above basis function, for example, with respect to a hydrogen atom, the orbitals from 1s to 3s are considered, and with respect to a carbon atom, the orbitals from 1s to 4s and from 2p to 4p are considered. Further, in order to improve the calculation accuracy, as a polarization basis class, a p function is added to the hydrogen atom and a d function is added to the atoms other than the hydrogen atom.

[0139] Further, as a quantum chemical calculation program, Gaussian 09 was used. The calculation was performed using a high-performance computer (manufactured by SGI Inc., Altix 4700).

[0140] First, the HOMO level and the LUMO level were calculated with respect to the DBq monomer, the TPA monomer, and the dimer of DBq and TPA. FIG. 64 The HOMO level and the LUMO level are shown, and FIG. 65(A1), 65(A2), 65(B1), 65(B2), 65(C1), and 65(C2) show the distribution of the HOMO and the LUMO.

[0141] FIG. 65(A1) shows the distribution of the LUMO of the DBq monomer, FIG. 65(A2) shows the distribution of the HOMO of the DBq monomer, FIG. 65(B1) shows the distribution of the LUMO of the TPA monomer, FIG. 65(B2) shows the distribution of the HOMO of the TPA monomer, FIG. 65(C1) shows the distribution of the LUMO of the dimer of DBq and TPA, and FIG. 65(C2) shows the distribution of the HOMO of the dimer of DBq and TPA.

[0142] As FIG. 64As shown, the dimer of DBq and TPA forms an exciplex of DBq and TPA, influenced by the LUMO level of DBq (-1.99 eV) which is deeper (lower) than the LUMO level of TPA, and the HOMO level of TPA (-5.21 eV) which is shallower (higher) than the HOMO level of DBq. In fact, it is known from FIG. 65(C1) and FIG. 65(C2) that the LUMO of the dimer of DBq and TPA is distributed on the DBq side, and the HOMO is distributed on the TPA side.

[0143] Next, the excitation energy obtained from the optimal molecular structure of S1 and T1 of the DBq monomer is shown. Here, the excitation energy of S1 and T1 corresponds to the wavelength of the fluorescence and phosphorescence emitted by the DBq monomer, respectively. The excitation energy of S1 of the DBq monomer is 3.294 eV, and the fluorescence wavelength is 376.4 nm. Further, the excitation energy of T1 of the DBq monomer is 2.460 eV, and the phosphorescence wavelength is 504.1 nm.

[0144] In addition, the excitation energy obtained from the optimal molecular structure of S1 and T1 of the TPA monomer is shown. Here, the excitation energy of S1 and T1 corresponds to the wavelength of the fluorescence and phosphorescence emitted by the TPA monomer, respectively. The excitation energy of S1 of the TPA monomer is 3.508 eV, and the fluorescence wavelength is 353.4 nm. Further, the excitation energy of T1 of the TPA monomer is 2.610 eV, and the phosphorescence wavelength is 474.7 nm.

[0145] Further, the excitation energy obtained from the optimal molecular structure of S1 and T1 of the dimer of DBq and TPA is shown. The excitation energy of S1 and T1 corresponds to the wavelength of the fluorescence and phosphorescence emitted by the dimer of DBq and TPA, respectively. The excitation energy of S1 of the dimer of DBq and TPA is 2.036 eV, and the fluorescence wavelength is 609.1 nm. Further, the excitation energy of T1 of the dimer of DBq and TPA is 2.030 eV, and the phosphorescence wavelength is 610.0 nm.

[0146] As is apparent from the above, either of the DBq monomer and the TPA monomer has a phosphorescence wavelength which is shifted to the long wavelength side by about 100 nm compared to the fluorescence wavelength. This result shows the same tendency as the CBP described above (actual measurement value), and this result supports the effectiveness of the calculation.

[0147] On the other hand, the fluorescence wavelength of the dimer of DBq and TPA is on the long wavelength side compared to the fluorescence wavelength of either the DBq monomer or the TPA monomer. This result shows the same tendency as the example described later (actual measurement value), and this result supports the effectiveness of the calculation. In addition, the difference between the fluorescence wavelength and the phosphorescence wavelength of the dimer of DBq and TPA is only 0.9 nm, and the wavelengths are almost equal.

[0148] From the above results, it can be said that the exciplex can integrate the singlet excited state energy and the triplet excited state energy at approximately the same energy. Thus, as described above, the exciplex can efficiently transfer energy from both the singlet excited state and the triplet excited state to the phosphorescent compound.

[0149] This effect is a special effect obtained by using the exciplex as a medium for energy transfer. In general, energy transfer from the singlet excited state or the triplet excited state of the host material to the phosphorescent compound is considered. On the other hand, one embodiment of the present application is greatly different from the prior art in that an exciplex (an exciplex of the first organic compound and the second organic compound) is first formed by the host material and another material, and energy transfer from the exciplex is used. Moreover, the above difference can provide high emission efficiency that has not been achieved in the past.

[0150] In addition, in general, when an exciplex is used for a light-emitting layer of a light-emitting element, the emission color can be suppressed, but the emission efficiency is generally greatly reduced. Thus, it has been considered that an exciplex is not suitable for obtaining a light-emitting element with high efficiency. However, as described in one embodiment of the present application, the present inventors have found that by using an exciplex as a medium for energy transfer to a phosphorescent compound, the emission efficiency can be improved to the limit. This is a technical idea contrary to the existing fixed concept.

[0151] In order to make the emission spectrum of the exciplex and the absorption spectrum of the guest material sufficiently overlap, it is preferable that the difference between the energy of the peak of the emission spectrum and the energy of the peak of the absorption band on the lowest energy side of the absorption spectrum be within 0.3 eV. It is more preferable that the difference be within 0.2 eV, and it is most preferable that the difference be within 0.1 eV.

[0152] In one embodiment of the present application, an exciplex is formed using a singlet exciton of the first organic compound or the second organic compound.

[0153] In the light-emitting element of one embodiment of the present application, the basic process of forming an exciplex is to form a singlet exciton in one of the first organic compound and the second organic compound and then interact with the other in the ground state. As described above, since the emission spectrum of the exciplex overlaps with the absorption spectrum of the phosphorescent compound to a large extent, the energy transfer efficiency can be improved. Thus, a light-emitting element with high external quantum efficiency can be realized.

[0154] Since the excited lifetime of the singlet exciton is short (τ is small) as described above, there is a problem in that part of the excited energy is deactivated (emission or thermal deactivation) before the excited energy is transferred to the guest material (Φ ETtendency). However, in one embodiment of the present application, since singlet excitons rapidly form an exciplex, the deactivation of this excitation energy can be suppressed. Further, it is considered that since the excitation lifetime of an exciplex is long, energy transfer efficiency Φ ET Thus, by applying one embodiment of the present application, deactivation of singlet excitation energy of a host material can be suppressed, which affects not only the efficiency but also the lifetime of a device, and thus a long-lifetime light-emitting element can be realized.

[0155] In one embodiment of the present application, it is also preferable that excitation energy of an exciplex be sufficiently transferred to a phosphorescent compound, and that luminescence from the exciplex be substantially not observed. Thus, it is preferable that energy be transferred from an exciplex to a phosphorescent compound so that the phosphorescent compound emits phosphorescence.

[0156] From the concept of energy transfer described above, one embodiment of the present application is effective when at least one of a first organic compound and a second organic compound is a fluorescent compound (i.e., a compound that easily causes luminescence from a singlet excited state or thermal deactivation). Thus, it is preferable that at least one of a first organic compound and a second organic compound be a fluorescent compound.

[0157] Further, when a phosphorescent compound is used as an organic compound for a host material, the organic compound itself easily emits light, and energy is not easily transferred to a guest material. In this case, it is sufficient that the organic compound efficiently emit light, but since the organic compound for a host material has a problem of concentration quenching, it is difficult to achieve high light-emitting efficiency. Thus, it is preferable that the organic compound be a fluorescent compound and that energy be transferred by the above structure.

[0158] Further, in one embodiment of the present application, it is preferable that a phosphorescent compound be an organometallic complex.

[0159] The exciplex utilized in one embodiment of the present application is described below in detail.

[0160] <Exciplex>

[0161] An exciplex is formed through interaction between different molecules in an excited state. It is generally known that an exciplex is easily formed between a material having a relatively deep LUMO level and a material having a relatively shallow HOMO level.

[0162] The luminescence wavelength depends on the energy difference between the HOMO level and the LUMO level. The luminescence wavelength is shorter when the energy difference is large, and the luminescence wavelength is longer when the energy difference is small.

[0163] In this specification, one embodiment of the present application is applied to a first organic compound and a second organic compound whose HOMO level and LUMO level are different. Specifically, the energy levels are in the following order from low to high: the HOMO level of the first organic compound < the HOMO level of the second organic compound < the LUMO level of the first organic compound < the LUMO level of the second organic compound (see FIG. 17 ).

[0164] In the formation of an exciplex using the above two organic compounds, the LUMO level of the exciplex is derived from the first organic compound, and the HOMO level is derived from the second organic compound (see FIG. 17 ). Thus, the energy difference of the exciplex is smaller than the energy difference of the first organic compound and the energy difference of the second organic compound. In other words, the emission wavelength of the exciplex is longer than each of the emission wavelengths of the first organic compound and the second organic compound.

[0165] The formation process of the exciplex used in one embodiment of the present application is roughly divided into two processes.

[0166] Electroplex

[0167] In this specification, the term "electroplex" means that an exciplex is directly formed using a first organic compound in a ground state and a second organic compound in a ground state.

[0168] As described above, generally, when an electron and a hole are recombined in a host material, excitation energy is transferred from the host material in an excited state to a guest material, so that the guest material is in an excited state and emits light.

[0169] Here, before excitation energy is transferred from the host material to the guest material, the host material itself emits light or excitation energy is converted into thermal energy, so that excitation energy is partially deactivated. In particular, when the host material is in a singlet excited state, deactivation of singlet excitation energy easily occurs because the excited lifetime is short as compared with the case where the host material is in a triplet excited state. Deactivation of excitation energy is one of reasons for reduction in the lifetime of a light-emitting element.

[0170] However, in one embodiment of the present application, since an electroplex is formed using a first organic compound and a second organic compound each of which has a carrier (cation or anion), formation of a singlet excimer whose excited lifetime is short can be inhibited. In other words, there is a process in which an exciplex is directly formed in a state where a singlet excimer is not formed. Thus, deactivation of singlet excitation energy described above can also be inhibited. Accordingly, a light-emitting element with a long lifetime can be realized.

[0171] For example, in the case where the first organic compound is a compound having an electron-trapping property and the second organic compound is a compound having a hole-trapping property, an exciplex is directly formed using an anion of the first organic compound and a cation of the second organic compound. There has been no concept of inhibiting generation of a singlet excited state of a host material in the above manner and transferring energy from the exciplex to a guest material to obtain a light-emitting element with high emission efficiency. Furthermore, it is considered that an exciplex is directly formed because generation of a triplet excited state of a host material is also inhibited, and energy is transferred from the exciplex to a guest material. This mechanism has also been unknown.

[0172] Furthermore, the emission spectrum of the formed exciplex is on the long-wavelength side compared with each of the emission wavelengths of the first organic compound and the second organic compound.

[0173] The emission spectrum of the exciplex overlaps with the absorption spectrum of the phosphorescent compound more than the emission spectrum of the first organic compound (or the second organic compound) overlaps with the absorption spectrum of the phosphorescent compound. The light-emitting element of one embodiment of the present application has high energy transfer efficiency because energy is transferred by the overlap between the emission spectrum of the exciplex and the absorption spectrum of the phosphorescent compound. Thus, one embodiment of the present application can achieve a light-emitting element with high external quantum efficiency.

[0174] "Formation of Exciplex by Exciton"

[0175] As another process, a basic process in which a singlet exciton is formed in one of the first organic compound and the second organic compound and then interacts with the other in the ground state to form an exciplex can be considered. Unlike an exciplex, in this case, a singlet excited state of the first organic compound or the second organic compound is temporarily generated, but since the singlet excited state is rapidly converted into an exciplex, deactivation of singlet excitation energy can be inhibited. Thus, deactivation of excitation energy of the first organic compound or the second organic compound can be inhibited. Thus, one embodiment of the present application can achieve a light-emitting element with a long lifetime. Furthermore, it is considered that a triplet excited state of a host material is also rapidly converted into an exciplex, and energy is transferred from the exciplex to a guest material.

[0176] The emission spectrum of the formed exciplex is on the long-wavelength side compared with each of the emission wavelengths of the first organic compound and the second organic compound.

[0177] The emission spectrum of the exciplex overlaps with the absorption spectrum of the phosphorescent compound more than the emission spectrum of the first organic compound (or the second organic compound) overlaps with the absorption spectrum of the phosphorescent compound. The light-emitting element of one embodiment of the present application has a high energy transfer efficiency because energy is transferred using the overlap between the emission spectrum of the exciplex and the absorption spectrum of the phosphorescent compound. Thus, one embodiment of the present application can achieve a light-emitting element with high external quantum efficiency.

[0178] For example, in the case where the first organic compound is a compound having electron-trapping property and the second organic compound is a compound having hole-trapping property, when the difference in HOMO level and the difference in LUMO level of these compounds are large (specifically, the difference is greater than or equal to 0.3 eV), electrons are selectively injected into the first organic compound and holes are selectively injected into the second organic compound. In this case, it is considered that the process of forming an electrically generated exciplex is preferentially performed compared to the process of forming an exciplex through a singlet excitation.

[0179] This embodiment mode can be combined as appropriate with other embodiment modes.

[0180] (Embodiment 2)

[0181] In this embodiment, reference is made to FIG. 16A to FIG. 16C A light-emitting element of one embodiment of the present application is described.

[0182] FIG. 16A FIG. 1 is a diagram illustrating a light-emitting element including an EL layer between a first electrode and a second electrode. FIG. 16A The light-emitting element in FIG. 1 includes a hole-injection layer 701, a hole-transport layer 702, a light-emitting layer 703, an electron-transport layer 704, an electron-injection layer 705, and a second electrode 108 which are stacked in this order over a first electrode 103.

[0183] The first electrode 103 is preferably formed using any of a metal, an alloy, a conductive compound, a mixture thereof, or the like having a high work function (specifically, 4.0 eV or more). Specifically, for example, indium tin oxide (ITO), indium tin oxide containing silicon or silicon oxide, indium zinc oxide (Indium Zinc Oxide), indium oxide containing tungsten oxide and zinc oxide (IWZO), or the like can be given. These conductive metal oxide films are generally formed by a sputtering method, but can also be formed using a sol-gel method or the like. For example, by a sputtering method, using a target in which 1 wt% to 20 wt% of zinc oxide is added to indium oxide, an indium zinc oxide film can be formed. Further, by a sputtering method, using a target in which 0.5 wt% to 5 wt% of tungsten oxide and 0.1 wt% to 1 wt% of zinc oxide are added to indium oxide, an IWZO film can be formed. Further, graphene, gold, platinum, nickel, tungsten, chromium, molybdenum, iron, cobalt, copper, palladium, or a nitride of a metal material (for example, titanium nitride), or the like can be given.

[0184] However, in the EL layer 102, when the layer formed in contact with the first electrode 103 is formed using a composite material formed by mixing an organic compound described later and an acceptor, any of a metal, an alloy, a conductive compound, a mixture thereof, or the like can be used as a material for the first electrode 103 without considering the size of the work function. For example, aluminum, silver, an aluminum-containing alloy (for example, Al-Si), or the like can also be used.

[0185] The first electrode 103 can be formed, for example, by a sputtering method or an evaporation method (including a vacuum evaporation method), or the like.

[0186] The second electrode 108 is preferably formed using any of a metal, an alloy, a conductive compound, a mixture thereof, or the like having a low work function (preferably 3.8 eV or less). Specifically, an element belonging to Group 1 or Group 2 of the periodic table, that is, an alkali metal such as lithium or cesium, an alkaline earth metal such as calcium or strontium, magnesium, an alloy containing the above metal (for example, Mg-Ag, Al-Li), a rare earth metal such as europium or ytterbium, an alloy containing the above metal, aluminum, silver, or the like can be used.

[0187] However, when the layer formed in contact with the second electrode 108 in the EL layer 102 is formed using a composite material formed by mixing an organic compound described later and a donor, any of various conductive materials, for example, Al, Ag, ITO, indium tin oxide containing silicon or silicon oxide, or the like can be used without considering the size of the work function.

[0188] In addition, a vacuum evaporation method or a sputtering method can be used when forming the second electrode 108. Further, in the case of using silver paste or the like, a coating method, an inkjet method, or the like can be employed.

[0189] The EL layer 102 has at least a light-emitting layer 703. Part of the EL layer 102 can use a known substance, and either of a low molecular compound and a high molecular compound can be used. In addition, the substance forming the EL layer 102 includes not only a substance composed of only an organic compound but also a substance in which a part of the substance includes an inorganic compound.

[0190] The EL layer 102 includes, in addition to the light-emitting layer 703, an appropriate combination of a hole-injection layer 701 including a substance having a high hole-injection property, a hole-transport layer 702 including a substance having a high hole-transport property, an electron-transport layer 704 including a substance having a high electron-transport property, and an electron-injection layer 705 including a substance having a high electron-injection property, as shown in FIG. 7. FIG. 16A

[0191] The hole-injection layer 701 is a layer including a substance having a high hole-injection property. As the substance having a high hole-injection property, a metal oxide such as molybdenum oxide, titanium oxide, vanadium oxide, rhenium oxide, ruthenium oxide, chromium oxide, zirconium oxide, hafnium oxide, tantalum oxide, silver oxide, tungsten oxide, or manganese oxide can be used. Further, a phthalocyanine-based compound such as phthalocyanine (abbreviation: H2Pc) or copper phthalocyanine (II) (abbreviation: CuPc) can be used.

[0192] Alternatively, an aromatic amine compound or the like of a low molecular organic compound 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), 3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole (abbreviation: PCzPCN1), or the like can be used.

[0193] ​In addition, a high molecular compound (e.g., an oligomer, a dendrimer, or a polymer) 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), poly[N,N'-bis(4-butylphenyl)-N,N'-bis(phenyl)benzidine] (abbreviation: Poly-TPD), and a high molecular compound to which an acid is added such as poly(3,4-ethylenedioxythiophene) / poly(styrenesulfonic acid) (PEDOT / PSS) or polyaniline / poly(styrenesulfonic acid) (PAni / PSS), and the like can be used.

[0194] A complex material formed by mixing an organic compound and an electron acceptor can be used for the hole injection layer 701. Such a complex material has excellent hole-injection property and hole-transport property because holes are generated in the organic compound by the electron acceptor. In this case, the organic compound is preferably a material (a substance having a high hole-transport property) that has excellent properties in transporting the generated holes.

[0195] As the organic compound used for the complex material, various compounds such as an aromatic amine compound, a carbazole derivative, an aromatic hydrocarbon, and a high molecular compound (e.g., an oligomer, a dendrimer, or a polymer) can be used. As the organic compound of the complex material, an organic compound having a high hole-transport property is preferably used. Specifically, a substance with a hole mobility of 10 -6 cm 2 or more is preferably used. Note that in addition to the above substances, any substance with a higher hole-transport property than an electron-transport property can be used. Specific examples of the organic compound that can be used for the complex material are given below.

[0196] As an organic compound which can be used for the composite material, for example, it can be an aromatic amine compound such as TDATA, MTDATA, DPAB, DNTPD, DPA3B, PCzPCA1, PCzPCA2, PCzPCN1, 4,4'-bis[N-(l-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB or α-NPD), N,N'-bis(3-methylphenyl)-N,N'-diphenyl-[l,r-biphenyl]-4,4'-diamine (abbreviation: TPD), 4-phenyl-4'-(9-phenylfluorene-9-yl)triphenylamine (abbreviation: BPAFLP); and a carbazole derivative 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), 1,4-bis[4-(N-carbazolyl)phenyl]-2,3,5,6-tetraphenylbenzene.

[0197] Other examples of the organic compound which can be used are aromatic hydrocarbon compounds such as 2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation: t-BuDNA), 2-tert-butyl-9,10-di(l-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-l-naphthyl)anthracene (abbreviation: DMNA), 9,10-bis[2-(l-naphthyl)phenyl)]-2-tert-butylanthracene, 9,10-bis[2-(l-naphthyl)phenyl]anthracene, 2,3,6,7-tetramethyl-9,10-di(l-naphthyl)anthracene.

[0198] Other examples of the organic compound which can be used are aromatic hydrocarbon compounds such as 2,3,6,7-tetramethyl-9,10-di(2-naphthyl)anthracene, 9,9'-bianthracene, 10,10'-diphenyl-9,9'-bianthracene, 10,10'-bis(2-phenylphenyl)-9,9'-bianthracene, 10,10'-bis[(2,3,4,5,6-pentaphenyl)phenyl]-9,9'-bianthracene, anthracene, tetracene, rubrene, perylene, 2,5,8,11-tetra(tert-butyl)perylene, pentacene, coronene, 4,4'-bis(2,2-diphenylvinyl)biphenyl (abbreviation: DPVBi), 9,10-bis[4-(2,2-diphenylvinyl)phenyl]anthracene (abbreviation: DPVPA), and the like.

[0199] Further, as the electron acceptor, there can be mentioned an organic compound such as 7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation: F4-TCNQ), chloranil, a transition metal oxide, and an oxide of a metal belonging to Groups 4 to 8 of the periodic table. Specifically, vanadium oxide, niobium oxide, tantalum oxide, chromium oxide, molybdenum oxide, tungsten oxide, manganese oxide, and rhenium oxide are preferably used because these metal oxides have high electron accepting properties. Among them, molybdenum oxide is particularly preferably used because it is stable in the air, has low hygroscopicity, and is easy to handle.

[0200] The above-described high molecular compound such as PVK, PVTPA, PTPDMA, or Poly-TPD and the above-described electron acceptor can be used to form a composite material, which is used for the hole injection layer 701.

[0201] The hole transport layer 702 is a layer containing a substance having high hole transport property. As the substance having high hole transport property, an aromatic amine compound such as NPB, TPD, BPAFLP, 4,4'-bis[N-(9,9-dimethylfluorene-2-yl)-N-phenylamino]biphenyl (abbreviation: DFLDPBi), or 4,4'-bis[N-(spiro-9,9'-difluorene-2-yl)-N-phenylamino]biphenyl (abbreviation: BSPB), or the like can be used. The substance described here is mainly a substance whose hole mobility is 10 -6 cm 2 or more. Note that, in addition to the above-described substances, any substance whose hole transport property is higher than its electron transport property can be used. Further, the layer containing a substance having high hole transport property is not limited to a single layer, and two or more layers each containing any of the above-described substances can be stacked.

[0202] As the hole transport layer 702, a carbazole derivative such as CBP, CzPA, PCzPA, or the like, or an anthracene derivative such as t-BuDNA, DNA, DPAnth, or the like can also be used.

[0203] As the hole transport layer 702, a high molecular compound such as PVK, PVTPA, PTPDMA, or Poly-TPD can also be used.

[0204] The light-emitting layer 703 is a layer containing a light-emitting substance. The light-emitting layer 703 of this embodiment has a phosphorescent compound, a first organic compound, and a second organic compound. The phosphorescent compound is a light-emitting substance (guest material). In the light-emitting layer 703, the compound having a higher content among the first organic compound and the second organic compound is a host material. Specifically, refer to Embodiment 1.

[0205] As the phosphorescent compound, an organic metal complex is preferably used, and an iridium complex is particularly preferably used. In addition, when considering the energy transfer of the above-mentioned Förster mechanism, the molar absorption coefficient of the absorption band on the longest wavelength side of the phosphorescent compound is preferably 2000 M -1 cm -1 Above, especially preferably 5000M -1 cm -1 above. Examples of compounds having such a large molar absorption coefficient include bis(3,5-dimethyl-2-phenylpyrazinato)(dipivaloylmethane)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)(dipivaloylmethane)iridium(III) (abbreviation: [Ir(tppr)2(dpm)]), (acetylacetonato)bis(6-methyl-4-phenylpyrimidinato)iridium(III) (abbreviation: [Ir(mppm)2(acac)]), and (acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III) (abbreviation: [Ir(tBuppm)2(acac)]). Especially when using a molar absorption coefficient of 5000M -1 cm -1 When the above materials are used, such as [Ir(dppm)2(acac)], a light-emitting element with an external quantum efficiency of about 30% can be obtained.

[0206] As the first organic compound and the second organic compound, a combination of any one of compounds which easily accept an electron (typically, heteroaromatic compounds) such as 2-[3-(dibenzo-thiophene-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation: 2mDBTPDBq-II), 2-[4-(3,6-diphenyl-9H-carbazol-9-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation: 2CzPDBq-III), 7-[3-(dibenzo-thiophene-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation: 7mDBTPDBq-II), 6-[3-(dibenzo-thiophene-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation: 6mDBTPDBq-II), 2-[4-(dibenzo-thiophene-4-yl)phenyl]-l-phenyl-lH-benzimidazole (abbreviation: DBTBIm-II), and any one of compounds which easily accept a hole (typically, aromatic amine compounds and carbazole compounds) such as 4,4'-bis[N-(l-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB or α-NPD), 4-phenyl-4'-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PCBA1BP), 4,4'-di(l-naphthyl)-4"-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PCBNBB), 4-(l-naphthyl)-4'-phenyltriphenylamine (abbreviation: αNBA1BP), 2,7-bis[N-(diphenylamino phenyl)-N-phenylamino]-spiro-9,9'-bifluorene (abbreviation: DPA2SF), 4,4',4"-tris[N-(l-naphthyl)-N-phenylamino]triphenylamine (abbreviation: l'-TNATA), 9-phenyl-9H-3-(9-phenyl-9H-carbazol-3-yl)carbazole (abbreviation: PCCP), and the like is preferable. By employing the above structures, not only an effect of improving luminous efficiency and lifetime by energy transfer from an exciplex but also an effect of improving luminous efficiency and lifetime by adjusting the balance of carriers of hole transport and electron transport in a light-emitting layer can be obtained. However, the present application is not limited to these compounds, as long as the combination can form an exciplex.

[0207] Further, an exciplex can also be formed in an interface between two layers. For example, when a layer containing a second organic compound and a layer containing a first organic compound are stacked, an exciplex is formed in the vicinity of the interface thereof. The two layers can be used as a light-emitting layer in one embodiment of the present application. In this case, a phosphorescent compound is added to the vicinity of the interface. Further, a phosphorescent compound can be added to one or both of the two layers.

[0208] The electron transport layer 704 is a layer containing a substance having high electron transportability. As the substance having high electron transportability, 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, bis[2-(2-hydroxyphenyl)benzoxazolato]zinc (abbreviation: Zn(BTZ)2), and the like can be given. Further, heteroaromatic compounds such as 2-(4-biphenyl)-5-(4-tert-butylphenyl)-l,3,4-oxadiazole (abbreviation: PBD), 1,3-bis[5-(p-tert-butylphenyl)-l,3,4-oxadiazol-2-yl]benzene (abbreviation: OXD-7), 3-(4-tert-butylphenyl)-4-phenyl-5-(4-biphenylyl)-l,2,4-triazole (abbreviation: TAZ), 3-(4-tert-butylphenyl)-4-(4-ethylphenyl)-5-(4-biphenylyl)-l,2,4-triazole (abbreviation: p-EtTAZ), bathophenanthroline (abbreviation: BPhen), bathocuproin (abbreviation: BCP), 4,4'-bis(5-methylbenzoxazol-2-yl)stilbene (abbreviation: BzO s), and the like can be used. Further, high molecular compounds such as poly(2,5-pyridinediyl) (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) can be used. The substances described here are mainly substances having an electron mobility of 10 -6 cm 2 or more. Note that another substance other than the above substances can be used as the electron transport layer as long as the hole transportability thereof is higher than the electron transportability.

[0209] In addition, the electron transport layer is not limited to a single layer, and can be a stack of two or more layers containing any of the above-described substances.

[0210] The electron injection layer 705 is a layer containing a substance having high electron injection property. The electron injection layer 705 can use 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 above-described substances for the electron transport layer 704.

[0211] Alternatively, a composite material formed by mixing an organic compound with an electron donor can also be used for the electron injection layer 705. This composite material has excellent electron injection and electron transport properties because electrons are generated in the organic compound by the electron donor. In this case, the organic compound is preferably a material with excellent performance in terms of transporting the generated electrons. Specifically, for example, any of the above-mentioned substances constituting the electron transport layer 704 (such as metal complexes or heteroaromatic compounds, etc.) can be used. A substance that shows electron donation to the organic compound can be used as an electron donor. Specifically, alkali metals, alkaline earth metals and rare earth metals, such as lithium, cesium, magnesium, calcium, erbium and ytterbium, are preferably used. In addition, any alkali metal oxide or alkaline earth metal oxide, such as lithium oxide, calcium oxide and barium oxide, and a Lewis base such as magnesium oxide or an organic compound such as tetrathiafulvalene (abbreviation: TTF) are preferably used.

[0212] Note that the hole injection layer 701, the hole transport layer 702, the light-emitting layer 703, the electron transport layer 704, and the electron injection layer 705 can be formed by evaporation (including vacuum evaporation), inkjet, coating, or the like.

[0213] like FIG. 16B As shown, a plurality of EL layers may be stacked between the first electrode 103 and the second electrode 108. In this case, it is preferable to provide a charge generation layer 803 between the stacked first EL layer 800 and the second EL layer 801. The charge generation layer 803 may be formed using the composite material described above. Alternatively, the charge generation layer 803 may have a stacked structure in which a layer containing the composite material and a layer containing other materials are stacked. In this case, as the layer containing the other material, a layer containing a substance with an electron-donating property and a substance with a high electron-transporting property, a layer composed of a transparent conductive film, etc. may be used. A light-emitting element having such a structure is less prone to problems such as energy transfer or quenching, and since the range of material selection can be expanded, it is easy to obtain a light-emitting element having both high luminous efficiency and a long life. In addition, it is easy to obtain a light-emitting element that emits phosphorescence from one EL layer and fluorescence from another EL layer. This structure can be used in combination with the structure of the above-mentioned EL layer.

[0214] Furthermore, by making the emission colors of each EL layer different, the light-emitting element as a whole can emit light of a desired color. For example, in a light-emitting element having two EL layers, by making the light emitted by the first and second EL layers complement each other, the light-emitting element as a whole can emit white light. This approach can also be applied to light-emitting elements having three or more EL layers.

[0215] like FIG. 16CAs shown, the EL layer 102 can also have a hole injection layer 701, a hole transport layer 702, a light emitting layer 703, an electron transport layer 704, an electron injection buffer layer 706, an electron relay layer 707, and a composite layer 708 in contact with the second electrode 108 between the first electrode 103 and the second electrode 108.

[0216] By providing the composite layer 708 in contact with the second electrode 108, damage to the EL layer 102 can be reduced, particularly when the second electrode 108 is formed using a sputtering method, and is therefore preferable. The composite layer 708 can be formed using a composite material in which the organic compound having high hole transport properties includes an acceptor material.

[0217] Further, by providing the electron injection buffer layer 706, the injection barrier between the composite layer 708 and the electron transport layer 704 can be reduced, and thus electrons generated in the composite layer 708 can be easily injected into the electron transport layer 704.

[0218] As the electron injection buffer layer 706, a material having high electron injection properties can be used, such as an alkali metal, an alkaline earth metal, a rare earth metal, and a compound thereof (e.g., an alkali metal compound (including oxides such as lithium oxide, halides, carbonates such as lithium carbonate or cesium carbonate), an alkaline earth metal compound (including oxides, halides, and carbonates), or a rare earth metal compound (including oxides, halides, and carbonates)).

[0219] When the electron injection buffer layer 706 includes a material having high electron transport properties and a donor material, the donor material is preferably added so that the mass ratio of the donor material to the material having high electron transport properties is 0.001:1 to 0.1:1. In addition, as the donor material, an organic compound such as tetrathianaphthacene (abbreviation: TTN), nickelocene, or decamethylnickelocene, and an alkali metal, an alkaline earth metal, a rare earth metal, and a compound thereof (e.g., an alkali metal compound (including oxides such as lithium oxide, halides, carbonates such as lithium carbonate or cesium carbonate), an alkaline earth metal compound (including oxides, halides, and carbonates), or a rare earth metal compound (including oxides, halides, and carbonates)) can be used. In addition, as the material having high electron transport properties, the same material as the electron transport layer 704 described above can be used.

[0220] Further, it is preferable to form an electron relay layer 707 between the electron injection buffer layer 706 and the composite layer 708. The electron relay layer 707 is not necessarily provided, but by providing an electron relay layer 707 having high electron transport properties, electrons can be rapidly transported to the electron injection buffer layer 706.

[0221] The structure in which the electron relay layer 707 is interposed between the composite layer 708 and the electron injection buffer layer 706 is a structure in which the acceptor substance contained in the composite layer 708 and the donor substance contained in the electron injection buffer layer 706 do not easily interact with each other and do not easily influence the respective functions. Thus, it is possible to prevent an increase in driving voltage.

[0222] The electron relay layer 707 contains a substance having high electron transportability, and the LUMO level of the substance having high electron transportability is set to be between the LUMO level of the acceptor substance contained in the composite layer 708 and the LUMO level of the substance having high electron transportability contained in the electron transport layer 704. In addition, when the electron relay layer 707 contains a donor substance, the donor level of the donor substance is also set to be between the LUMO level of the acceptor substance contained in the composite layer 708 and the LUMO level of the substance having high electron transportability contained in the electron transport layer 704. As to the specific values of the levels, it is preferable that the LUMO level of the substance having high electron transportability contained in the electron relay layer 707 be greater than or equal to -5.0 eV, and more preferably greater than or equal to -5.0 eV and less than or equal to -3.0 eV.

[0223] As the substance having high electron transportability contained in the electron relay layer 707, it is preferable to use a phthalocyanine-based material or a metal complex having a metal-oxygen bond and an aromatic ligand.

[0224] As the phthalocyanine-based material contained in the electron relay layer 707, specifically, it is preferable to use any one of CuPc; SnPc (Phthalocyanine tin(II) complex); ZnPc (Phthalocyanine zinc complex); CoPc (Cobalt(II) phthalocyanine, β-form); FePc (Phthalocyanine Iron); and PhO-VOPc (Vanadyl 2,9,16,23-tetraphenoxy-29H,31H-phthalocyanine).

[0225] As the metal complex having a metal-oxygen bond and an aromatic ligand included in the electron-relaying layer 707, a metal complex having a double bond of metal-oxygen is preferably used. Since the double bond of metal-oxygen has an acceptor property (a property of easily accepting electrons), movement (donation and acceptance) of electrons becomes easier. Further, it is considered that the metal complex having a double bond of metal-oxygen is stable. Thus, by using the metal complex having a double bond of metal-oxygen, the light-emitting element can be driven more stably at a low voltage.

[0226] As the metal complex having a metal-oxygen bond and an aromatic ligand, a phthalocyanine-based material is preferably used. Specifically, any of VOPc (Vanadyl phthalocyanine), SnOPc (Phthalocyanine tin(IV) oxide complex), and TiOPc (Phthalocyanine titanium oxide complex) is preferably used because the double bond of metal-oxygen is easily interacted with other molecules in the molecular structure and has a high acceptor property.

[0227] In addition, as the phthalocyanine-based material described above, a phthalocyanine-based material having a phenoxy group is preferably used. Specifically, a phthalocyanine derivative having a phenoxy group such as PhO-VOPc is preferably used. The phthalocyanine derivative having a phenoxy group can be dissolved in a solvent. Thus, there are advantages that the phthalocyanine derivative having a phenoxy group is easy to handle at the time of forming a light-emitting element and is easy to maintain an apparatus used for film formation.

[0228] The electron-relaying layer 707 can further include a donor substance. As the donor substance, an organic compound such as tetrathianaphthacene (abbreviation: TTN), nickelocene, or decamethylnickelocene, and an alkali metal, an alkaline earth metal, a rare earth metal, and a compound thereof (e.g., an alkali metal compound (including an oxide such as lithium oxide, a halide, lithium carbonate, or cesium carbonate; an alkaline earth metal compound (including an oxide, a halide, and a carbonate); and a rare earth metal compound (including an oxide, a halide, and a carbonate)) can be used. When such a donor substance is included in the electron-relaying layer 707, electrons are easily moved, and the light-emitting element can be driven at a lower voltage.

[0229] When the donor substance is contained in the electron relay layer 707, as a substance having high electron transportability, a substance whose LUMO level is higher than the acceptor level of the acceptor substance contained in the composite material layer 708 can be used in addition to the above-mentioned substances. Specifically, a substance whose LUMO level is higher than or equal to -5.0 eV, further preferably higher than or equal to -5.0 eV and lower than or equal to -3.0 eV is preferably used. As such a substance, for example, a perylene derivative, a nitrogen-containing condensed ring aromatic compound, or the like can be given. Further, since the nitrogen-containing condensed ring aromatic compound has high stability, it is preferable to use the electron relay layer 707.

[0230] As specific examples of the perylene derivative, the following substances can be given: 3,4,9,10-perylenetetracarboxylic dianhydride (abbreviation: PTCDA); 3,4,9,10- perylenetetracarboxylic bisbenzimidazole (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.

[0231] As specific examples of the nitrogen-containing condensed ring aromatic compound, the following substances can be given: pyrazino[2,3-f][l,10]phenanthroline-2,3-dicarbonitrile (abbreviation: PPDN), 2,3,6,7,10,11-hexacyano-l,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.

[0232] In addition to the above-mentioned substances, the following substances can be used: 7,7,8,8-tetracyanoquinodimethane (abbreviation: TCNQ); 1,4,5,8-naphthalenetetracarboxylic dianhydride (abbreviation: NTCDA), perfluoropentacene; copper hexadecafluorophthalocyanine (abbreviation: CuPc); N,N'-bis(2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-pentadecafluorooctyl)-l,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), methenylfulteropyrrolidine (e.g., [6,6]-phenyl C61- butyric acid methyl ester) and the like. 16 61 In addition to the above-mentioned substances, the following substances can be used: 7,7,8,8-tetracyanoquinodimethane (abbreviation: TCNQ); 1,4,5,8-naphthalenetetracarboxylic dianhydride (abbreviation: NTCDA), perfluoropentacene; copper hexadecafluorophthalocyanine (abbreviation: CuPc); N,N'-bis(2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-pentadecafluorooctyl)-l,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), methenylfulteropyrrolidine (e.g., [6,6]-phenyl C61- butyric acid methyl ester) and the like.

[0233] Further, when the electron relay layer 707 contains the donor substance, the electron relay layer 707 can be formed by, for example, a method in which a substance having high electron transportability and a donor substance are co-evaporated.​

[0234] 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 above-described materials.

[0235] By the above steps, the EL layer 102 of the present embodiment can be manufactured.

[0236] In the above light-emitting element, current flows due to a potential difference generated between the first electrode 103 and the second electrode 108, and light is emitted due to recombination of holes and electrons in the EL layer 102. Furthermore, the light is extracted to the outside through either or both of the first electrode 103 and the second electrode 108. Thus, either or both of the first electrode 103 and the second electrode 108 becomes an electrode having a light-transmitting property with respect to visible light.

[0237] Note that the structure of the layer provided between the first electrode 103 and the second electrode 108 is not limited to the above structure. A structure different from the above structure can be employed as long as a light-emitting region in which holes and electrons recombine is provided in a portion apart from the first electrode 103 and the second electrode 108, so that quenching due to proximity of the light-emitting region to a metal is prevented.

[0238] That is, there is no particular limitation on the stacked structure of the layers. Layers containing a substance having a high electron-transport property, a substance having a high hole-transport property, a substance having a high electron-injection property, a substance having a high hole-injection property, a bipolar substance (a substance having a high electron-transport property and a high hole-transport property), or a hole-blocking material, and the like can be freely combined with the light-emitting layer.

[0239] By using the light-emitting element described in the present embodiment, a passive matrix light-emitting device or an active matrix light-emitting device in which the driving of the light-emitting element is controlled by a transistor can be manufactured. Furthermore, the light-emitting device can be applied to electronic devices, lighting devices, or the like.

[0240] As described above, the light-emitting element of one embodiment of the present application can be manufactured.

[0241] The present embodiment can be combined with any of the other embodiments as appropriate.

[0242] [Example 1]

[0243] In this embodiment, one example of a combination of a first organic compound, a second organic compound, and a phosphorescent compound which can be applied to the light-emitting element of one embodiment of the present application will be described with reference to FIG. 1A and FIG. 1B [Example 2]

[0244] The phosphorescent compound used in this embodiment is bis(3,5-dimethyl-2-phenylpyrazine) (dipivaloylmethane) iridium (III) (abbreviation: [Ir(mppr-Me)2(dpm)]). The first organic compound used in this embodiment is 2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation: 2mDBTPDBq-II). The second organic compound used in this embodiment is 4,4'-di(1-naphthyl)-4"-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PCBNBB). The chemical formulas of the materials used in this embodiment are shown below.

[0245]

[0246] <Absorption spectrum>

[0247] FIG. 1A and FIG. 1B The following table shows the ultraviolet-visible absorption spectrum (hereinafter simply referred to as the absorption spectrum) of a dichloromethane solution of [Ir(mppr-Me)2(dpm)], a phosphorescent compound. The absorption spectrum was measured using a UV-visible spectrophotometer (V550, manufactured by JASCO Corporation) with the dichloromethane solution (0.093 mmol / L) placed in a quartz cell at room temperature.

[0248] <Emission Spectrum>

[0249] FIG. 1A and FIG. 1B Also shown are the emission spectra of a thin film of 2mDBTPDBq-II as the first organic compound (emission spectrum 1), the emission spectrum of a thin film of PCBNBB as the second organic compound (emission spectrum 2), and the emission spectrum of a thin film of a mixed material of 2mDBTPDBq-II and PCBNBB (emission spectrum 3). FIG. 1A In the figure, the horizontal axis represents the wavelength (nm), and the vertical axis represents the molar absorption coefficient ε (M -1 cm -1 ) and luminous intensity (arbitrary unit). FIG. 1B In the figure, the horizontal axis represents energy (eV) and the vertical axis represents the molar absorption coefficient ε (M -1 cm -1 ) and luminous intensity (arbitrary units).

[0250] from FIG. 1A The absorption spectrum of [Ir(mppr-Me)2(dpm)] shows that it has a broad absorption band around 520nm. This absorption band is considered to contribute significantly to luminescence.

[0251] In comparison with the emission spectrum 1, 2, the peak of the emission spectrum 3 is located on the long wavelength (low energy) side. Further, the peak of the emission spectrum 3 is located closer to the absorption band than the peaks of the emission spectrum 1, 2. From this, it is known that the emission spectrum having the largest overlap with the absorption band which is considered to contribute greatly to emission in the absorption spectrum is the emission spectrum 3. Specifically, the difference between the peak of the absorption band (the shoulder peak around 520 nm) in the absorption spectrum and the peak of the emission spectrum 3 is 0.04 eV. FIG. 1A and FIG. 1B From this, it is known that the emission spectrum having the largest overlap with the absorption band which is considered to contribute greatly to emission in the absorption spectrum is the emission spectrum 3. Specifically, the difference between the peak of the absorption band (the shoulder peak around 520 nm) in the absorption spectrum and the peak of the emission spectrum 3 is 0.04 eV.

[0252] In comparison with the emission spectrum of the monomer, the peak of the emission spectrum of the mixed material of 2mDBTPDBq-II and PCBNBB is located on the long wavelength (low energy) side. From this, it is known that an exciplex is formed by mixing 2mDBTPDBq-II and PCBNBB.

[0253] It is found that the emission spectrum of the mixed material has a large overlap with the absorption band which is considered to contribute greatly to emission in the absorption spectrum of [Ir(mppr-Me)2(dpm)]. Therefore, since the light-emitting element using the mixed material of 2mDBTPDBq-II and PCBNBB and [Ir(mppr-Me)2(dpm)] utilizes the overlap of the emission spectrum of the mixed material with the absorption spectrum of the phosphorescent compound, energy transfer, the energy transfer efficiency is high. Thus, a light-emitting element having a high external quantum efficiency can be obtained.

[0254] [Example 2]

[0255] In this embodiment, an example of a combination of a first organic compound, a second organic compound, and a phosphorescent compound which can be applied to one embodiment of the light-emitting element of the present application is described with reference to FIG. 2A and FIG. 2B In this embodiment, an example of a combination of a first organic compound, a second organic compound, and a phosphorescent compound which can be applied to one embodiment of the light-emitting element of the present application is described with reference to

[0256] The phosphorescent compound used in this embodiment is (acetylacetonato)bis(4,6-diphenylpyrimidinato)iridium(III) (abbreviation: [Ir(dppm)2(acac)]). The first organic compound used in this embodiment is 2mDBTPDBq-II. The second organic compound used in this embodiment is PCBNBB. The chemical formulas of the materials used in this embodiment are shown below. Note that the chemical formulas of the materials used in Example 1 are omitted.

[0257]

[0258] <Absorption spectrum>

[0259] FIG. 2A and FIG. 2BThe following shows the ultraviolet-visible absorption spectrum (hereinafter simply referred to as the absorption spectrum) of a dichloromethane solution of [Ir(dppm)2(acac)], a phosphorescent compound. The absorption spectrum was measured using a UV-visible spectrophotometer (V550, manufactured by JASCO Corporation) with a dichloromethane solution (0.093 mmol / L) placed in a quartz cell at room temperature.

[0260] <Emission Spectrum>

[0261] also, FIG. 2A and FIG. 2B The emission spectrum of a thin film of 2mDBTPDBq-II as the first organic compound (emission spectrum 4), the emission spectrum of a thin film of PCBNBB as the second organic compound (emission spectrum 5), and the emission spectrum of a thin film of a mixed material of 2mDBTPDBq-II and PCBNBB (emission spectrum 6) are shown. FIG. 2A In the figure, the horizontal axis represents the wavelength (nm), and the vertical axis represents the molar absorption coefficient ε (M -1 cm -1 ) and luminous intensity (arbitrary unit). FIG. 2B In the figure, the horizontal axis represents energy (eV) and the vertical axis represents the molar absorption coefficient ε (M -1 cm -1 ) and luminous intensity (arbitrary units).

[0262] from FIG. 2A The absorption spectrum of [Ir(dppm)2(acac)] shows that it has a broad absorption band around 520nm. This absorption band is believed to contribute significantly to luminescence.

[0263] The peak of emission spectrum 6 is located at the long wavelength (low energy) side compared to emission spectra 4 and 5. Moreover, the peak of emission spectrum 6 is located closer to the absorption band than the peaks of emission spectra 4 and 5. FIG. 2A and FIG. 2B It can be seen that the emission spectrum that has the greatest overlap with the absorption band that contributes significantly to luminescence in the absorption spectrum is emission spectrum 6. Specifically, the difference between the peak (515 nm) of this absorption band in the absorption spectrum and the peak of emission spectrum 6 is 0.02 eV.

[0264] Compared to the emission spectrum of the monomers, the peak of the emission spectrum of the mixed material of 2mDBTPDBq-II and PCBNBB is located at a longer wavelength (lower energy), indicating that the mixing of 2mDBTPDBq-II and PCBNBB forms an exciplex.

[0265] The peak of the mixed material's emission spectrum significantly overlaps with an absorption band in the [Ir(dppm)2(acac)] absorption spectrum, which is believed to contribute significantly to luminescence. Therefore, a light-emitting device comprising a mixed material of 2mDBTPDBq-II and PCBNBB and [Ir(dppm)2(acac)] utilizes the overlap between the emission spectrum of the mixed material and the absorption spectrum of the phosphorescent compound to transfer energy, resulting in high energy transfer efficiency. Consequently, a light-emitting device with high external quantum efficiency can be obtained.

[0266] [Example 3]

[0267] In this embodiment, referring to FIG. 15 A light-emitting element according to one embodiment of the present invention will be described. The chemical formulas of the materials used in this embodiment are shown below. The chemical formulas of the materials used in the above embodiments are omitted.

[0268]

[0269] The following describes a method for manufacturing the light-emitting element 1 of this example and the comparative light-emitting element 2.

[0270] (Light-emitting element 1)

[0271] First, a film of indium tin oxide containing silicon oxide (ITSO) was formed on a glass substrate 1100 by sputtering to form a first electrode 1101 serving as an anode. The thickness was set to 110 nm, and the electrode area was set to 2 mm x 2 mm.

[0272] Next, as pretreatment for forming a light-emitting element over the substrate 1100, the substrate surface was washed with water and baked at 200° C. for 1 hour, and then subjected to UV ozone treatment for 370 seconds.

[0273] Then, the substrate is placed in the inner portion of the device and the pressure is reduced to 10 -4 The substrate 1100 is vacuum-baked in a vacuum deposition apparatus at about 170° C. for 30 minutes in a heating chamber of the vacuum deposition apparatus, and then cooled for about 30 minutes.

[0274] Next, the substrate 1100 having the first electrode 1101 formed thereon is fixed on a substrate holder in a vacuum evaporation apparatus with the surface having the first electrode 1101 formed thereon facing downward, and the pressure in the vacuum evaporation apparatus is reduced to 10°C. -4Bathocuproin (abbreviation: BPhen) was deposited on the first electrode 1101 to a thickness of 20 nm to form a hole-transport layer 1112. The thickness of the hole-transport layer 1112 was set to 20 nm.

[0275] Next, BPhen was formed to a thickness of 20 nm over the hole-transport layer 1112 to form a second electron-transport layer 1114b.

[0276] Further, 2mDBTPDBq-II, PCBNBB, and [Ir(mppr-Me)2(dpm)] were deposited to form a light-emitting layer 1113 over the hole-transport layer 1112. Here, the weight ratio of 2mDBTPDBq-II, PCBNBB, and [Ir(mppr-Me)2(dpm)] was adjusted to be 0.8:0.2:0.05 (=2mDBTPDBq-II:PCBNBB:[Ir(mppr-Me)2(dpm)]). The thickness of the light-emitting layer 1113 was set to 40 nm.

[0277] Next, a 2mDBTPDBq-II film was formed to a thickness of 10 nm over the light-emitting layer 1113 to form a first electron-transport layer 1114a.

[0278] Next, a BPhen film was formed to a thickness of 20 nm over the first electron-transport layer 1114a to form a second electron-transport layer 1114b.

[0279] Then, a lithium fluoride (LiF) film was formed to a thickness of 1 nm over the second electron-transport layer 1114b to form an electron-injection layer 1115.

[0280] Finally, an aluminum film was formed to a thickness of 200 nm by deposition as a second electrode 1103 which served as a cathode, whereby the light-emitting element 1 of this embodiment was manufactured.

[0281] (Comparative Light-Emitting Element 2)

[0282] The light-emitting layer 1113 of the comparative light-emitting element 2 was formed by depositing 2mDBTPDBq-II and [Ir(mppr-Me)2(dpm)] together. Here, the weight ratio of 2mDBTPDBq-II and [Ir(mppr-Me)2(dpm)] was adjusted to be 1:0.05 (=2mDBTPDBq-II:[Ir(mppr-Me)2(dpm)]). The thickness of the light-emitting layer 1113 was set to 40 nm. The comparative light-emitting element 2 was manufactured in the same manner as in the light-emitting element 1, except for the light-emitting layer 1113.

[0283] Note that in the above evaporation process, resistance heating method was used for evaporation.

[0284] Table 1 shows the element structures of the light-emitting element 1 and the comparative light-emitting element 2 obtained through the above steps.

[0285] [Table 1]

[0286]

[0287] The light-emitting element 1 and the comparative light-emitting element 2 were sealed in a glove box in a nitrogen atmosphere in such a manner that the light-emitting elements were not exposed to the air, and then the operation characteristics of these light-emitting elements were measured. Note that the measurement was performed at room temperature (in an atmosphere kept at 25 °C).

[0288] FIG. 3 The current density-luminance characteristics of the light-emitting element 1 and the comparative light-emitting element 2 are shown. In FIG. 3 , the horizontal axis represents current density (mA / cm 2 ), and the vertical axis represents luminance (cd / m 2 ). In addition, FIG. 4 The voltage-luminance characteristics are shown. In FIG. 4 , the horizontal axis represents voltage (V), and the vertical axis represents luminance (cd / m 2 ). FIG. 5 The luminance-current efficiency characteristics are shown. In FIG. 5 , the horizontal axis represents luminance (cd / m 2 ), and the vertical axis represents current efficiency (cd / A). FIG. 6 The luminance-external quantum efficiency characteristics are shown. In FIG. 6 , the horizontal axis represents luminance (cd / m 2 ), and the vertical axis represents external quantum efficiency (%).

[0289] In addition, Table 2 shows the voltage (V), current density (mA / cm 2 ), CIE color coordinates (x, y), current efficiency (cd / A), power efficiency (lm / W), and external quantum efficiency (%) of the light-emitting element 1 and the comparative light-emitting element 2 at a luminance of about 1000 cd / m 2 .

[0290] [Table 2]

[0291]

[0292] FIG. 7 The emission spectra when a current of 0.1 mA was passed through the light-emitting element 1 and the comparative light-emitting element 2 are shown. In FIG. 7In the graph, the horizontal axis represents wavelength (nm), and the vertical axis represents luminous intensity (arbitrary unit). As shown in Table 2, the luminescent element 1 emits orange light at a luminance of 1200 cd / m 2 The CIE color coordinates of the luminescent element 1 at a luminance of 960 cd / m 2 The CIE color coordinates of the comparative luminescent element 2 at a luminance of 960 cd / m

[0293] From Table 2 and FIG. 3 to FIG. 6 It is known that the current efficiency, the power efficiency, and the external quantum efficiency of the luminescent element 1 are higher than those of the comparative luminescent element 2.

[0294] In the luminescent element 1 of the present embodiment, 2mDBTPDBq-II, PCBNBB, and [Ir(mppr-Me)2(dpm)] shown in Example 1 are used for the luminescent layer. From Example 1, it is known that the emission spectrum of the mixed material of 2mDBTPDBq-II and PCBNBB (the emission spectrum of the exciplex) greatly overlaps the absorption spectrum of [Ir(mppr-Me)2(dpm)] compared with the emission spectrum of 2mDBTPDBq-II or PCBNBB monomer. The luminescent element 1 of the present embodiment is considered to have high energy transfer efficiency because energy transfer is performed using the overlap, and thus the external quantum efficiency is higher than that of the comparative luminescent element 2.

[0295] From the above results, it is known that, by applying one embodiment of the present application, a luminescent element having high external quantum efficiency can be realized.

[0296] Next, a reliability test of the luminescent element 1 and the comparative luminescent element 2 was performed. FIG. 8 The results of the reliability test are shown. In the graph, the vertical axis represents normalized luminance (%) in the case where the initial luminance is 100%, and the horizontal axis represents the driving time (h) of the element. FIG. 8

[0297] In the reliability test, the initial luminance was set to 5000 cd / m 2 , and the luminescent element 1 and the comparative luminescent element 2 were driven under the condition where the current density was constant.

[0298] The luminance of the comparative luminescent element 2 after 120 hours was 58% of the initial luminance. The luminance of the luminescent element 1 after 630 hours was 65% of the initial luminance. From the above results, it is known that the lifetime of the luminescent element 1 is longer than that of the comparative luminescent element 2.

[0299] From the above results, it is known that, by applying one embodiment of the present application, a luminescent element having high reliability can be realized.​

[0300] [Example 4]

[0301] In this example, the light-emitting element of one embodiment of the present application is described with reference to FIG. 15 A light-emitting element of one embodiment of the present application is described. Since the materials used in this example are the materials used in the above examples, the chemical formulas are omitted.

[0302] A method for manufacturing the light-emitting element 3 of this example is described below.

[0303] (Light-emitting element 3)

[0304] First, an ITSO film was formed on a glass substrate 1100 by a sputtering method, whereby a first electrode 1101 serving as an anode was formed. The thickness thereof was set to 110 nm, and the electrode area thereof was set to 2 mm x 2 mm.

[0305] Next, as a pretreatment for forming a light-emitting element on the substrate 1100, after the substrate surface was washed with water and baked at 200 °C for 1 hour, UV ozone treatment was performed for 370 seconds.

[0306] Then, the substrate was put into a vacuum evaporation apparatus whose inside was reduced in pressure to 10 -4 Pa or lower, and after vacuum baking at 170 °C for 30 minutes in a heating chamber of the vacuum evaporation apparatus, the substrate 1100 was cooled for about 30 minutes.

[0307] Next, the substrate 1100 on which the first electrode 1101 was formed was fixed to a substrate holder in the vacuum evaporation apparatus so that the surface on which the first electrode 1101 was formed faced downward, the pressure in the vacuum evaporation apparatus was reduced to 10 -4 Pa or lower, and then BPAFLP and molybdenum (VI) oxide were co-evaporated on the first electrode 1101, whereby a hole-injection layer 1111 was formed. The thickness of the hole-injection layer 1111 was set to 40 nm, and the weight ratio of BPAFLP to molybdenum (VI) oxide was adjusted to 4:2 (= BPAFLP : molybdenum (VI) oxide).

[0308] Next, a BPAFLP film with a thickness of 20 nm was formed on the hole-injection layer 1111, whereby a hole-transport layer 1112 was formed.

[0309] Further, co-evaporation of 2mDBTPDBq-II, PCBNBB, and [Ir(dppm)2(acac)] is performed on the hole-transport layer 1112 to form a light-emitting layer 1113. Here, the weight ratio of 2mDBTPDBq-II, PCBNBB, [Ir(dppm)2(acac)] is adjusted to 0.8:0.2:0.05 (= 2mDBTPDBq-II:PCBNBB:[Ir(dppm)2(acac)]). The thickness of the light-emitting layer 1113 is set to 40 nm.

[0310] Next, a 2mDBTPDBq-II film having a thickness of 10 nm is formed on the light-emitting layer 1113 to form a first electron-transport layer 1114a.

[0311] Next, a BPhen film having a thickness of 20 nm is formed on the first electron-transport layer 1114a to form a second electron-transport layer 1114b.

[0312] Then, a LiF film having a thickness of 1 nm is formed on the second electron-transport layer 1114b by evaporation to form an electron-injection layer 1115.

[0313] Finally, an aluminum film having a thickness of 200 nm is formed by evaporation as a second electrode 1103 serving as a cathode, thereby manufacturing the light-emitting element 3 of this embodiment.

[0314] Note that in the above evaporation process, evaporation is performed by a resistance heating method.

[0315] Table 3 shows the element structure of the light-emitting element 3 obtained by the above steps.

[0316] [Table 3]

[0317]

[0318] The light-emitting element 3 is sealed in a glove box in a nitrogen atmosphere in such a manner that the light-emitting element is not exposed to the atmosphere, and then the operation characteristics of the light-emitting element are measured. Further, the measurement is performed at room temperature (in an atmosphere kept at 25 °C).

[0319] FIG. 9 The current density-luminance characteristics of the light-emitting element 3 are shown. In FIG. 9 , the horizontal axis represents current density (mA / cm 2 ), and the vertical axis represents luminance (cd / m 2 ). FIG. 10 The voltage-luminance characteristics are shown. In FIG. 10 , the horizontal axis represents voltage (V), and the vertical axis represents luminance (cd / m 2 ). FIG. 11 The luminance-current efficiency characteristics are shown. InFIG. 11 In the figure, the horizontal axis represents the brightness (cd / m 2 ), and the vertical axis represents the current efficiency (cd / A). FIG. 12 The brightness-external quantum efficiency characteristics are shown. FIG. 12 In the figure, the horizontal axis represents the brightness (cd / m 2 ), and the vertical axis represents the external quantum efficiency (%).

[0320] In addition, Table 4 shows that the light emitting element 3 has a brightness of 1100 cd / m 2 The voltage (V), current density (mA / cm 2 ), CIE color coordinates (x, y), current efficiency (cd / A), power efficiency (lm / W) and external quantum efficiency (%).

[0321] [Table 4]

[0322]

[0323] FIG. 13 The emission spectrum obtained when a current of 0.1 mA is applied to the light emitting element 3 is shown. FIG. 13 In the table, the horizontal axis represents wavelength (nm) and the vertical axis represents luminous intensity (arbitrary unit). As shown in Table 4, 1100 cd / m 2 The CIE color coordinates of the light-emitting element 3 at a brightness of 100 nm were (x, y) = (0.54, 0.46). From the above results, it can be seen that the light-emitting element 3 emits orange light derived from [Ir(dppm)2(acac)].

[0324] From Table 4 and FIG. 9 to FIG. 12 It can be seen that the current efficiency, power efficiency, and external quantum efficiency of the light emitting element 3 are high. In particular, 1100 cd / m 2 At a brightness of 100 nm, the external quantum efficiency of light-emitting element 3 was extremely high, at 28%. As mentioned above, the upper limit of external quantum efficiency is considered to be around 25%. However, this result exceeded this limit.

[0325] In the light-emitting element of this example, 2mDBTPDBq-II, PCBNBB, and [Ir(dppm)2(acac)], described in Example 2, were used in the light-emitting layer. As shown in Example 2, the emission spectrum of the mixed material of 2mDBTPDBq-II and PCBNBB (the emission spectrum of the exciplex) overlaps significantly with the absorption spectrum of [Ir(dppm)2(acac)] compared to the emission spectrum of either 2mDBTPDBq-II or PCBNBB alone. Because the light-emitting element of this example utilizes this overlap for energy transfer, it is believed that energy transfer efficiency is high, resulting in unprecedentedly high external quantum efficiency.

[0326] The results of Example 2 show that the peak on the absorption band on the longest wavelength side of the absorption spectrum of the guest material for the light-emitting element 3 is close to the peak of the emission spectrum, and the molar absorption coefficient of the peak is large (> 5000 M -1 cm -1 ). From the above results, it is found that the light-emitting element 3 has a very high energy transfer efficiency, and thus has a higher external quantum efficiency than ever achieved.

[0327] From the above results, it is found that, by applying one embodiment of the present application, an element with a high external quantum efficiency can be realized.

[0328] Next, a reliability test of the light-emitting element 3 was performed. FIG. 14 The results of the reliability test are shown. In FIG. 14 , the vertical axis represents a normalized luminance (%) in the case where the initial luminance is 100 %, and the horizontal axis represents the driving time (h) of the element.

[0329] In the reliability test, the initial luminance was set to 5000 cd / m 2 , and the light-emitting element 3 was driven under the condition where the current density was constant.

[0330] The light-emitting element 3 maintained 92 % of the initial luminance after being driven for 320 hours.

[0331] From the above results, it is found that, by applying one embodiment of the present application, an element with high reliability can be realized.

[0332] [Example 5]

[0333] In this embodiment, one example of a combination of a first organic compound, a second organic compound, and a phosphorescent compound which can be applied to one embodiment of the present application will be described with reference to FIG. 18A and FIG. 18B

[0334] The phosphorescent compound used in this embodiment is [Ir(dppm)2(acac)]. The first organic compound used in this embodiment is 2mDBTPDBq-II. The second organic compound used in this embodiment is 4-phenyl-4'-(9-phenyl-9H-carbazole-3-yl)diphenylamine (abbreviation: PCBA1BP). The chemical formulas of the materials used in this embodiment are shown below. Note that the chemical formulas of the materials used in the above embodiments are omitted.

[0335]

[0336] Absorption spectrum

[0337] FIG. 18A and FIG. 18B ​An ultraviolet-visible absorption spectrum (hereinafter, simply referred to as an absorption spectrum) of a dichloromethane solution of [Ir(dppm)2(acac)] as a phosphorescent compound was shown. In the measurement of the absorption spectrum, an ultraviolet-visible spectrophotometer (V550 manufactured by Japan Spectroscopic Co., Ltd.) was used, the dichloromethane solution (0.093 mmol / L) was placed in a quartz cell, and the measurement was performed at room temperature.

[0338] <emission spectrum>

[0339] Further, FIG. 18A and FIG. 18B An emission spectrum of a thin film of 2mDBTPDBq-II as the first organic compound (emission spectrum 7), an emission spectrum of a thin film of PCBA1BP as the second organic compound (emission spectrum 8), and an emission spectrum of a thin film of a mixed material of 2mDBTPDBq-II and PCBA1BP (emission spectrum 9) were shown. In the emission spectrum 7, the emission spectrum 8, and the emission spectrum 9, the horizontal axis represents wavelength (nm), and the vertical axis represents molar absorption coefficient ε (M-1cm-1) and luminous intensity (arbitrary unit). FIG. 18A FIG. 18B -1 ·cm -1 -1 ·cm -1

[0340] From the absorption spectrum, it was found that [Ir(dppm)2(acac)] has a wide absorption band around 520 nm. It was considered that this absorption band is an absorption band that contributes greatly to emission. FIG. 18A

[0341] Compared with the emission spectra 7 and 8, the emission spectrum 9 has a peak on the long wavelength (low energy) side. Further, the peak of the emission spectrum 9 is located closer to the absorption band than the peaks of the emission spectra 7 and 8. From this, it was found that the emission spectrum that has the greatest overlap with the absorption band that contributes greatly to emission in the absorption spectrum is the emission spectrum 9. Specifically, the difference between the peak of the absorption band (515 nm) in the absorption spectrum and the peak of the emission spectrum 9 is 0.02 eV. FIG. 18A FIG. 18B Compared with the emission spectra of the monomers, the emission spectrum of the mixed material of 2mDBTPDBq-II and PCBA1BP has a peak on the long wavelength (low energy) side. From this, it was found that by mixing 2mDBTPDBq-II and PCBA1BP, an exciplex is formed.

[0342] Compared with the emission spectra of the monomers, the emission spectrum of the mixed material of 2mDBTPDBq-II and PCBA1BP has a peak on the long wavelength (low energy) side. From this, it was found that by mixing 2mDBTPDBq-II and PCBA1BP, an exciplex is formed.

[0343] ​​​​​​The peak of the emission spectrum of the mixed material greatly overlaps with an absorption band in the absorption spectrum of [Ir(dppm)2(acac)], which can be considered to greatly contribute to luminescence. Therefore, the luminescent element using the mixed material of 2mDBTPDBq-II and PCBA1BP and [Ir(dppm)2(acac)] has a high energy transfer efficiency because of the overlap of the emission spectrum of the mixed material with the absorption spectrum of the phosphorescent compound. Thus, it is shown that a luminescent element having a high external quantum efficiency can be obtained.

[0344] [Example 6]

[0345] In this embodiment, the luminescent element of one embodiment of the present application is described with reference to FIG. 15 A luminescent element of one embodiment of the present application is described. Since the materials used in this embodiment are the materials used in the above embodiments, the chemical formulas are omitted.

[0346] A method for manufacturing the luminescent element 4 of this embodiment is described below.

[0347] (Luminescent element 4)

[0348] First, an ITSO film is formed on a glass substrate 1100 by a sputtering method, whereby a first electrode 1101 serving as an anode is formed. The thickness thereof is set to 110 nm, and the electrode area thereof is set to 2 mm x 2 mm.

[0349] Next, as a pretreatment for forming a luminescent element on the substrate 1100, after the substrate surface is washed with water and baked at 200 °C for 1 hour, UV ozone treatment is performed for 370 seconds.

[0350] Then, the substrate is put into a vacuum evaporation apparatus whose inside is reduced to a pressure of 10 -4 Pa, and vacuum baking at 170 °C for 30 minutes is performed in a heating chamber of the vacuum evaporation apparatus, and then the substrate 1100 is cooled for about 30 minutes.

[0351] Next, the substrate 1100 on which the first electrode 1101 is formed is fixed to a substrate holder in the vacuum evaporation apparatus so that the surface on which the first electrode 1101 is formed faces downward, and the pressure in the vacuum evaporation apparatus is reduced to a pressure of 10 -4 Pa, and then BPAFLP and molybdenum (VI) oxide are co-evaporated on the first electrode 1101, whereby a hole injecting layer 1111 is formed. The thickness of the hole injecting layer 1111 is set to 40 nm, and the weight ratio of BPAFLP to molybdenum (VI) oxide is adjusted to 4:2 (= BPAFLP : molybdenum (VI) oxide).

[0352] Next, a BPAFLP film having a thickness of 20 nm is formed on the hole injecting layer 1111, whereby a hole transporting layer 1112 is formed.

[0353] Further, co-evaporation of 2mDBTPDBq-II, PCBA1BP, and [Ir(dppm)2(acac)] is performed on the hole-transport layer 1112 to form a light-emitting layer 1113. Here, the weight ratio of 2mDBTPDBq-II, PCBA1BP, [Ir(dppm)2(acac)] is adjusted to 0.8:0.2:0.1 (= 2mDBTPDBq-II:PCBA1BP:[Ir(dppm)2(acac)]). The thickness of the light-emitting layer 1113 is set to 40 nm.

[0354] Next, a 2mDBTPDBq-II film having a thickness of 15 nm is formed on the light-emitting layer 1113 to form a first electron-transport layer 1114a.

[0355] Next, a BPhen film having a thickness of 15 nm is formed on the first electron-transport layer 1114a to form a second electron-transport layer 1114b.

[0356] Then, a LiF film having a thickness of 1 nm is formed on the second electron-transport layer 1114b by evaporation to form an electron-injection layer 1115.

[0357] Finally, an aluminum film having a thickness of 200 nm is formed by evaporation as a second electrode 1103 serving as a cathode, thereby manufacturing the light-emitting element 4 of this embodiment.

[0358] Note that in the above evaporation process, evaporation is performed by a resistance heating method.

[0359] Table 5 shows the element structure of the light-emitting element 4 obtained by the above steps.

[0360] [Table 5]

[0361]

[0362] The light-emitting element 4 is sealed in a glove box in a nitrogen atmosphere in such a manner that the light-emitting element is not exposed to the atmosphere, and then the operation characteristics of the light-emitting element are measured. Further, the measurement is performed at room temperature (in an atmosphere kept at 25 °C).

[0363] FIG. 19 The current density-luminance characteristics of the light-emitting element 4 are shown. In FIG. 19 , the horizontal axis represents current density (mA / cm 2 ), and the vertical axis represents luminance (cd / m 2 ). FIG. 20 The voltage-luminance characteristics are shown. In FIG. 20 , the horizontal axis represents voltage (V), and the vertical axis represents luminance (cd / m 2 ).FIG. 21 The luminance-current efficiency characteristics are shown. In FIG. 21 the horizontal axis indicates the luminance (cd / m 2 ), and the vertical axis indicates the current efficiency (cd / A). FIG. 22 The luminance-external quantum efficiency characteristics are shown. In FIG. 22 the horizontal axis indicates the luminance (cd / m 2 ), and the vertical axis indicates the external quantum efficiency (%).

[0364] Further, Table 6 shows the voltage (V), the current density (mA / cm 2 ), the CIE color coordinates (x, y), the current efficiency (cd / A), the power efficiency (lm / W), and the external quantum efficiency (%) of the light emitting element 4 at a luminance of 1100 cd / m 2

[0365] [Table 6]

[0366]

[0367] FIG. 23 The emission spectrum obtained when a current of 0.1 mA is applied to the light emitting element 4 is shown. In FIG. 23 the horizontal axis indicates the wavelength (nm), and the vertical axis indicates the emission intensity (arbitrary unit). As shown in Table 6, the CIE color coordinates of the light emitting element 4 at a luminance of 1100 cd / m 2 2 are (x, y) = (0.57, 0.43). From the above results, it is known that the light emitting element 4 shows orange light derived from [Ir(dppm)2(acac)].

[0368] From Table 6 and FIG. 19 to FIG. 22 , it is known that the current efficiency, the power efficiency, and the external quantum efficiency of the light emitting element 4 are high, respectively. Especially, the external quantum efficiency of the light emitting element 4 at a luminance of 1100 cd / m 2 is extremely high, i.e., 31%. As described above, the upper limit of the external quantum efficiency is considered to be about 25%. However, this time the result exceeds the limit.

[0369] In the light-emitting element of this embodiment, 2mDBTPDBq-II, PCBA 1 BP, and [Ir(dppm)2(acac)] shown in Example 5 were used for the light-emitting layer. As is clear from Example 5, the emission spectrum of the mixture of 2mDBTPDBq-II and PCBA 1 BP (the emission spectrum of the exciplex) has a large overlap with the absorption spectrum of [Ir(dppm)2(acac)] compared to the emission spectrum of the 2mDBTPDBq-II or PCBA 1 BP monomer. The light-emitting element of this embodiment can be considered to have a high energy transfer efficiency because of the use of the overlap, and can achieve an unprecedently high external quantum efficiency.

[0370] In the results of Example 5, the peak on the side of the longest wavelength of the absorption band of the guest material used in the light-emitting element 4 is close to the peak of the emission spectrum, and the molar absorption coefficient of the peak is large (> 5000 M -1 ·cm -1 ). From the above results, the light-emitting element 4 has an unprecedently high external quantum efficiency because of a particularly high energy transfer efficiency.

[0371] From the above results, it is clear that by applying one embodiment of the present application, a light-emitting element with a high external quantum efficiency can be realized.

[0372] Next, a reliability test of the light-emitting element 4 was performed. FIG. 24 The results of the reliability test are shown. In FIG. 24 , the vertical axis represents the normalized luminance (%) in the case where the initial luminance is 100%, and the horizontal axis represents the driving time (h) of the element.

[0373] In the reliability test, the initial luminance was set to 5000 cd / m 2 , and the light-emitting element 4 was driven under the condition where the current density was constant.

[0374] The light-emitting element 4 maintained 95% of the initial luminance after 170 hours of driving.

[0375] From the above results, it is clear that by applying one embodiment of the present application, a light-emitting element with high reliability can be realized.

[0376] [Example 7]

[0377] In this embodiment, one example of a combination of a first organic compound, a second organic compound, and a phosphorescent compound, which can be applied to a light-emitting element of one embodiment of the present application, is described with reference to FIG. 26A and FIG. 26B

[0378] ​The phosphorescent compound used in this example is [Ir(dppm)2(acac)]. The first organic compound used in this example is 2mDBTPDBq-II. The second organic compound used in this example is 4,4'-bis[N-(l-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB). The chemical formulas of the materials used in this example are shown below. Also, the chemical formulas of the materials used in the above examples are omitted.

[0379]

[0380] Absorption spectrum

[0381] FIG. 26A and FIG. 26B An ultraviolet-visible absorption spectrum (hereinafter, simply referred to as an absorption spectrum) of a dichloromethane solution of [Ir(dppm)2(acac)] as a phosphorescent compound is shown. In the measurement of the absorption spectrum, an ultraviolet-visible spectrophotometer (V550 manufactured by Japan Spectroscopic Co., Ltd.) was used, the dichloromethane solution (0.093 mmol / L) was placed in a quartz cell, and the measurement was performed at room temperature.

[0382] Emission spectrum

[0383] Further, FIG. 26A and FIG. 26B An emission spectrum of a thin film of 2mDBTPDBq-II as the first organic compound (emission spectrum 10) and an emission spectrum of a thin film of a mixed material of 2mDBTPDBq-II and NPB (emission spectrum 11) are shown. In the emission spectrum 10, the horizontal axis represents wavelength (nm), and the vertical axis represents molar absorption coefficient ε (M FIG. 26A -1 ·cm -1 ) and luminous intensity (arbitrary unit). In the emission spectrum 11, the horizontal axis represents energy (eV), and the vertical axis represents molar absorption coefficient ε (M FIG. 26B -1 ·cm -1 ) and luminous intensity (arbitrary unit).

[0384] From the absorption spectrum of FIG. 26A , it is found that [Ir(dppm)2(acac)] has a wide absorption band around 520 nm. It is considered that this absorption band is an absorption band that contributes greatly to emission.

[0385] ​​In comparison with the emission spectrum 10, the peak of the emission spectrum 11 is located on the long-wavelength (low-energy) side. Further, the peak of the emission spectrum 11 is located closer to the absorption band than the peak of the emission spectrum 10. In addition, it is known that the peak of the emission spectrum of NPB as the second organic compound is located around 430 nm. Thus, in comparison with the emission spectrum of NPB, the peak of the emission spectrum 11 is located on the long-wavelength (low-energy) side. Further, the peak of the emission spectrum 11 is located closer to the absorption band than the peak of the emission spectrum of NPB. From the above results, it is found that the emission spectrum having the largest overlap with the absorption band which contributes greatly to emission in the absorption spectrum is the emission spectrum 11. Specifically, the difference between the peak (515 nm) of the absorption band in the absorption spectrum and the peak of the emission spectrum 11 is 0.09 eV.

[0386] In comparison with the emission spectrum of the monomer, the peak of the emission spectrum of the mixed material of 2mDBTPDBq-II and NPB is located on the long-wavelength (low-energy) side. From this, it is found that an exciplex is formed by mixing 2mDBTPDBq-II and NPB.

[0387] The peak of the emission spectrum of the mixed material has a large overlap with the absorption band in the absorption spectrum of [Ir(dppm)2(acac)] which can be considered to greatly contribute to emission. Thus, since the light-emitting element using the mixed material of 2mDBTPDBq-II and NPB and [Ir(dppm)2(acac)] utilizes the overlap of the emission spectrum of the mixed material and the absorption spectrum of the phosphorescent compound, energy transfer, the energy transfer efficiency is high. Thus, a light-emitting element having a high external quantum efficiency can be obtained.

[0388] [Example 8]

[0389] In this embodiment, the chemical formulas of the materials used in this embodiment are shown below. Note that the chemical formulas of the materials used in the above embodiments are omitted. FIG. 15 A light-emitting element of one embodiment of the present application is described. The chemical formulas of the materials used in this embodiment are shown below. Note that the chemical formulas of the materials used in the above embodiments are omitted.

[0390]

[0391] The manufacturing method of the light-emitting element 5 and the comparative light-emitting element 6 of this embodiment is described below.

[0392] (Light-emitting element 5)

[0393] First, an ITSO film was formed over a glass substrate 1100 by a sputtering method, whereby a first electrode 1101 serving as an anode was formed. The thickness thereof was set to 110 nm and the electrode area thereof was set to 2 mm x 2 mm.

[0394] Next, as a pretreatment for forming the light-emitting element on the substrate 1100, after the substrate surface was washed with water and baked at 200°C for 1 hour, UV ozone treatment was performed for 370 seconds.

[0395] Then, the substrate was put into a vacuum evaporation apparatus whose inside was reduced in pressure to 10 -4 Pa or thereabout, and vacuum baking was performed 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.

[0396] Next, the substrate 1100 on which the first electrode 1101 was formed was fixed to a substrate holder in the vacuum evaporation apparatus with the surface on which the first electrode 1101 was formed facing downward, and the pressure in the vacuum evaporation apparatus was reduced to 10 -4 Pa or thereabout, and then 4,4',4"-(1,3,5-benzene triyl) tris(dibenzothiophene) (abbreviation: DBT3P-II) and molybdenum oxide (VI) were co-evaporated on the first electrode 1101, thereby forming a hole injection layer 1111. The thickness of the hole injection layer 1111 was set to 40 nm, and the weight ratio of DBT3P-II to molybdenum oxide was adjusted to 4:2 (=DBT3P-II:molybdenum oxide).

[0397] Next, a BPAFLP film with a thickness of 20 nm was formed over the hole injection layer 1111, so that a hole transport layer 1112 was formed.

[0398] Further, 2mDBTPDBq-II, NPB, and [Ir(dppm)2(acac)] were co-evaporated to form a light-emitting layer 1113 over the hole transport layer 1112. Here, the weight ratio of 2mDBTPDBq-II, NPB, and [Ir(dppm)2(acac)] was adjusted to 0.8:0.2:0.05 (=2mDBTPDBq-II:NPB:[Ir(dppm)2(acac)]). The thickness of the light-emitting layer 1113 was set to 40 nm.

[0399] Next, a 2mDBTPDBq-II film with a thickness of 10 nm was formed over the light-emitting layer 1113, so that a first electron transport layer 1114a was formed.

[0400] Next, a BPhen film with a thickness of 20 nm was formed over the first electron transport layer 1114a, so that a second electron transport layer 1114b was formed.

[0401] Then, a LiF film with a thickness of 1 nm was formed over the second electron transport layer 1114b by evaporation, so that an electron injection layer 1115 was formed.

[0402] Finally, an aluminum film having a thickness of 200 nm was deposited by evaporation to form a second electrode 1103 serving as a cathode, thereby manufacturing the light-emitting element 5 of this embodiment.

[0403] (Comparative Light-Emitting Element 6)

[0404] The light-emitting layer 1113 of the comparative light-emitting element 6 was formed by co-evaporation of 2mDBTPDBq-II and [Ir(dppm)2(acac)]. Here, the weight ratio of 2mDBTPDBq-II and [Ir(dppm)2(acac)] was adjusted to 1:0.05 (= 2mDBTPDBq-II : [Ir(dppm)2(acac)]). The thickness of the light-emitting layer 1113 was set to 40 nm. The comparative light-emitting element 6 was manufactured in the same manner as the light-emitting element 5, except for the light-emitting layer 1113.

[0405] Note that in the above evaporation process, evaporation was performed by a resistance heating method.

[0406] Table 7 shows the element structures of the light-emitting element 5 and the comparative light-emitting element 6 obtained by the above steps.

[0407] [Table 7]

[0408]

[0409] The light-emitting element 5 and the comparative light-emitting element 6 were sealed in a glove box in such a manner that the light-emitting elements were not exposed to the air, and then the operation characteristics of these light-emitting elements were measured. Further, the measurement was performed at room temperature (in an atmosphere kept at 25 °C).

[0410] FIG. 27 FIG. 17 shows the current density-luminance characteristics of the light-emitting element 5 and the comparative light-emitting element 6. In FIG. 17, the horizontal axis represents current density (mA / cm2) and the vertical axis represents luminance (cd / m2). 2 2 FIG. 27

[0411] FIG. 28 FIG. 18 shows the voltage-luminance characteristics. In FIG. 18, the horizontal axis represents voltage (V) and the vertical axis represents luminance (cd / m2). 2 FIG. 28 FIG. 29 FIG. 19 shows the luminance-current efficiency characteristics. In FIG. 19, the horizontal axis represents luminance (cd / m2) and the vertical axis represents current efficiency (cd / A). 2 FIG. 29 FIG. 30 FIG. 20 shows the luminance-external quantum efficiency characteristics. In FIG. 20, the horizontal axis represents luminance (cd / m2) and the vertical axis represents external quantum efficiency (%). 2 FIG. 30 ​​​​​​​​​

[0412] In addition, Table 8 shows that the luminous element 5 and the comparative luminous element 6 have a luminance of about 1000 cd / m 2 The voltage (V), current density (mA / cm 2 ), CIE color coordinates (x, y), current efficiency (cd / A), power efficiency (lm / W) and external quantum efficiency (%).

[0413] [Table 8]

[0414]

[0415] FIG. 31 The emission spectra obtained when a current of 0.1 mA was applied to the light-emitting element 5 and the comparative light-emitting element 6 are shown. FIG. 31 In the table, the horizontal axis represents wavelength (nm) and the vertical axis represents luminous intensity (arbitrary unit). As shown in Table 8, 1100 cd / m 2 The CIE color coordinates of the light emitting element 5 at the brightness of 830 cd / m 2 The CIE color coordinates of the comparative light-emitting element 6 at a brightness of 100 nm were (x, y) = (0.56, 0.44). From the above results, it can be seen that the light-emitting element 5 and the comparative light-emitting element 6 exhibit orange light derived from [Ir(dppm)2(acac)].

[0416] From Table 8 and FIG. 27 to FIG. 30 It can be seen that the current efficiency, power efficiency, and external quantum efficiency of Light-emitting Element 5 are higher than those of Comparative Light-emitting Element 6.

[0417] In Light-Emitting Element 5, 2mDBTPDBq-II, NPB, and [Ir(dppm)2(acac)], described in Example 7, were used in the light-emitting layer. As shown in Example 7, the emission spectrum of the mixed material of 2mDBTPDBq-II and NPB (the emission spectrum of the exciplex) significantly overlaps with the absorption band of [Ir(dppm)2(acac)], which contributes significantly to luminescence, compared to the emission spectrum of 2mDBTPDBq-II alone. Because Light-Emitting Element 5 utilizes this overlap for energy transfer, it is believed that energy transfer efficiency is high, resulting in a higher external quantum efficiency than that of Comparative Light-Emitting Element 6.

[0418] In the results of Example 7, the peak of the absorption band on the longest wavelength side of the absorption spectrum of the phosphorescent compound used in light-emitting element 5 is close to the peak of the emission spectrum, and the molar absorption coefficient of the peak is large (>5000M -1 cm -1 ) As can be seen from the above results, Light-Emitting Element 5 has a high external quantum efficiency that has never been achieved due to its particularly high energy transfer efficiency.

[0419] From the above results, it is clear that by applying one embodiment of the present invention, a device with high external quantum efficiency can be realized.

[0420] [Example 9]

[0421] In this embodiment, referring to FIG. 32A and FIG. 32B An example of a combination of a first organic compound, a second organic compound, and a phosphorescent compound that can be applied to a light-emitting element according to one embodiment of the present invention will be described.

[0422] The phosphorescent compound used in this example is bis(2,3,5-triphenylpyrazine) (dipivaloylmethane) iridium(III) (abbreviated: [Ir(tppr)2(dpm)]). The first organic compound used in this example is 2mDBTPDBq-II. The second organic compound used in this example is NPB. The chemical formulas of the materials used in this example are shown below. The chemical formulas of the materials used in the above examples are omitted.

[0423]

[0424] <Absorption spectrum>

[0425] FIG. 32A and FIG. 32B The following shows the ultraviolet-visible absorption spectrum (hereinafter simply referred to as the absorption spectrum) of a dichloromethane solution of [Ir(tppr)2(dpm)], a phosphorescent compound. The absorption spectrum was measured using a UV-visible spectrophotometer (V550, manufactured by JASCO Corporation). The dichloromethane solution (0.094 mmol / L) was placed in a quartz cell and measured at room temperature.

[0426] <Emission Spectrum>

[0427] also, FIG. 32A and FIG. 32B The emission spectrum of a thin film of 2mDBTPDBq-II as the first organic compound (emission spectrum 12) and the emission spectrum of a thin film of a mixed material of 2mDBTPDBq-II and NPB (emission spectrum 13) are shown. FIG. 32A In the figure, the horizontal axis represents the wavelength (nm), and the vertical axis represents the molar absorption coefficient ε (M -1 cm -1 ) and luminous intensity (arbitrary units). FIG. 32B In the figure, the horizontal axis represents energy (eV) and the vertical axis represents the molar absorption coefficient ε (M -1 cm -1 ) and luminous intensity (arbitrary units).

[0428] fromFIG. 32A The absorption spectrum of [Ir(tppr)2(dpm)] has a wide absorption band around 530 nm. This absorption band can be considered to be an absorption band that contributes greatly to emission.

[0429] In comparison with the emission spectrum 12, the peak of the emission spectrum 13 is located on the long-wavelength (low-energy) side. Furthermore, the peak of the emission spectrum 13 is located closer to the absorption band than the peak of the emission spectrum 12. It is known that the peak of the emission spectrum of NPB as the second organic compound is located around 430 nm. Thus, in comparison with the emission spectrum of NPB, the peak of the emission spectrum 13 is located on the long-wavelength (low-energy) side. Furthermore, the peak of the emission spectrum 13 is located closer to the absorption band than the peak of the emission spectrum of NPB. From the above results, it is found that the emission spectrum that overlaps most with the absorption band that can be considered to contribute greatly to emission in the absorption spectrum is the emission spectrum 13. Specifically, the difference between the peak of the absorption band (the shoulder around 530 nm) in the absorption spectrum and the peak of the emission spectrum 13 is 0.01 eV.

[0430] In comparison with the emission spectrum of the monomer, the peak of the emission spectrum of the mixed material of 2mDBTPDBq-II and NPB is located on the long-wavelength (low-energy) side. From this, it is found that an exciplex is formed by mixing 2mDBTPDBq-II and NPB.

[0431] The peak of the emission spectrum of the mixed material greatly overlaps with the absorption band that can be considered to contribute greatly to emission in the absorption spectrum of [Ir(tppr)2(dpm)]. Thus, since the light-emitting element using the mixed material of 2mDBTPDBq-II and NPB and [Ir(tppr)2(dpm)] utilizes the overlap between the emission spectrum of the mixed material and the absorption spectrum of the phosphorescent compound, the energy transfer efficiency is high. Thus, a light-emitting element with high external quantum efficiency can be obtained.

[0432] [Example 10]

[0433] In this embodiment, the light-emitting element of one embodiment of the present application will be described with reference to FIG. 15 Since the materials used in this embodiment are the materials used in the above embodiments, the chemical formulae are omitted.

[0434] The manufacturing method of the light-emitting element 7 and the comparative light-emitting element 8 of this embodiment will be described below.

[0435] (Light-emitting element 7)

[0436] The light-emitting layer 1113 of the light-emitting element 7 was formed by co-evaporation of 2mDBTPDBq-II, NPB, and [Ir(tppr)2(dpm)]. Here, the weight ratio of 2mDBTPDBq-II, NPB, and [Ir(tppr)2(dpm)] was adjusted to 0.8:0.2:0.05 (= 2mDBTPDBq-II: NPB: [Ir(tppr)2(dpm)]). The thickness of the light-emitting layer 1113 was set to 40 nm. The light-emitting element 7 was produced in the same manner as that of the light-emitting element 5 described in Embodiment 8, except for the light-emitting layer 1113.

[0437] (Comparative Light-Emitting Element 8)

[0438] The light-emitting layer 1113 of the comparative light-emitting element 8 was formed by co-evaporation of 2mDBTPDBq-II and [Ir(tppr)2(dpm)]. Here, the weight ratio of 2mDBTPDBq-II and [Ir(tppr)2(dpm)] was adjusted to 1:0.05 (= 2mDBTPDBq-II: [Ir(tppr)2(dpm)]). The thickness of the light-emitting layer 1113 was set to 40 nm. The comparative light-emitting element 8 was produced in the same manner as that of the light-emitting element 5 described in Embodiment 8, except for the light-emitting layer 1113.

[0439] Table 9 shows the element structures of the light-emitting element 7 and the comparative light-emitting element 8 obtained by the above steps.

[0440] [Table 9]

[0441]

[0442] The light-emitting element 7 and the comparative light-emitting element 8 were sealed in a glove box under a nitrogen atmosphere in such a manner that the light-emitting elements were not exposed to the atmosphere, and then the operation characteristics of these light-emitting elements were measured. Further, the measurement was performed at room temperature (in an atmosphere kept at 25°C).

[0443] FIG. 33 FIG. 17 shows the current density-luminance characteristics of the light-emitting element 7 and the comparative light-emitting element 8. In FIG. 17, the horizontal axis represents current density (mA / cm2) and the vertical axis represents luminance (cd / m2). 2 2 FIG. 33 FIG. 18 shows the voltage-luminance characteristics. In FIG. 18, the horizontal axis represents voltage (V) and the vertical axis represents luminance (cd / m2). 2 FIG. 34 FIG. 19 shows the luminance-current efficiency characteristics. In FIG. 19, the horizontal axis represents luminance (cd / m2) and the vertical axis represents current efficiency (cd / A). 2 FIG. 34 FIG. 35 FIG. 35 FIG. 36 ​​​​​​​The luminance-external quantum efficiency characteristics are shown. In FIG. 36 the horizontal axis indicates the luminance (cd / m 2 ), and the vertical axis indicates the external quantum efficiency (%).

[0444] Further, Table 10 shows the voltage (V), the current density (mA / cm 2 ), the CIE color coordinates (x, y), the current efficiency (cd / A), the power efficiency (lm / W), and the external quantum efficiency (%) of the light emitting element 7 and the comparative light emitting element 8 at a luminance of about 1000 cd / m 2

[0445] [Table 10]

[0446]

[0447] FIG. 37 The emission spectrum obtained when a current of 0.1 mA is applied to the light emitting element 7 and the comparative light emitting element 8 is shown. In FIG. 37 the horizontal axis indicates the wavelength (nm), and the vertical axis indicates the emission intensity (arbitrary unit). Further, as shown in Table 10, the CIE color coordinates of the light emitting element 7 at a luminance of 1100 cd / m 2 are (x, y) = (0.66, 0.34), and the CIE color coordinates of the comparative light emitting element 8 at a luminance of 1000 cd / m 2 are (x, y) = (0.66, 0.34). From the above results, it is known that the light emitting element 7 and the comparative light emitting element 8 show red light derived from [Ir(tppr)2(dpm)].

[0448] From Table 10 and FIG. 33 to FIG. 36 , it is known that the current efficiency, the power efficiency, and the external quantum efficiency of the light emitting element 7 are higher than those of the comparative light emitting element 8.

[0449] In the light emitting element 7, 2mDBTPDBq-II, NPB, and [Ir(tppr)2(dpm)] shown in Example 9 are used for the light emitting layer. From Example 9, it is known that the emission spectrum of the mixed material of 2mDBTPDBq-II and NPB (the emission spectrum of the exciplex) overlaps greatly with the absorption band in the absorption spectrum of [Ir(tppr)2(dpm)] which can be considered to greatly contribute to light emission, as compared with the emission spectrum of the 2mDBTPDBq-II monomer. The light emitting element 7 can be considered to have a high energy transfer efficiency because of energy transfer by the overlap, and thus has a high external quantum efficiency as compared with the comparative light emitting element 8.

[0450] From the above results, it is known that, by applying one embodiment of the present application, a light emitting element having a high external quantum efficiency can be realized.

[0451] ​Next, reliability tests of the light-emitting element 7 and the comparative light-emitting element 8 were performed. FIG. 38 The results of the reliability tests are shown in FIG. 8. FIG. 38 In FIG. 8, the vertical axis represents normalized luminance (%) in the case where the initial luminance is 100 %, and the horizontal axis represents the driving time (h) of the element.

[0452] In the reliability test, the initial luminance was set to 5000 cd / m 2 and the light-emitting element 7 and the comparative light-emitting element 8 were driven under conditions where the current density was constant.

[0453] The luminance of the comparative light-emitting element 8 after 97 hours was 63 % of the initial luminance. The luminance of the light-emitting element 7 after 98 hours was 87 % of the initial luminance. From the above results, it is found that the light-emitting element 7 has a longer lifetime than the comparative light-emitting element 8.

[0454] From the above results, it is found that a high-reliability element can be obtained by applying one embodiment of the present application.

[0455] [Example 11]

[0456] In this embodiment, one example of a combination of a first organic compound, a second organic compound, and a phosphorescent compound which can be applied to a light-emitting element of one embodiment of the present application is described with reference to FIG. 39A and FIG. 39B In this embodiment, one example of a combination of a first organic compound, a second organic compound, and a phosphorescent compound which can be applied to a light-emitting element of one embodiment of the present application is described with reference to

[0457] The phosphorescent compound used in this embodiment is (acetylacetonato)bis(6-methyl-4-phenylpyrimidinato)iridium (III) (abbreviation: [Ir(mppm)2(acac)]). The first organic compound used in this embodiment is 2mDBTPDBq-II. The two kinds of second organic compounds used in this embodiment are PCBA1BP and 4-(1-naphthyl)-4'-phenyltriphenylamine (abbreviation: αNBA1BP). The chemical formulas of the materials used in this embodiment are shown below. Note that the chemical formulas of the materials used in the above embodiments are omitted.

[0458]

[0459] <Absorption spectrum>

[0460] FIG. 39A and FIG. 39B An ultraviolet-visible absorption spectrum (hereinafter simply referred to as an absorption spectrum) of a dichloromethane solution of [Ir(mppm)2(acac)], which is a phosphorescent compound, is shown. In the measurement of the absorption spectrum, an ultraviolet-visible spectrophotometer (V550 manufactured by JASCO Corporation) was used, the dichloromethane solution (0.10 mmol / L) was placed in a quartz cell, and measurement was performed at room temperature.

[0461] <emission spectrum>

[0462] Further, FIG. 39A and FIG. 39B An emission spectrum of a thin film of 2mDBTPDBq-II as the first organic compound (emission spectrum 14), an emission spectrum of a thin film of PCBA1BP as the second organic compound (emission spectrum 15), an emission spectrum of a thin film of aNBA1BP as the second organic compound (emission spectrum 16), an emission spectrum of a thin film of a mixture of 2mDBTPDBq-II and PCBA1BP (emission spectrum 17), and an emission spectrum of a thin film of a mixture of 2mDBTPDBq-II and aNBA1BP (emission spectrum 18) are shown. In FIG. 39A , the horizontal axis represents wavelength (nm), and the vertical axis represents molar absorption coefficient ε (M -1 ·cm -1 ) and luminous intensity (arbitrary unit). In FIG. 39B , the horizontal axis represents energy (eV), and the vertical axis represents molar absorption coefficient ε (M -1 ·cm -1 ) and luminous intensity (arbitrary unit).

[0463] From the absorption spectrum of FIG. 39A , it is found that [Ir(mppm)2(acac)] has a wide absorption band around 490 nm. It is considered that this absorption band is an absorption band that contributes greatly to emission.

[0464] The peak of the emission spectrum of the mixture of 2mDBTPDBq-II and PCBA1BP (emission spectrum 17) and the peak of the emission spectrum of the mixture of 2mDBTPDBq-II and aNBA1BP (emission spectrum 18) are located on the long wavelength (low energy) side, compared with the emission spectrum of the monomer. From this, it is found that an exciplex is formed by mixing 2mDBTPDBq-II and PCBA1BP. Further, from this, it is found that an exciplex is formed by mixing 2mDBTPDBq-II and aNBA1BP.

[0465] The peak of the emission spectrum of the above-described mixture is largely overlapped with the absorption band that is considered to contribute greatly to emission in the absorption spectrum of [Ir(mppm)2(acac)]. Therefore, the light emitting element using the mixture of 2mDBTPDBq-II and PCBA1BP and [Ir(mppm)2(acac)] and the light emitting element using the mixture of 2mDBTPDBq-II and aNBA1BP and [Ir(mppm)2(acac)] utilize the overlap of the emission spectrum of the mixture and the absorption spectrum of the phosphorescent compound to transfer energy, so that the energy transfer efficiency is high. Thus, a light emitting element with high external quantum efficiency can be obtained.

[0466] In comparison with the emission spectrum 17, the peak of the emission spectrum 18 is located on the short wavelength (high energy) side. Further, the peak of the emission spectrum 18 is located closer to the absorption band than the peak of the emission spectrum 17. Specifically, the difference between the peak of the absorption band in the absorption spectrum (the shoulder peak around 490 nm) and the peak of the emission spectrum 17 is 0.15 eV, and the difference between the peak of the absorption band in the absorption spectrum (the shoulder peak around 490 nm) and the peak of the emission spectrum 18 is 0.01 eV.

[0467] The difference between the peak of the emission spectrum 17 and the peak of the emission spectrum 18 can be considered to be based on the difference between the HOMO level of PCBA1BP and the HOMO level of aNBA1BP. Specifically, the HOMO level of PCBA1BP is -5.43 eV, and the HOMO level of aNBA1BP is -5.52 eV (these values were calculated by cyclic voltammetry (CV)). The HOMO level of aNBA1BP is lower (deeper) than the HOMO level of PCBA1BP, and thus it can be considered that the peak of the emission spectrum 18 is located on the short wavelength (high energy) side in comparison with the emission spectrum 17.

[0468] [Example 12]

[0469] In this embodiment, reference is made to FIG. 15 A light-emitting element of one embodiment of the present application is described. Since the materials used in this embodiment are the materials used in the above embodiments, the chemical formulas are omitted.

[0470] The manufacturing methods of the light-emitting element 9 and the light-emitting element 10 of this embodiment are described below.

[0471] (Light-emitting element 9)

[0472] First, an ITSO film was formed by a sputtering method over a glass substrate 1100, whereby a first electrode 1101 serving as an anode was formed. The thickness thereof was set to 110 nm, and the electrode area thereof was set to 2 mm x 2 mm.

[0473] Next, as a pretreatment for forming a light-emitting element over the substrate 1100, after the substrate surface was washed with water and baking was performed at 200 °C for 1 hour, UV ozone treatment was performed for 370 seconds.

[0474] Then, the substrate was put into a vacuum evaporation apparatus whose inside was reduced to a pressure of 10 -4 Pa, and vacuum baking was performed 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.

[0475] Next, the substrate 1100 on which the first electrode 1101 is formed is fixed to a substrate holder in a vacuum evaporation device with the first electrode 1101 facing downward, and the pressure in the vacuum evaporation device is reduced to 10 -4 BPAFLP and molybdenum (VI) oxide are co-evaporated on the first electrode 1101, thereby forming a hole injection layer 1111. The thickness of the hole injection layer 1111 is set to 40 nm, and the weight ratio of BPAFLP to molybdenum oxide is adjusted to 4:2 (= BPAFLP : molybdenum oxide).

[0476] Next, a BPAFLP film having a thickness of 20 nm is formed on the hole injection layer 1111, thereby forming a hole transport layer 1112.

[0477] Further, 2mDBTPDBq-II, PCBA1BP, and [Ir(mppm)2(acac)] are co-evaporated on the hole transport layer 1112 to form a light-emitting layer 1113. Here, the weight ratio of 2mDBTPDBq-II, PCBA1BP, and [Ir(mppm)2(acac)] is adjusted to 0.8:0.2:0.05 (= 2mDBTPDBq-II : PCBA1BP : [Ir(mppm)2(acac)]). The thickness of the light-emitting layer 1113 is set to 40 nm.

[0478] Next, a 2mDBTPDBq-II film having a thickness of 10 nm is formed on the light-emitting layer 1113, thereby forming a first electron transport layer 1114a.

[0479] Next, a BPhen film having a thickness of 20 nm is formed on the first electron transport layer 1114a, thereby forming a second electron transport layer 1114b.

[0480] Then, a LiF film having a thickness of 1 nm is evaporated on the second electron transport layer 1114b, thereby forming an electron injection layer 1115.

[0481] Finally, an aluminum film having a thickness of 200 nm is evaporated as a second electrode 1103 serving as a cathode, thereby manufacturing the light-emitting element 9 of the present embodiment.

[0482] (Light-emitting element 10)

[0483] Light-emitting layer 1113 of light-emitting element 10 was formed by co-evaporation of 2mDBTPDBq-II, αNBA1BP, and [Ir(mppm)2(acac)]. The weight ratio of 2mDBTPDBq-II, αNBA1BP, and [Ir(mppm)2(acac)] was adjusted to 0.8:0.2:0.05 (= 2mDBTPDBq-II:αNBA1BP:[Ir(mppm)2(acac)]). The thickness of light-emitting layer 1113 was set to 40 nm. Except for light-emitting layer 1113, the process was similar to that of light-emitting element 9.

[0484] Note that in the above evaporation processes, resistance heating method is used for evaporation.

[0485] Table 11 shows the device structures of Light-Emitting Element 9 and Light-Emitting Element 10 obtained through the above steps.

[0486] [Table 11]

[0487]

[0488] Light-emitting elements 9 and 10 were sealed in a nitrogen glove box to prevent exposure to the atmosphere, and their operating characteristics were measured at room temperature (in an atmosphere maintained at 25°C).

[0489] FIG. 40 The current density-luminance characteristics of light-emitting element 9 and light-emitting element 10 are shown. FIG. 40 The horizontal axis represents the current density (mA / cm 2 ), while the vertical axis represents brightness (cd / m 2 ). FIG. 41 The voltage-luminance characteristics are shown. FIG. 41 In the figure, the horizontal axis represents voltage (V) and the vertical axis represents brightness (cd / m 2 ). FIG. 42 The brightness-current efficiency characteristics are shown. FIG. 42 In the figure, the horizontal axis represents the brightness (cd / m 2 ), and the vertical axis represents current efficiency (cd / A). FIG. 43 The brightness-external quantum efficiency characteristics are shown. FIG. 43 In the figure, the horizontal axis represents the brightness (cd / m 2 ), and the vertical axis represents the external quantum efficiency (%).

[0490] In addition, Table 12 shows that the luminous element 9 and the luminous element 10 have a luminance of about 1000 cd / m 2 The voltage (V), current density (mA / cm 2), CIE color coordinates (x, y), current efficiency (cd / A), power efficiency (lm / W), and external quantum efficiency (%).

[0491] [Table 12]

[0492]

[0493] FIG. 44 The emission spectrum obtained when a current of 0.1 mA was applied to the light-emitting element 9 and the light-emitting element 10 is shown. In FIG. 44 , the horizontal axis represents wavelength (nm) and the vertical axis represents emission intensity (arbitrary unit). As shown in Table 12, the emission spectrum of the light-emitting element 9 and the light-emitting element 10 was similar to the emission spectrum of the mixed material of 2mDBTPDBq-II and PCBA1BP or αNBA1BP. 2 The CIE color coordinates of the light-emitting element 9 at a luminance of 1100 cd / m 2 The CIE color coordinates of the light-emitting element 10 at a luminance of 860 cd / m 2 were (x, y) = (0.43, 0.56). From the above results, it was found that the light-emitting element 9 and the light-emitting element 10 showed yellowish-green light derived from [Ir(mppm)2(acac)].

[0494] From Table 12 and FIG. 40 to FIG. 43 , it was found that the current efficiency, the power efficiency, and the external quantum efficiency of the light-emitting element 9 and the light-emitting element 10 were high, respectively.

[0495] In the light-emitting element 9 and the light-emitting element 10, PCBA1BP or αNBA1BP, 2mDBTPDBq-II, and [Ir(mppm)2(acac)] shown in Example 11 were used for the light-emitting layer. From Example 11, it was found that the emission spectrum of the mixed material of 2mDBTPDBq-II and PCBA1BP or αNBA1BP (the emission spectrum of the exciplex) and the absorption spectrum of [Ir(mppm)2(acac)] had a large overlap in an absorption band which can be considered to greatly contribute to light emission. Since the light-emitting element 9 and the light-emitting element 10 perform energy transfer by utilizing the overlap, it is considered that the energy transfer efficiency is high, and thus the external quantum efficiency is high.

[0496] From the above results, it was found that by applying one embodiment of the present application, a light-emitting element with high external quantum efficiency can be realized.

[0497] Next, a reliability test of the light-emitting element 9 and the light-emitting element 10 was performed. FIG. 45 The results of the reliability test are shown. In FIG. 45 , the vertical axis represents normalized luminance (%) in the case where the initial luminance is 100 %, and the horizontal axis represents the driving time (h) of the element.

[0498] In the reliability test, the initial luminance was set to 5000 cd / m 2and the light-emitting element 10 was driven at a constant current density.

[0499] The luminance of the light-emitting element 9 after 260 hours was 74 % of the initial luminance. Further, the luminance of the light-emitting element 10 after 260 hours was 75 % of the initial luminance. From the above results, it is found that the light-emitting element 9 and the light-emitting element 10 have long lifetimes.

[0500] From the above results, it is found that a highly reliable element can be obtained by using one embodiment of the present application.

[0501] [Example 13]

[0502] In this embodiment, a combination of a first organic compound, a second organic compound, and a phosphorescent compound which can be applied to a light-emitting element of one embodiment of the present application is described with reference to FIG. 46A and FIG. 46B A combination of a first organic compound, a second organic compound, and a phosphorescent compound which can be applied to a light-emitting element of one embodiment of the present application is described with reference to

[0503] The phosphorescent compound used in this embodiment is (acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium (III) (abbreviation: [Ir(tBuppm)2(acac)]). The first organic compound used in this embodiment is 2mDBTPDBq-II. The two kinds of second organic compounds used in this embodiment are NPB and 2,7-bis[N-(diphenylamino)phenyl-N-phenylamino]-spiro-9,9'-bifluorene (abbreviation: DPA2SF). The chemical formulas of the materials used in this embodiment are shown below. Note that the chemical formulas of the materials used in the above embodiments are omitted.

[0504]

[0505] <Absorption Spectrum>

[0506] FIG. 46A and FIG. 46B An ultraviolet-visible absorption spectrum (hereinafter simply referred to as an absorption spectrum) of a dichloromethane solution of [Ir(tBuppm)2(acac)], which is a phosphorescent compound, is shown. In the measurement of the absorption spectrum, an ultraviolet-visible spectrophotometer (V550 manufactured by JASCO Corporation) was used, the dichloromethane solution (0.093 mmol / L) was placed in a quartz cell, and measurement was performed at room temperature.

[0507] <Emitted Spectrum>

[0508] Further, FIG. 46A and FIG. 46BAn emission spectrum of a thin film of 2mDBTPDBq-II as the first organic compound (emission spectrum 19), an emission spectrum of a thin film of DPA2SF as the second organic compound (emission spectrum 20), an emission spectrum of a thin film of a mixed material of 2mDBTPDBq-II and DPA2SF (emission spectrum 21), and an emission spectrum of a thin film of a mixed material of 2mDBTPDBq-II and NPB (emission spectrum 22) are shown. In FIG. 46A the horizontal axis indicates a wavelength (nm), and the vertical axis indicates a molar absorption coefficient ε (M -1 ·cm -1 ) and a luminous intensity (arbitrary unit). In FIG. 46B the horizontal axis indicates an energy (eV), and the vertical axis indicates a molar absorption coefficient ε (M -1 ·cm -1 ) and a luminous intensity (arbitrary unit).

[0509] From the absorption spectrum of FIG. 46A , it is found that [Ir(tBuppm)2(acac)] has a wide absorption band around 490 nm. It is considered that this absorption band is an absorption band that contributes greatly to emission.

[0510] The peak of the emission spectrum of the mixed material of 2mDBTPDBq-II and DPA2SF (emission spectrum 21) is located on the long wavelength (low energy) side compared with the emission spectrum of the monomer. From this, it is found that an exciplex is formed by mixing 2mDBTPDBq-II and DPA2SF.

[0511] It is known that the peak of the emission spectrum of the NPB monomer is located around 430 nm. From FIG. 46A , it is found that since the peak of the emission spectrum of the mixed material of 2mDBTPDBq-II and NPB (emission spectrum 22) is located on the long wavelength side than 430 nm, an exciplex is formed by mixing 2mDBTPDBq-II and NPB.

[0512] The peak of the emission spectrum of the above-described mixed material greatly overlaps with the absorption band that is considered to greatly contribute to emission in the absorption spectrum of [Ir(tBuppm)2(acac)]. Therefore, since the light-emitting element using the mixed material of 2mDBTPDBq-II and DPA2SF and [Ir(tBuppm)2(acac)] and the light-emitting element using the mixed material of 2mDBTPDBq-II and NPB and [Ir(tBuppm)2(acac)] utilize overlap of the emission spectrum of the mixed material and the absorption spectrum of the phosphorescent compound, energy transfer efficiency is high. Thus, a light-emitting element with high external quantum efficiency can be obtained.

[0513] In comparison with the emission spectrum 21, the peak of the emission spectrum 22 is located on the short wavelength (high energy) side. Further, the peak of the emission spectrum 22 is located closer to the absorption band than the peak of the emission spectrum 21. As described above, from the fact that the emission spectrum of the mixed material of 2mDBTPDBq-II and NPB and [Ir(tBuppm)2(acac)] is shifted to the short wavelength side in comparison with the emission spectrum of the mixed material of 2mDBTPDBq-II and DPA2SF and [Ir(tBuppm)2(acac)], it is considered that the energy transfer efficiency is higher. FIG. 46A and FIG. 46B It is understood that the emission spectrum 22, which overlaps the absorption band that contributes greatly to emission in the absorption spectrum the most, is the emission spectrum 22. Specifically, the difference between the peak of the absorption band in the absorption spectrum and the peak of the emission spectrum 21 is 0.39 eV, and the difference between the peak of the absorption band in the absorption spectrum and the peak of the emission spectrum 22 is 0.19 eV.

[0514] The difference between the peak of the emission spectrum 21 and the peak of the emission spectrum 22 can be considered to be based on the difference between the HOMO level of DPA2SF used as the second organic compound and the HOMO level of NPB. Specifically, the HOMO level of DPA2SF is -5.09 eV, and the HOMO level of NPB is -5.38 eV (these values are calculated by cyclic voltammetry (CV)). The HOMO level of NPB is lower (deeper) than the HOMO level of DPA2SF, and thus it is considered that the peak of the emission spectrum 22 is located on the short wavelength (high energy) side in comparison with the emission spectrum 21.

[0515] As described above, since the light-emitting element using the mixed material of 2mDBTPDBq-II and NPB and [Ir(tBuppm)2(acac)] has a higher energy transfer efficiency due to a larger overlap of the emission spectrum of the mixed material and the absorption spectrum of the phosphorescent compound by energy transfer, a light-emitting element with a higher external quantum efficiency can be obtained.

[0516] Further, it is understood from this embodiment that the overlap of the emission spectrum of the mixed material of the first organic compound and the second organic compound and the absorption spectrum of the phosphorescent compound can be increased by changing only the second organic compound without changing the first organic compound. In other words, in the case where the emission color of the phosphorescent compound (the position of the absorption band that contributes greatly to emission in the absorption spectrum) is changed, only the second organic compound is changed, a combination that increases the overlap can be obtained, and thus a light-emitting element with a high external quantum efficiency and a long lifetime can be obtained.

[0517] [Embodiment 14]

[0518] In this embodiment, a light-emitting element of one embodiment of the present application will be described with reference to FIG. 15 Since the materials used in this embodiment are the materials used in the above embodiments, the chemical formulas are omitted.

[0519] The following shows a manufacturing method of the light emitting element 11 and the light emitting element 12 of the present embodiment.

[0520] (Light emitting element 11)

[0521] First, an ITSO film is formed on the glass substrate 1100 by a sputtering method, thereby forming a first electrode 1101 serving as an anode. In addition, the thickness thereof is set to 110 nm, and the electrode area thereof is set to 2 mm x 2 mm.

[0522] Next, as a pretreatment for forming the light emitting element on the substrate 1100, after washing the substrate surface with water and performing baking at 200°C for 1 hour, UV ozone treatment is performed for 370 seconds.

[0523] Then, the substrate is put into a vacuum evaporation device whose inside is reduced to a pressure of 10 -4 Pa or thereabout, and vacuum baking at 170°C for 30 minutes is performed in the heating chamber of the vacuum evaporation device, and then the substrate 1100 is cooled for about 30 minutes.

[0524] Next, the substrate 1100 on which the first electrode 1101 is formed is fixed to a substrate holder inside the vacuum evaporation device in a manner that the surface on which the first electrode 1101 is formed faces downward, and the pressure inside the vacuum evaporation device is reduced to 10 -4 Pa or thereabout, and then DBT3P-II and molybdenum (VI) oxide are co-evaporated on the first electrode 1101, thereby forming a hole injection layer 1111. The thickness of the hole injection layer 1111 is set to 40 nm, and the weight ratio of DBT3P-II to molybdenum oxide is adjusted to 4:2 (= DBT3P-II : molybdenum oxide).

[0525] Next, a BPAFLP film having a thickness of 20 nm is formed on the hole injection layer 1111, thereby forming a hole transport layer 1112.

[0526] Further, 2mDBTPDBq-II, DPA2SF, and [Ir(tBuppm)2(acac)] are co-evaporated on the hole transport layer 1112 to form a light emitting layer 1113. Here, the weight ratio of 2mDBTPDBq-II, DPA2SF, [Ir(tBuppm)2(acac)] is adjusted to 0.8:0.2:0.05 (= 2mDBTPDBq-II : DPA2SF : [Ir(tBuppm)2(acac)]). The thickness of the light emitting layer 1113 is set to 40 nm.

[0527] Next, a 2mDBTPDBq-II film having a thickness of 10 nm is formed on the light emitting layer 1113, thereby forming a first electron transport layer 1114a.

[0528] Next, a BPhen film having a thickness of 20 nm was formed over the first electron-transport layer 1114a to form a second electron-transport layer 1114b.

[0529] Then, a LiF film having a thickness of 1 nm was formed over the second electron-transport layer 1114b by evaporation to form an electron-injection layer 1115.

[0530] Finally, an aluminum film having a thickness of 200 nm was formed by evaporation as the second electrode 1103 serving as a cathode, whereby the light-emitting element 11 of this embodiment was produced.

[0531] (Light-emitting element 12)

[0532] The light-emitting layer 1113 of the light-emitting element 12 was formed by co-evaporation of 2mDBTPDBq-II, NPB, and [Ir(tBuppm)2(acac)]. Here, the weight ratio of 2mDBTPDBq-II, NPB, and [Ir(tBuppm)2(acac)] was adjusted to 0.8:0.2:0.05 (= 2mDBTPDBq-II: NPB: Ir(tBuppm)2(acac)). The thickness of the light-emitting layer 1113 was set to 40 nm. The light-emitting element 12 was produced in the same manner as the light-emitting element 11 except for the light-emitting layer 1113.

[0533] Note that in the above evaporation process, evaporation was performed by the resistance heating method.

[0534] Table 13 shows the element structures of the light-emitting element 11 and the light-emitting element 12 obtained by the above steps.

[0535] [Table 13]

[0536]

[0537] The light-emitting element 11 and the light-emitting element 12 were sealed in a glove box in a nitrogen atmosphere without exposure to the air, and then the operation characteristics of these light-emitting elements were measured. Further, the measurement was performed at room temperature (in an atmosphere kept at 25 °C).

[0538] FIG. 47 The current density-luminance characteristics of the light-emitting element 11 and the light-emitting element 12 are shown in FIGS. 16A and 16B, respectively. FIG. 47 In FIGS. 16A and 16B, the horizontal axis represents current density (mA / cm 2 ), and the vertical axis represents luminance (cd / m 2 ). FIG. 48 The voltage-luminance characteristics are shown in FIGS. 17A and 17B, respectively. FIG. 48 In FIGS. 17A and 17B, the horizontal axis represents voltage (V), and the vertical axis represents luminance (cd / m 2 ). FIG. 49Luminance-current efficiency characteristics are shown. In FIG. 49 the horizontal axis indicates luminance (cd / m 2 ), and the vertical axis indicates current efficiency (cd / A). FIG. 50 Luminance-external quantum efficiency characteristics are shown. In FIG. 50 the horizontal axis indicates luminance (cd / m 2 ), and the vertical axis indicates external quantum efficiency (%).

[0539] Further, Table 14 shows voltage (V), current density (mA / cm 2 ), CIE color coordinates (x, y), current efficiency (cd / A), power efficiency (lm / W), and external quantum efficiency (%) of the light emitting element 11 and the light emitting element 12 at luminance of about 1000 cd / m 2 .

[0540] [Table 14]

[0541]

[0542] FIG. 51 Emission spectra obtained when a current of 0.1 mA was applied to the light emitting element 11 and the light emitting element 12 are shown. In FIG. 51 the horizontal axis indicates wavelength (nm), and the vertical axis indicates emission intensity (arbitrary unit). As shown in Table 14, CIE color coordinates of the light emitting element 11 at luminance of 890 cd / m 2 were (x, y) = (0.43, 0.56), and CIE color coordinates of the light emitting element 12 at luminance of 820 cd / m 2 were (x, y) = (0.42, 0.57). From the above results, it was found that the light emitting element 11 and the light emitting element 12 showed yellowish green light derived from [Ir(tBuppm)2(acac)].

[0543] From Table 14 and FIG. 47 to FIG. 50 , it was found that the current efficiency, the power efficiency, and the external quantum efficiency of the light emitting element 11 and the light emitting element 12 were high, respectively.

[0544] In the light-emitting element 11 and the light-emitting element 12, DPA2SF or NPB, 2mDBTPDBq-II, [Ir(tBuppm)2(acac)] described in Embodiment 13 was used for the light-emitting layer. As is apparent from Embodiment 13, the emission spectrum of the mixture of 2mDBTPDBq-II and DPA2SF or NPB (the emission spectrum of the exciplex) has a large overlap with the absorption band in the absorption spectrum of [Ir(tBuppm)2(acac)], which is considered to greatly contribute to light emission. The light-emitting element 11 and the light-emitting element 12 have high energy transfer efficiency and high external quantum efficiency because of energy transfer by the overlap. In particular, the emission spectrum of the mixture of 2mDBTPDBq-II and NPB has a larger overlap with the absorption band than the emission spectrum of the mixture of 2mDBTPDBq-II and DPA2SF. Thus, the light-emitting element 12 has higher energy transfer efficiency and higher external quantum efficiency than the light-emitting element 11 because of energy transfer by the larger overlap. Furthermore, it is preferable that the difference between the energy value of the peak of the emission spectrum of the exciplex and the energy value of the peak of the absorption band on the lowest energy side in the absorption spectrum be within 0.3 eV, as is apparent from the results in Embodiment 13.

[0545] In this embodiment, a light-emitting element with higher external quantum efficiency can be obtained by changing only the second organic compound (using NPB instead of DPA2SF) without changing the first organic compound (corresponding to 2mDBTPDBq-II).

[0546] As is apparent from the above results, a light-emitting element with high external quantum efficiency can be realized by application of one embodiment of the present application.

[0547] [Embodiment 15]

[0548] In this embodiment, an example of a combination of a first organic compound, a second organic compound, and a phosphorescent compound, which can be applied to a light-emitting element of one embodiment of the present application, is described with reference to FIG. 52A and FIG. 52B In this embodiment, an example of a combination of a first organic compound, a second organic compound, and a phosphorescent compound, which can be applied to a light-emitting element of one embodiment of the present application, is described with reference to

[0549] The phosphorescent compound used in this embodiment is [Ir(mppr-Me)2(dpm)]. The two kinds of first organic compounds used in this embodiment are 2mDBTPDBq-II and 2-[4-(dibenzothiophene-4-yl)phenyl]-l-phenyl-lH-benzimidazole (abbreviation: DBTBIm-II). The second organic compound used in this embodiment is 4,4',4"-tris[N-(l-naphthyl)-N-phenylamino]triphenylamine (abbreviation: l'-TNATA). The chemical formulas of the materials used in this embodiment are shown below. Note that the chemical formulas of the materials used in the above embodiments are omitted.

[0550]

[0551] <Absorption spectrum>

[0552] FIG. 52A and FIG. 52B The following shows the ultraviolet-visible absorption spectrum (hereinafter simply referred to as the absorption spectrum) of a dichloromethane solution of [Ir(mppr-Me)2(dpm)], a phosphorescent compound. The absorption spectrum was measured using a UV-visible spectrophotometer (V550, manufactured by JASCO Corporation). The dichloromethane solution (0.093 mmol / L) was placed in a quartz cell and measured at room temperature.

[0553] <Emission Spectrum>

[0554] also, FIG. 52A and FIG. 52B The emission spectrum of a thin film of 2mDBTPDBq-II as the first organic compound (emission spectrum 23), the emission spectrum of a thin film of DBTBIm-II as the first organic compound (emission spectrum 24), the emission spectrum of a thin film of 1'-TNATA as the second organic compound (emission spectrum 25), the emission spectrum of a thin film of a mixed material of 2mDBTPDBq-II and 1'-TNATA (emission spectrum 26), and the emission spectrum of a thin film of a mixed material of DBTBIm-II and 1'-TNATA (emission spectrum 27) are shown. FIG. 52A In the figure, the horizontal axis represents the wavelength (nm), and the vertical axis represents the molar absorption coefficient ε (M -1 cm -1 ) and luminous intensity (arbitrary units). FIG. 52B In the figure, the horizontal axis represents energy (eV) and the vertical axis represents the molar absorption coefficient ε (M -1 cm -1 ) and luminous intensity (arbitrary units).

[0555] from FIG. 52A The absorption spectrum of [Ir(mppr-Me)2(dpm)] shows that it has a broad absorption band around 520nm. This absorption band is considered to contribute significantly to luminescence.

[0556] The peak of the emission spectrum of the mixed material of 2mDBTPDBq-II and 1'-TNATA (emission spectrum 26) and the peak of the emission spectrum of the mixed material of DBTBIm-II and 1'-TNATA (emission spectrum 27) are respectively located on the long wavelength (low energy) side as compared with the emission spectrum of the monomer. From this, it is known that an excimer is formed by mixing 2mDBTPDBq-II and 1'-TNATA. In addition, an excimer is formed by mixing DBTBIm-II and 1'-TNATA.

[0557] The peak of the emission spectrum of the above mixed material largely overlaps with the absorption band in the absorption spectrum of [Ir(mppr-Me)2(dpm)] which can be considered to greatly contribute to emission. Therefore, the luminescent element using the mixed material of 2mDBTPDBq-II and 1'-TNATA and [Ir(mppr-Me)2(dpm)], and the luminescent element using the mixed material of DBTBIm-II and 1'-TNATA and [Ir(mppr-Me)2(dpm)] utilize the overlap of the emission spectrum of the mixed material with the absorption spectrum of the phosphorescent compound for energy transfer, and thus the energy transfer efficiency is high. As a result, a luminescent element having a high external quantum efficiency can be obtained.

[0558] The peak of the emission spectrum 27 is located on the short wavelength (high energy) side as compared with the emission spectrum 26. Furthermore, the peak of the emission spectrum 27 is located at a position closer to the absorption band as compared with the peak of the emission spectrum 26. As described above, from FIG. 52A and FIG. 52B It is known that the emission spectrum which largely overlaps with the absorption band in the absorption spectrum which greatly contributes to emission is the emission spectrum 27. Specifically, the difference between the peak of the absorption spectrum of the absorption band (the shoulder peak around 520 nm) and the peak of the emission spectrum 26 is 0.35 eV, and the difference between the peak of the absorption spectrum of the absorption band (the shoulder peak around 520 nm) and the peak of the emission spectrum 27 is 0.01 eV.

[0559] The difference between the peak of the emission spectrum 26 and the peak of the emission spectrum 27 can be considered to be based on the difference between the LUMO level of 2mDBTPDBq-II used as the first organic compound and the LUMO level of DBTBIm-II. Specifically, the LUMO level of 2mDBTPDBq-II is -2.95 eV, and the LUMO level of DBTBIm-II is -2.52 eV (these values are calculated by cyclic voltammetry (CV)). The LUMO level of DBTBIm-II is higher (shallower) than the LUMO level of 2mDBTPDBq-II, and thus it is considered that the peak of the emission spectrum of the mixed material in which 2mDBTPDBq-II is mixed with 1'-TNATA having a high HOMO level is not located at a longer wavelength (in other words, the peak of the emission spectrum 27 is located on the short-wavelength side as compared to the emission spectrum 26).

[0560] As described above, the light-emitting element using the mixed material of DBTBIm-II and 1'-TNATA and [Ir(mppr-Me)2(dpm)] has a higher energy transfer efficiency than the light-emitting element using the mixed material of 2mDBTPDBq-II and 1'-TNATA and [Ir(mppr-Me)2(dpm)], because of the larger overlap between the emission spectrum of the mixed material and the absorption spectrum of the phosphorescent compound. Thus, a light-emitting element with a higher external quantum efficiency can be obtained.

[0561] Further, it is found from this embodiment that the overlap between the emission spectrum of the mixed material of the first organic compound and the second organic compound and the absorption spectrum of the phosphorescent compound can be increased by changing only the first organic compound without changing the second organic compound. In other words, even when the emission color of the phosphorescent compound (the position of an absorption band that contributes greatly to emission in the absorption spectrum) is changed, a combination that increases the overlap can be obtained by changing only the first organic compound, and thus a light-emitting element with a high external quantum efficiency and a long lifetime can be obtained.

[0562] [Embodiment 16]

[0563] In this embodiment, the light-emitting element of one embodiment of the present application is described with reference to FIG. 15 A light-emitting element of one embodiment of the present application is described. Since the materials used in this embodiment are the materials used in the above embodiments, the chemical formulae are omitted.

[0564] The manufacturing method of the light-emitting element 13 and the light-emitting element 14 of this embodiment is described below.

[0565] (Light-emitting element 13)

[0566] First, an ITSO film was formed on the glass substrate 1100 by a sputtering method, thereby forming a first electrode 1101 serving as an anode. In addition, the thickness thereof was set to 110 nm, and the electrode area thereof was set to 2 mm x 2 mm.

[0567] Next, as a pretreatment for forming a light emitting element on the substrate 1100, after the substrate surface was washed with water and baked at 2000C for 1 hour, UV ozone treatment was performed for 370 seconds.

[0568] Then, the substrate was put into a vacuum evaporation device whose inside was reduced to a pressure of 10 -4 Pa or less, and vacuum baking was performed at 1700C for 30 minutes in a heating chamber of the vacuum evaporation device, and then the substrate 1100 was cooled for about 30 minutes.

[0569] Next, the substrate 1100 on which the first electrode 1101 was formed was fixed to a substrate holder in the vacuum evaporation device with the surface on which the first electrode 1101 was formed facing downward, and the pressure in the vacuum evaporation device was reduced to 10 -4 Pa or less, and then BPAFLP and molybdenum (VI) oxide were co-evaporated on the first electrode 1101, thereby forming a hole injection layer 1111. The thickness of the hole injection layer 1111 was set to 40 nm, and the weight ratio of BPAFLP to molybdenum (VI) oxide was adjusted to 4:2 (= BPAFLP : molybdenum (VI) oxide).

[0570] Next, a BPAFLP film having a thickness of 20 nm was formed on the hole injection layer 1111, thereby forming a hole transport layer 1112.

[0571] Further, 2mDBTPDBq-II, 1'-TNATA, and [Ir(mppr-Me)2(dpm)] were co-evaporated on the hole transport layer 1112 to form a light emitting layer 1113. Here, the weight ratio of 2mDBTPDBq-II, 1'-TNATA, and [Ir(mppr-Me)2(dpm)] was adjusted 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 set to 20 nm.

[0572] Next, a 2mDBTPDBq-II film having a thickness of 30 nm was formed on the light emitting layer 1113, thereby forming a first electron transport layer 1114a.

[0573] Next, a BPhen film having a thickness of 20 nm was formed on the first electron transport layer 1114a, thereby forming a second electron transport layer 1114b.

[0574] Next, a LiF film was formed to a thickness of 1 nm by vapor deposition on the second electron-transport layer 1114 b to form the electron-injection layer 1115 .

[0575] Finally, a 200 nm-thick aluminum film was formed by vapor deposition as the second electrode 1103 functioning as a cathode, thereby manufacturing the light-emitting element 13 of this example.

[0576] (Light-emitting element 14)

[0577] The light-emitting layer 1113 of light-emitting element 14 was formed by co-evaporation of DBTBIm-II, 1'-TNATA, and [Ir(mppr-Me)2(dpm)]. The weight ratio of DBTBIm-II, 1'-TNATA, and [Ir(mppr-Me)2(dpm)] was adjusted to 0.8:0.2:0.05 (DBTBIm-II:1'-TNATA:Ir(mppr-Me)2(dpm)). The thickness of light-emitting layer 1113 was 20 nm.

[0578] The first electron-transport layer 1114a of the light-emitting element 14 was formed using a 30-nm-thick DBTBIm-II film. The light-emitting element 14 was manufactured in the same manner as the light-emitting element 13 except for the light-emitting layer 1113 and the first electron-transport layer 1114a.

[0579] Note that in the above evaporation processes, resistance heating method is used for evaporation.

[0580] Table 15 shows the element structures of the light-emitting element 13 and the light-emitting element 14 obtained through the above steps.

[0581] [Table 15]

[0582]

[0583] The light-emitting elements 13 and 14 were sealed in a nitrogen glove box to prevent exposure to the atmosphere, and their operating characteristics were measured at room temperature (in an atmosphere maintained at 25° C.).

[0584] FIG. 53 The current density-luminance characteristics of the light-emitting element 13 and the light-emitting element 14 are shown. FIG. 53 The horizontal axis represents the current density (mA / cm 2 ), while the vertical axis represents brightness (cd / m 2 ). FIG. 54 The voltage-luminance characteristics are shown. FIG. 54 In the figure, the horizontal axis represents voltage (V) and the vertical axis represents brightness (cd / m 2 ). FIG. 55Luminance-current efficiency characteristics are shown. In FIG. 55 the horizontal axis indicates luminance (cd / m 2 ), and the vertical axis indicates current efficiency (cd / A). FIG. 56 Luminance-external quantum efficiency characteristics are shown. In FIG. 56 the horizontal axis indicates luminance (cd / m 2 ), and the vertical axis indicates external quantum efficiency (%).

[0585] Further, Table 16 shows voltage (V), current density (mA / cm 2 ), CIE color coordinates (x, y), current efficiency (cd / A), power efficiency (lm / W), and external quantum efficiency (%) of the light emitting element 13 and the light emitting element 14 at a luminance of 860 cd / m 2 .

[0586] [Table 16]

[0587]

[0588] FIG. 57 Emission spectra obtained when a current of 0.1 mA was applied to the light emitting element 13 and the light emitting element 14 are shown. In FIG. 57 the horizontal axis indicates wavelength (nm), and the vertical axis indicates emission intensity (arbitrary unit). As shown in Table 16, CIE color coordinates of the light emitting element 13 and the light emitting element 14 at a luminance of 860 cd / m 2 were (x, y) = (0.53, 0.46). From the above results, it was found that the light emitting element 13 and the light emitting element 14 showed orange light derived from [Ir(mppr-Me)2(dpm)].

[0589] From Table 16 and FIG. 53 to FIG. 56 , it was found that the current efficiency, the power efficiency, and the external quantum efficiency of the light emitting element 13 and the light emitting element 14 were high, respectively.

[0590] In the light-emitting element 13 and the light-emitting element 14, 2mDBTPDBq-II or DBTBIm-II shown in Example 15, 1'-TNATA, and [Ir(mppr-Me)2(dpm)] were used for the light-emitting layer. As is clear from Example 15, the emission spectrum of the mixture of 2mDBTPDBq-II or DBTBIm-II and 1'-TNATA (the emission spectrum of the exciplex) greatly overlaps with an absorption band in the absorption spectrum of [Ir(mppr-Me)2(dpm)] which can be considered to greatly contribute to light emission. The light-emitting element 13 and the light-emitting element 14 can be considered to have high energy transfer efficiency and high external quantum efficiency because of energy transfer by the overlap. In particular, the emission spectrum of the mixture of DBTBIm-II and 1'-TNATA greatly overlaps with the absorption band compared to the emission spectrum of the mixture of 2mDBTPDBq-II and 1'-TNATA. Thus, the light-emitting element 14 can be considered to have high energy transfer efficiency and high external quantum efficiency compared to the light-emitting element 13 because of energy transfer by the greater overlap. Furthermore, it is preferable that the difference between the energy value of the peak of the emission spectrum of the exciplex and the energy value of the peak of the absorption band on the lowest energy side in the absorption spectrum be within 0.3 eV, which is clear from the results of Example 15.

[0591] In this embodiment, a light-emitting element with higher external quantum efficiency can be obtained by changing only the first organic compound (use of DBTBIm-II instead of 2mDBTPDBq-II) without changing the second organic compound (corresponding to 1'-TNATA).

[0592] From the above results, it is clear that a light-emitting element with high external quantum efficiency can be achieved by application of one embodiment of the present application.

[0593] [Example 17]

[0594] In this embodiment, an example of a combination of a first organic compound, a second organic compound, and a phosphorescent compound which can be applied to a light-emitting element of one embodiment of the present application is described with reference to FIG. 58A and FIG. 58B In this embodiment, an example of a combination of a first organic compound, a second organic compound, and a phosphorescent compound which can be applied to a light-emitting element of one embodiment of the present application is described with reference to

[0595] The phosphorescent compound used in this embodiment is [Ir(mppr-Me)2(dpm)]. The first organic compound used in this embodiment is 2mDBTPDBq-II. The two kinds of second organic compounds used in this embodiment are PCBNBB and 9-phenyl-9H-3-(9-phenyl-9H-carbazol-3-yl)carbazole (abbreviation: PCCP). The chemical formulas of the materials used in this embodiment are shown below.

[0596] Note that the chemical formulas of the materials used in the above embodiments are omitted.

[0597]

[0598] <Absorption spectrum>

[0599] FIG. 58A and FIG. 58B The following shows the ultraviolet-visible absorption spectrum (hereinafter simply referred to as the absorption spectrum) of a dichloromethane solution of [Ir(mppr-Me)2(dpm)], a phosphorescent compound. The absorption spectrum was measured using a UV-visible spectrophotometer (V550, manufactured by JASCO Corporation). The dichloromethane solution (0.093 mmol / L) was placed in a quartz cell and measured at room temperature.

[0600] <Emission Spectrum>

[0601] also, FIG. 58A and FIG. 58B The emission spectrum of a thin film of 2mDBTPDBq-II as the first organic compound (emission spectrum 28), the emission spectrum of a thin film of PCBNBB as the second organic compound (emission spectrum 29), the emission spectrum of a thin film of PCCP as the second organic compound (emission spectrum 30), the emission spectrum of a thin film of a mixed material of 2mDBTPDBq-II and PCBNBB (emission spectrum 31), and the emission spectrum of a thin film of a mixed material of 2mDBTPDBq-II and PCCP (emission spectrum 32) are shown. FIG. 58A In the figure, the horizontal axis represents the wavelength (nm), and the vertical axis represents the molar absorption coefficient ε (M -1 cm -1 ) and luminous intensity (arbitrary unit). FIG. 58B In the figure, the horizontal axis represents energy (eV) and the vertical axis represents the molar absorption coefficient ε (M -1 cm -1 ) and luminous intensity (arbitrary units).

[0602] from FIG. 58A The absorption spectrum of [Ir(mppr-Me)2(dpm)] shows that it has a broad absorption band around 500nm. This absorption band is considered to contribute significantly to luminescence.

[0603] Compared to the emission spectra of the monomers, the peaks of the emission spectra of the mixed material of 2mDBTPDBq-II and PCBNBB (emission spectrum 31) and the peaks of the emission spectra of the mixed material of 2mDBTPDBq-II and PCCP (emission spectrum 32) are both located on the longer wavelength (lower energy) side. This indicates that the mixing of 2mDBTPDBq-II and PCBNBB forms an exciplex. Furthermore, the mixing of 2mDBTPDBq-II and PCCP also forms an exciplex.

[0604] The peak of the emission spectrum of the above-described mixed material greatly overlaps with an absorption band in the absorption spectrum of [Ir(mppr-Me)2(dpm)], which can be considered to greatly contribute to luminescence. Therefore, the luminescent element using the mixed material of 2mDBTPDBq-II and PCBNBB and [Ir(mppr-Me)2(dpm)], and the luminescent element using the mixed material of 2mDBTPDBq-II and PCCP and [Ir(mppr-Me)2(dpm)] have high energy transfer efficiency because of energy transfer due to overlap of the emission spectrum of the mixed material with the absorption spectrum of the phosphorescent compound. Thus, a luminescent element with high external quantum efficiency can be obtained.

[0605] Further, it is known from this embodiment that, as one of the first organic compound and the second organic compound forming the exciplex, a carbazole compound can be used in addition to the aromatic amine compound.

[0606] [Embodiment 18]

[0607] In this embodiment, reference is made to FIG. 15 A luminescent element of one embodiment of the present application is described. Since the materials used in this embodiment are the materials used in the above embodiments, the chemical formulas are omitted.

[0608] A method for manufacturing the luminescent element 15 and the luminescent element 16 of this embodiment is described below.

[0609] (Luminescent element 15)

[0610] First, an ITSO film is formed on a glass substrate 1100 by a sputtering method, whereby a first electrode 1101 serving as an anode is formed. The thickness thereof is set to 110 nm, and the electrode area thereof is set to 2 mm x 2 mm.

[0611] Next, as a pretreatment for forming a luminescent element on the substrate 1100, after the substrate surface is washed with water and baked at 200 °C for 1 hour, UV ozone treatment is performed for 370 seconds.

[0612] Then, the substrate is put into a vacuum evaporation apparatus whose inside is reduced in pressure to 10 -4 Pa, and vacuum baking at 170 °C for 30 minutes is performed in a heating chamber of the vacuum evaporation apparatus, and then the substrate 1100 is cooled for about 30 minutes.

[0613] Next, the substrate 1100 on which the first electrode 1101 is formed is fixed to a substrate holder in the vacuum evaporation apparatus with the surface on which the first electrode 1101 is formed facing downward, and the pressure in the vacuum evaporation apparatus is reduced to 10 -4BPAFLP and molybdenum (VI) oxide were co-evaporated on the first electrode 1101 to form a hole injection layer 1111, with a thickness of 40 nm, at a weight ratio of BPAFLP to molybdenum (VI) oxide of 4:2 (= BPAFLP : molybdenum (VI) oxide) and at a pressure of about 1 Pa.

[0614] Next, a BPAFLP film with a thickness of 20 nm was formed on the hole injection layer 1111 to form a hole transport layer 1112.

[0615] Further, 2mDBTPDBq-II, PCBNBB, and [Ir(mppr-Me)2(dpm)] were co- evaporated on the hole transport layer 1112 to form a light-emitting layer 1113. Here, the weight ratio of 2mDBTPDBq-II, PCBNBB, and [Ir(mppr-Me)2(dpm)] was adjusted to 0.8:0.2:0.05 (= 2mDBTPDBq-II : PCBNBB : [Ir(mppr-Me)2(dpm)]). The thickness of the light-emitting layer 1113 was set to 20 nm.

[0616] Next, 2mDBTPDBq-II, PCBNBB, and [Ir(mppr-Me)2(dpm)] were co- evaporated on the light-emitting layer 1113 to form a first electron transport layer 1114a on the light-emitting layer 1113. Here, the weight ratio of 2mDBTPDBq-II, PCBNBB, and [Ir(mppr-Me)2(dpm)] was adjusted to 0.8:0.2:0.05 (= 2mDBTPDBq-II : PCBNBB : [Ir(mppr-Me)2(dpm)]). Further, the thickness of the first electron transport layer 1114a was set to 40 nm.

[0617] Next, a BPhen film with a thickness of 10 nm was formed on the first electron transport layer 1114a to form a second electron transport layer 1114b.

[0618] Then, a LiF film with a thickness of 1 nm was evaporated on the second electron transport layer 1114b to form an electron injection layer 1115.

[0619] Finally, an aluminum film with a thickness of 200 nm was evaporated as a second electrode 1103 serving as a cathode to manufacture the light-emitting element 15 of the present embodiment.

[0620] (Light-emitting element 16)

[0621] Light-emitting layer 1113 of light-emitting element 16 was formed by co-evaporation of 2mDBTPDBq-II, PCCP, and [Ir(mppr-Me)2(dpm)]. The weight ratio of 2mDBTPDBq-II, PCCP, and [Ir(mppr-Me)2(dpm)] was adjusted to 0.8:0.2:0.05 (= 2mDBTPDBq-II:PCCP:Ir(mppr-Me)2(dpm)). The thickness of light-emitting layer 1113 was set to 20 nm. Except for light-emitting layer 1113, the process was similar to that of light-emitting element 15.

[0622] Note that in the above evaporation processes, resistance heating method is used for evaporation.

[0623] Table 17 shows the element structures of the light-emitting element 15 and the light-emitting element 16 obtained through the above steps.

[0624] [Table 17]

[0625]

[0626] The light-emitting elements 15 and 16 were sealed in a nitrogen glove box to prevent exposure to the atmosphere, and then their operating characteristics were measured at room temperature (in an atmosphere maintained at 25° C.).

[0627] FIG. 59 The current density-luminance characteristics of the light-emitting element 15 and the light-emitting element 16 are shown. FIG. 59 The horizontal axis represents the current density (mA / cm 2 ), while the vertical axis represents brightness (cd / m 2 ). FIG. 60 The voltage-luminance characteristics are shown. FIG. 60 In the figure, the horizontal axis represents voltage (V) and the vertical axis represents brightness (cd / m 2 ). FIG. 61 The brightness-current efficiency characteristics are shown. FIG. 61 In the figure, the horizontal axis represents the brightness (cd / m 2 ), and the vertical axis represents the current efficiency (cd / A). FIG. 62 The brightness-external quantum efficiency characteristics are shown. FIG. 62 In the figure, the horizontal axis represents the brightness (cd / m 2 ), and the vertical axis represents the external quantum efficiency (%).

[0628] In addition, Table 18 shows that the light emitting element 15 and the light emitting element 16 have a brightness of 1200 cd / m 2 The voltage (V), current density (mA / cm 2), CIE color coordinates (x, y), current efficiency (cd / A), power efficiency (lm / W), and external quantum efficiency (%).

[0629] [Table 18]

[0630]

[0631] FIG. 63 The emission spectrum obtained when a current of 0.1 mA was applied to the light-emitting element 15 and the light-emitting element 16 is shown. In FIG. 18, the horizontal axis represents wavelength (nm) and the vertical axis represents emission intensity (arbitrary unit). As shown in Table 18, the emission spectrum of the light-emitting element 15 and the emission spectrum of the light-emitting element 16 were similar to each other. FIG. 63 The CIE color coordinates of the light-emitting element 15 at a luminance of 1200 cd / m 2 were (x, y) = (0.54, 0.45). The CIE color coordinates of the light-emitting element 16 at a luminance of 1200 cd / m 2 were (x, y) = (0.54, 0.46). From the above results, it was found that the light-emitting element 15 and the light-emitting element 16 exhibited orange light derived from [Ir(mppr-Me)2(dpm)].

[0632] From Table 18 and FIGS. 59-62 , it was found that the current efficiency, the power efficiency, and the external quantum efficiency of the light-emitting element 15 and the light-emitting element 16 were high.

[0633] In the light-emitting element 15 and the light-emitting element 16, 2mDBTPDBq-II, PCBNBB, or PCCP shown in Example 17, and [Ir(mppr-Me)2(dpm)] were used for the light-emitting layer. From Example 17, it was found that the emission spectrum of the mixture of 2mDBTPDBq-II and PCBNBB or PCCP (the emission spectrum of the exciplex) and the absorption band which can be considered to greatly contribute to light emission in the absorption spectrum of [Ir(mppr-Me)2(dpm)] were largely overlapped. Since the light-emitting element 15 and the light-emitting element 16 perform energy transfer by the overlap, it is considered that the energy transfer efficiency is high and the external quantum efficiency is high.

[0634] In addition, from this embodiment, it was found that a carbazole compound (PCCP) can be used to form an exciplex in addition to an aromatic amine compound (PCBNBB) and a light-emitting element with high external quantum efficiency can be obtained.

[0635] From the above results, it was found that a light-emitting element with high external quantum efficiency can be obtained by application of one embodiment of the present application.

[0636] (Reference Example 1)

[0637] The following shows the organometallic complex used in the above examples, namely (acetylacetonato)bis(4,6-diphenylpyrimidinato)iridium(III) (also known as bis[2-(6-phenyl-4-pyrimidinyl-κN3)phenyl-κC](2,4-pentanedione-κ 2

[0065] Synthesis Example of [Ir(dppm)2(acac)] (abbreviation: [Ir(dppm)2(acac)]). The structure of [Ir(dppm)2(acac)] is shown below.

[0638]

[0639] <Step 1: Synthesis of 4,6-diphenylpyrimidine (abbreviation: Hdppm)>

[0640] First, 5.02g of 4,6-dichloropyrimidine, 8.29g of phenylboronic acid, 7.19g of sodium carbonate, 0.29g of bis(triphenylphosphine) palladium(II) dichloride (abbreviation: Pd(PPh 3 ) 2Cl 2 ), 20mL of water and 20mL of acetonitrile were placed in a recovery flask equipped with a reflux line, and the air in the flask was replaced with argon. The reaction vessel was heated by irradiating microwaves (2.45GHz, 100W) for 60 minutes. Here, 2.08g of phenylboronic acid, 1.79g of sodium carbonate, 0.070g of Pd(PPh 3 ) 2Cl 2 , 5mL of water and 5mL of acetonitrile were further placed in the flask, and the mixture was heated by irradiating microwaves (2.45GHz, 100W) for 60 minutes again. Then, water was added to the solution and the organic layer was extracted with dichloromethane. The resulting extract was washed with water and dried over magnesium sulfate. After drying, the solution was filtered. The solvent of this solution was distilled off, and the resulting residue was subsequently purified by silica gel column chromatography using dichloromethane as a developing solvent. As a result, the pyrimidine derivative Hdppm (yellow-white powder, 38% yield) was obtained. Note that for microwave irradiation, a microwave synthesis apparatus (manufactured by CEM Corporation, Discover) was used. The synthesis scheme (a-1) of Step 1 is shown below.

[0641]

[0642] <Step 2: Synthesis of di-μ-chloro-bis[bis(4,6-diphenylpyrimidinyl)iridium(III)] (Abbreviation: [Ir(dppm)2Cl]2)>

[0643] Then, in the recovery flask that reflux line is housed, put into 15mL 2-ethoxyethanol, 5mL water, 1.10g Hdppm that obtains in the above-mentioned steps 1 and 0.69g iridium chloride hydrate (IrCl H O), and the air in the flask is replaced with argon gas.Then, carry out microwave (2.45GHz, 100W) irradiation 1 hour to cause reaction.Solvent is distilled out and resulting resistates is filtered and washed to obtain binuclear complex [Ir (dppm) Cl] (reddish-brown powder, productive rate 88%).The synthetic scheme (a-2) of step 2 is shown below.

[0644]

[0645] <Step 3: Synthesis of (acetylacetonato)bis(4,6-diphenylpyrimidinato)iridium(III) (Abbreviation: [Ir(dppm)2(acac)])>

[0646] Furthermore, 40mL 2-ethoxyethanol, 1.44g [Ir (dppm) 2Cl] 2, 0.30g acetylacetone and 1.07g sodium carbonate were placed in a recovery flask equipped with a reflux tube, and the air in the flask was replaced with argon. Then, microwave (2.45GHz, 120W) irradiation was performed for 60 minutes to cause a reaction. The solvent was distilled off, the residue obtained was dissolved in dichloromethane, and filtered to remove insoluble matter. The obtained filtrate was washed with water and then with saturated brine, and dried over magnesium sulfate. After drying, the solution was filtered. The solvent of the solution was distilled off, and the obtained residue was subsequently purified by silica gel column chromatography using dichloromethane: ethyl acetate = 50: 1 (volume ratio) as a developing solvent. Then, recrystallization was performed with a mixed solvent of dichloromethane and hexane to obtain an orange powder (yield 32%) as the target. The synthesis scheme (a-3) of step 3 is shown below.

[0647]

[0648] The NMR spectrum of the orange powder obtained in step 3 above ( 1 The results of H NMR analysis are shown below. From the above results, it can be seen that the organometallic complex [Ir(dppm)2(acac)] was obtained.

[0649] 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).

[0650] (Reference Example 2)

[0651] The following shows the organometallic complex used in the above examples, namely (acetylacetonato)bis(6-methyl-4-phenylpyrimidinato)iridium(III) (also known as bis[2-(6-methyl-4-pyrimidinyl-κN3)phenyl-κC](2,4-pentanedionato-κ 2

[0065] Synthesis Example of [Ir(mppm)2(acac)] (abbreviation: [Ir(mppm)2(acac)]). The structure of [Ir(mppm)2(acac)] is shown below.

[0652]

[0653] <Step 1: Synthesis of 4-methyl-6-phenylpyrimidine (abbreviation: Hmppm)>

[0654] First, in the recovery flask that reflux line is housed, put into 4.90g 4-chloro-6-methylpyrimidine, 4.80g phenylboric acid, 4.03g sodium carbonate, 0.16g bis(triphenylphosphine) palladium (II) dichloride (abbreviation: Pd(PPh ) Cl ), 20mL water and 10mL acetonitrile, and the air in the flask is replaced with argon.By heating for 60 minutes by irradiating microwave (2.45GHz, 100W) to this reaction vessel.Heat at this, in flask, further put into 2.28g phenylboric acid, 2.02g sodium carbonate, 0.082g Pd(PPh ) Cl , 5mL water and 10mL acetonitrile, and by heating for 60 minutes by using microwave (2.45GHz, 100W) to irradiate mixture again.Then, water is added in this solution and extracted with dichloromethane. The resulting extract was washed with a saturated aqueous sodium carbonate solution, water, and subsequently with saturated brine, and dried over magnesium sulfate. After drying, the solution was filtered. The solvent of the solution was distilled off, and the resulting residue was then purified by silica gel column chromatography using dichloromethane:ethyl acetate = 9:1 (volume ratio) as a developing solvent. As a result, the target pyrimidine derivative Hmppm (orange oily substance, yield 46%) was obtained. Note that for microwave irradiation, a microwave synthesis apparatus (manufactured by CEM, Discover) was used.

[0655] The synthetic scheme (b-1) of Step 1 is shown below.

[0656]

[0657] <Step 2: Synthesis of di-μ-chloro-bis[bis(6-methyl-4-phenylpyrimidinyl)iridium(III)] (Abbreviation: [Ir(mppm)2Cl]2)>

[0658] Then, in the recovery flask that reflux line is housed, put into 15mL 2-ethoxyethanol, 5mL water, 1.51g Hmppm that obtains in the above-mentioned steps 1 and 1.26g iridium chloride hydrate (IrCl H O), and the air in the flask is replaced with argon gas.Then, carry out microwave (2.45GHz, 100W) irradiation 1 hour to cause reaction.The solvent is distilled off, again with resulting resistates washing with ethanol and filter to obtain binuclear complex [Ir (mppm) Cl] (dark green powder, productive rate 77%).The synthetic scheme (b-2) of step 2 is shown below.

[0659]

[0660] <Step 3: Synthesis of (acetylacetonato)bis(6-methyl-4-phenylpyrimidinato)iridium(III) (Abbreviation: [Ir(mppm)2(acac)])>

[0661] Furthermore, 40mL 2-ethoxyethanol, 1.84g of the binuclear complex [Ir(mppm)2Cl]2 obtained in the above step 2, 0.48g of acetylacetone and 1.73g of sodium carbonate were placed in a recovery flask equipped with a reflux tube, and the air in the flask was replaced with argon. Then, microwave (2.45GHz, 120W) irradiation was performed for 60 minutes to cause a reaction. The solvent was distilled off, the obtained residue was dissolved in dichloromethane, and filtered to remove insoluble matter. The obtained filtrate was washed with water and then with saturated brine, and dried over magnesium sulfate. After drying, the solution was filtered. The solvent of the solution was distilled off, and the obtained residue was subsequently purified by silica gel column chromatography using dichloromethane: ethyl acetate = 4:1 (volume ratio) as a developing solvent. Then, recrystallization was performed with a mixed solvent of dichloromethane and hexane to obtain a yellow powder (yield 22%) as the target. The synthesis scheme (b-3) of step 3 is shown below.

[0662]

[0663] The NMR spectrum of the yellow powder obtained in step 3 above ( 1 The results of H NMR analysis are shown below. From the above results, it can be seen that an organometallic complex [Ir(mppm)2(acac)] was obtained.

[0664] 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).

[0665] (Reference Example 3)

[0666] The following shows the organometallic complex used in the above examples, namely (acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidino)iridium(III) (also known as bis[2-(6-tert-butyl-4-pyrimidino-κN3)phenyl-κC](2,4-pentanedione-κ 2

[0065] Synthesis Example of [Ir(tBuppm)2(acac)] (abbreviation: [Ir(tBuppm)2(acac)]). The structure of [Ir(tBuppm)2(acac)] is shown below.

[0667]

[0668] <Step 1: Synthesis of 4-tert-butyl-6-phenylpyrimidine (abbreviation: HtBuppm)>

[0669] First, 22.5g of 4,4-dimethyl-1-phenylpentane-1,3-dione and 50g of formamide were placed in a recovery flask equipped with a reflux tube, and the air in the flask was replaced with nitrogen. The reaction vessel was heated to reflux the reaction solution for 5 hours. Then, the solution was injected into an aqueous sodium hydroxide solution, and the organic layer was extracted with dichloromethane. The obtained organic layer was washed with water, saturated brine and dried over magnesium sulfate. After drying, the solution was filtered. The solvent of the solution was distilled off, and the obtained residue was subsequently purified by silica gel column chromatography using hexane: ethyl acetate = 10: 1 (volume ratio) as a developing solvent. As a result, a pyrimidine derivative HtBuppm (colorless oily substance, yield 14%) was obtained. The synthesis scheme (c-1) of step 1 is shown below.

[0670]

[0671] <Step 2: Synthesis of di-μ-chloro-bis[bis(6-tert-butyl-4-phenylpyrimidinyl)iridium(III)] (Abbreviation: [Ir(tBuppm)2Cl]2)>

[0672] Then, in the recovery flask that reflux line is housed, put into 15mL 2-ethoxyethanol, 5mL water, 1.49g HtBuppm that obtains in above-mentioned steps 1 and 1.04g iridium chloride hydrate (IrCl H O), and the air in the flask is replaced with argon gas.Then, carry out microwave (2.45GHz, 100W) irradiation 1 hour to cause reaction.Solvent is distilled out, and subsequently with resulting resistates with ethanol suction filtration and washing to obtain binuclear complex [Ir (tBuppm) Cl] (yellow-green powder, productive rate 73%).The synthetic scheme (c-2) of step 2 is shown as follows.

[0673]

[0674] <Step 3: Synthesis of (acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III) (Abbreviation: [Ir(tBuppm)2(acac)])>

[0675] Furthermore, 40 mL of 2-ethoxyethanol, 1.61 g of the binuclear complex [Ir(tBuppm)2Cl]2 obtained in Step 2 above, 0.36 g of acetylacetone, and 1.27 g of sodium carbonate were placed in a recovery flask equipped with a reflux tube, and the air in the flask was replaced with argon. Microwave irradiation (2.45 GHz, 120 W) was then performed for 60 minutes to induce a reaction. The solvent was distilled off, and the resulting residue was filtered with ethanol and washed with water and ethanol. The solid was dissolved in dichloromethane, and the mixture was filtered through a filter aid consisting of diatomaceous earth (Wako Pure Chemical Industries, Ltd., Catalog No. 531-16855), alumina, and diatomaceous earth layered in that order. The solvent was distilled off, and the resulting solid was recrystallized from a mixed solvent of dichloromethane and hexane to obtain the target compound as a yellow powder (yield 68%). The synthetic scheme (c-3) of step 3 is shown below.

[0676]

[0677] The NMR spectrum of the yellow powder obtained in step 3 above ( 1 The results of H NMR analysis are shown below. From the above results, it can be seen that the organometallic complex [Ir(tBuppm)2(acac)] was obtained.

[0678] 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).

[0679] (Reference Example 4)

[0680] The following describes a method for synthesizing 2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation: 2mDBTPDBq-II) used in the above examples.

[0681]

[0682] Synthesis of 2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation: 2mDBTPDBq-II)

[0683] Synthetic scheme (d-1) of 2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation: 2mDBTPDBq-II) is shown below.

[0684]

[0685] First, 5.3 g (20 mmol) of 2-chlorodibenzo[f,h]quinoxaline, 6.1 g (20 mmol) of 3-(dibenzothiophen-4-yl)phenylboronic acid, 460 mg (0.4 mmol) of tetrakis(triphenylphosphine)palladium(0), 300 mL of toluene, 20 mL of ethanol, and 20 mL of 2M aqueous potassium carbonate solution were placed in a 2L three-necked flask. The mixture was degassed by stirring under reduced pressure, and the air in the three-necked flask was replaced with nitrogen. The mixture was stirred at 100°C for 7.5 hours under a stream of nitrogen. After cooling to room temperature, the resulting mixture was filtered to obtain a white material. The resulting residue was washed with water and ethanol in this order, and then dried. The resulting solid was dissolved in about 600 mL of hot toluene, and then suction-filtered through diatomaceous earth and Florisil, thereby obtaining a colorless transparent filtrate. The resulting filtrate was concentrated and purified by column chromatography using about 700 mL of silica gel. The chromatography was performed using hot toluene as a developing solvent. Acetone and ethanol were added to the solid thus obtained, and then irradiated with ultrasonic waves. Subsequently, the resulting suspended solid was filtered and the resulting solid was dried to obtain 7.85 g of a white powder, with a yield of 80%.

[0686] The above objective material is a material that is relatively soluble in hot toluene, but easily precipitates upon cooling. In addition, the material is not easily soluble in other organic solvents such as acetone and ethanol. Thus, by utilizing these different solubilities, the synthesis can be performed with a high yield by the above simple method. Specifically, after the end of the reaction, the mixture is returned to room temperature, and the precipitated solid is collected by filtration, whereby most of the impurities can be easily removed. In addition, by column chromatography using hot toluene as a developing solvent, the objective material that easily precipitates can be easily purified.

[0687] The resulting 4.0 g of white powder was purified by gradient sublimation. Purification was performed by heating the white powder at 300°C under conditions where the pressure was 5.0 Pa and the argon flow rate was 5 mL / min. After purification, 3.5 g of a white powder was obtained as the objective material, with a yield of 88%.

[0688] Nuclear magnetic resonance spectrum (1H NMR) of the objective material was measured using a nuclear magnetic resonance spectrometer (JNM-LA400 manufactured by JEOL Ltd.). 1H NMR) confirmed that the compound was the desired 2-[3-(dibenzo-thiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation: 2mDBTPDBq-II).

[0689] The following shows the H NMR data of the obtained substance 1 H NMR data.

[0690] 1 H NMR (CDC13, 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).

[0691] Explanation of symbols

[0692] 102 EL layer

[0693] 103 First electrode

[0694] 108 Second electrode

[0695] 701 Hole injection layer

[0696] 702 Hole transport layer

[0697] 703 Emission layer

[0698] 704 Electron transport layer

[0699] 705 Electron injection layer

[0700] 706 Electron injection buffer layer

[0701] 707 Electron relay layer

[0702] 708 Composite layer

[0703] 800 First EL layer

[0704] 801 Second EL layer

[0705] 803 Charge generation layer

[0706] 1100 Substrate

[0707] 1101 First electrode

[0708] 1103 Second electrode

[0709] 1111 hole injection layer

[0710] 1112 hole transport layer

[0711] 1113 light emitting layer

[0712] 1114a first electron transport layer

[0713] 1114b second electron transport layer

[0714] 1115 electron injection layer

[0715] This application is based on Japanese Patent Application No. 2011-031462 filed on February 16, 2011 with the Japan Patent Office, the entire contents of which are hereby incorporated by reference.

Claims

1. A light-emitting element comprising: a first electrode; a light-emitting layer over the first electrode; and a second electrode over the light-emitting layer, wherein the light-emitting layer comprises a first organic compound, a second organic compound, and a guest material, wherein the first organic compound is a heteroaromatic compound, wherein the second organic compound is a carbazole compound, wherein the first organic compound and the second organic compound form an exciplex, wherein a peak of an emission spectrum of the exciplex overlaps with an absorption band on the longest wavelength side in an absorption spectrum of the guest material, wherein a difference between an energy value of the peak of the emission spectrum of the exciplex and an energy value of a peak of the absorption band on the longest wavelength side in the absorption spectrum is 0.2 eV or less, and wherein a triplet excitation energy level of the first organic compound and a triplet excitation energy level of the second organic compound are each higher than a triplet excitation energy level of the guest material.

2. A light-emitting element comprising: a first electrode; a light-emitting layer over the first electrode; and a second electrode over the light-emitting layer, wherein the light-emitting layer comprises a first organic compound, a second organic compound, and a guest material, wherein the first organic compound is a heteroaromatic compound, wherein the second organic compound is a carbazole compound, wherein the first organic compound and the second organic compound form an exciplex, wherein a peak of an emission spectrum of the exciplex overlaps with an absorption band on the longest wavelength side in an absorption spectrum of the guest material, wherein a difference between an energy value of the peak of the emission spectrum of the exciplex and an energy value of a peak of the absorption band on the longest wavelength side in the absorption spectrum is 0.1 eV or less, and wherein a triplet excitation energy level of the first organic compound and a triplet excitation energy level of the second organic compound are each higher than a triplet excitation energy level of the guest material.

3. The light-emitting element according to claim 1 or 2, wherein the triplet excitation energy level of the first organic compound, the triplet excitation energy level of the second organic compound, and the triplet excitation energy level of the guest material are each calculated using a time-dependent density functional method.

4. The light-emitting element according to claim 1 or 2, wherein the absorption band on the longest wavelength side corresponds to a direct transition from a singlet ground state to a triplet excited state.

5. The light-emitting element according to claim 1 or 2, wherein the carbazole compound comprises a carbazol-3-yl group.

6. The light-emitting element according to claim 1 or 2, wherein the emission spectrum of the exciplex is a fluorescence spectrum.

7. An electronic device comprising the light-emitting element according to claim 1 or 2.

8. A lighting device comprising the light-emitting element according to claim 1 or 2. ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ wherein ​ ​ wherein, ​ ​ wherein ​ ​ wherein, ​ ​ ​

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