Organic electroluminescent element, display device, and lighting device

Abstract

The present invention addresses the problem of providing an organic electroluminescent element which is capable of emitting light at a low applied voltage, while utilizing an excitation complex that is formed between a donor material and an acceptor material, and which has excellent drive stability. A solution for the problem disclosed herein is an organic electroluminescent element (1) which is provided with a positive electrode (3), a hole transport layer (5), a light emitting layer (6), and a negative electrode (8) in this order, wherein: the hole transport layer (5) contains a first organic compound that is a donor material having a small ionization potential; the light emitting layer (6) contains a second organic compound that is an acceptor material having a large electron affinity; and an excitation complex is formed between the first organic compound and the second organic compound. This organic electroluminescent element (1) is characterized in that the first organic compound is a thermally activated delayed fluorescent material.

Description

Organic electroluminescence element, display device, and lighting device

[0001] The present invention relates to an organic electroluminescence (hereinafter, electroluminescence (field emission) may be abbreviated as "EL") element, a display device, and a lighting device.

[0002] In recent years, efforts have been actively made to make organic electroluminescent devices emit light at low applied voltages. For example, it has been reported that when rubrene, a tetracene derivative, is used as a donor and fullerene, which has a high electron affinity, is used as an acceptor, the energy of the exciplex formed at the interface between them can be extracted as light emission from the singlet excited state of rubrene via triplet-triplet annihilation in an organic electroluminescent device (see, for example, Non-Patent Documents 1 and 2).

[0003] In addition, compounds that can utilize triplet-triplet annihilation include anthracene derivatives, tetracene derivatives, and the like, which have excellent electronic properties and exhibit high luminescence quantum yields, and are therefore used as light-emitting materials for organic electroluminescence devices that emit light at low driving voltages (see, for example, Non-Patent Documents 1 to 5).

[0004] In this low-voltage organic electroluminescent device, the charge recombination region and the light-emitting region are concentrated at the interface between the donor material and the acceptor material. It has been reported that the concentration of the charge recombination region and the light-emitting region at the interface generally shortens the device life of the organic electroluminescent device. Although light emission at a low voltage can be achieved by utilizing an exciplex formed at the interface between the donor material and the acceptor material, there is no prospect of extending the device life for practical use (see, for example, Non-Patent Document 6).

[0005] Sebastian Engmann and five others, "Nature Communications," Vol. 10, 2019, p. 227, doi. org / 10.1038 / s41467-018-08075-z. Seiichiro Izawa and three others, DOI: 10.26434 / chemrxiv.14685417. v1Xiangyang Tang, Qing Bai, Tong Shan, Jinyu Li, Yu Gao, Futong Liu, Hui Liu, Qiming Peng, Bing Yang, Feng Li, and Ping Lu, Advanced Functional Materials, 28:1705813, DOI: 10.1002 / adfm. 201705813 Futong Liu, 8 others, "Journal of Materials Chemistry C", Volume 7, 2019, p14881, DOI: 10.1039 / c9tc05040jYoun Jue Bae, and 9 others, “Journal of the American chemical society”, 2018, Volume 140, p15140, DOI: 10.1021 / jacs. 8b07498Yifan Zhang and 2 others, "Nature Communications", Volume 5, 2014, p5008, doi. org / 10.1038 / ncomms6008

[0006] As described above, in Non-Patent Documents 1 and 2, light emission is obtained at a low applied voltage by using rubrene, a tetracene derivative, as a donor material. However, since the charge recombination region and light-emitting region are concentrated at the interface, there is room for improvement in the device lifetime. To extend the device lifetime, a fundamental review of the material composition used for the donor and acceptor is necessary.

[0007] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an organic electroluminescence device that utilizes an exciplex formed between a donor material and an acceptor material, and that emits light at a low applied voltage and has excellent driving stability. Another object of the present invention is to provide a display device and a lighting device that include such an organic electroluminescence device, and that have a low driving voltage and excellent driving stability.

[0008] The present inventors have investigated various combinations of donor materials and acceptor materials, and have come to the conclusion that the above-mentioned problems can be solved by using a material that exhibits thermally activated delayed fluorescence as the donor material, thereby completing the present invention. That is, the gist of the organic electroluminescence element, display device, and lighting device of the present invention that solve the above-mentioned problems is as follows.

[0009] [1] An organic electroluminescence device comprising an anode, a hole transport layer, an emitting layer, and a cathode in this order, wherein the hole transport layer contains a first organic compound that is a donor material with a small ionization potential, and the emitting layer contains a second organic compound that is an acceptor material with a large electron affinity, and an exciplex is formed between the first organic compound and the second organic compound, wherein the first organic compound is a thermally activated delayed fluorescence material.

[0010] [2] The organic electroluminescence element according to [1], wherein the first organic compound has a calculated ionization potential (IP) of 4.8 eV or less as calculated using density functional theory, or a measured ionization potential (IP) of 5.5 eV or less as determined by ultraviolet photoelectron spectroscopy.

[0011] [3] The organic electroluminescence element according to [1] or [2], wherein the second organic compound has a calculated electron affinity (EA) of 2.1 eV or more as calculated using density functional theory, or a measured electron affinity (EA) of 2.8 eV or more as determined by low-energy inverse photoelectron spectroscopy.

[0012] [4] The organic electroluminescence element according to any one of [1] to [3], wherein the first organic compound has a dimethylacridine skeleton represented by the following formula (1-1), a phenoxazine skeleton represented by the following formula (1-2), or a carbazole skeleton represented by the following formula (1-3): [In formulas (1-1), (1-2), and (1-3), X represents a bond to another atom.]

[0013] [5] The organic electroluminescence device according to any one of [1] to [4], wherein the light-emitting layer further contains a phosphorescent material.

[0014] [6] The organic electroluminescence device according to any one of [1] to [4], wherein the light-emitting layer further contains a fluorescent material.

[0015] [7] The organic electroluminescence device according to any one of [1] to [4], wherein the light-emitting layer further contains a thermally activated delayed fluorescent material.

[0016] [8] A display device comprising the organic electroluminescence element according to any one of [1] to [7].

[0017] [9] A lighting device comprising the organic electroluminescence element according to any one of [1] to [7].

[0018] According to the present invention, it is possible to provide an organic electroluminescence device that utilizes an exciplex formed between a donor material and an acceptor material, and that emits light at a low applied voltage and has excellent driving stability. Furthermore, according to the present invention, it is possible to provide a display device and a lighting device that require a low driving voltage and have excellent driving stability.

[0019]

[0023] FIG. 1 is a schematic diagram showing an example of the structure of an organic EL element of the present invention.

[0024] FIG. 2 is a schematic diagram showing energy levels in an example of an organic EL element of the present invention.

[0025] FIG. 3 is a schematic diagram showing light emission from a singlet excited state of a material in an emitting layer, utilizing energy transfer from an exciplex and triplet-triplet annihilation, which is an example of the light-emitting mechanism of an organic EL element using an exciplex.

[0026] FIG. 4 is a schematic diagram showing light emission from a singlet excited state of a material in an emitting layer, utilizing energy transfer from a triplet excited state to a singlet excited state due to reverse intersystem crossing in a thermally activated delayed fluorescent material, energy transfer from the singlet excited state of the thermally activated delayed fluorescent material to a material in the emitting layer, energy transfer from an exciplex, and triplet-triplet annihilation, which is an example of the light-emitting mechanism of an organic EL element of the present invention.

[0026] FIG. 5 is a schematic diagram showing another example of the structure of an organic EL element of the present invention.

[0027] FIG. 6 is a graph showing the decay process of light emission from 2-SF-PHX under a nitrogen atmosphere and an air atmosphere.

[0028] FIG. 7 is a graph showing the decay process of light emission from 2Cz-DMAC under a nitrogen atmosphere and an air atmosphere.

[0023] FIG. 1 is a schematic diagram showing light emission from the singlet excited state of an emitting dopant, utilizing energy transfer from a triplet excited state to a singlet excited state due to reverse intersystem crossing in a thermally activated delayed fluorescent material, energy transfer from the singlet excited state of the thermally activated delayed fluorescent material to the material of the emitting layer, energy transfer from the exciplex, triplet-triplet annihilation, and energy transfer from the singlet excited state of the material of the emitting layer to the emitting dopant, which is another example of the light-emitting mechanism of the organic EL device of the present invention.

[0024] FIG. 1 is a graph showing the results of measuring (a) the current density-voltage characteristics, (b) the luminance-voltage characteristics, and (c) the EL spectrum of various organic EL devices produced in Examples 1, 2, and Comparative Example 1.

[0025] Each of the organic EL devices produced in Examples 1, 2, and Comparative Example 1 was measured at an initial luminance of 100 cd / m ２ 1 is a graph showing the change in luminance when the organic EL elements produced in Examples 3, 4, and Comparative Example 2 are continuously driven from an initial luminance of 100 cd / m. 2 is a graph showing the change in luminance when the organic EL elements are continuously driven from an initial luminance of 100 cd / m. 3 is a graph showing the change in luminance when the organic EL elements are continuously driven from an initial luminance of 100 cd / m. 4 is a graph showing the change in luminance when the organic EL elements are continuously driven from an initial luminance of 100 cd / m. 5 is a graph showing the change in luminance when the organic EL elements are continuously driven from an initial luminance of 100 cd / m. 6 is a graph showing the change in luminance when the organic EL elements are continuously driven from an initial luminance of 100 cd / m. 7 is a graph showing the change in luminance when the organic EL elements are continuously driven from an initial luminance of 100 cd / m. 8 is a graph showing the change in luminance when the organic EL elements are continuously driven from an initial luminance of 100 cd / m. 9 is a graph showing the change in luminance when the organic EL elements are continuously driven from an initial luminance of 100 cd / m. 10 is a graph showing the change in luminance when the organic EL elements are continuously driven from an initial luminance of 100 cd / m. 11 is a graph showing the change in luminance when the organic EL elements are continuously driven from an initial luminance of 100 cd / m. 12 is a graph showing the change in luminance when the organic EL elements are continuously driven from an initial luminance of 100 cd / m. 13 is a graph showing the change in luminance when the organic EL elements are continuously driven from an initial luminance of 100 cd / m. 14 is a graph showing the change in lumin ２ 10 is a graph showing a change in luminance when continuously driven from 100°C to 120°C.

[0020] The organic electroluminescence element, the display device, and the lighting device of the present invention will be described in detail below based on their embodiments. Note that a combination of two or more of the individual preferred embodiments of the present invention described below is also a preferred embodiment of the present invention.

[0021] <Organic Electroluminescent Device> The organic electroluminescent device of the present invention comprises an anode, a hole transport layer, an emitting layer, and a cathode, in this order, wherein the hole transport layer contains a first organic compound that is a donor material with a small ionization potential, and the emitting layer contains a second organic compound that is an acceptor material with a large electron affinity. The organic electroluminescent device of the present invention is characterized in that an exciplex is formed between the first organic compound and the second organic compound, and the first organic compound is a thermally activated delayed fluorescence material.

[0022] The thermally activated delayed fluorescent material is a material that emits thermally activated delayed fluorescence (TADF). The thermally activated delayed fluorescence refers to fluorescence observed from the delayed singlet excited state, which is thermally excited from a triplet excited state to a singlet excited state, causing reverse intersystem crossing. The donor material is a material that can donate electrons to adjacent molecules or atoms, and the smaller the ionization potential (IP), the easier it is to donate electrons. The acceptor material is a material that can attract electrons from adjacent molecules or atoms, and the larger the electron affinity (EA), the easier it is to attract electrons. The donor material having a small ionization potential and the acceptor material having a large electron affinity refers to the fact that the ionization potential is small and the electron affinity is large enough to form an exciplex between the donor material (first organic compound) and the acceptor material (second organic compound), and the energy difference between them is small.

[0023] In the organic electroluminescent device of the present invention, since the energy difference between the ionization potential (IP) of the first organic compound as the donor material and the electron affinity (EA) of the second organic compound as the acceptor material is small, by using the first organic compound as a donor material in the hole transport layer and the second organic compound as an acceptor material in the light-emitting layer, and arranging the first organic compound as a donor material and the second organic compound as an acceptor material adjacent to each other so as to form an exciplex between them, it is possible to obtain light emission at a low applied voltage. In addition, in the organic electroluminescent device of the present invention, since the first organic compound as a donor material is a thermally activated delayed fluorescent material and the energy difference between its singlet excited state and triplet excited state is small, the lifetime of the unstable excited state can be shortened, thereby enabling a long lifetime.

[0024] Next, one embodiment of the organic electroluminescent device of the present invention will be described in detail with reference to the drawings. Fig. 1 is a schematic diagram showing an example of the structure of the organic EL device of the present invention. The organic EL device 1 shown in Fig. 1 has a laminated structure in which an anode 3, a hole injection layer 4, a hole transport layer 5, a light-emitting layer 6, an electron injection layer 7, and a cathode 8 are formed in this order on a substrate 2.

[0025] 2 shows the energy levels within the organic electroluminescent device 1 shown in FIG. 1 . In the organic EL device 1, electrons injected into the light-emitting layer 6 containing a second organic compound that is an acceptor material with a large electron affinity and holes injected into the hole-transport layer 5 containing a first organic compound that is a donor material with a small ionization potential form an exciplex, causing a current to flow and resulting in light emission. The voltage required for light emission is determined by the energy difference between the electron affinity (EA) of the material used in the light-emitting layer 6 and the ionization potential (IP) of the material used in the hole-transport layer 5. However, if the material used in the light-emitting layer 6 has a large electron affinity and the material used in the hole-transport layer 5 has a small ionization potential, this energy difference becomes small, and the externally applied voltage required for light emission decreases.

[0026] Next, the process of energy transfer from the formation of an exciplex to light emission is shown in Fig. 3. Also, Fig. 4 shows the process of energy transfer when a thermally activated delayed fluorescent material is used in the hole transport layer 5.

[0027] As shown in Figure 3, the small external voltage required to form an exciplex means that the energy of the light from the resulting exciplex is not large. However, if the triplet energy of the material (e.g., anthracene derivatives, etc.) used in the light-emitting layer 6 involved in the formation of the exciplex is smaller than the energy of the excited state of this exciplex, the energy of the exciplex is transferred to the triplet excited state of the material used in the light-emitting layer 6. It is well known that anthracene derivatives, etc., can generate a singlet excited state by utilizing triplet-triplet annihilation, and this property can be utilized to generate a singlet excited state and obtain light emission from it. Here, the light emission from the singlet excited state has greater energy than the light from the exciplex.

[0028] 3 , when a general donor material is used as the material for the hole transport layer 5, it is expected that as the voltage applied to the device increases, excited states (singlet excited state, triplet excited state) will be formed in the material for the hole transport layer 5 and the material for the light-emitting layer 6. In this case, the triplet excited state of the material for the light-emitting layer 6 can be obtained as light emission by utilizing triplet-triplet annihilation, but the triplet excited state of the material for the hole transport layer 5 remains unstable, which causes a decrease in luminous efficiency and deterioration of the organic EL element 1.

[0029] In contrast, as shown in Fig. 4, when a thermally activated delayed fluorescent material is used in the hole transport layer 5, the energy difference between the singlet excited state and the triplet excited state of the thermally activated delayed fluorescent material is small, and therefore the energy of the triplet excited state is transferred to the singlet excited state by reverse intersystem crossing. Furthermore, this singlet excited state undergoes energy transfer to the material of the light-emitting layer 6, and light is emitted from the light-emitting layer 6. By enabling such an energy transfer process, the lifetime of the triplet excited state of the material of the hole transport layer 5 generated upon voltage application can be shortened, thereby enabling the organic EL device 1 to have a longer lifetime.

[0030] The organic EL element 1 of this embodiment may be an organic EL element having an inverted structure in which a cathode 8 is provided on a substrate 2, as shown in Fig. 5. The organic EL element 1 shown in Fig. 5 has a layered structure in which a cathode 8, an electron injection layer 7, a light-emitting layer 6, a hole transport layer 5, a hole injection layer 4, and an anode 3 are formed in this order on a substrate 2.

[0031] The organic EL element 1 of this embodiment may also be an organic-inorganic hybrid organic electroluminescent element (HOILED element) in which some of the layers constituting the organic EL element are formed using inorganic compounds.

[0032] "Substrate" Examples of materials for the substrate 2 include resin materials and glass materials. Examples of resin materials used for the substrate 2 include polyethylene terephthalate, polyethylene naphthalate, polypropylene, cycloolefin polymer, polyamide, polyethersulfone, polymethyl methacrylate, polycarbonate, and polyarylate. When a resin material is used as the material for the substrate 2, an organic EL element 1 with excellent flexibility can be obtained, which is preferable. Examples of glass materials used for the substrate 2 include quartz glass and soda glass.

[0033] When the organic EL element 1 is a bottom-emission type, a transparent substrate is used as the material for the substrate 2. When the organic EL element 1 is a top-emission type, not only a transparent substrate but also an opaque substrate may be used as the material for the substrate 2. Examples of opaque substrates include a substrate made of a ceramic material such as alumina, a substrate in which an oxide film (insulating film) is formed on the surface of a metal plate such as stainless steel, and a substrate made of a resin material.

[0034] The average thickness of the substrate 2 can be determined depending on the material of the substrate 2, and is preferably 0.1 to 30 mm, and more preferably 0.1 to 10 mm. The average thickness of the substrate 2 can be measured using a digital multimeter and vernier calipers.

[0035] "Anode" The anode 3 shown in Fig. 1 is formed on the substrate 2 in direct contact with it, but in the case of an organic EL element having an inverted structure as shown in Fig. 5, it does not have to be formed on the substrate 2 in direct contact with it. Materials for the anode 3 include ITO (indium tin oxide), IZO (indium zinc oxide), FTO (fluorine tin oxide), In ２ O ３ , SnO ２ , Sb-containing SnO ２ Examples of conductive materials include oxides such as Al-containing ZnO. Among these, ITO, IZO, and FTO are preferably used as the material for the anode 3. There are no particular limitations on the average thickness of the anode 3, but it is preferably 10 to 500 nm, and more preferably 100 to 200 nm. The average thickness of the anode 3 can be measured using a stylus profilometer or spectroscopic ellipsometry.

[0036] "Hole injection layer" The hole injection layer 4 may be made of an inorganic material or an organic material. Inorganic materials are more stable than organic materials, and therefore, it is easier to obtain high resistance to oxygen and water compared to when an organic material is used. The inorganic material is not particularly limited, but examples thereof include vanadium oxide (V ２ O ５ ), molybdenum oxide (MoO ３ ), ruthenium oxide (RuO ２ One or more metal oxides such as dipyrazino[2,3-f:2',3'-h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile (HAT-CN) and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) can be used as the organic material. Polymer materials such as poly(3,4-ethylenedioxythiophene) (PEDOT) can also be used. The average thickness of the hole injection layer 4 is not particularly limited, but is preferably 1 to 1,000 nm, and more preferably 5 to 50 nm. The average thickness of the hole injection layer 4 can be measured during film formation using a quartz crystal film thickness meter.

[0037] "Hole transport layer" As described above, the hole transport layer 5 contains a first organic compound that is a donor material with a small ionization potential, and the first organic compound is a thermally activated delayed fluorescence material. That is, the hole transport layer 5 contains an organic compound that has a small ionization potential and exhibits thermally activated delayed fluorescence. The hole transport layer 5 may be composed only of the first organic compound, or may contain other components.

[0038] Specifically, the first organic compound used in the hole transport layer 5 is preferably a material having a molecular structure with high donor properties, such as a triarylamine skeleton, a carbazole skeleton, a phenoxazine skeleton, a phenazine skeleton, an acridine skeleton, an azadibenzopyrene skeleton, or a julolidine skeleton.

[0039] The first organic compound preferably has a dimethylacridine skeleton represented by the following formula (1-1), a phenoxazine skeleton represented by the following formula (1-2), or a carbazole skeleton represented by the following formula (1-3).

[0040] In the above formulas (1-1), (1-2), and (1-3), X represents a bond bonded to another atom, and various substituents can be bonded to the end of the bond. Furthermore, various substituents can be bonded to the dimethylacridine skeleton of formula (1-1), the phenoxazine skeleton of formula (1-2), and the carbazole skeleton of formula (1-3) in place of hydrogen atoms in these skeletons. Examples of the substituent include an alkyl group having 1 to 10 carbon atoms, an alkoxy group having 1 to 10 carbon atoms, an alkylthio group having 1 to 10 carbon atoms, an alkylamino group having 1 to 10 carbon atoms, an acyl group having 2 to 10 carbon atoms, an aralkyl group having 7 to 20 carbon atoms, a substituted or unsubstituted aromatic hydrocarbon group having 6 to 40 carbon atoms, and a substituted or unsubstituted 6-membered aromatic heterocyclic group having 3 to 40 carbon atoms.

[0041] More specifically, the first organic compound is preferably 2-SF-PHX, 2Cz-DMAC, or the like, as shown below.

[0042] Whether or not a donor material exhibits thermally activated delayed fluorescence can be determined by observing the decay process of luminescence in both a nitrogen atmosphere and an air atmosphere. As an example, a method for determining whether or not 2-SF-PHX and 2Cz-DMAC exhibit thermally activated delayed fluorescence and the results thereof will be described below.

[0043] First, 2-SF-PHX and 2Cz-DMAC were vacuum-deposited onto a quartz substrate to form a 100 nm film, and the decay process of the light emission was observed using a streak camera manufactured by Hamamatsu Photonics KK The results are shown in Figures 6 and 7.

[0044] For both materials, delayed fluorescence components were observed in the results of measurements under a nitrogen atmosphere, whereas the delayed fluorescence components were significantly weaker in the results of measurements under an air atmosphere. This indicates that the delayed fluorescence is extinguished by oxygen in the air. These results confirmed that 2-SF-PHX and 2Cz-DMAC exhibit thermally activated delayed fluorescence.

[0045] The first organic compound contained in the hole transport layer 5 preferably has a calculated ionization potential (IP) of 4.8 eV or less as calculated using density functional theory [B3LPY / 6-31G (d,p)] or a measured ionization potential (IP) of 5.5 eV or less as determined by ultraviolet photoelectron spectroscopy (UPS), and more preferably has a calculated ionization potential (IP) of 4.72 eV or less as calculated using density functional theory or a measured ionization potential (IP) of 5.4 eV or less as determined by ultraviolet photoelectron spectroscopy. When the calculated ionization potential (IP) of the first organic compound contained in the hole transport layer 5 is 4.8 eV or less as calculated using density functional theory or a measured ionization potential (IP) of 5.5 eV or less as determined by ultraviolet photoelectron spectroscopy, the energy difference with the electron affinity (EA) of the second organic compound, which is an acceptor material described later, becomes even smaller, making it possible to obtain light emission at an even lower applied voltage.

[0046] The calculated ionization potentials (IP) of 2-SF-PHX and 2Cz-DMAC calculated using density functional theory and the measured ionization potentials (IP) determined by ultraviolet photoelectron spectroscopy (UPS) are shown in Table 1. For reference, the calculated and measured ionization potentials (also referred to as "ionization energies") of α-NPD and TPDI are also shown.

[0047]

[0048] 2-SF-PHX and 2Cz-DMAC have a small ionization potential equivalent to that of N,N'-di-1-naphthyl-N,N'-diphenylbenzidine (α-NPD), a common hole-transporting material, and yet exhibit thermally activated delayed fluorescence. Therefore, they are expected to emit light stably for a long period of time at a low applied voltage.

[0049] The average thickness of the hole transport layer 5 is not particularly limited, but is preferably 10 to 150 nm, and more preferably 20 to 100 nm. The average thickness of the hole transport layer 5 can be measured, for example, by a stylus profilometer or spectroscopic ellipsometry.

[0050] "Light-Emitting Layer" As described above, the light-emitting layer 6 contains a second organic compound that is an acceptor material with a high electron affinity, and the second organic compound can form an exciplex with the first organic compound. The light-emitting layer 6 may be composed solely of the second organic compound, or may contain other components (e.g., a dopant). The light-emitting layer 6 is primarily formed from a material that can generate a singlet excited state through triplet-triplet annihilation, and preferably contains a tetracene derivative such as a rubrene derivative, an anthracene derivative, or the like. In other words, the second organic compound is preferably a tetracene derivative such as a rubrene derivative, an anthracene derivative, or the like. Here, rubrene derivative refers to a compound in which rubrene and a hydrogen atom in its molecular structure are substituted with a substituent. Furthermore, tetracene derivative refers to a compound having a tetracene skeleton (tetracene ring), and includes tetracene and a compound in which a hydrogen atom in its molecular structure is substituted with a substituent. An anthracene derivative refers to a compound having an anthracene skeleton (anthracene ring) and includes anthracene and compounds in which hydrogen atoms in the molecular structure are substituted with substituents. Examples of the substituent include a cyano group, an alkyl group having 1 to 10 carbon atoms, an alkoxy group having 1 to 10 carbon atoms, an alkylthio group having 1 to 10 carbon atoms, an alkylamino group having 1 to 10 carbon atoms, an acyl group having 2 to 10 carbon atoms, an aralkyl group having 7 to 20 carbon atoms, a substituted or unsubstituted aromatic hydrocarbon group having 6 to 30 carbon atoms, and a substituted or unsubstituted aromatic 6-membered heterocyclic group having 3 to 30 carbon atoms.

[0051] The second organic compound preferably has a calculated electron affinity (EA) of 2.1 eV or more as calculated using density functional theory [B3LPY / 6-31G(d,p)], or a measured electron affinity (EA) of 2.8 eV or more as measured using low-energy inverse photoemission spectroscopy (LEIPS). When the calculated electron affinity (EA) of the second organic compound contained in the light-emitting layer 6 is 2.1 eV or more as calculated using density functional theory, or the measured electron affinity (EA) of the second organic compound contained in the light-emitting layer 6 is 2.8 eV or more, the energy difference with the ionization potential of the first organic compound, which is the donor material, becomes even smaller, and light emission can be obtained at an even lower applied voltage.

[0052] In the light-emitting layer 6, it is possible to extract light from a singlet excited state generated by triplet-triplet annihilation as shown in Fig. 4, while it is also possible to extract light from another light-emitting dopant by using a second organic compound (exciplex) as a host for the light-emitting layer 6 as shown in Fig. 8. In this case, a phosphorescent material such as an iridium complex or a platinum complex, a fluorescent material, a thermally activated delayed fluorescent material, or the like may be mixed into the light-emitting layer 6 as a dopant for the host. The mixing ratio of the dopant with respect to the second organic compound (exciplex) is preferably 20% by mass or less, more preferably 10% by mass or less.

[0053] In one embodiment, the light-emitting layer 6 further contains a phosphorescent material in addition to the second organic compound. As the phosphorescent material, a metal complex containing a metal such as Ir or Pt can be used. As an iridium complex, factorys(2-phenylpyridine)iridium (Ir(ppy) ３ ), fac-tris(3-methyl-2-phenylpyridinato-N,C2′-)iridium(III) (Ir(mppy) ３ ), tris[1-phenylisoquinoline]iridium(III) (Ir(piq) ３ Examples of platinum complexes include 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphineplatinum(II).

[0054] In another embodiment, the light-emitting layer 6 further contains a fluorescent material in addition to the second organic compound. The fluorescent material is 8-hydroxyquinoline aluminum (Alq ３ ), tris(4-methyl-8-quinolinolato)aluminum(III) (Almq ３ ), 8-hydroxyquinoline zinc (Znq ２ ), various metal complexes such as benzene-based compounds such as distyrylbenzene (DSB) and diaminodistyrylbenzene (DADSB); naphthalene-based compounds such as naphthalene and Nile Red; phenanthrene-based compounds such as phenanthrene; chrysene-based compounds such as chrysene and 6-nitrochrysene; perylene-based compounds such as perylene and N,N'-bis(2,5-di-t-butylphenyl)-3,4,9,10-perylene-di-carboximide (BPPC); coronene-based compounds such as coronene; anthracene-based compounds such as anthracene and bisstyrylanthracene; pyrene-based compounds such as pyrene and BD-1; pyran-based compounds such as 4-(di-cyanomethylene)-2-methyl-6-(para-dimethylaminostyryl)-4H-pyran (DCM); acridine-based compounds such as acridine; stilbene-based compounds such as stilbene; thiophene-based compounds such as 2,5-dibenzoxazolethiophene; benzoxazole-based compounds such as benzoxazole; benzimidazole-based compounds such as benzimidazole; benzothiazole-based compounds such as 2,2'-(para-phenylenedivinylene)-bisbenzothiazole; butadiene-based compounds such as bistyryl(1,4-diphenyl-1,3-butadiene) and tetraphenylbutadiene; naphthalimide-based compounds such as naphthalimide; coumarin-based compounds such as coumarin; perinone-based compounds such as perinone; oxadiazole-based compounds such as oxadiazole; aldazine-based compounds; cyclopentadiene-based compounds such as 1,2,3,4,5-pentaphenyl-1,3-cyclopentadiene (PPCP); quinacridone-based compounds such as quinacridone and quinacridone red; pyridine-based compounds such as pyrrolopyridine and thiadiazolopyridine; and spiro compounds such as 2,2',7,7'-tetraphenyl-9,9'-spirobifluorene.

[0055] In yet another embodiment, the light-emitting layer 6 further contains a thermally activated delayed fluorescent material in addition to the second organic compound. Examples of the thermally activated delayed fluorescence material include 2,4,5,6-tetra(9-carbazole)-isophthalonitrile (4CzIPN), 4,5-di(9-carbazole)-phthalonitrile (2CzPN), 3,4,5,6-tetra(9-carbazole)-phthalonitrile (4CzPN), 2,3,5,6-tetra(9-carbazole)-terephthalonitrile (4CzTPN), 2,3,5,6-tetra(3,6-dimethyl-9-carbazole)-terephthalonitrile (4CzTPN-Me), and 2,3,5,6-tetra(3,6-diphenyl-9-carbazole)-terephthalonitrile (4CzTPN-Ph). Further examples include compounds described in JP 2012-193352 A and WO 2011 / 070963 A.

[0056] The average thickness of the light-emitting layer 6 is not particularly limited, but is preferably 10 to 150 nm, and more preferably 20 to 100 nm. The average thickness of the light-emitting layer 6 may be measured using a stylus-type step profiler or may be measured using a quartz crystal film thickness meter during the deposition of the light-emitting layer 6.

[0057] "Electron Injection Layer" For the electron injection layer 7, any material having a low work function, such as alkali metals or alkaline earth metals, a compound containing a metal having a low work function, such as lithium quinoline, lithium fluoride, cesium carbonate, or calcium carbonate, a hexahydropyrimidopyrimidine compound having a structure represented by the following general formula (2), or a compound having a structure represented by the following general formula (3), can be used.

[0058] (In general formula (2), R １ represents an aromatic hydrocarbon group, an aromatic heterocyclic group, an aryl alkylene group, a divalent to tetravalent chain or cyclic hydrocarbon group which may have a substituent, a group formed by combining two or more of these groups, or a group formed by combining one or more of these groups with a nitrogen atom. １ is an integer from 1 to 4.

[0059] (In general formula (3), X １ , X ２ are the same or different and represent a nitrogen atom, an oxygen atom, a sulfur atom or a divalent linking group which may have a substituent. L represents a direct bond or a p-valent linking group. ２ represents the number 0 or 1, and p represents a number from 1 to 4. q represents the number 0 or 1, and when p is 1, q is 0. R ２ ~R ４ are the same or different and represent a monovalent substituent. １ ~m ３ are the same or different and represent a number from 0 to 3. ２ ~R ４ is X １ , X ２ may be bonded to form a ring structure. ２ If there are multiple R ２ may be bonded to form a ring structure. ３ If there are multiple R ３ may be bonded to form a ring structure. ４ If there are multiple R ４ may be bonded to form a ring structure.)

[0060] R in the above general formula (2) １represents an aromatic hydrocarbon group, an aromatic heterocyclic group, an aryl alkylene group, a divalent to tetravalent linear or cyclic hydrocarbon group which may have a substituent, a group formed by combining two or more of these groups, or a group formed by combining one or more of these groups with a nitrogen atom. The aromatic hydrocarbon group and aromatic heterocyclic group preferably have 3 to 30 carbon atoms, more preferably have 4 to 24 carbon atoms, and even more preferably have 5 to 20 carbon atoms. Examples of the aromatic hydrocarbon group include compounds consisting of only one aromatic ring such as benzene; compounds in which multiple aromatic rings are directly bonded via one carbon atom, such as biphenyl and diphenylbenzene; and groups formed by removing 1 to 4 hydrogen atoms from any of the aromatic rings of fused-ring aromatic hydrocarbon compounds such as naphthalene, anthracene, phenanthrene, and pyrene. Examples of aromatic heterocyclic groups include compounds consisting of only one aromatic heterocycle, such as thiophene, furan, pyrrole, oxazole, oxadiazole, thiazole, thiadiazole, imidazole, pyridine, pyrimidine, pyrazine, and triazine; compounds in which multiple compounds consisting of only one aromatic heterocycle are directly bonded via one carbon atom (e.g., bipyridine); and groups formed by removing 1 to 4 hydrogen atoms from any of the aromatic heterocycles of fused-ring heteroaromatic hydrocarbon compounds, such as quinoline, quinoxaline, benzothiophene, benzothiazole, benzimidazole, benzoxazole, indole, carbazole, dibenzofuran, dibenzothiophene, acridine, and phenanthroline. Examples of aryl alkylene groups include groups formed by combining the above-mentioned aromatic hydrocarbon groups with alkylene groups having 1 to 3 carbon atoms. Divalent to tetravalent linear or cyclic hydrocarbon groups preferably have 1 to 12 carbon atoms, more preferably 1 to 6 carbon atoms, and even more preferably 1 to 4 carbon atoms. The chain hydrocarbon group may be a straight chain or a branched chain. １ may be a group formed by combining two or more of the above-mentioned aromatic hydrocarbon groups, aromatic heterocyclic groups, aryl alkylene groups, and divalent to tetravalent chain hydrocarbon groups. １may be a group formed by combining one or more of the above-mentioned aromatic hydrocarbon groups, aromatic heterocyclic groups, aryl alkylene groups, and divalent to tetravalent chain hydrocarbon groups with a nitrogen atom. Examples of such groups include groups formed by removing 1 to 4 hydrogen atoms from trialkylamines such as trimethylamine or triphenylamine.

[0061] The aromatic hydrocarbon group, aromatic heterocyclic group, or arylalkylene group may have one or more monovalent substituents. Examples of the monovalent substituent include a fluorine atom; haloalkyl groups such as a fluoromethyl group, a difluoromethyl group, and a trifluoromethyl group; linear or branched alkyl groups having 1 to 20 carbon atoms such as a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, an isobutyl group, and a tert-butyl group; cyclic alkyl groups having 5 to 7 carbon atoms such as a cyclopentyl group, a cyclohexyl group, and a cycloheptyl group; linear or branched alkoxy groups having 1 to 20 carbon atoms such as a methoxy group, an ethoxy group, a propoxy group, an isopropoxy group, a butoxy group, an isobutoxy group, a tert-butoxy group, a pentyloxy group, a hexyloxy group, a heptyloxy group, and an octyloxy group; a nitro group; a cyano group; an alkylamino group having an alkyl group having 1 to 10 carbon atoms such as a methylamino group, an ethylamino group, a dimethylamino group, and a diethylamino group; a pyrrolidino group, a piperidino group, and the like. cyclic amino groups such as dino group and morpholino group; diarylamino groups such as diphenylamino group and carbazolyl group; acyl groups such as acetyl group, propionyl group and butyryl group; alkenyl groups having 2 to 30 carbon atoms such as styryl group; aryl groups having 5 to 20 carbon atoms which may be substituted with a halogen atom such as a fluorine atom, or an alkyl group, alkoxy group or amino group having 1 to 20 carbon atoms (specific examples of the aryl group are the same as those for the aromatic hydrocarbon groups described above); heterocyclic groups containing one or more of a nitrogen atom, sulfur atom and oxygen atom which may be substituted with a halogen atom such as a fluorine atom, or an alkyl group, alkoxy group or amino group having 1 to 20 carbon atoms (the heterocyclic group may be one consisting of only one ring, or may be a compound in which a plurality of compounds consisting of only one aromatic heterocycle are directly bonded together via one carbon atom, or may be a fused heterocyclic group).Specific examples of heterocyclic groups include specific examples of aromatic heterocyclic groups such as a thiophene ring, a furan ring, a pyrrole ring, a benzothiophene ring, a benzofuran ring, an indole ring, a dibenzothiophene ring, a dibenzofuran ring, a carbazole ring, a thiazole ring, a benzothiazole ring, an oxazole ring, a benzoxazole ring, an imidazole ring, a benzimidazole ring, a pyridine ring, a pyrimidine ring, a pyrazine ring, a pyridazine ring, a triazine ring, a quinoline ring, an isoquinoline ring, a quinoxaline ring, a benzothiadiazole ring, and a phenanthridine ring; an ester group, a thioether group, and the like. These groups may be substituted with a halogen atom, a hetero element, an alkyl group, an aromatic ring, or the like.

[0062] n in the above general formula (2) １ is an integer of 1 to 4, preferably 2 or 3.

[0063] X in the above general formula (3) １ , X ２ are the same or different and represent a nitrogen atom, oxygen atom, sulfur atom, or divalent linking group which may have a substituent. Examples of the divalent linking group include divalent hydrocarbon groups and groups in which a portion of the carbon atoms of a hydrocarbon group is substituted with a heteroatom such as a nitrogen atom, an oxygen atom, or a sulfur atom. The hydrocarbon group preferably has 1 to 6 carbon atoms, and more preferably has 1, 2, or 6 carbon atoms. The hydrocarbon group may be linear, branched, cyclic, or a combination thereof. The divalent hydrocarbon group may be an alkylene group which is a saturated hydrocarbon group, or an unsaturated hydrocarbon group such as an alkenylene group or an alkynylene group.

[0064] In the general formula (3), L represents a direct bond or a p-valent linking group. Note that L becomes a direct bond only when p is 2. Examples of p-valent linking groups include nitrogen atoms, oxygen atoms, sulfur atoms, and carbon atoms, as well as groups formed by removing p hydrogen atoms from hydrocarbon groups and groups in which some of the carbon atoms of a hydrocarbon group have been substituted with a heteroatom such as a nitrogen atom, an oxygen atom, or a sulfur atom. When the p-valent linking group has carbon atoms, it preferably has 1 to 30 carbon atoms. More preferably, it has 1 to 20 carbon atoms. The hydrocarbon group may be linear, branched, cyclic, or a combination thereof. The hydrocarbon group may be a saturated hydrocarbon group, an unsaturated hydrocarbon group, or an aromatic hydrocarbon group. Examples of aromatic hydrocarbon groups include groups formed by removing hydrogen atoms from aromatic compounds such as a benzene ring, a naphthalene ring, an anthracene ring, a tetracene ring, a pentacene ring, a triphenylene ring, a pyrene ring, a fluorene ring, and an indene ring.

[0065] R in the above general formula (3) ２ ~R ４ are the same or different and represent a monovalent substituent. １ ~m ３are the same or different and represent a number from 0 to 3. Examples of the monovalent substituent include a fluorine atom; a haloalkyl group such as a fluoromethyl group, a difluoromethyl group, or a trifluoromethyl group; a linear or branched alkyl group having 1 to 20 carbon atoms such as a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, an isobutyl group, or a tert-butyl group; a cyclic alkyl group having 5 to 7 carbon atoms such as a cyclopentyl group, a cyclohexyl group, or a cycloheptyl group; a methoxy group, an ethoxy group, a propoxy group, an isopropoxy group, a butoxy group, an isobutoxy group, a tert-butoxy group, a pentyloxy group, a hexyloxy group, linear or branched alkoxy groups having 1 to 20 carbon atoms, such as a heptyloxy group or an octyloxy group; a nitro group; a cyano group; an alkylamino group having an alkyl group having 1 to 10 carbon atoms, such as a methylamino group, an ethylamino group, a dimethylamino group or a diethylamino group; a cyclic amino group, such as a pyrrolidino group, a piperidino group or a morpholino group; a diarylamino group, such as a diphenylamino group or a carbazolyl group; an acyl group, such as an acetyl group, a propionyl group or a butyryl group; an alkenyl group having 2 to 30 carbon atoms, such as a styryl group; a halogen atom such as a fluorine atom or a substituted or unsubstituted alkyl group having 1 to 2 carbon atoms; an aryl group having 5 to 20 carbon atoms which may be substituted with an alkyl group, an alkoxy group, an amino group, or the like having 1 to 20 carbon atoms (specific examples of the aryl group are the same as those of the aromatic hydrocarbon group); a heterocyclic group containing one or more of a nitrogen atom, a sulfur atom, and an oxygen atom which may be substituted with a halogen atom such as a fluorine atom, an alkyl group, an alkoxy group, an amino group, or the like having 1 to 20 carbon atoms (the heterocyclic group may be one consisting of only one ring, or may be a compound in which a plurality of compounds consisting of only one aromatic heterocycle are directly bonded together at one carbon atom, and a fused heterocyclic group Specific examples of the heterocyclic group include specific examples of aromatic heterocyclic groups such as a thiophene ring, a furan ring, a pyrrole ring, a benzothiophene ring, a benzofuran ring, an indole ring, a dibenzothiophene ring, a dibenzofuran ring, a carbazole ring, a thiazole ring, a benzothiazole ring, an oxazole ring, a benzoxazole ring, an imidazole ring, a benzimidazole ring, a pyridine ring, a pyrimidine ring, a pyrazine ring, a pyridazine ring, a triazine ring, a quinoline ring, an isoquinoline ring, a quinoxaline ring, a benzothiadiazole ring, and a phenanthridine ring.ester group, thioether group, etc. These groups may be substituted with a halogen atom, a hetero element, an alkyl group, an aromatic ring, etc.

[0066] In the general formula (3), p represents a number from 1 to 4, and is preferably a number from 1 to 3. ２ represents the number 0 or 1, but is preferably 0.

[0067] The average thickness of the electron injection layer 7 is preferably 0.5 to 100 nm, and more preferably 1 to 10 nm. The electron injection layer 7 can be formed by applying a coating composition or by co-deposition using a vacuum deposition method. The average thickness of the electron injection layer 7 can be measured, for example, by a stylus profilometer or spectroscopic ellipsometry.

[0068] "Cathode" Examples of materials used for the cathode 8 include ITO, IZO, Au, Pt, Ag, Cu, Al, and alloys containing these. Among these, ITO, IZO, Au, Ag, and Al are preferably used as the material for the cathode 8. The average thickness of the cathode 8 is not particularly limited, but is preferably 10 to 1000 nm, and more preferably 30 to 150 nm. Even when an opaque material is used as the material for the cathode 8, the cathode can be used as a transparent cathode in a top-emission organic EL element by, for example, setting the average thickness to approximately 10 to 30 nm. The average thickness of the cathode 8 can be measured during the deposition of the cathode 8 using a quartz crystal film thickness meter.

[0069] In particular, in the organic EL element having an inverted structure shown in Figure 5, a layer made of an inorganic oxide formed on the cathode 8 is also treated as part of the cathode. The oxide used in this case is a semiconductor or insulator laminated thin film layer. Specifically, it may be a layer made of a single metal oxide, a layer made of a mixture of two or more metal oxides and a layer made of a single metal oxide, or both, or a layer made of a mixture of two or more metal oxides.

[0070] Metal elements constituting metal oxides that form inorganic oxides include magnesium, calcium, strontium, barium, titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, manganese, indium, gallium, iron, cobalt, nickel, copper, zinc, cadmium, aluminum, and silicon.

[0071] When the inorganic oxide layer includes a layer of a mixture of two or more metal oxides, it is preferable that at least one of the metal elements constituting the metal oxide is a layer made of magnesium, aluminum, calcium, zirconium, hafnium, silicon, titanium, or zinc.When the inorganic oxide layer is a layer made of a single metal oxide, it is preferable that it is a layer made of a metal oxide selected from the group consisting of magnesium oxide, aluminum oxide, zirconium oxide, hafnium oxide, silicon oxide, titanium oxide, and zinc oxide.

[0072] When the layer made of an inorganic oxide is a layer obtained by laminating either or both of a layer made of a mixture of two or more types of metal oxides and a layer made of a single metal oxide, or a layer made of a mixture of two or more types of metal oxides, examples of the layer include a laminated and / or mixed combination of two types of metal oxides selected from titanium oxide / zinc oxide, titanium oxide / magnesium oxide, titanium oxide / zirconium oxide, titanium oxide / aluminum oxide, titanium oxide / hafnium oxide, titanium oxide / silicon oxide, zinc oxide / magnesium oxide, zinc oxide / zirconium oxide, zinc oxide / hafnium oxide, zinc oxide / silicon oxide, and calcium oxide / aluminum oxide; a laminated and / or mixed combination of three types of metal oxides selected from titanium oxide / zinc oxide / magnesium oxide, titanium oxide / zinc oxide / zirconium oxide, titanium oxide / zinc oxide / aluminum oxide, titanium oxide / zinc oxide / hafnium oxide, titanium oxide / zinc oxide / silicon oxide, and indium oxide / gallium oxide / zinc oxide.

[0073] The inorganic oxide is IGZO (indium gallium zinc oxide), an oxide semiconductor that exhibits good properties as a special composition, and / or 12CaO.7Al, an electride. ２O ３ The average thickness of the inorganic oxide film is not particularly limited, but is preferably 1 to 1000 nm, and more preferably 2 to 100 nm. The average thickness of the inorganic oxide layer can be measured by a stylus profilometer or spectroscopic ellipsometry.

[0074] "Sealing" The organic EL element 1 may be sealed as necessary. For example, the organic EL element 1 may be sealed by a sealed container (not shown) having a recessed space for accommodating the organic EL element 1 and an adhesive for bonding the edge of the sealed container to the substrate 2. Alternatively, the organic EL element 1 may be housed in a sealed container and sealed by filling it with a sealant made of an ultraviolet (UV) curable resin or the like.

[0075] When the organic EL element 1 is sealed using a sealing container or a sealing member, a moisture-absorbing desiccant may be placed inside the sealing container or the sealing member. Furthermore, a moisture-absorbing material may be used as the sealing container or the sealing member. Furthermore, a space may be formed inside the sealed sealing container or the sealing member.

[0076] Resin materials, glass materials, etc. can be used as the material of the sealing container or sealing member used to seal the organic EL element 1. Examples of the resin materials and glass materials used for the sealing container or sealing member include the same materials as those used for the substrate 2.

[0077] "Method of Manufacturing Organic EL Element" Next, a method of manufacturing the organic EL element 1 will be described as an example of a method of manufacturing the organic EL element of the present invention.

[0078] To manufacture the organic EL element 1, first, an anode 3 is formed on a substrate 2. The anode 3 can be formed by a sputtering method, a vacuum deposition method, a sol-gel method, a spray pyrolysis (SPD) method, an atomic layer deposition (ALD) method, a vapor phase film formation method, a liquid phase film formation method, or the like. The anode 3 may also be formed by bonding a metal foil.

[0079] Next, a hole injection layer 4, a hole transport layer 5, a light-emitting layer 6, and an electron injection layer 7 are formed in this order on the anode 3. The method for forming the hole injection layer 4, the hole transport layer 5, the light-emitting layer 6, and the electron injection layer 7 is not particularly limited, and various conventionally known formation methods can be used as appropriate depending on the properties of the materials used for each of the hole injection layer 4, the hole transport layer 5, the light-emitting layer 6, and the electron injection layer 7. Specifically, methods for forming the hole injection layer 4, the hole transport layer 5, the light-emitting layer 6, and the electron injection layer 7 include a coating method in which an organic compound solution containing an organic compound to become the hole injection layer 4, the hole transport layer 5, the light-emitting layer 6, and the electron injection layer 7 is applied, a vacuum deposition method, an ESDUS (Evaporative Spray Deposition from Ultra-dilute Solution) method, and the like.

[0080] Next, the cathode 8 is formed on the electron injection layer 7. The cathode 8 can be formed, for example, in the same manner as the anode 3.

[0081] The order is reversed when fabricating an organic EL element having an inverted structure as shown in Fig. 5. In the organic EL element having an inverted structure as shown in Fig. 5, when a layer made of an inorganic oxide is formed on the cathode 8, the oxide is formed by a method such as spray pyrolysis, sol-gel, sputtering, or vacuum deposition. The organic EL element 1 is obtained by the above steps.

[0082] "Sealing Method" When sealing the organic EL element 1, it can be sealed using a method normally used for sealing organic EL elements.

[0083] <Display Device, Lighting Device> The organic EL element 1 of the present embodiment emits light by utilizing an exciplex formed between a first organic compound, which is a donor material that has a small ionization potential and exhibits thermally activated delayed fluorescence, and a second organic compound, which is an acceptor material that can form a singlet excited state by utilizing triplet-triplet annihilation, thereby enabling charge recombination with little energy. Therefore, the organic EL element 1 has a low driving voltage, and the lifetime of the unstable excited state can be shortened, resulting in excellent driving stability.

[0084] The organic EL element of the present invention can change the emission color by appropriately selecting materials for the light-emitting layer, etc., and can also obtain a desired emission color by using a color filter, etc. Therefore, the organic EL element of the present invention can be suitably used as a light-emitting portion of a display device or a lighting device.

[0085] The display device of the present invention has excellent productivity due to its simple element structure and includes the organic EL element of the present invention, which has a low driving voltage. Therefore, it is preferable as a display device. Such a display device of the present invention emits light stably for a long period of time at a low applied voltage. Furthermore, the lighting device of the present invention has excellent productivity due to its simple element structure and includes the organic EL element of the present invention, which has a low driving voltage. Therefore, it is preferable as a lighting device. Such a lighting device of the present invention emits light stably for a long period of time at a low applied voltage.

[0086] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples in any way.

[0087] Example 1 (Fabrication of Organic EL Element) An organic EL element 1 having the structure shown in FIG. 1 was fabricated by the method described below and evaluated.

[0088] [Step 1] A commercially available transparent glass substrate having an average thickness of 0.7 mm and a 3 mm-wide patterned electrode (anode 3) made of ITO was prepared as the substrate 2. The substrate 2 having the anode 3 was then subjected to ultrasonic cleaning in acetone and isopropanol for 10 minutes each, and then boiled in isopropanol for 5 minutes. The substrate 2 having the anode 3 was then removed from the isopropanol, dried by nitrogen blowing, and subjected to UV ozone cleaning for 20 minutes.

[0089] [Step 2] The substrate 2 on which the anode 3 cleaned in [Step 1] was formed was set on a spin coater, and a hole injection material "Clevios HIL1.3N" manufactured by Heraeus was spin coated as a hole injection layer 4, followed by a heat treatment in the atmosphere to form a hole injection layer 4 with a thickness of 10 nm.

[0090] [Step 3] Next, the substrate 2 on which each layer up to the hole injection layer 4 had been formed was fixed to a substrate holder of a vacuum deposition apparatus. －５ The pressure was reduced to 100 Pa. First, a hole transport layer 5 having the following structural formula: 2-SF-PHX represented by the formula (I) and NDP-9, a p-type dopant from Novaled, were vapor-deposited at a mass ratio of 9:1 to a thickness of 30 nm, and then 2-SF-PHX was vapor-deposited at a thickness of 10 nm.

[0091] Furthermore, the light-emitting layer 6 may comprise a compound represented by the following structural formula (4-1): Here, the electron affinity (EA) of the anthracene derivative represented by the structural formula (4-1) used in the light-emitting layer 6 was estimated to be 2.1 eV as calculated using density functional theory, and 2.8 eV as measured using low-energy inverse photoemission spectroscopy (LEIPS).

[0092] The anthracene derivative of structural formula (4-1) was synthesized as follows: First, 5-bromoisophthalic acid was used as a starting material, and its carboxyl group was chlorinated with oxalyl chloride to obtain a carboxylic acid chloride (acid chloride), which was then amidated with t-butylamine to obtain an amide compound, which was then nitrified with thionyl chloride to obtain a benzene derivative having a bromo group and two cyano groups, and the benzene derivative having a bromo group and two cyano groups was reacted with bis(pinacolato)diboron to introduce pinacolatoboryl groups, thereby obtaining an organoboron compound having two cyano groups. On the other hand, 2-(4-bromophenyl)-4,6-diphenyl-1,3,5-triazine was used as a starting material and subjected to Suzuki coupling with 9-anthraceneboronic acid to obtain an anthracene derivative having a triphenyltriazine moiety, which was then brominated with N-bromosuccinimide (NBS) to obtain an anthracene derivative having a triphenyltriazine moiety and a bromo group.The anthracene derivative of structural formula (4-1) was synthesized by Suzuki coupling the organoboron compound having two cyano groups obtained as described above with an anthracene derivative having a triphenyltriazine moiety and a bromo group.

[0093] After the light-emitting layer 6 was formed, a compound represented by the following structural formula (2-1) was deposited to a thickness of 3 nm as the electron-injecting layer 7 .

[0094] The compound represented by the structural formula (2-1) was synthesized according to the method described in T. Sasaki, M. Hasegawa, K. Inagaki, H. Ito, K. Suzuki, T. Oono, K. Morii, T. Shimizu and H. Fukagawa, Nature Communications, 12, pp. 2706.1, DOI: 10.1038 / s41467-021-23067-2.

[0095] Next, a cathode 8 made of aluminum and having a thickness of 100 nm was formed by vacuum deposition on the substrate 2 on which the electron injection layer 7 had been formed. The cathode 8 was formed using a stainless steel deposition mask so that the deposition surface would be a strip with a width of 3 mm, and the light-emitting area of ​​the fabricated organic EL element was 9 mm ２ It was decided.

[0096] [Step 4] Next, the substrate 2 on which each layer up to the cathode 8 had been formed was housed in a glass cap (sealing container) having a concave space, and sealed by filling it with a sealant made of an ultraviolet (UV) curable resin, thereby obtaining the organic EL element of Example 1.

[0097] Example 2 A material used for the hole transport layer 5 was selected from the group consisting of a compound represented by the following structural formula: An organic EL device of Example 2 was produced in the same manner as in Example 1, except that 2Cz-DMAC represented by the following formula was used:

[0098] 2Cz-DMAC was synthesized according to the following reaction scheme.

[0099] First, 3,6-dibromo-9-phenylcarbazole was used as the starting material, and Pd(PPh ３ ) ４ , K. ２ CO ３ The compound represented by formula (5) is then reacted with 9-phenylcarbazole-3-boronic acid in the presence of Pd ２ (dba) ３ , t-BuONa, [(t-Bu) ３ PH]BF ４ 2Cz-DMAC was synthesized by reacting 2Cz-DMAC with 9,9-dimethyl-9,10-dihydroacridine in the presence of

[0100] Comparative Example 1 A material used for the hole transport layer 5 was a compound represented by the following structural formula: An organic EL device of Comparative Example 1 was fabricated in the same manner as in Example 1, except that TPDI represented by the formula (1) was used. TPDI is a material having an ionization potential equivalent to that of 2-SF-PHX, but does not exhibit thermally activated delayed fluorescence.

[0101] Example 3 (Fabrication of Organic EL Device) An organic EL device was fabricated and evaluated by the following method.

[0102] [Step 1] A commercially available transparent glass substrate having an average thickness of 0.7 mm and a 3 mm-wide patterned electrode (anode 3) made of ITO was prepared as the substrate 2. The substrate 2 having the anode 3 was then subjected to ultrasonic cleaning in acetone and isopropanol for 10 minutes each, and then boiled in isopropanol for 5 minutes. The substrate 2 having the anode 3 was then removed from the isopropanol, dried by nitrogen blowing, and subjected to UV ozone cleaning for 20 minutes.

[0103] [Step 2] The substrate 2 on which the anode 3 cleaned in [Step 1] was formed was set on a spin coater, and a hole injection material "Clevios HIL1.3N" manufactured by Heraeus was spin coated as a hole injection layer 4, followed by a heat treatment in the atmosphere to form a hole injection layer 4 with a thickness of 10 nm.

[0104] [Step 3] Next, the substrate 2 on which each layer up to the hole injection layer 4 had been formed was fixed to a substrate holder of a vacuum deposition apparatus. －５ The pressure was reduced to 100 Pa. On the hole injection layer 4, a hole transport layer 5 was formed by first depositing 2-SF-PHX represented by the above structural formula and NDP-9, a p-type dopant from Novaled, at a mass ratio of 9:1 to a thickness of 30 nm, and then depositing 2-SF-PHX to a thickness of 10 nm. Furthermore, a light emitting layer 6 was formed by depositing a compound represented by the following structural formula (4-2): Here, the electron affinity (EA) of the anthracene derivative represented by the structural formula (4-2) used in the light-emitting layer 6 was estimated to be 2.73 eV as calculated using density functional theory, and 3.0 eV as measured using low-energy inverse photoemission spectroscopy (LEIPS).

[0105] The anthracene derivative of structural formula (4-2) was synthesized as follows: First, 5-bromoisophthalic acid was used as a starting material, and its carboxyl group was chlorinated with oxalyl chloride to obtain a carboxylic acid chloride (acid chloride), which was then amidated with t-butylamine to obtain an amide compound, which was then nitrified with thionyl chloride to obtain a benzene derivative having a bromo group and two cyano groups, and the benzene derivative having a bromo group and two cyano groups was reacted with bis(pinacolato)diboron to introduce pinacolatoboryl groups, thereby obtaining an organoboron compound having two cyano groups. On the other hand, a Grignard reagent having a cyanophenyl group is obtained by a metal-halogen exchange reaction between 4-iodobenzonitrile and iPrMgCl as a starting material, and the Grignard reagent having a cyanophenyl group is reacted with cyanuric chloride to obtain a triazine compound having two cyanophenyl groups and one chloro group. The triazine compound having two cyanophenyl groups and one chloro group is then subjected to Suzuki coupling with 4-chlorophenylboronic acid to obtain a triazine compound having two cyanophenyl groups and one chlorophenyl group. An anthracene derivative having a triazine moiety having two cyanophenyl groups and one phenyl group is obtained by Suzuki coupling the triazine compound having two cyanophenyl groups and one chlorophenyl group with 9-anthraceneboronic acid, and the anthracene derivative having a triazine moiety having two cyanophenyl groups and one phenyl group is brominated with N-bromosuccinimide (NBS) to obtain an anthracene derivative having a triazine moiety having two cyanophenyl groups and one phenyl group and a bromo group. The anthracene derivative of structural formula (4-2) was synthesized by Suzuki coupling the organoboron compound having two cyano groups obtained as described above with an anthracene derivative having a triazine moiety having two cyanophenyl groups and one phenyl group and a bromo group.

[0106] After the light-emitting layer 6 was formed, the compound represented by the structural formula (2-1) was deposited to a thickness of 3 nm as the electron-injecting layer 7 .

[0107] Next, a cathode 8 made of aluminum and having a thickness of 100 nm was formed by vacuum deposition on the substrate 2 on which the electron injection layer 7 had been formed. The cathode 8 was formed using a stainless steel deposition mask so that the deposition surface would be a strip with a width of 3 mm, and the light-emitting area of ​​the fabricated organic EL element was 9 mm ２ It was decided.

[0108] [Step 4] Next, the substrate 2 on which each layer up to the cathode 8 had been formed was housed in a glass cap (sealing container) having a concave space, and sealed by filling it with a sealant made of an ultraviolet (UV) curable resin, thereby obtaining an organic EL element of Example 3.

[0109] Example 4 An organic EL device of Example 4 was produced in the same manner as in Example 3, except that the material used for the hole transport layer 5 was 2Cz-DMAC represented by the above structural formula.

[0110] Comparative Example 2 An organic EL element of Comparative Example 2 was produced in the same manner as in Example 3, except that the material used for the hole transport layer 5 was TPDI represented by the above structural formula.

[0111] (Evaluation of Characteristics of Organic EL Devices) A ​​voltage was applied to the devices of the Examples and Comparative Examples obtained in this manner using a Keithley "2400 Type Source Meter," and the luminance was measured using a Konica Minolta "LS-100," to examine the relationship between the applied voltage and the luminance. The relationship between the applied voltage and the current density, and the emission spectrum were also examined. These results are shown in FIGS. 9 and 11.

[0112] Figure 9(a) shows the applied voltage-current density characteristics of Example 1, Example 2, and Comparative Example 1. It can be seen that in devices using TPDI or 2-SF-PHX, which have low ionization potentials, a slight current rises at a low voltage. Figures 9(b) and 9(c) show the applied voltage-luminance characteristics and EL spectra of Example 1, Example 2, and Comparative Example 1. It can be seen from Figures 9(b) and 9(c) that blue light emission can be obtained at a low driving voltage. As shown in Figures 2, 3, and 4, it is believed that an exciplex is formed at a low applied voltage, and the energy of the exciplex is extracted as blue light emission via triplet-triplet annihilation.

[0113] FIG. 10 shows the devices of Example 1, Example 2, and Comparative Example 1 at an initial luminance of 100 cd / m ２ The graph shows the change in luminance when the device is continuously driven from 0°C to 100°C. Compared to the device of Comparative Example 1, the devices of Examples 1 and 2 have a significantly longer lifetime. This is thought to be because, as shown in Figures 3 and 4, using a thermally activated delayed fluorescence material as the donor shortens the lifetime of the triplet excited state of the donor, leading to a longer lifetime.

[0114] FIG. 11(a) shows the applied voltage-current density characteristics of Examples 3, 4, and Comparative Example 2. It can be seen that in devices using TPDI or 2-SF-PHX, which have low ionization potentials, a slight current rises at low voltages. FIGS. 11(b) and 11(c) show the applied voltage-luminance characteristics and EL spectra of Examples 3, 4, and Comparative Example 2. It can be seen from FIGS. 11(b) and 11(c) that green light emission can be obtained at a low driving voltage of 2 V or less. As shown in FIGS. 2, 3, and 4, it is believed that an exciplex is formed at a low applied voltage, and the energy of the exciplex is extracted as green light emission via triplet-triplet annihilation.

[0115] FIG. 12 shows the devices of Example 3, Example 4 and Comparative Example 2 at an initial luminance of 100 cd / m ２ 3 and 4 show the change in luminance when the device is continuously driven from 1000 rpm. Compared to the device of Comparative Example 2, the devices of Examples 3 and 4 have a significantly longer lifetime. This is thought to be because, as shown in FIGS. 3 and 4, using a thermally activated delayed fluorescence material as the donor shortens the lifetime of the triplet excited state of the donor, leading to a longer lifetime.

[0116] 1: Organic EL element, 2: Substrate, 3: Anode, 4: Hole injection layer, 5: Hole transport layer, 6: Light-emitting layer, 7: Electron injection layer, 8: Cathode

Claims

1. An organic electroluminescence device comprising an anode, a hole transport layer, an emitting layer, and a cathode, in this order, wherein the hole transport layer contains a first organic compound which is a donor material having a small ionization potential, and the emitting layer contains a second organic compound which is an acceptor material having a large electron affinity, and an exciplex is formed between the first organic compound and the second organic compound, characterized in that the first organic compound is a thermally activated delayed fluorescent material.

2. The organic electroluminescence device according to claim 1, wherein the first organic compound has a calculated ionization potential (IP) of 4.8 eV or less as calculated using density functional theory, or an actual measured ionization potential (IP) of 5.5 eV or less as determined by ultraviolet photoelectron spectroscopy.

3. The organic electroluminescence element according to claim 1, wherein the second organic compound has a calculated electron affinity (EA) of 2.1 eV or more as calculated using density functional theory, or a measured electron affinity (EA) of 2.8 eV or more as measured by low-energy inverse photoelectron spectroscopy.

4. The organic electroluminescence device according to claim 1, wherein the first organic compound has a dimethylacridine skeleton represented by the following formula (1-1), a phenoxazine skeleton represented by the following formula (1-2), or a carbazole skeleton represented by the following formula (1-3). [In formula (1-1), formula (1-2), and formula (1-3), X represents a bond bonded to another atom.] 5. The organic electroluminescence device according to claim 1, wherein the light-emitting layer further comprises a phosphorescent material.

6. The organic electroluminescence device according to claim 1, wherein the light-emitting layer further comprises a fluorescent light-emitting material.

7. The organic electroluminescence device according to claim 1, wherein the light-emitting layer further comprises a thermally activated delayed fluorescent material.

8. A display device comprising an organic electroluminescence element according to any one of claims 1 to 7.

9. A lighting device comprising an organic electroluminescence element according to any one of claims 1 to 7.

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