Organic electroluminescent elements, display devices and lighting devices

Abstract

The objective of this invention is to provide an organic electroluminescent element that can emit light at a low applied voltage and has excellent driving stability while utilizing an excitation complex formed between a donor material and an acceptor material. The solution is an organic electroluminescent element (1) that sequentially includes an anode (3), a hole transport layer (5), an emitting layer (6), and a cathode (8). The hole transport layer (5) contains a donor material with a low ionization potential, i.e., a first organic compound. The emitting layer (6) contains an acceptor material with a high electron affinity, i.e., a second organic compound. An excitation complex is formed between the first organic compound and the second organic compound. The first organic compound is characterized in that it is a thermally activated delayed fluorescence material.

Description

Technical Field

[0001] This invention relates to an organic electroluminescent (hereinafter, electroluminescence (electroluminescence) is sometimes referred to as "EL") element, a display device, and a lighting device. Prior Technology

[0002] In recent years, there has been active effort to enable organic electroluminescent devices to emit light at low applied voltages. For example, reports have indicated that red fluorene, as a fused tetraphenylene derivative, is used as an donor, and fullerene, which has high electron affinity, is used as an acceptor. The energy of the exciplex formed at the interface is extracted in the organic electroluminescent device as the emission of light in the singlet excited state of red fluorene through triplet-triplet state quenching (see, for example, Patent Documents 1 and 2).

[0003] In addition, regarding compounds that can be extinguished by triplet-tritt state, compounds such as anthracene derivatives and condensed tetraphenyl derivatives have excellent electronic properties and exhibit high luminescence quantum yields, and are therefore used as luminescent materials for organic electroluminescent elements that emit light at low driving voltages (for example, see Non-Patent Literature 1 to Non-Patent Literature 5).

[0004] In this low-voltage organic electroluminescent element, the charge recombination region—the luminescent region—is concentrated at the interface between the donor and acceptor materials. It has been reported that if the charge recombination region—the luminescent region—is concentrated at the interface, the lifetime of the organic electroluminescent element is shortened. While luminescence at low voltage can be achieved using excitation complexes formed at the interface between the donor and acceptor materials, the prospect of long lifetime for practical application has not been realized (see, for example, Non-Patent Literature 6). [Existing Technical Documents] [Non-patent literature]

[0005] Non-patent literature 1: Sebastian Engmann, et al., Nature Communications, Vol. 10, 2019, p. 227, doi.org / 10.1038 / s41467-018-08075-z Non-patent literature 2: Seiichiro Izawa, et al., DOI: 10.26434 / chemrxiv.14685417.v1 Non-Patent Literature 3: Xiangyang 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 Non-patent literature 4: Futong Liu, et al., Journal of Materials Chemistry C, Vol. 7, 2019, p14881, DOI: 10.1039 / c9tc05040j Non-Patent Literature 5: Youn Jue Bae, et al., *Journal of the American Chemical Society*, 2018, Vol. 140, p. 15140, DOI: 10.1021 / jacs.8b07498 Non-patent literature 6: Yifan Zhang, et al., *Nature Communications*, Vol. 5, 2014, p. 5008, doi.org / 10.1038 / ncomms6008 Summary of the Invention

[0006] [The problem that the invention aims to solve]

[0007] As described above, in Non-Patent Documents 1 and 2, luminescence can be obtained at low applied voltages by using red fluorene, a condensed tetraphenylene derivative, as a donor material. However, the charge recombination region—the luminescence region—is concentrated at the interface, thus leaving room for improvement in device lifetime. A thorough re-evaluation of the material structure used in the donor-acceptor pair is needed to extend the lifetime of the device.

[0008] The present invention was made in view of the above circumstances, and its object is to provide an organic electroluminescent element that can obtain light emission at a low applied voltage and has excellent driving stability while utilizing an excitation complex formed between a donor material and an acceptor material. Furthermore, a further objective of the present invention is to provide a display device and a lighting device that include the aforementioned organic electroluminescent element, have a low driving voltage, and exhibit excellent driving stability. [Methods for solving problems]

[0009] The inventors conducted various studies on combinations of donor and acceptor materials and concluded that the aforementioned problem could be effectively solved by using a material exhibiting thermally activated delayed fluorescence as the donor material, thus achieving the present invention. That is, the main structure of the organic electroluminescent element, display device, and lighting device of the present invention that solves the aforementioned problems is as follows.

[0010] [1] An organic electroluminescent device, comprising, in sequence, an anode, a hole transport layer, a light-emitting layer, and a cathode, The hole transport layer contains a donor material with a low ionization potential, namely a first organic compound. The light-emitting layer contains an acceptor material with high electron affinity, namely a second organic compound. An excitation complex is formed between the first organic compound and the second organic compound, and the organic electroluminescent element is characterized in that... The first organic compound is a thermally activated delayed fluorescence material.

[0011] [2] The organic electroluminescent element as described in [1], wherein the ionizing potential (IP) of the first organic compound is calculated to be less than 4.8 eV by density functional method or measured to be less than 5.5 eV by ultraviolet photoelectron spectroscopy.

[0012] [3] An organic electroluminescent element as described in [1] or [2], wherein the electron affinity (EA) of the second organic compound is calculated to be 2.1 eV or higher using density functional method or measured to be 2.8 eV or higher using low-energy reflected electron spectrometry.

[0013] [4] An organic electroluminescent element as described in any one of [1] to [3], wherein the first organic compound has a dimethylacridine skeleton represented by formula (1-1), a phenoxazine skeleton represented by formula (1-2), or a carbazole skeleton represented by formula (1-3). [Chemistry 1] [In equations (1-1), (1-2), and (1-3), X represents the bond formed with other atoms.]

[0014] [5] An organic electroluminescent element as described in any one of [1] to [4], wherein the light-emitting layer further comprises a phosphorescent material.

[0015] [6] An organic electroluminescent element as described in any one of [1] to [4], wherein the light-emitting layer further comprises a fluorescent light-emitting material.

[0016] [7] An organic electroluminescent element as described in any one of [1] to [4], wherein the light-emitting layer further comprises a thermally activated delayed fluorescent material.

[0017] [8] A display device, characterized in that it comprises an organic electroluminescent element as described in any one of [1] to [7].

[0018] [9] A lighting device, characterized in that it comprises an organic electroluminescent element as described in any one of [1] to [7]. [The effects of the invention]

[0019] This invention provides an organic electroluminescent element that can emit light at a low applied voltage and exhibits excellent driving stability while utilizing an excitation complex formed between a donor material and an acceptor material. In addition, the present invention provides a display device and a lighting device with low driving voltage and excellent driving stability. Simple Explanation of the Diagram

[0020] Figure 1 is a schematic diagram showing an example of the structure of the organic EL element of the present invention. Figure 2 is a schematic diagram showing the energy levels within an example of the organic EL element of the present invention. Figure 3 is a schematic diagram illustrating, as an example, the luminescence mechanism of an organic EL element utilizing an excited complex, which effectively utilizes the energy transfer of the self-excited complex and the singlet excited state of the material of the self-luminescent layer, and triplet-triplet extinguishing. Figure 4 is a schematic diagram illustrating an example of the light-emitting mechanism of the organic EL element of the present invention, showing the efficient use of energy transfer from the triplet excited state to the singlet excited state caused by anti-intersystem crossing in the thermally activated delayed fluorescent material, energy transfer from the singlet excited state of the thermally activated delayed fluorescent material to the material of the light-emitting layer, energy transfer of the self-excited complex, and light emission from the singlet excited state of the material of the self-emitting layer after triplet-triple extinguishing. Figure 5 is a schematic diagram showing another example of the structure of the organic EL element of the present invention. Figure 6 is a graph showing the decay process of luminescence from 2-SF-PHX under nitrogen and atmospheric conditions, respectively. Figure 7 is a graph showing the decay process of luminescence from 2Cz-DMAC under nitrogen and atmospheric conditions, respectively. Figure 8 is a schematic diagram showing another example of the light-emitting mechanism of the organic EL element of the present invention, which effectively utilizes the energy shift from the triplet excited state to the singlet excited state caused by anti-intersystem crossing in the thermally activated delayed fluorescent material, the energy shift from the singlet excited state of the thermally activated delayed fluorescent material to the material of the light-emitting layer, the energy shift of the self-excited complex, the triplet-triplet extinguishing and the energy shift from the singlet excited state of the material of the self-emitting layer to the light-emitting dopant, and the light emission of the singlet excited state of the self-emitting dopant. Figures 9(a) to 9(c) are graphs showing the results of measuring (a) current density-voltage characteristics, (b) luminance-voltage characteristics, and (c) EL spectra of various organic EL elements manufactured in Examples 1, 2 and Comparative Example 1. Figure 10 is a graph showing the brightness change of each organic EL element manufactured in Example 1, Example 2 and Comparative Example 1 when continuously driven from an initial brightness of 100 cd / m2. Figures 11(a) to 11(c) are graphs showing the results of measuring (a) current density-voltage characteristics, (b) luminance-voltage characteristics, and (c) EL spectra of various organic EL elements manufactured in Examples 3, 4, and Comparative Example 2. Figure 12 is a graph showing the brightness change of each organic EL element manufactured in Example 3, Example 4 and Comparative Example 2 when continuously driven from an initial brightness of 100 cd / m2. Implementation

[0021] The organic electroluminescent element, display device, and lighting device of the present invention will be described in detail below based on the described embodiments. Furthermore, combining two or more of the preferred embodiments of the present invention described below is also a preferred embodiment of the present invention.

[0022] Organic electroluminescent elements The organic electroluminescent element of the present invention sequentially comprises an anode, a hole transport layer, a light-emitting layer, and a cathode. The hole transport layer contains a donor material with a low ionization potential, namely a first organic compound, and the light-emitting layer contains an acceptor material with high electron affinity, namely a second organic compound. Furthermore, the organic electroluminescent element of the present invention is characterized in that an excitation complex is formed between the first organic compound and the second organic compound, wherein the first organic compound is a thermally activated delayed fluorescence material.

[0023] The thermally activated delayed fluorescence material is a material that emits thermally activated delayed fluorescence (TADF). The so-called thermally activated delayed fluorescence refers to the fluorescence observed from the singlet excited state, which is generated by thermal excitation from the triplet excited state to the singlet excited state, resulting in antisystem crossing and delay. In addition, 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. In addition, the acceptor material is a material that can attract electrons from neighboring molecules or atoms. The greater the electron affinity (EA), the easier it is to attract electrons. Furthermore, the statement that the donor material has a low ionization potential and the acceptor material has a high electron affinity means that the ionization potential is low enough to form an excited complex between the donor material (first organic compound) and the acceptor material (second organic compound), the electron affinity is high enough to reach the stated level, and the energy difference between them is low enough to reach the stated level.

[0024] In the organic electroluminescent element of the present invention, the energy difference between the ionization potential (IP) of the donor material, i.e., the first organic compound, and the electron affinity (EA) of the acceptor material, i.e., the second organic compound, is small. Therefore, by using the donor material, i.e., the first organic compound, in the hole transport layer and the acceptor material, i.e., the second organic compound, in the light-emitting layer, and making them adjacent to each other in a manner that forms an excitation complex between the donor material, i.e., the first organic compound, and the acceptor material, i.e., the second organic compound, the light emission can be achieved at a low applied voltage. Furthermore, in the organic electroluminescent element of the present invention, the donor material, namely the first organic compound, is a thermally activated delayed fluorescent material. The energy difference between its singlet excited state and triplet excited state is small, thus shortening the existence lifetime of the unstable excited state and enabling a longer lifetime.

[0025] Next, an example of the organic electroluminescent (EL) element of the present invention will be described in detail with reference to the drawings. FIG1 is a schematic diagram showing an example of the structure of the organic electroluminescence (EL) element of the present invention. The organic EL element 1 shown in FIG1 has a multilayer 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 are sequentially formed on a substrate 2.

[0026] Furthermore, the energy levels within the organic electroluminescent element 1 shown in Figure 1 are illustrated in Figure 2. In the organic EL element 1, electrons injected into the light-emitting layer 6, which contains a second organic compound and an acceptor material with high electron affinity, form excited complexes with holes injected into the hole transport layer 5, which contains a first organic compound and a donor material with low ionization potential, causing current to flow and emitting light. At this time, the voltage required for emitting light 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. When the electron affinity of the material used in the light-emitting layer 6 is high and the ionization potential of the material used in the hole transport layer 5 is low, this energy difference becomes smaller and the externally applied voltage required for emitting light decreases.

[0027] Next, Figure 3 shows the energy transfer process from the formation of the self-excited complex to luminescence. Figure 4 shows the energy transfer process when a thermally activated delayed fluorescent material is used in the hole transport layer 5.

[0028] As shown in Figure 3, a low external voltage required to form the excited complex means that the energy of the light obtained from the excited complex is not high. However, if the triplet energy of the material used in the luminescent layer 6 (e.g., anthracene derivatives, etc.) related to the formation of the excited complex is lower than the energy of the excited state of the excited complex, the energy of the excited complex shifts to the triplet excited state of the material used in the luminescent layer 6. It is well known that anthracene derivatives, etc., generate a singlet excited state by utilizing triplet-triplet extinction; therefore, by utilizing this property, a singlet excited state can be generated and its light emission obtained. Here, the energy of the light emitted from the singlet excited state is higher than that of the light from the excited complex.

[0029] However, as shown in Figure 3, when a conventional donor material is used in the hole transport layer 5, it is anticipated that the excitation states (singlet state excitation state, triplet state excitation state) of the hole transport layer 5 material or the light-emitting layer 6 material will change as the applied voltage to the device increases. In this case, the triplet state excitation state of the light-emitting layer 6 material can be obtained in the form of light emission by utilizing triplet-triplet state extinguishing. Although the triplet state excitation state of the hole transport layer 5 material is unstable, it still exists as is, thus becoming a factor in reducing luminous efficiency or degrading the organic EL element 1.

[0030] In contrast, as shown in Figure 4, when a thermally activated delayed fluorescence (TEF) material is used in the hole transport layer 5, the energy difference between the singlet and triplet excited states of the TEF material is small. Therefore, through antisystem crossing, the energy of the triplet excited state moves to the singlet excited state. Furthermore, this singlet excited state energy moves to the material of the light-emitting layer 6, allowing the self-emissive layer 6 to emit light. By enabling this energy transfer process, the lifetime of the triplet excited state of the hole transport layer 5 material generated when a voltage is applied can be shortened, thus extending the lifetime of the organic EL element 1.

[0031] Furthermore, as shown in FIG5, the organic EL element 1 of this embodiment can also be configured as an organic EL element having a cathode 8 on the substrate 2 with the opposite structure. The organic EL element 1 shown in FIG5 has a multilayer structure on the substrate 2 having 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 formed in sequence.

[0032] Alternatively, the organic EL element 1 in this embodiment may also be an organic-inorganic hybrid organic field light-emitting element (organic-inorganic light-emitting diode, HOILED) element, which uses inorganic compounds to form part of the layer constituting the organic EL element.

[0033] "Substrate" Materials that can be used as substrate 2 include resin materials, glass materials, etc. Examples of resin materials used in substrate 2 include: polyethylene terephthalate, polyethylene naphthalate, polypropylene, cyclic olefin polymers, polyamide, polyethersulfone, polymethyl methacrylate, polycarbonate, and polyarylate. Using a resin material as the substrate 2 results in a more flexible organic EL element 1, which is therefore preferable. Examples of glass materials used in substrate 2 include quartz glass and soda glass.

[0034] In the case where the organic EL element 1 is a bottom-emitting type, a transparent substrate is used as the material for the substrate 2. When the organic EL element 1 is a top-emitting type, the substrate 2 can be made of either a transparent substrate or an opaque substrate. Examples of opaque substrates include: substrates containing ceramic materials such as alumina, substrates with an oxide film (insulating film) formed on the surface of a metal plate such as stainless steel, and substrates made of resin materials.

[0035] The average thickness of substrate 2 can be determined according to the material of substrate 2, preferably 0.1 mm to 30 mm, more preferably 0.1 mm to 10 mm. The average thickness of substrate 2 can be measured by a digital multimeter or vernier calipers.

[0036] "anode" As shown in Figure 1, the anode 3 is formed on the substrate 2 in direct contact. In the case of an organic EL element with the opposite structure as shown in Figure 5, it can also be formed on the substrate 2 without direct contact. Examples of conductive materials that can be used as the anode 3 include indium tin oxide (ITO), indium zinc oxide (IZO), fluorine tin oxide (FTO), In₂O₃, SnO₂, Sb-containing SnO₂, and Al-containing ZnO. Among these, ITO, IZO, and FTO are preferred as the anode 3. There is no particular limitation on the average thickness of anode 3, but it is preferably 10 nm to 500 nm, and more preferably 100 nm to 200 nm. The average thickness of anode 3 can be measured using a stylus step meter and a spectrophotometer.

[0037] "Electric hole injection layer" The hole injection layer 4 may contain either inorganic or organic materials. Compared to organic materials, inorganic materials are more stable, and therefore, it is easier to obtain high resistance to oxygen or water compared to the use of organic materials. As an inorganic material, there are no particular restrictions. For example, one or more metal oxides such as vanadium oxide (V₂O₅), molybdenum oxide (MoO₃), and ruthenium oxide (RuO₂) can be used. As organic materials, dipyrazino[2,3-f:2',3'-h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile (HAT-CN) or 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquino dimethane (F4-TCNQ) can be used. Alternatively, polymers 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 it is preferably 1 nm to 1000 nm, and more preferably 5 nm to 50 nm. The average thickness of the hole injection layer 4 can be measured during film formation using a crystal oscillator film thickness gauge.

[0038] "Hole transport layer" As described above, the hole transport layer 5 contains a donor material with a low ionization potential, namely a first organic compound, which is a thermally activated delayed fluorescence material. That is, the hole transport layer 5 contains an organic compound with a low ionization potential and exhibits thermally activated delayed fluorescence. Furthermore, the hole transport layer 5 may contain only the first organic compound, or it may contain other components.

[0039] The first organic compound used in the hole transport layer 5 is preferably a material with highly donor molecular structures, such as triarylamine skeleton, carbazole skeleton, phenoxazine skeleton, phenoxazine skeleton, acridine skeleton, aziridine skeleton, and 2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinazine (Julolidine) skeleton.

[0040] In addition, the first organic compound is preferably having a dimethylacridine skeleton represented by formula (1-1), a phenoxazine skeleton represented by formula (1-2), or a carbazole skeleton represented by formula (1-3). [Chemistry 2]

[0041] In formulas (1-1), (1-2), and (1-3), X represents a bond with other atoms, and various substituents can be bonded to the front 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) to replace the hydrogen atoms in these skeletons. Examples of substituents include: alkyl groups with 1 to 10 carbon atoms, alkoxy groups with 1 to 10 carbon atoms, alkylthio groups with 1 to 10 carbon atoms, alkylamino groups with 1 to 10 carbon atoms, acetyl groups with 2 to 10 carbon atoms, aralkyl groups with 7 to 20 carbon atoms, substituted or unsubstituted aromatic hydrocarbon groups with 6 to 40 carbon atoms, and substituted or unsubstituted aromatic 6-membered heterocyclic groups with 3 to 40 carbon atoms.

[0042] The first organic compound, more specifically, is preferably 2-SF-PHX, 2Cz-DMAC, etc. [Chemistry 3]

[0043] Furthermore, whether the donor material exhibits thermally activated delayed fluorescence can be determined by observing the decay process of luminescence under nitrogen and atmospheric conditions, respectively. As an example, this paper illustrates a method for confirming whether the 2-SF-PHX and 2Cz-DMAC exhibit thermally activated delayed fluorescence and the results thereof.

[0044] First, a 100 nm film of 2-SF-PHX and 2Cz-DMAC was fabricated on a quartz substrate by vacuum evaporation. The decay process of luminescence was observed using a streak camera from Hamamatsu Photonics. The results are shown in Figures 6 and 7.

[0045] For any material, delayed fluorescence was observed in the results measured under nitrogen atmosphere, while the delayed fluorescence component was significantly weakened in the results measured under atmospheric atmosphere. This indicates that the delayed fluorescence disappears due to oxygen in the atmosphere. Based on these results, it can be confirmed that 2-SF-PHX and 2Cz-DMAC exhibit thermally activated delayed fluorescence.

[0046] The first organic compound contained in the hole transport layer 5 preferably has a calculated ionization potential (IP) of less than 4.8 eV calculated using density functional theory [B3LPY / 6-31G (d,p)] or a measured ionization potential (IP) of less than 5.5 eV obtained by ultraviolet photoelectron spectroscopy (UPS). More preferably, the calculated ionization potential (IP) calculated using density functional theory is less than 4.72 eV or the measured ionization potential (IP) obtained by ultraviolet photoelectron spectroscopy is less than 5.4 eV. If the calculated value of the free potential (IP) of the first organic compound contained in the hole transport layer 5 using density functional theory is less than 4.8 eV or the measured value of the free potential (IP) obtained by ultraviolet photoelectron spectroscopy is less than 5.5 eV, then the energy difference with the electron affinity (EA) of the acceptor material, i.e. the second organic compound, becomes smaller, and thus luminescence can be obtained at a lower applied voltage.

[0047] Table 1 shows the calculated values ​​of the free potential (IP) of 2-SF-PHX and 2Cz-DMAC using density functional theory and the measured values ​​of the free potential (IP) obtained by ultraviolet photoelectron spectroscopy (UPS). Additionally, for reference, the calculated and measured values ​​of the free potential (also referred to as "free energy") of α-NPD and TPDI are also shown. [Chemistry 4]

[0048] [Table 1] Free potential (Calculated value, eV) Free potential (Measured value, eV) 2-SF-PHX 4.55 5.4 2Cz-DMAC 4.72 5.6 α-NPD 4.77 5.5 TPDI 4.66 5.3

[0049] 2-SF-PHX and 2Cz-DMAC exhibit thermally activated delayed fluorescence while having a small ionization potential similar to that of N,N'-di-1-naphthyl-N,N'-diphenyl benzidine (α-NPD), a common hole transport material. Therefore, long-term stable luminescence at low applied voltages can be expected.

[0050] The average thickness of the hole transport layer 5 is not particularly limited, but it is preferably 10 nm to 150 nm, and more preferably 20 nm to 100 nm. The average thickness of the hole transport layer 5 can be measured, for example, by a stylus step meter or a spectrophotometer.

[0051] "Emitting layer" As described above, the luminescent layer 6 contains a second organic compound, which is an acceptor material with high electron affinity and can form an excitation complex with the first organic compound. Furthermore, the luminescent layer 6 may contain only the second organic compound, or it may contain other components (e.g., dopants). The luminescent layer 6 is mainly formed of a material that can generate a singlet excited state through triplet-triplet state quenching, preferably containing fused tetraphenyl derivatives such as fluorescein derivatives or anthracene derivatives. In other words, the second organic compound is preferably a fused tetraphenyl derivative such as fluorescein derivatives or anthracene derivatives. Here, the term "fluorene derivative" refers to compounds in which fluorene and its hydrogen atoms in the molecular structure are substituted with substituents. Similarly, "fused tetraphenyl derivative" refers to compounds having a fused tetraphenyl skeleton (fused tetraphenyl ring), including compounds in which fused tetraphenyl and its hydrogen atoms in the molecular structure are substituted with substituents. Anthracene derivative refers to compounds having an anthracene skeleton (anthracene ring), including compounds in which anthracene and its hydrogen atoms in the molecular structure are substituted with substituents. Examples of substituents include: cyano, alkyl groups with 1-10 carbon atoms, alkoxy groups with 1-10 carbon atoms, alkylthio groups with 1-10 carbon atoms, alkylamino groups with 1-10 carbon atoms, acetyl groups with 2-10 carbon atoms, aralkyl groups with 7-20 carbon atoms, substituted or unsubstituted aromatic hydrocarbon groups with 6-30 carbon atoms, and substituted or unsubstituted aromatic 6-membered heterocyclic groups with 3-30 carbon atoms.

[0052] The second organic compound preferably has an electron affinity (EA) of 2.1 eV or higher calculated using density functional theory [B3LPY / 6-31G (d,p)] or an electron affinity (EA) of 2.8 eV or higher measured using low-energy inverse photoemission spectroscopy (LEIPS). If the electron affinity (EA) of the second organic compound contained in the luminescent layer 6 is 2.1 eV or higher calculated using density functional theory or 2.8 eV or higher measured using low-energy inverse photoemission spectroscopy, the energy difference between it and the ionization potential of the first organic compound, which is the donor material, becomes smaller, thus enabling luminescence at a lower applied voltage.

[0053] As shown in FIG. 4, luminescence from the singlet excited state generated by triplet-triplet state extinguishing can also be extracted from the luminescent layer 6. On the other hand, as shown in FIG. 8, luminescence from other luminescent dopants can also be extracted by using the second organic compound (excitation complex) as the host of the luminescent layer 6. In this case, phosphorescent materials, fluorescent materials, thermally activated delayed fluorescent materials, etc., such as iridium complexes or platinum complexes, can be mixed into the luminescent layer 6 as dopants relative to the host. The mixing ratio of dopants relative to the second organic compound (excitation complex) is preferably 20% by mass or less, more preferably 10% by mass or less.

[0054] In one embodiment, the light-emitting layer 6, in addition to the second organic compound, further comprises a phosphorescent material. As this phosphorescent material, metal complexes containing metals such as Ir and Pt can be used. Examples of iridium complexes include tris(2-phenylpyridine)iridium (Ir(ppy)3), tris(3-methyl-2-phenylpyridine-N,C2'-)iridium(III) (Ir(mppy)3), and tris[1-phenylisoquinoline]iridium(III) (Ir(piq)3). Examples of platinum complexes include 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin platinum(II).

[0055] In another embodiment, the light-emitting layer 6, in addition to the second organic compound, further comprises a fluorescent light-emitting material. Examples of such fluorescent light-emitting materials include: various metal complexes such as aluminum 8-hydroxyquinoline (Alq 3), tris(4-methyl-8-hydroxyquinoline)aluminum(III) (Almq 3), and zinc 8-hydroxyquinoline (Znq 2); benzene compounds such as distyrylbenzene (DSB) and diaminodistyrylbenzene (DADSB); and naphthalene and Nile red. Naphthalene compounds such as red; phenanthrene compounds such as phenanthrene; chrysene, 6-nitro chrysene, and other phenanthrene compounds; perylene compounds such as perylene and N,N'-bis(2,5-di-t-butylphenyl)-3,4,9,10-perylene-di-carboxyimide (BPPC); and cardamom compounds such as cardamom. Anthracene compounds such as anthracene and bis(styryl)anthracene; pyrene compounds such as BD-1; pyran compounds such as 4-(di-cyanomethylene)-2-methyl-6-(para-dimethylaminostyryl)-4H-pyran (DCM); acridine compounds such as acridine; and stilbene compounds such as stilbene. Thiophene compounds such as 2,5-dibenzoxazole-thiophene; benzoxazole and other benzoxazole compounds; benzimidazole and other benzoimidazole compounds; 2,2'-(p-phenylenediene)-bisbenzothiazole and other benzothiazole compounds; butadiene compounds such as bis(1,4-diphenyl-1,3-butadiene) and tetraphenylbutadiene; naphthiadimethylimide and other naphthiadimethylimide compounds; coumarin and other coumarin compounds; violetone and other violetone compounds; oxadiazole and other oxadiazole compounds. Compounds; aldazine compounds; cyclopentadiene compounds such as 1,2,3,4,5-pentaphenyl-1,3-cyclopentadiene (PPCP); quinacridone compounds such as quinacridone red; pyridine compounds such as pyrrolopyridine and thiadiazopyridine; spirocyclic compounds such as 2,2',7,7'-tetraphenyl-9,9'-spirodifluorene, etc.

[0056] In another embodiment, the light-emitting layer 6, in addition to the second organic compound, further includes a thermally activated delayed fluorescent material. Examples of thermally activated delayed fluorescence materials include 2,4,5,6-tetra(9-carbazole)-isophthalonitrile (4CzIPN), 4,5-di(9-carbazole)-o-phthalonitrile (2CzPN), 3,4,5,6-tetra(9-carbazole)-o-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). Furthermore, compounds described in Japanese Patent Application Publication No. 2012-193352 and International Publication No. 2011 / 070963 can also be cited.

[0057] The average thickness of the light-emitting layer 6 is not particularly limited, but it is preferably 10 nm to 150 nm, and more preferably 20 nm to 100 nm. The average thickness of the light-emitting layer 6 can be measured by a stylus step gauge or by a crystal oscillator film thickness gauge during the film formation of the light-emitting layer 6.

[0058] "Electron injection layer" The electron injection layer 7 may be made of materials with low work functions, such as alkali metals or alkaline earth metals, metal-containing compounds with low work functions such as lithium quinoline, lithium fluoride, cesium carbonate, and calcium carbonate, hexahydropyrimidine compounds having the structure represented by the following general formula (2), or compounds having the structure represented by the following general formula (3).

[0059] [Chemistry 5] (In general formula (2), R1 represents an aromatic hydrocarbon group, aromatic heterocyclic group, aryl alkyl group, divalent to tetravalent chain or cyclic hydrocarbon group, or 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; n1 is an integer from 1 to 4)

[0060] [Chemistry 6] (In general formula (3), X1 and X2 represent nitrogen atoms, oxygen atoms, sulfur atoms or divalent linkages that may have substituents, whether they are the same or different; L represents a direct bond or a p-valent linkage; n2 represents a number of 0 or 1, p represents a number of 1 to 4; q represents a number of 0 or 1, and when p is 1, q is 0; R2 to R4 represent monovalent substituents, whether they are the same or different; m1 to m3 represent a number of 0 to 3, whether they are the same or different; R2 to R4 can bond with X1 and X2 to form a ring structure; when there are multiple R2s, multiple R2s can bond to form a ring structure; in addition, when there are multiple R3s, multiple R3s can bond to form a ring structure; in addition, when there are multiple R4s, multiple R4s can bond to form a ring structure)

[0061] In the general formula (2), R1 represents an aromatic hydrocarbon group, an aromatic heterocyclic group, an aryl alkyl group, a divalent to tetravalent chain or cyclic hydrocarbon group, or 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. As an aromatic hydrocarbon group or an aromatic heterocyclic group, it is preferably one with 3 to 30 carbon atoms, more preferably one with 4 to 24 carbon atoms, and even more preferably one with 5 to 20 carbon atoms. As aromatic hydrocarbon groups, they can be categorized from any of the aromatic rings of the following compounds by removing one to four hydrogen atoms: compounds containing only one aromatic ring, such as benzene; compounds with multiple aromatic rings directly bonded together by a single carbon atom, such as biphenyl and diphenylbenzene; and condensed cyclic aromatic hydrocarbon compounds such as naphthalene, anthracene, phenanthrene, and pyrene. As aromatic heterocyclic groups, groups formed by removing one to four hydrogen atoms from any of the aromatic heterocycles of the following compounds can be listed: thiophene, furan, pyrrole, oxazole, oxadiazole, thiazole, thiadiazole, imidazole, pyridine, pyrimidine, pyrazine, triazine, etc., compounds containing only one aromatic heterocycle; compounds formed by directly bonding multiple of these compounds containing only one aromatic heterocycle to each other via a single carbon atom (bipyridine, etc.); condensed cyclic heteroaromatic hydrocarbon compounds such as quinoline, quinoxaline, benzothiophene, benzothiazole, benzimidazole, benzoxazole, indole, carbazole, dibenzofuran, dibenzothiophene, acridine, phenobarbital, etc. Examples of aryl alkyl groups include those formed by combining the aromatic hydrocarbon group with an alkyl group having 1 to 3 carbon atoms. The divalent to tetravalent chain or cyclic hydrocarbon group is preferably composed of 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 can be straight-chain or branched. In addition, R1 can be a group formed by combining two or more of the aromatic hydrocarbon group, aromatic heterocyclic group, aryl alkyl group, and divalent to tetravalent chain hydrocarbon group. Furthermore, R1 can also be a group formed by combining one or more of the aforementioned aromatic hydrocarbon group, aromatic heterocyclic group, arylalkyl group, or divalent to tetravalent chain hydrocarbon group with a nitrogen atom. Examples of such groups include those formed by removing one to four hydrogen atoms from trialkylamines such as trimethylamine or triphenylamine.

[0062] The aromatic hydrocarbon group, aromatic heterocyclic group, or aryl alkyl group may have one or more monovalent substituents. Examples of monovalent substituents include: fluorine atoms; halogenated alkyl groups such as fluoromethyl, difluoromethyl, and trifluoromethyl; straight-chain or branched alkyl groups with 1 to 20 carbon atoms such as methyl, ethyl, propyl, isopropyl, butyl, isobutyl, and tributyl; cyclic alkyl groups with 5 to 7 carbon atoms such as cyclopentyl, cyclohexyl, and cycloheptyl; and methoxy, ethoxy, propoxy, isopropoxy, butoxy, isobutoxy, tributoxy, pentoxy, hexoxy, heptoxy, and octoxy groups with 1 to 20 carbon atoms. 20 straight-chain or branched alkoxy groups; nitro groups; cyano groups; alkylamino groups with 1 to 10 carbon atoms, such as methylamino, ethylamino, dimethylamino, and diethylamino; cyclic amino groups such as pyrrolidyl, piperidinyl, and morpholinyl; diarylamino groups such as diphenylamino and carbazole; acetyl, propionic, and butyryl; alkenyl groups with 2 to 30 carbon atoms, such as styrene; and those that can be substituted by halogen atoms such as fluorine atoms or alkyl, alkoxy, or amino groups with 1 to 20 carbon atoms. The aryl group having 5 to 20 carbon atoms (specific examples of aryl groups are the same as those for aromatic hydrocarbon groups); a heterocyclic group having 4 to 40 carbon atoms containing one or more nitrogen, sulfur, or oxygen atoms, which may be substituted by halogen atoms such as fluorine atoms or alkyl, alkoxy, or amino groups having 1 to 20 carbon atoms (the heterocyclic group may contain only one ring, or it may be a compound containing only one aromatic heterocycle formed by direct bonding of multiple rings through one carbon atom, or it may be a condensed heterocyclic group); Specific examples of heterocyclic groups may include thiophene rings, furan rings, pyrrole rings, benzothiophene rings, benzofuran rings, indole rings, dibenzothiophene rings, dibenzofuran rings, carbazole rings, thiazole rings, benzothiazole rings, oxazole rings, benzoxazole rings, imidazole rings, benzimidazole rings, pyridine rings, pyrazine rings, pyridazine rings, triazine rings, quinoline rings, isoquinoline rings, quinoxaline rings, benzothiadiazole rings, phenazine rings, etc. (specific examples of aromatic heterocyclic groups); ester groups, thioether groups, etc. Furthermore, these groups may also be substituted with halogen atoms or heteroelements, alkyl groups, aromatic rings, etc.

[0063] In the general formula (2), n1 is an integer from 1 to 4, preferably 2 or 3.

[0064] In the general formula (3), X1 and X2 represent nitrogen atoms, oxygen atoms, sulfur atoms or divalent linkages that may have substituents, whether they are the same or different. Examples of divalent linking groups include: divalent hydrocarbon groups and groups formed by substituting a portion of the carbon atom of a hydrocarbon group with any heteroatom of nitrogen, oxygen, or sulfur. As a hydrocarbon group, it is preferred to have 1 to 6 carbons, and more preferably 1, 2 or 6 carbons. The hydrocarbon group can be straight-chain, branched, cyclic, or a combination thereof. Divalent hydrocarbon groups can be saturated hydrocarbon groups, i.e., alkyl groups, or unsaturated hydrocarbon groups such as alkenyl and alkyne groups.

[0065] In the general formula (3), L represents a direct bond or a p-valent linkage. Furthermore, L is a direct bond only when p is 2. As a p-valent linkage group, in addition to nitrogen, oxygen, sulfur, and carbon atoms, it can also be a group formed by removing p hydrogen atoms from a group formed by substituting a portion of the carbon atom of a hydrocarbon group with any heteroatom of nitrogen, oxygen, or sulfur. When the p-valent linkage has a carbon atom, it is preferable to have 1 to 30 carbon atoms. More preferably, it is preferable to have 1 to 20 carbon atoms. The hydrocarbon group can be straight-chain, branched, cyclic, or a combination thereof. As a hydrocarbon group, it can be any of a saturated hydrocarbon group, an unsaturated hydrocarbon group, or an aromatic hydrocarbon group. As aromatic hydrocarbon groups, examples include groups formed by removing hydrogen atoms from aromatic compounds such as benzene rings, naphthalene rings, anthracene rings, fused tetraphenyl rings, fused pentaphenyl rings, triphenyl rings, pyrene rings, fluorene rings, and indene rings.

[0066] In the general formula (3), R2 to R4 represent monovalent substituents, which may be the same or different. In addition, m1 to m3 represent numbers from 0 to 3, which may be the same or different. Examples of monovalent substituents include: fluorine atoms; halogenated alkyl groups such as fluoromethyl, difluoromethyl, and trifluoromethyl; straight-chain or branched alkyl groups with 1 to 20 carbon atoms such as methyl, ethyl, propyl, isopropyl, butyl, isobutyl, and tributyl; cyclic alkyl groups with 5 to 7 carbon atoms such as cyclopentyl, cyclohexyl, and cycloheptyl; and methoxy, ethoxy, propoxy, isopropoxy, butoxy, isobutoxy, tributoxy, pentoxy, hexoxy, heptoxy, and octoxy groups with 1 to 20 carbon atoms. 20 straight-chain or branched alkoxy groups; nitro groups; cyano groups; alkylamino groups with 1 to 10 carbon atoms, such as methylamino, ethylamino, dimethylamino, and diethylamino; cyclic amino groups such as pyrrolidyl, piperidinyl, and morpholinyl; diarylamino groups such as diphenylamino and carbazole; acetyl, propionic, and butyryl; alkenyl groups with 2 to 30 carbon atoms, such as styrene; and those that can be substituted by halogen atoms such as fluorine atoms or alkyl, alkoxy, or amino groups with 1 to 20 carbon atoms. The aryl group having 5 to 20 carbon atoms (specific examples of aryl groups are the same as those for aromatic hydrocarbon groups); a heterocyclic group having 4 to 40 carbon atoms containing one or more nitrogen, sulfur, or oxygen atoms, which may be substituted by halogen atoms such as fluorine atoms or alkyl, alkoxy, or amino groups having 1 to 20 carbon atoms (the heterocyclic group may contain only one ring, or it may be a compound containing only one aromatic heterocycle formed by direct bonding of multiple rings through one carbon atom, or it may be a condensed heterocyclic group); Specific examples of heterocyclic groups may include thiophene rings, furan rings, pyrrole rings, benzothiophene rings, benzofuran rings, indole rings, dibenzothiophene rings, dibenzofuran rings, carbazole rings, thiazole rings, benzothiazole rings, oxazole rings, benzoxazole rings, imidazole rings, benzimidazole rings, pyridine rings, pyrazine rings, pyridazine rings, triazine rings, quinoline rings, isoquinoline rings, quinoxaline rings, benzothiadiazole rings, phenazine rings, etc. (specific examples of aromatic heterocyclic groups); ester groups, thioether groups, etc. Furthermore, these groups may also be substituted with halogen atoms or heteroelements, alkyl groups, aromatic rings, etc.

[0067] In the general formula (3), p represents a number from 1 to 4, preferably a number from 1 to 3. In addition, n 2 in the general formula (3) represents a number of 0 or 1, preferably 0.

[0068] The average thickness of the electron-injected layer 7 is preferably 0.5 nm to 100 nm, more preferably 1 nm to 10 nm. The electron-injected layer 7 can be formed by coating a coating composition or by co-deposition using a vacuum evaporation method. The average thickness of the electron injection layer 7 can be measured, for example, by a stylus step meter or a spectrophotometer.

[0069] "cathode" Materials used in the cathode 8 may include: ITO, IZO, Au, Pt, Ag, Cu, Al, or alloys containing these. Among these, ITO, IZO, Au, Ag, and Al are preferred as materials for the cathode 8. The average thickness of the cathode 8 is not particularly limited, but is preferably 10 nm to 1000 nm, and more preferably 30 nm to 150 nm. In addition, even when using an opaque material as the material of the cathode 8, for example, it can be used as a transparent cathode in a top-emitting organic EL element by setting the average thickness to about 10 nm to 30 nm. The average thickness of cathode 8 can be measured during the film formation of cathode 8 using a crystal oscillator film thickness gauge.

[0070] In particular, in the organic EL element with the opposite structure shown in Figure 5, the layer containing inorganic oxides formed on the cathode 8 is also processed as part of the cathode. The oxide used here is a layer of semiconductor or insulating volume layer film. Specifically, it can be a layer of metal oxide containing monomers, a layer formed by laminating any one or two layers of a layer containing two or more metal oxides and a layer containing monomers, or any layer of a layer containing two or more metal oxides.

[0071] Metallic elements that constitute 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.

[0072] In the case where the layer containing inorganic oxides includes a layer containing two or more metal oxides, it is preferred that at least one of the metal elements constituting the metal oxide is magnesium, aluminum, calcium, zirconium, hafnium, silicon, titanium, or zinc. When the layer containing inorganic oxides is a layer containing monomeric metal oxides, it is more preferably a layer containing metal oxides selected from the group consisting of magnesium oxide, aluminum oxide, zirconium oxide, hafnium oxide, silicon oxide, titanium oxide, and zinc oxide.

[0073] When the layer containing inorganic oxides is formed by laminating one or two layers of a layer containing two or more metal oxides and a layer containing a monomeric metal oxide, or when the layer contains two or more metal oxides, examples include combinations and / or mixtures of two metal oxides selected from titanium oxide / zinc oxide, titanium oxide / magnesium oxide, titanium oxide / zirconia, titanium oxide / aluminum oxide, titanium oxide / hafnium oxide, titanium oxide / silicon oxide, zinc oxide / magnesium oxide, zinc oxide / zirconia, zinc oxide / hafnium oxide, zinc oxide / silicon oxide, and calcium oxide / aluminum oxide; and combinations and / or mixtures of three metal oxides selected from titanium oxide / zinc oxide / magnesium oxide, titanium oxide / zinc oxide / zirconia, 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, etc.

[0074] Inorganic oxides may also include oxide semiconductors exhibiting good properties as a special component, namely indium gallium zinc oxide (IGZO) and / or 12CaO∙7Al 2O 3 as an electride. There is no particular limitation on the average thickness of inorganic oxide films, but it is preferably 1 nm to 1000 nm, and more preferably 2 nm to 100 nm. The average thickness of the layer containing inorganic oxides can be measured using a stylus step meter or a spectrophotometer.

[0075] "seal" Organic EL element 1 can be sealed as needed. For example, the organic EL element 1 can be sealed by a sealed container (not shown) having a concave space for accommodating the organic EL element 1 and an adhesive that bonds the edge of the sealed container to the substrate 2. Alternatively, the organic EL element 1 can be housed in a sealed container and filled with a sealing material containing an ultraviolet (UV) curing resin or the like for sealing.

[0076] When sealing the organic EL element 1 using a sealed container or sealing member, a moisture-absorbing desiccant can be placed inside the sealed container or inside the sealing member. Alternatively, a moisture-absorbing material can also be used as the sealed container or sealing member. Furthermore, a space can be formed inside the sealed container or inside the sealing member.

[0077] The materials used as sealing containers or sealing components for sealing organic EL element 1 can be resin materials, glass materials, etc. Examples of resin and glass materials used as sealing containers or sealing components that are the same as those used in substrate 2 can be cited.

[0078] Manufacturing method of organic EL devices Next, as an example of the manufacturing method of the organic EL element of the present invention, the manufacturing method of the organic EL element 1 will be described.

[0079] In manufacturing organic EL element 1, firstly, an anode 3 is formed on substrate 2. The anode 3 can be formed by sputtering, vacuum evaporation, sol-gel deposition, spray pyrolysis deposition (SPD), atomic layer deposition (ALD), vapor phase deposition, or liquid phase deposition. When forming the anode 3, a method of bonding metal foil can be used.

[0080] Next, the hole injection layer 4, the hole transport layer 5, the light emission layer 6, and the electron injection layer 7 are sequentially formed on the anode 3. There are no particular limitations on the formation methods of the hole injection layer 4, hole transport layer 5, light emission layer 6, and electron injection layer 7. Various existing well-known formation methods can be used appropriately according to the characteristics of the materials used in each of the hole injection layer 4, hole transport layer 5, light emission layer 6, and electron injection layer 7. Specifically, methods for forming the hole injection layer 4, hole transport layer 5, light emission layer 6, and electron injection layer 7 include: coating with an organic compound solution containing the organic compound that forms the hole injection layer 4, hole transport layer 5, light emission layer 6, and electron injection layer 7; vacuum evaporation; and evaporative spray deposition from ultra-dilute solution (ESDUS).

[0081] Next, a cathode 8 is formed on the electron injection layer 7. The cathode 8 can be formed in the same way as the anode 3, for example.

[0082] In the case of fabricating an organic EL element with the opposite structure shown in Figure 5, the order is reversed. Furthermore, in the organic EL element with the opposite structure shown in Figure 5, when a layer containing inorganic oxides is formed on the cathode 8, the oxides are formed, for example, by methods such as spray pyrolysis, sol-gel method, sputtering method, vacuum evaporation method, etc. Through the above steps, organic EL element 1 can be obtained.

[0083] "Sealing Method" When sealing the organic EL element 1, the usual sealing methods used in the sealing of organic EL elements can be used.

[0084] Display devices, lighting devices In the organic EL element 1 of this embodiment, luminescence is achieved by utilizing an excitation complex formed between a first organic compound, a donor material with a low ionization potential and exhibiting thermally activated delayed fluorescence, and a second organic compound, an acceptor material, which can form a singlet excited state through triplet-triplet state extinguishing. Therefore, charge recombination can be achieved with low energy. Consequently, this results in an organic EL element 1 with a low driving voltage, a shorter lifespan of the unstable excited state, and excellent driving stability.

[0085] The organic EL element of the present invention can change the emission color by appropriately selecting materials such as the light-emitting layer, and can also obtain the desired emission color by using color filters or the like. Therefore, the organic EL element of the present invention can be preferably used as the light-emitting part of a display device or an illumination device.

[0086] The display device of the present invention includes the organic EL element of the present invention, which has excellent manufacturability and low driving voltage due to its simple element structure. Therefore, it is preferred as a display device. The display device of the present invention emits light stably over a long period of time under low applied voltage. Furthermore, the lighting device of the present invention includes the organic EL element of the present invention, which has excellent manufacturability and low driving voltage due to its simple element structure. Therefore, it is preferred as a lighting device. The lighting device of the present invention emits light stably over a long period of time under a low applied voltage. [Example]

[0087] The present invention will be described in more detail below with examples, but the present invention is not limited to any of the examples described below.

[0088] Example 1 (Fabrication of organic EL components) The organic EL element 1 with the structure shown in Figure 1 was manufactured and evaluated using the method described below.

[0089] [Step 1] As substrate 2, a commercially available transparent glass substrate with an average thickness of 0.7 mm is prepared, having an electrode (anode 3) patterned with ITO and a width of 3 mm. Then, the substrate 2 with anode 3 was ultrasonically cleaned in acetone and isopropanol for 10 minutes each, and boiled in isopropanol for 5 minutes. Afterward, the substrate 2 with anode 3 was removed from the isopropanol, dried by nitrogen blowing, and then subjected to UV ozone cleaning for 20 minutes.

[0090] [Step 2] The substrate 2, which has an anode 3 after being cleaned in [Step 1], is placed in a spin coater and a hole injection material "Clevios HIL1.3N" manufactured by Heraeus is spin-coated as the hole injection layer 4. The 10 nm hole injection layer 4 is then formed by heat treatment in the atmosphere.

[0091] [Step 3] Next, the substrate 2, with each layer formed up to the hole injection layer 4, is fixed to the substrate holder of the vacuum evaporation apparatus. The pressure inside the vacuum evaporation apparatus is reduced to 1×10⁻⁵ Pa. On the hole injection layer 4, firstly, 2-SF-PHX represented by the following structural formula and Novaled's NDP-9 as a p-type dopant are deposited at a mass ratio of 9:1 for 30 nm. Then, 2-SF-PHX is deposited for 10 nm as the hole transport layer 5. [Chemistry 7]

[0092] Furthermore, a film formed by evaporating 30 nm of an anthracene derivative represented by the following structural formula (4-1) is used as the light-emitting layer 6. [Chemistry 8] Here, the electron affinity (EA) of the anthracene derivative of the structure (4-1) used in the luminescent layer 6 is estimated to be 2.1 eV using density functional theory and 2.8 eV using low-energy inverse photoemission spectroscopy (LEIPS).

[0093] Furthermore, the anthracene derivative with structural formula (4-1) is synthesized in the following manner. First, using 5-bromoisophthalic acid as the starting material, its carboxyl group is chlorinated with oxalic acid to obtain a carboxylic acid chloride (oxalic acid). The carboxylic acid chloride is then oxalamidated with tert-butylamine to obtain an oxalamine compound. The oxalamine compound is then nitrified with thionyl chloride to obtain a benzene derivative having a bromine group and two cyano groups. This benzene derivative having a bromine group and two cyano groups is then reacted with bis(pinacolyl)diboron to introduce a pinacolylboron group, thereby obtaining an organoboron compound having two cyano groups. On the other hand, using 2-(4-bromophenyl)-4,6-diphenyl-1,3,5-triazine as a starting material, it is coupled with 9-anthraboronic acid via Suzuki coupling to obtain an anthracene derivative having a triphenyltriazine site. This anthracene derivative having a triphenyltriazine site is then brominated with N-bromosuccinimide (NBS) to obtain an anthracene derivative having both a triphenyltriazine site and a bromo group. An anthracene derivative of structural formula (4-1) was synthesized by Suzuki coupling of an organoboron compound having two cyano groups obtained in the manner described above and an anthracene derivative having a triphenyltriazine site and a bromine group.

[0094] After the light-emitting layer 6 is formed, the compound represented by the following structural formula (2-1) is vapor-deposited for 3 nm as the electron injection layer 7. [Chemistry 9]

[0095] Furthermore, 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. Ohno, K. Morii, T. Shimizu and H. Fukagawa, Nature Communications, 12, pp.2706.1, DOI:10.1038 / s41467-021-23067-2.

[0096] Next, on the substrate 2 to which the electron injection layer 7 is formed, a cathode 8 containing aluminum with a film thickness of 100 nm is formed by vacuum evaporation. Furthermore, the cathode 8 is formed by using a stainless steel vapor deposition mask in a strip shape with a vapor deposition surface of 3 mm in width, and the light-emitting area of ​​the organic EL element is set to 9 mm².

[0097] [Step 4] Next, the substrate 2, on which the layers up to the cathode 8 are formed, is housed in a glass cover (sealed container) with a concave space, and filled with a sealing material containing ultraviolet (UV) curing resin to seal it, thereby obtaining the organic EL element of Example 1.

[0098] Example 2 The material used in the hole transport layer 5 is set to 2Cz-DMAC as represented by the following structural formula. Otherwise, the organic EL element of Example 2 is fabricated in the same manner as in Example 1. [Chemistry 10]

[0099] Furthermore, 2Cz-DMAC is synthesized according to the following reaction procedure. [Chemistry 11]

[0100] First, 3,6-dibromo-9-phenylcarbazole was used as the starting material and reacted with 9-phenylcarbazole-3-boric acid in the presence of Pd(PPh 3) 4 and K 2CO 3 to obtain the compound represented by formula (5). Next, the compound represented by formula (5) was reacted with 9,9-dimethyl-9,10-dihydroacridine in the presence of Pd 2(dba) 3, t-BuONa, and [(t-Bu) 3PH]BF 4 to synthesize 2Cz-DMAC.

[0101] Comparative Example 1 The material used in the hole transport layer 5 is TPDI as represented by the following structural formula. Otherwise, the organic EL element of Comparative Example 1 is fabricated in the same manner as in Example 1. [Chemistry 12] Furthermore, although TPDI is a material with the same ionization potential as 2-SF-PHX, it does not exhibit thermally activated delayed fluorescence.

[0102] Example 3 (Fabrication of organic EL components) Organic EL elements were manufactured and evaluated using the methods shown below.

[0103] [Step 1] As substrate 2, a commercially available transparent glass substrate with an average thickness of 0.7 mm is prepared, having an electrode (anode 3) patterned with ITO and a width of 3 mm. Then, the substrate 2 with anode 3 was ultrasonically cleaned in acetone and isopropanol for 10 minutes each, and boiled in isopropanol for 5 minutes. Afterward, the substrate 2 with anode 3 was removed from the isopropanol, dried by nitrogen blowing, and then subjected to UV ozone cleaning for 20 minutes.

[0104] [Step 2] The substrate 2, which has an anode 3 after being cleaned in [Step 1], is placed in a spin coater and a hole injection material "Clevios HIL1.3N" manufactured by Heraeus is spin-coated as the hole injection layer 4. The 10 nm hole injection layer 4 is then formed by heat treatment in the atmosphere.

[0105] [Step 3] Next, the substrate 2, with each layer formed up to the hole injection layer 4, is fixed to the substrate holder of the vacuum evaporation apparatus. The pressure inside the vacuum evaporation apparatus is reduced to 1×10⁻⁵ Pa. On the hole injection layer 4, firstly, 2-SF-PHX represented by the above structure and NDP-9 from Novaled, which is a p-type dopant, are deposited at a mass ratio of 9:1 for 30 nm. Then, 2-SF-PHX is deposited for 10 nm as the hole transport layer 5. Furthermore, a film formed by evaporating 30 nm of anthracene derivative represented by the following structural formula (4-2) is formed as the light-emitting layer 6. [Chemistry 13] Here, the electron affinity (EA) of the anthracene derivative of the structure (4-2) used in the luminescent layer 6 is estimated to be 2.73 eV using density functional theory and 3.0 eV using low-energy inverse photoemission spectroscopy (LEIPS).

[0106] Furthermore, the anthracene derivatives of structural formula (4-2) are synthesized in the following manner. First, using 5-bromoisophthalic acid as the starting material, its carboxyl group is chlorinated with oxalic acid to obtain a carboxylic acid chloride (oxalic acid). The carboxylic acid chloride is then oxalamidated with tert-butylamine to obtain an oxalamine compound. The oxalamine compound is then nitrified with thionyl chloride to obtain a benzene derivative having a bromine group and two cyano groups. This benzene derivative having a bromine group and two cyano groups is then reacted with bis(pinacolyl)diboron to introduce a pinacolylboron group, thereby obtaining an organoboron compound having two cyano groups. On the other hand, using 4-iodobenzonitrile as a starting material, a cyanophenyl genistein reagent is obtained by metal-halogen exchange reaction with iPrMgCl. This cyanophenyl genistein reagent is then reacted with cyanuric chloride to obtain a triazine compound having two cyanophenyl groups and one chloro group. This triazine compound is then Suzuki coupled with 4-chlorophenylboronic acid to obtain a triazine compound having two cyanophenyl groups and one chlorophenyl group. This triazine compound is then Suzuki coupled with 9-anthrabenzic acid to obtain an anthracene derivative having a triazine site containing two cyanophenyl groups and one phenyl group. This anthracene derivative with a triazine site containing two cyanophenyl groups and one phenyl group is then brominated using N-bromobutyldiamide (NBS) to obtain an anthracene derivative having a triazine site containing two cyanophenyl groups and one phenyl group, and a bromo group. An anthracene derivative of structural formula (4-2) is synthesized by Suzuki coupling of an organoboron compound having two cyano groups obtained in the manner described above and an anthracene derivative having a triazine site containing two cyanophenyl groups and one phenyl group and a bromine group.

[0107] After the light-emitting layer 6 is formed, the compound represented by the structural formula (2-1) is vapor-deposited for 3 nm as the electron injection layer 7.

[0108] Next, on the substrate 2 to which the electron injection layer 7 is formed, a cathode 8 containing aluminum with a film thickness of 100 nm is formed by vacuum evaporation. Furthermore, the cathode 8 is formed by using a stainless steel vapor deposition mask in a strip shape with a vapor deposition surface of 3 mm in width, and the light-emitting area of ​​the organic EL element is set to 9 mm².

[0109] [Step 4] Next, the substrate 2, on which the layers up to the cathode 8 are formed, is placed in a glass cover (sealed container) with a concave space, and a sealing material containing ultraviolet (UV) curing resin is filled in to seal it, thereby obtaining the organic EL element of Example 3.

[0110] Example 4 The material used in the hole transport layer 5 is set to 2Cz-DMAC as represented by the structure, and the organic EL element of Example 4 is fabricated in the same manner as in Example 3.

[0111] Comparative Example 2 The material used in the hole transport layer 5 is TPDI as represented by the above structural formula. Otherwise, the organic EL element of Comparative Example 2 is fabricated in the same manner as in Example 3.

[0112] (Characteristics evaluation of organic EL devices) For the components of the embodiments and comparative examples obtained in the manner described above, a voltage was applied using a Keithley 2400 digital source meter, and the brightness was measured using a Konica Minolta LS-100 meter. The relationship between the applied voltage and the brightness was investigated. Furthermore, the relationship between the applied voltage and the current density, as well as the emission spectrum, were investigated. These results are shown in Figures 9(a) to 9(c) and Figures 11(a) to 11(c).

[0113] The applied voltage-current density characteristics of Example 1, Example 2 and Comparative Example 1 are shown in Figure 9(a). In the use of TPDI or 2-SF-PHX with small ionization potential, it can be confirmed that although the current rises slightly, it is still very small at low voltage. Furthermore, Figures 9(b) and 9(c) show the applied voltage-brightness characteristics and EL spectra of Examples 1, 2, and Comparative Example 1. Based on Figures 9(b) and 9(c), it can be confirmed that blue emission can be obtained at a low driving voltage. As shown in Figures 2, 3, and 4, it is believed that an excited complex is formed at a low applied voltage, and the energy of this excited complex is extracted in the form of blue emission through triplet-triplet state extinguishing.

[0114] Figure 10 shows the brightness changes of the elements of Example 1, Example 2, and Comparative Example 1 as they are continuously driven from an initial brightness of 100 cd / m². Compared with the element of Comparative Example 1, the elements of Example 1 and Example 2 exhibit significantly longer lifetimes. This is believed to be because, as shown in Figures 3 and 4, when a thermally activated delayed fluorescent material is used as the donor, the lifetime of the triplet excited state of the donor is shortened, thus resulting in a longer lifetime.

[0115] Furthermore, the applied voltage-current density characteristics of Examples 3, 4, and Comparative Example 2 are shown in Figure 11(a). In devices using TPDI or 2-SF-PHX with small ionization potentials, it can be confirmed that although the current rises slightly, it is still very small at low voltages. Furthermore, Figures 11(b) and 11(c) show the applied voltage-brightness characteristics and EL spectra of Examples 3, 4, and Comparative Example 2. Based on Figures 11(b) and 11(c), it can be confirmed that green emission can be obtained at a low driving voltage of 2 V or less. As shown in Figures 2, 3, and 4, it is believed that an excited complex is formed at a low applied voltage, and the energy of this excited complex is extracted in the form of green emission through triplet-triplet state extinguishing.

[0116] Figure 12 shows the brightness changes of the elements of Examples 3, 4, and Comparative Example 2 as they are continuously driven from an initial brightness of 100 cd / m². Compared with the element of Comparative Example 2, the elements of Examples 3 and 4 exhibit significantly longer lifetimes. This is believed to be because, as shown in Figures 3 and 4, when a thermally activated delayed fluorescent material is used as the donor, the lifetime of the triplet excited state of the donor is shortened, thus resulting in a longer lifetime.

[0117] 1: Organic EL elements (organic electroluminescent devices) 2:Substrate 3: Anode 4: Hole Injection Layer 5: Hole transport layer 6: Emissive layer 7: Electron Injection Layer 8: Cathode

Claims

1. An organic electroluminescent element, comprising, in sequence, an anode, a hole transport layer, a light-emitting layer, and a cathode, wherein the hole transport layer comprises a donor material with a low ionization potential, namely a first organic compound, and the light-emitting layer comprises an acceptor material with high electron affinity, namely a second organic compound, wherein an excitation complex is formed between the first organic compound and the second organic compound, and wherein the organic electroluminescent element is characterized in that the first organic compound is a thermally activated delayed fluorescent material having a dimethylacridine skeleton represented by formula (1-1), a phenoxazine skeleton represented by formula (1-2), or a carbazole skeleton represented by formula (1-3), the second organic compound is a tetraphenylbenzene derivative or an anthracene derivative, and wherein the calculated electron affinity (EA) of the second organic compound, calculated using density functional theory, is 2.1 eV or higher, or the measured electron affinity (EA) of the second organic compound, determined by low-energy reflected electron spectrometry, is 2.8 eV or higher, [Chemical 1] In equations (1-1), (1-2), and (1-3), X represents the bond with other atoms.

2. The organic electroluminescent element as claimed in claim 1, wherein, In the first organic compound, the calculated value of the free potential (IP) using density functional theory is less than 4.8 eV, or the measured value of the free potential (IP) obtained by ultraviolet photoelectron spectroscopy is less than 5.5 eV.

3. The organic electroluminescent element as claimed in claim 1, wherein, The light-emitting layer further comprises phosphorescent materials.

4. The organic electroluminescent element as claimed in claim 1, wherein, The light-emitting layer further comprises fluorescent light-emitting materials.

5. The organic electroluminescent element as claimed in claim 1, wherein, The light-emitting layer further comprises a thermally activated delayed fluorescent material.

6. A display device, characterized in that it comprises an organic electroluminescent element as described in any one of claims 1 to 5.

7. A lighting device, characterized in that it comprises an organic electroluminescent element as described in any one of claims 1 to 5.

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