Organic el element

Abstract

An organic EL element 10 includes a laminate sandwiched between a pair of electrodes 3, 4. The laminate has an acceptor layer 1 and a donor layer 2 forming an interface with the acceptor layer 1. The acceptor layer 1 includes acceptor molecules. The donor layer 2 includes donor molecules causing triplet-triplet annihilation. The acceptor molecules and donor molecules meet, e.g., certain energy level requirements.

Description

organic EL element

[0001] The present invention relates to an organic EL device.

[0002] An organic electroluminescence element (organic EL element) is an element that includes one or more organic semiconductor layers sandwiched between a pair of electrodes, and emits light by itself when a voltage is applied between the electrodes.

[0003] In recent years, research into improving the luminous efficiency and luminance of organic EL elements has been actively conducted. For example, the present inventors have proposed an organic EL element having a predetermined configuration and excellent luminous efficiency and luminance (see Patent Document 1).

[0004] International Publication No. 2022 / 211041

[0005] However, as a result of investigations by the present inventors, it has become clear that there is room for improvement in the light-emitting efficiency and light-emitting brightness of conventional organic EL elements such as those disclosed in Patent Document 1.

[0006] Therefore, an object of the present invention is to provide an organic EL element that is excellent in luminous efficiency and luminance.

[0007] In view of the above circumstances, the present inventors have conducted extensive research and have found an organic EL element having the following configuration. That is, the organic EL element of the present invention is an organic EL element comprising a pair of electrodes and a laminate sandwiched between the electrodes, wherein the laminate has an acceptor layer and a donor layer forming an interface with the acceptor layer, wherein the acceptor layer contains acceptor molecules, and wherein the donor layer contains donor molecules that cause triplet annihilation, wherein the HOMO level of the acceptor molecule is lower than the HOMO level of the donor molecule and the LUMO level of the acceptor molecule is lower than the LUMO level of the donor molecule, wherein the excited triplet level of the donor molecule is smaller than the value obtained by adding 0.1 eV to the energy difference between the HOMO level of the donor molecule and the LUMO level of the acceptor molecule, and wherein the excited triplet level of the acceptor molecule is larger than the energy difference between the HOMO level of the donor molecule and the LUMO level of the acceptor molecule, The energy difference between the HOMO level of the donor molecule and the LUMO level of the acceptor molecule is smaller than the energy difference between the HOMO level and the LUMO level of the donor molecule by 0.4 eV or more. However, the following may be excluded: the acceptor layer is composed of an acceptor molecule represented by formula (A), the donor layer is composed of a first layer forming an interface with the acceptor layer and a second layer laminated on the first layer, the first layer being composed of a donor molecule represented by formula (B) and a dopant molecule represented by formula (C), and the second layer being composed of a donor molecule represented by formula (B); the acceptor layer is composed of an acceptor molecule represented by formula (A), and the donor layer (which may be a single layer or composed of multiple layers) is composed of a donor molecule represented by formula (B) and a dopant molecule represented by formula (C); the acceptor layer is composed of an acceptor molecule represented by formula (A), and the donor layer (which may be a single layer or composed of multiple layers) contains a donor molecule represented by formula (B); and the acceptor layer is composed of an acceptor molecule represented by formula (A).

[0008] Such an organic EL element has excellent luminous efficiency and luminance. The reason for this is not entirely clear, but the inventors' considerations will be explained with reference to FIGS.

[0009] Fig. 1 is a conceptual diagram showing the mechanism by which light is emitted by the organic EL element of the present invention. In Fig. 1, an organic EL element 10 has an acceptor layer 1, a donor layer 2 that forms an interface with the acceptor layer 1, a first electrode 3 formed on the acceptor layer 1 side, and a second electrode 4 formed on the donor layer 2 side. In the explanation here, as in the examples, the acceptor layer 1 corresponds to an electron transport layer, the donor layer 2 corresponds to an emitting layer, the first electrode 3 corresponds to a cathode, and the second electrode 4 corresponds to an anode, respectively.

[0010] When electrons (-) are injected from the cathode and holes (+) are injected from the anode into the organic EL element 10, electron (-)-hole (+) pairs form a charge transfer (CT) state at the interface between the acceptor layer 1 and the donor layer 2. Charge recombination in the CT state causes the triplet state T of the donor molecule in the donor layer 2 to be formed. １Ｄ is generated. Triplet-triplet annihilation (TTA) occurs in the donor layer 2, resulting in a high-energy excited state (S １ ) is generated. When no dopant molecules are present in the donor layer 2, light emission originates from the donor molecules. When dopant molecules are present in the donor layer 2, energy transfer occurs from the donor molecules to the dopant molecules, resulting in light emission originating from the dopant molecules.

[0011] 2 is a diagram showing the energy levels of rubrene, perylene, NDI-HF, and NDI-Cy used in the examples. NDI-HF and NDI-Cy correspond to acceptor molecules, and rubrene and perylene correspond to donor molecules. The HOMO level of the acceptor molecule is lower than the HOMO level of the donor molecule, the LUMO level of the acceptor molecule is lower than the LUMO level of the donor molecule, and the energy difference between the HOMO level of the donor molecule and the LUMO level of the acceptor molecule is 0.4 eV or more smaller than the energy difference between the HOMO level and the LUMO level of the donor molecule. Therefore, it is believed that electron (-)-hole (+) pairs injected from the electrode can form a CT state at the interface between the acceptor layer 1 and the donor layer 2.

[0012] 3A is a schematic diagram showing the energy transfer mechanism up to light emission in the organic EL element of the present invention. The energy difference between the HOMO level of the donor molecule and the LUMO level of the acceptor molecule is CT. ３ In FIG. 3A, the excited triplet energy level T １Ａ is larger than CT (for example, T １Ａ’ ), the excited triplet level T of the donor molecule １Ｄ is smaller than the value obtained by adding 0.1 eV to CT. On the other hand, as shown in FIG. 3B, when the excited triplet level of the acceptor molecule is smaller than CT (for example, T １Ａ’’ The energy efficiency of the organic EL element of the present invention is inferior to that of the organic EL element of the present invention. The reason for this is that, as shown in FIG. 3B, the excited triplet state T １Ｄ and energy transfer T to the excited triplet state of the acceptor molecule. １Ａ’’ and compete with each other, resulting in non-radiative decay of the acceptor to the ground state S ０ On the other hand, as shown in FIG. 3A, the excited triplet level T １Ａ T is greater than CT １Ａ’In the case of the organic EL element of the present invention, it is considered that non-luminescent deactivation via the above route is sufficiently prevented, and energy efficiency is improved. １Ｄ is smaller than the value obtained by adding 0.1 eV to CT, it is considered that the electron (−)-hole (+) pair can pass through the CT state and generate a triplet state of the donor molecule in the donor layer 2. Note that, although an example in which light emission originates from the donor molecule is described in FIG. 3A , when dopant molecules are present in the donor layer 2, energy transfer occurs from the donor molecule to the dopant molecule, resulting in light emission originating from the dopant molecule.

[0013] Excited triplet level T of the donor molecule １Ｄ is preferably equal to or smaller than CT from the viewpoint of further improving energy efficiency.

[0014] From the viewpoint of further improving the luminous efficiency, the donor layer preferably further contains dopant molecules.

[0015] The donor layer may have a first layer that forms an interface with the acceptor layer and a second layer stacked on the first layer, wherein the first layer contains a donor molecule that causes triplet triplet annihilation, and the second layer contains the donor molecule and a dopant molecule.

[0016] The donor layer may also have a first layer that forms an interface with the acceptor layer, a second layer stacked on the first layer, and a third layer stacked on the second layer, wherein the first layer contains a donor molecule that causes triplet annihilation, the second layer contains the donor molecule and a dopant molecule, and the third layer contains the donor molecule.

[0017] According to the present invention, it is possible to provide an organic EL element having excellent luminous efficiency and luminous brightness.

[0018] 1A and 1B are conceptual diagrams showing the mechanism by which light is emitted by the organic EL element of the present invention. The diagrams show the energy levels of rubrene, perylene, NDI-HF, and NDI-Cy used in the examples. (A) is a schematic diagram showing the energy transfer mechanism up to light emission by the organic EL element of the present invention, and (B) is a schematic diagram showing the energy transfer mechanism by an organic EL element outside the scope of the present invention. The excited triplet level T of NDI-HF. １ 1 is a diagram showing the measurement results of the excited triplet level T of NDI-Cy. １ FIG. 1 is a diagram showing the measurement results of the applied voltage-luminance characteristics of the organic EL elements of Examples 1 and 2 and Comparative Example 1A. FIG. 2 is a diagram showing the measurement results of the applied voltage-luminance characteristics of the organic EL elements of Reference Example 1 and Comparative Example 1B. FIG. 3 is a diagram showing the measurement results of the external quantum efficiency (EQE) of the organic EL elements of Examples 1 and 3 and Reference Example 1. FIG. 4 is a diagram showing the measurement results of the applied voltage-luminance characteristics of the organic EL elements of Example 4 and Comparative Example 2.

[0019] An embodiment of the present invention will be described in detail below, although the present invention is not limited to the following embodiment.

[0020] The organic EL element of this embodiment includes a laminate sandwiched between a pair of electrodes.

[0021] The laminate has an acceptor layer containing acceptor molecules and a donor layer containing donor molecules, and the acceptor layer and the donor layer form an interface. The laminate may have an organic semiconductor layer other than the acceptor layer and the donor layer, and may have an organic semiconductor layer such as molybdenum trioxide (MoO ３ The organic electroluminescent device may further include an inorganic compound layer such as a lithium fluoride layer (electron injection layer) or a lithium fluoride layer (hole transport layer or electron injection layer) between the electrodes.

[0022] The acceptor layer may be formed only from acceptor molecules, or may contain materials other than acceptor molecules as long as the effects of the present invention are not significantly impaired.

[0023] The donor molecule is a material that causes triplet-triplet annihilation. The donor layer may be formed only from donor molecules, or may be formed only from donor molecules and dopant molecules, or may contain materials other than donor molecules and dopant molecules as long as the effects of the present invention are not significantly impaired.

[0024] The donor layer may be formed from multiple layers. For example, the donor layer may be a two-layer donor layer having a first layer that forms an interface with the acceptor layer and a second layer stacked on the first layer (the second layer facing the acceptor layer across the first layer), or a three-layer donor layer having a first layer that forms an interface with the acceptor layer, a second layer stacked on the first layer (the second layer facing the acceptor layer across the first layer), and a third layer stacked on the second layer (the third layer facing the first layer across the second layer). When the donor layer is formed from multiple layers, each layer contains the same donor molecule.

[0025] When the donor layer contains donor molecules and dopant molecules, the donor layer preferably has an intermediate layer (hereinafter also referred to as "intermediate layer") that does not contain dopant molecules but contains donor molecules between the layer containing donor molecules and dopant molecules and the acceptor layer, from the viewpoint of further improving luminous efficiency. The reason why the luminous efficiency is improved by the donor layer having an intermediate layer is not necessarily clear, but one reason is thought to be that light emission occurs away from the interface between the donor layer and the acceptor layer, thereby preventing interface quenching of excitons.

[0026] When an intermediate layer is present, its thickness can be, for example, 1 to 10 nm, preferably 1 to 5 nm.

[0027] The above-mentioned two-layer donor layer may be, for example, such that the first layer is the intermediate layer and the second layer is made of the donor molecule and the dopant molecule.

[0028] The above-mentioned three-layer donor layer may have, for example, the first layer being the intermediate layer, the second layer being made of the donor molecule and the dopant molecule, and the third layer being made of the donor molecule.

[0029] The HOMO level of the acceptor molecule is lower than that of the donor molecule. From the viewpoint of highly preventing hole leakage and further improving luminous efficiency, the difference between the HOMO levels of the acceptor molecule and the donor molecule is preferably 0.5 eV or more. Note that the upper limit of the difference between the HOMO levels of the acceptor molecule and the donor molecule is not particularly limited, but can be, for example, 2 eV or less.

[0030] The LUMO level of the acceptor molecule is lower than that of the donor molecule. From the viewpoint of highly preventing electron leakage and further improving luminous efficiency, the difference between the LUMO levels of the acceptor molecule and the donor molecule is preferably 0.3 eV or more. Note that the upper limit of the difference between the LUMO levels of the acceptor molecule and the donor molecule is not particularly limited, but can be, for example, 2 eV or less.

[0031] Excited triplet level T of the donor molecule １Ｄ is less than CT plus 0.1 eV, and is preferably equal to or less than CT. １Ｄ The difference between CT and CT is preferably less than 0.8 eV, more preferably less than 0.7 eV, and even more preferably less than 0.6 eV. If these differences are small, the light emission start voltage can be reduced.

[0032] The CT is smaller than the energy difference between the HOMO level and the LUMO level of the donor molecule by 0.4 eV or more. The upper limit of the CT is not particularly limited, but can be set to, for example, 2 eV or less.

[0033] When the donor layer contains donor molecules and dopant molecules, it is preferable that the energy difference between the HOMO level and the LUMO level of the dopant molecules is smaller than the energy difference between the HOMO level and the LUMO level of the donor molecules. When the dopant molecules are used, energy transfer occurs from the donor molecules to the dopant molecules, and light emission originating from the dopant molecules can be realized.

[0034] The content of the dopant molecules in the donor layer can be, for example, 0.01 to 50% by volume, preferably 0.1 to 10% by volume, relative to 100% by volume of the total amount of the donor layer.

[0035] As the acceptor molecule, for example, a conventionally known electron transporting material can be used, and specific examples thereof include the compounds shown below.

[0036]

[0037]

[0038]

[0039]

[0040] As the donor molecule, for example, the following compound, which has been reported to generate TTA, can be used.

[0041]

[0042]

[0043] (Chem. Rev. 2015, 115, 395-465 reference)

[0044] The HOMO level, LUMO level, and excited triplet level T of these compounds １ The energy levels are specific to the material, and reference values ​​can be referred to.

[0045] As the dopant molecule, a conventionally known light-emitting material can be used. Specific examples of the dopant molecule include the compounds shown below.

[0046]

[0047]

[0048]

[0049] The organic EL element of the present embodiment may be formed from a pair of electrodes, an acceptor layer, and a donor layer, as shown in FIG. 1, or may include other conventionally known organic semiconductor layers, inorganic compound layers, etc.

[0050] The organic EL element of this embodiment may include, as layers between a pair of electrodes, a hole injection layer, an electron blocking layer, a hole transport layer, an emitting layer, an electron transport layer, a hole blocking layer, and an electron injection layer, in this order from the anode. Among these, the acceptor layer may be the electron transport layer, and the donor layer may be the emitting layer.

[0051] The functions of these layers are not strictly distinguished. For example, the light-emitting layer, which is a donor layer, may also function as a hole-transporting layer, and the hole-blocking layer may also function as an electron-injecting layer.

[0052] The order of the layers in the organic EL element is not limited to the above-mentioned order. For example, the hole injection layer may be located between the electron blocking layer and the hole transport layer, or the electron injection layer may be located between the hole blocking layer and the electron transport layer.

[0053] The layers in the organic EL element may include layers containing the same organic semiconductor material. For example, when the organic EL element contains rubrene as donor molecules in the donor layer (light-emitting layer), the organic EL element may include a hole-blocking layer formed from rubrene.

[0054] The organic EL device of this embodiment can be manufactured by forming an acceptor layer and a donor layer by a conventionally known method, such as vacuum deposition, chemical vapor deposition, sputtering, vapor deposition polymerization, spin coating, blade coating, bar coating, dip coating, or laminating. Specifically, the organic EL device of this embodiment can be manufactured by, for example, stacking a first electrode, an acceptor layer, a donor layer, a second electrode, and any other layers on a substrate. The method for forming each organic semiconductor layer can be appropriately selected depending on the compound. Examples of substrates that can be used include glass substrates, quartz substrates, sapphire substrates, plastic substrates, and film substrates.

[0055] The thickness of the acceptor layer and the donor layer in the organic EL element of this embodiment is not particularly limited, but is preferably 0.1 nm to 500 nm, more preferably 2 nm to 200 nm.

[0056] Organic EL elements are expected to be applied to, for example, organic EL displays, organic EL lighting, digital signage, light sources for photosensors, laser light sources, light sources for optical communications, and the like.

[0057] The present invention will be described in more detail below based on examples, but the present invention is not limited to these examples. The structures of the compounds used in the examples are shown below.

[0058]

[0059] (The excited triplet level T of NDI-HF and NDI-Cy １ Measurement of excited triplet level T １ was determined from the peak intensity of a phosphorescence spectrum measured at liquid nitrogen temperature using a spectrofluorometer (JASCO Corporation, Model FP-8650). When multiple peaks were detected, the peak on the higher energy side was used. For NDI-HF, a 50 nm-thick thin film was deposited on a quartz substrate to prepare a sample. For NDI-Cy, a 10 －５ A chloroform solution of 100 mol / L concentration was placed in a sample tube with a diameter of 5 mm to serve as a sample.１ The measurement results are shown in FIGS. 4 and 5, respectively.

[0060] Example 1: A MoO ３ Hole transport layer (10 nm), rubrene layer (undoped) (10 nm), rubrene layer (DBP-doped) (100 nm), rubrene layer (undoped) (2 nm), NDI-HF layer (50 nm), LiF electron injection layer (3 nm), MoO ３ An electron injection layer (3 nm) and an Al electrode (100 nm) were deposited in this order in a vacuum deposition system under high vacuum (∼10 －５ The device was thermally evaporated at a temperature of 1000 K (Pa). The device was sealed with a glass substrate and epoxy resin in a glove box to obtain an organic EL element. The rubrene layer (DBP doped) was formed by adding DBP as a dopant at 0.5% by volume relative to the entire rubrene layer and introducing DBP into the rubrene layer by co-evaporation. The mixed concentration was controlled by the ratio of the evaporation rates. The obtained organic EL element had the following configuration: ITO electrode / MoO ３ Hole transport layer / rubrene layer (undoped) / rubrene layer (DBP-doped) / rubrene layer (undoped) / NDI-HF layer / LiF electron injection layer / MoO ３ Electron injection layer / Al electrode Fig. 6 is a cross-sectional view of the organic EL element of Example 1. The organic EL element 100 shown in Fig. 6 includes a glass substrate 11, an ITO electrode 12, and a MoO ３ Hole transport layer 13, rubrene (undoped) layer 14A, rubrene (DBP-doped) layer 14B, rubrene (undoped) layer 14C, NDI-HF layer 15, LiF electron injection layer 16, MoO ３ The organic EL element 100 has a structure in which an electron injection layer 17 and an Al electrode 18 are laminated in this order. In the organic EL element 100, the rubrene (undoped) layer 14A, the rubrene (DBP-doped) layer 14B, and the rubrene (undoped) layer 14C correspond to the donor layer 14, and the NDI-HF layer 15 corresponds to the acceptor layer.

[0061] Example 2 An organic EL element was fabricated in the same manner as in Example 1, except that an NDI-Cy layer (50 nm) was formed by thermal evaporation instead of the NDI-HF layer. This organic EL element had the following configuration: ITO electrode / MoO ３ Hole transport layer / rubrene layer (undoped) / rubrene layer (DBP-doped) / rubrene layer (undoped) / NDI-Cy layer / LiF electron injection layer / MoO ３ Electron injection layer / Al electrode

[0062] (Comparative Example 1A) PTCDI-C instead of NDI-HF layer ８ An organic EL device was fabricated in the same manner as in Example 1, except that a layer (50 nm) was formed by thermal evaporation. This organic EL device had the following configuration: ITO electrode / MoO ３ Hole transport layer / rubrene layer (undoped) / rubrene layer (DBP-doped) / rubrene layer (undoped) / PTCDI-C ８ Layer / LiF electron injection layer / MoO ３ Electron injection layer / Al electrode

[0063] (Comparative Example 1B) PTCDI-C ８ An organic EL device was fabricated in the same manner as in Comparative Example 1A, except that the rubrene layer (undoped) in contact with the ITO electrode layer was omitted. This organic EL device had the following configuration: ITO electrode / MoO ３ Hole transport layer / rubrene layer (undoped) / rubrene layer (DBP doped) / PTCDI-C ８ Layer / LiF electron injection layer / MoO ３ Electron injection layer / Al electrode

[0064] <Evaluation of Organic EL Elements 1> The applied voltage-luminance characteristics of the organic EL elements of Examples 1 and 2 and Comparative Example 1A were measured using a source measure unit (B2902A manufactured by Keysight Technologies) and a luminance meter (BM-9 manufactured by Topcom). The results are shown in Figure 7. As is clear from Figure 7, the organic EL elements of Examples 1 and 2 have superior emission luminance compared to the organic EL element of Comparative Example 1A.

[0065] (Example 3: Investigation of film thickness of intermediate layer) An organic EL element was fabricated in the same manner as in Example 1, except that the film thickness of the intermediate layer (rubrene layer (undoped)) corresponding to the first layer of the donor layer was changed from 2 nm to 4 nm.

[0066] (Reference Example 1: Study on the Case Where an Intermediate Layer is Not Present) An organic EL element was fabricated in the same manner as in Example 1, except that the intermediate layer (rubrene layer (undoped)) corresponding to the first layer of the donor layer was omitted. The obtained organic EL element had the following configuration: ITO electrode / MoO ３ Hole transport layer / rubrene layer (undoped) / rubrene layer (DBP-doped) / NDI-HF layer / LiF electron injection layer / MoO ３ Electron injection layer / Al electrode

[0067] <Evaluation of Lifetime> The device lifetimes (assessed as LT95, the time it takes for the brightness to decrease to 95%, assuming the initial brightness to be 100%) of the organic EL elements of Reference Example 1 and Comparative Example 1B are shown in Figure 8. As is clear from Figure 8, the organic EL element of Reference Example 1 had an LT95 of 20 hours or more, which is about twice as long as the organic EL element of Comparative Example 1B, which had an LT95 of 10 hours.

[0068] <Evaluation of Organic EL Elements 2> The external quantum yields (EQEs) of the organic EL elements of Examples 1 and 3 and Reference Example 1 were measured using a calibrated high-sensitivity broadband spectrometer (AvaSpec-UV / VIS / NIR, manufactured by Avantes). The results are shown in Figure 9. As is clear from Figure 9, the luminous efficiency was highest when the thickness of the intermediate layer was 2 nm.

[0069] Example 4: A MoO ３ Hole transport layer (10 nm), perylene layer (undoped) (50 nm), NDI-HF layer (50 nm), LiF electron injection layer (3 nm), MoO ３ An electron injection layer (3 nm) and an Al electrode (100 nm) were deposited in this order in a vacuum deposition system under high vacuum (∼10 －５The device was thermally evaporated at 2000 K (Pa). The device was encapsulated with a glass substrate and epoxy resin in a glove box to obtain an organic EL element. The obtained organic EL element had the following configuration: ITO electrode / MoO ３ Hole transport layer / perylene layer (undoped) / NDI-HF layer / LiF electron injection layer / MoO ３ Electron Injection Layer / Al Electrode In the organic EL device of Example 4, the "perylene layer (undoped)" corresponds to the donor layer, and the NDI-HF layer corresponds to the acceptor layer.

[0070] (Comparative Example 2) Instead of the NDI-HF layer, a PTCDI-C ８ An organic EL device was fabricated in the same manner as in Example 4, except that a layer (50 nm) was formed by thermal evaporation. This organic EL device had the following configuration: ITO electrode / MoO ３ Hole transport layer / perylene layer (undoped) / PTCDI-C ８ Layer / LiF electron injection layer / MoO ３ Electron injection layer / Al electrode

[0071] <Evaluation of Organic EL Elements 3> The applied voltage-luminance characteristics of the organic EL elements of Example 4 and Comparative Example 2 were measured using a source measure unit (B2902A manufactured by Keysight Technologies) and a luminance meter (BM-9 manufactured by Topcom). The results are shown in Figure 10. As is clear from Figure 10, the organic EL element of Example 4 is superior in luminance compared to the organic EL element of Comparative Example 2.

[0072] REFERENCE SIGNS LIST 1 acceptor layer, 2 donor layer, 3 first electrode, 4 second electrode, 10, 100 organic EL element, 11 glass substrate, 12 ITO electrode, 13 MoO ３ Hole transport layer, 14... donor layer, 14A, 14C... rubrene (undoped) layers, 14B... rubrene (DBP-doped) layer, 15... NDI-HF layer, 16... LiF electron injection layer, 17... MoO ３ Electron injection layer, 18...Al electrode.

Claims

1. An organic EL device comprising a pair of electrodes and a laminate sandwiched between the electrodes, wherein the laminate has an acceptor layer and a donor layer forming an interface with the acceptor layer, the acceptor layer contains acceptor molecules, the donor layer contains donor molecules that undergo triplet annihilation, the HOMO level of the acceptor molecules is lower than the HOMO level of the donor molecules, and the LUMO level of the acceptor molecules is lower than the LUMO level of the donor molecules, the excited triplet level of the donor molecules is smaller than the energy difference between the HOMO level of the donor molecules and the LUMO level of the acceptor molecules plus 0.1 eV, and the excited triplet level of the acceptor molecules is larger than the energy difference between the HOMO level of the donor molecules and the LUMO level of the acceptor molecules, An organic EL element in which the energy difference between the HOMO level of the donor molecule and the LUMO level of the acceptor molecule is smaller by 0.4 eV or more than the energy difference between the HOMO level and the LUMO level of the donor molecule (excluding the case where the acceptor layer is made of an acceptor molecule represented by the following formula (A), the donor layer is made of a first layer forming an interface with the acceptor layer and a second layer laminated on the first layer, the first layer is made of a donor molecule represented by the following formula (B) and a dopant molecule represented by the following formula (C), and the second layer is made of a donor molecule represented by the following formula (B)).

2. The organic EL device according to claim 1, wherein the excited triplet level of the donor molecule is equal to or smaller than the energy difference between the HOMO level of the donor molecule and the LUMO level of the acceptor molecule.

3. The organic EL device according to claim 1 or 2, wherein the donor layer further contains dopant molecules.

4. The organic EL device according to claim 1 or 2, wherein the donor layer comprises a first layer that forms an interface with the acceptor layer, and a second layer laminated on the first layer, the first layer containing donor molecules that cause triplet annihilation, and the second layer containing the donor molecules and dopant molecules.

5. The organic EL device according to claim 1 or 2, wherein the donor layer comprises a first layer that forms an interface with the acceptor layer, a second layer laminated on the first layer, and a third layer laminated on the second layer, wherein the first layer contains donor molecules that cause triplet annihilation, the second layer contains the donor molecules and dopant molecules, and the third layer contains the donor molecules.

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