Organic EL element

Abstract

This organic electroluminescent element 10 comprises a plurality of organic semiconductor layers sandwiched between a pair of electrodes 3, 4. The organic semiconductor layers include: a first organic semiconductor layer 1 containing a first organic semiconductor material; and a second organic semiconductor layer 2 containing a second organic semiconductor material and a third organic semiconductor material. The first organic semiconductor layer and the second organic semiconductor layer form a joining surface, and the first organic semiconductor material and the second organic semiconductor material satisfy, inter alia, requirements regarding a prescribed energy level.

Description

[Technical Field]

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

[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 devices has been actively conducted. For example, Non-Patent Document 1 discloses a method for manufacturing a rubrene layer and a C 60 An energy up-converted organic EL device having a structure in which layers are stacked in this order has been proposed. [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] Ajay K. Pandey, Scientific Reports 2015,5, 7787. Summary of the Invention [Problem to be solved by the invention]

[0005] Incidentally, as a result of investigations by the present inventors, it has become clear that conventional organic EL elements such as those disclosed in Non-Patent Document 1 have room for improvement in terms of luminous efficiency and luminance.

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

[0007] In view of the above circumstances, the present inventors have conducted extensive research and have come up with 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 plurality of organic semiconductor layers sandwiched between a pair of electrodes, the organic semiconductor layer has a first organic semiconductor layer containing a first organic semiconductor material and a second organic semiconductor layer containing a second organic semiconductor material and a third organic semiconductor material; the first organic semiconductor layer and the second organic semiconductor layer form a junction surface; the HOMO level of the first organic semiconductor material is lower than the HOMO level of the second organic semiconductor material, and the LUMO level of the first organic semiconductor material is lower than the LUMO level of the second organic semiconductor material; the second organic semiconductor material is a material that undergoes triplet-triplet annihilation; an excited triplet level T1 of the second organic semiconductor material is smaller than the energy difference between the HOMO level of the second organic semiconductor material and the LUMO level of the first organic semiconductor material; an energy difference between the HOMO level of the second organic semiconductor material and the LUMO level of the first organic semiconductor material is smaller than an energy difference between the HOMO level and the LUMO level of the second organic semiconductor material by 0.5 eV or more; In the second organic semiconductor layer, the second organic semiconductor material is a host material and the third organic semiconductor material is a dopant; The second organic semiconductor material has an emission spectrum with a maximum wavelength that is longer than the absorption spectrum of the first organic semiconductor material.

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

[0009] Figure 1 is a conceptual diagram showing the mechanism by which light is emitted by the organic EL element of the present invention. In Figure 1, an organic EL element 10 has a first organic semiconductor layer 1, a second organic semiconductor layer 2 that forms an interface (junction surface) with the first organic semiconductor layer 1, a first electrode 3 formed on the first organic semiconductor layer 1 side, and a second electrode 4 formed on the second organic semiconductor layer 2 side. In the explanation here, as in the examples, the first organic semiconductor layer 1 corresponds to an electron transport layer, the second organic semiconductor 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 (-) and holes (+) are injected into the organic EL device 10 from the cathode and the anode, respectively, electron (-)-hole (+) pairs form a charge transfer (CT) state at the interface between the first organic semiconductor layer 1 and the second organic semiconductor layer 2. Charge recombination in the CT state generates a triplet state (T1) of the second organic semiconductor material (host material) in the second organic semiconductor layer 2. Triplet-triplet annihilation (TTA) occurs in the second organic semiconductor layer 2, generating a high-energy excited state (S1). Energy transfer from the second organic semiconductor material to the third organic semiconductor material (dopant) in the second organic semiconductor layer 2 results in light emission from the third organic semiconductor material.

[0011] FIG. 2 shows the energy levels of rubrene, PTCDI-C8, and C60 used in the examples. PTCDI-C8 and C60 correspond to the first organic semiconductor material, and rubrene corresponds to the second organic semiconductor material. The HOMO level of the first organic semiconductor material is lower than that of the second organic semiconductor material, the LUMO level of the first organic semiconductor material is lower than that of the second organic semiconductor material, and the energy difference between the HOMO level of the second organic semiconductor material and the LUMO level of the first organic semiconductor material is at least 0.5 eV smaller than the energy difference between the HOMO level and the LUMO level of the second organic semiconductor material. Therefore, it is believed that electron (-)-hole (+) pairs injected from the electrode can form a CT state at the interface between the first organic semiconductor layer 1 and the second organic semiconductor layer 2.

[0012] Figure 3 is a schematic diagram showing the energy transfer mechanism leading to light emission in an organic EL device according to an embodiment, which uses PTCDI-C8 as the first organic semiconductor material, rubrene as the second organic semiconductor material, and DBP as the third organic semiconductor material. In Figure 3, the energy level of the CT state (CT = 1.5 eV) corresponds to the energy difference between the HOMO level of the second organic semiconductor material and the LUMO level of the first organic semiconductor material, and the excited triplet level T1 of the second organic semiconductor material is 1.1 eV. Because the excited triplet level T1 of the second organic semiconductor material is smaller than the energy difference between the HOMO level of the second organic semiconductor material and the LUMO level of the first organic semiconductor material, it is believed that electron (-)-hole (+) pairs can pass through the CT state and generate the triplet state of the second organic semiconductor material in the second organic semiconductor layer 2.

[0013] Furthermore, since the maximum wavelength of the emission spectrum of the second organic semiconductor material is longer than the maximum wavelength of the absorption spectrum of the first organic semiconductor material, it is possible to prevent the first organic semiconductor material from absorbing the light emitted from the organic EL element and thereby preventing a decrease in the light emission intensity.

[0014] Although the case where the first organic semiconductor layer 1 is an electron transport layer has been described here, the first organic semiconductor layer 1 may also be an electron injection layer or a hole blocking layer. [Effects of the Invention]

[0015] The organic EL device of the present invention is excellent in luminous efficiency and luminance. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 1 is a conceptual diagram illustrating the mechanism by which light is emitted by the organic EL element of the present invention. [Figure 2] FIG. 1 shows the energy levels of rubrene, PTCDI-C8, and C60 used in the examples. [Figure 3] FIG. 2 is a schematic diagram showing the energy transfer mechanism up to light emission in the organic EL element of the example. [Figure 4] FIG. 1 shows the absorption or emission spectra of the compounds used in the examples. [Figure 5] FIG. 1 shows the PL intensity for a single rubrene layer. [Figure 6] 1A is a graph showing the V-luminance characteristics of the organic EL device of Example 1 and the like, and FIG. 1B is a graph showing the external quantum yield (EQE) of the organic EL device of Example 1 and the like. [Figure 7] FIG. 10 is a graph showing the V-luminance characteristics of the organic EL element of Example 2. [Figure 8] FIG. 10 is a diagram showing the V-luminance characteristics of the organic EL element of Example 3 etc. [Figure 9] FIG. 1 is a graph showing the external quantum efficiency (EQE) of the organic EL device of Example 4 etc. [Figure 10] 1A is a graph showing the EL emission spectrum of the organic EL device of Example 5 and the like, and FIG. 1B is a graph showing the V-luminance characteristics of the organic EL device of Example 5 and the like. [Figure 11] FIG. 10 is a diagram showing the V-luminance characteristics of the organic EL element of Example 6 etc. [Figure 12] 1A is a graph showing the V-luminance characteristics of the organic EL device of Example 7 and the like, and FIG. 1B is a graph showing the external quantum yield (EQE) of the organic EL device of Example 7 and the like. [Figure 13] FIG. 1 shows the absorption spectrum of NDI-bis-HFI. [Figure 14] FIG. 10 is a diagram showing the V-luminance characteristics of the organic EL element of Example 8 etc. DETAILED DESCRIPTION OF THE INVENTION

[0017] Preferred embodiments of the present invention will be described in detail below, but the present invention is not limited to the following embodiments.

[0018] The organic EL device of this embodiment includes multiple organic semiconductor layers sandwiched between a pair of electrodes. The organic EL device of this embodiment may further include an inorganic compound layer, such as a molybdenum trioxide (MoO) layer (hole injection layer) or a lithium fluoride layer (electron injection layer), between the electrodes.

[0019] The organic semiconductor layer has a first organic semiconductor layer containing a first organic semiconductor material and a second organic semiconductor layer containing a second organic semiconductor material and a third organic semiconductor material, and the first organic semiconductor layer and the second organic semiconductor layer form a junction surface.

[0020] The first organic semiconductor layer may be formed only from the first organic semiconductor material, or may contain materials other than the first organic semiconductor material, as long as the effects of the present invention are not significantly impaired. The second organic semiconductor layer may be formed only from the second organic semiconductor material and the third organic semiconductor material, or may contain materials other than the second organic semiconductor material, as long as the effects of the present invention are not significantly impaired.

[0021] The HOMO level of the first organic semiconductor material is lower than that of the second organic semiconductor material. The difference between the HOMO levels of the first organic semiconductor material and the second organic semiconductor material is preferably 0.5 eV or more from the viewpoint of highly preventing hole leakage and further improving luminous efficiency. The upper limit of the difference between the HOMO levels of the first organic semiconductor material and the second organic semiconductor material is not particularly limited, but can be, for example, 2 eV or less.

[0022] The LUMO level of the first organic semiconductor material is lower than that of the second organic semiconductor material. The difference between the LUMO levels of the first organic semiconductor material and the second organic semiconductor material is preferably 0.5 eV or more from the viewpoint of highly preventing electron leakage and further improving luminous efficiency. The upper limit of the difference between the LUMO levels of the first organic semiconductor material and the second organic semiconductor material is not particularly limited, but can be, for example, 2 eV or less.

[0023] The second organic semiconductor material is a material that undergoes triplet-triplet annihilation, and the excited triplet level T1 of the second organic semiconductor material is smaller than the energy difference between the HOMO level of the second organic semiconductor material and the LUMO level of the first organic semiconductor material. The difference between the excited triplet level T1 of the second organic semiconductor material and the energy difference between the HOMO level of the second organic semiconductor material and the LUMO level of the first organic semiconductor material is preferably less than 0.8 eV, more preferably less than 0.65 eV, and even more preferably less than 0.5 eV. If these differences are small, the light emission onset voltage can be reduced.

[0024] The energy difference between the HOMO level of the second organic semiconductor material and the LUMO level of the first organic semiconductor material is smaller than the energy difference between the HOMO level and the LUMO level of the second organic semiconductor material by at least 0.5 eV, and preferably by at least 0.7 eV. The upper limit of the energy difference between the HOMO level of the second organic semiconductor material and the LUMO level of the first organic semiconductor material is not particularly limited, but can be, for example, 2 eV or less.

[0025] In the second organic semiconductor layer, the second organic semiconductor material is a host material, and the third organic semiconductor material is a dopant. It is preferable that the energy difference between the HOMO level and the LUMO level of the third organic semiconductor material is smaller than the energy difference between the HOMO level and the LUMO level of the second organic semiconductor material. The maximum wavelength of the emission spectrum of the third organic semiconductor material, which is a dopant, is longer than the maximum wavelength of the emission spectrum of the second organic semiconductor material, which is a host material. The use of a third organic semiconductor material allows energy transfer from the second organic semiconductor material to the third organic semiconductor material, resulting in light emission originating from the third organic semiconductor material.

[0026] The content of the third organic semiconductor material in the second organic semiconductor layer can be, for example, 0.01 to 50% by volume, and preferably 0.1 to 10% by volume, relative to 100% by volume of the total amount of the second organic semiconductor layer.

[0027] The maximum wavelength of the emission spectrum of the second organic semiconductor material is longer than the maximum wavelength of the absorption spectrum of the first organic semiconductor material, thereby suppressing excess absorption loss of light emitted by layers other than the second organic semiconductor layer, such as the first organic semiconductor layer.

[0028] As the first organic semiconductor material, for example, a conventionally known electron transporting material can be used, specific examples of which include the compounds shown below.

[0029] [ka]

[0030] [ka]

[0031] [ka]

[0032] [ka]

[0033] As the second organic semiconductor material, for example, the following compound, which has been reported to produce TTA, can be used.

[0034] [ka]

[0035] [ka]

[0036] [ka] (Chem. Rev. 2015, 115, 395-465 reference)

[0037] The energy levels of these compounds, such as the HOMO level, LUMO level, and excited triplet level T1, are specific to the material, and reference values ​​can be used.

[0038] As the third organic semiconductor material, a conventionally known light-emitting material can be used. Specific examples of the third organic semiconductor material include the compounds shown below.

[0039] [ka]

[0040] [ka]

[0041] [ka]

[0042] The organic EL element of the present embodiment may be formed from a pair of electrodes, a first organic semiconductor layer, and a second organic semiconductor layer, as shown in FIG. 1, or may include other conventionally known organic semiconductor layers, inorganic compound layers, etc.

[0043] The organic EL device of this embodiment may have layers between a pair of electrodes, such as 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. Of these, the first organic semiconductor layer may be the electron transport layer, and the second organic semiconductor layer may be the emitting layer.

[0044] The functions of these layers are not strictly distinguished. For example, the light-emitting layer, which is the second organic semiconductor layer, may also function as a hole-transporting layer, and the hole-blocking layer may also function as an electron-injecting layer.

[0045] The order of the layers in the organic EL element is not limited to the above. 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.

[0046] The layers in the organic EL device may contain the same organic semiconductor material. For example, when the organic EL device contains rubrene as the second organic semiconductor material in the second organic semiconductor layer (light-emitting layer), the organic EL device may have a hole-blocking layer formed from rubrene.

[0047] The organic EL device of this embodiment can be manufactured by forming the first and second organic semiconductor layers 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 laminating a first electrode, a first organic semiconductor layer, a second organic semiconductor 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.

[0048] The thickness of the first and second organic semiconductor layers in the organic EL device of this embodiment is not particularly limited, but is preferably 0.1 nm to 500 nm, more preferably 2 nm to 200 nm.

[0049] 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. [Example]

[0050] 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.

[0051] [ka]

[0052] <Measurement of absorption (ABS) spectrum and emission (PL) spectrum> Rubrene, PTCDI-C8, C60, or DBP was evaporated under high vacuum (~10 -5 The thin films were thermally evaporated onto quartz substrates at 1000 kJ / cm² (Pa) to form single-layer thin films. The thickness of each film was approximately 50 nm. PL spectra were measured for the rubrene and DBP thin films, and ABS spectra were measured for the PTCDI-C8 and C60 thin films. The results are shown in Figure 4. The ABS spectrum of NDI-bis-HFI is also shown in Figure 13. The absorption spectrum was measured using a spectrometer (V-570, manufactured by Jasco). The emission spectrum was measured using a spectrofluorometer (Fluorolog, manufactured by HORIBA).

[0053] As is clear from Fig. 4, the maximum wavelength (about 565 nm) of the PL spectrum of rubrene (the second organic semiconductor material) is on the longer wavelength side than the maximum wavelengths (about 490 nm and about 345 nm, respectively) of the ABS spectra of PTCDI-C8 and C60 (the first organic semiconductor materials). Also, the maximum wavelength (about 605 nm) of the PL spectrum of DBP (the third organic semiconductor material) is on the longer wavelength side than the maximum wavelength of the PL spectrum of rubrene. Also, as shown in Fig. 13, the maximum wavelength of the ABS spectrum of NDI-bis-HFI is about 305 nm, which is on the shorter wavelength side than the maximum wavelength of the PL spectrum of rubrene.

[0054] <Measurement of PL intensity> For a single-layer rubrene layer, samples without a dopant (0 vol%), with 0.2 vol%, 0.5 vol%, 1 vol%, and 5 vol% of DBP added as a dopant to the entire rubrene layer were prepared, and the emission intensity (PL intensity) was measured for each. The rubrene layer was formed by thermal evaporation onto a quartz substrate under high vacuum (~10 -5 Pa) in a vacuum evaporation system. DBP was introduced by co-evaporation when the rubrene layer was evaporated, and the mixing concentration was controlled by the ratio of the evaporation rates. The PL intensity was measured using an absolute PL quantum yield measurement device (Quantaurus-QY, manufactured by Hamamatsu Photonics). The results are shown in Fig. 5. As is clear from Fig. 5, when DBP was added as a dopant, emission was confirmed around about 605 nm derived from DBP, and energy transfer from rubrene to DBP was confirmed.

[0055] <Measurement of fluorescence quantum yield (PL QY)> For the various rubrene layers prepared in the above measurement of the PL intensity, the fluorescence quantum yield (PL QY) was measured using an absolute PL quantum yield measurement device (Quantaurus-QY, manufactured by Hamamatsu Photonics). The results are shown in Table 1.

[0056]

Table 1

[0057] As is clear from Table 1, the fluorescence quantum yield was higher when a dopant (DBP) was added than when no dopant was added, and the highest fluorescence quantum yield (72.6%) was obtained when the amount of DBP added was 0.5 vol%.

[0058] Example 1 Glass substrate coated with indium tin oxide (ITO) (ITO thickness: 150 nm, sheet resistance: 10.3 Ω) -1 On the substrate (Technoprint Co., Ltd.), a MoO hole injection layer (10 nm, 0.01 nm / s), a rubrene layer (50 nm, 0.1 nm / s), a PTCDI-C8 layer (50 nm, 0.1 nm / s), a LiF electron injection layer (0.2 nm, 0.001 nm / s), and an Al electrode (70 nm, 0.3 nm / s) were deposited in this order in a vacuum deposition system under high vacuum (~10 -5 The rubrene layer was thermally evaporated at a temperature of 1000 K (Pa). The device was then sealed with a glass substrate and epoxy resin in a glove box to obtain an organic EL device. DBP was added as a dopant to the rubrene layer at 0.5% by volume relative to the entire rubrene layer. DBP was introduced by co-evaporation when the rubrene layer was evaporated, and the mixed concentration was controlled by the ratio of the evaporation rates. The obtained organic EL device had the following configuration. ITO electrode / MoO3 hole injection layer / Rubrene layer (DBP doped) / PTCDI-C8 layer / LiF electron injection layer / Al electrode

[0059] (Comparative Example 1) Except for not adding DBP, an organic EL device was produced in the same manner as in Example 1. The obtained organic EL device had the following configuration. ITO electrode / MoO3 hole injection layer / Rubrene layer (undoped) / PTCDI-C8 layer / LiF electron injection layer / Al electrode

[0060] <Evaluation of organic EL elements> The V-brightness characteristics of the organic EL devices of Example 1 and Comparative Example 1 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 6(A). The external quantum yields (EQEs) of the organic EL devices of Example 1 and Comparative Example 1 were measured using a calibrated high-sensitivity broadband spectrometer (AvaSpec-UV / VIS / NIR, manufactured by Avantes), and the results are shown in Figure 6(B). As is clear from FIGS. 6(A) and 6(B), when compared with the case where the dopant was present (Example 1) and the case where it was not present (Comparative Example 1), the luminance was improved by up to 9.89 times under the same voltage condition, and the external quantum yield (EQE) was improved by up to 28.9 times and at least 3.19 times under the same current density condition.

[0061] (Example 2: Insertion of a hole-blocking layer) An organic EL device was fabricated in the same manner as in Example 1, except that a BCP layer (10 nm, 0.05 nm / s) was formed by thermal evaporation between the PTCDI-C8 layer and the LiF electron injection layer. This organic EL device had the following configuration. ITO electrode / MoO3 hole injection layer / Rubrene layer (DBP doped) / PTCDI-C8 layer / BCP layer / LiF electron injection layer / Al electrode The V-brightness characteristics of the obtained organic EL element (rubDBP / BCP) were measured by the above-mentioned method, and the results are shown in Figure 7 together with the measurement results of the organic EL element (rubDBP) of Example 1. As is clear from Figure 7, the insertion of the BCP layer improves the luminance in the high-voltage region.

[0062] (Example 3: Insertion of an electron blocking layer) Two types of organic EL devices were fabricated in the same manner as in Example 1, except that a rubrene layer (10 nm, 0.1 nm / s) (Example 3A) or an NPD layer (10 nm, 0.1 nm / s) (Example 3B) was formed by thermal evaporation between the MoO hole injection layer and the rubrene layer (DBP-doped). These organic EL devices had the following configurations. Example 3A: ITO electrode / MoO3 hole injection layer / Rubrene layer (undoped) / Rubrene layer (DBP doped) / PTCDI-C8 layer / LiF electron injection layer / Al electrode Example 3B: ITO electrode / MoO3 hole injection layer / NPD layer / Rubrene layer (DBP doped) / PTCDI-C8 layer / LiF electron injection layer / Al electrode The V-brightness characteristics of the two organic EL devices obtained (rub / rubDBP, NPD / rubDPB) were measured by the method described above, and the results are shown in Figure 8 together with the measurement results for the organic EL device (rubDBP) of Example 1. As is clear from Figure 8, the insertion of a rubrene layer or an NPD layer improves the luminance in the high-voltage region.

[0063] (Example 4: Study of the first organic semiconductor layer) Three types of organic EL devices were fabricated in the same manner as in Example 3A, except that a PTCDI-C6 layer (50 nm, 0.1 nm / s) (Example 4A), a PTCDI-C13 layer (50 nm, 0.1 nm / s) (Example 4B), or a C60 layer (50 nm, 0.1 nm / s) (Example 4C) was formed by thermal evaporation instead of the PTCDI-C8 layer. These organic EL devices had the following configurations. Example 4A: ITO electrode / MoO3 hole injection layer / Rubrene layer (undoped) / Rubrene layer (DBP doped) / PTCDI-C6 layer / LiF electron injection layer / Al electrode Example 4B: ITO electrode / MoO3 hole injection layer / Rubrene layer (undoped) / Rubrene layer (DBP doped) / PTCDI-C13 layer / LiF electron injection layer / Al electrode Example 4C: ITO electrode / MoO3 hole injection layer / Rubrene layer (undoped) / Rubrene layer (DBP doped) / C60 layer / LiF electron injection layer / Al electrode The external quantum yield (EQE) of the three organic EL devices obtained was measured by the above-mentioned method. The results are shown in FIG. 9 together with the measurement results of the organic EL device of Example 3A. Furthermore, for Examples 3A and 4A to 4C, comparative organic EL devices were fabricated using a 60 nm-thick rubrene layer (undoped) instead of the "rubrene layer (undoped) / rubrene layer (DBP-doped)" structure, and measurements were carried out in the same manner. The results are shown in FIG. 9. As is clear from FIG. 9, regardless of which electron transport layer was used, the luminous efficiency was improved by about 10 times when the rubrene layer (DBP-doped) was used, but the luminous efficiency was highest when the PTCDI-C8 layer was used.

[0064] (Example 5: Examination of dopants) An organic EL device was fabricated in the same manner as in Example 3A, except that DCJTB (0.5% by volume) was used as the dopant in the rubrene layer (DBP-doped) instead of DBP. This organic EL device had the following structure. ITO electrode / MoO3 hole injection layer / Rubrene layer (undoped) / Rubrene layer (doped with DCJTB) / PTCDI-C8 layer / LiF electron injection layer / Al electrode The EL emission spectrum of the obtained organic EL device (DCJTB) was measured using a high-sensitivity broadband spectrometer (AvaSpec-UV / VIS / NIR, manufactured by Avantes). The results are shown in Figure 10(A) together with the measurement results of the organic EL device of Example 3A (DBP) and Comparative Example 1 (rub). As is clear from these results, when a dopant (DBP or DCJTB) was used, energy transfer from rubrene to DBP or DCJTB occurred, quenching the emission of rubrene, and emission derived from DBP or DCJTB was observed. The V-brightness characteristics of the resulting organic EL device (DCJTB) were measured using the method described above. The results are shown in Figure 10(B) together with the measurement results for the organic EL device (DBP) of Example 3A. As is clear from Figure 10(B), the luminance is higher when doped with DBP.

[0065] (Example 6: Study of film thickness of electron transport layer) An organic EL device was fabricated in the same manner as in Example 3A, except that the thickness of the PTCDI-C8 layer was changed from 50 nm to 20 nm. The V-brightness characteristics of the obtained organic EL device were measured by the above-mentioned method, and the results are shown in Figure 11 together with the measurement results of the organic EL device of Example 3A. As is clear from Figure 11, the light emission brightness is high even when the film thickness is 20 nm.

[0066] (Example 7: Study of film thickness of light-emitting layer) Organic EL devices were fabricated in the same manner as in Example 3A, except that the thickness of the rubrene layer (doped with DBP) was changed from 50 nm to 20 nm, 100 nm, 150 nm, or 200 nm. The V-luminance characteristics of the obtained organic EL device were measured by the above-mentioned method. The results are shown in FIG. 12(A) together with the measurement results of the organic EL device of Example 3A, and the results of measuring the external quantum yield (EQE) are shown in FIG. 12(B) together with the measurement results of the organic EL device of Example 3A. As is clear from FIG. 12(A), the luminance is high regardless of the film thickness. As is clear from FIG. 12(B), the thicker the film thickness (200 nm), the larger the EQE (maximum 2.91% @ 30 mA / cm). 2 ).

[0067] (Example 8: Study of the first organic semiconductor layer, part 2) An organic EL device was fabricated in the same manner as in Example 3A, except that a NDI-bis-HFI layer (50 nm) was formed by thermal evaporation instead of the PTCDI-C8 layer. This organic EL device had the following configuration. ITO electrode / MoO3 hole injection layer / Rubrene layer (undoped) / Rubrene layer (DBP doped) / NDI-bis-HFI layer / LiF electron injection layer / Al electrode The V-brightness characteristics of the obtained organic EL element were measured by the above-mentioned method, and the results are shown in Figure 14 together with the measurement results of the organic EL element of Example 3A. As is clear from Figure 14, the organic EL element of Example 8 can achieve high emission brightness (380 cd / m) even at a low voltage (1.5 V). [Explanation of symbols]

[0068] 1...first organic semiconductor layer, 2...second organic semiconductor layer, 3...first electrode, 4...second electrode, 10...organic EL element.

Claims

1. An organic EL element comprising a plurality of organic semiconductor layers sandwiched between a pair of electrodes, the organic semiconductor layer has a first organic semiconductor layer containing a first organic semiconductor material, and a second organic semiconductor layer containing a second organic semiconductor material and a third organic semiconductor material; the first organic semiconductor layer and the second organic semiconductor layer form a junction surface; the HOMO level of the first organic semiconductor material is lower than the HOMO level of the second organic semiconductor material, and the LUMO level of the first organic semiconductor material is lower than the LUMO level of the second organic semiconductor material; the second organic semiconductor material is a material that undergoes triplet-triplet annihilation; The excited triplet level T of the second organic semiconductor material １ is smaller than the energy difference between the HOMO level of the second organic semiconductor material and the LUMO level of the first organic semiconductor material, an energy difference between the HOMO level of the second organic semiconductor material and the LUMO level of the first organic semiconductor material is smaller than an energy difference between the HOMO level and the LUMO level of the second organic semiconductor material by 0.5 eV or more; In the second organic semiconductor layer, the second organic semiconductor material is a host material and the third organic semiconductor material is a dopant; An organic EL element, wherein the maximum wavelength of the emission spectrum of the second organic semiconductor material is longer than the maximum wavelength of the absorption spectrum of the first organic semiconductor material.

2. 2. The organic EL device according to claim 1, wherein the difference between the HOMO levels of the first organic semiconductor material and the second organic semiconductor material is 0.5 eV or more.

3. 3. The organic EL device according to claim 1, wherein a difference between the LUMO level of the first organic semiconductor material and the LUMO level of the second organic semiconductor material is 0.5 eV or more.

4. The excited triplet level T of the second organic semiconductor material １ and an energy difference between the HOMO level of the second organic semiconductor material and the LUMO level of the first organic semiconductor material is less than 0.8 eV.

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