Organic el element

Abstract

An organic EL element is an organic EL element (10) provided with a plurality of organic semiconductor layers held between a pair of electrodes (3), (4), the organic semiconductor layers having 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 forming a junction surface, the first organic semiconductor material and the second organic semiconductor material satisfying a prescribed condition related to energy levels, and the like.

Description

Technical Field

[0001] This invention relates to organic EL elements. Background Technology

[0002] Organic electroluminescent devices (organic EL devices) are devices that have one or more organic semiconductor layers sandwiched between a pair of electrodes and emit light autonomously by applying a voltage between the electrodes.

[0003] In recent years, research has been actively conducted to improve the luminous efficiency and brightness of organic EL devices. For example, Non-Patent Literature 1 proposes a device with a rubrene layer and C... 60 Organic EL elements with an energy-up-converted structure consisting of layers stacked one on top of the other.

[0004] Existing technical documents

[0005] Non-patent literature

[0006] Non-patent literature 1: Ajay K. Pandey, Scientific Reports 2015, 5, 7787. Summary of the Invention

[0007] The problem that the invention aims to solve

[0008] Furthermore, through the research of the inventors, it has been shown that there is room for improvement in terms of luminous efficiency and luminous brightness for conventional organic EL elements such as those in Non-Patent Document 1.

[0009] Therefore, the object of the present invention is to provide an organic EL element with excellent luminous efficiency and luminous brightness.

[0010] Methods for solving problems

[0011] In view of the above, the inventors conducted in-depth research and discovered an organic EL element having the following structure.

[0012] That is, the organic EL element of the present invention is the following organic EL element,

[0013] It has multiple organic semiconductor layers sandwiched between a pair of electrodes.

[0014] The organic semiconductor layer has a first organic semiconductor layer comprising a first organic semiconductor material, and a second organic semiconductor layer comprising a second organic semiconductor material and a third organic semiconductor material.

[0015] The first organic semiconductor layer and the second organic semiconductor layer form a bonding surface.

[0016] The HOMO energy level of the first organic semiconductor material is lower than that of the second organic semiconductor material, and the LUMO energy level of the first organic semiconductor material is lower than that of the second organic semiconductor material.

[0017] The second type of organic semiconductor material is one that undergoes triplet-triplet annihilation.

[0018] The excited triplet energy level T1 of the second organic semiconductor material is less than the energy difference between the HOMO energy level of the second organic semiconductor material and the LUMO energy level of the first organic semiconductor material.

[0019] The energy difference between the HOMO level of the second organic semiconductor material and the LUMO level of the first organic semiconductor material is more than 0.5 eV smaller than the energy difference between the HOMO level and the LUMO level of the second organic semiconductor material.

[0020] In the second organic semiconductor layer, the second organic semiconductor material is the host material, and the third organic semiconductor material is the dopant.

[0021] The maximum wavelength of the emission spectrum of the second organic semiconductor material is closer to the long wavelength side than the maximum wavelength of the absorption spectrum of the first organic semiconductor material.

[0022] The organic EL element exhibits excellent luminous efficiency and brightness. While the reasons for this may not be entirely clear, it is based on... Figures 1-3 To illustrate the research conducted by the inventors, etc.

[0023] Figure 1 A conceptual diagram illustrating the mechanism by which light emission occurs through the organic EL element of the present invention. Figure 1 In this embodiment, the organic EL element 10 has a first organic semiconductor layer 1, a second organic semiconductor layer 2 forming an interface (bonding surface) with the first organic semiconductor layer 1, a first electrode 3 formed on the side of the first organic semiconductor layer 1, and a second electrode 4 formed on the side of the second organic semiconductor layer 2. As in the embodiments, the first organic semiconductor layer 1 corresponds to an electron transport layer, the second organic semiconductor layer 2 corresponds to a light-emitting layer, the first electrode 3 corresponds to a cathode, and the second electrode 4 corresponds to an anode.

[0024] Specifically, when electrons (-) are injected into the organic EL element 10 from the cathode and holes (+) are injected into the organic EL element 10 from the anode, electron (-) / hole (+) pairs form charge transfer (CT) states at the junction of the first organic semiconductor layer 1 and the second organic semiconductor layer 2. Charge recombination occurs in the CT states, thereby generating a triplet state (T1) of the second organic semiconductor material (host material) in the second organic semiconductor layer 2. A high-energy excited state (S1) is generated by inducing triplet-triplet annihilation (TTA) in the second organic semiconductor layer 2. Light emission originating from the third organic semiconductor material occurs in the second organic semiconductor layer 2 through energy transfer from the second organic semiconductor material to the third organic semiconductor material (dopant).

[0025] Figure 2 The diagram illustrates the energy levels of rubrene, PTCDI-C8, and C60 used in the embodiments. PTCDI-C8 and C60 correspond to the first organic semiconductor material, and rubrene corresponds to the second organic semiconductor material. The HOMO energy level of the first organic semiconductor material is lower than that of the second organic semiconductor material, and the LUMO energy level of the first organic semiconductor material is lower than that of the second organic semiconductor material. Furthermore, the energy difference between the HOMO energy level of the second organic semiconductor material and the LUMO energy level of the first organic semiconductor material is more than 0.5 eV smaller than the energy difference between the HOMO energy level and the LUMO energy 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 junction of the first organic semiconductor layer 1 and the second organic semiconductor layer 2.

[0026] Figure 3 This is a schematic diagram illustrating the energy transfer mechanism of an organic EL element based on an embodiment using PTCDI-C8 as the first organic semiconductor material, rubrene as the second organic semiconductor material, and DBP as the third organic semiconductor material, up to the point of luminescence. Figure 3 In this model, the energy level of the CT state (CT = 1.5 eV) corresponds to the energy difference between the HOMO energy level of the second organic semiconductor material and the LUMO energy level of the first organic semiconductor material, and the excited triplet energy level T1 of the second organic semiconductor material is 1.1 eV. Since the excited triplet energy level T1 of the second organic semiconductor material is smaller than the energy difference between the HOMO energy level of the second organic semiconductor material and the LUMO energy level of the first organic semiconductor material, it is assumed that electrons (−) and holes (+) can pass through the CT state to generate the triplet state of the second organic semiconductor material in the second organic semiconductor layer 2.

[0027] Furthermore, since the maximum wavelength of the emission spectrum of the second organic semiconductor material is closer to the long wavelength side than the maximum wavelength of the absorption spectrum of the first organic semiconductor material, it is possible to suppress the situation where the emission intensity of the first organic semiconductor material decreases due to the absorption of the emission of the organic EL element.

[0028] It should be noted that the case where the first organic semiconductor layer 1 is an electron transport layer has been described here, but the first organic semiconductor layer 1 can also be an electron injection layer or a hole blocking layer.

[0029] Invention Effects

[0030] Furthermore, the organic EL element of this invention exhibits excellent luminous efficiency and luminous brightness. Attached Figure Description

[0031] Figure 1 A conceptual diagram illustrating the mechanism by which light emission occurs through the organic EL element of the present invention.

[0032] Figure 2 A diagram illustrating the energy levels of rubrene, PTCDI-C8, and C60 used in the examples.

[0033] Figure 3 This is a schematic diagram illustrating the energy transfer mechanism of an organic EL element based on an embodiment until it emits light.

[0034] Figure 4 This is a graph showing the absorption or emission spectra of the compounds used in the examples.

[0035] Figure 5 A graph representing the PL intensity of a single-layer fluorene layer.

[0036] Figure 6 (A) is a graph showing the V-luminance characteristics of the organic EL element of Example 1, etc. Figure 6 (B) is a graph showing the external quantum efficiency (EQE) of the organic EL element of Example 1, etc.

[0037] Figure 7 A graph showing the V-luminance characteristics of the organic EL element in Example 2.

[0038] Figure 8 A graph showing the V-luminance characteristics of the organic EL element of Example 3, etc.

[0039] Figure 9 A graph showing the external quantum efficiency (EQE) of the organic EL element in Example 4, etc.

[0040] Figure 10 (A) is a graph showing the EL emission spectrum of the organic EL element of Example 5, etc. Figure 10(B) is a graph showing the V-brightness characteristics of the organic EL element of Example 5, etc.

[0041] Figure 11 A graph showing the V-luminance characteristics of the organic EL element of Example 6, etc.

[0042] Figure 12 (A) is a graph showing the V-luminance characteristics of the organic EL element of Example 7, etc. Figure 12 (B) is a graph showing the external quantum efficiency (EQE) of the organic EL element of Example 7, etc.

[0043] Figure 13 This is a graph showing the absorption spectrum of NDI-bis-HFI.

[0044] Figure 14 A graph showing the V-luminance characteristics of the organic EL element of Example 8, etc. Detailed Implementation

[0045] The preferred embodiments of the present invention will now be described in detail. However, the present invention is not limited to the following embodiments.

[0046] The organic EL device of this embodiment has multiple organic semiconductor layers sandwiched between a pair of electrodes. Furthermore, the organic EL device of this embodiment may further have inorganic compound layers such as a molybdenum trioxide (MoO3) layer (hole injection layer) and a lithium fluoride layer (electron injection layer) between the electrodes.

[0047] 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, wherein the first organic semiconductor layer and the second organic semiconductor layer form a bonding surface.

[0048] The first organic semiconductor layer may be formed solely of the first organic semiconductor material, or it may include materials other than the first organic semiconductor material, to a extent that does not significantly impair the effects of the present invention. The second organic semiconductor layer may be formed solely of the second and third organic semiconductor materials, or it may include materials other than the second organic semiconductor material, to a extent that does not significantly impair the effects of the present invention.

[0049] The HOMO energy level of the first organic semiconductor material is lower than that of the second organic semiconductor material. From the viewpoint of effectively preventing hole leakage and further improving luminous efficiency, the difference between the HOMO energy levels of the first and second organic semiconductor materials is preferably 0.5 eV or higher. It should be noted that there is no particular upper limit to the difference between the HOMO energy levels of the first and second organic semiconductor materials; for example, it can be set to below 2 eV.

[0050] The LUMO energy level of the first organic semiconductor material is lower than that of the second organic semiconductor material. From the viewpoint of effectively preventing electron leakage and further improving luminous efficiency, the difference between the LUMO energy levels of the first and second organic semiconductor materials is preferably 0.5 eV or higher. It should be noted that there is no particular upper limit to the difference between the LUMO energy levels of the first and second organic semiconductor materials; for example, it can be set to below 2 eV.

[0051] The second organic semiconductor material is a triplet-triplet annihilation material, and the excited triplet energy level T1 of the second organic semiconductor material is less than the energy difference between the HOMO energy level of the second organic semiconductor material and the LUMO energy level of the first organic semiconductor material. The energy difference between the excited triplet energy level T1 of the second organic semiconductor material and the energy difference between the HOMO energy level of the second organic semiconductor material and the LUMO energy 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 the above difference is small, the emission initiation voltage can be reduced.

[0052] 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, preferably at least 0.7 eV smaller. There is no particular upper limit to the energy difference between the HOMO level of the second organic semiconductor material and the LUMO level of the first organic semiconductor material; for example, it can be set to below 2 eV.

[0053] In the second organic semiconductor layer, the second organic semiconductor material is the host material, and the third organic semiconductor material is the dopant. The energy difference between the HOMO and LUMO levels of the third organic semiconductor material is preferably smaller than the energy difference between the HOMO and LUMO levels in the second organic semiconductor material. The maximum wavelength of the emission spectrum of the third organic semiconductor material, as the dopant, is closer to the longer wavelength side than the maximum wavelength of the emission spectrum of the second organic semiconductor material, which is the host material. Using the third organic semiconductor material results in energy transfer from the second organic semiconductor material to the third organic semiconductor material, enabling light emission originating from the third organic semiconductor material.

[0054] Regarding the content of the third organic semiconductor material in the second organic semiconductor layer, for example, it can be set to 0.01 to 50% by volume relative to 100% of the total amount of the second organic semiconductor layer, preferably 0.1 to 10% by volume.

[0055] The maximum wavelength of the emission spectrum of the second organic semiconductor material is closer to the longer wavelength side than the maximum wavelength of the absorption spectrum of the first organic semiconductor material. Therefore, it is possible to suppress unnecessary light absorption losses caused by layers other than the second organic semiconductor layer, such as the first organic semiconductor layer.

[0056] As the first organic semiconductor material, for example, existing known electron transport materials can be used. Specific examples include the compounds shown below.

[0057] [Chemical Formula 1]

[0058]

[0059] [Chemical Formula 2]

[0060]

[0061] [Chemical Formula 3]

[0062]

[0063] [Chemical Formula 4]

[0064]

[0065] As a second organic semiconductor material, for example, the following compounds that have been reported to produce TTA can be used.

[0066] [Chemical Formula 5]

[0067]

[0068] [Chemical Formula 6]

[0069]

[0070] [Chemical Formula 7]

[0071]

[0072] (Refer to Chem. Rev. 2015, 115, 395-465)

[0073] The HOMO, LUMO, and excited triplet T1 energy levels of these compounds are inherent to the material and can be found in references.

[0074] As a third organic semiconductor material, existing known luminescent materials can be used. Specific examples of the third organic semiconductor material include the compounds shown below.

[0075] [Chemical Formula 8]

[0076]

[0077] [Chemical Formula 9]

[0078]

[0079] [Chemical Formula 10]

[0080]

[0081] like Figure 1 As shown, the organic EL element of this embodiment can be formed from a pair of electrodes, a first organic semiconductor layer and a second organic semiconductor layer, or it can have other known organic semiconductor layers, inorganic compound layers, etc.

[0082] As an organic EL element in this embodiment, the layers that can be provided between a pair of electrodes include, for example, a hole injection layer, an electron blocking layer, a hole transport layer, a light-emitting layer, an electron transport layer, a hole blocking layer, and an electron injection layer, in sequence from the anode. Among these, the first organic semiconductor layer can be an electron transport layer, and the second organic semiconductor layer can be a light-emitting layer.

[0083] It should be noted that the functions of these layers are not strictly distinguished. For example, the light-emitting layer, which is the second organic semiconductor layer, can also function as a hole transport layer, and the hole blocking layer can also function as an electron injection layer.

[0084] The order of the layers in an organic EL device is not limited to the above order. For example, the hole injection layer may be located between the electron blocking layer and the hole transport layer, and the electron injection layer may be located between the hole blocking layer and the electron transport layer.

[0085] In an organic EL device, layers may contain the same organic semiconductor material. For example, in an organic EL device, when fluorene is included as the second organic semiconductor material in the second organic semiconductor layer (light-emitting layer), a hole-blocking layer formed of fluorene may also be present.

[0086] The organic EL element of this embodiment can be manufactured by forming the first and second organic semiconductor layers using existing known methods, such as vacuum evaporation, chemical vapor deposition, sputtering, vapor deposition polymerization, spin coating, blade coating, rod coating, dip coating, and lamination. Specifically, for example, the organic EL element of this embodiment can be manufactured by stacking a first electrode, a first organic semiconductor layer, a second organic semiconductor layer, a second electrode, and optionally other layers on a substrate. The method for forming each organic semiconductor layer can be appropriately selected depending on the compound. As the substrate, for example, a glass substrate, a quartz substrate, a sapphire substrate, a plastic substrate, or a film substrate can be used.

[0087] The thickness of the first and second organic semiconductor layers in the organic EL element of this embodiment is not particularly limited, but is preferably 0.1 nm to 500 nm, and more preferably 2 nm to 200 nm.

[0088] For example, organic EL components can be expected to be used in organic EL displays, organic EL lighting, digital signage, light sources for light sensors, laser light sources, and light sources for optical communication.

[0089] Example

[0090] The present invention will now be described in more detail with reference to the embodiments, but the present invention is not limited to the embodiments in any way. It should be noted that the structures of the compounds used in the embodiments are shown below.

[0091] [Chemical Formula 11]

[0092]

[0093] <Determination of Absorption (ABS) and Luminescence (PL) Spectra>

[0094] Within a vacuum evaporation system, under high vacuum (~10 -5 A monolayer thin film was formed by thermally depositing fluorene, PTCDI-C8, C60, or DBP onto a quartz substrate. The thickness of each layer was approximately 50 nm. PL spectra were measured for the fluorene and DBP films, and ABS spectra were measured for the PTCDI-C8 and C60 films, respectively. The results are presented in... Figure 4 In addition, the results of NDI-bis-HFI determination of ABS spectra are presented below. Figure 13 .

[0095] The absorption spectra were measured using a spectrometer (V-570, manufactured by Jasco).

[0096] The emission spectra were measured using a spectrophotometer (Fluorolog, HORIBA).

[0097] like Figure 4 As indicated, the maximum wavelength of the PL spectrum of rubrene (the second organic semiconductor material) (approximately 565 nm) is closer to the long wavelength side than the maximum wavelengths of the ABS spectra of PTCDI-C8 and C60 (the first organic semiconductor material) (approximately 490 nm and approximately 345 nm, respectively). Furthermore, the maximum wavelength of the PL spectrum of DBP (the third organic semiconductor material) (approximately 605 nm) is closer to the long wavelength side than the maximum wavelength of the PL spectrum of rubrene. Additionally, as... Figure 13 As shown, the maximum wavelength of the ABS spectrum of NDI-bis-HFI is approximately 305 nm, which is closer to the lower wavelength side than the maximum wavelength of the PL spectrum of rubrene.

[0098] <Determination of PL Strength>

[0099] For monolayer fluorene layers, the following were prepared: a dopant-free (0 vol%) fluorene layer; and fluorene layers with 0.2 vol%, 0.5 vol%, 1 vol%, and 5 vol% DBP added as dopants relative to the total fluorene layer. The luminescence intensity (PL intensity) of each was measured. The fluorene layers were deposited in a vacuum evaporation system under high vacuum (~10 °C). -5 The PL (polypropylene) layer was formed by thermal evaporation on a quartz substrate. For DBP, it was introduced during the evaporation of the fluorene layer via a co-evaporation method, and the mixing concentration was controlled by the ratio of the evaporation rate. The PL intensity was measured using an absolute PL quantum yield measuring instrument (Quantaurus-QY, manufactured by Hamamatsu Photonics KK). The results are shown below. Figure 5 .like Figure 5 As indicated, when DBP was added as a dopant, luminescence from DBP around 605 nm was confirmed, and energy transfer from rubrene to DBP was confirmed.

[0100] <Determination of Fluorescent Quantum Yield (PL QY)>

[0101] For the various fluorene layers prepared in the above PL intensity determination, the fluorescence quantum yield (PL QY) was measured using an absolute PL quantum yield measuring apparatus (Quantaurus-QY, manufactured by Hamamatsu Photonics KK). The results are shown in Table 1.

[0102] [Table 1]

[0103] 465nm excitation PL QY (%) Dopant-free 29.1 DBP 0.2% by volume 40.8 DBP 0.5% by volume 72.6 DBP 1 volume % 65.5 DBP 5% by volume 62.4

[0104] As shown in Table 1, the fluorescence quantum yield is higher when a dopant (DBP) is added compared to the undoped case, especially when the amount of DBP added is 0.5 vol%, the highest fluorescence quantum yield (72.6%) is obtained.

[0105] (Example 1)

[0106] Within a vacuum evaporation system, under high vacuum (~10 -5 Pa), on a glass substrate coated with indium tin oxide (ITO) (ITO thickness: 150 nm, sheet resistance: 10.3 Ω). -1On a substrate manufactured by Techno Print Co., Ltd., a MoO3 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 sequentially thermally vapor-deposited. The device was then encapsulated in a glove box using a glass substrate and epoxy resin to obtain an organic EL device. It should be noted that DBP was added as a dopant to the rubrene layer at a concentration of 0.5% by volume relative to the total rubrene layer. The DBP was introduced during the rubrene deposition using a co-evaporation method, and the mixing concentration was controlled by the ratio of the deposition rate.

[0107] The resulting organic EL element has the following structure.

[0108] ITO electrode / MoO3 hole injection layer / fluorene layer (DBP doped) / PTCDI-C8 layer / LiF electron injection layer / Al electrode

[0109] (Comparative Example 1)

[0110] The organic EL element was fabricated in the same manner as in Example 1, except that DBP was not added. The resulting organic EL element has the following structure.

[0111] ITO electrode / MoO3 hole injection layer / fluorene layer (undoped) / PTCDI-C8 layer / LiF electron injection layer / Al electrode

[0112] <Evaluation of Organic EL Components>

[0113] The V-luminance characteristics of the organic EL elements of Example 1 and Comparative Example 1 were measured using a source measurement unit (B2902A, manufactured by Keysight Technologies) and a luminance meter (BM-9, manufactured by Topcom). The results are shown below. Figure 6 (A).

[0114] The external quantum efficiency (EQE) of the organic EL devices of Example 1 and Comparative Example 1 was measured using a calibrated high-sensitivity broadband spectrometer (AvaSpec-UV / VIS / NIR, manufactured by Avantes). The results are presented below. Figure 6 (B).

[0115] like Figure 6As shown in (A) and (B), the case with dopant (Example 1) showed an improvement over the case without dopant (Comparative Example 1) at the same voltage conditions, with a maximum luminous intensity of 9.89 times and an external quantum efficiency (EQE) of 28.9 times and a minimum of 3.19 times, respectively.

[0116] (Example 2: Insertion of a hole-blocking layer)

[0117] A BCP layer (10 nm, 0.05 nm / s) was formed between the PTCDI-C8 layer and the LiF electron injection layer by thermal evaporation. Otherwise, the organic EL device was fabricated in the same manner as in Example 1. The organic EL device has the following structure.

[0118] ITO electrode / MoO3 hole injection layer / fluorene layer (DBP doped) / PTCDI-C8 layer / BCP layer / LiF electron injection layer / Al electrode

[0119] For the obtained organic EL element (rubDBP / BCP), the V-luminosity characteristics were measured using the method described above. The results are presented together with the measurement results of the organic EL element (rubDBP) in Example 1. Figure 7 .like Figure 7 As demonstrated, the luminous intensity of the high-voltage region is increased by inserting the BCP layer.

[0120] (Example 3: Insertion of an electron blocking layer)

[0121] By thermal evaporation, a fluorene layer (10 nm, 0.1 nm / s) (Example 3A) or an NPD layer (10 nm, 0.1 nm / s) (Example 3B) is formed between the MoO3 hole injection layer and the fluorene layer (DBP doped). Otherwise, two organic EL devices are fabricated in the same manner as in Example 1. These organic EL devices have the following structures.

[0122] Example 3A:

[0123] ITO electrode / MoO3 hole injection layer / fluorene layer (undoped) / fluorene layer (DBP doped) / PTCDI-C8 layer / LiF electron injection layer / Al electrode

[0124] Example 3B:

[0125] ITO electrode / MoO3 hole injection layer / NPD layer / Red fluorene layer (DBP doped) / PTCDI-C8 layer / LiF electron injection layer / Al electrode

[0126] For the two types of organic EL elements (rub / rubDBP, NPD / rubDPB), the V-luminosity characteristics were measured using the method described above. The results are presented together with the measurement results of the organic EL element (rubDBP) in Example 1. Figure 8 .like Figure 8 As demonstrated, the luminescence brightness in the high-voltage region is increased by inserting a red fluorene layer or an NPD layer.

[0127] (Example 4: Study of the first organic semiconductor layer)

[0128] Instead of the PTCDI-C8 layer, a PTCDI-C6 layer (50 nm, 0.1 nm / s) (Example 4A), a PTCDI-C13 layer (50 nm, 0.1 nm / s) (Example 4B), and a C60 layer (50 nm, 0.1 nm / s) (Example 4C) were formed by thermal evaporation. Otherwise, three organic EL elements were fabricated using the same procedure as in Example 3A. These organic EL elements have the following structures.

[0129] Example 4A:

[0130] ITO electrode / MoO3 hole injection layer / fluorene layer (undoped) / fluorene layer (DBP doped) / PTCDI-C6 layer / LiF electron injection layer / Al electrode

[0131] Example 4B:

[0132] ITO electrode / MoO3 hole injection layer / fluorene layer (undoped) / fluorene layer (DBP doped) / PTCDI-C13 layer / LiF electron injection layer / Al electrode

[0133] Example 4C:

[0134] ITO electrode / MoO3 hole injection layer / fluorene layer (undoped) / fluorene layer (DBP doped) / C60 layer / LiF electron injection layer / Al electrode

[0135] For the three organic EL elements obtained, the external quantum efficiency (EQE) was measured using the method described above. The results are presented together with the measurement results of the organic EL element in Example 3A. Figure 9 Furthermore, for Examples 3A and 4A-4C, a 60 nm thick undoped rubrene layer was used instead of the "rubrene layer (undoped) / rubrene layer (DBP doped)" to fabricate comparative organic EL devices, and measurements were performed in the same manner. The results are shown below. Figure 9 .like Figure 9As shown, regardless of the type of electron transport layer used, the luminescence efficiency was increased by about 10 times when a red fluorene layer (DBP doped) was used, with the highest luminescence efficiency achieved when a PTCDI-C8 layer was used.

[0136] (Example 5: Study of dopants)

[0137] For the fluorene layer (DBP doped), DCJTB (0.5 vol%) was used instead of DBP as the dopant. Otherwise, the organic EL device was fabricated in the same manner as in Example 3A. The organic EL device has the following structure.

[0138] ITO electrode / MoO3 hole injection layer / fluorene layer (undoped) / fluorene layer (doped with DCJTB) / PTCDI-C8 layer / LiF electron injection layer / Al electrode

[0139] The obtained organic EL element (DCJTB) was subjected to EL emission spectra measured using a high-sensitivity broadband spectrometer (AvaSpec-UV / VIS / NIR, manufactured by Avantes). The results are presented together with those of the organic EL element (DBP) of Example 3A and Comparative Example 1 (rub). Figure 10 (A). As this result indicates, with the use of dopants (DBP or DCJTB), an energy transfer occurs from rubrene to DBP or DCJTB, thereby quenching the luminescence of rubrene and observing luminescence originating from DBP or DCJTB.

[0140] Furthermore, the V-luminance characteristics of the obtained organic EL element (DCJTB) were measured using the method described above, and the results are presented together with the measurement results of the organic EL element (DBP) of Example 3A. Figure 10 (B). For example Figure 10 As shown in (B), the luminescence brightness is high when doped with DBP.

[0141] (Example 6: Study on the film thickness of the electron transport layer)

[0142] The thickness of the PTCDI-C8 layer was changed from 50 nm to 20 nm, and the organic EL device was fabricated in the same manner as in Example 3A.

[0143] For the obtained organic EL element, the V-luminance characteristics were measured using the method described above, and the results are presented together with the measurement results of the organic EL element of Example 3A. Figure 11 .like Figure 11 As shown, even when the film thickness is set to 20 nm, the luminous brightness is high.

[0144] (Example 7: Study on the film thickness of the light-emitting layer)

[0145] The thickness of the rubrene layer (doped with DBP) was changed from 50 nm to 20 nm, 100 nm, 150 nm or 200 nm, and the organic EL device was fabricated in the same manner as in Example 3A.

[0146] For the obtained organic EL element, the V-luminance characteristics were measured using the method described above, and the results are presented together with the measurement results of the organic EL element of Example 3A. Figure 12 (A) The results obtained by measuring the external quantum efficiency (EQE) are shown together with the measurement results of the organic EL element in Example 3A. Figure 12 (B). For example Figure 12 As shown in (A), the luminescence brightness is high regardless of the film thickness. Figure 12 As shown in (B), the EQE is larger (maximum 2.91% @ 30 mA / cm) when the film thickness is thicker (200 nm). 2 ).

[0147] (Example 8: The second part of the study on the first organic semiconductor layer)

[0148] Instead of the PTCDI-C8 layer, an NDI-bis-HFI layer (50 nm) was formed by thermal evaporation. Otherwise, the organic EL element was fabricated in the same manner as in Example 3A. The organic EL element has the following structure.

[0149] ITO electrode / MoO3 hole injection layer / fluorene layer (undoped) / fluorene layer (DBP doped) / NDI-bis-HFI layer / LiF electron injection layer / Al electrode

[0150] For the obtained organic EL element, the V-luminance characteristics were measured using the method described above, and the results are presented together with the measurement results of the organic EL element of Example 3A. Figure 14 .like Figure 14 As demonstrated, the organic EL element of Example 8 can achieve high luminous brightness (380 cd / m) even at a low voltage (1.5V).

[0151] Explanation of reference numerals in the attached figures

[0152] 1…First organic semiconductor layer

[0153] 2…Second organic semiconductor layer

[0154] 3…Electrode 1

[0155] 4…Electrode 2

[0156] 10… Organic EL components

Claims

1. An organic EL device having a plurality of organic semiconductor layers sandwiched between a pair of electrodes, The organic semiconductor layer has a first organic semiconductor layer comprising a first organic semiconductor material, and a second organic semiconductor layer comprising a second organic semiconductor material and a third organic semiconductor material. The first organic semiconductor layer and the second organic semiconductor layer form a bonding surface. At the junction between the first organic semiconductor layer 1 and the second organic semiconductor layer 2, electron (-) / hole (+) pairs form a charge transfer state, i.e., a CT state. The HOMO energy level of the first organic semiconductor material is lower than that of the second organic semiconductor material, and the LUMO energy level of the first organic semiconductor material is lower than that of the second organic semiconductor material. The second organic semiconductor material is a material in which the CT state undergoes charge recombination to generate an excited triplet state, and triplet-triplet annihilation occurs. The excited triplet energy level T1 of the second organic semiconductor material is less than the energy difference between the HOMO energy level of the second organic semiconductor material and the LUMO energy level of the first organic semiconductor material. The energy difference between the HOMO level of the second organic semiconductor material and the LUMO level of the first organic semiconductor material is more than 0.5 eV smaller than the energy difference between the HOMO level and the LUMO level of the second organic semiconductor material. The energy difference between the HOMO and LUMO levels of the third organic semiconductor material is smaller than that between the HOMO and LUMO levels of the second organic semiconductor material. In the second organic semiconductor layer, the second organic semiconductor material is the host material, and the third organic semiconductor material is the dopant. The wavelength at which the intensity of the emission spectrum of the second organic semiconductor material reaches its maximum is closer to the longer wavelength side than the wavelength at which the intensity of the absorption spectrum of the first organic semiconductor material reaches its maximum. The wavelength at which the intensity of the emission spectrum of the third organic semiconductor material reaches its maximum is closer to the longer wavelength side than the wavelength at which the intensity of the emission spectrum of the second organic semiconductor material, which is the host material, reaches its maximum.

2. The organic EL element according to claim 1, wherein The difference between the HOMO energy level of the first organic semiconductor material and the HOMO energy level of the second organic semiconductor material is greater than 0.5 eV.

3. The organic EL element according to claim 1, wherein The difference between the LUMO energy level of the first organic semiconductor material and the LUMO energy level of the second organic semiconductor material is greater than 0.5 eV.

4. The organic EL element according to any one of claims 1 to 3, wherein The energy difference between the excited triplet level T1 of the second organic semiconductor material and 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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