Organic el element

Abstract

This organic EL element 1 includes a first organic semiconductor layer 13 sandwiched between a pair of electrodes, and a second organic semiconductor layer 14 forming an interface 15 with the first organic semiconductor layer 13. The second organic semiconductor layer 14 includes an organic semiconductor material 17 causing triplet-triplet annihilation, and a guest material 18 emitting light by movement of energy generated by triplet-triplet annihilation by the organic semiconductor material 17. The guest material 18 includes nitrogen (N) as a hetero atom, and the HOMO level of the guest material 18 is -5.22 eV or less.

Description

organic EL element

[0001] The present disclosure relates to an organic electroluminescent device.

[0002] A technology is known in which an organic EL element having an emissive layer and an electron transport layer can emit light by applying a low voltage between a pair of electrodes sandwiching the emissive layer and the electron transport layer (see, for example, Patent Document 1). The organic EL element described in Patent Document 1 has a first organic semiconductor layer (corresponding to an electron transport layer) containing a first organic semiconductor material, and a second organic semiconductor layer (corresponding to an emissive layer) containing a second organic semiconductor material and a guest material and forming a junction surface with the first organic semiconductor layer. The second organic semiconductor material contained in the second organic semiconductor layer is a material that causes triplet-triplet annihilation, and the guest material contained in the second organic semiconductor layer is a material that emits light.

[0003] In the organic EL element described in Patent Document 1, when a voltage is applied between a pair of electrodes, a charge transfer state is formed at the junction surface between the first organic semiconductor layer and the second organic semiconductor layer. The charges formed at the junction surface recombine to generate a triplet state in the second organic semiconductor material. The second organic semiconductor material in which the triplet state is generated undergoes triplet-triplet annihilation, generating a high-energy excited state. Energy transfer occurs from the excited second organic semiconductor material to the guest material, resulting in light emission originating from the guest material.

[0004] The organic EL element described in Patent Document 1 can reduce the voltage applied when light emission originating from the guest material occurs by optimizing the relationship between the LUMO level of the first organic semiconductor material and the HOMO level and excited triplet level of the second organic semiconductor material.

[0005] Furthermore, Non-Patent Document 1 describes an organic EL element that employs TbPe, which has a peak wavelength of 462 nm, as a guest material. By employing TbPe as a guest material, the organic EL element described in Non-Patent Document 1 is able to emit fluorescence having a peak wavelength of 462 nm, which is included in the blue range of 430 nm to 490 nm.

[0006] International Publication No. 2022 / 211041

[0007] “Blue organic light-emitting diode with a turn-on voltage of 1.47 V”, S. Izawa et al., Nature Communications (2023) 14:5494

[0008] However, the fluorescence emitted by TbPe, which is used as a guest material in the organic EL element described in Non-Patent Document 1, has a wide full width at half maximum (FWHM), and is therefore blue light with low color purity.

[0009] The present disclosure is intended to solve such problems, and has an object to provide an organic EL element capable of emitting blue light with high color purity.

[0010] The organic EL element according to the present disclosure is an organic EL element including a first organic semiconductor layer sandwiched between a pair of electrodes and a second organic semiconductor layer forming an interface with the first organic semiconductor layer, wherein the second organic semiconductor layer contains an organic semiconductor material that undergoes triplet-triplet annihilation and a guest material that emits light due to the transfer of energy generated by triplet-triplet annihilation in the organic semiconductor material, the guest material containing nitrogen (N) as a heteroatom, and the HOMO level of the guest material is −5.22 eV or less.

[0011] Furthermore, in the organic EL element according to the present disclosure, the HOMO level of the guest material is preferably lower than the HOMO level of the organic semiconductor material.

[0012] Furthermore, in the organic EL device according to the present disclosure, the guest material is preferably selected from the group consisting of QAO, tbQAO, tbCZ2CO, DICZ, and pf-QAO.

[0013] Furthermore, in the organic EL device according to the present disclosure, the organic semiconductor material is preferably selected from the group consisting of 1,2-ADN and PCAN.

[0014] The organic EL element according to the present disclosure can emit blue light with high color purity.

[0015] 1 is a diagram showing a light-emitting device having an organic EL element according to an embodiment; 2 is a diagram showing the light-emitting mechanism of the organic EL element shown in FIG. 1; and 3 is a diagram showing the energy transition accompanying light emission shown in FIG. 2. (a) is a diagram (part 1) showing the light-emission onset voltages of the organic EL elements according to Example 1 and Comparative Examples 1 to 3, (b) is a diagram (part 2) showing the light-emission onset voltages of the organic EL elements according to Example 1 and Comparative Examples 1 to 3, (c) is a diagram showing the light-emission onset voltages of the organic EL element according to Example 5, and (d) is a diagram showing the light-emission onset voltages of the organic EL element according to Example 6. (a) is a diagram showing the light-emission onset voltages of the organic EL elements according to Examples 1 to 3, and (b) is a diagram showing the light-emission onset voltages of the organic EL elements according to Example 4 and Comparative Examples 4 and 5. (a) is a diagram showing the emission spectra of the organic EL elements according to Example 1 and Comparative Examples 1 to 3, and (b) is a diagram showing the emission spectra of the organic EL elements according to Examples 1 to 3. (a) is a diagram showing the emission spectra of the organic EL devices according to Example 4 and Comparative Examples 4 to 5, (b) is a diagram showing the emission spectrum of the organic EL device according to Example 5, and (c) is a diagram showing the emission spectrum of the organic EL device according to Example 6. (a) is a diagram showing the IP of ferrocene, (b) is a diagram showing the IP of QAO, (c) is a diagram showing the IP of tbCZ2CO, (d) is a diagram showing the IP of tbPe, (e) is a diagram showing the IP of tbQAO, (f) is a diagram showing the IP of DICZ, (g) is a diagram showing the IP of pf-QAO, and (h) is a diagram showing the IP of tb-DABNA. (a) is a diagram showing the color of fluorescence emitted by compounds employed as guest materials in the organic EL devices according to Examples 1 to 6, and (b) is a diagram showing the fluorescence of tbQAO contained as a guest material in the organic EL devices according to Examples 1 and 4. FIG. 1C is a diagram showing the fluorescence of QAO contained as a guest material in the organic EL element according to Example 2, and FIG. 1D is a diagram showing the fluorescence of tbCZ2CO contained as a guest material in the organic EL element according to Example 3.

[0016] The organic EL device according to the present disclosure will be described below with reference to the drawings. However, it should be noted that the technical scope of the present invention is not limited to the embodiments, but extends to the inventions set forth in the claims and their equivalents.

[0017] (Configuration and Function of Organic EL Element According to the Embodiment) FIG. 1 is a diagram showing a light emitting device having an organic EL element according to the embodiment.

[0018] The light emitting device 100 has a power supply 101 and an organic EL element 1, and is applied to organic EL displays, organic EL lighting, digital signage, light sources for photosensors, laser light sources, light sources for optical communications, etc. The power supply 101 supplies a desired current to the organic EL element 1 in response to instructions from a control device (not shown). The configuration and function of the power supply 101 are well known, so a detailed description thereof will be omitted here.

[0019] The organic EL element 1 has a pair of electrodes, a first electrode 11 and a second electrode 12, a first organic semiconductor layer 13 also referred to as an electron transport layer, and a second organic semiconductor layer 14 also referred to as an emitting layer, which forms an interface 15 with the first organic semiconductor layer 13. The organic EL element 1 emits blue light in response to the application of a predetermined light emission start voltage between the first electrode 11 and the second electrode 12, which sandwich the first organic semiconductor layer 13 and the second organic semiconductor layer 14, and the supply of a current I flowing from the second electrode 12 to the first electrode 11 within the organic EL element 1.

[0020] The first electrode 11 is formed of a conductive material such as aluminum, and a first organic semiconductor layer 13 is laminated on one surface. The film thickness of the first electrode 11 is 5 nm or more and 500 nm or less. The second electrode 12 is a thin film formed of a light-transmitting conductive material such as indium tin oxide (ITO), and is formed on a transparent substrate (not shown) such as glass. A second organic semiconductor layer 14 is laminated on the surface of the second electrode 12 opposite to the surface facing the transparent substrate. The film thickness of the second electrode 12 is 5 nm or more and 500 nm or less.

[0021] The first organic semiconductor layer 13 is disposed between the first electrode 11 and the second organic semiconductor layer 14, and contains a first organic semiconductor material 16 that is an acceptor that transports electrons. The film thickness of the first organic semiconductor layer 13 is preferably 0.1 nm or more and 500 nm or less, and more preferably 2 nm or more and 100 nm or less. The first organic semiconductor layer 13 may be formed only from the first organic semiconductor material 16, or may contain materials other than the first organic semiconductor material 16.

[0022] A conventionally known electron transporting material can be used as the first organic semiconductor material 16. Examples of organic semiconductors that can be used as the first organic semiconductor material 16 include the compounds shown below.

[0023]

[0024] More preferred first organic semiconductor materials 16 include derivatives of perylene tetracarboxylic diimide (PTCDI-C6, PTCDI-C8, PTCDI-C13, etc.) and derivatives of naphthalene tetracarboxylic diimide (NDI-HF, NDI-C6, NDI-Cy, etc.).

[0025] The second organic semiconductor layer 14 is disposed between the second electrode 12 and the first organic semiconductor layer 13. The film thickness of the second organic semiconductor layer 14 is preferably 0.1 nm or more and 500 nm or less, and more preferably 2 nm or more and 100 nm or less. The second organic semiconductor layer 14 contains a second organic semiconductor material 17 that is a donor that transports holes and a guest material 18 that emits blue fluorescence. The second organic semiconductor layer 14 may be formed only from the second organic semiconductor material 17 and the guest material 18, or may contain materials other than the second organic semiconductor material 17 and the guest material 18. The second organic semiconductor layer 14 may also have a pair of non-doped layers formed from the second organic semiconductor material 17, and a doped layer that includes the second organic semiconductor material 17 and the guest material 18 and is disposed between the pair of non-doped layers. By including a pair of non-doped and doped layers, the second organic semiconductor layer 14 can improve its luminous efficiency.

[0026] The second organic semiconductor material 17 is a material that causes triplet-triplet annihilation (TTA). Examples of organic semiconductors that can be used as the second organic semiconductor material 17 include the following compounds.

[0027]

[0028] The second organic semiconductor material 17 is preferably a compound having an anthracene ring because it has an emission spectrum suitable for blue light emission, more preferably a compound having an anthracene ring with aromatic substituents at the 9th and 10th positions because it blocks substitution positions with high reactivity, and most preferably a compound selected from the group consisting of 1,2-ADN and PCAN.

[0029] The first organic semiconductor material 16 and the second organic semiconductor material 17 are selected so that the LUMO level of the first organic semiconductor material 16 is lower than the LUMO level of the second organic semiconductor material 17. From the viewpoint of highly preventing electron leakage and further improving luminous efficiency, the difference between the LUMO levels of the first organic semiconductor material 16 and the second organic semiconductor material 17 is preferably 0.5 eV or more. Note that 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.

[0030] Hereinafter, the energy difference between the HOMO level of the second organic semiconductor material 17 and the LUMO level of the first organic semiconductor material 16 is also referred to as the "CT level." The second organic semiconductor material 17 is selected so that the excited triplet level of the second organic semiconductor material 17 is smaller than the CT level. The difference between the excited triplet level of the second organic semiconductor material 17 and the CT level is preferably less than 0.8 eV, more preferably less than 0.65 eV, and even more preferably less than 0.5 eV. By reducing the difference between the excited triplet level of the second organic semiconductor material 17 and the CT level, the fluorescence onset voltage, which is the voltage at which the organic EL element 1 starts to emit fluorescence, can be reduced.

[0031] Hereinafter, the energy difference between the HOMO level and the LUMO level of a material is also referred to as the "band gap" of that material. The CT level is preferably 0.5 eV or more smaller than the band gap of the second organic semiconductor material 17, and more preferably 0.7 eV or more smaller. The upper limit of the energy difference between the CT level and the band gap of the second organic semiconductor material 17 is not particularly limited, but is, for example, 2 eV or less.

[0032] The guest material 18 is a material that emits blue fluorescence. The FWHM of the guest material 18 is preferably 10 nm to 40 nm, and more preferably 10 nm to 30 nm. The guest material 18 emits blue fluorescence due to Förster resonance energy transfer (FRET) from the second organic semiconductor material 17. Organic semiconductors that can be used as the guest material 18 are materials that emit blue light due to FRET from the second organic semiconductor material 17, and are compounds that contain nitrogen (N) as a heteroatom and have a HOMO level of −5.22 eV or less. Examples include QAO, tbQAO, tbCZ2CO, DICZ, and pf-QAO shown below.

[0033]

[0034] When guest material 18 contains nitrogen (N) as a heteroatom, it can emit blue light as fluorescence with a narrow FWHM and high color purity. When guest material 18 has a HOMO level of −5.22 eV or lower, the applied voltage at which fluorescence begins is 1.8 V or lower, enabling low-voltage operation of organic EL element 1. Furthermore, by using a compound whose HOMO level is lower than the HOMO level of the second organic semiconductor material as guest material 18, organic EL element 1 can emit light using general primary and secondary batteries with a rated voltage of 1.5 V.

[0035] FIG. 2 is a diagram showing the light-emitting mechanism of the organic EL element 1, and FIG. 3 is a diagram showing the energy transition accompanying the light emission shown in FIG.

[0036] When a predetermined light emission start voltage is applied between the first electrode 11 and the second electrode 12, holes (+) are injected from the second electrode 12 into the second organic semiconductor layer 14, and electrons (-) are injected from the first electrode 11 into the first organic semiconductor layer 13 in response to the current supplied from the power source 101 to the organic EL element 1. The light emission start voltage applied between the first electrode 11 and the second electrode 12 is a voltage of 1.0 V or more and 2.0 V or less. The holes (+) injected into the second organic semiconductor layer 14 and the electrons (-) injected into the first organic semiconductor layer 13 form a charge transfer (CT) state at the interface 15 between the first organic semiconductor layer 13 and the second organic semiconductor layer 14, with pairs of electrons (-) and holes (+). 25% of the electrons (-) are in a singlet CT state (CT1), and 75% of the electrons (-) are in a triplet CT state (CT3). Charge recombination in the triplet CT state generates the second organic semiconductor material 17 in an excited triplet state (T1) in the second organic semiconductor layer 14. Triplet-triplet annihilation (TTA) of the second organic semiconductor material 17 in the excited triplet state (T1) generates the second organic semiconductor material 17 in a high-energy excited state (S1).

[0037] The second organic semiconductor material 17 in the high-energy excited state (S1) transfers energy by FRET to the guest material 18. The guest material 18 to which the energy has been transferred is in the high-energy excited state (S1) and emits blue light as fluorescence.

[0038] (Method of Manufacturing an Organic EL Element According to an Embodiment) First, in a substrate preparation step, a substrate having a transparent second electrode 12 made of ITO or the like formed on one surface is prepared. The substrate having the second electrode 12 formed on one surface is a transparent substrate, including a glass substrate, a quartz substrate, a sapphire substrate, a plastic substrate, and a film substrate. Next, in a second organic semiconductor layer lamination step, a second organic semiconductor layer 14 is laminated on the second electrode 12. In a first organic semiconductor layer lamination step, a first organic semiconductor layer 13 is laminated on the second organic semiconductor layer 14. The first organic semiconductor layer 13 and the second organic semiconductor layer 14 are laminated by a known lamination method such as vacuum deposition, sputtering, chemical vapor deposition, vapor deposition polymerization, spin coating, blade coating, bar coating, dip coating, or laminating. Then, in a first electrode lamination step, a first electrode 11 is laminated on the first organic semiconductor layer 13. The first electrode 11 is laminated by a known lamination method such as vacuum deposition, sputtering, or plating, to manufacture the organic EL element 1 .

[0039] (Operation and Effect of Organic EL Element According to the Embodiment) The organic EL element 1 can emit blue light with high color purity by the guest material 18 emitting blue fluorescence with a narrow FWHM.

[0040] Furthermore, the organic EL element 1 can operate at a low voltage by setting the HOMO level of the guest material 18 to −5.22 eV or lower. Furthermore, the organic EL element 1 can emit light using general primary and secondary batteries with a rated voltage of 1.5 V by setting the HOMO level of the guest material 18 to be lower than the HOMO level of the second organic semiconductor material 17.

[0041] (Modifications of Organic EL Element According to Embodiment) The organic EL element 1 includes a pair of electrodes, namely, a first electrode 11 and a second electrode 12, a first organic semiconductor layer 13, and a second organic semiconductor layer 14. However, the organic EL element according to the embodiment may further include other organic semiconductor layers or inorganic compound layers. For example, the organic EL element according to the embodiment may further include a hole transport layer that is disposed between the second organic semiconductor layer 14 and the second electrode 12 and transports holes. Furthermore, the organic EL element according to the embodiment may further include, in addition to the hole transport layer, a hole injection layer that is disposed between the hole transport layer and the second electrode 12 and injects holes. Furthermore, the organic EL element according to the embodiment may further include an electron injection layer that is disposed between the first organic semiconductor layer 13 and the first electrode 11 and injects electrons.

[0042] In the organic EL element 1, the first organic semiconductor layer 13 is an electron transport layer, but in the organic EL element according to the embodiment, the first organic semiconductor layer 13 may be a hole blocking layer. In the organic EL element according to the embodiment, the first organic semiconductor layer, which is a hole blocking layer, may also function as an electron injection layer, and the light-emitting layer, which is the second organic semiconductor layer, may also function as a hole transport layer.

[0043] The organic EL element 1 according to the embodiment may have multiple layers containing the same organic semiconductor material. For example, when the second organic semiconductor layer contains 1,2-ADN as the second organic semiconductor material, the organic EL element according to the embodiment may have a hole-blocking layer formed of 1,2-ADN.

[0044] (Method for measuring light-emitting characteristics) The light luminance, luminance intensity, and chromaticity were measured for each of the organic EL elements according to the examples and the organic EL elements according to the comparative examples. The luminance was measured using a source measure unit (B2902A, manufactured by Keysight Technologies) and a luminance meter (BM-9, manufactured by Topcom). The luminance intensity was measured using a spectrofluorometer (Fluorolog, manufactured by HORIBA). The chromaticity was measured using a spectrophotometer (C-7000, manufactured by Sekonic Corporation). Note that the "%" in the guest molecule content refers to volume %.

[0045] (Method of Measuring Energy Levels) The HOMO level, LUMO level, and band gap of a compound were calculated from the ionization potential (IP) measured by cyclic voltammetry (CV) using the following equations (1) and (2). CV was performed using tetrabutylammonium hexafluorophosphate (C 16 H 36 The CV was carried out at 25°C using dimethyl carbonate (DMC) containing 0.1 M of FNP. The working electrode was a platinum disk electrode, the counter electrode was a platinum wire electrode, and the reference electrode was an Ag / Ag electrode. + It is an electrode.

[0046]

[0047] Here, E HOMO indicates the HOMO level, and E LUMO denotes the LUMO level, Eg denotes the band gap, e denotes the elementary charge, and φ OX indicates the oxidation onset potential, and φ(FC + / FC) indicates the redox potential of ferrocene.

[0048] (Organic EL element according to Example 1) The organic EL element according to Example 1 is formed by forming a MoO ３ The layer was formed by sequentially stacking a hole injection layer made of pyhpp2 (2,6-Bis(1,3,4,6,7,8-tetrahydro-2H-pyrimido[1,2-a]pyrimidin-1-yl)pyridine), a light-emitting layer corresponding to the second organic semiconductor layer, an electron transport layer corresponding to the first organic semiconductor layer, an electron injection layer made of pyhpp2 (2,6-Bis(1,3,4,6,7,8-tetrahydro-2H-pyrimido[1,2-a]pyrimidin-1-yl)pyridine), and an Al electrode. The hole injection layer, light-emitting layer, electron transport layer, electron injection layer, and Al electrode were deposited in a vacuum deposition system at a concentration of 1×10 -4 The organic EL device was laminated by thermal evaporation under high vacuum of 100 Pa. The formed organic EL device was encapsulated in a glove box with a glass substrate and epoxy resin. The ITO-coated glass substrate was manufactured by Technoprint Co., Ltd., and the ITO thickness was 150 nm and the sheet resistance was 10.3 Ω sq. -1The hole injection layer had a thickness of 10 nm and was formed at a film formation rate of 0.01 nm / s. The Al electrode had a thickness of 70 nm.

[0049] The light-emitting layer includes a pair of non-doped layers having 1,2-ADN as a second organic semiconductor material, and a doped layer having 1,2-ADN as a second organic semiconductor material and tbQAO as a guest material, disposed between the pair of non-doped layers. Each of the pair of non-doped layers had a thickness of 5 nm and was deposited at a deposition rate of 0.1 nm / s. The doped layer had a tbQAO content of 1%. The doped layer had a thickness of 50 nm and was deposited at a deposition rate of 0.1 nm / s. The electron transport layer included NDI-HF as a first organic semiconductor material. The electron transport layer had a thickness of 50 nm and was deposited at a deposition rate of 1 nm / s. The electron injection layer had a thickness of 1 nm and was deposited at a deposition rate of 0.01 nm / s.

[0050] (Organic EL Devices of Examples 2 to 6) The organic EL device of Example 2 differs from the organic EL device of Example 1 in that it uses QAO as the guest material instead of tbQAO. The content of QAO in the doped layer is 1%. The thickness of the doped layer is 50 nm and it was formed at a film formation rate of 0.1 nm / s. The organic EL device of Example 3 differs from the organic EL device of Example 1 in that it uses tbCZ2CO as the guest material instead of tbQAO. The content of tbCZ2CO in the doped layer is 1%. The thickness of the doped layer is 50 nm and it was formed at a film formation rate of 0.1 nm / s. The organic EL device of Example 4 differs from the organic EL device of Example 1 in that it uses PCAN as the second organic semiconductor material instead of 1,2ADN. The thickness of each of the pair of non-doped layers is 5 nm and they were formed at a film formation rate of 0.1 nm / s. The organic EL element of Example 5 differs from the organic EL element of Example 1 in that it uses DICZ as the guest material instead of tbQAO. The content of DICZ in the doped layer is 0.1%. The thickness of the doped layer is 50 nm, and it was formed at a film formation rate of 0.1 nm / s. The organic EL element of Example 6 differs from the organic EL element of Example 1 in that it uses pf-QAO as the guest material instead of tbQAO. The content of pf-QAO in the doped layer is 1%. The thickness of the doped layer is 50 nm, and it was formed at a film formation rate of 0.1 nm / s.

[0051] (Organic EL Elements According to Comparative Examples 1 to 5) The organic EL element according to Comparative Example 1 differs from the organic EL element according to Example 1 in that the emitting layer does not contain a guest material. The thickness of the emitting layer was 50 nm, and the layer was formed at a deposition rate of 0.1 nm / s. The organic EL element according to Comparative Example 2 differs from the organic EL element according to Example 1 in that tbPe is used as the guest material instead of tbQAO. The content of tbPe in the doped layer is 1%. The thickness of the doped layer was 50 nm, and the layer was formed at a deposition rate of 0.1 nm / s. The organic EL element according to Comparative Example 3 differs from the organic EL element according to Example 1 in that tbDABNA is used as the guest material instead of tbQAO. The content of tbPe in the doped layer is 1%. The thickness of the doped layer was 50 nm, and the layer was formed at a deposition rate of 0.1 nm / s. The organic EL element according to Comparative Example 4 differs from the organic EL element according to Comparative Example 2 in that it has PCAN as the second organic semiconductor material instead of 1,2ADN. The pair of non-doped layers each had a thickness of 5 nm and were formed at a film formation rate of 0.1 nm / s. The organic EL element according to Comparative Example 5 differs from the organic EL element according to Comparative Example 3 in that it has PCAN as the second organic semiconductor material instead of 1,2ADN. The pair of non-doped layers each had a thickness of 5 nm and were formed at a film formation rate of 0.1 nm / s.

[0052] (Evaluation of Light-Emitting Diodes and Blue Light-Emitting Characteristics) FIG. 4( a) is a diagram (part 1) showing the light-emitting voltages of the organic EL elements according to Example 1 and Comparative Examples 1 to 3, FIG. 4( b) is a diagram (part 2) showing the light-emitting voltages of the organic EL elements according to Example 1 and Comparative Examples 1 to 3, FIG. 4( c) is a diagram showing the light-emitting voltages of the organic EL element according to Example 5, and FIG. 4( d) is a diagram showing the light-emitting voltages of the organic EL element according to Example 6. FIG. 5( a) is a diagram showing the light-emitting voltages of the organic EL elements according to Examples 1 to 3, and FIG. 5( b) is a diagram showing the light-emitting voltages of the organic EL elements according to Example 4 and Comparative Examples 4 to 5. In FIGS. 4( a), 4(b), 4(c), 4(d), 5(a), and 5(b), the horizontal axis represents the applied voltage, and the vertical axis represents the luminance. In Fig. 4(a), C101 shows the luminance characteristics of the organic EL element according to Example 1, C102 shows the luminance characteristics of the organic EL element according to Comparative Example 1, C103 shows the luminance characteristics of the organic EL element according to Comparative Example 2, and C104 shows the luminance characteristics of the organic EL element according to Comparative Example 3. In Fig. 4(b), C201 shows the luminance characteristics of the organic EL element according to Example 1, C202 shows the luminance characteristics of the organic EL element according to Comparative Example 1, C203 shows the luminance characteristics of the organic EL element according to Comparative Example 2, and C204 shows the luminance characteristics of the organic EL element according to Comparative Example 3. In Fig. 4(c), C105 shows the luminance characteristics of the organic EL element according to Example 5, and in Fig. 4(d), C106 shows the luminance characteristics of the organic EL element according to Example 6. In Fig. 5(a), C301 shows the luminance characteristics of the organic EL element according to Example 1, C302 shows the luminance characteristics of the organic EL element according to Example 2, and C303 shows the luminance characteristics of the organic EL element according to Example 3. In FIG. 5( b), C401 indicated by a circle represents the luminance characteristics of the organic EL element according to Example 4, C402 indicated by a triangle represents the luminance characteristics of the organic EL element according to Comparative Example 4, and C403 indicated by a diamond represents the luminance characteristics of the organic EL element according to Comparative Example 5.

[0053] The organic EL elements according to Examples 1 to 3 had a light emission start voltage of 1.4 [V], the organic EL element according to Example 4 had a light emission start voltage of 1.6 [V], the organic EL element according to Example 5 had a light emission start voltage of 1.5 [V], and the organic EL element according to Example 6 had a light emission start voltage of 1.4 [V]. The organic EL element according to Comparative Example 1 had a light emission start voltage of 1.4 [V], the organic EL element according to Comparative Example 2 had a light emission start voltage of 1.1 [V], and the organic EL element according to Comparative Example 3 had a light emission start voltage of 2.5 [V]. The organic EL element according to Comparative Example 4 had a light emission start voltage of 1.5 [V], and the organic EL element according to Comparative Example 5 had a light emission start voltage of 2.0 [V].

[0054] FIG. 6(a) shows the emission spectra of the organic EL elements according to Example 1 and Comparative Examples 1 to 3. FIG. 6(b) shows the emission spectra of the organic EL elements according to Examples 1 to 3. FIG. 7(a) shows the emission spectra of the organic EL elements according to Example 4 and Comparative Examples 4 to 5. FIG. 7(b) shows the emission spectrum of the organic EL element according to Example 5. FIG. 7(c) shows the emission spectrum of the organic EL element according to Example 6. In FIGS. 6(a), 6(b), 7(a), 7(b), and 7(c), the horizontal axis represents wavelength, and the vertical axis represents normalized photon count. In FIG. 6(a), C501 represents the photon count of the organic EL element according to Example 1, C502 represents the photon count of Comparative Example 1, C503 represents the photon count of the organic EL element according to Comparative Example 2, and C504 represents the photon count of the organic EL element according to Comparative Example 3. In Fig. 6(b), C601 indicates the photon number of the organic EL element according to Example 1, C602 indicates the photon number of the organic EL element according to Example 2, and C603 indicates the photon number of the organic EL element according to Example 3. In Fig. 7(a), C701 indicates the EL intensity of the organic EL element according to Example 4, C702 indicates the EL intensity of the organic EL element according to Comparative Example 4, and C703 indicates the EL intensity of the organic EL element according to Comparative Example 5. In Fig. 7(b), C801 indicates the photon number of the organic EL element according to Example 5. In Fig. 7(c), C901 indicates the photon number of Example 6.

[0055] The peak wavelength of the organic EL element according to Example 1 is 473 nm, and the peak wavelength of the organic EL element according to Example 2 is 458 nm. The peak wavelength of the organic EL element according to Example 3 is 447 nm, the peak wavelength of the organic EL element according to Example 4 is 473 nm, the peak wavelength of the organic EL element according to Example 5 is 450 nm, and the peak wavelength of the organic EL element according to Example 6 is 473 nm. The peak wavelength of the organic EL element according to Comparative Example 1 is 450 nm, the peak wavelength of the organic EL element according to Comparative Example 2 is 462 nm, and the peak wavelength of the organic EL element according to Comparative Example 3 is 462 nm. The peak wavelength of the organic EL element according to Comparative Example 4 is 462 nm, and the peak wavelength of the organic EL element according to Comparative Example 5 is 462 nm.

[0056] The FWHM of the organic EL element according to Example 1 is 27 nm, and the FWHM of the organic EL element according to Example 2 is 34 nm. The FWHM of the organic EL element according to Example 3 is 20 nm, the FWHM of the organic EL element according to Example 4 is 27 nm, the FWHM of the organic EL element according to Example 5 is 13 nm, and the FWHM of the organic EL element according to Example 6 is 31 nm. The FWHM of the organic EL element according to Comparative Example 1 is 55 nm, the FWHM of the organic EL element according to Comparative Example 2 is 46 nm, and the FWHM of the organic EL element according to Comparative Example 3 is 27 nm. The FWHM of the organic EL element according to Comparative Example 4 is 46 nm, and the FWHM of the organic EL element according to Comparative Example 5 is 27 nm.

[0057] Table 1 shows the light-emitting characteristics of the organic EL elements according to Examples 1 to 6 and Comparative Examples 1 to 5. In Table 1, the "Second Organic Semiconductor Material" column shows the compound contained as the second organic semiconductor material in each of the organic EL elements according to Examples 1 to 6 and Comparative Examples 1 to 5. The "Guest Material" column shows the compound contained as the guest material in each of the organic EL elements according to Examples 1 to 6 and Comparative Examples 1 to 5. The "Light Emission Start Voltage" column includes the measured light-emission start voltage and its evaluation, the "Peak Wavelength" column includes the measured peak wavelength and its evaluation, and the "FWHM" column includes the measured FWHM and its evaluation. The "Evaluation" column in the "Light Emission Start Voltage" column indicates "Good" when the measured light-emission start voltage is less than 1.8 V, and indicates "Poor" when the measured light-emission start voltage is 1.8 V or more. The "Evaluation" column of the "Peak Wavelength" column is marked with "Good" when the peak wavelength is within the range of 430 nm to 490 nm, and marked with "Poor" when the peak wavelength is outside the range of 430 nm to 490 nm. The "Evaluation" column of the "FWHM" column is marked with "Good" when the FWHM is 40 nm or less, and marked with "Poor" when the FWHM exceeds 40 nm. The "Evaluation Result" column is marked with "Good" when the "Evaluation" columns of the "Light Emission Start Voltage", "Peak Wavelength", and "FWHM" columns are all marked with "Good", and marked with "Poor" when any of the "Evaluation" columns of the "Light Emission Start Voltage", "Peak Wavelength", and "FWHM" columns is marked with "Poor".

[0058]

[0059] The organic EL elements according to Examples 1 to 6 and Comparative Examples 1, 2, and 4 all had a light emission starting voltage of less than 1.8 V, and the "evaluation" column in the "light emission starting voltage" column was marked "Good." On the other hand, the organic EL elements according to Comparative Examples 3 and 5 had a light emission starting voltage of 1.8 V or more, and the "evaluation" column in the "light emission starting voltage" column was marked "Poor."

[0060] Table 2 shows the HOMO levels, LUMO levels, and band gaps of the second organic semiconductor materials and guest materials of the organic EL devices according to Examples 1 to 6 and Comparative Examples 1 to 5. In Table 2, the "HOMO" column shows the HOMO levels, the "LUMO" column shows the LUMO levels, and the "Eg" column shows the band gaps.

[0061]

[0062] The HOMO and LUMO levels shown in Table 2 were calculated using equations (1) and (2).

[0063] FIG. 8(a) shows the IP of ferrocene, FIG. 8(b) shows the IP of tbQAO, and FIG. 8(c) shows the IP of QAO. FIG. 8(d) shows the IP of tbCZ2CO, FIG. 8(e) shows the IP of tbPe, FIG. 8(f) shows the IP of DICZ, FIG. 8(g) shows the IP of pf-QAO, and FIG. 8(h) shows the IP of tb-DABNA. In FIGS. 8(a) to 8(h), the horizontal axis represents potential [eV], and the vertical axis represents current in arbitrary units. IP was measured using a HZ-5000 manufactured by Hokuto Denko Corporation. For the IPs of 1,2-ADN and PCAN, see Fig. S6 in "Electron Transfer Enhanced by Minimal Energy Offset at Organic Semiconductor Interface" (Hiroto Iwasaki et al., [online], ChemRxiv, Feb 28, 2024, Version 1, [retrieved May 29, 2024], Internet <URL: https: / / doi.org / 10.26434 / chemrxiv-2024-fx03q>).

[0064] The oxidation peak potential of ferrocene is 0.63535 eV, and the reduction peak potential of ferrocene is -0.33417 eV. The oxidation onset potential of tbQAO is 1.23493 eV, the oxidation onset potential of QAO is 1.41484 eV, and the oxidation onset potential of tbCZ2CO is 1.31485 eV. The oxidation onset potential of tbPe is 0.57523 eV, the oxidation onset potential of DICZ is 0.92998 eV, the oxidation onset potential of pf-QAO is 1.11517 eV, and the oxidation onset potential of tb-DABNA is 0.43545 eV.

[0065] The organic EL elements according to Comparative Examples 3 and 5, which employ tbDABNA as the guest material, have light-emission onset voltages of 2.5 V, 2.0 V, and 1.8 V or higher, respectively, and do not exhibit low-voltage light-emission characteristics. The HOMO level of tbDABNA is higher than the HOMO levels of the second organic semiconductor materials 1,2-ADN and PCAN, and tbDABNA employed as the guest material acts as a hole trap that traps holes injected into the second organic semiconductor layer 14. The organic EL elements according to Comparative Examples 3 and 5, which employ tbDABNA as the guest material, do not exhibit low-voltage light-emission characteristics because holes injected into the second organic semiconductor layer 14 are trapped by tbDABNA.

[0066] On the other hand, the organic EL elements according to Comparative Examples 2 and 4, which employ tbPe as the guest material, have light emission start voltages of 1.1 V and 1.6 V, respectively, less than 1.8 V, and thus exhibit low-voltage light emission characteristics. The HOMO level of tbPe is higher than the HOMO levels of the second organic semiconductor materials 1,2-ADN and PCAN. However, unlike tbDABNA, tbPe employed as the guest material does not trap holes injected into the second organic semiconductor layer 14. The organic EL elements according to Comparative Examples 2 and 4 exhibit low-voltage light emission characteristics because tbPe does not trap holes injected into the second organic semiconductor layer 14. Since the organic EL elements according to Comparative Examples 2 and 4 employ tbPe, which has a HOMO level of −5.22 eV, as the guest material, exhibit low-voltage light emission characteristics, the organic EL elements can exhibit low-voltage light emission characteristics by setting the HOMO level of the guest material to −5.22 eV or lower.

[0067] The organic EL devices according to Examples 1 to 6, which employ tbQAO, QAO, tbCZ2CO, DICZ, and pf-QAO as guest materials, have a light-emission onset voltage of 1.4 V to 1.6 V, which is less than 1.8 V, and exhibit low-voltage light-emission characteristics. The HOMO levels of the guest materials tbQAO, QAO, tbCZ2CO, DICZ, and pf-QAO are lower than the HOMO levels of the second organic semiconductor materials 1,2-ADN and PCAN. Because the HOMO levels of the guest materials are lower than the HOMO level of the second organic semiconductor material, holes injected into the second organic semiconductor layer 14 are not trapped by the guest material, and the organic EL devices according to Examples 1 to 6 exhibit low-voltage light-emission characteristics. The organic EL elements according to Examples 1 to 6 have a light emission start voltage of 1.4 V to 1.6 V, and can achieve low-voltage operation at a voltage at which light can be emitted using general primary and secondary batteries with a rated voltage of 1.5 V.

[0068] The peak wavelengths of the organic EL elements according to Examples 1 to 6 and Comparative Examples 1 to 5 are in the range of 430 nm to 490 nm, and the "Evaluation" column in the "Peak Wavelength" column is marked with "Good." Since the peak wavelengths of the organic EL elements according to Examples 1 to 6 and Comparative Examples 1 to 5 are in the range of 430 nm to 490 nm, the organic EL elements according to Examples 1 to 6 and Comparative Examples 1 to 5 emit blue fluorescence.

[0069] The organic EL elements according to Examples 1 to 6 and Comparative Examples 3 and 5 had FWHMs of 40 nm or less, and the "Evaluation" column in the "FWHM" column was marked "Good." The organic EL elements according to Examples 1 to 6 and Comparative Examples 3 and 5 had peak wavelengths in the range of 430 nm to 490 nm and FWHMs of 40 nm or less, and therefore emitted blue fluorescence with high color purity.

[0070] The organic EL elements according to Comparative Examples 1, 2, and 4 had FWHMs exceeding 40 nm, and the "Evaluation" column in the "FWHM" column was marked with "X." The organic EL elements according to Comparative Examples 1, 2, and 4 had peak wavelengths within the range of 430 nm to 490 nm, but because their FWHMs exceeded 40 nm, they emitted blue fluorescence with low color purity.

[0071] Fig. 9(a) is a diagram showing the colors of fluorescence emitted by compounds employed as guest materials in the organic EL devices according to Examples 1 to 6, and Fig. 9(b) is a diagram showing the fluorescence of tbQAO contained as a guest material in the organic EL devices according to Examples 1 and 4. Fig. 9(c) is a diagram showing the fluorescence of QAO contained as a guest material in the organic EL device according to Example 2, and Fig. 9(d) is a diagram showing the fluorescence of tbCZ2CO contained as a guest material in the organic EL device according to Example 3. Fig. 9(a) is a CIE xy chromaticity diagram defined by JIS Z8110.

[0072] The tbQAO contained as a guest material in the organic EL devices according to Examples 1 and 4 emits fluorescence having a dominant wavelength of 473 nm, and the QAO contained as a guest material in the organic EL device according to Example 2 emits fluorescence having a dominant wavelength of 458 nm. The tbCZ2CO contained as a guest material in the organic EL device according to Example 3 emits fluorescence having a dominant wavelength of 447 nm. The DICZ contained as a guest material in the organic EL device according to Example 5 emits fluorescence having a dominant wavelength of 450 nm. The pf-QAO contained as a guest material in the organic EL device according to Example 6 emits fluorescence having a dominant wavelength of 473 nm. The dominant wavelengths of tbQAO, pf-QAO, QAO, tbCZ2CO, and DICZ employed as guest materials in the organic EL elements according to Examples 1 to 6 are within the blue range of 430 nm to 490 nm, and therefore it was confirmed that all of the organic EL elements according to Examples 1 to 6 emit blue fluorescence.

Claims

1. An organic EL device comprising a first organic semiconductor layer sandwiched between a pair of electrodes and a second organic semiconductor layer forming an interface with the first organic semiconductor layer, wherein the second organic semiconductor layer contains an organic semiconductor material that undergoes triplet-triplet annihilation and a guest material that emits light due to the transfer of energy generated by triplet-triplet annihilation in the organic semiconductor material, wherein the guest material contains nitrogen (N) as a heteroatom, and the HOMO level of the guest material is -5.22 eV or less.

2. The organic EL device according to claim 1, wherein the HOMO level of the guest material is lower than the HOMO level of the organic semiconductor material.

3. The organic EL device according to claim 2, wherein the guest material is selected from the group consisting of QAO, tbQAO, tbCZ2CO, DICZ, and pf-QAO.

4. The organic EL device according to any one of claims 1 to 3, wherein the organic semiconductor material is selected from the group consisting of 1,2-ADN and PCAN.

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