近赤外有機EL素子に関する研究
キーワード：近赤外有機EL
2022.04.

有機発光分子における光物理過程
キーワード：光物理
2016.04.

高性能蛍光有機EL素子に関する研究
キーワード：有機EL
2015.01.

JST ERATO　安達分子エキシトン工学プロジェクト 2013.12～2018.03
2013.12～2019.03, 安達千波矢.

主要著書

主要原著論文

1.	Nishiki Yamada, Hajime Nakanotani, Akitsuna Takagi, Masashi Mamada, Umamahesh Balijapalli, Tatsuya Ichikawa, Eiji Hirata, Shun Kaizu, Akira Tanaka, Kazuichiro Itonaga, Chihaya Adachi, Three-dimensional sensing of surfaces by projection of invisible electroluminescence from organic light-emitting diodes, Science Advances, 10.1126/sciadv.adj6583, 2024.01, Organic light-emitting diodes (OLEDs) that efficiently emit near-infrared (NIR) light and consume little power will create valuable applications for OLEDs beyond just displays. Here, we report such a NIR-OLED with high operational stability that can be used as a light source for three-dimensional sensing of object’s surfaces. Using a narrow-energy-gap material as a host for producing NIR hyperfluorescence system, we fabricated a NIR-OLED exhibiting intense emission at 930 nm with a high external electroluminescence quantum efficiency of more than 1% at a current density of 100 milliamperes per square meter without any degradation even after more than 300 hours of operation. The NIR-OLEDs were integrated with dense complementary metal-oxide semiconductor circuits to make a micro-NIR-OLED projector (0.21 inch, 230,400 pixels). By actively driving the projector on a pixel by pixel and projecting their emission onto objects, we successfully scanned and sensed the surfaces in three dimensions with invisible NIR..
2.	Shintaro Kohata, Hajime Nakanotani, Youhei Chitose, Takuma Yasuda, Youichi Tsuchiya, Chihaya Adachi, Anti-Stokes Luminescence in Multi-Resonance-Type Thermally- Activated Delayed Fluorescence Molecules, Angew. Chem. Int. Ed., doi.org/10.1002/anie.202312326, 2023.09, Photon-upconversion in organic molecular systems is one of the promising technologies for future energy harvesting systems because these systems can generate excitons that possess higher energy than excitation energy. The photon-upconversion caused by absorbing ambient heat as additional energy is particularly interesting because it could ideally provide a light-driving cooling system. However, only a few organic molecular systems have been reported. Here, we report the anti-Stokes photoluminescence (ASPL) derived from hot-band absorption in a series of multi-resonance-type thermally-activated delayed fluorescence (MR-TADF) molecules. The MR-TADF molecules exhibited an anti-Stokes shift of approximately 0.1 eV with a high PL quantum yield in the solution state. The anti-Stokes shift corresponded well to the 1–0 vibration transition from the ground state to the excited singlet state, and we further evaluated a correlation between the activation energy for the ASPL intensity and the TADF process. Our demonstration underlines that MR-TADF molecules have become a novel class of ASPL materials for various future applications, such as light-driving cooling systems..
3.	Yamanaka, Takahiko; Nakanotani, Hajime; Nakamoto, Katsuhiro; Adachi, Chihaya, Electron Lifetime of Over One Month in Disordered Organic Solid-State Films, ADVANCED MATERIALS, 10.1002/adma.202210335, 2023.01, Understanding intrinsic carrier lifetime in disordered organic solid-state semi- conductors is essential for improving device performance in not only mole- cule-based optoelectronic devices such as organic solar cells (OSC) but also photocatalysts used for producing solar fuel cells. Carriers in disordered films are generally thought to have short lifetimes on a scale ranging from nano- seconds to milliseconds. These short carrier lifetimes cause loss of charges in OSCs and low quantum yields in photocatalysts and impede the future application of organic semiconductors to, for example, charge-storage-based memory devices. This study reports an ultralong intrinsic carrier lifetime of more than one month in a disordered film of an organic semiconductor stored at room temperature without external power. This extraordinary life- time, which is several orders of magnitude longer than that generally believed possible in conventional organic semiconductors, arises from carrier stabili- zation by spontaneous orientation polarization, excited spin-triplet recycling, and blocking of recombination processes in disordered films..
4.	Masaki Tanaka, Morgan Auffray, Hajime Nakanotani & Chihaya Adachi , Spontaneous formation of metastable orientation with well-organized permanent dipole moment in organic glassy films, NATURE MATERIALS, 10.1038/s41563-022-01265-7, 21, 7, 819-+, 2022.07, The performance of organic optoelectronic and energy-harvesting devices is largely determined by the molecular orientation and resultant permanent dipole moment, yet this property is difficult to control during film preparation. Here, we demonstrate the active control of dipole direction—that is, vector direction and magnitude—in organic glassy films by physical vapour depo- sition. An organic glassy film with metastable permanent dipole moment orientation can be obtained by utilizing the small surface free energy of a trifluoromethyl unit and intramolecular permanent dipole moment induced by functional groups. The proposed molecular design rule could pave a way toward the formation of spontaneously polarized organic glassy films, leading to improvement in the performance of organic molecular devices..
5.	Yamanaka, Takahiko; Nakanotani, Hajime; Adachi, Chihaya, Significant role of spin-triplet state for exciton dissociation in organic solids, SCIENCE ADVANCES, 10.1126/sciadv.abj9188, 8, 9, 2022.03, Clarification of the role of the spin state that initiates exciton dissociation is critical to attaining a fundamental understanding of the mechanism of organic photovoltaics. Although an excited spin-triplet state with an energy lower than that of excited spin-singlet state is disadvantageous in exciton dissociation, a small electron exchange integral results in small singlet-triplet energy splitting in some material systems. This energy splitting leads to a nearly isoenergetic alignment of both excited states, raising a question about the role of excited spin states in exciton dissociation. Herein, we show that the spin-triplet rather than the spin-singlet plays a critical role in the exciton dissociation that leads to the formation of free carriers. This result indicates that the spin-triplet inherently acts as an intermediate, leading to exciton dissociation. Thus, our demonstration provides a fundamental under- standing of the role of excited spin states of organic molecular systems in photoinduced charge-carrier generation..
6.	Umamahesh Balijapalli, Ryo Nagata, Nishiki Yamada, Hajime Nakanotani, Masaki Tanaka, Anthony D'Aléo, Virginie Placide, Masashi Mamada, Youichi Tsuchiya, Chihaya Adachi, Highly Efficient Near‐Infrared Electrofluorescence from a Thermally Activated Delayed Fluorescence Molecule, Angewandte Chemie International Edition, 10.1002/anie.202016089, 60, 15, 8477-8482, 2021.04, Near-IR organic light-emitting diodes (NIR-OLEDs) are potential light-sources for various sensing applications as OLEDs have unique features such as ultra-flexibility and low-cost fabrication. However, the low external electroluminescence (EL) quantum efficiency (EQE) of NIR-OLEDs is a critical obstacle for potential applications. Here, we demonstrate a highly efficient NIR emitter with thermally activated delayed fluorescence (TADF) and its application to NIR-OLEDs. The NIR-TADF emitter, TPA-PZTCN, has a high photoluminescence quantum yield of over 40 % with a peak wavelength at 729 nm even in a highly doped co-deposited film. The EL peak wavelength of the NIR-OLED is 734 nm with an EQE of 13.4 %, unprecedented among rare-metal-free NIR-OLEDs in this spectral range. TPA-PZTCN can sensitize a deeper NIR fluorophore to achieve a peak wavelength of approximately 900 nm, resulting in an EQE of over 1 % in a TADF-sensitized NIR-OLED with high operational device durability (LT95>600 h.)..
7.	Chin-Yiu Chan, Masaki Tanaka, Yi-Ting Lee, Yiu-Wing Wong, Hajime Nakanotani, Takuji Hatakeyama, Chihaya Adachi, Stable pure-blue hyperfluorescence organic light-emitting diodes with high-efficiency and narrow emission, Nature Photonics, 10.1038/s41566-020-00745-z, 15, 3, 203-207, 2021.03.
8.	Lin Song Cui, Alexander J. Gillett, Shou Feng Zhang, Hao Ye, Yuan Liu, Xian Kai Chen, Ze Sen Lin, Emrys W. Evans, William K. Myers, Tanya K. Ronson, Hajime Nakanotani, Sebastian Reineke, Jean Luc Bredas, Chihaya Adachi, Richard H. Friend, Fast spin-flip enables efficient and stable organic electroluminescence from charge-transfer states, Nature Photonics, 10.1038/s41566-020-0668-z, 14, 10, 636-642, 2020.10, [URL].
9.	Jong Uk Kim, In Seob Park, Chin Yiu Chan, Masaki Tanaka, Youichi Tsuchiya, Hajime Nakanotani, Chihaya Adachi, Nanosecond-time-scale delayed fluorescence molecule for deep-blue OLEDs with small efficiency rolloff, Nature communications, 10.1038/s41467-020-15558-5, 11, 1, 2020.04, [URL].
10.	Thanh Ba Nguyen, Hajime Nakanotani, Takuji Hatakeyama, Chihaya Adachi, The Role of Reverse Intersystem Crossing Using a TADF-Type Acceptor Molecule on the Device Stability of Exciplex-Based Organic Light-Emitting Diodes, Advanced Materials, 10.1002/adma.201906614, 32, 9, 2020.01, [URL], Exciplex system exhibiting thermally activated delayed fluorescence (TADF) holds a considerable potential to improve organic light‐emitting diode (OLED) performances. However, the operational lifetime of current exciplex‐based devices, unfortunately, falls far behind the requirement for commercialization. Herein, rationally choosing a TADF‐type electron acceptor molecule is reported as a new strategy to enhance OLEDs' operating lifetime. A comprehensive study of the exciplex system containing 9,9′,9′′‐triphenyl‐9H,9′H,9′′H‐3,3′:6′,3′′‐tercarbazole (Tris‐PCz) and triazine (TRZ) derivatives clarifies the relationship between unwanted carrier recombination on acceptor molecules, TADF property of acceptors, and the device degradation event. By employing a proposed “exciton recycling” strategy, a threefold increased operational lifetime can be achieved while still maintaining high‐performance OLED properties. In particular, a stable blue OLED that employs this strategy is successfully demonstrated. This research provides an important step for exciplex‐based devices toward the significant improvement of operational stability..
11.	Takahiko Yamanaka, Hajime Nakanotani, Chihaya Adachi, Slow recombination of spontaneously dissociated organic fluorophore excitons, Nature communications, 10.1038/s41467-019-13736-8, 10, 1, 2019.12, [URL], The harvesting of excitons as luminescence by organic fluorophores forms the basis of light-emitting applications. Although high photoluminescence quantum yield is essential for efficient light emission, concentration-dependent quenching of the emissive exciton is generally observed. Here we demonstrate generation and accumulation of concentration-dependent “long-lived” (i.e., over 1 h) photo-generated carriers and the successive release of their energy as electroluminescence in a solid-state film containing a polar fluorophore. While fluorophore excitons are generally believed to be stable because of their high exciton binding energies, our observations show that some of the excitons undergo spontaneous exciton dissociation in a solid-state film by spontaneous orientation polarization even without an external electric field. These results lead to the reconsideration of the meaning of “luminescence quantum yield” for the solid films containing polar organic molecules because it can differ for optical and electrical excitation..
12.	Hiroki Noda, Xian Kai Chen, Hajime Nakanotani, Takuya Hosokai, Momoka Miyajima, Naoto Notsuka, Yuuki Kashima, Jean Luc Brédas, Chihaya Adachi, Critical role of intermediate electronic states for spin-flip processes in charge-transfer-type organic molecules with multiple donors and acceptors, Nature Materials, 10.1038/s41563-019-0465-6, 18, 10, 1084-1090, 2019.10, [URL], Spin-flip in purely organic molecular systems is often described as a forbidden process; however, it is commonly observed and utilized to harvest triplet excitons in a wide variety of organic material-based applications. Although the initial and final electronic states of spin-flip between the lowest singlet and lowest triplet excited state are self-evident, the exact process and the role of intermediate states through which spin-flip occurs are still far from being comprehensively determined. Here, via experimental photo-physical investigations in solution combined with first-principles quantum-mechanical calculations, we show that efficient spin-flip in multiple donor–acceptor charge-transfer-type organic molecular systems involves the critical role of an intermediate triplet excited state that corresponds to a partial molecular structure of the system. Our proposed mechanism unifies the understanding of the intersystem crossing mechanism in a wide variety of charge-transfer-type molecular systems, opening the way to greater control over spin-flip rates..
13.	Hiroki Noda, Hajime Nakanotani, Chihaya Adachi, Highly efficient thermally activated delayed fluorescence with slow reverse intersystem crossing, Chemistry Letters, 10.1246/cl.180813, 48, 2, 126-129, 2019.01, [URL].
14.	Ryo Nagata, Hajime Nakanotani, William J. Potscavage, Chihaya Adachi, Exploiting Singlet Fission in Organic Light-Emitting Diodes, Advanced Materials, 10.1002/adma.201801484, 30, 33, 2018.08, [URL], Harvesting of both triplets and singlets yields electroluminescence quantum efficiencies of nearly 100% in organic light-emitting diodes (OLEDs), but the production efficiency of excitons that can undergo radiative decay is theoretically limited to 100% of the electron–hole pairs. Here, breaking of this limit by exploiting singlet fission in an OLED is reported. Based on the dependence of electroluminescence intensity on an applied magnetic field, it is confirmed that triplets produced by singlet fission in a rubrene host matrix are emitted as near-infrared (NIR) electroluminescence by erbium(III) tris(8-hydroxyquinoline) (ErQ3) after excitonic energy transfer from the “dark” triplet state of rubrene to an “emissive” state of ErQ3, leading to NIR electroluminescence with an overall exciton production efficiency of 100.8%. This demonstration clearly indicates that the harvesting of triplets produced by singlet fission as electroluminescence is possible even under electrical excitation, leading to an enhancement of the quantum efficiency of the OLEDs. Electroluminescence employing singlet fission provides a route toward developing high-intensity NIR light sources, which are of particular interest for sensing, optical communications, and medical applications..
15.	Hiroki Noda, Hajime Nakanotani, Chihaya Adachi, Excited state engineering for efficient reverse intersystem crossing, Science Advances, 10.1126/sciadv.aao6910, 4, 6, 2018.06, [URL], Reverse intersystem crossing (RISC) from the triplet to singlet excited state is an attractive route to harvesting electrically generated triplet excitons as light, leading to highly efficient organic light-emitting diodes (OLEDs). An ideal electroluminescence efficiency of 100% can be achieved using RISC, but device lifetime and suppression of efficiency roll-off still need further improvement. We establish molecular design rules to enhance not only the RISC rate constant but also operational stability under electrical excitation. We show that the introduction of a second type of electron-donating unit in an initially donor-acceptor system induces effective mixing between charge transfer and locally excited triplet states, resulting in acceleration of the RISC rate while maintaining high photoluminescence quantum yield. OLEDs using our designed sky-blue emitter achieved a nearly 100% exciton production efficiency and exhibited not only low efficiency roll-off but also a marked improvement in operational stability..
16.	Takuya Hosokai, Hiroyuki Matsuzaki, Hajime Nakanotani, Katsumi Tokumaru, Tetsuo Tsutsui, Akihiro Furube, Keirou Nasu, Hiroko Nomura, Masayuki Yahiro, Chihaya Adachi, Evidence and mechanism of efficient thermally activated delayed fluorescence promoted by delocalized excited states, Science advances, 10.1126/sciadv.1603282, 3, 5, 2017.05, [URL].
17.	Ryo　Nagata, Nakanotani Hajime, CHIHAYA ADACHI, Near-Infrared Electrophosphorescence up to 1.1 mu m using a Thermally Activated Delayed Fluorescence Molecule as Triplet Sensitizer, ADVANCED MATERIALS, 10.1002/adma.201604265, 29, 5, 2017.02.
18.	Lin-Song Cui, Jong Uk Kim, Hiroko Nomura, Nakanotani Hajime, CHIHAYA ADACHI, Benzimidazobenzothiazole-based Bipolar Hosts to Harvest Nearly All of the Excitons from Blue Delayed Fluorescence and Phosphorescent Organic Light-Emitting Diodes, Angewandte Chemie International Edition, 10.1002/anie.201601136, 2016.05.
19.	Nakanotani Hajime, Taro Furukawa, Kei Morimoto, CHIHAYA ADACHI, Long-range coupling of electron-hole pairs in spatially separated organic donor-acceptor layers, Science Advances , 10.1126/sciadv.1501470, 2016.03, Understanding exciton behaviour in organic semiconductor molecules is crucial for the development of organic semiconductor-based excitonic devices like organic light-emitting diodes and organic solar cells, and the tightly bound electron–hole pair forming an exciton is normally assumed to be localized on an organic semiconducting molecule. Here we report the observation of long-range coupling of electron–hole pairs in spatially separated electron-donating and -accepting molecules across a 10-nm-thick spacer layer. We found that the exciton energy can be tuned over 100 meV and the fraction of delayed fluorescence can be increased by adjusting the spacer layer thickness. Furthermore, increasing the spacer layer thickness produced an organic light-emitting diode with an electroluminescence efficiency nearly eight times higher than that of a device without a spacer layer. Our results demonstrate the first example of a long-range coupled charge-transfer state between electron-donating and -accepting molecules in a working device..
20.	Hajime Nakanotani, Taro Furukawa, Chihaya Adachi, Light Amplification in an Organic Solid-State Film with the Aid of Triplet-to-Singlet Upconversion, ADVANCED OPTICAL MATERIALS, 10.1002/adom.201500236, 3, 10, 1381-1388, 2015.10, Organic laser dyes can be optically excited to achieve light amplification. The buildup of an excessive population of triplets is generally believed to limit the duration of the light amplification because of optical losses through excited-state absorption, so triplet excitons are usually eliminated by using a triplet quencher. However, destroying the triplets limits the electroluminescence efficiency of organic materials under electrical pumping and is counterproductive to realizing electrically-pumped organic laser devices. Herein, we report light amplification that constructively uses triplet states in an optically pumped organic film. In our system, the triplets are converted to singlets by reverse intersystem crossing in a “triplet harvester”, and then the singlets are resonantly transferred to the singlet state of the laser dye. Since this approach permits the constructive use of triplets, we observed not only gain-narrowed emission but also enhanced electroluminescence efficiency, indicating that the threshold current density for lasing might be reduced..
21.	Takahiro Higuchi, Hajime Nakanotani, Chihaya Adachi, High-Efficiency White Organic Light-Emitting Diodes Based on a Blue Thermally Activated Delayed Fluorescent Emitter Combined with Green and Red Fluorescent Emitters, ADVANCED MATERIALS, 10.1002/adma.201404967, 27, 12, 2019-2023, 2015.03, A new device architecture for highly efficient white OLEDs using a molecule exhibiting blue thermally activated delayed fluorescence as a common source of singlet excitons for molecules emitting red and green light based on conventional fluorescence is proposed. The device with an optimum combination of materials shows a maximum external quantum efficiency of over 12% without using phosphorescent emitters..
22.	Shuzo Hirata, Yumi Sakai, Kensuke Masui, Hiroyuki Tanaka, Lee Sae Youn, Nomura Hiroko, Nakamura Nozomi, Yasumatsu Mao, Hajime Nakanotani, Zhang Qisheng, Shizu Katsuyuki, Miyazaki Hiroshi, Chihaya Adachi, Highly efficient blue electroluminescence based on thermally activated delayed fluorescence, NATURE MATERIALS, 10.1038/NMAT4154, 14, 3, 330-336, 2015.03.
23.	Kohei Hayashi, Nakanotani Hajime, Munetomo Inoue, Kou Yoshida, Mikhnenko, Oleksandr, Thuc-Quyen Nguyen, Chihaya Adachi, Suppression of roll-off characteristics of organic light-emitting diodes by narrowing current injection/transport area to 50 nm, Appl. Phys. Lett., 10.1063/1.4913461, 106, 9, 2015.03.
24.	Taro Furukawa, Hajime Nakanotani, Chihaya Adachi, Dual enhancement of electroluminescence efficiency and operational stability by rapid upconversion of triplet excitons in OLEDs, SCIENTIFIC REPORTS, 10.1038/srep08429, 5, 8429, 2015.02, Recently, triplet harvesting via a thermally activated delayed fluorescence (TADF) process has been established as a realistic route for obtaining ultimate internal electroluminescence (EL) quantum efficiency in organic light-emitting diodes (OLEDs). However, the possibility that the rather long transient lifetime of the triplet excited states would reduce operational stability due to an increased chance for unwarranted chemical reactions has been a concern. Herein, we demonstrate dual enhancement of EL efficiency and operational stability in OLEDs by employing a TADF molecule as an assistant dopant and a fluorescent molecule as an end emitter. The proper combination of assistant dopant and emitter molecules realized a “one-way” rapid Förster energy transfer of singlet excitons from TADF molecules to fluorescent emitters, reducing the number of cycles of intersystem crossing (ISC) and reverse ISC in the TADF molecules and resulting in a significant enhancement of operational stability compared to OLEDs with a TADF molecule as the end emitter. In addition, we found that the presence of this rapid energy transfer significantly suppresses singlet-triplet annihilation. Using this finely-tuned rapid triplet-exciton upconversion scheme, OLED performance and lifetime was greatly improved..
25.	Hajime Nakanotani, Takahiro Higuchi, Taro Furukawa, Kensuke Masui, Kei Morimoto, Masaki Numata, Hiroyuki Tanaka, Yuta Sagara, Takuma Yasuda, CHIHAYA ADACHI, High-efficiency organic light-emitting diodes with fluorescent emitters, NATURE COMMUNICATIONS, 10.1038/ncomms5016, 5, 2014.05, Fluorescence-based organic light-emitting diodes have continued to attract interest because of their long operational lifetimes, high colour purity of electroluminescence and potential to be manufactured at low cost in next-generation full-colour display and lighting applications. In fluorescent molecules, however, the exciton production efficiency is limited to 25% due to the deactivation of triplet excitons. Here we report fluorescence-based organic light-emitting diodes that realize external quantum efficiencies as high as 13.4–18% for blue, green, yellow and red emission, indicating that the exciton production efficiency reached nearly 100%. The high performance is enabled by utilization of thermally activated delayed fluorescence molecules as assistant dopants that permit efficient transfer of all electrically generated singlet and triplet excitons from the assistant dopants to the fluorescent emitters. Organic light-emitting diodes employing this exciton harvesting process provide freedom for the selection of emitters from a wide variety of conventional fluorescent molecules..
26.	Nakanotani, Hajime; Masui, Kensuke; Nishide, Junichi; Shibata, Takumi; Adachi, Chihaya, Promising operational stability of high-efficiency organic light-emitting diodes based on thermally activated delayed fluorescence, SCIENTIFIC REPORTS, 10.1038/srep02127, 3, 2013.07.
27.	Hajime Nakanotani, Chihaya Adachi, Amplified Spontaneous Emission and Electroluminescence from Thiophene/Phenylene Co-Oligomer-Doped p-bis(p-Styrylstyryl)Benzene Crystals, ADVANCED OPTICAL MATERIALS, 10.1002/adom.201200066, 1, 6, 422-427, 2013.06.

主要学会発表等

特許出願・取得

所属学会名

学協会役員等への就任

学会大会・会議・シンポジウム等における役割

学術論文等の審査