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Nakanotani Hajime Last modified date:2019.12.05

Graduate School
Undergraduate School
Other Organization
Administration Post

Academic Degree
Ph.D. in Engineering
Field of Specialization
Organic Electronics
Outline Activities
Researches on organic optoelectronic materials and devices. We develop organic electronic devices such as organic light emitting diode (OLED), organic solar cells, organic transistors, organic laser diodes. We design and snthesis novel electronic materials and also clarify the device physics.
Research Interests
  • Photophysical process in organic semiconducting molecules
    keyword : Photophysics
  • High efficient fluorescence organic light-emitting diodes
    keyword : organic light-emitting diodes
Academic Activities
1. 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, 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..
2. 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.
3. 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, 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..
4. Hiroki Noda, Hajime Nakanotani, Chihaya Adachi, Excited state engineering for efficient reverse intersystem crossing, Science Advances, 10.1126/sciadv.aao6910, 4, 6, 2018.06, 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..
5. 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.
6. 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.
7. 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..
8. 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.
9. 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.
10. 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.
11. 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..
12. 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..
13. 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..
14. 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.
15. 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.