||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, A 10-fold improvement in the external electroluminescence quantum efficiency of NIR electrophosphorescence compared to the devices with conventional fluorescent materials as host by applying the concept of thermally activated delayed fluorescence to achieve an emitter layer capable of efficient spin conversion is demonstrated. This device architecture can maximize the performance of an organic-semiconductor-based NIR light source..
||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..
||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.
||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.
||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.
||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..
||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..
||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..
||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.
||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.