||Chuanjiang Qin, Toshinori Matsushima, William J. Potscavage, Atula S. D. Sandanayaka, Matthew R. Leyden, Fatima Bencheikh, Kenichi Goushi, Fabrice Mathevet, Benoit Heinrich, Go Yumoto, Yoshihiko Kanemitsu, Chihaya Adachi, Triplet management for efficient perovskite light-emitting diodes, NATURE PHOTONICS, 10.1038/s41566-019-0545-9, 14, 2, 70-+, 2020.02, Careful harvesting of triplet excitons allows the realization of efficient green-emitting quasi-2D perovskite LEDs.Perovskite light-emitting diodes are promising for next-generation lighting and displays because of their high colour purity and performance(1). Although the management of singlet and triplet excitons is fundamental to the design of efficient organic light-emitting diodes, the nature of how excitons affect performance is still not clear in perovskite(2-4) and quasi-two-dimensional (2D) perovskite-based devices(5-9). Here, we show that triplet excitons are key to efficient emission in green quasi-2D perovskite devices and that quenching of triplets by the organic cation is a major loss path. Employing an organic cation with a high triplet energy level (phenylethylammonium) in a quasi-2D perovskite based on formamidinium lead bromide yields efficient harvesting of triplets. Furthermore, we show that upconversion of triplets to singlets can occur, making 100% harvesting of electrically generated excitons potentially possible. The external quantum and current efficiencies of our green (527 nm) devices reached 12.4% and 52.1 cd A(-1), respectively..
||Ryota Ieuji, Kenichi Goushi, Chihaya Adachi, Triplet-triplet upconversion enhanced by spin-orbit coupling in organic light-emitting diodes, NATURE COMMUNICATIONS, 10.1038/s41467-019-13044-1, 10, 1, 2019.11, Triplet-triplet upconversion, in which two triplet excitons are converted to one singlet exciton, is a well-known approach to exceed the limit of electroluminescence quantum efficiency in conventional fluorescence-based organic light-emitting diodes. Considering the spin multiplicity of triplet pairs, upconversion efficiency is usually limited to 20%. Although this limit can be exceeded when the energy of a triplet pair is lower than that of a second triplet excited state, such as for rubrene, it is generally difficult to engineer the energy levels of higher triplet excited states. Here, we investigate the upconversion efficiency of a series of new anthracene derivatives with different substituents. Some of these derivatives show upconversion efficiencies close to 50% even though the calculated energy levels of the second triplet excited states are lower than twice the lowest triplet energy. A possible upconversion mechanism is proposed based on the molecular structures and quantum chemical calculations..
||Atula S. D. Sandanayaka, Toshinori Matsushima, Fatima Bencheikh, Shinobu Terakawa, William J. Potscavage, Chuanjiang Qin, Takashi Fujihara, Kenichi Goushi, Jean-Charles Ribierre, Chihaya Adachi, Indication of current-injection lasing from an organic semiconductor, APPLIED PHYSICS EXPRESS, 10.7567/1882-0786/ab1b90, 12, 6, 2019.06, In this study, we investigate the lasing properties of 4,4'-bis[(N-carbazole) styryl] biphenyl thin films under electrical pumping. The electroluminescent devices incorporate a mixed-order distributed feedback SiO2 grating into an organic light-emitting diode structure and emit blue lasing. The results provide an indication of lasing by direct injection of current into an organic thin film through selection of a high-gain organic semiconductor showing clear separation of the lasing wavelength from significant triplet and polaron absorption and design of a proper feedback structure with low losses at high current densities. This study represents an important advance toward a future organic laser diode technology. (C) 2019 The Japan Society of Applied Physics.
||Atula S. D. Sandanayaka, Toshinori Matsushima, Fatima Bencheikh, Kou Yoshida, Munetomo Inoue, Takashi Fujihara, Kenichi Goushi, Jean-Charles Ribierre, Chihaya Adachi, Toward continuous-wave operation of organic semiconductor lasers, SCIENCE ADVANCES, 10.1126/sciadv.1602570, 3, 4, e1602570, 2017.04, The demonstration of continuous-wave lasing from organic semiconductor films is highly desirable for practical applications in the areas of spectroscopy, data communication, and sensing, but it still remains a challenging objective. We report low-threshold surface-emitting organic distributed feedback lasers operating in the quasicontinuous-wave regime at 80 MHz as well as under long-pulse photoexcitation of 30 ms. This outstanding performance was achieved using an organic semiconductor thin film with high optical gain, high photoluminescence quantum yield, and no triplet absorption losses at the lasing wavelength combined with a mixed-order distributed feedback grating to achieve a low lasing threshold. A simple encapsulation technique greatly reduced the laser-induced thermal degradation and suppressed the ablation of the gain medium otherwise taking place under intense continuous-wave photoexcitation. Overall, this study provides evidence that the development of a continuous-wave organic semiconductor laser technology is possible via the engineering of the gain medium and the device architecture..
||Hiroki Uoyama, Kenichi Goushi, Katsuyuki Shizu, Hiroko Nomura, Chihaya Adachi, Highly efficient organic light-emitting diodes from delayed fluorescence, NATURE, 10.1038/nature11687, 492, 7428, 234-+, 2012.12, The inherent flexibility afforded by molecular design has accelerated the development of a wide variety of organic semiconductors over the past two decades. In particular, great advances have been made in the development of materials for organic light-emitting diodes (OLEDs), from early devices based on fluorescent molecules(1) to those using phosphorescent molecules(2,3). In OLEDs, electrically injected charge carriers recombine to form singlet and triplet excitons in a 1:3 ratio(1); the use of phosphorescent metal-organic complexes exploits the normally non-radiative triplet excitons and so enhances the overall electroluminescence efficiency(2,3). Here we report a class of metal-free organic electroluminescent molecules in which the energy gap between the singlet and triplet excited states is minimized by design(4), thereby promoting highly efficient spin up-conversion from non-radiative triplet states to radiative singlet states while maintaining high radiative decay rates, of more than 10(6) decays per second. In other words, these molecules harness both singlet and triplet excitons for light emission through fluorescence decay channels, leading to an intrinsic fluorescence efficiency in excess of 90 per cent and a very high external electroluminescence efficiency, of more than 19 per cent, which is comparable to that achieved in high-efficiency phosphorescence-based OLEDs(3)..
||Kenichi Goushi, Chihaya Adachi, Efficient organic light-emitting diodes through up-conversion from triplet to singlet excited states of exciplexes, APPLIED PHYSICS LETTERS, 10.1063/1.4737006, 101, 2, 2012.07, Enhanced electroluminescence efficiency is achieved in organic light-emitting diodes through delayed fluorescence of the exciplex state formed between 4,40,4 ''-tris[3-methylphenyl(phenyl) amino]triphenylamine (m-MTDATA) as an electron-donating material and 2,8-bis(diphenylphosphoryl) dibenzo[b,d]thiophene (PPT) as an electron-accepting material. The devices exhibited maximum external electroluminescence quantum and power efficiencies of 10.0% and 47.0 lm/W, respectively. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4737006].
||Kenichi Goushi, Kou Yoshida, Keigo Sato, Chihaya Adachi, Organic light-emitting diodes employing efficient reverse intersystem crossing for triplet-to-singlet state conversion, NATURE PHOTONICS, 10.1038/nphoton.2012.31, 6, 4, 253-258, 2012.04, Light emission from organic light-emitting diodes that make use of fluorescent materials have an internal quantum efficiency that is typically limited to no more than 25% due to the creation of non-radiative triplet excited states. Here, we report the use of electron-donating and electron-accepting molecules that allow a very high reverse intersystem crossing of 86.5% between non-radiative triplet and radiative singlet excited states and thus a means of achieving enhanced electroluminescence. Organic light-emitting diodes made using m-MTDATA as the donor material and 3TPYMB as the acceptor material demonstrate that external quantum efficiencies as high as 5.4% can be achieved, and we believe that the approach will offer even higher values in the future as a result of careful material selection..
||Kenichi Goushi, Toshiki Yamada, Akira Otomo, Excitation Intensity Dependence of Power-Law Blinking Statistics in Nanocrystal Quantum Dots, JOURNAL OF PHYSICAL CHEMISTRY C, 10.1021/jp811448k, 113, 47, 20161-20168, 2009.11, We present new information that requires explanation in the study of possible mechanisms for fluorescence blinking in single nanocrystal (NC) quantum dots. By using pulse laser excitation, we investigated the excitation intensity dependence of fluorescence blinking statistics in NC quantum dots embedded in polymer matrices. Under strong excitation intensity, we observed an unexpected excitation intensity dependence of the power-law distribution in the blinking statistics. To explain the new information on the blinking statistics in the framework of the Tang-Marcus model, we propose a diffusion-controlled model based upon anharmonic potentials that originate from a nonlinear response of polarization to the electric field and is caused by an increase in the dielectric constant of host media with the excitation intensity. The validity of the proposed model is investigated by comparing the blinking statistics for two host matrices..
||Y. KAWAMURA, K. GOUSHI, J. BROOKS, J. J. BROWN, H. SASABE, C. ADACHI, 100% phosphorescence quantum efficiency of Ir(III) complexes in organic semiconductor films, APPLIED PHYSICS LETTERS, 10.1063/1.1862777, 86, 071104, 2005.02.