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Kenichi Goushi Last modified date：2024.04.13

Assistant Professor / Applied Chemistry / Department of Applied Chemistry / Faculty of Engineering


Papers
1. Highly efficient light harvesting of a Eu(iii) complex in a host–guest film by triplet sensitization
Very high photoluminescence intensity of a Eu(iii) complex is achieved using a host–guest system. The intensity is 400 times larger than that of the Eu(iii) complex itself..
2. Yi-Ting Lee, Chin-Yiu Chan, Masaki Tanaka, Masashi Mamada, Kenichi Goushi, Xun Tang, Youichi Tsuchiya, Hajime Nakanotani, Chihaya Adachi, Tailor-Made Multi-Resonance Terminal Emitters toward Narrowband, High-Efficiency, and Stable Hyperfluorescence Organic Light-Emitting Diodes, ADVANCED OPTICAL MATERIALS, 10.1002/adom.202200682, 10, 17, 2022.09, Multi-resonance emitters (MREs) are a promising candidate for fulfilling the harsh requirements of display applications due to their unique photophysical properties. Recently, MREs have been widely used as a terminal emitter in hyperfluorescence organic light-emitting diodes (HF-OLEDs); however, since MREs are always thermally activated delayed fluorescence (TADF)-active, possessing long triplet lifetimes in millisecond order, they result in severe chemical degradation. Instead of shortening the delayed lifetime of MREs by molecular design, herein, a low-triplet pyrene unit is introduced into an MRE scaffold to achieve narrowband emission and quick removal of triplets in MREs simultaneously. Blue HF-OLED based on the non-TADF MRE demonstrates a high external quantum efficiency of 20% and a tenfold improvement in stability, compared to those of the HF-OLEDs with standard MREs. An opposite direction of molecular design of MREs for the sophisticated exciton generation and transfer system in HF-OLEDs is proposed to enhance device lifetime without sacrificing color purity and efficiency..
3. Masashi Mamada, Hiroshi Katagiri, Chin-Yiu Chan, Yi-Ting Lee, Kenichi Goushi, Hajime Nakanotani, Takuji Hatakeyama, Chihaya Adachi, Highly Efficient Deep-Blue Organic Light-Emitting Diodes Based on Rational Molecular Design and Device Engineering, ADVANCED FUNCTIONAL MATERIALS, 10.1002/adfm.202204352, 32, 32, 2022.08, There is increasing interest in thermally activated delayed fluorescence (TADF) in materials, and to understand its mechanism in the excited state dynamics. Recent challenges include color purity, efficient deep-blue emission, fast exciton decay lifetimes, high reverse intersystem crossing rates (k(RISC)), low-efficiency roll-off in organic light-emitting diodes (OLEDs), and long device lifetimes. Here, a series of compounds having benzonitrile and carbazole rings are examined, that provide a detailed understanding of the excited states, and a guideline for high-performance TADF. A dense alignment of the excited states with several different characters within a small energy range results in high k(RISC) of >2 x 10(6) s(-1), while maintaining radiative rate constants (k(r)) >10(7) s(-1). OLEDs based on the optimum compound exhibit a low-efficiency roll-off and a CIEy (y color coordinate of Commission Internationale de l'eclairage)
4. Chin-Yiu Chan, Yi-Ting Lee, Masashi Mamada, Kenichi Goushi, Youichi Tsuchiya, Hajime Nakanotani, Chihaya Adachi, Carbazole-2-carbonitrile as an acceptor in deep-blue thermally activated delayed fluorescence emitters for narrowing charge-transfer emissions, CHEMICAL SCIENCE, 10.1039/d2sc02478k, 13, 26, 7821-7828, 2022.07, This work reports a new acceptor for constructing donor-acceptor type (D-A type) blue thermally activated delayed fluorescence (TADF) emitters with narrowed charge-transfer (CT) emissions. A new acceptor core, carbazole-2-carbonitrile (CCN), is formed by the fusion of carbazole and benzonitrile. Three D-A type TADF emitters based on the CCN acceptor, namely 3CzCCN, 3MeCzCCN, and 3PhCzCCN, have been successfully synthesized and characterized. These emitters show deep-blue emissions from 439 to 457 nm with high photoluminescence quantum yields of up to 85% in degassed toluene solutions. Interestingly, all CCN-based deep-blue TADF emitters result in narrow CT emissions with full-width at half-maximums (FWHMs) of less than 50 nm in toluene solutions, which are pretty narrower compared with those of typical D-A type TADF emitters. Devices based on these emitters show high maximum external quantum efficiencies of up to 17.5%..
5. A. Lennart Schleper, Kenichi Goushi, Christoph Bannwarth, Bastian Haehnle, Philipp J. Welscher, Chihaya Adachi, Alexander J. C. Kuehne, Hot exciplexes in U-shaped TADF molecules with emission from locally excited states, NATURE COMMUNICATIONS, 10.1038/s41467-021-26439-w, 12, 1, 2021.10, Fast emission and high color purity are essential characteristics of modern opto-electronic devices, such as organic light emitting diodes (OLEDs). These properties are currently not met by the latest generation of thermally activated delayed fluorescence (TADF) emitters. Here, we present an approach, called "hot exciplexes" that enables access to both attributes at the same time. Hot exciplexes are produced by coupling facing donor and acceptor moieties to an anthracene bridge, yielding an exciplex with large T-1 to T-2 spacing. The hot exciplex model is investigated using optical spectroscopy and quantum chemical simulations. Reverse intersystem crossing is found to occur preferentially from the T-3 to the S-1 state within only a few nanoseconds. Application and practicality of the model are shown by fabrication of organic light-emitting diodes with up to 32 % hot exciplex contribution and low efficiency roll-off.Molecular designs improving the performance of thermally activated delayed fluorescence (TADF) emitters are greatly desired for optoelectronic applications. The authors propose a molecular geometry with donor and acceptor moieties facing each other separated by an anthracene bridge, giving rise to hot exciplex states producing color pure and fast TADF emission..
6. Chathuranganie A. M. Senevirathne, Seiya Yoshida, Morgan Auffray, Masayuki Yahiro, Buddhika S. B. Karunathilaka, Fatima Bencheikh, Kenichi Goushi, Atula S. D. Sandanayaka, Toshinori Matsushima, Chihaya Adachi, Recycling of Triplets into Singlets for High-Performance Organic Lasers, ADVANCED OPTICAL MATERIALS, 10.1002/adom.202101302, 10, 1, 2021.10, Achieving continuous-wave (CW) lasing in organic semiconductors is known to be a difficult task because long-lived triplets quench radiative singlets via singlet-triplet annihilation (STA). To avoid STA and operate organic lasers in CW or long-pulse photoexcitation, the triplets need to be removed from an organic laser gain medium. However, this triplet removal leads to a loss of excitons. In addition to removing the detrimental triplets, here it is reported a triplet recycling process, which includes triplet scavenging and successive triplet upconversion via triplet-triplet annihilation (TTA) to regenerate emissive singlet excitons in a laser medium. An anthracene derivative of 9-(1-naphthalenyl)-10-(4-(2-naphthalenyl)phenyl)anthracene (NaNaP-A) and a laser dye of 4,4 '-bis[4-(diphenylamino)styryl]biphenyl (BDAVBi) are used as the triplet recycling sensitizer and the emitting laser dye, respectively. In this laser system, NaNaP-A can efficiently scavenge the triplets formed on BDAVBi because the triplet level is deeper for NaNaP-A than for BDAVBi, and then NaNaP-A successively recycles the triplets into the BDAVBi's singlet state via TTA. The TTA compensates and overcomes the STA in this laser system. Hence, these laser devices can be operated with long pulse widths of up to 10 ms. This unique triplet recycling behavior is confirmed by transient photoluminescence (PL) and electroluminescence (EL) studies..
7. Chathuranganie A. M. Senevirathne, Atula S. D. Sandanayaka, Buddhika S. B. Karunathilaka, Takashi Fujihara, Fatima Bencheikh, Chuanjiang Qin, Kenichi Goushi, Toshinori Matsushima, Chihaya Adachi, Markedly Improved Performance of Optically Pumped Organic Lasers with Two-Dimensional Distributed-Feedback Gratings, ACS PHOTONICS, 10.1021/acsphotonics.0c01728, 8, 5, 1324-1334, 2021.05, Continuous-wave (CW) lasing is still difficult to realize in organic laser dyes, one reason being the thermal degradation caused by intense photoexcitation. A decrease in laser threshold suppresses the thermal degradation and, therefore, leads to long-lasting lasing from organic laser dyes. Here, we show that it is possible to decrease the laser thresholds by combining the organic laser dye 4,4-bis[(N-carbazole)styryl]biphenyl, which has a small spectral overlap between the laser emission and the excited-state triplet absorption, with two-dimensional (2D) distributed-feedback (DFB) gratings. We used second-order 2D cross double and square lattice DFB gratings, which offer light feedback in two orthogonal directions, and second-order and mixed-order circular DFB gratings, which offer light feedback in radial directions. Among these grating structures, the mixed-order circular DFB grating structure led to the lowest lasing threshold of similar to 0.015 mu J cm(-2) under short-pulse photoexcitation because of the excellent optical feedback. Moreover, a low average-power threshold of 10 W cm(-2) (a peak-power threshold of 1 kW cm-2) was obtained when laser devices with this grating structure were operated under 1 s of long-pulse photoexcitation with a repetition rate of 0.01 Hz. Additionally, using the mixed-order circular DFB grating structure improved the laser stability under CW photoexcitation since shorter exciton lifetimes in the optical resonator suppress the chemical decomposition. These results demonstrate the importance of choosing an optical resonator structure for improving organic laser performance..
8. Masashi Mamada, Kenichi Goushi, Ryota Nakamura, Hironori Kaji, Chihaya Adachi, Synthesis and Characterization of 5,5 '-Bitetracene, CHEMISTRY LETTERS, 10.1246/cl.200909, 50, 4, 800-803, 2021.04, A tetracene dimer, 5,5'-bitetracene (55BT), has successfully been synthesized for the first time and its structure was confirmed by X-ray single crystal analysis. The electrochemical and optical properties of 55BT were investigated and compared to those of tetracene. Although the photoluminescence quantum yield of 55BT was slightly higher than that of tetracene, the results of the transient absorption spectroscopy suggested triplet formation by singlet fission in solution..
9. Reiko Aoki, Ryutaro Komatsu, Kenichi Goushi, Masashi Mamada, Soo Young Ko, Jeong Weon Wu, Virginie Placide, Anthony D'Aleo, Chihaya Adachi, Realizing Near-Infrared Laser Dyes through a Shift in Excited-State Absorption, ADVANCED OPTICAL MATERIALS, 10.1002/adom.202001947, 9, 6, 2021.03, The development of near-infrared (NIR) light sources has attracted much interest due to their attractive applications, such as biosensing and light detection and ranging (LiDAR). In particular, organic semiconductor laser diodes with NIR emission are emerging as a next generation technology. However, organic NIR emitters have generally suffered from a low quantum yield, which has resulted in only a few examples of organic solid-state NIR lasers. In this study, the authors demonstrate a highly efficient NIR emitter based on a boron difluoride curcuminoid structure, which shows a high photoluminescence (PL) quantum yield (phi(PL)) at >700 nm and a high fluorescence radiative rate constant in a solid-state film. Amplified spontaneous emission and lasing occurs at >800 nm with very low thresholds. The large redshift of the stimulated emission is attributed to the transition from the lowest excited state to the different vibrational levels of the ground state owing to the overlap between the emission and the singlet-singlet excited-state absorption..
10. Buddhika S. B. Karunathilaka, Umamahesh Balijapalli, Chathuranganie A. M. Senevirathne, Seiya Yoshida, Yu Esaki, Kenichi Goushi, Toshinori Matsushima, Atula S. D. Sandanayaka, Chihaya Adachi, Suppression of external quantum efficiency rolloff in organic light emitting diodes by scavenging triplet excitons, NATURE COMMUNICATIONS, 10.1038/s41467-020-18292-0, 11, 1, 2020.10, Large external quantum efficiency rolloff at high current densities in organic light-emitting diodes (OLEDs) is frequently caused by the quenching of radiative singlet excitons by long-lived triplet excitons [singlet-triplet annihilation (STA)]. In this study, we adopted a triplet scavenging strategy to overcome the aforementioned STA issue. To construct a model system for the triplet scavenging, we selected 2,6-dicyano-1,1-diphenyl-lambda (5)sigma (4)-phosphinine (DCNP) as the emitter and 4,4-bis[(N-carbazole)styryl]biphenyl (BSBCz) as the host material by considering their singlet and triplet energy levels. In this system, the DCNP's triplets are effectively scavenged by BSBCz while the DCNP's singlets are intact, resulting in the suppressed STA under electrical excitation. Therefore, OLEDs with a 1wt.%-DCNP-doped BSBCz emitting layer demonstrated the greatly suppressed efficiency rolloff even at higher current densities. This finding favourably provides the advanced light-emitting performance for OLEDs and organic semiconductor laser diodes from the aspect of the suppressed efficiency rolloff. Though organic light-emitting diodes are highly desirable for display and lighting applications, efficiency roll-off at high current densities limits its applicability. Here, the authors report a triplet scavenging strategy for improved triplet exciton management and reduced efficiency roll-off..
11. Buddhika S. B. Karunathilaka, Umamahesh Balijapalli, Chathuranganie A. M. Senevirathne, Yu Esaki, Kenichi Goushi, Toshinori Matsushima, Atula S. D. Sandanayaka, Chihaya Adachi, An Organic Laser Dye having a Small Singlet-Triplet Energy Gap Makes the Selection of a Host Material Easier, ADVANCED FUNCTIONAL MATERIALS, 10.1002/adfm.202001078, 30, 30, 2020.07, Quenching of singlets by long-lived triplets is a serious issue for lasing from organic laser dyes, especially under long pulse excitation. As a strategy to scavenge or manage unnecessary triplets, an organic laser dye is dispersed into a host material having high singlet and low triplet energy levels [a large singlet-triplet energy gap (Delta E-ST)]. However, finding such a host material having a triplet scavenging capability is limited. In this study, an organic laser dye, 2,6-dicyano-1,1-diphenyl-lambda(5)sigma(4)-phosphinine (DCNP), having a small Delta E-ST of approximate to 0.44 eV is synthesized, and thus 4-4 '-bis[(N-carbazole)styryl] biphenyl (BSBCz) can be employed as a triplet scavenging host, i.e., the triplets formed on DCNP are easily transferred to BSBCz. A 1 wt%-DCNP-doped BSBCz film is formed on a mixed-order distributed feedback grating, showing lasing with a low threshold value of approximate to 0.86 mu J cm(-2) and a full-width-at-half-maximum value of approximate to 0.5 nm. Because of the suppressed singlet-triplet annihilation, DCNP-based laser devices operating under a continuous-wave regime, with a low threshold of 72 W cm(-2) and a long laser half-lifetime of approximate to 3 min, are demonstrated. These results indicate a possibility of the wider selection of host materials, easing a material design strategy of fabricating high-performance laser devices in future..
12. 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..
13. Takashi Kobayashi, Daisuke Kawate, Akitsugu Niwa, Takashi Nagase, Kenichi Goushi, Chihaya Adachi, Hiroyoshi Naito, Intersystem Crossing Rate in Thermally Activated Delayed Fluorescence Emitters, PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE, 10.1002/pssa.201900616, 217, 3, 2020.02, For a better understanding of the exciton decay process in thermally activated delayed fluorescence (TADF) emitters, the intersystem crossing rate, k(ISC), is one of the important physical constants that have to be determined. Herein, a method to calculate the k(ISC) value from photoluminescence (PL) measurements is reconsidered. The k(ISC) value can be determined at very low temperatures where delayed fluorescence (DF) is completely suppressed, as well as around room temperature where triplet excitons mainly decay into the ground state by emitting DF. However, there is a temperature range where the k(ISC) value cannot be determined accurately because the influences of nonradiative decay paths can be neither ignored nor corrected. For such a temperature range, an alternative approach, which utilizes the temperature dependence of an observed PL decay rate, is presented. In this way, k(ISC) values from 300 to 10 K are determined for thin films of two TADF emitters, i.e., 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene and 1,2-bis(carbazol-9-yl)-4,5-dicyanobenzene, which are known as 4CzIPN and 2CzPN, respectively..
14. Naoto Notsuka, Hajime Nakanotani, Hiroki Noda, Kenichi Goushi, Chihaya Adachi, Observation of Nonradiative Deactivation Behavior from Singlet and Triplet States of Thermally Activated Delayed Fluorescence Emitters in Solution, JOURNAL OF PHYSICAL CHEMISTRY LETTERS, 10.1021/acs.jpclett.9b03302, 11, 2, 562-566, 2020.01, Excited states of emissive organic molecules undergo various kinds of quenching phenomena such as vibration-coupled quenching depending on their environmental conditions. Because bright singlet excitons in purely organic molecules exhibiting thermally activated delayed fluorescence (TADF) can access dark triplet excited states, photogenerated singlet excitons can decay nonradiatively through both singlet and triplet excited states. Here, we investigated nonradiative decay behavior, including internal and external exciton quenching processes, of various types of TADF materials in solution. Under air-saturated conditions, both the lowest singlet and triplet excited states of almost all of the TADF materials showed oxygen quenching. We considered the effect of oxygen quenching for both spin states to develop a method for determination of the triplet contribution to the total photoluminescence quantum yield from the transient photoluminescence profiles. Furthermore, we observed a clear energy gap law for the internal nonradiative processes..
15. 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..
16. Takashi Kobayashi, Daisuke Kawate, Akitsugu Niwa, Takashi Nagase, Kenichi Goushi, Chihaya Adachi, Hiroyoshi Naito, Intersystem Crossing Rate in Thermally Activated Delayed Fluorescence Emitters, Physica Status Solidi (A) Applications and Materials Science, 10.1002/pssa.201900616, 2019.10, For a better understanding of the exciton decay process in thermally activated delayed fluorescence (TADF) emitters, the intersystem crossing rate, kISC, is one of the important physical constants that have to be determined. Herein, a method to calculate the kISC value from photoluminescence (PL) measurements is reconsidered. The kISC value can be determined at very low temperatures where delayed fluorescence (DF) is completely suppressed, as well as around room temperature where triplet excitons mainly decay into the ground state by emitting DF. However, there is a temperature range where the kISC value cannot be determined accurately because the influences of nonradiative decay paths can be neither ignored nor corrected. For such a temperature range, an alternative approach, which utilizes the temperature dependence of an observed PL decay rate, is presented. In this way, kISC values from 300 to 10 K are determined for thin films of two TADF emitters, i.e., 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene and 1,2-bis(carbazol-9-yl)-4,5-dicyanobenzene, which are known as 4CzIPN and 2CzPN, respectively..
17. Time-resolved photoluminescence spectra of 2CzPN with sky blue emission.
18. 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.
19. Matthew R. Leyden, Shinobu Terakawa, Toshinori Matsushima, Shibin Ruan, Kenichi Goushi, Morgan Auffray, Atula S. D. Sandanayaka, Chuanjiang Qin, Fatima Bencheikh, Chihaya Adachi, Distributed Feedback Lasers and Light-Emitting Diodes Using 1-Naphthylmethylamnonium Low-Dimensional Perovskite, ACS PHOTONICS, 10.1021/acsphotonics.8b01413, 6, 2, 460-466, 2019.02, This work investigates the feasibility of using low-dimensional perovskites for electrically driven lasers given the current status of perovskite light-emitting diodes and optically pumped lasers. In our progress toward electrically driven lasers, we performed a variety of measurements on bulk and low-dimensional perovskite films to give a baseline for expectations. This included the measurement of amplified spontaneous emission, lasing, and near-infrared light-emitting diodes operated at low and high current density. We considered power density thresholds needed for amplified spontaneous emission and lasing and compared this to light-emitting diodes operated at high current density to speculate on the future of electrically driven perovskite lasers. We concluded that our current perovskite devices will need current densities of similar to 4 to 10 kA/cm(2) to achieve lasing. Future devices will most significantly benefit from architectures that accommodate higher current, but meaningful reductions in threshold may also come from improved film quality and confinement..
20. Toshinori Matsushima, Chuanjiang Qin, Kenichi Goushi, Fatima Bencheikh, Takeshi Komino, Matthew Leyden, Atula S. D. Sandanayaka, Chihaya Adachi, Enhanced Electroluminescence from Organic Light-Emitting Diodes with an Organic-Inorganic Perovskite Host Layer, ADVANCED MATERIALS, 10.1002/adma.201802662, 30, 38, 2018.09, The development of host materials with high performance is essential for fabrication of efficient and stable organic light-emitting diodes (OLEDs). Although host materials used in OLEDs are typically organics, in this study, it is shown that the organic-inorganic perovskite CH3NH3PbCl3 (MAPbCl(3)) can be used as a host layer for OLEDs. Vacuum-evaporated MAPbCl(3) films have a wide band gap of about 3 eV and very high and relatively balanced hole and electron mobilities, which are suitable for the host material. Photoluminescence and electroluminescence take place through energy transfer from MAPbCl(3) to an organic emitter in films. Incorporation of an MAPbCl(3) host layer into OLEDs leads to a reduction of driving voltage and enhancement of external quantum efficiency as compared to devices with a conventional organic host layer. Additionally, OLEDs with an MAPbCl(3) host layer demonstrate very good operational stability under continuous current operation. These results can be extensively applied to organic- and perovskite-based optoelectronics..
21. Akitsugu Niwa, Shota Haseyama, Takashi Kobayashi, Takashi Nagase, Kenichi Goushi, Chihaya Adachi, Hiroyoshi Naito, Triplet-triplet annihilation in a thermally activated delayed fluorescence emitter lightly doped in a host, APPLIED PHYSICS LETTERS, 10.1063/1.5025870, 113, 8, 2018.08, Triplet-triplet annihilation (TTA) will change the ratio between fluorescence and phosphorescence in the photoluminescence spectrum of a thermally activated delayed fluorescence emitter at very low temperature. Using the resultant spectral blueshift, this study investigated the nature of TTA in 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) doped in a host material. The spectral blueshift is not caused by singlet-triplet annihilation and the emitter saturation effect and is less influenced by the emitter aggregates, particularly for the case of a lower doping concentration. Using these features, it is possible to focus on TTA. For 4CzIPN, the spectral blueshift due to TTA is recognized even in samples with the doping concentration as low as 1 wt. %. Published by AIP Publishing..
22. Takashi Kobayashi, Akitsugu Niwa, Shota Haseyama, Kensho Takaki, Takashi Nagase, Kenichi Goushi, Chihaya Adachi, Hiroyoshi Naito, Emission properties of thermally activated delayed fluorescence emitters: analysis based on a four-level model considering a higher triplet excited state, JOURNAL OF PHOTONICS FOR ENERGY, 10.1117/1.JPE.8.032104, 8, 3, 2018.07, A four-level model consisting of a higher triplet excited state (T-2), the lowest singlet and triplet excited states (S-1 and T-1), and the ground state was previously used to understand emission properties of a thermally activated delayed fluorescence (TADF) emitter, 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN). In this report, we discuss the four-level model in more detail and apply to the other two TADF emitters, i.e., 1,2,4,5-tetrakis(carbazol-9-yl)-3,6-dicyanobenzene (4CzTPN) and 1,2-bis(carbazol-9-yl)-4,5-dicyanobenzene(2CzPN), in order to determine their excited-state structures. It is suggested that in all the emitters T-2 lies between S-1 and T-1 and play an essential role in the emitting process. In 4CzTPN, phosphorescence from T-2 is clearly observed around 100 K as in 4CzIPN. Compared to the other two emitters, 2CzPN has a wider energy gap between S-1 and T-1 so that delayed fluorescence at room temperature is thought to be mixed with phosphorescence. Because of this mixing, the spectrum characteristic of phosphorescence from T-2 in 2CzPN cannot be identified. (C) 2018 Society of Photo-Optical Instrumentation Engineers (SPIE).
23. Kohei Osada, Kenichi Goushi, Hironori Kaji, Chihaya Adachi, Hisao Ishii, Yutaka Noguchi, Observation of spontaneous orientation polarization in evaporated films of organic light-emitting diode materials, ORGANIC ELECTRONICS, 10.1016/j.orgel.2018.04.026, 58, 313-317, 2018.07, The molecular orientation in organic semiconductor films determines device performances. In particular, the spontaneous orientation of a permanent dipole moment (PDM) along the surface normal direction induces a polarization charge at the hetero-interfaces of stacked multilayer devices, and the interface charge dominates the charge accumulation and injection properties. Spontaneous orientation polarization (SOP) has been observed in the "randomly oriented" films of several organic semiconductor materials, and is potentially inherent in many common materials. Herein, we report that 11 additional molecules of organic light-emitting diode materials, including thermally activated delayed fluorescence emitters, and horizontally oriented emitters and electron transporters, exhibit SOP in their evaporated films. The experimental results clearly indicate that SOP frequently occurs in "horizontally oriented" films as well as "randomly oriented" films. The factors contributing to SOP formation are discussed in terms of the figure of merit per PDM. We found that strong intermolecular interactions tend to reduce the figure of merit. Moreover, we suggest the impact of SOP on device performances..
24. Kenichi Goushi, Takeshi Fukuda, Atsushi Kubono, Atsushi Miura, Kei Noda, Hiroaki Usui, Ichiro Yamashita, Yuji Yoshida, Recent Advances in Molecular Electronics and Bioelectronics FOREWORD, JAPANESE JOURNAL OF APPLIED PHYSICS, 10.7567/JJAP.57.03E001, 57, 3, 2018.03.
25. Naoto Notsuka, Ryota Kabe, Kenichi Goushi, Chihaya Adachi, Confinement of Long-Lived Triplet Excitons in Organic Semiconducting Host-Guest Systems, ADVANCED FUNCTIONAL MATERIALS, 10.1002/adfm.201703902, 27, 40, 2017.10, Long-lived triplet excitons on organic molecules easily deactivate at room temperature because of the presence of thermally activated nonradiative pathways. This study demonstrates long-lived phosphorescence at room temperature resulting from suppression of the nonradiative deactivation of triplet excitons in conventional organic semiconducting host-guest systems. The nonradiative deactivation pathway strongly depends on the triplet energy gap between the guest emitting molecules and the host matrices. The triplet energy gap required to confine the long-lived triplet excitons (approximate to 0.5 eV) is much larger than that of conventional host-guest systems for phosphorescent emitters. By effectively confining the triplet excitons, this study demonstrates long-lived room-temperature phosphorescence under optical and electrical excitation..
26. Masatoshi Kitamura, Kenichi Goushi, Takao Ishida, Tatsuo Mori, Hiroyoshi Naito, Masaya Nogi, Kazuhito Tsukagoshi, Yukiharu Uraoka, Hirobumi Ushijima, Tsukasa Yoshida, Flexible and Printed Electronics, JAPANESE JOURNAL OF APPLIED PHYSICS, 10.7567/JJAP.56.05E001, 56, 5, 2017.05.
27. Shota Haseyama, Akitsugu Niwa, Takashi Kobayashi, Takashi Nagase, Kenichi Goushi, Chihaya Adachi, Hiroyoshi Naito, Control of the Singlet-Triplet Energy Gap in a Thermally Activated Delayed Fluorescence Emitter by Using a Polar Host Matrix, NANOSCALE RESEARCH LETTERS, 10.1186/s11671-017-2012-1, 12, 1, 2017.04, The photoluminescence properties of a thermally activated delayed fluorescence emitter, 1,2-bis(carbazol-9-yl)-4,5-dicyanobenzene (2CzPN), doped in a host matrix consisting of 1,3-bis(9-carbazolyl) benzene and a polar inert molecule, camphoric anhydride (CA), in various concentrations have been investigated. It is found that the addition of CA stabilizes only the lowest singlet excited state (S-1) of 2CzPN without changing the energy level of the lowest triplet excited state (T-1), leading to a reduction in the energy gap between S-1 and T-1. The maximum reduction of energy gap achieved in this work has been determined to be around 65 meV from the shift of the fluorescence spectrum and the temperature dependence of the photoluminescence decay rate..
28. 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..
29. Takashi Kobayashi, Akitsugu Niwa, Kensho Takaki, Shota Haseyama, Takashi Nagase, Kenichi Goushi, Chihaya Adachi, Hiroyoshi Naito, Contributions of a Higher Triplet Excited State to the Emission Properties of a Thermally Activated Delayed-Fluorescence Emitter, PHYSICAL REVIEW APPLIED, 10.1103/PhysRevApplied.7.034002, 7, 3, 2017.03, The temperature dependences of photoluminescence (PL) decay rates and the PL spectrum of a thermally activated delayed-fluorescence emitter, 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN), are investigated. It is found that not only the lowest singlet (S-1) and triplet ( T-1) excited states but also an additional triplet excited state ( T-n) lying between S-1 and T-1 play an important role in the exciton decay process, particularly around 100 K. At around this temperature, some of the triplet excitons are thermally activated into Tn but not up to S-1, and they then decay into the ground state (S-0) with phosphorescence emission. Therefore, two kinds of phosphorescence, originating from T-n and T-1, are observed. The temperature dependence of the PL decay rates of 4CzIPN can be explained by a four-level model consisting of S-1, T-1, T-n, and S-0, and its energy gaps between T-n and T-1 and between S-1 and T-1 are determined to be 45 +/- 5 meV and 135 +/- 10 meV, respectively..
30. SANDANAYAKA SANGARANGE, Kou Yoshida, Munetomo Inoue, QIN CHUANJIANG, Kenichi Goushi, JEAN CHARLES MAURICE RIBIERRE, Toshinori Matsushima, Chihaya Adachi, Quasi-Continuous-Wave Organic Thin-Film Distributed Feedback Laser, Advanced optical materials, 10.1002/adom.201600006, 4, 6, 834-839, 2016.06.
31. Atula S. D. Sandanayaka, Kou Yoshida, Munetomo Inoue, Chuanjiang Qin, Kenichi Goushi, Jean-Charles Ribierre, Toshinori Matsushima, Chihaya Adachi, Quasi-Continuous-Wave Organic Thin-Film Distributed Feedback Laser, ADVANCED OPTICAL MATERIALS, 10.1002/adom.201600006, 4, 6, 834-839, 2016.06.
32. Atula S. D. Sandanayaka, Kou Yoshida, Munetomo Inoue, Chuanjiang Qin, Kenichi Goushi, Jean-Charles Ribierre, Toshinori Matsushima, Chihaya Adachi, Quasi-Continuous-Wave Organic Thin-Film Distributed Feedback Laser, ADVANCED OPTICAL MATERIALS, 10.1002/adom.201600006, 4, 6, 834-839, 2016.06.
33. Atula S. D. Sandanayaka, Kou Yoshida, Munetomo Inoue, Chuanjiang Qin, Kenichi Goushi, Jean-Charles Ribierre, Toshinori Matsushima, Chihaya Adachi, Quasi-Continuous-Wave Organic Thin-Film Distributed Feedback Laser, ADVANCED OPTICAL MATERIALS, 10.1002/adom.201600006, 4, 6, 834-839, 2016.06.
34. Atula S. D. Sandanayaka, Kou Yoshida, Munetomo Inoue, Chuanjiang Qin, Kenichi Goushi, Jean-Charles Ribierre, Toshinori Matsushima, Chihaya Adachi, Quasi-Continuous-Wave Organic Thin-Film Distributed Feedback Laser, ADVANCED OPTICAL MATERIALS, 10.1002/adom.201600006, 4, 6, 834-839, 2016.06.
35. Atula S. D. Sandanayaka, Kou Yoshida, Munetomo Inoue, Chuanjiang Qin, Kenichi Goushi, Jean-Charles Ribierre, Toshinori Matsushima, Chihaya Adachi, Quasi-Continuous-Wave Organic Thin-Film Distributed Feedback Laser, ADVANCED OPTICAL MATERIALS, 10.1002/adom.201600006, 4, 6, 834-839, 2016.06.
36. Atula S. D. Sandanayaka, Kou Yoshida, Munetomo Inoue, Chuanjiang Qin, Kenichi Goushi, Jean-Charles Ribierre, Toshinori Matsushima, Chihaya Adachi, Quasi-Continuous-Wave Organic Thin-Film Distributed Feedback Laser, ADVANCED OPTICAL MATERIALS, 10.1002/adom.201600006, 4, 6, 834-839, 2016.06.
37. Chihaya Adachi, Saeyoun Lee, Tetsuya Nakagawa, Katsuyuki Shizu, Kenichi Goshi, Takuma Yasuda, William J. Potscavage, Organic light-emitting diodes (OLEDs) Materials, photophysics, and device physics, Organic Electronics Materials and Devices, 10.1007/978-4-431-55654-1_2, 43-73, 2015.09, Currently, organic light-emitting diodes (OLEDs) have reached the stage of commercialization, and there has been an intense drive to use them in various applications from small-and medium-sized mobile devices to illumination equipment and large television screens. In particular, room-temperature phosphorescent materials have become core OLED components as alternatives to conventionally used fluorescent materials because devices made with phosphorescent materials exhibit excellent light-emitting performance with internal electroluminescence efficiencies (ηint) of nearly 100 %. However, phosphorescent materials have several intrinsic problems, such as being limited to metal-organic compounds containing rare metals, for example, Ir, Pt, Au, and Os, and difficulty in realizing stable blue light emission. As a result, researchers have attempted to develop new materials for use as emissive dopants in OLEDs that overcome these limitations. In this chapter, first we briefly review the progress of OLED materials and device architectures mainly based on fluorescent (first-generation) and phosphorescent (secondgeneration) emitters. Then, we discuss third-generation OLEDs that use a new light-emitting mechanism called thermally activated delayed fluorescence (TADF). Recently, highly efficient TADF, which had been difficult to realize with conventional molecular design, has been achieved by very sophisticated molecular structures, allowing access to the unlimited freedom of molecular design using carbon-based materials. This has led to the production of ultimate OLEDs that are made of common organic compounds without precious metals and can convert electricity to light at ηint of nearly 100 %..
38. Toshiya Yonehara, Kenichi Goushi, Tomoaki Sawabe, Isao Takasu, Chihaya Adachi, Comparison of transient state and steady state exciton-exciton annihilation rates based on Forster-type energy transfer, JAPANESE JOURNAL OF APPLIED PHYSICS, 10.7567/JJAP.54.071601, 54, 7, 071601, 2015.07, We investigated differences between the transient state and steady state exciton-exciton annihilation rates based on Forster-type energy transfer. The exciton-exciton annihilation rate of an organic semiconductor is usually determined by transient state photoluminescence measurements using a pulsed laser or steady state photoluminescence measurements using a continuous wave laser. However, it is unclear that the respective annihilation rates determined by their rate equations are the same. In calculations with platinum-octaethylporphyrin (PtOEP) parameters, Monte Carlo simulations gave two different annihilation rates for the transient state and the steady state. The analytical models based on Forster-type energy transfer also showed the same result. These results indicate that the exciton-exciton annihilation rates in transient state and steady state are distinguished. (C) 2015 The Japan Society of Applied Physics.
39. Yuta Sagara, Katsuyuki Shizu, Hiroyuki Tanaka, Hiroshi Miyazaki, Kenichi Goushi, Hironori Kaji, Chihaya Adachi, Highly Efficient Thermally Activated Delayed Fluorescence Emitters with a Small Singlet-Triplet Energy Gap and Large Oscillator Strength, CHEMISTRY LETTERS, 10.1246/cl.141054, 44, 3, 360-362, 2015.03, A strategy for designing highly efficient thermally activated delayed fluorescence (TADF) emitters was reported. TADF emitters with donor-acceptor-donor (D-A-D)-type structures showed highly efficient TADF because of their small singlet-triplet energy gap and large oscillator strength. An organic light-emitting diode containing a D-A-D-type TADF emitter, cis-BOX2, exhibited a high external electroluminescence quantum efficiency of 17.6%..
40. Yutaka Noguchi, Hyung-Jun Kim, Ryuta Ishino, Kenichi Goushi, Chihaya Adachi, Yasuo Nakayama, Hisao Ishii, Charge carrier dynamics and degradation phenomena in organic light-emitting diodes doped by a thermally activated delayed fluorescence emitter, ORGANIC ELECTRONICS, 10.1016/j.orgel.2014.12.009, 17, 184-191, 2015.02, We report on the charge carrier dynamics and their degradation phenomena in an organic light-emitting diode (OLED) doped by a thermally activated delayed fluorescence emitter; (4s, 6s)-2,4,5,6-tetra(9H-carbazol-9-yl) isophthalonitrile (4CzIPN). Displacement current measurement (DCM) data revealed the presence of negative interface charge originating from the spontaneous orientation polarization of the electron transport layer (ETL), 1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl) benzene. The negative interface charge acts as a hole reservoir and thus confines the recombination zone near the emission layer (EML); 4,4'-bis(N-carbazolyl)-1,1'-biphenyl (CBP):4CzIPN (5 wt%)/ETL interface. By keeping the recombination zone far from the hole transport layer, 4,40-bis[N-(1-naphthyl)-N-phenylamino]- biphenyl (alpha-NPD), a better electroluminescence efficiency is expected because the Dexter energy transfer from the triplet state of 4CzIPN to that of alpha-NPD is suppressed. Moreover, we found an excellent linear relation between the accumulated hole amount and luminance losses owing to device aging. The results are consistent with hole traps originating at the degradation products of CBP as the main factor of device degradation. We suggest device architectures based on the charge carrier dynamics for better efficiency and lifetime. (C) 2014 Elsevier B.V. All rights reserved..
41. Gabor Mehes, Kenichi Goushi, William J. Potscavage, Chihaya Adachi, Influence of host matrix on thermally-activated delayed fluorescence: Effects on emission lifetime, photoluminescence quantum yield, and device performance, ORGANIC ELECTRONICS, 10.1016/j.orgel.2014.05.027, 15, 9, 2027-2037, 2014.09, The influence of the host molecules on the photoluminescent (PL) and electroluminescent (EL) properties of organic light-emitting diode (OLED) emitters showing efficient thermally-activated delayed fluorescence (TADF) has yet to be investigated in detail. Here we demonstrate that the choice of host can cause large variations in the PL quantum yield (Phi(PL) similar to 15-70%) and delayed PL transient decay (tau(del) similar to 2-70 ms) of a spiro-acridine-based TADF guest. We show that the effect of exciplex formation on Phi(PL) must be considered even at low concentrations of the TADF guest. Using the same TADF guest but changing the host layer, we are able to greatly vary the PL transient decay time from similar to 4 to similar to 70 ms while maintaining a high Phi(PL) similar to 70%, which can lead to new applications. Detailed spectral characterization during PL decay reveals a gradually increasing singlet-triplet energy gap (Delta E-ST) as the origin of these observations. The time-varying Delta E-ST is explained based on dipole interactions between the host and guest molecules. Finally, PL and electrical considerations for host selection are discussed based on the performance of OLED devices. (C) 2014 Elsevier B.V. All rights reserved..
42. Akitsugu Niwa, Takashi Kobayashi, Takashi Nagase, Kenichi Goushi, Chihaya Adachi, Hiroyoshi Naito, Temperature dependence of photoluminescence properties in a thermally activated delayed fluorescence emitter, APPLIED PHYSICS LETTERS, 10.1063/1.4878397, 104, 21, 2014.05, Using steady-state and time-resolved photoluminescence (PL) spectroscopy, we have investigated the temperature dependence of PL properties of 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN), which have a small energy gap between its singlet and triplet excited states and thus exhibits efficient thermally activated delayed fluorescence [H. Uoyama et al., Nature 492, 235 (2012)]. Below around 100 K, PL quantum efficiency of 4CzIPN thin films is largely suppressed and strong photoexcitation intensity dependence appears. These features can be explained by using rate equations for the densities of singlet and triplet excited states considering a triplet-triplet annihilation process. (C) 2014 AIP Publishing LLC..
43. Munetomo Inoue, Kenichi Goushi, Kuniaki Endo, Hiroko Nomura, Chihaya Adachi, Reduced amplified spontaneous emission threshold in organic semiconductor laser structure with relaxed roll-off characteristics under high current densities, JOURNAL OF LUMINESCENCE, 10.1016/j.jlumin.2013.04.001, 143, 754-758, 2013.11, We reduced the amplified spontaneous emission (ASE) threshold in an organic semiconductor laser structure by the introduction of an exciton blocking layer adjacent to the cathode layer. A phenyldipyrenylphosphine oxide (POPy2) layer with the function of an electron transport layer and the exciton blocking layer were inserted between an active layer of 4,4-bis[N-(carbazole)styrylbiphenyll (BSB-Cz) and a metal cathode layer. The electron injection barrier from POPy2 to BSB-Cz is downward, indicating no net energy barrier, and the hole injection barrier from the BSB-Cz layer into the POPy2 layer is also small because of the similarity between the highest occupied molecular orbital levels of the materials. Therefore, the proposed device structure can avoid extra carrier accumulations at the heterointerface, leading to compatibility in terms of both relaxation of the exciton-polaron annihilation and blocking of the energy transfer from the BSB-Cz layer into the cathode layer. (C) 2013 Elsevier B.V. All rights reserved..
44. Munetomo Inoue, Kenichi Goushi, Kuniaki Endo, Hiroko Nomura, CHIHAYA ADACHI, Reduced amplified spontaneous emission threshold in organic semiconductor laser structure with relaxed roll-off characteristics under high current densities, Journal of Luminescence, http://authors.elsevier.com/sd/article/S0022231313002123, 2013.04.
45. 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)..
46. 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].
47. Qisheng Zhang, Takeshi Komino, Shuping Huang, Shigeyuki Matsunami, Kenichi Goushi, Chihaya Adachi, Triplet Exciton Confinement in Green Organic Light-Emitting Diodes Containing Luminescent Charge-Transfer Cu(I) Complexes, ADVANCED FUNCTIONAL MATERIALS, 10.1002/adfm.201101907, 22, 11, 2327-2336, 2012.06, The temperature dependence of luminescence from [Cu(dnbp)(DPEPhos)]BF4 (dnbp = 2,9-di-n-butylphenanthroline, DPEPhos = bis[2-(diphenylphosphino)phenyl]ether) in a poly(methyl methacrylate) (PMMA) film indicates the presence of long-life green emission arising from two thermally equilibrated charge transfer (CT) excited states and one non-equilibrated triplet ligand center (3LC) excited state. At room temperature, the lower triplet CT state is found to be the predominantly populated excited state, and the zero-zero energy of this state is found to be 2.72 eV from the onset of its emission at 80 K. The tunable emission maximum of [Cu(dnbp)(DPEPhos)]BF4 in various hosts with different triplet energies is explained in terms of the multiple triplet energy levels of this complex in amorphous films. Using the high triplet energy charge transport material as a host and an exciton-blocking layer (EBL), a [Cu(dnbp)(DPEPhos)]BF4 based organic light-emitting diode (OLED) achieves a high external quantum efficiency (EQE) of 15.0%, which is comparable to values for similar devices based on Ir(ppy)3 and FIrpic. The photoluminescence (PL) and electroluminescence (EL) performance of green emissive [Cu(mu I)dppb]2 (dppb = 1,2-bis[diphenylphosphino]benzene) in organic semiconductor films confirmed its 3CT state with a zero-zero energy of 2.76 eV as the predominant population excited state..
48. 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..
49. Toshiharu Arai, Kenichi Goushi, Hiroko Nomura, Tomohiko Edura, Chihaya Adachi, Two-dimensional orientation control of organic semiconducting amorphous films by mechanical brushing, Appl. Phys. Lett., 99, 053303, 2011.08.
50. Toshiharu Arai, Kenichi Goushi, Hiroko Nomura, Tomohiko Edura, Chihaya Adachi, Two-dimensional orientation control of organic semiconducting amorphous films by mechanical brushing, APPLIED PHYSICS LETTERS, 10.1063/1.3608315, 99, 5, 2011.08, We demonstrate that horizontally oriented organic semiconducting molecules in amorphous films can be oriented uniaxially by mechanical brushing. The absorption of polarized light in the direction parallel to the brushing direction was higher than the absorption in the perpendicular direction. Thus, simultaneous uniaxial and in-plane molecular orientation in amorphous films was realized by brushing. In addition, we demonstrate a polarized electroluminescence from the films treated by mechanical brushing. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3608315].
51. Shuzo Hirata, Korefumi Kubota, Heo Hyo Jung, Osamu Hirata, Kenichi Goushi, Masayuki Yahiro, Chihaya Adachi, Improvement of Electroluminescence Performance of Organic Light-Emitting Diodes with a Liquid-Emitting Layer by Introduction of Electrolyte and a Hole-Blocking Layer, ADVANCED MATERIALS, 10.1002/adma.201003505, 23, 7, 889-+, 2011.02, Organic light-emitting diodes containing a liquid-emitting layer that exhibit improved external electroluminescence quantum efficiency (Phi(EL)) and maximum luminance are presented. Doping the liquid-emitting layer with an electrolyte significantly decreases the turn-on voltage for electroluminescence. Insertion of a TiO2 hole-blocking layer between the liquid-emitting layer and indium tin oxide cathode improves Phi(EL)..
52. 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..
53. Toshiki Yamada, Kenichi Goushi, Akira Otomo, Time-correlated single photon counting system and light-collection system for studying fluorescence emitters under high-vacuum conditions: Use of immersion objective and ionic liquid, THIN SOLID FILMS, 10.1016/j.tsf.2009.07.002, 518, 2, 432-436, 2009.11, In this paper, we introduce a time-correlated single photon counting system and an efficient light-collection system for studying fluorescence emitters under high-vacuum conditions; the latter system has an immersion objective, and an ionic liquid is used as a refractive index matching medium. The ionic liquid is positioned in high vacuum. The time-correlated single photon counting system has modified photomultiplier tubes that act as photon detectors. The light-collection system is designed to be simple, compact, and easy to use. In order to verify the performance of these systems, the optical properties of colloidal semiconductor nanocrystals (CdSe/ZnS) and fluorescent dye molecules are studied as examples. (C) 2009 Elsevier B.V. All rights reserved..
54. Yasuo Kimura, Michio Niwano, Naohiko Ikuma, Kenichi Goushi, Kingo Itaya, Organic Field Effect Transistor Using Pentacene Single Crystals Grown by a Liquid-Phase Crystallization Process, LANGMUIR, 10.1021/la900647y, 25, 9, 4861-4863, 2009.05, Nearly perfect pentacene single crystals with wide terraces several micrometers in width were grown by crystallization from a pentacene-containing trichlorobenzene solution. Organic Field-effect transistors (OFETs) were fabricated with the pentacene single crystals and characterized for their electrical properties. The field effect mobility was found to be in the range of 0.4-0.6 cm(2)/V.s, which is comparable to that of OFETs fabricated with pentacene single crystals prepared by a physical vapor-phase growth method. The results described in this paper clearly demonstrate that the crystallization of organic semiconductors from solution is a promising chemical method for device processing of OFETs..
55. Toshiki Yamada, Kenichi Goushi, Xingsheng Xu, Akira Otomo, Time-correlated single photon counting and optical measurement systems for studying single fluorescent emitters under high vacuum conditions, THIN SOLID FILMS, 10.1016/j.tsf.2008.09.029, 517, 4, 1507-1511, 2008.12, In this paper, we introduce a modified inverted microscope system in which an objective with high NA (0.95) is located under high-vacuum conditions, and a time-correlated single photon counting system is used along with a modified photomultiplier tube for the characterization of single emitters under high-vacuum conditions. The modified inverted microscope system is designed to be simple, compact, and easy to handle. As an example, the optical properties of individual colloidal semiconductor nanocrystals (CdSe/ZnS) were studied by using these systems. (c) 2008 Elsevier B.V. All rights reserved.
56. Toshinori Matsushima, Kenichi Goushi, Chihaya Adachi, Charge-carrier injection characteristics at organic/organic heterojunction interfaces in organic light-emitting diodes, CHEMICAL PHYSICS LETTERS, 10.1016/j.cplett.2007.01.010, 435, 4-6, 327-330, 2007.02, Organic light-emitting diodes (OLEDs) having various guest molecules doped in an organic host matrix layer are fabricated [the OLED structure is anode/hole-transporting layer (HTL)/guest-host emitting layer/hole-blocking layer/electron-transporting layer/cathode], and the dependence of current density-voltage (J-V) characteristics of the OLEDs on highest occupied molecular orbital (HOMO) levels of guest molecules are investigated. From the J-V characteristics of these OLEDs, we find two important results: (1) J-V characteristics of the OLEDs are controlled by the direct hole injection from the neighboring HTL to guest molecules, and (2) HOMO level alignment between the HTL and guest molecules provides efficient hole injection at this interface. (c) 2007 Elsevier B.V. All rights reserved..
57. Y Kawamura, K Goushi, J Brooks, JJ Brown, H Sasabe, C Adachi, Phosphorescence quantum efficiency and intermolecular interaction of Iridium(III) complexes in co-deposited films with organic semiconducting hosts, Organic and Nanocomposite Optical Materials, 846, 73-78, 2005.08, We accurately measured the absolute photoluminescence (PL) quantum efficiency (eta(PL)) Of organic solid-state thin films by using an integrating sphere. We measured the eta(PL) Of conventional organic materials used in organic light emitting diodes, such as a tris (8-quinolionolato)aluminum(III) complex (Alq(3)), and a phosphorescent 1.5mol%-fac-tris(2-phenylpyridyl)iridium(III):4,4'-bis(carbazol-9-yl)-2,2'-biphenyl [Ir(ppy)(3):CBP] co-deposited film. Alq(3) and lr(ppy)(3):CBP showed a eta(PL) = 20 +/- 1% and 97 +/- 2%, which corresponded well to external electroluminescence efficiency using these materials. We also measured red emitting bis[2-(2'-benzothienyl)pyridinato-N,C-3'] (acetylacetonato) iridium(III) [BtP(2)Ir(acac)] with CBP, and the blue complex, bis[(4,6-difluorophenyl)pyridinato -N, C-2](picolinato)iridium(III)[FIrpic], with m-bis(N-carbazolyl)benzene. The maximum eta(PL) values for BtP(2)Ir(acac), and FIrpic were 51 +/- 1 % (at 1.4mol%), and 99 +/- 1% (1.2mol%), respectively. These results suggest that the TIEL of red phosphorescent OLEDs using BtP(2)Ir(acac) as the dopant can be as high as 10%, and blue devices using FIrpic can reach the theoretical limit of 20%..
58. T. AIMONO, Y. KAWAMURA, K. GOUSHI, H. YAMAMOTO, H. SASABE C. ADACHI, 100% fluorescence efficiency of 4,4’-bis[(N-carbazole)styryl]biphenyl in a solid film and the very low amplified spontaneous emission threshold, APPLIED PHYSICS LETTERS, 10.1063/1.1867555, 86, 7, 071110, 2005.02.
59. 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, 7, 071104, 2005.02.
60. Goushi Kenichi, Kawamura Yuichiro, Sasabe Hiroyuki, Adachi Chihaya, Unusual Phosphorescence Characteristics of Ir(ppy)3 in a Solid Matrix at Low Temperatures, Japanese journal of applied physics. Pt. 2, Letters, 10.1143/JJAP.43.L937, 43, 7, L937-L939, 2004.07, We observed unusual radiative decay characteristics of a (fac-tris(2-phenylpyridine)iridium [Ir(ppy)3]) triplet-excited state doped into 4, 4$'$-N, N$'$-dicarbazole-biphenyl (CBP) and polymethylmethacrylate (PMMA) host matrices. From a measurement of the temperature dependence on the phosphorescence intensity and the lifetime of Ir(ppy)3 in these hosts, we observed that phosphorescence intensities are independent of temperature, while the lifetimes significantly increase at temperatures below $T sim 50$ K. These results lead us to conclude that the non-radiative transition rate ($k_{ ext{nr } }$) from the triplet excited state is very small compared with the radiative transition rate ($k_{ ext{r } }$) and that the phosphorescence quantum yield ($phi_{ ext{PH } }$) is nearly 100% even at room temperature. Based on our experimental results, we propose a possible decay mechanism to rationalize the characteristic temperature dependence on the transient phosphorescence..
61. K. GOUSHI, R. KWONG, J. J. BROWN, H. SASABE, C. ADACHI, Triplet exciton confinement and unconfinement by adjacent hole-transport layers, JOURNAL OF APPLIED PHYSICS, 10.1063/1.1751232, 95, 12, 7798 -7802 , 2004.06.
62. High Efficiency White Electrophosphorescence Mechanism with Two Phosphorescent Dopants
High efficiency white organic light emitting diodes (WOLED) have been expected for applications of lighting, illumination and back-light sources for LCD. Recently, since organic phosphors demonstrated very high electroluminescence (EL) quantum efficiency of ˜100%, high efficiency WOLED with the organic phosphors has been anticipated. In this study, we employed an emitter layer which is doped with blue and red phosphorescence molecules. In our optimized device structure, we obtained maximum EL quantum efficiency of 4%. However, the shapes of the EL spectrum were strongly dependent on current density. With analysis of exciton decay process under optical and electrical excitations, we conclude that triplet-triplet annihilation occurs in blue and red phosphors independently, resulted in a significant change of the EL spectrum. Thus, the coincidence of the lifetimes of the blue and red phosphors is indispensable to escape change of EL spectrum with current density..
63. Y. KAWAMURA, H. YAMAMOTO, K. GOUSHI, H. SASABE C. ADACHI, Ultraviolet amplified spontaneous emission from thin films of 4,4 '-bis(9-carbazolyl)-2,2 '-biphenyl and the derivatives, APPLIED PHYSICS LETTERS, 10.1063/1.1695091, 84, 15, 2724-2726, 2004.04.
64. H. INOMATA, K. GOUSHI, T. MASUKO, T. KONNO, T. IMAI, H. SASABE, J. J. BROWN, C. ADACHI, High-efficiency organic electrophosphorescent diodes using 1, 3, 5-triazine electron transport materials, CHEMISTRY OF MATERIALS, 10.1021/cm034689t, 16, 7, 1285-1291, 2004.04.


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