Directory of Researchers

ASAHI Shigeo
Graduate School of Engineering / Department of Electrical and Electronic Engineering
Associate Professor
Electro-Communication Engineering
Last Updated :2022/08/04

Researcher Profile and Settings

Affiliation

  • <Faculty / Graduate School / Others>

    Graduate School of Engineering / Department of Electrical and Electronic Engineering
  • <Related Faculty / Graduate School / Others>

    Faculty of Engineering / Department of Electrical and Electronic Engineering

Teaching

  • Faculty of Engineering / Department of Electrical and Electronic Engineering, 2021, Practice on Electromagnetic Fields and Waves

Research at Kobe

Research Activities

Research Interests

  • 太陽電池
  • 量子ドット
  • 光物性
  • 電気・電子材料工学
  • 半導体物性
  • 分子線エピタキシー

Research Areas

  • Manufacturing technology (mechanical, electrical/electronic, chemical engineering) / Electric/electronic material engineering
  • Nanotechnology/Materials / Crystal engineering

Committee Memberships

  • Apr. 2021 - Nov. 2021, 34th International Microprocesses and Nanotechnology Conference (MNC2021), Program Committee
  • Apr. 2021 - Present, 第33回太陽光発電国際会議, プログラム委員会委員
  • Apr. 2020 - Present, 電子材料シンポジウム(EMS), 実行委員(会場委員)
  • Apr. 2020 - Nov. 2020, 33rd International Microprocesses and Nanotechnology Conference (MNC 2020), Program Committee
  • Apr. 2019 - Nov. 2019, 32nd International Microprocesses and Nanotechnology Conference (MNC2019), Program Committee
  • Apr. 2017 - Sep. 2019, 7th International Workshop on Epitaxial Growth and Fundamental Properties of Semiconductor Nanostructures (SemiconNano2019), Steering Committee

Awards

  • Jun. 2020 47th IEEE Photovoltaic Specialists Conference (PVSC47), PVSC 47 Best Student Paper Award, Up-converted photocurrent enhancement in modulation-doped two-step photon up-conversion

    Kohei Watanabe, Shigeo Asahi, Takashi Kita

  • Dec. 2015 第26回光物性研究会, 奨励賞

  • Aug. 2014 日本材料学会半導体エレクトロニクス部門委員会, 平成26年度第1回研究会学生優秀講演賞

Published Papers

  • Yaxing Zhu, Shigeo Asahi, Naoya Miyashita, Yoshitaka Okada, Takashi Kita

    AIP Publishing, 28 Sep. 2021, Journal of Applied Physics, 130 (12), 124505 - 124505, English

    [Refereed]

    Scientific journal

  • Kohei Watanabe, Shigeo Asahi, Yaxing Zhu, Takashi Kita

    Corresponding, AIP Publishing, 28 Aug. 2021, Journal of Applied Physics, 130 (8), 085701 - 085701, English

    [Refereed]

    Scientific journal

  • Yaxing Zhu, Shigeo Asahi, Kohei Watanabe, Naoya Miyashita, Yoshitaka Okada, Takashi Kita

    AIP Publishing, 21 Feb. 2021, Journal of Applied Physics, 129 (7), 074503 - 074503, English

    [Refereed]

    Scientific journal

  • Takahiko Murata, Shigeo Asahi, Stefano Sanguinetti, Takashi Kita

    Springer Science and Business Media LLC, Dec. 2020, Scientific Reports, 10 (1)

    [Refereed]

    Scientific journal

  • Noriyuki Kinugawa, Shigeo Asahi, Takashi Kita

    Corresponding, American Physical Society (APS), 06 Jul. 2020, Physical Review Applied, 14 (1), English

    [Refereed]

    Scientific journal

  • Yukihiro Harada, Shigeo Asahi, Takashi Kita

    IOP Publishing, 01 Dec. 2019, Applied Physics Express, 12 (12), 125008 - 125008

    [Refereed]

    Scientific journal

  • Yukihiro Harada, Naoto Iwata, Shigeo Asahi, Takashi Kita

    IOP Publishing, 01 Sep. 2019, Semiconductor Science and Technology, 34 (9), 094003 - 094003

    Scientific journal

  • ASAHI SHIGEO, KAIZU TOSHIYUKI, KITA TAKASHI

    We studied the dynamics of electrons generated by two-step photoexcitation in an intermediate-band solar cell (IBSC) comprising InAs/GaAs/Al0.3Ga0.7As dot-in-well (DWELL) structure using time-resolved photocurrent (TRPC) measurement. The examined IBSC exhibited considerably slower photocurrent decay than a conventional InAs/GaAs quantum dot IBSC, which is due to the extraordina

    Lead, Nature Publishing Group, May 2019, Scientific Reports, 9, 7859 - 1-8, English

    [Refereed]

    Scientific journal

  • Mar. 2019, Physica E: Low-dimensional Systems and Nanostructures

    [Refereed]

  • ASAHI SHIGEO, KITA TAKASHI

    Nature Publishing Group, Feb. 2019, Nature Communications, 10, 956 - 1-3, English

    [Refereed]

    Scientific journal

  • Takashi Kita, Yukihiro Harada, Shigeo Asahi

    Springer Singapore, 2019, Green Energy and Technology

    [Refereed]

  • Shigeo Asahi, Kazuki Kusaki, Yukihiro Harada, Takashi Kita

    Development of high-efficiency solar cells is one of the attractive challenges in renewable energy technologies. Photon up-conversion can reduce the transmission loss and is one of the promising concepts which improve conversion efficiency. Here we present an analysis of the conversion efficiency, which can be increased by up-conversion in a single-junction solar cell with a hetero-interface that boosts the output voltage. We confirm that an increase in the quasi-Fermi gap and substantial photocurrent generation result in a high conversion efficiency.

    Lead, Nature Publishing Group, 01 Dec. 2018, Scientific Reports, 8 (1), pp. 872 - 1-8, English

    [Refereed]

    Scientific journal

  • Y. Harada, T. Matsuo, S. Asahi, T. Kita

    Nov. 2018, Proceedings of the 35th European Photovoltaic Solar Energy Conference and Exhibition, 126 - 128, English

    International conference proceedings

  • Hot-Carrier Generation in a Solar Cell Containing InAs/GaAs Quantum-Dot Superlattices as a Light Absorber

    DAIKI WATANABE, NAOTO IWATA, SHIGEO ASAHI, YUKIHIRO HARADA, KITA TAKASHI

    Jul. 2018, Applied Physics Express, Vol. 11 (No. 1), pp.082303 - 1-4, English

    [Refereed]

    Scientific journal

  • Kazuki Hirao, Shigeo Asahi, Toshiyuki Kaizu, Takashi Kita

    We studied intermediate-band solar cells (IBSCs) incorporating highly homogeneous InAs/GaAs quantum dot superlattices (QDSLs). The extra photocurrent generated by two-step photon absorption markedly increases at the fundamental state (FS) because the FS miniband has been successfully formed in the QDSL-IBSC by controlling the QD size homogeneity. Here, the carriers excited into the miniband spatially separate in the internal electric field, and the long-lived electrons in the intermediate states of the miniband increase the inter-subband absorption strength. The two-step photocurrent response, therefore, extends toward the longer-wavelength side corresponding to the QDSL-FS at approximately 1.2 mu m. (C) 2018 The Japan Society of Applied Physics

    IOP PUBLISHING LTD, Jan. 2018, APPLIED PHYSICS EXPRESS, 11 (1), pp. 012301 - 1-4, English

    [Refereed]

    Scientific journal

  • S. Asahi, T. Kita

    We have proposed a two-step photon up-conversion solar cell (TPU-SC), which is a single junction solar cell comprising a wide gap semiconductor (WGS) and a narrow gap semiconductor (NGS) to break through the Shockley-Queisser limit for the single-junction solar cells. In the TPU-SC, below-gap photons of WGS excite the NGS and accumulate electrons at the WGS / NGS hetero-interface. The accumulated electrons at the hetero-interface are easily excited towards the WGS barrier by the low-energy photons, resulting in the efficient two-step up-conversion (TPU). We have experimentally demonstrated highly efficient current generation by the TPU. In this paper, we present the concept of the TPU-SC, theoretical prediction of the conversion efficiency of the TPU-SC, and experimental result of efficient photocarrier collection attributable to the TPU phenomenon.

    SPIE, 2018, Proceedings of SPIE - The International Society for Optical Engineering, 10527, English

    [Invited]

    International conference proceedings

  • T. Kada, S. Asahi, T. Kaizu, Y. Harada, R. Tamaki, Y. Okada, T. Kita

    We studied the effects of the internal electric field on two-step photocarrier generation in InAs/GaAs quantum dot superlattice (QDSL) intermediate-band solar cells (IBSCs). The external quantum efficiency of QDSL-IBSCs was measured as a function of the internal electric field intensity, and compared with theoretical calculations accounting for interband and intersubband photoexcitations. The extra photocurrent caused by the two-step photoexcitation was maximal for a reversely biased electric field, while the current generated by the interband photoexcitation increased monotonically with increasing electric field intensity. The internal electric field in solar cells separated photogenerated electrons and holes in the superlattice (SL) miniband that played the role of an intermediate band, and the electron lifetime was extended to the microsecond scale, which improved the intersubband transition strength, therefore increasing the two-step photocurrent. There was a trade-off relation between the carrier separation enhancing the two-step photoexcitation and the electric-field-induced carrier escape from QDSLs. These results validate that long-lifetime electrons are key to maximising the two-step photocarrier generation in QDSL-IBSCs.

    NATURE PUBLISHING GROUP, Jul. 2017, SCIENTIFIC REPORTS, 7, pp. 5865 - 1-10, English

    [Refereed]

    Scientific journal

  • Sho Watanabe, Shigeo Asahi, Tomoyuki Kada, Kazuki Hirao, Toshiyuki Kaizu, Yukihiro Harada, Takashi Kita

    We studied the effects of miniband formation on the photocurrent generated by two-step intersubband absorption in an intermediate-band solar cell incorporating an InAs/GaAs quantum dot superlattice (QDSL). The two-step photocarrier generation increases with the electronic state coupling of InAs QDSLs in the intrinsic layer. Because carriers that are excited into the superlattice minibands spatially separate in an internal electric field, the electron-hole recombination rate for the photoexcited carriers decreases, and therefore, the electron lifetime increases. The long-lived electrons in the intermediate states of the QDSL miniband increase the intersubband absorption strength. We confirmed a systematic sensitive change in the two-step photocurrent generation depending on the miniband formation controlled by the temperature. Published by AIP Publishing.

    Corresponding, AMER INST PHYSICS, May 2017, APPLIED PHYSICS LETTERS, 110 (19), pp. 193104 - 1-5, English

    [Refereed]

    Scientific journal

  • Shigeo Asahi, Haruyuki Teranishi, Kazuki Kusaki, Toshiyuki Kaizu, Takashi Kita

    Reducing the transmission loss for below-gap photons is a straightforward way to break the limit of the energy-conversion efficiency of solar cells (SCs). The up-conversion of below-gap photons is very promising for generating additional photocurrent. Here we propose a two-step photon up-conversion SC with a hetero-interface comprising different bandgaps of Al0.3Ga0.7As and GaAs. The below- gap photons for Al0.3Ga0.7As excite GaAs and generate electrons at the hetero-interface. The accumulated electrons at the hetero-interface are pumped upwards into the Al0.3Ga0.7As barrier by below- gap photons for GaAs. Efficient two-step photon up-conversion is achieved by introducing InAs quantum dots at the hetero-interface. We observe not only a dramatic increase in the additional photocurrent, which exceeds the reported values by approximately two orders of magnitude, but also an increase in the photovoltage. These results suggest that the two-step photon up-conversion SC has a high potential for implementation in the next-generation high-efficiency SCs.

    NATURE PUBLISHING GROUP, Apr. 2017, NATURE COMMUNICATIONS, 8, pp. 14962 - 1-9, English

    [Refereed]

    Scientific journal

  • 半導体材料・デバイスの最新の進展 3. 太陽電池の変換効率限界を引き上げる半導体材料設計

    ASAHI SHIGEO, KITA TAKASHI

    Mar. 2017, 材料 別冊, 66 (3), pp. 244~249, Japanese

    [Refereed][Invited]

    Scientific journal

  • T. Tanibuchi, T. Kada, S. Asahi, D. Watanabe, T. Kaizu, Y. Harada, T. Kita

    We studied time-resolved photocarrier transport through InAs/GaAs quantum dot superlattice (QDSL) solar cells (SCs) using time-of-flight spectroscopy with an optical probe QD structure beneath the QDSL. Carriers optically pumped in the top p-GaAs layer were transported through the intrinsic layer, including the QDSLs, before arriving at the probe QDs. The photoexcited carrier density significantly influenced the time-resolved photoluminescence (PL) of the QDSLs and probe QDs. The time-resolved PL profile of the probe QDs indicated that excitation densities in excess of 25nJ/cm(2) drastically decreased the rise time, suggesting rapid carrier transport through the QDSLs. This was also confirmed by QDSL carrier transport dynamics, for which the PL intensity of the excited states decayed rapidly above this excitation power density, 25nJ/cm(2), while the ground state remained constant. These results demonstrate that filling the ground states of QDSLs and starting to populate the excited state miniband accelerates carrier transport in QDSL SCs. Furthermore, according to two-step photon absorption measurements taken with a 1.3-mu m infrared laser light source, electrons play a key role in the generation of extra photocurrent by sub-band-gap photon irradiation.

    AMER PHYSICAL SOC, Nov. 2016, PHYSICAL REVIEW B, 94 (19), pp. 195313 - 1 -9, English

    [Refereed]

    Scientific journal

  • Tomoyuki Kada, Shigeo Asahi, Yukihiro Harada, Takashi Kita

    We studied effects of the internal electric field on the two-step photocurrent generation in quantum dot superlattice (QDSL) solar cells. We calculated the quantum efficiency of intersubbad photoexcited carriers in QDSL as a function of the internal electric field. In our calculation, we proposed a model of a QDSL structure in which electrons created by the interband transition are excited by subbandgap photons corresponding to the intersubband transition. We found that extra photocurrent caused by the two-step photoexcitation shows the maximum at a reverse biased electric field, whereas current generated by only the interband photoexcitation increases monotonically with increasing the electric field. The internal electric field of the solar cell can separate photocreated electron and hole in the SL miniband, and electron lifetime is extended, which improve the intersubband transition strength, and, therefore, the two-step photocurrent increases. Thus, the calculated result unveils that there is a trade-off relation between carrier separation in the SL miniband and electric-field induced carrier escape from QDSL. These results clarify that long electron lifetime extended by carrier separation is a key maximizing the two-step photocurrent generation in a QDSL solar cell.

    Society of Materials Science Japan, 01 Sep. 2016, Zairyo/Journal of the Society of Materials Science, Japan, 65 (9), 647 - 651, Japanese

    [Refereed]

    Scientific journal

  • 量子ドット超格子太陽電池における2段階光励起電流生成ダイナミクスの電界依存特性

    加田 智之, 朝日 重雄, HARADA YUKIHIRO, KITA TAKASHI

    Sep. 2016, 日本材料学会会誌「材料」, 65 (9), pp. 647 - 651, Japanese

    [Refereed]

    Scientific journal

  • Shigeo Asahi, Haruyuki Teranishi, Naofumi Kasamatsu, Tomoyuki Kada, Toshiyuki Kaizu, Takashi Kita

    We studied in detail the photocurrent generation process in two-step photon absorption in intermediate-band solar cells, including InAs quantum dots embedded in Al0.3Ga0.7As/GaAs quantum wells at room temperature. The photocurrent generated by the two-step photon absorption exhibited saturation as the interband excitation intensity increased in strength. On the other hand, as the intersubband excitation intensity increased, the two-step photoexcitation current deviated from a power law. Furthermore, the two-step photoexcitation current exhibiting saturation and deviation strongly depended on both the interband and intersubband excitation intensities. To interpret these phenomena, we performed a theoretical simulation of the two-step photoexcitation current. The results suggest that the photocurrent saturation and deviation were caused by filling of the intermediate states with electrons. Furthermore, our calculated results indicate that the electron-recombination lifetime in the intermediate states is extremely long. The results of the temperature dependence of the two-step photoexcitation current and the excitation intensity dependence of photoluminescence suggest that efficient electron-hole separation extends electron lifetime.

    IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Mar. 2016, IEEE JOURNAL OF PHOTOVOLTAICS, 6 (2), 465 - 472, English

    [Refereed]

    Scientific journal

  • Kasidit Toprasertpong, Taizo Tanibuchi, Hiromasa Fujii, Tomoyuki Kada, Shigeo Asahi, Kentaroh Watanabe, Masakazu Sugiyama, Takashi Kita, Yoshiaki Nakano

    Institute of Electrical and Electronics Engineers ({IEEE}), Nov. 2015, IEEE Journal of Photovoltaics, 5 (6), 1613 - 1620, English

    [Refereed]

    Scientific journal

  • Shigeo Asahi, Haruyuki Teranishi, Naofumi Kasamatsu, Tomoyuki Kada, Toshiyuki Kaizu, Takashi Kita

    We systematically studied two-step photocurrent generation as functions of the excitation intensities for the interband and inter-subband transitions in an InAs/GaAs/Al0.3Ga0.7As dot-in-well (DWELL) intermediate-band solar cell. The two-step photoexcitation current shows saturation as the inter-band excitation intensity becomes strong, and we found that the inter-band excitation intensity showing the current saturation strongly depends on the inter-subband excitation intensity. To interpret the current-saturation behavior, we proposed a model and carried out theoretical simulation. Simulated results excellently reproduce the experimental observations. It has been clarified that the photocurrent saturation is caused by filling the intermediate states with electrons. Furthermore, the recombination lifetime in DWELL was pointed out to be extremely long. Our results suggest that this carrier lifetime is an important key to realize strong enhancement of two-step photoexcitation.

    Society of Materials Science Japan, 01 Sep. 2015, Zairyo/Journal of the Society of Materials Science, Japan, 64 (9), 690 - 695, Japanese

    [Refereed]

    Scientific journal

  • Kasidit Toprasertpong, Naofumi Kasamatsu, Hiromasa Fujii, Tomoyuki Kada, Shigeo Asahi, Yunpeng Wang, Kentaroh Watanabe, Masakazu Sugiyama, Takashi Kita, Yoshiaki Nakano

    In this study, we propose a carrier time-of-flight technique to evaluate the carrier transport time across a quantum structure in an active region of solar cells. By observing the time-resolved photoluminescence signal with a quantum-well probe inserted under the quantum structure at forward bias, the carrier transport time can be efficiently determined at room temperature. The

    {AIP} Publishing, Jul. 2015, Applied Physics Letters, 107 (4), 043901 - 043901, English

    [Refereed]

    Scientific journal

  • T. Kada, S. Asahi, T. Kaizu, Y. Harada, T. Kita, R. Tamaki, Y. Okada, K. Miyano

    We studied the two-step photon absorption (TSPA) process in InAs/GaAs quantum-dot superlattice (QDSL) solar cells. TSPA of subband-gap photons efficiently occurs when electrons are pumped from the valence band to the states above the inhomogeneously distributed fundamental states of QDSLs. The photoluminescence (PL)-excitation spectrum demonstrates an absorption edge attributed to the higher excited states of the QDSLs in between the InAs wetting layer states and the fundamental states of QDSLs. When the absorption edge of the excited state was resonantly excited, the superlinear excitation power dependence of the PL intensity demonstrated that the electron and hole created by the interband transition separately relax into QDSLs. Furthermore, time-resolved PL measurements demonstrated that the electron lifetime is extended by thereby inhibiting recombination with holes, enhancing the second subband-gap absorption.

    AMER PHYSICAL SOC, May 2015, PHYSICAL REVIEW B, 91 (20), pp. 201303 - 1-6, English

    [Refereed]

    Scientific journal

  • Ultrafast Photocarrier Transport Dynamics in InAs/GaAs Quantum Dot Superlattice Solar Cell

    Taizo Tanibuchi, Tomoyuki Kada, Naofumi Kasamatsu, Takuya Matsumura, Shigeo Asahi, Takashi Kita

    We studied time-resolved carrier transport through InAs/GaAs quantum dot superlattice (QDSL) solar cells, using time-of-flight spectroscopy with an optical probe structure lying beneath the QDSL. The density of photoexcited carriers in the top p-GaAs layer significantly influences the time-resolved photoluminescence (TRPL) of probe while TRPL of QDSL keeps unchanged. Also, the PL intensity of probe showed exponential increase as the excitation pulse energy increased, which may indicate that the dynamics of holes rule the dynamics observed in TRPL. The induced filling of QD states by strong excitation leads to the condition where carries travel over the QDSL and reach the probe faster.

    IEEE, 2015, 2015 IEEE 42ND PHOTOVOLTAIC SPECIALIST CONFERENCE (PVSC), English

    [Refereed]

    International conference proceedings

  • Saturable Two-step Photo current Generation in Intermediate-band Solar Cells Including InAs Quantum Dots Embedded in Al0.3Ga0.7As/GaAs Quantum Wells

    Shigeo Asahi, Haruyuki Teranishi, Naofumi Kasamatsu, Tomoyuki Kada, Toshiyuki Kaizu, Takashi Kita

    We have studied detailed photocurrent generation process in the two-step photon absorption in intermediate-band solar cells including InAs quantum dots embedded in Al0.3Ga0.7As/GaAs quantum wells and influence of thermal carrier escape at room temperature. The photocurrent generated by the two-step photon absorption shows saturation as the inter-band excitation intensity becomes strong. and the inter-band excitation intensity showing the saturation behavior strongly depends on the inter-subband excitation intensity. To interpret this phenomenon. we carried out a theoretical simulation based on carrier dynamics considering carrier generation. energy relaxation and thermal carrier escape. The results indicate that the photocurrent saturation is caused by filling the intermediate states with electrons. The shift of the saturation point depending on the inter-subband excitation intensity is caused by the shift of the quasi-Fermi level for the intermediate states.

    IEEE, 2015, 2015 IEEE 42ND PHOTOVOLTAIC SPECIALIST CONFERENCE (PVSC), English

    [Refereed]

    International conference proceedings

  • Comparison of Electron and Hole Mobilities in Multiple Quantum Well Solar Cells Using a Time-of-Flight Technique

    Kasidit Toprasertpong, Taizo Tanibuchi, Hiromasa Fujii, Tomoyuki Kada, Shigeo Asahi, Kentaroh Watanabe, Masakazu Sugiyama, Takashi Kita, Yoshiaki Nakano

    A difficulty of carrier transport in multiple quantum well ( MQW) solar cells is one critical issue that limits their cell performance. Here, direct measurement of electron and hole transport times across InGaAs/GaAsP MQWs has been carried out using our proposed time-of-flight measurement technique on p-on-n and n-on-p MQW structures, respectively. The corresponding effective mobilities are determined, allowing us to approximate the MQW region as a quasi-bulk material with smaller carrier mobilities than a bulk crystal. The result shows similar effective electron and hole mobilities. This results in the similar tendency of cell performance in p-on-n and n-on-p MQW solar cells.

    IEEE, 2015, 2015 IEEE 42ND PHOTOVOLTAIC SPECIALIST CONFERENCE (PVSC), English

    [Refereed]

    International conference proceedings

  • Tomoyuki Kada, Taizo Tanibuchi, Shigeo Asahi, Toshiyuki Kaizu, Yukihiro Harada, Takashi Kita, Ryo Tamaki, Yoshitaka Okada, Kenjiro Miyano

    We studied the two-step photon absorption (TSPA) process in InAs/GaAs quantum-dot superlattice (QDSL) solar cells. The photoluminescence (PL) and its excitation spectrum (PLE) showed the contribution of the higher excited states (ESs) forming the miniband of the QDSLs above the in homogeneously distributed ground states (GSs). TSPA of subbandgap photons efficiently occurs when electrons are pumped from the valence band (VB) to the higher ESs. When the higher ESs were resonantly excited, the superlinear excitation power dependence of the PL intensity appeared. Moreover, time-resolved PL showed that the electron lifetime is extended. These results demonstrate that the excited electron and hole separately relax into QDSLs, and thereby, enhancing the second sub-bandgap absorption.

    IEEE, 2015, 2015 IEEE 42ND PHOTOVOLTAIC SPECIALIST CONFERENCE (PVSC), English

    [Refereed]

    International conference proceedings

  • S. Asahi, H. Teranishi, N. Kasamatsu, T. Kada, T. Kaizu, T. Kita

    We have studied detailed carrier generation process in the two-step photon absorption and influence of thermal carrier escape in quantum-dot intermediate-band solar cells (QD-IBSC). The photocurrent created by the two-step photon absorption shows saturation as the inter-band excitation intensity becomes strong, and the inter-band excitation intensity showing the saturation behavior strongly depends on the inter-subband excitation intensity. To interpret this phenomenon, we carried out a theoretical simulation based on carrier dynamics considering carrier generation, energy relaxation and thermal carrier escape. The results indicate that the photocurrent saturation is caused by filling the intermediate states. The shift of the saturation point depending on the inter-subband excitation intensity is caused by the shift of the quasi-Fermi level for the intermediate states.

    SPIE-INT SOC OPTICAL ENGINEERING, 2015, PHYSICS SIMULATION AND PHOTONIC ENGINEERING OF PHOTOVOLTAIC DEVICES IV, 9358, English

    International conference proceedings

  • Kasidit Toprasertpong, Naofumi Kasamatsu, Hiromasa Fujii, Tomoyuki Kada, Shigeo Asahi, Yunpeng Wang, Kentaroh Watanabe, Masakazu Sugiyama, Takashi Kita, Yoshiaki Nakano

    Institute of Electrical and Electronics Engineers ({IEEE}), Nov. 2014, IEEE Journal of Photovoltaics, 4 (6), 1518 - 1525, English

    [Refereed]

    Scientific journal

  • S. Asahi, H. Teranishi, N. Kasamatsu, T. Kada, T. Kaizu, T. Kita

    We investigated the effects of an increase in the barrier height on the enhancement of the efficiency of two-step photo-excitation in InAs quantum dot (QD) solar cells with a dot-in-well structure. Thermal carrier escape of electrons pumped in QD states was drastically reduced by sandwiching InAs/GaAs QDs with a high potential barrier of Al0.3Ga0.7As. The thermal activation energy increased with the introduction of the barrier. The high potential barrier caused suppression of thermal carrier escape and helped realize a high electron density in the QD states. We observed efficient two-step photon absorption as a result of the high occupancy of the QD states at room temperature. (C) 2014 AIP Publishing LLC.

    AMER INST PHYSICS, Aug. 2014, JOURNAL OF APPLIED PHYSICS, 116 (6), pp. 063510 - 1-5, English

    [Refereed]

    Scientific journal

  • Yaxing Zhu, Shigeo Asahi, Naoya Miyashita, Yoshitaka Okada, Takashi Kita

    Abstract We elucidate a photocarrier collection mechanism in intermediate band solar cells (IBSCs) with InAs-quantum dots (QDs)-in-an-Al0.3Ga0.7As/GaAs-quantum well structures. When the Al0.3Ga0.7As barrier is excited, the device electrical output can be varied by additional infrared light for the electron intraband optical transition in QDs. The photocurrent in IBSC with a single QDs-in-a-well structure shows a monotonic increase with the intraband-excitation density. Conversely, IBSC with a multilayered QDs-in-a-well structure exhibits a photocurrent reduction when electrons in QDs are optically pumped out. The simultaneously measured photoluminescence spectra proved that the polarity of QD states changes depending on the intraband-excitation density. We discuss the drift and diffusion current components and point out that the hole diffusion current is significantly influenced by carriers inside the confinement structure. Under strong intraband excitations, we consider an increased hole diffusion current occurs by blocking hole-capture in the quantum structures. This causes unexpected photocurrent reduction in the multilayered device.

    IOP Publishing, 01 Jul. 2022, Japanese Journal of Applied Physics, 61 (7), 074002 - 074002

    Scientific journal

MISC

  • Shigeo Asahi, Takashi Kita

    Society of Materials Science Japan, 01 Mar. 2017, Zairyo/Journal of the Society of Materials Science, Japan, 66 (3), 244 - 249, Japanese

    Book review

  • キャリア走行時間測定法による量子構造太陽電池内のダイナミクスの直接観測

    TOPRASERTPONG KASIDIT, KASAMATSU, NAOFUMI, FUJII HIROMASA, KADA TOMOYUKI, ASAHI SHIGEO, WANG YUNPENG, WATANABE KENTARO, SUGIYAMA MASAKAZU, KITA TAKASHI, NAKANO YOSHIAKI

    01 Sep. 2014, 応用物理学会秋季学術講演会講演予稿集(CD-ROM), 75th, ROMBUNNO.17P-A28-1, Japanese

  • プローブ構造を用いた量子構造太陽電池におけるキャリア走行時間の測定

    TOPRASERTPONG KASIDIT, KASAMATSU, NAOFUMI, FUJII HIROMASA, KADA TOMOYUKI, ASAHI SHIGEO, WANG YUNPENG, WATANABE KENTARO, SUGIYAMA MASAKAZU, KITA TAKASHI, NAKANO YOSHIAKI

    03 Mar. 2014, 応用物理学会春季学術講演会講演予稿集(CD-ROM), 61st, ROMBUNNO.19P-D7-11, Japanese

Books etc

  • Energy Conversion Efficiency of Solar Cells

    KITA Takashi, HARADA Yukihiro, ASAHI Shigeo

    Joint work, Springer, Aug. 2019, ISBN: 9789811390890

Presentations

  • Route to High Conversion Efficiency Solar Cell

    Shigeo Asahi, Yukihiro Harada, Takashi Kita

    The 6th International Conference on New Energy Future Energy Systems (NEFES 2021), 02 Nov. 2021, English

    [Invited]

    Invited oral presentation

  • 2段階フォトンアップコンバージョン太陽電池における 断熱的バンド内光励起による擬フェルミ準位分裂

    朝日重雄, 渡辺航平, Y. Zhu, 喜多隆

    第40回電子材料シンポジウム, 11 Oct. 2021, Japanese

    Poster presentation

  • On the Simulation of Two-Step Photocurrent Generation in an InAs Quantum Dot -in-Well Intermediate Band Solar Cell

    Y. Zhu, S. Asahi, T. Kita

    第82回応用物理学会秋季学術講演会, 11 Sep. 2021, English

    Oral presentation

  • 正孔のアップコンバージョンを利用した2段階フォトンアップコンバージョン太陽電池のバンド内遷移過程

    朝日重雄, M, P .Nielsen, 池田一真, N. J. Ekins-Daukes, 喜多隆

    第82回応用物理学会秋季学術講演会, 10 Sep. 2021, Japanese

    Oral presentation

  • Two-step photon up-conversion solar cells: recent progress and future directions

    Shigeo Asahi, Takashi Kita

    8th International Workshop on Epitaxial Growth and Fundamental Properties of Semiconductor Nanostructures (SemiconNano2021), 03 Sep. 2021, English

    [Invited]

    Invited oral presentation

  • Efficiency Compensation from Intraband Transitions of Opposite Carrier in a Quantum Dot-in-Well Intermediate Band Solar Cell

    Y. Zhu, S. Asahi, T. Kita

    第68回応用物理学会春季学術講演会, 17 Mar. 2021, English

    Oral presentation

  • 変調ドープした二段階フォトンアップコンバージョン太陽電池における電圧上昇効果

    渡辺航平, 朝日重雄, 喜多隆

    第68回応用物理学会春季学術講演会, 17 Mar. 2021, Japanese

    Oral presentation

  • 高効率2段階フォトンアップコンバージョン太陽電池の基礎動作検証

    朝日重雄

    第4回フロンティア太陽電池セミナー, 26 Nov. 2019, Japanese

    [Invited]

    Invited oral presentation

  • Strong Voltage-Boost Effect in Two-Step Photon-Up Conversion Solar Cells

    SHIGEO ASAHI, KITA TAKASHI

    46th IEEE Photovoltaic Specialists Conference, Jul. 2019, English, Chicago, International conference

    Oral presentation

  • Hot-Carrier Extraction in InAs/GaAs Quantum Dot Superlattice Solar Cells

    HARADA YUKIHIRO, IWATA NAOTO, WATANABE DAIKI, ASAHI SHIGEO, KITA TAKASHI

    46th IEEE Photovoltaic Specialists Conference, Jul. 2019, English, Chicago, International conference

    Oral presentation

  • Reciprocal Relationship Between Photoluminescence and Photocurrent in Two-Step Photon Up-Conversion Solar Cell

    NORIYUKI KINUGAWA, SHIGEO ASAHI, KITA TAKASHI

    46th IEEE Photovoltaic Specialists Conference, Jun. 2019, English, Chicago, International conference

    Oral presentation

  • Efficient Hot-Carrier Generation in InAs/GaAs Quantum Dot Superlattices

    HARADA YUKIHIRO, IWATA NAOTO, WATANABE DAIKI, ASAHI SHIGEO, KITA TAKASHI

    Optics&Photonics International Congress 2019, Apr. 2019, English, Yokohama, International conference

    Oral presentation

  • InAs/GaAs量子ドット超格子を用いたホットキャリア型太陽電池における開放電圧の向上

    HARADA YUKIHIRO, 岩田 尚之, ASAHI SHIGEO, KITA TAKASHI

    第66回応用物理学会春季学術講演会, Mar. 2019, Japanese, Domestic conference

    Oral presentation

  • Voltage boost effect in two-step photon up-conversion solar cell with partial absorptivity

    Y. Harada, S. Asahi, T. Kita

    第37回電子材料シンポジウム, Oct. 2018, Japanese, Domestic conference

    Poster presentation

  • Optimal Band Gap Energies for Two-Step Photon Up-Conversion Solar Cells with Partial Absorptivity

    YUKIHIRO HARADA, TETSUHIRO MATSUO, SHIGEO ASAHI, KITA TAKASHI

    35th European PV Solar Energy Conference and Exhibition, Sep. 2018, English, Belgium, International conference

    Oral presentation

  • InAs/GaAs量子ドット超格子太陽電池におけるホットキャリア電流取り出し特性

    岩田 尚之, 渡部 大樹, HARADA YUKIHIRO, ASAHI SHIGEO, KITA TAKASHI

    第79回応用物理学会秋季学術講演会, Sep. 2018, Japanese, Domestic conference

    Oral presentation

  • 2段階フォトンアップコンバージョン太陽電池における理論変換効率の入射光スペクトル形状依存性

    HARADA YUKIHIRO, 松尾 哲弘, ASAHI SHIGEO, KITA TAKASHI

    第79回応用物理学会秋季学術講演会, Sep. 2018, Japanese, Domestic conference

    Oral presentation

  • 超高効率フォトンアップコンバージョン太陽電池の提案と基礎検証

    朝日重雄

    2018 年度 応用物理学会中国四国支部・若手半導体研究会, 05 Aug. 2018, Japanese

    [Invited]

    Invited oral presentation

  • Carrier Collection Efficiency of Intraband-Excited Carriers in Two-Step Photon Up-Conversion Solar Cells

    ASAHI SHIGEO, K. Nishimura, KAIZU TOSHIYUKI, KITA TAKASHI

    The 7th edition of the World Conference on Photovoltaic Energy Conversion, Jun. 2018, English, Hawaii, International conference

    Oral presentation

  • 入射光スペクトル形状を考慮した2段階フォトンアップコンバージョン太陽電池の理論変換効率

    MATSUO TETSUHIRO, HARADA YUKIHIRO, ASAHI SHIGEO, KITA TAKASHI

    応用物理学会関西支部平成30年度第1回講演会, May 2018, Japanese, 神戸大学, Domestic conference

    Poster presentation

  • Polarization Dependent Photocurrent in InAs/GaAs Quantum Dot Superlattice Solar Cells

    HARADA YUKIHIRO, J. Yamada, D. Watanabe, ASAHI SHIGEO, KITA TAKASHI

    International Conference on Nanophotonics and Nano-optoelectronics 2018, Apr. 2018, English, Yokohama, International conference

    Oral presentation

  • Extraction Efficiency of Up-Converted Electrons in Two-Step Photon Up-Conversion Solar Cells

    ASAHI SHIGEO, K. Nishimura, KAIZU TOSHIYUKI, KITA TAKASHI

    International Conference on Nanophotonics and Nano-optoelectronics 2018, Apr. 2018, English, Yokohama, International conference

    Oral presentation

  • 吸収率を考慮した2段階フォトンアップコンバージョン太陽電池の理論変換効率

    HARADA YUKIHIRO, ASAHI SHIGEO, KITA TAKASHI

    第65回応用物理学会春季学術講演会, Mar. 2018, Japanese, Domestic conference

    Oral presentation

  • InAs/GaAs量子ドット超格子を用いたホットキャリア型太陽電池動作実証

    岩田 尚之, 渡部 大樹, HARADA YUKIHIRO, ASAHI SHIGEO, KITA TAKASHI

    第65回応用物理学会春季学術講演会, Mar. 2018, Japanese, Domestic conference

    Oral presentation

  • InAs/GaAs量子ドット超格子を用いたホットキャリア型太陽電池の動作評価

    岩田 尚之, 渡部 大樹, HARADA YUKIHIRO, ASAHI SHIGEO, KITA TAKASHI

    応用物理学会関西支部平成29年度第3回講演会, Feb. 2018, Japanese, Domestic conference

    Poster presentation

  • Two-Step Photon Up-Conversion Solar Cell: Propose and Demonstration

    ASAHI SHIGEO, KITA TAKASHI

    SPIE Photonics West, Jan. 2018, English, San Francisco, International conference

    [Invited]

    Invited oral presentation

  • Two-Step Photon Up-Conversion Solar Cell

    KITA TAKASHI, ASAHI SHIGEO

    MTSA2017-OptoX Nano-TeraNano8, Nov. 2017, English, Okayama, International conference

    Oral presentation

  • Two-Step Photo-Excitated Electrons with Extremely-Long Lifetime in Intermedeate-band Solar Cells Using Dot-in Well Strucyure

    ASAHI SHIGEO, H. Teranishi, KAIZU TOSHIYUKI, KITA TAKASHI

    The 27th Photovoltaic Science and Engineering Conference, Nov. 2017, English, Otsu, International conference

    Oral presentation

  • Two-Step Photn Up-Conversion Solar Cells Incorporating a Voltage Booster Hetero-Interface

    ASAHI SHIGEO, K. Kusaki, HARADA YUKIHIRO, KITA TAKASHI

    The 27th Photovoltaic Science and Engineering Conference, Nov. 2017, English, Otsu, International conference

    Oral presentation

  • Infrared Absorption Characteristics in Two-Step Photon Up-Conversion Solar Cells

    K. Kusaki, ASAHI SHIGEO, KAIZU TOSHIYUKI, R. Tamaki, Y. Okada, KITA TAKASHI

    The 27th Photovoltaic Science and Engineering Conference, Nov. 2017, English, Otsu, International conference

    Oral presentation

  • Efficient Two-Step Photocurrent in Intermediate Band Solar Cells Using Highly Homogeneous InAs/GaAs Quantum-Dot Superlattice

    K. Hirao, ASAHI SHIGEO, KAIZU TOSHIYUKI, HARADA YUKIHIRO, KITA TAKASHI

    The 27th Photovoltaic Science and Engineering Conference, Nov. 2017, English, Otsu, International conference

    Oral presentation

  • 低温キャップInAs/GaAs量子ドット超格子中間バンド型太陽電池における熱脱出の抑制

    平尾 和輝, ASAHI SHIGEO, KAIZU TOSHIYUKI, HARADA YUKIHIRO, KITA TAKASHI

    第78回応用物理学会秋季学術講演会, Sep. 2017, Japanese, Domestic conference

    Oral presentation

  • 低温キャップInAs/GaAs量子ドット超格子中間バンド型太陽電池における2段階光吸収の増強

    平尾 和輝, ASAHI SHIGEO, KAIZU TOSHIYUKI, HARADA YUKIHIRO, KITA TAKASHI

    日本材料学会半導体エレクトロニクス部門委員会平成29年度第1回研究会, Sep. 2017, Japanese, Domestic conference

    Oral presentation

  • Photon Up-Converted Photocurrent in a Single Junction Solar Cell with a Hetero-Interface

    K. Kusaki, ASAHI SHIGEO, KAIZU TOSHIYUKI, KITA TAKASHI

    33rd European Photovoltaic Solar Energy Conference and Exhibition, Sep. 2017, English, Netherlands, International conference

    Oral presentation

  • Increasing Photovoltage Boosted by Photon Up-Conversion in a Single-Junction Solar Cell with a Hetero-Interface

    ASAHI SHIGEO, K. Kusaki, KAIZU TOSHIYUKI, KITA TAKASHI

    33rd European Photovoltaic Solar Energy Conference and Exhibition, Sep. 2017, English, Netherlands, International conference

    Oral presentation

  • InAs/GaAs量子ドット超格子太陽電池におけるバンド内遷移の偏光特性

    HARADA YUKIHIRO, 山田 淳也, 渡部 大樹, ASAHI SHIGEO, KITA TAKASHI

    第78回応用物理学会秋季学術講演会, Sep. 2017, Japanese, Domestic conference

    Oral presentation

  • Extended Optical Response of Two-Step Photoexcitation in InAs/GaAs Quantum-Dot Superlattice Intermediate Band Solar Cells

    K. Hirao, ASAHI SHIGEO, KAIZU TOSHIYUKI, KITA TAKASHI

    33rd European Photovoltaic Solar Energy Conference and Exhibition, Sep. 2017, English, Netherlands, International conference

    Oral presentation

  • Carrier Dynamics in Photon Up-Conversion Solar Cells

    KITA TAKASHI, ASAHI SHIGEO

    SemiconNano 2017: 6th International Workshop Epitaxial Growth and Fundamental Properties of Semiconductor Nanostructures, Sep. 2017, English, Milano, International conference

    Oral presentation

  • Increasing Current Generation by Photon Up-Conversion in a Single-Junction Solar Cell with a Hetero-Interface

    ASAHI SHIGEO, K. Kusaki, KAIZU TOSHIYUKI, KITA TAKASHI

    2017 IEEE Photovoltaic Specialists Conference, Jun. 2017, English, Washington D.C., International conference

    Oral presentation

  • InAs/GaAs量子ドット中間バンド型太陽電池における電子の熱脱出過程の解明

    平尾 和輝, 渡辺 翔, ASAHI SHIGEO, KAIZU TOSHIYUKI, HARADA YUKIHIRO, KITA TAKASHI

    第77回応用物理学会秋季学術講演会, Sep. 2016, Japanese, Domestic conference

    Oral presentation

  • Thermal carrier-escape process from the intermediate band in InAs/GaAs quantum dot solar cells

    K. Hirao, S. Asahi, S. Watanabe, T. Kaizu, Y. Harada, T. Kita

    第35回電子材料シンポジウム, Jul. 2016, Japanese, Domestic conference

    Poster presentation

  • Polarization-Insensitive Intraband Transition in InAs/GaAs Quantum Dot Superlattices

    HARADA YUKIHIRO, J. Yamada, D. Watanabe, S. Asahi, KITA TAKASHI

    UK Semiconductors 2016, Jul. 2016, English, Sheffield, International conference

    Oral presentation

  • Extended Electron Lifetime in Intermediate-Band Solar Cells Using Dot-in-Well Structure

    S. Asahi, H. Teranishi, S. Watanabe, T. Kada, KAIZU TOSHIYUKI, KITA TAKASHI

    32nd European Photovoltaic Solar Energy Conference and Exhibition, Jun. 2016, English, Munich, International conference

    Oral presentation

  • Enhancement of Two-Step Photon Absorption Due to Miniband Formation in InAs/GaAs Quantum Dot Superlattice Solar Cell

    S. Watanabe, S. Asahi, T. Kada, KAIZU TOSHIYUKI, HARADA YUKIHIRO, KITA TAKASHI

    32nd European Photovoltaic Solar Energy Conference and Exhibition, Jun. 2016, English, Munich, International conference

    Oral presentation

  • 2段階フォトンアップコンバージョン太陽電池における ヘテロ界面のバンド内遷移の光吸収率

    朝日 重雄, H.Mahamu, 喜多 隆

    第69回応用物理学会春季学術講演会, 23 Mar. 2022, Japanese

    Oral presentation

  • 正孔フォトンアップコンバージョン太陽電池の赤外光照射による光電流制御

    豊友太, 朝日重雄, H. Mahamu, M, P. Nielsen, N, J. Ekins-Daukes, 喜多隆

    第69回応用物理学会春季学術講演会, 23 Mar. 2022, Japanese

    Oral presentation

Association Memberships

  • IEEE Electron Devices Society Membership

    Jan. 2018 - Present
  • IEEE Membership

    Jan. 2018 - Present
  • 応用物理学会 正会員

    Jan. 2017 - Present

Research Projects

  • Control of intraband optical transition sensitized in infrared wavelength region and application for ultra-high sensitive infrared sensor

    喜多 隆, 原田 幸弘, 朝日 重雄

    Japan Society for the Promotion of Science, Grants-in-Aid for Scientific Research Grant-in-Aid for Challenging Research (Exploratory), Grant-in-Aid for Challenging Research (Exploratory), Kobe University, Jul. 2020 - Mar. 2022

  • Realization of high-efficiency energy conversion in solar cells using photon up-conversion

    朝日 重雄

    Japan Society for the Promotion of Science, Grants-in-Aid for Scientific Research Grant-in-Aid for Early-Career Scientists, Grant-in-Aid for Early-Career Scientists, Kobe University, Apr. 2020 - Mar. 2023

  • Novel development of infrared sensitized photovoltaics based on control of intraband optical transition dipole

    喜多 隆, 原田 幸弘, 朝日 重雄

    Japan Society for the Promotion of Science, Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research (A), Grant-in-Aid for Scientific Research (A), Kobe University, 01 Apr. 2019 - 31 Mar. 2023

  • フォトンアップコンバージョン太陽電池における高効率エネルギー変換の実現

    公益財団法人ひょうご科学技術協会, 学術研究助成, Apr. 2019 - Mar. 2020, Principal investigator

  • 喜多 隆

    学術研究助成基金助成金/国際共同研究加速基金(国際共同研究強化(B)), Oct. 2018 - Mar. 2021

    Competitive research funding

  • 朝日 重雄

    科学研究費補助金/研究活動スタート支援, Aug. 2018 - Mar. 2020, Principal investigator

    Competitive research funding

  • 半導体ヘテロ接合を有する単接合型太陽電池における光アップコンバージョンによる光起電力の増強

    公益財団法人 小笠原科学技術振興財団, 国際研究集会出張助成, Nov. 2017 - Nov. 2017

  • ヘテロ接合を有する単接合型太陽電池の光アップコンバージョンによる電流生成の増大

    公益財団法人 関西エネルギー・リサイクル科学技術振興財団, 国際交流活動助成(渡航), Jun. 2017 - Jun. 2017

  • ドットインウェル構造を用いた中間バンド型太陽電池における、電子寿命の伸長

    公益財団法人 村田学術振興財団, 海外派遣援助, Apr. 2016 - Apr. 2016

  • 公益財団法人 関西エネルギー・リサイクル科学技術振興財団, 国際交流活動助成(海外渡航), Al0.3Ga0.7As/GaAs量子井戸に埋め込まれたInAs量子ドットを使用した中間バンド型太陽電池の2段階光キャリア生成の飽和, Jun. 2015 - Jun. 2015, Principal investigator

Industrial Property Rights

  • 高変換効率太陽電池およびその調製方法

    KITA TAKASHI, HARADA YUKIHIRO, ASAHI SHIGEO, 渡部 大樹

    特願2014-113313, 30 May 2014, 大学長, 特許6385720, 17 Aug. 2018

    Patent right