Research activity information

• Sep. 2022 9th International Symposium on Control of Semiconductor Interfaces (ISCSI-XI), Young Researcher Award, Photoluminescence Characteristics of InAs Quantum Dots in the Doubled-heterointerface of AlGaAs/GaAs-based Two-step Photon Up-conversion Solar Cells

  H. Mahamu, S. Asahi, T. Kita

  共同研究・競争的資金等の業績IDIRI


• 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

• Nonlinear photoexcitation processes at a MAPbBr3/GaAs heterointerface

  Hambalee Mahamu, Matthias Bourzier, Shigeo Asahi, Takashi Kita

  Abstract While a significant part of the solar energy lies in the infrared range, common semiconductors cannot absorb this part of the solar irradiance by direct band-to-band transitions, because the corresponding photon energies are below the bandgap energy. Two-step photon up-conversion (TPU) is one of the processes that allows us to harvest energy in the region below the bandgap, and one possible approach to realize a TPU-based solar cell is to use an AlGaAs/GaAs heterointerface with quantum dots in order to induce additional intraband transitions. On the other hand, here we report on the TPU phenomenon at a methylammonium lead bromide/gallium arsenide (MAPbBr₃/GaAs) heterointerface without quantum dots. For this heterojunction, we observed high-energy photoemission by low-energy photoinjection, demonstrating the TPU. By using photoluminescence (PL) and time-resolved PL measurement techniques, we elucidate the mechanism of the PL emission from MAPbBr₃ observed from MAPbBr₃/GaAs samples. Through the comparisons of the experimental PL and TRPL results between the MAPbBr₃/GaAs and MAPbBr₃/Glass-substrate samples, we successfully distinguish the TPU phenomenon from the ordinal two-photon absorption of MAPbBr₃. Our findings in the TPU at the MAPbBr₃/GaAs heterointerface may help to realize quantum-dot-free photon up-conversion solar cells.

  Springer Science and Business Media LLC, Jan. 2025, Journal of Optics

  [Refereed]

  Scientific journal

  Link to Kobe Univ. Repository (Kernel)


• The basic monochromatic photovoltaic properties of a photodiode for laser power conversion

  Ryota Hanakuma, Shigeo Asahi, Yukihiro Harada, Takashi Kita

  Abstract We studied the photovoltaic properties of a conventional silicon photodiode under monochromatic illumination conditions to clarify the loss mechanisms that are important for application as a laser power converter. While the short-circuit current increases linearly with the excitation power, the power dependence of the open-circuit voltage consists of two regions with different slopes as a result of the Joule heating. At higher excitation power densities, thermal effects play a key role in the current–voltage characteristics, and therefore the maximum conversion efficiency is achieved at a certain excitation-power density. Furthermore, the optimum excitation wavelength shifts towards longer wavelengths as the excitation power density increases, because the optimum value is determined by a trade-off between the optical absorption strength and the excitation power density.

  IOP Publishing, Jan. 2025, Japanese Journal of Applied Physics, 64(1) (1), 014001 - 014001

  [Refereed]

  Scientific journal

  Link to Kobe Univ. Repository (Kernel)


• Conduction Band Resonant State Absorption for Quantum Dot Infrared Detectors Operating at Room Temperature

  Stefano Vichi, Shigeo Asahi, Sergio Bietti, Artur Tuktamyshev, Alexey Fedorov, Takashi Kita, Stefano Sanguinetti

  American Chemical Society (ACS), Dec. 2024, ACS Photonics, 12(1) (1), 447 - 456

  [Refereed]

  Scientific journal


• Intraband transitions at a CsPbBr3/GaAs heterointerface in a two-step photon upconversion solar cell

  Hambalee Mahamu, Shigeo Asahi, Takashi Kita

  Abstract Two-step photon upconversion solar cells (TPU-SCs) based on III–V semiconductors can achieve enhanced sub-bandgap photon absorption because of intraband transitions at the heterointerface. From a technological aspect, the question arose whether similar intraband transitions can be realized by using perovskite/III–V semiconductor heterointerfaces. In this article, we demonstrate a TPU-SC based on a CsPbBr₃/GaAs heterointerface. Such a solar cell can ideally achieve an energy conversion efficiency of 48.5% under 1-sun illumination. This is 2.1% higher than the theoretical efficiency of an Al_0.3Ga_0.7As/GaAs-based TPU-SC. Experimental results of the CsPbBr₃/GaAs-based TPU-SC show that both the short-circuit current J_SC and the open-circuit voltage V_OC increase with additional illumination of sub-bandgap photons. We analyze the excitation power dependence of J_SC for different excitation conditions to discuss the mechanisms behind the enhancement. In addition, the observed voltage-boost clarifies that the J_SC enhancement is caused by an adiabatic optical process at the CsPbBr₃/GaAs heterointerface, where sub-bandgap photons efficiently pump the electrons accumulated at the heterointerface to the conduction band of CsPbBr₃. Besides the exceptional optoelectronic properties of CsPbBr₃ and GaAs, the availability of a CsPbBr₃/GaAs heterointerface for two-step photon upconversion paves the way for the development of high-efficiency perovskite/III–V semiconductor-based single-junction solar cells.

  Springer Science and Business Media LLC, Nov. 2024, Scientific Reports, 14(1) (1)

  [Refereed]

  Scientific journal

  Link to Kobe Univ. Repository (Kernel)


• Electric Field Enhancement Effect by Localized Surface Plasmon Resonance in Heavily-Doped InAs/GaAs Quantum Dots

  Mizuto KAWAKAMI, Yukihiro HARADA, Shigeo ASAHI, Takashi KITA

  Society of Materials Science, Japan, Feb. 2024, Journal of the Society of Materials Science, Japan, 73(2) (2), 178 - 182

  Scientific journal


• 量子ドットを内包するヘテロ界面を利用した2段階フォトンアップコンバージョン太陽電池

  朝日重雄, 原田幸弘, 喜多隆

  Lead, 公益社団法人応用物理学会, Sep. 2023, 応用物理, 92(9) (9), 550 - 554, Japanese

  [Refereed]


• Multi-step photon upconversion in quantum-dot-based solar cells with a double-heterointerface structure

  Hambalee Mahamu, Shigeo Asahi, Takashi Kita

  Photon upconversion (PU) is a process where an electron is excited from the valence band to the conduction band of a wide-gap semiconductor by the sequential absorption of two or more photons via real states. For example, two-step PU can generate additional photocurrent in the so-called intermediate-band solar cells. In this work, we consider two- and three-step processes; we study multi-step PU in a quantum dot (QD)-based single-junction solar cell with a double-heterointerface structure. The solar cell consists of three different absorber layers: Al_0.7Ga_0.3As, Al_0.3Ga_0.7As, and GaAs, which form two heterointerfaces. Just beneath each heterointerface, an InAs/GaAs QD layer was inserted. After band-to-band excitation, electrons accumulate at each heterointerface, and then, below-bandgap photons excite a certain fraction of these electrons above the barrier energy. The photoluminescence spectra of the InAs QDs reveal slightly different QD size distributions at the two heterointerfaces. We show that the external quantum efficiency is improved by additional irradiation with below-bandgap infrared photons, which suggests a multi-step PU process that involves the two heterointerfaces. The dependence of the photocurrent on the infrared excitation power density only shows a superlinear behavior when the GaAs layer is excited but the Al_0.3Ga_0.7As layer is not. These data demonstrate a multi-step PU process that consists of one intraband transition at each of the two heterointerfaces and one interband transition in GaAs.

  AIP Publishing, Mar. 2023, Journal of Applied Physics, 133(12) (12), 124503 - 124503

  [Refereed]

  Scientific journal


• Diffusion photocurrent influenced by intraband excitation in an intermediate band solar cell with type-I band alignment

  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-Al_0.3Ga_0.7As/GaAs-quantum well structures. When the Al_0.3Ga_0.7As 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, Jul. 2022, Japanese Journal of Applied Physics, 61(7) (7), 074002 - 074002, English

  [Refereed]

  Scientific journal


• Erratum: Two-photon photocurrent spectra of InAs quantum dot-in-well intermediated-band solar cells at room temperature (Journal of Applied Physics (2021) 130 (124505) DOI: 10.1063/5.0060569)

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

  American Institute of Physics Inc., Nov. 2021, Journal of Applied Physics, 130(17) (17), English

  Scientific journal


• Two-photon photocurrent spectra of InAs quantum dot-in-well intermediated-band solar cells at room temperature

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

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

  [Refereed]

  Scientific journal


• Voltage boost effects in two-step photon upconversion solar cells with a modulation-doped structure

  Kohei Watanabe, Shigeo Asahi, Yaxing Zhu, Takashi Kita

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

  [Refereed]

  Scientific journal

  Link to Kobe Univ. Repository (Kernel)


• Collection of Photocarriers Varied by Effective Electron Intraband Excitation in an InAs Quantum Dot-in-Well Intermediate Band Solar Cell

  Yaxing Zhu, Shigeo Asahi, Takashi Kita

  Institute of Electrical and Electronics Engineers Inc., Jun. 2021, Conference Record of the IEEE Photovoltaic Specialists Conference, 1786 - 1788, English

  International conference proceedings


• Two-step excitation induced photovoltaic properties in an InAs quantum dot-in-well intermediate-band solar cell

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

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

  [Refereed]

  Scientific journal


• Infrared photodetector sensitized by InAs quantum dots embedded near an Al0.3Ga0.7As/GaAs heterointerface

  Takahiko Murata, Shigeo Asahi, Stefano Sanguinetti, Takashi Kita

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

  [Refereed]

  Scientific journal

  Link to Kobe Univ. Repository (Kernel)


• Reciprocal Relation Between Intraband Carrier Generation and Interband Recombination at the Heterointerface of Two-Step Photon Up-Conversion Solar Cells

  Noriyuki Kinugawa, Shigeo Asahi, Takashi Kita

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

  [Refereed]

  Scientific journal

  Link to Kobe Univ. Repository (Kernel)


• Intensive-Light-Induced Backtracking Voltage Phenomenon: An Insight into Intermediate-Band Solar Cell Output Performance

  Yaxing Zhu, Shigeo Asahi, Takashi Kita

  Institute of Electrical and Electronics Engineers Inc., Jun. 2020, Conference Record of the IEEE Photovoltaic Specialists Conference, 2020-, 0146 - 0148, English

  International conference proceedings


• Up-converted photocurrent enhancement in modulation-doped two-step photon up-conversion solar cells

  Kohei Watanabe, Shigeo Asahi, Yaxing Zhu, Takashi Kita

  Institute of Electrical and Electronics Engineers Inc., Jun. 2020, Conference Record of the IEEE Photovoltaic Specialists Conference, 2020-, 0902 - 0904, English

  International conference proceedings


• Bound-to-continuum intraband transition properties in InAs/GaAs quantum dot superlattice solar cells

  Yukihiro Harada, Shigeo Asahi, Takashi Kita

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

  [Refereed]

  Scientific journal


• Hot-carrier generation and extraction in InAs/GaAs quantum dot superlattice solar cells

  Shigeo Asahi

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

  Scientific journal


• Reciprocal relationship between photoluminescence and photocurrent in two-step photon up-conversion solar cell

  Noriyuki Kinugawa, Shigeo Asahi, Takashi Kita

  Institute of Electrical and Electronics Engineers Inc., Jun. 2019, Conference Record of the IEEE Photovoltaic Specialists Conference, 2623 - 2626, English

  International conference proceedings


• Strong voltage-boost effect in two-step photon-up conversion solar cells

  Shigeo Asahi, Takashi Kita

  Institute of Electrical and Electronics Engineers Inc., Jun. 2019, Conference Record of the IEEE Photovoltaic Specialists Conference, 2597 - 2599, English

  International conference proceedings


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

  Yukihiro Harada, Naoto Iwata, Daiki Watanabe, Shigeo Asahi, Takashi Kita

  Institute of Electrical and Electronics Engineers Inc., Jun. 2019, Conference Record of the IEEE Photovoltaic Specialists Conference, 3004 - 3006, English

  International conference proceedings


• Adiabatic two-step photoexcitation effects in intermediate-band solar cells with quantum dot-in-well structure

  Shigeo Asahi

  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

  Link to Kobe Univ. Repository (Kernel)


• Luminescence study of Si/SiC nano-particles embedded in SiOxCy matrix deposited using O-Cat-CVD

  Shigeo Asahi

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

  [Refereed]

  Scientific journal


• Reply to: “Thermal artefacts in two-photon solar cell experiments”

  Shigeo Asahi

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

  [Refereed]

  Scientific journal

  Link to Kobe Univ. Repository (Kernel)


• The Conversion Efficiency of a Solar Cell as Determined by the Detailed Balance Model

  Takashi Kita, Yukihiro Harada, Shigeo Asahi

  Springer Verlag, 2019, Green Energy and Technology, 55 - 79, English

  In book


• Actual Calculation of Solar Cell Efficiencies

  Takashi Kita, Yukihiro Harada, Shigeo Asahi

  Springer Verlag, 2019, Green Energy and Technology, 81 - 137, English

  In book


• Photons from the Sun

  Takashi Kita, Yukihiro Harada, Shigeo Asahi

  Springer Verlag, 2019, Green Energy and Technology, 15 - 24, English

  In book


• Fundamentals of Semiconductors

  Takashi Kita, Yukihiro Harada, Shigeo Asahi

  Springer Verlag, 2019, Green Energy and Technology, 157 - 202, English

  In book


• Application Limits for the Ideal Conditions

  Takashi Kita, Yukihiro Harada, Shigeo Asahi

  Springer Verlag, 2019, Green Energy and Technology, 139 - 156, English

  In book


• “Graphical Solution” for the Solar Cell Conversion Efficiency in the Completely Ideal Case

  Takashi Kita, Yukihiro Harada, Shigeo Asahi

  Springer Verlag, 2019, Green Energy and Technology, 25 - 42, English

  In book


• Influences of Carrier Generation and Recombination on the Solar Cell Conversion Efficiency

  Takashi Kita, Yukihiro Harada, Shigeo Asahi

  Springer Verlag, 2019, Green Energy and Technology, 43 - 54, English

  In book


• The Solar Cell and the Electrochemical Cell

  Takashi Kita, Yukihiro Harada, Shigeo Asahi

  Springer Verlag, 2019, Green Energy and Technology, 1 - 13, English

  In book


• Energy Conversion Efficiency of Solar Cells

  Takashi Kita, Yukihiro Harada, Shigeo Asahi

  Springer Singapore, 2019, Green Energy and Technology

  [Refereed]


• Increasing conversion efficiency of two-step photon up-conversion solar cell with a voltage booster hetero-interface

  Shigeo Asahi, Kazuki Kusaki, Yukihiro Harada, Takashi Kita

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

  [Refereed]

  Scientific journal

  Link to Kobe Univ. Repository (Kernel)


• Carrier collection efficiency of intraband-excited carriers in two-step photon up-conversion solar cells

  Shigeo Asahi, Kenta Nishimura, Toshiyuki Kaizu, Takashi Kita

  Institute of Electrical and Electronics Engineers Inc., Nov. 2018, 2018 IEEE 7th World Conference on Photovoltaic Energy Conversion, WCPEC 2018 - A Joint Conference of 45th IEEE PVSC, 28th PVSEC and 34th EU PVSEC, 3447 - 3450, English

  International conference proceedings


• Optimal band gap energies for two-step photon up-conversion solar cells with partial absorptivity

  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) (No. 1), pp.082303 - 1-4, English

  [Refereed]

  Scientific journal


• Two-step photon up-conversion solar cell: Propose and demonstration

  S. Asahi, T. Kita

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

  [Invited]

  International conference proceedings


• Two-step photocurrent generation enhanced by the fundamental-state miniband formation in intermediate-band solar cells using a highly homogeneous InAs/GaAs quantum-dot superlattice

  Kazuki Hirao, Shigeo Asahi, Toshiyuki Kaizu, Takashi Kita

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

  [Refereed]

  Scientific journal

  Link to Kobe Univ. Repository (Kernel)


• Efficient two-step photocarrier generation in bias-controlled InAs/GaAs quantum dot superlattice intermediate-band solar cells

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

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

  [Refereed]

  Scientific journal

  Link to Kobe Univ. Repository (Kernel)


• Two-step photocurrent generation enhanced by miniband formation in InAs/GaAs quantum dot superlattice intermediate-band solar cells

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

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

  [Refereed]

  Scientific journal

  Link to Kobe Univ. Repository (Kernel)


• Two-step photon up-conversion solar cells

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

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

  [Refereed]

  Scientific journal

  Link to Kobe Univ. Repository (Kernel)


• 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) (3), pp. 244~249, Japanese

  [Refereed][Invited]

  Scientific journal


• Photocarrier transport dynamics in InAs/GaAs quantum dot superlattice solar cells using time-of-flight spectroscopy

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

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

  [Refereed]

  Scientific journal

  Link to Kobe Univ. Repository (Kernel)


• Field-dependent characteristics of two-step photoexcited current generation in a quantum-dot superlattice solar cell

  Tomoyuki Kada, Shigeo Asahi, Yukihiro Harada, Takashi Kita

  Society of Materials Science Japan, Sep. 2016, Zairyo/Journal of the Society of Materials Science, Japan, 65(9) (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

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

  [Refereed]

  Scientific journal


• Saturable Two-Step Photocurrent 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

  Mar. 2016, IEEE JOURNAL OF PHOTOVOLTAICS, 6(2) (2), 465 - 472, English

  [Refereed]

  Scientific journal


• 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

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

  [Refereed]

  Scientific journal


• Analyses of saturable behavior of two-step photoexcitation in InAs/GaAs/Al0.3Ga0.7As intermediate-band solar cells

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

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

  [Refereed]

  Scientific journal


• Microscopic observation of carrier-transport dynamics in quantum-structure solar cells using a time-of-flight technique

  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) (4), 043901 - 043901, English

  [Refereed]

  Scientific journal

  Link to Kobe Univ. Repository (Kernel)


• Two-step photon absorption in InAs/GaAs quantum-dot superlattice solar cells

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

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

  [Refereed]

  Scientific journal

  Link to Kobe Univ. Repository (Kernel)


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

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

  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

  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

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

  [Refereed]

  International conference proceedings


• Two-Step Photocarrier Generation in InAs/GaAs Quantum Dot Superlattice Intermediate Band Solar Cell

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

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

  [Refereed]

  International conference proceedings


• Suppression of thermal carrier escape and enhanced two-step photon absorption in quantum-dot intermediate-band solar cells with a high-potential barrier

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

  2015, PHYSICS SIMULATION AND PHOTONIC ENGINEERING OF PHOTOVOLTAIC DEVICES IV, 9358, English

  International conference proceedings


• Carrier Time-of-Flight Measurement Using a Probe Structure for Direct Evaluation of Carrier Transport in Multiple Quantum Well Solar Cells

  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) (6), 1518 - 1525, English

  [Refereed]

  Scientific journal


• Suppression of thermal carrier escape and efficient photo-carrier generation by two-step photon absorption in InAs quantum dot intermediate-band solar cells using a dot-in-well structure

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

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

  [Refereed]

  Scientific journal

• Recent advancement of semiconductor materials and devices: III: Design of semiconvuctor materials to improve conversion-efficiency limit of solar cells

  Shigeo Asahi, Takashi Kita

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

• Energy Conversion Efficiency of Solar Cells

  KITA Takashi, HARADA Yukihiro, ASAHI Shigeo

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

• Spectroscopic characterization of silicon laser-power converter

  Ryota Hanakuma, Shigeo Asahi, Yukihiro Harada, Takashi Kita

  The 72nd Japanese Society of Applied Physics Spring Meeting, Mar. 2025, Japanese

  Oral presentation


• All-inorganic CsPbBr3–xClx Perovskite Nanocrystals, Potential Semiconductors for Bifacial Two-step Photon Upconversion Solar Cells

  Hambalee Mahamu, Soma Ueno, Shigeo Asahi, Takashi Kita

  The 72nd Japanese Society of Applied Physics Spring Meeting, Mar. 2025, English

  Oral presentation


• Efficient intraband transition in up-conversion solar cells using double tunnel-junction

  Shigeo Asahi, Koichiro Matsuzawa, Takashi Kita

  The 72nd Japanese Society of Applied Physics Spring Meeting, Mar. 2025, Japanese

  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バンド内赤外光学遷移の量子ドットによる増強特性

  山本祥, 永井大地, 朝日重雄, 喜多隆

  第85回応用物理学会秋季学術講演会, Sep. 2024, Japanese

  Oral presentation


• MAPbI₃/Siヘテロ構造を利用した二段階フォトンアップコンバージョン太陽電池

  徳永隼也, 朝日重雄, 喜多隆

  第85回応用物理学会秋季学術講演会, Sep. 2024, Japanese


• Intraband Transitions Induced by Below-Bandgap Photoexcitation at CsPbBr₃/GaAs Heterointerface

  H. Mahamu, S. Asahi, T. Kita

  The 85th JSAP Autumn Meeting 2024, Sep. 2024, English


• Voltage Boost Effects in Solar Cells Utilizing Adiabatic Photon Up-Conversion with a Double Tunnel Junction

  Koichiro Matsuzawa, Shigeo Asahi, Takashi Kita

  52nd IEEE Photovoltaic Specialists Conference (PVSC52), Jun. 2024, English

  Poster presentation


• Two-Step Photon Upconversion Solar Cells Based on CsPbBr3/GaAs Heterointerface

  Hambalee Mahamu, Koichiro Yoshida, Shigeo Asahi, Takashi Kita

  52nd IEEE Photovoltaic Specialists Conference (PVSC52), Jun. 2024, English

  Poster presentation


• 極性制御した2段階アップコンバージョン太陽電池におけるアップコンバージョン特性の変化

  西琢朗, 豊友太, 朝日重雄, 喜多隆

  第71回応用物理学会春季学術講演会, Mar. 2024, Japanese

  Oral presentation


• Optimization of the Morphological Structure of Spin-Coated MAPbBr₃ on p-GaAs Substrates for Perovskite/GaAs-based Photon Up-conversion Solar Cells

  H. Mahamu, M. Bourzier, S. Asahi, T. Kita

  第70回応用物理学会春季学術講演会, Mar. 2023, English

  Oral presentation


• ドープされたInAs/GaAs量子ドットにおける局在表面プラズモン共鳴による電場増強効果

  川上瑞人, 原田幸弘, 朝日重雄, 喜多隆

  日本材料学会2022年度半導体エレクトロニクス部門委員会第3回研究会, Jan. 2023, Japanese

  Oral presentation


• 量子ドットを内包する半導体ヘテロナノ構造を利用した量子型赤外光検出素子の暗電流制御

  錦見海地, 朝日重雄, 喜多隆

  日本材料学会2022年度半導体エレクトロニクス部門委員会第2回研究会, Nov. 2022, Japanese

  Oral presentation


• Broadband Enhancement of Intraband Transition in Two-Step Photon Up-Conversion Solar Cells with a Doubled-Heterointerface Strucure

  Hambalee Mahamu, Shigeo Asahi, Takashi Kita

  33rd International Photovoltaic Science and Engineering Conference (PVSEC-33), English

  Oral presentation


• Intraband Transition in Two-Step Photon Up-Conversion Solar Cells

  Y. Harada, S. Asahi, T. Kita

  第41回電子材料シンポジウム（EMS41), Oct. 2022


• Thermal Activation Process at the Heterointerface in Photon Up-Conversion Solar Cells Using hole Up-Conversion

  Y. Toyo, S. Asahi, H. Mahamu, T. Kita

  第41回電子材料シンポジウム（EMS41), Oct. 2022, Japanese

  Oral presentation


• High Absorptivity of Intraband Transition Occurring at Heterointerface in Two-Step Photon Up-Conversion Solar Cells

  S. Asahi, H. Mahamu, T. Kita

  8th World Conference on Photovoltaic Energy Conversion (WCPEC-8), English

  Poster presentation


• Electrical Properties of AlGaAs/GaAs-Based Two-Step Photon Up-Conversion Solar Cells with Doubled Heterointerfaces

  H. Mahamu, S. Asahi, T. Kita

  The 83rd JSAP Autumn Meeting 2022, Sep. 2022, English

  Oral presentation


• Photoluminescence Characteristics of InAs Quantum Dots in the Doubled-heterointerface of AlGaAs/GaAs-based Two-step Photon Up-conversion Solar Cells

  H. Mahamu, S. Asahi, T. Kita

  9th International Symposium on Control of Semiconductor Interfaces(ISCSI-IX), English

  Oral presentation


• Intraband Absorptivity in Two-Step Photon Up-Conversion Solar Cells

  Y. Harada, K. Kusaki, S. Asahi, T. Kita

  International Conference on the Physics of Semiconductors (ICPS 2022), English

  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, 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, Mar. 2022, Japanese

  Oral presentation


• 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), 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, 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, 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, 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), 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, 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, 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, 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, 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

• 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

• ヘテロ界面に挿入した量子ドットによる赤外増感型光電変換の実現

  喜多 隆, 朝日 重雄, 原田 幸弘

  日本学術振興会, 科学研究費助成事業 基盤研究(B), 基盤研究(B), 神戸大学, Apr. 2023 - Mar. 2026


• ダブルトンネル接合を利用したラチェット型アップコンバージョン太陽電池

  朝日 重雄

  日本学術振興会, 科学研究費助成事業 基盤研究(C), 基盤研究(C), 神戸大学, Apr. 2023 - Mar. 2026


• 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

  p型とn型に挟まれたダイオード構造の真性層に、AlGaAs/GaAsヘテロ界面にInAs量子ドットを挿入した量子ナノ構造を作製し、電子のみを蓄積したヘテロ界面において価電子バンド－伝導バンド間光学遷移とバンド内光学遷移の連続した2段階の遷移による電子のエネルギーをアップコンバージョンによって増強されたバンド内光吸収に基づく赤外波長域に応答する光電変換特性を実現する。本年度は以下のように実施した。 （１）量子ナノ構造の作製と基礎光学特性評価：分子線エピタキシー技術を利用してGaAs(001)基板上にAlGaAs/GaAsヘテロ界面にInAs量子ドットを挿入した量子ナノ構造を内包するダイオード構造を作製し、ヘテロ界面にSi変調ドーピングによって電子密度を制御した量子構造を作製した。変調ドーピングによる界面電界の制御を明らかにして、光電流取り出し特性を詳細に明らかにした。 （２）バンド内光学遷移分極の制御：バンド内光学遷移強度は、光電場で誘起される電子分極の大きさと遷移始状態の電子占有率に比例する。量子ドットのサイズ、密度による光学応答への影響を明らかにするため電子で満たされた量子ドットにおけるプラズモン共鳴特性を明らかにして中赤外領域における光アンテナ効果を理論的に予測した。具体的には、量子ドットの形状・サイズに依存したプラズモン共鳴特性を計算し、8～16マイクロメートルの広範囲にわたる制御性を明らかにするとともに、量子ドット間のプラズモン共鳴状態の結合によるモード分散を明らかにした。 （３）光吸収係数の定量評価とアップコンバージョン光電流の最大化：デバイスを試作して光近赤外から中赤外に広がる広い波長範囲で光応答特性を明らかにし、ヘテロ界面電子濃度に依存すると予想されるバンド内光学遷移を確認した。


• 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

  現在の太陽電池の主流である単接合型太陽電池の変換効率は理論的に約30%に制限される。その理論変換効率を超える太陽電池のアイデアがいくつか提案されているが、我々は2個の低エネルギー光子から1個の高エネルギー電子を生成する2段階アップコンバージョン（TPU）現象を利用した、2段階フォトンアップコンバージョン太陽電池（TPU-SC）を提案した。本研究では、アップコンバージョンによる電流上昇効率を現状からさらに10倍向上し、変換効率向上に寄与することを目的とする。 1年目は変調ドーピングによるアップコンバージョン効率の向上を実証することを行った。その結果、約3倍のアップコンバージョン効率の向上に成功した。また、アップコンバージョンによるヘテロ界面での擬フェルミ準位分裂の実験観測に成功した。これは、ヘテロ界面における赤外線吸収により、低エネルギーフォトンの光エネルギー変換が可能であることを示している。2年目は、ヘテロ界面に挿入した量子ドットの位置によるアップコンバージョンへの変化を調べた。その結果、量子ドットの位置を最適化することでアップコンバージョン効率がさらに3倍以上向上することを見出した。これにより、当初の目標であったアップコンバージョン効率の10倍向上が見えてきた。一方、ナローギャップ半導体に正孔のバリア層を設けることで、電子と正孔の分離を促し、ヘテロ界面に蓄積する電子密度の向上を図ったが、このバリアによりヘテロ界面での電子密度が減少することが分かり、この構造は適切ではないことが分かった。 一方、ホールのアップコンバージョンを利用したアップコンバージョン太陽電池の電流減少メカニズムも継続して取り組んでいる。アップコンバージョン遷移後の状態で、実効的なエネルギーバリアにより、正孔の取り出しが阻害される可能性を見出した。来年度も引き続き解明に取り組む。


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

  Kita Takashi

  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

  We have developed an infrared sensing device with a quantum nano-structure of a heterointerface containing quantum dots. This device exhibits effective intraband photo-absorption at the hertointerface which is enhanced by quantum dots. The central part of our device is an Al0.3Ga0.7As/GaAs heterostructure containing InAs/GaAs quantum dots. In this device, the electrons that have been accumulated at the heterointerface are transferred to the conduction band of the Al0.3Ga0.7As barrier by absorbing infrared photons and the following drift due to the electric field at the interface. These intraband transitions at the heterointerface are sensitized by the quantum dots. According to theoretical calculations, strong localized plasmon modes enhances the electro-magnetic wave of the infrared signal light.


• 量子ナノ構造を利用して根本的に変換効率を向上させる太陽電池技術の開発

  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


• 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


• 2段階フォトンアップコンバージョン太陽電池のヘテロ界面における電圧ブーストの増強

  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

• 高効率太陽電池

  朝日 重雄, 喜多 隆, 松沢 光一郎

  特願2023-142969, 04 Sep. 2023, 国立大学法人神戸大学

  Patent right

  IRI


• 量子型赤外線センサ

  喜多 隆, 朝日 重雄, 村田 貴彦

  特願2019-138335, 26 Jul. 2019, 特開2021-022662, 18 Feb. 2021, 特許7291942号, 18 Jun. 2023

  Patent right


• 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