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朝日 重雄
大学院工学研究科 電気電子工学専攻
准教授

研究者基本情報

■ 学位
  • 博士（工学）, 神戸大学
■ 研究ニュース
■ 研究キーワード
  • 半導体物性
  • 太陽電池
  • 光物性
  • 量子ドット
  • 分子線エピタキシー
  • 電気・電子材料工学
■ 研究分野
  • ナノテク・材料 / 結晶工学
  • ものづくり技術（機械・電気電子・化学工学） / 電気電子材料工学
■ 委員歴
  • 2024年04月 - 2024年11月, 第37回マイクロプロセス・ナノテクノロジー国際会議（MNC2024）, 論文委員セクションヘッド
  • 2022年03月 - 2024年02月, 電子材料シンポジウム（EMS）, 実行委員会 企画展示委員
  • 2023年04月 - 2023年11月, 第36回マイクロプロセス・ナノテクノロジー国際会議（MNC2023）, 論文委員
  • 2022年04月 - 2022年11月, 第35回マイクロプロセス・ナノテクノロジー国際会議（MNC2022）, 論文委員
  • 2021年04月 - 2022年11月, 第33回太陽光発電国際会議, プログラム委員会委員
  • 2020年04月 - 2022年02月, 電子材料シンポジウム（EMS）, 実行委員（会場委員）
  • 2021年04月 - 2021年11月, 34th International Microprocesses and Nanotechnology Conference (MNC2021), 論文委員
  • 2020年04月 - 2020年11月, 33rd International Microprocesses and Nanotechnology Conference (MNC 2020), 論文委員
  • 2019年04月 - 2019年11月, 32nd International Microprocesses and Nanotechnology Conference (MNC2019), 論文委員
  • 2017年04月 - 2019年09月, 7th International Workshop on Epitaxial Growth and Fundamental Properties of Semiconductor Nanostructures (SemiconNano2019), 現地実行委員

研究活動情報

■ 受賞
  • 2022年09月 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ID対象リソースIRI

  • 2020年06月 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

  • 2015年12月 第26回光物性研究会, 奨励賞

  • 2014年08月 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

■ 論文
  • 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, 2025年01月, Journal of Optics
    [査読有り]
    研究論文（学術雑誌）

  • 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, 2025年01月, Japanese Journal of Applied Physics, 64(1) (1), 014001 - 014001
    [査読有り]
    研究論文（学術雑誌）
    神戸大学リポジトリ（Kernel）へのリンク

  • Stefano Vichi, Shigeo Asahi, Sergio Bietti, Artur Tuktamyshev, Alexey Fedorov, Takashi Kita, Stefano Sanguinetti
    American Chemical Society (ACS), 2024年12月, ACS Photonics, 12(1) (1), 447 - 456
    [査読有り]
    研究論文（学術雑誌）

  • 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 Al0.3Ga0.7As/GaAs-based TPU-SC. Experimental results of the CsPbBr3/GaAs-based TPU-SC show that both the short-circuit current JSC and the open-circuit voltage VOC increase with additional illumination of sub-bandgap photons. We analyze the excitation power dependence of JSC for different excitation conditions to discuss the mechanisms behind the enhancement. In addition, the observed voltage-boost clarifies that the JSC 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, 2024年11月, Scientific Reports, 14(1) (1)
    [査読有り]
    研究論文（学術雑誌）
    神戸大学リポジトリ（Kernel）へのリンク

  • Mizuto KAWAKAMI, Yukihiro HARADA, Shigeo ASAHI, Takashi KITA
    Society of Materials Science, Japan, 2024年02月, Journal of the Society of Materials Science, Japan, 73(2) (2), 178 - 182
    研究論文（学術雑誌）

  • 朝日重雄, 原田幸弘, 喜多隆
    公益社団法人応用物理学会, 2023年09月, 応用物理, 92(9) (9), 550 - 554, 日本語
    [査読有り]

  • 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: Al0.7Ga0.3As, Al0.3Ga0.7As, 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 Al0.3Ga0.7As 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, 2023年03月, Journal of Applied Physics, 133(12) (12), 124503 - 124503
    [査読有り]
    研究論文（学術雑誌）

  • 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, 2022年07月, Japanese Journal of Applied Physics, 61(7) (7), 074002 - 074002, 英語
    [査読有り]
    研究論文（学術雑誌）

  • Yaxing Zhu, Shigeo Asahi, Naoya Miyashita, Yoshitaka Okada, Takashi Kita
    This article was originally published online on 29 September 2021 with author Takashi Kita incorrectly spelled. The name is correct as it appears above. All online versions of this article were corrected on 5 October 2021 the article is correct as it appears in the printed version of the journal. AIP Publishing apologizes for this error.
    American Institute of Physics Inc., 2021年11月, Journal of Applied Physics, 130(17) (17), 英語
    研究論文（学術雑誌）

  • Yaxing Zhu, Shigeo Asahi, Naoya Miyashita, Yoshitaka Okada, Takashi Kita
    The generation of two-photon photocurrent plays an essential role in realizing intermediate-band solar cells (IBSCs) with high conversion efficiencies. This current generation process strongly depends on the photocarrier dynamics in the intermediate levels, which can sometimes give rise to a deficient output current unexpectedly. In this work, we investigated the two-photon photocurrent generation process in InAs quantum dot (QD)-in-well intermediate-band solar cells. The two-photon photocurrent is generated by an interband transition in the structure (for example, in the well or the QD) and a subsequent intraband transition in the QD, and we used two different light sources to separately control these transitions. We found that, in the case of QD interband excitation in a sample with multiple wells, the carrier collection efficiency does not simply increase with the intraband excitation intensity; in the range from about 0.08 to 0.5 W/cm2, the collection efficiency decreases with increasing intraband excitation density. A comparison between samples with different numbers of wells revealed that the repetition of carrier trapping and detrapping during the transport in a multi-well structure can effectively modulate the recombination rate. This modulation induces a reduction of the current yield under certain illumination conditions. We propose a model to explain this phenomenon and verify it by investigating the bias dependence of the two-photon photocurrent from the QD.
    AIP Publishing, 2021年09月, Journal of Applied Physics, 130(12) (12), 124505 - 124505, 英語
    [査読有り]
    研究論文（学術雑誌）

  • Kohei Watanabe, Shigeo Asahi, Yaxing Zhu, Takashi Kita
    AIP Publishing, 2021年08月, Journal of Applied Physics, 130(8) (8), 085701 - 085701, 英語
    [査読有り]
    研究論文（学術雑誌）
    神戸大学リポジトリ（Kernel）へのリンク

  • Yaxing Zhu, Shigeo Asahi, Takashi Kita
    Nanostructured intermediate band solar cells have been proposed as one of the promising candidates to surpass the limited efficiency of single-junction photovoltaic devices, while challenges still remain for realizing their practical implementation due to intricate carrier behaviors occurring in confined states. In this report, we have elucidated a detailed photo-carrier collection mechanisms in a quantum dot-in-well intermediate band solar cell, which is varied by effective electron intraband excitation process. Results from the photoluminescence response of quantum dots are discussed to examine the occupation state inside confined levels. Detailed analyses point out that diffusion of holes plays an important role in modulating the electrical generations.
    Institute of Electrical and Electronics Engineers Inc., 2021年06月, Conference Record of the IEEE Photovoltaic Specialists Conference, 1786 - 1788, 英語
    研究論文（国際会議プロシーディングス）

  • Yaxing Zhu, Shigeo Asahi, Kohei Watanabe, Naoya Miyashita, Yoshitaka Okada, Takashi Kita
    AIP Publishing, 2021年02月, Journal of Applied Physics, 129(7) (7), 074503 - 074503, 英語
    [査読有り]
    研究論文（学術雑誌）

  • Takahiko Murata, Shigeo Asahi, Stefano Sanguinetti, Takashi Kita
    Springer Science and Business Media LLC, 2020年12月, Scientific Reports, 10(1) (1)
    [査読有り]
    研究論文（学術雑誌）
    神戸大学リポジトリ（Kernel）へのリンク

  • Noriyuki Kinugawa, Shigeo Asahi, Takashi Kita
    American Physical Society (APS), 2020年07月, Physical Review Applied, 14(1) (1), 英語
    [査読有り]
    研究論文（学術雑誌）
    神戸大学リポジトリ（Kernel）へのリンク

  • Yaxing Zhu, Shigeo Asahi, Takashi Kita
    Intermediate-band solar cell is one of the promising concepts to jump out the dilemma of limited efficiency for traditional single junction photovoltaic devices. Albeit fruitful explorations have been done on revealing its working principle and promoting output performance, optical and electrical match of outer system can sometimes get it complicated because of extra below-bandgap photon absorptions. In this study, we reported a voltage backtracking behavior under intensive bi/uni-color irradiation circumstance within this kind of cell. Two different mechanisms are separately disclosed here for the sake of deep understanding how it operates and by which it can better work.
    Institute of Electrical and Electronics Engineers Inc., 2020年06月, Conference Record of the IEEE Photovoltaic Specialists Conference, 2020-, 0146 - 0148, 英語
    研究論文（国際会議プロシーディングス）

  • Kohei Watanabe, Shigeo Asahi, Yaxing Zhu, Takashi Kita
    Two-step photon up-conversion solar cell is a newly-proposed concept for high-conversion efficiency photovoltaics, in which a heterointerface of different semiconductor materials is delicately merged inside common single-junction cell structure. Recently, we have reported an enhancement of its photocurrent under a reverse bias condition due to the boosted carrier collection efficiency. To push it further, in this study, modulation-doping method is employed in order for a better elucidation of two-step photon up-conversion at the heterointerface. It is observed that the difference of the external quantum efficiency with additional infrared light irradiation on the TPU-SCs can be greatly enlarged by changing doping concentration selectively reflecting the importance of the electric field at the heterointerface.
    Institute of Electrical and Electronics Engineers Inc., 2020年06月, Conference Record of the IEEE Photovoltaic Specialists Conference, 2020-, 0902 - 0904, 英語
    研究論文（国際会議プロシーディングス）

  • Yukihiro Harada, Shigeo Asahi, Takashi Kita
    IOP Publishing, 2019年12月, Applied Physics Express, 12(12) (12), 125008 - 125008
    [査読有り]
    研究論文（学術雑誌）

  • Shigeo Asahi
    IOP Publishing, 2019年09月, Semiconductor Science and Technology, 34(9) (9), 094003 - 094003
    研究論文（学術雑誌）

  • Noriyuki Kinugawa, Shigeo Asahi, Takashi Kita
    Two-step photon up-conversion solar cell (TPU-SC) we have recently proposed is a single-junction solar cell containing a hetero-interface of different semiconductor materials. Although efficient two-step photon up-conversion is achieved in the TPU-SC, the detailed mechanism of the significant intraband photoexcitation occurring at the hetero-interface is still unclear. In this study, we performed simultaneous measurements of photoluminescence and photocurrent as a function of the applying bias voltage in the TPU-SC. We experimentally demonstrate the reciprocity relationship between the radiative recombination and the photocurrent of the TPU-SC.
    Institute of Electrical and Electronics Engineers Inc., 2019年06月, Conference Record of the IEEE Photovoltaic Specialists Conference, 2623 - 2626, 英語
    研究論文（国際会議プロシーディングス）

  • Shigeo Asahi, Takashi Kita
    We have recently proposed a solar cell (SC) structure called two-step photon up-conversion SC (TPU-SC). In this SC, we have observed an obvious enhancement in the open- circuit voltage as well as the short-circuit current. In this study, we investigated the excitation intensity dependence of TPU. We find that the extra-photocurrent caused by TPU shows a sublinear relationship with the interband excitation intensity. In addition, the enhancement of the open-circuit voltage by TPU is larger than the value estimated from the short-circuit current, suggesting a voltage-boost at the hetero-interface.
    Institute of Electrical and Electronics Engineers Inc., 2019年06月, Conference Record of the IEEE Photovoltaic Specialists Conference, 2597 - 2599, 英語
    研究論文（国際会議プロシーディングス）

  • Yukihiro Harada, Naoto Iwata, Daiki Watanabe, Shigeo Asahi, Takashi Kita
    We demonstrated hot-carrier (HC) extraction in GaAs solar cells containing InAs/GaAs quantum dot superlattices (QDSLs) functioning as a light absorber at 15 K. The short-circuit current density and the open-circuit voltage in the QDSL solar cells show step-wise changes as a function of the excitation photon density because of state filling under below-bandgap excitation. Furthermore, the short-circuit current density and the open-circuit voltage originated from the HC extraction were enhanced by increasing the period of the QDSL due to the improved absorptivity.
    Institute of Electrical and Electronics Engineers Inc., 2019年06月, Conference Record of the IEEE Photovoltaic Specialists Conference, 3004 - 3006, 英語
    研究論文（国際会議プロシーディングス）

  • 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
    Nature Publishing Group, 2019年05月, Scientific Reports, 9, 7859 - 1-8, 英語
    [査読有り]
    研究論文（学術雑誌）
    神戸大学リポジトリ（Kernel）へのリンク

  • Shigeo Asahi
    2019年03月, Physica E: Low-dimensional Systems and Nanostructures, 英語
    [査読有り]
    研究論文（学術雑誌）

  • Shigeo Asahi
    Nature Publishing Group, 2019年02月, Nature Communications, 10, 956 - 1-3, 英語
    [査読有り]
    研究論文（学術雑誌）
    神戸大学リポジトリ（Kernel）へのリンク

  • Takashi Kita, Yukihiro Harada, Shigeo Asahi
    In the previous chapter, we discussed the conditions that apply to the actual solar cell but were not considered for the ideal solar cell. In order to derive the maximum output power of the actual solar cell, we have to discuss the balance between absorption (resulting in electrical current) and emission (due to the established voltage). Therefore, the present chapter discusses the balanced condition of voltage and current by employing the current–voltage relation determined in the previous chapter. Based on that, the conversion efficiency of the actual solar cell is derived. Then, we discuss the contributions of the different types of losses that occur in a solar cell with a single band gap.
    Springer Verlag, 2019年, Green Energy and Technology, 55 - 79, 英語
    論文集(書籍)内論文

  • Takashi Kita, Yukihiro Harada, Shigeo Asahi
    In Chap. 5, we derived the single-junction solar cell conversion efficiency within the framework of the detailed balance theory and computed the solar spectrum by employing Planck’s law for black-body radiation. As explained in Sect. 2.2, the solar spectrum that passes the atmosphere differs according to the amount of air passed. This amount can be expressed as air mass (AM). In this chapter, we do not rely on the black-body radiation for the solar spectrum, but employ the actual spectral data for the sunlight that reaches the Earth after passing the atmosphere. With this data, we calculate the conversion efficiencies of several different solar cell structures. We first recalculate the conversion efficiency of the single-junction solar cell in Sect. 6.1 by using the real solar spectrum. In Sect. 6.2, we determine the conversion efficiency of a solar cell under concentrated sunlight, which can be obtained, for example, by using a lens. Additionally, in Sects. 6.3–6.6 we discuss four selected solar cell structures that are recently attracting attention as novel types of solar cells with the ability to overcome the S–Q limit. To exceed the S–Q limit means that the transmission and thermalization losses, which we declared as “being unavoidable,” have to “be avoided somehow” by using novel approaches. For the reduction of the losses and conversion of the sunlight into electrical power to the best extent possible, we can either increase the wavelength range that is employed by the solar cell for conversion into electrical power, or the opposite, and we can reshape the solar spectrum itself in such a way that it can be efficiently converted into electrical power by the solar cell. As examples for the first-mentioned approach, we discuss the multi-junction solar cell (which can also be called tandem solar cell) in Sect. 6.3 and the intermediate-band solar cell in Sect. 6.4. Multi-junction solar cells comprise a multilayer structure where each of the stacked layers is a semiconductor junction with different Eg. This method enables capturing the light transmitted through one solar cell by another solar cell in the layer below. Therefore, the light that would usually be lost through transmission can be used for electricity generation, and this allows us to exceed the S–Q limit of the single junction. This solar cell architecture has already been practically implemented, and the multi-junction type is the current world record holder in terms of solar cell performance. Unfortunately, the series connection between the junctions constitutes a drawback for the multi-junction solar cells. The tandem solar cells consist of stacked junctions that are serially connected. Therefore, the electricity generation is already completely suppressed if one of the stacked junctions does not operate. For example, a certain spectral part of the light may hardly reach the solar cell due to the scattering and absorption in clouds on a cloudy day. If due to this the electricity generation of any solar cell in the stack drops, the performance of the whole device is affected because the current that flows through the directly affected cell is reduced and becomes the bottleneck of the whole device, even if the solar cells in the other layers work fine. On the other hand, the intermediate-band solar cell has attracted attention as a solar cell architecture that promises high conversion efficiencies and can also lift the restrictions imposed by the series connection of the multi-junction tandem solar cells. This architecture reduces the transmission losses by implementing a new band (the so-called intermediate band) within the band gap of the solar cell. Because the direct absorption of the light via a transition between the conduction and the valence bands (a so-called interband transition) occurs in parallel with the stepwise absorption via the intermediate band, the intermediate-band solar cell’s electricity generation cannot be completely suppressed under clouded conditions as was the case for the multi-junction solar cells. In the intermediate-band solar cell, by absorbing a below-gap photon, an electron transits from the valence band to the intermediate band. Upon absorbing another below-gap photon, the electron is further excited into the conduction band. There transitions widely cover the solar spectrum. This two-step excitation (two-step photon up-conversion) process following the absorption of two below-gap photons produces additional photocurrent without degrading the photovoltage. Such two-step photoexcitation is known to occur at heterointerfaces. In Sect. 6.5, we deal with a solar cell containing a heterointerface in the intrinsic layer so-called two-step photon up-conversion solar cell. The conversion efficiency of the solar cell utilizing interband and intraband transitions strongly depends on the absorptivity of these transitions. Detailed influences of the absorptivity of photovoltaic materials on the conversion efficiency are also discussed. Section 6.6 discusses one example for the reduction of losses by altering the solar spectrum. To reduce the transmission losses, it is necessary to convert the light in the long-wavelength region of the solar spectrum into light with shorter wavelengths. Similarly, to reduce losses through thermalization, it is necessary to convert the light in the short-wavelength region of the solar spectrum into light with longer wavelengths. We analyze how much the conversion efficiency can be improved by down-converting the short-wavelength light (high photon energies) to long-wavelength light (small photon energies). Besides, the influences of changing weather conditions on the conversion efficiency of various types of solar cells are introduced in detail in Sect. 6.7. Up to Sect. 6.7, we proceed to discuss the solar cell at a constant temperature of 300 K. However, since concentration of sunlight is an important technique to enhance conversion efficiencies, the influence of the temperature has to be investigated. Due to the strong concentration of sunlight, the temperature of the concentrator solar cell device increases. If the temperature increases, the semiconductor band gap becomes smaller and furthermore, a higher the electron–hole recombination rate (stronger black-body radiation) is observed as explained in Sect. 4.2 this causes a decrease in the conversion efficiency. In Sect. 6.8, we analyze the change in the single-junction solar cell efficiency that occurs upon a change in the solar cell temperature. The solar cell is the photovoltaic device used as a solar energy converter. When indoor light such as white light-emitting diode (LED) is harvested, the optimized energy structure of the photovoltaic materials is different from the results obtained for the sunlight. We touch the energy conversion efficiency of indoor photovoltaic cells in Sect. 6.9.
    Springer Verlag, 2019年, Green Energy and Technology, 81 - 137, 英語
    論文集(書籍)内論文

  • Takashi Kita, Yukihiro Harada, Shigeo Asahi
    The solar cell is a device that converts the energy of light into electrical energy. Consequently, a prerequisite for the derivation of the conversion efficiency of a solar cell is to understand the nature of light energy. In the present chapter, we clarify what energy is contained in light and explain the characteristics of the light that is emitted from the sun.
    Springer Verlag, 2019年, Green Energy and Technology, 15 - 24, 英語
    論文集(書籍)内論文

  • Takashi Kita, Yukihiro Harada, Shigeo Asahi
    The solar cells that are discussed in this book are based on semiconductors. The degree of difficulty in the explanation of the properties of semiconductors greatly depends on the starting point of the explanations. In this book, we start with the fact that “a semiconductor is formed by a valence band which is completely filled with electrons and an empty conduction band, both bands being separated by a characteristic energy gap.” First, we shall investigate the behavior of the bands in the representative semiconductor silicon (Si) to clarify the band-gap concept. Then, we provide explanations on the intrinsic semiconductors which contain no impurities, and the extrinsic semiconductors wherein impurity atoms with a different number of valence electrons are added. Within the extrinsic semiconductors, we distinguish between the n-type semiconductor with excess electrons that are delivered to the conduction band and the p-type semiconductor with the so-called holes (i.e., missing electrons) that are delivered to the valence band. Regarding these two classes of semiconductors with different electronic conditions, we clarify the details about the density of electrons and holes, their distributions, and the so-called Fermi level (which is the energy level corresponding to an electron occupation probability of 50%). We also consider the transport of electrons and holes inside the semiconductor. Finally, we derive the characteristics of the p–n junction, which connects an n-type semiconductor with a p-type semiconductor and is commonly used for solar cell devices. The present chapter provides detailed explanations from the viewpoint of the semiconductor electronics, which makes it accessible with the knowledge equivalent to that of a third-year undergraduate student. With respect to the structure of the present book, not all details mentioned in this chapter are required for the understanding of the other chapters. In other words, the calculation of the energy conversion efficiency of a solar cell, which is the target of the present book, is based on a relatively independent approach. The reader may omit this chapter, or regard it as a guide to find concrete ideas for improving the conversion efficiencies of semiconductor solar cells. In order to enable an independent study of this chapter on semiconductors, we again define all variables of the previous chapters.
    Springer Verlag, 2019年, Green Energy and Technology, 157 - 202, 英語
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  • Takashi Kita, Yukihiro Harada, Shigeo Asahi
    The solar cells covered in the previous chapters are idealized solar cells, which allowed us to evaluate the efficiency limits under various conditions. Of course, besides the conditions employed so far (like the transmission of photons with Ephoton < Eg), there exist plenty of other factors that lead to losses in the amount of generated electricity and thus are important for the actual development of solar cells. Due to these losses, the conversion efficiency of an actually fabricated solar cell is smaller than the theoretical efficiency limit. For example, if we use silicon (Si) for the solar cell absorber material, the detailed balance limit of the conversion efficiency is about 30%. This value can be obtained from Fig. 5.10 in Chap. 5 by looking up the efficiency for the curve (b) at 1.1 eV, which is the band gap of Si. On the other hand, when we look up the present values for actual Si p–n junction solar cells, we find that the single crystal type has reached a maximum of 26.3% and the multi-crystalline type has reached a maximum of 22.3%, both being well below the theoretical efficiency limit[1]. In Chap. 3, we discussed the trade-off relation between the transmission and thermalization losses in the single-junction solar cell and derived the conversion efficiency limit that is imposed by these two types of losses. In considerations on the solar cell performance, such trade-off relations exist for various parameters. As another example for a trade-off relation, in the following sections, we introduce the semiconductor light absorption characteristics, which differ for each material. With this new concept, we discuss the solar cell conversion efficiency from a viewpoint that is different from the detailed balance model elaborated in the previous chapters. The trade-off presented in the following sections is a trade-off between the thickness of the semiconductor that is required for sufficient light absorption and the distance that can be travelled by the generated electrons and holes. The latter property is essential for the efficient extraction of carriers at the electrodes. It is straightforward that a thicker semiconductor can absorb more light. On the other hand, if the semiconductor is too thick, the distance that has to be travelled by the photoexcited electrons and holes also becomes longer. If the path to be travelled is long, there is a high probability for the electron to collide with an obstacle before its electrical energy can be used. Then the energy is unfortunately lost in spite of the efforts to generate it. A thicker semiconductor is not necessarily better we have to consider the optimum thickness for efficient extraction of the electrical energy. After discussing the light absorption characteristics of semiconductors in Sect. 7.1, we explain the transport of the generated electrons and holes inside the semiconductor in Sect. 7.2. In the last section, we consider two specific semiconductor materials, Si and GaAs, as examples and discuss the optimum semiconductor thickness in a solar cell.
    Springer Verlag, 2019年, Green Energy and Technology, 139 - 156, 英語
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  • Takashi Kita, Yukihiro Harada, Shigeo Asahi
    The goal of this book is to derive the conversion efficiency under consideration of the equilibrium between the carrier generation and recombination that occurs in the solar cell (i.e., under consideration of the detailed balance principle). As a step toward this goal, we shall first determine the most basic conversion efficiency. The ideal efficiency curve u(νg, Ts), which is derived in this chapter, is the basis of all conversion efficiency curves that are presented in the following chapters and thus constitutes an important relation. Besides the mathematical derivation of the ideal efficiency curve u(νg, Ts), the present chapter also explains a visual approach, that is, a graphical solution for the efficiency curve, which provides a deeper understanding of u(νg, Ts). With this visual approach, the reader can intuitively estimate the value of the ideal solar cell conversion efficiency without the need of solving the equations. In this chapter, we first provide the definition of the solar cell conversion efficiency, and then explain details about the efficiency losses induced by the semiconductor band gap, and finally, we derive the energy conversion efficiency of the ideal solar cell.
    Springer Verlag, 2019年, Green Energy and Technology, 25 - 42, 英語
    論文集(書籍)内論文

  • Takashi Kita, Yukihiro Harada, Shigeo Asahi
    In the previous chapter, we determined the ideal conversion efficiency u(νg, Ts) of an ideal solar cell. Since we wanted to know the limiting efficiency in the most ideal case, we neglected effects like internal generation of radiation and luminescence. However, the relation between the voltage and the current in an actual solar cell that employs a semiconductor diode (i.e., a p–n junction) is incompatible with the assumptions in Sect. 3.4 (these assumptions enabled us to set the electromotive force equal to that provided by the potential Eg/e). In the present chapter, we analyze the conversion efficiency of a solar cell that possesses a nonzero temperature. By including losses that arise due to the very fact of a finite solar cell temperature, the relation between the current and voltage is derived, and then, we determine the open-circuit voltage and the short-circuit current.
    Springer Verlag, 2019年, Green Energy and Technology, 43 - 54, 英語
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  • Takashi Kita, Yukihiro Harada, Shigeo Asahi
    We derive the conversion efficiencies of solar cells under various conditions in the chapters following Chap. 3. For our discussions on the solar cell conversion efficiency, it is extremely important to first understand the principle of electricity generation in a solar cell. In contrast to the electrochemical cell, which is usually introduced in high-school grade chemistry, the opportunities to study the operation principle of solar cells are almost zero except for some specialized schools. Most solar cells consist of a semiconductor p–n junction, which is a connection between a semiconductor n-layer with many electrons and a semiconductor p-layer containing plenty of holes in the semiconductor crystal. Even if high-school lectures on solar cells could be established, we have in mind that the explanation of how the electricity generation exactly works in such solar cells often will have to be skipped and summarized as “knowledge belonging to the category of semiconductor engineering.” On hearing the rather famous word semiconductor, one may think of realms of science completely different from the electrochemical cell. Since there are too many things that have to be learned regarding semiconductor materials, many of those who try studying the operation principle of solar cells may have difficulties to immediately grasp all essential relations.In this chapter, we explain how the solar cell converts the energy supplied by the Sun into electrical energy. For this, we employ the example of the well-known electrochemical cell, in particular, the “voltaic pile” which was most likely studied first in high school, and compare it with the solar cell from the viewpoint of the redox reactions. In order to easily grasp the idea of how current flows, the explanations especially emphasize the electron movement that occurs inside the solar cell.After explanation of the operation principle of the voltaic pile on a high-school chemistry level in Sect. 1.1, we explain the principle of electricity generation in a solar cell while outlining the parallels to the electrochemical cell. Finally, we clarify the unique qualities of the solar cell by discussing the similarities and differences between these types of “cells,” i.e., the electrochemical cell and the solar cell.
    Springer Verlag, 2019年, Green Energy and Technology, 1 - 13, 英語
    論文集(書籍)内論文

  • Takashi Kita, Yukihiro Harada, Shigeo Asahi
    Springer Singapore, 2019年, Green Energy and Technology
    [査読有り]

  • 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.
    Nature Publishing Group, 2018年12月, Scientific Reports, 8(1) (1), pp. 872 - 1-8, 英語
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    研究論文（学術雑誌）
    神戸大学リポジトリ（Kernel）へのリンク

  • Shigeo Asahi, Kenta Nishimura, Toshiyuki Kaizu, Takashi Kita
    Recently, we have proposed a new type solar cells utilizing photon up-conversion phenomenon, called two-step photon up-conversion solar cells for realizing high conversion efficiency solar cells. Here, achieving efficient intraband photoexcitation is indispensable. This solar cell has a simple single- j unction structure containing a hetero-interface. In this study, we investigated the applying electric field dependence of two-step photon up-conversion occurring at the hetero-interface. We found that dense electron accumulation and an appropriate electric field enable to dramatically increase the intraband excitation strength and the carrier collection efficiency at the hetero-interface.
    Institute of Electrical and Electronics Engineers Inc., 2018年11月, 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, 英語
    研究論文（国際会議プロシーディングス）

  • Y. Harada, T. Matsuo, S. Asahi, T. Kita
    2018年11月, Proceedings of the 35th European Photovoltaic Solar Energy Conference and Exhibition, 126 - 128, 英語
    研究論文（国際会議プロシーディングス）

  • Hot-Carrier Generation in a Solar Cell Containing InAs/GaAs Quantum-Dot Superlattices as a Light Absorber
    D. Watanabe, N. Iwata, S. Asahi, Y. Harada, T. Kita
    2018年07月, Applied Physics Express, Vol. 11(No. 1) (No. 1), pp.082303 - 1-4, 英語
    [査読有り]
    研究論文（学術雑誌）

  • 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年, Proceedings of SPIE - The International Society for Optical Engineering, 10527, 英語
    [招待有り]
    研究論文（国際会議プロシーディングス）

  • 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, 2018年01月, APPLIED PHYSICS EXPRESS, 11(1) (1), pp. 012301 - 1-4, 英語
    [査読有り]
    研究論文（学術雑誌）
    神戸大学リポジトリ（Kernel）へのリンク

  • 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, 2017年07月, SCIENTIFIC REPORTS, 7, pp. 5865 - 1-10, 英語
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    研究論文（学術雑誌）
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  • 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.
    AMER INST PHYSICS, 2017年05月, APPLIED PHYSICS LETTERS, 110(19) (19), pp. 193104 - 1-5, 英語
    [査読有り]
    研究論文（学術雑誌）
    神戸大学リポジトリ（Kernel）へのリンク

  • 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, 2017年04月, NATURE COMMUNICATIONS, 8, pp. 14962 - 1-9, 英語
    [査読有り]
    研究論文（学術雑誌）
    神戸大学リポジトリ（Kernel）へのリンク

  • 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
    朝日 重雄, 喜多 隆
    2017年03月, 材料 別冊, 66(3) (3), pp. 244~249, 日本語
    [査読有り][招待有り]
    研究論文（学術雑誌）

  • 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, 2016年11月, PHYSICAL REVIEW B, 94(19) (19), pp. 195313 - 1 -9, 英語
    [査読有り]
    研究論文（学術雑誌）
    神戸大学リポジトリ（Kernel）へのリンク

  • 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, 2016年09月, Zairyo/Journal of the Society of Materials Science, Japan, 65(9) (9), 647 - 651, 日本語
    [査読有り]
    研究論文（学術雑誌）

  • 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, 原田 幸弘, 喜多 隆
    日本材料学会, 2016年09月, 日本材料学会会誌「材料」, 65(9) (9), pp. 647 - 651, 日本語
    [査読有り]
    研究論文（学術雑誌）

  • 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, 2016年03月, IEEE JOURNAL OF PHOTOVOLTAICS, 6(2) (2), 465 - 472, 英語
    [査読有り]
    研究論文（学術雑誌）

  • 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}), 2015年11月, IEEE Journal of Photovoltaics, 5(6) (6), 1613 - 1620, 英語
    [査読有り]
    研究論文（学術雑誌）

  • 朝日 重雄, 寺西 陽之, 笠松 直史, 加田 智之, 海津 利行, 喜多 隆
    We systematically studied two-step photocurrent generation as functions of the excitation intensities for the inter-band and inter-subband transitions in an InAs/GaAs/Al<sub>0.3</sub>Ga<sub>0.7</sub>As 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 th
    The Society of Materials Science, Japan, 2015年09月, 材料, 64(9) (9), 690 - 695, 日本語
    [査読有り]
    研究論文（学術雑誌）

  • 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, 2015年07月, Applied Physics Letters, 107(4) (4), 043901 - 043901, 英語
    [査読有り]
    研究論文（学術雑誌）
    神戸大学リポジトリ（Kernel）へのリンク

  • 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, 2015年05月, PHYSICAL REVIEW B, 91(20) (20), pp. 201303 - 1-6, 英語
    [査読有り]
    研究論文（学術雑誌）
    神戸大学リポジトリ（Kernel）へのリンク

  • 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年, 2015 IEEE 42ND PHOTOVOLTAIC SPECIALIST CONFERENCE (PVSC), 英語
    [査読有り]
    研究論文（国際会議プロシーディングス）

  • 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年, 2015 IEEE 42ND PHOTOVOLTAIC SPECIALIST CONFERENCE (PVSC), 英語
    [査読有り]
    研究論文（国際会議プロシーディングス）

  • 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年, 2015 IEEE 42ND PHOTOVOLTAIC SPECIALIST CONFERENCE (PVSC), 英語
    [査読有り]
    研究論文（国際会議プロシーディングス）

  • 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年, 2015 IEEE 42ND PHOTOVOLTAIC SPECIALIST CONFERENCE (PVSC), 英語
    [査読有り]
    研究論文（国際会議プロシーディングス）

  • 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年, PHYSICS SIMULATION AND PHOTONIC ENGINEERING OF PHOTOVOLTAIC DEVICES IV, 9358, 英語
    研究論文（国際会議プロシーディングス）

  • 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}), 2014年11月, IEEE Journal of Photovoltaics, 4(6) (6), 1518 - 1525, 英語
    [査読有り]
    研究論文（学術雑誌）

  • 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, 2014年08月, JOURNAL OF APPLIED PHYSICS, 116(6) (6), pp. 063510 - 1-5, 英語
    [査読有り]
    研究論文（学術雑誌）

■ MISC
  • Shigeo Asahi, Takashi Kita
    Society of Materials Science Japan, 2017年03月01日, Zairyo/Journal of the Society of Materials Science, Japan, 66(3) (3), 244 - 249, 日本語
    書評論文,書評,文献紹介等

  • 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, 笠松直史, 藤井宏昌, 加田智之, 朝日重雄, WANG Yunpeng, 渡辺健太郎, 杉山正和, 喜多隆, 中野義昭
    2014年09月01日, 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, 日本語

  • 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, 笠松直史, 藤井宏昌, 加田智之, 朝日重雄, WANG Yunpeng, 渡辺健太郎, 杉山正和, 喜多隆, 中野義昭
    2014年03月03日, 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, 日本語

■ 書籍等出版物
  • Energy Conversion Efficiency of Solar Cells
    朝日 重雄
    共著, Springer, 2019年08月, ISBN: 9789811390890

■ 講演・口頭発表等
  • Siレーザーパワーコンバーターの分光評価
    花熊亮汰, 朝日重雄, 原田幸弘, 喜多隆
    第72回応用物理学会春季学術講演会, 2025年03月, 日本語
    口頭発表（一般）

  • 全無機CsPbBr3–xClxペロブスカイトナノ結晶, 両面2段階フォトンアップコンバージョン太陽電池のための半導体の可能性
    Hambalee Mahamu, Soma Ueno, Shigeo Asahi, Takashi Kita
    第72回応用物理学会春季学術講演会, 2025年03月, 英語
    口頭発表（一般）

  • ダブルトンネル接合を利用したアップコンバージョン太陽電池の効率的バンド内遷移
    朝日重雄, 松沢光一郎, 喜多隆
    第72回応用物理学会春季学術講演会, 2025年03月, 日本語
    口頭発表（一般）

  • 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, 2024年09月, 日本語
    口頭発表（一般）

  • 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, 2024年09月, 日本語

  • Intraband Transitions Induced by Below-Bandgap Photoexcitation at CsPbBr₃/GaAs Heterointerface
    H. Mahamu, S. Asahi, T. Kita
    第85回応用物理学会秋季学術講演会, 2024年09月, 英語

  • 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), 2024年06月, 英語
    ポスター発表

  • 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), 2024年06月, 英語
    ポスター発表

  • 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, 2024年03月, 日本語
    口頭発表（一般）

  • 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, 2023年03月, 英語
    口頭発表（一般）

  • 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, 2023年01月, 日本語
    口頭発表（一般）

  • 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, 2022年11月, 日本語
    口頭発表（一般）

  • 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), 英語
    口頭発表（一般）

  • 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), 2022年10月

  • 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), 2022年10月, 日本語
    口頭発表（一般）

  • 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), 英語
    ポスター発表

  • Electrical Properties of AlGaAs/GaAs-Based Two-Step Photon Up-Conversion Solar Cells with Doubled Heterointerfaces
    H. Mahamu, S. Asahi, T. Kita
    第83回応用物理学会秋季学術講演会, 2022年09月, 英語
    口頭発表（一般）

  • 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), 英語
    口頭発表（一般）

  • 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), 英語
    口頭発表（一般）

  • 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, 2022年03月, 日本語
    口頭発表（一般）

  • 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, 2022年03月, 日本語
    口頭発表（一般）

  • 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), 2021年11月, 英語
    [招待有り]
    口頭発表（招待・特別）

  • 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, 2021年10月, 日本語
    ポスター発表

  • 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, 2021年09月, 英語
    口頭発表（一般）

  • 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, 2021年09月, 日本語
    口頭発表（一般）

  • 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), 2021年09月, 英語
    [招待有り]
    口頭発表（招待・特別）

  • 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, 2021年03月, 英語
    口頭発表（一般）

  • 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, 2021年03月, 日本語
    口頭発表（一般）

  • 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, 2019年11月, 日本語
    [招待有り]
    口頭発表（招待・特別）

  • Strong Voltage-Boost Effect in Two-Step Photon-Up Conversion Solar Cells
    S. Asahi, T. Kita
    46th IEEE Photovoltaic Specialists Conference, 2019年07月, 英語, Chicago, 国際会議
    口頭発表（一般）

  • Hot-Carrier Extraction in InAs/GaAs Quantum Dot Superlattice Solar Cells
    Y. Harada, N. Iwata, D. Watanabe, S. Asahi, T. Kita
    46th IEEE Photovoltaic Specialists Conference, 2019年07月, 英語, Chicago, 国際会議
    口頭発表（一般）

  • Reciprocal Relationship Between Photoluminescence and Photocurrent in Two-Step Photon Up-Conversion Solar Cell
    N. Kinugawa, S. Asahi, T. Kita
    46th IEEE Photovoltaic Specialists Conference, 2019年06月, 英語, Chicago, 国際会議
    口頭発表（一般）

  • Efficient Hot-Carrier Generation in InAs/GaAs Quantum Dot Superlattices
    Y. Harada, N. Iwata, D. Watanabe, S. Asahi, T. Kita
    Optics&Photonics International Congress 2019, 2019年04月, 英語, Yokohama, 国際会議
    口頭発表（一般）

  • 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
    原田 幸弘, 岩田 尚之, 朝日 重雄, 喜多 隆
    第66回応用物理学会春季学術講演会, 2019年03月, 日本語, 国内会議
    口頭発表（一般）

  • 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, 2018年10月, 日本語, 国内会議
    ポスター発表

  • Optimal Band Gap Energies for Two-Step Photon Up-Conversion Solar Cells with Partial Absorptivity
    Y. Harada, T. Matsuo, S. Asahi, T. Kita
    35th European PV Solar Energy Conference and Exhibition, 2018年09月, 英語, Belgium, 国際会議
    口頭発表（一般）

  • 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, 原田 幸弘, 朝日 重雄, 喜多 隆
    第79回応用物理学会秋季学術講演会, 2018年09月, 日本語, 国内会議
    口頭発表（一般）

  • 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
    原田 幸弘, 松尾 哲弘, 朝日 重雄, 喜多 隆
    第79回応用物理学会秋季学術講演会, 2018年09月, 日本語, 国内会議
    口頭発表（一般）

  • 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, 2018年08月, 日本語
    [招待有り]
    口頭発表（招待・特別）

  • Carrier Collection Efficiency of Intraband-Excited Carriers in Two-Step Photon Up-Conversion Solar Cells
    S. Asahi, K. Nishimura, T. Kaizu, T. Kita
    The 7th edition of the World Conference on Photovoltaic Energy Conversion, 2018年06月, 英語, Hawaii, 国際会議
    口頭発表（一般）

  • 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
    松尾 哲弘, 原田 幸弘, 朝日 重雄, 喜多 隆
    応用物理学会関西支部平成30年度第1回講演会, 2018年05月, 日本語, 神戸大学, 国内会議
    ポスター発表

  • Polarization Dependent Photocurrent in InAs/GaAs Quantum Dot Superlattice Solar Cells
    Y. Harada, J. Yamada, D. Watanabe, S. Asahi, T. Kita
    International Conference on Nanophotonics and Nano-optoelectronics 2018, 2018年04月, 英語, Yokohama, 国際会議
    口頭発表（一般）

  • Extraction Efficiency of Up-Converted Electrons in Two-Step Photon Up-Conversion Solar Cells
    S. Asahi, K. Nishimura, T. Kaizu, T. Kita
    International Conference on Nanophotonics and Nano-optoelectronics 2018, 2018年04月, 英語, Yokohama, 国際会議
    口頭発表（一般）

  • 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
    原田 幸弘, 朝日 重雄, 喜多 隆
    第65回応用物理学会春季学術講演会, 2018年03月, 日本語, 国内会議
    口頭発表（一般）

  • 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, 原田 幸弘, 朝日 重雄, 喜多 隆
    第65回応用物理学会春季学術講演会, 2018年03月, 日本語, 国内会議
    口頭発表（一般）

  • 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, 原田 幸弘, 朝日 重雄, 喜多 隆
    応用物理学会関西支部平成29年度第3回講演会, 2018年02月, 日本語, 国内会議
    ポスター発表

  • Two-Step Photon Up-Conversion Solar Cell: Propose and Demonstration
    S. Asahi, T. Kita
    SPIE Photonics West, 2018年01月, 英語, San Francisco, 国際会議
    [招待有り]
    口頭発表（招待・特別）

  • Two-Step Photon Up-Conversion Solar Cell
    T. Kita, S. Asahi
    MTSA2017-OptoX Nano-TeraNano8, 2017年11月, 英語, Okayama, 国際会議
    口頭発表（一般）

  • Two-Step Photo-Excitated Electrons with Extremely-Long Lifetime in Intermedeate-band Solar Cells Using Dot-in Well Strucyure
    S. Asahi, H. Teranishi, T. Kaizu, T. Kita
    The 27th Photovoltaic Science and Engineering Conference, 2017年11月, 英語, Otsu, 国際会議
    口頭発表（一般）

  • Two-Step Photn Up-Conversion Solar Cells Incorporating a Voltage Booster Hetero-Interface
    S. Asahi, K. Kusaki, Y. Harada, T. Kita
    The 27th Photovoltaic Science and Engineering Conference, 2017年11月, 英語, Otsu, 国際会議
    口頭発表（一般）

  • Infrared Absorption Characteristics in Two-Step Photon Up-Conversion Solar Cells
    K. Kusaki, S. Asahi, T. Kaizu, R. Tamaki, Y. Okada, T. Kita
    The 27th Photovoltaic Science and Engineering Conference, 2017年11月, 英語, Otsu, 国際会議
    口頭発表（一般）

  • Efficient Two-Step Photocurrent in Intermediate Band Solar Cells Using Highly Homogeneous InAs/GaAs Quantum-Dot Superlattice
    K. Hirao, S. Asahi, T. Kaizu, Y. Harada, T. Kita
    The 27th Photovoltaic Science and Engineering Conference, 2017年11月, 英語, Otsu, 国際会議
    口頭発表（一般）

  • 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, 朝日 重雄, 海津 利行, 原田 幸弘, 喜多 隆
    第78回応用物理学会秋季学術講演会, 2017年09月, 日本語, 国内会議
    口頭発表（一般）

  • 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, 朝日 重雄, 海津 利行, 原田 幸弘, 喜多 隆
    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, 2017年09月, 日本語, 国内会議
    口頭発表（一般）

  • Photon Up-Converted Photocurrent in a Single Junction Solar Cell with a Hetero-Interface
    K. Kusaki, S. Asahi, T. Kaizu, T. Kita
    33rd European Photovoltaic Solar Energy Conference and Exhibition, 2017年09月, 英語, Netherlands, 国際会議
    口頭発表（一般）

  • Increasing Photovoltage Boosted by Photon Up-Conversion in a Single-Junction Solar Cell with a Hetero-Interface
    S. Asahi, K. Kusaki, T. Kaizu, T. Kita
    33rd European Photovoltaic Solar Energy Conference and Exhibition, 2017年09月, 英語, Netherlands, 国際会議
    口頭発表（一般）

  • 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
    原田 幸弘, 山田 淳也, 渡部 大樹, 朝日 重雄, 喜多 隆
    第78回応用物理学会秋季学術講演会, 2017年09月, 日本語, 国内会議
    口頭発表（一般）

  • Extended Optical Response of Two-Step Photoexcitation in InAs/GaAs Quantum-Dot Superlattice Intermediate Band Solar Cells
    K. Hirao, S. Asahi, T. Kaizu, T. Kita
    33rd European Photovoltaic Solar Energy Conference and Exhibition, 2017年09月, 英語, Netherlands, 国際会議
    口頭発表（一般）

  • Carrier Dynamics in Photon Up-Conversion Solar Cells
    T. Kita, S. Asahi
    SemiconNano 2017: 6th International Workshop Epitaxial Growth and Fundamental Properties of Semiconductor Nanostructures, 2017年09月, 英語, Milano, 国際会議
    口頭発表（一般）

  • Increasing Current Generation by Photon Up-Conversion in a Single-Junction Solar Cell with a Hetero-Interface
    S. Asahi, K. Kusaki, T. Kaizu, T. Kita
    2017 IEEE Photovoltaic Specialists Conference, 2017年06月, 英語, Washington D.C., 国際会議
    口頭発表（一般）

  • 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, 朝日 重雄, 海津 利行, 原田 幸弘, 喜多 隆
    第77回応用物理学会秋季学術講演会, 2016年09月, 日本語, 国内会議
    口頭発表（一般）

  • 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, 2016年07月, 日本語, 国内会議
    ポスター発表

  • Polarization-Insensitive Intraband Transition in InAs/GaAs Quantum Dot Superlattices
    Y. Harada, J. Yamada, D. Watanabe, S. Asahi, T. Kita
    UK Semiconductors 2016, 2016年07月, 英語, Sheffield, 国際会議
    口頭発表（一般）

  • Extended Electron Lifetime in Intermediate-Band Solar Cells Using Dot-in-Well Structure
    S. Asahi, H. Teranishi, S. Watanabe, T. Kada, T. Kaizu, T. Kita
    32nd European Photovoltaic Solar Energy Conference and Exhibition, 2016年06月, 英語, Munich, 国際会議
    口頭発表（一般）

  • Enhancement of Two-Step Photon Absorption Due to Miniband Formation in InAs/GaAs Quantum Dot Superlattice Solar Cell
    S. Watanabe, S. Asahi, T. Kada, T. Kaizu, Y. Harada, T. Kita
    32nd European Photovoltaic Solar Energy Conference and Exhibition, 2016年06月, 英語, Munich, 国際会議
    口頭発表（一般）

■ 所属学協会
  • IEEE Electron Devices Society Membership
    2018年01月 - 現在

  • IEEE Membership
    2018年01月 - 現在

  • 応用物理学会 正会員
    2017年01月 - 現在

■ 共同研究・競争的資金等の研究課題
  • 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, 2023年04月 - 2026年03月

  • 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, 2023年04月 - 2026年03月

  • バンド内光学遷移分極の制御を基盤とした赤外増感型光電変換の新展開
    喜多 隆, 原田 幸弘, 朝日 重雄
    日本学術振興会, 科学研究費助成事業 基盤研究(A), 基盤研究(A), 神戸大学, 2019年04月01日 - 2023年03月31日
    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。

  • アップコンバージョン現象を利用した太陽電池による高効率エネルギー変換の実現
    朝日 重雄
    日本学術振興会, 科学研究費助成事業 若手研究, 若手研究, 神戸大学, 2020年04月 - 2023年03月
    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。

  • 赤外増感型バンド内光学遷移の制御と超高感度量子型赤外線センサーへの応用
    喜多 隆, 原田 幸弘, 朝日 重雄
    日本学術振興会, 科学研究費助成事業 挑戦的研究(萌芽), 挑戦的研究(萌芽), 神戸大学, 2020年07月 - 2022年03月
    本研究では量子ドットとヘテロ界面を融合した量子ナノ構造を有する新しい赤外光センシングデバイスを開発した。このデバイスはヘテロ界面における高効率なバンド内光学遷移エンジニアリングを原理として駆動しており、量子ドット形状による光増感が特徴である。開発したデバイスはAl0.3Ga0.7As/GaAsヘテロ界面にInAs量子ドットを挿入した構造で、ヘテロ界面に蓄積される電子が赤外光で励起されて光電流を生じる量子型デバイスである。赤外光センシングは量子ドットによって顕著に増強した。理論計算により、量子ドット界面に局在する強いプラズモンモードによって赤外光の電場が増強していることが明らかになった。

  • 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)), 2018年10月 - 2021年03月
    競争的資金

  • 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, 2019年04月 - 2020年03月, 研究代表者

  • 朝日 重雄
    科学研究費補助金／研究活動スタート支援, 2018年08月 - 2020年03月, 研究代表者
    競争的資金

  • 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, 2017年11月 - 2017年11月

  • 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）, 2017年06月 - 2017年06月

  • 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, 2016年04月 - 2016年04月

  • 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, 2015年06月 - 2015年06月, 研究代表者

■ 産業財産権
  • 高効率太陽電池
    朝日 重雄, 喜多 隆, 松沢 光一郎
    特願2023-142969, 2023年09月04日, 国立大学法人神戸大学
    特許権
    外部リンク

  • 量子型赤外線センサ
    喜多 隆, 朝日 重雄, 村田 貴彦
    特願2019-138335, 2019年07月26日, 特開2021-022662, 2021年02月18日, 特許7291942号, 2023年06月18日
    特許権

  • 高変換効率太陽電池およびその調製方法
    喜多 隆, 原田 幸弘, 朝日 重雄, 渡部 大樹
    特願2014-113313, 2014年05月30日, 大学長, 特許6385720, 2018年08月17日
    特許権

研究シーズ

■ 研究シーズ
  • アップコンバージョン太陽電池–次世代太陽電池を目指して
    シーズカテゴリ：エネルギー, ものづくり技術（機械・電気電子・化学工業）
    研究キーワード：太陽電池, 量子ドット, アップコンバージョン
    研究内容：現在主流であるシリコン単接合型太陽電池は理論変換効率が最大約30%で、それ以上の変換効率を目指すには別の構造が必要です。そうした中で、シリコン単接合型太陽電池では吸収できなかった長波長の光子を2段階で吸収するアップコンバージョン太陽電池を提案しております。この太陽電池の理論変換効率は100倍集光下で50%を超えることが理論的に分かっており、この太陽電池の実現に向け取り組んでいます。
研究者連絡先、関連サイト
　　　　　
メールアドレス：https://www.research.kobe-u.ac.jp/eng-photonics/　※左記の「@」は画像ですので、ご利用の際はテキストの「@」を入力してください。
研究者紹介：https://kuid-rm-web.ofc.kobe-u.ac.jp/search/detail?systemId=f6b9bd62c178c09e520e17560c007669
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