Faculty Profiles - KUROKAWA Yasuyoshi

Papers - KUROKAWA Yasuyoshi

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Numerical Simulation Study for Analysis of Hydrogenated Amorphous Silicon/Crystalline Silicon Heterostructure by Reactive Molecular Dynamics Method Reviewed International journal

Proc. 50th IEEE Photovoltaic Specialists Conference page： 1 - 3 2023.12

Free-Standing Electrode and Fixed Surface Tiny Electrode Implemented Tiboelectric Nanogenerator with High Instantaneous Current Reviewed

Wang, HT; Kurokawa, Y; Wang, J; Cai, WT; Zhang, JH; Kato, S; Usami, N

SMALL page： e2308531 2023.12

Language：English Publishing type：Research paper (scientific journal) Publisher：Small

Conventional triboelectric nanogenerators (TENGs) face challenges pertaining to low output current density at low working frequencies and high internal impedance. While strategies, such as surface modification to enhance surface charge density, permittivity regulation of materials, and circuit management, have partially mitigated these issues. However, they have also resulted in increased complexity in the fabrication process. Therefore, there is an urgent demand for a universal and simplified approach to address these challenges. To fulfill this need, this work presents a free-standing electrode and fixed surface tiny electrode implemented triboelectric nanogenerator (FFI-TENG). It is fabricated by a straightforward yet effective method: introducing a tiny electrode onto the surface of the tribo-negative material. This approach yields substantial enhancements in performance, notably a more than tenfold increase in output current density, a reduction in effective working frequencies, and a decrease in matching resistance as compared to vertical contact-separation TENGs (CS-TENGs) or single-electrode TENGs (SE-TENGs). Simultaneously, a comprehensive examination and proposition regarding the operational mechanism of FFI-TENG, highlighting its extensive applicability are also offered. Significantly, FFI-TENG excels in mechanical energy harvesting even under ultra-low working frequencies (0.1 Hz), outperforming similar contact-separation models. This innovation positions it as a practical and efficient solution for the development of low-entropy energy harvesters.

DOI： 10.1002/smll.202308531

Web of Science

Scopus

PubMed

The effect of room-temperature plastic deformation in darkness on the photoluminescence properties of ZnS Reviewed

Ogura, Y; Nakamura, A; Kameyama, T; Kurokawa, Y; Tochigi, E; Shibata, N; Torimoto, T; Hoshino, S; Yokoi, T; Matsunaga, K

JOURNAL OF THE AMERICAN CERAMIC SOCIETY Vol. 107 ( 4 ) page： 2040 - 2047 2023.11

Language：English Publishing type：Research paper (scientific journal) Publisher：Journal of the American Ceramic Society

Inorganic semiconductors have been considered to be brittle at room temperature generally, but recently zinc sulfide (ZnS), a II–VI compound semiconductor, was found to exhibit extraordinarily large plasticity even at room temperature if it was deformed in darkness. Room-temperature plastic deformation can achieve high density of glide dislocations in the crystal without significantly changing the point defect structure, and these dislocations themselves have a potential to provide unique functional properties different from those of the bulk. In this study, therefore, undoped ZnS crystals were plastically deformed at room temperature in darkness to generate only a large number of dislocations, and the changes in luminescence properties resulting from this process were investigated for the first time. As a result, we found that ZnS deformed in darkness exhibits characteristic photoluminescence and persistent luminescence emissions with a visible green color. SEM-CL analyses also identified the emissions from the dislocations. Theoretical calculations indicated that individual dislocations in ZnS have the ability to trap photo-excited carriers. Such changes in luminescence properties due to room temperature plastic deformation in darkness can be considered to originate from dislocations rather than point defects. In other words, the dislocations themselves serve as the significant recombination centers, realizing visible light emission.

DOI： 10.1111/jace.19564

Web of Science

Scopus

Improvement of passivation performance of silicon nanocrystal/silicon oxide compound layer by two-step hydrogen plasma treatment Reviewed

M. Matsumi, K. Gotoh, M. Wilde, Y. Kurokawa, K. Fukutani and N. Usami

Solar Energy Materials and Solar Cells Vol. 262 page： 112538 2023.10

Nanoscale Size Control of Si Pyramid Texture for Perovskite/Si Tandem Solar Cells Enabling Solution-Based Perovskite Top-Cell Fabrication and Improved Si Bottom-Cell Response Reviewed

Li, YQ; Sai, H; Mcdonald, C; Xu, ZH; Kurokawa, Y; Usami, N; Matsui, T

ADVANCED MATERIALS INTERFACES Vol. 10 ( 35 ) 2023.9

Language：English Publishing type：Research paper (scientific journal) Publisher：Advanced Materials Interfaces

A monolithic perovskite/silicon tandem solar cell architecture is employed to surpass the single-junction efficiency limit. Recently, there is an increasing need for the double-sided textures in the Si bottom cell to be compatible with the solution-processed perovskite top cell from an industrial perspective. Herein, a silver-assisted alkaline etching method is applied to fabricate nanoscale Si pyramid textures, and the influence of varying pyramid size (400–900 nm) on the interface morphology and the performance of perovskite/Si tandem cells is investigated. It is demonstrated that electrical shunting starts to increase, and the open-circuit voltage (VOC) decreases when the texture size exceeds the perovskite thickness (~500nm) due to the non-uniform top-cell formation on a rough Si surface. However, when the texture size is reduced to 400–500 nm, all spin-coated perovskite top-cell component layers exhibit an even form over the nanopyramid Si, resulting in a high VOC and an enhanced Si bottom cell current (≈1.0 mA cm−2) due to the suppressed reflectance at the top/bottom cell interface without using optical couplers. The double-sided nanopyramid Si texture offers opportunities to increase tandem cell efficiency while reducing its production cost compared with the commonly used single-sided textured Si.

DOI： 10.1002/admi.202300504

Web of Science

Scopus

Impact of B<inf>2</inf>H<inf>6</inf>plasma treatment on contact resistivity in silicon heterojunction solar cells Reviewed

Gotoh K., Ozaki R., Morimura M., Tanaka A., Iseki Y., Nakamura K., Muramatsu K., Kurokawa Y., Ohshita Y., Usami N.

Japanese Journal of Applied Physics Vol. 62 ( SK ) 2023.8

Language：English Publishing type：Research paper (scientific journal) Publisher：Japanese Journal of Applied Physics

We investigated the effect of the B2H6 plasma treatment on p-type hydrogenated amorphous silicon (p-a-Si:H) surfaces for high-performance silicon heterojunction (SHJ) solar cells. Secondary ion mass spectroscopy measurements revealed that the boron concentration at the p-a-Si:H surface is increased by employing the B2H6 plasma treatment. Furthermore, specific contact resistance is decreased by about one-third after the B2H6 plasma treatment. No degradation of passivation performance is induced by the B2H6 plasma treatment. The power conversion efficiency of the SHJ solar cells with the B2H6 plasma treatment is improved by the increase in fill factor (FF) due to decreased series resistance and increased shunt resistance. From numerical simulations, the upward band bending is enhanced at the heterointerface between transparent conductive oxide (TCO) and p-a-Si:H by the B2H6 plasma treatment, which is responsible for the improved FF owing to facilitated tunneling holes from c-Si to p-a-Si:H layers and the TCO/p-a-Si:H heterointerface.

DOI： 10.35848/1347-4065/acc953

Scopus

Quantitative evaluation of implied open-circuit voltage after metal electrode deposition on TiO <inf>x</inf> /Si heterostructures by photoluminescence imaging: impact of metallization on passivation performance Reviewed

Fukaya S., Gotoh K., Matsui T., Sai H., Kurokawa Y., Usami N.

Japanese Journal of Applied Physics Vol. 62 ( SK ) 2023.8

Language：English Publishing type：Research paper (scientific journal) Publisher：Japanese Journal of Applied Physics

The degradation of surface passivation performance by metallization is a challenge in realizing highly efficient crystalline Si solar cells that use novel carrier-selective contacts. Here, we report on a simple method to study the effect of metallization on passivation of titanium oxide (TiOx)/Si heterostructures. We investigated the relationship between the implied open-circuit voltage (iV OC) and the photoluminescence (PL) intensity imaging of solar cell precursors before metallization. Based on the relationship obtained, the change of the iV OC before and after metallization on the TiOx was evaluated quantitatively. The results showed that the iV OC predicted by the PL measurement decreases by 23-104 mV after metal deposition and shows a good agreement with the measured V OC in the finished solar cells. These results demonstrate that the iV OC evaluation by PL measurement provides a good prediction of the V OC after metallization, which is useful in analyzing the passivation degradation induced by metallization.

DOI： 10.35848/1347-4065/acc813

Scopus

Fabrication of light trapping structures specialized for near-infrared light by nanoimprinting for the application to thin crystalline silicon solar cells Reviewed

Kimata Yuto, Gotoh Kazuhiro, Miyamoto Satoru, Kato Shinya, Kurokawa Yasuyoshi, Usami Noritaka

DISCOVER NANO Vol. 18 ( 1 ) page： 72 2023.5

Language：English Publishing type：Research paper (scientific journal) Publisher：Discover Nano

Vehicle-integrated photovoltaics (VIPV) are gaining attention to realize a decarbonized society in the future, and the specifications for solar cells used in VIPV are predicated on a low cost, high efficiency, and the ability to be applied to curved surfaces. One way to meet these requirements is to make the silicon substrate thinner. However, thinner substrates result in lower near-infrared light absorption and lower efficiency. To increase light absorption, light trapping structures (LTSs) can be implemented. However, conventional alkali etched pyramid textures are not specialized for near-infrared light and are insufficient to improve near-infrared light absorption. Therefore, in this study, as an alternative to alkaline etching, we employed a nanoimprinting method that can easily fabricate submicron-sized LTSs on solar cells over a large area. In addition, as a master mold fabrication method with submicron-sized patterns, silica colloidal lithography was adopted. As a result, by controlling silica coverage, diameter of silica particles (D), and etching time (t et), the density, height, and size of LTSs could be controlled. At the silica coverage of 40%, D = 800 nm, and t et = 5 min, the reduction of reflectance below 65% at 1100 nm and the theoretical short-circuit current gain of 1.55 mA/cm2 was achieved.

DOI： 10.1186/s11671-023-03840-6

Web of Science

Scopus

PubMed

Performance enhancement of droplet-based electricity generator using a CYTOP intermediate layer Reviewed

Japanese Journal of Applied Physics Vol. 62 ( SC ) page： SC1032 2023.1

Language：English Publishing type：Research paper (scientific journal) Publisher：Japanese Journal of Applied Physics

DOI： 10.35848/1347-4065/acaca7

Study on the Etching Characteristics and Thermal Conductivity of Porous Silicon Nanowire Arrays Using Highly Doped Silicon Reviewed

Li Haibin, Kato Shinya, Kurokawa Yasuyoshi, Soga Tetsuo

e-Journal of Surface Science and Nanotechnology Vol. advpub ( 0 ) 2022.12

Language：English Publishing type：Research paper (scientific journal) Publisher：The Japan Society of Vacuum and Surface Science

Silicon nanowires (SiNWs) fabricated by metal-assisted chemical etching (MACE) of highly doped silicon are promising thermoelectric materials because of their high conductivity and porosity. However, it is difficult to fabricate long SiNWs using highly doped silicon with doping concentrations higher than 1019 cm−3. In this study, we fabricated SiNWs up to 44 μm in length using highly doped silicon with a doping concentration of 1020 cm−3, analyzed the etching characteristics in SiNWs length and the morphology, and investigated the thermal conductivity. The phenomenon of continuous loss of SiNWs was found, which is a unique phenomenon indicating the presence of loss SiNWs throughout the whole etching process and difficulty of forming long SiNWs with highly doped silicon. On the other hand, the thermal conductivity results showed that the lowest thermal conductivity of the samples reached 2.98 W m−1 K−1, and the mean diameters of pores in these samples were 5−10 nm. Thus, the SiNWs using highly doped silicon is a potential material for thermoelectric device.

DOI： 10.1380/ejssnt.2023-006

CiNii Research

A novel self-separating silicon nanowire thin film and application in lithium-ion batteries Reviewed

Li H., Kato S., Ishii Y., Kurokawa Y., Soga T.

Nano Express Vol. 3 ( 4 ) 2022.12

Language：English Publishing type：Research paper (scientific journal) Publisher：Nano Express

Nano silicon structures are important materials for modern electronic devices and have been widely researched with regard to photoelectricity, thermoelectricity, and lithium-ion batteries. However, since the nano silicon structures fabricated by conventional methods cannot be separated from silicon substrates, reuse of the substrate is restricted. Here, we propose a simple fabrication method to separate the nano silicon structures from the silicon substrates, which allows the reuse of the substrates. The fabrication was processed at room temperature, which allows large-area fabrication and is not restricted by the substrate thickness. Honeycomb structures of different length scales observed on both the nano silicon structure and the substrate suggest that the separation occurred due to the amplification of the silicon crystal defects. The nano silicon structures comprised porous silicon with an excellent specific surface area of 480 m2 g−1 and a mean pore diameter of 5.7 nm. Moreover, the nano silicon structures show good potential as anode materials for lithium-ion batteries wherein the measured reversible capacity was 1,966 mAh g−1 after 100 cycles. Based on the proposed method and morphological characteristics, the fabricated nano silicon structures can be considered a low-cost material with suitable applications in the energy field.

DOI： 10.1088/2632-959X/acb807

Scopus

Performance enhancement of droplet-based electricity generator using a CYTOP intermediate layer Reviewed International journal

Haitao Wang, Yasuyoshi Kurokawa, Kazuhiro Gotoh, Shinya Kato, Shigeru Yamada, Takashi Itoh, Noritaka Usami

Ext. Abstr. 2022 International Conference on Solid State Devices and Materals page： 145 - 146 2022.9

Preparation and thermoelectric characterization of boron-doped silicon nanocrystals/silicon oxide multilayers Reviewed International journal

Keisuke Shibata, Shinya Kato, Masashi Kurosawa, Kazuhiro Gotoh, Satoru Miyamoto, Noritaka Usami, Yasuyoshi Kurokawa

Ext. Abstr. 2022 International Conference on Solid State Devices and Materals page： 147 - 148 2022.9

Nanopyramid Texture Formation by One-Step Ag-Assisted Solution Process for High-Efficiency Monocrystalline Si Solar Cells Reviewed

Li Y., Sai H., Matsui T., Xu Z., Nguyen V.H., Kurokawa Y., Usami N.

Solar RRL 2022.9

Language：English Publishing type：Research paper (scientific journal) Publisher：Solar RRL

In monocrystalline Si solar cells, pyramidal surface textures with the size of a few micrometers (micropyramids) are commonly applied for light in-coupling and trapping. Recently, there is a growing need to reduce the size of pyramids below 1 μm. Herein, an original method to fabricate nanometer-sized pyramids (nanopyramids) featuring an average size of ≈500 nm or even smaller by using a simple one-step Ag-assisted solution process is proposed. The texture size can be controlled by the concentration of AgNO3 in the alkaline etching solution where the generated Ag nanoparticles act as an etching mask as well as promoting the detachment of H2 bubbles from the reacting Si surface. This process enables the formation of uniformly distributed Si nanopyramids with a lower reflectance below 10% and a smaller etching margin less than 1.7 μm compared with the conventional micropyramids. Silicon heterojunction solar cells employing Si nanopyramids outperform those with the conventional micropyramids owing to the reduced optical reflectance while preserving excellent surface passivation and carrier transport properties. These results demonstrate the advantages of our method and the high potential of Si nanopyramids for various applications such as thin crystalline Si solar cells and perovskite/Si tandem solar cells.

DOI： 10.1002/solr.202200707

Scopus

Influence of post-oxidizing treatment on passivation performance on the spin-coated titanium oxide films on crystalline silicon Reviewed

Hao Luo, Van Hoang Nguyen, Kazuhiro Gotoh, Saya Ajito, Tomohiko Hojo, Yasuyoshi Kurokawa, Eiji Akiyama and Noritaka Usami

Thin Solid Films Vol. 764 page： 139597 2022.9

Language：English Publishing type：Research paper (scientific journal) Publisher：Solar RRL

DOI： 10.1016/j.tsf.2022.139597

Enhancement of reflectance reduction of solar cells by a silicon nanoparticle layer on a textured silicon substrate Reviewed

Shinya Kato, Yasuhoshi Kurakawa, Tetsuo Soga

Results in Optics Vol. 9 page： 100296 2022.9

Language：English Publishing type：Research paper (scientific journal) Publisher：Solar RRL

DOI： 10.1016/j.rio.2022.100296

Fabrication of Silicon Heterojunction Solar Cells with Light Trapping Structures Specialized for Near-Infrared Light by Nanoimprinting Reviewed International journal

Y. Kimata, K. Gotoh, S. Miyamoto, Y. Kurokawa, N. Usami

Proc. 8th World Conference on Photovoltaic Energy Conversion page： 230 - 233 2022.9

Improvement of Passivation Performance of Silicon Nanocrystal/Silicon Oxide Compound Layer by Hydrogen Plasma Treatment Reviewed International journal

M. Matsumi, K. Gotoh, M. Wilde, Y. Kurokawa, K. Fukutani, N. Usami

Proc. 8th World Conference on Photovoltaic Energy Conversion page： 402 - 405 2022.9

Improved conversion efficiency of p-type BaSi2/n-type crystalline Si heterojunction solar cells by a low growth rate deposition of BaSi2 Reviewed

Fujiwara Michinobu, Takahashi Kazuma, Nakagawa Yoshihiko, Gotoh Kazuhiro, Itoh Takashi, Kurokawa Yasuyoshi, Usami Noritaka

AIP ADVANCES Vol. 12 ( 4 ) 2022.4

Authorship：Corresponding author Language：English Publishing type：Research paper (scientific journal) Publisher：AIP Advances

The effect of low growth rate deposition (LGD) of BaSi2 on the film quality and performance of silicon heterojunction solar cells was investigated. The total thickness of the BaSi2 layer decreased with increasing LGD duration (tLGD). Analysis using Raman spectroscopy indicated that an amorphous Si (a-Si) phase existed on the surface of the BaSi2 layer. The a-Si on the surface was converted into BaSi2 by post-annealing owing to the diffusion of Ba and Si atoms. X-ray diffraction analysis revealed that LGD improved the rate of a-axis orientation and crystallinity. Post-annealing was also observed to have significantly improved these structural properties. Furthermore, the solar cell performance was observed to be strongly dependent on tLGD, and the highest conversion efficiency of 10.62% was achieved by the p-BaSi2/n-c-Si heterojunction solar cells at a tLGD of 6 min. The improved structure and solar cell properties are attributed to improved atom rearrangement during LGD.

DOI： 10.1063/5.0083812

Web of Science

Scopus

Fabrication of BaSi<inf>2</inf>homojunction diodes on Nb-doped TiO<inf>2</inf>coated glass substrates by aluminum-induced crystallization and two-step evaporation method Reviewed

Kurokawa Y., Yoshino T., Gotoh K., Miyamoto S., Usami N.

Japanese Journal of Applied Physics Vol. 61 ( SC ) 2022.2

Authorship：Lead author,　Corresponding author Language：Japanese Publishing type：Research paper (scientific journal) Publisher：Japanese Journal of Applied Physics

BaSi2 homojunction diodes on Nb-doped TiO2 (TiO2:Nb) coated glass substrates were fabricated using aluminum-induced crystallization (AIC) and a two-step evaporation method. From Raman scattering spectra, the growth of BaSi2 on TiO2:Nb was confirmed when the thickness of poly-Si grown by AIC (AIC-Si) was more than 150 nm. The partial formation of BaSi2 diodes was confirmed from the samples prepared at temperature during AIC TAIC = 475 °C-525 °C. The long-wavelength edge of photoresponsivity of the diodes was located around 950 nm, which corresponds to the bandgap of BaSi2 of 1.3 eV, suggesting that this photocurrent is derived from BaSi2 thin films. At TAIC = 500 °C, the maximum value of photoresponsivity was obtained. Since the largest grains in AIC-Si were also obtained at TAIC = 500 °C, these results suggest that larger grain of AIC-Si leads to the improvement of the quality of BaSi2 thin films themselves and the performance of BaSi2 diodes.

DOI： 10.35848/1347-4065/ac4077

Scopus

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Papers - KUROKAWA Yasuyoshi