Papers - Araki Kenji
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Hara T., Ogawa T., Liang J., Araki K., Kamioka T., Shigekawa N.
Japanese Journal of Applied Physics 57 ( 8 ) 2018.8
Language：English Publishing type：Research paper (scientific journal) Publisher：Japanese Journal of Applied Physics
© 2018 The Japan Society of Applied Physics. The electrical properties of GaAs//indium tin oxide (ITO)/Si junctions fabricated by surface-activated bonding (SAB) are investigated with emphasis on their dependence on the temperature of postbonding annealing. The current–voltage (I–V) characteristics of n+-GaAs//ITO/p+-Si and n+-GaAs//ITO/n+-Si junctions without annealing are linear. Those of p+-GaAs//ITO/p+-Si and p+-GaAs//ITO/n+-Si junctions without annealing are nonlinear. Although the interface resistance of all the junctions increases with increasing annealing temperature, the resistances of the respective junctions after the annealing at 400 °C are still smaller than the series resistance of the actual SAB-based InGaP/GaAs//Si hybrid triple-junction cells (>4 Ω cm2). The n+-GaAs//ITO/n+-Si junction reveals the lowest resistance among the investigated junctions after annealing. These results demonstrate that GaAs//ITO/Si junctions with an ITO intermediate layer could be effective for reducing series resistance in hybrid multijunction cells.
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Evaluation and optimization of coating for wide acceptance angle concentrator photovoltaic module Reviewed
Ahmad N., Ota Y., Araki K., Lee K.H., Yamaguchi M., Nishioka K.
Japanese Journal of Applied Physics 57 ( 8 ) 2018.8
Language：English Publishing type：Research paper (scientific journal) Publisher：Japanese Journal of Applied Physics
© 2018 The Japan Society of Applied Physics. Because of the reflection on the module surface, the irradiance received by the solar cells constituting the PV module is less than that received by the module surface. In this situation, an antireflection coating is crucial in improving the performance of the PV module. In this study, we experimented with and simulated a middle concentrator photovoltaic (MCPV) module with a large-acceptance-angle lens. The module was coated with a silica-based coating, which has an antisoiling and an antireflection function. Using the silica-based coating, the optical efficiency of the optics of the MCPV module increased. As a result, under outdoor conditions, the short circuit current and the conversion efficiency of the MCPV module with the silica-based coating was increased by 3 and 2.9%, respectively, compared with the module without the coating. The short circuit current of the MCPV module with a 100-nm-thick coating achieved a 5.6% increase in the optical simulation.
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Shigekawa N., Hara T., Ogawa T., Liang J., Kamioka T., Araki K., Yamaguchi M.
IEEE Journal of Photovoltaics 8 ( 3 ) 879 - 886 2018.5
Language：English Publishing type：Research paper (scientific journal) Publisher：IEEE Journal of Photovoltaics
© 2011-2012 IEEE. Effects of GaAs/indium tin oxide (ITO)/Si junctions on III-V-on-Si multijunction solar cells are examined by fabricating and characterizing InGaP/GaAs/ITO/Si triple-junction (3J) solar cells. The 3J cells are fabricated by evaporating ≈100-nm-thick ITO films on the surfaces of Si bottom cells and bonding the InGaP/GaAs double-junction (2J) subcells and the ITO films using surface-activated bonding technologies at room temperature. The current-voltage characteristics of 3J cells with p+-GaAs/ITO/ n+-Si and n+-GaAs/ITO/n+-Si junctions are compared with those of an InGaP/GaAs/Si 3J cell. The parasitic resistance of the respective 3J cells is estimated by analyzing their characteristics in the dark. We find that the 3J cell with an n+-GaAs/ITO/ n+-Si junction shows the lowest parasitic resistance, which is the origin of its lowest differential resistance at the open-circuit voltage and highest fill factor. This means that n+-GaAs/ITO/ n+-Si junctions are promising for improving the performances of III-V-on-Si hybrid multijunction cells. The spectral response characteristics of these cells indicate that the thickness of the ITO films must be optimized.
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Analysis of future generation solar cells and materials Reviewed
Yamaguchi M., Zhu L., Akiyama H., Kanemitsu Y., Tampo H., Shibata H., Lee K.H., Araki K., Kojima N.
Japanese Journal of Applied Physics 57 ( 4 ) 2018.4
Language：English Publishing type：Research paper (scientific journal) Publisher：Japanese Journal of Applied Physics
© 2018 The Japan Society of Applied Physics. The efficiency potentials of future generation solar cells such as wide bandgap chalcopyrite, Cu2ZnSnS4 (CZTS), Cu2ZnSn(S,Se)4 (CZTSSe), multi quantum well (MQW) and quantum dot (QD) solar cells are discussed on the basis of external radiative efficiency (ERE), open-circuit voltage loss, fill factor loss, and nonradiative recombination losses. CZTS and CZTSSe solar cells have efficiency potentials of more than 20% owing to the improvement in ERE from about 0.001 to 1%. MQW and QD cells have efficiency potentials of 24.8%, and 25.8% owing to the improvement in ERE from around 0.01 to 0.1%, and 1%, respectively. In this paper, the effects of nonradiative recombination on the properties of future generation solar cells are discussed.
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A review of recent progress in heterogeneous silicon tandem solar cells Reviewed
Yamaguchi M., Lee K., Araki K., Kojima N.
Journal of Physics D: Applied Physics 51 ( 13 ) 2018.3
Language：English Publishing type：Research paper (scientific journal) Publisher：Journal of Physics D: Applied Physics
© 2018 IOP Publishing Ltd. Silicon solar cells are the most established solar cell technology and are expected to dominate the market in the near future. As state-of-the-art silicon solar cells are approaching the Shockley-Queisser limit, stacking silicon solar cells with other photovoltaic materials to form multi-junction devices is an obvious pathway to further raise the efficiency. However, many challenges stand in the way of fully realizing the potential of silicon tandem solar cells because heterogeneously integrating silicon with other materials often degrades their qualities. Recently, above or near 30% silicon tandem solar cell has been demonstrated, showing the promise of achieving high-efficiency and low-cost solar cells via silicon tandem. This paper reviews the recent progress of integrating solar cell with other mainstream solar cell materials. The first part of this review focuses on the integration of silicon with III-V semiconductor solar cells, which is a long-researched topic since the emergence of III-V semiconductors. We will describe the main approaches - heteroepitaxy, wafer bonding and mechanical stacking - as well as other novel approaches. The second part introduces the integration of silicon with polycrystalline thin-film solar cells, mainly perovskites on silicon solar cells because of its rapid progress recently. We will also use an analytical model to compare the material qualities of different types of silicon tandem solar cells and project their practical efficiency limits.
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Electrical conduction of Si/indium tin oxide/Si junctions fabricated by surface activated bonding Reviewed
Liang J., Ogawa T., Hara T., Araki K., Kamioka T., Shigekawa N.
Japanese Journal of Applied Physics 57 ( 2 ) 2018.2
Language：English Publishing type：Research paper (scientific journal) Publisher：Japanese Journal of Applied Physics
© 2018 The Japan Society of Applied Physics. The electrical properties of n+-Si//indium tin oxide (ITO)/n+-Si, n+-Si//ITO/p+-Si, and p+-Si//ITO/n+-Si junctions fabricated by surface activated bonding (SAB) were investigated. The current-voltage (I-V) characteristics of n+-Si//ITO/n+-Si, n+-Si//ITO/p+-Si, and p+-Si//ITO/n+-Si junctions showed excellent linear properties. The interface resistances of n+-Si//ITO/n+-Si, n+-Si//ITO/p+-Si, and p+-Si//ITO/n+-Si junctions were found to be 0.030, 0.025, and 0.029 Ω•cm2, respectively, which are lower than required for concentrator photovoltaics. The interface resistances of all the junctions increased with increasing annealing temperature. The degradation of the interface resistance is lower in n+-Si//ITO/n+-Si junctions than in n+-Si//ITO/p+-Si and p+-Si//ITO/n+-Si junctions, when the annealing temperature is higher than 100 °C. These results demonstrate that the ITO thin film as an intermediate layer has high potential application for the connection of subcells in the fabrication of tandem solar cells.
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HOW DOES R&D ON THE CAR-ROOF PV IMPACT TO THE ENTIRE PV TECHNOLOGIES? Reviewed
Araki Kenji, Lee Kan-Hua, Kojima Nobuaki, Yamaguchi Masafumi
グランド再生可能エネルギー国際会議論文集 1 ( 0 ) 2018
Language：English Publishing type：Research paper (scientific journal) Publisher：特定非営利活動法人 再生可能エネルギー協議会
With the advancement of both vehicle and PV technologies, it will be possible that 70 % of the cars will run by the solar energy, 8 % of the Greenhouse gas emission will be cut, and 50 GW/year of the new PV market will be created. The R&D of the car-roof PV impacts to entire PV technologies. The progress of R&D of the car-roof PV impacts to various aspects of PV technologies including 3-D nowcasting leading to the establishment of the real-time market and smoothing PV output to the grid.
DOI： 10.24752/gre.1.0_39
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TOWARD STANDARDIZATION OF SOLAR TRACKERS, CONCENTRATOR PV, AND CAR-ROOF PV Reviewed
Araki Kenji, Ji Liang, Kelly George, Algora Carlos, Siefer Gerald, NIshioka Kensuke, Muller Mathew, Leutz Ralf, Carter Sam, Wang Shitao, Askins Steve
グランド再生可能エネルギー国際会議論文集 1 ( 0 ) 2018
Language：English Publishing type：Research paper (scientific journal) Publisher：特定非営利活動法人 再生可能エネルギー協議会
IEC TC82 WG7 works for standardization of concentrator modules, and their optics, mechanics and other advanced photovoltaic systems. These standards will be in the general areas of safety, photoelectric performance and environmental reliability tests. The standards ultimately produced should be universal and non-restrictive in their application, taking into account different environments and manufacturing technologies. In addition to the essential electrical and mechanical characteristics, standards will be written for other relevant factors such as thermal performance, high voltage performance, angular performance, mechanical precision, specific testing fault resistance and fault-tolerant design. Recently an expanded group started to discuss standardization of the car-roof PV.
DOI： 10.24752/gre.1.0_37
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SUPER-MULTIJUNCTION CELL, A NEW SOLAR CELL OVERCOMING THE SPECTRUM LOSS OF MULTIJUNCTION CELLS Reviewed
Araki Kenji, Lee Kan-Hua, Kojima Nobuaki, Yamaguchi Masafumi
グランド再生可能エネルギー国際会議論文集 1 ( 0 ) 2018
Language：English Publishing type：Research paper (scientific journal) Publisher：特定非営利活動法人 再生可能エネルギー協議会
Fluctuation of the atmospheric parameters substantially impacts the energy yield of multi-junction cells. The calculation considering the variation of atmospheric parameters showed a dramatic decline of the annual energy yield from the cells with more than five junctions. Super-multijunction solar cells were proposed, and optimization work was done. The top and the bottom junctions have lower bandgap for buffering to the mismatching and collecting a full range of the sunlight by robust energy conversion, and the annual energy substantially increases even by five or six junctions.
DOI： 10.24752/gre.1.0_45
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Herrero R., Antón I., Victoria M., Domínguez C., Askins S., Sala G., De Nardis D., Araki K.
Energy Science and Engineering 5 ( 5 ) 257 - 269 2017.10
Language：English Publishing type：Research paper (scientific journal) Publisher：Energy Science and Engineering
© 2017 The Authors. Energy Science & Engineering published by the Society of Chemical Industry and John Wiley & Sons Ltd. An inherent characteristic of high concentrator photovoltaics (HCPV) modules is a tight mechanical tolerance caused by the narrow angular transmission of the optical system, typically below or close to 1°. Misalignments in the modules caused during the assembly process in the production line will degrade not only the electrical but also the angular performance of the module. Moreover, dispersion in the electrical characteristics of the elementary units comprising a module would lead also to power loss. Quality control and data analysis on the production line is of great significance for adjusting the production line and preserving the angular tolerance and the electrical performance. This is particularly critical during the set-up and tuning of an automated production line. This paper presents the results of a pilot production line for HCPV modules carried out within the European funded ECOSOLE project. Several quality controls were established, which are the binning of the photovoltaic receivers, the measurement of misalignments among the elementary units within every module, and the indoor electrical characterization of the modules. Collected experimental data during the tuning phase of the pilot line were used to validate a module performance model based on production parameters. Monte Carlo method is lately applied to the model to assess the influence of production defects of diverse nature and the adequacy of quality controls, in several manufacturing scenarios beyond the specific constrains of the ECOSOLE experience.
DOI： 10.1002/ese3.178
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Efficiency potential and recent activities of high-efficiency solar cells Reviewed
Yamaguchi M., Yamada H., Katsumata Y., Lee K., Araki K., Kojima N.
Journal of Materials Research 32 ( 18 ) 3445 - 3457 2017.9
Language：English Publishing type：Research paper (scientific journal) Publisher：Journal of Materials Research
Copyright © Materials Research Society 2017 Â. The present status of R&D for various types of solar cells is presented by overviewing research and development projects for solar cells in Japan as the PV R&D Project Leader of the New Energy and Industrial Technology Development Organization (NEDO) and the Japan Science and Technology Agency (JST). Developments of high-efficiency solar cells such as 44.4% (under concentration) and 37.9% (under 1-sun) InGaP/GaAs/InGaAs 3-junction solar cells by Sharp, 26.6% crystalline Si heterojunction back-contact (HBC) solar cells by Kaneka, 22.3% CIGS solar cells by Solar Frontier have been demonstrated under the NEDO PV R&D Project. 15.0% efficiency has also been attained with 1 cm2 perovskite solar cell by NIMS under the JST Project. This article also presents analytical results for efficiency potential of high-efficiency solar cells based on external radiative efficiency (ERE), open-circuit voltage loss and fill factor loss. Crystalline Si solar cells, GaAs, III-V compound 3-junction and 5-junction, CIGSe, and CdTe solar cells have efficiency potential of 28.5%, 29.7%, 42%, 43%, 26.5%, and 26.5% under 1-sun condition, respectively, by improvements in ERE.
DOI： 10.1557/jmr.2017.335
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Towards creation of mobility society using solar energy Reviewed
Yamaguchi M., Araki K., Lee K., Kojima N., Masuda T., Kimura K., Satou A., Yamada H.
2017 5th IEEE International Conference on Smart Energy Grid Engineering, SEGE 2017 394 - 399 2017.9
Language：English Publishing type：Research paper (scientific journal) Publisher：2017 5th IEEE International Conference on Smart Energy Grid Engineering, SEGE 2017
© 2017 IEEE. This paper presents current status and future direction of Photovoltaic (PV) electric power system installations in the world. This article also overviews PV R&D activities in Japan as the PV R&D Project Leader of NEDO and JST. Present status of various solar cells efficiencies under NEDO and JST PV R&D projects are presented: 44.4% for concentrator III-V compound 3-junction solar cell, 37.9% for 1-sun III-V compound 3-junctiion cell, 26.7% for single-crystal Si cell, 22.3% for CIGS cell, and 14.0% for a-Si based 3-junction cell. Efficiency potential of various solar cells is also discussed. Prospects of PV and our recent approaches towards the creation of 'Mobility Society by using Solar Energy' are discussed.
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Design of the partial concentrator lens for III-V on Si static concentration Reviewed
Araki K., Ota Y., Lee K., Nishioka K., Yamaguchi M.
AIP Conference Proceedings 1881 2017.9
Authorship：Lead author Language：English Publishing type：Research paper (scientific journal) Publisher：AIP Conference Proceedings
© 2017 Author(s). To compete with the flat-plate crystalline Silicon cell module, III-V on Si structure is developed. However, it is clear that the situation of the higher cost of III-V cell relative to the Silicon cell will be unchanged. Then, it is preferred concentrating III-V cell for further savings. The partial concentrator is expanding the acceptance angle despite the higher concentration ratio. It is achieved by better performance balance of on-axis and high incidence angle. The new and generalized design method of the partial concentrator was developed. The profile function was constructed by selected Zernike's polynomial considering rotational symmetry. The full conditions of the calculation including the initial value and the radial and azimuthal degree of the function were examined. It was found that the recommended radial and azimuthal degree were 12 and 12.
DOI： 10.1063/1.5001412
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Quantifying the potential of III-V/Si partial concentrator by a statistical approach Reviewed
Lee K., Araki K., Ota Y., Nishioka K., Yamaguchi M.
AIP Conference Proceedings 1881 2017.9
Language：English Publishing type：Research paper (scientific journal) Publisher：AIP Conference Proceedings
© 2017 Author(s). We propose a theoretical framework for analyzing the energy yields of partial concentrators. A partial concentrator uses a concentrator cell to absorb the principal defracted or reflected light rays from its concentrator optics and a backplane cell to absorbs the diffused or defocused light. This concept can be applied to the concentrator system when accurate sun-tracking is not available, such as on a vehicle. This analysis framework provides a simplified way to describe the uncertainties of solar incidences dealt by partial concentrator. This help identified a clearer design criteria of partial concentrator in order to outperform the flat-panel PV or conventional CPV.
DOI： 10.1063/1.5001445
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Outdoor validation of the 30 minutes intermittent tracking of 100 x CPV Reviewed
Araki K., Ota Y., Lee K., Nishioka K., Yamaguchi M.
AIP Conference Proceedings 1881 2017.9
Language：English Publishing type：Research paper (scientific journal) Publisher：AIP Conference Proceedings
© 2017 Author(s). HCPV uses trackers, but their cost and reliability have raised problems. These problems may be solved by substantially increasing acceptance angle and mechanical tolerance. For a demonstration of the possibility of such substantial improvement, a 30 minutes intermittent tracking in high concentration applications was examined. To allow for 30 minutes intermittent tracking, both advanced optics and an advanced tracking control were investigated. For advanced optics, a 100 x refractive and dielectric concentrator was designed. It had about plus or minus 5°of acceptance angle. For advanced tracking control, a feed-forward control with a linear error correction was investigated. With combining both optimizations, it was shown that 30 minutes intermittent tracking was validated in outdoor experiments even for an as high as 100 x concentration.
DOI： 10.1063/1.5001401
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Optical and thermal simulation for wide acceptance angle CPV module Reviewed
Ahmad N., Ota Y., Araki K., Lee K., Yamaguchi M., Nishioka K.
AIP Conference Proceedings 1881 2017.9
Language：English Publishing type：Research paper (scientific journal) Publisher：AIP Conference Proceedings
© 2017 Author(s). Concentrator photovoltaic (CPV) technology has the potential to decrease the cost of systems in the near future by using less expensive optical elements in the system which replace the receiving surface aperture and concentrate the sunlight onto small solar cells. One of the main concerns of CPV is the need for high precision tracking system and the relation to the acceptance angle. In this paper, we proposed a CPV module with concentration ratio larger than 100 times and wide acceptance angle. An optical simulation for the module with S-TIM2 glass as a lens material was conducted to estimate the optical performance of the module. Thermal and electrical simulation was also conducted using COMSOL Multiphysics and SPICE respectively to evaluate the working temperature and electrical characteristics of the multijunction solar cell under concentration conditions.
DOI： 10.1063/1.5001400
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Monte Carlo simulation to analyze the performance of CPV modules Reviewed
Herrero R., Antón I., Sala G., De Nardis D., Araki K., Yamaguchi M.
AIP Conference Proceedings 1881 2017.9
Language：English Publishing type：Research paper (scientific journal) Publisher：AIP Conference Proceedings
© 2017 Author(s). A model to evaluate the performance of high concentrator photovoltaics (HCPV) modules (that generates current-voltage curves) has been applied together with a Monte Carlo approach to obtain a distribution of modules with a given set of characteristics (e.g., receivers electrical properties and misalignments within elementary units in modules) related to a manufacturing scenario. In this paper, the performance of CPV systems (tracker and inverter) that contain the set of simulated modules is evaluated depending on different system characteristics: inverter configuration, sorting of modules and bending of the tracker frame. Thus, the study of the HCPV technology regarding its angular constrains is fully covered by analyzing all the possible elements affecting the generated electrical power.
DOI： 10.1063/1.5001450
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Is it CPV? Yes, but it is a partial CPV Reviewed
Araki K., Ota Y., Lee K., Nishioka K., Yamaguchi M.
AIP Conference Proceedings 1881 2017.9
Language：English Publishing type：Research paper (scientific journal) Publisher：AIP Conference Proceedings
© 2017 Author(s). III-V on Si cell is one of the most promising candidates of the low-cost and high-efficiency solar cell beyond the limit of Si cells. However, it is likely that the cost of III-V cell will keep greater than that of Si cells. The partial concentrator technology was invented to concentrate the sunlight onto smaller III-V cells on the Si cell with the increased acceptance angle for static concentrator operation. The partial concentrator allows on-axis optical loss but expands the apparent acceptance angle often beyond the thermodynamic limit. The spilled light will be saved and collected by the bottom Si cell. It is not just placing the Si cell around the concentrated III-V cell, but its design philosophy is "partially" collecting the sunlight to the III-V cell to expand the acceptance. Depending on the values of cost parameters, the cost of the partial CPV module becomes minimum at 4 x - 10 x of concentration onto the top III-V cell, meaning that the area of expensive III-V cell can be saved to 1/4 - 1/10. In the partial concentrator. There may be a better optics than the ideal concentrator. The partial concentrator can be applied not only to III-V on Si cells but also III-V "by" Si cells. This lateral configuration does not have as excellent efficiency as the III-V on Si cell, but the usage of III-V cell is only several percents of the Si cell and has good similarity on the conventional Si flat-plate module technology.
DOI： 10.1063/1.5001439
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Araki K., Lee K., Yamaguchi M.
AIP Conference Proceedings 1881 2017.9
Language：English Publishing type：Research paper (scientific journal) Publisher：AIP Conference Proceedings
© 2017 Author(s). The spectrum influence on the CPV multi-junction cell is not only responsible for the air mass but significantly influenced by the fluctuation of the atmospheric parameters. The Monte Carlo simulation assuming the best and worst conditions of the distribution of parameters was examined to the CPV multi-junction cells. It was shown both aerosol density and water precipitation affects to the optimum bandgap design. The optimum bandgap set in the area of the worst aerosol density was significantly different from those of the best area. Even though the bandgap was designed maximizing the annual energy yield in a set of annual data, the fluctuation of the atmospheric parameters affects the annually mean efficiency to ±5 % (best area) and ± 9 % (worst area). Considering this considerable influence on the annual energy yield of the CPV multi-junction cell, with junctions connected in series, the best number of the junctions was 4 and more than five junctions were not better for the annual energy yield.
DOI： 10.1063/1.5001435
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Estimation of conversion efficiency for partially static concentrator with III-V on Si solar cell Reviewed
Ota Y., Araki K., Lee K., Yamaguchi M., Nishioka K.
AIP Conference Proceedings 1881 2017.9
Language：English Publishing type：Research paper (scientific journal) Publisher：AIP Conference Proceedings
© 2017 Author(s). We estimated the annual mean conversion efficiency of partially static concentrator (PSC) module. The PSC module consistes of static concentrator, small III-V top solar cell and large Si bottom solar cell. The large Si bottom solar cell absorbes the solar power that is not collected by the III-V top soler cell. The mean optical efficiency decreased with the increase of concentration ratio. The decreasing ratio of mean optical efficiency between 1X to 4X was 49.9%. By using PSC module structure, it was possible to boost the annually mean conversion efficiency. The decreasing ratio of annually mean conversion efficiency between 1X to 4X was 21.5%. The acceptance angle of conversion efficiency was larger than that of optical efficiency. The PSC module structure also improved acceptance angle of the conversion efficiency.
DOI： 10.1063/1.5001409