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Development of high-efficiency solar cell modules and new application fields are significant for the further development of photovoltaics (PV) and creation of new clean energy infrastructure based on PV. Especially, development of PV-powered EV applications is desirable and very important for this end. This paper shows analytical results for efficiency potential of various solar cells for choosing candidates of high-efficiency solar cell modules for automobile applications. As a result of analysis, Si tandem solar cells are thought to be some of their candidates. This paper also overviews efficiency potential and recent activities of various Si tandem solar cells such as III-V/Si, II-VI/Si, chalcopyrite/Si, perovskite/Si and nanowire/Si tandem solar cells. The III-V/Si tandem solar cells are expected to have a high potential for various applications because of high efficiency with efficiencies of more than 36% for 2-junction and 42 % for 3-junction tandem solar cells under 1-sun AM1.5 G, lightweight and low-cost potential. Recent results for our 28.2 % efficiency and Sharp’s 33% mechanically stacked InGaP/GaAs/Si 3-junction solar cell are also presented. Approaches to automobile application by using III-V/Si tandem solar cells and static low concentration are presented.
Efficiency potential of crystalline Si solar cells is analyzed by considering external radiative efficiency (ERE), voltage, and fill factor losses. Crystalline Si solar cells have an efficiency potential of more than 28.5% by realizing ERE of 20% from about 5% and normalized resistance of less than 0.05 from around 0.1. Nonradiative recombination losses in single-crystalline and multicrystalline Si solar cells are also discussed. Especially, nonrecombination and resistance losses in multicrystalline Si solar cells are shown to be higher than those of single-crystalline cells. Importance of further improvement of minority-carrier lifetime in crystalline Si solar cells is suggested for further improvement of crystalline Si solar cells. High efficiency of more than 28.5% will be realized by realizing high minority-carrier lifetime of more than 30 ms. Key issues for those ends are reduction in carbon concentration of less than 1 × 1014 cm−3, oxygen precipitated and dislocations even in single-crystalline Si solar cells, and reduction in dislocation density of less than 3 × 103 cm−2 in multicrystalline Si solar cells.
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.
III-V compound multi-junction solar cells have high efficiency potential of more than 50% due to wide photo response, while limiting efficiencies of single-junction solar cells are 31-32%. In order to realize high efficiency III-V compound multi-junction solar cells, understanding and controlling imperfections (defects) are very important. This paper reviews fundamentals of defects and defect management for III-V compound materials, single-junction, multi-junction, space and concentrator solar cells.
We review recent progresses on in-situ observation of lattice relaxation of III-V lattice-mismatched system and analyses of defect properties in III-V-N solar cell materials. We found that there were five phases during the InGaAs growth on GaAs substrate. The transition point of the dominant dislocation behavior could be determined precisely. We also found that compositionally step-graded InGaAs/GaAs(001) buffers with overshooting (OS) layers were effective to control the strain of the top layer from tensile to compression. To understand the defect properties that dominate the electrical property of CBE-grown GaAsN films, we characterized deep levels in CBE-grown GaAsN films by DLTS. In this characterization, a well-known electron trap E1 (Ec-0.33eV) center in n-GaAsN and p-GaAsN was confirmed to be non-radiative recombination center by using double-carrier pulse DLTS.
Studies on the electrical conductivity of molecular beam deposited carbon films after annealing of the carbon films have been carried out. Detailed temperature dependence of conductivity on the as-deposited and on annealed samples has been investigated. The results were interpreted in terms of a model which includes a variable range hopping and strongly scattering metallic components. A correlation between annealing behavior of the electrical conductivity and the results of x-ray photoelectron spectroscopy and Raman spectroscopy is presented.
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