Triple-Junction Thin-Film Silicon Solar Cells on W-Textured ZnO for Applications to Low-Concentration Photovoltaics
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Triple-Junction Thin-Film Silicon Solar Cells on W-Textured ZnO for Applications to Low-Concentration Photovoltaics Shunsuke Kasashima1, Yuki Moriya1, Porponth Sichanugrist1 and Makoto Konagai1,2 1 Department of Physical Electronics, Tokyo Institute of Technology, Japan 2 Photovoltaic Research Center (PVREC), Tokyo Institute of Technology, Japan
ABSTRACT We report for the first time the a-Si:H/c-Si:H/c-Si:H triple-junction solar cells fabricated on W-textured ZnO having a very high haze value which can improve light scattering effect. For further enhancement of light confinement effects, p-type a-SiOx:H and c-SiOx:H as wide-gap window-layers, n-type c-SiOx:H as intermediate layers and a back reflector were employed in these solar cells too. From theoretical analysis, we have found an advantage of aSi:H/c-Si:H/c-Si:H structure for an application to low-concentration photovoltaics. For the fabricated solar cells, a conversion efficiency of 8.86% at 1 sun and and 9.86% under 7.2 suns, and a total photocurrent from each subcell of 24.1 mA/cm2 were achieved although there was still a current mismatch among component subcells. INTRODUCTION Multi-junction thin-film silicon solar cells have hopeful prospects on the achievement of both low-cost and high-efficiency. Theoretical calculation has shown that triple-junction thinfilm silicon solar cells have a potential for high conversion efficiency [1]. The triple-junction solar cells with several types of structure have ever been reported [2-8], and among them, hydrogenated amorphous silicon (a-Si:H) and hydrogenated microcrystalline silicon (c-Si:H) have a large role as absorbers of the top and bottom cells, respectively. For the middle cell, one candidate is hydrogenated amorphous silicon germanium (a-SiGe:H) with a preferred band gap between a-Si:H and c-Si:H, and another is c-Si:H for high stability against light degradation [2-5]. The latter one seems to be matched for our purpose to apply the triple-junction thin-film silicon solar cells to low-concentration photovoltaics (LCPV) because the high illumination intensity which enhances an open-circuit voltage (Voc) might accelerate the light degradation. Most of reported triple-junction solar cells had a substrate configuration although it seems that a superstrate configuration is of practical advantage to lower production cost because of inexpensive glass substrates. It has another advantage to make incident light scattered effectively by using zinc oxide (ZnO) with textured surface morphology as transparent conductive oxide (TCO) films [9]. a-Si:H/c-Si:H/c-Si:H triple-junction cells require thinner top subcell and thicker bottom subcell in terms of current matching among subcells. The top and bottom subcells require ZnO with smooth surface and large feature size to reduce shunting and increase the light scattering [10], respectively. From this standpoint, it is suitable for the demands to use the W-textured ZnO with a very high haze value [11], which gives much help to increase the photo current against the red
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