Status of nc-Si:H Solar Cells at United Solar and Roadmap for Manufacturing a-Si:H and nc-Si:H Based Solar Panels
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0989-A15-01
Status of nc-Si:H Solar Cells at United Solar and Roadmap for Manufacturing a-Si:H and nc-Si:H Based Solar Panels Baojie Yan, Guozhen Yue, and Subhendu Guha United Solar Ovonic LLC, 1100 West Maple Road, Troy, MI, 48084
ABSTRACT This paper reviews the research and development of hydrogenated nanocrystalline silicon (nc-Si:H) solar cells at United Solar Ovonic LLC. We have been studying nc-Si:H solar cells since 2001 and have made significant progress. We have achieved an initial active-area cell efficiency of 15.1% using an a-Si:H/a-SiGe:H/nc-Si:H triple-junction structure, a stable activearea cell efficiency of 13.3% using an a-Si:H/nc-Si:H/nc-Si:H triple-junction structure, and a stable aperture-area (420 cm2) fully encapsulated module efficiency of 9.5% using an a-Si:H/ncSi:H double-junction structure. Although the cell efficiencies with nc-Si:H in the middle and/or bottom cells have exceeded the corresponding efficiencies achieved using a-Si:H and a-SiGe:H, we still need to address several critical issues before using nc-Si:H in photovoltaic manufacturing plants. First, the cell efficiency needs to be improved further to show a clear advantage over the conventional a-Si:H/a-SiGe:H/a-SiGe:H triple-junction cell structure. Second, we need to increase the deposition rate further to make the nc-Si:H based technology more cost effective. Third, we need to develop a machine design to overcome the large-area uniformity issue, especially for very high frequency glow discharge deposition. Fourth, we need to qualify nc-Si:H based solar cell product, especially with respect to long term reliability. We have been addressing these critical issues, and will discuss the roadmap for manufacturing a-Si:H and nc-Si:H based solar panels using the roll-to-roll technology. INTRODUCTION Since first reported by the Neuch‚tel group in 1994 [1], hydrogenated microcrystalline silicon (µc-Si:H) solar cell has attracted significant attention and has been studied widely. Because µc-Si:H materials contain nanometer-size grains and amorphous tissues, µc-Si:H is now more often called nanocrystalline silicon (nc-Si:H) to align with other nano-technologies. Compared to hydrogenated amorphous silicon (a-Si:H) and silicon germanium alloy (a-SiGe:H) solar cells, nc-Si:H solar cell has two advantages. First, its lower optical bandgap close to the value of crystalline silicon results in a high short-circuit current density (Jsc), because of enhanced long wavelength response. Second, optimized nc-Si:H solar cells show very little light-induced degradation. However, the major drawback of nc-Si:H cells is the indirect bandgap in the crystalline phase. The optical absorption coefficient in the short wavelength region is lower than that in a-Si:H and a-SiGe:H. Therefore, a thick nc-Si:H intrinsic layer of over one micrometer is normally needed to achieve high Jsc . In order to make nc-Si:H solar cell technology cost-effective for solar panel manufacturing, a high deposition rate is essential. The Neuch‚tel group also pioneered the use of ver
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