Efficiency Limitations of Multicrystalline Silicon Solar Cells Due to Defect Clusters
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Efficiency Limitations of Multicrystalline Silicon Solar Cells Due to Defect Clusters Bhushan Sopori1, Chuan Li1, S. Narayanan2, and D. Carlson2 1 National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 80401 2 BP Solar, Solarex Court, Fredrick, MD ABSTRACT Multicrystalline Si wafers used in commercial solar cell fabrication exhibit a tendency to form large “clusters” of defects, which remain laterally separated from each other. Defect clusters are also sites of impurity precipitation. Because precipitated impurities cannot be gettered by the conventional processes used in Si solar cell fabrication, defect clusters constitute low-performing regions in the cell. They shunt the device and constitute the primary efficiency limiting mechanism in current solar cells. We show that the efficiency loss caused by defect clusters can exceed 3 – 4 absolute points. INTRODUCTION One of the approaches to reduce the cost of commercial Si solar cells is to use lower-cost, multicrystalline Si (mc-Si) substrates instead of single-crystal wafers. This approach has gained increasing acceptance by the photovoltaic (PV) industry and, as a result, the use of mc-Si has grown steadily to about 60% of total Si-based solar cell production. The success of mc-Si as a cost-effective solar cell material is due primarily to the fact that advanced processing techniques, such as impurity gettering and hydrogen passivation, which are used in current solar cell fabrication, have worked very well in enhancing cell performance. These processing methods have led to efficiency of mc-Si solar cells exceeding 14%, which is slightly below that of commercial crystalline Si (c-Si) solar cells. However, further improvement in cell efficiency has proven difficult to achieve. Recent research has found that, in spite of using extensive gettering and passivation, mc-Si solar cells exhibit regions of low photoresponse and concomitant poor material quality. These local, low-response regions have a crystal structure that is not amenable to gettering, and constitute a new performance-limiting mechanism. We have analyzed the structure of these regions and found them to be a new type of defect configuration, now called a defect cluster [1]. This paper describes the nature of defect clusters, their formation mechanism, and their effects on solar cell performance. An electronic model of a solar cell containing defect clusters is described and verified experimentally. This model is used to assess the limitation in cell performance posed by defect clusters. Further improvements in cell efficiency require new gettering/passivation techniques, which can effectively getter defect clusters. CHARACTERIZATION OF DEFECT CLUSTERS Multicrystalline Si used for commercial solar cells is grown either as ribbons or cast in a crucible and then wafered by wire sawing. The PV industry has accepted two basic measures to lower the cost of mc-Si substrates. The first is substrates with high impurity content, which result from the use of cheaper, lower-grade feedstock (consist
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