Optically Assisted Metal-Induced Crystallization of Thin Si Films for Low-Cost Solar Cells
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Optically Assisted Metal-Induced Crystallization of Thin Si Films for Low-Cost Solar Cells Wei Chen, Bhushan Sopori, Kim Jones, and Robert Reedy, National Renewable Energy Laboratory, Golden, CO; N. M. Ravindra, New Jersey Institute of Technology, Newark, NJ; Roger Aparacio and Robert Birkmire, University of Delaware, Institute of Energy Conversion, Newark, DE; Scott Morrison, Ken Coats, and Arun Madan, MV Systems, Golden, CO ABSTRACT Optically assisted, metal induced crystallization (MIC) was used to convert amorphous Si films, deposited on Al coated glass substrates, into polycrystalline Si (pc-Si). The study investigated the effects of deposition temperature, process temperature, and film thickness on the grain orientation, grain size, and crystallization front of the processed films. Furthermore, we have attempted to examine the role of Al in MIC — in particular, whether the metal can be confined to the interface while grain enhancement occurs. INTRODUCTION A thin-film Si solar cell consists of a low-cost substrate, such as glass, that supports a very thin (~10 µm) Si solar cell. Because thin Si can absorb only a fraction of the useable solar spectrum, the absorption must be enhanced by suitable light-trapping structures. Recent calculations show that by including practical light-trapping designs, only about 10 µm of Si is needed to yield absorption comparable to that of thick wafers [1]. Such a film must also meet other requirements pertaining to the grain size and the grain boundary structure to make the device amenable to low carrier-recombination and concomitant high device efficiency. We have developed a thin Si cell design that incorporates a number of high-efficiency features and can be fabricated by low-cost processing [2]. The cell design is illustrated in Figure 1, where a glass substrate is coated with Al/Cr, followed by a layer of a-Si or fine-grain pc-Si. The rest of the structure is generic. One of the important fabrication issues in this structure is how to make a suitable crystalline Si film at temperatures compatible with the glass substrate. The layer of Al has multiple roles — to serve as a back reflector, provide a stress-relief interface so that the thick Si film does not peel off, and provide an intrinsic layer for impurity gettering. Because the cell configuration involves a low-cost glass substrate, the device processing must be performed below 550°C, which is the softening temperature of a typical soda-lime glass. There are several ways to obtain a thin pc-Si layer. One way is to start with an amorphous Si film and convert it into pc-Si by thermal annealing. However, thermal annealing requires long times, temperatures higher than 550°C, or both. Another disadvantage of this approach is that the grain size is typically less than 0.1 µm [3]. A more tempting approach for Figure 1. A schematic of the thin-film Si solar cell. D3.2.1
fabricating the proposed device is to use Al for converting a-Si into crystalline Si by MIC. This approach has been used quite successfully in display device te
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