SILICON AS A PHOTOVOLTAIC MATERIAL

The great majority of solar (photovoltaic) modules currently being installed worldwide are made from wafer-based silicon solar cells. The wafers used are typically 200-350um thick while the wafering process leads to a kerf loss of another 200-250um per wafer. For cells made using this approach, the cost of the starting wafer is approximately half the cost of the finished photovoltaic module. In order to achieve future cost reductions, a substantial reduction in the amount of silicon used per unit area of module is required. Several approaches are being considered to reduce the cost of the silicon. These include improvements to conventional wafering technology to produce thinner wafers with reduced kerf loss; novel wafering techniques; the growth of thin ‘silicon ribbon ’ material which does not require wafering; and the deposition of thin polycrystalline silicon films on foreign substrates. In this paper, some of these approaches are briefly discussed. We then focus on the so-called lift-off techniques, in which silicon films are epitaxially grown on a single crystal silicon substrate and subsequently detached. These techniques have attracted substantial interest in the past 5 years. In principle, they are an elegant way to achieve the dual aims of high material quality and low cost. We present several of these techniques and their current status. Finally, we discuss the practical challenges that need to be addressed for these approaches to become viable for solar module production