Silicon sheets for solar cells grown from silicon powder by the SSP technique

Abstract

The purpose of the SSP (silicon sheets from powder) project is to grow large area polycrystalline silicon sheets (100 × 100 mm²) for solar cells with high conversion efficiency ( > 13%). In the SSP process, a thin layer of silicon powder is converted into a coarse-grained sheet material by two consecutive melting steps using focussed incoherent light as the heat source. The feasibility of the technique was demonstrated by fabricating sheets up to 80 × 150 mm² with a thickness of only 350 μm. Recently, a new machine has been built which allows continuous processing of ribbons of 100 mm width. The same sequence of process steps and the same heating technique are used. The sheets are characterized by metallographic and electrical methods. Grain sizes extend to some cm in length and some mm in width, showing a predominance of 〈 211〉 orientation in growth direction. Strong variations in the etch-pit density (10³−10⁶ cm^-2) are correlated to variations in EBIC contrast. Test solar cells (20 × 20 mm² area) from these sheets demonstrate the quality of the SSP material, with conversion efficiencies of up to 13.1% achieved.