Recent advances in ribbon-to-ribbon crystal growth

Abstract

The ribbon-to-ribbon (RTR) crystal growth method is a promising approach for producing silicon sheet suitable for fabrication into photovoltaic cells. In this method, a polycrystalline silicon ribbon is first grown on a temporary substrate by chemical vapor deposition, and separated from it during cooling. A narrow molten zone is established across the ribbon, as it moves, by a pair of scanned laser beams. Large-grained silicon ribbon is crystallized from the molten zone, even at high growth rates. The area growth rate using a single swept laser (i.e. throughput) can be increased beyond the limit imposed by a single (wide) ribbon by multiple growth of wide ribbons. Ribbons grown at high rates can exhibit an unusual dendritic structure. By controlling the temperature profile across the width of the molten zone, the dendrites can be grown in a uniform array. Examination of the microstructure of a controlled dendritic ribbon reveals the most damaging defects to be in narrow areas between dendrites. Thus, a controlled dendritic structure could improve the overall photovoltaic efficiency of RTR ribbon.