Silicon ribbon growth using scanned lasers

Abstract

The recent drive for low-cost photovoltaic arrays has renewed the interest in growing silicon in ribbon form. Our approach is to use a pair of scanned focused CO(2) laser beams to establish a molten zone in a preformed fine-grained polycrystalline ribbon. Large-grained macrocrystalline silicon is then drawn from the molten zone at high rates. The coupling of the 10.6-microm laser beam to the melt zone is very low due to the high reflectivity of molten silicon. However, the coupling can be greatly increased by using a spherical-section reflector to reimage the reflected beam onto the molten zone. Silicon ribbon has been grown at rates up to 13.3 cm/min by this method. The best solar cell fabricated so far on this material had a conversion efficiency of 12.7%.