SOLAR-GRADE SILICON BY DIRECTIONAL SOLIDIFICATION IN CARBON CRUCIBLES

Directional solidification of silicon in carbon crucibles was achieved by using two variations of the Bridgman-Stockbarger method. One was a static technique wherein liquid silicon in a carbon crucible was positioned in a temperature gradient of about 35 degree C/cm, with the highest temperature at the top of the crucible. Solidification was achieved by lowering the system temperature at a rate of 4-5 degree C/min. The second technique entailed lowering a silicon-loaded carbon crucible through a fixed-rf coil at a rate of 0. 55 cm/min. Crack-free silicon was produced by both methods. The equilibrium grain structure was initiated by nucleation at the crucible walls, with surviving grains tending to grow in alignment with the temperature gradient to produce an axially columnar grain structure of mainly 110 orientation. The average grain diameter was 0. 11 cm; a typical length was 0. 7 cm. Solar cells made with this material gave an AMI conversion efficiency of 11. 4%.