Directionally solidified solar-grade silicon using carbon crucibles

Abstract Directional solidification of silicon, in carbon crucibles, was achieved by two variations of the Bridgman method. One is a static technique, wherein liquid silicon in a 5 cm diameter × 5 cm high carbon crucible was positioned in a temperature gradient of about 35°C/cm, with the temperature at the crucible top being hottest. Solidification was achieved by lowering the system temperature at a rate of 4–5°C/min. The second technique entailed lowering a 5 cm × 5 cm × 12 cm high carbon crucible, loaded with silicon, through a fixed RF-coil at a rate of 5.5 mm/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. This behavior dominated attempts to seed the growth at the crucible bottom. The average grain diameter was 1.08 mm and the typical length was 7 mm. Transmission electron microscopy was used to assess crystal perfection. A solar cell efficiency of 11.5% (AM1) was achieved using this material.