Removal of metal impurities by controlling columnar grain growth during directional solidification process

Silicon ingots with columnar grains and irregular grains as raw material for solar cells were obtained by directional solidification. The relationship between crystal morphology and concentration of metal impurities was studied. The result shows that columnar grains can improve the ratio of the high-purity area, where the concentration of metal impurities is less than 10 ppmw and more consistent with the calculated value by the Scheil’s equation. Meanwhile, the metal silicide precipitates at the irregular grain boundaries. Large temperature gradient at the solid-liquid interface dedicated to the stability of solid-liquid interface, which is conductive to the growth of the columnar grains. High concentration of metal impurities introduces a large constitutional supercooling to reduce the stability of solid-liquid interface, then further form a mushy zone in front of the solid-liquid interface, which leads to the growth of irregular grains.