Production of solar grade (SoG) silicon by refining liquid metallurgical grade (MG) silicon: Annual Report: June 10 1998--October 19, 1999

Pyro-metallurgical refining techniques are being developed for use with molten metallurgical-grade (MG) silicon so that directionally solidified refined MG silicon can be used as solar-grade (SoG) silicon feedstock for photovoltaic applications. The most problematic impurity elements are B and P because of their high segregation coefficients. Refining processes such as evacuation, formation of impurity complexes, oxidation of impurities, and slagging have been effective in removal of impurities from MG silicon. Charge sizes have been scaled up to 60 kg. Impurity analysis of 60-kg charges after refining and directional solidification has shown reduction of most impurities to <1 ppma and B and P to the 10-ppma level. It has been demonstrated that B and P, as well as other impurities, can be reduced from MG silicon. Further reduction of impurities will be necessary for use as SoG silicon. The procedures developed are simple and scaleable to larger charge sizes and carried out in a foundry or MG silicon production plant. Therefore, SoG silicon production using these procedures should be at low cost