Production of Solar-Grade Silicon by the SiF4 and Mg Reaction

Abstract

Because crystalline Si substrate still accounts for a significant portion of the total cost of solar cells, substantial efforts have been invested in the development of new processes that can significantly lower the production cost of solar-grade silicon. At SRI International, we developed a low-cost process of producing solar-grade silicon based upon the SiF 4 -Na reaction. This process has now been scaled up to pilot-plant level, and solar cells obtained using such silicon feedstock reached > 18% efficiencies. Due to continuing increases in demand and production capacity and the simultaneous drop in price of Mg metal, we decided to explore its potential as a reducing agent instead of Na metal. Experimental conditions for obtaining a fast and complete metallothermic reaction between SiF4 and Mg are identified and aspects of the reaction mechanism are discussed. The reaction involves both a heterogeneous liquid-gas phase reaction and a homogeneous gas-gas phase reaction. The separation of the products, Si and MgF 2 , is described. When pure Mg was used, Si obtained after melt separation had the potential to be suitable for solar cell applications.