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Production of solar-grade silicon by the SiF4 and Mg reaction

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 SiF4-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 MgF2, is described. When pure Mg was used, Si obtained after melt separation had the potential to be suitable for solar cell applications.