Production of Solar-Grade Silicon by the SiF_4 and Mg Reaction

摘要

Over 90 pct of the solar cells currently produced and installed are Si based, and this industrial dominance is expected to persist for the foreseeable future. The crystalline Si substrate accounts for a significant portion of the total cost of solar cells. In order to further reduce the cost of solar panels, there has been significant effort in producing inexpensive solar-grade Si, mainly through three paths: (1) modification of the Siemens process to lower production costs, (2) upgrading metallurgical-grade Si to reach solar-grade purity, and (3) by means of new metallurgical processes such as the reduction of a silicon halide, e.g., SiF_4 or SiCl_4, by a reactive metal such as Na or Zn. In this paper, we describe an alternative path that uses Mg to react with SiF_4 to produce low-cost solar grade Si. Experimental conditions for complete reaction and separation of the products, Si and MgF_2, as well as aspects of the reaction mechanism are described. The reaction involves both a heterogeneous liquid-gas phase reaction and a homogeneous gas-gas phase reaction. When pure Mg was used, the Si product obtained had sub-ppm levels of B and P impurities and is expected to be suitable for solar cell applications.