Production of solar-grade silicon: A comparison study of the reduction of SiF4 with Na, Mg, or Al metals

The process of producing solar-grade silicon by direct reduction of SiF
₄
with Na metal is well established and demonstrated to pilot-plant scale (50 MTY). The silicon obtained contains most impurities, including boron and phosphorus, at levels below 1 ppm. Such silicon has been used to produce solar cells with efficiencies over 18% using commercial manufacturing lines and processes. Recently, we have experimented using Mg and Al as reducing elements. We have shown that Mg may also be a good candidate, and its use may appreciably reduce material costs and total energy required for the solar-grade silicon production. The reduction with Al, although thermodynamically favored, takes place only at high temperatures with a relatively low rate and extent of reaction. In addition, this reaction is complicated by several chemical vapor transport mechanisms. The conclusion is that Al is not attractive reductant for solar silicon production. A comparison study of thermodynamics, the reaction behaviors, and the purity of silicon will be presented.