Xiaobing Xie

Low-cost solar-grade silicon: Purification and consolidation of silicon fines from wafering

More than 90% of commercial solar cells are produced using mono- and poly-crystalline silicon, with estimated about 15,000 MT of silicon feedstock used in 2008. Future silicon use is estimated to grow proportionally with the solar industry (30%/y). Substantial amounts of highly refined polycrystalline silicon material are wasted in the final stages of producing ingots and in wafer slicing. Recycling of wasted silicon in the form of fines generated in deposition reactors and in slicing operation can lower the production cost of solar-grade silicon feedstock to less than

Production of solar-grade silicon: A comparison study of the reduction of SiF 4 with Na, Mg, or Al metals

10/kg, and substantially shorten energy payback time. The SRI International developed technology is based on the prepurification and simultaneous melt-consolidation and further purification of silicon fines from wafering operations. The consolidated product in form of granules and ingots with a resistivity range of 1 to 5 Ω·cm can be used in the melt growth of both single-crystal and polysilicon ingots for use in photovoltaic cells.

Methods for producing consolidated materials

The process of producing solar-grade silicon by direct reduction of SiF4 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.