Production of low-cost solar-grade silicon by reduction of SiF4 gas with sodium: Technical and industrial developmental status

To meet increasing demand for electrical power using solar photovoltaics, millions of tons of solar-grade silicon costing <$20/kg will be needed. Low-cost solar-grade silicon can be mass produced by burning Na metal in an atmosphere of pure SiF
4
gas—an exothermic, fast, and complete reaction. SRI International routinely uses reactors that can produce at rates >20 metric tons per year (mty), and has licensed the technology to several companies. The Si product is completely separated from the by-product NaF by leaching or melt separation. With the direct use of industrially available Na, B and P levels in the Si product are <1 ppm. Using purified Na results in Si with dopant levels as low as 20 ppb. Experimental ingots grown have resistivity of 1 to 30 Ω·cm, with >100 Ω·cm when purified reactants were used. The purity, electronic parameters, and solar cell efficiency of 15% indicate that this silicon is of solar grade and can be a key contributor in developing solar power markets.