Assessment of silicon purification possibility by chemical vapor transport reaction with zinc sulfide

The demand for renewable energy sources, including solar, is increasing every year, stimulating researchers to develop innovative technological solutions for obtaining material for photovoltaic modules - solar silicon. The article discusses a new process for the vapor transport of silicon in the form of sulfide compounds, which can serve as the basis for a halogen-free technology for producing high-purity silicon for photovoltaic batteries. Considering the well-known properties of silicon di- and monosulfide, it is proposed to use zinc sulfide as a carrier reagent, the presence of which in the Si-ZnS system first provides silicon sulfidization with the formation of gaseous products Zn (g) and SiS (g), and then the reduction of monosulfide to elemental silicon. The possibility of a chemical vapor transport reaction of silicon with zinc sulfide at a temperature above 1000 °C and a Si/ZnS ratio of 1 was justified by the method of the thermodynamic simulation of interactions in the Si-ZnS system in the temperature range 500-1500 °C. Based on the obtained equilibrium models of the interaction of zinc sulfide with technical silicon (grade Kr 2), the separation coefficients of (α) silicon from impurity elements that affect the electrophysical properties of silicon, in particular, reduce the lifetime of excess charge carriers, are calculated. The selectivity of this transport reaction and the prospects for its use for refining metallurgical silicon are estimated. It has been shown that the use of the silicon transfer reaction of zinc sulfide, for example, at 1100 °C, can provide deep purification of silicon from Fe, Ca, Ti, V, Cr, Mn and Cu (α ~ 108-1012), as well as Mg and Al (α ~ 104-106). The process is less effective for removing P and B (α ~ 102) and is not applicable for alkali metals in the entire studied temperature range. It is theoretically possible to improve the refining indexes by lowering the reaction temperature, but the necessary sulfur concentration in the gas phase for the complete conversion of silicon to SiS (g) is achieved only above 1050-1100 oC due to thermal dissociation of ZnS.