Plasma-refining process to provide solar-grade silicon

A new purification process has been developed to refine metallurgical silicon using a plasma torch blowing at the surface of the silicon to be purified. An inductive system has been designed to maintain the silicon in a liquid state, control the shape of its free surface and to provide a strong electromagnetic stirring, ensuring a rapid transfer of pollutants from the bulk liquid to its surface. A numerical model is used to control the design of the induction system and to look at its potential evolutions. It is shown that even with a reduced induction power, the stirring is sufficient to provide a very rapid mass transfer in the liquid, compared to the reaction rate at the surface (deduced from experiments). The effect of the power and frequency of the induction power is also discussed, showing that it gives the possibility to control the heating power and the shape of the surface, and even to push the liquid away from any contact with the crucible. The material provided by the refining process operating in such conditions was used to produce solar cells that reached a conversion efficiency of 12.7%. (C) 2002 Elsevier Science B.V. All rights reserved.