Preparation of photovoltaic silicon by purification of metallurgical grade silicon with a reactive plasma process

Abstract

The main methods of silicon purification involve physicochemical techniques of separation. They yield an ultrapure metal, but its price is not compatible with photovoltaic central power generation. The plasma zone melting process consists in the horizontal displacement of a melted zone under an argon-hydrogen-oxygen plasma. The melt zone is characterized by a vertical gradient of concentration and temperature which drains impurities toward the upper surface of the bar. Moreover, the displacement of the zone from the head to the tail of the bar ensures a horizontal drainage of impurities toward the tail. This technique has been used with success for purification of metallurgical grade silicon; for example, iron was reduced from 3500 ppm to 0.5 ppm at a displacement rate between 20 and 40 cm/h. The final impurity level was less than 1 ppm. The mass transfer was independent of the displacement rate at 40 cm/h, but dependent on the heat transfer rate. The segregation of the metals is caused by drainage toward the tail of the bar, while the plasma vaporizes impurities since the metallic and metalloidic impurities have high vapor pressures (e.g., S, P, C, O, Mn). Acid (HF-HNO3) is used to dissolve the impurities between each passage of the zone. Boron elimination depends directly on the plasma oxygen concentration. Classical mathematic models of zone melting are not valid for plasma melting zone processes.