Effects of the DC bias in a molten silicon bath on its purification and hydrogenation by RF thermal plasma

The aim of this work is the purification and the hydrogenation of metallurgical grade silicon for photovoltaic applications using a thermal plasma process. In addition to the plasma treatment, a DC bias of the liquid silicon was used to increase the kinetics of impurity extraction and hydrogenation reaction. On-line measurement by OES (Optical Emission Spectroscopy), ex-situ analysis by LIBS (Laser Induced Breakdown Spectroscopy) and ICP techniques demonstrated that elimination of metallic impurities and boron depends on the applied bias, and/or on the plasma gas composition. The DC bias was found to significantly increase the hydrogenation rate of liquid silicon, when it is attributed to electrochemical and/or chemical reactions in the electrical plasma sheath.