Directional solidification of silicon under the influence of travelling magnetic field

Abstract

Silicon is the most widely used semiconductor for photovoltaic solar cells, and also the most extensively studied. The need for cost reduction lead solar cell producers to look for cheaper raw silicon, allowing a higher impurity content in the charge used as input for crystallization and making it very different from the extensively studied electronic-grade silicon. In order to keep a good efficiency of the solar cells, the impurities that have a huge impact over the electrical properties of the material should be removed from the final ingot. Controlling the segregation of impurities during the solidification is therefore mandatory.A study of the solidification of metallurgical grade silicon under controlled solidification parameters such as growth rate, thermal gradient and forced convection flow was conducted in a Bridgman set-up. Forced convection was induced by a travelling magnetic field (TMF) during the solidification, and resulted in a mixing of excess impurities in the liquid, removing them from the vicinity of the solidification front. As a result the purity of the final ingot is increased. The impact of the forced convection on the segregation of metallic impurities and on the orientation of grain boundaries during the silicon solidification is presented in this paper.