Solar Grade Silicon

Impurities in metallurgical grade silicon can drastically modify the material properties relevant for solar cells. For example, transition metal impurities such as iron, copper, and chromium can shorten the lifetime of excited carriers in the silicon solar cell and are thus detrimental to the cell efficiency. Their removal from silicon is therefore an important issue in silicon solar cell fabrication. Solvent refining has been investigated as a potential cost effective purification process that relies on preferential segregation of impurities to a liquid, from which high purity solid silicon crystals can be grown, as discussed in a paper by Yoshikawa and Morita, JOM, vol. 64, no. 8, pp. 946–951, 2012.

Thermo-Calc can be used to predict the solubility of impurity elements in pure Si as a function of temperature and composition as shown in the figure here. The software can also be used to predict the segregation coefficient between the solid and liquid phase, which is relevant to the solvent refining process.