Solvent refining of silicon for solar cells - some practical aspects

Solvent refinement has been suggested as an energy- and cost-efficient process to produce high purity silicon for photovoltaic solar cells. In this process, metallurgical grade silicon is dissolved in a liquid metal e.g. Al at high temperature. High purity silicon crystals can then be precipitated by decreasing the temperature and extracted from the melt. Residual aluminium is then removed by leaching, and after remelting and directional solidification, the purified silicon can be used as silicon for solar cells. Metallurgical grade silicon (99 wt%Si) with iron as the main impurity has been dissolved to 40 wt% in liquid aluminium at 1000 degrees C. Silicon crystals have been extracted slightly above the eutectic temperature. The crystals have then been leached in 10% hydrochloric acid at room temperature. Silicon crystal have been sorted into three size fraction which have been melted separately and thereafter analyzed by light optical- and electron microscopy with EDS analysis. After leaching, extracted material has been remelted together with silicon oxide in order to oxidize residual aluminium. The extracted material contains significant amounts of residual Al-Si eutectic. The aluminium has not been removed completely by the leaching and insoluble metal rich intermetallic compounds have been found in the smallest size fraction. The residual aluminium can be further reduced by oxidation by silicon oxide and sedimentation of the aluminium oxide.