Purification of silicon for photovoltaic applications

Solar grade silicon, as a starting material for crystallization to produce solar cells, is discussed here in terms of impurities whose maximum content is estimated from recent literature and conferences. A review of the production routes for each category of solar-grade silicon (undoped, compensated or heavily compensated) is proposed with emphasis on the metallurgical route.Some recent results are proposed concerning segregation, showing that directional solidification systems can be used for solidification even at high solidification rate (15 cm/h). Results on inductive plasma purification, where boron is evacuated as HBO in a gas phase blown from an inductive plasma torch, are shown to apply as well to arc plasmas and purification by moist gas.Special attention is paid to the history of impurities in the purification processes, showing that impure auxiliary phases (silicon tetrachloride, slag, aluminum, etc.) often need their own purification process to enable their recycling, which has to be considered to evaluate the cost (financial, energetic and environmental) of the purification route.

► Review of existing processes to produce solar grade silicon.
► Chemical purification route with auxiliary steps to recycle the tetrachlorosilane.
► Metallurgical purification route: path of impurities, and silicon yield.
► Recent results on segregation at high solidification rate (15 cm/h).
► Boron evacuation as HBO in moist gas and arc plasmas as well as in inductive plasma.