Electrorefining of Silicon

Abstract  The marked for silicon based solar cells has grown rapidly along with the increased focus on renewable energy sources. Earlier on, the solar grade Si (SoG Si) feedstock was based on scrap from the electronic industry. The demand for solar grade silicon or “poly silicon” is now larger than this source can supply, and there is therefore a need for new and less costly production methods. Traditionally, metals such as magnesium and aluminium that are not suitable for carbothermic reduction are produced by molten salt electrolysis.  With reasonable yield and production rates, an electrochemical process might also be a simple and low cost alternative to chemical routes to SoG Si. In the eighties there was a general “boom” in research for new Si electrodeposition processes. Uri Cohen1, using a dissolving silicon anode, successfully deposited dense coherent silicon films from alkaline fluoride melts with K2SiF6. Also Rao et al. seemed to come quite far with their electrolysis process of K2SiF6 in LiF-KF2. They obtained coherent uniform films with  99.999% purity. Taking a ”rougher” approach to the above mentioned systems, our objective was to electrorefine metallurgical grade silicon (MG Si) by electrodeposition in molten alkaline fluoride electrolytes. Using a dissolving MG Si anode, the process was carried out at higher current densities producing Si powder – renouncing the idea of direct deposition of dense thin film on wafer substrate. A simplified cell design without the restrictions associated with operating a closed furnace with controlled atmosphere was employed.  Some of the results will be presented at the conference. 1.  Uri Cohen: “ Some Prospective Applications of Silicon Electrodeposition from Molten Fluorides to Solar Cell Fabrication”, Journal of Electronic Materials, Vol. 6, No 6, 1977. 2.   Gopalakrishna M. Rao, Dennis Elwell and Robert S. Feigelson, “Electrowinning of Silicon from K2SiF6- Molten Fluoride Systems”, Journal of Electrochemical Soci