Purification of Metallurgical Grade Silicon to Solar Grade for Use in Solar Cell Wafers

A pyrometallurgical process for manufacturing solar grade silicon (SOG-Si) from metallurgical grade silicon (MG-Si) was developed. Metallic elements in MG-Si such as iron, titanium and aluminum, which show low partition coefficients, were removed by directional solidification practice. Phosphorus removal was carried out by evaporation for high vapor pressure, and boron was changed to boron oxide effectively by water vapor addition, because partition coefficients of phosphorus and boron are close to unity. To satisfy the impurity specification of SOG-Si, each purification process was combined as follows: (first process) dephosphorization by electron beam melting-first step directional solidification-(second process) oxidation of boron and carbon during plasma practice-deoxidation-second step directional solidification. MG-Si (purity level; 99 mass%) was successfully purified to the solar grade impurity level, that is, 6N (99.9999 mass%) except carbon and oxygen elements, by 20 kg scale and 150-300 kg scale equipments. The manufactured ingot was directly sliced to wafers and prepared to solar cells of multicrystalline silicon through the NEDO standard cell production process. The maximum conversion efficiency of solar cells attained to above 14%, which is almost equal to that of solar cells on the market.