Research Of Purification Of Multicrystalline Silicon By Directional Solidification

In recent years, with the rapid development of photovoltaic industry, solar grade silicon (SoG-Si) feedstock, which mainly comes from scraps, cutting and rejects of the electronic chip industry, is in short of supply. Researcher put their attention to the SoG-Si feedstock fabrication which was independent of the electronic industry value chain. Because metallurgical method used large amount and low price metallurgical grade silicon (MG-Si) as feedstock to fabricate SoG-Si, and it has short production cycle, small pollution and low cost, it has become the research and development focus of each country. Directional solidification can not only be used to produce SoG-Si ingot for solar cells, but also be used to remove the metal impurities with small segregation coefficient, so it has become the research emphases of purification of MG-Si by the metallurgical method. It is very important to investigate and optimize the directional solidification process to obtain a high purity and integrated ingot.Using self-designed vacuum induction melting furnace, directional solidification purification experiment in medium scale was carried out with different initial impurity contents, pulling rates and temperature gradient. Resistivity distribution of multicrystalline silicon ingot, solid-liquid interface during directional solidification process, the effect of initial impurity contents, pulling rates and temperature gradient on the purification of multicrystalline silicon were investigated by analyzing the structure, composition, electrical property and temperature field. The results show that:On the vertical section of the silicon ingot prepared by directional solidification with MG-Si as feedstock, the resistivity increased along the solidified height initially, and reached to maximum at the polarity transition position, then decreased rapidly along the solidified height and tended to zero on the top of the ingot where impurities accumulated. The variation of resistivity in the vertical section of the ingot was deeply relevant to the distribution of the content of A1,B and P in the growth direction.The microstructure, resistivity and minority carrier lifetime distribution in the radius direction were corresponding to the solid-liquid interface, and they can be used to analyze the variation of solid-liquid interface during directional solidification process. The solid-liquid interface in the bottom part of the silicon ingot was concave; the curvature decreased with increasing the solidified height, the solid-liquid interface can be plane on the top part of the ingot for the sample with 0.12mm/min pulling rate. The solid-liquid interface curvature in the same solidified height decreased with lowering the pulling rate. The cellular interface caused the effective segregation coefficient increase. The concave interface caused the impurity content in the center part was higher than that on the side.The impurity content in the ingot decreased with decreasing the initial impurity content, when 20 percent of the ingot on the top was removed, the maximum removal ratio of Fe is 98.5%, and that of A1 is 94.9%. With decreasing the initial content of Fe, the effective segregation coefficient decreased at first and then increased again. However, the effective segregation coefficient of A1 decreased with decreasing the initial content of Al.The impurity content in the ingot decreased with decreasing the growth rate, when the growth rate was 0.116mm/min, the lowest content of Fe can be on the 10-6 order of magnitude. The purification area can reach to 80%.The liquid temperature gradient increased with increasing the solidified height can inhibit the happening of constitutional supercooling. When the liquid temperature gradient reached to 0.95℃/mm, the position taking place constitutional supercooling can be over 90% of the solidified height.