Reduction of Carbon and Oxygen Impurities in mc-Silicon Ingot Using Molybdenum Gas Shield in Directional Solidification Process

Abstract

Transient global heat transfer simulations are done to analyse the impact of molybdenum shield on carbon and oxygen impurity formation in mc-silicon grown by Directional Solidification (DS) process. Carbon and oxygen are the two main impurities that have a direct impact on affecting the conversion efficiency of the solar cell. The molybdenum shield effectively guides the argon gas flow within the furnace and carries SiO away from the melt free surface and the incorporation of carbon into the melt from the graphite furnace elements is also reduced. The molybdenum shield within the furnace reduces the incorporation of carbon and oxygen concentration in the ingot thereby enhancing the quality of the ingot. For the ingot grown in the conventional furnace, the higher concentration region of carbon ranges between 2.45 × 1017 atom/cm3 to 2.25 × 1017 atom/cm3 and the lower concentration region of carbon ranges between 6.77 × 1016 atom/cm3 to 8.56 × 1015 atom/cm3. For the ingot grown with molybdenum shield, the higher concentration region of carbon ranges between 1.06 × 1017 atom/cm3 to 9.76 × 1016 atom/cm3 and the lower concentration region of carbon ranges between 4.32 × 1016 atom/cm3 to 1.61 × 1016 atom/cm3. Similarly, for the ingot grown in the conventional furnace, the higher concentration region of oxygen ranges between 2.51 × 1017 atom/cm3 to 2.31 × 1017 atom/cm3 and the lower concentration region of oxygen ranges between 6.85 × 1016 atom/cm3 to 7.67 × 1015 atom/cm3. For the ingot grown with molybdenum shield, the higher concentration region of oxygen ranges between 2.49 × 1017 atom/cm3 to 2.25 × 1017 atom/cm3 and the lower concentration region of oxygen ranges between 8.75 × 1016 atom/cm3 to 9.10 × 1015 atom/cm3. Simulations are done for the whole DS growth process for the conventional furnace having no molybdenum shield and for the furnaces having molybdenum shield fixed at three different positions. The molybdenum shield incorporation in DS furnace shows a significant impact on uniformity and reduction of carbon, oxygen impurity distribution in the as-grown mc-silicon ingot.