Effects of liquid convection on the purification of silicon by directional solidification for photovoltaic applications

Abstract

Photovoltaic energy rapidly grows in utilization and demand every year. It has become one of the most important energies of the European electric system and its generation increased by 44% in Brazil between 2015 and 2016 alone. There are estimates that 1.2 million new photovoltaic generators will be installed in Brazil by 2024. Approximately 90% of the photovoltaic energy is generated by solar panels made of mono or polycrystalline solar-grade silicon. The increasing demand for solar-grade silicon has triggered the research for new low-cost and high-efficiency production processes. The processes that rely on the purification of the metallurgical-grade silicon make up the metallurgical route, which is one of the most promising routes. Directional solidification of metallurgical-grade silicon is one of the most important steps to remove metallic impurities in the metallurgical route. During directional solidification, impurities are segregated to one extreme of the ingot (macrosegregation), whereas the remainder of the ingot is purer and with an impurity content acceptable for the production of solar-grade silicon. The main objective of the present research project is to investigate the effects of forced convection of liquid silicon and heat transfer on the macrosegregation of impurities during the directional solidification of metallurgical-grade silicon. Directional solidification experiments will be carried out under liquid silicon convection, imposed by a disc immersed in the liquid at constant rotation velocity. The micro and macrostructures of the resulting ingots will be examined and the macrosegregation of impurities will be revealed by chemical analysis of parts of the ingot as a function of the distance to the ingot base. A mathematical model of directional solidification of the metallurgical-grade silicon under the effect of forced convection will be implemented to assist in this investigation. Experimental and model results will enable the identification of important processing variables and the definition of more efficient purification conditions for solar-grade silicon production. (AU)