Simulation and Experimental Approach to Investigate the Annealing Effect on mc-Si Ingot Grown by Directional Solidification Process for PV Application

Abstract

A two-dimensional, numerical simulation using finite volume method has been made on directional solidification of multi-crystalline silicon ingot. Two identical simulations have been carried out to grow silicon ingots in which one of them was maintained at 1523K after the growth process for about an hour to study the effect of annealing process on the quality enhancement of the grown ingot. The thermal field inside the growth chamber has been investigated during and after the growth process. The thermal stress and dislocation density of as grown and annealed ingots have been investigated numerically. The simulation results show that annealing the grown ingot reduces the maximum thermal stress up to 8 MPa, and decreases dislocation density throughout the ingot. Based on the simulation results, an experiment on the growth of mc-Si ingot on G1 directional solidification furnace has been made. Different stages of directional solidification process such as melting, crystallization and annealing have been discussed. the minority carrier lifetime measurement has been carried out on mc - Si bricks as well as on wafers. The lifetime of the minority carriers is found to be higher (75µs) in the central region of the ingot. The resistivity of the wafers taken from different positions of the grown ingot has been analyzed to find the solid – liquid interface during the growth process and compared with simulated interface. The results show that, the thermal stress and the dislocation density can be significantly reduced and the minority carrier lifetime can be improved by annealing the grown ingot at 1523K. The results may be useful for the mass production of mc-Si ingots for photovoltaic application.