Optimization on reduction of impurities and thermal stress induced dislocations for improved performance of multi crystalline silicon INGOT grown by directional solidification

Abstract

The heat and mass transport occurring within the Directional Solidification furnace during the growth of multi-crystalline silicon ingot is optimized by numerical simulation. The solar cell efficiency of the multi-crystalline silicon wafer is mainly affected by the minority carrier recombination caused by metal impurities and dislocation densities caused by thermal stress which reduces the minority diffusion length thereby reducing the photoelectric conversion efficiency. Carbon, Oxygen and Nitrogen are the major impurities that directly affect the conversion efficiency of solar cells. Electrical activity and density of dislocation in multi-crystalline silicon hugely depend on carbon concentration which leads to the formation of new grains and ohmic shunts that degrades solar cell performance. The mechanical strength of multi-crystalline silicon wafer is mainly affected by the oxygen precipitation during thermal annealing which acts as a gettering site for impurities. Oxygen-boron pairs formed in the boron-doped P-type silicon wafers cause light induced degradation (LID). The optimization of the crystal/melt interface during the solidification of the ingot is essential because it plays a key role in determining the quality of the ingot. Uniform temperature distribution is essential during the growth of multi-crystalline silicon in which the thermal stress caused by temperature gradient can be reduced for growing high-quality ingot with less dislocation density. By introducing various modifications in DS furnace, transient heat and mass transfer simulations were done to investigate and optimize thermal fields, interface shape, impurity distribution and thermal stress-induced dislocations while growing multi-crystalline newline