Influence of helium gas flow under the retort bottom to control the impurities in grown mc-Si ingot by DS process for photovoltaic application: Numerical simulation

•To improve the bottom heat extraction by helium gas flow under the bottom retort.•The convexity of the melt crystal interface was maintained for the whole growth.•The impurity concentration of the grown mc-Si is not sufficient to form the LID effect and SiC formation.•The uniform temperature gradient and distributions are maintained by the helium gas flow under the retort.