Iron Gettering in CZ Silicon during the Industrial Solar Cell Process

We investigated the impact of using low quality feedstock such as recycled silicon and simplified pulling condition on the performance of CZ silicon solar cells. Groups of wafers carefully chosen from different ingots were analyzed after different solar cell process steps by minority carrier lifetime measurements, by measurements of the interstitial iron content and by measurements of the total impurity content using NAA. Our results show that the main electronic properties of the ingots, namely the carrier lifetime, interstitial iron content and base resistivity are strongly affected by feedstock quality. Surprisingly, high solar cell efficiencies were achieved using highly contaminated silicon. These positive results are due to the beneficial effect of impurity segregation gettering by phosphorous diffusion and aluminum alloying. Post-diffusion gettering by an additional annealing step was demonstrated to enhance the charge carrier lifetime.