Cast Silicon of Varying Purity for High Efficiency PERC Solar Cells

In this work we investigate the effect of the bulk metal contamination level in cast silicon on the electrical performance of PERC (passivated emitter and rear cell) solar cells [1] . The bulk contamination is varied in two different ways. In the first approach, cast silicon crystallized with high purity silicon feedstock and reduced metal contamination from crucible and coating is used. Second, an extended gettering step is introduced in the PERC process flow. The interstitial iron concentration is taken as a measure of the bulk metal contamination of the investigated cells. Both the higher purity cast silicon and the gettering step significantly lower the interstitial iron concentration in the resulting PERC cells. This clear positive effect is also mirrored in solar cell parameters such as the open circuit voltage UOC. In this experiment, the impact of gettering is larger than the effect which is reached by reducing the bulk metal concentration during the crystallization process. Device simulations show that most of the difference in UOC can be explained by the different interstitial iron levels while other recombination active defects might also be present.