Low Temperature Internal Gettering of Bulk Defects in Silicon Photovoltaic Materials

Abstract

Multicrystalline silicon (mc-Si) substrates are widely used for photovoltaic cells. The minority carrier lifetime in mc-Si is affected by recombination associated with metallic impurities in many forms, such as point-like defects, precipitates and bound to or precipitated at structural defects such as dislocations. We have studied the effect of low temperature annealing on the lifetime and bulk iron concentration in as-received mc-Si wafers from different locations within a block. Lifetime measurements are made using a temporary iodine-ethanol surface passivation technique to minimize the occurrence of bulk hydrogenation which often occurs from dielectric films. In good wafers from the middle of the block the lifetime is reduced by annealing at 400 °C and 500 °C in a way which does not correlate with changes in bulk iron concentration. Lifetime improvements occur in relatively poor samples from the top and bottom of the block annealed at 300 °C, and also in samples from the bottom annealed at 400 °C. The improvement in bottom wafers correlates with iron loss from the bulk. Our work shows that under some conditions the lifetime in relatively poor as-grown wafers can be improved by low temperature internal gettering.