External and internal gettering of interstitial iron in silicon for solar cells

The removal of dissolved iron from the wafer bulk is important for the performance of p-type multicrystalline silicon solar cells. In this paper we review some recent progress in understanding both external and internal gettering of iron. Internal gettering at grain boundaries and dislocations occurs naturally during ingot cooling, and can also be driven further during cell processing, especially by moderate temperature anneals (usually below 700 degrees C). Internal gettering at intra-grain defects plays key a role during such precipitation annealing. External gettering to phosphorus diffused regions is crucial in reducing the dissolved iron concentration during cell processing, although its effectiveness depends strongly on the diffusion temperature and profile. Gettering of Fe by boron and aluminum diffusions is also found to be very effective under certain conditions.