High net doping concentration responsible for critical diode breakdown behavior of upgraded metallurgical grade multicrystalline silicon solar cells

Solar cells made of upgraded metallurgical grade silicon (UMG-Si) feedstock material generally show an increased reverse current at a relatively low reverse bias, which is thought to be disadvantageous for the use in solar modules. In the solar cells used in this study, the reverse current flows through many soft breakdown sites at recombination active defects. The onset voltage of the soft breakdown is decreased primarily due to the increased net doping concentration in the wafer base of UMG-Si wafers compared to standard multicrystalline silicon. We attribute this behavior to the enhancement of the electric field around metal precipitates which form Schottky junctions with the surrounding semiconductor. (C) 2010 American Institute of Physics. [doi:10.1063/1.3463332]