Cause of increased currents under reverse-bias conditions of upgraded metallurgical grade multicrystalline silicon solar cells

Solar cells made of upgraded metallurgical grade (UMG) silicon in general suffer from increased reverse currents at relatively low reverse bias easily attainable in standard solar cell modules. We show that the high net doping concentration, often inherent to UMG-Si feedstock, causes the reduction of the diode breakdown voltage of the soft breakdown. This type of breakdown occurs at recombination active defects which are omnipresent in multicrystalline silicon. We explain this behavior by the enhancement of the electric field around metal precipitates forming Schottky junctions with the surrounding semiconductor, which is consistent with known diode breakdown mechanisms based on large electric fields in the space charge region of the pn junction.