Understanding junction breakdown in multicrystalline solar cells

Extensive investigations on industrial multicrystalline silicon solar cells have shown that, for standard 1 Ω cm material, acid-etched texturization, and in absence of strong ohmic shunts, there are three different types of breakdown appearing in different reverse bias ranges. Between −4 and −9 V there is early breakdown (type 1), which is due to Al contamination of the surface. Between −9 and −13 V defect-induced breakdown (type 2) dominates, which is due to metal-containing precipitates lying within recombination-active grain boundaries. Beyond −13 V we may find in addition avalanche breakdown (type 3) at etch pits, which is characterized by a steep slope of the I-V characteristic, avalanche carrier multiplication by impact ionization, and a negative temperature coefficient of the reverse current. If instead of acid-etching alkaline-etching is used, all these breakdown classes also appear, but their onset voltage is enlarged by several volts. Also for cells made from upgraded metallurgical grade material these classes can be distinguished. However, due to the higher net doping concentration of this material, their onset voltage is considerably reduced here.