Physical mechanisms of breakdown in multicrystalline silicon solar cells

We have identified at least five different local breakdown mechanisms according to the temperature coefficient (TC) and slope of their characteristics and electroluminescence (EL) under reverse bias. These are (1) early pre-breakdown (strongly negative TC, low slope), (2) edge breakdown (positive TC, low slope, no EL), (3) weak defect-induced breakdown (zero or weakly negative TC, moderate slope, 1550 nm defect luminescence), (4) strong defect-induced breakdown (zero or weakly negative TC, moderate slope, no or weak defect luminescence), and (5) avalanche breakdown at dislocation-induced etch pits (negative TC, high slope). The latter mechanism usually dominates at high reverse bias. In addition to the local breakdown sites there is evidence of an areal reverse current between the dominant breakdown sites showing a positive TC. Since defect-induced breakdown shows a zero or weakly negative TC and also leads to weak avalanche multiplication, we propose defect level-induced avalanche instead of trap-assisted tunneling to be responsible for this breakdown mechanism.