Review of Induced Degradation Phenomena Affecting PV Modules

Abstract

Degradation phenomena of solar modules occurring under operating conditions in the field may cause severe power losses for a PV system. To avoid or at least minimize the negative effects correlated with these phenomena, a good understanding of the nature and origin of the degradation is crucial. In this contribution the state of knowledge of four different important degradation phenomena is reviewed, and mitigation strategies are discussed. The underlying defects of these induced degradation phenomena form at operating conditions of the solar cell under injection, (elevated) temperature and realistic ambient conditions. Under these conditions, the quasi-Fermi level in ptype based Si wafers changes under injection and may cause defects to change their charge state. This has implications for some of the degradation phenomena, since charge states of species determine the underlying reaction rates and degradation/regeneration kinetics in general. BO-related light induced degradation seems to be well understood and mitigation strategies are available, although the exact microscopic picture of the defect itself is still unclear. Potential induced degradation of the shunt type is only occurring under specific polarity and high voltage drop in the string of interconnected modules. The formation mechanism seems to be clear and mitigation strategies exist as well. In contrary, for light and elevated temperature induced degradation the underlying defect is still not known, but a lot of empirical findings are available, pointing strongly towards the involvement of H being present in the crystalline Si bulk. Mitigation strategies exist, but seem to cause extra steps/costs in solar cell production. Degradation of surface passivation quality was recently observed for dielectric layer systems being used in new cell architectures allowing high efficiencies. Again, the exact nature of the underlying defect(s) is still unclear, but H may play a role in here, too.