Moisture induced degradation in field-aged multicrystalline silicon photovoltaic modules

Moisture ingress is one of the key fault mechanisms responsible for photovoltaic (PV) devices degradation. Understanding moisture induced degradation (MID) mechanisms in field-aged PV modules is more reflective of the reality in the field. In the present work, MID products of reclaimed solar cells from 20-year-old field-aged silicon PV modules is investigated. The defective areas in the PV modules were identified using visual inspection, electroluminescence (EL), ultraviolet fluorescence (UV-F), and infrared thermal (IR-T) techniques. SEMEDS analysis is used to elucidate the role of moisture on the observed degradation mechanisms. Degradation of the ethylene vinyl acetate (EVA) encapsulation produces acetic acid, carbon dioxide, phosphorus, sulfur, fluorine, and chlorine. Migration of metal ions under the influence of moisture ingress makes the formation of oxides, hydroxides, sulfides, phosphates, acetates, and carbonates of silver, lead, tin, copper, zinc, and aluminum feasible. Also, other competing reactions can lead to the formation of stannates of copper, silver, sodium, and zinc. Another observation is that, in the presence of MID species, Pb is preferentially corroded (to form lead acetate complexes) instead of the expected sacrificial Sn in the solder. These MID species account for different defects and fault modes that lead to parasitic resistance losses. This is witnessed by the 1.2%/year degradation in the Pmax of the PV module.