Degradation of copper-plated silicon solar cells with damp heat stress

Crystalline silicon solar cells with copper-plated contacts are fabricated, encapsulated in ethylene-vinyl acetate (EVA), and subject to extended damp heat stress (85 degrees C and 85% relative humidity). We source cell precursors from several different cell manufacturers and employ several different patterning methods of the silicon nitride layer and deposit a plated front contact stack of nickel, copper, and tin using light-induced plating. Across different Cu-plated samples, we find similar degradation that impacts both series resistance and diode quality of the cells, indicating that there is some degradation of the p-n junction. The overall degradation is on the order of 15%-20% of maximum power (P-MP), and roughly half of this degradation is attributable to degradation of the p-n junction. Control samples with silver-screenprinted contacts do not exhibit the same degradation, and p-n junction degradation in copper-plated samples is prevented by changing the encapsulant from EVA to a polyolefin. The degradation mode is hypothesized to be the diffusion of copper from the contact, followed by the transport of this copper into the silicon cell via some mechanism facilitated by the degraded EVA encapsulant.