Second-stage potential-induced degradation of n-type front-emitter crystalline silicon photovoltaic modules and its recovery

Abstract

We investigate the second-stage potential-induced degradation (PID) of n-type front-emitter (n-FE) crystalline silicon (c-Si) photovoltaic (PV) modules. The PID of n-FE c-Si PV modules is known to occur in three stages under negative bias stress. The second-stage PID is characterized by a reduction in fill factor (FF), due to the invasion of sodium (Na) into the depletion region of a p+–n junction and the resulting increase in recombination current. The second-stage PID shows a curious independence from a negative bias voltage for the PID stress. This may indicate that the Na inducing the FF reduction comes not from the cover glass but originally existed on and/or near the cell surface. The FF reduction is recovered quite rapidly, within a few seconds, by applying a positive bias to the degraded cell. The recovered n-FE c-Si PV modules show more rapid degradation if they receive the negative bias stress again, which can be explained by Na remaining in the p+ emitter.