Local Corrosion of Silicon as Root Cause for Potential-Induced Degradation at the Rear Side of Bifacial PERC Solar Cells

Industrial bifacial PERC solar cells are exposed to a high‐voltage stress at the rear side. The tested cells show a potential‐induced degradation (PID) in Voc and Isc. From a front side current-voltage measurement, a power loss of about 12%rel is observed, however, without a significant change in the fill factor. The degradation is traced back to micron‐sized hole‐shaped damages of the rear surface, which correlate with localized regions of increased recombination. A focused ion beam (FIB) cross‐section through a hole‐shaped damage is performed for subsequent transmission electron microscopy (TEM) and energy‐dispersive X‐ray spectroscopy (EDXS). EDXS shows an accumulation of Na, K, and Ca impurities in the cavity. Thus, it is assumed that corrosion of the Si surface occurs beneath the AlOx/SiNy passivation layer stack and is accompanied by the formation of a SiO2 layer.