In-depth analysis of potential-induced degradation in a commercial CIGS

Abstract

A post-mortem analysis is conducted after potential-induced degradation (PID) of a commercial copper-indium-gallium-selenide (CIGS) photovoltaic module. After PID, the conversion efficiency of the total module decreased by 62%. Electroluminescence images of the module show that the edges of the modules were much more affected by the PID than the middle part of the module. Coring samples were prepared of the different areas and chemical compositional information of the various areas was combined with electrical characterisation, cell modelling and luminescence data to obtain an overall perspective on the root cause of degradation in these modules during high voltage stress. Consistent with earlier studies on cell level, the module analysis shows the occurrence of alkali migration. From current-voltage modelling, it was concluded that the degradation of the most affected areas is due to an increase in bulk and CdS/CIGS interface defects, likely induced by ion migration. Further degradation on the same samples occurred when they are taken out of the argon-filled glovebox and stored under ambient conditions. Remarkably, the PID-degraded areas show stronger degradation when left in ambient atmosphere, as well as a stronger Na redistribution. These new results show that ion migration not only causes the immediate degradation but also strongly affects the longer-term stability of the cells in ambient atmosphere. This indicates that PID degradation makes CIGS devices more vulnerable to hermeticity problems, which are most prominent at the module edges.