A comparison of evolution of adhesion mechanisms and strength post damp-heat aging for a range of VA content in EVA encapsulant with photovoltaic backsheet

Ethylene vinyl acetate (EVA) is an effective encapsulant to adhere various layers in the photovoltaic (PV) module. In this study, we have fabricated EVA films containing different vinyl acetate (VA) content (18%, 24%, 33% and 40%) and laminated between two layers of backsheet (BS). These laminates have been subjected to damp-heat (DH) ageing at 85OC and 85% relative humidity (RH) for 1000 h to evaluate the change in adhesion strength at EVA/BS interface using the T-peel test. The adhesion strength for EVA containing 18% VA content has been found to improve but it deteriorates for EVA with 33 and 40% VA content after ageing. However, adhesion strength for EVA containing 24% VA content is found unaffected after ageing. The degree of cross-linking and VA content have been determined through gel content measurement and thermo-gravimetric analysis (TGA) respectively. The degree of cross-linking and VA content increases significantly in EVA18 but decreases in EVA40 due to DH ageing. The chemical changes have been examined by the Fourier transform infrared spectroscopy (FTIR) and the X-ray photoelectron spectroscopy (XPS). The failure mode (cohesive or delamination) at the interface and its correlation with the adhesion strength has been explored using XPS. The failure mode at the EVA18/BS interface alters from delamination type in pristine condition to the cohesive type due to DH ageing. However, the failure mode for EVA24 and EVA40 is found to be cohesive in both the pristine and aged condition.