Characterization of ethylene vinyl acetate (EVA) encapsulant: Effects of thermal processing and weathering degradation on its discoloration

Abstract

Ethylene vinyl acetate (EVA) copolymer used as an encapsulant in photovoltaic (PV) modules turns from clear to yellow-brown after four or more years of weathering. EVA films on PV modules stored in the dark at ambient temperature for six years have not degraded. Various analytical methods were used to analyze and characterize the degraded EVA and to determine the cause of discoloration. As a result of degradation, the gel content increased greatly while the UV absorber, Cyasorb UV 531, decreased concomitantly. The increase in the gel content indicates that a cross-linking reaction was dominant in EVA's structural change. Degradation of the polymer near the cover glass surface took place more rapidly and to a greater extent than that next to the solar cells. While EVA remaines clear when it retains more than 70% of the initial Cyasorb UV 531 concentration, a moderate increase in the gel content indicates that some degradation occurred without producing the yellow color. Further depletion of Cyasorb during the weathering process resulted in the yellow to brown color in the EVA due to formation of polyconjugated (C=C)n double bonds of various lengths (n>3). The discoloration deepens as the extent of degradation increases. Acetic acid and other volatile organic components are also produced from the photothermal decomposition of EVA. The EVA discoloration significantly reduces the electrical performance of solar cells.