Depth profiling of chemical and mechanical degradation of UV-exposed PV backsheets

The properties of the multilayer PV backsheets, including their interfaces, during weathering are not well-known. In this study, a commercial PPE (polyethylene terephthalate (PET)/PET/ethylene vinyl acetate (EVA)) backsheet films was selected as a model system for a depth profiling study of chemical and mechanical properties of a backsheet film during UV exposure. Cryo-microtomy was used to obtain cross-sectional PPE samples. The NIST SPHERE (Simulated Photodegradation via High Energy Radiant Exposure) was used for the accelerated laboratory exposure of the materials with UV at 85°C and two relative humidities (RH) of 5 % (dry) and 60 % (wet). Chemical and mechanical depth profiling of the aged and unaged samples was conducted by Raman microscopic mapping, nanoindentation and atomic force microscopy in quantitative nanomechanical mapping mode. The results indicated that non-uniform degradation took place across the thickness of the PPE backsheet with severe chemical and mechanical degradation observed on the outer pigmented PET layer, two adhesive layers, and the pigmented-EVA layer. The regions with the increase in the modulus detected by nanoindetation were consistent with those showing clear chemical degradation in Raman and Attenuated Total Reflection Fourier Transform Infrared Spectroscopy (ATR-FTIR). This depth profiling study brings new understanding to the mechanisms of failures observed in the backsheets during weathering.