Depth profiling of degradation of multilayer photovoltaic backsheets after accelerated laboratory weathering: I. Raman imaging
Chiao-Chi Lin, Peter J. Krommenhoek, Stephanie S. Watson, Xiaohong Gu
In this study, Raman imaging was used to depth profiling the chemical degradation of a commercial PPE (polyethylene terephthalate (PET)/PET/ethylene vinyl acetate (EVA)) backsheet films during exposure with UV irradiance of 170 W/m2 (300 nm to 400 nm) at 85°C, 5% relative humidity (RH, dry) and 85°C, 60% RH (wet). Cryo-microtomy was used to obtain cross-sectional PPE samples with a flat surface parallel to thickness. The multilayer structures and chemical properties of each layer were examined by laser confocal microscopy, Raman spectroscopic imaging and atomic force microscopy (AFM). The Raman images were constructed using intensities of both background fluorescent emission and characteristic Raman peaks after baseline correction, providing the chemical degradation profiles based on the spatial distribution of fluorescence and specific chemical groups across the thickness, respectively. The results revealed that the PPE backsheet films were comprised of five main layers, including layers of pigmented-PET outer, PET core, EVA inner, pigmented-EVA reflection and EVA outer, along with two adhesives between PET outer/PET core and PET core/EVA inner. Non-uniform degradation was observed in the backsheets, and both UV and moisture appeared to affect the degradation profiles of the multilayers. It was found that severe degradation with high fluorescence occured on the outmost region of the pigmented PET outer layer, and the degradation gradient extended to approximately 20 µm to the bulk. Both adhesive layers were severely deteriorated and the degradation of pigmented-EVA was enhanced in wet condition. The relationship between the sharp (non-uniform) degradation, the resultant internal stress, and failures of cracking and delamination was also discussed.