Depth profiling of degradation of multilayer photovoltaic backsheets after accelerated laboratory weathering: Cross-sectional Raman imaging

Abstract

Understanding of the durability of each individual layer and their interfaces in a multilayered photovoltaic (PV) backsheet is critical to the design and selection of materials for making reliable and high performance PV modules. In this study, Raman imaging was used to depth profile the chemical degradation of a multilayer commercial backsheet film exposed to ultra-violet (UV) radiation at 85 °C, 5% relative humidity (RH, dry) and 85 °C, 60% RH (humid) on the NIST (National Institute of Standards and Technology) SPHERE (Simulated Photodegradation via High Energy Radiant Exposure). The backsheet film was a multipart laminate comprising of a pigmented polyethylene terephthalate (PET)-based outer layer, PET core layer and three ethylene vinyl acetate (EVA) layers having different vinyl acetate (VA) contents, along with two inner adhesive layers between PET outer and PET core layers, and PET core and EVA layers. Cross-sectional samples were prepared by cryo-microtomy for various characterizations. The multilayer structures were examined by laser scanning confocal and atomic force microscopies, while their chemical degradation profiles were obtained by Raman spectroscopic imaging. Non-uniform degradation was observed in the aged backsheet film, and both UV and moisture appeared to significantly affect the degradation profiles of the multilayers. Severe degradation, indicated by high fluorescence, occurred in the outermost region of the pigmented PET outer layer, and the degradation gradient extended to approximately 20 μm to the bulk. It was also found that the inner adhesive layers were severely deteriorated under moist condition, indicating that the long-term adhesion between the layers could be a major area of concern for multilayer backsheets used in a humid environment. The relationship between the sharp (non-uniform) degradation profile, resultant internal stress, and ultimate failures (cracking and delamination) was discussed as well.