Long-term UV Durability of Laminated Glass/Transparent Backsheet Coupons for Bifacial Photovoltaics: Backsheet Side Exposure

Bifacial modules with glass/transparent backsheet (GB) structure offer many advantages over traditional glass/glass (GG) modules, such as lighter weight, smaller heat capacity, and higher hail resistance. However, research on long-term durability of transparent backsheets and their usage in bifacial modules is lacking. In this study, durability of GB laminated coupons constructed with three fluoropolymer-based transparent backsheets were investigated, along with GG coupons laminated with either polyolefin elastomer (POE) or ethylene vinyl acetate (EVA) encapsulants. Laboratory accelerated weathering was performed using the NIST SPHERE (Simulated Photo degradation via High Energy Radiant Exposure) under UV/65 °C/50% relative humidity (RH), followed by thermal cycling. Optical and chemical properties of the specimens were characterized by UV-visible spectroscopy and attenuated total reflectance Fourier transform infrared spectroscopy with microscope (micro-ATR-FTIR) periodically during UV exposure. The non-destructive depth profiling of yellowing in GB coupons was performed by laser scanning confocal microscope-based fluorescence mapping. Results indicated that the long-term performances of the GB coupons strongly depended on the materials and structures of the transparent backsheets. The backsheet cracking was observed in a fluoroethylene vinyl ether (FEVE)/ polyethylene terephthalate (PET)/ EVA-based transparent GB coupons after UV exposure and subsequential thermal cycling. The UV durability of PET core layer appeared to be critical to the application of GB structure for bifacial modules. Additional characterizations on aged GB and GG coupons are on-going. This study has provided a scientific basis for material choice and product development for a more reliable bifacial PV technology.