Transparent Backsheets for Bifacial PV Modules: Material Characterization and Weathering

Bifacial PV modules are promising to achieve a lower levelized cost of electricity (LCOE) compared to monofacial modules because light absorption occurs from both sides of the devices. The glass/transparent backsheet structure offers many advantages over the glass/glass structure for bifacial modules, such as lighter weight and higher hail resistance. However, research on the weatherability and long-term reliability of transparent backsheet materials and bifacial modules under their service environments is lacking. In this study, the material property and the durability of three fluoropolymer-based transparent backsheets and their laminated coupons have been investigated. The accelerated laboratory tests were carried out on the NIST SPHERE (Simulated Photodegradation via High Energy Radiant Exposure) under UV/75 °C/50 % RH and UV/65 °C/50 % RH for transparent films and coupons, respectively. Optical and chemical properties of specimens were characterized by UV-visible spectroscopy, attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy and Raman microscopy. Nano-indentation was used to evaluate changes in surface modulus and scratch-resistance of the transparent backsheets. Preliminary results indicate that different backsheets perform differently after aging, and potential cracking could occur once films become brittle. The cracking of polyethylene terephthalate (PET) core layer was observed, as well as the cracking of the ethylene vinyl-acetate (EVA) inner layer. This study will provide a scientific basis for material choice and product development for a more reliable bifacial PV technology.