Transparent backsheets for bifacial photovoltaic (PV) modules: Material characterization and accelerated laboratory testing

Interest in bifacial modules has rapidly increased over the past decade due to their ability to generate more power than conventional monofacial photovoltaic (PV) technology as they can absorb sunlight from both sides of the module. Compared to the traditional glass/glass bifacial modules, glass/backsheet modules show many advantages including lighter weight, high light transmittance, and high corrosion resistance. However, research on the weatherability and long-term reliability of transparent backsheet materials and their usage in bifacial modules under service environments is lacking. In this study, accelerated weather testing using the NIST SPHERE (Simulated Photodegradation via High Energy Radiant Exposure) was conducted to investigate the durability of three fluoropolymer-based transparent backsheets and their laminated coupon counterparts. The transparent backsheets were exposed at 75 degrees C/50% relative humidity (RH), while the coupons were exposed at 65 degrees C/50% RH, both subjected to UV irradiance of approximately 140 W/m(2) for up to 2000 h. Results indicate that a fluoroethylene vinyl ether (FEVE)/polyethylene terephthalate (PET)/ethylene-vinyl acetate (EVA)-based transparent backsheet (CB3) exhibited substantial chemical, optical, thermal, and mechanical changes and ultimate cracking after 1200 h (approximate to 600 MJ/m(2)). The other two backsheets, polyvinyl fluoride (PVF)/PET/FEVE-based (CB1) and polyvinylidene fluoride (PVDF)/PET/FEVE-based (CB2) backsheets, showed no obvious signs of cracking up to 2000 h of UV exposure (approximate to 1000 MJ/m(2)). For all three backsheets, the PET core layer demonstrated the greatest material property changes after UV exposure indicating that this layer is the most susceptible to UV degradation. Results indicate that the application of transparent backsheets for bifacial modules is promising. However, proper design of the layers of the backsheets for increasing the stability of PET core layer under UV exposure is critical. This study will provide a scientific basis for material choice and product development for a more reliable bifacial PV technology.