Long-Term Durability of Transparent Backsheets for Bifacial Photovoltaics: An In-Depth Degradation Analysis

Abstract

Bifacial modules with a glass/transparent backsheet (G/CB) structure offer many advantages over their glass/glass (G/G) counterparts. However, compared to traditional opaque backsheets, transparent backsheets can be more susceptible to degradation because UV light can more easily go through the transparent outer layer into the core and inner layers. A better understanding of the degradation mechanism and long-term durability of emerging transparent backsheets is needed. In this study, the G/CB laminated coupons constructed with silica glass, polyolefin elastomer encapsulant (POE) and three types of fluoropolymer-based transparent backsheets were subjected to accelerated weathering under ultraviolet (UV) light at 65 °C and 50% relative humidity (RH) for up to 3600 h with additional 200 thermal cycles. The maximum UV dose (1800 MJ/m2) is equivalent to approximately 45 years of field exposure in Arizona, assuming a 12% albedo. Cross-sectional characterizations of optical, chemical, and mechanical degradation were conducted on the transparent backsheets of the aged and unaged coupons using confocal fluorescence microscopy, microscale infrared spectroscopy, and nanoindentation, respectively. Results showed that a superficial outer layer cracking occurred on coupons constructed with polyvinyl fluoride (PVF)-based backsheet (G/CB1), and local delamination appeared near a corner of polyvinylidene fluoride (PVDF)-based backsheet (G/CB2). However, the coupon containing fluoroethylene vinyl ether (FEVE)/polyethylene terephthalate (PET)/ethylene-vinyl acetate (EVA)-based transparent backsheet (G/CB3) showed the greatest yellowing as well as substantial backsheet cracking. This study suggests that, although the application of transparent backsheet is promising, careful considerations of the long-term UV durability are needed when designing and selecting backsheets for 50-year bifacial modules.

Keywords: Transparent backsheets, Degradation, Bifacial modules, accelerated laboratory testing, fluorescence mapping