Long-term durability of transparent backsheets for bifacial photovoltaics: An in-depth degradation analysis

• Published in: Solar energy materials and solar cells 2023-07, Vol.256, p.112309, Article 112309
• Main Authors: Smith, Soshana, Mitterhofer, Stefan, Moffitt, Stephanie L., Jhang, Song-Syun, Watson, Stephanie S., Sung, Li-Piin, Gu, Xiaohong
• Format: Article
• Language: English
• Subjects: Accelerated laboratory testing; Bifacial modules; Fluorescence mapping; NIST SPHERE; Transparent backsheets
• ISSN: 0927-0248; 1879-3398
• DOI: 10.1016/j.solmat.2023.112309

Bifacial modules with a glass/transparent backsheet (G/CB) structure offer many advantages, such as lighter weight, smaller heat capacity, and higher anti-soiling ability, 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 backsheet core and inner layers. Therefore, 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 for reflected light on the backside of a PV module. The transmittance of the coupons was characterized by UV–visible spectroscopy periodically during UV exposure. Cross-sectional characterization of optical, chemical, and mechanical degradation was conducted on the transparent backsheets of the aged and unaged coupons using confocal fluorescence microscopy, microscale infrared spectroscopy, and nanoindentation, respectively. Results showed that superficial outer layer cracking occurred on coupons constructed with a polyvinyl fluoride (PVF)-based backsheet (G/CB1) at the end of the exposure.Local delamination appeared near a corner of the coupon with a polyvinylidene fluoride (PVDF)-based backsheet (G/CB2). However, the coupon containing a 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. Significant degradation was not only observed on the FEVE outer layer but also in the PET core layer of CB3. The depth-dependent degradation was seen in fluorescence, micro-IR and nanoindentation mapping of the aged CB3. 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. •Confocal fluorescence microscopy can identify areas of high