Impact of the Material Combination on the Barrier Properties and their Stability in the Course of Accelerated Weathering

Polymer films (encapsulants and backsheets) serve as an important barrier against water ingress into a PV module both in a vapor and liquid state. Once in the module, water firstly initiates degradation of its polymeric components, and thus alters their properties including the barrier properties, leading to various failure modes such as delamination, discoloration, cracking, and corrosion. This results not only in reduced module lifetimes, frequent invertor shutdowns, and associated lower yields, but also poses safety risks. Therefore, the barrier properties, and especially their long-term stability must be thoroughly scrutinized. The long-term stability of polymeric films depends on the polymer type, its additivation, and also interactions between individual layers. In this contribution, 6 different backsheets, based on poly(ethylene terephtalate) (PET), poly(propylene) (PP), and poly(lactic acid) (PLA), 2 different encapsulants (Ethylene-Vinyl acetate Copolymer (EVA) and polyolefin elastomer (POE)) as well as glass-encapsulantbacksheet coupons, based on them, were prepared and exposed to three different accelerated weathering conditions for 2000 h. The weathering tests encompassed a classical damp heat (DH) (85°C 85% relative humidity (RH)), combined DH with a UV irradiation (85°C 60% RH), and UV irradiation in dry conditions (IEC 62788-7-2 A3). The water ingress was monitored non-destructively using near infrared spectroscopy (NIR) every 250 h. The water absorbance data, measured by NIR, were found to positively correlate with the solubility and water vapor transmission rates (WVTR), found by permeation measurements. Further, the crack formation could be directly observed in the NIR signal. The PPbased samples featured the best barrier properties initially and in the course of accelerated weathering, whereas PLA samples exhibited the worst performance. Encapsulants enhanced the barrier properties (with the only exemption being EVA for the PP backsheets) and functioned as a UV filter, protecting backsheets from cracking. However, the compatibility and interactions between the encapsulants and backsheets must be additionally considered, since discoloration could be observed for a number of polymer combinations. The backsheet cracking could be correlated with post-crystallization, assessed by DSC, leading to embrittlement. Additional tests were performed in the liquid water, mimicking the wet-leakage tests, enabling an assessment of isolation on the material level, which can be used as pre-qualification of module’s BOMs. Further, an NMR sensor was employed for the first time to study the water ingress into the polymeric coupons over the sample cross-section with a resolution of 50 µm. A water-content gradient could be observed within the polymeric layers.