Long Term Accelerated Weathering Tests on “Coupons” to Develop New Classes of Backsheets

Renolit N.V. has developed a new class of Backsheet on base of Polypropylene and Polyamide with lower WVTR and higher acetic acid permeability than PET based backsheet. The backsheet is white pigmented in the bulk which allows for an higher Solar Reflectance than PET based transparent core backsheet. Photovoltaic module constructions using such backsheet show some discoloration (yellowing) in Damp Heat test (85°C / 85 % RH) contrary to the same constructions with PET based backsheet. It is important to understand the root cause of observed yellowing and its significance. In the present case, yellowing in Damp Heat is the consequence of the oxidation of the anti-oxidants added to the backsheet to extend its durability. It doesn’t occur in use, i.e. under sunlight exposure. The mechanism of yellowing is discussed. By reaction with residual peroxides of the encapsulation films (EVA), phenolic anti-oxidant are oxidized to yellow dyes derived from Quinonemethide. Under light, such molecules are reduced against to regenerate colorless anti-oxidants, able to further neutralize residual peroxides. This explains why the same coupons (Glass / EVA / mini-cell / EVA / backsheet) which heavily turn yellow after 1.000 hours of Damp Heat, remain unchanged under 2-Suns weathering test even after 21.000 hours, an equivalent of close to 40 years Florida sun exposure. No sign of corrosion neither of the grid fingers nor of the ribbons is observed in such weathering test. As additives extending the lifetime of polymers and reacting with residual peroxides have the tendency to produce yellow dyes in the dark, reduction of additive package of polymeric films to minimize yellowing in Damp Heat test may in fact lead to durability reduction. The present paper enhances the importance of developing cost effective encapsulation and backsheet solutions for Photovoltaic modules on base of weathering tests on coupons. To improve Photovoltaic module durability, the barrier properties of the backsheet should also be taken into account.