Backsheet with dramatically enhanced durability for photovoltaic modules

In this paper, the durability (weatherability) of a novel backsheet material used in photovoltaic (PV) modules is reported. The backsheet (code: FF-backsheet) consists of a special-grade poly(ethylene terephthalate) (PET) film and protective layers with different functionalities, which are directly coated onto the surface of the PET film without adhesives. The durability of the backsheet was evaluated through accelerated degradation tests (ADTs) under extreme conditions, including intense UV irradiation, high temperature, and high humidity by an international certification authority (China Quality Certification Centre). After an ADT for 4000 h, the backsheet partly maintained its original properties of elongation at break, tensile strength, yellowness index, breakdown voltage, and water vapor transmission rate. Consequently, the FF-backsheet was certified to have the highest level of durability (class 1). In contrast, conventional backsheets, in which protective films are attached to a normal-grade PET film using adhesives, were prominently embrittled. The outstanding durability of the FF-backsheet was also certified through a more severe ADT by another international certification authority (TUV SUD China Shanghai). Here, we propose the reaction mechanisms for the hydrolytic and photoinduced degradation of the PET layer and discuss the relationship between the toughness of the film and its yellowing behavior. Module-imitated samples were prepared via lamination of the backsheets with glass plates and were evaluated their durability via an original ADT (125 degrees C, 100% relative humidity). We used conventional backsheets as reference samples, all of which exhibited many cracks in the PET layers after the ADT. In contrast, no damage (cracking, foaming, and delamination) was observed in the PET layers of the FF-backsheet. These results suggested that the FF-backsheet has unprecedentedly high durability.