UV lamp spectral effects on the aging behavior of encapsulants for photovoltaic modules

Polymer encapsulants used in photovoltaic (PV) modules experience severe UV aging during more than 25 years of module's operation. However, the current standard qualification tests cannot provide a sufficient solution able to mimic the UV exposure in the field. IEC 61215 defines the requirements for UV stability of PV modules with very low cumulative UV dosage. While the new IEC 62788-7-2 imposes higher UV dosages on polymers films, the used UV testing procedure allows the use of either UV-A fluorescence lamps or xenon lamps, yielding different spectra. The given work compares the effect of different lamp spectra, based on IEC 62788-7-2, on the aging behavior of the encapsulant. For this purpose, coupons (glass/encapsulant/busbar/encapsulant/backsheet) using the common encapsulant ethylene-vinyl acetate copolymer (EVA) and the emerging encapsulants polyolefine (POE) and thermoplastic polyolefine (TPO) were manufactured. The samples were weathered under different UV conditions for a total duration of 2500 h with extraction times of 250 h. Photodegradation was observed for all encapsulants and weathering types. However, in general the weathering with UV-A lamps inflicted significantly less damage. The formation of unsaturated carbon bonds because of polymer backbone decomposition was only reported for xenon lamp weathering. The carried out experiments suggest that xenon lamps, providing more realistic accelerated aging, should be predominately used in the UV weathering standards in the PV industry, whose current versions are to be accordingly reconsidered.