Developing methodology for service life prediction of PV materials: Quantitative effects of light intensity and wavelength on discoloration of a glass/EVA/PPE laminate

Abstract

Service life prediction (SLP) is important for the long-term performance and reliability evaluation of materials used in photovoltaics (PV). This paper has introduced our first work on developing methodology for SLP of PV materials using a glass/EVA/PPE backsheet (EVA: ethylene-vinyl acetate, PPE: polyethylene terephthalate (PET)-PET-EVA) laminated system. The NIST SPHERE (Simulated Photodegradation via High Energy Radiant Exposure) was used for the accelerated laboratory exposure. Specifically, the quantitative effects of spectral UV irradiances ranging from 60.5 W/m2 to 142.5 W/m2 and different spectral wavelengths centered at 306 nm, 326 nm, 354 nm and 452 nm, respectively, on discoloration of the laminates were systematically investigated. Applicability of the reciprocity law to the discoloration response of this system was carefully validated by three mathematic methods, suggesting that a linear correlation can be established between the light intensity and the discoloration of the laminates. Moreover, an exponential relationship was found between the UV wavelength and discoloration. A shorter UV wavelength led to a greater discoloration, and photobleaching effects occurred under a longer wavelength centered at 452 nm. Fluorescence emission was also recorded along with the discoloration for samples exposed to different light intensities. These results have indicated the importance of selecting appropriate irradiance levels and wavelength ranges for an accelerated light source to ensure a similar degradation mechanism with the outdoor exposure. This quantitative study has laid crucial groundwork for developing accelerated laboratory testing and building predictive models for service lives of PV materials.