Surface photooxidation of polypropylene-based photovoltaic backsheets: A comprehensive spectroscopic investigation

Published

December 8, 2024

Author(s)

Karissa Jensen, Ashlee Aiello, Stefan Mitterhofer, Chiara Barretta, Gernot Oreski, Christopher Stafford, Xiaohong Gu

Abstract

The prospect of cost reduction, enhanced performance, and improved sustainability has been driving innovation in the development of new backsheet materials for photovoltaic (PV) modules in recent years. Among the materials of interest, polypropylene (PP) has emerged as a promising alternative to fluoropolymer backsheets. As backsheets serve as a principal barrier providing electrical insulation and environmental protection to the sensitive electrical components of PV modules, comprehensive understanding of the durability of PP-based backsheets in service conditions is essential to ensure module reliability in the field. In this study, free-standing coextruded polypropylene backsheets were subjected to artificial weathering to further elucidate the degradation behavior of this material in environmental conditions. The UV exposure was performed on the NIST SPHERE (Simulated Photodegradation via High Energy Radiant Exposure) under three different environmental conditions: 75°C and 50% RH, 75°C and 20% RH, and 65°C and 20% RH. The total UV dose (295 nm – 400 nm) for each film was approximately 1710 MJ/m2, equivalent to approximately 50 years in Arizona, assuming 10% albedo. Chemical and optical changes were monitored periodically during the exposure via attenuated total reflectance-Fourier transform infrared (ATR-FTIR), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and colorimetry. Morphological changes were investigated with laser scanning confocal microscopy (LSCM), and the cracking propensity of the PP backsheet was evaluated by the fragmentation testing. Peak resolving analysis was applied to the FTIR results to obtain functional group concentration results with respect to exposure for all exposure conditions. The results indicate that the greatest chemical and physical changes occur after initial exposure (100 MJ/m2 - 220 MJ/m2), and reach a saturation point during the exposure. Observed chemical changes indicate minor accumulation of oxidation products and emergence of a potential polymer additive at the back (not exposed to light) surface. Consequently, Raman spectroscopy showed a slight increase in crystallinity of the surface layers of the backsheets with exposure. Mechanical testing revealed low cracking propensity, with minor shallow cracks occurring under high strains.

Citation

Polymer Degradation and Stability

Volume

232

Pub Type

Journals

Download Paper

https://doi.org/10.1016/j.polymdegradstab.2024.111132

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Keywords

FTIR spectroscopy, accelerated aging, backsheets, photovoltaics (PVs), polypropylene, reliability, polyolefin, photooxidation

Citation

Jensen, K. , Aiello, A. , Mitterhofer, S. , Barretta, C. , Oreski, G. , Stafford, C. and Gu, X. (2024), Surface photooxidation of polypropylene-based photovoltaic backsheets: A comprehensive spectroscopic investigation, Polymer Degradation and Stability, [online], https://doi.org/10.1016/j.polymdegradstab.2024.111132, https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=958614 (Accessed April 16, 2025)