Highly Accelerated UV Stress Testing for Transparent Flexible Frontsheets

Abstract

For flexible photovoltaic (PV) applications, the dominant material for the frontsheet is poly(ethylene-co-tetrafluoroethylene). As a fluoropolymer, it resists soiling by letting the water run off easily, is resistant to degradation by exposure to ultraviolet light, and is more mechanically durable than most fluoropolymers. To keep costs down, less expensive alternative polymers are desirable. In this study, highly accelerated ultraviolet light and heat stresses are applied to candidate materials, and the degradation kinetics are determined to provide information to evaluate their suitability for use in a PV application. Because of the uncertainty in service life prediction, the acceleration parameters are instead used primarily to evaluate the relevance of the applied stresses. Here, we find that the best materials are fluoropolymer based and that even when exposed to high irradiance at high temperatures, relatively little degradation is seen. For the 15 materials tested here, we found the Arrhenius activation energy for various degradation processes to be 39 ± 22 kJ/mol with a power law dependence on irradiance of 0.49 ± 0.22 with a negative correlation coefficient of −0.606 (i.e., more highly thermally activated processes are less dependent on the irradiance level). For frontside exposure, the most severe conditions used here (4 W/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /nm @340 nm, 70 °C, for 4000 h) were on average equal to about 11.4 y in Riyadh, Saudi Arabia when mounted with insulation on the backside. Thus, to get relevant amounts of ultraviolet exposure with unmodified commercial equipment (∼0.8 W/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /nm @340 nm) requires extraordinarily long exposure times, especially if conducted at lower irradiance levels.