Degradation of Hydrophobic Anti-soiling Coatings: Correlating Indoor and Outdoor Testing

Abstract

One of the most important challenges faced by solar asset managers is the accumulation of soiling and cementation on the photovoltaic module glass cover. Soiling attenuates the incident light and can severely reduce the output power. Hydrophobic coatings applied to the cover glass have the potential to reduce the amount of soiling by reducing surface energy and adhesion. Their presence should make the modules easier to clean. However, there are concerns that the currently available coatings degrade in an unacceptably short time of service in the field. It is important that the mechanisms of degradation are understood, with limited time to perform outdoor testing on hydrophobic coatings, accelerated laboratory-based environmental exposure tests are also conducted. This study evaluates the effectiveness of these accelerated environmental exposure tests and compares the degradation mechanisms observed in ultraviolet (UV) and damp heat (DH) exposure, to how they compare and correlate with long-term outdoor counterparts. The results from surface chemical characterisation show that all forms of testing result in a decrease in fluorine. Conversely oxygen and silicon increase over time as the thickness of the hydrophobic film is reduced and more of the glass substrate is exposed as observed in X-ray Photoelectron Spectroscopy. UV exposure was found to cause free radicalization leading to chain scission and detachment of fluorinated functional groups. Likewise, damp heat caused chain scission via hydrolysis. UV degradation was the primary factor of outdoor exposure with added factors such as environmental abrasion causing accelerated damage of the hydrophobic coating.