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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.

The cover glass on solar modules provides protection for the underlying solar cells but also leads to two forms of power loss: reflection losses and soiling losses. In this work we explore the addition of a thin hydrophobic layer of refractive index n=1.35 to the outer surface of a broadband multilayer anti-reflection (MAR) coating, comparing modelling with the actual performance of the coating. Systems with hydrophobic layers from 5nm to 300nm in thickness deposited on the surface of a broadband AR coating have been modelled, with reflectance curves and weighted average reflection (WAR) calculations showing that the total reflection stays below that of uncoated glass at all thicknesses. However, the optimal coating is determined to be ∼5nm in thickness. Addition of the hydrophobic layer increases the water contact angle of the MAR coating from 7° to 114°, with a significant increase in anti-soiling properties. This provides proof of principle of the benefits of combining a high performance AR coating with a hydrophobic anti-soiling coating on module cover glass.

Glass-free, lightweight, photovoltaic modules have the potential to enable new uses of solar in building integrated and vehicle integrated applications. Glass-free modules have the advantages of reduced weight, lower-cost mounting solutions, and reduced transportation costs. Ethylene tetrafluoroethylene (ETFE) is usually considered a suitable candidate to replace glass as the front cover sheet material in module fabrication. In this article, we report on its optical, chemical, self-cleaning, and morphological properties. Durability is a key issue, especially for building-integrated applications since harsh conditions are prevalent outdoors. We report on the effect of sequences of environmental stress tests (UV exposure, and Damp Heat). The module cover material is also subject to regular cleaning, which can cause surface damage. We report on the use of linear abrasion testing to assess the abrasion resistance of ETFE to a variety of abrasive materials. The results provide useful data on the advantages and disadvantages of replacing glass with a durable polymer for front sheets in solar modules.