The performance and durability of anti-reflective and anti-soiling coatings on solar cover glass

In recent years, huge advances have been made in the development of photovoltaic (PV) technology. Anti-reflective (AR) and anti-soiling (AS) coatings have demonstrably improved the energy output of PV modules and are establishing commercial viability. However, the durability of such coatings has not been sufficiently addressed and remains a key concern. In this thesis, a range of testing methods have been identified and evaluated for the purpose of assessing the performance, durability, environmental stability and wear resistance of AR and AS hydrophobic coatings. A comprehensive set of performance parameters and characterisation tools were identified to assess the degradation mechanisms of several coating formulations. Comparative results were generated for coatings before and after being subjected to a range of stress tests. New test rigs and procedures, including a sand impact and raindrop simulation test, were also designed and implemented to provide further insight into the degradation of coating performance following exposure to sand and rain water. The durability of two types of single layer (ARC1 and ARC2) and a multilayer anti-reflective (MAR) coatings were investigated. The reflectance obtained from single layer AR coatings after damp heat and abrasion tests suggest poor durability and inadequate useful lifetime despite an initial performance increase in transmittance when applied to PV cover glass. The test results on the MAR coating, designed and produced in CREST, showed excellent durability against all environmental and mechanical stresses. Six variants of a hydrophobic coating, ASC1 were stress tested during the development of an anti-soiling coating for PV cover glass application. The degradation mechanisms of each variant were used to improve the performance and durability of the coating. In addition, the ASC2 hydrophobic coating, that is commercially available for application on PV cover glass, was durability tested under same stresses as ASC1 coating for comparison. The ASC2 coating showed good resistance to mechanical stress and UV exposure. However, damp heat testing of the ASC2 coating lead to a significant performance drop revealing that it is not durable in high temperature and humidity conditions. The results indicate that better performance could be achieved if these types of coatings are designed and developed for specific climatic conditions. Three hydrophobic coatings commercially available for other applications such as on ophthalmic lenses and smart phone displays, ASC3, ASC4 and ASC5 coatings were subjected to a range of stress tests to demonstrate suitability for use in PV applications. The hydrophobic properties of ASC3 and ASC5 coatings showed good durability against mechanical abrasion, but both failed to perform well against exposure to UV light. There was no visual or performance degradation observed after both abrasion resistance and sand impact test on both ASC3 and ASC5 coatings. Scanning electron microscope (SEM) and transmission electron microscope (TEM) analysis of ASC5 showed that the nanoparticles responsible for the hydrophobic properties of the coating degraded under UV light with the loss of fluorine. Nevertheless, the ASC5 coating showed excellent stability against damp heat and thermal cycling stress. Furthermore, the ASC4 coating showed good durability against temperature cycling and UV light exposure. The coating also retained its hydrophobicity after 1000 hours of damp heat exposure. In addition, the coating showed a variable abrasion resistance against different abrasive materials. The test results suggest that ASC4 coating has good adhesion to the glass substrate with good durability against cheesecloth and felt pad abrasion. However, a harder scour pad damaged the surface of ASC4 severely. The test results suggest that all three hydrophobic coatings (ASC3, ASC4 and ASC5) have their strengths and weaknesses to withstand certain environmental and mechanical stresses due to their design for different applications. The durability of these coatings against UV light and abrasion resistance would need to be improved if they are to be applied to PV cover glass.