Anti-soiling coatings for solar cell cover glass: Climate and surface properties influence

The objective of this study has been twofold: i) to investigate different strategies for CPV module glass surface modification, in particular preparing hydrophilic and hydrophobic coatings in order to reduce the dust accumulation (soiling) on the module surface; ii) to perform a joint comparative soiling testing in Italy, Spain and Brazil in order to understand the limit and advantages of the proposed anti-soiling coatings in different climate condition. Two TiO₂/SiO₂ films with different titanium content have been synthesized and benchmarked against pure TiO₂ in relation to transparency and hydrophilicity. Moreover, a hydrophobic antireflective material based on functionalized-SiO₂ thin film was also investigated. All these coatings have been deposited over low iron float glass substrates by sol-gel dip-coating and electron-beam evaporation technique. TiO₂/SiO₂ and functionalized-SiO₂ films showed higher transmittance in visible range than pure TiO₂. TiO₂/SiO₂ films showed a persistent superhydrophilic character with water contact angles near to 0° while functionalized-SiO₂ presented hydrophobic property. The joint comparative soiling tests showed the importance of setting anti-soiling strategies in region characterized by more dry climate: in Brazil, which during the soiling test was characterized by a long dry period, the anti-soiling coatings were effective in reducing the soiling deposition and in the removal of the contaminants by rainwater; in Spain and Italy, the more frequent rain precipitation made the soiling effect less relevant, however, the deposition of anti-soiling coating on the module cover glass allowed to fully recover the initial transmittance after rain washing. A chemical and mineral characterization of the soiling has been carried out revealing the dependence of the contaminants from the environment conditions (e.g. car traffic, presence of industries, amount of rain and local minerals in the ground).