Self-cleaning surfaces have excelled in recent years in energy and environmental fields. In particular, in solar energy area, these surfaces are used to avoid soiling accumulation on photovoltaic (PV) modules. So far TiO 2 has been widely used due to its photocatalytic activity and photo-induced superhydrophilicity. However, this oxide has some limitations since it reduces the glass transmittance and it rapidly reestablish the water contact angle in dark environments. In order to circumvent these limitations, composites TiO 2 /SiO 2 have been proposed. For photovoltaic application, besides a good transparency in the wavelength region 300-1800 nm and self-cleaning properties, the coating should also present long durability and adequate adhesion to endure the outdoor conditions. Aiming at developing a coating with these properties, in this work, TiO 2 /SiO 2 composites containing different titanium content have been synthesized and compared with pure TiO 2 films in relation to adhesion, transparency and hydrophilicity. Both films have been deposited over low iron float glass substrates by sol-gel dipcoating technique and different calcination temperatures (400, 500, 600°C) and Si/Ti molar rates (Si 86 Ti 14 , Si 40 Ti 60 ) have been considered. TiO 2 /SiO 2 films showed higher transmittance in visible range compared with pure TiO 2 . TiO 2 /SiO 2 films showed superhydrophilic character before and after ultraviolet irradiation, with water contact angles near to 0°. Furthermore, as predicted, TiO 2 /SiO 2 films could keep the superhydrophilic character in dark environments, in contrast with pure TiO 2 films. Both TiO 2 and TiO 2 /SiO 2 films exhibited good adherence and it is shown that higher calcination temperatures and higher titanium content enhance such property. All films presented abrasion resistant property in contact with sponge and detergent. It has been demonstrated that high transmittance, self-cleaning and adherent composite has been obtained by a simple sol-gel route presenting good potential to be applied on photovoltaics systems.

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 TiO 2 /SiO 2 films with different titanium content have been synthesized and benchmarked against pure TiO 2 in relation to transparency and hydrophilicity. Moreover, a hydrophobic antireflective material based on functionalized-SiO 2 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. TiO 2 /SiO 2 and functionalized-SiO 2 films showed higher transmittance in visible range than pure TiO 2. TiO 2 /SiO 2 films showed a persistent superhydrophilic character with water contact angles near to 0°, while functionalized-SiO 2 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).

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