Hydrophobic, Antireflective, Self-Cleaning, and Antifogging Sol-Gel Coatings: An Example of Multifunctional Nanostructured Materials for Photovoltaic Cells

Antireflective, photocatalytic (self-cleaning), water repellent, and high water-wetting (anti fogging) properties were combined for the first time into a sol gel coating deposited onto glass substrates. Such an original multifunctional coating was obtained by sol gel liquid deposition of two successive oxide layers. The First coating is composed of a hybrid methyl-functionalized nanoporous SiO2 material that exhibits high transparency, high water resistance, close to null water adsorption, and fairly high mechanical stability (transversal Young Modulus: 1.5 GPa). Thickness and refractive index can be controlled by selecting proper chemical and processing conditions so as to adjust the anti-reflectivity properties. The second layer is an ultrathin crystalline TiO2 nanoperforated layer that was deposited on top of the previous antireflective layer. Its thickness and refractive index were adjusted around 12 nm and n approximate to 1.8 respectively. This hard TiO2 top layer acts as a protecting barrier toward mechanical aggressions and assures high water wetting (antifogging) and photocatalysis (self-cleaning) at the surface. Indeed, this bilayer system shows an excellent capability to photodecompose organic species that were adsorbed into the Anti-Reflective (AR) layer porosity. We show that the decomposition of model pollutants takes place inside and on the surface of the layer, which is likely due to diffusion of the pollutants toward TiO2 and/or diffusion of the radical species toward the pollutants. These systems are easy to produce on a large scale at low cost and exhibit high mechanical and chemical durability. They are thus serious candidates to be used as antireflective, self-cleaning coatings for photovoltaic cells.