Surface modification of porous antireflective coatings for solar glass covers

The nanoporous silica films are usually rich in residual silanol groups (Si–OH). Theses groups are very reactive and induce the adsorption of water vapour and contaminants under humidity conditions, resulting in a deterioration of the optical properties of the antireflective (AR) films, among others. This adsorption increases the refractive index of the films, and the antireflective property disappears. If the film surface is modified exchanging the active hydroxyl groups by inert organosilyl groups, the reactivity of the surface film can be decreased and then the durability of its properties will be increased. In this paper, the application of a surface modification process for increasing the durability of the films has been studied. This process has been performed by immersing the AR coated glasses in a hexamethyldisilazane (HMDS)/hexane solution. The contact angle measurements show that the hydrophobicity increases with both concentration of HMDS and treatment time. The influence of outdoor natural soiling in the transmittance spectra of the films has been also studied, at different light incidence angles. The results obtained suggest that, for the same exposure conditions, the uncoated glass becomes dirtier than the AR coated glass.