Optical and mechanical properties of sol-gel antireflective films for solar energy applications

Abstract

Antireflective films on glass were prepared by a dip-coating method from a sol-gel, which consisted of 50 nm monodisperse silica particles mixed with ethanol and water. The solar transmittance increased by 5.4% for the best made sample. A visible transmittance as high as 99.2%, with a peak transmittance of 99.5%, was achieved. A super insulated triple glazed window was made up of two low emissive glazings and an intermediate glazing which was either a standard float glass or an antireflective coated low iron one. At low angels of incidence the visible transmittance is more than 10% higher for the unit with the antireflective glazing, but at high angles of incidence they become almost identical. The scratch resistance of the film and the adhesion between the film and the substrate was considerably improved by baking the film at elevated temperatures. A slight decrease in the transmittance is noticed after the baking and the peak transmittance is displaced towards shorter wavelengths. It is believed that the particles fuse together and to the substrate upon baking. Transmission electron microscopy was used to study the baking process and preliminary results support the fusion theory. One large scale sample was tempered in a commercial oven with strongly improved mechanical properties as a result. The minimum reflectance was slightly increased to 1.5% from the initial 1%, but on the whole the antireflective properties were preserved.