Preparation of single- and double-layer antireflective coatings by sol-gel method

Antireflective (AR) SiO2 coatings were developed in this work to increase the transmittance of glass in the visible light range of 400-800 nm, due to their potential applications on solar energy systems. Also, double-layer AR coatings were prepared by covering AR SiO2 layer with a dense TiO2 layer with the aim of protecting AR layer from environmental effects. SiO2 coatings were obtained with sol-gel and dip coating methods, by using acid- and base-catalyzed silica sols as precursors. Only 1% transmittance increase was gained with acid- catalyzed SiO2 coating due to its dense structure. On the contrary, SiO2 coatings formed by base-catalyzed sol have transmittance > 99% in the 460-660 nm range and of 99.8% at 550 nm, corresponding to the wavelength at maximum intensity of solar spectrum. This high values were attributed to the porous structure of the film revealed by SEM and AFM analysis, and to the optimized thickness of coating achieved at 90-120 mm/min withdrawal speeds. Transmittance of base-catalyzed SiO2 coatings decreased to an average value of 96.2% after coated with high refractive index TiO2 layer. However, still a 5-6% achievement in transmittance of glass in 400-800 nm range was attained by optimizing the thickness of each layer using different withdrawal speeds, and concentration and types of the precursor sols. Characterization of the coatings was performed with SEM, AFM, EDS and FTIR analysis.