Sol-gel deposition and optical characterization of multilayered SiO2/Ti1-xSixO2 coatings on solar collector glasses
Thin films of silicon oxide and silicon titanium mixed oxides are deposited on solar collector glazing in a sol–gel dip-coating process based on alcoxide precursors. Spectrophotometry is used to characterize the relation of film thickness and withdrawal speed for the precursor solutions, and to determine the refractive index of individual layers of the mixed oxides. The inferred dispersion relations n(λ) are compared to the predictions of effective medium theories. Based on the knowledge of the optical properties of individual layers, multilayer interference stacks are designed. Multilayered samples of superior quality are deposited by sol–gel dip-coating in a particle-free environment. The final optical performance of the multilayer stacks are characterized in terms of the visible reflectance RVIS, CIE color coordinates, and the solar transmission Tsol. Values of up to 2.4 have been attained for the energy efficiency of the colored reflection M=RVIS/(100%-Tsol). The produced coatings combine a bright colored reflection with an acceptable solar transmittance, and are thus well suited for the application in colored glazed thermal solar collectors. This novel type of colored glazing opens up new perspectives for the architectural integration of thermal solar collectors, e.g. as solar active glass facades.