Antireflective properties of porous silica thin films obtained with the dip-coating process

In solar energy plants, suitable spectral selectivity may minimize the dispersed solar light and decrease heat losses. Nowadays antireflective coatings are used to increase the solar transmittance of glass equipments. In this study, porous silica thin films on glass substrates were prepared by a sol-gel dip-coating technique with silica colloidal sol prepared from basic catalysis of tetra ethyl orthosilicate (TEOS) plus ammonia in ethanol; after different sol aging, hexamethyldisilazane (HDMS) was added to improve the porosity of the film. Thin films with a very low refractive index (1.15 to 1.18) were obtained through fast thermal treatments up to 450°C; when deposited on the glass surface with thick of 140 nm about, allow to increase the solar transmittance from 91.5 % to about 96.8%. Furthermore, dip-coating parameters as age of colloidal sol, substrate withdrawal speed, number of dips and thermal treatment conditions were correlated with the obtained optical properties such as thickness and refractive index.