Functional nano-textured titania-coatings with self-cleaning and antireflective properties for photovoltaic surfaces

Photoactive TiO2-only transparent coatings having self-cleaning and antireflection (AR) properties were prepared by forming first a nanosol through controlled hydrolysis of tetraisopropyl orthotitanate (TIPT), followed by deposition of this nanosol on glass substrates by dip-coating with a final calcination step to form the surface nano-textured thin film. The samples were characterized in terms of nanostructure and -texture by X-ray diffraction, UV-vis spectroscopy, scanning electron microscopy and atomic force microscopy, while AR properties were investigated by transmittance measurements. Self-cleaning properties were analyzed by measuring the changes of water contact angle, and by photocatalytic degradation of a dye. The aim was to analyze how to prepare these materials and the relation of the properties of titania with the surface nano-texture, particularly in relation to obtain the properties required to their use as functional coatings for PV cells. Films with good optical characteristics and high transmittance (<1% loss in transmittance) can be obtained at low speed of dip-coating (6 mm/s) and high nitric acid concentration (0.5 M). Under optimized conditions, calcination at low temperature (400 C) may already be sufficient to produce coatings with good functional properties, making the procedure compatible with the use of some flexible substrates. A preliminary mechanism of formation of the surface nano-texturing is also proposed.