Optical and morphological characterisation of low refractive index materials for coatings on solar collector glazing
Nanostructured coatings based on the elements Si, O, Mg, and F have been deposited as thin films by solegel dip-coating in a particle-free atmosphere. The refractive index of the prepared SiO2 and quaternary MgeFeSieO thin films has been determined from spectrophotometric and ellipsometric data. The morphology of those thin films has been observed by TEM. Nanoporous SiO2 coatings with a pore size smaller than 3 nm (TEM) and a pore volume fraction of 30% (as inferred from ellipsometric measurements) have been achieved. They are characterised by significantly lower refractive index values (approx. 1.32 at 550 nm) than compact SiO2 (approx. 1.46). Quaternary MgeFeSieO thin films are characterised by a surprisingly low refractive index (approx. 1.26 at 550 nm), even lower than that of dense MgF2 coatings (approx. 1.38). Preliminary results of transmission electron microscopy suggest that these films are of nanocomposite nature. In both cases, highly transparent samples have been produced in a single dip-coating step followed by simple thermal annealing in air. Broad spectral transmittance maxima are observed exceeding values of 98.5% (nanoporous SiO2) and 99.5% (quaternary MgeFeSieO films). The quaternary films might exhibit a higher ageing stability than porous SiO2 films with respect to pore-filling and could therefore be a promising alternative for single-layered anti-reflection coatings as well as for multi-layered coloured coatings on solar collector glazing.