Optimization of TiAlN/TiAlON/Si3N4 solar absorber coatings

A procedure to optimize a solar absorber coating stack with layers of TiAlN, TiAlON, and Si3N4, prepared using direct current magnetron sputtering on stainless steel with a Mo buffer layer, is presented. As input for this procedure the optical constants of individual layers of TiAlN, TiAlON, and Si3N4, deposited on Si substrates were obtained using spectroscopic ellipsometry and reflectance spectra for each material were calculated by the transfer matrix method using the obtained optical constants, showing good agreement with measured spectra. The optical constants were then used to design an optimized three-layer absorber coating by numerical simulations. Focused-Ion-Beam cross-sectioning showed that the individual layers of an actual fabricated three-layer absorber had a total thickness of 255 nm (92 nm of TiAlN, 78 nm of TiAlON, and 85 nm of Si3N4, for individual layers) with a 100-nm-thick Mo buffer layer. For this experimental geometry we found a solar absorptance and thermal emittance of 94.6% and 5.2% at 400 K by analyzing the experimental reflectance spectrum of the whole stack.