Solar Energy Materials and Solar Cells, Vol.176, 157-166, 2018
Enhancing spectrally selective response of W/WAl/WAlON/Al2O3 - Based nanostructured multilayer absorber coating through graded optical constants
In the field of concentrating solar power (CSP) technologies, multilayer absorber coatings are widely being investigated. The spectral properties of selective coatings can be tailored by carefully adjusting the composition and thickness of each layer. Based on the extensive analysis using the transmission electron microscopy (TEM), phase modulated spectroscopic ellipsometry along with computational study, we demonstrate how we can engineer the optical constants (refractive index and extinction coefficient) of individual layer to successfully achieve the spectrally selective properties in W/WAlN/WAlON/Al2O3 - based multilayer absorber coating. This coating exhibits a high absorptance of 0.958 and a low emittance of 0.08. The spectroscopic ellipsometry study confirmed the variation in metallic and optical properties of single layer of WAlN, WAlON and Al2O3 films, deposited on stainless steel substrates. This study also revealed the presence of intermediate layers of 26% WAlN - 74% WAlON at WAlN/WAlON interface and 60% WAlON - 40% Al2O3 at WAlON/Al2O3 interface. The Tauc - Lorentz dispersion model could effectively interpret the ellipsometry data of single layers of WAlN and Al2O3, while Cauchy absorbent model was useful for WAlON coating. Bruggeman effective medium approximation was used to describe the optical functions of intermediate layers. Investigation on optical constants reveals that the refractive index and extinction coefficient of each layer decrease from substrate to surface. The computational predictions of the reflectance properties corroborate well with the experimental results. In summary, the careful engineering of the optical properties in W/WAlN/WAlON/Al2O3 enables it to be an exceptional spectrally selective absorber coating.