Optimization of W/WAlSiN/SiON/SiO2 tandem absorber consisting of double layer anti-reflection coating with broadband absorption in the solar spectrum region

A novel spectrally selective tandem stack of W/WAlSiN/SiON/SiO₂ was deposited on stainless steel 304 and silicon substrates using a four-cathode reactive unbalanced magnetron sputtering system. The coatings were deposited by sputtering of W, Al and Si targets in Ar, Ar + N₂, Ar + N₂ + O₂, and Ar + O₂ plasmas. The process parameters were optimized by studying the optical properties of the individual layers using UV-VIS-NIR spectrophotometer and Fourier transform infrared spectroscopy measurements. The high spectral selectivity of the tandem stack was achieved by varying the reactive gas flow rates of N₂ and O₂ and thickness of individual layers. In the tandem stack, W layer acts as an IR reflector, WAlSiN acts as the main absorber layer, SiON, and SiO₂ layers act as anti-reflecting layers. The tandem stack was designed based on graded refractive indices of individual layers with a double layer anti-reflection coating. The tandem stack exhibits superior spectral selectivity with a high solar absorptance of 0.955 in the broadband solar spectrum region and low thermal emissivity of 0.10 in the infrared region. The coating was found to be thermally stable up to 600 °C in vacuum for 200 h under cycling heating conditions.