On the origin of spectrally selective high solar absorptance of TiB2-based tandem absorber with double layer antireflection coatings

Despite significant efforts in developing tandem ceramic coatings for photo-thermal conversion applications, the underlying physics behind spectrally selective high absorptance and thermal stability of the ceramic absorbers remains to be addressed. In this perspective, TiB2/Ti(B,N)/SiON/SiO2 and Ti/TiB2/Ti(B,N)/SiON/SiO2 films were developed on stainless steel substrates (SS 304) using pulsed direct current and radio frequency magnetron sputtering. The addition of double layer antireflection coating not only increases the solar absorptance, but also the thermal stability of the tandem absorber. The amorphous coatings exhibit a high solar absorptance of 0.981 at room temperature and an acceptable thermal emissivity of 0.15 at 82 °C. Variable angle spectroscopic ellipsometry analysis reveals a gradient in the refractive indices of the individual layers from the substrate to the top surface. The coatings with and without titanium interlayer were found to be stable for 2 h in air up to 450 °C and 400 °C, respectively. Under vacuum, the coatings exhibited good thermal stability up to 250 h at 500 °C. We make an attempt to explain the high absorptance of the tandem absorber in terms of the composition and gradient in refractive indices across the stack. •TiB2/Ti(B,N)/SiON/SiO2 and Ti/TiB2/Ti(B,N)/SiON/SiO2 based absorber coating has been developed.•The coatings exhibit a high solar absorptance of 0.981 at room temperature.•Gradient in the refractive indices of the individual layers across the stack is observed.•SiON/SiO2 double layer antireflection coating increases the absorptance and thermal stability.•The coating exhibited good thermal stability for 250 h at 500 °C in vacuum.