Spectral Selectivity Enhancement in Solar Absorber Multilayers through Titanium Oxynitride Thin Film Modulation

Broadband photothermal materials with spectral selectivity and stability in harsh environments are crucial for high-temperature solar-thermal systems. Additionally, achieving broadband absorption without relying on metamaterials remains a challenge. Refractory titanium nitride (TiN) and oxynitride (TiON), with their high infrared (IR) reflectance and tunable optical/plasmonic properties, are promising candidates for such applications. However, their susceptibility to oxidation complicates synthesis. Here, a straightforward approach is demonstrated to synthesize and tune the optical/plasmonic properties of TiN/TiON thin films by simply controlling the oxygen pressure during room-temperature pulsed laser deposition. Specifically, it is shown that highly metallic Ti(O)N films, as well as TiON films exhibiting double-epsilon-near-zero (D-ENZ) behavior in the optical region, can be obtained. This tunability enabled the design and fabrication of a nitride-based multilayer with optimized solar-selective absorption. In particular, a highly metallic TiN film was employed as the bottom layer, while a TiON ultrathin film exhibiting D-ENZ behavior was used as the absorbing layer. The resulting device achieved 91% solar absorption, 80% mid-IR reflectance, and maintained broadband absorption at incident angles up to 70°. These findings establish a lithography-free, thermally untreated route to broadband, spectrally selective absorbers, based on tunable Ti(O)N films, opening new opportunities for next-generation high-temperature energy harvesting applications.