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

ABSTRACT 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.