Single layer broadband spectrally selective SiON coatings for passive radiative cooling

Engineering the spectral response of materials in infrared (IR) wavelength range can enable effective radiative cooling. Current configurations of IR emitters usually adopt complicated photonic designs and engineered structures because pure/singular materials such as dielectrics and polymers can hardly realize broadband and spectrally selective emissivity simultaneously. Herein, we propose single-layer silicon oxynitride (SiON) coatings with visible transparency and outstanding broadband spectral emissivity of 0.8-0.9 in the wavelength range of 8-14 μm. The single layered composite coating has a thickness below 2 μm, and its mid-IR broadband absorption characteristics can be tuned by adjusting the chemical intermixing of SiO₂ and SiN as well as proportion of the Si-O and Si-N bonds during the physical and chemical vapor deposition processes, respectively. When deposited on solar reflective substrate, the composite coating with highest selective emissivity of 0.9 shows a promising net cooling power of ≥ 120 W m⁻², when the ambient temperature is 27 °C. In an outdoor experiment, the selective emitter exhibits 4.5-5 °C lower temperature when compared to the ambient temperature inside the chamber, thus achieving the sub-ambient daytime cooling. Therefore, absorption bandwidth extension of single material provides a feasible cooling solution for many applications, including solar cells, and building and automobile windows.