Inorganic Multilayer Film with High Infrared Selective Emission Enhanced by SiNx for Daytime Radiative Cooling

Inorganic multilayer films of radiative cooling can overcome issues like photothermal degradation and lack of selectivity in the infrared spectrum, which are common limitations of widely used radiative cooling organic materials. However, there are few reports about inorganic multilayer films with either excellent cooling performance or a simple structure that is easy to prepare and suitable for microdevices. In this work, we propose an inorganic multilayer film strategy that adopts a simple film structure, namely, SiNx/(L/H/L/H)/SiNx/Ag/glass(substrate) (where L and H represent low and high refractive index materials, respectively), to achieve high performance of daytime radiative cooling. The complexity of one-dimensional photonic crystal structures was reduced for the first time using SiNx. The film structure utilizes impedance mismatch to enhance solar reflection and complementary spectral contributions between different materials to enhance emissivity in the atmospheric window. The effectiveness of this design was demonstrated by using a multilayer film composed of SiO2, TiO2, and SiNx. This film possesses a high reflectivity of 91.6% in the solar spectrum and selective radiation in the atmospheric window (8–13 μm) with an emissivity of 90.9%, which should have been the highest emissivity in the reported radiative cooling inorganic multilayer films so far. Our calculation shows that the film temperature can achieve a steady-state temperature of 10 K below the ambient temperature under common solar radiation. The prepared multilayer film is tested under real outdoor temperature conditions, demonstrating significant subambient cooling effects. This study thus introduces an effective strategy for daytime radiative cooling using inorganic multilayer films with high performance.