High-Performance Daytime Radiative Cooler and Near-Ideal Selective Emitter Enabled by Transparent Sapphire Substrate.

Daytime radiative cooling serving as a method to pump heat from objects on Earth to cold outer space is an attractive cooling option that does not require any energy input. Among radiative cooler structures, the multilayer- or photonic-structured radiative cooler, composed of inorganic materials, remains one of the most complicated structures to fabricate. In this study, transparent sapphire-substrate-based radiative coolers comprising a simple structure and selective emitter-like optical characteristics are proposed. Utilizing the intrinsic optical properties of the sapphire substrate and adopting additional IR emissive layers, such as those composed of silicon nitride thin film or aluminum oxide nanoparticles, high-performance radiative coolers can be fabricated with a low mean absorptivity (3-4%) at 0.3-2.5 µm and a high mean emissivity of over 90% at 8-13 µm. Experiments show that the fabricated radiative coolers reach temperature drops of ≈10 °C in the daytime. From the theoretical calculations of radiative cooling performance, the sapphire-substrate-based radiative coolers demonstrate a net cooling power as high as 100 Wm.