Fluorescence-Enabled Colored Bilayer Subambient Radiative Cooling Coatings

Abstract

Passive daytime radiative cooling has emerged as a promising green technology for the thermal management of buildings, vehicles, textiles, and electronics. Typically, both high solar reflectance and high thermal emissivity are prerequisites to achieve sufficient daytime cooling. However, colored radiative cooling materials are facing the dilemma of introducing visible light absorption, leading to challenges in balancing cooling and aesthetic demands. Here, three colored bilayer radiative cooling coatings, each comprised of a white base layer and a colored top layer with fluorescence enhancement are fabricated. Three phosphors (Sr2Si5N8:Eu2+, Y3Al5O12:Ce3+, and (Ba,Sr)SiO4:Eu2+) are employed with respective photoluminescence quantum yields (PLQYs) of 81%, 95.8%, and 91.0% as the colored pigment in the top layer. To mitigate the contradiction between coloration and solar reflectance, SiO2 microspheres are introduced into the top layer and utilize their Mie-resonance-based multiple scattering to increase the photoluminescent (PL) properties of the phosphors, which jointly boosts the effective solar reflectance (ESR) of the top layer. As a result, the three bilayer coatings exhibit soft colors while achieving subambient cooling with temperature drops of up to 1.5 °C. This fluorescence-enhancement strategy may pave the way for preparing highly efficient radiative cooling coatings with tunable colors.