Fluorescence-enhanced light-blue bilayer radiative cooling coatings

Daytime passive radiative cooling devices expel heat into the cold outer space by thermal radiation, offering an energy-efficient strategy for cooling terrestrial objects. Colored coolers are more desirable than white ones in real-world applications under the demands of aesthetics and reducing light pollution. The recently proposed fluorescence-assisted methods based on phosphor powders have simultaneously achieved sub-ambient cooling performance and bright colors, ascribed to the fluorescence-induced heat gain offset and consequent high-effective solar reflectance. However, the color gamut of radiative coolers is still limited due to the lack of blue colors, attributed to the absorption of the majority of visible light. In this work, we propose and prepare a light-blue radiative cooling coating composed of a ZrO₂ white bottom layer with a remarkable high solar reflectance of >98% and an ultra-thin top layer consisting of a blue phosphor (SrO·Al₂O₃: Eu). This bilayer coating reveals a high effective solar reflectance of 94% with a vivid light-blue appearance and achieves a sub-ambient temperature reduction of 3-4.4 °C under a peak solar intensity of >800 W m⁻² and an average humidity of 30% in Hong Kong. Our work paves the way for fabricating highly efficient radiative cooling coatings with a whole gamut of tunable colors.