All-Color Sub-ambient Radiative Cooling Based on Photoluminescence

Ingenious solutions to reduce energy consumption and improve power efficiency are the need of the hour to tackle climate change. Radiative cooling is being extensively studied as a potential route for passive cooling and energy harvesting. However, to achieve almost complete solar reflection necessary for daytime sub-ambient cooling, the visual appearance of radiative coolers proposed so far has primarily been limited to being either diffusive white or mirror-like, which may be undesirable in real applications in terms of aesthetics, safety, and daytime light pollution. In this work, we demonstrate that optimally designed wavelength conversion in the visible spectral region allows the realization of radiative coolers with any desired color in the entire absorptive color space, including black and other dark colors, while still achieving daytime sub-ambient cooling. To obtain these results, we propose a general spectral design method based on metamerism that can find the ideal wavelength conversion spectrum with minimal solar absorption under strict color-matching constraints. Through our work, we provide guidelines for designing wavelength conversion profiles of suitable photoluminescent colorants and set performance boundaries for radiative coolers based on them.