Effects of Stokes shift and Purcell enhancement on fluorescence-assisted radiative cooling

Passive daytime radiative cooling has been widely investigated ascribed to the great potential in combatting energy crisis and global warming. Polymeric coatings comprising functional fillers with solar reflectance and infrared emissivity both over 0.9 have proven to be particularly effective for sub-ambient daytime radiative cooling. However, the intrinsic absorption of many functional fillers with optical bandgaps within the solar spectrum reduces the cooling potential and consequently limits material choices. It has recently been demonstrated that introducing fluorescent materials into polymeric coatings can convert the absorbed sunlight to fluorescent emissions and hence increase the effective solar reflectance and cooling performance. In this work, we carry out a systematic experimental investigation on the influence of Stokes shift and Purcell enhancement in such fluorescence-mediated radiative cooling. We find that using phosphors with smaller Stokes shifts and nanoparticle fillers of appropriate sizes can significantly enhance the effective solar reflectance. We show that matching the emission wavelengths of two phosphors with the Mie scattering resonance wavelengths of the TiO₂ nanoparticle fillers shortens their fluorescence lifetimes by 61% and 23%, respectively, indicating Purcell enhancement factors of 2.6 and 1.3 and ultimately increasing the effective solar reflectance of the fluorescent coatings by up to similar to 4% in field tests. The fluorescence enhancement approach demonstrated here provides an effective strategy for making radiative cooling coatings compatible with commercially available inexpensive engineering materials and potential for realizing colored coatings.