Bilayer fluorescent colored radiative cooling coatings for building energy saving

Fluorescent colored radiative cooling coatings present a scalable alternative to ultra-white coatings, addressing both light pollution mitigation and architectural aesthetic requirements. However, effective solar reflectance (ESR) of fluorescent coatings that involve fluorescent wavelength conversion cannot be characterized by the conventional spectrometric test designed for non-fluorescent coatings. This complexity makes it challenging to investigate the effects of key parameters on optical properties of fluorescent coatings, as well as tailoring the spectral properties to achieve high ESR with vivid color, and high infrared emittance for energy-efficient buildings. Herein, a bilayer fluorescent colored radiative cooling coating (BFC-RCC) was proposed, using a polymer matrix, which comprises a bottom TiO2 layer and a thin colored top phosphor/Y2O3 hybrid layer. A modified Monte Carlo method was developed to simulate the spectral properties of these coatings and investigate the influence of crucial parameters like coating thickness, nanoparticle concentration, and photoluminescence quantum yield (PLQY) of fluorescent pigments on their optical performance. It was shown that a 270 μm-thick BFC-RCC with a 250 μm-thick bottom layer containing 25 vol% TiO2 nanoparticles (NPs) and a 20 μm-thick colored top layer with optimal Y2O3 NPs and phosphor concentrations could achieve an excellent ESR of 0.946 and 0.959 for red and yellow colors, respectively, and a high infrared emittance of 0.95. Furthermore, it could demonstrate superior daytime cooling, i.e., 1.2 and 3.9 °C cooler than non-fluorescent coatings for yellow and red colors, respectively. This BFC-RCC shows an average energy saving of 59.14 % across diverse climatic regions, demonstrating its building energy-saving potential.