Metal-free radiative cooling polymer films containing high bandgap materials employing a tandem approach

Abstract

Achieving sustainable and clean energy in a completely passive manner is conceivable through radiative cooling process that pumps heat from the terrestrial objects deep into the cold outer space. Various designs of radiative coolers, multilayer, and photonic radiators, have been explored in the past, but most of them require optimization for up-scaling and cost-reduction. Herein, we report a facile method of preparing a radiative cooling film using a one-step solution processing technique. The concept relies on the choice of materials that emit in the atmospheric window (8 µm -13 µm) using the tandem approach to maximize the thermal radiation without incorporating any metallic reflective components. The film prepared by dispersing silicon dioxide and silicon nitride microparticles in a polymer matrix shows excellent emission spanning the entire atmospheric window, resulting in efficient light management and thus daytime radiative cooling. The cooling efficiency of the system is studied by varying the ratio of the particles and comparing it with those prepared using individual particles dispersed in the matrix. The film, thus optimized with an equal ratio of particles in the matrix, devoid of any reflective layer, exhibits a maximum cooling of 7.8 °C below ambient temperatures with an average cooling of 6.2 °C, on par with the cooling efficiencies obtained from conventional architectures. The observed effect is attributed to forward scattering, high bandgap, and phonon-polariton resonances associated with the microparticles dispersed in the polymer. As a prototype, an umbrella fabric laminated with the film exhibits a sub-ambient cooling of 7.6 °C and 5.4 °C in controlled and field conditions, respectively, during daytime. Thus, radiative cooling metal-free films that employ a tandem approach via a facile solution processing technique are highly scalable and effective for daytime cooling applications.