Optimal design and preparation of high-performance multilayered thin film daytime radiative coolers

Radiative cooling is a passive cooling strategy that can radiate heat to outer space through the 8 to 13 mu m waveband (atmospheric window) and is now widely used for buildings, wearable fabric, solar cells, and electronic devices. Daytime radiative cooling requires both high reflection in the solar spectrum and high absorption/emission in the 8 to 13 mu m range. Previous multilayered structures were optimized by changing the thickness ratio of the layers, but the optical properties of multilayer thin films, such as absorptivity, transmissivity, and reflectivity, are determined by complex factors. In this work, several initial multilayer structures were selected and then the thickness of each layer was globally optimized; the theoretically smallest thickness with the best absorption performance was achieved in the 8 to 13 mu m range, which significantly improved the cooling performance and reduced costs. We developed an optimized SiO2-Ta2O5 alternating multilayer photonic radiative cooling thin film and fabricated it using ion source assisted electron beam evaporation with an average emissivity of 0.876 within the 8 to 13 mu m range and an average reflectivity of 0.963 in the 0.3 to 2.5 mu m waveband; it achieved an average temperature reduction of 20.1 degrees C lower than the uncoated substrate and 3.2 degrees C lower than the ambient temperature under direct sunlight with an average solar power of 859.3 W/m(2).