Highly reflective multilayer solar reflectors for daytime radiative cooling

Passive radiative cooling can occur even during the day if sunlight is highly reflected, and thermal radiation occurs in the mid-infrared region. To realize such a daytime radiative cooler, a simple silver film is used as a solar reflector due to its high solar reflectance. However, the silver film has a drawback of poor ultraviolet (UV) reflectance, which is crucial for achieving a high-performance daytime radiative cooler. This study demonstrates compensation for the low UV reflectance of the silver film by combining it with a distributed Bragg reflector (DBR) made of silica and tantalum dioxide multilayers with physical separation. The sample exhibit a high solar reflectance of 0.987 and thermal emittance of 0.852. Moreover, adding a polymer on the DBR can improve thermal emittance to 0.926 with a slight decrease in solar reflectance by 0.007. Outdoor measurements conducted in summer in Japan confirm the radiative cooling performance of the samples. The high solar reflectance and broad thermal emissivity are particularly advantageous for above-ambient radiative cooling. Furthermore, concise multilayer designs can be scaled up, paving the way for large-area applications.