Flexible Polymer Photonic Films with Embedded Microvoids for High-Performance Passive Daytime Radiative Cooling

Passive radiative cooling, a promising strategy for energy savings and sustainability, enables cooling of the ambient temperature by synchronously reflecting sunlight and dissipating heat to the ultracold outer space through the atmospheric transparency window. While designed photonic structures have shown intriguing passive radiative cooling performance, the implementation of such photonic radiators remains challenging due to complex nanoscale lithography/synthesis and rigidity. Here, we experimentally demonstrate a simple and versatile approach of fabricating flexible polydimethylsiloxane radiator thin films with built-in three-dimensional microvoid (inverse-opal-like) arrays for highly efficient daytime radiative cooling. The microvoid-embedded polymer radiator film with tailored spectral responses shows an optimized total reflectivity of ∼93.4% in the sunlight region and a strong infrared emissivity of ∼94.6% within the atmospheric window, respectively. Through such remarkable solar reflection and infrared thermal radiation, the structural polymer radiator achieves subambient cooling of ∼9.8 °C during the night and ∼5.8 °C under direct sunlight in a nonvacuum setup. The three-dimensionally embedded microvoid arrays in our engineered photonic polymer films efficiently backscatter the incident solar radiation and simultaneously enhance the absorption/emissivity in mid-infrared wavelengths, leading to continuous subambient all-day cooling. Our findings provide an effective pathway toward a low-cost, high-performance flexible photonic radiative cooler for passive daytime cooling.