Scalable and Flexible Multi-Layer Prismatic Photonic Metamaterial Film for Efficient Daytime Radiative Cooling

To maintain a comfortable indoor living environment in low latitude or tropical regions, humans consume significant amounts of electrical energy in air conditioning, leading to substantial CO₂ emissions. Passive daytime radiative cooling (PDRC) allows objects to cool down during the daytime without any energy consumption by dissipating heat through the atmospheric transparency window (8-13 µm) to outer space, which has garnered significant attention. However, the practical applications of common PDRC materials are hindered by their poor optical selectivity and high-reflective silver backing. Additionally, the availability of artificial photon emitters with complex structures and excellent performance is also limited by their high cost. Herein, a novel multilayer prismatic photonic metamaterial film without any silver reflector, easily scalable and produced by a roll-to-roll method is demonstrated, which exhibits ≈96.4% sunlight reflectance (0.3-2.5 µm) and ≈97.2% emissivity in mid-infrared (IR) (8-13 µm). At an average solar intensity of ≈920 W m^-2, it is on average 6.8 °C below ambient temperature during the day and theoretically yields a radiative cooling power of 88.9 W m^-2. Furthermore, the film exhibits excellent hydrophobicity, superior flexibility, and robust mechanical strength, providing an attractive and viable pathway for practical applications addressing the pressing challenges of climate and energy issues.