A UV‐Reflective Organic–Inorganic Tandem Structure for Efficient and Durable Daytime Radiative Cooling in Harsh Climates

Abstract Radiative cooling shows great promise in eco‐friendly space cooling due to its zero‐energy consumption. For subambient cooling in hot humid subtropical/tropical climates, achieving ultrahigh solar reflectance (≥96%), durable ultraviolet (UV) resistance, and surface superhydrophobicity simultaneously is critical, which, however, is challenging for most state‐of‐the‐art scalable polymer‐based coolers. Here an organic–inorganic tandem structure is reported to address this challenge, which comprises a bottom high‐refractive‐index polyethersulfone (PES) cooling layer with bimodal honeycomb pores, an alumina (Al 2 O 3 ) nanoparticle UV reflecting layer with superhydrophobicity, and a middle UV absorption layer of titanium dioxide (TiO 2 ) nanoparticles, thus providing thorough protection from UV and self‐cleaning capability together with outstanding cooling performance. The PES‐TiO 2 ‐Al 2 O 3 cooler demonstrates a record‐high solar reflectance of over 0.97 and high mid‐infrared emissivity of 0.92, which can maintain their optical properties intact even after equivalent 280‐day UV exposure despite the UV‐sensitivity of PES. This cooler achieves a subambient cooling temperature up to 3 °C at summer noontime and 5 °C at autumn noontime without solar shading or convection cover in a subtropical coastal city, Hong Kong. This tandem structure can be extended to other polymer‐based designs, offering a UV‐resist but reliable radiative cooling solution in hot humid climates.