Correlating Mid-Infrared Absorption With Radiative Efficiency in Passive Daytime Radiative Cooling Systems

Passive daytime radiative coolers (PDRCs) are physical systems capable, on the Earth's surface, of achieving a net irradiation of energy toward outer space even under direct solar illumination. In order to achieve this amazing feat, PDRCs require tailored photonic properties all the way from the near-ultraviolet to the mid-infrared (mid-IR). In particular, PDRCs require high reflectivity over the solar-radiation range and selective absorbance (or equivalently emittance) in the mid-IR atmospheric transparency window. In this study we fabricated PDRCs composed of silica-loaded poly(4-methyl-1-pentene) thin films hot-pressed onto Ag-coated stainless-steel slabs, and we correlated the photonic response of these systems with their thermodynamic radiative efficiency. We clearly show that the presence of a strong emittance across the mid-IR range corresponds to a higher radiative power around room temperature, thereby establishing a clear quantitative criterion for PDRC assessment.