Colloidal deposition of colored daytime radiative cooling films using nanoparticle-based inks

In this study, we fabricated easily applicable and processible colored passive daytime radiative cooling (PDRC) films using a solution process with colloidal nanoparticle-based inks. White PDRC films were prepared using hollow silica nanoparticle (H–SiO₂)-based colloidal inks and polymeric binders and spray coating. The films have an average reflectivity of 97.2% and emissivity of 94.3% and their temperature is 6.12 °C lower than the ambient temperature during the daytime at outdoor measurement. We also fabricated colored PDRC films by depositing Cu-based quantum dots (QDs) on white PDRC films. The Cu-based QDs partially absorb light in the visible spectrum, allowing yellow, red, and brown colors, that are highly efficient in preventing heat generation. The absorbed energy is converted into another wavelength and emitted as photons based on the photoluminescence effect. Based on the wavelength conversion, the yellow, red, and brown PDRC films can re-emit powers of 14.06, 28.36, and 43.92 W/m², respectively, resulting in the prevention of heating. The results of outdoor measurements confirm that the temperature of the yellow and red PDRC films decreases by 3.25 and 0.51 °C, respectively, compared with the ambient temperature. Furthermore, we numerically and experimentally determined the daytime cooling performance of two brown PDRC films with different quantum efficiencies. Our results confirm that these easily processable colored PDRC films are more efficient daytime cooling than commercial paint color films on various substrates.