Free-standing, colored, polymer film with composite opal photonic crystal structure for efficient passive daytime radiative cooling

The daytime radiative coolers offer a sustainable way to passively cool objects under direct sun exposure with the merits of zero energy consumption and zero pollutant generation. However, the ivory or silvery glare in the color of these coolers consequently restricts their practical applications, particularly for functional and aesthetic situations. Here, we present a free-standing, colored, polymer film with composite opal photonic crystal structure that simultaneously achieves color and efficient passive daytime radiative cooling. The polymer optical film composed of flexible inexpensive polydimethylsiloxane (PDMS) thin film with embedded three-dimensional (3D), inverse-opal-like TiO2 skeleton nanostructures behaves as a composite opal photonic crystal structure within the visible regime for reflecting specific visible light to generate desired full colorization based on Bragg diffractions. Meanwhile, the embedded TiO2 skeleton nanostructures can also efficiently scatter the rest of unwanted incoming visible light, resulting in high total reflectivity in the sunlight band. Importantly, all developed colored optical films (e.g., red, green, and blue) exhibit strong infrared emissivity both within the second (8–13 μm) and the third (16–26 μm) atmospheric window regions. Consequently, all colored optical films could realize desired subambient temperature drop, and achieve maximum subambient cooling of ∼4.1 °C under strong solar radiation of 854 W m−2. The present work provides a conceptually generic route for both coloration and efficient radiative cooling in a simple and scalable way.