Radiative cooling materials allow spontaneous temperature drop by a net positive radiative heat transfer to deep space and offer a sustainable and energy-saving venue of cooling even under sunlight. To achieve daytime radiative cooling, it is important to minimize absorption in the solar spectrum and maximize the emission in the atmospheric transparent window (8 to 13 µm). Such materials typically have either a white or silvery appearance with maximized solar energy reflection. However, color is often desirable to improve visual appearance in realistic applications. Coloration is typically provided by dyes and pigments that partially absorb within visible range, leading to an inevitable heating-up of the material. In this work, we simultaneously demonstrate striking color and sub-ambient radiative cooling with self-assembled Cellulose Nanocrystal (CNCs)-based films. By modulating light interference at selective wavelengths with chiral nematic structures in CNC, we circumvent this dilemma between high reflection and coloration. The obtained CNCs films feature a low solar absorption of 2.8-3.8% but a high emittance within the atmospheric transparent window of ~91-92% that minimizes the solar heating while allowing significant heat radiation into the deep space. To further minimize solar transmission towards high-performance cooling, composite thin films composed of colored CNC films and highly scattering porous ethylcellulose base layer with archetypical whiteness were demonstrated. At the interface of CNC and ethylcellulose, the incident light is diffusively backscattered while the color rendered by the structure of CNC is retained. Daytime sub-ambient cooling of 2-4 °C was demonstrated in the field test for both suspended CNC films and composite films. Finally, we demonstrate the potential scalability of the process by manufacturing meter-size composite films via roll-to-roll deposition that empowers a scalable, low-cost, and sustainable solution for the colored radiative cooling system.

Colored radiative cooler by self-assembled cellulose nanocrystal

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