Micro-structured polyethylene film as an optically selective and self-cleaning layer for enhancing durability of radiative coolers

Abstract

Passive daytime radiative cooling (PDRC) as a zero-energy cooling technology that reflects most of sunlight and emits infrared thermal radiation to outer space, has attracted much attention. However, most PDRC materials suffer dust accumulation problem during long-term use, seriously detrimental to their cooling performance. Here, we demonstrate a micro-structured polyethylene film fabricated through a scalable hot embossing lithography (named HELPE), enables good superhydrophobic property and therefore excellent self-cleaning performance as a universal protective layer for most PDRC materials. Specifically, the precisely designed three-dimensional periodic micron columns on polyethylene film allow for high water droplet contact angle of 151°, and the intrinsic molecular bindings of polyethylene endow low solar absorption (A = 3.3%) and high mid-infrared transmission (T = 82.3%) for negligible optical impacts on underlying PDRC materials. Taking polyvinylidene fluoride (PVDF) radiative cooler as an example, when covered with the HELPE film the net cooling performance maintains unchanged (7.5°C in daytime and 4.5°C in nighttime) compared to that without HELPE film. After 12days continuous outdoor experiment, none of obvious dust accumulation can be observed on the radiative cooler covered with HELPE film. Our work offers a universal pathway for most PDRC materials toward practical applications with minimal maintenance need.