Energy-Efficient Stacks—Covellite (CuS) on Polyethylene Terephthalate Film: A Sustainable Solution to Heat Management

Regulating the temperature of buildings has the highest expenditure of energy (35–40%); thus, transparent heat reflectors (THRs) have been at the forefront of passive cooling technology due to its ability to reflect near-infrared (NIR) wavelengths without compromising optical transmission. Current designs like metallic thin films and dielectric–metal–dielectric are expensive and face complications with material incompatibility, oxidation, and interdiffusion, which hamper their performance. A cost-effective THR using a single-layer coating has not yet been achieved due to the susceptibility of metals to oxidation. In this work, a layer of CuS was fabricated onto flexible polyethylene terephthalate (PET) via radio frequency magnetron sputtering (RFMS), a high throughput method known for its conformal large-area fabrication. The 150 nm-thick CuS-PET films demonstrated a visible transmission T₅₅₀ ₙₘ of 64.1% and NIR reflectance of 50% that resulted in 8 °C passive cooling. Additionally, wetting the surface resulted in a total of 11 °C of passive cooling due to its hydrophilicity, giving each household an estimated 20% of cost saving per year. Moreover, CuS-PET films are stable in ambient conditions due to instantaneous surface passivation, preventing it from being oxidized and retaining its performance even after three months. Through the optimization of the RFMS fabrication of CuS-PET films, the realization of a cost-effective, single-layer, flexible, and industrially scalable THR is possible.