One-dimensional thin-film structure for efficient radiative cooling window

Abstract

Passive radiative cooling has been actively studied for achieving space cooling without the need for energy-intensive cooling systems. To achieve efficient passive radiative cooling, it is crucial to reflect sunlight, which can serve as a heat source. Additionally, having high thermal emissivity within the mid-IR range ($3-24 \mu m$), especially in the atmospheric window ($8-13 \mu m$), is essential for effectively dissipating heat to the surrounding environment. However, most of the previous structures are not suitable for use in windows due to their low transmittance in the visible range ($400-700 nm$). The radiative cooling window with high transmittance in the visible range has recently drawn attention. In this work, we theoretically designed the one-dimensional thin-film photonic structure for transparent passive radiative cooling windows. This structure consists of tantalum pentoxide ($Ta_2O_5$), silicon dioxide ($SiO_2$), and silver (Ag) multilayers. In order to improve the mid-IR thermal emission, we added a layer of polydimethylsiloxane (PDMS) to the structure. We simulated the optical properties of our structure using a transfer matrix method (TMM). Our simulations demonstrated that our structure exhibits high a visible transmittance ($85 %$), a high near-IR reflectance ($88 %$), and a high mid-IR emissivity ($95 %$). To validate our simulations, we fabricated radiative cooling windows using lithography-free processes, including electron-beam evaporation, thermal evaporation, and spin coating. Subsequently, we conducted daytime outdoor temperature experiments to experimentally demonstrate the cooling efficiency of our structure. The results confirmed that our structure can lower indoor temperatures by approximately $3^\circ C$ compared to conventional glass windows.