# Transparent metafilms for enhanced thermal regulation in energy-efficient windows

## TL;DR

This paper shows that placing transparent metafilms on the interior of windows improves thermal cooling and energy efficiency by enhancing mid-infrared radiation.

## Contribution

The study introduces a five-layer metafilm structure and demonstrates how its placement affects mid-infrared emissivity and radiative cooling.

## Key findings

- Interior-coated metafilms show high emissivity (0.8) in the 8–14 μm range, enhancing radiative cooling.
- Exterior-coated metafilms have low emissivity (0.01) in the same range, reducing cooling effectiveness.
- Interior placement maintains high visible transmittance and NIR reflectance while improving thermal dissipation.

## Abstract

Transparent metafilms with spectrally selective properties have shown great potential in energy-efficient window systems. Most previous studies focused on optimizing materials and thicknesses to enhance visible transmittance and near-infrared (NIR) reflectance. However, few have considered how the position of the metafilms on the glass affects overall optical and thermal performance, especially in the mid-infrared (MIR) range critical for radiative cooling. In this work, we propose and analyze a five-layer TiO2/Ag/TiO2/Ag/TiO2 structure and systematically evaluate its performance under two typical installation scenarios. Numerical simulations based on the transfer matrix method show that both configurations maintain a high visible transmittance (∼0.88) and an effective NIR reflectance (∼0.98). Notably, a substantial difference is observed within the atmospheric transparency window 8–14 μm, where the interior-coated configuration possesses a high emissivity of 0.8. This value significantly exceeds the average emissivity of 0.01 found for the exterior-coated configuration, thereby resulting in superior passive radiative cooling capability. Moreover, we also compared the net radiative cooling power under the two configurations. These findings reveal that the position of the transparent metafilms critically influences MIR radiation. Coating placement on the interior surface not only maintains favorable solar modulation but also markedly enhances the thermal dissipation. This study offers theoretical guidance and practical insight into the design and implementation of metafilms in energy saving window systems aimed at reducing energy consumption, especially in regions with hot climates.