Cost-Effective Inorganic Multilayer Film for High-Performance Daytime Radiative Cooling

Inorganic multilayer films for radiative cooling have garnered significant attention due to their exceptional resistance to photothermal degradation. However, the design and fabrication of structurally simple and cost-effective inorganic multilayer films remain challenging due to limitations in material properties and the preparation process. This study develops a structurally simple inorganic multilayer film (Si₃N₄/SiO₂/Al₂O₃/Si₃N₄/Al) for daytime radiative cooling. Instead of the conventional periodic alternation of high and low refractive indices (H-L…H-L), this work proposes a H-L-L-H symmetric multilayer film structure to achieve improved radiative cooling performance. The fabricated multilayer film demonstrates superior radiative cooling properties and lower thickness than that in the current studies using Al as the reflective layer, achieving a solar reflectance of 89.57%, an atmospheric window (8–13 μm) emissivity of 83.41%, and a net cooling power of 63.38 W·m⁻². Under direct sunlight, the multilayer film demonstrated a maximum temperature reduction of approximately 3 °C compared to the reference sample. By employing a thermal treatment process for the Si₃N₄ layer, the poor adhesion between the Al layer and the Si₃N₄ layer is successfully addressed without compromising optical performance. The underlying physical mechanisms are also elucidated. This work provides an effective strategy for developing daytime radiative cooling inorganic multilayer films suitable for large-scale industrial production.