A Thermo‐Responsive Refractive Index‐Matching Film for Solar Modulation of Smart Windows

ABSTRACT The architectural trend of incorporating large‐area glazing, while enhancing natural lighting and aesthetics, significantly increases building energy consumption due to its poor thermal performance. This motivates research on smart windows for photothermal modulation. However, passive thermo‐responsive methods lack precise, temperature‐dependent solar control and often require a design trade‐off that sacrifices essential daylighting for greater modulation. This paper introduces a novel polymer composite film for thermo‐responsive smart windows using dynamic refractive index‐matching. The film modulates Mie scattering by exploiting the temperature‐dependent refractive contrast between a polydimethylsiloxane (PDMS) matrix and embedded calcium fluoride (CaF 2 ) particles, enabling a monotonic variation in solar transmittance with ambient temperature. The composite structure was optimized through systematic optical simulations. The resulting simple blade‐coated film achieves a substantial solar‐band transmittance modulation (Δ T sol ∼ 18%) while maintaining high luminous transmittance ( T lum > 65%) for natural daylighting. Both indoor and outdoor temperature tests have demonstrated the film's effective solar modulation and radiative sky cooling capabilities. Furthermore, the film's simple fabrication method shows potential for large‐scale industrial application in building energy conservation.