Concentration gradient engineering optimizes the phase transition optical response of W-VO2 thin films for high-performance thermochromic smart windows

The development of thermochromic smart windows is a crucial topic for sustainable and energy-saving buildings. W-VO2 is a promising thermochromic material, as W doping lowers the phase transition temperature (Tc) to near room temperature, thereby improving its suitability for building smart window applications. However, W-doping compromises its luminous transmittance (Tlum) and solar energy modulation ability (ΔTsol). In this study, a W-VO2&PVP/PMMA/W-VO2&PVP multilayer structure was proposed to enhance Tlum and ΔTsol simultaneously. This configuration establishes a VO2 concentration gradient while utilizing PMMA as an interlayer to prevent mutual dissolution between the two VO2 composite layers. The Tlum of the W-VO2&PVP/PMMA/W-VO2&PVP composite film-based smart windows was increased by 16.29% (from 50.33% to 58.53%) compared with the W-VO2&PVP, and the solar modulation (ΔTsol) was increased 5.45% (from 15.76% to 16.62%). This enhancement is primarily attributed to the concentration gradient, which extends the effective propagation depth of light and forms a refractive index gradient, significantly enhancing the Tlum. Meanwhile, the deepened light penetration intensifies the localized surface plasmon resonance absorption, resulting in a higher ΔTsol. This study demonstrates that 0.22%–0.56% concentration-gradient engineering provides an efficient strategy for simultaneously improving Tlum and ΔTsol, offering valuable guidance for the development of energy-saving smart window materials.