Active W-VO2-based Kirigami-structured films with digital control for energy efficient smart window

Smart window promises to enhance the building energy efficiency by dynamically controlling the indoor solar irradiation. In this study, a new thermochromic smart films for smart window applications were proposed. This film is based on polyvinylpyrrolidone (PVP)-coated tungsten-doped VO2 (W-VO2) and polyurethane acrylate (PUA). It is demonstrated that the PVP-coating technique plays a vital role in improving the optical properties of the film and the dispersion of W-VO2 in PUA. The optimized 3D-printed Kirigami-structured films exhibit favorable tensile properties and improved optical properties. Furthermore, an innovative device has been developed to enable automatic adjustment of the film stretch rate in response to the angle of sunlight, ensuring the smart window's dynamic response to sunlight. The film's solar transmittance varies between 48.50 % and 78.56 % during stretching, accompanied by a solar modulation up to 30.05 %. In the indoor and outdoor demo experiments, the temperature drops by 5.2 °C and 4.8 °C, respectively, compared to a PUA film window. The work developed a new film for energy-efficient smart windows and provides a systematic technological route from materials synthesis, structural optimization, system digital control, to practical demo assessment, which promises to facilitate the development of VO2-based materials and energy-efficient windows.