Spectrally Selective Smart Window with High Near-Infrared Light Shielding and Controllable Visible Light Transmittance

Smart windows with high near-infrared (NIR) light shielding and controllable visible light transmittance are highly sought after for cooling energy saving in buildings. Herein, we present a rationally designed spectrally selective smart window which is capable of shielding 96.2% of the NIR irradiation from 800 to 2500 nm and at the same time permitting acceptable visible light (78.2% before and 45.3% after its optical switching) for indoor daylighting. The smart window synergistically integrates the highly selective and effective NIR absorption based photothermal conversion of cesium tungsten bronze (Cs_xWO₃) with the transparent thermoresponsive poly(N-isopropyl acrylamide) (PNIPAM) microgel-polyacrylamide (PAM) hydrogel. Optical switching of the smart window is a direct result of the phase transition of PAM-PNIPAM hydrogel, which in turn is induced by the photothermal effect of Cs_xWO₃ under sunlight irradiation. The smart window exhibits fast optical switching, shows long-term operational stability, and can be made highly flexible. Under the experimental conditions in this work, the indoor temperature with the smart window is ∼21 °C lower than that with a regular single-layered glass window under one sun irradiation. The smart window design in this work is meaningful for further development of effective smart windows for energy saving in the build environment.