Multiband Light Absorption Hydrogel with Adjustable Light Transmission for Smart Window

Thermochromic smart windows dynamically regulate sunlight transmittance in response to temperature changes, thereby suppressing indoor temperature increases. The near-infrared light in sunlight is the main band that causes the indoor temperature to rise. However, the smart window materials in existing research have significant limitations in the absorption of this band. To optimize the modulation efficiency of the infrared band, upconversion nanoparticles (UCNPs) were introduced into the poly(N-isopropylacrylamide) (PNIPAm) matrix. This approach effectively blocks sunlight when natural visible light is obstructed due to an increase in temperature, thereby supplementing indoor brightness with the NIR-to-visible light conversion capability of UCNPs. The 1 wt % microcrystalline composite hydrogel exhibits the best solar spectral modulation performance: the visible light transmittance is 80.1% at room temperature, and the value is reduced to 34.7% after phase transition. In addition, the smart window achieves a temperature drop of 23 °C compared to that of the standard glass window during all-day modulation. It provides a novel approach to developing thermochromic smart windows with broadband solar spectrum modulation capability and excellent durability.