Tri–Band and Dual–Response Smart Windows With Asymmetric Mid–Infrared Emissivity for Year–Round Energy Conservation

Building energy consumption accounts for 20%-40% of global energy usage, with windows among the least energy-efficient components. For this issue, thermochromic smart windows have emerged as a cost-effective, stimuli-responsive approach to enhance building energy performance. However, achieving long-term improvements in the indoor lighting environment remains a significant challenge for this technology. In this work, we developed a dual-responsive hydrogel that combines photochromic and thermochromic functionalities, exhibiting excellent shrinkage resistance, tunable spectral characteristics, and robust color-changing behavior. By integrating these hydrogels with solar selective and indium tin oxide films, we designed a tri-band regulation smart window featuring asymmetric mid-infrared emissivity, capable of simultaneously managing visible and near-infrared light to effectively reduce indoor heat gain and loss. The resulting smart window demonstrated superior spectral performance, with a solar modulation rate (ΔT_sol) of 40.8%, visible light modulation (ΔT_lum) of 61.4%, near-infrared blocking of 71.7%, and an emissivity contrast between the inner and outer surfaces (Δε) of 66%. These outstanding properties translate into annual energy savings of 14.2%-24.4% for curtain wall buildings globally, excluding polar regions. The concept of tri-band dual-response regulation significantly enhances the energy saving potential of smart windows and broadens their prospects for practical application.