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

ABSTRACT 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.