Synergistic Effects in Multistate Black Electro-Thermochromic Smart Windows for Energy-Efficient Modulation of Solar Heating.

Electrochromic and thermochromic smart windows are mature technologies that offer complementary strategies for regulating solar transmittance. To enhance their overall performance and mitigate key limitations associated with each approach, we present hybrid electro-thermochromic (ETC) smart windows. By integrating a black electrochromic layer composed of three complementarily-colored, redox-matched viologen molecules with a paraffin-particle-based thermochromic film, these hybrid devices deliver multiple synergistic functionalities: voltage-controlled, multimodal operation enabling both passive and active solar modulation; photothermal-assisted activation of the thermochromic layer driven by the strong solar absorption of the colored electrochromic state; efficient regulation of solar heat gain with a ten-fold reduction in electrical energy consumption for the electrochromic component; and near-complete suppression of sunlight transmission by simultaneous activation of both layers thanks to their complementary spectral responses. In energy-saving experiments in hot days, ETC windows achieved a maximum reduction of 8.5°C in solar heat gain relative to clear glass, in agreement with building-energy simulations predicting up to a 23% decrease in annual energy usage in low-latitude regions (e.g., Honolulu). Collectively, these results demonstrate the practical energy-saving potential of ETC smart windows, which offer a versatile and energy-efficient solution for dynamic solar control across diverse climatic conditions.