Smart window with high solar blocking capability: Integrating electrochromic TiO₂-viologen composite film and thermochromic electrolyte

Yanan He, Hanyu Xu, Fangyuan Sun, Yanqing Tian^*

Department of Materials Science and Engineering, Southern University of Science and Technology, Shenzhen 518055, China

*Email: tianyq@sustech.edu.cn

    Conventional electrochromic (EC) devices suffer from limited optical modulation, especially in the near-infrared (NIR) region. This research reports a dual-responsive electrochromic-thermochromic smart window which can achieve ultra-low optical transmittance (below 1% in the range of 400-2500 nm) by integrating a novel EC film with a thermochromic (TC) electrolyte. The EC film is composed of π-extended viologen derivative (FPB) and TiO₂ nanoparticles with selected NIR absorption. Furthermore, the thermochromic poly(OEGMA-co-MEO₂MA) copolymers with tunable lower critical solution temperatures (32-58 ℃) have reversible transparent-to-opaque transitions. The assembled smart window possesses four switchable working modes: bleached, colored, opaque, and colored-opaque states. The colored EC device performed a high optical contrast of 67.2% at 574 nm. In the opaque and colored-opaque states, the device reduced the indoor temperature by approximately 5.0 ℃ compared with a normal window under simulated solar irradiation, presenting a great prospect for application in building solar thermal management.

Fig. 1 (a) Schematic illustration of the smart window in four working modes：bleached, colored, opaque, and colored-opaque states; (b) Vis-NIR transmittance spectra of the device under the four different working states; (c) Indoor temperature as a function of irradiation time under continuous solar illumination.