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

Abstract

Electrochromic (EC) smart windows offer significant potential for building energy efficiency through dynamic solar transmittance regulation. However, current EC technologies face a critical challenge in achieving complete optical blocking. Here, we present a dual-responsive device capable of achieving 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, composed of TiO₂ nanoparticles and carefully designed extended viologen derivative with phosphoric moieties (FPB), could enable selective solar blocking through reversible color changes. Furthermore, we synthesized ethylene glycol-containing polymethacrylates as TC copolymers (O_xM_y) for cooperative regulation of solar transmittance, which exhibited a wide tunable phase transition temperature range (32–58 °C) to accommodate diverse practical requirements. The resulting device demonstrated four distinct working modes including bleached, colored, opaque, and colored-opaque states. Experiments on house model revealed that the smart window in its colored-opaque state exhibited superior thermal management capabilities, achieving a 5.0 °C reduction in indoor temperature compared to conventional windows. Notably, the device could also function as a battery during operation, generating energy that can be utilized for indoor lighting. In summary, this work provides a promising new approach for developing next-generation smart windows.