Unveiling the Electrochromic Switch: The π Spacer's Pivotal Role in Fluorinated D-A Copolymer for High Color Efficiency and Energy-Saving Smart Windows

Donor-Acceptor (D-A) copolymers are critical for cost-effective and energy-efficient electrochromic devices (ECDs) and smart windows. In this study, a type of copolymers with dual-introduction of alkyl thiophene as π spacer and fluorine are designed for highly efficient ECDs, contributing to superior color efficiency (CE). Through π spacer modification, the twist angle as well as the optical bandgap of the copolymers (PDT-Th-BT) are minimized via blocking the F…O noncovalent repulsive interaction. ECDs based on PDT-Th-BT1F are achieved with high CE of 1440   cm²/C, along with electrochromic contrast up to 41.6%. This CE is the best yet among ECDs based on fluorination D-A type copolymers according to literature. Furthermore, smart window made from PDT-Th-BT1F contribute a 5 ℃ cooling down compared to the glass in the building models and an energy saving of 631,257.14 kWh in a 26,404.54 m 2 building model, greatly conducive to environmental protection. This work presents a viable strategy for the development of highly efficient ECDs and smart windows.