Robust fast-switching black electrochromic windows based on solution-processed n-doped transparent organic conductor

Electrochromic (EC) windows face trade-offs in cost, switching speed, color neutrality, and durability. We present solution-processed n-doped poly(benzodifurandione) (n-PBDF) as a robust organic conductor for black EC windows overcoming these limitations. Utilizing an engineered solvent ink and ultrasonic spray coating, we achieve uniform large-area n-PBDF deposition under ambient conditions without additives. n-PBDF EC electrodes show unprecedented weathering durability (maintaining performance under simultaneous exposure to light (including UV), heat, and humidity for >768 h), addressing a key barrier for organic EC materials. The electrodes exhibit deep black coloration with color neutrality, rapid switching (<2 s), and remarkable cycling stability (>20,000 cycles). Large-area EC devices demonstrate uniform switching performance, confirming scalable fabrication. Building energy simulations of the EC window reveal significant HVAC savings potential across diverse transitional climates. This work establishes n-PBDF as a scalable, high-performance alternative to conventional inorganic EC systems, advancing the viability of solution-processable smart windows for sustainable architecture. Electrochromic materials are widely explored in energy-saving smart windows, yet combining fast switching, neutral black coloration, and robust long-term durability remains challenging. Here the authors report a solution processed n-doped poly(benzodifurandione) affording an electrochromic black window that overcomes these limitations.