Synergistic LSPR and interfacial built-in electric field in an Au/WO ₃ film for a high-performance flexible electrochromic smart window

Electrochromic (EC) technology enables reversible optical modulation under external electric stimuli, emerging as a cornerstone for next-generation smart windows and low-power displays. However, inorganic EC materials, such as tungsten oxide (WO₃), generally suffer from slow response speed, low coloration efficiency and poor flexibility arising from dense microstructure and brittleness, which greatly constrain their practical deployment. Herein, we report an Au nanoparticle (AuNP)/WO₃ heterofilm that synergistically harnesses the localized surface plasmon resonance (LSPR) effect of AuNPs and the built-in electric field at the AuNPs/WO₃ heterointerface. This synergy greatly boosts the electron/ion transport kinetics and light-matter interaction, delivering an exceptional EC performance with ultrafast switching (2.0 and 0.7 s for coloration and bleaching, respectively), remarkable coloration and bleaching efficiencies of 187 and 680 cm² C^-1, respectively, as well as 90% optical contrast retention after 1000 cycles. A 6 cm × 6 cm flexible EC smart window assembled with this heterofilm exhibits dynamic transmittance modulation and excellent mechanical robustness, with negligible performance loss after 100 bending cycles. This work provides a facile, scalable plasmonic-interface engineering route to overcome the intrinsic limitations of inorganic EC materials, extendable to other transition metal oxide systems for high-performance flexible optoelectronics.