Scalable flexible electrochromic devices with WO3-based ion storage layer for enhanced optical modulation and stability

• WO 3 serves as a protective layer and ion storage in flexible electrochromic devices. • m-PEDOT/WO 3 structure improves optical modulation, switching times and efficiency. • Devices maintain better optical performance and stability after 500 redox cycles. • Flexible large area (1170 cm 2 ) electrochromic devices demonstrate scalability. Photo-electrochemical stability, rapid switching time, and mechanical durability are key factors in achieving high-performance flexible and large-area electrochromic (EC) devices. However, the typically used counter electrode of indium tin oxide (ITO) are prone to degradation after repeated redox cycling and reiterative bending stress. This study introduces a heterogeneous structure that incorporates modified poly(3,4-ethylenedioxythiophene) (m-PEDOT) as the organic electrochromic material and a tungsten trioxide (WO 3 ) film in the counter electrode of ITO, which serves as both an ion storage layer and a protective layer. The WO 3 -based EC devices achieved a maximum optical modulation (ΔT) of 51 % at 631 nm, significantly higher than the 17 % (ΔT) observed in devices without WO 3 . Additionally, the device demonstrated fast switching times of 1.2 s for bleaching and 6.3 s for coloring, with a high coloring efficiency of 474 cm 2 /C. Durability tests revealed a 98 % recovery in optical modulation after 500 cycles, and strong mechanical stability after 100 bending cycles. The fabrication of a 1170 cm 2 flexible device confirms the practical viability of this approach, enhancing both electrochemical and optical properties for large-scale applications such as smart windows.