Electrochemical Structural Dynamics of Transparent Capacitive Ion Storage Electrodes for Electrochromic Polymer Displays

Transparent capacitive ion storage layers are essential for electrochromic devices (ECDs) because they determine charge balance, electrochemical switching kinetics, optical neutrality, and bistability. However, their electrochemical structural dynamics and their relationship with ECD performance remain insufficiently understood compared with that of the active electrochromic material itself. Here, we investigate nanoengineered indium tin oxide (ITO) nanoparticles and PEDOT:PSS as transparent capacitive ion storage layers for p-type polymer-based ECDs. ITO nanoparticles show robust, optically neutral double-layer capacitance for fast ECD responses with high coloration efficiency, whereas PEDOT:PSS exhibits pseudocapacitive mixed ionic/electronic transport with pronounced optical memory. By combining electrochemical analysis on morphological and crystalline dynamics, we reveal that ITO nanoparticles maintain stable morphology and crystallinity, while PEDOT:PSS undergoes electrochemically induced lamellar densification without surface degradation during electrochemical operation. PEDOT:PSS is further utilized as a capacitive transparent working electrode after polar-solvent vapor annealing, which provides high electrical conductivity and a mixed-conducting capacitive interface adjacent to the electrochromic polymers. When combined with interfacial charge-buffering PEDOT:PSS electrodes and optically passive nanoengineered ITO, the polymeric ECDs exhibit large optical density change, fast coloration and bleaching responses, and high optical memory.