Thickness Variation of the Polymer Electrode in Hybrid Flexible Electrochromic Devices: Impact of Charge Balancing on Performance

Polymeric electrochromic devices (ECDs) can be used in a wide range of applications like smart windows or displays. For ideal electrochromic performance and stability, the charge balancing in these devices is crucial. In this study, roll‐to‐roll slot‐die coating is used to prepare three film thicknesses of the in situ polymerized sidechain‐modified poly(3,4‐ethylenedioxythiophene) derivative PEDOT‐EthC6 on indium tin oxide coated polyethylene terephthalate, ranging from approx. 100 nm to 170 nm. The PEDOT‐EthC6 electrodes show transmittance modulations varying from τ v = 31% ↔ 78% to τ v = 13% ↔ 68%. Constant current constant voltage measurements reveal that the volumetric charge density increases with film thickness from 0.15 C cm −3 to 0.18 C cm −3 . The three films are used in hybrid ECDs with Ni oxide as the counter electrode, where the PEDOT‐EthC6 electrode is either under‐dimensioned, matching, or over‐dimensioned. The latter shows the largest transmittance modulation of τ v = 12% ↔ 55%. Using three‐electrode cells, it is found that the over‐dimensioned PEDOT‐EthC6 electrode limits the potential range of this electrode, preventing side reactions. Simultaneously, the potential window is widened at the Ni‐oxide electrode, enabling it to be fully switched. The ECD shows good cycling stability over 5000 cycles at 25 °C and 65 °C.