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 τv = 31% ↔ 78% to τv = 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 τv = 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.