Improving Electrochemical Cycling Stability of Conjugated Yellow-to-Transmissive Electrochromic Polymers by Regulating Effective Overpotentials

Yellow-to-transmissive electrochromic polymers (ECP-Yellow) are a primary component in color-mixing-based electrochromic displays. However, like blue LEDs, ECP-Yellow is on the difficult end of the spectrum, and general approaches that stabilize ECP-Yellow over long cycling remain elusive. Here we demonstrate an effective strategy to improve the cycling stability of ECP-Yellow and introduce the general concept of the effective overpotential (ηeff), defined as the difference between the oxidation onset potential and the open-circuit potential (ηeff = EOX,onset – EOC). We show that it is the lower ηeff, instead of popular strategies of lowering EOX,onset alone, that leads to stable cycling of ECP-Yellow. As a proof of concept, classical 3,4-propylenedioxythiophene-alt-phenylene yellow copolymer (PProDOT-Ph) is demonstrated with an unprecedented 3000 full-contrast switching cycles in a three-electrode liquid cell and a record high of 20 000 switching cycles in solid-state devices. The strategy has been successfully extended to other electrochromic polymers that have a high-energy absorption. This work suggests a generalizable method for stabilizing organic mixed ionic–electronic conductors.