Improving interactions at the electrochromic polymer-transparent oxide electrode interface using alkyl phosphonic acid modifiers

Abstract

As new synthetic methods for the preparation of solution processable electrochromic polymers are explored, including increasing synthetic scale beyond that of the research laboratory, it is expected that polymer intermolecular and intramolecular interactions will be affected. In this study, we explore the use of four different alkyl phosphonic acids of differing chain lengths as an interfacial treatment on ITO transparent electrodes to improve the polymer-electrode interactions to mitigate the loss of film integrity and resulting electrochromic properties (current density, optical properties, and effective switch rates) during repeated oxidation/reduction and swelling/deswelling of the film. It was found that the phosphonic acid layer allows for a compatibilization of the polarity of the electrode surface with the polymer layer while also improving surface energy uniformity. We evaluated two electrochromic polymers (ECPs), and while a near complete delamination was observed on untreated ITO, film integrity was maintained beyond 25 repeated cycles, with polymer optical contrast maintained at all switching rates when coated onto dodecylphosphonic acid. Additionally, we show that electrochromic polymer film integrity is maintained over a range of film thicknesses. This method can be extended to applications using a variety of solution processable electroactive polymers in contact with metal oxide surfaces.