A minimally coloured dioxypyrrole polymer as a counter electrode material in polymeric electrochromic window devices

Abstract

As the colour palette of solution processable electrochromic conjugated polymers has extended from hues of purple and blue to vibrant colours such as yellow, orange, red, green, and cyan, the need for a counter electrode material that provides charge balance in electrochromic devices, but lends minimal colour, has become increasingly necessary. Nowhere is this more true than in absorptive/transmissive window-type devices where a high level of transparency and near colourlessness is needed in the device bleached state. This argument can also be extended to absorptive/reflective display-type devices where an “on” (coloured) and “off” (bleached or white) state is desired. Here, we report the development of a highly electroactive, low colouration efficiency propylenedioxypyrrole-based polymer that exhibits a low redox potential and minimal colour saturation in both fully neutralised and fully oxidised states. Thin films of the polymer, spray-cast onto transparent electrodes exhibited relatively low composite colouration efficiencies at 555 nm of 35 cm²C⁻¹, contrasted with the the high colouration efficiency ECP-Magenta that exhibits 633 cm²C⁻¹. We further demonstrate the utilisation of this polymer as an effective charge balancing counter electrode material in an absorptive/transmissive window-type device wherein the vibrantly coloured state and highly transmissive, near colourless bleached state of ECP-Magenta is maintained giving an overall device contrast of 61% at 540 nm.