Colouration efficiency measurements in electrochromic polymers: The importance of charge density

A poly(3,4-ethylenedioxythiopene) (PEDOT)/polyaniline (PANI) electrochromic device (ECD) was fabricated for the purpose of examining colouration efficiency (CE) as a function of charge density ingress/egress. CE is typically measured in cm2/C with the aim being to produce as large an optical density change as possible with the least amount of charge being consumed. Results indicate that CE is not constant but is highly dependent on charge density insertion and the switching voltage. At a switching voltage of 1.9 V the maximum CEmax was 1186 cm2/C, recorded at 60% of the full optical switch where as the CE95% was 302 cm2/C at 95% of the full optical switch. Furthermore, CEmax varied depending on the switching voltage from a high of 2212 cm2/C at a switching voltage of 1.2 V, 1528 cm2/C at 1.6 V and down to 1186 cm2/C at 1.9 V. The results highlight the fact that the current practice of quoting CE as a single-valued number may not reveal enough detail about the performance of ECDs and polymers.