Faradaic charge corrected colouration efficiency measurements for electrochromic devices
We report a new technique for measuring colouration efficiency (CE) for electrochromic polymers and devices. Faradaic charge associated with redox activity leading to chromic change was extracted from the total charge flow during a device switch. Typically, Faradaic charge is extracted from the total charge by first measuring a blank device (i.e. no polymer deposited). This is used as the baseline and subtracted from subsequent measurements with the polymer deposited into the device. The method is open to inaccuracies if the surface area before and after polymer deposition is different or if the inherent capacitance of the device is altered by the addition of the polymer(s). The new technique does not suffer from this drawback as it dispenses with the need to measure blank devices. To investigate the utility of this method a poly(3,4-ethylenedioxthiophene) (PEDOT) and polyaniline (PANI) electrochromic cell was tested. CE measurements at an optical density change of 95% (A = 555 nm) were recorded for the following three conditions: uncorrected, CEUC = 388 cm2/C; background corrected, CEBC = 391 cm2/C and; Faradaic corrected, CEFC = 2173 cm2/C. The result highlights the fact that the CE value of chromic polymers or devices may be higher than what is currently being reported.