Interfacial electrosynthesis was initiated using a double potential step In situ UV/vis absorbance measurements were performed using the parallel beam configuration that is illustrated in Supplementary Fig. Conductivity measurements. Ex situ (in plane) conductivity measurements were made on a four-strip conductivity electrode (ALS, Japan, see optical images in Supplementary Fig. ) using the method described elsewhere. The four-strip method was favoured over the conventional four-point-probe setup due to the better contact achieved with the former as the probe tips caused significant tearing of the PEDOT thin film sample.