Influence of addition of lithium salt solution into PEDOT:PSS dispersion on the electrochemical and spectroscopic properties of film electrodes

The present work has studied electrochemical and optical properties of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) film electrodes drop-casted from commercial PEDOT:PSS aqueous dispersion with preliminary addition and without addition of LiClO4 electrolyte (further denoted as PEDOT:PSS/LiClO4 and PEDOT:PSS). Cyclic voltammetry measurements showed the significant increase in capacitance of PEDOT:PSS/LiClO4 film electrodes in comparison to PEDOT:PSS. Furthermore, the improved charge transport in PEDOT:PSS/LiClO4 films was demonstrated by electrochemical impedance spectra. In situ spectroelectrochemical measurements revealed that preliminary addition of LiClO4 into PEDOT:PSS aqueous dispersion allows to increase amount of free charge carriers (polaron and bipolaron states) in the resulting film during electrochemical oxidation in LiClO4 propylene carbonate solution. This increase was attributed to ion-induced charge screening between positively charged PEDOT and negatively charged PSS in polyelectrolyte structure, which was supported by structural investigations of both types of film electrodes by using FTIR, SEM, and XPS measurements. Charge screening results from a more open structure that allows conformational relaxation of PEDOT molecules during charge transport, which leads to partial separation of oppositely charged PSS and PEDOT molecules and facilitating the increase of electrochemical activity.