Mass and Charge Transport Kinetics in an Organic Mixed Ionic–Electronic Conductor

Understanding the mass uptake, morphological changes, and charge transport in organic mixed ionic–electronic conductors (OMIECs) during device operation is crucial for applications in energy, actuators, and bioelectronics. In this work, we quantify the chemical composition and rheological properties of a model OMIEC material, acid-treated poly(3,4-ethylenedioxythiophene)/polystyrene sulfonate (PEDOT/PSS), during electrochemical cycling using electrochemical quartz crystal microbalance (EQCM) and elemental analysis techniques. We find an asymmetry in the de- and redoping mass transport kinetics and attribute this process to subsecond ion migration and slower ion reorganization. Furthermore, the kinetic constants from the EQCM measurements are compared to those from organic electrochemical transistors and from changes in structural packing by normalizing the corresponding RC time constants across experiments. This multimodal investigation allows us to deduce a sequence of mass, charge, and structure kinetics in OMIEC materials during the de- and redoping processes. The kinetics of processes in acid-treated PEDOT/PSS in response to step voltages can be clustered into three main subprocesses, namely, fast polarization, charge carrier population kinetics and macroscale transport, and slow relaxation. These findings provide a basis for future OMIEC design by determining the factors that affect response time and short-term stability of OMIEC devices.