Anion-Dependent Doping and Charge Transport in Organic Electrochemical Transistors

We study the effects of different electrolyte anions on the mixed ionic/electronic transport properties of organic electrochemical transistors (OECTs) based on poly(3-hexylthiophene-2,5-diyl). We show that the transport properties depend on the anion present in the electrolyte, with greater source-drain currents resulting from the use of molecular anions such as hexafluorophosphate and trifluoromethanesulfonylimide than from the use of smaller atomic anions such as fluoride or chloride. Using spectroelectrochemistry, we show the maximum doping level that can be achieved in an aqueous environment is also anion-dependent. Furthermore, we find that the average electronic carrier mobility at a given doping level depends on the chemistry of the compensating counterion. We further investigate this dependence by electrochemical quartz crystal microbalance measurements, showing the solvation of the dopant anions within the polymer is drastically different depending on the choice of the anion. Surprisingly, we find that the kinetics of the doping process in these OECTs is faster for bulkier anions. Finally, we use electrochemical strain microscopy to resolve ion-dependent differences in doping and local swelling at the nanoscale, providing further insight into the coupling between local structure and ion uptake. These measurements demonstrate that the identity of the compensating ion and its interaction with the polymer and solvent are important considerations for benchmarking and designing polymer materials for mixed ionic/electronic conduction applications.