Multimodal Probes of Charge Transport in Organic Mixed Ionic-Electronic Conductors—Interplay of Polymer Structure and Counterion Chemistry

Organic semiconductor-based mixed ionic-electronic conductors are being explored in applications from biosensors based on electrochemical transistors, to energy storage and neuromorphic computing. In many of these applications, understanding the fundamental factors controlling transport of coupled ion/electronic polaron pairs is key to achieving high performance (where performance may mean large currents, or fast switching, or both). Here, we combine multimodal and in situ probes of polymer structure and electrochemistry to better understand the interplay of polymer side-chain chemistry, polymer morphology, and ionic and electronic charge transport in organic semiconductors. Using a combination of <i>in situ, in operando</i> scanning probe measurements to probe morphology and swelling in the presence of different counterions, we study both seemingly hydrophobic n-type material and the more familiar hydrophilic ethylene-glycol-side-chain p-type materials, such as ethylene-glycol-side-chain polythiophenes. Although the initial hydration state of the polymer can have a profound effect on the initial electrochemical doping mechanism – with p-doping occurring in some cases by expulsion of cations rather than the typical picture of anion injection to compensate hole polarons – we generally find faster kinetics and higher volumetric capacitances when using larger, more polarizable (more chaotropic), and when the organic semiconductors have higher densities of ethylene glycol side chains. We compare and contrast this behavior with that observed for high-performance n-type materials.