Organic mixed ionic-electronic conductors (OMIECs) have recently risen as a promising material choice for bioelectronic devices due to their low impedance, soft mechanical properties, and ability to transduce ionic signals to electronic currents. These properties have enabled the development of high-performance devices for electrophysiological recordings, chemical sensing, cell monitoring, and neuromorphic devices. The unique behaviour of OMIECs arises from “electrochemical doping” where ion intercalation through the bulk of the material modifies the oxidation state, and therefore charge carrier concentration, of the conjugated polymer. However, the current understanding of electrochemical gating remains limited. To address this knowledge gap, we use a newly developed hyperspectral differential transmission microscopy technique to probe electrochemical gating in operando.

First, I will present a comprehensive drift-diffusion model to describe mixed ionic-electronic transport in OMIECs using the data collected from in operando microscopy which captures the complex behaviours of electrochemical doping. The proposed model captures several features observed for electrochemical (de)doping of OMIECs including diffusion-like rather than drift-like ion transport and the voltage-dependence of ion kinetics. The results suggest that diffusion of electronic carriers (holes) down electrochemical potential gradients plays a significant role in operation of OMIEC-based devices. In the second part of the presentation, I will show that electrochemical doping consists of two distinct kinetic regimes. Generally, electrochemical doping is assumed to be limited by motion of ions due to their large mass compared to electrons/holes. However, in a state-of-the-art polythiophene, electrochemical doping speeds can be limited by poor hole transport at low doping levels, leading to substantially slower switching speeds than expected. We show that the timescale of hole-limited doping can be controlled by the degree of microstructural heterogeneity, enabling the design of OMIECs with improved electrochemical performance. The framework for understanding the driving forces and kinetics of mixed ionic-electronic transport provides guidance for device design as well as optimization of new materials for improved performance in bioelectronic devices.