Phase Separation in Organic Mixed Ionic-Electronic Conductors and Their Interface with Bioelectronics—A Multiphysics Approach

Organic mixed ionic-electronic conductors (OMIECs) are ideal material candidates for bioelectronics owning to their unique capability of mixed conduction and biocompatibility. When integrated to organic electrochemical transistors (OECTs), they enable interaction between electrochemical devices and living matter, such as transmitting and accepting neuron signals and monitoring sweat as a skin sensor. Of all types, two-phase OMIECs exhibit exceptional performance due to their high stretchability and balanced ionic-electronic conduction. However, the electron-conducting phase may segregate from the ion-conducting phase in a two-phase OMIEC, changing the conducting path and eventually leading to degraded performance and dysfunction of the devices. In this work, we study the mechanics and electrochemistry of a two-phase OMIEC channel undergoing phase separation in an OECT model. The computational model captures the concurrent transport of charge carriers, mechanical swelling, and phase separation in the OMIEC and replicates the transfer curves of an OECT which agree well with the experiments. More specifically, we reveal the origin of the volumetric capacitance as the accumulation of charge carriers at the two-phase interfaces. We examine the parametric space to elucidate experimental observations such as the molecular size-dependent conductivity and substrate-dependent phase separation. The swelling behavior and the transfer curves of OECTs under stretched, free, and constrained states are compared, demonstrating the effects of deformation on the phase dynamics and the electron-conducting behavior. This work provides a theoretical basis for the mechanics and electrochemistry of two-phase OMIECs for biological interfaces and sheds light on synthetic and processing principles.