Electrochemical QCM-D for insights into organic mixed ionic–electronic conductors and transistors (OECTs)

Abstract

Recent progress in the synthesis of high-performance organic mixed ionic–electronic (semi)conductors (OMIECs) has given impetus to the development of organic electrochemical transistors (OECTs), which are key components for next-generation bioelectronics, neuromorphic devices, and flexible sensors. To ensure operational stability and enhanced performance of OECTs, it is essential to gain insight into the internal processes occurring within the transistor layers during electrochemical doping/de-doping, including mass exchange between the OMIEC and the electrolyte, interaction of the OECT channel with the substrate, its morphological changes, and so on. One method to address this problem in situ is an electrochemical quartz microbalance with dissipation monitoring (EQCM-D). This review aims to demonstrate how EQCM-D can facilitate the study of OMIECs and their interfacial interactions relevant to the operation and further development of OECT devices. EQCM-D allows real-time investigation of ion injection and charge carrier transport, water uptake, morphological changes and swelling (both passive and active), ion-to-electron coupling, the impact of oxygen reduction reactions, adhesion, degradation mechanisms, adsorption, OECT channel and gate electrode functionalisation, thereby shedding light on the mechanisms underlying electrochemical processes in the OMIEC–electrolyte system and OECT operation. This review unifies existing knowledge and offers guidance for effective use of EQCM-D, pointing to future interdisciplinary opportunities in OMIEC and OECT studies.