Guidelines on Measuring Volumetric Capacitance in Organic Electrochemical Transistors

Organic electrochemical transistors (OECTs) have been widely used in a diverse array of applications. This is primarily attributed to their remarkable ion-to-electron transduction capabilities, which are rooted in the ion uptake by the OECT channel, leading to doping/dedoping processes of the active layer, and are physically characterized by volumetric capacitance (C*). In this study, we present a comparative analysis of the two primary techniques used to measure the volumetric capacitance in the OECTs. Although electrochemical impedance spectroscopy (EIS) is commonly employed for this purpose, it does not allow us to describe the dependency of C* on the drain voltage, thereby overlooking potential doping/dedoping inhomogeneities across the channel. To address this limitation, we explore the use of the transient current technique (TCT), which provides a straightforward approach to evaluate the volumetric capacitance and its relationships with the doping/dedoping profiles across the channel. We concluded that for V G ≫ V D, i.e., V D ≈ 0 V, both EIS and TCT can be considered for accurately measuring C*. However, for V D ≠ 0 V and V G ≈ V D, only TCT offers ways to correctly evaluate C*. Considering the diversity of the OECT community, encompassing researchers from various backgrounds, we aimed to establish comprehensive guidelines for the accurate measurement and reporting of C* in the OECTs, ensuring a standardized and rigorous approach.