Hysteresis in Organic Electrochemical Transistors: Distinction of Capacitive and Inductive Effects

Abstract

Organic electrochemical transistors (OECTs) are effective devices for neuromorphic applications, bioelectronics, and sensors. Numerous reports in the literature show persistent dynamical hysteresis effects in the current–voltage curves, attributed to the slow ionic charging of the channel under the applied gate voltage. Here we present a model that considers the dominant electrical and electrochemical operation aspects of the device based on a thermodynamic function of ion insertion. We identify the volume capacitance as the derivative of the thermodynamic function, associated with the chemical capacitance of the ionic–electronic film. The dynamical analysis shows that the system contains both capacitive and inductive hysteresis effects. The inductor response, which can be observed in impedance spectroscopy, is associated with ionic diffusion from the surface to fill the channel up to the equilibrium value. The model reveals the multiple dynamical features associated with specific kinetic relaxations that control the transient and impedance response of the OCET.

Transient Response and Ionic Dynamics in Organic Electrochemical Transistors

“…Despite these insights, few device models have comprehensively accounted for these factors in describing OECTs hysteresis phenomena. Bisquert et al identified four distinct relaxation phenomena that contributing to the complex dynamics of hysteresis in OECTs: time constants related to electronic and ionic currents, vertical and lateral ion diffusion, and the effects of electrolyte resistance and film capacitance [ 54 ]. They also distinguish between capacitive and inductive hysteresis, associated with ion diffusion in the organic film, which manifest as counterclockwise and clockwise loops, respectively, in the transfer current [ 54 ].…”

“…Bisquert et al identified four distinct relaxation phenomena that contributing to the complex dynamics of hysteresis in OECTs: time constants related to electronic and ionic currents, vertical and lateral ion diffusion, and the effects of electrolyte resistance and film capacitance [ 54 ]. They also distinguish between capacitive and inductive hysteresis, associated with ion diffusion in the organic film, which manifest as counterclockwise and clockwise loops, respectively, in the transfer current [ 54 ]. Koch et al reproduced the forward–backward hysteresis curves by developing a drift–diffusion simulation model that incorporates an incomplete ionization approach, leveraging Poisson–Boltzmann statistics for accurate simulation of charge densities, and electrostatic properties of OECTs.…”