Coupled Ionic–Electronic Transport in Vertical OECTs: A Combined Experimental and Simulation Study

ABSTRACT Organic electrochemical transistors (OECTs) uniquely couple ionic and electronic transport, enabling high transconductance and low‐voltage operation for bioelectronic applications. While the Bernards–Malliaras model successfully describes lateral OECTs, it fails to capture the coupled space‐ and time‐dependent processes that govern vertical OECTs (vOECTs), particularly for disordered semiconductors and high ion concentrations. Here, we present a 2D numerical simulation that self‐consistently couples ion transport and electronic charge dynamics, validated against experimental data from n‐type poly(benzimidazobenzophenanthroline) (BBL) vOECTs. The simulations reproduce steady‐state and transient characteristics, revealing key physical mechanisms including diffusion‐dominated electronic transport, contact tunneling, energy loss at the semiconductor/electrolyte interface, and gate‐induced ion acceleration via band bending. The simulation also quantifies geometry‐dependent mobility discrepancies and anisotropic ionic transport between vertical and lateral architectures, consistent with recent reports on mixed ionic–electronic conductors. By bridging microscopic mechanisms with experimental observables, this work provides a predictive framework for vOECT operation and offers design guidelines for high‐performance, high‐density bioelectronic systems.