Unveiling the Hidden Geometry of OECTs: Contact‐Semiconductor Overlap Driven Errors in Volumetric Capacitance

ABSTRACT Organic electrochemical transistors (OECTs) are key mixed‐conducting platforms enabling efficient transduction between ionic and electronic signals. As device architectures advance toward sub‐micrometer dimensions, accurate determination of their performance parameters become critical. Volumetric capacitance (C*), which governs the ionic charging capacity of the mixed conductor and contributes to the transconductance through µC* parameter, is commonly extracted from impedance spectroscopy and transient‐response methods. However, to determine the transistor's volume, most methods use the channel length, width, and film thickness, neglecting contact‐semiconductor overlaps. Here, it is demonstrated that the contact‐semiconductor overlap, typically neglected in OECT analyses, can contribute to the effective volume when the channel length and overlap ratio is <10. Systematic measurements reveal that overlap‐induced geometric errors lead to large, material‐independent deviations in C*. Incorporating the overlap region into the channel volume yields consistent, intrinsic C* values. Transient response shows the same dependency, whereas the transconductance is independent of the overlap. By applying this corrective framework to the widely used materials, this work establishes reliable C* values and provides guidelines for accurate capacitance extraction in future reports. These findings highlight a previously overlooked geometric parameter that impacts OECT characterization and set the bases for reliable benchmarking and developing next‐generation organic electrochemical materials.