Organic Electrochemical Transistors: Current Debates and Open Questions

Organic electrochemical transistors (OECTs) based on organic mixed ionic-electronic conductors (OMIECs) are attracting growing interest for bioelectronics and neuromorphic computing because of their low-voltage operation, high transconductance, and coupling between ionic and electronic transport. In this talk, I will discuss two fundamental questions that challenge conventional descriptors used for evaluating OECT materials and devices.

First, I will revisit whether volumetric capacitance (C*) truly represents the doping capacity of OMIECs. Conventional models assume that all injected ions generate electronic charge carriers, whereas non-Faradaic processes and inefficient ion–backbone coupling can make this assumption invalid. We therefore introduce doping efficiency (η) and effective volumetric capacitance (Ceff = Cη) as descriptors of the true electrochemically generated carrier density. Using a side-chain-cleavable polymer, we show that reducing steric and electrostatic interference improves ion–backbone accessibility, drives η toward unity, and simultaneously enhances charge mobility, thereby overcoming the conventional mobility–capacitance trade-off.

Second, I will examine whether the widely used memory window provides an intrinsic measure of ion trapping in neuromorphic OECTs. Because memory window strongly depends on sweep range and scan rate, we propose trap efficiency (ηtrap), defined by the fraction of injected ions retained in the channel, as a measurement-condition-independent descriptor. Validation across operating conditions and polymer–electrolyte systems shows that ηtrap enables quantitative comparison of ion trapping and provides a framework for correlating ion retention with neuromorphic characteristics. Together, these results highlight the importance of replacing phenomenological device metrics with physically grounded descriptors for the rational design of high-performance OMIECs and neuromorphic OECTs.