Understanding the Working Mechanism of Organic Electrochemical Transistors*

The organic electrochemical transistor (OECT) is a key element for the field of organic bioelectronics [Nat. Rev. Mat. 3, 17086, 2018]. OECTs are versatile and can be functionalized for a wide range of analytes, including various metabolites, hormones, or neurotransmitters.
Operation of OECTs is often described by a 1D model based on standard TFT theory [Adv. Func. Mat. 17, 3538, 2007]. Still, although the model is successful in explaining general trends of device operation, a quantitative analysis is still challenging.
In this presentation, limits of current OECT device models are discussed. It is argued that the current model implicitly neglects lateral ion migration inside the transistor channel, which leads to non-equilibrium solution. To address this problem, a 2D numerical simulation is presented that solves the continuity equation of holes and cations consistently. It is argued that cations accumulate at the drain electrode, which leads to an additional potential drop at the contacts. This potential drop can be explained by an additional contact resistance, which increases with applied gate potential. The model predictions are confirmed experimentally by measuring the channel potential and the contact resistance of OECTs.