Electrolyte-gated organic transistors (EGOTs) emerged as a foundational platform for organic bioelectronics [1]. Their gating mechanism is governed by the electronic structure and the ion permeability of the organic semiconductor (OSC) [2]. In ion-permeable OSCs, electrolyte ions penetrate the semiconductor bulk and (de-)dope the active material, giving rise to the mixed ionic-electronic conductivity of organic mixed ionic–electronic conductors (OMIECs) [3]. EGOTs based on OMIECs are known as organic electrochemical transistors (OECTs) [4], in which the channel conductivity is modulated by gate-driven ionic diffusion.

The workhorse material for OECTs is poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), a p-type OMIEC operating in depletion mode under positive gate–source biases (VGS). The current-modulation mechanism commonly invoked for PEDOT:PSS relies on an electrochemical (de-)doping process in which monovalent cations (M+) diffuse from the aqueous electrolyte into the OSC microstructure. There, cations compensate the negative charge of the PSS- dopant counterions, with PEDOT+ that is reduced to its neutral state PEDOT0 [5]. As a result, the number of holes in the channel decreases, leading to a drop in the drain current, and the device reaches the OFF state [4].

The present work expands this picture by showing that the chemical identity of the monovalent alkali cation governs the gating effect. We describe the electrolyte/PEDOT:PSS interface as the concurrent interplay of three coupled processes: an acid-base equilibrium, a complexation reaction between the cations and the deprotonated sulfonate groups, and a cation-proton exchange at the protonated sulfonate groups. Solving this system, we derive a closed-form expression for ΔVT versus cation concentration, whose fit to the experimental data provides the cation-specific complexation constant (Kc). Across the alkali series, Kc follows a non-monotonic, V-shaped trend. When plotted against the hydrated mass of the cation, defined as the ion with its first hydration shell upon entering the polymer [6], this trend collapses onto a linear correlation.

Furthermore, we extended the study to the interaction between the PEDOT:PSS and the hydronium ion (viz., in acidic media). We demonstrated that the acid association constant between H3O+ and PSS- is consistent with the complexation-constant sequence established for the hydrated alkali cations. Notably, below pH 2, the hydrogen evolution reaction (HER) sets in, accompanied by the (over)oxidation of PEDOT:PSS channel. The resulting faradaic current markedly reshapes the transfer characteristics, confirming that redox-probe-triggered faradaic reactions at the PEDOT:PSS channel distort the transfer characteristics and transconductance (gm) profiles, as already demonstrated [7].

Taken together, these results broaden the purely faradaic description of OECT operation into a framework that rationalizes ion–polymer interactions, identifying ion hydration as the physicochemical descriptor that governs the complexation between alkali cations and the sulfonate groups of PEDOT:PSS.