Ionic-electronic transistors small signal AC admittance: Theory and experiment

The transient behaviour of organic electrochemical transistors (OECT) is complex due to mixed ionic-electronic properties that play a central role in bioelectronics, sensing and neuromorphic applications. We investigate the impedance response of ion-controlled transistors using a model that combines electronic motion along the channel and vertical ion diffusion by insertion from the electrolyte, depending on the chemical capacitance, the diffusion coefficient of ions, the electronic transit time, and the series impedance components due to the electrolyte and its interfaces. Based on transport and charge conservation equations, we show that the vertical impedance produces a standard result of diffusion in intercalation systems, while the transversal impedance (drain current vs gate voltage) contains the electronic parameters of hole accumulation and transport along the channel. We establish the spectral shapes of drain and gate currents and the complex admittance spectra by reference to equivalent circuit models for the vertical and transversal impedances, that describe well the measurements of a PEDOT:PSS OECT. We obtain new insights to the determination of mobility by the relationship between drain and gate currents.