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Organic electrochemical transistors (OECTs) transduce ionic into electric signals, which makes them a promising candidate for a wide range of bio-electronic applications. However, despite their promise, the influence of their device geometry on performance is still not fully understood. Here, two different device geometries—top contact and bottom contact OECTs—are compared in terms of their contact resistance, reproducibility, and switching speed. It is shown that bottom contact devices have faster switching times, while their top-contact counterparts are superior in terms of slightly reduced contact-resistance and increased reproducibility. The origin of this trade-off between speed and reproducibility is discussed, which provides optimization guidelines for a particular application. 

Abstract Organic doping is widely used for defining the majority charge carriers of organic thin films, tuning the Fermi level, and improving and stabilizing the performance of organic light‐emitting diodes and organic solar cells. However, in contrast to inorganic semiconductors, the doping concentrations commonly used are quite high (in the wt% range). Such high concentrations not only limit the scope of doping in organic field‐effect transistors (OFETs), but also limit the doping process itself resulting in a low doping efficiency. Here, the mechanism of doping at ultralow doping concentrations is studied. Doped C₆₀metal‐oxide‐semiconductor (MOS) junctions are used to study doping at the 100 ppm level. With the help of a small‐signal drift‐diffusion model, it is possible to disentangle effects of traps at the gate dielectric/organic semiconductor interface from effects of doping and to determine the doping efficiency and activation energy of the doping process. Doped C₆₀OFETs with an ultralow operation voltage of 800 mV and an excellent on/off ratio of up to 10⁷are realized. The devices have low subthreshold swing in the range of 80 mV dec⁻¹and a large transconductance of up to 8 mS mm⁻¹.