Organic electrochemical transistors (OECTs) operate at very low voltages, transduce ions into electronic signals, and reach extremely large transconductance values, making them ideally suited for bio-sensing applications. However, despite their importance and promising performance, an incomplete understanding of their working mechanism is currently precluding a targeted design of OECTs and it is still challenging to formulate precise design rules guiding materials development in this field. Here, it is argued that the current capacitive model doesn't capture the full working mechanism of an OECT, and in particular fails to describe the transient response, its equilibrium states, and the dependence of transconductance on device geometry and applied voltages correctly.
In this dissertation, current scaling laws for transconductance are revised in the light of a 2D device model that adequately accounts for drift and diffusion of ions inside the polymer channel. It is shown that the maximum transconductance of the devices is found at the transition between the depletion and accumulation region of the transistors. Furthermore, the switching is found to be strongly influenced by lateral ion currents. A consistent treatment of ion and hole currents leads to a dependency of time constants on the applied drain potential, and a complex dependency of the response time constants on the detailed device geometry.
In addition to improving the understanding of the device physics of OECTs, studies on the influence of their device geometry are presented. 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 contact-resistance and reproducibility. The origin of this trade-off between speed and reproducibility is discussed, which provides optimization guidelines for a particular application. Finally, biochemical (glucose, glutamate, acetylcholine, dopamine) sensors after functionalization of OECT using catalyst, and/or enzymes are realized. The relation of settling time with the concentration of analyte is presented.
Overall, the dissertation provides an improved understanding of the working mechanisms of OECTs, and facilitates design rules to optimize OECT performance further.