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Organic Electrochemical Transistors (OECTs) are versatile bio-sensors. However, despite their importance for the field of organic bio-electronics, 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, an extensive experimental and theoretical study is presented to clarify the working mechanism of OECTs.It is observed that drift and diffusion of ionic species inside the channel of the transistor lead to systematic discrepancies between experiments and previous device models. A 2D simulation model is implemented to understand the origins of the shortcoming of previous device models and an improved description of the device operation is proposed. It is found that ions move inside the transistor channel to reach an equilibrium state, in which ions are accumulated at the drain contact. The accumulation of ions leads to the large potential drop, which can be described by a voltage dependent contact resistance. This 2D model is validated by a comparison of the calculation results to measured potential profiles inside the channel as well as to transfer characteristics of the OECT for different channel lengths. The voltage dependent contact resistance is extracted from the output characteristics of the OECT in the linear region using transmission line method (TLM), and shown to follow similar trends as the numeric model. It is furthermore proposed that the voltage dependent contact resistance is responsible for the often observed transconductance peak at a particular gate voltage.The stability of the device is studied using a lateral device geometry. A crosslinked ionic liquid is used instead of an aqueous electrolyte. A hysteresis in the transfer characteristic and a shift in the transfer characteristic due to a constant gate bias stress are observed. It is found that these instabilities are due to slowly moving ions inside the channel and that they can be minimized with an optimized device geometry. Furthermore, the effect of gate bias stress is found to be reversible. In addition to studying the device physics of OECTs, possible applications are successfully demonstrated. It is found that the device can be used to sense e.g. ion concentrations, lactic acid concentrations, or glucose concentrations. Finally, a flexible OECT based on a dielectric elastomer is realized. This is a promising device with very high on-currents and a high transconductance, which opens new applications in the field of soft robotics, flexible electronics, or pressure sensors. |
