Date of Award
Doctor of Philosophy (PhD)
George G. Malliaras
Robert R. McLeod
Sean E. Shaheen
Organic electrochemical transistors (OECTs) are hybrid ionic/electronic devices capable of high-gain signal amplification and have shown exceptional performance in numerous applications such as logic circuits, neuromorphic elements, and biosensing platforms. The work in this thesis improves understanding of OECT behavior and demonstrates how to improve OECT performance in several applications. Prior to our work, researchers understood OECTs in terms of standard models for metal-oxide-semiconductor field-effect transistors (MOSFETs). While these models yielded insight about OECT behavior, they fell short of providing accurate quantitative predictions for two reasons. First, organic semiconductors in OECTs are disordered materials and conduct electricity in fundamentally different ways than crystalline semiconductors in MOSFETs. Secondly, ionic charge storage in OECTs endows them with channel capacitances several orders of magnitude greater than that of typical MOSFETs.
We address these differences between OECTs and MOSFETs, and we derive predictive models for OECT behavior. First, we report combined optical and electrical measurements of OECTs, yielding evidence that, unlike the conductivity of crystalline semiconductors, the conductivity of polymer semiconductors in OECTs has a non-linear dependence on charge carrier concentration. We derive a model that explains this behavior and enables up to 125% improvement of signal amplification with OECTs. Next, we address a ubiquitous, yet unexplained characteristic of OECTs – the non-monotonic relationship between signal amplification and applied voltage. We show this is due to material disorder in OECTs, and we explain this behavior with a model that fits experimental data for two different types of OECTs. Finally, we provide a model for the transient response of OECTs. Our model predicts that although OECTs are typically slow because of large channel capacitances, they can respond much faster than the RC time constant. We demonstrate that at a particular voltage, OECTs respond >30 times faster than the RC time constant, reaching steady state in <20 𝜇s.
Altogether, our work enables OECTs to operate at higher speeds and with higher gains, and it allows more accurate interpretation of OECT-based sensor measurements.
Friedlein, Jacob T., "Device Physics and Material Science of Organic Electrochemical Transistors" (2017). Electrical, Computer & Energy Engineering Graduate Theses & Dissertations. 151.