Unraveling The Bistability of Solid-State OECTs

Organic electrochemical transistors (OECTs) are excellent building blocks for a new class of neuromorphic devices. Owing to a peculiar switching mechanism based on a redox reaction, the ions of an electrolyte couple with the electronic charge carriers of the active material and unique properties absent in other types of thin-film transistors emerge. Among these is often a pronounced switching hysteresis, an attractive feature widely applied as a non-volatile memory. However, its physical origin has hardly been studied.<br/><br/>Using a specific electrolyte with the benchmark channel material PEDOT:PSS, we are able to report on OECTs of significantly enhanced hysteresis, as well as an off-state lowered to the regime of nA. Both of these properties, previously sought through complex material modifications, can be achieved here through the simple use of the commercially available ionic liquid [EMIM][EtSO₄], which does not require any further material processing. Based on a thermodynamic framework, we can describe this hysteretic behavior as a bistability and trace it back to the dominance of enthalpic effects over entropy. We substantiate this concept with experiments demonstrating the theoretically predicted scenario in which the subthreshold swing exhibits a non-monotonic dependence on temperature. This behavior is contrary to what is expected from classical transistor theory. Going further, we set out to investigate the microscopic origin of the bistability, that is, the interactions underlying the enthalpic effects. To this end, we have turned to spectroscopic methods, including <i>ex situ</i> XPS, GIWAXS, UV-Vis-NIR and Raman spectroscopy, which allow us to decipher the unique interplay between PEDOT:PSS and [EMIM][EtSO₄]. We find that the ionic liquid introduces significant changes to the channel material, as it removes excess dopant and induces an ion exchange, both of which contribute to the device’s exceptional electrical performance. To further study this performance <i>in situ</i>, we employ spectroelectrochemistry, which reveals a unique de-/doping behavior compared to other electrolyte systems, and which we can quantify by fitting the spectral signals with a vibronic model. As such, we can draw a conclusive picture of the OECT performance, ranging from the broad ensemble-picture of thermodynamics to the narrow view on microscopic interactions underlying the bistability.<br/><br/>We are convinced that this work will not only expand the understanding of OECT physics, but also provide simple access to devices of remarkable performance based on the mechanisms revealed herein.