Subthreshold Behavior in Organic Electric Chemical Transistors—Beyond the Thermodynamic Limit

Organic Electrochemical Transistors (OECTs) are important building blocks of next generation biosensing and neuromorphic circuits. By leveraging the volumetric ionic-electronic coupling inherent in Organic Mixed Ionic Electronic Conductors (OMIECs), OECTs produce low-voltage high-gain devices that can directly transduce and amplify ionic biotic signals, or exploit ionic-electronic coupling to induce memory behavior for synaptic-like behavior, amongst other applications. This has made OECTs often the device of choice in neuromorphics, biohybrid systems, and smart sensing. Improving the understanding of the physics of OECTs and OMIECs has enabled rapid advances. Most interest has focused on device gain (transconductance) or material mixed transport figure of merit (μC*); with contact resistance, energy levels/threshold voltages, and disorder also receiving study and attention. Much less attention has been given to OECT subthreshold behavior, that is the degree of abruptness of the initial turn-on of OECT electrical conductance. Neglect of this topic is unfortunate, as study of subthreshold behavior is important for fundamental understanding of OECT functioning, as well subthreshold being a regime of device operation that manifests ultra-low power and ultra-high device stability. Understanding and rationalization of OECT behavior often relies on making analogies to organic field effect transistor (OFET) physics. OFET models, following their MOSFET predecessors, impose a strict thermodynamic lower limit on the subthreshold swing (S.S.) of 60 mV/dec at room temperature. This analogy has been so widely held that a S.S. approaching 60 mV/dec in OECTs has often been interpreted as evidence of device “ideality”. However, the applicability of such a limit to OECTs is not self-evident.<br/><br/>We have investigated a series of oligoethylene glycol bithiophene chalcogenophene alternating copolymer OMIECs as channel materials in OECTs. Remarkably we found that the tellurophene-bithiophene copolymer OMIEC based OECTs broke the apparent thermodynamic limit, manifesting a subthreshold slope of 44 mV/dec at room temperature, well below the widely accepted 60 mV/dec limit at room temperature. We compared this OMIEC with analogous polymers with varied chalcogenophene heteroatom substitution using AC and DC OECT measurements, spectroelectrochemistry, and in situ and ex situ X-ray scattering. These investigations revealed an apparent structural origin to the anomalously low S.S. (and concomitant improved electronic transport characterizations), namely highly ordered and oriented three-dimensional crystallites that persist even during electrochemical cycling in aqueous environments. The thorough characterization of the OMIEC series reveal the delicate balance between OMIEC microstructure, subthreshold behavior, frontier molecular orbital energy level, OECT threshold voltage, bulk electronic transport, and bulk ionic transport. This elucidates clear design rules to engineer desire subthreshold behavior, including achieving S.S. below the thermodynamic limit. The theoretical sources of an anomalously low S.S. below the thermodynamic limit are considered, including the electrochemical D.O.S., charge density dependent electronic carrier mobility, and the impact of non-Faradaic gates. Further, a meticulous review of the literature reveals inadvertent reports of S.S. below 50 mV/dec going back multiple decades. These results demand a reconsideration of the fundamental device physics of OECTs, show the unique ways OECTs exceed the performance of more traditional electronics, and open new avenues for OMIEC based OECTs to be leveraged in neuromorphic, biohybrid, and smart sensing applications.