Bistable Organic Electrochemical Transistors: Thermodynamics and Symmetry Breaks

Lukas Matthias Bongartz

AutorIn

Lukas Matthias Bongartz

Titel

Bistable Organic Electrochemical Transistors: Thermodynamics and Symmetry Breaks

Zitierfähige Url:

https://nbn-resolving.org/urn:nbn:de:bsz:14-qucosa2-977878

Erstveröffentlichung

2025

Datum der Einreichung

16.01.2025

Datum der Verteidigung

27.06.2025

Abstract (EN)

The demand for computing power is growing at an accelerating rate. Evolving paradigms such as artificial intelligence and big data analytics require computer chips with ever-increasing efficiency, while conventional transistor devices approach their physical limits. This situation has sparked an extensive search for alternative hardware platforms. In this context, organic electrochemical transistors (OECTs) have emerged as a particularly interesting device class. OECTs couple the dynamics of electronic and ionic charge carriers and, being functionally similar to biological neurons, are intriguing for both designing neuromorphic computing hardware and directly interfacing with biological substrates. Yet, the traditional understanding of OECTs draws heavily on frameworks from conventional field-effect transistor physics, which falls short in capturing their complex functionalities. This includes, among others, the hysteretic behavior observed in the switching characteristics of many systems. While considered an undesirable artifact in conventional electronics, such state dependence can serve as a memory functionality in neuromorphic computing. Here, I present a solid-state OECT system that, besides exhibiting exceptional transistor metrics, displays a pronounced and reproducible hysteresis in its transfer curve. This feature is studied extensively as a function of various factors, revealing it as a phenomenon beyond what traditional OECT models can explain. Challenged by this circumstance, the OECT operation is described using a thermodynamic framework. This approach reveals that bistable device operation can emerge from the interplay of enthalpic and entropic driving forces, which is experimentally verified on the solid-state system. Among other findings, I derive and demonstrate a non-monotonic progression of the subthreshold swing with temperature, representing a phenomenon diverging from conventional Boltzmann statistics. Building on this reasoning, I realize the functionality of a comparator circuit in the form of a single-OECT Schmitt trigger, leveraging the intrinsic dynamics of the OECT as a thermodynamics-based computing unit. After establishing a macroscopic understanding of the bistability through thermodynamics, I uncover its microscopic origin. Using various spectroscopic techniques and a vibronic transition model, the bistability is found to arise from a peculiar effect where the electronic charge carriers of the semiconductor couple with the ionic charge carriers of the electrolyte. This coupling introduces a symmetry break in the energetics of doping and dedoping, fundamentally underlying the bistable switching behavior.

Freie Schlagwörter (DE)

Organisch elektrochemische Transistoren, Thermodynamik, Bistabilität

Freie Schlagwörter (EN)

Organic electrochemical transistors, thermodynamics, bistability

Klassifikation (DDC)

540

Klassifikation (RVK)

VN 6057

GutachterIn