Materials that efficiently transport and couple ionic and electronic charge are key to advancing a host of technological developments for next-generation bioelectronic, optoelectronic and energy ...storage devices. Here we highlight key progress in the design and study of organic mixed ionic-electronic conductors (OMIECs), a diverse family of soft synthetically tunable mixed conductors. Across applications, the same interrelated fundamental physical processes dictate OMIEC properties and determine device performance. Owing to ionic and electronic interactions and coupled transport properties, OMIECs demand special understanding beyond knowledge derived from the study of organic thin films and membranes meant to support either electronic or ionic processes only. We address seemingly conflicting views and terminology regarding charging processes in these materials, and highlight recent approaches that extend fundamental understanding and contribute to the advancement of materials. Further progress is predicated on multimodal and multi-scale approaches to overcome lingering barriers to OMIEC design and implementation.
Organic electrochemical transistors (OECTs) are thin-film transistors that have shown great promise in a range of applications including biosensing, logic circuits, and neuromorphic engineering. The ...device physics of OECTs are determined by the interaction between ionic and electronic charge carriers. This interaction sets OECTs apart from conventional transistor technologies and has necessitated the development of device models for the unique behavior of OECTs. In this Review, we discuss existing models for OECTs and provide a framework for understanding these models. Moreover, we show how the insight from these models inform device optimization. Finally, we discuss details of OECT operation that are not well-understood and that provide exciting opportunities for future research.
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•Review article on device physics of organic electrochemical transistors.•Geometry scaling for response time and transconductance.•Understanding ionic/electronic mixed conduction.•Design strategies for mitigation of parasitic resistance.•Density of states influence on charge transport.
Organic mixed conductors have garnered significant attention in applications from bioelectronics to energy storage/generation. Their implementation in organic transistors has led to enhanced ...biosensing, neuromorphic function, and specialized circuits. While a narrow class of conducting polymers continues to excel in these new applications, materials design efforts have accelerated as researchers target new functionality, processability, and improved performance/stability. Materials for organic electrochemical transistors (OECTs) require both efficient electronic transport and facile ion injection in order to sustain high capacity. In this work, we show that the product of the electronic mobility and volumetric charge storage capacity (µC*) is the materials/system figure of merit; we use this framework to benchmark and compare the steady-state OECT performance of ten previously reported materials. This product can be independently verified and decoupled to guide materials design and processing. OECTs can therefore be used as a tool for understanding and designing new organic mixed conductors.
The organic electrochemical transistor (OECT) represents a versatile and impactful electronic building block in the areas of printed electronics, bioelectronics, and neuromorphic computing. ...Significant efforts in OECTs have focused on device physics, new active material design and synthesis, and on preliminary implementation of individual transistors as proof-of-concept components for sensing and computation. However, as most of the current studies are based on single devices, the integration of OECTs into circuits or high-level systems has lagged. In this review, we focus on recent efforts to incorporate individual OECTs into digital, analog, and neuromorphic circuits, and lay out important considerations relevant for (hybrid) systems integration. We summarize the operation principles and the functions of OECT-based circuits and discuss the approaches for wireless power and data transmission for practicality in biological and bio-inspired applications. Finally, we comment on the future directions and challenges facing OECT circuits from both a fundamental and applied perspective.
•Organic electrochemical transistor (OECT)-based digital, analog and neuromorphic circuit for biological applications.•Wireless power and data transmission for OECT-based circuits.•Novel fabrication techniques and materials enable new circuit concepts.•Promising potential of OECT-based circuit in the field of bioelectronics.
Associative learning, a critical learning principle to improve an individual's adaptability, has been emulated by few organic electrochemical devices. However, complicated bias schemes, high write ...voltages, as well as process irreversibility hinder the further development of associative learning circuits. Here, by adopting a poly(3,4-ethylenedioxythiophene):tosylate/Polytetrahydrofuran composite as the active channel, we present a non-volatile organic electrochemical transistor that shows a write bias less than 0.8 V and retention time longer than 200 min without decoupling the write and read operations. By incorporating a pressure sensor and a photoresistor, a neuromorphic circuit is demonstrated with the ability to associate two physical inputs (light and pressure) instead of normally demonstrated electrical inputs in other associative learning circuits. To unravel the non-volatility of this material, ultraviolet-visible-near-infrared spectroscopy, X-ray photoelectron spectroscopy and grazing-incidence wide-angle X-ray scattering are used to characterize the oxidation level variation, compositional change, and the structural modulation of the poly(3,4-ethylenedioxythiophene):tosylate/Polytetrahydrofuran films in various conductance states. The implementation of the associative learning circuit as well as the understanding of the non-volatile material represent critical advances for organic electrochemical devices in neuromorphic applications.
Electrochemical aptamer-based sensors are typically deployed as individual, passive, surface-functionalized electrodes, but they exhibit limited sensitivity especially when the area of the electrode ...is reduced for miniaturization purposes. We demonstrate that organic electrochemical transistors (electrolyte gated transistors with volumetric gating) can serve as on-site amplifiers to improve the sensitivity of electrochemical aptamer-based sensors. By monolithically integrating an Au working/sensing electrode, on-chip Ag/AgCl reference electrode, and Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) counter electrode - also serving as the channel of an organic electrochemical transistor- we can simultaneously perform testing of organic electrochemical transistors and traditional electroanalytical measurement on electrochemical aptamer-based sensors including cyclic voltammetry and square-wave voltammetry. This device can directly amplify the current from the electrochemical aptamer-based sensor via the in-plane current modulation in the counter electrode/transistor channel. The integrated sensor can sense transforming growth factor beta 1 with 3 to 4 orders of magnitude enhancement in sensitivity compared to that in an electrochemical aptamer-based sensor (292 μA/dec vs. 85 nA/dec). This approach is believed to be universal, and can be applied to a wide range of tethered electrochemical reporter-based sensors to enhance sensitivity, aiding in sensor miniaturization and easing the burden on backend signal processing.
From established to emergent technologies, doping plays a crucial role in all semiconducting devices. Doping could, theoretically, be an excellent technique for improving repressively low ...transconductances in n-type organic electrochemical transistors - critical for advancing logic circuits for bioelectronic and neuromorphic technologies. However, the technical challenge is extreme: n-doped polymers are unstable in electrochemical transistor operating environments, air and water (electrolyte). Here, the first demonstration of doping in electron transporting organic electrochemical transistors is reported. The ammonium salt tetra-n-butylammonium fluoride is simply admixed with the conjugated polymer poly(N,N'-bis(7-glycol)-naphthalene-1,4,5,8-bis(dicarboximide)-co-2,2'-bithiophene-co-N,N'-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide), and found to act as a simultaneous molecular dopant and morphology-additive. The combined effects enhance the n-type transconductance with improved channel capacitance and mobility. Furthermore, operational and shelf-life stability measurements showcase the first example of water-stable n-doping in a polymer. Overall, the results set a precedent for doping/additives to impact organic electrochemical transistors as powerfully as they have in other semiconducting devices.
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