Microscopic Determination of Carrier Density and Mobility in Working Organic Electrochemical Transistors

Nanoscale Operando Characterization of Electrolyte-Gated Organic Field-Effect Transistors Reveals Charge Transport Bottlenecks

Charge transport in electrolyte-gated organic field-effect transistors (EGOFETs) is governed by the microstructural property of the semiconducting thin film that is in direct contact with the electrolyte. Therefore, a comprehensive nanoscale operando characterization of the active channel is crucial to pinpoint various charge transport bottlenecks for rational and targeted optimization of the devices. Here, the local electrical properties of EGOFETs are systematically probed by in-liquid scanning dielectric microscopy (in-liquid SDM) and a direct picture of their functional mechanism at the nanoscale is provided across all operational regimes, starting from subthreshold, linear to saturation, until the onset of pinch-off. To this end, a robust interpretation framework of in-liquid SDM is introduced that enables quantitative local electric potential mapping directly from raw experimental data without requiring calibration or numerical simulations. Based on this development, a straightforward nanoscale assessment of various charge transport bottlenecks is performed, like contact access resistances, inter- and intradomain charge transport, microstructural inhomogeneities, and conduction anisotropy, which have been inaccessible earlier. Present results contribute to the fundamental understanding of charge transport in electrolyte-gated transistors and promote the development of direct structure-property-function relationships to guide future design rules.

A guide for the characterization of organic electrochemical transistors and channel materials

The organic electrochemical transistor (OECT) is one of the most versatile devices within the bioelectronics toolbox, with its compatibility with aqueous media and the ability to transduce and amplify ionic and biological signals into an electronic output. The OECT operation relies on the mixed (ionic and electronic charge) conduction properties of the material in its channel. With the increased popularity of OECTs in bioelectronics applications and to benchmark mixed conduction properties of channel materials, the characterization methods have broadened somewhat heterogeneously. We intend this review to be a guide for the characterization methods of the OECT and the channel materials used. Our review is composed of two main sections. First, we review techniques to fabricate the OECT, introduce different form factors and configurations, and describe the device operation principle. We then discuss the OECT performance figures of merit and detail the experimental procedures to obtain these characteristics. In the second section, we shed light on the characterization of mixed transport properties of channel materials and describe how to assess films' interactions with aqueous electrolytes. In particular, we introduce experimental methods to monitor ion motion and diffusion, charge carrier mobility, and water uptake in the films. We also discuss a few theoretical models describing ion-polymer interactions. We hope that the guidelines we bring together in this review will help researchers perform a more comprehensive and consistent comparison of new materials and device designs, and they will be used to identify advances and opportunities to improve the device performance, progressing the field of organic bioelectronics.

Wearable bioelectronic masks for wireless detection of respiratory infectious diseases by gaseous media

Respiratory infectious diseases (H1N1, H5N1, COVID-19, etc.) are pandemics that can continually spread in the air through micro-droplets or aerosols. However, the detection of samples in gaseous media is hampered by the requirement for trace amounts and low concentrations. Here, we develop a wearable bioelectronic mask device integrated with ion-gated transistors. Based on the sensitive gating effect of ion gels, our aptamer-functionalized transistors can measure trace-level liquid samples (0.3 μL) and even gaseous media samples at an ultra-low concentration (0.1 fg/mL). The ion-gated transistor with multi-channel analysis can respond to multiple targets simultaneously within as fast as 10 min, especially without sample pretreatment. Integrating a wireless internet of things system enables the wearable mask to achieve real-time and on-site detection of the surrounding air, providing an alert before infection. The wearable bioelectronic masks hold promise to serve as an early warning system to prevent outbreaks of respiratory infectious diseases.

Micro- and nano-devices for electrochemical sensing

Electrode miniaturization has profoundly revolutionized the field of electrochemical sensing, opening up unprecedented opportunities for probing biological events with a high spatial and temporal resolution, integrating electrochemical systems with microfluidics, and designing arrays for multiplexed sensing. Several technological issues posed by the desire for downsizing have been addressed so far, leading to micrometric and nanometric sensing systems with different degrees of maturity. However, there is still an endless margin for researchers to improve current strategies and cope with demanding sensing fields, such as lab-on-a-chip devices and multi-array sensors, brain chemistry, and cell monitoring. In this review, we present current trends in the design of micro-/nano-electrochemical sensors and cutting-edge applications reported in the last 10 years. Micro- and nanosensors are divided into four categories depending on the transduction mechanism, e.g., amperometric, impedimetric, potentiometric, and transistor-based, to best guide the reader through the different detection strategies and highlight major advancements as well as still unaddressed demands in electrochemical sensing.

Graphical Abstract

Direct Electrochemical Signaling in Organic Electrochemical Transistors Comprising High-Conductivity Double-Network Hydrogels

In composite hydrogels, the high electrical performance of poly(3,4-ethylenedioxythiophene) complexed with poly(styrenesulfonate) (PEDOT:PSS) is integrated with complementary structural and electrochemical functions via a rationally designed poly(acrylamide) second network incorporating phenylboronic acid (PBA). Free-standing double-network hydrogels prepared by a simple one-pot radical polymerization exhibit state-of-the-art electrical conductivity (∼20 S cm^-1 in phosphate buffered saline) while retaining a degree of hydration similar to that of biological soft tissues. Low resistance contacts to Au electrodes are formed via facile thermo-mechanical annealing and demonstrate stability over a month of continuous immersion, thus enabling hydrogels to serve as channels of organic electrochemical transistors (OECTs). Despite thicknesses of ∼100 μm, gating of hydrogel OECTs is efficient with transconductances g_m ∼ 40 mS and on/off ratios of 10³ in saturation mode operation, whereas sufficiently high conductivity enables linear mode operation (g_m ∼ 1 mS at -10 mV drain bias). This drives a shift of sensing strategy toward detection of electrochemical signals originating within the bulky channel. A kinetic basis for glucose detection via diol esterification on PBA is identified as the coupling of PBA equilibrium to electrocatalyzed O₂ reduction occurring on PEDOT in cathodic potentials. Hydrogel OECTs inherently amplify this direct electrochemical signal, demonstrating the viability of a new class of soft, structural biosensors.

Finding the equilibrium of organic electrochemical transistors

Organic Electrochemical Transistors are versatile sensors that became essential for the field of organic bioelectronics. However, despite their importance, an incomplete understanding of their working mechanism is currently precluding a targeted design of Organic Electrochemical Transistors and it is still challenging to formulate precise design rules guiding materials development in this field. Here, it is argued that current capacitive device models neglect lateral ion currents in the transistor channel and therefore fail to describe the equilibrium state of Organic Electrochemical Transistors. An improved model is presented, which shows that lateral ion currents lead to an accumulation of ions at the drain contact, which significantly alters the transistor behavior. Overall, these results show that a better understanding of the interface between the organic semiconductor and the drain electrode is needed to reach a full understanding of Organic Electrochemical Transistors.

Though existing capacitive models describe the electronic transport in organic electrochemical transistors, these models do not provide an accurate steady-state solution. Here, the authors report a numerical model that describes the distribution and accumulation of ions in the transistor channel.