This review summarises high performing n-type polymers for use in organic thin film transistors, organic electrochemical transistors and organic thermoelectric devices with a focus on stability issues arising in these electron transporting materials.

Conjugated polymers achieve redox activity in electrochemical devices by combining redox-active, electronically conducting backbones with ion-transporting side chains that can be tuned for different electrolytes. In aqueous electrolytes, redox activity can be accomplished by attaching hydrophilic side chains to the polymer backbone, which enables ionic transport and allows volumetric charging of polymer electrodes. While this approach has been beneficial for achieving fast electrochemical charging in aqueous solutions, little is known about the relationship between water uptake by the polymers during electrochemical charging and the stability and redox potentials of the electrodes, particularly for electron-transporting conjugated polymers. We find that excessive water uptake during the electrochemical charging of polymer electrodes harms the reversibility of electrochemical processes and results in irreversible swelling of the polymer. We show that small changes of the side chain composition can significantly increase the reversibility of the redox behavior of the materials in aqueous electrolytes, improving the capacity of the polymer by more than one order of magnitude. Finally, we show that tuning the local environment of the redox-active polymer by attaching hydrophilic side chains can help to reach high fractions of the theoretical capacity for single-phase electrodes in aqueous electrolytes. Our work shows the importance of chemical design strategies for achieving high electrochemical stability for conjugated polymers in aqueous electrolytes.

Recent research demonstrates the viability of organic electrochemical transistors (OECTs) as an emergent technology for biosensor applications. Herein, a comprehensive summary is provided, highlighting the significant progress and most notable advances within the field of OECT‐based biosensors. The working principles of an OECT are detailed, with specific attention given to the current library of organic mixed ionic‐electronic conductor (OMIEC) channel materials utilized in OECT biosensors. The application of OECTs for metabolite, ion, neuromorphic, electrophysiological, and virus sensing as well as immunosensing is reported, detailing the breadth and scope of OECT‐based biosensors. Furthermore, an outlook and perspective on synthetic molecular design of future channel materials, specifically designed for OECT biosensors, is provided. The development of optimized channel materials, creative device architectures, and operational nuances will set the stage for OECT‐based biosensors to thrive and accelerate their clinical prevalence in the near future.

A series of fully fused n-type mixed conduction lactam polymers p(g 7 NC n N) , systematically increasing the alkyl side chain content, are synthesized via an inexpensive, nontoxic, precious-metal-free aldol polycondensation. Employing these polymers as channel materials in organic electrochemical transistors (OECTs) affords state-of-the-art n-type performance with p(g 7 NC 10 N) recording an OECT electron mobility of 1.20 × 10 –2 cm 2 V –1 s –1 and a μ C * figure of merit of 1.83 F cm –1 V –1 s –1 . In parallel to high OECT performance, upon solution doping with (4-(1,3-dimethyl-2,3-dihydro-1 H -benzoimidazol-2-yl)phenyl)dimethylamine (N-DMBI), the highest thermoelectric performance is observed for p(g 7 NC 4 N) , with a maximum electrical conductivity of 7.67 S cm –1 and a power factor of 10.4 μW m –1 K –2 . These results are among the highest reported for n-type polymers. Importantly, while this series of fused polylactam organic mixed ionic–electronic conductors (OMIECs) highlights that synthetic molecular design strategies to bolster OECT performance can be translated to also achieve high organic thermoelectric (OTE) performance, a nuanced synthetic approach must be used to optimize performance. Herein, we outline the performance metrics and provide new insights into the molecular design guidelines for the next generation of high-performance n-type materials for mixed conduction applications, presenting for the first time the results of a single polymer series within both OECT and OTE applications.

On-site signal amplification for bioelectronic sensing is a desirable approach to improving recorded signal quality and to reducing the burden on signal transmission and back-end electronics. While organic electrochemical transistors (OECTs) have been used as local transducers of bioelectronic signals, their current output presents challenges for implementation. OECT-based circuits offer new opportunities for high-performance signal processing. In this work, we introduce an active sensing node based on cofacial vertical OECTs forming an ambipolar complementary inverter. The inverter, which shows a voltage gain of 28, is composed of two OECTs on opposite side walls of a single active area, resulting in a footprint identical to a planar OECT. The inverter is used as an analog voltage preamplifier for recording electrocardiogram signals when biased at the input voltage corresponding to peak gain. We further demonstrate compatibility with nontraditional fabrication methods with potential benefits for rapid prototyping and large-area printed electronics.

A nonfullerene acceptor, isoIDITC, capable of exhibiting fibril-like morphology, is utilized as a third component in organic photovoltaics (OPVs). A power conversion efficiency (PCE) of 19% is achieved in ternary...

Propylene and butylene glycol oligoether chains have been employed as alternatives to ethylene glycol in thiophene based semiconductors for OECTs. Their impact on electrochemical, microstructure, and swelling properties are discussed.

Electron-transporting (n-type) conjugated polymers have recently been applied in numerous electrochemical applications, where both ion and electron transport are required. Despite continuous efforts to improve their performance and stability, n-type conjugated polymers with mixed conduction still lag behind their hole-transporting (p-type) counterparts, limiting the functions of electrochemical devices. In this work, we investigate the effect of enhanced backbone coplanarity on the electrochemical activity and mixed ionic-electronic conduction properties of n-type polymers during operation in aqueous media. Through substitution of the widely employed electron-deficient naphthalene diimide (NDI) unit for the core-extended naphthodithiophene diimide (NDTI) units, the resulting polymer shows a more planar backbone with closer packing, leading to an increase in the electron mobility in organic electrochemical transistors (OECTs) by more than two orders of magnitude. The NDTI-based polymer shows a deep-lying lowest unoccupied molecular orbital level, enabling operation of the OECT closer to 0 V vs Ag/AgCl, where fewer parasitic reactions with molecular oxygen occur. Enhancing the backbone coplanarity also leads to a lower affinity toward water uptake during cycling, resulting in improved stability during continuous electrochemical charging and ON–OFF switching relative to the NDI derivative. Furthermore, the NDTI-based polymer also demonstrates near-perfect shelf-life stability over a month-long test, exhibiting a negligible decrease in both the maximum on-current and transconductance. Our results highlight the importance of polymer backbone design for developing stable, high-performing n-type materials with mixed ionic-electronic conduction in aqueous media.