Organic electrochemical transistors are bioelectronic devices that exploit the coupled nature of ionic and electronic fluxes to achieve superior transducing abilities compared to conventional organic field effect transistors. In particular, the operation of organic electrochemical transistors relies on a channel material capable of conducting both ionic and electronic charge carriers to ensure bulk electrochemical doping. This review explores the various types of organic semiconductors that are employed as channel materials, with a particular focus on the past 5 years, during which the transducing abilities of organic electrochemical transistors have witnessed an almost tenfold increase. Specifically, the structure–property relationships of the various channel materials employed are investigated, highlighting how device performance can be related to functionality at the molecular level. Finally, an outlook on the field is provided, in particular toward the design guidelines of future materials and the challenges ahead in the field.

This work presents a soluble oligo(ether)-functionalized propylenedioxythiophene (ProDOT)-based copolymer as a versatile platform for a range of high-performance electrochemical devices, including organic electrochemical transistors (OECTs), electrochromic displays, and energy-storage devices. This polymer exhibits dual electroactivity in both aqueous and organic electrolyte systems, redox stability for thousands of redox cycles, and charge-storage capacity exceeding 80 F g −1 . As an electrochrome, this material undergoes full colored-to-colorless optical transitions on rapid time scales (<2 s) and impressive electrochromic contrast (Δ%T > 70%). Incorporation of the polymer into OECTs yields accumulation-mode devices with an I ON/OFF current ratio of 10 5 , high transconductance without post-treatments, as well as competitive hole mobility and volumetric capacitance, making it an attractive candidate for biosensing applications. In addition to being the first ProDOT-based OECT active material reported to date, this is also the first reported OECT material synthesized via direct(hetero)arylation polymerization, which is a highly favorable polymerization method when compared to commonly used Stille cross-coupling. This work provides a demonstration of how a single ProDOTbased polymer, prepared using benign polymerization chemistry and functionalized with highly polar side chains, can be used to access a range of highly desirable properties and performance metrics relevant to electro chemical, optical, and bioelectronic applications. Electroactive Polymers[+] Present address:

Replacing the alkyl side chains on conventional semiconducting polymers with ethylene glycol (EG) based chains is a successful strategy in the molecular design of mixed conduction materials for bioelectronic devices, including organic electrochemical transistors (OECTs). Such polymers have demonstrated the capability to conduct both ionic and electronic charges and can offer superior performance compared to the most commonly used active material, poly(3,4ethylenedioxythiophene):poly(styrenesulfonate). While many research efforts have been dedicated to optimizing OECT performance through the engineering of the semiconducting polymers' conjugated backbones, variation of the EG chain length has been investigated considerably less. In this work, a series of glycolated polythiophenes with pendant EG chains spanning two to six EG repeat units was synthesized and the electrochemical and structural characteristics of the resulting films were characterized by experimental means and molecular dynamics simulations. OECTs were fabricated and tested, and their performance showed a strong correlation to the length of the EG side chain length, thereby elucidating important structure-property guidelines for the molecular design of future channel materials. Specifically, a careful balance in the EG length must be struck during the design of EG functionalized conjugated polymers for OECTs. While minimizing the EG side chain length appears to boost both the capacitive and charge carrier transport properties of the polymers, the chosen EG side chain length must be kept sufficiently long to induce solubility for processing, and allow for the necessary ion interactions with the conjugated polymer backbone.

Copolymerization of alkoxy-functionalized 3,4-propylenedioxythiophenes (ProDOTs) with unfunctionalized 3,4-ethylenedioxythiophenes (EDOTs) in varying ratios using direct arylation yields a series of solution processable polymers with highly tunable optical and electronic properties. Within this series, we have identified ProDOT−EDOT 2 , a copolymer containing 67% EDOT compositionally, that combines the low oxidation potential, the redox behavior, and the deep-blue neutral color that are characteristic of PEDOT with the high solubility, exceptional electrochromic contrast, and color neutrality in the oxidized state characteristic of alkoxy-functionalized PProDOTs.

The processability and electronic properties of conjugated polymers (CPs) have become increasingly important due to the potential of these materials in redox and solid-state devices for a broad range of applications. To solubilize CPs, side chains are needed, but such side chains reduce the relative fraction of electroactive material in the film, potentially obstructing π−π intermolecular interactions, localizing charge carriers, and compromising desirable optoelectronic properties. To reduce the deleterious effects of side chains, we demonstrate that postprocessing side chain removal, exemplified here via ester hydrolysis, significantly increases the electrical conductivity of chemically doped CP films. Beginning with a model system consisting of an ester functionalized ProDOT copolymerized with a dimethylProDOT, we used a variety of methods to assess the changes in polymer film volume and morphology upon hydrolysis and resulting active material densification. Via a combination of electrochemistry, X-ray photoelectron spectroscopy, and charge transport models, we demonstrate that this increase in electrical conductivity is not due to an increase in degree of doping but an increase in charge carrier density and reduction in carrier localization that occurs due to side chain removal. With this improved understanding of side chain hydrolysis, we then apply this method to high-performance ProDOT-alt-EDOT x copolymers. After hydrolysis, these ProDOT-alt-EDOT x copolymers yield exceptional electrical conductivities (∼700 S/cm), outperforming all previously reported oligoether-/glycol-based CP systems. Ultimately, this methodology advances the ability to solution process highly electrically conductive CP films.

An organic soluble precursor polymer was prepared by direct (hetero)­arylation polymerization of 3,4-ethylenedioxythiophene (EDOT) with a solubilizing, 3,4-propylenedioxythiophene (ProDOT) derivative bearing ester-functionalized side chains. Chemical defunctionalization of the polymer, using base to hydrolyze the esters, yields a conjugated polyelectrolyte (CPE) that is readily soluble in water. This aqueous soluble CPE can then be processed using high-throughput coating methods from water-based inks. Postprocessing functionalization of the polymer film using dilute acid creates a solvent resistant film that is compatible with both organic and aqueous electrolyte systems for redox switching. The introduction of an unfunctionalized EDOT unit results in a soluble polymer that has a low oxidation potential and that is highly electroactive and pseudocapacitive in a wide voltage range (2 V in propylene carbonate-based electrolytes and 1.55 V in aqueous electrolytes) making it an attractive material for lightweight and flexible supercapacitors. Films of this copolymer demonstrate exceptionally rapid redox switching (10 V/s) and higher mass capacitance in aqueous electrolyte solutions than in organic solutions. Supercapacitors incorporating the solvent resistant copolymer exhibit symmetric charge/discharge behavior at currents of up to 20 A/g (1 s discharge) and are able to maintain >75% of the initial capacitance over 175 000 cycles using 0.5 M NaCl/water as the device electrolyte. Rapid electrochromic switching (∼0.2 s) from vibrant blue to colorless is also maintained in this salt–water electrolyte. The versatility of this polymer is further shown in a series of organic and aqueous electrolyte systems, including biologically compatible electrolytes (NaCl/water, Ringer’s solution, and human serum) and even sport drinks (Gatorade and Powerade), demonstrating the robustness of this polymer to differing ionic conditions. Based on these results, it is apparent that this polymer and similar systems have great potential in multiple electrochemical applications such as electrochromic devices, supercapacitors, and biocompatible devices.

We demonstrate the electrically controlled and reversible plasmonic signature of hybrid polymer–metal nanostructures composed of core/shell nanostructures: gold nanocubes (AuNCs) coated with electrochromic polyaniline (PANI) shells. A reversible tuning of the localized surface plasmon resonance (LSPR) peak of the AuNC core was obtained by applying an electrical potential that caused a reversible oxidation state change in the electroactive PANI nanoshell. A significant shift of the main LSPR peak was achieved with high reversibility and electrochemical stability due to the interplay of the local decay of the electromagnetic field and the controlled thickness of the surrounding polymer shell. Here, the PANI shell acts as an electroactive medium as well as a physical spacer to prevent uncontrollable plasmonic coupling. The most efficient LSPR shift can be induced by the refractive index change of nanoscale PANI shell thickness lower than the electromagnetic field decay length of the given gold nanoparticles. Single particle studies showed that coupling of plasmon resonances of densely packed nanocubes with thicker PANI shells was prevented and the extinction signature of individual core/shell nanocubes remained mostly unchanged after their assembly into densely packed aggregates. Therefore, these core/shell structures could preserve the original plasmonic signature of individual nanostructure by damping plasmonic coupling between AuNC cores.

The unique ability of combined ionic and electronic transport in conjugated, semiconducting polymers has resulted in the emergence of a variety of redox-based technologies ranging from energy storage and conversion, to bioelectronics, to on-demand color control. Although conjugated polymers have been extensively studied for decades, the recent revival of organic bioelectronics, in particular, has demonstrated that there needs to be a better understanding of the interplay between mixed ion and electron transport and the underlying film morphology. Many of the conjugated polymers that are effectively doped electrochemically and that exhibit a combination of high capacitance, fast and reversible redox switching, and exceptional stability lack long-range order making it more challenging to evaluate how the morphology evolves as a function of oxidation state. Here, we demonstrate how readily accessible electrochemical and spectroscopic techniques can offer a great deal of insight into ion and electron transport in redox-active conjugated polymers regardless of their degree of order. Furthermore, we show how numerous redox properties, including onset of oxidation, capacitance, and conductance profile, of five dioxythiophene-based copolymers can be manipulated by the size and polarity of the functional groups that are incorporated to provide solution processability.