Effects of Side-Chain Length and Functionality on Polar Poly(dioxythiophene)s for Saline-Based Organic Electrochemical Transistors

Abstract

Understanding the impact of side chains on the aqueous redox properties of conjugated polymers is crucial to unlocking their potential in bioelectrochemical devices, such as organic electrochemical transistors (OECTs).Here, we report a series of polar propylenedioxythiophene-based copolymers functionalized with glyme side chains of varying lengths as well as an analogue with short hydroxyl side chains. We show that long polar side chains are not required for achieving high volumetric capacitance (C*), as short hydroxy substituents can afford facile doping and high C* in saline-based electrolytes. Furthermore, we demonstrate that varying the length of the polar glyme chains leads to subtle changes in material properties. Increasing the length of glyme side chain is generally associated with an enhancement in OECT performance, doping kinetics, and stability, with the polymer bearing the longest side chains exhibiting the highest performance ([μC*] OECT = 200 ± 8 F cm −1 V −1 s −1 ). The origin of this performance enhancement is investigated in different device configurations using in situ techniques (e.g., time-resolved spectroelectrochemistry and chronoamperometry). These studies suggest that the performance improvement is not due to significant changes in C* but rather due to variations in the inferred mobility. Through a thorough comparison of two different architectures, we demonstrate that device geometry can obfuscate the benchmarking of OECT active channel materials, likely due to contact resistance effects. By complementing all electrochemical and spectroscopic experiments with in situ measurements performed within a planar OECT device configuration, this work seeks to unambiguously assign material design principles to fine-tune the properties of poly(dioxythiophene)s relevant for application in OECTs.

Ion Size-Dependent Electrochromism in Air-Stable Napthalenediimide-Based Conjugated Polymers

“…Moreover, incorporating electronically insulating (long) alkyl side chains are known to increase the ioninsertion energetics, most likely by obstructing inter-and intrachain electronic charge transport. 26 This is evident from Figure 2a and b, where a significant decrease in overpotentials (η) by ∼350 mV for Li + and ∼420 mV for Na + is seen, supporting this hypothesis.…”

“…To our surprise, the multiple reductions noted for Na + electrochemical doping are accompanied by three distinct color changes (Video S2). The origin of multiple redox peaks for Na + insertion could have resulted from the coexistence of film domains with varying extents of order. − Apart from the contrasting redox behaviors for Li + and Na + ion insertion, a similar trend in lowering of the energetic barriers for ion insertion is noticeable for both Li + and Na + when hydrophilic TEG side chains were attached to the backbone.…”

“…Therefore, we assume the changes in LUMO level from approximately −3.80 eV (2NDI-2OD) to approximately −4.15 eV (2NDI-OD-TEG and 2NDI-2TEG) for the poly-NDIs are due to the increase in the average permittivity of the polymer environment enabled by glycol chains. Moreover, incorporating electronically insulating (long) alkyl side chains are known to increase the ion-insertion energetics, most likely by obstructing inter- and intrachain electronic charge transport . This is evident from Figure a and b, where a significant decrease in overpotentials (η) by ∼350 mV for Li + and ∼420 mV for Na + is seen, supporting this hypothesis.…”

Crystallinity Determines Ion Injection Kinetics and Local Ion Density in Organic Mixed Conductors

“…The side chain chemistry of conjugated polymers significantly affects ion transport within the OMIEC matrix. For instance, the presence of oligoglycol side chains , significantly boosts the ion injection rate compared to polymers with hydrophobic side chains. Adding side chains with radicals, alcohol groups, and charged groups, , also improves OMIEC performance.…”

“…For instance, the presence of oligoglycol side chains , significantly boosts the ion injection rate compared to polymers with hydrophobic side chains. Adding side chains with radicals, alcohol groups, and charged groups, , also improves OMIEC performance. The electrolyte selection also affects OMIEC operation.…”