4:15 PM - EQ03.20.03
Synthetic Nuances to Maximize N-Type Organic Electrochemical Transistor and Thermoelectric Performance in Fused Lactam Polymers
Adam Marks1,2,Xingxing Chen3,Ruiheng Wu4,Reem Rashid4,Wenlong Jin5,Bryan Paulsen4,Maximilian Moser1,Xudong Ji4,Sophie Griggs1,Dilara Meli4,Helen Bristow1,Nicola Gasparini6,Simone Fabiano5,Jonathan Rivnay4,Iain McCulloch1
University of Oxford1,Stanford University2,King Abdullah University of Science and Technology3,Northwestern University4,Linköping University5,Imperial College London6
Organic electrochemical transistors (OECTs) have attracted significant attention in recent years as promising devices for bioelectronic applications due to their low operation voltages, high sensitivity, good biocompatibility, and operability in aqueous environments, as well as excellent signal amplification characteristics.[1,2] The core component of an OECT is an organic mixed ionic-electronic conductor (OMIEC) channel material, commonly a conjugated polymer, which is able to efficiently conduct both electronic and ionic charge carriers.
This talk focuses on the application and design rationale of the OMIEC, presenting a novel, high-performing series of fully fused n-type mixed conduction lactam polymers p(g7NCnN), systematically increasing the alkyl side chain content, as an example. The vast majority of channel material polymers reported for OECTs are p-type (hole transport) materials, whilst n-type (electron-transporting) counterparts lag far behind, both in performance and availability.[3,4] This is mainly due to the difficulty in achieving both stability and a high transconductance in aqueous conditions, for n-type polymers. Many current n-type OMIECs are donor-acceptor (D-A) co-polymers, constituting alternating electron-rich and electron-deficient units, synthesized via a transition metal-mediated cross-coupling polymerization. Whilst the D-A structure offers synthetic advantages, two shortcomings of this motif are the single bond connections, allowing rotational torsion, which gives rise to conformational disorder, and the localization of the lowest unoccupied molecular orbital (LUMO) on the electron-deficient unit, both of which inhibit electron mobility. In addition, residual toxic metallic species arising from polymerization present a limitation for the polymers to be applied in bioelectronic devices.
We exemplify the use of an acceptor-acceptor (A-A) motif by presenting the design of a high electron mobility polymer series that comprises only electron-deficient repeat units locked in-plane by double bond linkers, conferring a delocalized LUMO, and eliminating torsional twists along the backbone. On account of this structure, the series offers low-lying LUMO energy levels and is polymerized via a metal-free benign aldol condensation, improving biological compatibility. Employing these polymers as channel materials in organic electrochemical transistors affords state-of-the-art n-type performance with p(g7NC10N) recording OECT electron mobility of 1.20 × 10-2 cm2 V-1 s-1 and a µC* 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-1H-benzoimidazol-2-yl)phenyl)dimethylamine (N-DMBI) the highest thermoelectric performance is observed for p(g7NC4N), with a maximum electrical conductivity of 7.17 S cm-1 and a power factor of 10.5 µW m-1 K-2. These results are among the highest reported for n-type polymers. Importantly, this series of fused OMIECs highlights that synthetic molecular design strategies to bolster OECT performance can be translated to also achieve high organic thermoelectric (OTE) performance, however, a nuanced synthetic approach must be taken 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 a single polymer series within both OECT and OTE applications. Whereby the bulk nature of transport renders OECTs a more attractive platform for OTE research compared to OTFTs.
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