Synthetic Nuances to Maximize n-Type Organic Electrochemical Transistor and Thermoelectric Performance in Fused Lactam Polymers

Abstract

A series of fully fused n-type mixed conduction lactam polymers p(g<sub>7</sub>NC<sub>n</sub>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<sub>7</sub>NC<sub>10</sub>N) recording an OECT electron mobility of 1.20 x 10<sup>-2</sup> cm<sup>2</sup> V<sup>-1</sup> s<sup>-1</sup> and a μC* figure of merit of 1.83 F cm<sup>-1</sup> V<sup>-1</sup> s<sup>-1.</sup> 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(g<sub>7</sub>NC<sub>4</sub>N), with a maximum electrical conductivity of 7.67 S cm<sup>-1 </sup>and a power factor of 10.4 μWm<sup>-1</sup> K<sup>-2</sup>. 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.