Sequential Cyanation of Polythiophenes: Tuning Charge Carrier Polarity in Organic Electrochemical Transistors

Polythiophenes, built on head-to-head (HH) linked 3,3′-oligomer ethylene glycol-2,2′-bithiophene (gT2), have simple chemical structure, good backbone planarity, and relatively high-lying the lowest unoccupied molecular orbital (LUMO) energy levels, hence showing excellent p-type performance in organic electrochemical transistors (OECTs) with µC* of several hundred F cm V s. In this study, sequential cyano substitution is utilized to enable transformation of charge carrier polarity from p-type to n-type in OECTs, based on the parent polythiophene g4T2-T2. With the increase of cyano group number, the polythiophenes exhibit gradually lowered LUMO levels from −2.55 to −3.90 eV. As a result, from g4T2-T2 to CNg4T2-CNT2, the p-type performance dramatically diminishes accomplished by the enhancement of n-type one when applied in OECTs. To the authors' delight, polymer CNg4T2-CNT2 with the highest content of cyano groups exhibits a remarkable n-type µC* of 27.01 F cm V s and high g of 6.75 S cm with negligible p-type character. This study demonstrates that sequential cyano substitution provides a powerful approach for developing high-performance n-type polymers for OECT applications.