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

Abstract

Abstract 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−1 V−1 s−1. 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−1 V−1 s−1 and high gm,norm of 6.75 S cm−1 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.

Keywords

• charge carrier polarity
• cyanation
• organic electrochemical transistors
• organic mixed ionic–electronic conductors
• polythiophene