Side-Chain-Length-Regulating Ambipolarity of Organic Electrochemical Transistors Based on Naphthalenediimide–Dialkoxybithiazole Copolymers

Abstract

Ambipolar organic mixed ionic-electronic conductor (OMIEC) materials are ideal channel materials for organic electrochemical transistor (OECT)-based complementary inverters, which are fundamental components of logic circuits and are widely used in many applications because they can significantly reduce the complexity and manufacturing costs. However, the reported design strategies for ambipolar OMIECs that enable efficient cation/anion and electron–hole transport and coupling remain scarce. Herein, we report that the ambipolarity of donor–acceptor (D-A)-based OMIEC materials could be enhanced by regulating the alkyl side-chain length, without altering the conjugated backbone. We designed and synthesized three naphthalenediimide (NDI)-alt-dialkoxybithiazole (2Tz)-based conjugated polymers with alkyl side chains of different lengths on the dialkoxybithiazole unit and a polar hepta(ethylene glycol) side chain on the NDI unit. The resulting three OMIEC materials, P-7O–C4, P-7O–C6, and P-7O–C8, exhibit similar n-type OECT performance; however, they begin to exhibit p-type OECT performance as the lengths of the alkyl chains on the dialkoxybithiazole decrease. This occurs because shorter alkyl chains benefit anion penetration into the polymer film, thereby enhancing p-type doping. Finally, we successfully fabricated single-component ambipolar OECT-based complementary inverters. The P-7O–C4-based inverter exhibited the lowest switching voltage because of the best p-type performance among these polymers. This investigation emphasizes the critical role of the side chains of D–A conjugated polymers in ambipolar OECTs and offers unique insight into a fundamental understanding of the relationship between polymer molecular design and ambipolar OMIEC material performance, which provides guidance for the future rational design of efficient ambipolar OMIEC materials.