Achieving Stable Ambipolar and Unipolar Transport in NDI-DPP-Thiophene Copolymers for Organic Transistors

Donor–acceptor (D-A) conjugated copolymers are promising for organic field-effect transistors (OFETs) due to tunable ambipolar and unipolar transport. We report the synthesis of novel copolymers combining three building blocks with varying electron affinities: strong acceptor naphthalenediimide (NDI), weak donor/acceptor diketopyrrolopyrrole (DPP), and strong donor thiophene. By precisely controlling the copolymerization ratio of NDI:DPP:thiophene, we successfully modulated charge carrier polarity from unipolar hole transport to ambipolar and unipolar electron transport. At the 1:9 and 9:1 copolymer ratios, minority carriers were effectively suppressed, resulting in an ideal unipolar transistor behavior. This is attributed to the distinct crystalline orientations of the two building blocks─edge-on for DPP (hole transport) and face-on for NDI (electron transport)─which likely hinder charge pathways for minority carriers. Additionally, a small fraction of donor or acceptor units may act as traps for minority carriers, shifting the transport from ambipolar to unipolar. Unlike typical low band gap ambipolar materials, where minority carriers reduce device stability, these copolymers exhibit excellent stability under repeated bias sweeps. Density functional theory (DFT), atomic force microscopy (AFM), and grazing-incidence X-ray diffraction (GIXRD) reveal the molecular and morphological origins of transport modulation. Stable n-channel OFETs processed from environmentally friendly, nonchlorinated solvents demonstrate potential for sustainable organic electronics.