Coplanar Donor-Acceptor Semiconducting Copolymers to Achieve Better Conjugated Structures: Side-Chain Engineering

Abstract

It is reported on the allocation effects of branched alkyl chains, when used as solubility and ordering enhancers of the conjugated donor–accepter (D–A) copolymer backbones, on the ordering and π–π overlapping of the copolymers, that drastically affect the electrical properties of organic field-effect transistors (OFETs). Triisopropylsilylethynyl-benzo[1,2-b:4,5-b′]dithiophene (TIPSBDT) and diketopyrrolopyrrole (DPP)-based copolymers, which have two linear alkyl spacers (methylene (C1) or butylene (C4)) between the DPP and side-substituent (C10H21)CH(C8H17), are synthesized by Suzuki cross-coupling. These copolymer films are spun cast onto a polymer-treated SiO2 dielectric surface, and some are further thermally annealed. The longer spacer, C4, is found to efficiently enhance the coplanarity and conjugation of the D–A backbone, while the C1 does not. The resulting C4-bridged TIPSBDT-DPP-based copolymer readily develops a superior π-extended layer on the dielectric surface; the edge-on chains with randomly oriented side chains can be closely packed with a short π-planar distance (d(010)) of 3.57 A. Its properties are superior to those of the short spacer C1 system with d(010) ≈3.93 A. The C4-bridged TIPSBDT-DPP copolymer films yield a field-effect mobility up to 1.2 cm2 V−1 s−1 in OFETs, 12 times as higher than that of the C1 spacer system.