Effect of alkyl chain spacer on charge transport in n-type dominant polymer semiconductors with a diketopyrrolopyrrole-thiophene-bithiazole acceptor–donor–acceptor unit†
Although the effects of alkyl chain spacers have been extensively studied for p-type conjugated polymers with a donor (D)–acceptor (A) repeating unit, few studies have examined their effects for n-type polymers with an A–D–A architecture having two different acceptors in the repeating unit. Herein, diketopyrrolopyrrole-thiophene-bithiazole A–D–A type polymer semiconductors (DPPBTz) containing branched alkyl chains of 24-alkyl with C1 spacer (P-24-DPPBTz) and 29-alkyl with C6 spacer (P-29-DPPBTz) were designed and synthesized to elucidate the effect of the alkyl chain branching position on the electron-dominant charge transport system. Due to the strong electron deficiency and trans-planar conformation of the bithiazole group, DPPBTz-based polymer semiconductors exhibit n-type dominant electrical properties with a high electron mobility of up to 1.87 cm2 V−1 s−1. Systematic studies on the photophysical properties, thin-film microstructures, and electrical properties of the DPPBTz polymers revealed that upon modification of the branching position, A–D–A n-type dominant semiconductors differently behave compared to D–A p-type dominant semiconductors. This is attributed to the relatively weaker intermolecular interactions in A–D–A type semiconductors, making the C1 spacer more efficient for electron transport. These findings reveal the molecular design rule of alkyl side-chains in A–D–A n-type-dominant conjugated polymers for the first time.