Effect of alkyl chain spacer on charge transport in n-type dominant polymer semiconductors with a diketopyrrolopyrrole-thiophene-bithiazole acceptor-donor-acceptor unit

Abstract

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.