High-Lamellar Ordering and Amorphous-Like π-Network in Short-Chain Thiazolothiazole−Thiophene Copolymers Lead to High Mobilities

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Abstract

Owing to their superior transport properties, poly(alkylthiophenes) and their derivatives emerged as one of the most widely studied semiconducting polymers with potential applications in organic electronics. It is now generally acknowledged that one of the particularly effective ways to increase the carrier mobility in these materials is by increasing the length of the conjugated backbones. Some recent reports suggest also that carrier mobilities can be further enhanced by highly crystalline arrangement (and interdigitation) of alkyl side chains possibly because it promotes the formation of extensive layered structures favorable for carrier transport. Results presented here demonstrate that, surprisingly, none of these factors are actually necessary for good electronic performance of polythiophene-like systems. Thiophene-based semiconducting polymers bearing thiazolothiazole unit (PTzQT) described here were shown to have very high carrier mobilities (∼0.3 cm2/Vs) despite their low molecular weight and uneven spacing of alkyl side chains, which suppressed high side chain crystallinity/interdigitation as revealed by thermal analysis and X-ray scattering. The highly disordered nature of these materials extended to the nanoscale level as evident from atomic force microscopy images, which have shown only the presence of small domains packed into isotropic amorphous-like superstructures with lateral correlation lengths increasing with the length of alkyl side chains. The observed concomitant increase of carrier mobilities points to the possible role of characteristic length of surface roughness as the key parameter controlling carrier transport in disordered, noninterdigitating systems.

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