Understanding Effects of Alkyl Side Chain Density on Polaron Formation via Electrochemical Doping in Thiophene Polymers.

Polarons exist when charges are injected into organic semiconductors due to their strong coupling with the lattice phonons, significantly affecting electronic charge transport properties. Understanding the formation and (de)localisation of polarons is therefore critical for further developing organic semiconductors as a future electronics platform. However, there have been very few studies reported in this area. In particular, there has been no direct in situ monitoring of polaron formation and identification of its dependence on molecular structure and impact on electrical properties, limiting further advancement in organic electronics. Herein we demonstrate how a minor modification of side chain density in thiophene-based conjugated polymers affects the polaron formation via electrochemical doping, changing the polymers' electrical response to the surrounding dielectric environment for gas sensing. We find that the reduction in side chain density results in a multistep polaron formation, leading to an initial formation of localised polarons in thiophene units without side chains. Reduced side chain density also allows the formation of a high density of polarons with less polymer structural changes. More numerous but more localised polarons generate a stronger analyte response but without the selectivity between polar and non-polar solvents, which is different from the more delocalised polarons that show clear selectivity. Our results provide important molecular understanding and design rules for the polaron formation and its impact on electrical properties. This article is protected by copyright. All rights reserved.