Understanding Effects of Alkyl Side‐Chain Density on Polaron Formation Via Electrochemical Doping in Thiophene Polymers

Abstract

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

Interplay between Side Chain Density and Polymer Alignment: Two Competing Strategies for Enhancing the Thermoelectric Performance of P3HT Analogues

“…On the other hand, polymer alignment by high-temperature rubbing proved less efficient with decreasing side chain density. 26 This is evidenced by decreasing dichroic ratios measured on the aligned films using polarized ultraviolet−visible (UV−vis) spectroscopy. As a consequence, electrical conductivity parallel to the direction of alignment was not clearly correlated to the side chain density due to the simultaneous decrease in melting temperature and lower degree of crystallinity with decreasing side chain density of the neutral polymer films.…”

“…The effect of randomly altering the side chain density in polythiophenes is well documented to improve the charge carrier mobility; however, the effect on the thermoelectric properties is not so established. − For the non-aligned films, the electrical conductivity after doping with F4TCNQ increased gradually with the decreasing side chain density, affording a sixfold increase for the highest thiophene content ( x = 24) compared to P3HT. On the other hand, polymer alignment by high-temperature rubbing proved less efficient with decreasing side chain density . This is evidenced by decreasing dichroic ratios measured on the aligned films using polarized ultraviolet–visible (UV–vis) spectroscopy.…”