Impact of the alkyl side-chain length on solubility, interchain packing, and charge-transport properties of amorphous π-conjugated polymers

Increasing the length of alkyl side chains is a typical way to improve the solubility of π-conjugated polymers designed for use in solution-processed devices. However, these modifications have also been reported to alter the film morphology. Given that the mechanism leading to improved solubility is not well documented yet and the nanoscale (local) morphologies of amorphous π-conjugated polymer films are difficult to characterize experimentally, here, we combine molecular dynamics simulations and long-range corrected density functional theory calculations to examine at the molecular scale the impact that the alkyl side-chain length has on polymer solubility and film morphologies. As a representative example, we consider poly(thieno[3,4-c]pyrrole-4,6-dione-alt-3,4-difluorothiophene) (PTPD[2F]T) with two different lengths of the alkyl side chains on the thieno[3,4-c]pyrrole-4,6-dione (TPD) moieties, i.e., 2-hexyldecyl (2HD) and 2-decyltetradecyl (2DT). A detailed analysis of polymer–solvent and polymer–polymer interactions provides a picture that describes the underlying mechanism for improved solubility in going from 2HD to 2DT. We then underline an intrinsic characteristic that decreasing the side-chain length brings a greater extent of backbone planarity and lesser side chain-TPD interactions, which leads to higher interchain π-π packing density and order, while the interchain π-π packing patterns remain similar in the two films. These morphologies are discussed in terms of the charge-transport properties between neighboring PTPD[2F]T chains, which point to a higher electron mobility in the PTPD[2F]T films with shorter alkyl side chains. Overall, our findings offer guidance in the field of solution-processed electronic devices by pointing out that the polymer alkyl side-chain length could be minimized to improve carrier mobility while ensuring polymer solubility.