Changing the Charge Transport Mechanism in Conjugated Polymers via Post-Processing Side Chain Removal

As the processability of electroactive conjugated polymers (CPs) is essential to their application in a range of devices, balancing solubility, processability, and film properties has become a key point of research. Solubilization of the conjugated backbone is typically achieved by introduction of relatively long branched hydrocarbon or oligoether side chains, which allows many degrees of conformational freedom. Such side chains, while enabling a wide range of processing conditions, reduce the relative fraction of active material in the film and obstruct π-π intermolecular interactions, leading to increased localization of charge carriers and compromise many of the desirable properties of the material (such as electrical conductivity). To counteract the deleterious effects introduced by side chains, we report that post-processing side chain removal, exemplified here via ester hydrolysis, significantly increases the electrical conductivity of chemically doped CP films with a corresponding decrease in Seebeck coefficient. In a model alkyl ester functionalized 3,4-propylenedioxythiophene (ProDOT) system, we found that the electrical conductivity of oxidatively doped films increased 10-fold upon side chain removal via hydrolysis. A combination of X-ray photoelectron spectroscopy (XPS), profilometry, and grazing incidence wide angle X-ray scattering (GIWAXS) results show that improvement in electrical conductivity and reduced Seebeck values are not due to an increase in carrier ratio (i.e., number of charge carriers per repeat unit ring), but instead results from an increase in charge carrier density (i.e., number of charge carriers per unit volume) due to densification and increased ordering of active polymer backbone material following side chain removal. This increase in charge carrier density leads to a reduction in charge localization, thereby improving charge transport, as contextualized using the newly developed semi-localized transport (SLoT) model. Applying this method to 3,4-ethylenedioxythiophene (EDOT) copolymer and homopolymer structures, synthesized via direct arylation polymerization, significantly higher average electrical conductivities are attained, with values ranging from 450 to ~1200 S/cm, depending on the repeat unit structure, in thick (> 600 nm) films. In an all EDOT-based system, we observe a change in transport mechanism, from a low electrical conductivity and thermally activated (hopping-like) regime to a high electrical conductivity and thermally deactivated (metal-like) regime. Finally, these results are compared to those of matching oligoether functionalized polymers for a direct comparison of the differences in charge transport between these two reported methods of conductivity enhancement.