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Conducting polymers are of interest for a broad range of applications from bioelectronics to thermoelectrics. The factors that govern their complex charge transport physics include the structural disorder present in these highly doped polymer films and the Coulombic interactions between the electronic charge carriers and the dopant counterions. Previous studies have shown that at low doping levels carriers are strongly trapped in the vicinity of the counterions, while at high doping levels charge transport is not limited by Coulombic trapping, which manifests itself in the conductivity being independent of the size of the dopant counterion. Here we use a recently developed ion exchange doping method to investigate the ion size dependence of a semi-crystalline polythiophene-based model system across a wide range of conductivities. We find that the regime, in which the charge and thermoelectric transport is not or only weakly dependent on ion size, extends to surprisingly low conductivities. We explain this surprising observation by a heterogeneous doping that involves doping of the amorphous domains to high doping levels first before doping of the ordered, crystalline domains occurs. Our study provides new insights into how the thermoelectric physics of conducting polymers evolves as a function of doping level.

Heavy heteroatom substitution of the backbone is an effective strategy to improve molecular packing and charge delocalization in polymer semiconductors. Such a backbone modification also facilitates oxidative doping as a result of reduced ionization potential. Here, we explore the effect of single-atom selenium substitution on doping and charge transport properties of a class of polythiophene copolymers. The room temperature conductivities of the doped polymers are significantly enhanced by the selenium substitution for both molecular doping and ion exchange doping. The enhanced conduction is rationalized by the better crystallinity of the selenium-containing system, which can be reinforced by a chain-extended ribbon-phase morphology induced by thermal annealing, which is robust towards doping. The resulting increase in the charge delocalization of the doped selenium-containing system is evidenced by temperature-dependent conductivities. In ion exchange doped films we achieve the maximum conductivity of ~700 S/cm and a high thermoelectric power factor of 46.5 μW m-1 K-2 for the doped selenophene polymer and we observed signatures of a Metal-Insulator transition that are characteristics for heterogeneous conduction systems. Our results show that single-atom selenium substitution is an effective molecular design approach for improving the charge transport and thermoelectric properties of conjugated polymers.