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.