The design of a conjugated polymer backbone influences the charge transport and doping efficiency; however, the construction of the polymer backbone for maximized thermoelectric performance is still unclear. Herein, a novel synthesis of a donor-acceptor (D-A) conjugated polymer with an extended planar backbone, C6-ICPDPP, is reported for the investigation of the thermoelectric behavior. The C6- ICPDPP comprises a combination of fused heterocyclic compounds, both electron donor and acceptor groups, which show p-doping characteristics with FeCl3 owing to the excellent planarization. The C6-ICPDPP films are formed from two types of solvents with different boiling points, both of which are doped with FeCl3, showing good thermoelectric properties; however, the thin-film structure, dopant diffusion, and thickness of the C6-ICPDPP films were different between the two solvents. With an increase in the doping concentration of FeCl3, the polarity of the majority charge carriers changes from positive to negative, indicating that the C6-ICPDPP exhibits both p- and n-type electrical behaviors with FeCl3 doping. The optimized power factors are 1.32 μW/mK2 (p-type) and 0.410 μW/mK2 (n-type), respectively, which are consistent with the change in the polaronic features at the given FeCl3 doping concentration. This study provides a design strategy for D-A conjugated polymers with extended planarization and electrical behavior suitable for organic thermoelectrics.