Tuning the molecular rearrangement and oxidation level has been proven to be effective strategies for optimizing the thermoelectric (TE) performance of PEDOT. It is difficult to achieve these effects simultaneously via a one-step process, however. In this work, we combined vapor phase polymerization (VPP) and H2SO4 post-treatment to obtain a highly conductive PEDOT film. A novel strategy using polyethylenemine (PEI) as an effective reducing agent was employed to enhance the thermopower of the PEDOT film. Grazing-Incidence Wide-Angle X-ray Scattering analysis and the changes in the oxidation level allow us to elucidate the role of PEI and its transport mechanism. It is demonstrated that the thermopower of well-ordered crystallites in the PEDOT film significantly increases more than five times (from 11 to 59 mu V K-1) by the PEI-DMF solution immersion process, while the electrical conductivity is maintained at 100 S cm(-1). The promising method connecting VPP, H2SO4, and PEI shows great potential for effectively tuning the thermopower of organic TE materials. (c) 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 257-265