Vapor-phase polymerization (VPP) is an important method for the fabrication of high-quality conducting polymers, especially poly(3,4-ethylenedioxythiophene) (PEDOT). In this work, the effects of additives and post-treatment solvents on the thermoelectric (TE) performance of VPP-PEDOT films were systematically investigated. The use of 1-butyl-3-menthylinidazolium tetrafluoroborate ([BMIm][BF4], an ionic liquid) was shown to significantly enhance the electrical conductivity of VPP-PEDOT films compared with other additives. The VPP-PEDOT film post-treated with mixed ethylene glycol (EG)/[BMIm][BF4] solvent displayed the high power factor of 45.3 mu W m(-1) K-2 which is 122% higher than that prepared without any additive or post-treatment solvent, along with enhanced electrical conductivity and Seebeck coefficient. This work highlighted the superior effect of the [BMIm][BF4] additive and the EG/[BMIm][BF4] solvent post-treatment on the TE performance of the VPP-PEDOT film. These results should help with developing the VPP method to fabricate high-performance PEDOT films. (C) 2017 Wiley Periodicals, Inc.