Thermoelectric (TE) properties of flexible and freestanding poly(3,4-ethylenedioxythiophene) (PEDOT) films synthesized via galvanostatic polymerization of 3,4-ethylenedioxythiophene in propylene carbonate containing sulfated poly(beta-hydroxyethers) (S-PHE) as polymer electrolyte were elaborately studied. Both electrical conductivities (sigma) and Seebeck coefficients (S) of the PEDOT: S-PHE films were increased by decreasing the temperature (T) or by increasing the current density (J) during electrosynthesis. Possible reasons for the lack of a trade-off relation commonly observed between sigma and S are discussed on the basis of SEM and oxidation-level measurements. Preparation of the PEDOT: S-PHE films was optimized with respect to T and J. In addition, the oxidation level of the PEDOT: S-PHE films was controlled by potential and the change of their TE performances was discussed in conjunction with the change of chemical species involved. The power factor (PF=sigma S-2) of the PEDOT: S-PHE films reached 7.9 mu Wm(-1) K-2, leading to a dimensionless TE figure- of-merit (ZT) of 0.013. (C) 2017 Wiley Periodicals, Inc.