The electrical conductivity and thermopower of poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) filled carbon black (CB) were investigated with dimethyl sulfoxide, ethylene glycol, and 1-butyl-3-methylimidazolium tetrafluoroborate as second dopants. The electrical conductivity (608 S m(-1)) and thermopower (6.0 mu V K-1) of PEDOT:PSS-filled CB increased three and six times over pure CB, respectively. Dimethyl sulfoxide exhibits a better effect on the enhancement of electrical conductivity (2034 S m(-1)), while 1-butyl-3-methylimidazolium tetrafluoroborate has a greater influence on thermopower (8.3 mu V K-1) of PEDOT:PSS-filled CB. Electrical conductivity and thermopower depend on second dopant contents and 30 vol% dopants gain the highest power factor (0.75-0.10 mu W m(-1) K-2) for PEDOT:PSS-filled CB based on electrical conductivity and thermopower. The second dopants represent a great effect on the electtical conductivity of PEDOT:PSS-filled CB, but less influence on thermopower. (C) 2015 Elsevier B.V. All rights reserved.