In this work, the electrical conductivity of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) films was effectively enhanced by binarysecondary doping. Initially, doping with 5 vol.% dimethyl sulfoxide (DMSO)improved the electrical conductivity from 0.3 S cm−1 to 437 S cm−1 and a furtherincrease to 950 S cm−1 was achieved by adding LiClO4. The conductivity value wereport here is one of the highest reported for pretreated PEDOT:PSS films. Theobtained maximum electrical conductivity is almost 3000 times higher than thatshown by pristine PEDOT:PSS films. The increase in the electrical conductivity isascribed to the synergistic effect of the two dopants. Fourier transform infraredspectra indicated the absence of any changes to the chemical structure ofPEDOT:PSS. Atomic force microscopy images demonstrate an increased surfaceroughness and suggest the occurrence of conformational changes of PEDOT chainsfrom the coiled to coil-extended one, which is the key reason for the electricalconductivity enhancement. The pretreatments we propose here are rapid, simple andeffective for the large-scale preparation of high-conductivity PEDOT:PSS films.