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Poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS) is one of the most widely used hole transport layer (HTL) in polymer solar cells (PSCs). However, the improving of the conductivity and transparency of PEDOT:PSS thin films is still needed. To solve this problem, we here introduce 2-methoxyethanol (EGME) and dimethyl sulfoxide (DMSO) mixed secondary solvent to PEDOT:PSS solution as a novel additive to the best of our knowledge. We determined that the EGME-DMSO doped PEDOT:PSS layer provides better energy level alignment, conductivity and morphology with the help of methods of UV-Vis spectroscopy, atomic force microscopy, etc. The addition of 15% (v/v.) volume of EGME-DMSO mixed co-solvent improves the efficiency from 2.8% of control device to 3.9%. The significant enhancement of the short-circuit current density (Jsc) of 13.7 to 16.5 mA cm−2 is the main reason for this increase of performance due to better charge transport properties. This suggests that the EGME-DMSO mixed co-solvent doping into PEDOT:PSS solution is a simple approach to fabricate highly efficient PSCs.