Poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT: PSS) films were post-treated with polar solvent dimethylsulfoxide (DMSO) by spin-coating method and polymer solar cells (PSCs) based on poly [N-9 '']-hepta-decany1-2,7-carbazole-alt-5,5-(4',7'-di-2-thie nyl-2',1',3'-benzothiadiazole) (PCDTBT) : [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) were fabricated to investigate the effect of the treatment. By post-modifying the PEDOT: PSS layer, the conductivity of the PEDOT: PSS film was largely improved. Using electrochemical impedance spectroscopy (IS) we observed that the series resistance of the device decreased greatly after the treatment. With DMSO-treated PEDOT: PSS transport layer, the power conversion efficiency (PCE) of the PSC based on PCDTBT: PC71BM raised from 5.95% to 6.52% with both increase in Jsc and FF. We systematically studied charge transport property via space-charge-limited -current (SCLC) and our results suggest that the increment in device efficiency can be attributed to the increased hole-mobility and thus more balanced charge transport benefits the enhancement of polymer solar cell efficiency. We also noted that if measured without a shadow mask much more overestimation will take place in the DMSO-treated device as a result of lateral electrical conduction. We suggest that when we apply the highly conductive PEDOT: PSS layer in the PSCs, careful measurement should be carried out to avoid inaccuracy. (C) 2016 Elsevier Ltd. All rights reserved.