In this paper, bulk heterojunction solar cells with poly-(3-hexylthiophene) (P3HT): [6,6]-phenyl-C61-butyric-acid-methylester (PCBM) as an active layer and modified poly (3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) as a buffer layer are fabricated. The buffer layer is modified by adding 1% to 5% dimethyl sulfoxide (DMSO) into PEDOT: PSS solution before spin-coating. The conductivity of modified PEDOT:PSS and the performance of solar cells with modified PEDOT:PSS are measured. The highest conductivity of modified PEDOT:PSS with 4% DMSO can achieve 89.693 S/cm. The performance of organic solar cell with PEDOT:PSS modified by 4% DMSO is the best. The 4% DMSO-modified-PEDOT:PSS cell has a power conversion efficiency of 3.34%, V _oc of 5.7 V, J _sc of 14.56 mA/cm² and filling factor (FF) of 40.34%.