The effect of dimethyl sulfoxide (DMSO) doping concentration on the performance of polymer solar cells (PSCs) based on poly(3-hexylthiophene) (P3HT)∶(6,6)-phenyl C60 butyric acid methyl ester (PCBM) as the active layer was investigated. The results suggest that the doping of DMSO can improve short-circuit current density (Jsc) and fill factor (FF) of the PSCs. The cell with 3% DMSO exhibits enhanced Jsc (7.88 mA·cm-2), and FF (55.5%). The optimized power conversion efficiency (PCE) arrived to 2.54%, which is 17% higher than that of the cell without DMSO doping. The Fourier Transform infrared spectroscopy (FTIR) is used to demonstrate the effect of DMSO doping into P3HT∶PCBM on chemical properties. The presence of FTIR suggests that the chemical properties of P3HT and PCBM have no changes. To investigate the causes of the PCE improvement after addition of DMSO, an enhanced light harvesting and charge carriers transport properties of electroluminescence devices were observed by UV-Visible spectra and J-V characteristics. The absorption peaks of P3HT∶PCBM∶DMSO thin film show a distinguished red shift and strong absorption compared to P3HT∶PCBM thin films in the visible light range. It was considered that the increase of the Jsc was supported by this phenomenon of UV-Visible absorption. The charge carrier mobility change of the P3HT∶DMSO films is studied by employing the donor-only devices. The increased performance should be attributed to the enhanced charge carrier mobility and widened absorption spectra of P3HT∶PCBM through doping DMSO.