We fabricated organic solar cells (OSCs) using poly[2,6(4,4'bis(ethylhexyl)dithieno[3,2-b:2 ',3 '-d]silole)-alt-(1,3-(5-octyl-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione))] (PD-TSTPD) and (6,6)-phenyl-C-71 butyric acid methyl ester (PC71BM) active materials. PDTSTPD and PC71BM solutions for the active layer were mixed at concentration ratios of 1:1, 1:2, 1:3, and 1:4wt%, respectively. The effects of OSCs at various concentrations of active layer on the electrical and morphological properties of the device were investigated. The performance of the OSC at a concentration ratio of 1:3wt% showed the best electrical properties among the prepared samples, indicating that the maximum short circuit current density (J(sc)), open circuit voltage (V-oc) and power conversion efficiency (PCE) values were about 6.9mA/cm(2), 0.849V and 2.3%, respectively. PDTSTPD:PC71BM active layer with a concentration ratio of 1:3wt% showed smooth film roughness compared to the other prepared samples, which may lead to an improvement of the electron injection efficiency into the cathode electrode.