Flexible organic solar cells were fabricated on a polyethylene terephthalate (PET) substrate using a phenylene-thiophene oligomer, 3,7-bis[5-(4-n-hexylphenyl)-2-thienyl]dibenzothiophene-5,5-dioxide (37HPTDBTSO), as a photoactive donor. In bulk-heterojunction (BHJ) solar cells, many factors such as blend weight ratio, and solution and substrate temperatures have great effects on the photovoltaic performances. The flexible solar cell fabricated from the blend solution with a weight ratio of 1:2 of 37HPTDBTSO to phenyl-C61-butyric acid methyl ester (PCBM) and a temperature of 50 degrees C, exhibited a high open-circuit voltage of 0.74V and the highest power conversion efficiency of 0.26%.