Poly(3-hexylthiphene) (P3HT):[6,6]-phenyl C-60-butyric acid methyl ester (PCBM) bulk heterojunction solar cells doped with a small molecule dye of rubrene were fabricated. The effect of different doping concentration on the performance of solar cells was investigated. The results showed that by doping 3.5 wt% rubrene, the short circuit current and the power conversion efficiency (PCE) of the cell were improved by 23.1% and 23.4%, respectively. The increased light absorption and effective charge separation of blend films contributed to the enhancement of PCE in doping devices. Moreover, by simulating the photocurrent of solar cells using Onsager-Braun theory, it was found that doping rubrene was responsible for the enhancement of charge transfer generation and dissociation, thereby improving the performance of P3HT: PCBM solar cells. (C) 2013 Elsevier B.V. All rights reserved.