In this study, the enhanced performance of a conventional bulk heterojunction (BHJ) system based on poly({4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl}) (PTB7):[6,6]-phenyl C71-butyric acid methyl ester (PC71BM) is realized by adding a high charge mobility and low-cost small-molecular material of alpha-hexathienyl (alpha-6T) as an electron donor. Both the short circuit current (J(SC)) and fill factor (FF) are facilitated by adding 2 wt% alpha-6T to form a ternary system, resulting in a 22.2% improvement of power conversion efficiency (PCE) compared with the control binary device. The state-of-art BHJ layer was characterized by UV-Vis absorption, steady-state photoluminescence, space-charge-limited current, atomic force microscopy, and impedance spectra, and the enhanced device performance is mainly attributed to the Forster resonance energy transfer effect from alpha-6T to PTB7 and the increased charge mobility. The result also showed that the blend morphology can be modulated by the introduction of alpha-6T electron donor. Moreover, the incorporation of alpha-6T can contribute to the formation of more potential pathways for charge transportation in the active layer, and decrease the resistance of device.