In 1995,Yu et al.[1] first reported bulk-heterojunction (BHJ) solar cells with a conjugated polymer donor and a fullerene acceptor as the active materials.From then on,BHJ organic solar cells (OSCs) have attracted academic and industrial interests due to the advantages like lightweight,flexibility and roll-to-roll fabrication.Nowadays,17% power conversion efflciencies (PCEs) have been achieved in the state-of-the-art OSCs [2,3].The remarkable progress in OSCs relies on the continuously emerging new materials and device fabrication technologies,and the understanding on film morphology and device physics [4,5].