A selenophene-modified PTB7, PBDTSe-TT, is reported. The structure adjustment carried out by alkylselenophene substitution on the BDT building block is shown to slightly affect the polymers electronic property, and an enlarged V-OC of the resulting photovoltaic device is observed. More importantly, the PBDTSe-TT:PC71BM bulk-heterojunction thin film morphology can be optimized through this modification. As a result, an efficient PCE of 8.8% is achieved without using any solvent additive or special interfacial layer. In addition, the PBDTSe-TT-based device is relatively stable under thermal stress, making it a good candidate for fabricating stacking cells. Finally, a similar to 10% PCE tandem device is demonstrated by using identical PBDTSe-TT:PC71BM subcells.