Bulk-heterojunction solar cells are reported with an enhanced power conversion efficiency (PCE) based on a newly designed semiconducting selenophene-thienopyrrolodione (TPD) copolymer blended with [6,6]-phenyl C71 butyric acid methyl-ester. The solar cells are fabricated using simple solution processing (implying low-cost fabrication). The relatively deep highest occupied molecular orbital (HOMO) level leads to a correspondingly high open-circuit voltage of 0.88 V. The PCE approaches 5.8% when Clevious P VP AI4083 is used as the hole-transport interlayer, with an optimized active layer thickness of approximately 95 nm, and a donor-acceptor blend ratio of 1:1. A fill factor (FF) of 0.62 is achieved. The use of additives does not seem to be beneficial in this blended system, due to the achievement of proper phase separation in the as-cast films. Also, the BHJ devices with a 3% ratio of a 1-chloronaphthalene (CN) additive exhibit much more severe oxidative degradation from the decreased FF with a high series resistance than BHJ devices without additive. The selenophene-TPD based BHJ solar cell is a promising candidate for high-performance single cells with a low-cost additive-free fabrication and a long-term stable operation.