Three selenophene-incorporated quaterchalcogenophene-based donor-acceptor copolymers PFBT2Th2Se, PFBT2Se2Th, and PFBT4Se are designed and synthesized. To systematically fine-tune the molecular properties and investigate the chalcogen effect, PFBT2Th2Se and PFBT2Se2Th hybridize two thiophenes and two selenophenes as the donor with different isomeric main-chain placement while thiophene-free PFBT4Se uses quaterselenophene as the donor. On account of the selenophene's advantageous features such as higher quinoidal population and higher molecular polarizability, the three polymers show good light-harvesting ability, strong intermolecular interactions, high crystallinity, and high charge mobilities. Bulk-heterojunction solar cells incorporating these selenophene-containing polymers have exhibited promising photovoltaic performance with impressive current densities over 20 mA/cm2. The device with the PFBT2Se2Th:PC71BM blend showed a PCE of 9.02% with a Jsc of 21.02 mA/cm2. In addition, the device using quaterselenophene-based PFBT4Se:PC71BM blend exhibited a PCE of 8.92% with a superior Jsc of 22.63 mA/cm2 which represents one of the highest current densities from polymer:fullerene-based solar cells reported in the literature.