Conjugated donor (D)-π-acceptor (A) copolymers, PBDT-TPD, PBDT-ttTPD, PBDTT-TPD, and PBDTT-ttTPD, based on a benzodithiophene (BDT) donor unit and thieno[3,4-c]pyrrole-4,6(5H)-dione (TPD) acceptor unit were designed and synthesized with different π bridges via Pd-catalyzed Stille-coupling. The π bridges between BDT and TPD were thiophene in PBDT-TPD and PBDTT-TPD, and 6-alkylthieno[3,2-b]thiophene in PBDT-ttTPD and PBDTT-ttTPD. The effects of the π bridges on the optical, electrochemical, and photovoltaic properties of the polymers were investigated, in addition to the film crystallinities and carrier mobilities. Copolymers with the 6-alkylthieno[3,2-b]thiophene π-bridge exhibited high crystallinity and hole mobility. Improved Jsc and FF were obtained to increase the overall power conversion efficiencies (PCE) in inverted single organic photovoltaic cells. A PCE of 6.81% was achieved from the inverted single device fabricated using the PBDTT-ttTPD:PC71BM blend film with 3 vol% 1,8-diiodooctane. A tandem photovoltaic device comprising the inverted PBDTT-ttTPD cell and a PTB7-based cell as the bottom and top cell components, respectively, showed a maximum PCE of 9.35% with a Voc of 1.58 V, Jsc of 8.00 mA/cm2, and FF of 74% under AM 1.5 G illumination at 100 mW/cm2. The obtained PCE of the bottom cell and FF of the tandem cell are, to the best of our knowledge, the highest reported to date for a tandem OPV device. This work demonstrates that PBDTT-ttTPD may be very promising in applications in tandem solar cells. Furthermore, 6-alkylthieno[3,2-b]thiophene π-bridge systems in medium bandgap polymers can improve the performance of tandem organic photovoltaic cells.