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 J_sc 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 V_oc of 1.58 V, a J_sc of 8.00 mA cm⁻², and a FF of 74% under AM 1.5 G illumination at 100 mW cm⁻². 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 for 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.

We have synthesized a series of conjugated D–π–A copolymers, PT-ttTPD and PBT-ttTPD, based on a (5-hexyltridecyl)-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione (ttTPD) acceptor unit in order to develop better photovoltaic polymers based on the TPD moiety: an ε-branched alkyl side chain on the TPD unit was coupled with 6-alkyl-thieno[3,2-b]thiophene (tt) π-bridge molecules. The Stille polymerization of the brominated ttTPD and stannylated simple thiophene (T) finally gave a promising PT-ttTPD polymer showing well-ordered inter-chain orientation in the BHJ active layer. PT-ttTPD-based OPVs exhibited a highest power conversion efficiency (PCE) of 9.21% (V_OC = 0.86 V, J_SC = 15.30 mA cm⁻², FF = 70%).