Improving Photovoltaic Performance of a Fused-Ring Azepinedione Copolymer via a D-A-A Design.

Two conjugated copolymer donors, PTTABDT and PBTTABDT, based on a fused-ring azepinedione acceptor unit, 5-(2-octyldodecyl)-4H-thieno[2',3':4,5]thieno[3,2-c]thieno[2',3':4,5]thieno[2,3-e]azepine-4,6(5H)-dione (TTA), are prepared. PTTABDT possesses a conventional donor-acceptor (D-A) structure with one TTA in the repeat unit, while PBTTABDT has a D-A-A structure with two TTAs in the repeat unit. Compared with PTTABDT, PBTTABDT shows a deeper highest occupied molecular orbital (HOMO) level, a narrower bandgap, and a higher hole mobility, and exhibits better performance in bulk heterojunction solar cells. Power conversion efficiencies of 6.18% and 7.81% are achieved from PTTABDT:PC71 BM and PBTTABDT:PC71 BM solar cells, respectively. The higher performance of PBTTABDT:PC71 BM solar cells results from the enhanced open-circuit voltage (V oc ) and short-circuit current density (  J sc ).