Synergistic effects of an alkylthieno[3,2-b]thiophene π-bridging backbone extension on the photovoltaic performances of donor-acceptor copolymers

The synergistic effects of a thiophene-based pi-bridging backbone extension on the intrinsic and photovoltaic properties of electron donor-acceptor (D-A) copolymers were systematically investigated. A series of alternating D-A copolymers (PBTs) based on 4,8-bis(5-ethylhexylselenophen-2-yl)benzo[1,2-b:4,5-b']dithiophene (EHSeBDT) and 5-(2-butyloctyl)-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione (BOTPD), which featured thiophene-based pi-bridges, were synthesized: PBT without a pi-bridge, PBT with a 3-octylthiophene (OT) pi-bridge (PBT-OT), and PBT with a 3-octylthieno[3,2-b]thiophene (OTT) pi-bridge (PBT-OTT). The light absorption and charge transport properties were significantly enhanced upon incorporation of the OTT pi-bridge. The enhancements resulted from the strong pi-pi intermolecular interactions using the OTT pi-bridging backbone extension between neighboring polymer chains. PBT-OTT was most miscible in PC71BM. As a result, the photoactive layers prepared using PBT-OTT and PC71BM formed a well-mixed bulk-heterojunction morphology and yielded organic solar cells (OSCs) with a high power conversion efficiency of 7.21%. Transient absorption analysis suggested that the mu-electrons were further delocalized along the copolymer after incorporation of the OTT pi-bridge, and the charge separation efficiency increased. These results suggested that incorporating OTT pi-bridges into D-A copolymers provides a useful strategy for developing highly efficient OSCs.