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 π-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 π-bridges, were synthesized: PBT without a π-bridge, PBT with a 3-octylthiophene (OT) π-bridge (PBT-OT), and PBT with a 3-octylthieno[3, 2-b]thiophene (OTT) π-bridge (PBT-OTT). The light absorption and charge transport properties were significantly enhanced upon incorporation of the OTT π-bridge. The enhancements resulted from the strong π-π intermolecular interactions using the OTT π-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 π-electrons were further delocalized along the copolymer after incorporation of the OTT π-bridge, and the charge separation efficiency increased. These results suggested that incorporating OTTπ-bridges into D–A copolymers provides a useful strategy for developing highly efficient OSCs.