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

Abstract

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 OTT π-bridges into D–A copolymers provides a useful strategy for developing highly efficient OSCs.