The asymmetric thienobenzodithiophene (TBD) structure is first systematically compared with the benzo[1,2-b:4,5-b']dithiophene (BDT) and dithieno[2,3-d:2',3'-d']benzo[1,2-b:4,5-b']dithiophene (DTBDT) units in donor-acceptor (D-A) copolymers and applied as the central core in small molecule acceptors (SMAs). Specific polymers including PBDT-BZ, PTBD-BZ, and PDTBDT-BZ with different macromolecular conformations are synthesized and then matched with four elaborately designed acceptor-donor-acceptor (A-D-A) SMAs with structures comparable to their donor counterparts. The resulting polymer solar cell performance trends are dramatically different from each other and highly material-dependent, and the active layer morphology is largely governed by polymer conformation. Because of its more linear backbone, the PTBD-BZ film has higher crystallinity and more ordered and denser π-π stacking than those of the PBDT-BZ and PDTBDT-BZ films. Thus, PTBD-BZ shows excellent compatibility with and strong independence on the SMAs with varied structures, and PTBD-BZ-based cells deliver high power conversion efficiency (PCE) of 10-12.5%, whereas low PCE is obtained by cells based on PDTBDT-BZ because of its zigzag conformation. Overall, this study reveals control of molecular conformation as a useful approach to modulate the photovoltaic properties of conjugated polymers.
Keywords: asymmetrical backbone; molecular conformation; morphology; nonfullerene solar cell; power conversion efficiency.