Modulation of Donor Alkyl Terminal Chains with the Shifting Branching Point Leads to the Optimized Morphology and Efficient All-Small-Molecule Organic Solar Cells

Terminal group modification is one of the most influential factors for small-molecular donors compared with their polymer counterparts, resulting in an opportunity to optimize the morphology of all-small-molecule organic solar cells (ASM-OSCs). In this article, we report three novel small-molecular donors with branching points at the 1-, 2-, and 3-positions in alkyl terminal chains, called BSCl-C1, BSCl-C2, and BSCl-C3, respectively. Using IDIC-4Cl as the acceptor, the subtle branching position shift achieves a dramatic disparity in photovoltaic parameters, as indicated by the short circuit current (Jsc) changing from 4.9 to 20.1 to 14.2 mA cm–2 and the fill factor varying from 33.9 to 71.3 to 67.0% for BSCl-C1, BSCl-C2, and BSCl-C3, respectively. The best device performance of 12.40% is obtained by the BSCl-C2:IDIC-4Cl system, which not only ranks among the top values reported to date but also exhibits low energy loss in systems that use IDIC as acceptors. The notable device performance based on BSCl-C2 is attributed to the optimized phase morphology caused by the strong molecular crystallinity and suitable intermolecular interaction with IDIC-4Cl. These results demonstrate that suitably tuning the branching position of terminal groups could promote the high performance of ASM-OSCs.