Stepwise Molecular Engineering on BODIPY-Cored Nonfused Electron Acceptors for Organic Solar Cells

Taking advantages of boron dipyrromethene (BODIPY) cores and nonfused molecular skeletons, four A-D-A '-D-A-type nonfused-ring electron acceptors (NFREAs) are designed for organic solar cells, in which cyclopentadithiophene (CPDT) and halogenated dicyanoindanone are involved as electron-donating (D) pi-bridges and electron-accepting (A) terminal groups, respectively. Stepwise design strategies of twisting the molecular skeleton by steric hindrance, modifying pi-pi stacking by side chain engineering, and adjusting intermolecular interactions by different halogenation effects are employed. The planar BODIPY-cored NFREA of B-C8-F is first designed to show modest power conversion efficiency (PCE) of 4%. Steric tetramethyl is subsequently introduced onto the BODIPY core, resulting in up-shifted energy levels and blue-shifted and narrower absorption, whereas enhanced film state results in absorptivity for twisted MB series NFREAs. Remarkably improved PCE to 7.14% is achieved for MB-C8-F due to the reduced crystallinity for better morphological parameters and increased open-circuit voltage by similar to 0.3 V than planar B-C8-F. Successively, shortening the alkyl side chains in CPDT from 2-ethylhexyl to n-hexyl further increases the PCE to 10.67% for MB-C6-F, owing to improved intermolecular stacking intensity and more favorable blend morphology. The chlorine-terminated MB-C6-Cl demonstrates a slightly declined PCE of 9.03%. Further study on carrier dynamics reveals that the significantly higher PCE of twisted NFREAs than the planar analogue is attributed to the more efficient exciton dissociation and charge transport. Our work illuminates the promising prospects of the BODIPY unit in constructing efficient electron acceptors. In addition, in contrast with the commonly adopted coplanar geometry, the outperformed twisted NFREAs in this work may provoke specific considerations in developing NFREAs by manipulating the molecular planarity and configurations.