Moving Alkyl-Chain Branching Point Induced a Hierarchical Morphology for Efficient All-Small-Molecule Organic Solar Cells

The optimization of bulk heterojunction morphology is one of the most challenging topics in all-small-molecule organic solar cells. Herein, three small molecular donors based on dithieno[2,3-d;2 ',3 '-d ']benzo[1,2-b;4,5-b ']dithiophene (DTBDT) unit by systematically moving the branching point of the alkyl chain have been designed, synthesized, and applied in organic solar cells. Modifying the branching points enables the properties of the aggregation state to be tuned, and an efficient nanofiber-based hierarchical morphology is successfully demonstrated by combining with different nonfullerene acceptors. The molecules with far branching points can form nanofibers in active layers, and theses nanofibers help the charge separation and charge transport in a large donor-rich or acceptor-rich domain of approximately 100 nm. Using nonfullerrene Y6 as an acceptor, the highest power conversion efficiency of 14.78% is obtained, which is one of the highest efficiencies in all-small-molecule organic solar cells. The strategy of modification of alkyl side chain branching points can be a practical way to actualize crystallinity control and active layer morphology for improving the performance of all-small-molecule organic solar cells.