Optimized vertical phase separation via systematic Y6 inner side-chain modulation for non-halogen solvent processed inverted organic solar cells

Organic solar cells (OSCs) have entered a new phase of development with the advent of a high-performance non-fullerene acceptor known as Y6. Considering that the molecular curvature shape of Y6 allows for dense intermolecular packing with a grid-like packing structure, the modification of branched alkyl chains located on the pyrrole motif of Y6 is expected to have a significant impact on morphological properties, such as molecular packing/orientation behavior and vertical phase separation. In this work, we fine-tune the branched alkyl chain attached to nitrogen atoms of the Y6 main backbone and systematically investigate the relationship between the morphological features and photovoltaic performance of OSCs with a PM6 donor polymer. The systematic alkyl chain engineering effectively reduces the aggregation of Y6-analogs and substantially improves solubility, thereby providing solution processability in a non-halogenated solvent. Additional in-depth analyses reveal that the modification of branched alkyl chains allows the formation of an optimized bulk heterojunction morphology, which is advantageous for an inverted device architecture. In particular, the PM6:Y6-HU-based inverted OSC utilizing o-xylene as a processing solvent achieves a power conversion efficiency of 17.4% with an outstanding fill factor of 77.9 % under the binary blend system and single non-halogen solvent processing.