Realizing Over 20% Efficiency in Nonhalogenated Solvent-Processed Organic Solar Cells Via Linking Site Engineering of Giant Molecule Acceptors

Organic solar cells (OSCs) processed by nonhalogenated solvents often suffer from excessive aggregation of acceptors and poor donor–acceptor phase separation, which limits their power conversion efficiency (PCE). Changing the linking sites between bridge linking units and small molecule acceptor (SMA) subunits of giant molecule acceptors (GMAs) can modulate intermolecular interactions to suppress excessive molecular self-aggregation. In this work, we systematically synthesized three isomers of GMA, GMA-α, GMA-β, and GMA-γ to investigate the impact of linking site isomerization on the molecular properties and aggregation behaviors. By precisely tuning the linking site, the end groups of SMA subunits in GMA-α are driven deep into the space occupied by the inner alkyl chains of the SMA subunits, introducing significant steric hindrance that induces a more twisted backbone conformation, which effectively suppresses the excessive molecular self-aggregation. Thus, GMA-α-based OSCs processed by o-xylene exhibit more favorable phase separation and higher PCE than those of GMA-β- and GMA-γ-based ones. Furthermore, the ternary OSCs based on PM6:BTP-eC9:GMA-α yield the enhanced PCE of 20.20%, which is one of the highest PCE values for the nonhalogenated solvent-processed OSCs. This study highlights the significance of linking site isomerization as a powerful molecular design strategy of GMAs to fine-tune morphology and enhance the photovoltaic performance of the OSCs processed by nonhalogenated solvent.