Cosolvent-Modulated Donor Preaggregation Enhances Molecular Order in 20% Efficient Bilayer Organic Solar Cells.

Solvent selection critically modulates the aggregation behavior within the active layer, ultimately determining the performance of bilayer organic solar cells (OSCs). However, good solvents (e.g., chloroform and chlorobenzene) for polymer donors typically exhibit moderate boiling points, which suppress rapid aggregation, resulting in a smaller aggregate size and ultimately affecting charge transport properties. Herein, we demonstrate that trace DCM addition to CF modulates donor preaggregation, which enables precise fiber size control, increases the crystallinity, and enhances molecular ordering, thereby facilitating efficient charge transport. Therefore, bilayer OSCs processed with a cosolvent based on CF and DCM suppress bimolecular recombination, reduce trap density, extend carrier lifetime, and enhance carrier mobility, thereby improving charge extraction efficiency, achieving a notable 20.0% efficiency, and establishing it as one of the most efficient binary bilayer OSCs reported to date. The present strategy sheds light on manipulating donor preaggregation states to achieve superior molecular organization, laying the groundwork for advancing bilayer OSC performance.