Suppressing Molecular Aggregation via Alkyl Chain Steric Hindrance Enabled High-Performance Self-Assembly Monolayer-Based Organic Solar Cells.

Interfacial inhomogeneity and carrier recombination caused by molecular aggregation in self-assembled monolayers (SAMs) remain critical challenges for organic solar cells. Herein, we report an interfacial engineering strategy utilizing octylphosphonic acid (8PA) as a morphology-modulating additive. By blending 8PA with [4-(3,6-diphenyl-9H-carbazol-9-yl)butyl]phosphonic acid (Ph-4PACz), we achieve molecular-level miscibility via compatible phosphonic acid anchoring groups, while the extended alkyl chain of 8PA introduces steric hindrance that effectively suppresses π-π molecular aggregation. Experimental results demonstrate that this strategy significantly inhibits the self-agglomeration of Ph-4PACz, thereby enhancing the homogeneity and surface coverage of the resultant monolayer and optimizing molecular ordering. Devices incorporating the 8PA-modified SAM exhibit uniform phase separation morphology, reduced recombination centers, and facilitated charge transport. Consequently, the optimized device achieves a champion power conversion efficiency (PCE) of 19.8%, representing a significant improvement over the 18.2% observed in control devices. This work provides an effective pathway for enhancing the performance of organic photovoltaic devices through additive-driven SAM morphology control.