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

Abstract 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.