Nanoparticle-Reinforced Self-Assembled Molecular Interfaces Enable Mechanically Robust Flexible Organic Solar Cells
Self-assembled molecular interlayers (SAMs) are promising hole-selective contacts for high-efficiency organic solar cells (OSCs) due to their well-defined energy alignment and minimal parasitic absorption. However, their intrinsically limited mechanical robustness often leads to structural degradation and performance loss under mechanical deformation, restricting their application in flexible devices. Here, we report a nanoparticle-reinforced self-assembled composite interface that simultaneously enhances mechanical reliability and optoelectronic performance. Uniformly dispersed SiO2 nanoparticles are introduced as high-modulus reinforcing building blocks without disturbing molecular self-assembly. In contrast to NiOx nanoparticles, which suffer from aggregation and parasitic absorption, SiO2 nanoparticles exhibit excellent dispersion and optical transparency, enabling formation of a structurally compatible hybrid interface. Mechanistic studies reveal that SiO2 nanoparticles redistribute interfacial stress and form dynamic hydrogen-bond networks with phosphonic acid groups of 2PACz, providing efficient energy dissipation during cyclic deformation. Meanwhile, modulation of interfacial polarity extends the crystallization time window of the active layer, resulting in enhanced molecular ordering and improved charge transport. As a result, devices based on the SiO2/2PACz composite interface achieve a power conversion efficiency of 20.14% for rigid devices and 19.30% for flexible devices, placing the flexible devices among the highest-performing flexible OSCs reported to date, while retaining over 90% of their initial efficiency after repeated bending cycles. This work establishes a general strategy for overcoming the trade-off between electronic selectivity and mechanical robustness in ultrathin self-assembled molecular interfaces, providing design insights for high-performance flexible organic optoelectronics.