Modulating Buried Interfaces via Co‐Assembled Monolayers With Siloxane‐Mediated Intermolecular Interactions for Perovskite/Silicon Tandem Solar Cells

Conventional phosphonic acid‐based SAMs, exemplified by [4‐(3,6‐dimethoxy‐9H‐carbazol‐9‐yl)butyl]phosphonic acid (MeO‐4PACz), readily aggregate, leading to non‐uniform buried‐interface coverage and constrained device output. Here, we engineer co‐assembled SAMs (Co‐SAMs) by co‐assembling MeO‐4PACz with trimethoxysilane‐anchored terminal siloxanes—epoxy (GOPS), thiol (TMSPT) and isocyanate (IPTMS)—to homogenize the NiO x /perovskite contact. The resulting Co‐SAMs form denser and more uniform interfacial layers on NiO x , boosting the open‐circuit voltage ( V oc ) and fill factor (FF) of 1.68 eV wide‐bandgap perovskite solar cells, with IPTMS‐based Co‐SAMs delivering the largest improvement. Performance correlates with a distinct evolution of siloxane‐mediated intermolecular interactions. We demonstrate that the evolution of intermolecular interactions from physical blending (MeO4‐GOPS) and specific hydrogen bonding (MeO4‐TMSPT) to covalent bridging (MeO4‐IPTMS) significantly enhances interfacial quality. Specifically, the phosphonate–carbamate covalent bridge formed in MeO4‐IPTMS most effectively suppresses aggregation and delivers synergistic defect passivation. Leveraging this strategy, monolithic perovskite/silicon tandem cells deliver a champion power conversion efficiency (PCE) of 33.74% (certified 33.37%). Furthermore, the devices exhibit exceptional stability, showing negligible degradation after 4000 h of dark storage and retaining 87.12% of their initial PCE after 1000 h of maximum power point tracking (MPPT) under continuous one‐sun illumination, highlighting the potential of this interfacial strategy for highly stable tandem photovoltaics.