Buried Interface Toughening and Multi-Site Defect Passivation for Thermally Stable Wide-Bandgap Perovskite Solar Cells

Wide-bandgap (WBG) perovskites offer distinct advantages in constructing efficient perovskite/silicon tandem devices owing to their favorable spectral matching. However, weak adhesion at the perovskite-substrate-buried interface often leads to inferior mechanical stability of WBG devices under prolonged light and thermal stress. Herein, a functional bridging molecule 2,2'-bipyridine-5-carboxylic acid (BCA) is introduced at the buried interface to simultaneously toughen the interface and enable multisite defect passivation. The pyridine and carboxyl groups in BCA synergistically passivate undercoordinated Pb²⁺ defects of the perovskite layer, while the aromatic bipyridine framework establishes strong π-π interactions with the self-assembled monolayer (SAM). This approach significantly reduces interfacial voids, suppresses nonradiative recombination, and optimizes carrier extraction, thereby boosting the efficiency of WBG perovskite solar cells (PSCs) from 20.30% to 21.83%. Besides, the reinforced interfacial shear strength (from 0.95 to 2.05 MPa) endows the devices with superior thermal stability, retaining 80% of their initial efficiency after continuous thermal aging at 85 °C for 560 h. Our findings provide insights on designing interface toughening layers toward high-efficiency and mechanically stable perovskite devices.