Multifunctional Cellulose Derivative Enables Efficient and Stable Wide‐Bandgap Perovskite Solar Cells by Inhibiting Ion Migration

Wide‐bandgap (>1.65 eV) perovskites, characterized by their high bromine content, are plagued by severe photo‐induced phase segregation, which inflicts irreversible damage on device performance and stability. A primary origin of this degradation is the migration of iodide ions (I − ) and their subsequent conversion to molecular iodine (I 2 ) under photothermal stress, leading to irreversible performance loss. To address this critical challenge, we report a novel hydrophobically modified cellulose derivative, L12‐CMCNa, that through interfacial engineering, markedly enhances the performance and stability of 1.68 eV wide‐bandgap perovskite solar cells (PSCs). The functional groups (‐COO − and ‐OH) of L12‐CMCNa anchor at the interface, suppressing iodide ion migration by passivating both uncoordinated Pb 2 + defects and iodide vacancies. Concurrently, L12‐CMCNa reacts with residual PbI 2 to form an in situ low‐dimensional perovskite capping layer. This dual‐passivation strategy, combining defect anchoring with the formation of a low‐dimensional perovskite barrier, synergistically immobilizes ions at the interface. Consequently, the L12‐CMCNa‐modified devices deliver a champion power conversion efficiency (PCE) of 23.25% with a fill factor (FF) of 83.80%. Furthermore, the modified devices exhibit exceptional stability. In accordance with ISOS protocols, the unencapsulated devices maintained over 90% of their initial efficiency after 1000 h of continuous one‐sun operation (ISOS‐L‐1I), 800 h of storage at 30% relative humidity (ISOS‐D‐1), and 1000 h of aging at 65°C (ISOS‐D‐2I), placing them among the most stable 1.68 eV WBG PSCs reported to date.