Synergistic Ion Engineering for Wide-Bandgap Perovskite Enables Perovskite–Organic Tandem Solar Cells with a Record Certified Voltage

Mixed-halide wide-bandgap (WBG) perovskites suffer from defect-induced open-circuit voltage (VOC) loss and photoinduced phase segregation. Herein, we introduce rubidium formate (RbFa) as a single dopant to mitigate both challenges through synergistic ion engineering. The Rb+ cation enters the lattice to induce contraction, while the formate anion (Fa–) modulates crystallization kinetics and passivates halide vacancies. This cooperative interaction maximizes residual compressive strain, thermodynamically raising the ion-migration barrier, while vacancy passivation kinetically blocks migration pathways. Consequently, we achieve a high VOC of 1.371 V and enhanced photostability for 1.86 eV WBG perovskite solar cells (PSCs). Monolithic perovskite–organic tandem solar cells deliver an efficiency of 26.03% with a record certified VOC of 2.189 V and retain over 90% of their initial efficiency after 1000 h under continuous 1-sun illumination.