A Universal Strategy to Stabilize Crystal Lattice and Composition of Wide‐Bandgap Perovskites for Efficient Tandem Solar Cells

ABSTRACT The film quality of wide‐bandgap (WBG) perovskites is critical for achieving high‐efficiency perovskite/organic tandem solar cells (POTSCs). However, the Br‐rich WBG perovskites often suffer from inhomogeneous crystallization, leading to severe phase‐segregation and substantial non‐radiative energy losses. Here, cyanates are rationally designed to modulate the crystallization of WBG perovskites. RbOCN is successfully incorporated into the perovskite crystal lattice, optimizing the cation‐anion composition distribution, reducing the lattice constant, and inducing a blue‐shift in the band edge. These synergistic effects produce highly crystalline, phase‐stable WBG perovskites, yielding an impressive efficiency of 22.45% for a 1.73 eV perovskite device (0.09 cm 2 ). Moreover, RbOCN exhibits broad applicability across WBG perovskites with varying band gaps (1.79 eV, 1.85 eV, and 1.92 eV). These optimized sub‐cells are subsequently integrated with organic sub‐cells to fabricate POTSCs. Benefiting from well‐aligned spectral responses, an exceptional efficiency of 26.75% is achieved for POTSCs (0.09 cm 2 ) based on 1.85 eV perovskite sub‐cells. Notably, the strategy demonstrates excellent scalability, delivering an impressive efficiency of 25.37% and a record open‐circuit voltage of 2.22 V for 1 cm 2 POTSCs. This study establishes a robust approach for mitigating inhomogeneous crystallization and stabilizing the crystal lattice in WBG perovskites, thereby advancing the development of high‐performance TSCs.