Precursor‐Level Molecular Coupling Enables High‐Fill‐Factor Wide‐Bandgap Perovskite Solar Cells and 29.63% All‐Perovskite Tandems

ABSTRACT Wide‐bandgap (WBG) perovskites with a ∼1.78 eV bandgap are essential for monolithic all‐perovskite tandem cells, yet their efficiency and stability are limited by complex precursor chemistry, uncontrolled crystallization, defect formation, and photoinduced halide segregation. Here, we report 4‐(3‐Carbamimidoylguanidino)benzoic acid hydrochloride (CGBA) as a precursor‐level regulator for Cs/formamidinium (CsFA)‐based Br‐rich WBG perovskites. Through dynamic associations with lead–halide species and organic cations, CGBA modifies the local precursor environment and alters the stage‐dependent film‐formation behavior during spin coating and annealing. This regulation moderates crystallization, promotes texture development, reduces trap‐mediated nonradiative recombination, and suppresses photoinduced halide segregation. Consequently, the optimized 1.78 eV single‐junction device achieves a power conversion efficiency (PCE) of 21.43% and a fill factor (FF) of 85.89%, placing it among the highest reported FF values for 1.75–1.79 eV single‐junction WBG perovskite solar cells. The strategy is also transferable across multiple compositions and conditions, yielding efficiencies of 27.32% (1.53 eV), 23.91% (1.68 eV), 18.75% (1.85 eV), and 26.08% (1.58 eV, antisolvent‐free). Unencapsulated target devices retain 93% of their initial efficiency after 4500 h in N 2 . When integrated into a monolithic all‐perovskite tandem, the CGBA‐optimized WBG subcell enables a laboratory‐measured PCE of 29.63%. This work establishes precursor‐level regulation as a transferable strategy for efficient and stable WBG perovskites and all‐perovskite tandems.