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

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 N2. 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.