Stoichiometry-Regulated Crystallization and Halide Homogenization in Wide-Bandgap Perovskites for Efficient Solar Cells and Tandems.

Control over crystallization pathways and halide distribution is essential for realizing efficient and stable wide-bandgap (WBG) perovskite solar cells (PSCs), yet most advances rely on additives while the intrinsic role of precursor stoichiometry remains insufficiently understood. Here we show that precursor stoichiometry, specifically the balance between FAX and PbX2 (X = I, Br), serves as an additive-free chemical lever that regulates precursor supersaturation and nucleation kinetics, thereby directing crystallization and halide homogenization in FA0.83Cs0.17Pb(IxBr1-x)3 perovskites. We identify a stoichiometric regime with a PbI2-to-total PbX2 ratio of ∼55-75% that minimizes supersaturation, increases nucleation energy barrier, and slows crystal formation, yielding perovskite films with full coverage, preferred crystallographic orientation, and reduced residual strain. The optimized chemistry enables efficient and photostable WBG PSCs across a broad bandgap range (∼1.67, 1.79, 1.85, and 1.93 eV), achieving champion power conversion efficiencies of ∼24.19%, 21.64%, 19.98%, and 16.83%, respectively. Furthermore, this strategy enables monolithic all-perovskite tandem solar cells that deliver ∼29.10% efficiency with >80% retention over 400 h of maximum-power-point tracking. By establishing mechanistic links between precursor stoichiometry, crystallization pathway, electronic homogeneity, and device stability, this work provides a rational and broadly applicable chemical framework for advancing WBG perovskites in single-junction and tandem photovoltaics.