Tailoring Precursor‐Solvent Coordination Controls the Crystallization Kinetics and Nuclei Growth for Phase Homogenization in Wide‐Bandgap Perovskite Solar Cells

Volatile ammonium chloride (AC) regulates precursor‐solvent coordination and destabilizes hexagonal polytypes, promoting cubic‐phase formation and in situ defect self‐elimination in 1.73 eV wide‐bandgap perovskites. AC induces a de‐intercalated high‐valence iodoplumbate complex that suppresses the sol–gel state and balances nucleation, while Cl‐rich intermediates and NH4+−Cs+/FA+$N{H}_{4}^{+}-C{s}^{+}/F{A}^{+}$ cation exchange slow crystal growth, yielding devices with improved performance and excellent photostability. Abstract Wide‐bandgap (WBG) perovskites are promising top‐cell absorbers for tandem photovoltaics but suffer from unbalanced nucleation‐growth driven by strong coordination between polar aprotic solvents and perovskite precursors. This promotes the formation of hexagonal intermediate polytypes, stacking defects, halide‐cation migration, and phase segregation. Although long‐chain alkyl ammonium chlorides are used to control crystallization, the role of volatile ammonium chloride (AC) in altering the precursor chemistry and crystallization pathways in WBG perovskites remains unexplored. Present study, shows that AC weakens precursor‐solvent coordination and destabilizes undesired hexagonal polytypes. In situ characterizations indicate that AC induces high‐valence, de‐intercalated solvated iodoplumbate complexes that inhibit the sol–gel state and balance nucleation‐growth kinetics. Concurrently, Cl‐rich intermediates provide heterogeneous nucleation sites and, via cation exchange between NH4+ and Cs+/FA+ ions, retard uncontrolled crystal growth. The combined effect suppresses the formation of undesired phases, promotes transformation to the cubic perovskite phase, and enables defect self‐elimination during crystallization, yielding more homogeneous, higher‐quality films. As a result, AC‐treated perovskite films yield high‐quality 1.73 eV WBG perovskite solar cells with ≈18% PCE and a high Voc of 1.22 V, along with enhanced photostability. Volatile ammonium chloride (AC) regulates precursor-solvent coordination and destabilizes hexagonal polytypes, promoting cubic-phase formation and in situ defect self-elimination in 1.73 eV wide-bandgap perovskites. AC induces a de-intercalated high-valence iodoplumbate complex that suppresses the sol–gel state and balances nucleation, while Cl-rich intermediates and NH4+−Cs+/FA+$N{H}_{4}^{+}-C{s}^{+}/F{A}^{+}$ cation exchange slow crystal growth, yielding devices with improved performance and excellent photostability. Abstract Wide-bandgap (WBG) perovskites are promising top-cell absorbers for tandem photovoltaics but suffer from unbalanced nucleation-growth driven by strong coordination between polar aprotic solvents and perovskite precursors. This promotes the formation of hexagonal intermediate polytypes, stacking defects, halide-cation migration, and phase segregation. Although long-chain alkyl ammonium chlorides are used to control crystallization, the role of volatile ammonium chloride (AC) in altering the precursor chemistry and crystallization pathways in WBG perovskites remains unexplored. Present study, shows that AC weakens precursor-solvent coordination and destabilizes undesired hexagonal polytypes. In situ characterizations indicate that AC induces high-valence, de-intercalated solvated iodoplumbate complexes that inhibit the sol–gel state and balance nucleation-growth kinetics. Concurrently, Cl-rich intermediates provide heterogeneous nucleation sites and, via cation exchange between NH 4 + and Cs + /FA + ions, retard uncontrolled crystal growth. The combined effect suppresses the formation of undesired phases, promotes transformation to the cubic perovskite phase, and enables defect self-elimination during crystallization, yielding more homogeneous, higher-quality films. As a result, AC-treated perovskite films yield high-quality 1.73 eV WBG perovskite solar cells with ≈18% PCE and a high V oc of 1.22 V, along with enhanced photostability. Advanced Science, Volume 12, Issue 48, December 29, 2025.