Title : Intermediate phase engineering in wide bandgap perovskites by tailoring precursor solvent interaction

Wide-bandgap (WBG) perovskites are highly attractive as top-cell absorbers for tandem photovoltaics; however, their crystallization is often governed by strong precursor–solvent coordination that induces unbalanced nucleation and growth, leading to hexagonal intermediate polytypes, stacking defects, ion migration, and phase segregation. Here, we demonstrate a volatile ammonium chloride (AC)-assisted crystallization strategy to regulate precursor chemistry and phase evolution in 1.73 eV WBG perovskites. Unlike conventional long-chain alkyl ammonium chlorides, AC dynamically modulates precursor–solvent interactions by weakening solvent coordination and destabilizing undesired hexagonal intermediates. In situ investigations reveal that AC promotes the formation of high-valence, de-intercalated solvated iodoplumbate complexes, thereby suppressing the persistent sol–gel state and balancing nucleation-growth kinetics. Simultaneously, transient Cl-rich intermediates act as heterogeneous nucleation centers, while cation exchange between NH₄⁺ and Cs⁺/FA⁺ retards uncontrolled crystal propagation and facilitates defect self-elimination during crystallization. Consequently, the formation of secondary phases is significantly suppressed, enabling rapid transformation into the desired cubic perovskite phase and yielding highly homogeneous, compact, and defect-minimized films. Devices fabricated using AC-engineered WBG perovskites deliver a power conversion efficiency approaching 18% with a high open-circuit voltage of 1.22 V, together with enhanced photostability. This work establishes volatile ammonium chloride engineering as an effective route for controlling intermediate-phase chemistry and crystallization pathways toward high-performance, stable WBG perovskite photovoltaics.

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