Ligand‐Enabled Chemical‐Potential Modulation of A‐Site Cation Reactivity Stabilizes Tin Halide Perovskite Nanocrystals

ABSTRACT Tin halide perovskite nanocrystals offer a lead‑free platform for optoelectronics, but their practical use is severely hampered by rapid oxidation degradation. However, conventional strategies that strength Sn 2+ participation and stability inevitably face a trade‐off between lattice Sn vacancies (V Sn ) formation and surface Sn 2+ exposure. Here, we move beyond Sn 2+ ‐centric strategies to an A‐site reactivity regulation strategy. By selectively suppressing A‐site cation reactivity, Sn 2+ efficiently incorporates during their crystallization, thereby simultaneously suppressing V Sn formation and lowering surface Sn 2+ exposure. Using CsSnBr 3 nanocrystal as model system, this strategy enables precise control over product composition and crystallization kinetics, yielding nanocrystal films can preserve > 92% of the perovskite phase and 81% of initial emission after 60 h of air exposure. Ligand exchange experiment decouples the surface and lattice effect, confirming the dominant role of lattice V Sn in determining air stability. Extending this strategy to CsSnCl 3 , FASnBr 3 and CsSnI 3 nanocrystals demonstrated its generality across A/X‑site chemistries. This work establishes a new strategy to control vacancy formation and surface redox processes. The ligand‐enabled chemical‐potential modulation provides a general and predictive means to access this control.