Light‐Induced Lattice Coherence and Emission Enhancement in PTM‐Passivated CsSnI ₃ Perovskites

Abstract Metal halide perovskites continue to lead in optoelectronic applications, but the toxicity of lead has driven efforts to identify environmentally benign alternatives. Cesium tin iodide (CsSnI 3 ) is one such, with a direct bandgap and near‐infrared emission, though its performance is limited by instability. We show that phthalimide (PTM) passivation during single crystal growth enhances optical output and stability. Under continuous excitation, PTM‐passivated microscale crystals show up to one order of magnitude increase in photoluminescence (PL) quantum yield, accompanied by reversible sharpening of a low‐frequency Raman mode associated with Cs⁺ rattling. This reveals dynamic, light‐induced lattice reordering that passivates trap states and enhances radiative recombination. Mechanical grinding yields nanocrystals with redshifted, narrowed PL, consistent with a relaxed polymorph and reduced inhomogeneous broadening. Despite increased surface area, PTM remains effective in preserving near‐infrared emission in nanocrystals. Power‐dependent PL reveals distinct carrier dynamics: microcrystals show redshift due to bandgap renormalization, while nanocrystals show blueshift and elevated carrier temperatures (300–1900 K), consistent with hot‐carrier recombination. Extended illumination reveals reversible optical changes, including PL modulation, reflecting dynamic light–matter interactions and evolving defect landscapes. These results identify PTM‐passivated CsSnI 3 as an ideal platform for probing morphology‐dependent carrier relaxation and light‐induced vibrational coherence in lead‐free perovskites.