Stabilization of Quantum‐Confined Anisotropic CsPbI₃ Nanoplatelets by Solid‐Phase Metal Iodide Crude Reaction for Color‐Pure Red Emission

Quantum‐confined CsPbI 3 perovskite nanoplatelets (NPLs) are highly desirable for optoelectronic applications owing to their anisotropic electronic properties that substantially boost the light outcoupling efficiency in light‐emitting diodes (LEDs). However, the structural instability of the emissive CsPbI 3 phases makes it degrade rapidly to the non‐emissive δ ‐phase under ambient conditions. Here, the study presents a synthetic approach to produce spectrally stable CsPbI 3 nanoplatelets (NPLs) through solid‐phase crude reactions with metal iodide powders, MI 2 (M 2+ = Mn 2+ or Zn 2+ ). The synthesized NPLs exhibit narrow and color‐pure red emission with high photoluminescence (PL) quantum yields (QYs, η PL ) of up to 85%. Systematic investigations into the surface chemistry of NPLs reveal that metal iodide treatment stabilizes anisotropic CsPbI 3 NPLs via surface passivation with metal and halide ions, substantially hindering from the formation of non‐emissive yellow phase. The anisotropic NPLs display signatures of spontaneous self‐assembly in spin‐casted films, which yield strong emission anisotropy, with up to 82% of the transition dipoles being horizontally oriented with respect to the substrate, as revealed by the back focal plane (BFP) imaging. The results presented here shed light on solid‐phase approaches for the preparation of quantum‐confined nanocrystals (NCs) with desirable geometry, which boost the light outcoupling efficiency in LEDs.