Bright Structural‐Phase‐Pure CsPbI3 Core‐PbSO4 Shell Nanoplatelets With Ultra‐Narrow Emission Bandwidth of 77 meV at 630 nm

Achieving a narrow emission bandwidth is long pursued for display applications. Among all primary colors, obtaining pure red emission with high visual perception is the most challenging. In this work, CsPbI3 halide perovskite nanoplatelets (NPLs) with rigorously controlled 2D [PbI6]4− octahedron layer number (n) are demonstrated. A perovskite core‐PbSO4 shell structure is designed to prevent aggregation and fusion between NPLs, enabling consistent thickness and quantum confinement strength for each NPL. Consequently, exact n = 4 CsPbI3 NPLs are demonstrated, exhibiting emission peaks around 630 nm, with very narrow spectral bandwidths of