Bright Structural‐Phase‐Pure CsPbI ₃ Core‐PbSO ₄ Shell Nanoplatelets With Ultra‐Narrow Emission Bandwidth of 77 meV at 630 nm

Abstract 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, CsPbI ₃ halide perovskite nanoplatelets (NPLs) with rigorously controlled 2D [PbI ₆ ] ⁴⁻ octahedron layer number ( n ) are demonstrated. A perovskite core‐PbSO ₄ 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 CsPbI ₃ NPLs are demonstrated, exhibiting emission peaks around 630 nm, with very narrow spectral bandwidths of <24 nm and high absolute photoluminescence quantum yields up to 85%. The emission of n = 4 NPLs falls exactly within the pure‐red region, closely aligning with the International Telecommunication Union Recommendation BT.2020 standard. Measurements suggest predominant stability and color homogeneity compared to traditional red‐emitting CsPbI _x Br _{3− x} nanocrystals. Finally, proof‐of‐concept pure‐red emissive light‐emitting diodes (LEDs) are demonstrated by integrating n = 4 CsPbI ₃ NPLs films with a blue LED chip, showing an excellent external quantum efficiency of 18.3% and high brightness exceeding 3 × 10 ⁶ nits. Stringent requirements for future display technologies, are satisfied based on the high color purity, stability, and brightness of CsPbI ₃ NPLs.