Highly Emissive Blue Quantum Dots with Superior Thermal Stability via In Situ Surface Reconstruction of Mixed CsPbBr(3)–Cs(4)PbBr(6) Nanocrystals

Although metal halide perovskites are candidate high‐performance light‐emitting diode (LED) materials, blue perovskite LEDs are problematic: mixed‐halide materials are susceptible to phase segregation and bromide‐based perovskite quantum dots (QDs) have low stability. Herein, a novel strategy for highly efficient, stable cesium lead bromide (CsPbBr(3)) QDs via in situ surface reconstruction of CsPbBr(3)–Cs(4)PbBr(6) nanocrystals (NCs) is reported. By controlling precursor reactivity, the ratio of CsPbBr(3) to Cs(4)PbBr(6) NCs is successfully modulated. A high photoluminescence quantum yield (PLQY) of >90% at 470 nm is obtained because octahedron CsPbBr(3) QD surface defects are removed by the Cs(4)PbBr(6) NCs. The defect‐engineered QDs exhibit high colloidal stability, retaining >90% of their initial PLQY after >120 days of ambient storage. Furthermore, thermal stability is demonstrated by a lack of heat‐induced aggregation at 120 °C. Blue LEDs fabricated from CsPbBr(3) QDs with reconstructed surfaces exhibit a maximum external quantum efficiency of 4.65% at 480 nm and excellent spectral stability.