Eu‐Doped CsSrCl ₃ Large Nanocrystal Clusters with Self‐Reduction Effect and Near‐Unity Quantum Yield

Europium halide perovskites have emerged as promising candidates for environmental‐friendly blue‐emitting materials. However, their development is hindered by low photoluminescence quantum yields (PLQY, e.g. ∼2%–5% for intrinsic CsEuCl 3 ) and poor stability against air. Here, a one‐step procedure is introduced for synthesizing Eu 2+ ‐doped CsSrCl 3 large nanocrystal clusters (CsSrCl 3 :Eu LNCs) with the effect of self‐reduction, eliminating the use of conventional reductant oleylamine (OAm) and ensuring phase purity. CsSrCl 3 :Eu LNCs show photoluminescence emission centered at 430 nm with a full width at half maximum (FWHM) of 25 nm and a PLQY of ∼40%, which can be further enhanced to ∼97% after passivating the surface defects by adding trioctylphosphine (TOP), the highest among all reported lead‐free blue‐emitting perovskite nanocrystals. The stability of CsSrCl 3 :Eu can also be improved significantly by epitaxially growing a ZnS shell on the surface. This work will shed more light on lanthanide and alkaline‐earth metal (AEM)‐based perovskites for nontoxic light‐emitting materials.