Epitaxial Growth of Lead-Free Double Perovskite Shell for CsPbX₃/Cs₂SnX₆ (X = Cl, Br, and I) Core/Shell Perovskite Nanocrystals with Enhanced Photoelectric Properties and Stability

All-inorganic lead halide perovskite nanocrystals (NCs) have great optoelectronic properties with promising applications in light-emitting diodes (LEDs), lasers, photodetectors, solar cells, and photocatalysis. However, the intrinsic toxicity of Pb and instability of the NCs impede their broad applications. Shell-coating is an effective method for enhanced environmental stability while reducing toxicity by choosing non-toxic shell materials such as metal oxides, polymers, silica, etc. However, multiple perovskite NCs can be encapsulated within the shell material and a uniform epitaxial-type shell growth of well-isolated NCs is still challenging. In this work, lead-free vacancy-ordered double perovskite Cs₂SnX₆ (X = Cl, Br, and I) shells are epitaxially grown on the surface of CsPbX₃ NCs by a hot-injection method. The effectiveness of the non-toxic double perovskite shell protection is demonstrated by the enhanced environmental and phase stability against UV illumination and water. In addition, the photoluminescence quantum yields (PL QYs) increase for the CsPbCl₃ and CsPbBr₃ NCs after shelling because of the type I band alignment of the core/shell materials, while enhanced charge transport properties obtained from CsPbI₃/Cs₂SnI₆ core/shell NCs are due to the efficient charge separation in the type II core/shell band alignment.