Herein, the synthesis of CsPbX3 nanocrystals (NCs) is reported, and the effect of humidity during the synthesis of perovskite NCs is investigated. Generally, lead halide perovskite NCs degrade very quickly in the presence of moisture. Contrary to previous observations, it is found that the CsPbX3 (X = Cl, Br, I) NCs synthesized in the presence of an optimal amount of humidity show higher stability and significantly improved optical properties. At optimum humidity, it is found that water vapor helps in healing the surface defects commonly present on the CsPbX3 NCs, reduces the nonradiative pathways, and leaves behind the nearly perfect unit cell of CsPbX3 in the solution. Due to the reduction in the surface states, this simple protocol increases photoluminescence quantum yield (PLQY) of NCs by more than 90%. CsPbBr3 NCs synthesized at 30% relative humidity (RH) conditions retain their phase, shape, and size for more than 1 year with negligible reduction in PLQY under 65% RH conditions. Similar findings are observed for CsPbI3 NCs, where the NCs are stable for 2 months. This synthesis approach offers a new pathway to synthesize high‐quality perovskite NCs with excellent optical properties and outstanding stability for optoelectronic applications.

Herein we demonstrate a facile approach for the synthesis of all inorganic cesium lead halide perovskite nanocrystal composites CsPbX3 (X = Cl, Br, I) with high quantum yield by post-synthetic modulation of zero dimensional Cs4PbBr6 nanocrystals with ZnX2 salts.

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