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Hybrid lead halide perovskites have generated increasing attention for their high performance in optoelectronic applications. One main drawback of these materials is the sensitivity to moisture, which contributes heavily to the ultimate decomposition. Here, this study takes advantage of the reported representative white‐light emitting lead bromide perovskite and three‐dimensional (3D) perovskite as the subject of water‐stability study. It is reported here that other than observed phase transformation in one case, the majority of these perovskites break down in water rapidly, precipitating out as an inorganic salt compound Pb3Br6[H2O]·H2O. As these materials are studied in detail, it is discovered that better coverage of the hydrophobic organic cation enhances the water stability. Taking this idea further, it is demonstrated here that five new one‐dimensional (1D) lead bromide compounds have superior water stability, where the longest stability when completely immersed in water is more than one year and counting. This work provides a new mechanism regarding the degradation process when encountering moisture and general guidelines to increase the water stability for the lead bromide hybrids. [ABSTRACT FROM AUTHOR]

Core–shell perovskite nanocrystals (NCs) have emerged as a promising class of materials for optoelectronic applications due to their unique properties and stability. Although much research has been conducted on the shell growth mechanism and properties of the core–shell NCs, little is known about the role of shell thickness in determining their optical properties. Herein, a heteroepitaxial method is employed to prepare a series of core–shell FAPbBr3/CsPbBr3 NCs with different shell thicknesses. Through temperature‐dependent photoluminescence measurement, it is found that the electronic structure of core–shell FAPbBr3/CsPbBr3 NCs evolves from quasi‐type‐II to type‐I with the increase in shell thickness. This abnormal transition can be attributed to the thickened gradient alloy layer FA1‐xCsxPbBr3, which effectively relieves the lattice mismatch and releases strain in the NCs, resulting in variations in the bandgap between the core and the shell. Furthermore, the biexciton Auger lifetimes in these samples exhibit a non‐monotonic dependence on shell thickness, indicating the electronic structure transition. These results provide valuable insights into the relationship between shell thickness, electronic structure, and optical properties in core–shell perovskite NCs, which may offer guidance for the design of high‐performance optoelectronic devices. [ABSTRACT FROM AUTHOR]