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While the layered hybrid Ruddlesden-Popper (RP) halide perovskites have already established themselves as the frontrunners among the candidates in optoelectronics, their all-inorganic counterparts remain least explored in the RP-type perovskite family. Herein, we study and compare the optoelectronic properties of all-inorganic CsPbBr 3 perovskite nanocrystals (PNCs) with and without RP planar faults. We find that the RP-CsPbBr 3 PNCs possess both higher exciton binding energy and longer exciton lifetimes. The former is ascribed to a quantum confinement effect in the PNCs induced by the RP faults. The latter is attributed to a spatial electron-hole separation across the RP faults. A striking difference is found in the up-conversion photoluminescence response in the two types of CsPbBr 3 PNCs. For the first time, all-inorganic RP-CsPbBr 3 PNCs are tested in light-emitting devices and shown to significantly outperform the non-RP CsPbBr 3 PNCs.

Lead halide perovskites provide a test bed for exploring nonlinear optical properties. Although the underlying centrosymmetric crystal structure of 3D lead halide perovskites precludes the phenomenon of second harmonic generation, the third and higher-order harmonic generation are allowed. In this work, we probe the third harmonic generation (THG) from CsPbBr 3 nanocrystals (NCs) and compare it to the THG from CsPbBr 3 NCs with Ruddlesden-Popper planar faults (RP-CsPbBr 3 ), formed via postsynthetic fusion-growth. The THG from CsPbBr 3 NCs is negligible compared with that of RP-CsPbBr 3 NCs within a wide range of femtosecond excitation wavelengths. We further compare the THG from a thin film of RP-CsPbBr 3 with that of a single crystal of methylammonium lead bromide (MAPbBr 3 ). The THG efficiency of RP-CsPbBr 3 is found to be three times greater than that of MAPbBr 3 . An effective third-order susceptibility of the order of 10 -18 m 2  V -2 is obtained for a RP-CsPbBr 3 film, opening up the prospect of inorganic halide perovskite NCs with planar defects for a range of nonlinear optical applications.

To evaluate the role of planar defects in lead-halide perovskites-cheap, versatile semiconducting materials-it is critical to examine their structure, including defects, at the atomic scale and develop a detailed understanding of their impact on electronic properties. In this study, postsynthesis nanocrystal fusion, aberration-corrected scanning transmission electron microscopy, and first-principles calculations are combined to study the nature of different planar defects formed in CsPbBr 3 nanocrystals. Two types of prevalent planar defects from atomic resolution imaging are observed: previously unreported Br-rich [001](210)∑5 grain boundaries (GBs) and Ruddlesden-Popper (RP) planar faults. The first-principles calculations reveal that neither of these planar faults induce deep defect levels, but their Br-deficient counterparts do. It is found that the ∑5 GB repels electrons and attracts holes, similar to an n-p-n junction, and the RP planar defects repel both electrons and holes, similar to a semiconductor-insulator-semiconductor junction. Finally, the potential applications of these findings and their implications to understand the planar defects in organic-inorganic lead-halide perovskites that have led to solar cells with extremely high photoconversion efficiencies are discussed., (© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)