The temperature-dependent photoluminescence (PL) spectra and lifetimes, and thermal stability of ordered assemblies, of methylammonium lead iodide (CH3NH3PbI3, MAPI) and CsPbX3 (X=Cl, Br, I) nanocrystals were studied. These nanocrystals generally exhibit decreased PL lifetimes and decreased emission energy with lower temperature, unlike most semiconductors. Luminescent MAPI nanocrystals with cuboidal shape and tetragonal crystal structure are terminated by (110) and (002) facets based on grazing-incidence wide-angle scattering (GIWAXS) and high resolution transmission electron microscopy (TEM) and electron diffraction. In situ GIWAXS and GISAXS of the MAPI superlattices heated under nitrogen exhibit a phase transition from tetragonal-to-cubic structure at 60oC, similar to bulk MAPI. The MAPI nanocrystals start to decompose to PbI2 at about 90oC, which is 10oC lower reported for bulk MAPI thin films, and at slightly higher temperatures, the coexistence of hexagonal and rhombohedral PbI2 is observed. As PbI2 precipitates, there is simultaneously fusion of the MAPI nanocrystals in the superlattice. The thermal stability of superlattices of γ-orthorhombic CsPbI3 nanocrystals capped with either oleic acid (OA) or phosphinic acid (PA) were also studied. The capping ligands influences the orientation of the nanocrystals on the substrate, with OA and PA capping yielding nanocrystals oriented with (001)SL and (002)NC planes, or (110)NC and (002)NC planes, respectively, on the substrate. Heating the CsPbI3 nanocrystal assemblies to 300°C induced a transition from the γ-orthorhombic to the δ–orthorhombic (yellow phase) and the nanocrystals capped with PA were found to be more thermally stable than those capped with OA.

Temperature-dependent photoluminescence and stability of perovskite nanocrystal superlattices

2019 Spring National Meeting

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