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CsPbBr3 nanocrystals passivated with short molecular ligands and deposited on a substrate were annealed from room temperature to 400 °C in inert atmosphere. Chemical, structural, and morphological transformations were monitored in situ and ex situ by different techniques, while optoelectronic properties of the film were also assessed. Annealing at 100 °C resulted in a 1 order of magnitude increase in photocurrent and photoresponse as a result of partial sintering of the NCs and residual solvent evaporation. Beyond 150 °C the original orthorhombic NCs were partially transformed into tetragonal CsPb2Br5 crystals, due to the desorption of weakly bound propionic acid ligands. The photocurrent increased moderately until 300 °C although the photoresponse became slower as a result of the formation of surface trap states. Eventually, annealing beyond 350 °C removed the strongly bound butylamine ligands and reversed the transition to the original orthorhombic phase, with a loss of photocurrent due to the numerous defects induced by the stripping of the passivating butylamine.

Films of colloidal CsPbX3 (X = I, Br or Cl) nanocrystals, prepared by solution drop-casting or spin-coating on a silicon substrate, were exposed to a low flux of X-rays from an X-ray photoelectron spectrometer source, causing intermolecular C═C bonding of the organic ligands that coat the surface of the nanocrystals. This transformation of the ligand shell resulted in a greater stability of the film, which translated into the following features: (i) Insolubility of the exposed regions in organic solvents which caused instead complete dissolution of the unexposed regions. This enabled the fabrication of stable and strongly fluorescent patterns over millimeter scale areas. (ii) Inhibition of the irradiated regions toward halide anion exchange reactions, when the films were exposed either to halide anions in solution or to hydrohalic vapors. This feature was exploited to create patterned regions of different CsPbIxBryClz compositions, starting from a film with homogeneous CsPbX3 composition. (iii) Resistance of the films to degradation caused by exposure to air and moisture, which represents one of the major drawbacks for the integration of these materials in devices. (iv) Stability of the film in water and biological buffer, which can open interesting perspectives for applications of halide perovskite nanocrystals in aqueous environments.

Methylammonium lead iodide (MAPI) has excellent properties for photovoltaic applications, although it typically shows low photoluminescence quantum yield. Here, we report on vacuum-deposited MAPI perovskites obtained by modifying the methylammonium iodide (MAI) to PbI2 ratio during vacuum deposition. By studying the excitation density dependence of the photoluminescence lifetime, a large concentration of trap states was deduced for the stoichiometric MAPI films. The use of excess MAI during vacuum processing is capable of passivating these traps, resulting in luminescent films which can be used to fabricate planar light-emitting diodes with quantum efficiency approaching 2%.