Metal halide perovskite nanocrystals offer a range of interesting properties and are being studied extensively for applications in solar cells, photodetectors and light-emitting devices. This perspective provides a number of best practices for the synthesis, purification, and characterization of metal halide perovskite nanocrystals, with detailed discussion of CsPbI3, CsPbBr3, CH3NH3PbI3 (MAPI), and Cs2AgBiBr6 as examples. The choice of reactants and ligands for hot-injection reactions are discussed, as well as how various reaction conditions, including temperature and time, affect yield, uniformity, and crystal phase. We extensively discuss the use of antisolvent precipitation methods for purification, since ligand coordination to most perovskite nanocrystals is weak and the nanocrystals are sensitive to degradation. Finally, we discuss some of the strategies for imaging these nanocrystals using transmission electron microscopy (TEM).

We report a detailed study on APbX3 (A = formamidinium (FA+), Cs+; X = I–, Br–) perovskite quantum dots (PQDs) with combined A- and X-site alloying that exhibits both a wide bandgap and high open-circuit voltage (V oc) for the application of a potential top cell in tandem junction photovoltaic (PV) devices. The nanocrystal alloying affords control over the optical bandgap and is readily achieved by solution-phase cation and anion exchange between previously synthesized FAPbI3 and CsPbBr3 PQDs. Increasing only the Br– content of the PQDs widens the bandgap but results in shorter carrier lifetimes and associated V oc losses in devices. These deleterious effects can be mitigated by replacing Cs+ with FA+, resulting in wide-bandgap PQD absorbers with improved charge-carrier mobility and PVs with higher V oc. Although further device optimization is required, these results demonstrate the potential of FA1–x Cs x Pb­(I1–x Br x )3 PQDs for wide-bandgap perovskite PVs with high V oc.

The incorporation of monovalent silver (Ag + ) cations into methylammonium lead bromide (CH 3 NH 3 PbBr 3 ) perovskite films leads to a strongly preferred (001) crystallographic orientation on a wide variety of substrates, ranging from glass to mesoporous TiO 2 . CH 3 NH 3 PbBr 3 films deposited without Ag + exhibit only a weakly preferred (011) orientation. Compositional maps and depth profiles from time-of-flight secondary ion mass spectrometry (TOF-SIMS) reveal Ag + segregated to grain boundaries and interfaces. In photovoltaic devices (PVs), addition of Ag + to MAPBr films resulted in poorer device performance, most likely because of the observed Ag + segregation in the films.

Light-induced changes in photophysical and electronic properties in metal halide perovskites can affect their performance in photovoltaic devices, light-emitting diodes, and other applications. Here we reveal that light induces a slow, reversible enhancement in photoluminescence (PL) lifetime and intensity in films of perovskite-phase CsPbI3 nanocrystals. When films of CsPbI3 nanocrystals stored in air are photoexcited, their PL lifetime and intensity increase by as much as a factor of 5 over the course of 20–30 min. Several hours later, without additional light excitation, the initial PL lifetime and intensity return. Placing the films under vacuum or nitrogen for several minutes was also found to increase the PL lifetime and intensity. We propose a model of slow, humidity- and light-sensitive surface states in perovskite-phase CsPbI3 nanocrystals.

Color-changing materials have a variety of applications, ranging from smart windows to sensors. Here, we report deliquescent chromism of thin, color neutral films of nickel­(II) iodide (NiI2) that are less than 10 μm thick. This behavior does not occur in the bulk material. Dark brown thin films of crystalline NiI2 turn clear when exposed to humidity and can be switched back to the dark state when mildly heated (>35 °C). This optical transition between dark and clear states of an NiI2 thin film is reversible with thermal cycling.