In this topical review, we have focused on the recent advances made in the studies of lead-free perovskites in the bulk form and as nanocrystals. We highlight how nanocrystals can serve as model systems to explore the schemes of cationic exchange, doping and alloying for engineering the electronic structure of double perovskites.

Lead-free double perovskites are studied as an optional replacement to lead halide perovskites in optoelectronic applications. Recently, double-perovskite materials in which two divalent lead cations are replaced with an Ag + and a trivalent cation have been demonstrated. The presence of a reactive silver cation and observations of metallic silver nanodecorations raised concerns regarding the stability and applicability of these materials. To better understand the nucleation and crystal growth of lead-free double perovskites, we explore the origin and role that metallic silver nanoparticles (NPs) play in the Ag-based Pb-free double-perovskite nanocrystal (NC) systems such as Cs 2 AgInCl 6 , Cs 2 AgSbCl 6 , Cs 2 AgBiCl 6 , and Cs 2 AgBiBr 6 . With major focus on Cs 2 AgInCl 6 NCs, we show evidence supporting growth of the NCs through heterogeneous nucleation on preexisting metallic silver seeds. The silver seeds nucleate prior to injection of halide through reduction of the Ag + ion by the aminic ligand. The presence of preexisting silver NPs is supported by a localized surface plasmon resonance (LSPR). The injection of halide precursor into the reaction mixture step initiates a fast nucleation and growth of the perovskite NC on the silver seed. The change in the dielectric medium at the interface of the silver NP results in a quantifiable red shift of the LSPR peak. In addition, we demonstrate charge transfer from the perovskite to the silver NP through photoinduced electrochemical Ostwald ripening of the silver NPs via UV irradiation. The ripened perovskite–metal hybrid nanocrystal exhibits modified optical properties in the form of quenched emission and enhanced plasmonic absorption. Future development of Ag-based double-perovskite NC applications depends on the ability to control Ag + reduction at all synthetic stages. This understanding is critical for delivering stability and functionality for silver-based lead-free perovskite nanocrystals.

Double perovskites are considered for future photovoltaic and electro-optic applications as a toxic-free alternative to lead halide perovskites. Alas, due to the lower efficiency of lead-free devices, material properties need to improve to compete. In this work, the self-healing and annealing of crystal voids is reported. Experiments are conducted on nanocrystals and in situ a transmission electron microscopy (TEM) microscope. The setup enables creation of crystal voids and to monitor their dynamics in real time. Void trajectories and velocities are calculated for TEM videos. An inaccessible, protected volume for migration near the nanocrystal outer surface is discovered, confining the migration of voids to inner crystal parts. Once surface passivation in the form of organic ligands is removed, void dynamics changes, to enable annealing of the voids and self-healing of the crystal. It is determined that surface ligand protection against void migration is extending several atomic layers below the crystal surface. Modeling based on these results predict equilibrium positions for the voids, which are discovered in the data. The study suggests that tuning of organic ligand density influences structural stability and crystal defect tolerance in double perovskites. Engineering surfaces with inherent self-healing properties may increase efficiencies in future devices based on these materials.

Metal halide perovskite interfaces and heterostructures are crucial for engineering future technologies based on these new classes of semiconductors. The structure–function role of the CsPbBr3 and Cs4PbBr6 as mixed phases and their synergistic contribution to emission and efficiency are intensely debated. We show a clear connection between the growth of the competing Cs4PbBr6 phase and the presence of Br vacancies, which serve as the growth nucleation sites. Our understanding is fuelled by a unique cryogenic ultrafast time-resolved cathodoluminescence (TRCL) spectroscopy study of CsPbBr3 and mixed-phase microcrystals. This method precisely pinpoints the spatial location of emission centers and analyzes them spectrally and temporally, unveiling their defect-based origin. Bromide vacancies act as trap states at cryogenic temperatures, resulting in an apparent spectral split easing their detection. We find nonheteroepitaxial growth at the interface of the two phases CsPbBr3/Cs4PbBr6 and agglomeration of precipitants that are bromide-depleted. Our data connects the underpassivated bromide vacancy states at the interface to the enhanced emission from the CsPbBr3/Cs4PbBr6 heterostructures.

Double-perovskite (elpasolite) structures with Cs2AgBiBr6 composition are suggested as emerging inorganic semiconductors for solar energy conversion. We show how colloidal synthesis provides a methodological basis for investigating single monolayer two-dimensional (2D) materials. We then use the monolayers as building blocks for a more stable bilayer structure (quasi 2D) and thicker nanoplates. Each derivative’s structure, composition, and morphology are studied, and a growing mechanism for the three-dimensional (3D) nanoplates is hypothesized. High-resolution powder X-ray diffraction (HR-PXRD) synchrotron data reveal that the unit cell volume contracts by ∼2% when transitioning from a monolayer to a bilayer structure. The monolayer’s and bilayer’s thermal stability and thermal expansion coefficients are investigated using in situ temperature-dependent X-ray diffraction (XRD) measurements. Our colloidal approach to two-dimensional perovskites enables the use of high-resolution transmission electron microscopy (HRTEM) to detect structural defects. We found a typical structural defect in Cs2AgBiBr6 nanoplates with big lateral dimensions in the form of elongated voids. We hypothesize that these defects are reminiscent of an oriented attachment formation step accentuated in the final annealing step of the synthesis. The colloidal approach is essential for improving the properties of bismuth-based lead-free double perovskites, bringing them one step closer to real-life photovoltaic (PV) implementation.