Synthesis and properties of lead-free formamidinium bismuth bromide perovskites

In the past few years, Pb-free metal halide perovskites have been recognized as a promising material for various optoelectronic applications because of some of their unique features, such as direct and tunable bandgap, visible light emission, narrow emission spectra, lower toxicity level, and easy solution processability. Recently, several Bi-based perovskite-like single crystals (SCs) and nanocrystals (NCs) were reported, which are mostly suffering from their poor structural stability and lower emission intensity. Here, we report the growth of millimeter-sized formamidinium bismuth bromide (FA₃Bi₂Br₉) perovskite SCs via slow solvent evaporation method. They crystallized into a trigonal crystal structure and exhibit an indirect bandgap of 2.71 eV. These results are supported by the first-principle density-functional theory studies. We have also synthesized nanometer-sized spherical blue-emitting FA₃Bi₂Br₉ NCs by solvent ligand-assisted reprecipitation method and achieved a maximum photoluminescence quantum yield of 22%. We observe that the addition of excess ligands into the FA₃Bi₂Br₉ NCs solution before the purification step significantly improves the optical and colloidal stability of the NCs.