Room temperature synthesis of lead-free FASnI3 perovskite nanocrystals with improved stability by SnF2 additive

Tin halide perovskites are among the candidates for replacing lead-based ones for less toxicity and comparable optical properties. However, stability remains a challenge due to the easier oxidation of Sn2+ than Pb2+. Here, for the first time, we applied the ligand-assisted reprecipitation method to synthesize CH(NH2)2SnI3 (FASnI3) orthorhombic perovskite nanocrystals with an average diameter of 7.7 nm and a photoluminescence emission at 825 ± 2 nm (1.5 eV). The influence of synthesis parameters, including precursor solvent, precipitation media, temperature, and time on optical properties of nanocrystals, was studied. By incorporating SnF2, the stability of the nanocrystals was improved, and the oxidation from FASnI3 to FA2SnI6 was significantly delayed, which was quantitively demonstrated and confirmed by observing the characteristic diffraction peaks of the perovskite phase using x-ray diffraction at various exposure time to air. The addition of SnF2 is optimized to be 6%. The FASnI3 nanocrystals stayed stable for at least 265 days under N2 storage at room temperature and relative humidity of 20%.