Metal halide perovskite nanocrystals are a promising material for use in optoelectronic devices due to their outstanding properties, such as broad optical absorption, high photoluminescence quantum yield (PL QY), size-tunable bandgap, and relatively long excited state lifetime. The most well-studied materials in this class of compounds contain lead, which presents a challenge to their widespread use due to their toxicity. Tin halide perovskites are a promising alternative. CsSnI3 for example has a band gap of 1.3 eV, making it potentially useful for a wide range of applications, including solar cells and displays. However, CsSnI3 nanocrystals suffer from instability due to the oxidation of Sn2+ to Sn+4 in the presence of air and humidity. This results in the loss of the optically active black perovskite phase to a non-perovskite, indirect band gap layered compound, or furthermore, chemical decomposition. Capping ligands can stabilize the perovskite phase of the nanocrystals. Films of ligand-capped CsSnI3 nanocrystals were studied under various environmental conditions to determine their stability compared to bulk films and their lead halide perovskite counterpart, CsPbI3. When synthesizing CsSnI3 nanocrystals by ionic co-precipitation, the [Sn]:[Cs] molar ratio plays an important role in eliminating Sn2+ oxidation. Stoichiometric excesses of Sn in the reaction (i.e., [Sn]:[Cs] > 5) and the addition of trioctylphosphine (TOP) to oleylamine (OAm) and oleic acid (OA) capping ligand mixtures increases the stability of the nanocrystals. TOP addition also reduces the synthesis temperature needed to produce the nanocrystals. The addition of a zwitterionic ligand, lecithin, was also explored as a means to improve optical properties and stability of the CsSnI3 nanocrystals.
