Synthesis of Tin-based Mixed-Halide Perovskite Under Ambient Condition and Stabilization via Surface Functionalization

Presently, lead halide perovskite solar cells remain as a promising class of materials in photovoltaic technology. With 24.2% as the current highest reported power conversion efficiency, perovskite-based solar cells are the first low-cost, solution- processable materials to reach comparable performance with multicrystalline and thin- film silicon. However, the presence of toxic lead remains to be a hindrance to the complete, large-scale deployment of this technology. Substitution of lead by tin, which is also a Group 14 metal, has been successfully done, but the instability of its relevant ionic form, Sn2+ in air severely affects the integrity of the perovskite structure and, by extension, the overall stability of the device. Another common problem with perovskite solar cells is that their fabrication requires controlled conditions and an inert atmosphere. In this study, we investigate the synthesis of tin-based mixed-halide perovskite in ambient air, under relative humidity (RH) of 50-60%, using SnCl2·2H2O as an air- stable Sn2+ precursor. XRD confirmed the formation of mixed perovskite crystals, CH3NH3SnI(3-x)Clx and CH3NH3SnCl3. Their band gap energy was estimated to be 2.03 eV using UV-Vis and their flake-like morphology was viewed using SEM. XRD also revealed the oxidation of the crystals to (CH3NH3)2SnX6 (X = Cl or I) in less than an hour when RH=50% or higher. Meanwhile, there were no drastic changes in the XRD pattern when the crystals were stored under RH=25% up to a day after synthesis. Surface functionalization with alkylammonium cations and its subsequent effect on stability were also investigated. Stability of the crystals was observed to improve when an additional alkylammonium cation (n-butylammonium, nBAI; i-butylammonium, iBAI; or benzylammonium, BzAI) is mixed to the methylammonium iodide and tin chloride precursor solution at low concentrations, that is one for every 5 or 10 moles of methylammonium ions. Specifically, the iBAI-functionalized tin mixed-halide perovskite with 1:5 iBAI:MAI ratio showed the greatest stability, maintaining more than 75% of its structural integrity even when exposed to ~50% RH for almost 24 hrs. Change in semiconducting properties as displayed by lower band gap energy of the functionalized tin perovskites was also observed