Quantifying Colloid Sizes in Purified Tin(II) Iodide Precursors for Superior Tin-Halide Perovskite Films

Perovskites are notable semiconductors for photovoltaic devices, achieving power conversion efficiencies (PCEs) exceeding 26%. However, concerns over commercialization persist due to lead-based precursors. Tin-based perovskites present a promising alternative, sharing similar band structures, band gaps, and lattice properties, but they suffer from lower stability and reduced PCEs due to the oxidation of tin(II) to tin(IV) and rapid crystallization during casting. Literature suggests that uncontrolled crystallization results from the poor solubility of tin(II) iodide (SnI₂), a key precursor in tin perovskite synthesis, in aprotic solvents. This leads to defect trap states, iodide vacancies, and cascading surface oxidation of uncoordinated tin(II). Our study aimed to quantify SnI₂ solubility by measuring colloidal size distribution at varying temperatures and concentrations to optimize crystallization and improve stability.

Initially, to account for the spontaneous oxidation of SnI₂, we performed sublimation to separate tin(II) iodide from tin(IV) impurities by leveraging the differences in their sublimation temperatures. Differential scanning calorimetry (DSC) indicated tin(IV) iodide sublimes at 150°C, while tin(II) iodide sublimes at 330°C. X-ray photoelectron spectroscopy (XPS) confirmed a significant reduction in tin(IV) impurities after separation, validating sublimation as an effective purification method.

Following purification, we used dynamic light scattering (DLS) to study the effect of varying SnI₂ concentrations in N,N-Dimethylformamide (DMF) solvent. Increasing the concentration from 0.8M to 0.9M led to a 0.357% increase in particle hydrodynamic radius, from 0.546nm to 0.548nm. A further increase to 1M resulted in a 1.30% rise, reaching 0.555nm, thus indicating a positive correlation between concentration and particle size. We also examined temperature effects on colloidal size distribution. At 25˚C, 0.9M SnI₂ solutions showed two particle size groups with radii of 0.364nm and 46,500nm. After placing 0.9M SnI₂ initially at room temperature in a 50˚C bath, the larger particles shrank to 10,300nm in 3 minutes and disappeared completely after 10 minutes. We hypothesize that these large particles are colloidal agglomerates formed at room temperature, which gradually dissipate as temperature increases, leading to their disappearance from DLS measurements. These agglomerates could result in defects and poor crystallization of the film during casting.

In the future, we plan to synthesize tin perovskite films using SnI₂ precursors prepared at different temperatures and concentrations. Through X-ray diffraction and time-resolved photoluminescence characterization, we aim to identify the optimal SnI₂ precursor conditions to control crystallization and maximize film quality.