Uncovering the Subtleties of Solvation and Speciation of Organic Halide Salts in Organic Solvents

Despite the importance of solution chemistry in the design of synthesis and processing routes for lead halide perovskite thin films and crystals, a detailed molecular-level understanding of key solution species prior to the addition of lead salts remains unclear. Here we study solutions comprised of an organic cation (CH 3 NH 3 + (MA + ) or CHN 2 H 4 + (FA + )) and a halide (Cl –, Br –, or I – ) in one of four organic solvents: γ-butyrolactone (GBL), N, N -dimethylformamide (DMF), dimethyl sulfoxide (DMSO), or tetrahydrothiophene 1-oxide (THTO). We interrogate speciation and solvation dynamics through a combination of 1 H nuclear magnetic resonance (NMR) spectroscopy and room temperature ab initio molecular dynamics (AIMD) simulations. We elucidate the roles of each species in directing cation-halide ion pair formation, cation–solvent hydrogen bonding, and the intramolecular conformational freedom of the organic cation. In doing so, we identify trends in the propensity of different solvents to favor the formation of contact ion pairs (DMSO and THTO) versus solvent-separated ion pairs (DMF and GBL as well as THTO). Using insights from AIMD simulations, we propose factors that govern the solubility of the organic halide salt in organic solvent that are consistent with experimental observations. AIMD simulations additionally reveal a “halide-hopping” phenomenon in certain systems. This experimental-computational investigation has defined the key solution interactions that exist in perovskite precursor solutions prior to the addition of lead salts, thus laying the foundation for future experiments investigating how solvation and solution speciation direct the crystallization pathway of lead halide perovskites. The nuance and subtlety of the effects of changing the cation, halide, or solvent is surprisingly rich, and not easily categorized in simplistic correlations.