Sulfur-Donor Solvents Strongly Coordinate Pb²⁺in Hybrid Organic-Inorganic Perovskite Precursor Solutions

Strong coordination between Lewis-basic processing additives and the Lewis-acidic lead halide in hybrid organic-inorganic perovskite (HOIP) precursor solutions is required to solubilize the lead halide, and subsequently access the appropriate crystallization kinetics and attain the desired morphology of perovskite active layers. While oxygen-donor solvents and additives, such as dimethylformamide and dimethyl sulfoxide, are widely used for perovskite processing, we demonstrate that "soft"sulfur-donor solvents exhibit stronger coordination to the "borderline soft"Lewis acid Pb2+ center of PbI2 relative to "hard"O-donor solvents in the precursor solution. The stronger coordination of S-donor solvents compared to O-donor solvents to Pb2+ implies that such compounds can be useful additives to HOIP precursor solutions. Density-functional calculations of the enthalpy change resulting from the coordination of solvents to Pb2+ provide direct numerical comparison of the strength of O-donor and S-donor coordination with Pb2+ and expands the library of candidate S-donor compounds. Our results provide a roadmap for processing additive selection and expand the previously limited choice of perovskite processing additives to include strongly coordinating S-donor compounds.