Comparative computational study of sulfur-donor additives for stabilization of FAPbI₃ perovskites

A multiscale computational investigation integrating density functional theory (DFT) and molecular dynamics (MD) simulations was conducted to elucidate the mechanisms through which sulfur-containing donor molecules stabilize the photoactive α-phase of formamidinium lead iodide (FAPbI₃) perovskites. The binding energetics, charge-transfer behavior, and hydrogen-bonding interactions of thiourea (TU), thiosemicarbazide (TSC), thiocyanate (SCN^-), and diethyldithiocarbamate (DTC) were systematically analyzed. DFT results revealed pronounced Pb-S coordination and multidentate hydrogenbonding, with binding energies following the trend SCN->TSC>DTC>TU. Thermodynamic analysis demonstrated that these additives lower the Gibbs free energy difference, thereby stabilizing the black α-phase, with TSC, TU, and to a lesser extent DTC exhibitingthe most pronounced effects. Projected density of statesanalysis confirmed that TU and DTC effectively suppressed trap states near the band edges without introducing midgap defects. MD simulations demonstrated preferential adsorption of all S-donors on FAPbI₃ (001) surfaces, forming three to four hydrogen bonds per frame and achieving adsorption energies up to -52 kJ·mol^-1. These findings reveal a direct correlation between coordination strength, electronic coupling, and thermodynamic stabilization, establishing TU, TSC, and DTC as promising additives for improving the phase stability and electronic performance of FAPbI₃-based perovskite solar cells.