Molecular Mechanisms of Thiourea in FAPbI3 Perovskites: Phase Stabilization and Defect Passivation

Thiourea (TU) has recently attracted attention as an effective additive for the improvement of phase stability (including defect passivation), recombination reduction, and increase of the lifetime of charge carriers in MAPbI3/FAPbI3 perovskites (MA–methylammonium, FA–formamidinium), but the molecular-level mechanisms of these processes are not yet fully clarified. In this work, we employ a multiscale theoretical approach, combining density functional theory (DFT) and molecular dynamics (MD), to investigate the role of TU in the stabilization of the black α-phase and the passivation of surface defects in formamidinium lead iodide. Calculations reveal that TU forms stable complexes with PbI2 and the [PbI6]4- octahedral fragment via Pb–S coordination and establishes stronger hydrogen bonds with FA+ cations than with MA+ cations. The adsorption energy of TU on the α-FAPbI3(001) surface is higher than on the α-MAPbI3(001) surface, indicating stronger binding and more effective defect passivation. The density of states analysis confirms the elimination of mid-gap trap states through passivation of undercoordinated Pb and I atoms. Also, TU reduces the free-energy difference between α- and δ-phases of FAPbI3 by 5.79 kJ/mol per formula unit, thereby thermodynamically favoring the photoactive α-phase of FAPbI3. MD simulations also demonstrate that TU slows down the nucleation and aggregation of PbI6 and promotes the growth of larger grains. Thus, TU acts both as a phase stabilizer and as an electronic passivator, thus providing prospects for the rational design of additives aimed at enhancing the stability and efficiency of perovskite solar cells.