Thermal Effects and Halide Mixing of Hybrid Perovskites: MD and XPS Studies

Thermal effects in organo-metal halide perovskites are studied by ab initio molecular dynamics (MD) simulations performed at effective temperatures of 293 and 383 K and by X-ray photoelectron spectroscopy (XPS). We find that the cause of thermal instability in this class of perovskites is the rotation of the methylammonium (MA) groups that destroy the rigid lattice of pure compounds (MAPbI₃ and MAPbBr₃). When the Pb-I lattice is initially distorted by partial replacement of the I with Cl or Br, this not only prevents formation of PbI₂ seeds but also improves lattice flexibility and stability against the temperature-induced motion and rotation of MA groups.