Stabilization of the Perovskite Phase of Formamidinium Lead Triiodide by Methylammonium, Cs, and/or Rb Doping

In this work we perform a computational study comparing the influence of monovalent cation substitution by methylammonium (MA⁺), cesium (Cs⁺), and rubidium (Rb⁺) on the properties of formamidinium lead triiodide (FAPbI₃)-based perovskites. The relative stability of the desired, photoactive perovskite α phase ("black phase") and the nonphotoactive, nonperovskite δ phase ("yellow phase") is studied as a function of dopant nature, concentration and temperature. Cs⁺ and Rb⁺ are shown to be more efficient in the stabilization of the perovskite α phase than MA⁺. Furthermore, varying the dopant concentration allows changing the relative stability at different temperatures, in particular stabilizing the α phase already at 200 K. Upon Cs⁺ or Rb⁺ doping, the corresponding onset of the optical spectrum is blue-shifted by 0.1-0.2 eV with respect to pure FAPbI₃.