Local Order and Rotational Dynamics in Mixed A-Cation Lead Iodide Perovskites

Halide perovskites containing a mixture of formamidinium (FA⁺), methylammonium (MA⁺) and cesium (Cs⁺) cations are the actual standard for obtaining record-efficiency perovskite solar cells. Although the compositional tuning that brings to optimal performance of the devices has been largely established, little is understood on the role of even small quantities of MA⁺ or Cs⁺ in stabilizing the black phase of FAPbI₃ while boosting its photovoltaic yield. In this paper, we use Car-Parrinello molecular dynamics in large supercells containing different ratios of FA⁺ and either MA⁺ or Cs⁺, in order to study the structural and kinetic features of mixed perovskites at room temperature. Our analysis shows that cation mixing relaxes the rotational disorder of FA⁺ molecules by preferentially aligning their axis toward ⟨100⟩ cubic directions. The phenomenon stems from the introduction of additional local minima in the energetic landscape, which are absent in pure FAPbI₃ crystals. As a result, a higher structural order is achieved, characterized by a pronounced octahedral tilting and a lower vibrational activity for the inorganic framework. We show that both MA⁺ and Cs⁺ are qualified for this enhancement, with Cs⁺ being particularly effective when diluted within the FAPbI₃ perovskite.