Influence of the Halide Ion on the A‑Site Dynamics in FAPbX₃ (X = Br and Cl)

The optoelectronic properties and ultimately photovoltaic performance of hybrid lead halide perovskites are inherently related to the dynamics of the organic cations. Here, we report the dynamics of the formamidinium (FA) cation in FAPbX₃ perovskites for chloride and bromide varieties, as studied by neutron spectroscopy. Elastic fixed window scans showed the onset of the reorientational motion of FA cations in FAPbCl₃ to occur at a considerably higher temperature compared to that in FAPbBr₃. In addition, we observed two distinct dynamical transitions only in the chloride system, suggesting a significant variation in the reorientational motions of the FA cation with temperature. Quasielastic neutron scattering data analysis of FAPbCl₃ showed that in the low-temperature orthorhombic phase, FA cations undergo twofold jump reorientations about the C–H axis, which evolve into an isotropic rotation in the intermediate tetragonal and high-temperature cubic phases. Comparing the results with those from FAPbBr₃ reveals that the time scale and barrier to reorientation and the geometry of the reorientational motion of the FA cations are significantly different for the two halides. We note that this dependence of the dynamic properties of the A-site cation on the halide is unique to the FA series; the geometry of methylammonium (MA) cation dynamics in MAPbX₃ is known to be insensitive to different halides.