Geometry Distortion and Small Polaron Binding Energy Changes with Ionic Substitution in Halide Perovskites*

Solution-processed organometallic perovskites have demonstrated remarkable performances in optoelectronic devices and applications. Despite the extraordinary progress associated with perovskite materials, many questions about the fundamental photophysical processes taking place in these devices, remain open. Here we report the results from an in-depth computational study of small polaron formation, electronic structure, charge density, and reorganization energies using isolated structures. Local lattice symmetry, electronic structure, and electron phonon coupling are interrelated in polaron formation in hybrid halide perovskites. To illustrate these aspects, first principles calculations are performed on CsPbI3, CsSnI₃, CsPbBr₃, MAPbI₃, FAPbI₃, MAPbBr₃, FAPbBr₃, MASnI₃, and FASnBr₃. This study will focus on how ionic substitution changes the polaron binding energy in the material. It is found that in all cases that hole polaron formation is associated with lattice contraction, while electron polaron formation is associated with lattice expansion.