Room temperature dynamic correlation between methylammonium molecules in lead-iodine based perovskites: An ab-initio molecular dynamics perspective

摘要

The high efficiency of lead organo-metal-halide perovskite solar cells hasraised many questions about the role of the methylammonium (MA) molecules inthe Pb-I framework. Experiments indicate that the MA molecules are able to'freely' spin around at room temperature even though they carry an intrinsicdipole moment. We have performed large supercell (2592 atoms) finitetemperature ab-initio molecular dynamics calculations to study the correlationbetween the molecules in the framework. An underlying long rangeanti-ferroelectric ordering of the molecular dipoles is observed. The dynamicalcorrelation between neighboring molecules shows a maximum around roomtemperature in the mid-temperature phase. In this phase, the rotations are slowenough to (partially) couple to neighbors via the Pb-I cage. This results in acollective motion of neighboring molecules in which the cage acts as themediator. At lower and higher temperatures the motions are less correlated.