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

TL;DR

This study uses ab-initio molecular dynamics to investigate how methylammonium molecules in lead-iodine perovskites dynamically correlate at room temperature, revealing a collective behavior mediated by the Pb-I cage.

Contribution

First large-scale ab-initio molecular dynamics simulation showing temperature-dependent correlation and collective motion of methylammonium molecules in perovskites.

Findings

Long-range anti-ferroelectric ordering of molecular dipoles

Maximum dynamical correlation occurs at room temperature

Cage-mediated collective motion of neighboring molecules

Abstract

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