Thermal Effects on CH₃NH₃PbI₃ Perovskite from <i>Ab Initio</i> Molecular Dynamics Simulations

TLDR

The study investigates thermal effects on CH₃NH₃PbI₃ perovskite, including finite‑size impacts on optical properties, using ab initio molecular dynamics. Ab initio molecular dynamics simulations were carried out on 8‑ and 27‑unit‑cell models for 40 ps each, employing density‑functional‑theory forces. The smaller 8‑cell model shows over‑correlation due to long‑range electrostatics, whereas the 27‑cell model yields a more realistic anisotropic rotation of methylammonium; the HOMO–LUMO gap fluctuates by ~0.1 eV, and the projected density of states places the Fermi level on I p orbitals and the lowest virtual state on Pb p orbitals throughout the trajectory.

Abstract

We present a molecular dynamics simulation study of CH3NH3PbI3 based on forces calculated from density functional theory. The simulations were performed on model systems having 8 and 27 unit cells, and for a total simulation time of 40 ps in each case. Analysis of the finite size effects, in particular the mobility of the organic component, suggests that the smaller system is over-correlated through the long-range electrostatic interaction. In the larger system, this finite size artifact is relaxed, producing a more reliable description of the anisotropic rotational behavior of the methylammonium molecules. The thermal effects on the optical properties of the system were also analyzed. The HOMO–LUMO energy gap fluctuates around its central value with a standard deviation of approximately 0.1 eV. The projected density of states consistently place the Fermi level on the p orbitals of the I atoms and the lowest virtual state on the p orbitals of the Pb atoms throughout the whole simulation trajectory.