Effect of pressure on the dynamics of iodine defects in MAPI: An atomistic simulation

TL;DR

This study uses molecular dynamics simulations to explore how pressure influences iodine defect diffusion in MAPI, revealing that compression can significantly reduce defect mobility and potentially slow degradation in solar cells.

Contribution

It demonstrates the effect of pressure on iodine defect diffusion in MAPI, highlighting compression as a strategy to mitigate material degradation, with detailed analysis of diffusion coefficients and simulation methods.

Findings

Diffusion coefficients decrease under compressive strain.

Tensile strain increases iodine vacancy diffusion.

Compression can reduce MAPI degradation rates.

Abstract

The diffusion of iodine defects has been considered the most important degradation mechanism of methylammonium lead iodine (MAPI) in solar cells. The present study demonstrates the importance of the pressure inside this material on the dynamics of iodine defects, using molecular dynamics simulations. It is known that the diffusion coefficient of an iodine vacancy is an order of magnitude higher than that of interstitial iodine. We show that this difference systematically increases with increased tensile strain and that both diffusion coefficients tend to zero when a compressive strain is applied. This result suggests that compression of the MAPI can be a good solution to reduce its degradation rate. %Also, different methods of determining the diffusion coefficient, including mean squared displacement (MSD) and velocity auto-correlation function, are discussed and applied to the case of ${\rm V}_{\rm I}$. Besides, the statistical aspect of deriving the diffusion coefficient from the mean squared displacement (MSD) is discussed in terms of the initial conditions (positions and velocities) of the atoms and the simulation time, considering different seeds of the pseudo-random number generator used in the simulations performed with the LAMMPS software.