Caution! Static Supercell Calculations of Defect Migration in Higher Symmetry ABX 3 Perovskite Halides May Be Unreliable: A Case Study of Methylammonium Lead Iodide.

Activation energies of defect migration in ABX 3 perovskite halides are widely obtained through static supercell calculations with the nudged-elastic-band method. Taking methylammonium lead iodide (CH 3 NH 3 PbI 3 , MAPbI 3 ) as an example, we demonstrate that such calculations are unreliable for the higher symmetry structures adopted by the material at temperatures relevant to device operation (tetragonal and cubic MAPbI 3 ) because, in addition to ion relaxation around the point defects, local structural modifications characteristic of the ground-state (orthorhombic) structure occur. In this way, we offer a simple explanation of why calculated activation energies of defect migration in MAPbI 3 suffer from surprisingly large scatter. We propose a robust test to determine whether static supercell calculations of point-defect processes in ABX 3 perovskite systems are reliable.