Strong Dependence of Point Defect Properties in Metal Halide Perovskites on Description of van der Waals Interaction

Weaker than ionic
and covalent bonding, van der Waals (vdW) interactions
can have a significant impact on structure and function of molecules
and materials, including stabilities of conformers and phases, chemical
reaction pathways, electro-optical response, electron-vibrational
dynamics, etc. Metal halide perovskites (MHPs) are widely investigated
for their excellent optoelectronic properties, stemming largely from
high defect tolerance. Although MHPs are primarily ionic compounds,
we demonstrate that vdW interactions contribute ∼5% to the
total energy, and that static, dynamics, electronic and optical properties
of point defects in MHPs depend significantly on the vdW interaction
model used. Focusing on widely studied CsPbBr3 with the
common Br vacancy and interstitial defects, we compare the PBE, PBE+D3,
PBE+TS, PBE+TS/HI and PBE+MBD-NL models and show that vdW interactions
strongly alter the global and local geometric structure, and change
the fundamental bandgap, midgap state energies and electron-vibrational
coupling. The vdW interaction sensitivity stems from involvement of
heavy and highly polarizable chemical elements and the soft MHP structure.