Molecular-orientation-dependent bandgap deformation potentials in hybrid perovskites

Different from many conventional semiconductors, the presence of dipolar organic molecules (e.g., MA: CH₃NH₃) in hybrid perovskites introduces a new degree of freedom during deformation. It remains elusive how the molecule interacts with the inorganic lattice while deforming and how this perturbs the electronic structure of hybrid perovskites. In this study, we employ first-principles density functional theory to calculate the bandgap deformation potentials in prototypical hybrid perovskites (MAPbX₃, X=I, Br) with different MA orientations under biaxial strain. We show that the molecular orientation plays a critical role in the bandgap variation during deformation, which may in turn explain an experimental puzzle of larger bandgaps at the hybrid perovskite surface than in bulk. Our comparative study of the all-inorganic counterparts (CsPbX₃, X= I, Br) further demonstrates that the anomalous bandgap deformation in hybrid perovskites is mediated by the organic molecules and is an intrinsic feature of these hybrid materials. The present work improves our understanding of the bandgap deformation potentials in novel hybrid semiconductors, and opens a new degree of freedom in bandgap engineering.