Strain Tuning of Tin–Halide and Lead–Halide Perovskites: A First-Principles Atomic and Electronic Structure Study

Using density functional theory (DFT)-based calculations, we investigate the effects of biaxial strain and the accompanying structural distortions on the energy landscapes, band gaps, and band edges of the perovskite photovoltaic materials CsSnI3 and CsPbI3. We show that biaxial strains within ±3% of the respective cubic lattice parameters can alter band gaps by several tenths of an electronvolt, mainly through the tuning of antibonding interactions in the valence band maximum, while temperature-controlled octahedral rotations further widen band gaps. Notably, we predict that biaxial strain, particularly tensile strain at low temperature, has the potential to induce ferroelectric polarization in these materials. Throughout this work, we rationalize trends in electronic structure based on the character and symmetry of band-edge crystal orbitals and discuss their implications with respect to broader classes of materials.