Tuning the Band Gaps of Oxide and Halide Perovskite Compounds via Biaxial Strain in All Directions

Perovskite compounds in the oxide and halide families have generated interest as light absorbers for photocatalytic and photovoltaic applications, respectively. In both of these classes, biaxial strain can be used to tune structural distortions and, consequently, band gaps and solar energy conversion efficiencies. While strain has usually been explored perpendicular to the cubic perovskite unit cell axis (i.e., the 001 direction), strain in other crystallographic directions presents opportunities for qualitatively different changes to the electronic structure or even a range of local band gaps within the same material. For oxide (BaTiO3) and halide (CsGeX3) perovskites with polar ferroelectric distortions at room temperature, the present work explores how and why strain in all crystallographic directions tunes the band gap and band-edge orbitals. It is determined that, for reasons traceable to the interactions of atomic orbitals at the band edges, the band gaps of both oxide and halide compounds under achievable compressive biaxial strains vary by several tenths of an eV depending on the direction of strain. Notably, the range of band gaps accessible in CsGeI3 is predicted to span the most intense regions of the solar spectrum.