Bandgap Pressure Coefficient of a CH₃NH₃PbI₃ Thin Film Perovskite

Recent scientific interest in examining the bandgap evolution of a MAPbI₃ hybrid perovskite by applying hydrostatic pressure has mostly focused on a room-temperature tetragonal phase. In contrast, the pressure response of a low-temperature orthorhombic phase (OP) of MAPbI₃ has not been explored and understood. In this research, we investigate for the first time how hydrostatic pressure alters the electronic landscape of the OP of MAPbI₃. Pressure studies using photoluminescence combined with calculations within density functional theory at zero temperature allowed us to identify the main physical factors affecting the bandgap evolution of the OP of MAPbI₃. The negative bandgap pressure coefficient was found to be strongly dependent on the temperature (α_120K = -13.3 ± 0.1 meV/GPa, α_80K = -29.8 ± 0.1 meV/GPa, and α_40K = -36.3 ± 0.1 meV/GPa). Such dependence is related to the changes in the Pb-I bond length and geometry in the unit cell as the atomic configuration approaches the phase transition as well as the increasing phonon contribution to octahedral tilting as the temperature increases.