Bandgap Pressure Coefficient of a CH3NH3PbI3 Thin Film Perovskite

This work shows experimental and theoretical studies of the pressure-induced evolution of the electronic structure of the orthogonal phase (OP) of MAPbI3. We have shown that the bandgap of the MAPbI3 in OP can be tuned in a broader range of pressure (≲3.5 GPa) when compared to the tetragonal phase (TP), in which a red-shift of PL peak is observed only up to ∼0.3 GPa. Such behavior results from the better stability of the crystal lattice against octahedral distortions in the low-temperature regime due to the reduced phonon contribution and different Pb−I bond geometry. We found the pressure coefficient of the OP to be negative and dependent on the temperature. A negative deformation potential results from the antibonding character of the bottom of the conduction band and the top of the valence band, while dependence on the temperature can be explained by changes in the unit cell volume and geometry as the atomic configuration approaches the temperature-induced phase transition as well as phonon contribution. On the whole, our results advance the understanding of the behavior of the low-temperature MAPbI3 phase under extreme conditions and show that hydrostatic pressure allows fine-tuning of structural and optical properties, resulting in the possibility of realizing materials with the desired properties for technological applications.