Bandgap Pressure Coefficient of a CH₃NH₃PbI₃ Thin Film Perovskite

Recent scientific interest in examining the bandgap evolutionofa MAPbI(3) hybrid perovskite by applying hydrostatic pressurehas mostly focused on a room-temperature tetragonal phase. In contrast,the pressure response of a low-temperature orthorhombic phase (OP)of MAPbI(3) has not been explored and understood. In thisresearch, we investigate for the first time how hydrostatic pressurealters the electronic landscape of the OP of MAPbI(3). Pressurestudies using photoluminescence combined with calculations withindensity functional theory at zero temperature allowed us to identifythe main physical factors affecting the bandgap evolution of the OPof MAPbI(3). The negative bandgap pressure coefficient wasfound to be strongly dependent on the temperature (& alpha;(120K) = -13.3 & PLUSMN; 0.1 meV/GPa, & alpha;(80K) = -29.8 & PLUSMN; 0.1 meV/GPa, and & alpha;(40K) = -36.3 & PLUSMN;0.1 meV/GPa). Such dependence is related to the changes in the Pb-Ibond length and geometry in the unit cell as the atomic configurationapproaches the phase transition as well as the increasing phonon contributionto octahedral tilting as the temperature increases.