Band Gap Renormalization at Different Symmetry Points in Perovskites

Using ultrafast broad-band transient absorption (TA) spectroscopy of photoexcited MAPbBr3 thin films with probe continua in the visible and the mid- to deep-ultraviolet (UV) ranges, we capture the ultrafast renormalization at the fundamental gap at the R symmetry point of the Brillouin zone (BZ) and a higher energy gap at the M symmetry point. Advanced global lifetime analysis and lifetime density distribution analysis are applied to extract quantitative information. Our work confirms the similarity of the response at both high-symmetry points, which indicates a band edge renormalization that rises within the instrument response function (IRF, similar to 250 fs) and decays in ca. 400-600 fs, undergoing an energy red shift of 90-150 meV. The reported time scale corresponds to the decay of free carriers into neutral excitons. The ability to monitor different high-symmetry points in photoexcited perovskites opens exciting prospects for the characterization of a large class of materials and for photonic applications.