Probing Lead Halide Perovskites with High Resolution Ultrafast Spectroscopy

High time resolution broadband pump-probe experiments on CH3NH3PbI3 and CH3NH3PbBr3 films which delineate instantaneous and delayed relaxation effects on sample absorption are described. Analysis of the data reveals that photoexcitation high in the inter-band continuum partially bleaches and red shifts the exciton band instantaneously. Increased pump intensity saturates the exciton bleach and progressively reduces inter-band absorption in a broad range extending from the band edge to higher photon energies. Both effects are attributed to reduced Coulomb enhancement due to hot carrier screening. The spectral extent of the inter-band absorption attenuation provides estimated binding energies of 20-30 meV in both materials. Sub-picosecond carrier cooling reverses the initial exciton transition red shift and induces transmission near the band edge due to state filling and stimulated emission. These results demonstrate that both inter-band and exciton absorptions are essential for unraveling photo-induced dynamics in these materials, and that insights obtained from many-body theoretical analysis of dynamic screening are essential for correctly assigning the recorded spectral evolution. Finally very recent experiments with sub 10 fsec pulses will be presented which are sensitive to the ultrafast breakup of hot excitons, high above the band gap.