New insights into exciton binding and relaxation from high time resolution ultrafast spectroscopy of CH3NH3PbI3 and CH3NH3PbBr3 films

Abstract

High time resolution broadband pump-probe experiments on CH₃NH₃PbI₃ and CH₃NH₃PbBr₃ films are described. The improved time resolution delineates instantaneous and delayed relaxation related effects on sample absorption and assists in clarifying controversial assignment of the underlying mechanisms. Analysis of the data in terms of finite difference spectra and spectral band integrals reveals that photoexcitation is high in the inter-band continuum leading to partial bleaching and red-shifts of the exciton band just below the absorption-edge 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 in the range 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. Finally, 1–100 ps signals are dominated by reverse state filling due to non-geminate recombination. 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.