Charge Trapping Dynamics Revealed in CH₃NH₃PbI₃ by Ultrafast Multipulse Spectroscopy

The presence of defects in hybrid organic–inorganic perovskite films has been shown to be detrimental to photovoltaic device performance. A better understanding of trap state energy levels and their effects on charge carrier dynamics can help in forming more effective passivation strategies. Here we report on the dynamics of photoexcited conduction band electron trapping and the recombination of trapped electrons with valence band holes in CH₃NH₃PbI₃ films by using a combination of subgap pump and pump-push-probe transient absorption spectroscopy. By varying the energy of our push pulse, we find that hot carrier dynamics can be differentiated from changes in trapped and free electron populations. We apply a quantitative kinetic model to fit the recombination dynamics following the push pulse. Based on our fitting results, we conclude that pump pulses with photon energies larger than the bandgap energy can, in addition to exciting band-to-band free carrier transitions, directly induce transitions from electrons deeper within the valence band to trap states within the gap. These results suggest that the common assumption that all absorbed photons with energies above the band gap lead directly to free charge generation in these materials may not always be valid. This finding has serious implications for analyzing and interpreting transient absorption measurements, which have become a ubiquitous means of studying charge carrier dynamics in the perovskite photovoltaics literature.