Ultrafast carrier dynamics in methylammonium lead bromide perovskite

The high open-circuit voltage of perovskite solar cell based on CH₃NH₃PbBr₃ is suitable for a tandem system. It is important to understand the carrier dynamics to aid the optimization of solar devices that are efficient in extracting the photogenerated carriers before they recombine. This work reports the ultrafast carrier dynamics in CH₃NH₃PbBr₃ and test structures characterized by ultrafast transient absorption spectroscopy in the time scale of femto- and picoseconds. After laser excitation, the transient absorption signal at 534 nm is attributed to ground-state bleaching. The rise process with a time constant of hundreds of femtoseconds indicates fast cooling of hot carriers. The carrier population in the conduction band decreases subsequently, and the decay has a fast and a slow component, which are ascribed to phonon assisted recombination and free electron-hole recombination, respectively. The shallow trap states result in a weak negative band in the low energy side of the band gap. Two weak positive features at ∼507 and ∼715 nm are assigned to excited state absorptions due to carriers and excitons, respectively. With a compact TiO₂ (c-TiO₂ ) electron transport layer, an increase in the light absorption is observed due to better quality of the CH₃NH₃PbBr₃ film, resulting in higher photogenerated carrier density. We also elucidate the effective extraction of electrons by the c-TiO₂ and estimate the electron transport time at CH₃NH₃PbBr₃ /c-TiO₂ interface to be 0.68 ns.