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 similar to 507 and similar to 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.