Ultrafast Carrier Dynamics in Wide Band Gap Mixed-Cation Perovskites: Influence of the Cs Cation

Mixed-cation perovskites exhibit outstanding performance as next-generation solar cells and for optoelectronic applications. One pathway to increase the quality and stability of these materials is adding Cs to the A-site of mixed-cation formamidinium/methylammonium in the perovskite APbX3. Here, we use femtosecond transient absorption spectroscopy to study the effect of Cs on the dynamics of a mixed-cation wide band gap bromide-based perovskite. Negligible changes in the optical spectra are observed between the two films, indicating a similar localization of the charge carriers at the band edge. However, adding Cs reduces the non-radiative recombination sites and increases the lifetime of the photogenerated charge carriers in the perovskite film (from 291 to 355 ns). Furthermore, the Cs cation slows down the cooling of hot carriers through an efficient hot-phonon bottleneck, which is observed by increasing the excitation power from 9.7 to 648 μJ/cm2 [the lifetime of the fast component (τ1) increases from 0.21 to 0.91 ps]. Understanding the mechanism of charge dynamics in perovskite thin films is critical for the fabrication of high-performance devices.