Impact of Cesium/Rubidium Incorporation on the Photophysics of Multiple-Cation Lead Halide Perovskites

Incorporating cesium (Cs) or rubidium (Rb) cations into multiple-cation lead mixed-halide perovskites (FA0.83MA0.17Pb(I0.83Br0.17)3) increases their photovoltaic performance. In this study, the fundamental photophysics of perovskites are investigated by steady-state and transient optical spectroscopy and the reasons for the performance increase are revealed. Cs/Rb-cation incorporation slightly increases the bandgap, while exciton binding energies remain in the range of a few meV. Urbach energies are reduced, suggesting improved perovskite microstructure upon Cs/Rb incorporation. Carrier density-induced broadening of the photo-bleaching following the Burstein-Moss model is observed, and the effective carrier masses are determined to be a few tenths of the electron rest mass. From fits of the high-energy tail of the perovskite's photo bleach to Boltzmann's distribution, sub-picosecond hot carrier cooling is revealed, implying strong carrier-phonon coupling. Importantly, the charge carrier recombination dynamics indicate that Cs/Rb-incorporation reduces both the first-order (trap-assisted) and the second-order (radiative) recombination, which appears to be the main reason for the observed performance increase upon Cs/Rb-cation incorporation. Overall, this work presents a detailed study of the photophysics of multiple-cation mixed halide lead perovskites and develops a concise picture of the impact of cesium/rubidium incorporation on the photophysics and device performance.