Slow Carrier Cooling in Hybrid Pb–Sn Halide Perovskites

Low band gap perovskites, such as mixed Pb–Sn systems, are essential to make bottom cells in all perovskite tandem photovoltaics. However, currently the fundamental dynamics of carriers in these materials are not well explored. Here we use ultrafast broad-band pump–probe spectroscopy to probe the two-stage carrier cooling dynamics in CH3NH3Pb1–x Sn x I3 perovskites and show that the cooling is slower than that in pure Pb systems. The first-stage of cooling is associated with Fröhlich interactions between carriers and metal–iodide bonds that slow down monotonically with increasing Sn content. The second-tage of cooling involves the decay of an optical phonon into acoustic phonons and is slowest for 60% Sn content due to an increasing gap between the optical and acoustical phonon branches. Our results provide first insights into carrier cooling dynamics in hybrid Pb–Sn halide perovskites and pave the way for further understanding of the fundamental nature of Fröhlich interactions and the decay of optical phonons in perovskite systems.