Carrier Diffusion Lengths of over 500 nm in Lead-Free Perovskite CH₃NH₃SnI₃ Films

The dynamics of photoexcited lead-free perovskite films, CH 3 NH 3 SnI 3, were studied using broadband transient absorption and time-resolved fluorescence spectroscopy. Similar to its lead analogue CH 3 NH 3 PbI 3, we show that free carrier (electrons and holes) recombination is also the dominant relaxation pathway in CH 3 NH 3 SnI 3 films. The slow hot carrier relaxation time is 0.5 ps. Long carrier diffusion lengths for electrons (279 ± 88 nm) and holes (193 ± 46 nm) were obtained from fluorescence quenching measurements. We also show that SnF 2 doping in the CH 3 NH 3 SnI 3 film effectively increases the fluorescence lifetime up to 10 times and gives diffusion lengths exceeding 500 nm. These results suggest that the photophysics of CH 3 NH 3 SnI 3 perovskite are as favorable as those of CH 3 NH 3 PbI 3, demonstrating that it is a promising nontoxic lead-free replacement for lead iodide perovskite-based solar cells.