Photocarrier Recombination and Injection Dynamics in Long-Term Stable Lead-Free CH₃NH₃SnI₃ Perovskite Thin Films and Solar Cells

We investigated the near-band-edge optical responses and photocarrier dynamics of encapsulated long-term stable CH₃NH₃SnI₃ (MASnI₃) thin films and solar-cell devices. The MASnI₃ thin film prepared with SnF₂ exhibited a bandgap of 1.25 eV, while the film without SnF₂ had a significantly blueshifted absorption edge. On the contrary, the PL peak energies were not influenced by the addition of SnF₂. These observations indicate that the blueshift of the absorption edge in the SnF₂-free MASnI₃ sample is due to the Burstein–Moss shift induced by a significant unintentional hole doping. Furthermore, time-resolved photoluminescence measurements revealed that by adding SnF₂ the photocarrier lifetime of the film increased by one order of magnitude, which enables improved device performance of solar cells. We clarified that in the MASnI₃ solar cells the short-circuit current stays significantly below the ideal value due to a large nonradiative recombination rate in the perovskite layer, resulting in a small photocarrier-injection efficiency into the charge-transport layers