Impact of Tin Fluoride Additive on the Properties of Mixed Tin‐Lead Iodide Perovskite Semiconductors

Mixed tin‐lead halide perovskites are promising low‐bandgap absorbers for all‐perovskite tandem solar cells that offer higher efficiencies than single‐junction devices. A significant barrier to higher performance and stability is the ready oxidation of tin, commonly mitigated by various additives whose impact is still poorly understood for mixed tin‐lead perovskites. Here, the effects of the commonly used SnF 2 additive are revealed for FA 0 . 83 Cs 0 . 17 Sn x Pb 1− x I 3 perovskites across the full compositional lead‐tin range and SnF 2 percentages of 0.1–20% of precursor tin content. SnF 2 addition causes a significant reduction in the background hole density associated with tin vacancies, yielding longer photoluminescence lifetimes, decreased energetic disorder, reduced Burstein–Moss shifts, and higher charge‐carrier mobilities. Such effects are optimized for SnF 2 addition of 1%, while for 5% SnF 2 and above, additional nonradiative recombination pathways begin to appear. It is further found that the addition of SnF 2 reduces a tetragonal distortion in the perovskite structure deriving from the presence of tin vacancies that cause strain, particularly for high tin content. The optical phonon response associated with inorganic lattice vibrations is further explored, exhibiting a shift to higher frequency and significant broadening with increasing tin fraction, in accordance with lower effective atomic metal masses and shorter phonon lifetimes.