Comment on “Interplay of Structural and Optoelectronic Properties in Formamidinium Mixed Tin‐Lead Triiodide Perovskites”

Abstract Studying optoelectronic properties in FAPb_1−xSn_xI₃ and in FA_0.83Cs_0.17Pb_1−xSn_xI₃ perovskites as a function of the lead:tin content, Parrott et al. (2018) and Savill et al. (2020) observed the broadest luminescence linewidth and the largest Stokes shift in mixed compositions with Sn <25% and with >85%. It is in contrast to the intuitive expectation of the largest effects of alloy disorder for the 50:50 composition. This comment addresses the alloy disorder caused by statistical local spatial fluctuations of the alloy composition and shows that the largest effects of alloy disorder for perfectly random fluctuations in FAPb_1−xSn_xI₃ and FA_0.83Cs_0.17Pb_1−xSn_xI₃ are, in fact, expected for x < 0.25 and for x > 0.85. It can be one of the reasons why Pb‐rich and Sn‐rich Sn‐Pb perovskites typically show shorter photoluminescence (PL) lifetimes, broader emission, increased Stokes shifts, reduced PL quantum yield, and higher Urbach tails, compared with their lead‐only counterparts.