Studying optoelectronic properties inFAPb 1−x Sn x I 3 and in FA 0.83 Cs 0.17 Pb 1−x Sn x I 3 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−x Sn x I 3 and FA 0.83 Cs 0.17 Pb 1−x Sn x I 3 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.

Studying optoelectronic properties in FAPb1−xSnxI3 perovskites as a function of the lead:tin content, Parrott et al. observed the broadest luminescence linewidth and the largest luminescence Stokes shift in mixed compositions with Sn < 25% and with > 0.85%. Since the largest effects of alloy disorder were expected for the 50:50 composition, it was concluded that the revealed disorder effects might arise from extrinsic factors that can be eliminated upon further crystal growth optimization. This comment shows that the largest effects of alloy disorder for perfectly random fluctuations in FAPb1−xSnxI3 perovskite are, in fact, expected for x < 0.25 and for x > 0.85. Therefore, further crystal growth optimization is futile.