Disorder-order transition–induced unusual bandgap bowing effect of tin-lead mixed perovskites

Owing to the predominant merit of tunable bandgaps, tin-lead mixed perovskites have shown great potentials in realizing near-infrared optoelectronics and are receiving increasing attention. However, despite the merit, there is still a lack of fundamental understanding of the bandgap variation as a function of Sn/Pb ratio, mainly because the films are easy to segregate in terms of both composition and phase. Here, we report a fully stoichiometric synthesis of monocrystalline FAPb1−xSnxI3 nanocrystals as well as their atomic-scale imaging. On the basis of the systematic measurements of the monocrystalline materials, strain and Coulomb interaction–induced atomic ordering was revealed to be responsible for the unusual discontinuous bandgap jumping near x = 0.5 from the expected bowing effect. As a result, both FAPb0.6Sn0.4I3 and FAPb0.4Sn0.6I3 have the lowest bandgaps of around 1.27 electron volts, while that of FAPb0.5Sn0.5I3 is 1.33 electron volts. Correspondingly, their based light-emitting diodes can emit infrared lights with the wavelengths reaching 930 nanometers.