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 FAPb_1-SnI₃ 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 FAPb_0.6Sn_0.4I₃ and FAPb_0.4Sn_0.6I₃ have the lowest bandgaps of around 1.27 electron volts, while that of FAPb_0.5Sn_0.5I₃ is 1.33 electron volts. Correspondingly, their based light-emitting diodes can emit infrared lights with the wavelengths reaching 930 nanometers.