Composition Engineering‐Induced Phase Transition and Nonlinear Bandgap Behavior of Tin‐Lead Mixed Perovskites for Near‐Infrared Light‐Emitting Diodes

Organic‐inorganic hybrid perovskite materials, particularly methylammonium lead iodide (MAPbI 3 ), have demonstrated significant potential in solar cells, light‐emitting diodes and lasers owing to their exceptional optoelectronic properties. However, their inherent structural instability and the limited bandgap tunability (typically within the visible range) restrict their developments in near‐infrared (NIR) photonic devices. Here, MAPb 1−x Sn x I 3 ( x = 0.0–1.0) polycrystalline films are stoichiometrically synthesized via partial substitution of lead (Pb) with tin (Sn). This study reveals that Sn doping drives changes in lattice parameters and even induces phase transition from tetragonal ( x < 0.57) to pseudo‐cubic ( x ≥ 0.57) symmetry. The bandgap exhibits the nonlinear dependence with Sn content, varying from 1.60 eV ( x = 0.0) to 1.30 eV ( x = 1.0). The bandgap reaches a prominent minimum of 1.26 eV at x = 0.75 during the compositional evolution. Infrared light‐emitting diodes (LEDs) based on MAPb 1−x Sn x I 3 polycrystalline films emitted at 960 nm when driven at 5.5 V, achieving an external quantum efficiency (EQE) of 1.2% and a turn‐on voltage of 1.55 V. The result provides an effective strategy and experimental foundation for regulating the phase stability of perovskite materials through metal substitution and facilitates their application in environmentally friendly NIR light‐emitting devices.