Combining Triple‐Cation Engineering and Bifunctional Molecular Passivation Enables Ultralow Threshold Near‐Infrared Amplified Spontaneous Emission in Perovskite Films

Hybrid organic–inorganic perovskites have emerged as promising gain media for coherent light sources, yet defect‐mediated losses and phase instability limit their practical applications. Here, we synergistically integrate triple‐cation compositional engineering (Cs 0.05 FA 0.85 MA 0.1 PbI 3 ) with piperazine hydroiodide (PI) bifunctional molecular passivation to achieve ultralow‐threshold near‐infrared amplified spontaneous emission (ASE). Structural analyses reveal that this dual strategy suppresses PbI 2 impurities, enlarges grain sizes, and reduces surface roughness. Optical characterization demonstrates photoluminescence enhancement and reduced Urbach energy, confirming effective defect passivation. Under 532 nm excitation, PI‐treated film exhibits a record‐low ASE threshold of 1.05 µJ cm −2 . Besides, a 1.7‐fold higher net gain (143.8 cm −1 ) and 65.8% reduced optical loss (1.15 cm −1 ) are also observed compared with the Control film. Furthermore, femtosecond transient spectroscopy results indicate prolonged optical gain lifetimes (762 ps vs. 468 ps). Power‐dependent time‐resolved photoluminescence analysis shows that PI‐treated perovskite films exhibit a lower trap‐assisted monomolecular recombination constant and an increased bimolecular recombination constant, which is responsible for the enhancement of its radiative recombination efficiency and thereby the significantly positive role in promoting ASE. Meanwhile, the passivated triple‐cation perovskite films can maintain an ultra‐stable ASE output. This work establishes an effective strategy for designing high‐performance perovskite lasers through synergistic compositional and interfacial optimization.