Low‐Threshold Amplified Spontaneous Emission in Quasi‐2D Perovskite Films Enabled by 3‐Phosphonopropanoic Acid Additives

Quasi‐two‐dimensional (quasi‐2D) perovskites are promising for light‐emitting and lasing devices due to their strong excitonic properties and quantum‐well structures; however, their performance is limited by phase inhomogeneity and defect‐related nonradiative losses. Here, we incorporate 3‐phosphonopropanoic acid (3‐PPA) as an additive into the quasi‐2D (PEA) 2 Cs n −1 Pb n Br 3 n + 1 film to improve film quality and optical gain characteristics. The 3‐PPA‐treated films exhibit improved crystallization and significantly inhibit low‐dimensional impurity phases. The introduction of 3‐PPA extended the fluorescence lifetime to 203.63 ps and accelerated the stimulated emission process, shortening the amplified spontaneous emission (ASE) lifetime to 2.82 ps. Transient absorption and nonlinear optical measurements reveal longer gain lifetimes, enhanced two‐photon absorption, and lower saturation intensity, which collectively promote more efficient population inversion. Finally, the quasi‐2D perovskite film treated with 3‐PPA exhibited stronger emission and a reduced ASE threshold of 2.25 μJ/cm 2 . This work demonstrates an effective additive engineering approach that enhances the potential of quasi‐2D perovskites for efficient low‐threshold photonic devices.