Active Phase Transfer Enhances Amplified Spontaneous Emission and Mechanical Deformability in FAPbBr ₃ Perovskite Film

ABSTRACT Formamidine lead bromide perovskite (FAPbBr 3 ) is considered an ideal lasing gain material owing to its high photoluminescence quantum yield (PLQY) and optical gain. Nevertheless, the high‐concentration bulk defects and deficient mechanical deformability significantly limit the progress of flexible optoelectronics. Herein, we developed a cryogenic‐engineered precursor solution chemistry strategy for synthesizing FAPbBr 3 perovskite films with low defect density and good mechanical robustness. Under nanosecond pulsed laser excitation, the FAPbBr 3 films synthesized under low temperature (FAPbBr 3 @LT) yield enhanced amplified spontaneous emission (ASE) performance, with fivefold lower for threshold and four times higher for gain coefficient than the FAPbBr 3 film prepared at room temperature (FAPbBr 3 @RT). Notably, our FAPbBr 3 @LT film maintains better optical amplification performance after mechanical bending tests, demonstrating improved mechanical deformability. The core of this cryogenic‐engineered method is to activate the tetragonal‐phase‐induced crystallization of the FAPbBr 3 for the target cubic phase formation, which is conducive to contribute to its low trap density and strong electrostatic interactions, highlighting its promise for the development of high‐performance flexible electrically pumped perovskite lasers.