High Optical Gain of Solution‐Processed Mixed‐Cation CsPbBr3 Thin Films towards Enhanced Amplified Spontaneous Emission

Abstract A solution‐processed thin film made of all‐inorganic CsPbBr 3 perovskite is a promising candidate for low‐cost and flexible green‐color lasers. However, the amplified spontaneous emission (ASE) of solution‐processed CsPbBr 3 films still experiences a high threshold owing to poor morphology and insufficient optical gain. Here, a multiple‐cation doping strategy is demonstrated to develop compact, smooth thin films of Cs 0.87 (FAMA) 0.13 PbBr 3 /(NMA) 2 PbBr 4 (FA: formamidinium; MA: methylammonium; NMA: naphthylmethylammonium) with a record high net modal optical gain of ≈ 3030 cm −1 and low propagation loss of 1.0 cm −1 . The FA and MA cations improve the crystallization kinetics to form continuous films, and the NMA cations reduce the grain dimension, increase film dispersibility/uniformity, and enhance spatial confinement to promote optical gain. Room‐temperature ASE is demonstrated under a low threshold of ≈ 3.8 µ J cm −2 without degradation after four months of storage in glove box or excitation by 3 × 10 7 laser pulses. These findings provide insights into enhancing the optical gain and lowering the threshold of perovskite lasers in terms of molecular synthesis and microstructure engineering.