Solution-processed perovskites have shown great potential for coherent light emission, i.e. lasing, owing to their high optical gain and tunable gain profile over a broad range. For all-inorganic perovskite films, however, morphology-control is still a challenge to suppress the optical loss. Herein, we report a facile one-step solution-based approach to grow CsPbX3 (X = Br and I) perovskite films with a pinhole-free and ultra-smooth surface, by incorporating long-chain octylammonium halide as an additive. The addition of octylammonium halide hardly affects the Auger recombination rate under high excitation. The optimal perovskite films exhibit roughness less than 2 nm and low scattering loss. These merits facilitate wavelength tunable amplified spontaneous emission (ASE) with a low threshold of 14.9 μJ cm−2 and a high net gain coefficient of 380 cm−1.
