Smooth CsSnI3 Films for Low-Threshold Optically Pumped Near-Infrared Lasers.

Tin halide perovskites, such as CsSnI3, offer a solution-processable platform for near-infrared (NIR) lasers, but rapid crystallization typically yields defective, rough films with high optical loss. Here, we introduce a bulky phosphoric acid molecule (11BR) that coordinates with Sn2+ through -P═O and -OH groups, retarding crystallization and suppressing non-radiative recombination. The resulting CsSnI3 films are highly dense and smooth (root-mean-square roughness of 2.1 nm) and exhibit a low amplified spontaneous emission threshold of 5.0 μJ cm-2. With integration with a second-order distributed feedback grating, the 11BR-regulated films achieve room-temperature NIR lasing (peaking at 943 nm) with an ultralow threshold of 0.95 μJ cm-2 and a high quality factor of 10 400. The devices also exhibit excellent operational stability, retaining 50% of the initial lasing intensity after 19 million pump pulses. This work demonstrates that single-additive molecular regulation can transform rapidly crystallizing tin perovskites into high-quality gain media, opening a practical route toward integrable, lead-free, solution-processed NIR laser platforms.