Carrier Recombination and Diffusion in Wet-Cast Tin Iodide Perovskite Layers under High Intensity Photoexcitation

Tin iodide perovskite CH₃NH₃SnI₃ is often considered as a replacement for toxic lead halide perovskites. Tin iodide is not only suitable for production of solar cells, but also it emits in the near-infrared spectral region, which is unique among the metal halide perovskites. On the downside, the CH₃NH₃SnI₃ layers tend to be of high unintentional p-type doping, which significantly limits the solar cell efficiency. On the other hand, it is little known how this doping could affect other optical and electrical properties important for light-emitting applications. Here, we present an optical study of carrier diffusion and recombination pathways by time-resolved photoluminescence, differential transmission, and light induced transient grating techniques at excitations close to the lasing regime. We investigate several CH₃NH₃SnI₃ layers formed by a solvent bathing method and using different antisolvents, causing different structural quality and doping level of the layers. We observe the amplified spontaneous emission with a threshold excitation as low as 5 μJ/cm² however, the threshold is sensitive to structural quality and increases significantly in the layers with larger surface roughness. We present an all-optical method to determine the equilibrium density of holes, which varies in the range of 0.7-5.0 × 10¹⁸ cm^-3, depending on the antisolvent used for production of a particular layer. Finally, we observe band-like diffusion of carriers with high values of ambipolar diffusion coefficient: it grows from 0.5 to 1.5 cm²/s with excitation due to carrier degeneracy. High diffusivity, large quantum yield even at low densities, and low stimulated emission threshold allow us to argue that unintentional p-type doping can be beneficial for light emitting applications.