Two-dimensional halide perovskites are very promising for applications in optoelectronics^[1,2]. In recent years, research has been mostly motivated by the advance of device performance of halide perovskite solar cells. Here we focus on the 2D halide perovskite PEA₂SnI₄ (PEA: Phenethylammonium) that is synthesized via sequential physical vapor deposition, and we investigate the growth mechanism, the surface composition as well as the structural and optoelectronic properties.

We show that a stoichiometric layer of PEA₂SnI₄ can be grown via physical vapor deposition at room temperature. In the first step, a thin layer of SnI₂ was grown via evaporation, followed by an exposure to PEAI in the same vacuum chamber. The surfaces of both types of samples were analyzed with atomic force microscopy and the results showed a conversion to PEA₂SnI₄ with an increase in the grains size by a factor of 5. In X-ray diffraction, the characteristic peak related to the (002) plane was observed at 5.25°, validating the 2D structure of PEA₂SnI_4. The conversion of SnI₂ to PEA₂SnI₄ is a self-limited process that takes place only at the top 25nm. X-Ray photoelectron spectroscopy depth profile measurements corroborate a complete conversion of the near surface region with an expected iodine to tin ratio of 4, which decreases to 2 when moving deeper inside the material. Regarding the optoelectronic properties, PEA₂SnI₄ is a very highly luminescent semiconductor with a strong excitonic photoluminescence signal at 1.95 eV, in accordance with other reports for PEA₂SnI₄^[3], while high absorption and PLQY values of approximately 0.3% were measured for thicknesses of 25-30 nm.

Moreover, increasing the exposure time of PEAI in the second step of the sequential deposition was found to improve the quality of the surface and passivating defects, leading to an increase in the photoluminescence yield. The increase in PEAI exposure time did however not increase conversion depth of SnI₂ to PEA₂SnI₄, which was restricted to the top 25-30 nm.

Our results show that PEA₂SnI₄ can be a very promising candidate for optoelectronic applications, due to its high absorption and photoluminescence yield. Stoichiometric PEA₂SnI₄ can be grown in an easy, clean and reproducible way and only a few nanometers thick films are enough to achieve high and stable PLQY. The sequential room temperature process, that is self-limiting to about 25nm could be used to produce clean well defined 2D/3D halide perovskite solar cells.