Unravelling the Formation of 2-Phenylethylammonium 2D Perovskites by In Situ GIWAXS

Hybrid lead halide perovskites are considered a highly promising material for future advancements of photovoltaic technologies due to their outstanding optoelectronic properties. Nevertheless, their instability under ambient conditions hinders the large-scale adoption of perovskite solar cells. Two-dimensional (2D) perovskites have been proposed to address the issue of instability, but these structures display anisotropic optoelectronic properties related to their layered structure. Moreover, their crystallization pathways are not well understood. Here we present insight into the crystallization process of 2-phenyethylammonium (PEA+) Ruddlesden-Popper 2D perovskites with iodide (I−) and bromide (Br−) anions by performing in situ grazing incidence wide-angle X-ray scattering (GIWAXS) measurements of thin films with various compositions during their fabrication via spin-coating and thermal annealing. The GIWAXS data reveals the structure of coexisting 2D perovskite phases and their orientation with respect to the substrate for different precursor compositions. The bromide-based compositions exhibited crystallization already during spin-coating, while iodide-based compositions required thermal annealing to induce the crystallization process. For the latter ones, additional polymorphs were found, suggesting the intrinsic differences for compositions with different halides.