Direct assessment of structural order and evidence for stacking faults in layered hybrid perovskite films from X-ray scattering measurements

Abstract

Ruddlesden–Popper layered perovskites have emerged as a promising solution for overcoming the moisture instability of three-dimensional hybrid perovskite materials. Given that the optoelectronic properties of these layered perovskites strongly depend on the dimensionality (n) of the phases present, understanding of the microstructure and order in such materials is important. Typically, the dimensionality of phases present is inferred from optical measurements rather than diffraction measurements which are a more direct probe of structural order. Here we use a combination of grazing-incidence transmission wide-angle X-ray scattering and transmission wide-angle X-ray scattering techniques to probe the in-plane microstructure of highly textured Ruddlesden–Popper hybrid perovskite films with a target dimensionality of n = 4. By being able to resolve the in-plane diffraction peaks corresponding to the periodic repeating of layered phases that are often obscured by the substrate horizon in grazing incidence measurements, we are able to directly characterise structural order in such films. Despite having a target dimensionality of n = 4, only diffraction peaks corresponding to n = 2, 3 and ∞ phases are observed. Observations from X-ray measurements are combined with optical absorption measurements to show that there is increasing structural disorder in low-n phases with increasing dimensionality. Further analysis of peak positions and peak widths in the X-ray scattering patterns indicate that stacking fault defects are the origin of structural disorder in layered perovskite films, with increasing disorder with increasing n. We also present a complete indexing of grazing-incidence wide-angle X-ray scattering patterns of highly textured layered perovskite films and confirm the structurally ordered n = 3 and ∞ phases as the dominant crystalline phases in such films. This work provides guidance for the reliable characterisation of structural order and orientation in layered perovskite thin films from X-ray scattering measurements which will assist with assessing n-phase purity and photovoltaic device optimisation.