XPS depth profiles of organo lead halide layers and full perovskite solar cells by variable-size argon clusters

Organic and inorganic materials are more and more frequently combined in high-performance hybrid electronic and photonic devices. For such multilayered stacks, the identification of layers and interface defects by depth profile analysis is a challenging task, especially because of the possible ion beam induced modifications. This is particularly true for perovskite solar cells stacks that in a mesoscopic structure usually combine a metal electrode, a mesoscopic conductive oxide layer, an intrinsically hybrid light absorber, an organic hole extraction layer and a metal counter electrode. While depth profile analysis with X-ray photoelectron spectroscopy (XPS) was already applied to investigate these devices, the X-ray and ion beam induced modifications on such hybrid layers have not been previously investigated. In this work we compare the profiles obtained with monatomic Ar⁺ beam at different energies, with the ones obtained with argon ion clusters (Ar_n ⁺) with different sizes (150<n<1000) and energies (up to 8 keV). A systematic study is performed on full mesoscopic perovskite (CH₃NH₃PbI₃) solar cells and on model hybrid samples ((FA_xCs_1-xPbI₃)0.85 (MAPbBr₃)_0.15)/TiO₂). The results show that for monatomic beams, the implantation of positively charged atoms induces the surface diffusion of free iodine species from the perovskite which modifies the I/Pb ratio. Moreover, lead atoms in the metallic state (Pb⁰) are found to accumulate at the bottom of the perovskite layer where the Pb⁰ /Pb_tot fraction reaches 50%. With argon clusters, the ion beam induced diffusion of iodine is reduced only when the etch rate is sufficiently high to ensure a profile duration comparable with low-energy Ar⁺. Convenient erosion rates are obtained only for n=300 and n=500 clusters at 8 keV, which have also the advantage of preserving the TiO₂ surface chemistry. However, with argon cluster ions, Pb⁰ particles in the perovskite are less efficiently sputtered which leads to the increase of the Pb⁰ /Pb_tot fraction (up to 75%) at the perovskite/TiO₂ interface. Finally, ion beam and X-ray induced artifacts on perovskite absorbers can be reasonably neglected for fast analysis conditions in which the exposure time is limited to few hours.