Jean-Jacques Pireaux

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 (Arn+) 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 ((FAxCs1-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 /Pbtot 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 /Pbtot 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.

Combined AFM and ToF-SIMS analyses for the study of filaments in organic resistive switching memories

The resistive switching mechanism in organic and hybrid resistive memories has been intensively studied in the last years. A particular interest have been directed to solution processed resistive layers based on an organic or polymer compounds for which convincing direct and indirect evidences indicated that the switching mechanism is based on the formation of conductive filaments (CFs) bridging the two metal electrodes. However, the CF composition, formation and rupture dynamics and evolution during the prolonged cycling are still poorly explored. Experiments are rare because of the well-known challenges in characterizing nanoscale filaments. In this work, we combine time-of-flight secondary ion mass spectrometry (ToF-SIMS) 3D imaging and in-situ atomic force microscopy (AFM), acquired at different profile depths, to characterize the CF composition and dynamics in high-performance and environmental stable crossbar Ag/parylene C/Ag printed memories. The results allow characterizing the filaments composition, their formation mechanism by electrochemical metallization and their evolution upon cycling. Moreover, the AFM images allow for a more clear interpretation of ToF-SIMS 3D reconstructions of molecular ions and to highlight artifacts arising from the different sputtering rate of metals as compared to the organic material.