Perovskite solar cells had a meteoric rise in the solar cell community by going from unstable sub 5% efficiency cells to having stable efficiencies over 20%. The compositions have developed from simple MAPbI₃ to more complex mixed-ion and halide compositions and the ability of perovskites to restructure themselves through diffusion of ions has also been studied by a variety of methods [1]. However, it is clear that the interface between the perovskite and the selective contacts is vital for the performance of the cell. Using XPS enables us to perform in-situ studies of the chemical compositions of the perovskite in the interface with selective contacts.

In X-ray photoelectron spectroscopy (XPS) monochromatised X-ray photons eject core or valence electrons from a material, and the kinetic energy of these electrons can then be measured. Due to the known energy of the incident photons, this allows us to determine the binding energies of the electrons in the material. We use this to study the elemental and chemical composition of perovskites as well as the valence structure of the material. By using the variable photon energy available at synchrotrons we are able tune the probing depth to study buried interface between perovskite and metal/selective layer [2].

Using this methodology, we studied the chemistry at interfaces between perovskites and films of a variety of metals and selective contacts. We found evidence of diffusion of certain metals into the perovskite and the diffusion of iodide through selective contacts to the metal back contacts. In contrast, when metals, which are easily oxidised were deposited on the perovskite surface, no migration was observed but instead the formation of an oxide layer. This layer formation was typically accompanied by significant changes in the chemical composition of the perovskite layer close to the interface. In this presentation I will summarise our findings and the implications these have on perovskite solar cell design.

[1]         U. B. Cappel et al., “Partially Reversible Photoinduced Chemical Changes in a Mixed-Ion Perovskite Material for Solar Cells,” ACS Appl. Mater. Interfaces, vol. 9, no. 40, pp. 34970–34978, Oct. 2017.

[2]         B. Philippe et al., “Chemical Distribution of Multiple Cation (Rb ⁺ , Cs ⁺ , MA ⁺ , and FA ⁺ ) Perovskite Materials by Photoelectron Spectroscopy,” Chem. Mater., vol. 29, no. 8, pp. 3589–3596, Apr. 2017.