Direct observation of an inhomogeneous chlorine distribution in CH3NH3PbI3−xClx layers: surface depletion and interface enrichment

Abstract

We have used hard X-ray photoelectron spectroscopy (HAXPES) at different photon energies and fluorescence yield X-ray absorption spectroscopy (FY-XAS) to non-destructively investigate CH₃NH₃PbI_3−xCl_x perovskite thin films on compact TiO₂. This combination of spectroscopic techniques allows the variation of information depth from the perovskite layer surface to the top-most part of the underlying compact TiO₂ layer. We have taken advantage of this to understand the distribution of chlorine throughout the perovskite/TiO₂ layer stack. No Cl is detected using HAXPES, indicating surface depletion of Cl and allowing us to place an upper limit on the amount of Cl in the perovskite layer: x < 0.07 and x < 0.40 to depths of ∼10 nm and ∼26 nm, respectively, beneath the perovskite film surface. Our FY-XAS results, however, demonstrate that there is a higher average concentration of Cl throughout the perovskite layer than at the surface (x > 0.40) consistent with both enhanced concentrations of Cl deep beneath the perovskite film surface and near the CH₃NH₃PbI_3−xCl_x perovskite/TiO₂ interface. The consequences of this distribution of Cl in the CH₃NH₃PbI_3−xCl_x perovskite layer on device performance are discussed.