Impact of White Light Illumination on the Electronic and Chemical Structures of Mixed Halide and Single Crystal Perovskites

This study investigates the effect of white light illumination on the electronic and chemical properties of mixed halide perovskite (CH3NH3PbI3-xClx) thin films and CH3NH3PbI3 single crystals using photoelectron and absorption spectroscopy. The pristine materials' surfaces are found to be n-type because of surface band bending due to the presence of donor levels, likely consisting of reduced lead (Pb-0) that acts as surface traps. When illuminating the sample with white light (up to 1 sun), the valence features shifted to lower binding energy due to surface photovoltage, i.e., the bulk of the materials is much less n-type. However, the surface photovoltage is only partially reversible and vanishes for prolonged illumination time. Concomitantly, a high concentration of metallic Pb-0 is found, which induces strong Fermi-level pinning and quenching of the surface photovoltage. This is accompanied also by the formation of PbI2 defects. Similar experiments on single crystals reveal the presence of a high concentration of reduced (metallic) Pb-0 at the sample surface after cleaving. The present findings indicate that the chemical and electronic properties of perovskite films are very sensitive to white light illumination. Accounting for these light-induced material changes is important to fully understand its photophysical properties and for improving the lifetime of perovskite-based devices.