Intrinsic Behavior of CH3NH3PbBr3 Single Crystals under Light Illumination

Single crystal CH 3 NH 3 PbBr 3 samples are exposed to light illumination, with a light intensity about seven times stronger than the sun while under ultrahigh vacuum (UHV) conditions, in order to investigate their chemical and structural stability from prolonged light illumination. X‐ray photoemission spectroscopy measurements show that within 10 h of illumination, about half of the initial C, N, and Br elemental concentrations leave the surface and about half of the perovskite's Pb is converted into metallic Pb. Light exposures while in the UHV system also significantly roughen the surface, and surprisingly, empty voids form ≈1 to 3 µm down in the light exposed region. A framework based on the Kirkendall effect is put forward to explain the observed void formation. This proposed model may be relevant to the slow degradation of perovskite solar cells, which is sometimes attributed to irreversible chemical reactions from undesired diffusion. These measurements and observations reveal the intrinsic behavior of the CH 3 NH 3 PbBr 3 single crystals under light illumination while in a UHV system where volatile species are free to leave, in contrast to existing device studies on the photostability of perovskite solar cells.