Abstract: With the development of photovoltaic devices, organic-inorganic hybrid perovskite solar cells (PSCs) have been promising devices that have attracted significant attention in the fields of industrial and scientific research. Currently, the photoelectric conversion efficiency (PCE) of PSCs has been improved to 25.2%, and they are considered to be the primary alternative to silicon-based solar cells. However, the environmental stability of PSCs is unsatisfactory; they are prone to degradation under exposure to moisture, oxygen, elevated temperature, or even light illumination, which restricts their wide application in industrial production. Previous studies have elucidated that understanding the ultraviolet (UV)-induced degradation mechanism of organic-inorganic PSCs is of great importance for the improvement of light stability in PSCs. However, until now, there has been almost no comprehensive investigation on the decay process of PSCs under UV light illumination nor on the corresponding evolution of their microstructure. In this study, focused ion beam scanning electron microscopy (FIB-SEM) and aberration-corrected transmission electron microscopy (TEM) were used to comprehensively study changes in the performance and the evolution of the microstructure of PSC devices. The experimental results show that a built-in electric field developed under UV light illumination, which drove the diffusion of iodide ions (I^-) from the CH₃NH₃PbI₃ (MAPbI₃) layer to the hole transfer layer (HTL, Spiro-OMeTAD). Together with the photo-excited holes in the HTL, the I^- ions reacted with the Au electrode, and the Au atoms were oxidized into Au⁺ ions. Furthermore, Au⁺ ions preferred to diffuse across the HTL and the perovskite layer into the interface between the SnO₂ and MAPbI₃ layers. SnO₂ is known to be a good electron transfer layer (ETL), which should collect the photo-excited electrons to reduce the Au⁺ ions into metallic Au clusters; this is why the Au electrode was destroyed and Au clusters aggregated at the SnO₂-MAPbI₃ interface under the UV light illumination. Meanwhile, the Au clusters would accelerate the degradation of the perovskite. In addition, as the PSC performance declined (as determined by the PCE, open-circuit voltage (V_oc), and short-circuit current (J_sc)), the decomposition of tetragonal MAPbI₃ into hexagonal PbI₂ was observed at the interface between Spiro-OMeTAD and MAPbI₃, along with a widening of the grain boundaries in the perovskite layer. All of these factors play critical roles in the UV-induced degradation of PSCs. This is the first study to elucidate the light-induced migration of Au from the metal electrode to the interface between SnO₂/MAPbI₃, which reveals that the UV-induced degradation of PSCs may be mitigated by finding new ways to restrain the interdiffusion of Au⁺ and I^- ions.

Key words: Perovskite solar cell, Ultraviolet, Degradation mechanism, Electron microscopy, Gold migration