Influence of a Hole-Transport Layer on Light-Induced Degradation of Mixed Organic–Inorganic Halide Perovskite Solar Cells

Organic–inorganic halide perovskites (OIHPs) have been intensively studied in recent years for use in solar cells. Many studies have reported the light-induced phenomena of OIHP materials and their solar cells. In this study, we investigated the influence of a hole-transport layer (HTL) on light-induced degradation (LID) of mixed OIHP solar cells, especially at the OIHP/HTL interface, by hard X-ray photoelectron spectroscopy (HAXPES) and impedance spectroscopy. HAXPES shows accumulation of iodine and metallic lead in the vicinity of the OIHP/HTL and electron-transport layer/OIHP interfaces, respectively, after LID. Under light illumination and in the dark after LID, characteristic impedance responses are observed for a sample with a highly damaged OIHP/HTL interface as an intermediate frequency arc and negative capacitance, respectively, because of the electrochemical reaction in the vicinity of the OIHP/HTL interface. Overall, the results indicate that light-induced iodide diffusion to the OIHP/HTL interface and the electrochemical reaction to form iodine molecules are important factors for LID of OIHP solar cells.