In situ XPS study of the surface chemistry of MAPI solar cells under operating conditions in vacuum

The most limiting factor for the commercialization of perovskite solar cells is the lack of long-term stability under operating conditions. To examine the intrinsic stability of the perovskite film, we investigated the chemical and electronic properties of methylammonium lead triiodide (CH3NH3PbI3) perovskite by in situ X-ray photoelectron spectroscopy (XPS). In particular, XPS data were collected under dark conditions, at applied voltage, under illumination, at open circuit, and under operating conditions. In addition, operation in ambient air atmosphere was analysed by XPS. It was observed that the chemical properties of methylammonium lead triiodide change upon illumination under open-circuit condition by formation of metallic lead species and then by conversion into PbI2 due to evaporation of the organic compounds. These changes, however, can be restricted by applying an extraction voltage to the device contacts that will extract the photogenerated charges from the absorber. As these results were obtained in vacuum, i.e., in an inert atmosphere, experiments prove that the photogenerated charge carriers intrinsically induce changes in chemical and electronic properties if they are not extracted from the absorber. By illuminating CH3NH3PbI3 in ambient air, the metallic lead species react with oxygen and form lead oxides and lead complexes, which passivate the film and remove the in-gap states.