evidence, only few experimental works succeeded in the direct quantitative characterization of aging effects in PSCs. In this work, we apply state-of-the-art X-ray photoelectron spectroscopy (XPS) depth profile analysis and time-of-flight secondary ion mass spectrometry (ToF-SIMS) 3D imaging to investigate the light-induced degradation of layers and interfaces in reference (Au/Spiro-OMeTAD/CH3NH3PbI3/m-TiO2/cTiO2/FTO) and interface-engineered mesoscopic PSCs in which graphene flakes are added into the mesoscopic TiO2
layer and a solution-processed 2H-MoS2 flakes buffer layer is added at the Spiro-OMeTAD/CH3NH3PbI3 interface. Results show that the graphene addition into the mesoscopic TiO2 layer improves the stability of the PSC by reducing the locally-inhomogeneous light-induced back-conversion of the CH3NH3PbI3 layer into PbIxand PbOx species and the consequent release of iodine species, which
diffuse across the interfaces and causes the modifications at the gold electrode (Au-I bonding) and themesoscopic TiO2(Ti-I bonding) interfaces. Moreover, where the CH3NH3PbI3 layer is preserved the gold diffusion across the entire device structure is strongly reduced even after the aging. The 2H-MoS2 flakes
buffer layer allows limiting the localized diffusion of gold and the iodine diffusion in as-prepared PSCs while it is rather ineffective in preventing light-induced aging effects. Overall, thanks to the lower average degradation of the layers and interfaces, interface engineered PSCs could retain ~60% of their
initial PCE after the aging respect to less than ~25% in the reference cells
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