Light Induced Passivation in Triple Cation Mixed Halide Perovskites: Interplay between Transport Properties and Surface Chemistry.

Mixed halide perovskites have attracted a strong interest in the photovoltaic community as a result of their high power conversion efficiency and the solid opportunity to realise a low-cost and industry-scalable technology. Light soaking represents one of the most promising approach to reduce non-radiative recombination processes and thus to optimise device performances. Here, we investigate the effects of 1 sun illumination on state-of-the-art triple cation halide perovskite thin films Cs0.05(MA0.14, FA0.86)0.95 Pb (I0.84, Br0.16) by a combined optical and chemical characterization. Competitive passivation and degradation effects on perovskite transport properties have been analyzed by spectrally and time-resolved quantitative imaging luminescence analysis and by X-ray photoemission spectroscopy (XPS). We notice a clear improvement of the optoelectronic properties of the material, with a net increase of the Quasi Fermi Level Splitting and a corresponding decrease of methylammonium MA+ for short (up to one hour) light soaking time. However, after 5 hours of light soaking phase segregation and in-depth oxygen penetration lead to a decrease of the charge mobility.