Surface stability of ionic-liquid-passivated mixed-cation perovskite probed with in situ photoelectron spectroscopy

In recent times, mixed-cation metal halide perovskites have shown promising photovoltaic performance, and the long-term stability of these metal halide perovskites has also been considerably improved by incorporating additives into the perovskite precursor. Here, the role of ionic liquid additives in improving the stability of perovskite is investigated by in situ surface sensitive studies. A small amount (0.3 mol%) of 1-octyl-3-methylimidazolium chloride ionic liquid (IL) is incorporated into FA0.9Cs0.1PbI3 (FACs) (where FA represents the formamidinium cation, CH(NH2)2+). The thermal- and moisture-induced decomposition of FACs and IL-FACs is investigated using near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS). A comparative study of the pristine and IL-incorporated FACs compositions shows that the IL additive prevents the out-diffusion of organic ions (FA+) from the lattice for temperatures up to 100 °C under 9 mbar water vapour and up to 150 °C under UHV conditions. Both compositions exhibit better stability under 9 mbar water vapour (equivalent to ∼30% relative humidity) compared with conventional methylammonium lead iodide (MAPbI3). The champion device fabricated with IL additive exhibits an improved power conversion efficiency (PCE) of 16% compared with the 13% PCE of the pristine FACs sample. Overall, the results suggest that the IL additive acts to improve the device performance as well as the stability of perovskites under thermal annealing in dry environments, but that a careful choice of IL will be necessary for full passivation in wet environments.