Air-stable caesium tin iodide double perovskite (Cs2SnI6) thin films have been fabricatedviaaerosol-assisted chemical vapour deposition (AACVD).

Near-ambient-pressure X-ray photoelectron spectroscopy enables the study of the reaction of in situ-prepared methylammonium lead iodide (MAPI) perovskite at realistic water vapour pressures for the first time. We show that MAPI decomposes directly to PbI, HI and NH without formation of methylammonium iodide, allowing us to distinguish between alternative mechanisms for the atmospheric degradation reaction.

Perovskite solar cells (PSCs) based on organic–inorganic hybrid perovskites containing a small fraction of substituted alkali-metal cations have shown remarkable performance and stability. However, the role of these cations is unclear. The thermal- and moisture-induced degradation of FA1–x Cs x PbI3 and (FA1–x Cs x )1–y Rb y PbI3 (where FA represents formamidinium, x, y = 0.1, 0.05) is investigated using in situ photoelectron spectroscopy (PES). Both compositions exhibit superior moisture stability compared with methylammonium lead iodide under 9 mbar of water vapor. Ga Kα hard X-ray PES is used to investigate the composition of the perovskites at depths up to 45 nm into the surface. This allows more accurate quantification of the alkali-metal distribution than is possible using conventional X-ray PES. The addition of RbI results in a fairly homogeneous distribution of both Cs+ and Rb+ in the surface layers (in contrast to surface Cs depletion seen in its absence), together with a marked reduction in surface iodide vacancies. Overall, RbI is found to play a critical role in increasing the thermal stability of FA1–x Cs x PbI3 by providing a source of I– that fills iodine vacancy sites in the perovskite lattice, while Rb+ is not substantially incorporated into the perovskite. We suggest that the concomitant increase in ion migration barriers in the surface layers is key to improved PSC performance and long-lasting stability.