The role of alkali cations in stabilizing mixed cation perovskites to thermal stress and moisture conditions

Suresh Maniyarasu, Jack Chun-Ren Ke, Ben Spencer, Alex Walton, Andrew Thomas, Wendy Flavell

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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-xCsxPbI3 and (FA1-xCsx)1-yRbyPbI3 (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-xCsxPbI3 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.

Original languageEnglish
Pages (from-to)43573–43586
JournalACS Applied Materials and Interfaces
Issue number36
Publication statusPublished - 31 Aug 2021


  • hard X-ray photoelectron spectroscopy
  • methylammonium-free perovskites
  • mixed-cation perovskites
  • near-ambient pressure X-ray photoelectron spectroscopy
  • perovskites

Research Beacons, Institutes and Platforms

  • Henry Royce Institute


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