Role of Alkali Cations in Stabilizing Mixed-Cation Perovskites to Thermal Stress and Moisture Conditions

• Published in: ACS applied materials & interfaces Vol. 13; no. 36; pp. 43573 - 43586
• Main Authors: Maniyarasu, Suresh, Ke, J. Chun-Ren, Spencer, Ben F, Walton, Alex S, Thomas, Andrew G, Flavell, Wendy R
• Format: Journal Article
• Language: English
• Published: American Chemical Society 15.09.2021
• Subjects: Surfaces, Interfaces, and Applications; perovskites; methylammonium-free perovskites; hard X-ray photoelectron spectroscopy; mixed-cation perovskites; near-ambient pressure X-ray photoelectron spectroscopy
• ISSN: 1944-8244; 1944-8252
• DOI: 10.1021/acsami.1c10420

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.