Intrinsic Light Instabilities in Metal Halide Perovskites

Solution-processed metal halide perovskite solar cells are currently under the spotlight due to their high-power conversion efficiencies and easy fabrication. However, the commercialization might be hampered by certain drawbacks such as device degradation and hysteresis [1]. Several hypotheses have been formulated concerning the physical origins of the rapid deterioration of the perovskite absorbers, such as vacancies, ion migration, and phase segregation [2-3]. Recent reports in the literature, investigating the perovskite degradation by X-ray photoelectron spectroscopy suggest that X-ray exposure triggers the release of the organic cations as well as the dissociation of PbI₂ into metallic lead Pb(0) and I₂ [4-8]. However, <b><i>a fundamental understanding of the role of low energy light (white light) on the intrinsic degradation mechanism taking place in metal halide perovskite is still lacking</i></b>.<br/><br/>In this work, we combined scanning probe microscopies (STM, AFM, KPFM), and X-ray photoelectron spectroscopy (XPS) to systematically investigate the effect of light, X-ray, and temperature on the intrinsic stability of the metal-halide (FAPbI₃)_0.97(MAPbBr₃)_0.03 perovskite interface. In contrast to what is usually admitted [6-8], white light has a stronger effect on perovskite degradation as compared to X-rays. Importantly, we show that cooling down the sample helps reduce the formation of Pb(0) and prevents the release of the organic cation during sample analysis. Kelvin Probe microscopy measurements allowed us how to link the losses of the organic cations and iodine to changes in the work function of several hundred meV. The STM measurements reveal drastic changes in the local density of states. We do observe a bandgap shrinkage and strong lateral variations after light exposure. A model, explaining the compositional and electronic changes at the surface will be presented. In addition to providing useful insights into the intrinsic light-induced degradation of perovskite, our findings also offer useful guidelines for more accurate and non-invasive XPS analyses of this highly sensitive material system.<br/><br/><b>References:</b><br/>[1] P Wang <i>et al.</i>, <b>Adv. Funct. Mater.</b> 29 (47), 1807661 (2019).<br/>[2] S.G. Motti <i>et al.</i>, <b>Nature Commun. </b>12, 6955 (2021).<br/>[3] W. Zhou <i>et al.</i>, <b>J. Phys. D: Appl. Phys. </b>54, 063001 (2021).<br/>[4] J. Hieulle, <i>et al.</i>, <b>J. Am. Chem. Soc.</b>, 141, 8, 3515–3523 (2019).<br/>[5] A. Jamshaid, <i>et al.</i>, <b>Energy Environ. Sci.</b>, 14, 4541–4554 (2021).<br/>[6] S. Svanström, <i>et al.</i>, <b>Phys. Chem. Chem. Phys.</b>, 23, 12479–12489 (2021).<br/>[7] W.-C. Lin, <i>et al.</i>, <b>npj Materials Degradation</b> 5:13 (2021).<br/>[8] M. E. Stuckelberger, <i>et al.</i>, <b>J. Phys. Chem. C</b>, 124, 17949–17956 (2020).