Unraveling Photoelectrochemical Degradation Pathways in Lead Mixed Halide Perovskites with Near Ambient Pressure XPS

Long-term stability in metal halide perovskites has often been connected to defects in the electonic band gap, but rarely their (electro)chemical, physical and dynamic nature, as well as ion reactivity and mobility are considered. Interactions with O₂ and light excitation (i.e. from X-rays to UV-visible regions) have been suggested as culprits for bulk and interfacial degradation of perovskites and have been considered critical in the improvement of operational stability of photovoltaic devices and X-ray detectors.^[1-3] Possible explanations for this irreversible degradation process, however, range from intrinsic defects and phase segregation to electrochemical reactions as well as electronic and interfacial property changes.^[4-6]<br/>In this work, <i>in situ</i> degradation of device-relevant triple cation mixed halide perovskite films using dry O₂ gas using a near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) characterization method reveals the formation of a weakly coordinated form of Pb (relative to corner-sharing [PbI₆]^4- octahedra). Simultaneously, oxidized iodide species (I₃^-) are formed in the illuminated near-surface region causing the bromide enrichment, consistent with “de-mixing” under stress. Once UHV conditions are restored, a slow return to the initial perovskite stoichiometry is observed, suggesting “self-healing” of defects when the low stress environment is reestablished. In non-stoichiometric films degradation rates accelerate for FAI-rich stoichiometries, but, more importantly, PbI₂-rich stoichiometries show reduced rates.<br/>This NAP-XPS study demonstrates an important first step in a systematic approach to understanding the role(s) of ambient gases in conjunction with above band-gap illumination and other stressors that limit device stability. Prior to investing resources in high-throughput device construction, this NAP-XPS characterization technique will be an important asset in the investigation of interface engineering strategies for top contact (and bottom) modifications used as mitigation strategies in device optimization with focus on photoelectrochemical stability and energy level alignment if combined with UPS. Additionally, this work extends towards other semiconductive materials or material blends, where <i>in situ</i> chemical characterization will be essential to design and support next-generation electronics.<br/><br/>[1] N. Aristidou, C. Eames, I. Sanchez-Molina, X. Bu, J. Kosco, M. S. Islam, S. A. Haque, <i>Nature Communications</i> <b>2017</b>, 8, 15218.<br/>[2] A. Senocrate, T. Acartürk, G. Y. Kim, R. Merkle, U. Starke, M. Grätzel, J. Maier, <i>Journal of Materials Chemistry A</i> <b>2018</b>, 6, 10847.<br/>[3] D. Bryant, N. Aristidou, S. Pont, I. Sanchez-Molina, T. Chotchunangatchaval, S. Wheeler, J. R. Durrant, S. A. Haque, <i>Energy and Environmental Science</i> <b>2016</b>, 9, 1655.<br/>[4] K. Higgins, M. Lorenz, M. Ziatdinov, R. K. Vasudevan, A. V. Ievlev, E. D. Lukosi, O. S. Ovchinnikova, S. V. Kalinin, M. Ahmadi, <i>Advanced Functional Materials</i> <b>2020</b>, 30, 2001995.<br/>[5] R. Guo, D. Han, W. Chen, L. Dai, K. Ji, Q. Xiong, S. Li, L. K. Reb, M. A. Scheel, S. Pratap, N. Li, S. Yin, T. Xiao, S. Liang, A. L. Oechsle, C. L. Weindl, M. Schwartzkopf, H. Ebert, P. Gao, K. Wang, M. Yuan, N. C. Greenham, S. D. Stranks, S. V. Roth, R. H. Friend, P. Müller-Buschbaum, <i>Nature Energy</i> <b>2021</b>, 6, 977.<br/>[6] M. Ralaiarisoa, I. Salzmann, F. S. Zu, N. Koch, <i>Advanced Electronic Materials</i> <b>2018</b>, 4, 1800307.