Reveal Activated Corrosion Pathways in Lead Mixed-Halide Perovskites Using X-Ray-Based Methods

Long-term stability in lead halide perovskites has been connected to defect concentrations in their electonic band gap, but rarely their (electro)chemical, physical and dynamic nature, as well as their ion reactivity and mobility. Many culprits (such as O₂ and light excitation) for interfacial and ultimately bulk degradation of perovskites have been considered critical for the operational stability of photovoltaic devices and X-ray detectors.^1-3 Explanations for the irreversible degradation process, however, range from defects and phase segregation to electrochemical reactions as well as electronic and interfacial property changes.^4-6
Here, in situ degradation experiments of device-relevant triple cation mixed halide perovskite films using dry O₂ gas in a near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) method defines the formation of a weakly coordinated form of Pb (relative to Pb in [PbI₆]^4- octahedra). Simultaneously, oxidized iodide species (I₃^-) are formed in the illuminated near-surface region causing bromide enrichment, consistent with “de-mixing” under stress. Once UHV conditions the initial perovskite stoichiometry is slowly restored, 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, which is explained based on the defect distributions established using our electrochemical approach.^7-9
Additionally, operando methodologies based on systematic potential modulation that polarizes the interface of any device stack in combination with structural analysis using synchrotron-based X-ray diffraction suggest a similar “de-mixing” process under moderate electrochemical bias. A larger electrochemical bias induces a irreversible structure change, which is dependent on the buried interface.
Both studies demonstrate an important first step in a systematic approach to understanding the role(s) of environmental and electrical stressors in conjunction with above band-gap illumination that limit device stability. Prior to investing resources in high-throughput device construction, these stypes of studies 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. Additionally, this work extends towards other semiconductive materials or material blends, where operando chemical characterization will be essential to design and support next-generation electronics.

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