Degradation and Self-Healing of Halide Perovskites Under X-Ray Irradiation

Halide perovskites have emerged as a highly promising class of materials across various optoelectronic applications, encompassing solar cell technology, light-emitting diodes (LEDs), lasers, and X-ray detection. However, the advancement of perovskite X-ray detector technology remains in its early stages, necessitating further research to comprehensively elucidate the properties of these materials and their responses to external stimuli. Furthermore, given the prevalent utilization of X-rays in fundamental chemical surface analysis techniques like X-ray photoelectron spectroscopy (XPS), a thorough understanding of the effects of X-ray exposure on halide perovskites is imperative for the accurate interpretation of surface characterizations via XPS and related methodologies. This imperative is accentuated in the context of operando experiments, wherein materials endure prolonged exposure to high doses of X-rays.<br/><br/>We propose a comprehensive, multi-scale, and multi-technique approach by integrating X-ray photoelectron spectroscopy with steady-state photoluminescence imaging to investigate the detrimental effects and subsequent self-healing of Formamidinium Lead Bromide (FAPbBr₃) under ion bombardment and X-ray irradiation¹. We observed that short and low flux irradiation with synchrotron light causes local decomposition of FAPbBr₃ into PbBr₂. Concomitantly, metallic lead clusters are created in the decomposed sites by further decomposition of PbBr₂, causing pinning of the Fermi level close to the conduction band. Conversely, prolonged exposure to higher-flux irradiation triggers local self-healing mechanims, which aid in restoring optoelectronic properties. Ion migration is key to "heal" the surface as the lost Br atoms on the surface are replenished by Br atoms coming from the underlying material.<br/><br/>Subsequently, our investigation extends to triple-cation double-halide Cs_0.05(MA_0.14, FA_0.86)_0.95 Pb(I_0.84, Br_0.16)₃ perovskite thin films, employing a comparable experimental framework². Employing an array of optical imaging techniques, including spectrally and temporally resolved methodologies such as hyperspectral imaging and time-resolved fluorescence imaging (TR-FLIM), in combination with surface chemistry analysis via XPS, we delineate two principal degradation pathways within the perovskite layer. At lower X-ray fluences, we observe minor alterations in surface chemistry composition alongside the emergence of electronic defects. A second degradation pathway, occurring at higher fluence, involves the evaporation of organic cations and results in the formation of an iodine-deficient perovskite, which exhibits a faster carrier decay time. Leveraging insights derived from localized variations in optoelectronic properties, we propose a kinetic model elucidating the underlying mechanisms governing these degradation phenomena.<br/><br/>[1] V. Milotti, S. Cacovich, D. R. Ceratti, D. Ory, J. Barichello, F. Matteocci, A. di Carlo, P. M. Sheverdyaeva, P. Schulz, P. Moras. Degradation and Self-Healing of FAPbBr₃ Perovskite under Soft- X-Ray Irradiation. <i>Small Methods</i>, 23002222, 2023.<br/><br/>[2] Vidon, P. Dally, M. Al-Katrib, D. Ory, M. Kim, E. Soret, E. Rangayen, M. Legrand, A. Blaizot, P. Schulz, J.-B. Puel, D. Suchet, J.-F. Guillemoles, A. Etcheberry, M. Bouttemy, S. Cacovich. The impact of X-ray radiation on chemical and optical properties of triple-cation lead halide perovskite: from the surface to the bulk. <i>Advanced Functional Materials</i>, 2304730, 2023.