Examinando por Autor "Reyes-Lillo, S.E."
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Ítem Excited-state vibrational dynamics toward the polaron in methylammonium lead iodide perovskite(Nature Publishing Group, 2018) Park, M.; Neukirch, A.J.; Reyes-Lillo, S.E.; Lai, M.; Ellis, S.R.; Dietze, D.; Neaton, J.B.; Yang, P.; Tretiak, S.; Mathies, R.A.Hybrid organic-inorganic perovskites have attractive optoelectronic properties including exceptional solar cell performance. The improved properties of perovskites have been attributed to polaronic effects involving stabilization of localized charge character by structural deformations and polarizations. Here we examine the Pb-I structural dynamics leading to polaron formation in methylammonium lead iodide perovskite by transient absorption, time-domain Raman spectroscopy, and density functional theory. Methylammonium lead iodide perovskite exhibits excited-state coherent nuclear wave packets oscillating at ~20, 43, and 75 cm-1 which involve skeletal bending, in-plane bending, and c-axis stretching of the I-Pb-I bonds, respectively. The amplitudes of these wave packet motions report on the magnitude of the excited-state structural changes, in particular, the formation of a bent and elongated octahedral PbI6 4- geometry. We have predicted the excited-state geometry and structural changes between the neutral and polaron states using a normal-mode projection method, which supports and rationalizes the experimental results. This study reveals the polaron formation via nuclear dynamics that may be important for efficient charge separation. © 2018 The Author(s).Ítem Revealing Nanoscale Chemical Heterogeneities in Polycrystalline Mo-BiVO4 Thin Films(Wiley-VCH Verlag, 2020-09) Eichhorn, J.; Reyes-Lillo, S.E.; Roychoudhury, S.; Sallis, S.; Weis, J.; Larson, D.M.; Cooper, J.K.; Sharp, I.D.; Prendergast, D.; Toma, F.M.The activity of polycrystalline thin film photoelectrodes is impacted by local variations of the material properties due to the exposure of different crystal facets and the presence of grain/domain boundaries. Here a multi-modal approach is applied to correlate nanoscale heterogeneities in chemical composition and electronic structure with nanoscale morphology in polycrystalline Mo-BiVO4. By using scanning transmission X-ray microscopy, the characteristic structure of polycrystalline film is used to disentangle the different X-ray absorption spectra corresponding to grain centers and grain boundaries. Comparing both spectra reveals phase segregation of V2O5 at grain boundaries of Mo-BiVO4 thin films, which is further supported by X-ray photoelectron spectroscopy and many-body density functional theory calculations. Theoretical calculations also enable to predict the X-ray absorption spectral fingerprint of polarons in Mo-BiVO4. After photo-electrochemical operation, the degraded Mo-BiVO4 films show similar grain center and grain boundary spectra indicating V2O5 dissolution in the course of the reaction. Overall, these findings provide valuable insights into the degradation mechanism and the impact of material heterogeneities on the material performance and stability of polycrystalline photoelectrodes. © 2020 The Authors. Published by Wiley-VCH GmbH