Proceedings of Online nanoGe Fall Meeting 20 (OnlineNFM20)
Publication date: 4th October 2020
We study ion migration in 2D lead halide perovskites of varying dimensionality using scanning-Kelvin probe microscopy. We perform potentiometry on micron-scale lateral junctions in the absence of injected charge and we compare how ion motion varies between prototypical two-dimensional n-butylammonium lead iodide perovskites (BA2PbI4, n=1), and methylammonium-incorporated quasi-2D perovskites (BA2MA3Pb4I13, ~<n>=4) both in the dark and under illumination. For pure 2D BA2PbI4 films (n=1) under applied bias, we observe symmetric potential profiles with charges migrating towards the anode and cathode (the charging process), and then away from the anode and cathode when the electric field is removed (the discharging process), both in the dark and under illumination. In contrast, we observe asymmetric charging and discharging potential profiles for quasi-2D BA2MA3Pb4I13 films in the dark, which become symmetric under illumination. We attribute such a difference to the n=1 film being intrinsic and the n=4 film being self p-doped, on which the electric field is then screened by photogenerated carriers. We also measure the relaxation of the bias-induced ionic charge distributions at different temperatures to extract the activation energies associated with the ionic motion in each case. The relaxation dynamics during the discharging of both positive and negative potentials are similar for the n=1 film, but vary significantly for the n=4 film. Finally, we propose an explanation for these phenomena by hypothesizing that ion motion in purely 2D BA2PbI4 perovskite films is dominated by paired halide and halide vacancy motion, whereas for quasi-2D BA2MA3Pb4I13 films, the ion motion is a combination of both halide and methylammonium (vacancy) migration. These data show that dimensionality in these systems plays a critical role in the ion dynamics.
This work is primarily sponsored by the Department of Energy (DOE-SC0013957). We acknowledge support by the State of Washington through the University of Washington Clean Energy Institute and the Washington Research Foundation. Part of this work was conducted at the Molecular Analysis Facility, a National Nanotechnology Coordinated Infrastructure site at the University of Washington that is supported in part by the National Science Foundation (grant no. ECC-1542101), the University of Washington, the Molecular Engineering & Sciences Institute, the Clean Energy Institute and the National Institutes of Health. F.M. was additionally supported by the German Academic Exchange Service (DAAD) with funds from the German Federal Ministry of Education and Research (BMBF) and the European Union (FP7-PEOPLE-2013-COFUND - grant agreement No. 605728).
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