Microscopic (Dis)order and Dynamics of Cations in Mixed FA/MA Lead Halide Perovskites

Helen Grüninger; Menno Bokdam; Nico Leupold; Paul Tinnemans; Ralf Moos; Gilles A De Wijs; Fabian Panzer; Arno P M Kentgens

doi:10.1021/acs.jpcc.0c10042

Microscopic (Dis)order and Dynamics of Cations in Mixed FA/MA Lead Halide Perovskites

J Phys Chem C Nanomater Interfaces. 2021 Jan 28;125(3):1742-1753. doi: 10.1021/acs.jpcc.0c10042. Epub 2021 Jan 15.

Authors

Helen Grüninger¹, Menno Bokdam^{2

3}, Nico Leupold⁴, Paul Tinnemans¹, Ralf Moos⁴, Gilles A De Wijs¹, Fabian Panzer^{4

5}, Arno P M Kentgens¹

Affiliations

¹ Institute for Molecules and Materials, Radboud University, Heyendaalseweg 135, 6525 AJ Nijmegen, The Netherlands.
² Faculty of Physics and Center for Computational Materials Sciences, University of Vienna, Sensengasse 8/12, 1090 Vienna, Austria.
³ Faculty of Science and Technology and MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands.
⁴ Department of Functional Materials, University of Bayreuth, Universitätsstraße 30, 95447 Bayreuth, Germany.
⁵ Soft Matter Optoelectronics, University of Bayreuth, Universitätsstraße 30, 95447 Bayreuth, Germany.

Abstract

Recent developments in the field of high efficiency perovskite solar cells are based on stabilization of the perovskite crystal structure of FAPbI₃ while preserving its excellent optoelectronic properties. Compositional engineering of, for example, MA or Br mixed into FAPbI₃ results in the desired effects, but detailed knowledge of local structural features, such as local (dis)order or cation interactions of formamidinium (FA) and methylammonium (MA), is still limited. This knowledge is, however, crucial for their further development. Here, we shed light on the microscopic distribution of MA and FA in mixed perovskites MA_1-x FA _x PbI₃ and MA_0.15FA_0.85PbI_2.55Br_0.45 by combining high-resolution double-quantum ¹H solid-state nuclear magnetic resonance (NMR) spectroscopy with state-of-the-art near-first-principles accuracy molecular dynamics (MD) simulations using machine-learning force-fields (MLFFs). We show that on a small local scale, partial MA and FA clustering takes place over the whole MA/FA compositional range. A reasonable driving force for the clustering might be an increase of the dynamical freedom of FA cations in FA-rich regions. While MA_0.15FA_0.85PbI_2.55Br_0.45 displays similar MA and FA ordering as the MA_1-x FA _x PbI₃ systems, the average cation-cation interaction strength increased significantly in this double mixed material, indicating a restriction of the space accessible to the cations or their partial immobilization upon Br^- incorporation. Our results shed light on the heterogeneities in cation composition of mixed halide perovskites, helping to exploit their full optoelectronic potential.