Issue 9, 2023

Disorder to order: how halide mixing in MAPbI3−xBrx perovskites restricts MA dynamics


Mixed-halide lead perovskites are of particular interest for the design of tandem solar cells currently reaching record efficiencies. While halide phase segregation upon illumination of mixed perovskites is extensively studied, the effect of halide disorder on A cation dynamics is not well understood, despite its importance for charge carrier diffusion and lifetime. Here, we study the methylammonium (MA) reorientational dynamics in mixed halide MAPbI3−xBrx perovskites by a combined approach of experimental solid-state NMR spectroscopy and molecular dynamics (MD) simulations based on machine-learning force-fields (MLFF). 207Pb NMR spectra indicate the halides are randomly distributed over their lattice positions, whereas PXRD measurements show that all mixed MAPbI3−xBrx samples are cubic. The experimental 14N spectra and 1H double-quantum (DQ) NMR data reveal anisotropic MA reorientations depending on the halide composition and thus associated disorder in the inorganic sublattice. MD calculations allow us to correlate these experimental results to restrictions of MA dynamics due to preferred MA orientations in their local Pb8I12−nBrn “cages”. Based on the experimental and simulated results, we develop a phenomenological model that correlates the 1H dipolar coupling and thus the MA dynamics with the local composition and reproduces the experimental data over the whole composition range. We show that the dominant interaction between the MA cations and the Pb–X lattice that influences the cation dynamics is the local electrostatic potential being inhomogeneous in mixed halide systems. As such, we generate a fundamental understanding of the predominant interaction between the MA cations and the inorganic sublattice, as well as MA dynamics in asymmetric halide coordinations.

Graphical abstract: Disorder to order: how halide mixing in MAPbI3−xBrx perovskites restricts MA dynamics

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Supplementary files

Article information

Article type
21 Nov 2022
01 Feb 2023
First published
01 Feb 2023
This article is Open Access
Creative Commons BY license

J. Mater. Chem. A, 2023,11, 4587-4597

Disorder to order: how halide mixing in MAPbI3−xBrx perovskites restricts MA dynamics

K. Fykouras, J. Lahnsteiner, N. Leupold, P. Tinnemans, R. Moos, F. Panzer, G. A. de Wijs, M. Bokdam, H. Grüninger and A. P. M. Kentgens, J. Mater. Chem. A, 2023, 11, 4587 DOI: 10.1039/D2TA09069D

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