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

Kostas Fykouras; Jonathan Lahnsteiner; Nico Leupold; Paul Tinnemans; Ralf Moos; Fabian Panzer; Gilles A. de Wijs; Menno Bokdam; Helen Grüninger; Arno P. M. Kentgens

doi:10.1039/D2TA09069D

Disorder to order: how halide mixing in MAPbI_3−xBr_x perovskites restricts MA dynamics†

Kostas Fykouras,^a Jonathan Lahnsteiner,^b Nico Leupold,

^b Paul Tinnemans,

^c Ralf Moos,

^b Fabian Panzer,

^d Gilles A. de Wijs,

^c Menno Bokdam,

*^a Helen Grüninger

*^ce and Arno P. M. Kentgens

^c

Author affiliations

* Corresponding authors

^a Faculty of Science and Technology and MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, Netherlands
E-mail: m.bokdam@utwente.nl

^b Department of Functional Materials, University of Bayreuth, Universitätsstraße 30, 95447 Bayreuth, Germany

^c Radboud University, Institute for Molecules and Materials, Heyendaalseweg 135, 6525 AJ Nijmegen, Netherlands

^d Soft Matter Optoelectronics, University of Bayreuth, Universitätsstraße 30, 95447 Bayreuth, Germany

^e Inorganic Chemistry III and Northern Bavarian NMR Centre, University of Bayreuth, Universitätsstraße 30, 95447 Bayreuth, Germany
E-mail: helen.grueninger@uni-bayreuth.de

Abstract

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 MAPbI_3−xBr_x 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 MAPbI_3−xBr_x 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 Pb₈I_12−nBr_n “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.

This article is part of the themed collection: #MyFirstJMCA

Journal of Materials Chemistry A

Disorder to order: how halide mixing in MAPbI_3−xBr_x perovskites restricts MA dynamics†

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Journal of Materials Chemistry A

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

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

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Disorder to order: how halide mixing in MAPbI_3−xBr_x perovskites restricts MA dynamics†

Disorder to order: how halide mixing in MAPbI_3−xBr_x perovskites restricts MA dynamics