title: Organic Cation Rotation and Immobilization in Pure and Mixed Methylammonium Lead-Halide Perovskites.
abstract: Three-dimensional lead-halide perovskites have attracted a lot of attention due to their ability to combine solution processing with outstanding optoelectronic properties. Despite their soft ionic nature these materials demonstrate a surprisingly low level of electronic disorder resulting in sharp band edges and narrow distributions of the electronic energies. Understanding how structural and dynamic disorder impacts the optoelectronic properties of these perovskites is important for many applications. Here we combine ultrafast two-dimensional vibrational spectroscopy and molecular dynamics simulations to study the dynamics of the organic methylammonium (MA) cation orientation in a range of pure and mixed trihalide perovskite materials. For pure MAPbX3(X = I, Br, Cl) perovskite films, we observe that the cation dynamics accelerate with decreasing size of the halide atom. This acceleration is surprising given the expected strengthening of the hydrogen bonds between the MA and the smaller halide anions, but can be explained by the increase in the polarizability with the size of halide. Much slower dynamics, up to partial immobilization of the organic cation, are observed in the mixed MAPb(ClxBr1-x)3and MAPb(BrxI1-x)3alloys, which we associate with symmetry breaking within the perovskite unit cell. The observed dynamics are essential for understanding the effects of structural and dynamical disorder in perovskite-based optoelectronic systems.
authors.affiliation: FOM Institute AMOLF , Science Park 104, Amsterdam 1098 XG, The Netherlands.
authors.forename: Oleg
authors.initials: O
authors.lastname: Selig
authors.affiliation: Cavendish Laboratory, University of Cambridge , JJ Thomson Ave, Cambridge CB30HE, United Kingdom.
authors.forename: Aditya
authors.initials: A
authors.lastname: Sadhanala
authors.affiliation: InnovationLab , Speyerer Strasse 4, 69115 Heidelberg, Germany.|Institut für Hochfrequenztechnik, TU Braunschweig , Schleinitzstr. 22, Braunschweig, Germany.
authors.forename: Christian
authors.initials: C
authors.lastname: Müller
authors.affiliation: InnovationLab , Speyerer Strasse 4, 69115 Heidelberg, Germany.|Institut für Hochfrequenztechnik, TU Braunschweig , Schleinitzstr. 22, Braunschweig, Germany.
authors.forename: Robert
authors.initials: R
authors.lastname: Lovrincic
authors.affiliation: LPEM-UMR 8213, ESPCI-ParisTech/CNRS/Université Pierre et Marie Curie, 10 Rue Vauquelin, 75005 Paris, France.
authors.forename: Zhuoying
authors.initials: Z
authors.lastname: Chen
authors.affiliation: FOM Institute AMOLF , Science Park 104, Amsterdam 1098 XG, The Netherlands.
authors.forename: Yves L A
authors.initials: YL
authors.lastname: Rezus
authors.affiliation: Department of Chemistry, University of Bath , Claverton Down, Bath BA2 7AY, United Kingdom.
authors.forename: Jarvist M
authors.initials: JM
authors.lastname: Frost
authors.affiliation: Zernike Institute for Advanced Materials, University of Groningen , Nijenborgh 4, 9747 AG Groningen, The Netherlands.
authors.forename: Thomas L C
authors.initials: TL
authors.lastname: Jansen
authors.affiliation: Cavendish Laboratory, University of Cambridge , JJ Thomson Ave, Cambridge CB30HE, United Kingdom.|Department of Chemistry, Imperial College London , London SW7 2AZ, United Kingdom.
authors.forename: Artem A
authors.identifier: 0000-0002-3998-2000
authors.initials: AA
authors.lastname: Bakulin