DOI: https://doi.org/10.29363/nanoge.hpatom.2022.005
Publication date: 30th October 2021
An important question with regards to hybrid organic-inorganic perovskites (HOIPs) concerns the organic cation dynamics, which in earlier studies have been invoked for explaining several of the HOIPs’ material properties [1]. A powerful technique for studying the organic cation dynamics in HOIPs is quasielastic neutron scattering (QENS) as it is particularly sensitive to H (present in the ends of the organic cations) and allows to probe the dynamic response in both space and time through the momentum and energy transfer, respectively. In the past, this has been clearly demonstrated in studies of methylammonium (MA) based systems, such as MAPbI3 [2]. However, the currently best performing HOIPs, with a view towards technological applications, are built on mixed cation systems including the formamidinium (FA) organic cation, which have been shown to have a contrasting dynamical behavior as compared to MA organic cations in APbBr3 [3]. Compared to the much more studied MA based systems, the dynamical behavior of the mixed variants is less understood.
In this contribution, we will present QENS results pertaining to the organic cation dynamics in pure and MA-doped formamidinium lead iodide perovskites (FAPbI3). The results show that, in the high-temperature cubic phase of FAPbI3, the FA cations undergo nearly isotropic rotations with a relatively low activation energy of about 24 meV. In the intermediate-temperature tetragonal phase (140 K < T < 280 K), the FA cation performs reorientational motions between preferred orientations determined by tetragonal symmetry of the surrounding perovskite cage. However, in the low-temperature tetragonal phase (T < 140 K) of FAPbI3, the data indicate that the FA cation dynamics are more complex, and possibly featured by a distribution of relaxation times and activation energies related to the formation of an orientational glass of FA below 140 K [4]. Investigations of the mixed-cation system FA0.6MA0.4PbI3 show that the MA cation dynamics become drastically faster in the low-temperature phase of FA0.6MA0.4PbI3, as compared to in pure MAPbI3, which is coupled to a slowing-down of the FA cation dynamics. Our results thus unravel the strikingly different cations dynamics of MA and FA in FA0.6MA0.4PbI3 compared to the parent phases MAPbI3 and FAPbI3. Lastly, the connection between the observed organic cation dynamics in FA1-xMAxPbI3 and its structural properties and phonons will be discussed.
This research was funded by the Swedish Research Council (Grant No. 2016-06958). The authors thank the Institut Laue-Langevin for access to neutron beam facilities.
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