Rotational Dynamics of Organic Cations in Formamidinium Lead Iodide Perovskites
Rasmus Lavén, Michael Marek Koza, Lorenzo Malavasi, Adrien Perrichon, Markus Appel, Maths Karlsson
Abstract
High Resolution Image Download MS PowerPoint Slide We report results from quasi-elastic neutron scattering studies on the rotational dynamics of formamidinium (HC[NH 2 ] 2 +, FA) and methylammonium (CH 3 NH 3 +, MA) cations in FA 1– x MA x PbI 3 with x = 0 and 0.4 and compare it to the dynamics in MAPbI 3 . For FAPbI 3, the FA cation dynamics evolve from nearly isotropic rotations in the high-temperature ( T > 285 K) cubic phase through reorientations between preferred orientations in the intermediate-temperature tetragonal phase (140 K < T ⩽ 285 K) to an even more complex dynamics, due to a disordered arrangement of the FA cations, in the low-temperature tetragonal phase ( T ⩽ 140 K). For FA 0.6 MA 0.4 PbI 3, the dynamics of the respective organic cations evolve from a relatively similar behavior to FAPbI 3 and MAPbI 3 at room temperature to a different behavior in the lower-temperature phases where the MA cation dynamics are a factor of 50 faster as compared to those of MAPbI 3 . This insight suggests that tuning the MA/FA cation ratio may be a promising approach to tailoring the dynamics and, in effect, optical properties of FA 1– x MA x PbI 3 .