Organic Cation Dynamics in the Layered Lead Iodide Perovskites BA₂PbI₄ and PEA₂PbI₄

We report on a quasielastic neutron scattering (QENS) study of the rotational dynamics of organic cations in the optoelectronic layered metal halide perovskites BA2PbI4 (BA = butylammonium, CH3(CH2)3NH3) and PEA2PbI4 (PEA = phenethylammonium, C6H5(CH2)2NH3). For BA2PbI4, the measurements reveal highly temperature dependent dynamics. Between 100 and 260 K, the dynamics are ascribed to rotational dynamics of the -CH3 and -NH3 groups of the BA cation, whereas above 300 K we also observe the additional presence of full rotations of the whole BA cation around its long molecular axis. For PEA2PbI4, the measurements reveal dynamics that are more spatially restricted in nature, i.e., rotational dynamics of the -NH3 groups of the PEA cations for temperatures above 200 K, with no other dynamical processes observed at higher temperatures. A correlation of the dynamics results to literature data on the optical properties of the materials suggests that the more restricted organic cation dynamics in PEA2PbI4 are related to its longer lifetime and diffusion lengths for charge carriers, as compared to BA2PbI4.