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.