Influence of Inorganic Layer Thickness on Methylammonium Dynamics in Hybrid Perovskite Derivatives

The layered Ruddlesden–Popper derivatives of CH₃NH₃PbI₃ have recently emerged as high-performing materials for photovoltaics with improved stability. The inclusion of organic molecules within the inorganic framework provides additional dynamic degrees of freedom that influence the optoelectronic properties. The rotational dynamics of CH₃NH₃⁺ influence dielectric behavior and electronic excited-state dynamics in CH₃NH₃PbI₃; however, the influence of cation dynamics on properties in the layered derivatives has not yet been determined. We employ quasi-elastic neutron scattering to study the rotational dynamics of methylammonium (CH₃NH₃⁺, MA) and deuterated n-butylammonium (CD₃(CD₂)₃NH⁺₃, d-nBA) in (d-nBA)₂(MA)ₙ₋₁PbₙI₃ₙ₊₁ (n = 2, 3). (d-nBA)₂(MA)₂Pb₃I₁₀ exhibits shorter residence times of the CH₃NH₃⁺ and CD₃(CD₂)₃NH⁺₃ reorientational motions, which are attributed to the larger volumes that the cations occupy in the inorganic framework and to the dimensionality of the inorganic layer by way of dielectric screening between the organic cations. Discontinuities in the mean-squared displacement of overall hydrogen motion determined by fixed-window elastic neutron scattering are consistent with phase transitions observed by differential scanning calorimetry and time-resolved microwave conductivity signals. Determining how the dimensionality of the inorganic layer influences the organic cation rotational dynamics provides fundamental chemical insight into how the electronic dynamics vary between n-members.