DOI: https://doi.org/10.29363/nanoge.hpatom.2022.012
Publication date: 30th October 2021
Commonly employed cations in hybrid perovskites (CH(NH2)2+ - FA, CH3NH3+ - MA, and, C(NH2)3+ - GUA) differ in size, symmetry, dipole moment, and associated dynamics. These properties significantly contribute to the overall optoelectronic properties of the material. In particular, the long charge-carrier lifetimes of hybrid perovskites, which are an important parameter for their use in solar cell applications, have been attributed to the fast rotational dynamics of the cations. Solid state NMR has been used widely within the perovskite community to study cation incorporation, phase segregation, halide mixing, disorder, and dynamics. Here, we show how quadrupolar (2H and 14N) solid-state NMR relaxometry can be employed to determine the cation dynamics in hybrid perovskites, focusing on FA and GUA dynamics in FAPbI3 and GUAxMA1-xPbI3 perovskite compositions. In particular, we simultaneously fit the 2H and 14N T1 constants with an anisotropic rotational diffusion model to determine the activation energies (Ea) and correlation times (τc) corresponding to each of the three principal axes of the cation. This contrasts with the majority of previous studies for which an average rotational rate is considered. Overall, this methodology provides a comprehensive understanding of the cation dynamics in hybrid perovskites.
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