Finite-temperature, ab initio simulations of ion transport in lead–iodide perovskites

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Lead–halide perovskites show great promise as the active layers for next-generation opto-electronic devices. Perovskite-based devices exhibit, however, significant degradation in performance—both reversible and irreversible—over a broad range of timescales, and this is currently the major impediment to their large-scale application. The degradation phenomena are believed to be intimately related to the fact that, unlike conventional inorganic semiconductors used in opto-electronics, lead–halide perovskites exhibit significant ionic conductivity. Although much research has been much carried out on ion transport in such materials, the fact that there are multiple mobile species, as well as the existence of a strong dependence of device characteristics on fabrication method, hinders detailed understanding and further development. While density-functional-theory (DFT) calculations have been used to disentangle the different aspects of this complex problem, the calculated data are highly inconsistent. This situation has recently been explained in terms of the incorrect application of static methods (i.e. at zero Kelvin) to the study of ion transport in LHP phases that are only stable at finite temperatures. In this project, we will perform finite-temperature simulations, on the basis of DFT calculations, to study ion transport both in the ground-state, low-symmetry phases of lead–halide perovskites, and also in the higher symmetry phases that they adopt at device operating temperatures. Specifically, we will combine molecular dynamics simulations and accelerated free-energy sampling methods to obtain reliable diffusion coefficients for all constituent ionic species as a function of temperature. By comparing our results with literature data, we aim to bring clarity to the subject. Our dynamical simulations will also provide insights into various fundamental open questions related to ion transport in lead–halide perovskites, such as the role of the symmetry, and the influence of the rotational dynamics of organic cations in hybrid organic–inorganic compounds.