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

卤化铅钙钛矿作为下一代光电器件的有源层具有很大的前景。然而，基于钙钛矿的器件在大范围的时间尺度上表现出明显的性能下降（可逆和不可逆），这是目前其大规模应用的主要障碍。降解现象被认为与以下事实密切相关：与光电子中使用的传统无机半导体不同，卤化铅钙钛矿表现出显著的离子导电性。尽管对此类材料中的离子输运进行了大量的研究，但由于存在多种可移动物种，以及器件特性对制造方法的强烈依赖，阻碍了详细的理解和进一步的发展。虽然密度泛函理论（DFT）计算已被用于解开这个复杂问题的不同方面，但计算出的数据是高度不一致的。这种情况最近被解释为不正确地应用静态方法（即零开尔文）来研究仅在有限温度下稳定的LHP相中的离子输运。在本项目中，我们将在DFT计算的基础上进行有限温度模拟，以研究卤化铅钙钛矿在基态、低对称性相以及在器件工作温度下采用的高对称性相中的离子输运。具体来说，我们将结合分子动力学模拟和加速自由能采样方法来获得所有组成离子物种作为温度函数的可靠扩散系数。通过将我们的结果与文献数据进行比较，我们的目标是使这一主题更加清晰。我们的动力学模拟也将提供与铅卤化钙钛矿中离子传输相关的各种基本开放问题的见解，例如对称性的作用，以及有机-无机杂化化合物中有机阳离子旋转动力学的影响。

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.