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子器件的有源层显示出巨大的前景。然而，基于钙钛矿的器件在宽范围的时间尺度上表现出显著的性能退化（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：与光电子学中使用的常规无机半导体不同，卤化铅钙钛矿表现出显著的离子电导率。尽管已经对这种材料中的离子输运进行了大量研究，但是存在多个移动的物种的事实，以及器件特性对制造方法的强烈依赖性的存在，阻碍了详细的理解和进一步的发展。虽然密度泛函理论（DFT）计算已被用来解开这个复杂问题的不同方面，计算数据是高度不一致的。这种情况最近被解释为静态方法（即在零开尔文）的不正确的应用，在LHP阶段，只有在有限的温度下稳定的离子传输的研究。在这个项目中，我们将进行有限温度模拟，DFT计算的基础上，研究离子输运在基态，低对称相的铅卤化物钙钛矿，也在较高的对称相，他们在设备工作温度下采用。具体而言，我们将结合联合收割机分子动力学模拟和加速自由能采样方法，以获得可靠的扩散系数的所有组成离子物种作为温度的函数。通过将我们的结果与文献数据进行比较，我们的目标是澄清这个问题。我们的动力学模拟还将深入了解与铅卤化物钙钛矿中离子传输相关的各种基本开放问题，例如对称性的作用，以及有机阳离子在混合有机-无机化合物中的旋转动力学的影响。

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.