Cs_(2)SnI_(6)低指数晶面稳定性的第一性原理计算研究

Stability of Low-index Surfaces of Cs_(2)SnI_(6) Studied by First-principles Calculations

Cs_(2)SnI_(6)是一种稳定且环保的卤化物钙钛矿材料,在光伏和光电应用方面具有巨大潜力。虽然表面性质对于光电器件的制备至关重要,但目前尚没有对该材料开展相关的理论研究。利用密度泛函理论计算结合SCAN+rVV10泛函,本工作研究了Cs_(2)SnI_(6)的(001)、(011)和(111)表面以揭示其热力学稳定性。针对每个表面,研究考虑了具有不同截断的模型,包括两个沿(001)方向(分别为CsI_(2)和SnI_(4)终止的表面),两个沿(011)方向(分别为I_(4)和Cs_(2)SnI_(2)终止的表面)和三个沿(111)方向(分别为非化学计量比的CsI_(3)、Sn和满足化学计量比的CsI_(3)终止的表面)。由于大多数表面模型是非化学计量比的,它们的相对稳定性取决于实验制备条件,因此需要考虑组成元素的化学势。通过确定允许的化学势区域,研究分析了这些表面的热力学稳定性。结果表明,(001)和(011)面的表面能会受到化学势的影响,而满足化学计量比的CsI_(3)终止的(111)表面不受化学势影响,是Cs_(2)SnI_(6)最稳定的表面。该结果说明,近期实验普遍观察到的暴露(111)面的晶体是受热力学稳定性驱动形成的。

Cs_(2)SnI_(6) is a stable and environmentally friendly halide perovskite material with great potential for photovoltaic and optoelectronic applications.While the surface properties are of paramount importance for device fabrications,there have been no such theoretical studies on this material.Using density functional theory calculations with the SCAN+rVV10 functional,the(001),(011)and(111)surfaces of Cs_(2)SnI_(6) were studied to reveal their thermodynamic stability.We constructed seven models for these surfaces,including two along the(001)orientation(CsI_(2)-and SnI_(4)-terminated surfaces),two along the(011)orientation(I_(4-) and Cs_(2)SnI_(2)-terminated surfaces)and three along the(111)orientation(non-stoichiometric CsI_(3-),Sn-and stoichiometric CsI_(3)-terminated surfaces).Because most of the surfaces are non-stoichiometric,their relative stability depends on the experimental preparation condition,which is reflected by the chemical potentials of the constituent elements in the calculation.By determining the allowed chemical potential region,the thermodynamic stability of these Cs_(2)SnI_(6) surfaces is analyzed.The results show that the surface energies of the(001)and(011)surfaces are affected by the chemical potentials,while the stoichiometric CsI_(3)-terminated(111)surface is unaffected by the chemical potentials and is energetically the most stable surface of Cs_(2)SnI_(6).Thus,the observed exposure of(111)surface of Cs_(2)SnI_(6) crystals in several recent experiments is determined to be driven by thermodynamics.