Zibouche Nourdine, Islam M Saiful
Department of Chemistry, University of Bath, Bath BA2 7AY, U.K.
ACS Appl Mater Interfaces. 2020 Apr 1;12(13):15328-15337. doi: 10.1021/acsami.0c03061. Epub 2020 Mar 19.
Two-dimensional (2D) halide perovskites are receiving considerable attention for applications in photovoltaics, largely due to their versatile composition and superior environmental stability over three-dimensional (3D) perovskites, but show much lower power conversion efficiencies. Hence, further understanding of the structure-property relationships of these 2D materials is crucial for improving their photovoltaic performance. Here, we investigate by means of first-principles calculations the structural and electronic properties of 2D lead and tin Ruddlesden-Popper perovskites with general formula (BA)ABI, where BA is the butylammonium organic spacer, A is either methylammonium (MA) or formamidinium (FA) cations, B represents Sn or Pb atoms, and is the number of layers ( = 1, 2, 3, and 4). We show that the band gap progressively increases as the number of layers decreases in both Sn- and Pb-based materials. Through substituting MA by FA cations, the band gap slightly opens in the Sn systems and narrows in the Pb systems. The electron and hole carriers show small effective masses, which are lower than those of the corresponding 3D perovskites, suggesting high carrier mobilities. The structural distortion associated with the orientation of the MA or FA cations in the inorganic layers is found to be the driving force for the induced Rashba spin-splitting bands in the systems with more than one layer. From band alignment diagrams, the transfer process of the charge carriers in the 2D perovskites is found to be from smaller to higher number of layers for electrons and oppositely for holes, in excellent agreement with experimental studies. We also find that, when interfaced with 3D analogues, the 2D perovskites could function as hole transport materials.
二维（2D）卤化物钙钛矿因其在光伏领域的应用而备受关注，这主要归功于其多样的组成以及相对于三维（3D）钙钛矿而言更优异的环境稳定性，但其功率转换效率要低得多。因此，进一步了解这些二维材料的结构 - 性能关系对于提高其光伏性能至关重要。在此，我们通过第一性原理计算研究了通式为（BA）(_n)AB(_n)I(_3)的二维铅和锡的Ruddlesden - Popper钙钛矿的结构和电子性质，其中BA为丁基铵有机间隔基团，A为甲铵（MA）或甲脒（FA）阳离子，B代表Sn或Pb原子，(n)为层数（(n = 1)、2、3和4）。我们表明，在基于Sn和Pb的材料中，随着层数的减少，带隙逐渐增大。通过用FA阳离子取代MA，Sn体系中的带隙略有增大，而Pb体系中的带隙变窄。电子和空穴载流子的有效质量较小，低于相应的3D钙钛矿，这表明载流子迁移率较高。在具有多于一层的体系中，发现与无机层中MA或FA阳离子取向相关的结构畸变是诱导Rashba自旋分裂带的驱动力。从能带对齐图中可以发现，二维钙钛矿中电荷载流子的转移过程对于电子而言是从层数较少的向层数较多的方向，对于空穴则相反，这与实验研究结果非常吻合。我们还发现，当与3D类似物界面接触时，二维钙钛矿可以用作空穴传输材料。
