二维卤化钙钛矿半导体及其合金光电性质的理论研究

Two dimensional Halide Perovskites are now emerging as a new family of 2D semiconductor materials with unique structures and interesting optoelectronic properties, which are promising candidates for photovoltaic application. However, their band gap is still far from visible light absorption for photovoltaic applications, and high lead content is also of both environment and health concern. Thus, seeking lead-free hybrid perovskites with high photon-to-electron conversion efficiency are highly desired. In this project, we propose that there could be abundance of 2D halide perovskites by by varying the constituent elements and forming alloys (one based on isovalent element substitution and the other based on nonisovalent element mutation), moreover, their electronic and optical properties can be widely tuned to satisfy the requirements of the practical applications. Using the first principle calculations, we systematically study the following properties of the 2D halide perovskites and their alloys, including their structural stabilities, electronic, optical, and transport properties. Moreover, we will illuminate the relationships between the electronic and optical properties and the crystal structure, the element components, and the alloy element ratio. Based on the charge analysis, orbital splitting, the physical mechanism of the alloy tuning the semiconductor properties will be given. Eventually, we will be able to predict some new 2D halide perovskites with high power-conversion efficiency, which can provide useful information for the further experimental research.

二维卤化钙钛矿作为一种新型半导体材料，有优异的光电性质有望在太阳能电池领域中发挥重要作用。然而，二维卤化钙钛矿的带隙不在太阳能光伏需要的可见光吸收区，且含有高浓度Pb重金属元素对环境和人体都有很严重的危害。因此，迫切需要发展和寻找新型、无毒、具有高光电转化效率的二维钙钛矿卤化物光伏材料。本项目提出通过对二维钙钛矿材料的化学成分进行元素替换和合金化（同价元素替换法和异价元素置换法）途径对其半导体性质的细致调节。利用第一性原理计算方法进行高通量搜索，系统研究该类化合物及其合金的结构稳定性、电子性质及化学趋势、光吸收谱及光电转化效率，以及载流子输运性质。同时建立该类化合物及其合金的电子结构和光吸收谱与结构构型、构成元素以及合金元素比例的关联。通过分析体系的电荷转移，轨道杂化变化，得到调制该类材料半导体性质的内在的物理本质。最终，预测出若干新型高效二维钙钛矿光伏材料，为实验开发提供理论数据参考。

通过异价元素置换的方法，模拟计算出了12种结构，仔细的计算了它们的性质，讨论了它们的各种性质，包括二维卤化钙钛矿半导体A4MM’X8的稳定性及结构性质，二维卤化钙钛矿半导体A4MM’X8的电子结构性质和二维卤化钙钛矿半导体A4MM’X8的光吸收特性。用软件作出了材料的结构图和替换原子的示意图，计算了这12种结构的具体晶格参数，然后计算了各种物质的形成焓，讨论了它们各自的稳定性，其中最稳定的结构为Cs4AgSbI8和Cs4InBiBr8。然后我们计算出了各种结构的能带结构图，分析了各种置换方式对能带结构的影响，分析了它们带隙的大小，优秀的半导材料的带隙最好是处于1.0～1.7 eV的中等带隙，因此Cs4CuSbBr8，Cs4CuSbI8，Cs4InBiBr8和Cs4InBiI8具有比较合适的带隙，并借此预测了一下各种结构在光电器件中的应用。最后我们计算了各种结构的光吸收系数和入射光子能量间的关系，并对每一种结构的光系数系数的特点进行分析比较。最终大致掌握了以上材料的基本性质，即它们的结构参数，结构稳定性，能带结构，带隙大小，光吸收系数和入射光子能量关系等。经过以上的各种性质的计算，Cs4InBiBr8具有较好的稳定性，合适的带隙和较好的光吸收性质，因此可以进一步研究其他的性质，可能在光电器件中有所应用。