双钙钛矿卤化物Cs_(2)AgFeX_(6)(X=Cl, Br, I)电子结构和光学性质的第一性原理研究

First-principles study on electronic structures and optical properties of double - peovskite halides Cs_(2)AgFeX_(6)(X=Cl, Br, I)

近年来,银卤化物双钙钛矿作为铅基杂化卤化物钙钛矿的潜在环保替代品得到了广泛的研究.最近实验上合成的新型无铅双钙钛矿单晶材料Cs_(2)AgFeCl_(6)是一种立方结构半导体,吸收光谱可拓宽至800 nm.本论文采用密度泛函理论的第一性原理方法,对Cs_(2)AgFeX_(6)(X=Cl, Br, I)的电子结构和光学性质进行研究,讨论了二者之间的内在关联,并分析了X位元素的改变对材料性质的影响.电荷密度计算结果显示,由Cl到I,Fe-X键逐渐减弱,即原子对电荷的束缚能力减弱.另一方面,三种材料的带隙宽度是逐渐减小的,Cs_(2)AgFeCl_(6)和Cs_(2)AgFeBr6带隙分别为1.40 eV和0.91 eV,而Cs_(2)AgFeI6则呈现金属性质. Cs_(2)AgFeX_(6)双钙钛矿的光谱特征峰随X位原子序数的增加明显红移,且在可见至红外光波段均有明显的光吸收:Cs_(2)AgFeCl_(6)在534 nm处的吸收系数达到21.28×10^(4)cm^(-1),Cs_(2)AgFeBr6在712 nm处的吸收系数为20.54×10^(4)cm^(-1),而Cs_(2)AgFeI6在1200 nm后的红外波段有一极宽的吸收峰,吸收系数可以达到10×10^(4)cm^(-1).本论文为Cs_(2)AgFeX_(6)(X=Cl, Br, I)在光电子器件领域的广泛应用提供了理论指导.

In recent years, silver halide double perovskites have been widely studied as potential environmentally friendly alternatives to lead-based hybrid halide perovskites. Recently, a novel lead-free double perovskite single crystal Cs_(2)AgFeCl_(6)was synthesized experimentally. It is a semiconductor with cubic structure, and its absorption spectrum can extend to 800 nm. In this work, the electronic structures and optical properties of Cs_(2)AgFeX_(6)(X = Cl, Br, I) were investigated by using the first-principles method based on the density functional theory, and the inherent relevances between the two were discussed. Also, the influences of the change of X-site element on the properties of the materials were analyzed. The charge density calculation results show that from Cl to I, Fe-X bond gradually weakens, namely the binding capacity of atoms to charges weakens. On the other hand, the band gap widths of the three materials gradually decrease. Cs_(2)AgFeCl_(6)and Cs_(2)AgFeBr6have band gaps of 1.40 eV and 0.91 eV, respectively, while Cs_(2)AgFeI6exhibits metallic nature. The spectral characteristic peaks of double perovskite Cs_(2)AgFeX_(6)is obviously redshifted with the increase of X-site atomic number, and the three materials all process remarkable light absorption in visible to infrared region. The absorption coefficient of Cs_(2)AgFeCl_(6)at 534 nm is 21.28×10~4 cm^(-1), and that of Cs_(2)AgFeBr6at 712 nm is 20.54×10~4 cm^(-1). Cs_(2)AgFeI6has an extremely wide absorption peak in the infrared band over 1200 nm, and the absorption coefficient can reach up to 10×10^(4)cm^(-1). This paper provides theoretical guidance for the wide application of Cs_(2)AgFeX_(6)(X = Cl, Br, I) in the field of optoelectronic devices.