First Principles Study Of A₂MX₆ Type Halide Perovskite Material

Lead-free metal halide perovskites have become the focus in the fields of solar cells,light-emitting diodes,and photodetectors due to their environmental protection and excellent optoelectronic properties.As a family of halide perovskite materials,A2MX6-type halide perovskite materials have been proved to be one of the photoelectric materials with wide application prospect by experiments in the past few years.This class of perovskite materials has the advantages of good optoelectronic properties,environmental friendliness and high stability,and is regarded as a potential substitute material for lead halide perovskites.In addition to experimental studies,theoretical methods such as first-principles calculations have played an important role in the study of A2MX6-type halide perovskite optoelectronic materials.In this paper,based on the first-principles calculation method in the framework of density functional theory,several A2MX6-type halide perovskites have been studied by using the CASTEP module in the Materials Studio software.Through the analysis of stability,elastic,electronic and optical properties,this paper expands the range of A2MX6-type halide perovskites that can be used as optoelectronic materials,which provides a certain theoretical explanation and reference value for the experimental studies of lead-free halide perovskite materials.The innovative research conclusions obtained in this paper are summarized as follows:（1）The calculated results show that the crystal structure parameters of Cs₂PtI₆ and Rb₂PtI₆ are in good agreement with the experimental data.The negative formation energy of Cs₂PtI₆ and Rb₂PtI₆ indicates that they are thermodynamically stable.Both Cs₂PtI₆ and Rb₂PtI₆ are indirect bandgap semiconductors with bandgap values of 1.29 eV and 1.15 eV,respectively,which are suitable for photosensitive materials for solar cells.For Cs₂PtI₆ and Rb₂PtI₆,the valence band maximum（VBM）is mainly contributed by the I-5p orbital,while the conduction band minimum（CBM）is dominated by the I-5p orbital along with the Pt-5d orbital.Cs₂PtI₆ and Rb₂PtI₆ exhibit excellent light absorption and low energy loss in the photon energy range of 0-12 eV.In general,the similar optoelectronic properties make Cs₂PtI₆ and Rb₂PtI₆ have similar optoelectronic application scope and potential.（2）According to the calculated values of tolerance factors and elastic stiffness constants of K₂PdCl₆ and K₂PdBr₆,it can be determined that they can form stable three-dimensional cubic crystal structures.In addition,K₂PdCl₆ and K₂PdBr₆ have thermodynamic stability.The calculated results show that K₂PdCl₆ material is brittle,while K₂PdBr₆ material exhibits ductility.Both K₂PdCl₆ and K₂PdBr₆ are indirect bandgap semiconductors with bandgap values of 2.151 eV and 1.368 eV,respectively,which are suitable for optoelectronic devices.Furthermore,the optical properties of K₂PdCl₆ and K₂PdBr₆ in the photon energy range of 0-12 eV further reveal the potential application of these compounds in single-junction and tandem solar cells as well as other optoelectronic devices.（3）The crystal structure,elastic,electronic and optical properties of Cs₂PdBr₆ under hydrostatic pressure of 0-6 GPa were studied with a step size of 0.5 GPa.The calculated results at zero pressure are in good agreement with the corresponding experimental values.When the hydrostatic pressure is applied,the crystal structure parameters of Cs₂PdBr₆,such as lattice constant and bond length,appear nonlinear changes,but it can still maintain a stable cubic crystal structure.With the increase of pressure,the bulk modulus,shear modulus and Young’s modulus of Cs₂PdBr₆ increase gradually,and its ductility also increase gradually.Hydrostatic pressure can narrow the gap between the conduction band and valence band of Cs₂PdBr₆,resulting in a smaller bandgap value,and the excited photoelectrons are easier to move to the conduction band,thus enhancing the photoelectric properties such as absorption and conductivity.In summary,hydrostatic pressure can change the band gap value of Cs₂PdBr₆,improve its photoelectric performance,and make it more suitable for use as the light absorption layer material of solar cells.