Theoretical Simulation Of Magnetic Properties And Ion Migration Of Metal Halide Perovskites

Perovskite is a crystal structure discovered by Russian mineralogists in 1840's in calcium titanate minerals.The compounds with this structure have very important contributions in the fields of light,magnetism,electricity,superconductivity and so on.In the past decade,metal halide perovskite materials have attracted a lot of attention due to their excellent photoelectric properties,especially in solar cells,light-emitting diodes and other applications.However,the stability of the material is poor.In order to meet the production requirements,its stability must be improved.The poor stability of the material is mainly due to long-range ion migrations.According to the electromagnetic induction,the ion migrations should be surpressed by magnetism.However,the research on the magnetic properties and the relationship between magnetic properties and ion diffusions of halide perovskites are still in the blank stage.Hence,in this dissertation,we will study the magnetic properties of halide perovskite and the relationship between magnetism and stability in the halide perovskite.Furthermore,we find that the effect of unpaired d electrons in transition metal ions to improve the stability is more significant compared to magnetism effect.On this basis,the stability of the material is greatly improved by doping transition metal ions and can maintain its excellent optical properties.In the first chapter,the crystal structure of perovskite and general situation of perovskite structure in the field of magnetism are briefly introduced,which provides some ideas for the study of the magnetic properties of metal halide perovskite materials.Then the great development potential of metal halide perovskite in the most important photovoltaics field is introduced.Finally,we introduce the stability problems that limit the commercialization of the materials and the general thought of improving stability through magnetism.In the second chapter the development of density functional thoery and several software packages of the first principle are briefly introduced.In the third chapter,the magnetic properties of Cs₂Ge MX₆(M=Ti,V,Cr,Mn,Fe,Co,Ni and Cu;X=Cl,Br and I)are studied.In the traditional metal halide perovskite,all the electrons are paired and the structure has no magnetism,so we add the fourth period transition metal into this structure in order to introduce magnetism.Based on the first principle calculations,it is found that this kind of materials can exist stably at room temperature,and V-,Mn-and Ni-based compounds have ferromagnetic ground states with larger spin splitting value,and the Curie temperatures are estimated to be higher than 470 K,which can realize the semiconductor properties and high-temperature ferromagnetism at the same time.The reason is that the magnetic ions of these compounds have a large nearest neighbor number,and the super-exchange coupling between magnetic ions is strong.The research of this chapter provides some ideas for the design of ferromagnetic semiconductors with desired properties,and provides an interesting direction for the development of metal halide perovskite materials.More importantly,it is found that only halide perovskite without Jahn-Taller distortion can present ferromagnetism,which lays a foundation for later research.In the fourth chapter,we study the ion transport energy barrier,bandgap and light absorption of ferromagnetic Cs₂MFe X₆(M=Na,K,Rb and Cs;X=Cl,Br and I).In the traditional metal halide perovskite materials,the stability of the materials is poor because of the low barrier of ion migration.Based on the existing Cs₂Na Fe F₆,we designed a new group of halide perovskite materials,and found that the materials have good stability,especially in the resistance to ion migration.This is because the local magnetic moment provided by Fe³⁺ions and the unpaired 3d electrons have double restrictions on the migration of halogen anions.And the bandgaps and light absorptions of this family of materials show that Cs₃Fe Cl₆is suitable for the top cell of four terminal solar cell.Through this study,we propose a new method to improve the stability of metal halide perovskite materials by introducing transition metal ions,and also predict a new perovskite solar cell material.In the fifth chapter,the method of the fourth chapter is introduced into the lead halide perovskite material to obtain highly stable halide perovskites with excellent optical properties.Through first principle calculations,we found that the doping Ni²⁺prefers to occupy B site,and doping strategy does not change the phenomenon that halogen vacancy has the lowest formation energy in halide perovskites.Moreover,we verified that the transition metal can be successfully doped into the perovskite Cs Pb Br₃,and the quantum yield of photoluminescence will not be greatly affected by the doping strategy,and the stability of the transition metal has been greatly improved in experiments.This chapter provides a new method for improving the stability of halide perovskite LEDs without losing efficiency,and promotes the industrialization process of halide perovskites.