First Principles Study Of The Crystal Structure And Optoelectronic Properties Of A₂B⁴⁺X₆ And A₂B⁺B³⁺X₆ Halide Perovskite

In recent years,the perovskite solar cells have attracted widespread attention from researchers with the advantages of high photoelectric conversion efficiency,low cost,and simple production process,which make that they become a research hotspot in the field of new energy materials.A₂B⁴⁺X₆and A₂B⁺B³⁺X₆-type halide perovskites are a new perovskite materials,which have good stability and can achieve lead free,and have good application prospects in the field of perovskite solar cells,so their research value is very high.The research of perovskite materials mainly improves the stability or eliminates toxicity by the doping or alloy.In addition,the first principle calculation based on the density functional theory has also accelerated the design and transformation process of these perovskite photovoltaic materials.The first-principle calculation is not limited by experimental conditions and can screen a large number of potential perovskite photovoltaic materials at the same time.Therefore,this paper will use the CASTEP module in Materials Studio software to study the photoelectric properties of several types of A₂B⁴⁺X₆and A₂B⁺B³⁺X₆halide perovskite materials,and optimize the design of chemical components,so as to provide certain guidance and theoretical basis for subsequent experiments.Firstly,the structural,elastic,electronic and optical properties of lead-free perovskite derivatives Cs₂SeX₆（X=Cl,Br,I）were studied based on first-principles.The results show that the structures of these perovskite derivatives are stable and they are all anisotropic materials.The three perovskite derivatives Cs₂SeX₆（X=Cl,Br,I）are all indirect band gap semiconductors,and the band gap values are 3.28 e V,2.66 e V and 1.69 e V,respectively.Among the three perovskite derivatives,Cs₂Se I₆has not only a suitable band gap,but also a smaller carrier mass,a better absorption spectrum and a larger dielectric function in the visible region.Therefore,the perovskite derivative Cs₂Se I₆has the best photoelectric properties.Due to the suitable bandgaps and excellent light absorption,the perovskite derivative Cs₂Se I₆can be used as potential photoelectric absorption layer materials of solar cells,while Cs₂Se Cl₆and Cs₂Se Br₆have the potential to develop as other optoelectronic devices.Then,we have studied in detail the structural,electronic and optical properties of lead-free double perovskites Rb₂Se I₆and K₂Se I₆based on first-principles calculations.Their negative binding energy（E_b）and formation energy（E_f）ensure the stability of the studied materials.Our calculations give band gap values of Rb₂Se I₆and K₂Se I₆to be 1.51 e V and 1.45e V,respectively.This illustrates that the two perovskite compounds belong to indirect band gap semiconductors,and are suitable for photosensitive materials of solar cells.The results indicate that the effective mass of electrons and holes in Rb₂Se I₆and K₂Se I₆is very small,and therefore they are favorable for carrier transport.Moreover,the optical parameters display that Rb₂Se I₆and K₂Se I₆have significant light absorption properties,especially in the visible light range.Due to the suitable band gap values and excellent light absorption,Rb₂Se I₆and K₂Se I₆have great application potential in the absorption layer of solar cells.Finally,the structural,elastic,electronic and optical properties of lead-free double perovskites Cs₂Cu BiX₆（X=I,Br,Cl）were studied by using first-principles calculations.The negative binding energies（E_b）and formation energies（E_f）show that Cs₂Cu BiX₆can be synthesized and they meet thermodynamic stability.The Cauchy’s pressures of double perovskites Cs₂Cu BiX₆are positive,which indicate that these three perovskites are ionic crystals.The calculated results of Pugh’s ratio（B/G）and Poisson’s ratio（υ）reveal that the three perovskite materials are all ductile compounds.Furthermore,these three perovskite materials are all anisotropic.The band gap values of double perovskites Cs₂Cu BiX₆（X=I,Br,Cl）are 0.78 e V,1.13 e V and 1.29 e V,respectively,which are appropriate for photosensitive materials of solar cells.In addition,the effective mass values of electrons and holes of these three perovskite materials are very small,which is conducive to the transport of carriers.In the study of optical properties,we found that these three perovskite materials have better absorption performance for visible light.Due to the appropriate band gaps and prominent light absorption,the lead-free double perovskites Cs₂Cu BiX₆can be potential candidates for lead-free photovoltaic materials.