The Research On The Photoelectric Performance Of Lead-free Inorganic Perovskite With First Principles Method

Green energy,especial solar energy,have a high profile for various fields in recent years.Perovskite solar cells have attracted the attention of researchers in the field of photovoltaic due to their rapid development,simple preparation and high light absorption coefficient of photoelectric performance.In recent decades,since the perovskite solar cell was developed,its photoelectric conversion efficiency has caught up with that of traditional silicon-based solar cells,and the laboratory test results of organic-inorganic hybrid perovskite solar cells have been even better.However,its applications still have many limitations,such as the organic waste liquid produced in the preparation,the poor stability of the material in the wet environment,the toxicity contained in the material,etc.Therefore,non-toxic stable perovskite materials become a new research focus.In this paper,three lead-free inorganic perovskite Cs₃Bi₂X₉?X:Cl,Br,I?were theoretically studied by the first-principles calculation method,and the photoelectric characteristics of crystal structure,surface orientation and unique properties of surface structure were obtained,which will provide theoretical support for the synthesis and application of materials.?1?After mechanical and electrical relaxation optimization,the band structure and absorption spectrum of Cs₃Bi₂X₉?X:Cl,Br,I?are qualitatively consistent with the experimental data,demonstrated computational reliability.The bandgap are 1.94eV,1.68eV and 1.64eV,respectively,which decreased with the increase of the molecular mass,and the corresponding absorption spectral peaks were redshifted.We found that the valence electron orbitals of Bi and coordination halogen atom X in the octahedron hybridized to form the valence band top and conduction band bottom.The charge density shows that the electrons in the octahedron are highly localized,and there are two kinds of Bi-X bonds,and the electrons around the Cs atom are completely delocalized.The optical properties of crystal irradiation in the non-polarized light were calculated by simulating the natural light with irradiation in different directions.The optical properties under the condition of simulating the multi-direction natural light are creatively studied.The visible light was fully absorbed by the three materials,which is suitable to be used as absorbing materials,but different results were shown in different directions.Cs₃Bi₂Cl₉ crystal has internal electrical polarization in the ultraviolet region and stronger absorption capacity.In c direction,Cs₃Bi₂I₉ could not absorb near-ultraviolet light,and the resonance of Cs₃Bi₂Br₉ and Cs₃Bi₂I₉ increased.Cs₃Bi₂Cl₉ is shown as isotropic,while others show anisotropic.?2?The study on the interface structure of materials is helpful to judge the fracture of the interface and to determine the influence of the surface structure on the photoelectric performance.The internal interaction of zero-dimensional structure Cs₃Bi₂I₉ is small,and all interface structure are stable,which also leads to the possibility of complex fracture interface.However,since the lattice and surface energy change very little,the physical properties of Cs₃Bi₂I₉ are less affected by different interfaces.Cs₃Bi₂Cl₉ is a one-dimensional interface structure.The more one-dimensional structure and complete,the more stable the lattice and surface energy,and the more simple the interface fracture mode.Cs₃Bi₂Br₉ is two-dimensional structure with large interlaminar forces,large two-dimensional surface activity and more abundant physical properties.?3?Perovskite is light-absorbing layer material.There are still gap in the influence of surface characteristics of microstructure on photoelectric performance.According to the requirements of stable interface and rich physical properties,the dimensional structure of the three materials was retained,and large vacuum layer were inserted to eliminate the influence of interlayer period.The bandgap of the surface structure increases,the greater the surface activity is,the higher the dimension is,the greater the increase is,and the hybrid orbital moves to the low energy region.The response range to light wavelength is widened,and the polarized surface leads to the emergence of subbands in Cs₃Bi₂Cl₉,which enhances the absorption of infrared.The resonance frequency of the other two materials on the non-polarized surface decreased due to the illumination in c direction,and the anisotropy was inhibited by the surface structure.It is worth noting that the near-ultraviolet light which cannot penetrate Cs3Bi2I9 in the c direction can penetrate the surface structure.The introduction of vacuum layer reduces the relative thickness of the material and makes the optical properties of the material more stable.