Influence Of Chemical Modification On Perovskite Stability And Band Gap Regulation

Organic-inorganic hybrid perovskites have the characteristics of long carrier diffusion length and tunable optical band gap,and they have great development potential as light absorbing materials for solar cells.However,since perovskites are susceptible to phase transitions under the influence of temperature,and are prone to deliquescence in the environment for a long time,this greatly affects their commercial development.Therefore,improving the stability of organic-inorganic hybrid perovskite materials is the key to solving the problem.In this paper,inspired by the strong stability of the experimentally prepared hexagonal phase C₃N₂H₅Pb I₃（ImPbI₃）,according to the perovskite tolerance factor and octahedral factor,a new perovskite phase ImPbI₃ is proposed,and its rationality is verified.In order to obtain materials with strong stability and excellent optoelectronic properties,the effects of inorganic cation Cs doping on the band gap and stability of the Im_（1-x）Cs_xPb I₃ system were studied in detail.Combined with the comprehensive analysis of the stability and optoelectronic properties of the doping system,a material with strong stability and good optoelectronic properties was screened out and verified by experiments.The adsorption mechanism of water molecules on the surface of Im_（1-x）Cs_xPb I₃ was studied,and a new type of passivation molecule was proposed to passivate the surface of the system to effectively improve the water resistance stability of the system surface.First,formation energy,ab initio molecular dynamics（AIMD）,mean square displacement（MSD）,and X-ray diffraction（XRD）calculations were performed for the proposed perovskite phase ImPbI₃,and the perovskite was determined from an energetic and kinetic point of view.The rationality of the existence of the mineral phase ImPbI₃.Based on the cation doping strategy,a new doping system Im_（1-x）Cs_xPb I₃（x=0.25,0.5,0.75）was proposed.The effects of Cs doping on the ImPbI₃ architecture of the perovskite phase were revealed through the analysis of geometry,tolerance factor and volume.To study the effect of Cs doping on electronic properties,the band gap,density of states and charge density distributions of Im_（1-x）Cs_xPb I₃ were calculated.Then,using the crystal orbital Hamiltonian population（COHP）method,the bonding mechanism of Pb-I bond in Im_（1-x）Cs_xPb I₃ system and the effect of Cs doping concentration on the stability of Pb-I were revealed.The dynamic stability of the doping system was further studied by AIMD and XRD.In order to study the photoelectric properties of Im_（1-x）Cs_xPb I₃,the absorption coefficient,refractive index,reflectivity and effective mass of carriers were calculated.The results show that the perovskite phase ImPbI₃ has strong thermodynamic and structural stability,and its optical band gap value is lower than that of the hexagonal phase.When the content of doped Cs does not exceed 50%,the stability of the perovskite phase ImPbI₃ is significantly improved.Based on the inspiration of theoretical calculations,a thin film of Im_0.5Cs_0.5Pb I₃ was synthesized.Comprehensive analysis showed that Im_0.5Cs_0.5Pb I₃showed strong stability and high light absorption coefficient.Then,the surface energy of the perovskite phase ImPbI₃ was calculated to determine its most stable surface.The adsorption mechanism of water molecules on the surface of Im_（1-x）Cs_xPb I₃（x=0,0.5,1）was studied,and the effect of water molecules on the surface was revealed by calculating the adsorption energy,energy band structure and density of states.Then,based on the surface passivation strategy,a new type of passivation molecule（tri-n-propyl phosphorus oxide）was proposed.The electrostatic potential distribution（ESP）and electric dipole moment distribution（EDM）proved that the passivation molecule has a strong polarity,which can passivate the perovskite surface.Next,the adsorption configuration of passivation molecules was constructed for structural optimization,and the effect of passivation molecules on the electronic properties of the surface was revealed by calculating the adsorption energy,charge density difference,energy band structure and density of states.The results show that with the increase of the number of adsorbed water molecules,the adsorption energy of water molecules increases,and the forbidden band width of the system increases gradually.The strong interaction between passivation molecules and the surface can successfully passivate the electronic defect state of the surface,which effectively prevent water molecules from contacting the surface and improve the water resistance stability of the material.