Theoretical Study On Defects In CsSnX₃ （X=Cl,Br,I） Perovskite

Inorganic lead-free perovskites are regarded as promising photovoltaic and optoelectronic materials due to their excellent stability,pollution-free properties and excellent photo-and electrical properties.Notably,the defects are difficult to eliminate during the inorganic perovskite preparation process.Studying and understanding the formation of defects in all-inorganic lead-free perovskites,the structural properties and the influence laws and mechanisms on the microstructure,and electrical and optical properties of perovskites can provide guidance and ideas for improving the performance of all-inorganic lead-free perovskite devices.Therefore,in this study,the all-inorganic lead-free perovskite CsSnX₃（X=Cl,Br,I）is used as the research object to systematically study the midpoint defects of all-inorganic lead-free perovskite materials CsSnX₃ by first principles.Firstly,by using the density functional theory-based first-principles software（VASP）to systematically obtain the intrinsic properties of defect-free CsSnX₃ perovskites,find that the obtained experimental data and other reported literature results approximate,which shows that the calculated parameters taken are reasonable.Then,the formation conditions of various defects were obtained by stability studies as well as the phase diagram of the complete stoichiometric perovskite.found that the phase diagrams of CsSnX₃ all grew in narrow intervals,which means that the growth environment should be strictly controlled during the perovskite preparation process.By regulating the different growth environments in the chemical potential simulation experiments,obtained the defect formation energy in the corresponding growth environment and the influence of the defects on the spatial structure,fermi level and photoelectric properties,such as orbital configuration diagram,energy diagram,energy level diagram,light absorption and so on.And find that:1.the spatial structural deformation of defects and the theory of molecular orbitals suggest that V_X defects are more likely to form deep energy levels in CsSnX₃.2.the calculation results of defect formation energy show that both Cs Sn Cl₃ and Cs Sn Br₃ have p-type conductivity due to E_F is pinned by of intrinsic defects under extreme growth conditions of lean Sn or rich Sn;Cs Sn I₃ may exhibit intrinsic n-type conductivity with low electron conductivity under Sn-rich growth conditions,but can be transformed into intrinsic p-type with high hole conductivity under the growth conditions of lean Sn.3.the calculation results of energy level transition positions for all intrinsic defects.Although the calculated results show that there are relatively many deep energy levels in CsSnX₃,these deep energy level defects have high defect formation energy in different growth environments,so CsSnX₃ has defect tolerance.4.The calculation of the optical parameters of the defect results in the observation that some defects（such as V_X defects）have additional absorption peaks in the infrared region（compared to the ideal crystalline absorption edge）,which may be attributed to the transition of electrons within the band gap resulting in a smaller band gap.Finally,the effects of defect concentration of major defects on perovskite structure,electrical properties,optical properties and mechanism are further investigated.And found that:1.As the concentration of X_i defects increases,the formation energy becomes positive and gradually becomes larger;whereas V_Sn,Cs _Sn defects have a huge negative formation energy at high concentrations,which means that there may be a large number of Sn atoms missing in the material under suitable conditions.2.It is observed that the defect concentration of V_Idefect becomes larger and almost does not change the Sn-I bond length;while the high concentration of Sn atom deletion makes the Sn-I bond length（2.915?）of Cs Sn Br₃ close to that of Cs Sn Br₃（2.913?）,which may lead to higher stability.3.Most of the CsSnX₃ major defects(V_Sn、V_Cs、X_i和Cs _Sn)widen the band gap with the increase of defect concentration,but the increase of band gap value of V_I defect concentration does not change significantly;the calculation of effective mass shows that the high concentration of X_i defect may reduce the electron and hole mobility of the material.4.the calculated results of light absorption show that most of the major defects of CsSnX₃(V_Sn、V_Cs、X_i和Cs _Sn)occur blue shift of the absorption edge with increasing defect concentration.This may be due to the widening of the band gap caused by the defect,resulting in the increase of the energy required to make the valence band top electron transition to the bottom of the guide band;correspondingly,no such absorption edge blue shift phenomenon was observed in the V_I defect.