Theoretical Calculation Of Bulk And Surface Of Perovskites

In recent years,with unique structures and exceptional electrical properties,organic-inorganic hybrid perovskite materials have been widely studied in the field of solar cells.Among them,MAPb I₃ perovskite has been significantly enhanced with its photoelectric conversion efficiency and carrier life.However,shortcomings of MAPb I₃,the toxicity of Pd and instability of MA⁺cation in high temperature and humid environment,limit its application.Therefore,it is of great significance to find alternative materials for MAPb I₃.In this paper,Cs Sn Br₃,an alternative to MAPb I₃,is calculated with band structure,exciton binding energy,the position of the valence band maximum relative to vacuum energy level and Schottky barrier height contacted with electrodes.Via QSGW method and ecalj software calculation,Cs Sn Br₃ has 1.962 e V of band gap,0.063 m₀ of carrier reduced effective mass and 23 me V of exciton binding energy.Compared to MAPb I₃,although the band gap width of Cs Sn Br₃ is 0.3 e V higher,Cs Sn Br₃ owns its own merits:lower effective mass and exciton binding energy.The exciton binding energy of Cs Sn Br₃ is 21%lower than that of MAPb I₃（29 me V）,which leads to higher separation efficiency of electron-hole pair at room temperature.Therefore,Cs Sn Br₃ is an excellent alternative to MAPb I₃.Via VASP software calculations,core level alignment and macroscopic averaging method,the position of Cs Sn Br₃ valence band maximum relative to vacuum energy level is calculated.The value obtained by the PBE functional is-4.9±0.1 e V,and the value obtained by the HSE06 functional is-5.2±0.1 e V.The latter one is in good agreement with the experimental measurement.Based on the core level alignment method,the Schottky barrier heights of junction between Au or C graphite and Cs Sn Br₃are the lowest,which means those are suitable for electrode materials of Cs Sn Br₃.The Schottky barrier height between Cs Sn Br₃ and the electrode is directly proportional to the electronegativity difference of those,which is confirmed by the measurement data of XPS.By calculating the effective electronegativity of compounds,this rule of thumb can be extended to the selection of perovskite inorganic transport layer materials.The transport layer materials with effective electronegativity between 2.5 and 2.7 could be the best choice to match perovskite theoretically.Based on the theoretical analysis of Cs Sn Br₃ perovskite,Ca²⁺doping process in MAPb Br₃ perovskite was calculated.With increasing of Ca²⁺content,the covalency of MAPb_xCa_yBr₃ system decreases while the band gap and exciton binding energy increase.When the content of Ca²⁺is between 25%and 50%,there would be two electron-hole pair separation and recombination paths,which greatly promotes the photoluminescence effect.Therefore,the effect of photoluminescence is the most preferable when the content of Ca²⁺is around 25%,which theoretically explained that the overall luminous effect of 30%extra inclusion film is better than that of 15% and 60%.