Investigating The Humidity Stability Of CH₃NH₃PbX₃（X=I,Br,Cl）Based Perovskite Solar Cells From First-principles

Essential in the modern civilized society,energy is more and more a part of our lives.The world’s energy output is still dominated by the three major fossil energies of oil,natural gas and coal.Fossil energies will continue to be the main energy source for human survival and development for a long period of time in the future.However,the exhaustion of fossil energies has become a major problem that restricts the development of human beings.Solar cells,which can directly convert solar energy into electrical energy through photoelectric or photochemical effects,have attracted tremendous attention due to their great significance for adjusting the energy structure,promoting energy production and consumption revolution.With the advantages of low-cost,high efficiency,simple preparation process and good flexibility,perovskite solar cells(PSCs)have become one of the fastest-growing novel photovoltaic devices.However,its long-term stability,especially humidity stability,severely limits the large-scale industrial of PSCs.Therefore,a systematic and in-depth understanding of the degradation mechanism of perovskite materials in humid environment from atomic scale is of great significance to improve the long-term stability of PSCs,thus promoting large-scale application of PSCs and solving the acute energy crisis.We have systematically investigated the humidity stability of CH3NH3PbX3(X=I,Br,Cl)perovskite materials using first-principles simulations based on density functional theory.By directly comparing the geometric stability,electronic structure,photo-absorption and excited state dynamics of CH3NH3PbX3(X=I,Br,Cl)perovskite slabs in vacuum and water environment,we have found that:1.Compared with the perovskite material with PbX2 as terminated surface,water molecules are easily to corrode perovskite materials with CH3NH3X as terminated surface,which is due to the stronger adsorption abilities of water molecules on the PbX2 surface(with adsorption eneries of～2 eV),making it difficult for water molecules to penetrate into the perovskite.2.The humidity stabilities of the three perovskite materials follow the laws of CH3NH3PbI3>CH3NH3PbBr3>CH3NH3PbCI3.As the atomic number increases,the electronegativity of the halogen atoms gradually weakens,and the bond energy of the Pb-X bond gradually decreases,which is easily broken by water molecules.Consequently,CH3NH3PbI3 is more susceptible to the influence of water molecules.3.Unexpectely,the immersion of water molecules can increase the excited state carrier lifetimes of CH3NH3PbX3 materials with CH3NH3X surface as termination,while water molecules hardly affect the excited state carrier lifetimes of PbX2-ended CH3NH3PbX3,which might be a result of the subtle influence of water on the geometrical stabilities of PbX2-ended CH3NH3PbX3.This thesis establishes a sharper physical image of how water molecules influence the geometrical structure,electronic structure,light absorption,and excited state carrier lifetime of CH3NH3PbX3(X=I,Br,Cl)perovskite materials with different termination surfaces from atomic scale,offering a readily comprehensible understanding of the degradation mechanism of CH3NH3PbX3(X=I,Br,Cl)perovskite materials in humid environment,providing important clues for future device design and optimization.