Compressive Structural Phase Transition And Photoelectric Properties Of Perovskite Material CH₃NH₃PbI₃

Perovskite structure material CH₃NH₃PbI₃ has attracted much attention because of its excellent application in many optoelectronic fields such as photovoltaics,photodiodes,sensors and catalysts.In the new solar cells,CH₃NH₃PbI₃ is used as a material for the absorption layer of perovskite solar cells.These solar cells have excellent photovoltaic performance and achieve higher photoelectric conversion efficiency,because of the excellent charge carrier mobility of the internal inorganic metal halide octahedral structural unit of CH₃NH₃PbI₃ and the plasticity introduced by the organic part.Therefore,such perovskite solar cells have been given high hopes as a new photovoltaic technology.It has been reported in the literature that CH₃NH₃PbI₃has different structural phases in different temperature ranges.Except the temperature,the pressure is also one of the fundamental and pure thermodynamic variables,which can alter the inter-atomic distance and change the bonding patterns of materials,leading to structural phase transitions.As studying the structural and optoelectronic properties of CH₃NH₃PbI₃ under pressure can be beneficial to explore unusual lattice structures,and to help to understand the new chemical and physical properties of this material.First-principles calculations based on density functional theory have been performed in this work to further investigate the crystal structures,electronic and optical properties of CH₃NH₃PbI₃ under pressure.The main research contents and conclusions are as follows:1.The crystal structures and band gaps of CH₃NH₃PbI₃ under different pressures were studied.Many experimental results have shown that the crystal structure of CH₃NH₃PbI₃ under room temperature and atmospheric pressure is tetragonal structure（space group I4/mcm）,while there are two possible structures,which are orthorhombic structure（space group Imm2）and cubic structure（space group Im 3）under high-pressure.Based on this,the above two phase transformation processes were studied to determine the effect of pressure on the internal structure of the crystals of CH₃NH₃PbI₃.It is found that as the pressure increases,the band gap for each of three structures decreases,while the phase transition is completed,the band gaps of two possible high-pressure structures decrease gradually with pressure.According to the experiments,the change of band gap with pressure is mainly due to the structural distortion of[PbI₆]^4-octahedrons.Thus the standard deviationσof Pb-I bonds in[PbI₆]^4-octahedrons were used for the first time to characterize the distortion degree of octahedrons in order to analyze the change of band gap under pressure.It is found that both the decrease of Pb-I bond lengths and the distortion of[PbI₆]^4-octahedrons in CH₃NH₃PbI₃ under pressure have significant effects on the band gap changes of tetragonal,cubic and orthorhombic structures of CH ₃NH₃PbI₃.2.The effect of pressure-induced phase transition on the photoelectric properties of CH₃NH₃PbI₃ was investigated.Through calculating the energy band structures and the density of state for the three structures of CH₃NH₃PbI₃ at normal pressure and two possible high-pressure phases.It can be clearly seen that the conduction band minimum（CBM）for the three structures of CH₃NH₃PbI₃ are composed of Pb-6p orbitals and the valence band maximum（VBM）of all these three phases are composed of I-5p and Pb-6s orbitals.Combining with the projection energy band,it is found that the proportion of each orbital is different for different structures.And there is no essential change in CH₃NH₃PbI₃ within the pressure range applied in this work,that is,CH₃NH₃PbI₃ is a direct band gap semiconductor material for both atmospheric and high-pressure structures.In terms of electrical properties,the amount of charge transfer of different structures for CH₃NH₃PbI₃ under pressure is also analyzed,and the electronic charge densities in the（110）plane of these structures is given as a supplementary explanation.We found that the interactions between Pb and I atoms in three structures have mixed covalent and ionic bonding characters,the covalent character of Pb-I bonds is quite weak in comparison with the classical C-N covalent bonds.Moreover,the optical properties including the complex dielectric function,refractive index,extinction coefficient,reflectivity and absorption coefficient for three CH₃NH₃PbI₃ structures under pressure were also determined.