First-Principles Study On The Modulation Of The Electronic Structure Of Organic-Inorganic Hybrid Perovskite Materials Under Pressure

Since 2009,organic-inorganic hybrid perovskites have attracted widespread attention as new light-absorbing solar cells.Theoretical and experimental results have proved that,the perovskites MAPbX₃ have almost all of the conditions for a perfect light-absorbing material:suitable direct bandgap,high absorption coefficient,excellent carrier transport properties,and high defect tolerance,which has become one of the hotspots in the field of renewable energy.However,due to the influence of the bandgap Eg of the perovskites,how to further effectively find a perovskite material with a narrower bandgap and stable chemical properties,has important application value and scientific significance.As a basic physics parameter,pressure is now well applied and developed in the fields such as materials science.The experimental study found that MAPbI₃ under pressure realized a coordinated regulation of the bandgap and carrier lifetime of the system,both of which are very important reference factors for improving photovoltaic performance.According to the experimental results,based on the first-principles calculations of density functional theory（DFT）,we use the VASP software package to perform the pressure simulation calculation by changing the crystal lattice of the perovskite MAPbX3（X=I,Br）.We systematically studied the changes of the electronic structure of the compressed perovskites,and analyzed the mechanism of the changes in bandgap under pressure.（1）For the cubic perovskites MAPbI₃,the first-principles calculation results show that the bandgap of the system under pressure will decrease first and then increase.Under the pressure of 2.772 GPa,the bandgap reaches a minimum value of 1.114eV.With the pressure increasing,the bandgap of MAPbI₃ will increase again,and gradually deviate from the high symmetry point R,changing from direct band gap to indirect band gap.When the pressure is increased to 80 GPa,MAPbI₃ can be converted into the crystal structure with zero bandgap.For MAPbI₃ under pressure,detailed structural and electronic characteristic analyses indicate that the changes of the bandgap can be described by the effects of the hydrogen bonding between the organic MA cation and the PbI₆ framework:the lattice contraction decreases its band gap while the PbI₆octahedral tilting increases it.Under pressure,I atoms respond by coming closer to the H atoms near the end of the ammonia,causing the rotations and tilts of the PbI₆octahedron in different degrees.The lattice symmetry of the system is destroyed,and the main Pb-I-Pb bond angle deviates from 180°expected in an ideal system,which changes the charge density function between Pb and I atoms,which in turn affects the changes of electronic structure and optical properties of MAPbI₃ under different pressures.（2）In order to verify whether the modulation of the bandgap of the pure perovskite system under pressure has universality,based on the calculations of MAPbI₃,we further select the cubic crystal structure of MAPbBr₃ at room temperature with a larger bandgap to perform the pressure calculation.Theoretical calculations show that the bandgap of the compressed perovskites MAPbBr₃ under certain pressure also decrease first and then increase.Under the pressure of 1.877 GPa,the bandgap of the system reaches a minimum value of 1.834eV.The variation characteristics of the electronic structure under pressure are basically consistent with the results of MAPbI₃,which indicates that the pressure is of great significance for the modulation of the bandgap of the perovskites.（3）The electronic structure of the semiconductors fundamentally determines the optical properties of the system.We calculated the absorption spectra of the perovskites MAPbX₃（X=I,Br）under different pressures by the first-principles calculations.The absorption edge of the system under pressure will significantly move towards the long wave range,which is due to the changes of the bandgap,corresponding to the transition of the electron from the valence band to bottom of conduction band.Additionally,the light absorption rate of the compressed perovskites MAPbBr₃ is significantly higher than that at ambient pressure.As a direct bandgap semiconductor,the perovskites under pressure exhibit good physical properties,which has significant guidance for further study of its optical properties.