The Structure Evolution And Photoelectric Properties Of Organometal Halide Perovskite CH₃NH₃PbI₃ Under High Pressure

Organometal halide perovskite material,as a new composite crystal material,has wide application prospects in perovskite solar cells and semiconductor devices.The organic components and inorganic components are hybridized with the molecular scale,and have excellent photoelectric properties.However,the organic-inorganic hybrid perovskite material is sensitive to the change of the external environment.The slight change of crystal structure can cause great changes in the optical properties of such materials.Stress is a physical parameter independent of temperature,chemical composition,increase the pressure can effectively shorten the distance between adjacent atoms,the electron orbital overlap degree increases,discovery of new material and the law did not have many of the atmospheric pressure,the formation of new theory,research space can greatly expand the material science.Therefore,exploring the effect of high pressure on the structural stability of organic inorganic hybrid perovskite batteries and the change of optical properties is of great scientific significance and application value.As a physical parameter independent of temperature and chemical composition,pressure can effectively shorten the distance between atoms and increase the orbital overlap degree of adjacent electrons.The hydrogen bonds in the material structure are regulated,then the crystal structure,electronic structure and intermolecular interaction of the matter are changed,so that the matter reaches the high pressure equilibrium steady state and forms a completely new material state.The research of high pressure science can find many new substances and laws,that have not been seen under atmospheric pressure,and form new theories,which can greatly expand the research space of material science.The regulation of high pressure on crystal structure and electronic state can not only study the changes of structure and properties,but also find novel structures and properties.Methylammonium lead iodide?CH₃NH₃PbI₃?is used as a star material in perovskite solar cells has very good optical properties.The starting point of this paper is how the pressure affects the structural stability and optical properties of such materials.At room temperature,used Diamond Anvil Cell?DAC?,combined with in situ high-pressure synchrotron radiation X ray diffraction spectroscopy technology and a variety of in situ high pressure spectroscopy systematically to study the CH₃NH₃PbI₃ phase transformation mechanism,the changes of crystal structure and optical properties of the organic cation relationship,high pressure behavior.The maximum pressure of the experiment was 7 GPa.The in situ high pressure synchrotron radiation X ray diffraction show that,at 0.3 GPa,CH₃NH₃PbI₃ has undergone a phase transformation and transformed from the tetragonal phase at atmospheric pressure to a high-pressure orthogonal phase.The phase transition mechanism is the pressure that causes the octahedral structure[PbI₆]^4-to be tilted and distorted.At 4 GPa,the structure of CH₃NH₃PbI₃ began to crystallize.After completely pressure released,the sample reversibly recovered to the normal pressure state.In situ high-pressure absorption and fluorescence spectroscopy experimental results show that,in the pressure range of 0-0.25 GPa,CH₃NH₃PbI₃ absorption edge and the fluorescence peak is red shifted,when higher than 0.25 GPa,the absorption edge and the fluorescence peak of the samples were from red to blue shift.The results are the same as that of XRD.The occurrence of phase transition makes the symmetry of crystal structure decrease and the band gap change.It is indicated that the change of crystal structure can regulate the size of band gap.After completely pressure released,the CH₃NH₃PbI₃ sample is restored to the state before pressure.In situ high pressure IR and Raman spectra show that the crystal structure of CH₃NH₃PbI₃,organic cation?CH₃NH₃⁺?between the octahedral structure of hydrogen bond N-H···I and C-H···I.Hydrogen bond strength increased with increased pressure,resulting in C-H and N-H bending modes and rocking modes of CH₃ and NH₃ groups are red shifted.After 2 GPa,the N-H stretching modes changed from blue shift to red shift.We also discuss the behavior of organic cations?CH₃NH₃⁺?under high pressure.