Study Of Electronic Structure Of Organic Halide Perovskite Single Crystals

In recent years,perovskite solar cells have been greatly developed,organic halide perovskites have long diffusion lengths,carrier lifetimes and strong ability to absorb light,which are conducive to the transport of carriers in devices,in addition,organic halide perovskites have the unique advantages of low exciton binding energy,abundant raw materials and easy fabrication,making it one of the most promising photovoltaic absorber materials.T he materials and preparation methods of perovskite solar cells are also emerging in an endless stream,and the efficiency of the cells is constantly being refreshed.H owever,there are still many problems need to be solved,for example,the lifespan of perovskite solar cells has not met the requirements of commercialization,and the decline mechanism of cells is not clear,efficient transport and extraction of charges at various interfaces of perovskite solar cells have major impacts on the efficiency and stability of devices.Therefore,it is necessary to further study the electronic structure of devices and related energy level alignment.In this paper,we mainly focus on the position of valence band edge electrons and density of states in organic halide perovskites,the main research contents are as follows:In this dissertation,organic halide perovskite solar cells were used as the research target,CH3NH3PbI3 and CH3NH3PbBr3 perovskite single crystals were prepared by temperature-raising crystallization method,the samples were characterized by ultraviolet photoelectron spectroscopy（UPS）and XPS with high resolution,considering the energy band structure at different symmetry points in the momentum space,the position of the valence band edge was obtained by extrapolating the linear intensity scale and logarithmic intensity scale.The results showed that the DOS distribution at point M is weaker,the position of the energy of the VB M extracted from the logarithmic intensity scale using the H e la light source was approximately 0.2 eV smaller than that obtained quantitatively from linear intensity scale.T he VB M s of CH3NH3PbI3 and CH3NH3PbBr3 single crystals（at M symmetry point）are located 1.13 eV and 1.29 eV away from the Fermi energy level,respectively,which shows that the VBMs of CH3NH3PbI3 and CH3NH3PbBr3 single crystals（at R symmetry point）are 0.86 eV and 0.89 eV,respectively.In order to further study the density of gap states（DOGS）of CH3NH3PbI3 single crystals,in this paper,UPS was also used to study the effects of structural decomposition of C H 3N H 3PbI3 single crystals on the position of the valence band edge and the density of gap states（DOGS）distribution after air exposure and strong light irradiation.The results showed that when the CH3NH3PbI3 single crystals were exposed to air,PbI2 could be decomposed,and the VBM moved slightly toward the region of high binding energy,with further decomposition of the sample,PbI2 gradually transformed into metallic Pb,however,this did not have a noticeable effect on the spectral shape of DOGS in CH3NH3PbI3.