Preparation And Photostriction Properties Of MAPbI_3-xBr_x Halide Perovskite Single Crystal

MAPbX₃-based halide perovskite material has the advantages of good light absorption,long carrier life and high photoelectric conversion efficiency.It has been widely concerned in the fields of solar cells and optical detection,and the photoelectric conversion efficiency of its solar cells reaches 25.2%.However,its application in other fields remains to be further studied.In this paper,high quality single crystal of MAPbI_3-xBr_x（x=0,1,2,3）was prepared by inversion crystallization method,and its structure,photoelectric properties and photoelasticity were systematically investigated.The main progress is as follows:（1）Four cm-grade MAPbI_3-xBr_x semiconductor single crystals with low defect concentration were developed.The technological conditions of single crystal preparation by inversion crystallization method were explored and optimized continuously.Finally,four kinds of high quality,centimeter size single crystal of MAPbI₃,MAPbI₂Br,MAPbIBr₂ and MAPbBr₃were successfully prepared.The single crystal of MAPbI_3-xBr_x exhibits good semiconductor properties.The bandgap widths of single crystals of MAPbI₃,MAPbI₂Br,MAPbIBr₂ and MAPbBr₃ are 1.51 e V,1.73 e V,1.88 e V and 2.21 e V,respectively.The I-V characteristic curve tested by SCLC method shows that the carrier mobility of MAPbI_3-xBr_x single crystal is large and the defect concentration is low,and the carrier mobility of MAPbI_3-xBr_x single crystal increases obviously with the decrease of temperature.（2）MAPbI_3-xBr_x single crystal has great bulk photostrictive strain and pm displacement accuracy.Under 532 nm and 536 m W/cm² light,the photostrictive strains of 0.5 mm thick single crystals of MAPbI₃,MAPbI₂Br,MAPbIBr₂ and MAPbBr₃ are 0.43%,0.35%,0.28%and 0.17%,respectively.There is a good linear relationship between photostrictive strain and light intensity.The photostrictive strain of single crystal increases with the decrease of single crystal thickness.The photostrictive strain of 50μm thick single crystal of MAPbI₃,MAPbIBr₂ and MAPbBr₃ is0.72%,0.491%and 0.256%,respectively,which is basically consistent with the lattice strain results measured by XRD.In addition,the single crystal of MAPbI_3-xBr_x also has good fatigue cycling stability and pm displacement accuracy.Under 532 nm,0.2 m W/cm² light-dark cycling,the expansion strain of MAPbI_3-xBr_x single crystal is as low as about 400 pm.（3）The photostrictive strain mechanism of MAPbI_3-xBr_x single crystal can be reasonably explained by the physical processes such as the generation of internal electric field by the long range diffusion of photogenerated carriers and the regulation of MA⁺polar cation.The incident light photon excitation produces a large number of electron-hole pairs,which weakens the bond strength of Pb-I bond,makes the Pb-I-Pb bond expand,increases the atomic spacing,and introduces crystal expansion in the surface layer.With the diffusion of photogenerated electrons and holes into the crystal,the different densities of electrons and holes in the local region cause diffusion potential and built-in electric field,so that the polarity of MA⁺cations can reorient along the built-in electric field in the local region,causing structural movement and lattice distortion.The study of photostrictive strain of halide perovskite materials not only reveals the photon-carrier-polar ion-lattice coupling,but also promotes the further development of picmeter-scale precision,micron scale travel,wireless,remote MEMS systems.