Structural Envolution And Optical Properties Research Of Bismuth Based Lead-Free Perovskites Under High Pressure

Metal halide perovskites have emerged as one of the most promising kind of optoelectronic materials with its exceptional properties,such as low-cost solution processing,spectral tunability,and high light absorption and photoluminescence efficiency.and have shown outstanding performance in the solar cell,photodetector,LED,laser,and other fields.Since lead-based perovskite was used as the lightabsorbing layer of solar cells for the first time in 2009,the performance of lead-based perovskites has improved rapidly in this decade,and a series of records have been achieved.Despite the tremendous progress made before,challenges of lead's toxicity still remain,which called for the substitution of the lead with none toxicity element.Environment friendly perovskites had been obtained by Bismuth substitution,However,its performance is hampered by the large band gap,as a potential single-component light emitters with broad-band emission,extending the spectral coverage of emission also remains a challenge to date.As an independent thermodynamic parameter,pressure is a clean method which could induce structural phase transition without element doping and substitution,and changes the electronic band structure,and thus profoundly affects the physical properties of materials.In this thesis,in-situ high-pressure research was conducted on the structural evolution,band gap and photoluminescence properties of Bismuth based lead-free perovskites?CH₃NH₃?₃Bi₂Br₉ and Cs₂AgBiBr₆,the main results are as follows: 1.High pressure exploration of?CH₃NH₃?₃Bi₂Br₉ is performed to narrow the band gap.With the idea that the compressed structure of semiconductor material under high pressure will introduce phase transition,affect the energy band property,the phase transition of?CH₃NH₃?₃Bi₂Br₉ from monoclinal phase to trigonal phase is realized through the high pressure,and the band gap of?CH₃NH₃?₃Bi₂Br₉ is partially narrowed.The physical mechanism of structural phase transition and energy band structure change is described by DFT calculation.2.The ultra-broad fluorescence of Cs₂AgBiBr₆ microcrystals was realized by high pressure for the first time.The self-trapped exciton?STE?emission in Bismuth based perovskite comes from the Jahn-Teller distortion in the octahedron framework of perovskite.The cubic Cs₂AgBiBr₆ with octahedral angular sharing mode underwent an octahedron tilling under pressure,which is appropriate to study the relationship between structure and STE optical properties.Guided by the idea that structure determines performance,high pressure induced phase transition from cubic to tetragonal was performed to study the relationship between the structural evolution and optical properties of STE.An ultra-broadband emission of Cs₂AgBiBr₆ is achieved by octahedron tilting under pressure for the first time while the optical band gap is partially narrowed.The relationship between the structural transition and STE energy levels can be extended to other new perovskite materials.