The Study Of Phase Transition And Optoelectronic Properties Regulation By Halogen Substitution In Organic-Inorganic Hybrid Perovskite Materials

Organic-inorganic hybrid perovskite phase transition materials have attracted extensive attention due to their inherent molecular flexibility and structural tunability.Especially in the exploration of multifunctional phase transition materials,halogen substitutions are of great significance for the regulation of phase transition and optoelectronic properties of hybrid materials.In this thesis,we synthesized a series of organic-inorganic hybrid perovskite compounds and characterized their crystal structures,phase transition behaviors,and photocurrent properties.The research content of this paper is divided into the following three parts:In the first part,seven organic-inorganic hybrid compounds were synthesized based on N,N-dimethyl pyrrolidine（DMP）,R/S/rac-N,N-dimethyl-3-fluoropyrrolidine（DMFP）and Cd X₂（X=Cl,Br）to explore the regulation of phase transitions by halogen substitution:[（DMP）Cd Cl₃]（1）,[（R/S/rac-DMFP）Cd Cl₃]（2/3/4）,[（DMP）Cd Br₃]（5）,[（R/S-DMFP）Cd Br₃]（6/7）.Differential scanning calorimetry（DSC）analysis results show that compounds 1-7 have reversible phase transitions.Among them,compounds 1,4,and 5 undergo phase transitions at 411,439,and 385 K,respectively.The fluorine-substituted chiral enantiomers 2,3 and 6,7 undergo dual phase transitions with phase transition temperatures of 367/460 K(T_2/3a/T_2/3b)and417/443 K(T_6/7a/T_6/7b),respectively.By comparison,it is found that the fluorine substitution on the organic cation and the halogen substitution of the X-position anion lead to significant changes in the phase transitions.In the second part,an organic-inorganic hybrid compound:[（TMIM）Pb Cl₃]（8）was synthesized,and its phase transition and nonlinear optical switching properties were explored.The experimental results show that compound 8 undergoes dual phase transition with temperatures of 345/358 K(T_8a/T_8b),accompanied by dielectric anomalies.The phase transition temperatures were increased by 120 and 30 K compared with the 225/328 K of the parent compound tetramethylamine lead chloride.The intramolecular halogen···halogen interaction causes obvious distortion of the inorganic framework,which affects the crystal packing,and induces the crystallization of compound 8 in the chiral space group P2₁2₁2₁.Thus,compound 8 shows strong second harmonic generation（SHG）signal and stable nonlinear optics（NLO）switching performance.In the third part,four organic-inorganic hybrid compounds were synthesized based on pyrrolidine（Py）,R/S-3-fluoropyrrolidine（FP）,3,3-difluoropyrrolidine（DFP）and Pb I₂ to investigate the modulation of the structure and optoelectronic properties by mono-/di-fluorine substitutions:[Py Pb I₃]（9）,[R/S-FPPb I₃]（10/11）,[（DFP）₂Pb I₄]（12）.The experimental results show that compounds 10 and 11 are one-dimensional（1D）hybrid perovskites that crystallize in the space group P2₁2₁2₁,and compound 12 is a two-dimensional（2D）hybrid perovskite that crystallizes in the space group Pbcn.The optical band gaps of compounds 10,11 and 12 are 2.77,2.75 and 2.09 e V,respectively,and compound 12 exhibits better photoelectric response properties than 10 and 11.In summary,the mono-fluorine substitution on the organic cation leads us to obtain enantiomeric organic-inorganic hybrid perovskites,and the di-fluorine substitution induces the expansion of the inorganic Pb-I framework from 1D to 2D,indirectly regulating the band gap and photoelectric switching ratios of hybrid perovskites.