The Role Of "Fluorine Substitution" In Lead,Tin Metals Based Organic-inorganic Hybrid Perovskites

Organic-inorganic hybrid perovskite is called the ideal choice for the next generation of flexible wearable electronic devices due to its inherent mechanical flexibility,structural tunability and ease of processing.However,there is still a huge obstacle in effectively adjusting the phase transition temperature of non-volatile storage elements,capacitors,and sensors.Using the"Fluorinated Effect"molecular design strategy,this means“using strong electronegative fluorine atom to replace any two hydrogen atoms on the benzylamine benzene ring at any two equal positions”,to get four bisfluoroamine compounds:2,3-Difluorobenzylamine(2,3-DFBA)?2,4-Difluorobenzylamine(2,4-DFBA)?2,5-Difluorobenzylamine(2,5-DFBA)?2,6-Difluorobenzylamine(2,6-DFBA),and with lead halide in a ratio of 2:1 to synthesize six bisfluoroamine lead-based organic-inorganic hybrid perovskites:[(2,3-DFBA)₂Pb Cl₄](1),[(2,4-DFBA)₂Pb Cl₄](2),[(2,4-DFBA)₂Pb Br₄](3)?[(2,5-DFBA)₂Pb Cl₄](4)?[(2,5-DFBA)₂Pb Br₄](5)and[(2,6-DFBA)₂Pb Cl₆](6),and determine its purity through infrared analysis and powder XRD;through DSC and dielectric analysis,it is found that compounds 1?5 are all reversible phase change compounds.The phase transition temperature of compound 4 is as high as 500 K,which is a rare high-temperature phase transition compound in lead-based organic-inorganic hybrid perovskites.Moreover,the room temperature crystal of compound 4 crystallizes in the non-central space group.The second-order nonlinear coefficient(SHG)analysis proves the transition of the space group of compound 4 from non-center to center before and after the phase transition,and proves that the"fluorination effect"can increase the polarity and dipole moment of the compound and make it crystallize in the non-central space group;through single crystal diffraction found that the single crystal microstructures of compounds 1?5 are typical(RNH₃)₂BX₄ two-dimensional structures,while compound 6 is a one-dimensional structure of ABX₅ configuration,which is due to the highly symmetrical position of the fluorine atom substitution for(2,6-DFBA),the narrowness and instability between the inorganic skeletons determines that compound 6 does not have phase transition properties;the estimated normal temperature ultraviolet optical band gap values of compounds 1?6 are:3.49e V?3.53 e V?2.90 e V?3.53 e V?2.90 e V?3.53 e V,respectively.It can be seen that the change of organic amine did not change its optical properties.In order to increase the energy barrier of molecular rotation,using a chlorine atom to replace a fluorine atom in the"difluoro substitution",to get two organic amine:4-chloro-2-fluorobenzylamine(4-Cl-2-FBA?2-chloro-4-fluorobenzylamine(2-Cl-4-FBA),then synthesize three compounds in the same ratio with lead halide and tin halide respectively:[(4-Cl-2-FBA)₂Pb Br₄](7)?[(4-Cl-2-FBA)₂Sn Br₆](8)?[(2-Cl-4-FBA)₂Sn Cl₆](9),and determine its purity through infrared analysis and powder XRD;through DSC and dielectric analysis,it is found that compounds 7?9 are all It is a reversible phase change compound,in which the phase transition temperature of compound 9 is as high as 507 K;through single crystal diffraction,it is found that the single crystal microstructure of compound 7 is the same as that of compounds 1?5,and they are all typical(RNH₃)₂BX₄ two-dimensional structures.Compounds 8 and 9are typical zero-dimensional structures;the estimated normal temperature UV optical bandgap values of compounds 7?9 are:2.85 e V?2.54 e V and 4.13 e V,respectively.In this thesis,based on the basic theory of the molecular design strategy of the"Fluorinated Effect",nine new types of lead and tin(?)-based organic-inorganic hybrid perovskites were synthesized,and their phase transition,dielectric,and microstructure findings were analyzed and studied,the substitution of fluorine atoms at suitable positions can effectively increase the phase transition temperature of the compound and increase the polarity and dipole moment of the compound,and the collocation substitution of chlorine and fluorine atoms can expand the research on tin(?)-based organic-inorganic hybrid perovskites field.