Design And Optimization Of Lead/Stannous-Based Halide Perovskite Ferroelectrics

As a new type of semiconductor materials,lead and stannous-based halogen perovskites have the advantages of strong light absorption coefficient,long photo-generated carrier life,high carrier mobility,high fluorescence quantum yield and low exciton binding energies,having very broad application prospects in the fields of solar cells,light-emitting diodes,photo-detectors and so on.For lead and stannous-based halogen perovskite ferroelectrics,the built-in electric field and ferroelectric domains caused by their spontaneous polarization will trigger the ferroelectric photovoltaic effect,then promoting the separation and transmission of photo-generated carriers,and thus further improving the power conversion efficiencies（PCEs）of solar cells,which has been widely studied in recent years.However,due to the dual restrictions on structural symmetry and the size of organic cations,it is extremely difficult to explore new lead and stannous-based halogen perovskite ferroelectrics.In particular,lead and stannous halogen perovskite ferroelectrics with excellent properties such as large saturation polarization（P_s）,high Curie transition temperature（T_c）,and narrow band gap（E_g）are rarely reported.In this thesis,based on the design concept of ferroelectrochemistry,we successfully designed several lead and stannous-based halogen perovskite ferroelectrics,in which their structural phase transition and ferroelectric performance has been systematically characterized and researched.（Ⅰ）In Chapter Two,2D halogen perovskite ferroelectric（CHA）₂PbBr₄ was used as a prototype for design reference.By replacing the C atom in the para position of the CHA⁺cation with the O atom,2D perovskite ferroelectric（ATHP）₂PbBr₄was successfully synthesized,showing a large piezoelectric-voltage coefficient g₃₃.Simultaneously,compared with the prototype phase（CHA）₂PbBr₄,T_c,P_s and piezoelectric properties of（ATHP）₂PbBr₄are significantly improved.（Ⅱ）In Chapter Three,2D halogen perovskite ferroelectric（BA）₂PbCl₄ was used as a prototype for design reference.By applying the molecular design strategy of H/F substitution,2D halogen perovskite ferroelectric（2-FBA）₂PbCl₄was successfully synthesized.The introduction of F atom successfully improves the T_cand P_s of fluorinated compound without destroying the 2D perovskite structure of the prototype compound,which has become an important and effective method to optimize the ferroelectric properties of materials.（Ⅲ）In Chapter Four,a narrow band-gap 2D lead-iodine perovskite ferroelectric（4,4-DFHHA）₂PbI₄was successfully designed by using the H/F substitution strategy.Detailed analysis of the variable-temperature crystal structures show that the weak C-F…H-C interaction formed by 4,4-DFHHA⁺cations plays a vital role in the stability of the 2D（PbI₄）_n^2-perovskite inorganic layer.Conversely,the space formed by the 2D inorganic framework has a confinement effect on the 4,4-DFHHA⁺cations,making them in an orderly frozen state,thereby triggering spontaneous polarization.These discoveries provide a new design inspiration for exploring new 2D halogen perovskite ferroelectrics.（Ⅳ）In Chapter Five,A high-T_cmultiaxial ferroelectric MPSnBr₃ with 3D perovskite structure was successfully synthesized by using MASnBr₃ as a prototype compound.In conclusion,compared with the unmodified prototype compounds,the modified lead and stannous-based halogen perovskite ferroelectrics have significantly improved in terms of their T_c,P_sand piezoelectric properties.