Preparations And Physical Property Studies Of Novel Lead-free Halide Perovskites And Perovskite Derivatives

Halide perovskites belong to a new class of semiconductor materials with a variety of excellent physical and chemical properties,such as adjustable band gap,large light absorption coefficient,long carrier lifetime,facile preparation,etc.Being applied in various optoelectronic devices,including solar cells,light-emitting diodes,photodetectors,etc.,halide perovskites show a great potential application value.However,lead ions in traditional lead-based perovskites are biologically toxic,and how to replace lead elements and obtain lead-free perovskite materials has become an important research direction.In this paper,a series of research work has been carried out around lead-free perovskites,and several novel lead-free perovskites and their derivatives have been explored and studied,demonstrasting the potential in electronic devices.The innovative results are as follows:1.Antisolvent method was used for the first time to prepare two-dimensional lead-free Bi-Agbased double perovskite derivative(PEA)4BiAgBr8 microplatelets with assistance of organic large-sized cations to segment inorganic layer.The microplatelets have a two-dimensional layered structure with regular morphology and an average size of 13.75 μm.Through the characterization of optical absorption spectrum,it is observed that(PEA)4BiAgBr8 has a direct bandgap feature(Eg=3.05 eV),and its bandgap value can be adjusted by halogen components.Back-gate bottom-contact field-effect transistors(FETs)were fabricated by drytransfer method,which is the first report of FETs based on two-dimensional Bi-Ag-based double perovskite derivative.Output and transfer characteristics of the FET show that the(PEA)4BiAgBr8 microplatelets exhibit p-type conductivity controlled by gate voltage.2.High-quality large-area lead-free double perovskite Cs2BiAgBr6 thin films were prepared by a post-low-pressure treatment,and their morphology,lattice structure and optical absorption were characterized.Based on the double perovskite film,stacked non-volatile resistive memories with a structure of Au/Cs2BiAgBr6/ITO were prepared.I-V characteristics of the device show an excellent bipolar resistive switching behavior,and space charge limited conduction is the main conduction mechanism.The set and reset voltage values are 1.3 and 0.34 V,respectively,and the switching ratio is between 103～104.I-V characterizations show reliable retention and endurance.The resistance switching mechanism is filamentary-type,which may involve the synergy of Ag vacancies and Br vacancies.The vacancies migrate under electric fields,forming or destroying conductive filaments,showing reproducible resistance switching behavior;in the absence of electric field,the immobile vacancies maintain the high and low resistance states.3.Mn2+doping in Sb3+-based perovskite derivative Cs3Sb2Cl9 was realized for the first time,and the regulation on luminescence properties of Cs3Sb2Cl9 crystals was studied experimentally and theoretically.In order to explain the broad photoluminescence peak(λ=613 nm)of Mn2+-doped Cs3Sb2Cl9,the crystal structure and electronic state were calculated by DFT.[MnC16]4-octahedron and two impurity levels in the forbidden band contributed by Mn2+ 3d orbital are observed.According to the coordination field theory,the energy transfer from Cs3Sb2Cl9 to the[MnCl6]4-octahedral coordination field occurs,and delectrons of Mn2+ undergo d-d radiations transition from the excited state 4T1 to the ground state 6A1.4.Zero-dimensional perovskite derivative Cs2ZnCl4 was selected as the host,and Sn2+ and Sb3+doped Cs2ZnCl4 crystals were prepared for the first time,with research of self-trapped exciton(STE)emission.It is found that doping has little effect on the lattice through microstructural characterizations,and the relationship between STE emission and temperature was studied through optical characterizations.Time-resolved photoluminescence find that the luminescence lifetimes of both doping situations are in μs order,which is consistent with the forbidden transition characteristics of STE emission.The crystal structures and electronic states of Sn2+and Sb3+doped Cs2ZnCl4 were calculated by DFT.[SnCl4]2-and[SbCl5]2-units are in disphenoidal and pyramid configurations with low symmetry,and impurity bands of 5s and 5p states corresponding to doping elements appear in the forbidden band.Analyses show that the STE emission is due to partially forbidden radiative transition from 3P1 to 1S0 of Sn2+and Sb3+.