Synthesis,Structure,and Optoelectronic Properties Of Bromine-Based Methylammonium Lead Hybrid Perovskites

Organic-inorganic perovskites,due to their high absorption coefficient,low defect density,large exciton binding energy and low Auger recombination rate,have broad application prospects in optoelectronic fields such as in solar cells,light-emitting diodes,lasers,photodetectors and and so on.In addition,the organic lead-halide perovskite is easy to prepare and can be grown on any substrate.In addition,the organic lead-halide perovskite is easy to prepare and can be grown on any substrate.Various morphologies including quantum dots,nanowires/sheets,microwires/sheets,microblocks,thin films and large single crystals in millimeter or even centimeter scales can be prepared by solution method at room temperature.Moreover,the absorption and emission wavelength can be continuously adjusted by changing the halogen composition or crystal size in the ultraviolet-visible to near infrared region.Nevertheless,there are still many problems to be solved urgently.Here,there are two main categories:one is methylammonia lead halide perovskite,which is an organic-inorganic hybrid semiconductor with complex energy level structure and unclear luminescence mechanism;the other is its poor stability,sensitive to changes in light,humidity and temperature.It is easy to decompose under strong light,high humidity or high temperature conditions,which greatly hinders its commercial application.In order to understand the photoelectric properties of methylammonia lead halide perovskite and improve its environmental stability,methylammonia lead-bromide perovskite（CH₃NH₃PbBr₃）was selected as the study targeted in this thesis.Firstly,the crystallization process of the CH₃NH₃PbBr₃ single crystal was carried out to clarify CH₃NH₃PbBr₃ nucleation and growth mechanism.On this basis,the photoluminescence properties of CH₃NH₃PbBr₃ single crystal were studied in detail.Meanwhile,the CH₃NH₃PbBr₃ based laser,milliwires photodetector and CH₃NH₃Pbr₃-ZnO heterojunction negative photoconductive devices were constructed.The deeply underlying crystal structure,optical and electrical properties of CH₃NH₃PbBr₃ perovskite and the carriers transport properties in the devices were put forward systematic discussions.The main research contents are as follows:1.The experimental scheme of preparing CH₃NH₃PbBr₃ single crystal was optimized in anti-solvent method.The growth mechanism of single crystal was systematically studied by analyzing the optical absorption characteristics of perovskite precursor solution and monitoring the morphology of perovskite at different growth stages,and a clear growth model was given.Furthermore,the controlled synthesis of perovskite from three-dimensional micron blocks to one-dimensional nanowires was realized by adjusting the reaction parameters.2.The optical properties of CH₃NH₃PbBr₃ bulk single crystal were studied.The spectral characteristics of CH₃NH₃PbBr₃ with two emission peaks were analyzed by means of power-dependent single photon excitation spectroscopy,temperature dependent spectroscopy and time-resolved spectroscopy.Based on perovskite temperature phase transition theory,it was determined that the two emission peaks originated from the coexistence of two phases in CH₃NH₃PbBr₃ single crystal.The two-photon excitation spectra further verify that the emission peak of CH₃NH₃PbBr₃at 537 nm belongs to the centrosymmetric cubic phase and the other peak at 557 nm is attributed to the non-centrosymmetric tetragonal phase.3.Based on the controllable growth of CH₃NH₃PbBr₃ single crystal,we have prepared a series of CH₃NH₃PbBr₃ single crystal microsheets and microwires,and realized the mode regulation from amplified spontaneous emission（ASE）to multimode and single mode lasing.In addition,we have introduced a detachable aluminum（Al）nanoparticle substrate.To combine with surface plasma effect of Al,the laser intensity of CH₃NH₃PbBr₃ was increased by more than 10 times,and the excitation threshold was reduced by about 27%.4.It is found that the DMF solution of lead bromide has the characteristics of self-assembly in air.CH₃NH₃PbBr₃ mm-line crystals were synthesized by two-step method using lead bromide millimeter-line as template.The CH₃NH₃PbBr₃ millimeter line has good optical properties,high luminescence intensity and carrier lifetime of more than 110 ns.Meanwhile,the individual photodetector constructed by CH₃NH₃PbBr₃ millimeter line shows excellent electrical properties and light response,and its humidity,temperature and light stability is strong.It can keep its photoelectric performance more than 200 days under atmospheric conditions,and the reasons for its stability are discussed in detail.5.CH₃NH₃PbBr₃-ZnO composite structure of methambroyl perovskite-coated zinc oxide microstrips was prepared by thermal evaporation method,and metal-semiconductor-metal photoelectric devices were constructed.It was found that the detector exhibited obvious negative photoconductivity under light conditions.Carrier transfer and recombination properties of CH₃NH₃PbBr₃ thin films and CH₃NH₃PbBr₃-ZnO surface interfaces have been systematically studied.When CH₃NH₃PbBr₃ is coated on ZnO,the holes in p-CH₃NH₃PbBr₃ and the electrons in n-ZnO continuously diffuse to the junction region and form depletion layer at the interface due to the carrier concentration gradient.Under 532 nm light,photogenerated electrons in CH₃NH₃PbBr₃ transfer to its surface and recombine with the intrinsic hole nonradiation.The addition of photogenerated holes in CH₃NH₃PbBr₃promotes the further diffusion of electrons in ZnO and holes in CH₃NH₃PbBr₃ to the interface of CH₃NH₃PbBr₃-ZnO,increasing the width of depletion layer,and reducing the concentration of effective conducting carriers.