Fluorescence Properties Of Two-dimensional Halide Perovskites

In recent years,three-dimensional（3D）halide perovskites are extensively applied in solar cells and other optoelectronic devices owing to the larger absorption coefficient,higher defect tolerance and long carrier lifetime.However,because of the stability problem of 3D perovskites,such as easily absorbing moisture and decomposing in the atmospheric environment,the practical application of 3D perovskite is largely hindered.Compared to the 3D structure,two-dimensional（2D）halide perovskites possess a higher quantum efficiency and better environmental stability,which make them excellent optoelectronic materials and receive extensive attention from researchers.With the development of information technology,the preparation and study of micro-and nano-scale devices become the development trend of future devices.Therefore,synthesizing high-quality thin 2D halide perovskite film is an important research topic.However,the nonradiative recombination caused by the defects in thin-layer 2D halide perovskite extensively influences the luminescence and photoelectric conversion efficiency of devices.Passivating the surface and deep energy defects in2D perovskite and improving the fluorescence quantum efficiency of thin-layer 2D halide perovskites is an urgent problem.Furthermore,the altered connection of the organic layer and inorganic layer in 2D halide perovskite forms the quantum well.2D halide perovskite optoelectronic properties change with the quantum well depth which is determined by the thickness of the inorganic layer（n value）.An energy funnel effect is also formed between 2D halide perovskites with different n values.However,the physical properties of 2D halide perovskite heterostructure with various n values and the underlying mechanisms require further investigations.This thesis focuses on the research of 2D halide perovskites.We have demonstrated the realization of the fluorescence enhancement of perovskite by constructing perovskite/Spiro-OMe TAD composites,the preparation of N1*/N2 heterostructure flakes and the study of their fluorescence properties.The specific research results are as follows:（1）We utilized the anti-solvent assistant method and successfully prepared 2D halide nano-flakes with various thicknesses.Characterizing samples by optical microscope,atomic force microscope（AFM）,scanning electron microscope（SEM）,and X-ray diffraction（XRD）,the results show that the surface of the perovskite nanosheets was smooth and flat with uniform thickness.Based on the anti-solvent assistant method,by adjusting the ratio of raw materials and optimizing the experimental conditions,we synthesized different n-value halide perovskite heterostructure nano-flakes.Similarly,through AFM and SEM characterization,the results show that the heterostructure is smooth and flat with clear boundaries.In addition,we prepared bulk 2D halide perovskite from n=1 to n=4 by supersaturation crystallization method,with the scale of those samples up to centimeters.（2）We developed a kind of surface defects passivating method of thin-layer2D halide perovskite method,and realized the increase of perovskites’fluorescence.To prepare the PEA₂Pb I₄/Spiro-OMe TAD complex,the spiro-OMe TAD was coated onto the surface of the perovskite flakes by the spin-coating method.The photoluminescence（PL）mapping of the complex displays that in the area covered by spiro-OMe TAD,the perovskite has stronger fluorescence intensity.Then,we performed PL and transient PL tests on the complex with different thicknesses.For thin-layer perovskite（below 100 nm）,the perovskite fluorescence intensity is enhanced by 3-8 times after being affected by spiro-OMe TAD,and the lifetime of all samples is elongated.The XPS data show that the chemistry state of perovskite in the complex changed.This phenomenon demonstrates the defect passivation effect of spiro-OMe TAD on thin-layer perovskites.We also compare the thicker perovskite（≥1μm）,which has better crystalline quality and lower defect density.In addition,PEA₂Pb I₄and spiro-OMe TAD form the II band arrangement.Therefore,thick perovskite has a weaker photoluminescence intensity and a shorter lifetime under the effect of spiro-OMe TAD.Moreover,we utilized mechanically exfoliated thin and thick perovskites as control experiments.Under the same experimental condition,mechanically exfoliated samples and directly grown perovskites exhibit the same results.This demonstrates the universality of defect passivation of spiro-OMe TAD for thin-layer perovskites.（3）By the solution method,we successfully synthesized 2D halide perovskite heterostructure nano-flakes,and studied their fluorescence properties.The optical imaging,PL spectrum,and absorbance spectrum demonstrate the preparation of longitudinal heterostructure nano-flakes with regular morphology.The top layer of the heterostructure is the mixed-phase n=1and n=2（n=1 dominates,marked as N1*）,while the bottom layer is the phase of n=2（N2）.Except for the intrinsic fluorescence peaks of n=1 and n=2,the PL spectrum of the heterostructure shows a luminescence peak located on the wavelength of 737 nm that demonstrates the interaction between the heterostructure.We further performed spatially resolved transient PL testing to explore the energy transfer between the heterostructure.The fluorescence decay curve of n=2shows an obvious rise kinetics,which demonstrates the energy transfer from the n=1perovskite phase to the n=2 perovskite phase.In addition,we performed a power dependence test on the N1*/N2 heterostructure,the results show that the fluorescence intensity varies linearly with the laser power in the n=1phase while various super-linearly in the n=2 phase.This phenomenon illustrates that carriers exist in the form of excitons in the n=1 perovskite,while the excitons and free carriers coexist in the n=2 phase.