Preparation Of Low-dimensional MAPbI₃ Perovskite And Investigations Of Their Phase Transitions Under Pressure

In recent years,organic-inorganic hybrid perovskites have attracted extensive attention as a photovoltaic material.MAPbI₃ has excellent photoelectric conversion efficiency,high quantum yield,low-cost fabrication and remarkable light absorption property,and has been widely used in light emitting diodes,lasers,solar cells and photodetectors.The photoelectric conversion characteristics of MAPbI₃ depend on its light absorption capacity.The smaller the band gap,the more photons in the sunlight that are larger than the forbidden band width,the wider the energy range of the absorbed photons,the stronger the light absorption capacity,and the better the photoelectric conversion characteristic.Therefore,further reducing the bandgap of the material can effectively improve its photoelectric conversion performance.High pressure can effectively compress the atomic distance and adjust the overlapping degree of adjacent electron orbits,which is another important parameter that can independently affect the structure and properties of substances in addition to temperature and chemical composition.Dimension is also an important parameter for regulating the band gap and properties of materials.Combining low-dimensional and high pressure technology,and study of the transformation of the structure and properties of MAPbI₃ can help to obtain MAPbI₃ nanomaterials with smaller bandgap and better photoelectric performance.The paper chooses organic-inorganic hybrid perovskite MAPbI₃ as the research object,optimizes the material synthesis conditions,and explores the morphologies transformation of MAPbI₃ perovskite and its influence on the performance of optoelectronic devices,and by high pressure study of the transformation of the structure and properties of MAPbI₃.The contents and results of this paper are as follows:?1?Through the modified solution deposition method to synthesize MAPbI₃ nanocrystals with different morphologies,mainly by regulating the perovskite precursor concentrations,dropping rate by mixing two solutions and the vacuum drying temperature of the sample,the MAPbI₃ perovskite nanoplates,nanocubes and nanorods with uniform morphology are synthesized,and the photophysical properties of three morphologies of perovskite nanocrystals are systematically studied.The studies indicate that the MAPbI₃ nanocrystals with three different morphologies all belonged to the tetragonal structure.Through the analysis of optical properties,the optical band gaps of the nanorods,nanocubes and nanoplates are 1.53 e V,1.58 e V and 1.56 e V,and the average carrier lifetimes are 50 ns,71 ns and 109 ns,respectively.In comparison,nanoplates show stronger photoluminescence intensity and longer carrier lifetime than nanocubes and nanorods.The differences in their optical properties may be mainly related to the different structural defects in these MAPbI₃ nanomaterials with different morphologies.These results indicate that morphology plays an important role in regulating the optical band gap,lifetime and photoluminescence intensity of MAPbI₃.?2?MAPbI₃ nanotriangles,nanocubes and nanorods with uniform morphology are prepared by a simple ligand-assisted deposition method.By adjusting the ratio of oleic acid and oleylamine during the experiment,with the increase of the amount of oleamine,the morphology of the sample can be changed from nanotriangles to nanocubes to nanorods.The growth mechanism and photophysical properties with three different morphologies of MAPbI₃ nanocrystals are analyzed in detail,and optimized nanocrystals with different morphologies are applied to photodetectors to study the effect of morphology regulation on the performance of their optoelectronic devices.According to the optical performance analysis,the optical band gaps of the nanotriangles,nanocubes and nanorods perovskites are 1.59 e V,1.43 e V and 1.36 e V,and the average carrier lifetimes are 1.33 ?s,7.28 ?s and 7.81 ?s,respectively.We used a simple template-free and easy-to-control strategy for preparing photodetectors.Through research on photodetectors based on different morphologies,we find that compared with based on nanotriangles and nanocubes photodetectors,the based on nanorods photodetector exhibits the better light photophysical properties and optoelectronic characteristics.It has a higher optical responsivity and detectivity?12 m A/W,2.67 × 1011 Jones?at 0.14 m W/cm2 405 nm laser bias voltage of 4V.In addition,the on/off ratio of the photodetector can reach more than 800,and the rise time and decay time constants of 18 ms and 25 ms,respectively.By comparison,the responsivity R and detectivity D* of based on MAPbI₃ nanorods photodetectors are 3 times and 4 times improvement that of the photodetector based on nanocubes,indicating that the perovskite nanocrystals of different morphologies and sizes has a certain effect on the performance of the photodetector.?3?The phase transition and optical properties of MAPbI₃ nanoplates and nanocubes under high pressure are studied by a diamond-anvil device?DAC?.Through in-situ high-pressure UV-visible absorption spectra,in the low pressure region,the absorption edge of the nanoplates and nanocubes both red-shifted with the increase of pressure.However,under continued pressure,the nanoplates and nanocubes start to blue-shift at 0.36 GPa and 0.25 GPa,respectively.Moreover,the bandgap of MAPbI₃ nanoplates and nanocubes decreased by 0.121 e V and 0.077 e V,respectively,during the pressurization process.Through in-situ high-pressure photoluminescence spectra,the photoluminescence intensity of MAPbI₃ nanoplates first increased and then weakened,while the photoluminescence peak intensity of MAPbI₃ nanocubes increased slightly during the pressurization process except for 1.10-1.76 GPa,and the rest of the pressure range is weakened.Through in-situ high-pressure raman spectroscopy measurements,the raman vibration mode of the nanoplates has a slight blue shift compared with the nanocubes,indicating that under high pressure,the interaction force between the organic cation CH3NH3+ and Pb octahedrons [Pb I6]4-in the nanoplates crystals is stronger than that of the nanocubes under pressure,making the MAPbI₃ nanoplates more difficult to be compressed.These results indicate that the difference between the optical properties of MAPbI₃ nanoplates and nanocubes may be related to the change of lattice structure,and the optical properties of perovskite under high pressure can be regulated by adjusting the morphology.