Quasi-two-dimensional Perovskite High-Efficiency Green Fluorescent Material And Light-Emitting Diode

Halide perovskites have attracted many researchers in the fields of solar cells,light-emitting diodes,lasers,photodetectors,etc.Quasi-two-dimensional?quasi-2D?perovskite materials are known for their luminescent efficiency,so they can be used in light-emitting diodes?LEDs?.The main works of this thesis are to study the luminescent properties of quasi-two-dimensional perovskites,PEA₂?FAPbBr₃?_n-1PbBr₄,and to improve the efficiency of perovskite light-emitting diodes.The PEA₂?FAPbBr₃?₂PbBr₄[n=3?FA?composition]perovskite with multiple quantum wells?MQWs?structure has a feature—lacking the widest bandgap component and enriching the sub-wide bandgap component,which makes it easy to balance charge confinement and injection.Thus it is a good luminescent material for LEDs.The difference in the emission wavelength of each quantum well is small for the n=3?FA?composition perovskite film,and the luminescence spectrum is stable.The half-width of the electroluminescence spectra of the LEDs based on the n=3?FA?film is about 23 nm,which is consistent with the common halide perovskite green LEDs.Although the quasi-2D perovskite is a multi-phase mixture,it still has the advantage of high purity of the halide perovskite LEDs.The LEDs based on the n=3?FA?film have an emission wavelength of 532 nm,the CIE?Commission Internationale de l'Eclairage?chromaticity coordinate is?0.21,0.75?,and the color purity is 95%.The highest current efficiency achieved was 69.5 cd A^-1 and the external quantum efficiency was 15.8%.The other research contents of this thesis can be summarized into the following three aspects.?1?The grain size of these quasi-2D perovskite films is about one tenth of that of the three-dimensional perovskite films,and the large number of grains means a very large grain boundary area.For the quasi-2D perovskite film,the surface is quite rough and has a large surface area.Thus the number of lattice defects will be considerable in the quasi-2D perovskite film.Lattice defects generally play the role of non-radiative recombination sites and carrier traps,so it is necessary to find efficient passivation methods to remove or fill defects in quasi-2D perovskite films.In this thesis,we improved the crystal quality of the quasi-2D perovskite film by adding methylammonium chloride,and used trioctylphosphine oxide to passivate surface defects on the perovskite films to further improve its luminescent efficiency.?2?This thesis has made a detailed discussion and analysis on the characteristics of the efficiency roll-off of quasi-2D perovskite LEDs.The quasi-2D perovskite LEDs always achieve the maximum efficiency at a relatively low current density while the current density and luminance of the devices are often in a stage of rapid growth,indicating that the device is relatively intact and has not been seriously damaged.Then the current efficiency of the devices decreases with increasing current density,which should be similar to the efficiency roll-off of LEDs based on nitride semiconductor MQWs.In a series of LEDs based on PEA₂?FAPbBr₃?₂PbBr₄[n=3?FA?composition]perovskite,the efficiency roll-off is generally triggered at the current density no more than 10 mA cm^-2,and the efficiency roll-off increases a lot after reducing the leakage current.This also reflects that the quasi-2D perovskite MQWs can efficiently concentrate the dispersed charge carriers into small luminescent regions,greatly increasing the carrier concentration in luminescent regions,which is beneficial to enhance the luminescence but will also cause serious efficiency roll-off at the same time.?3?In this thesis,it is found that after replacing formamidinium?FA?with methylammonium?MA?,some results become completely opposite.The widest bandgap component tends to become abundant,and the sub-wide bandgap component tends to disappear,which makes the quasi-2D perovskite with the average chemical formula of PEA₂?MAPbBr₃?₂PbBr₄[n=3?MA?composition]become a highly efficient photoluminescent material,with an absolute photoluminescence quantum yield as high as 85.3%.This benefits from the enhanced exciton recombination and light emission in deeper quantum wells.The inclusion of the widest bandgap component is beneficial for achieving high photoluminescence efficiency for the quasi-2D perovskite material.However,its luminescent efficiency in LEDs is unfortunately limited because the widest bandgap component hinders the injection of holes and the migration and uniform distribution of carriers.The distinction between quasi-2D perovskites based on FA and MA indicates that the feature and quantity of the wide bandgap component as the charge confinement layer and the narrow bandgap component as the luminescent region need to be carefully controlled to achieve different purposes—to prepare efficient LEDs or to obtain excellent photoluminescence efficiency.Ensuring sufficient luminescent components is a common premise for both of the above purposes.To achieve quasi-2D perovskite materials with ultra-high photoluminescence efficiency,we need to increase the ability of the wide bandgap confinement layer.In order to choose a quasi-2D perovskite material for high-efficiency LEDs,a moderately wide bandgap confinement layer is required,because enhancing charge confinement should be not at the expense of charge injection.The balance of charge injection and confinement is an important principle in making high-efficiency LEDs.In conclusion,the organic cations such as FA and MA have a significant adjustment effect on the self-organized MQWs structure in the quasi-2D perovskite film,which significantly influences the charge confinement effect and also the charge injection effect in LEDs.The PEA₂?FAPbBr₃?₂PbBr₄ perovskite is a high-efficiency electroluminescent material that does not require a high-performance LED structure.The PEA₂?MAPbBr₃?₂PbBr₄ perovskite is an excellent photoluminescence material and can be used as a fluorescent material and a stimulated emission material.Finally,we made a prospective analysis of the defect passivation treatment in the quasi-2D perovskite film,the stability problem of the halide perovskite LEDs and the realization of the blue light emission from perovskite materials.