Efficient And Stable FAPbBr₃ Perovskite Nanocrystals Via In Situ Method And Application In Light-Emitting Diodes

In recent years,perovskite materials have attracted great attention in display,photovoltaic and other fields due to their advantages of adjustable band gap,long exciton diffusion length and low cost.In the field of electroluminescence,the synthesis of stable,efficient and high-repeatable perovskite luminescent materials and the fabrication of perovskite light-emitting diodes（Pe LEDs）are still urgent problems.Firstly,the synthesis of perovskite light-emitting layer with less impurities,uniform grain size and dense film layer requires high repeatability.Secondly,it is important to reduce the energy consumption and cost and increase the productivity in the synthesis of perovskite light-emitting layer.Thirdly,especially for organic-inorganic hybrid perovskites,organic cations are easy to degrade as exposing to humidity,oxygen and ultraviolet light,leading to composition and structure changes.Therefore,researchers are now focusing on improving and developing new synthesis methods and perovskite material system,in order to prepare high-efficient and stable perovskite nanocrystals and Pe LEDs.In view of the above problems,this thesis focuses on the exploration of highly efficient and stable organic-inorganic hybrid perovskite light-emitting materials,and explores the optimization of synthesis method,material system,film-forming method and stability improvement.（1）Optimize the carbon chain length of organic amine ligands to fabricate high-performance devices via in situ method.Organic amine ligands,including 3,3-diphenyllethylamine bromide（DPPA-Br）and 2,2-diphenyllethylamine bromide（DPEA-Br）,are used to fabricate perovskite nanocrystals and light-emitting diodes.The optimal concentration of precursor solution is 0.2 M,and the best ratio of FABr:Pb Br:ligand is 1:1:0.4,respectively.As a result,the current efficiency is2.59 cd/A,which is 1.69 time higher than that of DPPA-Br.The better performance of Pe LEDs based on DPEA-Br ligand can be attributed to the enhanced film conductivity due to the shorter carbon chain length.Furthermore,the effect of film fabrication methods on film quality and device performance are investigated.Compared with one-step method,the perovskite films with higher crystallinity,more uniform grain size and denser packing are obtained by in situ method using antisolvents.The device efficiencies are 0.19 and 4.58 cd/A for Pe LEDs fabricated by one-step and in situ methods,respectively.（2）Optimize the perovskite material systems based on formamidinium-lead to further improve the device performance.Based on the results in Chapter 2,Pe LEDs based on FAPbBr₃ and FA_0.8Cs_0.2Pb Br₃ are prepared in situ based on DPEA-Br ligands.For the FA_0.8Cs_0.2Pb Br₃ system,there is a blueshift in the photoluminance spectra and smaller angle of the peak position in the XRD pattern,due to the smaller ionic radius of Cs⁺.Limited to the solubility of Cs Br in N,N-Dimethylformamide（DMF）solvent,the maximum precursor concentration of FA_0.8Cs_0.2Pb Br₃is only 0.15 M.Besides,the solution is not stable,and the perovskite film morphology and device repeatability are poor.Therefore,FAPbBr₃-Pe LEDs obtain the better device performance with a maximum current efficiency of2.32 cd/A.（3）Passivate the surface defects of FAPbBr₃ perovskite films to improve the film stability and device efficiency.The n-octylphosphonic acid（OPA）ligand is adopted in facricating FAPbBr₃perovskite nanocrystals,and the effect of its concentrations on the optical property,film morphology and device performance are characterized.Firstly,P-OH in OPA removes the coordination H and combines Pb²⁺in perovskite nanocrystals,leading to a fluorescence quenching center reduction and the surface defect passivation.The average lifetime of perovskite nanocrystals is improved from 42.55 to 71.51 ns,and the fluorescence quantum yield increases from 17.01%to47.05%.The maximum brightness and current efficiency of Pe LEDs based on OPA-FAPbBr₃perovskite nanocrystals films increase from 261 cd/m² and 2.91 cd/A to 3273 cd/m² and 17.70 cd/A,which are 12.5 and 6.1 times higher than those for the pristine device,respectively.In this thesis,the effect of synthesis method,material system and ligand passivation on the film quality and device performance of organic-inorganic hybrid perovskite materials have been investigated.The homogeneous and dense FAPbBr₃ perovskite nanocrystals films are fabircated by in situ method,and the stability are significantly improved by using phosphonic ligand.Finally,the device efficiency and stability of Pe LEDs are enhanced successfully.This thesis provides a feasible strategy for exploring high-efficient and stable organic-inorganic hybrid perovskite light-emitting layer and Pe LEDs.