Ligand Engineering And Electroluminescent Device Performance Study Of CsPbBr₃ Nanocrystals

In recent years,lead halide perovskites are regarded as perfect emitting materials for its superb properties such as solution processability,high luminescent color purity,high defect tolerance and tunable bandgap.Specially,when the dimension of perovskite materials decreases to nanometer level,perovskite nanocrystals have higher photoluminescence quantum yield（PLQY）compared with perovskite bulk materials,are more beneficial to realize highly efficient light emitting diode（LED）.Due to the special structure of PNCs,a large number of surface ligands are needed to maintain their dispersion and stability.However,the common perovskite surface ligands are generally non-conductive long-chain organic compounds,resulting in poor electrical properties of perovskite nanocrystals films.It will significantly increase the injection difficulty of electrons and holes in perovskite light emitting diode（Pe LED）and limit the further improvement of Pe LED performance based on PNCs.Therefore,it is very important to enhance the optical and electrical properties of perovskite thin films through surface modification and ligand engineering.In this paper,inorganic perovskite Cs Pb Br₃ nanocrystals（Cs Pb Br₃ NCs）are chosen as research object.This thesis focuses on the efficiency improvement of Cs Pb Br₃ nanocrystals-based Pe LED through surface modification and ligand engineering,the details are as follows:（1）The Cs Pb Br₃ nanocrystals are obtained at room temperature through ligand assisted precipitation method with surface ligands of long-chain organic ligands didodecyl dimethylammonium bromide（DDAB）and octanoic acid（OTAc）.Na⁺ions are introduced for surface modification,and the mechanism of Na⁺and its effect of properties of Cs Pb Br₃NCs.The test characterization shows that Na⁺ions are capped on the surface rather than incorporated into perovskite lattice,the surface defects are better passivated and the long-chain ligands are reduced.When 8%Na⁺are added,the PLQY shows a remarkable increase from 46.9%to 84.6%,the film morphology and charge transportation are improved and the maximum external quantum efficiency（EQE）of Pe LED increases from 5.59%to 7.33%.（2）Short organics FA⁺ions are introduced for surface modification.The influence of FA⁺on morphology,lattice and opto-electronic properties of Cs Pb Br₃ NCs are explored.Test characterization shows that FA⁺ions could replace part of long-chain ligands DDAB by attaching on the surface of Cs Pb Br₃ NCs,which achieves the effect of replacing long chain with short chain,and the charge transfer ability of the Cs Pb Br₃ NCs is enhanced.After adjusting the device structure and the amount of FA⁺,with FA⁺ions added,Cs Pb Br₃ NCs-based Pe LED shows a better performance with maximum EQE,current efficiency（CE）and luminance increasing from 5.59%,18.36 cd/A and 2509 cd/m²to 11.19%,37.83 cd/A and7611 cd/m²,respectively.（3）FA⁺and Na⁺ions are introduced simultaneously for improving the performance of Cs Pb Br₃ NCs.Through monitoring the dynamic process of ligands during synthesis and purification,Na⁺ions are uncovered to adsorb on the surface as metal ligands in the synthesis of Cs Pb Br₃ PNCs and undergo a desorption process in purification process.When FA⁺and Na⁺ions are introduced simultaneously,not only the amount of long-chain ligands are effectively reduced but also short-chain ligands FA⁺achieved boosted proportion through filling the sites of Na⁺.The surface of the resultant Cs Pb Br₃ PNCs successfully achieved a better ligand distribution of less long-chain ligands and more short-chain ligands,hence the optical and electrical properties enhanced a lot.The Pe LED device with FA⁺and Na⁺modifying achieved the greatly improved performance with a maximum EQE,CE and luminance increasing from 5.59%,18.36 cd/A and 2509 cd/m² to 22.5%,79.68 cd/A and13290 cd/m²,respectively.