Study On The Effect Of Doping And Interfacial Modification Of Hole Injection Layer On The Performance Of All-inorganic Perovskite Light-emitting Diodes

In recent years,halide perovskite has made great progress in the field of light-emitting diodes because of its close to 100%photoluminescent quantum yield（PLQY）,more than 10 cm² V^-1 s^-1 carrier mobility,widely adjustable emission wavelength and narrow emission bandwidth.The external quantum efficiencies of both red and green perovskite light-emitting diode（Pe LED）have exceeded20%.Compared with organic-inorganic hybrid perovskite materials,all inorganic perovskite has better thermal stability and excellent color purity.Especially Cs Pb Br₃,its physical and chemical properties remain stable for a long time in an environment with a relative humidity of 70%and under conditions of 250℃,making it a very promising and efficient green Pe LEDs material.At present,there is still significant room for improvement in the electroluminescent efficiency of Pe LEDs based on Cs Pb Br₃.The energy level matching between the work function of the traditional PEDOT:PSS hole injection layer（HIL）and the maximum valence band（VBM）of Cs Pb Br₃ needs to be improved,which affects the injection of holes and triggers a large number of non-radiative recombination at the interface.In addition,the surface coverage of Cs Pb Br₃ thin film on the PEDOT:PSS surface is low,leading to a large leakage current,and thus reducing device performance.Therefore,this work aims to reduce the hole injection barrier and improve the surface morphology of Cs Pb Br₃ by doping PEDOT:PSS and surface modification,thereby improving the luminescence efficiency of Pe LEDs.The detailed work is described as follows:（1）Doping EDTA-2Na in PEDOT:PSS to improve the overall performance of Pe LEDs.EDTA-2Na is a disodium salt of EDTA.EDTA-2Na was used instead of EDTA because of its better solubility in water.EDTA-2Na has six coordination atoms,which is a good coordination agent in chemistry.After doping EDTA-2Na（5 mg/m L）in PEDOT:PSS,the work function of the PEDOT:PSS layer increased from 5.06 e V to 5.16 e V,leading to the reduction of the hole injection barrier and improvement the hole injection efficiency.Moreover,after the doping of EDTA-2Na,the root mean square roughness of PEDOT:PSS increased from 1.75 nm to 3.05 nm.It implied that the perovskite layer is more likely to form a mosaic structure at the interface with the PEDOT:PSS layer,thereby improving the surface morphology of Cs Pb Br₃.The contact angle of perovskite precursor solution on PEDOT:PSS decreased from 25°to 22.5°,demonstrating the better wettability of perovskite precursor solution on the substrate.Therefore,the overall performance of Pe LEDs has been improved.The maximum luminous efficiency（LE）increased from 7.4 cd/A to 9.3 cd/A.（2）Modification on the surface of PEDOT:PSS HIL with Na Cl to improve the morphology of Cs Pb Br₃.Experimental results show that the surface roughness of PEDOT:PSS/Na Cl layer increases from 1.59 nm to 84.8 nm after the introduction of Na Cl interlayer.This is because Na Cl crystals are separated out and tiny particles are formed with the evaporation of solvent during the annealing process of Na Cl layer.The rough surface induces the growth of perovskite crystals to form various nucleation points,which greatly improves the morphology of Cs Pb Br₃.Cs Pb Br₃film with improved surface coverage,reduction of pinholes and decreased defect state density from 5.34×10¹⁸ cm^-3 to 3.95×10¹⁸cm^-3was obtained.Thus,the non-radiative recombination was inhibited,leading to increased photoluminescence（PL）yield and electroluminescence（EL）intensity.The maximum luminous efficiency of the device is increased from 7.4 cd/A to 12.8 cd/A.（3）Na Cl modified PEDOT:PSS doped EDTA-2Na HIL to further improve the performance of Pe LEDs.Experimental results showed that in combination of Na Cl modification the optimized doping concentration of EDTA-2Na was 2.5 mg/m L,based on which the LE of Pe LEDs was further improved from 12.8 cd/A to 15.8 cd/A.By analyzing the current density voltage curve（J-V）of single hole devices,it was found that when the doping concentration of EDTA-2Na was higher than 2.5 mg/m L,the current density of holes was greatly improved.Therefore,it can be speculated that excessive hole injection broke the injection balance of charge carriers and reduced device efficiency.This work reveals the importance of controlling carrier injection balance in high efficiency Pe LEDs devices,and can further improve the overall performance of Pe LEDs devices through the joint use of different methods.