MoO_x Based Hole Injection Layer To Construct Stable Green Quantum Dot Light Emitting Diodes

Quantum dot light-emitting diodes?QLED?have developed rapidly due to their characteristics in higher purity,better monochromaticity,wider spectral coverage,and longer lifetime.Therefore,it is expected to become the next generation of lighting and display technology,which is applied to intelligent terminal,ultra-high definition display,high-end lighting and other fields.Poly?3,4-ethylenedioxythiophene?:polystyrene sulfonate?PEDOT:PSS?has many advantages in high transparence,good conductivity,and solution preparation,which is most commonly used in QLED devices as hole injection layer?HIL?.However,PEDOT:PSS seriously reduces devices' stability due to its hygroscopic and acidic nature,which corrodes the ITO electrode and damages the adjacent functional layers.Therefore,PEDOT:PSS is replaced or modified by transition metal oxides?V₂O₅,NiO_x,MoO_x,WO₃,CuO_x?with suitable work function,solution preparation,high transmittance,and good environmental stability.Among them,molybdenum oxide?MoO_x?,not only has the common advantages of transition metal oxides,but also has the advantages of good environmental stability,non-toxicity,and easy preparation.The previous research results show that,compared with control QLED devices,although the stability of these transition metal oxide devices has been effectively improved,due to their poor charge transport capability and large energy barrier between light emitting layer and the metal oxides,the hole injection efficiency is low.To solve the problem of poor hole injection capability devices,MoO_x thin film is selected as the HIL because of the advantages of non-toxic,high work function and good environmental stability.The green Zn Cd Se S/Zn S QDs are used as emitting layer.This paper mainly discusses the application of MoO_x synthesized by sol-gel method as HIL in greenQLED.By adjusting the annealing temperature,UV-O₃ treatment time,thickness,V doping concentration of the MoO_x thin film,and selecting different hole transport layers?HTL?,the hole injection ability of the QLED device can be improved.And the purpose of improving the lifetime and efficiency of the device can be achieved at the same time.The detailed work of this paper can be summarized into the following three parts:?1?Preparation of MoO_x thin film by sol-gel method and its characterizationMoO_x solution was prepared by sol-gel method using ammonium molybdate as precursor and deionized water as solvent.The MoO_x thin films were prepared by spin coating method.The results show that the smooth and compact MoO_x thin films can be formed on ITO substrate after optimization for thickness,annealing temperature and UV-O₃ treatment time,and their roughness is less than 1.0 nm.The XPS results show that the MoO_x thin films are composed of Mo⁶⁺ and Mo⁵⁺.With the increase of annealing temperature and UV-O₃ treatment time,the ratio of Mo⁶⁺/Mo⁵⁺ increases.It can also be found that MoO_x thin films are transformed from amorphous to tetragonal with the increase of annealing temperature from130 ? to 400 ?.?2?MoO_x as hole injection layer to construct QLED devicesGreen QLED devices were constructed by using the prepared MoO_x as HIL by replace of PEDOT:PSS.The devices were optimized for the MoO_x layer,such as the thickness,annealing temperature,UV-O₃ treatment time.The results show that when the precursor solution concentration is 6% w/t with UV-O₃ treatment time of 10 minutes at the annealing temperature is 130 ?,the L_max and EQE_max of the device are 229400 cd/m² and 9.70%,respectively.Based on the above work,MoO_x/PEDOT:PSS was introduced as double hole injection layer,and five commonly used hole transport layer materials?TCTA,PVK,Poly-TPD,TFB,CBP?with different HOMO and hole mobility are investigated to construct QLED devices.Due to the high hole mobility of TFB HTL,the related QLED device has thebest performance with the L_max and EQE_max are 282300 cd/m² and 15.13%,respectively.The T95 lifetime of this optimized device is 5816 h at an initial luminance of 100 cd/m².Compared with the control device,the EQE_max and T95 lifetime of the QLED device are increased by19% and 37 times,respectively.The efficiency and lifetime of the QLED device are simultaneously improved by introducing MoO_x/PEDOT:PSS HIL based on TFB HTL.?3?V-MoO_x/PEDOT:PSS double hole injection layer to construct QLED devicesV-doped MoO_x?V-MoO_x?solution was synthesized by sol-gel method with ammonium molybdate and vanadium triisopropoxide oxide as precursors,and green QLED device was constructed by V-MoO_x HIL.Firstly,the experimental parameters such as the thickness of V-MoO_x,annealing temperature,and UV-O₃ treatment were optimized.Then,by adjusting the V doping amount,its energy level and charge transfer characteristics of the V-MoO_x thin film can be adjusted.When the doping volume ratio is Mo:V = 10:2.5,the conduction band of MoO_x thin film decreases to 4.93 e V,and the current density of the corresponding single carrier devices?electrons and holes?is the more balanced.Compared with MoO_x thin film,the current in micro-region of V-MoO_x films is also increased from 0.27 n A to 1.54 n A,which further proved that the charge transfer characteristics of MoO_x thin film can be improved by doping V.This improving is conducive to the electron hole injection balance in the QLED device.The L_max and EQE_max of the device is up to 356400 cd/m² and 18.66%,respectively.