Solution Processed MoO_X As Hole Injection Layer For High Stable Red QLEDs

As an emerging electroluminescent device,quantum dot light-emitting diodes（QLEDs）are considered one of the most promising technologies in the fields of display and lighting due to their various advantages,including high fluorescence quantum yield,narrow emission spectra,tunable emission spectra,and solution-processable fabrication,which are attributed to the unique properties of quantum dots.Poly（3,4-ethylenedioxythiophene）-polystyrene sulfonate（PEDOT:PSS）has been used as the hole injection layer（HIL）in QLEDs due to its high carrier mobility and appropriate energy level structure.However,due to the acidity and hygroscopicity of PEDOT:PSS,corrosion of the ITO electrode may occur,resulting in a reduce the operation lifetime for the device.Transition metal oxides have appropriate work functions and other advantages,such as solution-processable fabrication and high transparency of thin films,as an alternative to PEDOT:PSS as the HIL.Among various transition metal oxides,molybdenum oxide（MoO₃）not only has the aforementioned advantages,but also has the property of forming ohmic contact easily with metal electrodes.Previous research results have indicated that the MoO₃ films prepared by ammonium molybdate sol-gel process as HIL can effectively improve the lifetime of the light-emitting devices,but the poor charge transport capability leads to a lower device efficiency.However,the H_xMoO₃ nanoparticle thin film with better charge transport capability cannot be widely applied in the field of light-emitting devices due to the existence of residual oxidizing agents and agglomerated large particles.To improve the charge transport capability and device stability of MoO₃ films,this paper is based on the preparation of MoO_x thin films by solution method as the HIL material.At the same time,by improving the preparation method of MoO₃,the red Zn Cd Se/Zn S QDs were used as the light-emitting layer,the QLEDs devices with organic-inorganic hybrid structures were constructed.A purification step consisting of evaporation,constant volume,centrifugation,and filtration was proposed for the preparation of H_xMoO₃ nanoparticle systems by oxidation method.The H_xMoO₃ nanoparticle thin films with better charge transport capability and uniform particle size was achieved successfully.When the films were used as HIL,the efficiency and lifetime of red QLEDs devices were both improved.The main work of this paper can be summarized in two parts as followed:（1）Preparation of MoO3 thin film by sol-gel method to construct QLEDs devicesUsing ammonium molybdate solution as a precursor,MoO₃ thin film was prepared via sol-gel method.MoO₃ thin film was utilized as the replacement of PEDOT:PSS as HIL to construct red QLEDs device.The MoO₃ thin film were optimized by the post-treatment conditions,including precursor concentration,spin-coating speed,annealing temperature,and UV-ozone time.The final results show that the maximum luminance(L_max)and maximum external quantum efficiency(EQE_max)of the red device with the structure of ITO/MoO₃/TFB/QDs/Zn Mg O/Al are 27710 cd/m² and 11.85%,respectively.The T₉₅ lifetime of the device at the initial luminance of 100 cd/m² is 4742.47 h,which is 89 times longer than that of the control device based on PEDOT:PSS as HIL(T₉₅ lifetime of 52.81 h).（2）Preparation of Hx MoO3 nanoparticles by the oxidation reaction method to construct QLEDs devicesUsing ethanol as a buffer and solvent,the reaction rate of molybdenum powder was controlled in the oxidation process using hydrogen peroxide,resulting in the successful preparation of uniformly-sized H_xMoO₃ nanoparticles with a diameter of about 28 nm.The UPS results indicate that the film exhibits typical n-type semiconductor behavior,with a conduction band bottom energy level of 4.85 e V,a valence band top energy level of 8.17 e V,and a work function of 5.35 e V.H_xMoO₃ films were prepared by spin-coating method as HIL for constructing red QLEDs devices.The preparation conditions,including the amount of oxidant,solution concentration,annealing temperature and spin-coating speed,were adjusted and optimized.The conductive atomic force microscopy results demonstrate that the local current in the H_xMoO₃ film reached the level of nanoamperes under a bias voltage of 2 V.The red QLEDs device structure of ITO/H_xMoO₃/TFB/QDs/Zn Mg O/Al was constructed with H_xMoO₃ as HIL.The device exhibited L_max and EQE_max of 44100 cd/m² and 14.50%,respectively.The T₉₅ operation lifetime of the device at the initial brightness of 100 cd/m² was5749.97 h,which was 109 times higher than that of the control device.Compared with red QLEDs devices constructed based on ammonium molybdate system,it has been demonstrated that the use of H_xMoO₃ film as HIL can lead to further improvement in efficiency,brightness,and lifetime.