Investigation On Fabrication And Properties Of NiO-based Quantum Dot Light Emitting Diodes

The quantum dot light-emitting diode?Quantum Dot Light-Emitting Diodes,QLED?has attracted considerable interests in research field recently driven by rapid development of display industry,which is a kind of novel thin film display and solid-state light-emitting device with adjustable internal emission,high color purity and good stability etc.Although the organic material with excellent performance was commonly used as hole transporting layer of QLED,it possessed poor stability and its hole transporting rate is lower than that of inorganic material.Therefore,it is necessary to find suitable inorganic materials to replace the organic hole transporting layer.The commonly used inorganic materials are WO₃,MoO₃,V₂O₅ and Ni O,etc.Thereinto,the NiO?band width of 3.6-4.0eV?as the third generation P type semiconductor material has good chemical stability,excellent optical,electrical and magnetic properties due to its special electronic structure.It is widely used as a hole transporting layer in QLED field recent years.In order to explore the application of NiO as a hole transporting layer in QLED,the following experiments were carried out in this work:Firstly,Ni O nanocrystalline was prepared by solution method.The X-ray diffraction?XRD?test showed that the prepared nanocrystalline was NiO and it exhibited single crystalline phase.Transmission electron microscopy?TEM?test showed that the Ni O nanocrystalline particles had good dispersion,uniform size distribution and the averaged particle size was 3.1±0.1nm.The results of UV-Vis and UPS meant that the optical band gap width of NiO nanocrystal was 3.9 eV,the power function?and the valence band top level position(E_VBM)were 3.32eV and 5.22eV,respectively.Then,NiO nanocrystalline thin films were prepared by spin coating method.And they were applied to the inorganic QLED structure of ITO/NiO/QDs/ZnO/Ag as hole transporting layer material.The influence factors of the device performance,such as the clean of the substrate,the ultraviolet ozone treatment of NiO films,the compared selection of NiO and ZnO of concentration and spin coating rate,have been explored.And then,the optimum conditions for the preparation of the all inorganic device were determined.The best luminescence intensity and turn-on voltage of the best all inorganic devices was 598 cd/m²and 3.3V,respectively.Finally,in order to get better performance inorganic device,the device structure was optimized.The NiO was used as the hole transporting layer material in the QLED structure of ITO/NiO/PVK/QDs/ZnO/Ag.XPS results showed that the UV ozone treatment can induce Ni³⁺ions in intracell of Ni O.Through combining multiple spin coating and multiple UV ozone treatment innovatively,the NiO film with increased content of Ni³⁺ion and uniformed distribution was obtained.Furthermore,it couldenhance the ability of NiO thin film conduction hole effectively.The results showed that the luminous brightness,current efficiency and external quantum efficiency of the device showed a gradually improvement trend with the increasing spin coated NiO hole transporting layers.The QLED obtained the best performance when the spin coating times was 4,the best luminescence intensity increased from 184cd/m² to 4775cd/m²,the maximum current efficiency was 0.54cd/A and maximum external quantum the efficiency was 0.22%.compared with the single NiO based QLED,it was noteworthy that all the experiment results increased by more than 50 times.