Production And Performance Analysis Of Quantum-dot Light-emitting Diodes Based On WO₃/ZnO Double Layer Of Inorganic Charge-generation Layer

Quantum dots（QDs）are semiconductor materials with excellent optical properties,such as wide absorption,narrow emission,easy solution processing,and good photo-stability.Due to their high color purity,low cost,low power consumption,simple structure,and high contrast ratio,the light-emitting diodes based on QDs as the emissive layers（QLEDs）have become a strong competitor in the next-generation lighting and display candidate technologies.Since the first report of QLED in 1994,with the continuous advancement achieved by the researchers,QLEDs have gained rapid development in the display field.At present,the maximum external quantum efficiency of red,green and blue QLED devices has exceeded more than 20%,and the maximum brightness has reached 356000 cd/m²,614000 cd/m²and88900 cd/m²respectively.However,some parameters of QLED devices have not yet reached commercial requirements.Direct current（DC）QLED devices are usually driven by the injection of both electrons and holes.Therefore,the carrier injection in the device will greatly affect the performance of the device,which puts forward extremely high requirements on the matching between the electrode and the injection layer.In this paper,we design and construct a charge generation layer（CGL）in a conventional inverted QLED from the perspective of optimizing the charge injection,which overcomes the high dependence of the carrier injection on the electrode in the conventional device.Most of the previously reported CGLs are composed of organic/inorganic structure.While the commonly used p-type organic semiconductor layer Poly（3,4-ethylenedioxythiophene）-poly（styrenesulfonate）（PEDOT:PSS）is sensitive to the water and oxygen.Moreover,PEDOT:PSS can cause corrosion of the ITO electrode layer due to its acidity,resulting in rapid degradation of charge injection performance.In this paper,we designed and prepared a CGL consisting of double-layer inorganic oxides with a structure of WO₃/Zn O.This CGL is used to prepare QLEDs with an inverted device structure.The electrical characteristics and working mechanism of this CGL,as well as its effects on the performance of QLEDs were studied as follows:First,the electrical properties of CGL composed of WO₃/Zn O double-layer inorganic oxides were investigated.The charge generation efficiency of the CGL can reach 100%,which confirms it has a strong charge generation ability.It was then used as electron supplier in a conventional inverted QLED instead of the Zn O electron-transport layer（ETL）.The device structure is consisting of glass/ITO/WO₃（5～20 nm）/Zn O（30 nm）/QDs（～20 nm）/4,4’-Bis（N-carbazolyl）-1,1’-biphenyl（CBP）（60 nm）/Mo O₃（8 nm）/Al（100 nm）.Experiment results show that the maximum current efficiency of the device can reach 14.90 cd/A,which is nearly 30%higher than that of conventional Zn O ETL-based device.We find that the WO₃layer can effectively suppress the leakage currentand improve carrier injection balance.All the results verify the feasibility of the inorganic CGL composed of WO₃/Zn O in QLEDs..To further explore the working mechanism of the CGL,the impedance spectroscopy and transient electroluminescence spectroscopy of the CGL-based device were measured.We find that WO₃in CGL indeed has a charge storage effect.The stored charges（holes）in WO₃lead to rapid injection and extraction of carriers in the devices.Therefore,the CGL-based QLED can respond quickly to the applied voltage and the luminous intensity can be greatly improved within tens of microseconds from the device turned on.Finally,the shelf stability of CGL-based QLEDs is characterized.The results demonstrate that the CGL-based device exhibits better stability.The active pixel still shows intact electroluminescence emission after being stored in the air for 13.5 hours.In comparison,obvious dark spots are appeared for the conventional device.