V₂O₅ Based Hole Injection Layer To Construct Efficient And Stable Green Quantum Dot Light Emitting Diodes

Quantum dot light-emitting diodes?QLED?have the advantages of wide spectrum coverage,high purity and low energy consumption.These characteristics make it widely used in mobile devices,solid state lighting and flat panel displays especially in light emitting devices with wide color gamut.At present,the maximum external quantum efficiency?EQE?of red,green and blue QLED is 21.6%,22.9%and 19.8%respectively.The most commonly used hole injection material for QLED is poly?3,4-ethylenedioxythiophene?:poly?styrene-sulfonate??PEDOT:PSS?.However,PEDOT:PSS seriously reduces devices'stability due to its hygroscopic and acidic nature,which corrodes the indium tin oxide?ITO?electrode.To address these problems,transition metal oxides,such as NiO,MoO₃,V₂O₅ and WO₃have been used as substitutes for PEDOT:PSS in recent years.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 the QDs and the metal oxides,the hole injection efficiency is low.This imbalance of electron and hole injection leads to inferior QLED device efficiency when metal oxides are used as HILs.In order to solve the problem of low device efficiency caused by poor conductivity of transition metal oxide and high hole injection barrier,the V₂O₅ is selected because it has the most matching energy level with the HOMO energy levels of PEDOT:PSS.This paper mainly discusses the application of V₂O₅synthesized by sol-gel method as HIL in QLED devices with green ZnCdSeS/ZnS QDs as EML.By means of gradient energy level design of double HIL and Ti ion doping,the hole injection ability is improved,the effect of electron-hole injection balance is achieved,and the device performance and operation lifetime are both greatly improved.The detialed work of this paper can be summarized into the following three parts:?1?Synthesis of green QLED device by sol-gel method for synthesizing V₂O₅ as hole injection layerV₂O₅ was prepared by sol-gel method using vanadyl triisopropoxide as precursor and isopropanol as solvent.The V₂O₅ film was formed by a simple spin coating method and applied to green QLED device as hole injection layer?HIL?.The ratio of oxygen vacancies in V₂O₅ thin films is controlled by optimizing the film thickness,annealing temperature,ultraviolet ozone treatment time,etc.and the thickness of other functional layers is optimized to control the device performance.The results show that the volume ratio of the current displacement body to the solvent is 1:70,the V₂O₅ thin film prepared after annealing at 130?and ozone for 10 minute has a roughness of less than 1.0 nm.When it is used as a hole injection layer instead of PEDOT:PSS,the constructed QLED device has a maximum brightness of 81690 cd/m²,a maximum current efficiency and a maximum EQE of 36.11 cd/A and 9.09%,respectively.?2?V₂O₅/PEDOT:PSS double hole injection layer to construct QLED deviceBased on the work in Chapter 2,V₂O₅/PEDOT:PSS double hole injection layer is introduced,and V₂O₅ and PEDOT:PSS are used as reference to construct three different HIL QLED devices.The results of Ultroviolet Photoelectron Spectrometer?UPS?and Kelvin Probe Force Microscope?KPFM?show that the V₂O₅/PEDOT:PSS double hole injection layer structure can not only form a stepped energy level between the anode and the hole transport layer,reduce the hole injection barrier,thus improving the hole injection efficiency,but also inhibit the corrosion of the ITO anode by the acidity of PEDOT:PSS,thus improving the service life of the device.Compared with the standard device?PEDOT:PSS?,the maximum current efficiency and EQE of QLED device constructed with double hole injection layers are increased by 30.6%and 31.2%,respectively,reaching 71.62 cd/A and 18.09%.At the same time,the service life is also increased by 2 times,from 6771 h to 13355 h for standard devices.?3?Ti Doped V₂O₅ as hole injection layer to construct QLED deviceTi-doped V₂O₅?Ti-V₂O₅?was synthesized by a low-cost sol-gel method using vanadium triisopropoxide and tetraTitanium isopropoxide as precursors and isopropanol as solvent,and characterized by X-ray photoelectron spectroscopy?XPS?,X-ray diffraction analyzer?XRD?,ultraviolet photoelectron spectroscopy?UPS?etc.The doping amount of Ti is adjusted by controlling the mixing ratio of the two precursors,and the charge transport characteristics and electronic structure of Ti-V₂O₅ are adjusted.The results show that when the doping ratio of Ti is 1.0%,the conduction band position of V₂O₅ moved down to4.94 eV.Compared with V₂O₅ thin films,the micro-region current of Ti-V₂O₅ thin films increases from pA to nA at 6 V voltage.When Ti-V₂O₅ thin film is used as a QLED device constructed by HIL,the hole injection barrier can be effectively reduced and the hole injection capability can be improved.Through subsequent optimization of ozone treatment,annealing temperature and film thickness,the maximum current efficiency and EQE of QLED device are 52.28 cd/A and 13.35%respectively,and the working life reaches 45027 h.