Constructing And Performance Of QLED Using Li And F Co-doped ZnO As Electron Transport Layer

Quantum dot light emitting diode(QLED)as a new type of electroluminescence device shows an enormous potential application for next-generation solid-state illumination and displays.Compared with liquid crystal display(LCD)and organic light emitting diode(OLED).QLED exhibits unique advantages such as solution processed,high material stability,high color purity and tunable emission wavelength over the entire visible region.However,the research of QLED devices is still in the laboratory stage due to the low efficiency and short lifetime.ZnO nanoparticles(NPs)are widely used as the electron transport layer(ETL)in QLEDs owing to their suitable electrical properties.The injection of electrons and holes is unbalanced in QLEDs due to the excess electron injection.And because of the well-aligned conduction band levels,electrons in QDs can be spontaneously transferred to adjacent ZnO NPs,leading to severe excition dissociation,which reduces the proportion of radiative recombination and deteriorates the device efficiency.To solve these problems,researchers have adopted two strategies: One is to introduce a buffer layer between the QDs light-emitting layer(EML)and the ZnO electron transport layer(ETL)to block the injection of electrons and suppress quantum dot fluorescence quenching by avoiding of ZnO and QDs direct contaction.The other is to synthesize cation-doped ZnO as ETL to reduce the injection of electrons by adjusting the band gap of the ZnO NPs and increase the electron injection barrier.Additionally doping could inhibit the surface defect states of ZnO,which are the exciton quenching sites degrading the device performance.However,the current doping scheme is not enough to accurately control the band gap of the ZnO NPs thin film electron transport layer,and it is difficult to significantly inhibit the surface defect state of ZnO which causes the exciton quenching between the ZnO layer and the QD layer.In order to effectively control the electron injection and reduce the exciton quenching sites on the ZnO surface without increasing the complexity of the device,three parts of work have been carried out.(1)Preparation and performance of QLEDs using Li doped ZnO as electron transport layer.Firstly,Li doped ZnO(LZO)was synthesized by solution method and used as electron transport layer to construct green QLEDs.The LZO NPs were characterized by XRD,TEM,absorption spectrum,steady-state fluorescence spectrum,transient fluorescence spectrum,XPS,UPS.And the photoelectric properties of the green QLEDs and single carrier devices were tested.In a result,Li+ may partially replace Zn2+ into the lattice,which makes the band gap of ZnO wider and the position of the conduction band slightly shifted up.So the mobility of the carrier is decreased,and the ability of electron injection is suppressed.In addition,Li doping can reduce the concentration of oxygen vacancy on the ZnO surface,which acts as exciton quenching sites degrading the device performance.When Li concentration was 1%,the EQEmax of the green QLEDs achieved the greatest increase,from 13.39% of the control device to 15.55%.(2)Preparation and investigation on the performance of QLED using F doped ZnO as electron transport layer.Secondly,F doped ZnO(ZOF)was synthesized by solution method and used as electron transport layer to construct green QLEDs.According to the characterization of ZOF NPs and the photoelectric properties of the green QLEDs and single carrier devices,F-may partially replace O2-into the lattice,which makes the band gap of ZnO wider and the position of the conduction band shifted up.So the mobility of the carrier is obviously decreased and the ability of electron injection is suppressed.In addition,F doping can significantly reduce the concentration of oxygen vacancy on the ZnO surface,which acts as exciton quenching sites degrading the device performance.When doping concentration was 40%,the EQEmax of the green QLEDs achieved the greatest increase,from 12.78% of the control device to 21.55%.(3)Preparation and performance of QLEDs using Li and F co-doped ZnO as electron transport layer.Finally,Li and F co-doped ZnO(LZOF)synthesized via solution method and was used as electron transport layer to construct green QLEDs.According to the characterization of LZOF NPs and the photoelectric properties of the green QLEDs and single carrier devices,Li+ may partially replace Zn2+ and F-may partially replace O2-into the lattice,which makes the band gap of ZnO significantly wider and the position of the conduction band shifted up.So the mobility of the carrier is obviously decreased,and the ability of electron injection is suppressed.In addition,Li and F co-doping can obviously reduce the concentration of oxygen vacancy on the ZnO surface,which acts as exciton quenching sites degrading the device performance.When Li-doping concentration was 2% and F-doping concentration was 40%,the EQEmax of the green QLEDs achieved the greatest increase,from 13.29% of the control device to 22.89%.