Application Of Peie Interface Modification Layer In All-Solution-Processed Flexible Inverted Quantum Dot-Light-Emitting Device

Quantum dot light-emitting diode(QLED)has a very broad application prospect in the field of illumination and display due to its high color purity,good material stability,tunable emission wavelength over the entire visible region and solution processed.Based on the current literature reports,QLED devices are mainly divided into two types:normal structure and inverted structure.The normal devices are mostly used in the field of illumination.Due to it can be integrated with low-cost n-type metal oxide or amorphous silicon thin film transistors,the inverted QLEDs are more practical for display applications.In the inverted structure of the QLED devices,it can be fabricated by vacuum thermal evaporation method or all solution processed method.Compared with the thermal evaporation method,the QLED device with inverted structure constructed by all solution method has the advantages of simple operation process,low cost and large area preparation.However,the QLED devices with inverted constructure by all solution method currently have the following problems:(1)Quantum dots(QDs)light-emitting-layer(EML)will be dissolved by the solvent of hole transport layer(HTL);(2)Carrier injection imbalance caused by high hole injection barrier at HTL/EML interface;(3)At the interface of electron transport layer(ETL)/EML,exciton quenching due to high-density surface defects of ZnO nanoparticles(ZnO NPs)can lead to the performance degradation of the device.In this thesis,in view of the problems in this all solution processed inverted QLED devices,ethoxylated polyethyleneimine(PEIE)with an aliphatic amine group was selected as the interface modification in the inverted QLED device.The formation of the diopole layer causes a significant change in the work function of the emitting layer.The PEIE is inserted in the HTL/EML and ETL/EML interfaces.On the one hand,the PEIE is not only used to reduce the destruction of the QDs EML by the solvent of HTL,but also can increase the valence band of the QDs,thereby promoting hole injection from the HTL to the EML.On the other hand,the surface defects of the ZnO NPs are passivated,and the exciton quenching at the interface of ETL/EML is reduced.As a result,the external quantum efficiency(EQE)of the device is increased to about 10%.The high-performance transparent electrode composed of silver nanowires(AgNWs)and graphene was prepared by one-step method.The above optimized device was further construted on the flexible substrate to realize the flexible inverted QLED devices.The main research results obtained in this thesis are as follows:(1)The effect of QDs/PEIE on the performance of QLED devicesIn order to alleviate the destruction of the QDs EML by the solvent of hole transport layer and reduce the hole injection barrier,a novel structure was introduced in the inverted QLED—the PEIE intermediate layer is inserted at the interface of QDs(EML)/PVK(HTL).Through the adjustment of different solvents of PEIE,the morphology of the QDs layer is protected.Through adjusting the concentration,rotation speed and annealing temperature of the PEIE layer,the performance of the device is optimized also.The result indicated that the QDs layer is effectively protected by the solvent of the hole transport layer and the valence band edge of QDs is shifted by 0.35 eV.The stepped band structure from HTL to EML can effectively reduce the hole injection barrier,and the current efficiency of the device is increased from 25.67 cd/A to 34.22 cd/A,the power efficiency is increased from 11.32 lm/W to 14.79 lm/W,the external quantum efficiency is increased from 6.37%to 8.54%,which improved by 33.3%,30.6%and34.1%,respectively.(2)The effect of ZnO/PEIE/QDs/PEIE on the performance of QLED devicesIn order to solve the exciton quenching caused by the high-density defect state of ZnO NPs at the interface of ETL/EML,the PEIE intermediate layer is inserted at the interface of ZnO(ETL)/QDs(EML).Through the adjustment the concentration,rotation speed,annealing temperature and other parameters of PEIE,the performance of the device is increased also.The transient fluorescence spectroscopy result indicated that the fluorescence lifetime of quantum dots is increased from 6.66 ns to 7.17 ns,which proved that the PEIE layer effectively suppressed exciton quenching at the interface of ZnO/QDs.The current efficiency of the device is increased from 32.10 cd/A to 46.90 cd/A,the power efficiency is increased from13.62 lm/W to 19.20 lm/W,and the external quantum efficiency is increased from 8.02%to 11.64%,which increased by 46.1%,41.0%and 45.1%,respectively.(3)Preparation of AgNWs/Graphene flexible electrode and construction of flexible QLED deviceIn order to fabricate flexible inverted QLED,AgNWs combined with graphene with good conductivity and high transmittance are used as electrodes,at the same time,the UV Cureable Resin(NOA63)is used as the flexible substrate.The AgNWs are embedded in the interior of the resin to reduce the surface roughness,and flexible transparent electrode with high bending resistance,mechanical fatigue resistance and high oxidation resistance is prepared in one step methods.Through the adjustment of the spray concentration and the number of spray cycles of AgNWs,the transparent electrode with transmittance of 88.35%and square resistance of 14±3?/sq is obtained The flexible inverted QLED with a luminous intensity of 4342 cd/m~2 and an external quantum efficiency of 1.27%is obtained,which lays a foundation for wearable,foldable and flexible flexible display technology.In this thesis,PEIE was selected as the interface modification material,which could protect the QDs EML from the damage of the HTL solvent.On the other hand,the surface defects of ZnO NPs were passivated and the exciton quenching is suppressed at the ETL/EML interface.As a result,the external quantum efficiency of the device is increased from 6.37%to 11.64%.Forthermore,AgNWs/Graphene flexible transparent electrode was fabricated by a one-step transferring methods.And flexible inverted QLED device with an emission intensity of 4342 cd/m~2 and an external quantum efficiency of 1.27%is obtained.Construction of the full solution method inverted flexible QLED device can lay a foundation for the development of flexible display technology in the future.