Fabrication And Characterization Of Top-emitting White Quantum-dot Light-emitting Diodes

As a new type of semiconductor nanomaterials,quantom dots?QDs?have the advantages of tunable color,high color purity,good stability,high quantum yield,simple solution processability,and thus have been widely applied to solar cells,photodetectors,light-emitting diodes,flat panel displays,solid state lighting and biomarkers.Quantum dot light-emitting diodes?QLEDs?which have the advantages of self-emission,high device efficiency,and all-solution preparation,have broad application prospect in the new generation of low cost,low power,large area,ultra thin and flexible display.After years of research and development,the external quantum efficiency?EQE?of monochromatic QLEDs and bottom-emitting white QLEDs have exceeded 20%.However,most of the work currently focused on the study of bottom-emitting structure devices.Compared with the bottom-emitting devices,the top-emitting structure devices can significantly improve the aperture ratio of the displays.In addition,the white light devices combined with the color filter can achieve high-resolution and full-color display.Therefore,the top-emitting white?TW?QLEDs are more suitable for the active-matrix full-color display with high aperture ratio and high-resolution,however,there has been little progress in the research of TWQLEDs.In this thesis,I focus on studying the TWQLEDs.To obtain broadband white light emission,the microcavity effect in the top-emitting structure is supressed by using the aluminum?Al?as the reflective bottom electrode and the indium tin oxide?ITO?as the transparent top electrode.Through the selection of experimental materials and the optimization of the preparation process,TWQLEDs are successfully realized,and the research objective of this subject is preliminarily completed.In the optimization process of the bottom electrode,we found that the Al electrode was easily oxidized by oxygen in the air and consequently leads to the formation of alumina which blocks the hole injection.In order to solve this problem,a thin layer of molybdenum trioxide?MoO₃?or ITO were coated on the Al electrode,and the thickness and preparation process were adjusted.Then,the performance of the device was compared and analyzed,and the turn-on voltage was successfully reduced from 12 to 2 V.In the optimization process of the top electrode,the common silver?Ag?electrode structure was first tried,and an optical coupling layer was prepared on the surface of the Ag electrode to enhance its transmittance.However,due to the strong microcavity interference effect of the device,it was impossible for the device to emit a broadband white light spectrum.To solve this problem,we chose ITO with high transmittance as the top electrode,sputtered by low-power of 50 W,and incorporate polyvinylpyrrolidone?PVP?in the electron transport layer to improve its compactness and avoid the damage caused by ion bombardment during sputtering.Finally,TWQLEDs with the brightness of 18270 cd/m²,the current efficiency of7.792 cd/A,the EQE of 4.764%and the color coordinate of?0.33,0.32?were successfully prepared,which provided a basis for future research on top-emitting white light.