Research On Tin Dioxide-based Normal-structured Quantum Dot Light-emitting Diodes

Since zinc oxide(ZnO)was first applied to the electron transport layer of quantum dot light-emitting diodes(QLEDs),the efficiency and operating lifetime of QLEDs have made rapid progress.However,the active surface chemistry of ZnO nanoparticles(NPs)leads to QLEDs with positive aging and unacceptably poor shelf-stability,which greatly limit the commercial application of QLEDs.In this work,tin dioxide(SnO2)with lower chemical activity was used to replace ZnO,and a ligand-coated SnO2 strategy is proposed to solve the incompatibility between the traditional SnO2 nanoparticle dispersion system and the preparation of normal-structured QLEDs and reduce the injection barrier between SnO2 and quantum dots(QDs).On this basis,a normal-structured SnO2-based QLED device with excellent performance was prepared,which successfully eliminated the positive aging of the device and improved the overall stability of the device.This work provides a new method for realizing quantum dot light-emitting diodes with overall high stability.Details are as follows:(1)Synthesis of SnO2 nanoparticles that can be stably dispersed in alcoholic solvents:In order to apply SnO2 as electron transport layer in normal-structured QLEDs,we used tetramethylammonium hydroxide as the coating ligand,and improved the dispersion of SnO2 nanoparticles in ethanol by utilizing the strong coordination of tetramethylammonium hydroxide-SnO2 and the steric hindrance effect provided by the quaternary ammonium group.In addition,the intrinsic dipoles of the ligands can induce electron-level shifts that favor charge injection.The conduction band of SnO2 nanoparticles is shifted from-3.87 eV to-3.42 eV after coating,and even has a lower injection barrier than ZnO(Ec=-3.85 eV).(2)Preparation of SnO2-based quantum dot light-emitting diode devices with both operating stability and storage stability:First,a red QLED with normal structure was prepared with SnO2 as the electron transport layer.These devices exhibit not only the most efficient electroluminescence for ZnO-free quantum dot light-emitting diodes(EQE=13.7%)on the basis of no positive aging,but also an operational lifetime(T95)over 3200 h at 1000 cd m-2.Furthermore,the external quantum efficiency of 12.7%,the operating lifetime of 2000 h and the excellent shelf stability of the SnO2-based green device all indicate the universality of SnO2 to manufacture high-performance QLEDs with overall stability.