Study Of Electroluminescent Cesium Lead Halide Perovskite Quantum Dot LEDs

Solution-processed perovskite quantum dots?QDs?feature excellent emission properties,including tunable emission,narrow emission line-width and high emission quantum yield up to 90%.These compelling properties make perovskite QDs as one of the most promising materials for lighting and display applications.We synthesized a series of all-inorganic perovskite QDs?CsPbX₃,X=Cl,Br,I?with tunable bandgaps by adjusting their composition of halides and studied their light emitting diode?LED?applications.At the beginning of the research of this thesis,there were very few publications about perovskite QDs and LED applications,and the electroluminescence performances were poor.In addition,compared to traditional cadmium chalcogenide QD materials,perovskite QDs suffer poor solution stability during purification and storage.This thesis focuses on exploring the surface chemistry of CsPbX₃ QDs as well as the development of highly efficient LED devices.We modified the surface properties of QDs and engineered the LED device structure to get improved material properties and LED applications.First,since the compositions and density of surface ligands for QDs greatly affect the optoelectronic properties,we synthesized CsPbBr₃ QDs with controllable surface modifications.It is demonstrated that by adding didodecyl demethyl ammonium bromide?DDAB?,the surface ligand composition and density can be changed,and eventually leads to an improvement of the material quality.On the one hand,CsPbBr₃ QDs possess higher emission quantum yield?from 70%to 96%?and better storage stability?the solution kept clear after one month in air?,which is because the new ligand bonded to the surface atoms more strongly and the surface vacancies were effectively passivated.On the other hand,the QD films featured better carrier transport ability due to the decrease of surface long-chain ligand density.Emloying the modified QDs into designed LED,we obtained a 4-time enhancement in external quantum efficiency.Secondly,by inserting an interfacial layer of Mg-doped ZnO nanocrystal between electron transport layer and emission layer,a more stable interface was obtained.The incorporation of Mg decreases the oxygen vacancy sites on ZnO nanocrystal surface,which prevents the damage of the absorbed water on ZnO surface to perovskite upper layer.As a result,the CsPbBr₃ QD layer obtained a higher stability when depositing on Mg-doped ZnO layer.Besides,Mg-doped ZnO film features a wider bandgap,and the conduction band level shifts upside,leading to a decreased electron injection barrier and more efficient LED application.Thirdly,through a post treatment to CsPb?Br/I?₃ QDs using polyethyleneimine?PEI?,the surface defects of the perovskite QDs were effectively passivated,leading to an obvious increase on film emission quantum yield and radiative lifetime.Besides,the PEI layer suppresses QD charging effect.By employing the well-passivated perovskite QD into an inverted structure and optimizing the layer thickness for charge balance,we achieved electroluminescent LEDs with 6.3%of external quantum efficiency,which was one of the highest values at that time.Also,based on the best condition,we made LED devices emitting red,green and blue light by changing the halide composition of the emission perovskite layer.Finally,transparent LEDs with double-side emission were fabricated by employing a transparent multilayered anode MoO₃/Au/MoO₃.By fine tuning the thickness of each layer,the hole injection was enhanced,and the transparency of the whole device can be maximized while maintaining the electroluminescence performance.The transparent device exhibits a highest transmittance of 58%,and shows similar electroluminescence spectra from both sides,which demonstrates the potential of perovskite QDs on transparent display application.From the point view of material science and device engineering,we have improved the perovskite QD material properties and LED applications.This study may provide a reference for future work of perovskite optoelectronic applications.