Effects On The Performance Of Quantum Dot Light Emitting Diodes With Thiophenol Molecular Interface Modification

Quantum dot light-emitting diodes(QLED)have great potential application in flat-panel displays and solid-state lighting due to their unique characters,such as narrow spectral emission bandwidth,high brightness,tunable emission wavelength over the entire visible region,solution process,etc.Flat-panel displays based on QLED have excellent color gamut and color purity.However,the research of QLED is still limited in laboratory because of many existing problems including device architecture and preparation technology.One of the main factors is the imbalanced carrier injection,which comes from the large offset(more than 1 eV)between the highest occupied molecular orbital(HOMO)of hole transport materials and the valance band of quantum dots,thus restricts the hole injection efficiency.This thesis is mainly based on using thiophenol molecules as hole transport layer / quantum dot layer(HTL/QDs)interface modified materials,to fabricate the CdSe@ZnS quantum dot light-emitting diodes,then investigate the effects of hole injection efficiency,devices performance and identify the mechanism.The hole injection ability and overall efficiency of QLED devices have been enhanced after using thiophenol molecules as interface modified materials.The work can be summarized as follows:(1)Optimization of the process parameters of 4-methylthiophenol molecular interface modified layer:Using 4-methylthiophenol(MTP)as HTL/QDs interface modified materials for QLED,and the optimum technological conditions of the modified layer were determined by optimizing the process parameters such as the concentration,the rotational speed,the solvent and the annealing temperature.It clearly shows that,when toluene was used as a solvent for MTP molecular,a large contact area would be formed between MTP and TFB,which was more effective for the optimization of the device.The destruction of HTL did not affect the overall performance of the device through a blank solvent(toluene)treatment.(2)The effects and mechanism of 4-methylthiophenol in quantum dot light-emitting diodes:Different device structures were designed to investigate the influence of MTP molecules in different positions.Results shows that the best device performance was obtained when MTP was used as HTL/QDs interface modified molecular,exhibiting a peak current efficiency of 42.91 cd/A and external quantum efficiency of 11.93%,increased by 40.97% and 41.69% compared with control device,respectively.The steady state and transient fluorescence spectra show that the fluorescence intensity of the quantum dots or TFB was decreased significantly when the MTP molecules were mixed into the quantum dots or TFB,but the fluorescence lifetime of QDs was not affected.The ultraviolet photoelectron spectroscopy characterization shows that the Femi level of TFB was increased by 0.2 eV when the MTP was used as TFB/QDs interface modified material.This means that the hole injection barrier was reduced,which was confirmed by the current-voltage relationship of hole only devices.(3)The effects of thiophenol molecular interface modification on the hole injection capability:Organic molecules with different structures were selected as HTL/QDs interface modified materials via the research concepts of fundamental organic chemistry to explore the contribution degree of aromatic ring electron density and thiol groups in thiophenol molecular on device performance.It found that the devices had higher performance when using thiophenol substituted with electron donating groups as interface modified molecular,but a reduced performance by electron withdrawing groups substituted thiophenol.The improvement in device performance was observed in the device with either ferrocene(without thiol group but a electron rich conjugated structure)or 1-octanethiol(without conjugated structure but a thiol group)as interface modified molecular,but the magnitude is lower than the device with thiophenol structure unit.The relationship between thiophenol substituents,functional groups and the hole injection efficiency was confirmed by the hole only devices.