The Study On Anode And Charge Transport Layer Optimization Of Quantum Dot Light-emitting Diodes

Quantum-dot light-emitting diodes?QLEDs?developed over the past ten years have been demonstrated high external quantum efficiencies, saturated visible color emission, narrow-band infra-red emission, and can be fabricated with a scalable fabrication technique of high resolution. The unique size-dependent optical properties of QDs have motivated increasingly active research aimed at applying them in the next generation of optoelectronic and biomedical technologies.In a typical QLED standard structure,organic small molecules such as polyethylene dioxythiophene : polystyrene sulfonate?PEDOT:PSS? and Poly[N,N'-bis?4-butylphenyl?-N,N'-bis?phenyl?-benzi?Poly-TPD? are often used as hole inject layer?HIL? and hole transport layer?HTL? respectively, and1,3,5-tris?N-phenylbenzimidazol-2-yl?benzene?TPBI? as the electron transport layer?ETL?are widely used. Organic small molecules can effectively improve the external quantum efficiency?EQE? and luminance. However, the performance of device with organic materials are easily affectedby air condition and degraded over time, especially the anode indium tin oxide?ITO? which greatly reduced the life of devices. Besides, the organic materials also cause a wide range of problems, such as the thermal instability, moisture and oxygen-induced degradation. Above all, the high cost, complicated manufacture process and toxic organic materials can't be neglected for further application. Therefore, it is becoming more and more important to develop new material to replace organic materials.The Kelvin probe is a non-contact, non-destructive vibrating capacitor device for the determination of work function of conducting, and semi-conducting materials. It is a tool with wide application for both surface science, sub-surface measurements, and industrial uses. In our work, the Kelvin probe was used to measure work function difference?CPD? between a conducting specimen and a vibrating tip, and meanwhile, we fabricate different QLED structures to investigate the surface potential of organic monolayers on conducting or semiconducting solids and to probe charge-transfer properties of these multilayers on conductive and semi-conductive surfaces. Based on above, for the purpose of optimizing the QLED structure, we use Kelvin probe to investigate the photoelectricity properties of materials. So our goal is as follows.For the first part, it is known that the improvement of work function can help enhance the hole injection efficiency of anode of QLED, which can effectively improve the device performance. we utilize kelvin probe to research how to improve the work function of ITO by different surface treatment method, such as different anneal temperature in vacuum or in air condition, different illumination time at ultraviolet, and treatment with strong acid or alkali. By comparing different treatment method, we find that the ultraviolet treatment is much better than the anneal treatment and strong acid or alkali treatment. By anneal treatment in air condition, the work function of ITO is improved about 0.2eV, By ultraviolet treatment, it is improved about 0.4ev, while by strong acid or alkali, it is improved less than 0.1eV. We also investigated the surface photovoltaic properties of the metal oxide ZnO and ZnO doped with Mg, and a big difference is founded between ZnO and Zn_1-xMg_xO.For the second part, the alloyed Zn_1-xMg_xOand positive semiconductor NiO nanoparticles were synthesized and applied to QLED respectively. as we all know, with the introduction of inorganic ZnO, typically in the form of nanoparticles?NPs?, as an electron transport layer?ETL? material, the device performance of electrically driven colloidal quantum dot-light-emitting diodes has been rapidly improved. In our work, we synthesized the Zn_1-xMg_xO?Zn/Mg= 20,10,3? with sol-gel method at low temperature,andcharacterized with X Ray Diffraction?XRD?, Transmission Electron Microscope?TEM?, Kelvin Probe, ultraviolet and visible spectrophotometer, we find that with the proportion of Mg increased, the crystal form of Zn_1-xMg_xO was not changed, there are the same as the ZnO, and the absorption peak of Zn_1-xMg_xO was obviously blue-shifted. Then we fabricate light emitting diodes by applying alloyed Zn_1-xMg_xO?Zn/Mg=20,10,3? nanoparticle as electron transport layer. The luminance and efficiency of those QLEDs are found to be strongly dependent on the type of Zn_1-xMg_xO NP as ETL, resulting in the substantial improvements by means of alloyed Zn_1-xMg_xO ETL versus pure ZnO only and the external quantum efficiency is improved about two times than pure ZnO. Meaningwhile, in order to solve the problem that the anode indium tin oxide?ITO? are easily corroded by acid environment induced by PEDOT:PSS, we synthesized positive metal oxide NiO to replace PEDOT:PSS, as for the P-type semiconductor NiO, the top valence band of NiO is well aligned with the highest occupied molecular orbital?HOMO? levels of many typical p-type conjugated polymers. The unique electronic structure of NiO offers impressive charge selective for HTL, making NiO as an ideal inorganic materials which can replace the organic materials. By applying NiO as HTL, we demonstrated maximum external quantum efficiency of 1.46%, maximum brightness of 22000cd/m2, and peak current efficiency of 2.27cd/A. Besides, the devices showed betterstability under continuous bias.