Carrier Control And Stability Of Quantum Dots Light-Emitting Diodes

Quantum dot light-emitting diodes（QLEDs）are promising large-scale electroluminescent devices used for display and solid-state lighting applications,due to their high efficiency,high color purity,and simple yet cost-effective solution processibility.After the past twenty years of research,the performance of QLEDs has been significantly improved through the study and improvement of the core/shell structure of quantum dots（QDs）,the surface ligands of QDs,and the device structure engineering.However,the efficiency and stability of QLEDs still need to be improved.Moreover,the stability of the device under ultraviolet light and during the storage process also requires to be further enhanced before the industrialization of QLEDs.The thickness of each functional layer in QLEDs is only a few tens of nanometers,so the properties of the transport materials and the interfaces have an important impact on the carrier injection and transportation,exciton recombination and dissociation of QLEDs.In this thesis,it is found that the device performance of QLEDs is effectively improved by modifying the electron transport layer and the interface of the hole injection.In terms of electron injection,Mg doped ZnO and polyethylene pyrrolidone（PVP）were blended to act as the electron transport layer for the inverted QLEDs with gradient alloy QDs,thus the luminescence performance of QLEDs was greatly improved.Based on the PVP-mixed Zn Mg O electron transport layer,the maximum external quantum efficiency and T₅₀ lifetime of the red QLEDs reached 8.4%and 10064 h（under an estimated 1000 cd/m²）,which were 3.2times and 6.3 times as much as those of the ZnO-based QLEDs,respectively.It was found that the mobility of the electron transport layer（ETL）could be further modified to reduce the electron injection,thereby improving the carriers balance in the device,by blending the Mg-doped ZnO layer with PVP.Besides,the surface defect of Zn Mg O suppressed and the spontaneous charge transfer at the interface of ETL/QDs inhibited.Thus,blending the Mg-doped ZnO layer with PVP enhanced the electrical neutrality of QDs to reduce the non-radiative recombination process of QDs and improved luminescence performance of QLEDs.This study shows that blending is a simple and effective way to control the injection transport capacity and surface properties of the ETL,and the regulation of the electron side can effectively improve the performance of QLEDs.Then in the field of the hole injection interface,a thin intermediate layer 4,4’-bis（carbazol-9-yl）biphenyl（CBP）was introduced between the hole transport layer 4,4’,4’’-tris（carbazol-9-yl）-triphenylamine（Tc Ta）and the hole injection layer Mo O_x,to improve the hole injection of device.Thus,it enhanced the performance of QLEDs.Compared with the performance of QLEDs without CBP intermediate layer,the maximum external quantum efficiency,power efficiency and T₅₀ lifetime of the red QLEDs based on the CBP had increased by 28%,54%and72%,respectively.Under the estimated 1000 cd/m²,the T₅₀ lifetime had reached 15606 hours.It was found that the Fermi level of the commonly used hole injection material Mo O_x was pinned in the deep expanded state density of the organic hole transport layer Tc Ta.Therefore,the charge had to overcome these narrow state densities before injecting into the organic hole transport layer,which led to the obstruction of hole injection.Thus,inserting a thin intermediate of CBP with higher HOMO level could eliminate the energy level pinning between Tc Ta and Mo O_x layer,and effectively improve the hole injection capacity,resulting in the improvement of the carrier injection balance of QLEDs.Therefore,with the intermediate layer of CBP,the device exhibited a significantly lower driving voltage and improvement of performance in the QLEDs.Finally,this strategy was also applied to green and blue QLEDs,and achieved similar effects of improving performance.In terms of device stability,it was the first time to find that both ultraviolet light and the self-passivation of ZnO during device storage could improve the efficiency but also reduce the storage stability of the ZnO-based QLEDs device.Ultraviolet light excited the absorbed oxygen and the defect state on the surface of ZnO nanoparticles,resulting in the improvement of the electron injection and transportation in the device.Therefore,after 15 minutes of ultraviolet light treatment,the device exhibited better performance that the maximum external quantum efficiency and power efficiency were increased by 26%and 143%,respectively.Moreover,it was found that ZnO would also passivate during the storage process,which lead to enhancing the charge balance in the device and inhibiting the exciton quenching of QDs.This improved the performance of QLEDs,but also reduced the storage stability of the ZnO-based device.Here,a stable Sn O₂ layer was proposed to replace the unstable ZnO layer as the electron transport layer of QLEDs,which effectively improved ultraviolet illumination stability and storage stability of the QLEDs.It was found that the Sn O₂ nanoparticles were insensitive to ultraviolet light because of the larger band gap,and the Sn O₂ had better stability of water and oxygen,so the QLEDs based on Sn O₂ had better stability.