Study Of The Key Factors Of Efficiency Improvement Of The Quantum Dot Light Emitting Diodes And Devices Fabrication

Colloidal quantum dot light emitting diodes?QLEDs?have attracted considerable interest for application in the field of panel display and solid state lighting due to the high color saturation and color rendering index?CRI?.The size of quantum dots?QDs?can be adjusted to achieve spectral tunability in the visible-wavelength range due to the quantum confinement effect.In addition,the functional layers of QLEDs can be deposited by solution processed methods,such as spin-coating and inkjet printing,which is in favor of large scale production.Therefore,QLEDs have become an important direction for the development of lighting and display technology.However,there are some problems that hinder the practical application of QLEDs,such as device efficiency,production cost and device lifetime.The imbalance of charge injection and the quenching of QDs by adjacent metal oxides are the key factors affecting the performance of QLEDs.In this thesis,the materials and device structure are optimized to increase the efficiency of monochrome device based on the key factors affecting the performance of QLEDs.In addition,surface plasmon-enhanced QLEDs and all solution processed inverted QLEDs have been achieved.The major achievement of this work is summarized as follows:?1?It is studied that the effect of the energy band of electron transport layer?ETL?and hole transport layer?HTL?on the charge balance.In addition,it is found that the size of ZnO has great effect on its energy band,further influencing the performance of conventional device.Further,the relation between charge balance and device efficiency is explored.The bilayer structure of HTL and doped HTL have been demonstrated to reduce the hole transport barrier to promote the charge balance.According to the experimental results,the performance of QLEDs could be improved by decreasing the size of ZnO,because it could lead to the upshift of the conduction band,thus blocking the excess electron to balance the charge.In addition,the performance improvement of the device benefits from the suppressed exciton dissociation as a result of more favorable energy level alignment at the interface between smaller ZnO NPs and QDs layer.The current efficiency and power efficiency of the device using 2.9 nm ZnO as ETL are 19.7 cd/A and 18.6 lm/W with the enhancement of 95%and 82%,respectively.In addition,the bilayer structure of HTL and doped HTL are utilized to improve the device efficiency by 18%and 55%,benefitting from the increasing hole injection efficiency from 13.2%to 40.1%.?2?A type of inverted QLEDs with high efficiency is proposed.The effect of doping concentration of cesium azide?CsN₃?into ZnO on the energy band,electron transport,film quality and excition quenching are systematically studied.Consequently,the relation between doping concentration of cesium azide?CsN₃?into ZnO and the device performance is revealed.It is found that the doped ZnO could effectively reduce electron flow and balance charge injection.This is by virtue of the upshift of the conduction band of ZnO,which also inhibits the quenching of excitons and preserves superior emissive properties of the QDs by spectroanalysis.The demonstrated green QLEDs exhibit the peak current efficiency,power efficiency and external quantum efficiency of up to 43.1 cd/A,33.6lm/W and 9.1%.This doping strategy is also applied to the red and blue inverted QLEDs to improve their efficiency by 106%and 36%,respectively.?3?A novel way to fabricate all-solution processed inverted QLED is demonstrated based on the solvent modified PEDOT:PSS.The isopropanol?IPA?in PEDOT:PSS could improve the performance of device via facilitating the wetting of PEDOT:PSS on HTL and the charge transport.In addition,the efficiency of the QLEDs has been improved by 2 times due to the coupling between the excitons in QDs and the localized surface plasmon of metal nanoparticles by introducing Au nanoparticles in QLEDs.