Electroluminescence Performance Of Eco-friendly Quantum Dot Light-emitting Diodes Based On Hole Injection Enhancement

Quantum dot light-emitting diode（QLED）has the advantages of high luminous efficiency,tunable wavelength and low fabrication cost,is currently one of the most promising self-emitting displays.At present,external quantum efficiencies（EQEs）red,green and blue quantum dot（QD）devices have reached more than 20%,but only II-VI Cd Se QD based QLEDs can meet the commercialization standards.In recent years,with the appearance of metal halide perovskite quantum dots Cs Pb X₃（X=Cl,Br,I）,scientists also began to put a lot of research into the perovskite LEDs.However,the disadvantage is that the above QLEDs contain Pb,Cd heavy metals.Since June 4,2015 the EU’s ROGS 2.0 order restricts the content of Cd and Pb in electronic products,which makes cadmium-based,lead-based QLEDs face an important bottleneck.Therefore,in view of the great strategic significance of quantum dot technology for the national new display,the development of efficient and highly stable InP,Zn Se and other environmentally friendly displays and lighting tricolor light sources has become an urgent issue in the field of QLEDs.However,InP and Zn Se,as the most promising cadmium-free alternatives,are far behind cadmium-based and lead-based QLEDs in terms of electroluminescence performance.The main reasons are insufficient hole injection and non-radiative processes triggered by electron over-injection.Therefore,according to the energy level position of eco-friendly QDs,the enhancement of hole carrier injection is expected to improve the electroluminescence performance of eco-friendly QDs.In this thesis,the enhancement of hole injection at the interface of hole injection layer（HIL）/hole transport layer（HTL）and HTL/QD is used to improve the electroluminescence performance of the QLED devices through the interface engineering strategy.The main work of this Ph D dissertation is as follows:（1）Improving the electroluminescence performance of InP QLEDs through inorganic step hole injection layer:Since the efficiency and stability of InP QLEDs are still facing great challenges,in order to develop high-performance InP QLEDs,in this chapter,colloidal nickel oxide and magnesium-doped nickel oxide nanocrystals are used to prepare bilayer hole-injection layers to replace the classical poly（3,4-ethylenedioxythiophene）:poly（styrene sulfonate）（PEDOT:PSS）HIL to construct InP QLEDs with excellent performance.Compared with QLEDs with a single HIL of PEDOT:PSS,the bi-layered HIL enables the external quantum efficiencies of the QLEDs to increase from 7.6%to 11.2%,and the T₉₅lifetime（time that the device brightness decreases to 95%of its initial value）under a high brightness of 1000 cd m^-2to prolonge about 7 times.The improved performance of QLEDs is attributed to that the bi-layered HIL reduces the mismatched potential barrier of hole injection,and narrows the potential barrier difference of ITO/hole transport layer interface to promote carrier balance injection,and realize high-efficiency radiative recombination.The experimental results indicate that the use of bi-layered HILs with p-type Ni O_xmight be an efficient method for fabricating high-performance InP-based QLEDs.（2）Improvement of InP-based quantum dot light-emitting diode performance by dipole modulation of hole injection layer PEDOT:PSS ordering.Due to the poor conductivity of PEDOT:PSS HIL and the high hole injection barrier with HTL,which leads to insufficient hole injection in the device,in order to develop InP QLEDs with easy operation and high EQE and operation stability,this chapter proposes to use Mo O_xnanoparticles doped with PEDOT:PSS to achieve higher ordering of the HIL.Mo O_xdoping not only decreases the work function of PEDOT:PSS HIL and reduces the barrier mismatch,but also achieves higher conductivity and hole mobility.As a result,the InP QLEDs can obtain more balanced carriers and more efficient electron-hole recombination,thus the maximum EQE reaches 17.8%,and the T₉₅reaches 20.8 h,which is three times higher than that of the unprocessed devices.Notably,the device undergoes an efficiency roll-off almost above 10,000 cd m^-2brightness,which is an excellent performance for InP devices.The observed enhancement in electroluminescence performance indicates that PEDOT:PSS HIL films with a higher degree of ordering have the potential to be utilized in the development of efficient,stable,and low-cost QLED products.（3）Enhancing the electroluminescence performance of blue ZnTeSe QLEDs through dual dipole layers:Blue ZnTeSe-based QLEDs often suffer from poor hole injection,which significantly limits their performance.Herein,we introduced dual dipole layers consisting of（2,3,6,7,10,11 hexaazatriphenyl hexacarbonitrile）（HAT-CN）between the hole injection layer of PEDOT:PSS,and hole transport layer of polyvinylcarbazole（PVK）to enhance the performance of blue ZnTeSe-based QLEDs.The introduction of the HAT-CN layer created dual dipole layers,which reduce the hole injection barrier between PEDOT:PSS and PVK,decrease the hole defect density,increase the hole current,and improve the effective radiative recombination of charge carriers.The QLEDs with the HAT-CN layer exhibited a higher EQE,compared to those without the HAT-CN layer,increasing from 4.2%to10.1%.Moreover,the T₅₀lifetime almost doubled at a high luminance of 2000 cd m^-2.This work shows that introducing the HAT-CN layer to create dual dipole layers using the full solution method is an effective strategy to enhance the electroluminescence performance of ZnTeSe-based blue QLEDs.（4）Enhancing performance of all-inorganic InP QLEDs using wide bandgap ZnTeSe quantum dot interlayer:All-inorganic QLEDs with low roll-off efficiency have attracted great interest in recent years.However,the presence of a high density of defect states at the Ni O_x/QD interface,which is prone to fluorescence bursting of the QDs,and the excessive hole injection barrier,which limits the hole injection,seriously affect the QLED performance.In this study,wide bandgap ZnTeSe QDs are proposed as an interlayer to modify the Ni O_x/Ni Mg O_x/InP QDs（i.e.,HTL/emissive layer（EML））interface.This strategy effectively reduces the interfacial carrier quenching,while the Te ion concentration can be used to precisely regulate the position of the valence band maximum（VBM）of the ZnTeSe interlayer,regulate the hole injection barrier.On the other hand,the ZnTeSe interlayer has a high conduction band minimum（CBM）,which significantly suppresses the electron leakage from the InP emission layer to the Ni Mg O_xHTL interface.The maximum brightness of the all-inorganic InP QEDs is obtained to be 6657 cd m^-2with an external quantum efficiency of 6.4%.It is shown that the initial success of the interface engineering of the ZnTeSe interlayer provides a greater possibility for fully inorganic InP QLEDs for future outdoor and optical projection display applications.In summary,the prerequisites for the construction of efficient and stable QLEDs are to solve the charge transport properties at the HIL/HTL and HTL/QD interfaces,narrow the hole injection barriers,and enhance the hole injection capability to promote the balanced injection of electron holes into the QD-EML and realize the efficient radiative recombination.In this Ph D thesis,the electroluminescence performance of eco-friendly QLEDs is enhanced by inorganic bi-layered HILs,constructing dipoles to enhance HIL ordering,dual dipole layers,and ZnTeSe QD interlayer,respectively.These multiple device structure design and interface modulation are effective strategies in reducing the hole injection barrier,enhancing the hole injection capability,and promoting the efficient radiation recombination of electron-hole to QD-EML,which provides an extremely important reference for the further development of high-efficiency display and lighting devices.