| Due to the excellent properties,such as tunable band gaps,narrow emission spectra,small light scattering effect in the whole visible region,high photoluminescence quantum yield(PLQY)and solution-processing film fabrication,quantum dots(QDs)are known as the most potential next generation semiconductor materials.Quantum dot light-emitting diodes(QLEDs)are also expected to be used in the next generation displays and lighting.However,the commercialization of QLEDs still needs to solve the problems of low efficiency and stability.In this thesis,the lattice mismatch between core and shell is alleviated by alloying the core shell structural QD,and the surface defects are efficiently passivated by designing the surface ligand.Therefore,the QD materials with photoluminescence quantum yield(PLQY)of close to 100%were obtained.Moreover,Auger recombination(AR)can be effectively suppressed in alloyed QDs due to the gradual change of electric potential from the core to shell.Firstly,we used ZnCl2 as a ligand to replace oleic acid in the shell growth of CdZnSe/ZnSe/ZnSeS/CdZnS alloyed core shell QD.Compared with the insulated oleic acid ligand,ZnCl2 ligand can improve the surface conductivity of the ligand.Moreover,Cl-can further combine with Zn2+on the surface of the quantum dot to further passivate the dangling bonds on the surface of quantum dot,improving the optical properties of quantum dot.The emission peak of photoluminescence(PL)is 534 nm with the PLQY of 93.0%and the half-peak width of 20.6 nm,corresponding to its excellent optical properties.The external quantum efficiencies(EQE)and the device life(T50)of QLED are 11.7%and 23 h,respectively.Secondly,a quaternary alloy Cd0.194 Zn0.806 Se0.406 S0.594/ZnS QD was designed with a feature of large-sized core(about 10 nm)and thin shell(about three ZnS monolayers).It has an ultra-fast radiative transition rate(Kr=4.16×107 s-1),which reveals that the exciton of QD could be decayed to its ground state through a quickly radiative recombination,and therefore less non-radiative recombination.Even under a high pump power(254.65 μJ cm-2),it still has long biexciton decay lifetime(τxx=1.05ns)and high biexciton quantum yield(QYXX=25.1%),indicating that the AR process is repressed.Therefore the PLQY of QD is close 100%PLQY.The DDT ligand can not only be used as the S precursor but also as the surface ligand.The C12H25+ cation is used to passivate the S atoms and the DDT is used to passivate the Zn2+cations,affording QD with excellent optical properties.Meanwhile,the use of DDT tunes the VB level of QD,which is in favor of hole injection inside the emissive layer and give the QLED an excellent performance.The QLED exhibits a high luminance of 105 cd m-2,high EQE of 20.1%and long operation lifetime(T50)of 145 h,which demonstrates that it has a great opportunity in commercial applications. |
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