Green CdZnSe-based Quantum Dot Light-emitting Diodes With High Luminance And Long Lifetime Through Modulating The Shell Thickness And Surface Ligand

Quantum dot light-emitting diodes（QLEDs）have been one of the most competitive products in the new-generation high-end display technology due to the excellent features,such as high color purity,wide color gamut,and continuously adjustable emission spectrum.In the recent thirty years,with the continuous improvement in the synthesis technology of quantum dot（QD）emitting material and the optimization of device structure,the performance of QLEDs in terms of brightness,efficiency,and lifetime has essentially met the requirements of low brightness(100 cd m^-2)display applications.However,the low device efficiency and short lifetime under high luminance(≥1000 cd m^-2),for example,green QLEDs with a T₉₅ lifetime（the time for the luminance decrease to 95%of its initial luminance）is only～35%of that of red ones,which hindered its application in the field of high luminance display and solid-state lighting.To address this issue,this thesis aims to improve the energy level matching between the emissive layer and the charge transport layer by modulating the shell structure of CdZnSe alloy core-shell QDs,and combining with the surface ligands modification of QDs to enhance the carrier injection and recombination balance,and then reduce the probability of Auger recombination in QD emitting layers.After that,green QLEDs with simultaneous high luminance and long lifetime will be realized.The main contents in this work include the following aspects:（1）Fabrication of high-performance green QLEDs based on CdZnSe alloy core-shell QDs by modulating the shell thickness.As previously reported,increasing the ZnS shell thickness can effectively passivate the surface defects of QDs and suppress Forster resonance energy transfer（FRET）.However,excessively thick ZnS shells can not only lead to the formation of the core-shell interface defects in QDs,but also reduce the efficiency of carrier injection,and ultimately impair the performance of the QLED.To address this issue,based on CdZnSe alloy core QDs,CdZnSe/ZnSe/ZnS core-shell QDs with fluorescence quantum yields of more than 90%are obtained by regulating the thickness of the ZnSe intermediate shells and ZnS outer shells.And then,QLED based CdZnSe/ZnSe/ZnS QDs emitting layers are fabricated,and the impact of ZnS thickness on both the optical properties of the QDs and the performance of corresponding devices have been systematically studied.The results show that when the ZnSe intermediate shell layer is 7monolayers（ML）,and the ZnS outer shells is 2 ML,the highest fluorescence quantum yield of the QDs reaches 90%,with a full width at half maximum（FWHM）of 34 nm.Correspondingly,the QLED exhibits a maximum external quantum efficiency（EQE）of 21.05%at the voltage of 3.41 V,and the maximum luminance reaches 629480 cd m^-2.Moreover,and the T₅₀ lifetime under the initial brightness of 1000 cd m^-2is 116 hours.The improved device performance is due to the suitable shell layer thickness improving the carrier injection balance and reducing the probability of Auger recombination in QDs emitting layer under high luminance.（2）Fabrication of high-luminance and long-lifetime green QLEDs based on halide ion modified QDs.Long-chain organic ligands attached onto QDs surface can help to improve the stability of QDs in solution and prevent the formation of dangling bonds on the surface,thus enabling efficient fluorescence quantum yields.However,the insulating property of long-chain organic ligands can decrease the efficiency of carrier transfer within QDs layers,ultimately leading to a reduction in exciton recombination rate.As reported,halide ligands modified QDs not only passivates the surface defects but also elevates the valence band energy level.This will enhance the hole injection efficiency,facilitating carrier injection balance,and reducing the charge accumulation in the device,ultimately improving device performance.Based on the principle,we replaced the long-chain organic ligands of CdZnSe/ZnSe/ZnS QDs with Cl^-,and systematically studied the influence of halide ligands exchange on the optoelectronic properties of QDs and device performance based on the optimization of the ligand concentration and reaction time.The results show that the device have the best performance when the Cl^-concentration is 15 mg/m L and the reaction time is 20min.At this point,the device efficiency is maintained more than 20%with a maximum luminance of 770830cd m^-2,which is 1.2 times higher than that of devices before ligand exchange.Moreover,the luminance at the photon voltage is increased by 2.34 times and up to 1660 cd m^-2.In particular,at an initial brightness of1000 cd m^-2,the T₅₀ is increased by a factor of 1.41.Furthermore,we also investigated the effect of Br^-and I^-as ligands on device performance.The results show that devices based on Br^-and I^-modified QDs sustain the maximum of>700000 cd m^-2,suppress the efficiency roll-off and improve the lifetime.The above results provide some references for improving the luminance at low voltage and device lifetime through using halide modified QDs as emitting layers.