Improvement Of Luminescence Performance Of Quantum Dots With Thick Shell And Its Application In Led Devices

Semiconductor quantum dots（QDs）have received widespread attention due to their superior optical properties,and quantum dot light-emitting diode（QLED）devices have shown broad application prospects in displays and lightings.Recently,the luminescent performance of QDs and the efficiency of QLED have been greatly developed,among which thin-shell QDs in solution exhibit excellent optical properties.When thin-shell QD solid films are applied in actual LED,the nonradiative energy transfer（ET）significantly reduces its luminous efficiency and stability,thereby limiting further improvement of device performance.A thick-shell with wide bandgap can effectively enhance the luminescence performance of QD films on glass and in real devices,while significantly improving device efficiency and stability.However,the current luminescence performance of QDs with thick shell needs to be further improved,and the intrinsic correlation between QDs shell thickness and device performance also needs to be discussed in depth.In this dissertation,we design and prepare three primary colors QDs with thick shell based on II-VI compound semiconductors,and optimize the structure to significantly improve the luminescence performance.Then,it was further applied to LED to investigate the intrinsic correlation mechanism between shell thickness and device performance,and to enhance the efficiency and stability of thick-shell QDs-based LED.First,the design of thick-shell structure and the improvement of luminescence performance for CdSe-based red-green QDs are investigated.By designing the ternary alloy Cd_xZn_1-xS/CdSe/Cd_yZn_1-yS well-type structure and adjusting the CdSe emitting layer thickness and the elemental ratio of the surrounding CdZn S,type-I red-and green-emitting well-type QDs with thick shell are successfully prepared,corresponding photoluminescence quantum yield（PL QY）reach 97%and 90%at 7.5 nm and 5.6 nm shell thickness,respectively,and maintained nonblinking.Through structural analysis and spectroscopic characterization,it is found that the intermediate CdSe layer was coherently strained to fit the underlying lattice of Cd_xZn_1-xS seeds,thus allowing the defect-free growth of the thick Cd_yZn_1-yS shells,which significantly enhanced the PL QY of the thick-shell QDs.In addition,using steady-state and transient spectroscopic measurements combined with the F?rster resonance energy transfer mechanism,it is found that a thicker shell significantly reduce the nonradiative ET efficiency in QD films and enhance its luminescence efficiency,which lays the foundation for the subsequent realization of efficient and stable QLED.Next,a series of high crystalline quality and different sized Zn Se QDs are prepared,the emission wavelength covered 390-450 nm.Single Zn S shell and Zn Se_xS_1-x/Zn S gradient shell structures were designed with large-size Zn Se as the emitting cores.The effect of the shell structure and thickness on the surface/interface defect states of Zn Se-based blue QDs is investigated by time-resolved spectroscopic measurements,and it is found that the Zn Se_xS_1-x intermediate shell contributes to the lattice strain relaxation during the shell growth;the thick and wide bandgap Zn S shell enhances the luminescence performance of Zn Se-based blue QDs by passivating the surface defects of the emitting cores and improving the quantum confinement effect.Finally,we prepare the blue Zn Se/Zn Se_xS_1-x/Zn S QDs with a shell thickness of 3.95 nm,an emission wavelength of445 nm,and PL QY of 53%.The spectral broadening of CdSe based alloy QDs with thick shell has been investigated.The coherent strained structure and ion exchange strategy are used to reduce the light-heavy hole band splitting by eliminating the lattice mismatch strain between core and shell,thus suppressing the asymmetric broadening on the high-energy side of the spectrum.Besides,two samples of alloyed QDs synthesized with two temperatures are designed to investigate the relationship between alloying homogeneity and exciton-phonon coupling,and it is found that increasing the alloying homogeneity can significantly weaken the exciton-phonon coupling and thus reduce the homogeneous broadening contribution in single-dot spectrum.The synergy of these two strategies can further reduce the spectral linewidth of QD ensembles and single-dot.Finally,we obtain blue Cd_xZn_1-xSe/Zn Se/Zn S thick-shell QDs with a narrow ensemble spectral linewidth of10.1 nm（58.4 me V）.This work provides a new method for other semiconductor nanocrystals in controlling the spectral linewidth.A series of electroluminescent QLEDs and photoluminescent white light-emitting diodes（WLEDs）are constructed by selecting optimized well-type QDs with different shell thicknesses,and the intrinsic correlation mechanism between the shell thickness and device performance is investigated.By analyzing the spectral properties of QD films,it is found that the thicker CdZn S shell significantly improve the QLED efficiency and WLED spectral chromatic quality by suppressing the non-radiative ET process and reducing fluorescence quenching of QD films in the devices.By analyzing the relationship between the average exciton number in a single-QD and the luminance and external quantum efficiency（EQE）of QLED,it is found that the exciton-photon conversion efficiency is higher in a single-QD with a thicker shell even at a higher exciton generation rate;meanwhile,the efficiency roll-off caused by Auger recombination is significantly reduced during the operation period.Finally,we construct efficient（EQE=21.2%）,high brightness（185682.5 cd/m²）and stable red QLED,and a series of high color rendering（CRI>90）and stable WLEDs with adjustable color temperature（3000-22000K）based on thick-shell QDs.