Fabrication Of High-efficiency Blue Quantum Dot Light Emitting Diodes And Their Working Mechanism

Colloidal quantum dots（QDs）synthesized by the solution method have attracted a lot of attention because of their high photoluminescence quantum yield（PLQY）,good photochemical stability,high colour purity and tunable emission spectra.Compared with traditional electroluminescent technology,QDs-based light emitting diodes（QLED）have the advantages of good colour saturation,high luminous brightness,wide spectral tunability and low processing cost,making it show broad application prospects in the field of display and lighting.Over the past three decades,device performance has steadily improved with the continuous optimisation of QDs synthesis technology and device structures,as well as a deeper understanding of the physical mechanisms of the devices.The brightness,external quantum efficiency（EQE）and operating lifetime of red and green QLED are now comparable to those of organic light emitting diodes（OLED）.However,the overall performance of blue QLED still lags behind that of red and green QLED and is a major bottleneck in the commercialisation of QLED.Therefore,a deeper understanding of the physical mechanisms of blue QLED and the need to improve device performance through materials and device engineering are important prerequisites for the industrialisation of QLED.From the point of view of QDs materials,in order to improve the PLQY and stability of QDs,usually epitaxial growth of thicker Zn S shell layer on the nucleus and other wide band gap semiconductor materials.Therefore,the current blue QLED light-emitting layer of QDs are all core-shell structure.However,when QDs are used in QLED,the impact of the shell layer thickness on the device performance is very complex because the electroluminescence（EL）process is closely dependent on the PLQY and charge injection of the QDs.That is,both the PLQY and charge injection of QDs are closely related to the shell layer.Therefore,an in-depth understanding of the effect of shell layer thickness on the charge injection kinetics of blue QLED is necessary and urgent.In terms of device structure,most of the reported high-performance blue QLED are organic-inorganic hybrid devices.However,the deep valence band energy levels of blue QDs make them poorly matched with the hole transport layer（HTL）,leading to problems such as carrier injection imbalance in blue QLED.Although the performance of the devices has been greatly improved by optimising the apparent charge balance characteristics,there is still a lack of in-depth understanding of the charge carrier distribution and dynamics in blue QLED.In particular,the EL turn-on process is closely related to the charge carrier injection and distribution,energy transfer and charge injection.Therefore,the EL turn-on process can be studied to resolve the charge carrier dynamics in the device and thus provide a more effective strategy for further optimisation of the device performance.Based on the above considerations,this thesis investigates the working mechanism in blue QLED in terms of both the shell layer thickness of quantum dots and the EL turn-on mechanism of the device.The three main parts of the work are as follows.（1）Blue Zn_xCd_1-xSe_yS_1-y/Zn S QDs with different shell thicknesses were synthesised and their properties were characterised.Firstly,a series of Zn_xCd_1-xSe_yS_1-y gradient alloy core QDs with different emission wavelengths were synthesized by thermal injection method by regulating the ratio of anions and cations,and the optimal growth conditions for the cores were determined.Then,in order to further passivate the defects on the surface of the QDs,the epitaxial growth of wide band gap Zn S shell layers was continued at high temperature.Finally,blue Zn_xCd_1-xSe_yS_1-y/Zn S core-shell QDs with fluorescence peaks at 479-489 nm and shell thicknesses of 0.5 nm（thin）,1.25 nm（moderate）and 2.25 nm（thick）were synthesized.The PLQY of the QDs increases from 60%to 82%as the shell layer thickness increases.However,when the Zn S shell layer thickness is too thick,the PLQY decreases again to 79%.This is because an excessively thick Zn S shell layer leads to an increase in interfacial defects,which reduces the PLQY.（2）Inverted blue QLED were prepared using the above-mentioned blue QDs with different shell layer thicknesses as the light-emitting layer,and the mechanism of the effect of shell layer thickness on carrier injection and QLED performance was investigated by introducing a polyethyleneimine（PEI）electron-blocking layer in the Zn O/QDs interface.The devices based on moderate QDs with moderate shell layer thickness showed the best device performance with a maximum current efficiency of 7.19 cd/A,a peak EQE of 7.83%and a maximum luminance of 32530 cd/m².As the shell layer thickness continued to increase,the device performance showed a decline.Transient EL tests reveal the effect of shell layer thickness on electron injection kinetics:electron injection is more susceptible to QDs shell layer thickness than hole injection.（3）The EL turn-on mechanism of blue QLED was investigated by preparing blue QLED based on different HTL and introducing bis（4,6-difluorophenylpyridinato-N,C2）-picolinatoiridium（FIrpic）probe layers.The results show that the EL turn-on of blue QLED is highly dependent on the charge transport polarity of the HTL.The EL turn-on of blue QLED based on bipolar HTLs is always accompanied by an energy transfer process from the HTL to the blue QDs.In contrast,for unipolar HTL-based devices,the EL turn-on is achieved by direct charge injection.Also the performance of blue devices is governed by the HTL.QLED based on 2,2’-Bis（4-（carbazol-9-yl）phenyl）biphenyl（BCBP）show the best performance,with a maximum EQE of 13.18%,which is the highest efficiency available for inverted deep blue QLED.In contrast,for unipolar HTL-based devices,the large hole injection barrier leads to a severe accumulation of holes at the HTL/QDs interface,which in turn leads to severe electron leakage in the HTL,making the devices less efficient.This work shows that limiting carriers to quantum dots and reducing the hole injection barrier by optimising the QDs（including the shell layer and ligands outside the QDs）and the HTL（including hole mobility,energy level and electron-blocking capability）is a more desirable strategy to improve the performance of blue QLEDs than using electron-blocking layers to modulate charge transport.