Fabrication And Characterization Of Highly Efficient Blue Quantum Dot Light-Emitting Diodes

In recent years,light-emitting diodes based on colloidal quantum dots(QD)have attracted increasing attention because of their excellent properties such as high color saturation,tunable emission wavelength,favorable stability and economical solution-processability.These advantages mentioned above enable quantum dot light-emitting diodes(QLED)to provide reliable technical support for next-generation display.Nowadays,with the rapid development of QD materials and device structure,the performances of red and green devices have been significantly enhanced.For instance,QLEDs with external quantum efficiency(EQE)reaching 20.5%and 21%for red and green have been reported,which make QLEDs meet the requirement of practical applications.Nevertheless,there is still substantial room for promoting the efficiency and the stability of blue QLEDs.In this work,a fluorescence dye 4-(dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidin-4-yl-vinyl)-4H-pyran(DCJTB)doped into the carrier transport layer(CTL)was used as a sensor to detect the exciton recombination zone;the Forster resonant energy transfer process(FRET)between QD and probe was monitored to determine the location of the exciton recombination zone.During this process,we found that the electron transport layers(ETL)have great impact on the recombination zone.These are two conclusions:1.In QLEDs with ZnMgO ETL,its exciton recombination zone is restricted in the QD layer,and close to the interface of QD/hole transport layer(HTL),thus the emission are mainly contributed by the direct charge recombination;2.Unlike normal structure,in devices with 2,2',2"-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole(TPBi)ETL,the recombination zone extends to ETL,which indicates the photons are generated from both FRET and the direct carrier recombination.Based on our previous study on emission mechanism of blue QLEDs,we know that the hole injection is more difficult than electron injection and there is an imbalance of carrier injection.Then we systematically study the influence of various HTLs and their thickness on performance of devices,aiming to achieve carrier balance and enhance the efficiency of QLEDs.For normal structure,the devices with poly(9-vinylcarbazole)(PVK)exhibit the highest EQE of 8.6%and turn-on voltage of 3.2 V,while devices with poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4'-(N-(4-sec-butylphenyl)diphenylamine)](TFB)exhibit the lowest turn-on voltage of 2.4 V and EQE of 2.8%.By combining the advantages of PVK and TFB,devices with TFB/PVK bilayered HTL simultaneously exhibit a low driving voltage of 2.6 V and a high EQE of 5.9%.For inverted devices,we insert an interlayer 1,3-Bis(carbazol-9-yl)benzene(mcp)between emission layer and electron blocking layer to further prevent the excess electron.As a result,the leakage current is reduced and the performance of devices is enhanced.The optimized blue inverted QLEDs show a current efficiency of 4.2 cd/A and a maximum brightness over 20000 cd/m2.During the optimization of blue QLED,we also observe an exciplex emission at the interface of HTL/QDs.The intensity of exciplex emission can be tuned by modulating the hole injection.By using PVK doped with 25wt%poly(3-hexylthiophene)(P3HT)as HTL,exciplex emission is significantly enhanced at low driving voltage while QD emission is dominant at high driving voltage.By combining the exciplex emission and the QD emission,the emission color can be effectively tuned from red to blue when the driving voltage is changed from 2 to 10 V.