Study On High Performance Non-cavity And Microcavity Colloidal Quantum Dot Light-emitting Diodes

Due to their high luminance efficiency,narrow linewidth,saturated color,wide color range,and solution-processed fabrication,light-emitting diodes based on colloidal quantum dots（QLEDs）show great promise for the next generation displays and solid state lighting.Researchers pay much attention on QLEDs with the high development on the synthesis of quantum dots（QDs）and fabricated optimization of QLEDs.Recent years,electroluminescence（EL）performances are largely motivated by the incorporation thick-shell and alloyed-shell into the core/shell QDs which are used as the emitting layer（EML）in devices.The external quantum efficiency（EQE）was enhanced from 0.5%which reported in 2002 to over 20%for red QLEDs.The lifetimes of QLEDs were also been improved.So far,the reported device lifetimes at luminance of 100 cd/m² for red,green,and blue QLEDs are 300000,90000 and1000 hours,respectively.QLEDs show a good prospect in commercial application due to their excellent luminance efficiency and long operating lifetime.However,there are still problems need to solved,for example,the working mechanism of QLEDs.It is a critical problem that how to increase the device performance and color saturation which would limit the commercial development of QLEDs.In this thesis,the works were designed and conducted based on the above questions.The QLEDs were fabricated using low cost solution-process technology and their EL performances were well studied.Meanwhile,optical architecture—microcavity structure was incorporated into the structure of QLEDs.Based on a well-designed microcavity structure,the out-coupling efficiency of device can been enhanced.The current efficiency（CE）,luminance and color-saturation can be improved by well controlled effect of microcavity.Because the carrier transport mechanism and the principle of luminescence of QLEDs are still not clear enough,the structural design of microcavity device is very important for the study on high performance QLED.The main research results of this thesis are as follows:（1）Double hole injection layers（HIL,HAT-CN/MoO₃）were used to enhance the hole injection efficiency of green inverted QLEDs.By thickness optimization of the QLED structure,hole injection current density at voltage of 3 V was enhanced from 36.8 mA/cm² for single HAT-CN device to 86.4 mA/cm² for double hole injection layers device.The calculated hole injection efficiency of double hole injection layers device was over 1-fold higher than that of single HAT-CN device,which much higher than that of single MoO₃ device.The EQE and CE of the best performance device（2.5 nm HAT-CN/1.5 nm MoO₃）are 9.72%and 41.6 cd/A,respectively.（2）In the purpose of reducing the harmful effect of deep blue light to human eyes,we used high efficiency functional graded alloyed core/shell QDs as the emitting layer and different particle size Zn O nanoparticles（NPs）as electric transport layer（ETL）to fabricate high efficient blue QLEDs.By changing the details synthetic progress of Zn O NPs,high performance ZnO NP film was obtained,and surface morphology and mobility of the ZnO NPs layer were improved.The emission peaks of EL spectrum of best performance blue QLEDs is 468 nm.The device reached an EQE of 19.8%,to the best of our knowledge,it is the highest reported value up to now.The Commossion International de I‘Eclairage（CIE）1931color coordinates of the highest efficient QLED is（0.136,0.078）which is very close to the standard（0.14,0.08）of National Television System Committee（NTSC）1953.The color saturation property makes the blue QLEDs to be an important and promising materials for next-generation displays and lighting.（3）Microcavity structure is incorporated into the architecture of QLEDs.The resonant mode and electric field distribution of standing wave were calculated based on the transfer matrix method.Based on a device structure of Glass/DBR/ITO/HTL/QDs/ZnO NPs/Al,the recombination zone of QLEDs can be controlled by changing the position of the EML in the cavity,so as to achieve the purpose of modulating EL properties of QLEDs.The control of optical field of standing wave by microcavity structure greatly adjusting the EL performance of QDs and intensity of which can be largely enhanced.It was found that the peak wavelength of resonance spectrum coincides with EL spectrum of QLEDs without microcavity structure,the EL spectrum was largely narrowed.The full width at half maximum（FWHM）of red microcavity QLEDs is 14 nm,which is 1/3 of that of QLED without microcavity structure.（4）Red,green and blue（RGB）MQLEDs whose emission peak were 624,524and 448 nm,were designed and fabricated based on the structure of QLEDs:Glass/ITO/HIL/HIL/HTL/QDs/ETL/Al.The luminance,color saturation and current efficiency of RGB MQLEDs were much higher than the reference devices without the microcavity structure.These microcavity devices show maximum CE and EQE of 76.6 cd/A and 27.6%for red QLEDs,93.8 cd/A and 23.3%for green QLEDs,and2.69 cd/A and 10.2%for blue QLEDs,respectively.The study of devices with different cavity length shows that the current efficiency of the device deviates from the peak wavelength of the PL resonance peak was decreased.This is mainly due to the viewing function and difference of recombination zone and carrier recombination efficiency caused by different cavity length of the microcavity devices.Those RGB microcavity devices exhibit narrowed EL spectra with a full width at half maximum（FWHM）of 12-14 nm.The CIE 1931 color coordinates of RGB MQLEDs form a superior color gamut as high as 129%of the NTSC 1953 color space while those of normal QLEDs without microcavity structure（NQLEDs）constitute a color space equivalent to 111%of NTSC 1953.