Construction And Performance Research Of Quantum Dot Light-emitting Diodes Based On Thermal Cross-linked DV-CBP Hole Transport Layer

Featured with excellent properties of pure emission color,narrow emission linewidth,emission wavelength tunability,easy fabrication,and high photo-stability,quantum dots-based light-emitting diodes(QLEDs)have become one of the most potential candidates for next-generation display and solid-state lighting technologies.Although the luminance and external quantum efficiency(EQE)of QLED devices have been significantly improved,there are still some problems in hole transport layer(HTL).One is that a large amount of joule heat will be generated in QLED devices under high current density,the other is that quantum dots have similar solubility to HTL materials,which is very easy to occur in the process of device preparation.In order to solve these problems,people usually adopt the method of cross-linking in recent years.Cross-linking refers to the formation of new covalent bonds or relatively short chemical bond sequences that link polymers together under the stimulation of ultraviolet/heat,the cross-linked films have good thermal stability and solvent resistance.Cross-linking is divided into optical cross-linking and thermal cross-linking,compared with optical cross-linking,the advantage of thermal cross-linking is that the cross-linking reaction can be carried out without the need of additives and by-products,and the film after cross-linking has good flatness,it is more advantageous to the hole injection and transmission,and also improves the stability of the device.In this work,QLED device is fabricated using a low-temperature cross-linked small molecule with a deep HOMO energy level.Two vinylbenzyl ether groups containing carbazole derivatives(DV-CBP)are dissolved in chlorobenzene solution,and the film is annealed at 150℃for 30 min.The maximum EQE of DV-CBP-based red,green and blue QLED devices are 27.01%,20.52%and 17.94%respectively,which are significantly improved compared with PVK-based standard devices.The research content of this paper is mainly divided into the following two aspects:(1)The properties of cross-linked DV-CBP films were investigatedThe thermal properties of DV-CBP were analyzed by differential scanning calorimeter(DSC)and thermogravimetric analyzer(TGA).The results show that the films have good thermal stability.The cross-linked DV-CBP film was prepared by heat treatment,and the film was tested by optical microscope and UV-vis.The film was rinsed with chlorobenzene before and after cross-linking,and the cross-linked film had good solvent resistance.Then the film was cleaned with the common organic solvents in the laboratory.It is found that the cross-linked film has good resistance to the common organic solvents in the laboratory.Atomic Force Scanning probe microscopy(AFM)and contact angle measurements were performed on the films before and after crosslinking.The results show that the films after crosslinking have a good surface morphology and form a more compact film,moreover,it has good wettability to the solvents commonly used in quantum dots,which meets the basic requirements of QLED device preparation.The HOMO energy level of the cross-linked DV-CBP film was calculated to be-5.94 e V,which can effectively match the energy level of quantum dots and lower the injection barrier.(2)Application of cross-linked DV-CBP film in QLEDFirstly,we fabricated the blue QLED device based on DV-CBP with different concentration,different speed and different annealing temperature,and screened out the optimal condition,next,we fabricated the QLED devices with small batch of blue QDs.The maximum EQE was 17.94%and the maximum brightness was 13200 cd/m~2,which was obviously improved compared with the standard devices based on PVK.In order to investigate the main reasons of improving the performance of blue QLED devices,we fabricated the hole-only devices(HOD)based on DV-CBP and PVK and calculated the hole mobility of DV-CBP by using the space charge-limited current(SCLC)method,we found that the hole transfer of DV-CBP-based blue QLED devices is more efficient than PVK-based standard devices,the hole mobility of DV-CBP is about three orders of magnitude higher than that of PVK,the excellent hole transport ability of DV-CBP is further proved.The results of time-resolved photoluminescence spectroscopy(TRPL)and device lifetime tests show that DV-CBP doesn’t cause additional exciton quenching in QD layer after cross-linking,the lifetime of cross-linked devices has also been greatly enhanced.Then we fabricated the red and green QLED devices under the optimal conditions,and the maximum EQE can reach 27.01%and20.52%respectively,which was significantly improved compared with the PVK-based devices.