Construction Of High-performance Quantum Dot Light-Emitting Diodes Based On Photo-Thermal Synergistic Cross-Linked Hole Transport Layer

Quantum dot light-emitting diodes(QLED)showing promising prospects in next-generation display applications because of unique advantages such as tunable emission wavelength over the entire visible region,extremely narrow bandwidths,superior color purity,high material stability and low-cost solution processing technologies.In QLED applications,the multilayer device structures are mainly prepared by solution processing.However,during the solution processing,the previously deposited layer can be eroded by the solvents when depositing the next layer due to their similar solubility,which significantly reduce the performance of QLEDs.To solve this problem,use of cross-linkable materials is particularly promising because it can prepare films with good solvent resistance without limiting the solvents used in subsequent layer deposition.Besides,insoluble and high thermal stability films are obtained by forming new covalent bonds between molecules through thermal or light activation.An added advantage is that the crosslinked strategy can prepare films with good solvent resistance without limiting the solvents used in subsequent layer deposition,and the crosslinked hole transport layer(HTL)films usually has a smooth surface morphology,which can promote the hole injection into quantum dots(QDs)and process of the QD emitting layers.In this work,taking the advantage that vinyl polymerization produces no by-products,4,4`-bis(3-vinyl-9H-carbazol-9-yl)-1,1`-biphenyl(CBP-V)radical crosslinking is realized under a lower temperature by UV assisted initiation without photoinitiator.Here,the vinyl group in the molecule can effectively improve the solubility of the whole molecule and the cross-linked films were qualified with smooth surface morphology,excellent hydrophobicity and solvent resistance.Hence,red,green and blue QLEDs with photo-thermal synergistic cross-linked CBP-V as HTL achieved maximum external quantum efficiency(EQE)of 25.69%,24.42% and 16.51%,respectively,which are much better than the reference devices.The main contents of this thesis are as follows:(1)Properties of the photo-thermal synergistic cross-linked CBP-V films:Optical microscope observation,Fourier transform infrared spectroscopy(FT-IR)and Raman spectroscopy were employed to prove the cross-linking reaction was carried out.The surface morphology of the cross-linked CBP-V thin films were studied by atomic force microscope(AFM).The solvent resistance of the cross-linked thin films was evaluated by ultraviolet-visible absorption(UV-vis).The result indicated that the cross-linked CBP-V thin film exhibited smooth surface morphology,denser three-dimensional structure and excellent solvent resistance,and these properties will improve the quality of subsequent deposited QDs layer.The ultraviolet photoelectron spectroscopy(UPS)was employed to investigate the energy level of CBP-V polymer.The HOMO level of CBP-V is-6.0 e V,which is closer to the valence band of quantum dots,indicating that the cross-linked CBP-V as HTL can effectively improve the injection efficiency of hole carriers and improving the balance of charge injection.The CBP-V films had suitable surface properties for application as HTL in solution-processed QLEDs.(2)QLEDs based on the photo-thermal synergistic cross-linked CBP-V hole transport layer.:The CBP-V polymer has a unique deep HOMO energy level(-6.0 e V),which may be more suitable for blue quantum dots,so we first constructed the blue QLED devices.We optimize the thickness of the CBP-V film,and the highest maximum EQE of 16.51% was achieved.The improvement mechanism of the blue QLED devices was investigated.The surface morphology of the CBP-V thin films before and after crosslinked were studied by AFM.To investigate the charge transfer from the QDs layer to HTL,the time-resolved photoluminescence(TRPL)of QDs on different substrates were measured.The hole-only devices(HODs)were fabricated to explore the hole transport property of the crosslinked CBP-V films.The space-chargelimited current(SCLC)model was introduced to calculate the hole mobility of CBP-V.The lifetime of crosslinked CBP-V based blue QLED devices was tested using the QLED device lifetime testing system.In view of the excellent hole transport performance of CBP-V,we constructed the red and green QLED devices based on the cross-linked CBP-V hole transport layer.The red and green QLEDs based on the cross-linked HTLs achieved maximum EQE of 25.69% and 24.42% respectively,which are much better than the reference devices.Finally,combined with the advantages of photo-thermal synergistic cross-linked low temperature treatment,a flexible QLED device based on cross-linked CBP-V hole transport layer was constructed.