Improving Performance Of Quantum Dot Light-Emitting Diodes By Regulating Hole Transport Layers

Quantum dots?QDs?have attracted wide attention in the field of display due to their extraordinary attributes such as high fluorescence quantum yield,narrow emission spectra and tunable emission spectra.In particular,QDs can be used to manufacture low-cost,large-area and self-luminescent quantum dot light-emitting diode?QLED?display screen,which is expected to be one of the mainstream technologies of large-screen televisions.To become a commercialized technology,QLED faces two challenges including low efficiency and rapid roll-off.In most reported efficient QLEDs,the electron injection is more intensive than the hole injection,which limits the device efficiency and lifetime.Therefore,it is one of the most important subjects to improve device performance by regulating charge transport layers,especially by modulating hole transport layers?HTLs?.Firstly,we studied the impacts of the mixtures of the small-molecule and polymer hole transport materials on the performance of the QLED devices based on a single layer of HTL.In the inverted QLEDs,a small molecular material TAPC with high hole mobility was used to physically blend with polymer PVK by solution processing.The PVK:TAPC mixture was then used as the HTL for the inverted red QLEDs,aiming at optimizing the device performance.At the blend mass ratio of 2:1,the turn-on voltage of the device decreased from 4.0 V for standard device?with only PVK as HTL?to 2.4 V,the maximum current efficiency(CE_max)reached19.26 cd/A,which was 50%higher than that of the standard device.The maximum power efficiency(PE_max)was significantly increased from 5.18 lm/W to 16.72 lm/W.And the optimized device showed a significant improvement in lifetime.These are mainly attributed to the enhancement of hole injection,which contributes to charge balance in the inverted red QLED.Then,the PVK:TAPC?2:1?blended HTL was further applied into the inverted green/blue QLEDs.It could be observed that the turn-on voltages of the devices with PVK:TAPC?2:1?decreased significantly compared to those standard devices,whatever the emitting layer are red,green or blue QDs.Yet the improvement of current efficiency decreased with the blue shift of QDs.This is attributed to the difference of the energy band structures of the three types of QDs,which results in different electron and hole injection densities for the charge balance states of the corresponding QLEDs.In the normal QLEDs,three kinds of small molecular materials,TAPC,26DCzPPy and TCTA with deeper HOMO levels,were blended with the polymer poly-TPD with shallower HOMO level.The result showed that the device based on poly-TPD:26DCzPPy:TAPC?10:1:1,mass ratio?as HTL had a 14%increase in CE_maxax and 21.1%increase in PE_max compared to that of standard device?with only poly-TPD as HTL?.However,the QLED based on poly-TPD:TCTA as HTL showed an insignificantly performance improvement.This is mainly due to the intrinsic high hole mobility of poly-TPD,thus the small molecules have little effect on the hole injection enhancement.Secondly,we studied the impacts of the mixture of the blue phosphorescent small molecules and the polymer hole transport material on the performance of the QLED devices based on double layers of HTL.To fabricate poly-TPD/PVK double HTLs,1,4-dioxane with strong polarity was selected for the solvent for PVK,chlorobenzene for poly-TPD,avoiding the erosion of the underlying poly-TPD layer when spin coating PVK solution due to the orthogonal solvent effect.The fabricated poly-TPD/PVK co-HTLs showed a uniform surface morphology and stepwise energy levels,which facilitated hole transport.In addition,a blue phosphorescent dye FIrpic was incorporated into PVK,and the formed PVK:FIrpic mixture showed a deep HOMO level and sensitization property.At the blend mass ratio of 10:2,the CE_max and EQE_maxax of the red QLED reached 18.7 cd/A and 13.1%,respectively,almost two-fold higher than that of the standard device with only poly-TPD as HTL.Even compared to those with poly-TPD/PVK co-HTLs,the performance was increased by²0%.A series of photophysical tests showed that PVK:FIrpic?10:2?acted as a sensitizer for the red QDs,and the efficient non-radiative energy transfer occured between PVK:FIrpic and QDs,which is the reason for the further performance improvement.