Fabrication Of Highly Efficient Blue-Violet Quantum-Dot Light-Emitting Diodes Via Controlling The Surface/interfaces Of Organic Hole Transport Layers

Inorganic semiconductive fluorescence nanocrystals(quantum dots,QDs)exhibit the unique optoelectronic characteristics,including high photoluminescence(PL)quantum yield(QY),high color purity,size-controlled tunable emission spectra,high photochemical and thermal stability,low-cost solution-processability,and scalable production of high-class QDs.Therefore,light-emitting diodes based on QDs(QD-LEDs)have been the most promissing in the application of the solid-state lighting and next generation flat panel displays.Since the initial demonstration of QD-LEDs,the maximum external quantum efficiency(EQE)of QD-LEDs has been elevated to 23.68% from 0.01%,which is comparable with that of the state-of-the-art organic light-emitting diodes(OLEDs).This is attributed to great efforts on synthsizing high-quality QDs,developing proper device architeture and understanding the fundamental device physics.However,these QD-LEDs with excellent performance are fabricated in N2-filled glovebox due to the instability of organic hole transport layers(HTLs).Also,the influence of different ambient gases(N2 and air)on the performance of QD-LEDs has not been systematically studied.These factors limit the application in the actual large-scale production.Furthermore,the performance of blue-violet QD-LEDs still cannot meet the requierments of application,which is due to the lower PL QY of QDs,larger potential energy barrier between QDs and HTLs,and imbalance of hole-electron injection.On the basis of the above problems,the work in this paper is given as follows.(1)We studied the influence of the change of organic HTLs or HTL/QDs interfaces under the different ambient gas on the performance of blue QD-LEDs.Considering the intrinsic sensitivity of organic HTLs(poly(9,9-dioctylfluorene-co-N-(4-butylphenyl)diphenylamine,TFB)to atmospheric oxygen/moisture and the stability of inorganic QDs(ZnCdSe/ZnS)and Zn O in ambient air,the key points are narrowed down to mainly studying the effect of ambient gas on the TFB layer and/or TFB/QDs interfaces.Firstly,when TFB layer is alone treated in air,the devices exhibit a low current density,relative to the reference devices(that were fabricated in N2-filled glove box).Meanwhile,the maximum efficiency decreases by ~ 14%,but the maximum luminance of 10500 cd/m2 is still on a par with that of reference devices(10900 cd/m2).As the humidity increases from 30% to 70%,the average peak value of EQE decreases from 8.6% to 5.7%.Furthermore,in light of the influence of ambient gas on the performance of devices,QD-LEDs with films of TFB/QDs/ZnO fabricated in air were also tested.The results suggest that the maximum EQE decreases by 12% and turn-on voltage increases by 0.3 V.However,for the devices fabricated in air,relatively high efficiency could be maintained only at low voltages,because of the near balanced injection of carriers under low bias.By analyzing the surface potential,surface morphology and chemical composition of films,we attribute the reduced efficiency to the electron-hole injection imbalance,which originates from the changes such as the energy alignment of layers because of oxygen adsorption and relatively rough surface of films.These results would offer reasonable guidance for the application of QD-LEDs in actual large-scale production.(2)Highly efficient blue light-emitting diodes based on non-blinking QDs via modifying the interface using poly(methyl methacrylate)(PMMA).Blue non-blinking(>99% ‘on' time)ZnCdSe/ZnS//ZnS QDs with high absolute PL QY of 92%(?PL peak=472 nm)were synthesized via a low temperature nucleation and high temperature shell growth method.Such bright non-blinking ZnCdSe/ZnS//ZnS core/shell QDs exhibit not only good emission tunability in the blue-cyan range with corresponding wavelength from 450 to 495 nm,but also high absolute PL QY and superior chemical and photochemical stability.Highly efficient blue QD-LEDs have been demonstrated by using non-blinking ZnCdSe/ZnS//ZnS QDs as emissive layers and the crucial development we report here is the ability to dramatically enhance the efficiency through introducing an insulating layer of PMMA as the interface layer between electron transport layer(ETL)and QD layers,where this interface layer takes the role of moderately impeding electron injection and thus improving charge-injection balance within QD active layer.The best device displays remarkable features like maximum luminance of 14100 cd/m2,current efficiency of 11.8 cd/A and EQE of 16.2%.Such blue QD-LEDs show a 65.3% increase in EQE relative to QD-LEDs without PMMA.Importantly,the peak efficiency of the QD-LEDs with PMMA is achieved at about 1000 cd/m2,and high efficiency(EQE>12%)can be maintained in the range of 100 to 3000 cd/m2.Additionally,QD-LEDs utilizing an optimized device structure display the maximum EQE of 13.2% and 15.8% by adopting QDs with PL emissions centered at 454 and 495 nm,respectively.These results suggest that such outstanding performance of blue QD-LEDs would be promising in the application of the solid-state lighting and next generation flat panel displays.(3)Based on the high quality ZnSe/ZnS QDs,highly efficient and bright violet cadmium-free QD-LEDs have been obtained via controlling the interface potential barrier of organic HTLs.First,the performance of QD-LEDs based on the various hole transport materials has been studied,including poly(N,N?-bis(4-butylphenyl)-N,N?-bis(phenyl)benzidine)(poly-TPD),TFB,poly-N-vinylcarbazole(PVK)and the mixture of TFB and PVK.We find that higher brightness and low turn-on voltage violet QD-LEDs could be obtained when TFB was used as the HTLs,while,QD-LEDs with high efficiency but low brightness and high turn-on voltage could be obtained when PVK was used as the HTLs.Interestingly,the devices show an excellent performance(relatively high brightness and EQE and low turn-on voltage)when the mixture of PVK and TFB was adopted as the hole tranport layers.Then,the mixing ratio of TFB and PVK was further optimized.The results show that the best ratio of TFB is 10 wt.%,when the brightness and EQE of QD-LEDs reach up to 2856 cd/m2 and 7.39%,which represent 113-fold increase in brightness and 10-fold increase in device efficiency compared with previous reports.However,when TFB/PVK consecutive films were used as HTLs,the performance of QD-LEDs has not been obviously improved,which suggests that the performance enhancement of devices via using the TFB-PVK mixture as HTLs is arised from the higher hole mobility of TFB and lower HOMO of PVK in one film.These results offer a practicable platform for the realization of heavy-metal-free QD-based violet display and lighting.