Large-area Fabrication Of Quantum Dot Light-Emitting Diodes And The Performance Modification By Two-Dimensional Materials

Quantum dot light-emitting diodes(QD-LEDs)not only have the advantages of high color saturation,good stability,self-emission and easiness to adjust emitting color,but also can be fabricated on flexible substrates by solution methods in large area.Therefore,they have great potential to be the next generation of lighting and display technology,and show broad application prospects in the wearable electronics,health monitoring and so on.However,there are still many challenges in the large-area fabrication on flexible substrate and performance improvement of QD-LEDs.For example,due to the inhomogeneous thickness distribution,the commonly used spincoating method is difficult to prepare uniform film in large area,and the solution utilization rate is below 10%.The other solution processing methods,such as inkjet printing and spray coating,generally suffer from coffee-ring effect,therefore are difficult to obtain large-area uniform and compact films with smooth surface.In addition,the charge injection imbalance greatly affects the device performance.In order to solve the above problems,the centrifugal coating method was developed to fabricate large-area QD-LEDs on a flexible substrate,and the modification for the charge injection/transport layers of QD-LEDs by two-dimensional MoS2/C-QDs and graphene oxide(GO)was established to improve the charge injection balance,enabling the high efficiency QD-LED devices.The main results are as follows:(1)Centrifugal coating method was developed to achieve the large-area fabrication of QD-LEDs on flexible substrates.Due to the large equivalent gravity field and in-situ heating,centrifugal coating method can effectively reduce the thickness of liquid film and the contact angle with the substrate,to increase the solvent evaporation rate,which can effectively suppress the coffee-ring effect caused by solute particle migration and gathering.As a result,the large-area fabrication of various nanometer-thick films including poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate)(PEDOT:PSS),polyvinylcarbazole(PVK),ZnCdSe/ZnSeS/ZnCdS QDs and ZnO nanocrystalline films on flexible substrate was realized by centrifugal coating method.The solution utilization rate was up to 61.2%,and the films were uniform,compact and smooth.Furthermore,these thin films were stacked layer-by-layer by centrifugal coating method to produce large-area flexible QD-LEDs.The stacks showed high quality interfaces.The maximum current efficiency(CE),power efficiency(PE)and external quantum efficiency(EQE)of the obtained red QD-LEDs were 5.7 cd A-1,3.2 lm W-1 and 4.9%,respectively,which are comparable to the device fabricated by spin coating in ambient condition.The maximum working area can reach～115 cm2,which are～300 times higher than those flexible devices fabricated by spinning coating.Importantly,the passive matrix red/green QD-LED displays were realized by an automatic driving system to control the on/off of every emitting unit,which show excellent bending stability and uniform lighting.(2)Local high temperature and high pressure induced by cavitation effect of probe sonication were first proved effectively to cause the carbonization of the organic solvents such as N-methyl-2-pyrrolicone(NMP)to produce carbon quantum dots(CQDs).Taking advantage of this phenomenon,one pot preparation of hybrid MoS2/CQDs was realized through exfoliation of MoS2 with probe sonication in NMP.By introducing these hybrid MoS2/C-QDs into PEDOT:PSS hole injection layer,the hole injection efficiency of the device was improved obviously because MoS2 QDs and CQDs have high carrier mobility and work function,respectively.When the concentration of MoS2/C-QDs was 0.2 mg mL-1,the turn-on voltage of the device was reduced from 2.4 V to 2.3 V,and the maximum CE and EQE of the device were 25.7 cd A-1 and 17.6%,respectively,which were 10.3%and 17.3%higher than those of the device with PEDOT:PSS hole injection layer.(3)The GO dispersed in water was successfully transferred to ethanol by solvent replacement method.The monolayer GO dispersed in ethanol showed diameter distribution of 1～5 μm.The GO in ethanol maintains uniform dispersion and good structural integrality as that in water.Making use of the insulated and passivation nature of GO,the incorporation of GO into ZnO electron transport layer can effectively block the electron injection,which solves the problem that the electron injection efficiency is significantly higher than the hole injection from hole transport layer to balance the charge injection of QD-LEDs.When the concentration of GO was 0.1 mg mL-1,the maximum CE and EQE of the device achieved 34.1 cd A-1 and 25.3%,respectively,which were 36.4%and 36.8%higher than those of the device with ZnO electron transport layer.