Research On The Key Technologies For Solution-Processed Highly Efficient And Stable OLEDs

As the fourth-generation illumination sources,organic light-emitting diodes(OLEDs)are not only energy-saving,eco-friendly and naturally warm white,but also have the advantages of glare-free emission and no thermal management required.More importantly,the unique flexibility property together with the thinness and lightweight of OLEDs expands the displaying form of light,enabling the light to be vivid and aesthetic.They are promising candidates for major light sources in exhibition,medical treatment,vehicle illumination,aircraft cabins,indoor rooms,etc.The potential market value of OLEDs is approximately several billion dollars order of magnitude,thus studying OLEDs is of great economic and scientific significance.With the rapid developing of technology,cost is the primary restrain for OLEDs to apply widely.Solution-processed fabrication methods are effective approaches to lower the cost for their high utilization ratio of the materials,low invest on the production lines and mass production ability.However,compared to the vacuum-deposited methods,efficiency and lifetime of OLEDs based on solution-processed methods are relatively poor.Thus,this study focuses on the increase of the efficiency and lifetime of solution-processed OLEDs,making attempts from the aspects of the hole injection/transport materials,device structure and transparent electrodes,which are crucial to the efficiency,lifetime,and cost of the OLEDs.Specifically,the copper salts based hole injection/transport materials,organic/inorganic hybrid device structure and the graphene transparent electrodes are well studied here.To take better use of copper salts based hole injection/transport materials in OLEDs,a kind of CuSCN/CuI(copper thiocyanate/copper iodide)composite hole injection/transport material with balanced properties is proposed,in order to solve the low mobility drawback of CuSCN and poor interface properties issue of CuI.The effects of different blending ratios on crystal structure,optical transmittance,bandgap and interface properties of the composites are investigated.It is revealed that the blending mechanism of the composites is physical mixing.And the composites are found to have high transmittance,wide bandgap and tunable surface roughness.Moreover,hole-only devices based on CuSCN/CuI composite materials are fabricated.According to the Mott-Gurney law,the equivalent mobility of the composites is acquired from the space-charge-limited current region of hole-only devices'J-V curves.The work function of the composites is measured by Kelvin probe force microscope.The optimal blending ratio of CuSCN/CuI composite materials is explored and corresponding OLED devices are fabricated based on the optimal blending ratio.Consequently,the efficiency and lifetime of CuSCN/CuI based devices is 27%and 56%higher than that of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)(PEDOT:PSS)based devices respectively.The inverted hybrid OLEDs lacks flexibility in structure designing and its performance is limited by transport materials.In order to solve this problem,a non-inverted organic/inorganic hybrid structure is proposed,which utilizes the solution-processed CuSCN and zinc oxide nanoparticles(ZnO NPs)as the inorganic hole and electron transport layer respectively,and the organic layers are placed in between.The material properties of CuSCN and ZnO NPs including light absorption/transmission,bandgap,injection ability and work function are studied.The results show that CuSCN has low light absorption,high transmittance,wide bandgap,good injection ability and high work function while ZnO NPs have uniform size,good disperse ability and wide bandgap.Then,the energy level diagram and interface properties of the non-inverted hybrid structure are analyzed.It is demonstrated that the non-inverted hybrid structure realizes the confinement of excitons and yields high-quality inorganic transport layers by all-solution-processed method.In addition,OLED devices based on the non-inverted organic/inorganic hybrid structure are fabricated and their luminance and efficiency are as high as 110500 cd/m~2,14.6 cd/A respectively.The stability of such devices is also studied.Due to the synergetic effect of the novel device structure and inorganic transport materials,the lifetime of OLEDs with non-inverted hybrid structure is increased by~18.7 times compared with that of the standard PEDOT:PSS devices.In order to solve the high sheet resistance,low work function and poor wettability problems of graphene transparent electrodes,a method by which graphene is modified with a conductive polymer material PEDOT:PSS through interface engineering is proposed.After analyzing the SEM morphology of the as-grown graphene,it is found that one important reason causing the poor conductivity of graphene is its topological defects from the growth process.And the Raman spectrum of graphene shows a blueshift of the G peak,indicating that the graphene is slightly p-doped and its work function is altered by 0.2 eV.Then,graphene is used as the back gate to fabricate the field effect transistors,through which the carrier mobility and density of a single layer graphene sheet are acquired.The properties of the interface-engineered graphene are studied,showing that the wettability is strengthened,the roughness is lowered and the sheet resistance is decreased by 27.3%while the optical transmittance keeps almost the same with that of commercial indium tin oxide(ITO)electrodes.Flexible OLEDs based on the graphene composite electrodes are fabricated successfully,which have reached6377 cd/m~2 in luminance and 11.5 cd/A in efficiency.Particularly,the efficiency surpasses that of the graphene OLEDs of the same kind reported before by 1 order of magnitude.