The Effective Way To Suppress Efficiency Roll-Off In High Brightness Organic Light-Emitting Diodes

Recntly the organic light-emitting diodes(OLEDs)have attracted tremendous attention owing to their ultra-thin,large-area light sources,high luminance efficiency,color-tunability,and the high mechanical flexibility,and are generally considered as next-generation full-color displays and solid-state lighting sources.Currently,small OLED panels are already being utilized for cellular phones,however,future applicationos of OLEDs micro displays and general illumination require brightness levels that by far exceed typical values of mobile phone displays.Optimizing OLED performance at high brightness will thus most likely be critical to the success of the technology beyond conventional display applications.The important challenge for high-brightness applications is to reduce the efficiency roll-off of OLEDs.In this context,we develop fundamental and applicable research combining OLEDs theory,electroluminescent mechanism and process.Some results have been obtained as follows:In this thesis,a detailed description of the different mechanisms involved in efficiency roll-off to is firstly investigated.The dominant mechanisms leading to effi ciency roll-off at high brightness are different for fluorescent and phosphorescent emitters:in phosphorescent emitter systems,triplet-triplet annihilation(TTA)and charge carrier imbalance appear to be the most important.Fluorescent emitters instead suffer especially from singlet-triplet annihilation,and singlet-polaron annihilation,but loss of charge carrier balance at high currents can also be an issue in these systems.singlet-singlet annihilation,triplet-polaron annihilation,field-induced quenching,Joule heating,and changes in outcoupling were discussed as further loss mechanisms.In addition,we also discuss several routes for further reduction of the efficiency roll-off in OLEDs.TTA can be reduced by decreasing the exciton lifetime,reducing molecular aggregation,or broadening the recombination zone.Research should focus on highly conductive ambipolar materials,which can serve as transport and as matrix materials and which thus can lead to broader exciton formation profiles.More attention should also be paid to the charge carrier balance and to possible enhancement of this balance at high current densities.Then,the influence of sulfide-based double electron injection layers(D-EILs)on the roll-off has been studied for blue fluorescent inverted OLEDs.The D-EILs composed of metal sulfide and cesium carbonate(Cs2CO3)-doped 4,7-diphenyl-1,10-phenanthroline(Bphen)is presented.We demonstrate the effects of the thickness and species of sulfide film on the electroluminescence characteristics of blue florescent devices.By using(2-nm-thick metal sulfide)/Bphen:Cs2CO3 as D-EILs,the low operating voltage,high electron/photon conversion efficiency and high power efficient of the blue electroluminescence device are obtained.Notably,our device exhibits a surprisingly roll-up character and a beyond theoretical limited ?EQE of 6.9%at 1000 cd m-2,and 7.5%at 10 000 cd m-2.The formation of a favorable interfacial dipole layer at the metal sulfide-organic interface was shown to be the main reason for the improved performance of IOLED with D-EILs.Moreover,OLEDs with inverted architectures provide longer device lifetime because water-and oxygen-sensitive electron injection materials can be kept beneath the organic and metal layers.Considering molybdenum disulfide(MoS2)with graphene-like structure,we have systematically investigated the electrooptical properties of MoS2 and applied it in developing highly efficient and stable IOLEDs.We have demonstrated that MoS2-based IOLEDs can significantly improve the electron-injecting capacity.For the MoS2-based device on plastic substrate,we have achieved a very high external quantum efficiency of 7.3%at the luminance of 9141 cd m-2,which is the highest among the flexible blue fluorescent IOLEDs reported.Also,an approximately 1.8-fold improvement in power efficiency was obtained compared to the glass-based IOLED.We attributed the enhanced performance of flexible IOLED to MoS2 nanopillar arrays due to its light extraction effect.The van der Waals force played an important role in the formation of MoS2 nanopillar arrays by thermal evaporation.Notably,MoS2-based flexible IOLED exhibits an intriguing efficiency roll-up,that is,the current efficiency slightly increases from 14.0 to 14.6 cd A-1 with the luminance increasing from 100 to 5000 cd m-2.Besides,we have successfully fabricated a transparent flexible IOLED with low efficiency roll-off at high current density.Furthermore,the suppression of roll-off characteristics of electroluminescence at high brightness levels in phosphorescent OLEDs by using thermally activated delayed fluorescence material as host has been studied.A novel bipolar hosting material,11-(3-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-1 2,12-dimethyl-11,12-dihydroindeno[2,1-a]carbazole(DPDDC),was designed,synthesized,and characterized for green phosphorescent OLEDs.The DPDDC exhibits excellent hole and electron transport properties,superior thermal stability,high glass-transition temperature and small singlet-triplet energy gap for efficient reverse intersystem crossing from triplet to singlet,reducing triplet density of the host for phosphorescent OLEDs.The electrophosphorescence properties of the devices using DPDDC as the host and three green phosphorescent iridium(?)complexes,bis(2-(4-tolyl)pyridinato-N,C2')iridium(?)acetylacetonate,bis(2-phenylpyridine)iridium(?)acetylacetonate,and bis(4-methyl-2,5-diphenylpyridine)iridium(?)acetylacetonate[(mdppy)2Iracac]as the emitter were investigated.Green phosphorescent OLED with 5 wt%(mdppy)2Iracac presents an excellent performance,including high power efficiency of 92.3 lm W-1,high external quantum efficiency of 23.6%,current efficiency roll-off as low as 5.5%at 5000 cd m-2 and a twentyfold lifetime improvement(time to 90%of the 5000 cd m-2 initial luminance)over the reference electrophosphorescent device.Finally,research have focused on reduced efficiency roll-off in high-efficiency hybrid white OLEDs.We have discussed the method of achieving white OLEDs,however,typically consist of multiple emitter materials with different charge transport characteristics and different energy levels.Thus,we have successfully fabricated a highly efficient red phosphorescent OLEDs with low roll-off by using DPDDC as host.we have achieved a very high external quantum efficiency(EQE)of 20.7%,the efficiency slightly decreases from 20.7%to 18.3%with the luminance increasing from 10 to 3000 cd m-2.More importantly,we have demonstrated hybried white OLEDs based on RGB and RGBG structure.The RGBG-based white OLED exhibits an excellent performance,such as high EQE of 19.9%and roll-off as low as 3%at 3000 cd m-2.Even at high luminance,the EQE maintained still quire high and is comparable to that of the state of the hybrid white device,reaching 18.1%at 5000 cd m-2.In addition,we have also developed white OLEDs with roll-up characteristics by employing an exciton-sweeping layer.The current efficiency slightly increases from 43.7 to 51.5 cd A-1 with the luminance increasing from 100 to 5000 cd m-2.Even at ultra-high luminance of 10 000 cd m-2,the EQE still maintained at 48.2 cd A-1.Nevertheless,highly efficient,low roll-off in high-color quality white demands further investigations.