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Studies On Organic And Organic/Inorganic Hybrid Electroluminescence Devices

Posted on:2017-05-29Degree:DoctorType:Dissertation
Country:ChinaCandidate:J WangFull Text:PDF
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Since organic light-emitting diodes(OLEDs) and quantum-dot light-emitting diodes(QD-LEDs)were introduced, it had attracted more and more attentions due to their low energy consumption, high display quality, low-cost, and lightweight, and so on. So this technology of organic light emitting was thinked the most promising nest generation flat display technology to replace LCD. This paper is focus on the work mechanisms of the OLEDs and QD-LED by designing the device and materials structures. We investigate the mechanism of the exciton quenching in blue OLEDs based on FIrpic as the emission layer. The injection of the carriers and energy transfer in the devices were studied in detailed. In QD-LED, the interface for the exciton formation was investigated and the device performance was highly enhanced by controlling the carrier injection and distribution.Firstly, we focus on the phosphorescent OLEDs that enable harvesting both singlet and triplet excitons to achieve nearly 100% internal quantum efficiency for high efficiency OLEDs. However, the phosphorescent OLEDs suffer from a high efficiency roll-off under high operation current due to the exciton quenching, which is proportional to the square of triplet exciton density. This limits the device performance and leads to a severe efficiency roll-off. Generally, the doped OLEDs are more difficult to adapt for mass production processes than those based on non-doped ones considering the reproducibility of the optimum doping level. Blue phosphorescent organic light emitting devices(OLEDs) was fabricated with commonly used bis(3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl)iridium(FIrpic) doped 4,4-N,N-dicarbazole-biphenyl(CBP) as the emission layer. The efficiency roll-off was reduced with increasing the FIrpic concentration and the lowest efficiency roll-off was obtained with a neat FIrpic emission layer. In our study, it is surprising that the device with a neat 30 nm FIrpic can still maintain high efficiency, 4 7.76 cd/A with the HAT-CN as the hole injection layer.Secondly, we investigate the electroluminescence mechanism in the QD-LEDs. As we know, the electroluminescence(EL) mechanism of the inverted QD-LEDs with a ZnO nanoparticle electron injection/transport layer should be direct charge-injection from charge transport layers into the QDs. Based on this mechanism, we investigated and designed the hole transport layer and hole injection layer in the QD-LEDs. Highly efficient red QD-LEDs with a structure of ITO/Zn O/QDs/mCP/CBP/MoO3/Al were built. In the stepwise hole-transport layers consisting of CBP combined with N,N′-dicarbazolyl-3,5-benzene(mCP), the mCP layer plays two important roles in this kind of QD-LEDs. One is that it can block the electron to leak into the HTL due to its higher LUMO(LUMO = the lowest unoccupied molecular orbital) energy level than that of CBP; and the other is it can separate the carrier accumulation zone from the exciton formation interface, which is attributed to the stepwise hole-transport layer structure. Moreover, the lower HOMO(HOMO = the highest occupied molecular orbital) energy level of mCP decreases the hole-injection barrier from the HTL to the QD emitting layer, which improves the charge carrier balance injected into the QD layer, reducing the turn-on voltage of QD-LEDs fabricated with the stepwise HTL structure. In addition, the performance of QD-LEDs is significantly enhanced with ethanol treated(ET- treated) PEDOT:PSS as the hole injection layer(HIL). We demonstrate that the improvement of the conductivity of PEDOT:PSS film is due to the decreased PSSH amount after solvent treatment, reducing the thickness of PSSH insulating shell, leading to an efficient charge transport across the PEDOT chains.Finally,it has been demonstrated that the confined radiation field influences the emission lifetime of a fluorescent dye located near a metallic mirror greatly. Noting that this high reflective metal electrode in common QD-LEDs also acts as a mirror that reflects the light from the QDs, altering the distribution of the confined radiation field or local density of optical states(LDOS). In typical QD-LED architectures, the excitons are located within tens of nanometers from the high reflective electrode and the decay rate of excitons must be influenced by the nearby metallic mirror due to interactions with the reflected confined electric field. Time-resolved photoluminescence and electroluminescence measurements were used to explore the emission characteristics of excitons in QD-LEDs. It is found that the lifetime of excitons in the QDs can be varied by adjusting the distance between the excitons and metal Al mirror, which is due to the effect of local density of optical states(LDOS) on the exciton decay rate. The efficiency and efficiency roll-off in QD-LEDs were improved by optimizing the device structure.
Keywords/Search Tags:Organic light emitting diodes, Quantum-dot light emitting diodes Triplet-triplet exciton annihilation, Efficiency roll-off, Auger recombination, Localized density of optical states
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