Interface Behavior Of Organic Light-Emitting Diode And Its Effects On The Performance Of Devices

The Organic Light-Emitting Diodes (OLEDs) can be widely applied to the solid-state lighting, flat panel displays, as well as the LCD backlight source because its low power-consumption, fast response and surface-emitting characteristics. There are still some problems to be solved for industry applications of OLED, especially for those related to OLED efficiency and lifetime. The uses of light emitting material with high fluorescent efficiency, the multilayer structure controlling exciton recombination and interfacial modification through metal cathode or ITO anode, have been proven to be a very effective method to improve the efficiency of the device. Firstly, molecule-molecule interface in light-emitting layers has been studied. Through optimized device structure and improved carrier balance device, the study aims at maximizing the probability of exciton generation and recombination. Secondly, from the viewpoint of organic layer-organic layer interface, we have developed a device with a light emitting layer structure of the nanopillar, and improved light out-coupling to some degree. Finally, the characteristics and mechanism of device degradation due to interfacial degradation has been proposed to avoid the degradation of device driven under a long operation time.The main research contents include the following three aspects. By investigating on the carrier transporting ability and the light emitting characteristics of the phosphorescent material doped organic thin film, we have found that10%of the Ir(ppy)3doped hole-only device can improve the conductivity of holes, whereas it inhibits the transport of electrons. For the film of Ir(ppy)3doped CBP with different concentrations, its fluorescence lifetime and PL emission are varied, which is mainly caused by the exciton quenching. According to optimized device structure, we have prepared a low efficiency roll-off green phosphorescent OLED. At low luminance, the maximum current efficiency reached52.5cd/A, and at the high brightness of10000cd/m2the device efficiency is also higher than30.0cd/A. Further, the triplet exciton space barrier layer has been designed for achieving a Pt-4based phosphorescent OLED. We found that the species and thickness of the carrier transporting materials adjacent to light-emitting layer can effectively affect the transfer of the triplet energy to the adjacent function layers; thereby reducing the loss of energy and efficiency roll-off at high luminance. The triplet-triplet annihilation of exciton is different for different phosphorescent emttiers, in which that of Ir complex is heavier than that of Pt complex. Next, by patterning the light-emitting layer/carrier transporting layer interface with nanopillar for the device,ITO/NPB/CBP:Ir(ppy)3/TPBi/Alq3/LiF/Al, the enhancement in light extraction efficiency of the device has been demonstrated. Nanopillars were prepared on both sides of the light emitting layer of the device having two different nano-pillar array modes with varied number and height. We have found that the use of nanopillar could increase the exciton recombination zone, therefore improve the light extraction efficiency to some extent to. In addition, the improvement in light efficiency is the proportion of the height and spacing of the nanopillars.Finally, in order to study the impact factor of device deterioration, oblique cutting technology has been used by observing each oblique-cutting point of light-induced degradation. By testing the PL intensity of each point for operated device compared to device without operated, we found that except a slight degradation at the interface device between the light emitting layer and a hole transport layer, the device has no significant degradation. The author also studied the PL decay at70℃, the results showed that in addition to the carrier injection layer/transport layer interface, temperature had no effect on the device degradation. We concluded that the degradation of the device at high temperatures is mainly because the carrier accumulation at the interface with energy barrier, as well as a low glass transition temperature. The effects of heat-induced degradation characteristics and active light-emitting area have been studied too.