Study On The Performance Improvement Of Simplified White Organic Light-emitting Diodes

With twenty years of rapid development, organic light-emitting diodes(OLEDs) possess great potential application in flat-panel displays and solid-state lighting, due to their high-efficiency, active light emitting, large area, flexible property, highbrightness and high contrast ratio, and so on. They have condensed great tremendous attentions manpower and materials resources, and achieved great progress, so that OLEDs are known as the symbol in display area. However, there are lots of difficult problems in the commercialization, the main problems are the complicated structures, and the unmanageable manufacture process, which aggravate the cost of the devices fabrication. In this work, we studied the way to achieve high-efficiency, low efficiency roll-off and color stability of simplified white OLED. We also discussed the emission mechanism.Firstly, we fabricated simplified OLEDs based on double layers structure. The structure efficiently takes advantage of the better carriers transporting property of the blue phosphorescent material, which can obtain high efficiency and low efficiency roll-off. Meanwhile, the host material was also used as the hole transporting layer, avoiding the effects of the multi-interface to on the device turn-on voltage and the efficiency roll-off in the traditional device based on multiple emission layers. The maximum power efficiency and current efficiency of the yellow OLED obtains 46.5 lm/W and 51.5 cd/A, respectively. By changing the doped concentration of the yellow dye, we obtained white OLED with Commission Internationale de I’Eclairage coordinates of(0.39, 0.46), and current efficiency of 41.6 cd/A, which is highly improved compared to the conventional white device with two emission layers.Then, we fabricated efficient white OLEDs with excellent stable spectra based on an ultrathin non-doped emission layer and a doped emission layer. The emission layer consists of PO-01 ultrathin non-doped layer and FIr Pic doped layer, which are separated by spacer of CBP and mixed interlayer of CBP:TPBi. By changing the thickness of the CBP spacer or the mixed ratio of CBP and 2,20,200-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole)(TPBi) in the mixed interlayer, The resulting device achieves color stable white light emission with the color shift of(-0.005, +0.002) within a large luminance range. The maximum efficiency are 38.3 cd/A(40.1 lm/W).Finally, by optimizing the thickness of the non-doped ultrathin emission layer, we fabricated red, orange, green and blue monochrome OLEDs based on non-doped ultrathin structure, and the maximum efficiencies are 19.3 cd/A(17.3 lm/W), 45.7 cd/A(43.2 lm/W), 46.3 cd/A(41.6 lm/W) and 11.9 cd/A(9.2 lm/W), respectively. Moreover, by changing the thickness of spacer between the ultrathin emission layers, we fabricated highly efficient and color stable two-color, three-color and four-color white OLEDs, and the maximum efficiencies achieve 30.9 cd/A(27.7 lm/W), 30.3 cd/A(27.2 lm/W) and 28.9 cd/A(26.0 lm/W), respectively. And we also discussed the position of the exciton recombination zone and the emission mechanism of the designed monochrome and white devices. We concluded that the white OLED based on ultrathin emission layers can be regarded as the device with a single-emission layer doped with multiple dyes, and the incomplete energy transfer among the different emission dyes contributes to the color stable white light emission.