Tandem Organic Light-emitting Devices Based On New Charge Generating Unit

Because of their excellent properties, such as high luminance, low weight, wide viewangle, flexibility, et al. Organic light-emitting devices (OLEDs) have attractedconsiderable attention due to their potential applications in future full-color displayand the next generation of solid-state lighting. Over the last two decades, OLEDshave developed from fundamental research to commercial products. To further satisfythe requirement of commercial demand, more research should be made on theimprovement of OLEDs’ efficiency and lifetime. Tandem OLEDs, possessing highbrightness as well as high-efficiency at low current density, provide a promisingapproach to solve above problem. It has been demonstrate that efficient chargegenerating unit is the key to realize high-performance tandem OLEDs. To this end,this paper focuses on the p-type doping in organic semiconductor and color-stable andefficient tandem white OLEDs based on new type charge generating layer.We have demonstrated color-stable and efficient tandem organic light-emittingdevices using Bphen:LiF/m-MTDATA: MoOxas charge generating unit (CGU),which has the advantages of air stability and ease of fabrication, the workingmechanism of Bphen:LiF/m-MTDATA:MoOxis also discussed through analyzing theelectrical and spectral emission properties of tandem devices with different CGUs.The performance of tandem white OLED, comprising blue and yellowphosphorescent EL units, can be improved by optimizing the thickness of Bphen:LiFlayer. The device comprised of30nm Bphen:LiF layer has a maximum currentefficiency of38.7cd/A and it can still maintain24.6cd/A at the luminance of10370cd/m2. Moreover, the Commission Internationale de L’Eclairage (CIE) coordinates ofthe device are rather stable and the variation is only (±0.003,±0.007) over a widerange of luminance (100-13000cd/m2).Organic light-emitting devices based on molybdenum oxide (MoOx) doped4,4’-bis(carbazol-9-yl)biphenyl (CBP) as a p-type doping hole injection layer wasdemonstrated. The devices comprise the following structure:ITO/MoOx/CBP:MoOx/CBP/CBP:tris(2-phenylpyridine)iridium(III)(Ir(ppy)3)/4,7-diphenyl-1,10-phenanthroline(Bphen)/LiF/Al, where CBP acts as hole injection layer(HIL) and hole transport layer (HTL) as well as the host metarial of emitting layer(EML). The simple structure also has the the benefit of lowing the injection andtranspotring barrier of holes. With the thickness of HIL increasing, the current densityof device increases, indicating the p-type doping layer has a good performance ofenhancing the holes injection. The performance of the device can be improvedthrough optimizing the thickness of HIL and HTL. The maximum current efficiencyof optimized device is29.8cd/A, which can be attributed to the more balanceddistribution of carrier in EML. It is noteworthy that the efficiency roll-off ofoptimized device is only17.7%between maximum and20000cd/m2, comparing tothat of conventional device (62.1%). The result shows that MoOxis simple butefficient p-type doping material, which make it possible in the use of charge generating unit in tandem devices.