Tandem OLED stack is ultra-flexible and efficient
They devised a vertical stack of flexible tandem OLEDs using a modified flexible graphene anode instead of the more commonly used but brittle indium–tin oxide (ITO), as well as a unique lithium nitride-based charge generation layer sandwiched between the two OLED stacks.
This configuration increased the luminous current efficiency and external quantum efficiency over equivalent standalone devices. It also reduced the efficiency roll-off at high luminance.
They explain that the low-temperature processed charge generation layer not only provided efficient stacking of the two electroluminescent units, but it also enhanced the compatibility of the flexible device on a thin plastic substrate for in-line mass production. To reach their goal, the researchers formed the charge generation layer (CGL) through a stable co-deposition of the organic electron transporting layer and an n-type metal dopant in a single organic chamber.
They fabricated flexible green phosphorescent tandem OLEDs with p-type-doped four-layered graphene electrodes, on PET and glass substrates.
The tandem OLEDs with a 4LG anode on PET exhibited better luminescent characteristics than the OLEDs realized on glass. What’s more, thanks to the CGL generating and separately injecting opposite charges into the upper and lower OLED stacks (both OLEDs emitting light at the same time), the tandem OLEDs also had better luminescent characteristics at the same current density compared with single-unit OLEDs of the same stack.
The tandem OLEDs require much less current density than single-unit OLEDs to emit a same luminance, they found, with a lab prototype supporting a luminance of 10 000 cd m−2 at a low current densities of about 5.2 mA cm−2 (on PET) and 5.5 mA cm−2 (on glass). This is to compare with almost double the current density required by conventional single-unit OLEDs realized with the modified 4LG anode to output the same light. They could operate the flexible tandem OLEDs as they were wrapped around tight radius cylinders (only a few millimetres in diameter), demonstrating the graphene anode’s high flexibility and resilience even under a bending strain of nearly 7%.
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