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.