Pure Mg Cathode for Highly Efficient Single-Layer Organic Light-Emitting Diodes.

Highly efficient single-layer organic light-emitting diodes (OLEDs) are demonstrated by using a pure Mg cathode that is seeded with a small amount of Ag nucleation sites. Bis(4-phenylthieno[3,2-]pyridinato-,C2')(acetylacetonate)iridium(III) (PO-01)-doped devices with three-, two-, and one-region doping configurations exhibit maximum external quantum efficiency (EQE) values of 22.8%, 21.7%, and 11.2%, respectively, which are the highest reported to date for orange single-layer OLEDs. Moreover, the EQE of the one-region device can be raised to 18.2% when using a codopant to balance electron and hole traps. Charge transport characteristics are also investigated by single-carrier devices, which reveal that bipolar hosts with well-balanced hole and electron transport are not a prerequisite for achieving high-efficiency single-layer OLEDs; instead, dopants play a crucial role in determining the location of the recombination zone due to the charge trapping effect.