Inverted organic light-emitting devices using a charge-generation unit as an electron injector

Efficient green fluorescent and phosphorescent inverted organic light emitting devices (IOLEDs) with a non-doped charge generation unit (CGU) of NPB/MoO₃/Al/LiF as an electron injector have been developed. It is found that the electrons are generated effectively at the NPB/MoO₃ interface and injected from MoO₃ to the electron transporting layer via assistant electron injection layers of Al/LiF. More importantly, the performance of the devices shows insensitivity to the work function of the used cathode. Under the optimal structure of NPB(10 nm)/MoO₃(5 nm)/Al(3 nm)/LiF(1 nm), the green fluorescent IOLEDs present a maximum current efficiency (CE_max) of 3.5 cd/A. Furthermore, the green phosphorescent IOLEDs based on this CGU show a turn-on voltage of 3.8 V, a CE_max of 54.6 cd/A and a maximum external quantum efficiency of 15.8%. The results suggest that the introduction of non-doped CGU in IOLEDs provides a promising strategy to integrate IOLEDs with the n-channel thin film transistors for applications in active-matrix OLEDs.