Highly efficient bi-directional organic light-emitting diodes by strong micro-cavity effects

We report on highly enhanced light outcoupling of bi-directional red phosphorescent organic light-emitting diodes (BiOLEDs) by strong micro-cavity effects. Inserting additional metal layers between the indium tin oxide (ITO) anode and the hole transporting layer (HTL) and controlling the thickness of the counter metal electrode result in dramatic changes in electroluminescence spectra and angular distribution. We find that the external quantum efficiency (EQE) for both bottom and top emitting side can be significantly improved: the overall EQE reaches up to 24.24%.