Analysis of metal-oxide-based charge generation layers used in stacked organic light-emitting diodes

We study electron and hole injection in MoO3 charge generation layers (CGLs) commonly used for establishing balanced injection in multilayer stacked organic light-emitting diodes (SOLEDs). A compound CGL consisting of 100-A degrees-thick MoO3 and Li-doped 4,7-diphenyl-1,10-phenanthroline in a 1:1 molar ratio is demonstrated to have a high electron generation efficiency. Charge injection from the compound CGL is modeled based on a two-step process consisting of tunneling-assisted thermionic emission over an injection barrier of (1.2 +/- 0.2) eV and a trap level due to oxygen vacancies at (0.06 +/- 0.01) eV above the MoO3 valence band edge. Peak external quantum efficiencies (EQEs) of (10.5 +/- 0.2)%, (10.1 +/- 0.2)%, (8.6 +/- 0.2)%, and (8.9 +/- 0.2)% are obtained for tris-(phenylpyridine)iridium-based electrophosphorescent OLEDs with indium tin oxide (ITO) anode/CGL cathode, CGL anode/CGL cathode, CGL anode/Al cathode, and ITO anode/Al cathode contacts, respectively. Based on our analysis, a three-element green emitting electrophosphorescent SOLED is demonstrated with a peak forward-viewing EQE=(24.3 +/- 1.0)% and a power efficiency of (19 +/- 1) lm/W.