Study on MoO3−x films deposited by reactive sputtering for organic light-emitting diodes

The authors investigate the role of reduced molybdenum trioxide [MoO3−x (x≤1)] films in organic light-emitting diodes, particularly from the viewpoint of the oxidation state of Mo. MoO3−x films were deposited by reactive sputtering under a mixture of argon (Ar) and oxygen (O2). The O2 gas-flow ratio (GFR) [O2/(Ar+O2)] was adjusted between 10% and 100%. Mo with six, five, and four valence electrons was detected in MoO3−x film deposited with an O2 GFR of 10% and 12.5%, whereas, under higher O2 GFRs, only six valence electrons for Mo in the MoO3−x film were detected. N,N′-di(1-naphthyl)-N,N′-diphenylbenzidine (α-NPD) layer, hole-transport material, were deposited over the MoO3−x layer by subsequent vacuum evaporation. At the α-NPD/MoO3−x interface, it was found that α-NPD cations were generated and that MoO3−x was reduced, which provided evidence of charge transfer across the interface by Raman spectroscopy and x-ray photoelectron spectroscopy.