Hole Injection Enhancement of MoO₃/NPB/Al Composite Anode^*

Abstract

An ultra-thin molybdenum(VI) oxide (MoO3) modification layer can significantly improve hole injection from an electrode even though the MoO3 layer does not contact the electrode. We find that as the thickness of the organic layer between MoO3 and the electrode increases, the hole injection first increases and it then decreases. The optimum thickness of 5 nm corresponds to the best current improvement 70%, higher than that in the device where MoO3 directly contacts the Al electrode. According to the 4,4-bis[N-(1-naphthyl)-N-phenyl-amino] biphenyl (NPB)/MoO3 interface charge transfer mechanism and the present experimental results, we propose a mechanism that mobile carriers generated at the interface and accumulated inside the device change the distribution of electric field inside the device, resulting in an increase of the probability of hole tunneling through the injection barrier from the electrode, which also explains the phenomenon of hole injection enhanced by MoO3/NPB/Al composite anode. Based on this mechanism, different organic materials other than NPB were applied to form the composite electrode with MoO3. Similar current enhancement effects are also observed.