Impact of electrode contamination on the α-NPD/Au hole injection barrier

This paper addresses the effects of ambient exposure of a polycrystalline Au electrode prior to making contact with the hole-transport material N,N′-diphenyl-N,N′-bis (1-naphthyl)-1,1′-biphenyl-4,4′-diamine (α-NPD). Ultraviolet photoemission spectroscopy (UPS) and current-voltage (I-V) measurements are used to investigate the resulting hole barrier and charge injection characteristics. UPS measurements show that the hole barrier with the contaminated low work function Au electrode is reduced by 0.4-0.6 eV with respect to the barrier with the clean high work function Au electrode. The corresponding interface dipoles are 0.3 eV for the former and 1.3 eV for the latter. I-V measurements confirm this unexpected change in barrier. These results are accounted for using the induced density of interface state model, and considering the role of the contamination layer in reducing the direct interaction between metal and molecules.