Oxide insertion layer in organic semiconductor devices

We investigate the counterintuitive phenomenon of inserting a transition metal oxide layer to improve hole injection or extraction in organic semiconductor devices using ultraviolet photoemission (UPS), x-ray photoemission (XPS), and inverse photoemission spectroscopy. We observe that metal oxides, such as MoO3 and WO3, substantially increase the work function when deposited on indium tin oxide. The increase lifts up the highest occupied molecular orbital (HOMO) of the hole transport layer and therefore reduces the energy barrier between the HOMO and the Fermi level of the anode. The uplift creates an interface band-bending-like region that results in a drift electric field, which encourages the collection of holes at the anode. The optimum thickness for the oxide layer is estimated to be 2 nm. We also investigate the effects of air and O2 exposure of MoOx films. We observe that while most of the electronic energy levels of the oxide remain largely intact, the work-function reduction is substantial. The UPS and XPS data indicate that chemisorption is the major contributor of the work-function reduction. The reduction can be seen in two stages: initially dominated by oxygen adsorption at exposure <1013 L, and finally saturated by moisture at 1014 L.