Metal-Oxides Hole Injection/Extraction Layer in Organic Semiconductor Devices

We have investigated the counter intuitive phenomenon of inserting a metal oxide layer to improve hole injection or extraction in organic semiconductor devices using ultraviolet photoemission, x-ray photoemission, and inverse photoemission spectroscopy (UPS, XPS and IPES). We observed that metal oxides such as MoOx substantially increase the workfunction. The increase lifts up the highest occupied molecular orbital (HOMO) of the hole transport layer, therefore reduces the energy barrier between the HOMO and the Fermi level. The uplift creates an interface band bending region that results in a steering electric field that encourages the collection of holes at the anode. We have also investigated the effects of ambient and O2 exposure of MoOx. We observe that while most of the electronic energy levels of the oxide remain largely intact, the workfunction reduction is substantial. The effect of annealing MoOx thin films in air will also be discussed.