Manipulation of interface electronic structure by thin metal oxide films

Abstract

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 MoO3, substantially increase the work function when deposited on indium-tin-oxide (ITO). 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 of the anode. The uplift creates an interface band bending region that results in a drift electric field that encourages the collection of holes at the anode. The optimum thickness for the oxide layer is estimated to be 2 nm. We have also investigated the effects of ambient or O2 exposure of MoO3. We observed that while most of the electronic energy levels of the oxide remained largely intact, the work function reduction was significant. This opens a way for optimal energy level alignment by modifying the work function through exposure. Furthermore, we observed that the work function reduction by exposure could be reversed by proper annealing of the sample in vacuum. The investigations therefore point to manipulate the interface electronic structure and charge injection/extraction by thin metal oxide films.