Band alignment in organic devices: Photoemission studies of model oligomers on In2O3

Abstract

The interfaces of In2O3, a model for indium–tin–oxide (ITO), with benzene, thiophene, and benzaldehyde, models for technologically important organic molecules, are studied using angle resolved ultraviolet photoemission and work function measurements. Band alignment diagrams for hypothetical Al/organic/ITO devices have been drawn, using values determined from this work and previously published studies of these molecules on Al(111). The similarity between the bonding of benzene and thiophene on Al(111) and In2O3, i.e., largely electrostatic, leads to near identical alignment at both metal and oxide interfaces. This indicates that clean Al and ITO will make a very poor electron/hole injecting pair. We suggest that the apparent efficiency of Al as an electron injecting contact in real devices is due to the presence of oxygen at the Al/organic interface. For benzaldehyde the interaction with In2O3 is largely electrostatic, in contrast to the covalent bonds formed on Al(111). This leads to very different alignment at the Al and oxide interfaces, showing the importance of the particular organic–inorganic interaction in determining band alignment.