A combined photoelectron spectroscopy and capacity-voltage investigation of the aluminum oligothiophene interface

In order to better understand the electronic barriers at the metal contacts of organic light emitting devices (OLEDs), we investigated interfaces of thin films of endcapped sexithiophene (EC6T) with metal contacts by Ultraviolet and X-ray Photoelectron Spectroscopy (UPS/XPS). We compare the interface of an EC6T film deposited on an Ag(lll) surface and the interface obtained by deposition of Al on an EC6T film. The two situations differ in so far as a strong chemical reaction and diffusion of Al into the EC6T film occurs in the latter. For the EC6T/Ag(111) interface we derive the existence of an interface dipole leading to a "band-offset" directly at the interface. For Al/EC6T, the data suggest a common vacuum level for the reacted molecules and the Al. The band-offset is determined as 1.7 eV. These results are discussed in context with capacity-voltage measurements performed on Al/EC6T/indium-tin-oxide OLEDs. (C) 1999 Elsevier Science B.V. All rights reserved.