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 t hin films of endcapped sexithiophene (EC6T) with metal contacts by Ultravio let and X-ray Photoelectron Spectroscopy (UPS/XPS). We compare the interfac e of an EC6T film deposited on an Ag(lll) surface and the interface obtaine d 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 a n interface dipole leading to a "band-offset" directly at the interface. Fo r 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 dis cussed in context with capacity-voltage measurements performed on Al/EC6T/i ndium-tin-oxide OLEDs. (C) 1999 Elsevier Science B.V. All rights reserved.