Molecular orientation dependent energy levels at interfaces with pentacene and pentacenequinone N. Koch a, * , I. Salzmann a , R.L. Johnson b , J. Pflaum c , R. Friedlein d , J.P. Rabe a a Humboldt-Universita ¨ t zu Berlin, Institut fu ¨ r Physik, Newtonstrasse 15, 12489 Berlin, Germany b Universita ¨ t Hamburg, Institut fu ¨ r Experimentalphysik, 22761 Hamburg, Germany c Universita ¨ t Stuttgart, 3. Physikalisches Institut, 70550 Stuttgart, Germany d Department of Physics, Chemistry and Biology (IFM), Linko ¨ ping University, 581 83 Linko ¨ ping, Sweden Received 4 May 2006; received in revised form 26 July 2006; accepted 27 July 2006 Available online 22 August 2006 Abstract We used ultraviolet photoelectron spectroscopy (UPS) to investigate the energy level alignment at contacts between pentacene and Ag(1 1 1) in the presence of interfacial 6,13-pentacenequinone (PQ). Depending on the metal pre-coverage with PQ, we found evidence for three distinctly different interface morphologies and molecular orientations, accompanied by significant changes of the energy level alignment. Consequently, the hole injection barrier between pentacene and Ag(1 1 1) varied between 1.1 eV (pristine Ag) and 0.45 eV (5.4 nm PQ pre-coverage on Ag). In addition, our UPS results suggest that PQ can act as deep trap for electrons in a pentacene matrix. Depending on the exact mutual orientation of PQ and pentacene, the depth of these traps can be in the range of 0.2–0.75 eV. Ó 2006 Elsevier B.V. All rights reserved. PACS: 73.61.Ph; 73.40.Lq; 79.60.i Keywords: Organic semiconductor; Interface; Photoelectron spectroscopy; Electronic structure 1. Introduction Conjugated organic materials receive consider- able attention due to their huge potential for the use as active layers in novel (opto-)electronic appli- cations, such as thin film field effect transistors (FETs) [1–3] or light emitting diodes (LEDs) [4– 8]. In addition to application-oriented studies, sig- nificant efforts are directed towards an understand- ing of fundamental physical properties of organic molecular crystals and thin films, particularly with regard to the nature of charge transport and charge carrier mobility. In order to assess the intrinsic mobility of charge carriers, samples of exceptional high purity are needed [9]. Besides structural defects, chemical impurities can signifi- cantly complicate the evaluation of intrinsic mate- rial properties [10]. Particularly, chemical defects can act as deep charge carrier traps. Pentacene (PEN) is one of the most intensively studied molec- ular materials, since it holds great potential for application in FETs (due to its higher sublimation 1566-1199/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2006.07.010 * Corresponding author. Tel.: +49 30 2093 7819; fax: +49 30 2093 7632. E-mail address: norbert.koch@physik.hu-berlin.de (N. Koch). Organic Electronics 7 (2006) 537–545 www.elsevier.com/locate/orgel