Device relevant organic films and interfaces: A surface science approach G. Koller, S. Berkebile, J. Ivanco, F.P. Netzer, M.G. Ramsey * Institute of Physics, Karl-Franzens-University, A-8010 Graz, Austria Available online 6 July 2007 Abstract Here the morphology, molecular orientation and electronic structure of in situ prepared para-sexiphenyl (6P) and a-sexithiophene (6T) films studied with atomic force microscopy, near edge X-ray absorption fine structure spectroscopy and valence band photoemission are presented. Attention is given to the differences between different organic crystallite orientations and the pitfalls in the interpretation of area averaging surface sensitive techniques that can arise from inhomogeneities in the films, which commonly occur even on single crystal inorganic substrates. The growth of organic–organic heterostructures is then considered for sexithiophene films grown on homo- geneous upright (6P(0 0 1)) and lying (6P(2 0 3)) crystalline films. In both cases, the orientation of the substrate molecules is imposed on the molecules of the second species and thick films of upright-on-upright or lying-on-lying could be produced. The organic substrates are thus shown to be excellent templates for further organic film growth that do not require the stringent UHV conditions of inorganic templates. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Organic interfaces; Organic film growth; Oligothiophenes; Oligophenyls; Photoelectron spectroscopy; Near-edge X-ray absorption spectro- scopy 1. Introduction Driven by the wealth of the possible technological appli- cations conjugated organic materials have been gathering increasing interest worldwide. At present, light emitting and simple electronic devices are on the threshold of being commercially realised, while new applications, such as so- lar cells and the possibilities of organic lasers are being ex- plored. It is now generally recognised that the interfaces in organic devices are crucial to their performance, however, the basic questions of how and why, and to what extent they can be controlled are still open. To address these is- sues requires controlled/reproducible investigations and in the last few years there has been increasing activity in this area involving model molecules on single crystal sub- strates, that is, the surface science approach [1–6]. The fun- damental issues that are important are both electronic and geometric: the relative position of electronic levels (band alignment) is a prime determinant of the charge injection ability, molecular orientation and crystallinity control the light emission/absorption and charge transport, while film morphology is generally important to device construction and function. Moreover, different devices, such as field ef- fect transistors (OFETs) or opto-electronic devices, have different requirements as light emission/absorption is polarised parallel to the molecular axis, while charge trans- port is highest perpendicular to it, as illustrated in the sche- matic of Fig. 1. Thus, the understanding of organic film growth is important for the full potential of organic elec- tronics to be realised. Here, we concentrate on the larger single-linked chain like the molecules sexiphenyl and sexithiophene. These were chosen on the one hand because of the understanding of the interactions of their monomer and dimers obtained in the earlier work [2–6], while on the other hand, they are model molecules that are directly device relevant. Sex- ithiophene (6T) was the active material in the first organic field effect transistor (OFET) with useful characteristics [7], while sexiphenyl (6P) was one of the first blue organic light emitting diodes (OLED) [8]. We have been studying the electronic and geometric structure of films of these 0039-6028/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2007.06.070 * Corresponding author. Tel.: +43 316 380 5203; fax: +43 316 380 9816. E-mail address: michael.ramsey@uni-graz.at (M.G. Ramsey). www.elsevier.com/locate/susc Available online at www.sciencedirect.com Surface Science 601 (2007) 5683–5689