DOI: 10.1002/cphc.200900472 A High Molecular Weight Donor for Electron Injection Interlayers on Metal Electrodes Benjamin Brçker, [a] Ralf-Peter Blum, [a] Luca Beverina, [b] Oliver T. Hofmann, [c] Mauro Sassi, [b] Riccardo Ruffo, [b] Giorgio A. Pagani, [b] Georg Heimel, [a] Antje Vollmer, [d] Johannes Frisch, [a] Jürgen P. Rabe, [a] Egbert Zojer, [c] and Norbert Koch* [a] 1. Introduction In the field of organic electronics much progress has been made during the past decade for devices such as organic light emitting diodes (OLEDs), field effect transistors and photovol- taic cells. [1, 2] Common to most of these devices are the interfa- ces between metal or transparent conductive oxide electrodes and conjugated organic materials. These interfaces are of huge importance for device performance, because they are often the bottleneck for charge carrier injection. [2–4] In the case of OLEDs, the most simple device structure exhibits 1) a high work function (F) anode, 2) the active material, which is usual- ly sandwiched in between hole and electron transporting and/ or blocking layers, and 3) a low F cathode. [4] Since high F ma- terials are usually less chemically reactive than low F ones, there exists a tradeoff between (oxidation-related) lifetime and performance at the cathode side of the device. To obtain good electron injection despite high F cathode materials, several methods have been suggested. These include employing thin layers of alkali halides [5, 6] or using alkali/alkaline-earth metals [7, 8] or small molecular donors [9–11] as “electrical dopants” for the electron transport layers. The drawback of using alkali/alkaline- earth metal atoms is their tendency to diffuse throughout or- ganic layers, thus rendering devices unstable over time. [12] To circumvent this problem, a molecular donor layer at the cath- ode side may be used instead of alkali/alkaline-earth metal layers. For instance, it has been shown that the exposure of indium-tin-oxide electrodes to the vapor of tetrakis(dimethyl- amino)ethylene can yield electron injection barriers similar to those achieved with aluminium due to a strong interfacial chemical reaction. [13] The same molecule decreases F of poly- crystalline gold by 1.3 eV and an effective F of 3.9 eV was ob- tained. [14] In a previous report we have shown that a thin layer of methyl viologen (MV0) on metal single crystals can reduce F of a Au(111) surface to values of even 3.3eV. [15] This presents a promising way for using high F metal cathodes that are easy to handle but still provide efficient electron injection when an appropriate donor layer is applied. Herein, 9,9’-ethane-1,2-diylidene-bis(N-methyl-9,10-dihydro- acridine) [NMA] was chosen as molecular donor because its chemical structure exhibits pronounced similarities to the The molecular donor 9,9’-ethane-1,2-diylidene-bis(N-methyl- 9,10-dihydroacridine) (NMA) has been synthesized, and its elec- tronic properties were characterized both in solution using cyclic voltammetry and optical absorption spectroscopy, and at interfaces to metals with photoelectron spectroscopy (PES). The optical energy gap of NMA in solution increases by 0.10 eV when the compound is doubly oxidized. On the basis of quantum-chemical calculations, this ipsochromic effect is ra- tionalized by a change in geometry involving a severe torsion of the two acridinium moieties with respect to the central double bond, thus reducing conjugation upon oxidation. PES is reported for NMA deposited on Au(111), Ag(111), and Cu(111) single crystals. A decrease of the sample work function is observed that becomes larger with increasing molecular cov- erage and clearly exceeds values that would be expected for metal surface electron “push back” alone, confirming the elec- tron donating nature of NMA. The growth mode of NMA on all three surfaces is almost layer-by-layer (Frank–van der Merwe). For tris(8-hydroxyquinoline)aluminum (Alq 3 ) deposited on top of a NMA-modified Au(111) surface, the electron injection barri- er (EIB) is reduced by 0.25 eV compared to that on pristine Au(111). Furthermore, the EIB reduction depends linearly on F of the donor-modified Au(111) surface, adjustable by NMA pre- coverage. This enables continuous tuning of the EIB in organic electronic devices, in order to optimize device efficiency and performance. [a] B. Brçker, Dr. R.-P. Blum, Dr. G. Heimel, J. Frisch, Prof. Dr. J.P. Rabe, Dr. N. Koch Institut für Physik, Humboldt-Universität zu Berlin Newtonstrasse 15, 12389 Berlin (Germany) Fax: (+ 49) 30-209-37632 E-mail : n.koch@physik.hu-berlin.de [b] Dr. L. Beverina, Dr. M. Sassi, Dr. R. Ruffo, Prof. Dr. G. A. Pagani Department of Materials Science and INSTM State University of Milano-Bicocca Via Cozzi 53, 20125 Milano (Italy) [c] O. T. Hofmann, Prof. Dr. E. Zojer Institut of Solid-State Physics, Graz University of Technology Petersgasse 16, 8010 Graz (Austria) [d] Dr. A. Vollmer Helmholtz-Zentrum Berlin für Materialien und Energie GmbH Bessy II, Albert-Einstein-Str. 15, 12489 Berlin (Germany) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.200900472. ChemPhysChem 2009, 10, 2947 – 2954  2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 2947