A high molecular weight donor for electron injection interlayers on metal electrodes.

The molecular donor 9,9'-ethane-1,2-diylidene-bis(N-methyl-9,10-dihydroacridine) (NMA) has been synthesized, and its electronic 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 rationalized 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 coverage and clearly exceeds values that would be expected for metal surface electron "push back" alone, confirming the electron 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 barrier (EIB) is reduced by 0.25 eV compared to that on pristine Au(111). Furthermore, the EIB reduction depends linearly on Phi of the donor-modified Au(111) surface, adjustable by NMA precoverage. This enables continuous tuning of the EIB in organic electronic devices, in order to optimize device efficiency and performance.

Energy-level alignment at strongly coupled organic-metal interfaces.

Energy-level alignment at organic-metal interfaces plays a crucial role for the performance of organic electronic devices. However, reliable models to predict energetics at strongly coupled interfaces are still lacking. We elucidate contact formation of 1,2,5,6,9,10-coronenehexone (COHON) to the (1 1 1)-surfaces of coinage metals by means of ultraviolet photoelectron spectroscopy, x-ray photoelectron spectroscopy, the x-ray standing wave technique, and density functional theory calculations. While for low COHON thicknesses, the work-functions of the systems vary considerably, for thicker organic films Fermi-level pinning leads to identical work functions of 5.2 eV for all COHON-covered metals irrespective of the pristine substrate work function and the interfacial interaction strength.

"Soft" metallic contact to isolated C60 molecules.

C60 adsorbed on a monolayer of hexaazatriphenylene-hexanitrile (HATCN) on Ag(111) is investigated by ultraviolet photoelectron spectroscopy (UPS) and scanning tunneling microscopy. UPS and quantum-mechanical modeling show that HATCN chemisorbed on Ag(111) displays metallic character. This metallic molecular layer decouples C60 electronically from the Ag substrate and simultaneously acts both as template for the stable adsorption of isolated C60 molecules at room temperature and as "soft" metallic contact for subsequently deposited molecules.