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The structure and stability of dimers of quaterthiophene radical cations are studied using density functional calculations. Several dimer structures are identified that are metastable in vacuum, in which the cations are stacked coplanarly. Evidence for a weak chemical dimer bonding is found from the binding energy curves and the singlet-triplet splittings. Introducing solvent or solid-state effects by embedding the dimer in a dielectric medium results in stabilizing the dimer, as is observed experimentally. An infinite stack of quaterthiophene cations shows a weak spontaneous dimerization. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The energy level alignment in metal-organic and organic-organic junctions of the widely used materials tris-(8-hydroxyquinoline)aluminum (Alq3) and 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTCDA) is investigated. The measured alignment schemes for single and bilayer films of Alq3 and NTCDA are interpreted with the integer charge transfer (ICT) model. Single layer films of Alq3 feature a constant vacuum level shift of ∼0.2-0.4 eV in the absence of charge transfer across the interface. This finding is attributed to the intrinsic dipole of the Alq3 molecule and (partial) ordering of the molecules at the interfaces. The vacuum level shift changes the onset of Fermi level pinning, as it changes the energy needed for equilibrium charge transfer across the interface. [ABSTRACT FROM AUTHOR]

The atomic structure of interfaces between conducting electrodes and molecular organic materials varies considerably. Yet experiments show that pinning of the Fermi level, which is observed at such interfaces, does not depend upon the structural details. In this letter, we develop a general model to explain Fermi level pinning, and formulate simple expressions for the pinning levels, based upon integer charge transfer between the conductor and the molecular layer. In particular, we show that DFT calculations give good values for the pinning levels. [ABSTRACT FROM AUTHOR]

We develop an integer charge transfer model for the potential steps observed at interfaces between donor and acceptor molecular semiconductors. The potential step can be expressed as the difference between the Fermi energy pinning levels of electrons on the acceptor material and holes on the donor material, as determined from metal-organic semiconductor contacts. These pinning levels can be obtained from simple density functional theory calculations. [ABSTRACT FROM AUTHOR]