Conjugated Polymer Surfaces and Interfaces for Light-Emitting Devices

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rs can be used as the active component in LEDs.615 The PPV films provide relatively high quantum yield for electro- or photoluminescence in the yellow/ green portion of the visible spectrum.7'812 Although Tr-conjugated polymers may be used to provide light from somewhat different portions of the visible spectrum,6'10-111516 the present discussion is focused on PPV, which remains the most investigated and exploited polymer material. An important factor in determining the overall quantum efficiency of polymer LEDs is the injection of electrons and holes at the respective metalpolymer interfaces.1517 Typically but not exclusively, indium tin oxide (ITO)coated glass is used as the hole-injecting contact, one that also allows visible light to escape from the active light-generation medium. The counter electrode is usually a metal with low work function, enabling electron injection into the conduction band of the polymer medium. In this context, the injection efficiency of the minority carrier may be one major limiting factor in overall device efficiency. Over the past several years, the combined experiment-theory approach to the study of polymer surfaces and interfaces has been used in studies of a wide variety of ^-conjugated polymers and model molecules, as well as the early stages of metal-on-polymer (and modelmolecule) interface formation.1718 During the course of these studies, certain general trends have emerged. Generally

these studies have focused upon PPV," poly(2,5-diheptyl-l,4-phenylene vinylene),20'21poly(2,5,2',5'-tetrahexyloxy-8,7'dicano-diphenylvinylene),22 which is a CN-substituted PPV, and several alkylsubstituted polythiophenes.23"27 In addition, certain molecules have been studied in the condensed molecular state as model systems for the conjugated polymers: a diphenylpolyene model molecule for PPV—namely a, aj-diphenyltetradecaheptaene,28"30 abbreviated as DP7— and a model molecule for the polythiophenes— a-sexithiophene, or a6T.31 The metal atoms on surfaces of ultrathin polymer films or of the condensed molecular solids include aluminum, sodium, calcium, potassium, and rubidium. In all cases experiments were carried out using ultraviolet and x-ray photoelectron spectroscopy (UPS and XPS) in ultrahigh vacuum (UHV), for which the pressure P < 10"10 Torr.17'18'32 Quantumchemical modeling used in the interpretation of spectra was often carried out on the basis of the valence effective Hamiltonian (VEH)33'34 calculations. The VEH method has an excellent record of providing reliable estimates of ionization potentials, bandwidths, and bandgaps for a wide variety of conjugated polymers. In the works reviewed, the geometries used for the VEH calculations35 are obtained from full molecular-geometry optimizations with the Modified Neglect of Differential Overlap (MNDO) or Austin Model 1 (AMI) Hartree-Fock semiempirical techniques. In addition, because it is appropriate for treating metal atoms or ions, density functional theory is being used increasingly.36 Sodium on a Model Molecular System for PPV Although t

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