We demonstrate that charge-transport in partially-ordered thin films of poly(3- hexylthiophene) is strongly dependent on the charge-carrier density in the film, which we relate to the localized nature of the charge carriers. We base our conclusions on measurements in which the charge-carrier density is varied by electrostatic means (in a transistor geometry) and by in- situ thermal removal of oxygen dopants. For the doping levels investigated, we argue that the removal of oxygen does not induce structural changes in the conjugated polymer, and that it only shifts the Fermi-level of the film.

In order to investigate the capabilities and limitations of organic semiconductor devices high-quality single crystals of these materials have been grown and their properties have been investigated. The observation of the Integer and Fractional Quantum Hall effect in organic semiconductors reveals the similarity to conventional inorganic semiconductor materials, such as Si or GaAs. In addition, electronic as well as optoelectronic devices based on such crystals have been prepared leading to the demonstration of ambipolar field-effect transistors and electrically- pumped solid state lasers.

The interaction between thin films of ethylenedioxythiophene (EDOT) and polycrystalline copper and gold surfaces has been studied using photoelectron spectroscopy. Thick films of EDOT (∼100 Å) have been prepared by vapor deposition onto clean gold surfaces, which were cooled down to a temperature of 170 K during the deposition process. Monolayers were prepared by slowly heating the thick films up to 300 K. At 300 K most of the material has evaporated from the surface and about one monolayer remains chemisorbed on the gold surface. This shows that there is an interaction between EDOT and Au. This chemisorption causes a shift of around -0.5 eV of the binding energies for the core level electrons, presumably because of screening of the core-hole by the metal. An experimental and theoretical analysis of the valence level electrons suggests that two molecular orbitals, localized at the thiophene part of the molecule, are involved in the interaction with the metal atoms of the surface. The most likely orientation of the EDOT molecules is parallel to the Au surface. Upon adsorption the work function is changed from 5.2 eV for the clean gold surface to 4.0 eV for the EDOT monolayer. In the case of EDOT adsorbed on clean copper surfaces, no interaction was observed.

The organic highly conducting Langmuir-Blodgett (LB) film formed by a molecular association of BEDO-TTF and stearic acid shows the logarithmic decrease of DC conductivity and negative magnetoresistance at low temperature. These data are interpreted by the weak localization of two-dimensional (2D) electronic system inside the homogeneous conducting layer of BEDO-TTF molecules. The electronic length with phase memory has a mesoscopic scale. Strong evidence of the 2D coherent charge transport in the conducting LB film is provided for the first time.

Organic electroluminescent (EL) diodes based on fluorescent dyes and conducting polymers have attracted the interest of researchers, mainly because of their emission in the visible region and for application to large area portable flat panel display devices, driven at low voltages. Therefore, for the development of higher efficiency polymer EL diodes, the optimal combination of the merits of organic fluorescent dye molecules with that of conjugated polymer is an important approach. We report electroluminescence studies of polymer light emitting diodes (p-LEDs) fabricated with poly[2-(3-thienyl)ethanol n-butoxy carbonylmethyl urethane] (PURET) and its composite with 4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H pyran (DCM) dye. These materials have been chosen in view of the fact that PURET exhibits a small overlap between emission and absorption spectra whereas DCM has a good efficiency of trapping both electrons as well as holes. Polyaniline has been utilized as hole injecting layer whereas tris-8-hydroxyquinoline-aluminum as electron injecting layer. Enhanced electroluminescence with bright yellow color has been observed in p-LEDs by the addition of dye.