Investigation of exciton photodissociation, charge transport and photovoltaic response of poly(N-vinyl carbazole):TiO(2) nanocomposites for solar cell applications.

The photogeneration of charge carriers in spin-coated thin films of nanocrystalline (nc-)TiO(2) particles dispersed in a semiconducting polymer, poly(N-vinylcarbazole) (PVK), has been studied by photoluminescence and charge transport measurements. The solvent and the TiO(2) particle concentration have been selected to optimize the composite morphology. A large number of small domains leading to a large interface and an improved exciton dissociation could be obtained with tetrahydrofuran (THF). The charge transport mechanism and trap distribution at low and high voltage in ITO/nc-TiO(2):PVK/Al diodes in the dark could be identified by current-voltage measurements and impedance spectroscopy. The transport mechanism is space charge limited with an exponential trap distribution in the high voltage regime (1-4 V), whereas a Schottky process with a barrier height of about 0.9 eV is observed at low bias voltages (<1 V). The current-voltage characteristics under white illumination have shown a dramatic increase of the short circuit current density J(sc) and open circuit voltage V(oc) for a 30% TiO(2) volume content corresponding to the morphology exhibiting the best dispersion of TiO(2) particles. A degradation of the photovoltaic properties is induced at higher compositions by the formation of larger TiO(2) aggregates. A procedure has been developed to extract the physical parameters from the J-V characteristics in the dark and under illumination on the basis of an equivalent circuit. The variation of the solar cell parameters with the TiO(2) composition confirms that the photovoltaic response is optimum for 30% TiO(2) volume content. It is concluded that the photovoltaic properties of nc-TiO(2):PVK nanocomposites are controlled by the interfacial area between the donor and the acceptor material and are limited by the dispersion of the TiO(2) nanoparticles in the polymer.

Surface implantation treatments to prevent infection complications in short term devices.

Surface treatments of short term devices are actually evaluated to reduce the risk of infections, which in particular are one of the main causes of complications following catheter insertion. We have investigated the efficacy of ion beam techniques to reduce bacterial adhesion-or to induce bactericidal activity of different polymer materials: PVC, silicone rubber, poly(urethane) and poly(ethylene). Two routes have been evaluated, based on the production of non fouling surfaces, through the production of diamond-like surfaces upon irradiation with rare gases, or the implantation of silver, known for its bactericidal action. In this contribution we discuss more specifically the treatment of poly(ethylene), where a broad range of surface characterisation techniques could show that the biological activity resulted from the formation of metallic colloidal silver near the surface of the polymer, associated to the formation of a dense surface acting as a diffusion barrier. Reduction of the implantation energy to 10 keV, led to activity enhancement resulting from the easier accessibility of surface colloids evidenced by AFM microscopy. This study emphasises the specific processes induced by the formation of silver nano-particles at low energy implantation, which differs basically from Ion Beam Assisted Deposition (IBAD technique) leading to the formation of a continuous silver coating (Artif. Organs 18 (1994) 266; International Patent (PCT) WO 95/18637 (1995)).