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.

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Electron transport across rectangular barriers, made of 200 A thick GaAlAs layers embedded in GaAs, and triangular barriers at the (n+)GaAs-(n)GalnP interface has been studied. Current versus voltage and temperature characteristics have been analysed in order to extract the different mechanisms that induce this transport, and to determine the temperature and electric field range in which they apply. At low temperature and high field the current is driven by the Fowler-Nordheim regime. At higher temperatures the current is dominated by a defect-induced mechanism. This mechanism consists of the thermal emission of electrons into the barrier conduction band from defects lying in the barrier that can be refilled by tunnelling. The defect involved appears to be the deep state associated with the donor impurity, i.e. the DX centre. This study demonstrates that the apparent band offset depends strongly on the experimental conditions under which it is measured.

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