Investigation of exciton photodissociation, charge transport and photovoltaic response of poly(N-vinyl carbazole):TiO2 nanocomposites for solar cell applications

The photogeneration of charge carriers in spin-coated thin films of nanocrystalline (nc-)TiO2 particles dispersed in a semiconducting polymer, poly(N-vinylcarbazole) (PVK), has been studied by photoluminescence and charge transport measurements. The solvent and the TiO2 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-TiO2: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 Jsc and open circuit voltage Voc for a 30% TiO2 volume content corresponding to the morphology exhibiting the best dispersion of TiO2 particles. A degradation of the photovoltaic properties is induced at higher compositions by the formation of larger TiO2 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 TiO2 composition confirms that the photovoltaic response is optimum for 30% TiO2 volume content. It is concluded that the photovoltaic properties of nc-TiO2:PVK nanocomposites are controlled by the interfacial area between the donor and the acceptor material and are limited by the dispersion of the TiO2 nanoparticles in the polymer