Charge carrier transport in poly(N-vinylcarbazole):CdS quantum dot hybrid nanocomposites

Hybrid organic:inorganic materials have emerged as a novel class of electronic and optoelectronic media for a number of potential technological applications. However, very little fundamental understanding of charge carrier transport in such hybrid materials exists. A knowledge of the influence of nanoparticle doping on charge carrier mobility in nano composites becomes important in order to optimize properties for photorefractive and photovoltaic operations. We report here a study of the mobility of holes in a model nanoparticle-sensitized hybrid organic:inorganic system consisting of poly(N-vinylcarbazole) (PVK) doped with quantum dots of cadmium sulfide. The mobility of holes (dominant carriers in the PVK host materials) was measured using the conventional time-of-flight technique with injection of holes from a selenium layer. Though photocurrent transients exhibit features typical of dispersive transport in an amorphous semiconductor, certain deviations from the original Scher-Montroll theory are observed. Strong dependence of the carrier mobility on field and temperature indicate Poole - Frenkel-like activated hopping transport. A thickness dependence stronger than that suggested by the Scher - Montroll theory is found. Significant enhancement of the effective carrier mobility is noticed with the increase of nanoparticle concentration, still well below the percolation limit. A simple theoretical model based on time- and mean-free-path dependent mobility is proposed to account for this surprising result, which provides a good fit to the experimental data obtained.