Charge transport in hybrid nanorod-polymer composite photovoltaic cells

Charge transport in composites of inorganic nanorods and a conjugated polymer is investigated using a photovoltaic device structure. We show that the current-voltage $\mathrm{}(I-V)\mathrm{}$ curves in the dark can be modeled using the Shockley equation modified to include series and shunt resistance at low current levels, and using an improved model that incorporates both the Shockley equation and the presence of a space-charge limited region at high currents. Under illumination the efficiency of photocurrent generation is found to be dependent on applied bias. Furthermore, the photocurrent-light intensity dependence was found to be sublinear. An analysis of the shunt resistance as a function of light intensity suggests that the photocurrent as well as the fill factor is diminished as a result of increased photoconductivity of the active layer at high light intensity. By studying the intensity dependence of the open circuit voltage for nanocrystals with different diameters and thus band gaps, it was inferred that Fermi-level pinning occurs at the interface between the aluminum electrode and the nanocrystal.