Solar cells based on conjugated polymers are often designed to be nanostructured heterojunctions, with the aim of providing a large interface area for exciton dissociation. While such devices have been shown to be efficient at splitting excitons, the improvements in efficiency have not been as great as would be expected, with losses due to recombination being identified as a major problem. We therefore apply a simple drift-diffusion model to such a system to investigate how changing the interface area affects the short circuit current. We find that for a given system, there is a clear optimum device thickness and that increasing the interface area at the expense of device thickness past this value will lead to a decrease in solar cell efficiency. (c) 2005 Elsevier B.V. All rights reserved.