ZnO films consisting of either polydisperse or monodisperse aggregates of nanocrystallites were fabricated and studied as dye-sensitized solar-cell electrodes. The results revealed that the overall energy-conversion efficiency of the cells could be significantly affected by either the average size or the size distribution of the ZnO aggregates. The highest overall energy-conversion efficiency of similar to 4.4% was achieved with the film formed by polydisperse ZnO aggregates with a broad size distribution from 120 to 360nm in diameter. Light scattering by the submicrometer-sized ZnO aggregates was employed to explain the improved solar-cell performance through extending the distance travelled by light so as to increase the light-harvesting efficiency of photoelectrode film. The broad distribution of aggregate size provides the ZnO films with both better packing and an enhanced ability to scatter the incident light, and thus promotes the solar-cell performance.