External quantum efficiency in solar cells based on junctions between PbSe quantum dots (QDs) and thin ZnO films is increased by replacing the ZnO films with a vertically oriented array of single-crystalline ZnO nanowires, and infiltrating this array with colloidal QDs. When illuminated with 100 mW/cm2 of simulated solar light, QD-nanowire solar cells exhibited power conversion efficiencies approaching 2%, approximately three times higher than that achieved with thin-film ZnO devices constructed with the same amount of QDs. Significant photocurrent and power conversion improvement with increasing nanowire length is consistent with higher exciton and charge collection efficiencies.