Post-synthesis thermal treatment promotes crystallinity and decrease the defect d. in soln.-processed nanomaterials. In particular, the annealing atm. strongly influences the functional properties of ZnO nanowires (NWs) and specifically their performance as photoanodes in dye-sensitized solar cells (DSCs). The authors prepd. vertically aligned ZnO NWs by a low-cost, high-yield, and up-scalable hydrothermal method and studied the effect of the post-annealing atm. on their optoelectronic properties and on their performance as electrodes in DSCs. When annealing ZnO NWs under Ar, instead of air, significantly higher photoluminescence (PL) UV emission and relatively lower defects-related visible PL emission were obtained. At the same time, Ar-annealing rendered ZnO NWs with higher elec. conductivities, as obsd. from single NW measurements using conductive-at. force microscopy. Also, DSCs based on ZnO NWs annealed in Ar were characterized by 50% higher photocurrents than those obtained from air-annealed ZnO. As a result 30% efficiency increases were systematically obtained when using Ar as the annealing atm. These results are discussed within the framework of a multiple trapping model for transport and charge transfer, taking into account differences in the defect concn. introduced during the annealing.