Thin-film ZnO/Cu2O photovoltaic devices were prepared by electrochemical deposition in aqueous solutions, and significant improvements in the photovoltaic performances, especially external quantum efficiency (EQE), were achieved by rapid thermal annealing (RTA). ZnO were electrodeposited on Ga: ZnO-coated glass substrates from solutions containing zinc nitrate, and Cu2O were electrodeposited on the ZnO layer from solutions containing copper acetate and lactic acid. The photovoltaic performances were evaluated after annealing the ZnO/Cu2O layers at varying temperatures of 423-573 K under vacuum for 60 min, and a high EQE of nearly 100% was demonstrated by RTA at 423 K under vacuum. To elucidate the improvement effect of the RTA, structural characterizations were carried out with XRD and FESEM, and the optical and electrical properties were examined by UV-vis absorption spectroscopy and Hall effect measurements, respectively. The improvements in EQE was mainly attributed to the lessening of scattering defects in the light-absorbing Cu2O layer such as lattice dislocations and grain boundaries, which improved the charge collection efficiency. The photovoltaic performances deteriorated at elevated temperatures above 523 K due to the generation of small Cu particles on the Cu2O surface, which is associated with the generation of nanopores in the Cu2O bulk.