High quality organic–inorganic heterojunction photodiodes based on nanostructured copper (II) phthalocyanine (CuPc) and intrinsic zinc oxide (i-ZnO) have been fabricated. The i-ZnO thin films/layers were grown by RF magnetron sputtering on clean indium tin oxide (ITO) coated glass substrates. These films have been characterized by optical absorption and field emission scanning electron microscopy (FESEM). CuPc thin films deposited at room temperature on i-ZnO have exhibited a change in their surface morphology with the post-deposition annealing temperature under normal atmosphere. The electrical dark conductivity and the photoconductivity of ITO/i-ZnO/CuPc/Au sandwich structures have been measured under various photoexcitation intensities using a xenon light source. The devices have shown excellent reproducibility of their electrical characteristics and high rectification ratios. The highest rectification ratio is nearly 831 calculated above the threshold voltage at room temperature for the sample annealed at 250 °C (i.e. Pc 250). The effects of the annealing temperature of CuPc on the surface morphology, rectification ratio, and optical properties have been discussed.