A photodetector is a device that converts a light signal into an electrical signal. Most transparent ultraviolet (UV) photodetectors are composed of metal oxide materials having a wide energy bandgap. Here, we manufactured a high-performance transparent photodetector that specifies the UV region using a heterojunction structure of a metal oxide (n-type ZnO/p-type NiO). In particular, ZnO and NiO are less harmful to the human body, have a chemically stable structure, and are low cost due to abundant reserves. Our device can be applied to various fields of photoelectric devices, including solar cells and photodetectors, and human electronics. Owing to the photovoltaic effect of the ZnO/NiO device, the transparent photodetector can be operated in self-powered mode. To verify the optimum operation, bias tuning was applied to investigate the device’s performances of response time and photocurrent. The characteristics of devices and their effects on electrical changes inside the device, including an increase in the depletion region, improvements in carrier collection and carrier velocity, and an increase in the electric field intensity, were analyzed by reverse bias.