A photodiode comprising a photoactive spinel oxide layer is described. This photoactive spinel oxide layer forms a contact with both a light absorption layer of quantum dots, quantum wires, or quantum rods, and an inorganic substrate layer. In some embodiments, the inorganic substrate layer and the photoactive spinel oxide layer form an isotype junction. Methods of characterizing the photodiode are provided and demonstrate commercially relevant electrical and optoelectronic properties, particularly the ability to operate as a photodetector with a high photosensitivity. An economical process for preparing the photodiode is provided as well as applications.