A necessary condition for the occurrence of the photovoltaic effect is the presence of a film of oxide, halogen or sulfur compounds of metals on the clean metal surface of the illuminated electrode. It can be stated that most of the studied photosensitive layers are compounds related to the type of semiconductors, and the photovoltaic element can be considered as a system consisting of a semiconductor bordering a metal electrode on one side, and an electrolyte on the other. Thus, it is natural to compare the photovoltaic effect with the internal photoelectric effect characteristic of semiconductors. The advantage of titanium dioxide over copper oxide is its chemical inertness, which allows for a wide variation in the conditions of the medium bordering the electrode. Based on the spectral characteristics of the positive and negative signs, it can be concluded that photocurrents of both signs are caused by electronic processes in the semiconductor that are independent of each other.
A large amount of experimental material has been presented in works on photovoltaic phenomena. The elucidation of the general laws governing these phenomena is complicated by the fact that the photoelectric effect occurs in the "electrode-solution" system, where the slightest changes are manifested in a violation of the established potential. Due to these considerations, the results obtained by different authors for the same object are contradictory and often irreproducible. This article establishes a number of important laws (see article).
