Conventional photoconductors for electrophotographic light-sensitive compositions are comprised of various organic compounds and some of these compounds have considerably high light sensitivity. However, organic photoconductors for electrophotographic light-sensitive materials are not commonly used at present.
Organic photoconductors have many excellent properties as compared to inorganic photoconductors and may be applied to numbers of different techniques in the technical field of electrophotography. For example, organic photoconductors now make it possible to produce a transparent electrophotographic light-sensitive film, a flexible electrophotographic light-sensitive film, and a light-weight and easily handlable electrophotographic light-sensitive film. Also, organic photoconductors have properties, which cannot be expected from inorganic photoconductors or inorganic semi-conductors, such as the film-forming property and surface smoothness at the production of electrophotographic light-sensitive materials and the selectivity of charging polarity in an electrophotographic process.
In spite of having these excellent properties, organic photoconductors have not yet been able to sufficiently contribute to the technical field of electrophotography mainly due to their low light sensitivity and the brittleness of films or layers.
Preliminary studies of organic photoconductors were made on compounds such as low molecular heterocyclic compounds, nitrogen-containing aromatic compounds, and various high molecular type aromatic compounds. As a result of these studies, organic photoconductors having considerably high sensitivity were found. Recently, however, sensitizing methods have been investigated for obtaining higher sensitivity. This is because even an organic photosemiconductive compound having the highest sensitivity presently known does not have sufficient sensitivity for practical use of the compound without requiring the application of a sensitizing treatment. Accordingly, during the practical application of organic photoconductors it is necessary to select the most effective sensitizing method. The industrial value of an organic photoconductor depends substantially on how highly sensitized electrophotographic light-sensitive material can be made by using the organic photoconductor.
The most generally known sensitizing method is the addition of a sensitizing dye or the addition of Lewis acid. Such methods can be applied to almost all organic photoconductors. The former method provides sensitization by imparting the spectral absorption characteristics of a sensitizing dye to an organic photoconductor and the latter method provides sensitization by forming a donor-acceptor complex between the Lewis acid and an organic photoconductor to give a new spectral sensitivity.