"Sensitizers other than dyes have been found the more effective, particularly aromatic polycyclic compounds having at least three fused benzene rings and having an ionization energy of less than about 7.5 ev. Suitable such sensitizers are described in U.S. Pat. No. 4,069,054, and include anthracene, rubrene, perylene, phenanthrene, fluoranthene, and pyrene." . . . .