Xerography is a well-known process for the formation and development of images on the imaging surfaces of photoconductive materials by electrostatic means. The process involves forming a latent electrostatic image on the imaging surface of an imaging member by first uniformly electrostatically charging the imaging surface and then exposing the charged surface selectively to light The electrostatic charge is selectively dissipated in irradiated areas leaving a member having selected areas of charged surface. Thereafter, the image is rendered visible by development with a finely divided colored electroscopic material known in the art as a "toner" which is principally attracted to the charges areas of the surface.
A number of materials, hereinafter referred to as "persistent photoconductors", have been noted which are characterized by exhibiting a substantial lag in returning to their original state of dark conductivity after illumination. This permits the imaging sequence to be reversed from that of xerography, namely an uncharged imaging layer is initially selectively irradiated to render areas persistently conductive in an imagewise fashion and thereafter a charge is imposed on the entire surface, but is only selectively processed by the persistently conductive areas. Persistent photoconductors and their use are described in a number of references, including i.e., Persistent Conductivity, Electrophotography, R. M. Schoffert ed. (Foral Press 1965) pp. 70-77; Nisho and Inoue, Photo-Induced Memory Effect of Organic Photoconductor, Photographic Science & Engineering 22:194 (1978), 22:35 (1981) and 26:24 (1982); and Hanna and Inoue, The Design of An Organic Photoreceptor With A Charge-Acceptance Memory, Photographic Science & Engineering 25:209 (1981), 26:69 (1982), and 27:51 (1983) and also in a number of U.S. patents including U.S. Pat. Nos. 3,113,022, 3,519,966 and 3,879,201, the disclosure of all of which is hereby incorporated by reference.
A problem which has characterized persistent photoconductivity compositions heretofore is that the image quality is both generally insufficient and the quality has a tendency to degenerate as the speed of exposure is increased. Also, there is a low toner image density. A number of additives have been described which improve the persistent conductivity characteristics, that is, permit extremely short exposure times to produce relatively long persistent conductivity, but such additives have not been found to improve image quality. In the experimentation which led to the present invention, it was also found that some additives improved image quality but they caused a decrease in the persistent conductivity characteristics of the persistent photoconductive composition.
It is accordingly the object of this invention to provide new persistent photoconductive compositions which not only require only extremely short exposure times to achieve relatively high persistent conductivity, but also exhibit improved image quality. This and other objects of the invention will become apparent to those of ordinary skill in this art from the following detailed description.