1. Field of the Invention
This invention relates to organic polymeric photoconductive compositions and the use of such compositions in electrophotographic imaging members and methods. More specifically, this invention concerns itself with inherently photoconductive compositions which are characterized by their high extinction coefficients and good spectral response at wavelengths of from about 4,200 - 6,200 A.
2. Description of the Prior Art
The formation and development of images on the imaging surfaces of photoconductive materials by electrostatic means is well known. The best known of the commercial processes, more commonly known as xerography, involves forming a latent electrostatic image on the imaging surface of an imaging member by first uniformly electrostatically charging the surface of the imaging layer in the dark and then exposing this electrostatically charged surface to a light and shadow image. The light struck areas of the imaging layer are thus rendered conductive and the electrostatic charge selectively dissipated in these irradiated areas. After the photoconductor is exposed, the latent electrostatic image on this image bearing surface is rendered visible by development with a finely divided colored electroscopic material, known in the art as "toner". This toner will be principally attracted to those areas on the image bearing surface which retain the electrostatic charge and thus form a visible powder image.
The developed image can then be read or permanently affixed to the photoconductor where the imaging layer is not to be resued. This latter practice is usually followed with respect to the binder-type photoconductive films (e.g. ZnO) where the photoconductive imaging layer is also an integral part of the finished copy.
In so-called "plain paper" copying systems, the latent image can be developed on the imaging surface of a reusable photoconductor or transferred to another surface, such as a sheet of paper, and thereafter developed. When the latent image is developed on the imaging surface of a reusable photoconductor, it is subsequently transferred to another substrate and then permanently affixed thereto. Any one of a variety of well known techniques can be used to permanently affix the toner image to the copy sheet, including overcoating with transparent films, and solvent or thermal fusion of the toner particles to the supportive substrate.
In the above "plain paper" copying system, the materials used in the photoconductive layer should preferably be capable of rapid switching from insulating to conductive to insulating state in order to permit cyclic use of the imaging surface. The failure of a material to return to its relatively insulating state prior to the succeeding imaging sequence will result in a decrease in the maximum charge acceptance of the photoconductor. This phenomenon, commonly referred to in the art as "fatigue", has in the past been avoided by the selection of photoconductive materials possessing rapid switching capacity. Typical of the materials suitable for use in such a rapidly cycling system include anthracene, sulfur, selenium and mixtures thereof (U.S. Pat. No. 2,297,691); selenium being preferred because of its superior photosensitivity.
In addition to anthracene, other organic photoconductive materials, most notably, poly(N-vinylcarbazole), have been the focus of increasing interest in electrophotography U.S. Pat. No. 3,037,861. Until recently, neither of these organic materials have received serious consideration as an alternative to such inorganic photoconductors as selenium, due to fabrication difficulties and/or to their relative lack of speed and photosensitivity. The recent discovery that high loadings of 2,4,7-trinitro-9-fluorenone in polyvinylcarbazoles dramatically improves the photoresponsiveness of these polymers has led to a resurgence in interest in organic photoconductive materials, U.S. Pat. No. 3,484,237. Unfortunately, the inclusion of high loadings of such activators can and usually does result in phase separation of the various materials within such compositions. Thus, there will occur within these compositions regions having an excess of activator, regions deficient in activator and regions having the proper stoichiometric relationship of activator to photoconductor. The maximum amount of activator that may be added to most polymeric photoconductive materials without occasioning such phase separation generally will not exceed in excess of about 6 - 8 weight percent.
One method suggested for avoiding the problems inherent in the use of such activators in conjunction with polymeric photoconductors, is the direct incorporation of the activators into the polymeric backbone of the photoconductor, U.S. Pat. No. 3,418,116. In this patent is disclosed the copolymerization of a vinyl monomer having an aromatic and/or heterocyclic substituent capable of an electron donor function with a vinyl monomer having an aromatic and/or heterocyclic substituent capable of an electron acceptor function. The spatial constraint placed upon these centers of differing electron density favors their charge transfer interaction upon the photoexcitation of such a composition. These so-called "intramolecular" charge transfer complexes, more accurately designated "intrachain" charge transfer complexes, are believed to function substantially the same as charge transfer complexes formed between small activator molecules and a photoconductive polymer. The fact that the electron donor function and an electron acceptor function are on a common polymeric backbone does not apparently change the .pi. - .pi. charge transfer interaction, but merely increases the probability of it occurring. Unfortunately, the preparation of such copolymers from vinyl monomers having an electron donor center and vinyl monomers having an electron acceptor center is often beset with difficulty.
Accordingly, it is the object of this invention to provide a polymeric photoconductive composition which is highly photosensitive and yet relatively easy to prepare.
More specifically, it is the object of this invention to provide a polymeric photoconductive composition capable of an intramolecular charge transfer transition.
It is another object of this invention to provide a polymeric photoconductive composition wherein an electron donor and an electron acceptor center are contained within a single structural unit of the polymer.
It is yet another of the objects of this invention to provide a polymeric photoconductive composition having good spectral response to light in the visible region of the electromagnetic spectrum.