Radiation imagery chemistry: process – composition – or product th – Electric or magnetic imagery – e.g. – xerography,... – Process of making radiation-sensitive product
Reexamination Certificate
1998-03-25
2001-03-06
Nguyen, Nam (Department: 1753)
Radiation imagery chemistry: process, composition, or product th
Electric or magnetic imagery, e.g., xerography,...
Process of making radiation-sensitive product
C430S133000, C204S192230, C204S192260
Reexamination Certificate
active
06197471
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to electrophotographic image-forming members or photoreceptors comprising hydrogenated amorphous silicon (a-Si:H) formed onto a supporting conductive substrate. More particularly, this invention is directed to an improved a-Si:H photoreceptor and to a method for making the photoreceptor.
2. Discussion of the Prior Art
Electrophotography is a well-known image transduction art depending on the formation of an electrostatic latent image on a charge-sensitized photoconductor formed onto a suitable substrate. The latent image is typically produced by photo-induced discharge of the photoconductor in response to a light image projected onto the working surface of the photoconductor, and a visual image for transfer to the hard-copy medium is developed from the latent image by contacting it with charge-sensitive toner particles. The toned image is then the basis for a variety of further imaging processes. The versatility of electrophotography has permitted its application in systems for copying, duplicating, printing, plate making and color proofing, among others, and electrophotography is increasingly being applied in computer output devices in which lasers are used to produce the latent image. Commercial potential of such systems is directly affected by the performance and producibility of the photoconductor. Generally, the photoconductor must have good charge acceptance V
0
and a long dark decay &tgr;
D
, typically 10 to 20 seconds at minimum. In addition, fast photo-induced discharge is required, and the spectral response of the photoconductor must be compatible with the source. In critical applications, photoconductor fatigue or residual voltage may be limiting.
Considerable effort has been expended in development of prior-art photoconductors based on inorganic materials such as cadmium sulfide, zinc oxide, or selenium, as well as organic materials such as TNF-PVCz (the reaction product of 2,4,7-trinitro-9-fluorenone and poly-9-vinylcarbazole). Some prior-art photoconductors suffer well-known disadvantages such as low charge acceptance or short dark decays, poor thermal or environmental stability, poor mechanical strength or durability, or the potential for environmental contamination. Further, others lack good adhesion properties or are otherwise incompatible with use of flexible substrates required by large-format applications such as color proofing. In addition, many require formation temperatures too high to permit their use with plastic substrates.
High-quality, large-format electrophotography can be practiced through use of microcrystalline cadmium sulfide deposited onto thin conductive substrates (U.S. Pat. Nos. 4,025,339 and 4,269,919). A metallic member, or a plastic member coated with a metallic or an ohmic layer, may form such conductive substrate. Sputtered to thicknesses of 0.3 to 5 micra onto stainless-steel roll-stock up to one meter wide and about 0.1 mm thick, such anisotropic photoconductors have been adapted to provide flexible photoreceptors for an analog color-proofing application (U.S. Pat. Nos. 4,358,195 and 4,556,309). This application required that a latent image be retained almost two minutes between photoreceptor charging and development. The large-format (approximately 50 cm by 60 cm) photoreceptors were required to be reusable for thousands of operational cycles. During operation, at 105 seconds after corona charging these thin cadmium sulfide photoconductors demonstrate typical surface potentials V
105
=22 volts, they have linear photo-induced discharge, and they yield substantially zero residual voltage on complete discharge. When used with optimized liquid toner systems, these flexible low-voltage photoreceptors provide high-resolution four-color proofs. However, a potential environmental hazard due to manufacture or disposal of the cadmium sulfide photoconductor remains a concern.
The disadvantages of prior-art photoconductors has prompted investigation of amorphous silicon (a-Si) as the photosensitive material for use in electrophotographic photoreceptors. Amorphous silicon poses no environmental hazard and has good mechanical strength, adhesion, and durability, but demonstrates undesirable characteristics thought to originate in unsatisfied (or dangling) bonds in the silicon matrix. It has been shown that formation of amorphous silicon in presence of hydrogen provides a material (a-Si:H) with fewer dangling bonds and improved characteristics, the greatest improvement occurring for deposition substrate temperatures of approximately 230° C.
An extensive art based on a-Si:H materials has developed in the field of solar-energy conversion, in which thin a-Si:H layers are routinely deposited onto large-area flexible substrates; the internal resistance of such photovoltaic devices must be as low as possible (of the order of 100 ohms) for attractive power outputs, but the corresponding volume resistivities (about 10
6
ohm·cm) result in photoconductive properties ill-suited to electrophotographic applications. Other a-Si:H materials made to have higher resistivities exhibit attractive photoconductive properties, and by appropriate doping, such a-Si:H photoconductors can be made to accept positive charging, negative charging, or charging in either polarity. However, conventional a-Si:H photoreceptors are typically directed toward rapid-imaging systems for office use, the toner systems for which may require surface potentials of 100 volts or greater but the operational cycles for which seldom require dark decays longer than a few seconds. Consequently, the prior-art a-Si:H photoreceptors (e.g., U.S. Pat. No. 4,265,991) have demonstrated several characteristics which limit their usefulness as low-voltage electrophotographic photoreceptors. Included are the following significant disadvantages:
1. The low dark volume-resistivity (about 10
10
ohm·cm) of such a-Si:H photoconductors, and their resultant fast dark decays, require deposition of a high-voltage a-Si:H layer at least 10 (and usually 20 to 50) micra in thickness to achieve the surface potentials needed by many electrophotographic processes at toning; these thick photoconductive layers are both expensive to produce and poorly adapted to use with flexible substrates. As is known in the art, long dark decays require that a photoconductor have both a wide optical bandgap, which indicates a low density of thermally generated charge carriers, and a low drift mobility for such carriers. The optical bandgap of a-Si:H is known to increase with increasing hydrogen content, up to about 10% total hydrogen, and carrier mobilities in a-Si:H are known to decrease with addition of small amounts of neutral dopants such as oxygen or nitrogen. However, prior-art a-Si:H photoconductors based on either bandgap widening by hydrogen enrichment or mobility suppression by doping-induced trapping enhancement demonstrate degraded photoconductive properties and spatial inhomogeneities in the charge acceptance or toning response. In addition, the bulk properties of prior-art a-Si:H photoconductors are adversely affected by interface processes. When prior-art a-Si:H photoconductors are used in bilayer photoreceptors, carrier injection from the conductive substrate or charge transfer from the environment accelerates bulk dark-decay processes, further reducing applicability of such photoreceptors. Such processes have been partially overcome in the prior art by fabrication of multilayered photoreceptors in which thin (a few hundred nm or less) insulating layers are either deposited at the interface between the a-Si:H photoconductor and the conductive substrate, or topcoated over the photoconductor, or both.
2. Charge injection or impurity migration into the adherent surface of the photoconductor has been particularly limiting for a-Si:H photoconductors formed onto many conductive substrates. In the prior art, thin blocking or barrier layers are commonly deposited on the substrate surface prior to formation of the a-Si:H photoconduc
Dorer Gary L.
Graham Marshall Donnie
Alter Mitchell E.
Coulter International Corp.
Nguyen Nam
Ver Steeg Steven H.
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