The present embodiments relate generally to development systems for electrophotographic imaging and printing apparatuses and machines in which ghosting print defects are reduced, and more particularly, is directed to a method for reducing positive ghosting in such systems.
Electrophotographic imaging members, e.g., photoreceptors, photoconductors, and the like, typically include a photoconductive layer formed on an electrically conductive substrate. The photoconductive layer is an insulator in the substantial absence of light so that electric charges are retained on its surface. Upon exposure to light, charge is generated by the photoactive pigment, and under applied field charge moves through the photoreceptor and the charge is dissipated.
These photoreceptors have a target voltage which is the voltage that the photoreceptor surface becomes uniformly electrostatically charged. It is the optimum voltage for a xerographic system found through testing, and its determination depends on various systemic and environmental parameters. For example, the target voltage may be dependent on characteristics of the photoreceptor, such as the photoreceptor thickness, or characteristics of the development system, such as the type of toner and carrier. The target photoreceptor surface voltage also depends on the desired image quality, such as solid area density, line width, and avoidance of defects (such as background). The target P/R surface voltage is one of many variables that are optimized to achieve best overall performance.
Photoreceptors also have a maximum voltage, which is defined as the safe upper limit. Exceeding this value may cause damage to the photoreceptor due to dielectric breakdown and resulting holes that may form. The holes will cause spots in the reproduction prints. Likewise, the maximum photoreceptor surface voltage depends on the photoreceptor material and thickness. The target value is generally much lower then the maximum P/R surface voltage.
In electrophotography, also known as xerography, electrophotographic imaging or electrostatographic imaging, the surface of an electrophotographic plate, drum, belt or the like (imaging member or photoreceptor) containing a photoconductive insulating layer on a conductive layer is first uniformly electrostatically charged at the target surface voltage. The imaging member is then exposed to a pattern of activating electromagnetic radiation, such as light. Charge generated by the photoactive pigment moves under the force of the applied field. The movement of the charge through the photoreceptor selectively dissipates the charge on the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image. This electrostatic latent image may then be developed to form a visible image by depositing oppositely charged particles on the surface of the photoconductive insulating layer. The resulting visible image may then be transferred from the imaging member directly or indirectly (such as by a transfer or other member) to a print substrate, such as transparency or paper. The imaging process may be repeated many times with reusable imaging members.
An electrophotographic imaging member may be provided in a number of forms. For example, the imaging member may be a homogeneous layer of a single material such as vitreous selenium or it may be a composite layer containing a photoconductor and another material. In addition, the imaging member may be layered. These layers can be in any order, and sometimes can be combined in a single or mixed layer.
Typical multilayered photoreceptors or imaging members have at least two layers, and may include a substrate, a conductive layer, an optional charge blocking layer, an optional adhesive layer, a photogenerating layer (sometimes referred to as a “charge generation layer,” “charge generating layer,” or “charge generator layer”), a charge transport layer, an optional overcoating layer, an optional undercoat layer, and, in some belt embodiments, an anticurl backing layer. In the multilayer configuration, the active layers of the photoreceptor are the charge generation layer (CGL) and the charge transport layer (CTL). Enhancement of charge transport across these layers provides better photoreceptor performance.
Conventional imaging members, however, have exhibited drawbacks when implementing image forming methods. One common problem is that in that electrons tend to remain in the charge generating layer after holes are first inject into the electrophotographic photosensitive member, and the electrons act as a kind of memory causing variations in potential. This problem, associated with charge accumulation, is known as “ghosting.” Consequently, when a sequential image is printed, the accumulated charge results in image density changes in the current printed image that reveals the previously printed image. It is assumed that the electrons remaining in the charge generating layer advance for some reason to the boundary between the charge generating layer and the charge transporting layer, thereby reducing a barrier height for injecting holes in a vicinity of the boundary.
Ghosting can be described as developed image-forming patterns on a latent image-retaining member which are electrostatically transferred to a transfer material such as paper. These images become visual and the image formed can either be lighter than the background formed by toner deposition or darker than the background formed by toner deposition. In a situation where the ghost image is lighter than the background, the phenomenon is known as “negative ghosting.” In a situation where the ghost image is darker than the background, the phenomenon is known as “positive ghosting.”
Thus, as the demand for improved print quality in xerographic reproduction is increasing, there is a continued need for achieving improved performance, such as finding a way to minimize or eliminate charge accumulation in photoreceptors.