Photoreceptor

P. M. Borsenberger and D. S. Weiss, Organic Photoreceptors for Imaging Sjstems, Marcel Dekker, New York, 1993. 

The electrophotographic system (102,103) involves two key physicochemical elements a photoreceptor and a toner. The minimum requirements of the process are (/) to charge a photoconductive photoreceptor uniformly (2) to illuminate selectively the photoreceptor to form a latent electrostatic image and (J) to develop the image by applying charged toner. These steps are illustrated in Figure 17. 

Liquid toners are suspensions of toner particles in a fluid carrier. The carrier is typically a hydrocarbon. Dielectric, chemical, and mechanical properties of the Hquid must be compatible with the photoreceptor, the suspended toner particles, and the materials of the development equipment. Liquid toners are capable of producing higher resolution than dry toners because of the smaller (3—5 -lm) particle size achievable. Development of the latent image occurs as it passes through a bath of toner and the charged particles are attracted to the oppositely charged surface. 

Selenium recovery from electronic, xerographic, or other scrap for sale on the open market remains small. This was less than 100 t/yr in 1995. The xerographic industry recovers essentially all of the selenium it uses in photoreceptor dmms by extremely efficient recycling programs (see... 

HP arsenic is used in the manufacture of photoreceptor arsenic-selenium alloys for xerographic plain paper copiers (see Electrophotography). The level of arsenic maybe 0.5%, 5.0%, or 35% present as arsenic triselenide [1303-36-2] As2Se2. 

Fig. 8. Organic photoreceptors for electrophotographic use. (10), 4-[4-DimethylamiQo]phenyl]-2,6-diphenylthiapyryhumperchlorate [14039-00-0] ...

The most desirable characteristics of photoreceptors for use in electrophotography are the following ... 

The abihty to accept and hold the electrostatic charge in the darkness. The photoconductive layer should support a surface charge density of approximately 0.5-2 x 10 C/cm. The charge also has to be uniformly distributed along the surface, otherwise nonuniformities can print out as spot defects. The appHed surface potential should be retained on the photoreceptor until the time when the latent electrostatic image is developed and transferred to paper or, if needed, to an intermediate belt or dmm. In other words, the "dark decay" or conductivity in the dark must be very low. The photoconductor materials must be insulators in the dark. 

The photoreceptor must be stable in performance. In order to guarantee that the quaUty of copies stays the same after hundreds of thousands of produced copies, the photoreceptor should maintain the same charge acceptance. That is, the potential across the photoreceptor should be the same... 

Lastly, the photoreceptors must be inexpensive and easy to fabricate into defect-free, large-area thin films with uniform thickness of all layers. 

Fig. 2. Schematics of (a) single-layer photoreceptor, where the + signs represent the corona-deposited charge, D the photoconductor, and 1 the conductive substrate and (b), the CdS Sej (Katsuragawa) photoreceptor, where D represents the insulating layer, the CdS Sej, and I the...

Amorphous (vitreous) selenium, vacuum-deposited on an aluminum substrate such as a dmm or a plate, was the first photoconductor commercially used in xerography (6). It is highly photosensitive, but only to blue light (2). Its light absorption falls off rather rapidly above 550 nm. Because of the lack of photoresponse in the red or near infrared regions, selenium photoreceptors caimot be used in laser printers having He—Ne lasers (632.8 nm), or soHd-state lasers (680—830 nm). 

Some of these devices have a respectable quantum efficiency of charge generation and collection, approaching 0.4 (20). The nature of the polymeric binder has a large effect on the device performance (21), and so does the quaUty and source of the dye (22). Sensitivity to the environment and fabrication methods results in some irreproducibiUties and batch-to-batch variances. However, the main advantage of the ZnO-based photoreceptor paper is its very low cost. 

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