Photoreceptor Charging

For reviews of the xerographic process, see Berger et al. (1979), Scharfe (1984), Williams (1984), Burland and Schein (1986), Pai and Melnylk (1986, 

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For example, a 25-(xm-thick TL (in which there is no deep trapping) with (X = 10 cm /V-s at = 10 V/cm in a photoreceptor charged initially to 1000 V still retains a residual voltage of 20 V after 0.3 s, or 7 V after 1 s, even when enough incident light is used to photogenerate all carriers needed (i.e., CVJ to discharge the device. A device with (x = 10 cm /V-s at the same field will retain as much as 60 V at 1 s after exposure, which is probably unacceptable. 

Figure 15. Neutralization of toner and photoreceptor charge prior to cleaning. (Reproduced, with permission, from Ref. 84. Copyright 1980, Society of Photographic Scientists and Engineers.)...

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. 

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... 

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...

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. 

Fig. 7. Schematic of an organic layered photoreceptor, where the — signs represent the corona-deposited charge, which is typically negative D, the CTL ...

Fig. 9. (a) Schematic of charged area development (CAD), using toner charged oppositely to that of the photoreceptor and resulting in a positive document (b) discharged area development (DAD), where the toner and photoreceptor polarity are the same, resulting in a negative document. 

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