Dark discharge

Typical dark discharge characteristics for pure Se and Sb Sei photoreceptors are shown in Fig. 7.1 for compositions noted in the figure. It is apparent that for pure a-Se, the decay of the surface potential is relatively slow. Comparison of the respective characteristics for a-Sb -Sci with the dark discharge behavior of pure a-Se shows clearly that alloying a-Se with antimony increases the dark-decay rate. The discharge rates in a-Sb -Sei were not constant but decreased with time. 

There are several physical processes that can lead to the decay of the surface potential. The currently accepted model for the dark decay in a-Se-based films involves [3,14,15]  

With relatively thick films (L = 10-50 pm) and a good blocking contact between a-Se-based films and the preoxidized A1 substrate, the latter phenomenon dominates. 

In a series of experiments carried out on a composition series of glassy Sb cSei- c alloys, it was found that the time-dependent dark-decay rate of the potential to which 

It is found that in a-Sb cSei- c alloys, electrons (the mobile carrier species) are depleted (n-type system) during dark decay, leaving behind a deeply trapped positive space charge. Note that the same situation prevails in alkali-doped a-Se [16]. 

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Dark discharge rate must be sufficiently low to maintain an ample amount of charge on the photoreceptor during the exposure and development steps. A high dark decay rate will limit the available contrast potential. The residual potential remaining after the xerographic cycle must be small enough that it does not impair the quality... 

Figure 5.6 shows typical positive and negative dark discharge curves for pure a-Se films prepared under different conditions [2], where it can be seen that the dark discharge rate depends on substrate temperature. 

Figure 5.6 Dark discharge of surface potential on a-Se layers. A, B, and C involve a-Se deposited under different substrate temperature (Tj) conditions A and A at = 75°C B and B at Tj = 50-60°C C and C at Fs = 25-50°C and uncontrolled [2].

Figure 5.7 Typical log-log plot of the dark discharge rate versus time for an amorphous Se-Te film [13].

Figure 7.1 Dark discharge of surface potential on a-Sb Sej layers.

The development process usually requires that the surface is charged to about 400 V by the corona discharge. In order to prevent the dark discharge, three sources of cmrent must be avoided-the thermal generation of bulk carriers, injection from the back contact, and dielectric breakdown. A dielectric blocking layer is used to suppress injection and a slight boron doping of the photoconductor layer... 

During the past three decades, there have been many models of charge acceptance, dark discharge, and photoinduced discharge described in the litera-... 

Taylor et al. (1990) described the use of Langmuir-Blodgett techniques for the preparation of monolayers as blocking layers. Layers as thin as 0.002 (im were effective in suppressing the dark discharge and maximizing charge... 

The effects of the generation-layer fabrication variables on the sensitometry of a dual-layer photoreceptor prepared with bis(4-dimethylaminophenyl) squaraine (X = H in Appendix 2) have been extensively investigated by Law (1987). The charge acceptance, dark discharge, sensitivity, and the residual... 

Table 3 The Sensitivity and Dark Discharge of a Series of Dual-Layer Photoreceptors with Phthalocyanine Dispersion Generation Layers. (After Loutfy et al. 1988a) ...

Phthalocyanine Generation Layer Thickness ( J.m) Dark Discharge (V/s) e (ergs/cm2)... 

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