Xerographic Dark Decay and Photoinduced Effects

Analysis of the time and temperature dependent decay of the surface voltage on an amorphous film after charging, but prior to exposure (xerographic dark decay), and of residual decay after exposure can (in combination) be used to map the density of states. 

In xerographic measurements, as illustrated in Fig. 5.3, the sample is corona-charged to a voltage Vq and then exposed to a short wavelength (absorption depth S L) step illumination. At the end of the illumination, there is a measurable surface potential, termed the residual potential V because of the bulk trapped charges. If positive charging is used, then is due to trapped holes in the bulk of the specimen. 

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 

In the case of a-Se, these xerographic properties have been extensively studied. In addition to the magnitude of the saturated residual voltage, the rate of decay and the temperature dependence of the cycled-up residual potential are important considerations, because they determine the time required for the photoreceptor to regain its first-cycle xerographic properties. 

The origin of the deep localized states in the mobility gap that control the dark decay has been attributed to structural native thermodynamic defects [12]. Thermal cycling experiments show that the response of the depletion time to temperature steps is retarded, as would be expected when the structure relaxes toward its metastable liquid-like equilibrium state. As the structure relaxes toward the equilibrium state, t(j decreases further until the structure has reached equilibrium. The only possible inference is that must be controlled by structure-related thermodynamic defects. The generation of such defects is, therefore, thermally activated. We should note that because the depletion discharge mechanism involves the thermal emission of carriers 

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