Degradation of Amorphous Silicon Based Photoconductors by Corona Discharge
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DEGRADATION OF AMORPHOUS SILICON BASED PHOTOCONDUCTORS BY CORONA DISCHARGE R. A. C. M. M. VAN SWAAIJ, W. P. M. WILLEMS*, J. BEZEMER, H. J. P. LOKKER and W. F. VAN DER WEG, Department of Atomic and Interface Physics, Debye Institute, Utrecht University, P. 0. Box 80.000, NL-3508 TA Utrecht, The Netherlands, *Stork Colorproofing B.V., P. 0. Box 210, NL-5830 AE Boxmeer, The Netherlands.
ABSTRACT Thin three-layered a-Si:H photoconductive structures have been studied for application in high resolution electrophotography. The resolution of prints is determined by the a-Si:C:H' top layer. It is observed that the resolution of prints decreases from 100 to 30 lines/mm after about 25 charge cycles of corona discharge. The effect of corona charging on the surface properties of a-Si:C:H films with different carbon content is studied. Before and after corona exposure Fourier transform infrared absorption and lateral dark conductivity were measured. We observe that modes around 3300 cm-1, ascribed to H2 0 and -OH related modes, arise in combination with an increase of the lateral dark conductivity. This effect is not dependent on the film thickness, indicating that only the surface is affected by the corona discharge. The increase of the dark conductivity is interpreted as due to band bending at the surface, induced by the adsorption of H20 or the growth of a thin Si-OH layer at the surface.
INTRODUCTION The electrophotographic process [1], as used in photocopiers and laser printers, imposes a number of demands on the photoconductor. In particular, large area (paper size A3) high resolution (100 lines/mm) imaging requires several minutes of exposure time. The photoconductor should have a very slow dark decay, in order to retain the charge in the nonilluminated areas. On the other hand, the light decay must be very fast. Furthermore, it is desired that no residual potential arises after repeated cycles of charging and discharging. These requirements can be integrated in a three-layered a-Si:H photoconductor [2]. The ideal charge density profile corresponding to a single printed line and a single inverted (white, non-printed) line of 10 pm width is represented in Figure 1. In practice, the steep edges will become smoothened, as the surface charge carriers drift or diffuse due to the voltage and concentration gradients at the edges. A highly resistive a-Si:C:H top layer in the three layered structure provides a barrier against such charge redistribution. However, the (surface) lateral dark conductivity (LDC) is sensitive to changes at the surface. This paper reports on corona induced changes of the surface. During corona charging, the photoconductor is exposed to reactive species. As a result, the LDC at the surface is changed and this eventually affects the resolution of prints. To study this phenomenon, corona induced changes of amorphous silicon carbon films are measured by Fourier transform infrared absorption (FTIR) and correlated with dark conductivity measurements.
Mat. Res. Soc. Symp. Proc. Vol. 297. @1993 Materials Rese
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