Electrophotographic Characteristics of (HgI2 : CdS) Binder Layers

Abstract

The combination of a photoconductor and insulating resin provides a photoconductive insulator capable of retaining a surface charge in d a r k and discharging in light. These combinations have some unique aspects: (i) The photoconductive material need not have high d a r k resistivity (which is required to retain charge) and this allows for a wide choice of materials. (ii) The independent characteristics of several photoconductors can be incorporated in one layer by dispersing these as powders in the resin binder. Several studies have been reported on the b i n d e r -t y p e plates of Se (1), ZnO (2), anthracene (3), CdS (4), and their combinations (5-6).The present investigations deal with the surface charge characteristics of HgI2:CdS mixed system in polystyrene. Our earlier photoconductivity studies (6) have shown that the binder layers of CdS dispersed in polystyrene have Iphoto/Idark • 100 and a low speed of response, while the binder layers of HgI2 have /photo/ /dark : 150 and a fast speed of response. This suggested that the binder layers of HgI2: CdS mixed system might possess some characteristics more desirable for use in electrophotography. T h e experiments have shown that the layer containing HgI2 and CdS in the 60:40 ratio exhibits the m a x i m u m value of Iphoto/Idark (5 X 103 at 1750 l x ) . It was then felt worthwhile to study the surface charge characteristics.The surface charge studies were made as follows: The photoconducting layers backed by NESA coated glass plates were charged by using a scorotron unit and the resulting surface charge was measured in terms of surface potential by a vibrating condenser unit. A 100W tungsten filament lamp was used to discharge the layers. Figure 1 shows typical dark and light decay curves of the HgI2, CdS, and HgI2: CdS layers. It is seen that (i) m a x i m u m surface potential is retained by the HgI2 layers, (ii) dark and light decay are fastest in HgI2 layers, and (iii) HgI2: CdS layers show slow dark and fast light decay rates.The effect of binder percentage on the d a r k and light decay rates of the HgI2:CdS layers is shown in Fig. 2. It is seen that as the percentage of binder is increased, the d a r k and light decay rates slow down, and that the surface potential retained b y the layer increases. F r o m these characteristics it is evident that the binder layers containing HgI2:CdS in the 60:40 composition dispersed in 60% by weight of polystyrene show the optimum characteristics for use in electrophotography. For this layer, the surface potential decays only by 5% in a period of 5 sec in dark, while during the same period the light decay is about 70% for 100 Ix white illumination. These values indicate that a good image contrast can be obtained, which could be further improved by exposing the layer to higher intensities of light. Fig. 1. Dark and light decay curves. Dark decay: (a) 100% Hgl2, (b) 60:40 Hgl2:CdS, (c) 100% CdS. Light decay: (1) 100% Hgl2, (2) 60:40 Hgl2:CdS, (3) 100% CdS. Binder percentage is 40 in all the samples.Fig. 2. Effect of ...