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The physical phenomena controlling the operation of xerographic photoreceptors are reviewed with special emphasis on the changes required by the realization of the importance of field controlled photo-generation. Several transport-limited models for photoreceptor sensitivity are analyzed. A comparison is made of photoreceptors in which only one carrier moves with photoreceptors in which both carriers move easily. The factors affecting residual potential buildup, dark decay, and charging are also considered. The effects of traps are included in each of the sections.

A model has been developed which adequately describes the sublinear photoresponse to strongly absorbed light in sensitized CdS. This model does not involve electron or hole trapping but rather postulates monomolecular bulk recombination and bimolecular surface recombination. Sublinear photocurrents are produced in this model whenever the ambipolar diffusion length is more than five times the penetration depth for the radiation. The results of measurements on sensitized CdS for temperatures between 90°K and 300°K will be presented which agree very well with this model when the electron and hole mobilities and hole lifetime determined by Spear and Mort are used as parameters in the model. The model also predicts many features of the photoresponse of sensitized CdS to weakly absorbed light, and provides a quantitative correlation between the bulk sensitivity, the surface recombination rate, and the peak in the photoresponse observed at the absorption edge. For the samples investigated, copper-compensated sensitized CdS platelets, the ambipolar diffusion length was found to be 6.2X10 -4 cm at 300°K.

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