Mechanism of Photoconductivity in Microcrystalline Powders

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Abstract
Measurements on a variety of cadmium sulfide powder cells show that there are four different regions encountered as the applied voltage is varied. At low voltages an ohmic region is found in which barrier breakdown does not occur. At intermediate voltages a region is found in which barrier breakdown by a Process I occurs. At still higher voltages a region is found in which barrier breakdown by another Process II occurs in addition to that associated with Process I. At very high voltages another ohmic region is found where all barrier limitations have been removed. It is proposed, on the basis of the data, that Processes I and II are associated either with tunneling of electrons through interparticle barriers or with a reduction in barrier height assisted by the localized trapping of photo-excited or field-excited holes. A model constructed along these lines is applied to a variety of observations such as current-voltage curves, photocurrent lag after periods of no applied voltage or of opposite-polarity applied voltage, the existence of a semipermanent polarized set, ac properties of powders, and the hysteresis or storage effect.