Negative Differential Photovoltages in ZnS Crystals

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Abstract
The intensity dependence of the anomalous photovoltaic effect was measured on ZnS crystals at room temperature and at 77°K. A decrease in photovoltage caused by an increase in illumination intensity was observed in many crystals. For all the investigated crystals, the short-circuit current was found to increase linearly with light intensity. These results are explained in terms of the general theory of photovoltaic effects. It is shown that a decrease in photovoltage caused by an increase in carrier concentrations is to be expected to occur in photovoltaic effects observed under the following conditions: (1) sample is under high-level excitation so that the thermal equilibrium carrier concentrations are negligible in comparison with the excess carrier concentrations. (2) The conditions at both contacts are identical (i.e., no contact effects). The decreases in photovoltages can be explained by the tendency of all the elementary photocells to saturate at high carrier concentrations. These considerations also furnish an explanation for a shift in the wavelength of photovoltage reversal, which was observed in many crystals. Various models for the explanation of the anomalous photovoltaic effect in ZnS crystals are discussed, with reference to the results presented.