A photo-e.m.f. effect is observed in Tl2S produced by partial sulphurization of a thallium disk or of an evaporated thallium layer. The photosensitivity is observed to be as high as 6000·10−6 ampere/lumen. Both open-circuit-voltage and short-circuit-current are observed in some cells to be directly proportional to the incident light intensity at all temperatures. In other cells, especially those displaying high photo-response, the open-circuit-voltage and short-circuit-current tend to saturate at high light intensities. The spectral photosensitivity curve possesses a maximum at 9800A±200A with long and short wave thresholds at 15,000 and 6500A, respectively. The short-circuit-current increases with decreasing temperature down to about −45°C and then decreases continuously down to −180°C. The position of the maximum varies somewhat from cell to cell. The photoelectric output of the cells when measured as a function of frequency of interruption of the incident light displays a recession of 75 percent at 20,000 cycles.
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