A photo-e.m.f. effect is observed in Tl<sub>2</sub>S 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<sup>-6</sup> 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 shortcircuit-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 <i>increases with decreasing</i> 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.
FOSTER C. NIX and ARNOLD W. TREPTOW, "A Thallous Sulphide Photo-e.m.f. Cell," J. Opt. Soc. Am. 29, 457-462 (1939)