The Photo-Voltaic Effect

Author: J.B. Hoag

In contrast with photo-emissive tubes and photoconductive cells, which require a battery for their operation, photo-voltaic cells in themselves act like a battery when they are exposed to light. In one of the common commercial forms, the Weston Photronic cell, a thin film of properly annealed selenium is formed on a thick base of iron. When light passes through the thin layer of selenium and reaches the transition region between the two metals, it causes electrons to move from the iron to the selenium. In the conventional sense, the iron serves as the positive and the selenium serves as the negative terminal of a battery.

Fig. 20 J. Spectral distribution curves of photronic cells A, compared with the response (B and C) of the human eye to different colors. (From E. & N. P.)

Figure 20 J shows the output of this type of cell for different colors of light, all of the same intensity. Photronic cells can send as much as one-quarter of one milliampere through a low resistance load, even when the illumination is quite moderate, and hence can be used to operate fairly rugged meters and relays directly, without the use of vacuum tube amplifiers. Photoemissive cells have a high internal resistance but give only small current changes and require extremely delicate meters or vacuum tube amplifiers. On the other hand, photovoltaic cells have comparatively low internal resistance (500 to 6,000 ohms), and deliver quite large currents into low resistance loads. Photoconductive cells are usually intermediate between the other two cells.

If the external resistance or load of a photovoltaic cell is sufficiently low, say less than 100 ohms, the output currents are found to be directly proportional to the intensity of the incident light. But when the external resistance is greater than a few hundred ohms, and especially when the light is very bright, the output current falls short of the value it might be expected to have. This is due to a leakage of current in the cell itself. If the light is alternately turned on and off at increasing frequencies, the current is increasingly lost in the cell itself, due to its so-called internal capacity. The output becomes negligible at a few thousand cycles per second. Hence this type of cell cannot be used in " talkies."