Abstract

It has been shown previously that in order to explain the electronic processes in a whole class of aromatic solids starting from the aromatic molecular systems and running through condensed aromatics, baked carbons, and polycrystalline graphite to perfect graphite crystals, the existence of an energy gap between the filled π band and the conduction band has to be assumed, the energy gap decreasing gradually with the increase of molecular dimensions and disappearing in the limit for large graphite crystals. This process of the gradual decrease of the energy gap was later investigated for pyrolyzed cellophane films heat treated up to temperatures of 800°C by study of the position of the long-wavelength limit of the absorption. It was found that the position of the absorption limit does in fact move from the visible into the infrared with increase in molecular size (increase of maximum heat-treatment temperature). The possibility of an exact control in the position of the infrared limit and furthermore the possibility of shifting it as far into the infrared as necessary should make such substances of great value as photodetectors. In this paper, preliminary results of experiments performed with thin films of condensed aromatic substances are reported. The presence of a photoresponse with a sensitivity limit roughly corresponding to the absorption limit has been established using square-wave illumination with frequencies up to somewhat more than 2000 cycles per second. The current shows saturation within about one-twentieth of a second. The amplitude of the fundamental is given as a function of pulse frequency in Fig. 2. The signal-to-noise ratio is found to be considerably inferior to that shown by commercial PbS photocells. The simplicity of such cells is remarkable, however, and might be a great asset for some special applications. Cells with infrared limits of photoresponse up to somewhat more than 5μ were obtained, the extension into the farther infrared being prevented by the increase in the conductivity of the films. The photoresponse seems to be a complicated process involving a combination of a photoelectric excitation with an effect of essentially bolometric nature.

© 1954 Optical Society of America

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