Photo-Conductivity in Photographic Systems I. Dye-Sensitization of Photo-Conductivity

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Abstract
Gelatino‐silver halide photographic films increase in electrical conductivity on exposure to light at intensities corresponding to exposures within the normal photographic range, and above. The spectral distribution of this change in conductivity in films containing, respectively, photographic sensitizing dyes, and no dye, is found to be parallel with the spectral distribution of photographic sensitivity— the wave‐lengths of maximum and minimum sensitivity in the two effects are identical and coincide with absorption maxima and minima, and there is a general parallelism in the efficiency of a dye in sensitizing the two effects to light not absorbed by the silver halide. The conclusion is drawn that the observed changes in conductivity represent a photo‐conductivity originating in the photo‐liberation of electrons within the silver halide, the primary act of the photographic process. The photo‐currents are proportional to the applied field up to strengths of at least 140 volts per millimeter, and at intensities of illumination within the normal photographic range and somewhat above, are proportional to the intensity. At still higher light intensities, the photo‐currents verge towards a dependence on the square root of the intensity. The change in the intensity relation is interpreted as representing the passage from a situation, at low intensities, in which most of the electrons are captured by foreign traps to that, at high intensities, in which recombination with bromine atoms predominates. It is shown that a sensitizing dye acts by causing the liberation of an electron within the conduction band of the silver halide under the stimulus of light absorbed by the dye but whether the process involves direct electron‐transfer or energy‐transfer is left open.

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