Hole Motion in Anthracene Crystals

Abstract

If a photoconductive crystal such as CdS is excited in a thin layer adjacent to one of the electrodes a rectifying effect is observed. When the electrode adjacent to the excited layer is made negative, larger currents are observed than with the reverse polarity [1]. Such an inhomogeneous excitation can be obtained from a beam of alpha particles or ultraviolet light ; both of which are almost completely absorbed in a region very close to the surface. This rectifier effect was explained by assuming the electrons are the more mobile carrier in CdS and that the excited charges will move into the remaining unexcited region of the crystal. A similar effect was observed in ZnCdS [2] powders. However, with organic photoconductive crystals the reverse behavior was observed [3]. The larger current was obtained when the excited layer was adjacent to the positive electrode. For all the organic crystals investigated, Sentner [4] finds large differences in currents for powders as well as single crystals. The ratio of the currents obtained when the excited layer is adjacent to the positive to when it is to the negative electrode, is almost 100. The conclusion drawn from these results is that the positive holes are the predominant carriers of photoconductivity in organic crystals. It is still an open question whether the weak current observed with negative polarity at the excited layer is due to a small mobility of negative charges or due to a weak excitation of the whole crystal by the fluorescent light emitted from the excited layer. That this light is partially absorbed by the crystal has been shown previously [5]. Further Sentner [4] has shown that visible light in a wavelength range of the fluorescent light produced photoconductivity but without differences due to polarity because of the homogeneous excitation throughout the crystal.

This program was supported by the US. Air Force through the Office of Scientific Research of the Air Research and Development Command.