Drift Mobilities of Holes and Electrons in Naphthalene Single Crystals

The drift mobilities of holes and electrons in very pure single crystals of naphthalene were experimentally determined by measurement of transit times of transient space-charge-limited currents. The room-temperature values are in general agreement with published data except for holes in the ${\overrightarrow{\mathrm{c}}}^{\prime }$ direction where a larger value of 0.99 ${\mathrm{cm}}^2$/V sec was measured. In the temperature range 220-300 °K, the mobilities of holes and electrons were found to vary as $T^{-2.1\mathrm{}}$ and $T^{-0.9\mathrm{}}$, respectively, in the ${\overrightarrow{\mathrm{c}}}^{\prime }$ direction, while the mobility of holes was found to vary as $T^{-1.0\mathrm{}}$ for the a crystal direction and as $T^{-0.8\mathrm{}}$ for the $\overrightarrow{\mathrm{b}}$ crystal direction. While these results are consistent with band calculations based on the tight-binding approximation, the mobilities of electrons in the $\overrightarrow{\mathrm{a}}$ and $\overrightarrow{\mathrm{b}}$ crystal directions were found to be essentially independent of temperature, and hence cannot be accounted for by band theory. The experimental results are compared with predictions of the band and hopping models, and it appears that the transport mechanism is intermediate between these two modes of transport.