Volume Generation and Hall Mobility of Holes in Anthracene

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Abstract
The reversible chemisorption of iodine vapor on anthracene is shown to produce a space charge layer of trapped holes extending from 2 × 10−4 to 2 × 10−3 cm below the surface. The surface iodine level lies approximately 0.73 eV above the valence band. The Hall effect for free holes, generated in this layer by weakly absorbed light and moving parallel to the surface, was measured for currents along the b axis and magnetic field along the c′ axis. For eight samples, p-type signals were measured with mobilities 8–15 cm2 V−1·sec−1. The theoretically predicted sign reversal was not found. For both electrons and holes generated by strongly absorbed light in the singlet band, the measured mobilities exceed 20 cm2 V−1·sec−1. The order-of-magnitude difference between Hall mobilities in the region of generation and drift mobilities in the bulk (1 cm2 V−1·sec−1) is compatible with the existence of both narrow and broad bands for electrons and holes. The wide band mobility is calculated to be of the order of 200 cm2 V−1·sec−1. In a time-of-flight experiment, only the low-mobility component is observed. These results also eliminate the hypothesis that the Hall effect may be anomalous in the narrow bands only.