Thermally Stimulated Currents and Carrier Trapping in Anthracene Crystals

Abstract

Thermally stimulated currents in anthracene single crystals have been investigated. A fairly large current peak observed at a temperature near 0°C, is shown to be characteristic of anthracene and corresponds to a carrier trap level of depth 0.76±0.05 eV. The effects of temperature, applied voltage, and the time of illumination with uv light on trap filling were investigated. Illumination at low temperatures, with or without an applied field, generates charge carriers which are localized in the crystal. Below —60°C the rate of carrier release and/or recombination appears negligibly small, permitting charge storage for indefinitely long periods of time. In addition to thermal detrapping, it was found that carrier detrapping could also be brought about by optical radiation with wavelengths in the visible and near infrared region. The resulting optically stimulated currents depended linearly on the intensity of the long wavelength light. The results suggest that on uv illumination at low temperatures ion pairs are generated at dissociation centers, the electron being deeply trapped by the center. The liberated holes (majority carriers) are partially mobile at low temperatures and are eventually localized in traps. The observed thermally stimulated currents are attributed to thermal release of the majority carriers.