Intrinsic Photoconduction in Anthracene Crystals

Abstract

The dc photoconductance of ``pure'' anthracene crystals, anthracene crystals doped with tetracene and acridine, and ``pure'' anthracene melts in the uv region (2500–4000 Å) has been investigated. A novel conduction cell allowed crystals to be grown in situ between electrically conducting, semitransparent electrodes, and photocurrent measurements to be carried out at temperatures up to and above the anthracene melting point (>220°C). A photocurrent peak at ∼2800 Å, which was first reported by Castro and Hornig in pulsed-light experiments, has been confirmed. The photocurrent spectral-response data and the change in the spectral response, produced by adding the fluorescence quenchers tetracene or acridine to the anthracene, or by melting the crystals, demonstrate that 2800-Å light generates carriers by a direct optical transition to a conducting state of the crystal. The threshold for this direct ionization process (i.e., a band-to-band transition) occurs at ∼3100 Å or ∼4.0 eV. The quantum yield for carrier generation at 2800 Å is on the order of 10−4 or greater. Excitation in the 4000–3100-Å region generates carriers by a mechanism which involves singlet exciton states as intermediates, and the interaction of these excitons with surface or near-surface impurity sites. Some preliminary data on intrinsic photoconduction in pyrene crystals are also reported.