The Trapping of Photo-Carriers in Anthracene Crystals by Tetracene

The trapping of free electrons and holes in anthracene single crystals doped with tetracene has been measured by the pulsed photoconduction method. Tetracene behaves as a shallow hole trap located 0.40 eV ±0.02 eV above the hole band. At the same time it also acts both as a shallow and a deep electron traps. The shallow electron trap is located at 0.17±0.02 eV below the electron band. Both for electrons and holes the capture cross-section of shallow traps is proportional to the tetracene content. The quantum efficiency for photo-carrier generation in anthracene does not change by the existence of tetracene which is a remarkable quencher for photo-fluorescence. From this result it is deduced that the photo-carriers are generated by multi-exciton processes, and that the generation of carriers at the surface interacting with diffusing-out singlet excitons makes negligible contribution for photoconduction.