Photoconductivity spectra of stable and metastable single‐crystals of perylene

Abstract

AbstractA study was made of the photoconductive properties of plate‐shaped single crystals of perylene with aqueous electrodes applied to both sides of the crystal. Two modifications have been investigated, a stable modification (α‐perylene) and a metastable one (β‐perylene). Photoconductivity spectra have been measured by illuminating through the positive electrode. The spectra of α and β‐crystals show analogous systems of vibronic bands with a wavelength shift of about 200 cm−1 between the two band systems. In the case of α‐crystals the conductivity spectrum duplicates the optical absorption spectrum. For β‐crystals the agreement between the conductivity spectrum and the reported absorption spectrum is less satisfactory.The shape of the photoconductivity spectra and various other evidence indicates that the origin of all observed photocurrents is at the surface of the crystal in contact with the positive electrode. Here excited perylene molecules react with the electrode to give a mobile charge carrier (hole). To account for the observed quantum yields of the photoconduction process it must be assumed that excitation energy can migrate (in the form of excitons) from the interior of the crystal to its surface (the same picture is generally accepted in the photoconduction of anthracene crystals). The quantum efficiencies obtained with β‐crystals were considerably higher than those for α‐crystals. This can be correlated with a difference in the mean diffusion paths of the excitons in the two modifications. The relatively short diffusion path in the dimeric α‐crystals is attributed to the formation of immobile charge‐transfer states.