Photoconductivity of Anthracene. III

Abstract

The photoconductivity of anthracene has been studied using three types of crystals : large scintillation crystals, crystals grown from dimethylformamide, and sublimation flakes. The existence of a true bulk conductivity as well as a surface conductivity has been demonstrated. This bulk conductivity is not affected by oxygen. The space charge found by previous workers has been shown to be located at the electrodes and to be due to poor contacts. The dependence of both the surface and the bulk photocurrent on temperature, voltage, and wavelength and intensity of light, have been studied and found to be markedly different from each other. The surface photocurrent shows a non-ohmic voltage dependence. It is concluded that the formation of charge carriers is not a primary process of light absorption, but a secondary process, competing with fluorescence and thermal degradation of the absorbed light energy, which is free to move through the crystal in the form of excitons. A mechanism for the migration of charge carriers has been put forward to explain the main features of the photoconduction. A much higher effective carrier mobility on the surface than in the bulk of the crystal is postulated. The behavior of the surface photocurrent is interpreted in terms of the formation of carriers in the bulk, those that diffuse to the surface playing the greater part in carrying the current. The bulk current is considered as a recombination-limited photocurrent. In agreement with previous workers, it is concluded that most of the current is carried by positive holes.