Photocarrier generation mechanisms in anthracene crystals under ultra-high vacuum

Abstract

A detailed investigation of the transient photocurrents generated in anthracene single crystals has been undertaken. The variations of the measured currents with the wavelength and intensity of the incident light, the applied voltage, and the temperature of the crystal are given for crystals both before, and after, subjection to ultra-high vacuum. Removal of the surface by evaporation is seen to produce large changes in the observed photocurrents. A biphotonic generation process for both hole and electron currents at high light intensities is observed after prolonged evacuation. Theoretical considerations show this biphotonic process to be singlet-exciton/singlet-exciton collision annihilation with a rate constant of 1.8 x 10^-18±0.5 m³/s. Surface generation of holes is shown to decrease upon evacuation and to grow on readmission of oxygen. After prolonged evacuation, the bi-excitonic mechanism for hole generation almost completely swamps the "extrinsic" generation at the surface. However, the electron current becomes predominantly monophotonic with evaporation of the surface.