Electronic properties of aromatic hydrocarbons. IV. Photo-electric effects

Reliable measurements of photoconductivity in aromatic hydrocarbons are not easy to make, owing to rapid photo-oxidation in air and spurious space-charge effects with certain electrode systems. A technique of growing thin single crystals sealed between glass flats is described which overcomes both difficulties. The magnitude of the photoconductivity observed with these specimens does not vary greatly from one pure hydrocarbon to another. Absorption of light in the fundamental band of an aromatic hydrocarbon crystal results in fluorescence as well as photoconductivity. The light produces excitons which may revert to the ground state with emission of fluorescence, but do not transport charge. Additional energy is needed to generate photocarriers by ionizing the excited molecules, and it is not obvious where this energy comes from. An explanation based on the intensity dependence of photocurrent attributes the carriers to the decay of excitons, but unlike the unimolecular decay governing the fluorescence emission the interaction of two excitons is required to produce a single ionized molecule. Twice the fundamental energy thus becomes available to ionize one molecule. The theory requires photoconductivity and fluorescence to be directly proportional to the exciton density, so that both should be quenched equally by impurities which trap excitons. It was confirmed experimentally that the addition of an impurity which quenches the fluorescence of a hydrocarbon also reduces the photocurrent in the same ratio. This is true over a wide range of concentration and for impurity molecules with different quenching efficiencies. Using quantities derived in earlier parts of this series an approximate value of 5 x 10-3 cm2 V-1 S-1 is found for the mobility of the carriers in anthracene.