Kinetics of Photocurrent Rise and Decay in Molecular Crystalline Materials

Abstract

Vapour-grown crystals of tetracyanoquino-dimethane showed photocurrents whose rise and decay were first-order processes with lifetimes of 1.1 and 1.2 ms, in good agreement with the value of 1.2 ms deduced from variable-frequency chopped-light measurements. Solution-grown crystals gave longer lifetimes (4.2 ms) and smaller photo⁻ and dark-currents. For lead phthalocyanine single crystals the decay of photocurrent was slower than the rise in vacuum and ammonia but particularly in nitrogen dioxide. Both rise and decay involved at least two first-order processes whose rates increased as the applied field was increased. These observations are discussed in terms of effects of carrier trapping and de-trapping on rise and decay rates respectively. This interpretation is also supported by activation energies which were deduced from temperature dependence of rise and decay rates to be 0.08 eV and 0.22 − 0.28 eV respectively, and by the form and magnitude of transients observed in pulsed-light drift mobility experiments. The importance of these measurements in interpreting the effects of adsorbed gases on electronic processes in molecular crystals is discussed.