Evidence for the Triplet State in Photoconductivity in Anthracene

Publisher Website
Google Scholar
Abstract
Some evidence is presented indicating that the formation of a triplet state is involved as an intermediary step in the creation and transfer of charges in the photoconduction process in organic substances. On the basis of the simple potential curve picture, approximate solutions are obtained to the rate process equations. These equations predict that the density of the triplet states under thermal and optical equilibrium conditions will follow a Boltzmann exponential dependence upon temperature with an energy equal to a low multiple (1 or 2) of the vibrational spacing of the excited singlet state. The assumption is made that the photocurrent is proportional to the triplet state density. Measurements by the author and others of the photoconduction activation energy in a number of aromatic hydrocarbons are in very good agreement with this. The triplet state model also requires that the buildup and decay of the photocurrents be approximately monomolecular with time constants at low temperatures and intensities equivalent to the lifetime of the triplet state. The experimental results corroborate this. The model also suggests that direct absorption into the triplet state should cause photoconduction. The experimental results with anthracene are not conclusive on this point. The model of photoconduction in organic material is predicated on a molecular rather than an extended band model approach. The major contribution to the photocurrent comes from molecules which have undergone a spin conversion while in the excited state to the triplet state of the molecule. The long lifetime of this metastable state and the assumed partial localization of the uncoupled electrons increases the probability of an intermolecular quantum tunneling of the positive hole in the ground state orbital.

This publication has 18 references indexed in Scilit: