Electron Spin Resonance Study of Some Organic Photoconductors

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Abstract
In dye photoconductors which have a very high density of traps, such as rhodamine‐B and crystal violet, it is possible to build up a density of spins ∼1016/cm3 in strong illumination. The kinetics of rise and decay of these spins can be directly related to those of photoconductivity, and the spins are believed to be associated with trapped conduction electrons. Since the magnitude of the ESR signal is a measure of the number of electrons, the assumptions regarding mobility which have been made to account for the conductive behavior can be tested. In general, the conclusion that the mean or drift mobility has an exponential temperature dependence corresponding to a monoenergetic trap depth has been confirmed. The mean mobility of electrons is estimated to be ∼1× exp (— Et/kT) cm2/V·sec. The previously observed irregularities in the photoconductive behavior of rhodamine‐B arise from the existence of a considerable population of unpaired electrons which do not contribute directly to conduction, but interchange with those which do. Some salts of crystal violet which have appreciable semiconductivities were found to have spin densities ∼1017/cm3 in the dark.