Dark Conductivity of Cationic Dye Photoconductors

A theory is put forth to account for the dark conduction in cationic dyes, which assumes thermal excitation from ground state to conductive excited state, followed by trapping of the hole at the anion. It is applicable in principle to any case in which charge carriers are separated by an intrinsic thermal process which is followed by deep trapping of the minority carrier. Where such processes take place, measurements of thermally excited conduction will in general lead to one of three results when interpreted by simple equilibrium theory: the correct activation energy will be found, but the mobility inferred will be impossibly high; or, the mobility will be of the correct order of magnitude, but the activation energy will be too low; or both will be incorrect. The theory is applied to data for dark conduction in eight triphenylmethane dye halides; it is shown to account for the observed thermal activation energies in terms of optical energy levels, and to lead to mobilities which are similar and of acceptable magnitude.