Organic Photoconductors. V. A Model for Photoconductivity in Cationic Dyes

Abstract

It is shown that there is a close relationship between the excited state associated with the lowest optical transition of the molecule in solution and that associated with photoconductivity in the solid dye. The model considers the solid dye as a periodic array of potential wells having discrete bands of allowed energies corresponding to the ground state and excited state of the molecule. In the case of dyes whose conductive properties are strongly temperature‐dependent, it is assumed that an excited electron can move in the array by passing over the intermolecular barrier. Extension of this assumption to dyes in which the conductive properties depend only slightly on temperature leads to contradictions, and it is assumed in this case that electrons move through the array by tunneling. It is shown that in such a system the diffusivity will be very large and the mobility very small, and that the Einstein diffusivity‐mobility relationship does not hold. The model has properties which are in accord with those observed for the two classes of dyes.