CONDUCTION IN LADDER POLYMERS

Ladder polymers (i.e., conjugated polymers with at least two conduction paths in parallel) may-like the well-known polymers with a single conjugation path-be doped to high conductivity. Ladder polymers have been found to have the advantages of greater thermal stability and mechanical strength. It has been suggested that the existence of multiple conjugation paths would also result in higher conductivity because carriers could bypass a defect on one of the chains. In this paper we calculate the transmission of electrons through a site defect on a simple ladder polymer and compare it to that for a single-strand polymer. A noteworthy feature of the ladder-polymer transmission is the existence of scattering between wave functions of different symmetry belonging to different conduction bands. This scattering occurs also for incident-electron energies at which the different bands do not overlap, resulting in exponentially decaying portions of the transmitted and reflected waves. For a site defect in the simple-ladder polymer that we study, we find that, in general, the transmission is decreased. The decrease, however, is not as much as it is for the same defect in a single-strand polymer.