Photogeneration of polaron pairs in conducting polymers.

Abstract

It is usually assumed that when photogeneration in a conducting polymer results in an electron and hole on separate chains they form negative and positive polarons that can move independently of each other. We show, on the basis of the small carrier yield per photon seen in picosecond photoconductivity, the different behavior of photoinduced absorption (PA) in dilute solution and thin films, and the spectral distribution of the PA in thin films, that photogenerated positive and negative polarons in poly(p-phenylene vinylene), polythiophene, and polyacetylene are, for the most part, bound in pairs by their Coulomb attraction. We also show that PA data give evidence for a gap of 2.8 eV, and thus an exciton binding energy of 0.4 eV, in poly(p-phenylene vinylene).