Electronic and optical excitations in crystalline conjugated polymers

We calculate the electronic and optical excitations of crystalline polythiophene and poly(phenylene vinylene), using the GW approximation for the electron self-energy and including excitonic effects by solving the electron-hole Bethe-Salpeter equation. Surprisingly, we find optical gaps and exciton binding energies that are much smaller than experimental values and our earlier calculations on chains embedded in a dielectric medium. We attribute the disagreement to the absence of quantum mechanical coherence in most experimental situations and discuss possible reasons for this absence. An aggregate-forming ladder-type PPP appears to be an interesting exception. © 2003 Elsevier Science B.V. All rights reserved.