Suppression of electron-hole correlations in 3D polymer materials.

We present an ab initio study of the optical absorption spectra of isolated as well as crystalline trans-polyacetylene. We include excitonic effects by solving the Bethe-Salpeter equation for the electron-hole two-particle correlation function. We observe that the strength of the electron-hole interactions drastically reduces when going from an isolated polymer chain to a crystalline arrangement. This is not only a result of enhanced screening in the 3D material, but also of the increased spatial extent of the exciton perpendicular to the polymer chains. We point out that these findings apply to crystalline phases of conjugated polymers and molecular crystals in general.