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.

Figure 1

Electronic band structure of crystalline polyacetylene along high symmetry directions in the Brillouin zone. Panels (left) and (right) correspond to the $P{2}_1/a$ and $P{2}_1/n$ crystal structures, respectively.

Figure 2

The imaginary part of the DF for $a$-PA (left panel) and $n$-PA (right panel) for light polarized parallel to the polymer chain. The dashed lines are the RPA spectra; the solid lines correspond to the spin singlet excitations. The energy scale has been shifted with respect to the quasiparticle gap $E_0$ indicated by the vertical dotted line.

Figure 3

The electron distribution with respect to the hole (cross) of the $S_4$ state of $a$-PA in a plane perpendicular to the polymer chains and through a C atom. The unit cell axis $a$ (vertical) and $b$ (horizontal) are shown by dashed lines, and the projections of the C polymer backbone are indicated by the short lines.