Ab initio prediction of the electronic and optical excitations in polythiophene:: Isolated chains versus bulk polymer

We calculate the electronic and optical excitations of polythiophene using the GW (G stands for one-electron Green function, W for the screened Coulomb interaction) approximation for the electronic self-energy, and include excitonic effects by solving the electron-hole Bethe-Salpeter equation. Two different situations are studied: excitations on isolated chains and excitations on chains in crystalline polythiophene. The dielectric tensor for the crystalline situation is obtained by modeling the polymer chains as polarizable line objects, with a long wavelength polarizability tensor obtained from the ab initio polarizability function of the isolated chain. With this model dielectric tensor we construct a screened interaction for the crystalline case, including both intra- and interchain screening. In the crystalline situation both the quasiparticle band gap and the exciton binding energies are drastically reduced in comparison with the isolated chain. However, the optical gap is hardly affected. We expect this result to be relevant for conjugated polymers in general.