Ab-initio calculation of quasi-particle bandstructure, exciton binding energies and dielectric properties of polythiophene

We use the ab-initio many-body GW method to calculate the quasi-particle spectrum of polythiophene. For the isolated chain, we find a large increase of the gap compared to DFT-LDA calculations (1.2 eV). The result (4.1 eV) exceeds experimental values, due to the absence of long-range screening in 1D. 3D screening will reduce the gap. With dielectric constants ε∥ and ε⊥, calculated semi-classically, εeff becomes 3.0 and the exciton binding energy is found to be about 1.0 eV; the exciton size is about 2 unit cells, i.e. 4 monomers.